Sustainable hydrogel-based 3D printing ink incorporating biowaste and food waste powder

Abstract

The invention provides a method for providing a 3D-printed structure (2000), wherein the method comprises: (A) providing a gelled printing composition (1000); wherein the gelled printing composition (1000) comprises a first particulate material (110), a first polymer material (120), a first crosslinker (130), and a first solvent (140); wherein: (a) the first particulate material (110) comprises a first biological material; wherein at least 50% of the first particulate material (110) has a particle size (Dp) selected from the range of 6-500 pm; wherein the gelled printing composition (1000) comprises the first particulate material (110) in a particulate concentration (Cp) of 1-20 wt%; (b) the first polymer material (120) comprises one or more of an alginate-based polymer and a gelatin-based polymer; wherein the gelled printing composition (1000) comprises the first polymer material (120) in a polymer concentration (CM) of 2-15 wt%; and (c) the first crosslinker (130) is configured to partially crosslink the first polymer material (120); wherein the first polymer material (120) and the first crosslinker (130) are dissolved in the first solvent (140); (B) 3D-printing the gelled printing composition (1000) with a 3D-printer (400) to provide a precursor structure (210); wherein the 3D-printer (400) comprises a nozzle (410); wherein the nozzle (410) has a nozzle diameter (Dn) selected from the range of 0.4- 1.2 mm; wherein 3D-printing the gelled printing composition ( 1000) comprises extruding the gelled printing composition (1000) via the nozzle (410) at (i) a printing pressure (Pi) selected from the range of 40-250 kPa, (ii) a printing temperature (Tp) selected from the range of 18-30 °C, and (iii) a printing speed selected from the range of 4-12 mm / s; and (C) crosslinking the precursor structure (210) to provide a crosslinked structure (220); wherein crosslinking the precursor structure (210) comprises one or more of (i) exposing the precursor structure (210) to a second crosslinker (150), and (ii) exposing the precursor structure (210) to radiation (901); and removing at least part of the first solvent (140) from the crosslinked structure (220) to provide the 3D-printed structure (2000).

Description

[0001] Sustainable hydrogel-based 3D printing ink incorporating biowaste and food waste powder

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method for providing a 3D-printed structure. The invention further relates to a 3D-printed structure. Additionally, the invention relates to an object comprising the 3D-printed structure. Further yet, the invention relates to a use of the 3D-printed structure.

[0004] BACKGROUND OF THE INVENTION

[0005] 3D-printed structures are known in the art. For instance, Sauerwein et al. (2020), “Reprintable paste-based materials for additive manufacturing in a circular economy”, Sustainability, 12(19), 8032 describes the development of a reprintable bio-based composite material for extrusion paste printing. The material is derived from natural and abundant resources, i.e., ground mussel shells and alginate. The alginate in the printing paste is ionically cross-linked after printing to create a water-resistant material. The reaction can be reversed to retain a printable paste.

[0006] EP4233925A1 describes a bioink comprising coral particles and biocompatible polymer, wherein the concentration of coral particles in the bioink is 25 to 85 weight % of the total weight of the bioink.

[0007] WO2018169965A1 describes a biodegradable and biocompatible three dimensional construct comprising a combination of a nano silicate (e.g., laponite) and two different polymers, the two polymers each individually providing at least one covalently linked polymer chain and at least one ionically linked polymer chain, the polymeric chains forming a dual strengthening intertwined polymeric system. The bioink may also comprise cells or combination of cells.

[0008] EP3427697A2 describes computer implemented methods of producing a porous implant, including obtaining a 3-D image of an intended tissue repair site; generating a 3-D digital model of the porous implant based on the 3-D image of the intended tissue repair site. The method also includes determining an implant material and an amount of a porogen to add to an implant material to obtain a desired porosity of the porous implant. The desired porosity is based on a combination of macropores, micropores and / or nanopores structures. The 3-D digital model developed is stored on a database coupled to a processor, wherein the processorhas instructions for combining the implant material with the porogen based on the stored 3-D digital model and for instructing a 3-D printer to produce the porous implant.

[0009] SUMMARY OF THE INVENTION

[0010] 3D-printing has emerged as a new technique for the rapid production of components and products in a wide range of industries. 3D-printing may provide the benefit that complex shapes may be produced relatively easily, with the option to make small adjustments to the products without the need to, e.g., produce a new mold. There may be several types of 3D-printing processes, such as fused deposition modeling (FDM) and direct ink writing (DIW). Both FDM and DIW may print by extruding a 3D-printable material from a nozzle, however, DIW may use lower extrusion temperatures compared to FDM. Especially, DIW may allow 3D-printing at room temperature, and may thus require less energy than FDM.

[0011] The main materials used for 3D-printing may be (artificial or synthetic) polymers, such as polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS). However, in recent years, a trend towards the use of more natural materials may be observed, wherein the natural materials are preferably biodegradable, durable, and originating from sustainable sources and / or waste streams. For instance, prior art may describe a water-based paste with alginate as a binder and filler particles derived from ground and sieved natural materials, primarily mussel shells. The water-based paste was printed into an object using DIW, and the alginate was crosslinked after printing by ionic crosslinking using calcium ions. After drying of the crosslinked structure, during which time the crosslinked structure decreased in volume due to water evaporation, a solid object was obtained. However, such prior art water-based pastes (or “inks”) for DIW 3D-printing may have a relatively low viscosity, such that the layers deposited with DIW may start to spread prior to crosslinking. Hence, such inks may not be suitable for providing detailed or precise structures (comprising e.g. fine features). Further, as such inks may have a relatively low viscosity, the inks may not be suitable for printing larger (especially taller) structures, as the deposited layers may start to collapse under their own weight, thereby distorting the shape of the final product. Hence, it is an aspect of the invention to provide an alternative method for providing a 3D-printed structure, which preferably further at least partly obviates one or more of above-described drawbacks. The present invention may have as object to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0012] According to a first aspect, the invention provides a method for providing a 3D-printed structure. The method may comprise providing a gelled printing composition.Especially, the gelled printing composition may comprise a first particulate material, a first polymer material, a first crosslinker, and a first solvent. In embodiments, the first particulate material may comprise a first biological material. Further, at least 40%, such as at least 50%, of the first particulate material may have a particle size (Dp) selected from the range of 6-500 pm. In embodiments, the gelled printing composition may comprise the first particulate material in a particulate concentration (Cp) of 0.5-25 wt%, such as 1-20 wt% (relative to the total weight of the gelled printing composition). Further, the first polymer material may comprise one or more of an alginate-based polymer and a gelatin-based polymer. The gelled printing composition may comprise the first polymer material in a polymer concentration (CM) of 1-17 wt%, such as 2-15 wt%. Further, the first crosslinker may be configured to (at least) partially crosslink the first polymer material. In embodiments, the first polymer material and the first crosslinker may be (configured) (at least partially) dissolved in the first solvent. The method may further comprise 3D-printing the gelled printing composition with a 3D-printer to provide a precursor structure. The 3D-printer may comprise a nozzle. In embodiments, the nozzle may have a nozzle diameter (Dn) selected from the range of 0.2- 1.5 mm, such as from the range of 0.4- 1.2 mm. Further, 3D-printing the gelled printing composition may comprise extrading the gelled printing composition via the nozzle at (one or more of) (i) a printing pressure (Pi) selected from the range of 20-300 kPa, such as from the range of 40-250 kPa, (ii) a printing temperature (Tp) selected from the range of 15-35 °C, such as from the range of 18-30 °C, and (iii) a printing speed selected from the range of 2-15 mm / s, such as from the range of 4-12 mm / s. The method may further comprise (at least partially) crosslinking the precursor structure to provide a crosslinked structure. Especially, crosslinking the precursor structure may comprise one or more of (i) exposing the precursor structure to a second crosslinker, and (ii) exposing the precursor structure to radiation. Further, the method may comprise removing at least part of the first solvent from (i.e., drying) the crosslinked structure to provide the 3D-printed structure. Hence, in specific embodiments, the invention may provide a method for providing a 3D-printed structure, wherein the method comprises: (A) providing a gelled printing composition; wherein the gelled printing composition comprises a first particulate material, a first polymer material, a first crosslinker, and a first solvent; wherein: (a) the first particulate material comprises a first biological material; wherein at least 50% of the first particulate material has a particle size (Dp) selected from the range of 6-500 pm; wherein the gelled printing composition comprises the first particulate material in a particulate concentration (Cp) of 1-20 wt%; (b) the first polymer material comprises one or more of an alginate-based polymer and a gelatin-based polymer; wherein the gelled printing compositioncomprises the first polymer material in a polymer concentration (CM) of 2-15 wt%; and (c) the first crosslinker is configured to partially crosslink the first polymer material; wherein the first polymer material and the first crosslinker are dissolved in the first solvent; (B) 3D-printing the gelled printing composition with a 3D-printer to provide a precursor structure; wherein the 3D-printer comprises a nozzle; wherein the nozzle has a nozzle diameter (Dn) selected from the range of 0.4- 1.2 mm; wherein 3D-printing the gelled printing composition comprises extruding the gelled printing composition via the nozzle at (i) a printing pressure (Pi) selected from the range of 40-250 kPa, (ii) a printing temperature (Tp) selected from the range of 18-30 °C, and (iii) a printing speed selected from the range of 4-12 mm / s; and (C) crosslinking the precursor structure to provide a crosslinked structure; wherein crosslinking the precursor structure comprises one or more of (i) exposing the precursor structure to a second crosslinker, and (ii) exposing the precursor structure to radiation; and removing at least part of the first solvent from the crosslinked structure to provide the 3D-printed structure. Such a method may provide the benefit that a viscosity of the gelled printing composition may be relatively high, such that 3D-printed shapes and layers may hold their shape. Hence, such a method may facilitate 3D-printing complex shapes with fine features, e.g. for low tolerance manufacturing purposes. Especially, the gelled printing composition may be in a gel-form, wherein the gel-form may be a result of a partial crosslinking of a (precursor) printing composition or of the rheological properties of the first polymer material used (see also below). Further, crosslinking the precursor structure may provide a solid and stable structure which may be applied in e.g. agriculture or packaging. Such a method may further provide the benefit that the first particulate material and the first polymer material may (both) be biological (such as natural) materials, wherein the first particulate material may be selected from a waste stream material, thereby providing a sustainable gelled printing composition and a sustainable method for providing a 3D-printed structure.

[0013] Hence, the method may comprise providing a gelled printing composition. The gelled printing composition may comprise a first particulate material. The first particulate material may be configured embedded in the gelled printing composition. In embodiments, the first particulate material may comprise a first biological material. Further, the first particulate material may comprise a natural material. Especially, the first particulate material may comprise (as the first biological and / or natural material) one or more of wood chips, ground pit material, ground seed material, and ground nut shell material. Herein, the term “wood chips” may refer to (small) pieces of wood, preferably obtained from a waste stream. For instance, the wood chips may comprise, such as be, one or more of sawdust, (ground) wood shavings, and(ground) waste wood, e.g. from the production of wood products, construction, demolition, or transport (e.g. waste pallets). Further, the terms “ground pit material” and “ground seed material” may refer to particulate materials obtained by grinding the (dried) pits and / or seeds of trees, angiosperms, and gymnosperms. The terms “seed” and “pit” may be used interchangeably in general language usage, e.g. when referring to an avocado seed or pit. For instance, the ground pit material and / or ground seed material may comprise one or more of a ground olive pit (or seed), a ground date seed (or pit), a ground avocado seed (or pit), a ground peach pit (or seed), a ground apricot pit (or seed), a ground cherry pit (or seed), a ground nectarine pit (or seed), a ground mango pit (or seed), a ground plum seed (or pit), a ground (shell of a) argan pit (or seed), a ground apple seed (or pit), a ground (watermelon seed (or pit), a ground (shell of a) pumpkin seed (or pit), a ground papaya seed (or pit), and a ground (shell of a) sunflower seed (or pit). Of course, other types of ground pit material and / or ground seed material are herein not excluded. For instance, the ground pit material and / or ground seed material may comprise ground coffee beans, optionally originating from a waste stream (of e.g. the coffee industry). Further, the term “ground nut shell material” may refer to a particulate material obtained by grinding the shells surrounding nuts (or fruits). For instance, the ground nut shell material may comprise one or more of a ground almond shell, a ground walnut shell, a ground pistachio shell, a ground peanut shell, a ground macadamia shell, a ground hazelnut shell, a ground pecan shell, a ground Brazil nut shell, a ground coconut shell, a ground cashew shell, a ground pine nut shell, and a ground acorn shell. Hence, in specific embodiments, the first particulate material may comprise one or more of wood chips, ground pit material, ground seed material, and ground nut shell material. Such first particulate materials may generally originate from waste streams of the food industry, construction, demolition, transport, and / or production. Hence, using such first particulate materials may facilitate providing a valuable product from waste materials, thereby reducing the generation of waste and the need for (non-sustainable) fresh material for the production of a 3D-printable materials.

[0014] Additionally to the one or more of wood chips, ground pit material, ground seed material, and ground nut shell material, the first particulate material may comprise a ground mineral-based material. For instance, the ground mineral-based material may comprise one or more of ground gastropod shells, ground Bivalvia shells, ground scaphopod shells, ground monoplacophora shells, ground coral, ground pearls, ground (dried) clay, ground gypsum, ground speleothems, ground sedimentary rocks (e.g. limestone), and ground igneous rock. Especially, the ground mineral-based material may comprise ground mussel shells. The first particulate material may consist for < 50 vol%, such as < 30 vol%, especially < 10 vol%, of theground mineral-based material, with the remainder of the first particulate material comprising one or more of wood chips, ground pit material, ground seed material, and ground nut shell material. In other words, of the plurality of (first) particles in the first particulate material (see also below), < 50 vol, such as < 30 vol, especially < 10 vol, of the (first) particles may comprise, such as originate from, ground mineral-based material. Especially, 100 vol% of the first particulate material may consist of, and / or 100% of the (first) particles may originate from, one or more of wood chips, ground pit material, ground seed material, and ground nut shell material.

[0015] The first particulate material may comprise, especially consist of, a plurality of first particles. In embodiments, each of the first particles may have a particle size (Dp). Further, the first particulate material may have the particle size (Dp). The particle size (Dp) may especially be determined (such as measured) using scanning electron microscopy (SEM). Hence, the first particulate material may have a particle size (Dp), wherein the particle size (Dp) may be determined using SEM. In embodiments, at least 40%, such as at least 50%, especially at least 60%, like at least 70%, of the first particulate material (i.e., of the plurality of first particles) may have a particle size (Dp) (individually) selected from the range of 3-600 pm. Further, at least 40%, such as at least 50%, especially at least 60%, like at least 70%, of the first particulate material (i.e., of the plurality of first particles) may have a particle size (Dp) (individually) selected from the range of 6-500 pm. Further yet, at least 40%, such as at least 50%, especially at least 60%, like at least 70%, of the first particulate material (i.e., of the plurality of first particles) may have a particle size (Dp) (individually) selected from the range of 10-400 pm. Yet, in embodiments, at least 80%, such as at least 90%, especially at least 95%, including (essentially) 100%, of the first particulate material (i.e., of the plurality of first particles) may have a particle size (Dp) (individually) selected from the range of 6-500 pm. Herein, phrases like “at least x% of the first particulate material may have a particle size (Dp) selected from the range of’ may especially indicate that at least x% of the (first) particles of the first particulate material may have a particles size (Dp) selected from the indicated range.

[0016] In embodiments, the first particulate material may be (relatively) homogeneous. In such embodiments, the first particulate material may have a number average particle size (Dpa), wherein at least 66% of the first particulate material may have a particle size selected from the range of 0.8*Dpa- 1.2*Dpa, such as from the range of 0.9*Dpa- l.l*Dpa. Yet, alternatively, the first particulate material may have a (relatively) large distribution in particle size (Dp). In such embodiments, the first particulate material may have a number average particle size (Dpa), wherein at most 66% of the first particulate material may have a particle size selected from the range of 0.8*Dpa- 1.2*Dpa, such as from the range of 0.7*Dpa- 1.3*Dpa.Further, in embodiments, for a particle size (Dp, min) of a smallest first particle of the first particulate material and a particle size (Dp,max) of a largest first particle of the first particular material may apply that Dp,max / Dp,min > 10, such as Dp,max / Dp,min > 15, especially Dp max / Dp min > 20, like Dp,max / Dp,min > 25. Additionally or alternatively, Dp,max / Dp,min < 100 may apply, such aS Dp,max / Dp,min < 80, especially Dp,max / Dp,min < 60, like Dp,max / Dp,min < 40. The particle size may refer to a(n equivalent circular) diameter of the particles in the first particulate material.

[0017] Additionally or alternatively, the first particulate material may have a polydispersity index. The term “polydispersity index” may refer to a ratio of the weight average weight (of the first particulate material) to the number average weight (of the first particulate material), as is known to the person skilled in the art. In embodiments, the first particulate material may have a poly dispersity index of > 1.0, such as > 1.1, especially > 1.2. Additionally or alternatively, the first particulate material may have a poly dispersity index of < 2.1, such as < 2.0, especially < 1.9. Hence, in embodiments, the first particulate material may have a polydispersity index selected from the range of 1.0-2.1, such as from the range of 1.1-2.0, especially from the range of 1.2- 1.9.

[0018] The first particulate material may further have a particle morphology. That is, the first particles of the first particulate material may have a particle shape. In embodiment, the first particles may have a spherical-like shape (i.e., the first particles may each have a shape approximating a sphere). Alternatively, the first particles may have an elongated shape, such as a shape approximating a rectangle, ellipsoid, or cylinder. Especially, the first particulate material may comprise wood chips, wherein the first particles may have an elongated shape. Alternatively, the first particulate material may comprise one or more of ground pit material, ground seed material, and ground nut shell material, wherein the first particles may have a spherical-like shape. Herein, the phrase “the first particles have a shape approximating a sphere”, and similar phrases, may indicate that > 50%, such as > 70%, especially > 90%, including 100%, of the first particles may have the indicated shape.

[0019] The term “approximate” and its conjugations herein, such as in “to approximate a shape”, refers to being nearly identical to, especially identical to, the following term, for example nearly identical to a sphere. For example, a first particle may define a sphere but for a defect. In particular, an object approximating a first shape may herein refer to: a first shape realization encompassing the object, wherein the first shape realization has the first shape and is defined as the smallest encompassing shape of the object, wherein a ratio of the volume of the first shape realization to the volume of the object is < 1.2, especially < 1.1, such as < 1.05, like < 1.02, including 1. Further, the term “approximate” may refer to the object and the firstshape being superimposable, such that an intersection between the object and the first shape covers at least n% of the object and at least n% of the shape, wherein n is > 90%, such as > 95%, especially > 98%, such as > 99%, including 100%.

[0020] The gelled printing composition may comprise the first particulate material in a particulate concentration (Cp) (i.e., the gelled printing composition may have a particulate content (Cp)). The particulate concentration (Cp) may be selected from the range of > 0.5 wt%, such as from the range of > 1 wt%, especially from the range of > 1.5 wt% (relative to the total weight of the gelled printing composition). Additionally or alternatively, the particulate concentration (Cp) may be selected from the range of < 25 wt%, such as from the range of < 20 wt%, especially from the range of < 15 wt%, like from the range of < 10 wt% (relative to the total weight of the gelled printing composition). Hence, the particulate concentration (Cp) may be selected from the range of 0.5-25 wt%, such as from the range of 1-20 wt%, especially from the range of 1.5-15 wt% (relative to the total weight of the gelled printing composition). Further, the particulate concentration (Cp) may be selected from the range of > 2.5 wt%, such as from the range of > 4 wt%, especially from the range of > 5 wt% (relative to the total weight of the gelled printing composition). Additionally or alternatively, the particulate concentration (Cp) may be selected from the range of 2.5-10 wt%, such as from the range of 4-10 wt%, especially from the range of 5-10 wt% (relative to the total weight of the gelled printing composition). Hence, in specific embodiments, the particulate concentration (Cp) may be selected from the range of 2.5-10 wt%. Such a particulate concentration (Cp) may facilitate that the viscosity of the gelled printing composition may be low enough to allow 3D-printing with relatively small nozzles at relatively low printing pressures. Yet, such a particulate concentration (Cp) may be high enough to provide a 3D-printed part with high mechanical stability. Would the first particulate material comprise ground mineral-based material, the gelled printing composition may comprise the ground mineral-based material in a concentration of < 0.5*Cp, such as < 0.3*Cp, especially < 0.1*Cp.

[0021] The gelled printing composition may further comprise a first polymer material. The first polymer material may especially comprise one or more of an alginate-based polymer and a gelatin-based polymer. Herein, the term “alginate-based polymer” may refer to a polymer having the same or a similar backbone to an alginate polymer, wherein the alginate backbone may optionally be functionalized with one or more (types of) functional groups. Similarly, the term “gelatin-based polymer” may refer to a polymer having the same or a similar backbone to a gelatin polymer, wherein the gelatin backbone may optionally be functionalized with one or more (types of) functional groups. Further, the terms “alginate-based polymer” and “gelatin-based polymer” may indicate (block) co-polymers, wherein > 50%, such as > 65%, especially > 80%, of the (block) co-polymer may have the same or a similar backbone to respectively an alginate polymer and a gelatin polymer. The one or more (types of) functional groups (for the alginate-based polymer and / or for the gelatin-based polymer) may especially be selected from the group of cross-linkable groups, i.e., functional groups that may be crosslinked using one or more crosslinkers. For instance, the one or more (types of) functional groups may be selected from the group comprising acrylate groups, methacrylate groups, carboxyl groups, hydroxyl groups, carbonyl groups, ester groups, amide groups, amine groups, and thiol groups, though other suitable functional groups capable of being crosslinked are herein not excluded. A person skilled in the art will be capable of selecting a suitable functional group.

[0022] Further, the first polymer material may comprise one or more of a polyethylene-glycol-based polymer (“PEG-based polymer”) and a polyester-based polymer. Herein, the term “PEG-based polymer” may refer to a polymer having the same or a similar backbone to a polyethylene glycol (PEG) polymer, wherein the PEG backbone may optionally be functionalized with one or more (types of) functional groups. Similarly, the term “polyester-based polymer” may refer to a polymer having the same or a similar backbone to a polyester (polymer), wherein the polyester backbone may optionally be functionalized with one or more (types of) functional groups. In embodiments, the polyester-based polymer may especially be selected from the group comprising a polylactic-acid-based polymer (“PLA-based polymer”), a polyethylene-terephthalate-based polymer (“PET-based polymer”), and a polycaprolactone-based polymer (“PCL-based polymer”). Hence, in specific embodiments, the first polymer material may further comprise one or more of a polyethylene-glycol-based polymer and a polyester-based polymer. Such polymers may improve the biocompatibility (PEG-based polymer) and / or chemical and thermal resistance (polyester-based polymer) of the gelled printing composition. Further, such polymers may facilitate tuning e.g. the rheological properties of the gelled printing composition and / or the mechanical and physical properties of the 3D-printed structure.

[0023] As indicated above, the PEG-based polymer and / or the polyester-based polymer may comprise one or more (types of) functional groups, such as selected from the functional groups provided above in relation to the alginate-based polymer and / or gelatin-based polymer. Especially, one or more of the alginate-based polymer, gelatin-based polymer, PEG-based polymer, and polyester-based polymer may comprise a (meth)acrylate group. Further, each polymer (strand or molecule) of the first polymer material may comprise one or more functional groups. For instance, the polymer may be an acrylate polymer, a diacrylate polymer, or apolyacrylate polymer. Additionally or alternatively, one or more of the alginate-based polymer, gelatin-based polymer, PEG-based polymer, and polyester-based polymer may not comprise a functional group. That is, the first polymer material may comprise one or more of alginate, gelatin, PEG, and a polyester (e.g. PET). Especially, in specific embodiments, the first polymer material may comprise one or more of alginate, gelatin, gelatin methacrylate (gelMA), polyethylene glycol diacrylate (PEGDA), and a polyester. A first polymer material comprising one or more of such polymers may provide the benefit that the first polymer material may be crosslinked using ionic crosslinking (e.g. in the case of alginate) and / or using covalent crosslinking (e.g. in the case of gelMA). As ionic crosslinking may be reversed (more easily than covalent crosslinking), such a first polymer material may facilitate adjusting the degree to and ease with which the 3D-printed structure may be recycled.

[0024] In embodiments, the gelled printing composition may comprise the first polymer material in a polymer concentration (CM) (i.e., the gelled printing composition may have a polymer content (CM)). The polymer concentration (CM) may be selected from the range of > 1 wt%, such as from the range of > 2 wt%, especially from the range of > 3 wt%, like from the range of > 4 wt% (relative to the total weight of the gelled printing composition). Additionally or alternatively, the polymer concentration (CM) may be selected from the range of < 17 wt%, such as from the range of < 15 wt%, especially from the range of < 13 wt%, like from the range of < 12 wt% (relative to the total weight of the gelled printing composition). Further, the polymer concentration (CM) may be selected from the range of 1-17 wt%, such as from the range of 2-15 wt%, especially from the range of 3-13 wt%, like from the range of 4-12 wt% (relative to the total weight of the gelled printing composition). Hence, in specific embodiments, the polymer concentration (CM) may be selected from the range of 3-13 wt%. Such a polymer concentration (CM) may facilitate that the viscosity of the gelled printing composition may be low enough to allow 3D-printing with relatively small nozzles at relatively low printing pressures. Yet, such a polymer concentration (CM) may be high enough to provide a 3D-printed part with high mechanical stability. Especially, after 3D-printing and crosslinking, such a polymer concentration (CM) may provide a (stable) crosslinked polymer network configured to bind and / or trap the first particulate material in the crosslinked polymer network.

[0025] The gelled printing composition may further comprise the first crosslinker. The first crosslinker may especially be configured to (at least) partially crosslink the first polymer material. Hence, the first crosslinker may be configured to one or more of (i) facilitate a reaction between (side groups of) one or more polymer chains of the first polymer material, thereby crosslinking said polymer chain(s), and (ii) reacting or interacting with (side groups of) one ormore polymer chains of the first polymer material, thereby (covalently or ionically) crosslinking said polymer chain(s). In embodiments, the first crosslinker may comprise an ionic crosslinker. Especially, the first (ionic) crosslinker may comprise one or more of calcium ions (Ca2+), magnesium ions (Mg2+), barium ions (Ba2+), strontium ions ( Sr21), and aluminum ions (Al3+). In embodiments, the ionic crosslinker may be present as a free ion in the gelled printing composition. Yet, in embodiments, the ionic crosslinker may be added as a salt to the first (gelled) ink (see also below), wherein the salt may comprise one or more of calcium chloride (CaCh), magnesium chloride (MgCh), barium chloride (BaCh), strontium chloride (SrCh), aluminum sulfate ( AltSCfr);), calcium carbonate (CaCOj), calcium sulfate (CaSCL), calcium lactate, and calcium gluconate. Additionally or alternatively, the first crosslinker may comprise a covalent crosslinker. Especially, the first crosslinker may comprise a photo-initiator, such as one or more of ruthenium ions (Ru2+) and lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). In embodiments, Ru2+may be present as a free ion in the first crosslinker, especially in the gelled printing composition. Alternatively, Ru2+may be present as a complex, such as CpRuCl(PPhs)2 (pentamethylcyclopentadienylbis(triphenylphosphine)ruthenium(II) chloride). Hence, in specific embodiments, the first crosslinker may comprise one or more of Ca2+, Mg2+, Ba2+, Sr21, Al3+, RU2+, and lithium phenyl-2,4,6-trimethylbenzoylphosphinate. As indicated above, such first crosslinkers may facilitate one or more of ionic and covalent crosslinking of the first polymer material, thereby allowing adjusting the chemical stability and / or recyclability of the 3D-rpited structure prepared from the gelled printing composition.

[0026] In embodiments, the gelled printing composition may comprise the first crosslinker in a first crosslinker concentration (Cci) (i.e., the gelled printing composition may have a first crosslinker content (Cci))- The first crosslinker concentration (Cci) may be selected from the range of > 0.008*CM, such as from the range of > 0.01*CM, especially from the range of > 0.012*CM. Additionally or alternatively, the first crosslinker concentration (Cci) may be selected from the range of < 0.07*CM, such as from the range of < 0.065*CM, especially from the range of < 0.06*CM. That is, the first crosslinker concentration (Cci) may be selected from the range of 0.008*CM - 0.07*CM, such as from the range of 0.01 *CM - 0.065*CM, especially from the range of 0.012*CM - 0.06*CM. Hence, the concentration of the first crosslinker may be determined in dependence of the polymer concentration (CM).

[0027] The first crosslinker and the first polymer material may be configured (at least partially) dissolved in the first solvent. Hence, in embodiments, the gelled printing composition may comprise a first solvent. In embodiments, the first solvent may be a water-miscible solvent. Further, in embodiments, the first solvent may be protic solvent. Especially, the first solventmay comprise one or more of water and an alcohol. For instance, the first solvent may comprise one or more of water, methanol, ethanol, and propanol. Yet, especially, the first solvent may at least comprise water (and may optionally further comprise an alcohol, e.g. ethanol). Hence, in embodiments, the first solvent may comprise a mixture of water and an alcohol (e.g. ethanol). Especially, the first solvent may comprise > 10 vol%, such as > 30 vol%, especially > 50 vol%, water (with respect to the total volume of the first solvent). Further, the first solvent may comprise > 70 vol%, such as > 90 vol%, especially > 95 vol%, including 100 vol%, water (with respect to the total volume of the first solvent). Additionally or alternatively, the first solvent may comprise > 1 vol%, such as > 5 vol%, especially > 10 vol%, of an alcohol (e.g. ethanol) (with respect to the total volume of the first solvent). Further, the first solvent may comprise < 50 vol%, such as < 35 vol%, especially < 20 vol%, of an alcohol (e.g. ethanol) (with respect to the total volume of the first solvent). Hence, in specific embodiments, the first solvent may comprise one or more of water and ethanol. Such solvents may be especially safe and / or environmentally friendly to work with. In embodiments, the first solvent may further comprise a salt, configured dissolved in the first solvent. Especially, in embodiments, the first solvent may comprise a sodium chloride (NaCl) solution, such as a 0.5- 1.3 wt% NaCl solution, especially a 0.7- 1.1 wt% NaCl solution (in water), though other salt concentrations are herein not excluded.

[0028] In embodiments, the gelled printing composition may comprise the first solvent in a solvent concentration (Cs) (i.e., the gelled printing composition may have a solvent content (Cs)). The solvent concentration (Cs) may be selected from the range of > 42 wt%, such as from the range of > 45 wt%, especially from the range of > 55 wt% (relative to the total weight of the gelled printing composition). Additionally or alternatively, the solvent concentration (Cs) may be selected from the range of < 98 wt%, such as from the range of < 95 wt%, especially from the range of < 85 wt% (relative to the total weight of the gelled printing composition). Hence, the gelled printing composition may comprise the first solvent in a solvent concentration (Cs) selected from the range of 42-98 wt%, such as from the range of 45-95 wt%, especially from the range of 55-85 wt% (relative to the total weight of the gelled printing composition).

[0029] In embodiments, the gelled printing composition may comprise one or more further components. In embodiments, the gelled printing composition may further comprise an antifungal material (i.e., a material configured to prevent and / or inhibit the growth of fungal material). For instance, the gelled printing composition may comprise one or more of a copperbased antifungal material (e.g. copper sulfate and / or copper nanoparticles), a benzoate, a sorbate, a competing (beneficial) fungus (e.g. Trichoderma spp. chitosan, a tannin, acinnamon extract, and a neem extract (e.g. comprising azadirachtin). Additionally or alternatively, the gelled printing composition may comprise a flame retardant material. The flame retardant material may for instance be selected from the group comprising brominated flame retardants, chlorinated flame retardants, organophosphorus flame retardants, red phosphorus flame retardants, metal hydroxide flame retardants, melamine flame retardants, silicone-based flame retardants, and nanomaterial flame retardants. Especially, in embodiments, the flame retardant material may comprise one or more of an inorganic flame retardant material, a boron-based flame retardant material, and a biodegradable flame retardant material. For instance, the flame retardant material may comprise one or more of ammonium polyphosphate, aluminum hydroxide (A1(OH)3), magnesium hydroxide (Mg(OH)2), zinc borate, borax (i.e., sodium tetraborate), boric acid, a tannin, chitosan, eggshell, expandable graphite, and a melamine-based compound. Additionally or alternatively, the gelled printing composition may comprise an insect repellent. For instance, the insect repellent may comprise one or more of a tannin and Bacillus thuringiensis. Additionally or alternatively, the gelled printing composition may comprise a mineral. Especially, the gelled printing composition may comprise a mineral selected from the group comprising nitrogen, calcium, iron, copper, chlorine, phosphorus, magnesium, manganese, boron, potassium, sulfur, zinc, nickel, and molybdenum. Additionally or alternatively, the gelled printing composition may comprise a fertilizer. For instance, the fertilizer may comprise NPK 20-20-20, comprising nitrogen (N), phosphorus (P), potassium (K), and one or more of calcium carbonate and magnesium carbonate. Further, the gelled printing composition may comprise a second biological material. The second biological material may comprise one or more of a growth-promoting biological material, a fungal material, a plant material, a bacteria, and a cell material. The growthpromoting biological material may for instance comprise one or more of humic acid and fulvic acid. Herein, the term “fungal material” may refer to e.g. a spore of a fungus, but may also refer to (a piece of) a fungus or a fungal network. Similarly, the term “plant material” may refer to a seed, a bulb, a root, a cutting, a seedling, a sapling, or (a piece of) a plant. The term “cell material” may refer to any (biological) material consisting of or comprising a cell, such as e.g. a cell culture, an algae, a (piece of) tissue, etc.. Further, in embodiments, the bacteria may comprise a bacterial inoculant, e.g. one or more of Rhizobacteria, Bacillus subtilis, and Pseudomonas spp.. Hence, in specific embodiments, the gelled printing composition may further comprise one or more of: (i) an antifungal material, (ii) a flame retardant material, (iii) an insect repellant, (iv) a mineral, (v) a fertilizer, and (vi) a second biological material, wherein the second biological material may comprise one or more of a growth-promoting biologicalmaterial, a fungal material, a plant material, a bacteria, and a cell material. Such an addition to the gelled printing composition may facilitate cultivating e.g. a fungus, plant, or cell culture in the gelled printing composition and / or in the 3D-printed structure. Further, the addition of a flame retardant material and / or an antifungal material may improve the safety and / or lifetime of the 3D-printed structure.

[0030] In embodiments, the method may comprise preparing the gelled printing composition. Especially, the method may comprise preparing the gelled printing composition from a printing composition. That is, the printing composition may be (at least partially) gelated (or converted into a gel-form), thereby providing the gelled printing composition. Especially, the method may comprise preparing the gelled printing composition from the printing composition by one or more of (i) crosslinking the first polymer material with the first crosslinker, and (ii) gelation. Hence, preparing the gelled printing composition from the printing composition may comprise crosslinking (at least part of) the first polymer material with the first crosslinker. In such embodiments, the first crosslinker may especially comprise one or more of Ca2+, Mg2+, Ba2+, Sr2+, and Al3+. Further, in such embodiments, the method may comprise adding the first crosslinker to the printing composition (wherein the printing composition at least comprises the first polymer material), and allowing the first crosslinker to (partially) crosslink (at least part of) the first polymer material (such as especially the alginate-based polymer). Especially, in such embodiments, the first polymer material may especially comprise x (side) groups that may be crosslinked using the first crosslinker, wherein the first crosslinker may be configured to crosslink > 5%, such as > 10%, especially > 15%, of the x (side groups) (wherein two groups provide one crosslink). Additionally or alternatively, the first crosslinker may be configured to crosslink < 80%, such as < 60%, especially < 50%, of the x (side groups). Alternatively, preparing the gelled printing composition from the printing composition may comprise gelating the printing composition. In such embodiments, the method may comprise reducing a temperature of the printing composition, such as especially below a gelation temperature of (at least part of) the first polymer material. Further, in such embodiments, the first polymer material may especially comprise a gelatin-based polymer (having a gelation temperature of about 22.5-25 °C). Hence, in specific embodiments, preparing the gelled printing composition from the printing composition may comprise one or more of (i) adding the first crosslinker to the printing composition, and allowing the first crosslinker to crosslink the first polymer material, and (ii) reducing a temperature of the printing composition, wherein the first polymer material may comprise a gelatin-basedpolymer. Such methods of preparing the gelled printing composition may be relatively simple and / or may require relatively little energy.

[0031] In embodiments, the printing composition may have a first viscosity (ip). Further, the gelled printing composition may have a second viscosity (r|2) - The second viscosity may be determined by observing the stress on (or in) the gelled printing composition at different shear rates (i.e., different rotational speeds in a rheology measurement), see also below. The second viscosity (ip) may be a zero-shear viscosity. Further, the second viscosity (ip) may be deteimined using a stress-controlled rheometer equipped with parallel plate geometry, wherein experimental viscosity data is fitted to a (normalized) Carreau- Yasuda model to determine the second viscosity (ip). Hence, the second viscosity (ip) may be a zero-shear viscosity, wherein the second viscosity (ip) may be determined using a stress-controlled rheometer equipped with parallel plate geometry, and wherein experimental viscosity data is fitted to a (normalized) Carreau-Yasuda model to determine the second viscosity Cp). In embodiments, the second viscosity (ip) may be selected from the range of > 800 Pa-s, such as from the range of > 1000 Pa-s, especially from the range of > 1200 Pa-s. Additionally or alternatively, the second viscosity (ip) may be selected from the range of < 8000 Pa-s, such as from the range of < 7000 Pa-s, especially from the range of < 6000 Pa-s. That is, the second viscosity (ip) may be selected from the range of 800-8000 Pa-s, such as from the range of 1000-7000 Pa-s, especially from the range of 1200-6000 Pa-s. Hence, in specific embodiments, the second viscosity (ip) may be selected from the range of 1000-7000 Pa-s. Such a second viscosity (ip) may facilitate that the gelled printing composition may flow relatively smoothly from the nozzle of a 3D-printer, yet my further facilitate that the gelled printing composition may maintain its shape after 3D-printing (and before crosslinking the precursor structure).

[0032] The second viscosity (ip) may especially be larger than the first viscosity (ip). Especially, in embodiments, ip > 3*ip may apply, such as ip > 4*ip, especially ip > 5*ip, like ip > 6*ip. Additionally or alternatively, ip < 15*ip may apply, such as ip < 12*ip, especially ip < 10*ip, like ip < 8*ip. That is, in embodiments, 3*ip < ip < 15*ip may apply, such as 4*ip < ip < 12*ip, especially 5*ip < ip < 10*ip, like 6*ip < ip < 8*ip. Hence, in specific embodiments, the method may comprise preparing the gelled printing composition from a printing composition; wherein the printing composition may have a first viscosity (ip), wherein the gelled printing composition may have a second viscosity (ip), and wherein 4*ip < ip < 12*ip. Increasing the viscosity of the printing composition to prepare the gelled printing composition may provide the benefit that the relatively low viscosity of the printing composition may allow for easy mixing of the printing composition, while the relatively higherviscosity of the gelled printing composition may allow the gelled printing composition to maintain its shape after 3D-printing.

[0033] Hence, providing the gelled printing composition may comprise preparing the gelled printing composition from the printing composition. Further, providing the gelled printing composition may comprise dissolving the first polymer material in the first solvent to provide a first polymer solution. In embodiments, the first polymer material may be dissolved in the first solvent at room temperature (i.e., ~22 °C). Yet, in embodiments, the method may comprise dissolving the first polymer material in the first solvent at a temperature of > 25 °C, such as of > 30 °C, especially of > 40 °C. Additionally or alternatively, the method may comprise dissolving the first polymer material in the first solvent at a temperature of < 60 °C, such as of < 50 °C, especially of < 45 °C. Hence, in specific embodiments, the method may comprise dissolving the first polymer material in the first solvent at a temperature of > 30 °C. The first polymer material may have a relatively higher solubility at a temperature of > 30 °C (compared to room temperature). Further, especially for a gelatin-based polymer, the first polymer material may form a gel (rather than a solution), at temperatures of < 30 °C, such that heating may be required to provide the first polymer solution.

[0034] Providing the gelled printing composition may further comprise combining the first polymer solution with the first particulate material to form a first composition. Especially, the first polymer solution and the first particulate material may be mixed together into a homogeneous mixture, which mixture may be the first composition. In embodiments, the first polymer solution may be combined with the first particulate material at room temperature. Yet, in embodiments, (such as especially when the first polymer material comprises a gelatin-based polymer,) the first polymer solution may be combined with the first particulate material at a temperature of > 25 °C, such as of > 30 °C, especially of > 35 °C. Additionally or alternatively, the first polymer solution may be combined with the first particulate material at a temperature of < 55 °C, such as of < 50 °C, especially of < 45 °C.

[0035] Further, providing the gelled printing composition may comprise combining the first composition with the first crosslinker, to provide (after crosslinking and / or after cooling down) the gelled printing composition. Hence, in embodiments, the first composition may be the printing composition, which may turn into the gelled printing composition after addition of the first crosslinker (and crosslinking (at least part of) the first polymer material with the first crosslinker). Alternatively, combining the first composition with the first crosslinker may provide the printing composition, which may turn into the gelled printing composition upon reducing the temperature of the printing composition. In embodiments, the first crosslinker mayespecially be combined with the first composition at room temperature. Further, the first crosslinker may be combined with the first composition such, that a homogeneous first (gelled) ink in provided. Hence, in specific embodiments, providing the gelled printing composition may comprise: (a) dissolving the first polymer material in the first solvent to provide a first polymer solution; (b) combining the first polymer solution with the first particulate material to form a first composition; and (c) combining the first composition with the first crosslinker. Such a method for providing the gelled printing composition may be relatively simple (i.e., may not require specific equipment or knowledge).

[0036] The method may comprise 3D-printing the gelled printing composition. Especially, the gelled printing composition may be 3D-printed with a 3D-printer, thereby providing a precursor structure. In embodiments, the 3D-printer may comprise a reservoir (e.g. a container, a chamber, a syringe, etc.) configured to host the gelled printing composition. Further, the 3D-printer may comprise a nozzle (fluidically coupled with the reservoir). During 3D-printing, the gelled printing composition may especially be (flowed from the reservoir to the nozzle, and subsequently) extruded from the nozzle. The nozzle(, such as especially a nozzle opening through which the gelled printing composition may exit the 3D-printer,) may have a nozzle diameter (Dn). The nozzle diameter (Dn) may especially be determined in a direction perpendicular to a direction in which the gelled printing composition flows through the nozzle. In embodiments, the nozzle (opening) may be circular, wherein the nozzle diameter (Dn) may be a diameter of the circular nozzle (opening). Alternatively, the nozzle (opening) may have a non-circular shape, such as e.g. an elliptical shape or a simple (regular) polygonal shape, and the nozzle diameter (Dn) may be an equivalent circular diameter of the nozzle (opening). The equivalent circular diameter (or ECD) of an (irregularly shaped) two-dimensional shape is the diameter of a circle of equivalent area. For instance, the equivalent circular diameter of a square with side a is 2a / SQRT(7t). For a circle, the diameter D is the same as the equivalent circular diameter. Would a circle in an xy -plane with a diameter D be distorted to any other shape (in the xy-plane), without changing the area size, then the equivalent circular diameter of that shape would be D. Hence, the nozzle may have a nozzle diameter (Dn). The nozzle diameter (Dn) may be selected from the range of > 0.2 mm, such as from the range of > 0.4 mm, especially from the range of > 0.6 mm. Additionally or alternatively, the nozzle diameter (Dn) may be selected from the range of < 1.5 mm, such as from the range of < 1.2 mm, especially from the range of < 1 mm. That is, the nozzle diameter (Dn) may be selected from the range of 0.2- 1.5 mm, such as from the range of 0.4- 1.2 mm, especially from the range of 0.6-1 mm.As indicated above, the method may comprise extruding the gelled printing composition via the nozzle at a printing pressure (Pi). Further, the method may comprise extruding the gelled printing composition via the nozzle at a printing temperature (Tp). Further yet, the method may comprise extruding the gelled printing composition via the nozzle at a printing speed selected from the range of 4-12 mm / s. Yet, the method may comprise extrading the gelled printing composition via the nozzle at (all of) (i) a printing pressure (Pi), (ii) a printing temperature (Tp), and (iii) a printing speed. Hence, 3D-printing the gelled printing composition may comprise extrading the gelled printing composition via the nozzle. Especially, the gelled printing composition may be extraded via the nozzle at a printing pressure (Pi) selected from the range of > 20 kPa, such as from the range of > 40 kPa, especially from the range of > 60 kPa. Additionally or alternatively, the printing pressure (Pi) may be selected from the range of < 300 kPa, such as from the range of < 250 kPa, especially from the range of < 200 kPa. Hence, in embodiments, the gelled printing composition may be extraded via the nozzle at a printing pressure (Pi) selected from the range of 20-300 kPa, such as from the range of 40-250 kPa, especially from the range of 60-200 kPa.

[0037] Further, the gelled printing composition may be extraded via the nozzle at a printing temperature (Tp) selected from the range of > 15 °C, such as from the range of > 18 °C, especially from the range of > 20 °C. Additionally or alternatively, the printing temperature (Tp) may be selected from the range of < 35 °C, such as from the range of < 30 °C, especially from the range of < 28 °C. Hence, the gelled printing composition may be extraded via the nozzle at a printing temperature (Tp) selected from the range of 15-35 °C, such as from the range of 18-30 °C, especially from the range of 20-28 °C.

[0038] The gelled printing composition may further be extraded via the nozzle at a printing speed selected from the range of > 2 mm / s, such as from the range of > 4 mm / s, especially from the range of > 6 mm / s. Additionally or alternatively, the printing speed may be selected from the range of < 15 mm / s, such as from the range of < 12 mm / s, especially from the range of < 10 mm / s. Hence, the gelled printing composition may further be extruded via the nozzle at a printing speed selected from the range of 2-15 mm / s, such as from the range of 4-12 mm / s, especially from the range of 6-10 mm / s. Such a printing speed, nozzle diameter (Dn), and printing pressure (Pi) may facilitate extruding the gelled printing composition via the nozzle at a printing speed of > 1.5 min / cm3, such as > 2 min / cm3, especially > 2.5 min / cm3. Additionally or alternatively, the gelled printing composition may be extraded via the nozzle at a printing speed of < 4.5 min / cm3, such as < 4 min / cm3, especially < 3.5 min / cm3.3D-printing the gelled printing composition may comprise depositing the gelled printing composition layer by layer. In embodiments, each layer may be extruded separately, wherein extrusion of the gelled printing composition may be (temporarily) stopped between the printing of each layer. Alternatively, the different layers may be provided by a continuous extrusion (i.e., a single strand or stream) of gelled printing composition. The precursor structure may show a characteristic ribbed structure (originating from the deposition of the gelled printing composition in a layer-like fashion). In embodiments, the method may comprise smoothening the ribbed structure, which may lead to an essentially smooth surface. Alternatively, the ribbed structure may be maintained.

[0039] Hence, the precursor structure may comprise a plurality of 3D-printed layers ((each) comprising 3D-printed gelled printing composition). That is, the method may comprise generating the precursor structure with a plurality of 3D-printed layers. In embodiments, each layer may have a layer height (HL), wherein the layer height (HL) may especially be determined perpendicular to a stacking direction of the 3D-printed layers (and / or in a direction of gravity during 3D-printing). The layer height (HL) may be related to the nozzle diameter (Dn). Especially, the layer height (HL) may (for each layer individually) be selected from the range of > 0.65*Dn, such as from the range of > 0.75*Dn, especially from the range of > 0.85*Dn. Additionally or alternatively, the layer height (HL) may (for each layer individually) be selected from the range of < Dn, such as from the range of < 0.98*Dn, especially from the range of < 0.95*Dn. Hence, the layer height (HL) may (for each layer individually) be selected from the range of 0.65*Dn- Dn, such as from the range of 0.75*Dn- 0.98*Dn, especially from the range of 0.85*Dn- 0.95*Dn. Further, in embodiments, the layer height (HL) may (for each layer individually) be selected from the range of > 0.15 mm, such as from the range of > 0.25 mm, especially from the range of > 0.3 mm. Additionally or alternatively, the layer height (HL) may (for each layer individually) be selected from the range of < 1.1 mm, such as from the range of < 1 mm, especially from the range of < 0.9 mm. Hence, each layer may have a layer height (HL) selected from the range of 0.15-1.1 mm, such as from the range of 0.25-1 mm, especially from the range of 0.3-0.9 mm. In embodiments, each layer may have the same layer height (HL). Especially, each layer may have a layer height (HL) deviating < 7%, such as < 5%, especially < 2%, including 0%, from an average layer height (HL) of the plurality of 3D-printed layers. Hence, the layer height (HL) may in embodiments refer to an average layer height (HL) of the plurality of 3D-printed layers. Further, the layer height (HL) within each layer may deviate < 7%, such as < 5%, especially < 2%, including 0%, from an average layer height (HL) of said 3D-printed layer. Hence, in specific embodiments, the precursor structure may comprisea plurality of 3D-printed layers, wherein each layer may have a layer height (HL) selected from the range of 0.25-1 mm. Such a layer height may be small enough to allow the printing of small and / or detailed structures, yet may be large enough to facilitate relatively quickly 3D-printing 1 cm3of precursor structure.

[0040] The precursor structure may be (at least partially) crosslinked to provide a crosslinked structure. Hence, the method may comprise (at least partially) crosslinking the precursor structure to provide a crosslinked structure. In embodiments, crosslinking the precursor structure may comprise exposing the precursor structure to a second crosslinker. In such embodiments, the first crosslinker may especially (at least) comprise one or more of Ca2+, Mg2+, Ba2+, Sr2 -, and Al3+. The second crosslinker may be equal to the first crosslinker. Alternatively, the second crosslinker may be different from the first crosslinker. Especially, the second crosslinker may comprise one or more of Ca2+, Mg2+, Ba2+, Sr2Al3+, strontium acetate, glutaraldehyde, epichlorohydrin, and genipin. Hence, the second crosslinker may be a crosslinker facilitating the formation of ionic bonds between the second crosslinker and the first polymer material (or between (side) groups of (different) polymer strands within the first polymer material). In such embodiments, the second crosslinker may especially comprise one or more of Ca2+, Mg2+, Ba2+, Si21, Al3+, and strontium acetate. Alternatively, the second crosslinker may facilitate the formation of covalent bonds, either between the second crosslinker and (at least) two (side) groups of the first polymer material, or between (side) groups of (different) polymer strands within the first polymer material. In such embodiments, the second crosslinker may especially comprise one or more of glutaraldehyde, epichlorohydrin, and genipin. In embodiments, crosslinking the precursor structure may comprise spraying the precursor structure with a solution comprising the second crosslinker. Yet, especially, crosslinking the precursor structure may comprise submerging the precursor structure in a solution comprising the second crosslinker. The second crosslinker, such as especially one or more of Ca2+, Mg2+, Ba2+, Sr21, and Al3+, may be present as (part of) a (dissociated) salt in the solution. For instance, the second crosslinker may be present in the solution as dissociated calcium chloride (CaCh), magnesium chloride (MgCh), barium chloride (BaCh), strontium chloride (SrCh), aluminum sulfate (A1(SO4)3), calcium carbonate (CaCOj), calcium sulfate (CaSOfi, calcium lactate, and calcium gluconate. Further, the solution (for spraying and / or submerging) may especially comprise the second crosslinker in a second crosslinker concentration (Cc2) of > 150 mM, such as > 200 mM, especially > 250 mM. Additionally or alternatively, the second crosslinker concentration (Cc2) may be selected from the range of < 400 mM, such as from the range of < 350 mM, especially from the range of <300 mM. Further, the precursor structure may be submerged in (or sprayed with) the solution comprising the second crosslinker for at least 20 min, such as at least 40 min, especially at least 1 h. Additionally or alternatively, the precursor structure may be submerged in (or sprayed with) the solution comprising the second crosslinker for at most 24 h, such as at most 20 h, especially at most 16 h. Hence, in specific embodiments, the second crosslinker may comprise one or more of Ca2+, Mg2+, Ba2+, Sr2+, Al3+, strontium acetate, glutaraldehyde, epichlorohydrin, and genipin; wherein crosslinking the precursor structure may comprise submerging the precursor structure in a solution comprising the second crosslinker. Such a second crosslinker (especially one or more of Ca2+, Mg2+, and strontium acetate) may especially provide ionic bonds between (different) polymer chains, thereby providing a reversible bonding, facilitating a later recycling of the 3D-printed structure. Further, such a method of crosslinking the precursor structure may be relatively simple and safe.

[0041] Additionally or alternatively, crosslinking the precursor structure may comprise exposing the precursor structure to radiation. In such embodiments, the first crosslinker may especially comprise one or more of (a) Ru2+(complex) and LAP. In embodiments wherein the first crosslinker comprises (the) Ru2+(complex), the gelled printing composition may especially further comprise sodium persulfate. Alternatively (or additionally), crosslinking the precursor structure may comprise exposing the precursor structure to radiation, wherein the first crosslinker may comprise LAP. Especially, when crosslinking the precursor structure by exposing the precursor structure to radiation, the radiation may especially be selected from the group of blue radiation, violet radiation, and UV radiation. Herein, ultraviolet (UV) radiation may especially refer to radiation (or light) having a wavelength selected from the range of 190-380 nm, such as from the range of 200-380 nm. Further, the terms “violet radiation” or “violet light” may relate to light having a wavelength in the range of 380-440 nm. The terms “blue radiation” or “blue light”, may relate to light having a wavelength in the range of 440-490 nm. Especially, crosslinking the precursor structure may comprise exposing the precursor structure to UV radiation. In embodiments, the UV radiation may be incident on the precursor structure with an irradiance of > 10 mW / cm2, such as an irradiance of > 12 mW / cm2, especially an irradiance of > 15 mW / cm2. Additionally or alternatively, the UV radiation may be incident on the precursor structure with an irradiance of < 25 mW / cm2, such as an irradiance of < 20 mW / cm2, especially an irradiance of < 17 mW / cm2. Further, in embodiments, the precursor structure may be exposed to UV radiation for > 2 min, such as > 5 min, especially > 10 min. Additionally or alternatively, the precursor structure may be exposed to UV radiation for < 3 h, such as < 1 h, especially < 30 min. Hence, in specific embodiments, one of the followingmay apply: (i) the first crosslinker may comprise Ru2+, wherein the gelled printing composition may further comprise sodium persulfate, and (ii) the first crosslinker may comprise lithium phenyl-2,4,6-trimethylbenzoylphosphinate; wherein crosslinking the precursor structure may comprise exposing the precursor structure to UV radiation. Such a crosslinking method may especially provide covalent bonds between (different) polymer chains, thereby providing a more chemically and / or thermally stable 3D-printed structure. Further, crosslinking by exposure to UV radiation may be relatively faster than crosslinking using a second crosslinker.

[0042] As indicated above, the first polymer material may comprise a polyester (e.g. PET). In such embodiments, crosslinking the precursor structure (to provide the crosslinked structure) may further comprise heating the precursor structure at a temperature of > 60 °C, such as > 65 °C, especially > 70 °C. Additionally or alternatively, the precursor structure may be heated at a temperature of < 100 °C, such as < 90 °C, especially < 80 °C. Heating the precursor structure may facilitate thermal crosslinking (especially thermal polymerization) of the polyester-based polymer (e.g. PET).

[0043] In embodiments, the method may comprise 3D-printing the precursor structure, and subsequently crosslinking the precursor structure to provide the crosslinked structure. Yet, alternatively, the method may comprise crosslinking at least part of the precursor structure while 3D-printing the precursor structure. That is, the precursor structure and the crosslinked structure may be provided simultaneously. In such embodiment, the method may especially comprise crosslinking the part of the precursor structure that has already been 3D-printed, while simultaneously (continuing with) 3D-printing the precursor structure. In embodiments, crosslinking the precursor structure while 3D-printing may comprise one or more of (i) spraying the 3D-printed part of the precursor structure with a solution comprising the second crosslinker, and (ii) submerging the 3D-printed part of the precursor structure into a solution comprising the second crosslinker. That is, the precursor structure may be 3D-printed on a moveable printer bed, which may be lowered into the solution comprising the second crosslinker, or the solution comprising the second crosslinker may be (slowly) raised to submerge the (static) printer bed of the 3D-printer. Hence, in embodiments, the method may comprise 3D-printing the gelled printing composition to provide a precursor structure, wherein, during 3D-printing of the precursor structure, the method may comprise exposing at least part of the 3D-printed precursor structure to the second crosslinker to provide the crosslinked structure. Alternatively, the method may comprise 3D-printing the gelled printing composition to provide a precursor structure, wherein, during 3D-printing of the precursor structure, the method may comprise exposing at least part of the 3D-printed precursor structure to (UV)radiation to provide the crosslinked structure. Especially, the method may comprise providing (UV) radiation to the already-printed parts of the precursor structure, while simultaneously 3D-printing the precursor structure. Yet, especially, the method may comprise providing the precursor structure, and subsequently crosslinking the precursor structure to provide the crosslinked structure.

[0044] Hence, the precursor structure may be (at least partially) crosslinked to provide a crosslinked structure. In embodiments, for at least 80%, such as for at least 90%, especially for at least 95%, like for (essentially) 100%, of the (volume of the) crosslinked structure may apply that a distance to a nearest surface may be (at most) a first distance di. That is, from any point within said > 80%, such as > 90%, especially > 95%, like 100%, of the (volume of the) crosslinked structure, the shortest distance from that point to the nearest surface may be (at most) the first distance di. In embodiments, the first distance di may be selected from the range of < 15 mm, such as from the range of < 10 mm, especially from the range of < 8 mm. Additionally or alternatively, the first distance di may be selected from the range of > 0.1 mm, such as from the range of > 0.2 mm, especially from the range of > 0.5 mm. Hence, in specific embodiments, for at least 90% of the crosslinked structure may apply that a first distance di to a nearest surface may be selected from the range of < 10 mm. Such a first distance di may facilitate that the second crosslinker and / or the UV radiation may penetrate (fully) into the precursor structure in a relatively short period of time, thereby decreasing the time needed for (fully) crosslinking the precursor structure into the crosslinked structure. Hence, in embodiments, also for at least 80%, such as for at least 90%, especially for at least 95%, like for (essentially) 100%, of the (volume of the) precursor structure may apply that a distance to a nearest surface may be the first distance di, wherein the first distance di may be selected from the ranges provided above.

[0045] In embodiments, the first distance di may depend on one or more of the second crosslinker and the desired crosslinking time. As will be clear to a person skilled in the art, a smaller first distance di may allow reducing the crosslinking time, as e.g. the second crosslinker may not need to penetrate as deeply into the crosslinked structure as with a larger first distance di. Hence, the first distance di may be determined in view of the desired (maximum) crosslinking time for the crosslinked structure. Additionally or alternatively, the first distance di may, in case of crosslinking using exposure to UV radiation, further depend on the particulate concentration (Cp). Especially, the first particulate material may at least partially absorb the UV light received by the first particulate material, leading to a (linear or exponential) decrease in the intensity of the UV radiation with increasing distance from a nearest surface.At higher particulate concentrations (Cp), the decrease in the intensity of the UV radiation may be more rapid, such that a lower first distance di may be required to ensure a high enough intensity of UV radiation for crosslinking throughout the (precursor or) crosslinked structure. Yet, in further embodiments, the first distance di may be determined in view of the second crosslinker used. Especially, different second crosslinkers may have different diffusion coefficients for diffusing through the precursor structure, wherein a larger diffusion coefficient may allow for a larger first distance di (at the same crosslinking time). For instance, the second crosslinker glutaraldehyde may have a larger molecular size (~1 nm in water) than the second crosslinker Ca2+(< 0.2 nm in water), such that (at the same crosslinking time) the glutaraldehyde may penetrate less deeply into the precursor structure than the Ca2+. Hence, when crosslinking the precursor structure with glutaraldehyde as a second crosslinker, a lower first distance di may be selected then when crosslinking the precursor structure with Ca2+. It will be clear to the skilled person how to select a suitable first distance di in view of the crosslinking method (radiation-based or second-crosslinker-based), the desired (maximum) crosslinking time, and one or more of (i) the particulate concentration (Cp) (and extinction coefficient of the gelled printing composition) and (ii) the second crosslinker used.

[0046] In embodiments, the crosslinked structure may have one or more of a height, width, and length of < 2*di, such that the crosslinked structure may have the first distance di in said one or more of the height, width, and length. Hence, the crosslinked structure may be designed to have the first distance di (for > 80%, like > 90%, of the crosslinked structure). Especially, the crosslinked structure may have an outer surface (defined by one or more major faces of the crosslinked structure), wherein the outer surface may define one or more of the height, width, and length of the crosslinked structure. Note that, would the crosslinked structure be a hollow structure (e.g. a hollow tube), the inside wall of the hollow structure may be comprised by the outer surface. Yet, in embodiments, the (outside) dimensions of the crosslinked structure may (for at least 20% of the volume of the crosslinked structure) be > 2*di. In such embodiments, the crosslinked structure may comprise one or more (through) channels. The one or more (through) channels may be configured extending (inwards) from the outer surface. Alternatively, (at least one of) the one or more (through) channels may be an internal channel, wherein the internal channel may be configured separated from (the nearest part of) the outer surface by a distance of < di (e.g. separated by a (thin) layer of crosslinked gelled printing composition). In embodiments, the one or more (through) channels may be formed by selecting an infill percentage during 3D-printing, as is known to the person skilled in the art. Yet, especially, > 80%, such as > 90%, especially > 95%, including (essentially)100%, of the one or more (through) channels may be configured fluidically coupled with (such as in liquid contact with) an environment of the crosslinked structure. Further, each channel may be defined by a channel wall (lining the channel, and optionally physically coupled with the outer surface). In such embodiments, the nearest surface may be selected from the group of the outer surface and the one or more channel walls (of the one or more channels). That is, in such embodiments, for > 80%, such as for > 90%, especially for > 95%, like for (essentially) 100%, of the (volume of the) crosslinked structure may apply that a distance to one or more of (i) the outer surface, and (ii) at least one of the one or more channel walls, may be the first distance di. Hence, in specific embodiments, the crosslinked structure may comprise an outer surface; wherein the crosslinked structure may comprise one or more channels, wherein each channel may be defined by a channel wall; and wherein the nearest surface may be selected from the group of the outer surface and the one or more channel walls. Such one or more channels may facilitate providing a crosslinked structure with relatively large dimensions (i.e., > 2*di), wherein for > 80% of the crosslinked structure may still apply that a distance to the nearest surface may be < di.

[0047] In embodiments, each of the one or more channels may have a smallest cross-sectional diameter (Dc). In embodiments, the one or more channels may have a circular shape in said smallest cross-section, such that the cross-sectional diameter (Dc) may be the diameter of said circular shape. Alternatively, the one or more channels may have an elliptical or regular (simple) polygonal shape in said smallest cross-section, such that the cross-sectional diameter (Dc) may be an equivalent circular diameter. In embodiments, the cross-sectional diameter (Dc) may be selected from the range of > 0.1 mm, such as from the range of > 0.15 mm, especially from the range of > 0.2 mm. For relatively large cross-sectional diameters (Dc), e.g. Dc> 1 cm, the crosslinked structure may be considered as having a grid shape (“perforated shape”) or open shape, such that the crosslinked structure may not comprise the one or more channels, and such that the (inside) walls of the perforations or open shape may be comprised by the outer surface.

[0048] Hence, the method may comprise providing a gelled printing composition, 3D-printing the gelled printing composition to provide a precursor structure, and crosslinking the precursor structure to provide a crosslinked structure. Further, the method may comprise removing at least part of the first solvent from (i.e., drying) the crosslinked structure to provide the 3D-printed structure. In embodiments, removing at least part of the first solvent from the crosslinked structure may comprise allowing the first solvent to evaporate naturally at room temperature (i.e., natural drying). Yet, in embodiments, removing at least part of the first solvent from the crosslinked structure may comprise heating the crosslinked structure at atemperature of > 25 °C, such as > 30 °C, especially > 40 °C. Additionally or alternatively, removing at least part of the first solvent from the crosslinked structure may comprise heating the crosslinked structure at a temperature of < 70 °C, such as < 60 °C, especially < 50 °C. Further, removing at least part of the first solvent from the crosslinked structure may comprise providing an air flow to the crosslinked structure, especially wherein the air flow has < 80%, such as < 60%, especially < 40%, relative humidity. Yet, in embodiments, removing at least part of the first solvent from the crosslinked structure may comprise reducing an air pressure in the environment surrounding the crosslinked structure. For instance, the method may comprise removing at least part of the first solvent from the crosslinked structure at a pressure (of the environment surrounding the crosslinked structure) of < 1 bar, such as < 0.9 bar, especially < 0.8 bar (and optionally at an elevated temperature, see above). Yet, the method may comprise removing at least part of the first solvent from the crosslinked structure at a pressure (of the environment surrounding the crosslinked structure) of > 0.1 bar, such as > 0.2 bar, especially > 0.3 bar (and optionally at an elevated temperature, see above).

[0049] In embodiments, removing at least part of the first solvent from the crosslinked structure (to provide the 3D-printed structure) may comprise removing > 60%, such as > 65%, especially > 70%, of the first solvent (e.g. by evaporation or leaking) from the crosslinked structure. Additionally or alternatively, removing at least part of the first solvent from the crosslinked structure (to provide the 3D-printed structure) may comprise removing < 95%, such as < 90%, especially < 80%, of the first solvent from the crosslinked structure. The amount of first solvent removed from the crosslinked structure may especially be determined by weight. That is, a phrase like “removing > 60% of the first solvent” may also be read as “removing > 60 wt% of the first solvent”. The crosslinked structure may shrink while removing the at least part of the first solvent. Especially, the crosslinked structure may have a first volume (Vi), and the 3D-printed structure (obtained after removing at least part of the first solvent from the crosslinked structure) may have a second volume (V2). In embodiments, x*V2 = Vi may apply, wherein x > 1.25, such as x > 1.5, especially x > 2. Additionally or alternatively, x*V2 = Vi may apply, wherein x < 5.5, such as x < 5, especially x < 4.5. Hence, in embodiments, 1.25*V2 < Vi < 5.5*V2, such as 1.5*V2 < Vi < 5*V2, especially 2*N < Vi < 4.5*V2. Yet, in embodiments, the shape of the crosslinked structure may be maintained while removing the at least part of the first solvent. That is, the crosslinked structure may shrink relatively evenly over (and throughout) the (volume of the) crosslinked structure. Hence, in embodiments, while removing the at least part of the first solvent, the crosslinked structure may be scaled by a factor 1 / x to provide the 3D-printed structure. In other words, the 3D-printed structure, scaled by afactor x, may provide (the dimensions of) the crosslinked structure. Especially, the 3D-printed structure, scaled by a factor x, may have > 70%, such as > 80%, especially > 90%, like > 95%, including (essentially) 100%, overlap with the crosslinked structure. Further, as indicted, the gelled printing composition may maintain its shape after 3D-printing (and during crosslinking of the precursor structure. Hence, the crosslinked structure may have a relatively high shape fidelity. Herein, the term “shape fidelity” may indicate how much a 3D-printed structure may resemble the (computer) design of the 3D-printed structure used for 3D-printing the 3D-printed structure. Hence, in embodiments, the 3D-printed structure, scaled by a factor x, may (further) have > 70%, such as > 80%, especially > 90%, like > 95%, including (essentially) 100%, overlap with a design of the 3D-printed structure used for the 3D-printing. Hence, in specific embodiments, the crosslinked structure may have a first volume (Vi), wherein the 3D-printed structure may have a second volume (V2); wherein x*V2 = Vi, wherein x > 1.5; and wherein the 3D-printed structure, scaled by a factor x, may have > 90% overlap with a design of the 3D-printed structure used for the 3D printing. Such a factor x may provide the benefit that, to produce a 3D-printed structure with relatively fine features, said features may be 3D-printed at a larger size, as the even shrinking of the crosslinked structure may ensure the correct size of the features after removing at least part of the first solvent. As larger features may be 3D-printed more reliably than smaller features, such a method may improve the quality and reproducibility of the 3D-printed structures. Further, such an overlap between the 3D-printed structure and the design used for the 3D-printing may facilitate that a designer may design the 3D-printed structure without the need to account for correction factors and / or distortion (apart from the scaling factor).

[0050] Hence, the method may comprise providing a 3D-printed structure. Further, according to a second aspect, the invention may provide a 3D-printed structure. The 3D-printed structure may comprise a plurality of printed layers. Especially, each printed layer in the 3D-printed structure may correspond to a 3D-printed layer of the precursor structure. In embodiments, each printed layer may have a printed layer height (HLP) selected from the range of 0.06-1 mm, such as from the range of 0.08-0.8 mm. Further, at least one of the plurality of printed layers, such as especially each printed layer, may comprise a first particulate material and a crosslinked polymer matrix. Especially, the first particulate material may be configured embedded in the crosslinked polymer material. The first particulate material may comprise a first biological material. Further, at least 40%, such as at least 50%, of the first particulate material may have a particle size (Dp) selected from the range of 6-500 pm. In embodiments, the 3D-printed structure may comprise the first particulate material in a structure particulateconcentration (Cps) of 2-98 wt%, such as 4-95 wt%. The crosslinked polymer matrix may comprise one or more of a crosslinked alginate-based polymer and a crosslinked gelatin-based polymer. Further, the 3D-printed structure may comprise the crosslinked polymer matrix in a structure polymer concentration (CMS) of 4-90 wt%, such as 5-80 wt%. Further, in embodiments, Cps+ CMS > 20 wt% may apply, such as Cps+ CMS > 30 wt%. Hence, in specific embodiments, the invention may provide a 3D-printed structure, wherein: (A) the 3D-printed structure comprises a plurality of printed layers, wherein each printed layer has a printed layer height (HLP) selected from the range of 0.1-1 mm; (B) at least one of the plurality of printed layers comprises a first particulate material and a crosslinked polymer matrix; wherein the first particulate material is configured embedded in the crosslinked polymer material; (C) the first particulate material comprises a first biological material; wherein at least 50% of the first particulate material has a particle size (Dp) selected from the range of 6-500 pm; wherein the 3D-printed structure comprises the first particulate material in a structure particulate concentration (Cps) of 4-95 wt%; and (D) the crosslinked polymer matrix comprises one or more of a crosslinked alginate-based polymer and a crosslinked gelatin-based polymer; wherein the 3D-printed structure comprises the crosslinked polymer matrix in a structure polymer concentration (CMS) of 5-80 wt%; wherein Cps+ CMS > 20 wt%. Such a 3D-printed structure may especially be based on biodegradable materials. Further, especially the first particulate material may comprise or originate from a waste material, such that the 3D-printed structure may help in reducing waste. Further yet, such a 3D-printed structure may be relatively strong and versatile, and may be used for a variety of applications.

[0051] In embodiments, the 3D-printed structure may be obtainable using the method of the invention as defined herein. Especially, the 3D-printed structure of the method of the invention may be 3D-printed structure of the invention. Alternatively, the 3D-printed structure may be obtained using another suitable method. Yet, in specific embodiments, the 3D-printed structure may be obtainable using the method as defined herein.

[0052] The 3D-printed structure may comprise a plurality of printed layers. Especially, as indicated above, the 3D-printed structure may be obtained using the method of the invention, such that each printed layer in the 3D-printed structure may correspond to a 3D-printed layer of the precursor structure of the method. In embodiments, each printed layer (of the 3D-printed structure) may have a printed layer height (HLP). The printed layer height (HLP) may in embodiments be provided by HL*SL, wherein HL is the layer height of the corresponding 3D-printed layer in the precursor structure, and wherein SL is a shrinkage coefficient of the corresponding printed layer in the 3D-printed structure. In embodiments, SL may for each ofthe printed layers be selected from the range of 0.32-0.57, such as from the range of 0.34-0.55, especially from the range of 0.36-0.53, like from the range of 0.4-0.5. Additionally or alternatively, in embodiments, the printed layer height (HLP) may (for each layer individually) be selected from the range of > 0.06 mm, such as from the range of > 0.08 mm, especially from the range of > 0.1 mm. Additionally or alternatively, the printed layer height (HLP) may (for each layer individually) be selected from the range of < 1 mm, such as from the range of < 0.8 mm, especially from the range of < 0.5 mm. Hence, each printed layer may have a printed layer height (HLP) selected from the range of 0.06-1 mm, such as from the range of 0.08-0.8 mm, especially from the range of 0.1-0.5 mm. Especially, each printed layer may have the same printed layer height (HLP) (such as a printed layer height (HLP) deviating < 5% from an average printed layer height (HLP) of the plurality of printed layers).

[0053] At least one of the plurality of printed layers may comprise a first particulate material and a crosslinked polymer matrix. Further, > 20%, such as > 35%, especially > 50%, of the plurality of printed layers may comprise the first particulate material and the crosslinked polymer matrix. Further yet, > 65%, such as > 80%, especially > 90%, like > 95%, including 100%, of the plurality of printed layers may comprise the first particulate material and the crosslinked polymer matrix. Hence, in embodiments, each printed layer may comprise the first particulate material and the crosslinked polymer matrix. Would < 100% of the plurality of printed layers comprise the first particulate material and the crosslinked polymer matrix, the printed layers comprising the first particulate material and the crosslinked polymer matrix may be configured as a sub-structure within the 3D-printed structure (i.e., the printed layers may be adjacent to each other), yet may also be configured (homogeneously) distributed throughout the 3D-printed structure.

[0054] Hence, at least one of the plurality of printed layers, such as especially each printed layer, may comprise a first particulate material. The first particulate material of the 3D-printed structure of the invention may especially be selected from the first particulate materials described above in relation to the method of the invention. Hence, the first particulate material of the 3D-printed structure may comprise one or more of wood chips, ground pit material, ground seed material, and ground nut shell material. Further, at least part of the first particulate material of the 3D-printed structure may have the particle size (Dp) (and particle size distribution) as defined above. Especially, at least 40%, such as at least 50%, especially at least 60%, like at least 70%, of the first particulate material (i.e., of the plurality of first particles) may have the particle size (Dp). Further, at least 80%, such as at least 90%, especially at least 95%, including (essentially) 100%, of the first particulate material (i.e., of the plurality of firstparticles) may have the particle size (Dp). In embodiments, as indicated above, the particle size (Dp) may (for each first particle individually) be selected from the range of 3-600 pm, such as from the range of 6-500 pm, especially from the range of 10-400 pm.

[0055] The 3D-printed structure may comprise the first particulate material in a structure particulate concentration (Cps) of > 2 wt%, such as > 4 wt%, especially > 6 wt%, like > 8 wt% (relative to a total weight of the 3D-printed structure). Additionally or alternatively, the 3D-printed structure may comprise the first particulate material in a structure particulate concentration (Cps) of < 98 wt%, such as < 95 wt%, especially < 90 wt%, like < 85 wt% (relative to a total weight of the 3D-printed structure). Hence, the 3D-printed structure may comprise the first particulate material in a structure particulate concentration (Cps) of 2-98 wt%, such as 4-95 wt%, especially 6-90 wt%, like 8-85 wt%.

[0056] Further, the 3D-printed structure, such as at least one of the plurality of printed layers, especially each printed layer, may comprise a crosslinked polymer matrix. In embodiments, the crosslinked polymer matrix may be crosslinked via one or more of ionic bonds and covalent bonds. Further, in embodiments, the polymer matrices of adjacent printed layers may be crosslinked with each other, i.e., adjacent printed layers may be held or bound together via the crosslinked polymer matrix. Especially, the 3D-printed structure may comprise a (single) crosslinked polymer matrix, wherein the printed layers may each comprise a part of said crosslinked polymer matrix. In embodiments, the first particulate material may be configured embedded (such as suspended) in the crosslinked polymer matrix.

[0057] The crosslinked polymer matrix may comprise one or more of a crosslinked alginate-based polymer and a crosslinked gelatin-based polymer. In embodiments, the crosslinked polymer matrix may further comprise one or more of a crosslinked PEG-based polymer and a crosslinked polyester-based polymer (e.g. one or ore of a crosslinked PET-based polymer, a crosslinked PLA-based polymer, and a crosslinked PCL-based polymer). Further, the 3D-printed structure may comprise the crosslinked polymer matrix in a structure polymer concentration (CMS) of > 4 wt%, such as > 5 wt%, especially > 10 wt% (relative to a total weight of the 3D-printed structure). Additionally or alternatively, the 3D-printed structure may comprise the crosslinked polymer matrix in a structure polymer concentration (CMS) of < 90 wt%, such as < 80 wt%, especially < 70 wt% (relative to a total weight of the 3D-printed structure). Hence, the 3D-printed structure may comprise the crosslinked polymer matrix in a structure polymer concentration (CMS) of 4-90 wt%, such as 5-80 wt%, especially 10-70 wt%.

[0058] Further, in embodiments, Cps+ CMS > 20 wt% may apply, such as Cps+ CMS > 30 wt%, especially Cps+ CMS > 40 wt%, like Cps+ CMS > 50 wt%. Additionally or alternatively,in embodiments, Cps+ CMS < 100 wt% may apply, such as Cps+ CMS < 95 wt%, especially Cps+ CMs< 90 wt%, like Cps+ CMS < 85 wt%. Hence, in embodiments, 20 wt% < Cps+ CMS < 100 wt%, such as 30 wt% < Cps+ CMS < 95 wt%, especially 40 wt% < Cps+ CMS < 90 wt%, like 50 wt% < Cps+ CMs< 85 wt%.

[0059] In embodiments, the 3D-printed structure may comprise a second solvent. In embodiments, the second solvent may be equal to (such as selected from the same group as) the first solvent. Alternatively, the second solvent may comprise one or more of water and an alcohol (e.g. ethanol). In embodiments, the 3D-printed structure may comprise the second solvent in a solvent concentration of > 5 wt%, such as > 10 wt%, especially > 15 wt% (relative to a total weight of the 3D-printed structure). Additionally or alternatively, the 3D-printed structure may comprise the second solvent in a solvent concentration of < 30 wt%, such as < 25 wt%, especially < 20 wt% (relative to a total weight of the 3D-printed structure).

[0060] In embodiments, the 3D-printed structure may further comprise one or more of: (i) an antifungal material, (ii) a flame retardant material, (iii) a mineral, and (iv) a second biological material, wherein the second biological material may comprise one or more of a fungal material, a plant material, a bacteria, and a cell material (see also above). Especially, said one or more of an antifungal material, a flame retardant material, a mineral, and a second biological material may be configured embedded in the crosslinked polymer matrix.

[0061] The 3D-printed structure may be a viscoelastic structure, i.e., the 3D-printed structure may exhibit viscoelastic behavior. Especially, in a stress-strain measurement, wherein the 3D-printed structure is compressed while observing the stress and strain, the 3D-printed structure may exhibit viscoelastic behavior up to 15% strain, such as up to 20% strain, especially up to 25% strain. Further, in embodiments, at strains of > 15%, such as > 20%, especially > 25%, the 3D-printed structure may exhibit nonlinear stiffening. In embodiments, the 3D-printed structure may have a Young’s modulus at 10% strain (i.e., in the viscoelastic region). The Young’s modulus may be selected from the range of > 1 MPa, such as from the range of > 5 MPa, especially from the range of > 10 MPa. Further, the Young’s modulus may be selected from the range of > 25 MPa, such as from the range of > 50 MPa, especially from the range of > 55 MPa. Additionally or alternatively, the Young’s modulus may be selected from the range of < 120 MPa, such as from the range of < 100 MPa, especially from the range of < 90 MPa. Further, the Young’s modulus may be selected from the range of < 80 MPa, such as from the range of < 70 MPa, especially from the range of < 65 MPa. Hence, in embodiments, the Young’s modulus may be selected from the range of 1-120 MPa, such as from the range of 5-100 MPa, especially from the range of 10-90 MPa. Further, the Young’s modulus may beselected from the range of 25-80 MPa, such as from the range of 50-70 MPa, especially from the range of 55-65 MPa. Hence, in specific embodiments, the 3D-printed structure may have a Young’s modulus at 10% strain selected from the range of 5-100 MPa. A 3D-printed structure having such a Young’s modulus may be relatively sturdy, yet may still have some flexibility and / or compressibility. Hence, such a structure may be used to support another element, wherein the structure may slightly mold itself to the element it is supporting.

[0062] Further, the 3D-printed structure, such as especially the material of the 3D-printed structure, may have a density. In embodiments, the density may be selected from the range of < 1.1 g / cm3, such as from the range of < 1.0 g / cm3, especially from the range of < 0.9 g / cm3. Additionally or alternatively, the density may be selected from the range of > 0.4 g / cm3, such as from the range of > 0.5 g / cm3, especially from the range of > 0.55 g / cm3. That is, in embodiments, the density may be selected from the range of 0.4- 1.1 g / cm3, such as from the range of 0.5- 1.0 g / cm3, especially from the range of 0.55-0.9 g / cm3. Hence, in specific embodiments, the 3D-printed structure may have a density selected from the range of 0.5- 1.0 g / cm3. Such a density may facilitate providing a buoyant 3D-printed structure. Further, with such a density, a relatively large 3D-printed structure may be relatively light in weight.

[0063] As indicated above, the 3D-printed structure may be prepared using the method of the invention. Further, as indicated above, the crosslinked structure may have a first distance di, wherein for at least 80%, such as for at least 90%, of the crosslinked structure may apply that a distance to a nearest surface may be the first distance di. Similarly, the 3D-printed structure may have a second distance d2. In embodiments, for at least 80%, such as for at least 90%, especially for at least 95%, like for (essentially) 100%, of the (volume of the) 3D-printed structure may apply that a distance to a nearest structure surface may be (at most) the second distance d2. That is, from any point within said > 80%, such as > 90%, especially > 95%, like 100%, of the (volume of the) 3D-printed structure, the shortest distance from that point to the nearest structure surface may be (at most) the second distance dz. In embodiments, the second distance d2 may be selected from the range of < 6 mm, such as from the range of < 5 mm, especially from the range of < 4 mm. Additionally or alternatively, the second distance d2 may be selected from the range of > 0.02 mm, such as from the range of > 0.05 mm, especially from the range of > 0.1 mm. Hence, in specific embodiments, for at least 90% of the 3D-printed structure may apply that a second distance d2 to a nearest structure surface may be selected from the range of < 5 mm. Such a second distance dz may facilitate that the 3D-printed structure may either be relatively small or relatively porous. A relatively porous 3D-printed structure may have a reduced weight compared to a non-porous 3D-printed structure. Further, a relativelyporous 3D-printed structure may facilitate cultivating e.g. a plant in or on the 3D-printed structure, wherein the roots of the plant may extend into the porous 3D-printed structure.

[0064] In embodiments, the 3D-printed structure may have one or more of a height, width, and length of < 2*d2, such that the 3D-printed structure may have the second distance d2 in said one or more of the height, width, and length. Especially, the 3D-printed structure may have a structure outer surface (defined by one or more major faces of the 3D-printed structure), wherein the structure outer surface may define one or more of the height, width, and length of the 3D-printed structure. Note that, would the 3D-printed structure be a hollow structure (e.g. a hollow tube), the inside wall of the hollow structure may be comprised by the structure outer surface. Yet, in embodiments, the (outside) dimensions of the 3D-printed structure may be > 2*d2. In such embodiments, the 3D-printed structure may comprise one or more (through) channels. The one or more (through) channels may be configured extending (inwards) from the outer surface. Alternatively, (at least one of) the one or more (through) channels may be an internal channel, wherein the internal channel may be configured separated from (the nearest part of) the structure outer surface by a distance of < d2 (e.g. separated by a (thin) layer of 3D-printed structure material). As indicated above, the one or more (through) channels may be formed by selecting an infill percentage during 3D-printing. Each channel (in the 3D-printed structure) may be defined by a channel wall (lining the channel, and optionally physically coupled with the structure outer surface). In such embodiments, the nearest structure surface may be selected from the group of the structure outer surface and the one or more channel walls (of the one or more channels). That is, in such embodiments, for > 80%, such as for > 90%, especially for > 95%, like for (essentially) 100%, of the (volume of the) 3D-printed structure may apply that a distance to one or more of (i) the structure outer surface, and (ii) at least one of the one or more channel walls, may be the second distance d2. Hence, in specific embodiments, the 3D-printed structure may comprise a structure outer surface; wherein the 3D-printed structure may comprise one or more channels, wherein each channel may be defined by a structure channel wall; and wherein the nearest structure surface may be selected from the group of the structure outer surface and the one or more structure channel walls. Such one or more channels may facilitate providing a 3D-printed structure with relatively large dimensions (i.e., > 2*dz), wherein for > 80% of the 3D-printed structure may still apply that a distance to the nearest structure surface may be < d2.

[0065] In embodiments, each of the one or more channels (of the 3D-printed structure) may have a smallest structure cross-sectional diameter (Dcs). In embodiments, the one or more channels may have a circular shape in said smallest cross-section, wherein the structure cross-sectional diameter (Dcs) may be the diameter of said circular shape. Alternatively, the one or more channels may have an elliptical or regular (simple) polygonal shape in said smallest crosssection, wherein the structure cross-sectional diameter (Dcs) may be an equivalent circular diameter. In embodiments, Dcsmay be selected from the range of > 0.01 mm, such as from the range of > 0.02 mm, especially from the range of > 0.05 mm. For relatively large structure cross-sectional diameters (Dcs), e.g. Dcs> 1 cm, the 3D-printed structure may be considered as having a grid shape (“perforated shape”) or open shape, such that the 3D-printed structure may not comprise the one or more channels, and such that the (inside) walls of the perforations or open shape may be comprised by the structure outer surface.

[0066] In embodiments, the 3D-printed structure may be configured to absorb one or more third solvents, such as especially one or more of water and an alcohol (e.g. ethanol). Especially, when soaking the 3D-printed structure in the third solvent, a weight of the 3D-printed structure may increase by > 90%, such as by > 100%, especially by > 105%. Additionally or alternatively, when soaking the 3D-printed structure in the third solvent, the weight of the 3D-printed structure may increase by < 120%, such as by < 115%, especially by < 110%. Further, upon soaking the 3D-printed structure in the third solvent, a volume of the 3D-printed structure may increase by > 50%, such as by > 55%, especially by > 60%. Additionally or alternatively, upon soaking the 3D-printed structure in the third solvent, the volume of the 3D-printed structure may increase by < 75%, such as by < 70%, especially by < 65%. In embodiments, the 3D-printed structure may reach > 90%, such as > 95%, especially > 98%, including 100%, of its final weight and volume after soaking within 2 h of soaking.

[0067] In embodiments, the 3D-printed structure may be recycled, wherein especially a new 3D-printed structure may be produced from the recycled (or “old”) 3D-printed structure. Hence, the invention may provide a method for recycling the 3D-printed structure. The method may comprise grinding the 3D-printed structure to provide a powdered 3D-printed material. In embodiments, at least part of the powdered 3D-printed material may have a powdered particle size (Dp0). Especially, at least 40%, such as at least 50%, especially at least 60%, like at least 70%, of the powdered 3D-printed material may have the powdered particle size (Dpo). Further, at least 80%, such as at least 90%, especially at least 95%, including (essentially) 100%, of the powdered 3D-printed material may have the powdered particle size (Dpo). In embodiments, the powdered particle size (Dpo) may (for each particle individually) be selected from the range of 3-600 pm, such as from the range of 6-500 pm, especially from the range of 10-400 pm.

[0068] Further, the method of recycling the 3D-printed structure may comprise combining the powdered 3D-printed material with the first polymer solution as defined aboveto provide a recycled ink. In embodiments, the method may comprise combining the powdered 3D-printed material with the first polymer solution such, that the recycled ink may comprise > 5 wt%, such as > 10 wt%, especially > 15 wt%, of the powdered 3D-printed material (relative to the total weight of the recycled ink). Additionally or alternatively, the recycled ink may comprise < 30 wt%, such as < 25 wt%, especially < 20 wt%, of the powdered 3D-printed material. Hence, the recycled ink may further comprise 70-95 wt%, such as 75-90 wt%, especially 80-85 wt%, of the first polymer solution (relative to the total weight of the recycled ink). In embodiments, the recycled ink may be 3D-printed similarly to the gelled printing composition, to provide a recycled precursor structure. Further, the method may comprise crosslinking the recycled precursor structure to provide a recycled crosslinked structure by one or more of (i) exposing the recycled precursor structure to the second crosslinker, and (ii) exposing the recycled precursor structure to UV radiation, in the same way as described above for the precursor structure. In embodiments, the method may comprise crosslinking the recycled precursor structure by exposing the recycled precursor structure to UV radiation, wherein the method may comprise adding one or more of Ru2+and LAP to the recycled ink (prior to 3D-printing). Further yet, the method may comprise removing at least part of the first solvent from the recycled crosslinked structure to provide a recycled 3D-printed structure.

[0069] According to a further aspect, the invention may provide an object comprising the 3D-printed structure as defined herein. In embodiments, the object may be an agricultural material element. For instance, the agricultural material element may be a water management material, configured to be mixed into the ground to moderate the amount of water in the ground. Alternatively, the agricultural material element may be a seed(ling) hosting element, configured to host a seed(ling), and optionally to moderate the amount of water available to the seed(ling). Yet, in embodiments, the object may be a packaging material element. For instance, the packaging material element may be a box or pallet. In further embodiments, the object may be a culture medium element. For instance, the culture medium element may be a scaffold configured to host a (growing) cell culture or an artificial reef for the culturing and / or preservation of one or more of (marine) algae and invertebrates (e.g. barnacles, corals, and oysters). Yet, in embodiments, the object may be a biomaterial element. For instance, the biomaterial element may be an implant, a (skin) graft, or an artificial reef. Alternatively, the object may be a pharmaceutical material element. For instance, the pharmaceutical material element may be a bandage or pill box. Yet, in embodiments, the object may be a cosmetic material element. For instance, the cosmetic material element may be a make-up brash or (make-up and / or face) mask. Alternatively, in embodiments, the object may be a constructionmaterial element. For instance, the construction material element may be a (lightweight) filler material, an insulating material, a furniture element, or wall plate (e.g. for an external facade of a building). Additionally or alternatively, the object may be a (home) decor element, such as a decorative element (e.g. an accessory, applique, a trinket, an adornment, etc.). Hence, in specific embodiments, the invention may provide an object comprising the 3D-printed structure as defined herein, wherein the object is selected from the group comprising an agricultural material element, a packaging material element, a culture medium element, a biomaterial element, a pharmaceutical material element, a cosmetic material element, and a construction material element. An object comprising the 3D-printed structure may provide the benefit that at least part of the object may consist of a biodegradable material and / or a waste material. Hence, such an object may especially be sustainable.

[0070] According to a further aspect, the invention may provide a use of the 3D-printed structure as defined herein. Especially, the 3D-printed structure may be used in agriculture, e.g. to moderate a water level in the soil or to host seed(ling)s. Additionally or alternatively, the 3D-printed structure may be used in packaging, e.g. as (part of) a box or pallet. Additionally or alternatively, the 3D-printed structure may be used in pharmaceutics, e.g. as a bandage or pill box, or as a scaffold for growing cells. Additionally or alternatively, the 3D-printed structure may be used in cosmetics, e.g. as (part of) a make-up brash. Additionally or alternatively, the 3D-printed structure may be used in recycling, e.g. by recycling an old 3D-printed structure to produce a new 3D-printed structure. Additionally or alternatively, the 3D-printed structure may be used in construction, e.g. as an insulation material or wall plate. Hence, in specific embodiments, the invention may provide a use of the 3D-printed structure as defined herein in one or more of agriculture, packaging, pharmaceutics, cosmetics, recycling, and construction.

[0071] Hence, the 3D-printed structure may be used to host seed(ling)s, or the object comprising the 3D-printed structure may be a seed(ling) hosting element. As such, the 3D-printed structure may have a cylindrical shape, wherein the cylindrical shape may have a circular first face, a circular second face, and a curved side face configured bridging the first and second faces. Further, as such, the 3D-printed structure may have a depression in one (or more) of the first face and the second face. Especially, the (cylindrical shape of the) 3D-printed structure may have a through opening extending from the first face to the second face. Would the 3D-printed structure be used to host seed(ling)s, or would the object be a seed(ling) hosting element, the 3D-printed structure (such as the gelled printing composition used to make the 3D-printed structure) may comprise an antifungal agent and / or a fertilizer. Further, would the 3D-printed structure be used to host seed(ling)s, or would the object be a seed(ling) hostingelement, the 3D-printed structure may especially be obtainable by crosslinking the precursor structure by submerging the precursor structure in a solution of the second crosslinker, wherein the concentration (Cb) of the second crosslinker in the solution may be 0.1*Cc2 < Cb < 0.5* Cc2, such as 0.15*CC2 < Cb < 0.4* Cc2, especially 0.2*Cc2 < Cb < 0.3* Cc - Such a concentration (Cb) may facilitate that the resulting 3D-printed structure may have a higher flexibility, and / or may be softer, than 3D-printed structures crosslinked using a second crosslinker concentration of Cc - As such, a sprouting or growing seedling may be obstructed relatively little by the matrix of the 3D-printed structure. Hence, in embodiments, the 3D-printed structure may be used to host seed(ling)s. Additionally or alternatively, in embodiments, the object (comprising the 3D-printed structure) may be a seed(ling) hosting element.

[0072] Alternatively, the 3D-printed structure may be used as a scaffold for growing cells, or the object comprising the 3D-printed structure may be a culture medium element. As such, the 3D-printed structure may be sterilized by exposing the 3D-printed structure to UV irradiation (e.g. for 3x20 minutes). Further, as such, the 3D-printed structure may be submerged in a cell culture medium (e.g. a-MEM or “minimum essential medium”) for > 6 h, such as > 12 h, especially > 24. After removal of the 3D-printed structure from the cell culture medium, a solution comprising cells may be applied to the 3D-printed structure, and the 3D-printed structure may be incubated for 1-5 h, such as 2-4 h, to allow the cells to adhere to the 3D-printed structure. Consequently, the 3D-printed structure may be submerged in a cell culture medium (e.g. a-MEM), and the submerged 3D-printed structure may be incubated further.

[0073] BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: Fig. 1 schematically depicts an embodiment of the method of the invention; Fig. 2 schematically depicts an embodiment of the crosslinked structure and the 3D-printed structure; and Fig. 3 schematically depicts an embodiment of the object. The schematic drawings are not necessarily on scale.

[0075] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0076] Fig. 1 schematically depicts an embodiments of the method for providing a 3D-printed structure 2000. The method may comprise providing a gelled printing composition 1000. Especially, the gelled printing composition 1000 may comprise a first particulate material 110, a first polymer material 120, a first crosslinker 130, and a first solvent 140. The firstparticulate material 110 may comprise a first biological material. Further, at least 50% of the first particulate material 110 may have has a particle size Dp selected from the range of 6-500 pm. Especially, the gelled printing composition 1000 may comprise the first particulate material 110 in a particulate concentration Cp of 1-20 wt% (relative to the total weight of the gelled printing composition 1000). The first polymer material 120 may comprise one or more of an alginate-based polymer and a gelatin-based polymer. Further, the gelled printing composition 1000 may comprise the first polymer material 120 in a polymer concentration CM of 2-15 wt%. In embodiments, the first crosslinker 130 may be configured to (at least) partially crosslink the first polymer material 120. Especially, the first polymer material 120 and the first crosslinker 130 may be (configured) (at least partially) dissolved in the first solvent 140. The method may further comprise 3D-printing the gelled printing composition 1000 with a 3D-printer 400 to provide a precursor structure 210. The 3D-printer 400 may comprise a nozzle 410. Especially, the nozzle 410 may have a nozzle diameter Dnselected from the range of 0.4-1.2 mm. Further, 3D-printing the gelled printing composition 1000 may comprise extruding the gelled printing composition 1000 via the nozzle 410 at (i) a printing pressure Pi selected from the range of 40-250 kPa, (ii) a printing temperature Tp selected from the range of 18-30 °C, and (iii) a printing speed selected from the range of 4-12 mm / s. Further, the method may comprise at least partially) crosslinking the precursor structure 210 to provide a crosslinked structure 220. Crosslinking the precursor structure 210 may comprise one or more of (i) exposing the precursor structure 210 to a second crosslinker 150, and (ii) exposing the precursor structure 210 to radiation 901 (provided by a light source 900). The method may further comprise removing at least part of the first solvent 140 from (i.e., drying) the crosslinked structure 220 to provide the 3D-printed structure 2000.

[0077] Providing the gelled printing composition 1000 may especially comprise (as schematically depicted at the top of Fig. 1) dissolving the first polymer material 120 in the first solvent 140 to provide a first polymer solution 610. Further, providing the gelled printing composition 1000 may comprise combining the first polymer solution 610 with the first particulate material 110 to form a first composition 620. Finally, providing the gelled printing composition 1000 (or the printing composition 100) may comprise combining the first composition 620 with the first crosslinker 130. The method may further comprise preparing the gelled printing composition 1000 from a printing composition 100, such as especially by one or more of (i) crosslinking the first polymer material 120 with the first crosslinker 130, and (ii) gelation. Especially, preparing the gelled printing composition 1000 from the printing composition 100 may comprise increasing the first viscosity r|i of the printing composition 100to provide the gelled printing composition 1000 having a second viscosity r|2- Hence, r|2 > 4*r|i may apply.

[0078] The first particulate material 110 may especially comprise one or more of wood chips, ground pit material, ground seed material, and ground nut shell material. Further, the first polymer material 120 may further comprise one or more of a polyethylene-glycol-based polymer and a polyester-based polymer. Additionally, the first crosslinker 130 may comprise one or more of Ca2+, Mg2+, Ba2+, Sr2+, Al3+, Ru2+, and LAP. Further yet, the first solvent 140 may comprise one or more of water and ethanol.

[0079] The gelled printing composition 1000 may further comprise one or more of: (i) an antifungal material, (ii) a flame retardant material, (iii) a mineral, and (iv) a second biological material, wherein the second biological material may comprise one or more of a fungal material, a plant material, a bacteria, and a cell material. These additives may be added at any point during the preparation of the gelled printing composition 1000. Yet, especially, these additives may be added simultaneously with the first particulate material 110.

[0080] Fig. 1 further schematically depicts two methods for crosslinking the precursor structure 210. On the left, Fig. 1 schematically depicts crosslinking the precursor structure 210 using a second crosslinker 150, while on the right, Fig. 1 schematically depicts crosslinking the precursor structure 210 using radiation 901. Focusing on the method on the left, the first crosslinker 130 may comprise one or more of Ca2+, Mg2+, Ba2+, Sr2+, and Al3+. Further, in this method, the second crosslinker 150 may comprise one or more of Ca2+, Mg2+, Ba2+, Sr2+, Al3+, strontium acetate, glutaraldehyde, epichlorohydrin, and genipin. Here, crosslinking the precursor structure 210 may especially comprise submerging the precursor structure 210 in a solution comprising the second crosslinker 150. Alternatively, the precursor structure 210 may be sprayed with the solution comprising the second crosslinker 150.

[0081] Turning towards the crosslinking method depicted on the right, here, the first crosslinker 130 may comprise Ru2+, and the gelled printing composition 1000 may further comprise sodium persulfate. Additionally or alternatively, the first crosslinker 130 may comprise lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP). In this method, crosslinking the precursor structure 210 may especially comprise exposing the precursor structure 210 to UV radiation 901.

[0082] The precursor structure 210 (and the crosslinked structure 220) may comprise a plurality of 3D-printed layers 500 (comprising 3D-printed gelled printing composition 1000). In Fig. 1, (the strands of 3D-printed gelled printing composition 1000 in) the plurality of 3D-printed layers 500 are depicted as being deposited in a cross-wise manner, with (the strands of3D-printed gelled printing composition 1000 in) each 3D-printed layer 500 being deposited (and oriented) perpendicular to (the strands of 3D-printed gelled printing composition 1000 in) a lower (or “earlier deposited”) 3D-printed layer 500. Note that this need not be the case, and that in embodiments (the strands of 3D-printed gelled printing composition 1000 in) each 3D-printed layer 500 may be configured in parallel. Each (3D-printed) layer 500 may have a layer height HL. Especially, the layer height HL may (for each layer 500 individually) be selected from the range of 0.25-1 mm.

[0083] The precursor structure 210 may be crosslinked to form the crosslinked structure 220. Further, the crosslinked structure 220 may be dried to provide the 3D-printed structure 2000. The crosslinked structure 220 may shrink while removing the at least part of the first solvent 140. Especially, the crosslinked structure 220 may have a first volume Vi, and the 3D-printed structure 2000 may have a second volume V2. In embodiments, x*V2 = Vi may apply, wherein x > 1.5. The shrinkage of the crosslinked structure 220 may be homogeneous (see also above). Hence, the 3D-printed structure 2000, scaled by a factor x, may have > 90% overlap with a design of the 3D-printed structure 2000 used for the 3D printing. The 3D-printed structure 2000 may also comprise a plurality of printed layers 700, each having a printed layer height HLP.

[0084] Fig. 2 schematically depicts a further embodiment of the crosslinked structure 220. In embodiments, for at least 90% of the crosslinked structure 220 may apply that a first distance di to a nearest surface 2201 (of the crosslinked structure 220) may be selected from the range of < 10 mm. The crosslinked structure 220 may comprise an outer surface 2210. Further, the crosslinked structure 220 may comprise one or more (through) channels 2250. Especially, each channel 2250 may be defined by a channel wall 2260. In such embodiments, the nearest surface 2201 may be selected from the group of the outer surface 2210 and the one or more channel walls 2260.

[0085] Fig. 2 may further schematically depict an embodiment of the 3D-printed structure 2000 (ignoring the scaling factor x with regards to the crosslinked structure 220). The 3D-printed structure 2000 may comprise a plurality of printed layers 700. Especially, each printed layer 700 may have a printed layer height HLPselected from the range of 0.1-1 mm. Further, at least one of the plurality of printed layers 700, such as each printed layer 700, may comprise a first particulate material 110 and a crosslinked polymer matrix 710. The first particulate material 110 may especially be configured embedded in the crosslinked polymer matrix 710. The first particulate material 110 and the crosslinked polymer matrix 710 are schematically depicted for the bottom two printed layers 700. The first particulate material 110may comprise a first biological material. Further, at least 50% of the first particulate material 110 may have a particle size Dp selected from the range of 6-500 pm. The 3D-printed structure 2000 may especially comprise the first particulate material 110 in a structure particulate concentration Cpsof 4-95 wt%. Further, the crosslinked polymer matrix 710 may comprise one or more of a crosslinked alginate-based polymer and a crosslinked gelatin-based polymer. Especially, the 3D-printed structure 2000 may comprise the crosslinked polymer matrix 710 in a structure polymer concentration CMS of 5-80 wt%. Further, Cps+ CMS > 30 wt% may apply.

[0086] Similar to the crosslinked structure 220 above, for at least 90% of the 3D-printed structure 2000 may apply that a second distance d2 to a nearest structure surface 2001 may be selected from the range of < 5 mm. Further, the 3D-printed structure 2000 may comprise a structure outer surface 2010. The 3D-printed structure 2000 may further comprise one or more (through) channels 2050. Especially, each channel 2050 may be defined by a structure channel wall 2060. In such embodiments, the nearest structure surface 2001 may be selected from the group of the structure outer surface 2010 and the one or more structure channel walls 2060.

[0087] Fig. 2 may further schematically depict an embodiment of the object 4000 comprising the 3D-printed structure 2000 as defined herein. Especially, the object 4000 may be selected from the group comprising an agricultural material element, a packaging material element, a culture medium element, a biomaterial element, a pharmaceutical material element, a cosmetic material element, and a construction material element.

[0088] Fig. 3 schematically depicts a further embodiment of the 3D-printed structure 2000 and / or of the object 4000. Here, the (through) channels 2050 may be relatively large. Hence, in embodiments, the 3D-printed structure 2000 and / or the object 4000 may be described as having an open shape. In such embodiments, the 3D-printed structure 2000 and / or the object 4000 may not (be considered as) compris(e)(ing) the one or more channels 2050, and the one or more structure channel walls 2060 may be comprised by the structure outer surface 2010.

[0089] Experiments

[0090] Preparation of gelled printing composition

[0091] Unless specified otherwise, weight percentage (wt%) mentioned below are relative to the total weight of the gelled printing composition 1000.

[0092] Alginate-based gelled printing composition 1000

[0093] For the alginate-based gelled printing composition 1000, 3-12 wt% sodium alginate is dissolved in a 0.9% solution of NaCl in demineralized water at 30 °C, and stirred overnight to prepare the first polymer solution 610. Then, the first polymer solution 610 is mixed (by hand) with 1 wt%, 5 wt%, or 10 wt% of first particulate material 110 (groundhardwood) until a homogeneous mixture is achieved (generally after 1 min), which homogeneous mixture may be the first composition 620. Next, 15 pL of CaCfi (1 M in demineralized water) is added per 1 mL of the first composition 620, the resulting mixture is mixed for 30 s, and the resulting first (gelled) ink 100(0) is either used for 3D-printing directly, or stored at 4 °C until use. The alginate-based gelled printing composition 1000 may hereafter be referred to as “Ink A x-y”, wherein x may refer to the wt% of alginate in the gelled printing composition 1000, and wherein y may refer to the wt% of first particulate material 110 in the gelled printing composition 1000. Hence, an alginate-based gelled printing composition 1000 comprising 8 wt% alginate and 10 wt% first particulate material 110 may be Ink A 8-10.

[0094] Several further alginate-based gelled printing compositions 1000 were prepared by dissolving 4 wt%, 8 wt%, or 12 wt% sodium alginate in a 0.9% solution of NaCl in demineralized water at 40 °C. The solutions were stirred at 40 °C for 6-8 hours, and stored overnight at 4°C, to provide the first polymer solution 610. The overnight storage may help to stabilize the polymer network and eliminate air bubbles. After overnight storage, three types of first particulate material 110 were mixed into the first polymer solution 610 (by hand) until homogeneity in order to provide the first composition 620: ground hardwood, ground walnut shell, and ground olive pits. The concentration of the first particulate material 110 in the first composition 620 was in each case 10 wt%. Next, 15 pL of CaCh (1 M in demineralized water) is added per 1 mL of the first composition 620, and the resulting first (gelled) ink 100(0) is either used for 3D-printing directly, or stored at 4 °C until use. The alginate-based gelled printing composition 1000 may hereafter be referred to as “Ink A x-yz”, wherein x may refer to the wt% of alginate in the gelled printing composition 1000, wherein y may refer to the wt% of first particulate material 110 in the gelled printing composition 1000, and wherein z may refer to the first particulate material 110 used, with W indicating ground hardwood, S indicating ground walnut shell, and O indicating ground olive pits. Hence, an alginate-based gelled printing composition 1000 comprising 8 wt% alginate and 10 wt% ground hardwood may be Ink A 8- 10W.

[0095] Composite alginate-gelatine gelled printing composition 1000

[0096] For the composite alginate-gelatine gelled printing composition 1000, 5 wt% of sodium alginate, 3.2 wt% of gelatin (powder), and 5 mM sodium persulfate are dissolved in 20 mL of demineralized water at 50 °C, and the first polymer solution 610 is left overnight to ensure complete dissolution. The first polymer solution 610 is then heated to 40 °C, mixed with 10 wt% of first particulate material 110 (ground hardwood), and allowed to cool to room temperature. Subsequently, 15 pL of a ruthenium solution (66 mM in demineralized water) isadded per 1 mL of first composition 620. After the addition of the ruthenium solution, the material is stored in the dark at 4 °C for 2 hours. The composite alginate-gelatine gelled printing composition 1000 may hereafter be referred to as “Ink B”.

[0097] Composite gelatin methacrylate-PEG diacrylate gelled printing composition 1000

[0098] For the composite gelatin methacrylate-PEG diacrylate gelled printing composition 1000, 5 wt% of gelatin methacrylate and 2.5 wt% of PEG diacrylate are dissolved in demineralized water at 50 °C, and the first polymer solution 610 is left overnight to ensure complete dissolution. The first polymer solution 610 is then heated to 40 °C, mixed with 10 wt% of first particulate material 110, and allowed to cool to room temperature. Subsequently, 0.5 wt% of LAP is added, and the material is stored in the dark at 4 °C. The composite gelatin methacrylate-PEG diacrylate gelled printing composition 1000 may hereafter be referred to as “Ink C”.

[0099] Viscosities of gelled printing compositions 1000

[0100] The second viscosity r|2 of various gelled printing compositions 1000 is determined using rheological measurements. Especially, a stress-controlled rheometer (Discovery Hybrid Rheometer DHR-3, TA Instruments, USA) equipped with parallel plate geometry was used. The upper plate diameter was 20 mm, and the lower plate diameter 40 mm. The surfaces of both plates were sandblasted to prevent slippage during measurement. A fixed gap of 1000 pm was maintained across all the tests. Measurements were conducted at a controlled temperature of 25 °C. For each test, approximately 0.3 mL of printing composition 1000 was carefully deposited at the centre of the lower plate using a spatula to avoid introducing air bubbles. The upper plate was then lowered to the defined gap, and any excess printing composition 1000 was trimmed from the edges. Before data acquisition, the printing composition 1000 was allowed to equilibrate under zero shear for 100 seconds to minimize structural disturbance from loading. A linear stress ramp was then applied, increasing shear stress from 0 to 5000 Pa over 60 seconds. All the measurements were performed in stress-controlled mode, wherein the instrument imposes a defined torque corresponding to a specific shear stress, and the resulting angular velocity (to) is recorded.

[0101] The shear rate (y‘) was calculated based on parallel plate geometry using the equation y = (R * to) / / i, where R is the radius of the rotating plate, to is the angular velocity (rad / s), and h is the plate gap. The viscosity q was not measured directly, but calculated based on rotational speed (shear rate) using the equation q(y) = r / y, wherein r is the shear stress. With increasing shear rate, a minimal flow is observed at < 101s1shear rate, causing theviscosity (stress / shear rate) to increase with an increase in stress. It should be noted that the viscosity at minimal flow does not represent the true viscosity. Further, upon reaching the yield stress (at > 101s1shear rate), the gelled printing composition 1000 began to flow freely, exhibiting shear thinning behavior. For modeling purposes, the viscosity was assumed to transition from a plateau value (inflexion at ~101s"1) to a negligible value at infinite shear rate, justified by a two-order magnitude drop at a shear rate of 102s’1. Shear thinning behavior was

[0102]

[0103] fitted using the (normalized) Carreau-Yasuda model, assuming r|oo ~ 0: = (1 +

[0104] ’72

[0105]

[0106] wherein 2 is a time constant indicating the onset of shear-thinning, a is a dimensionless transition parameter (with higher values reflecting a sharper viscosity drop while lower values correspond to smoother transitions), n is the flow behaviour index, and r|oo is the viscosity at infinite shear rate. The second viscosity rp may thus correlate to a zero-shear viscosity. The second viscosities rp (with standard deviations) are obtained via scipy. optimize. curve Jit in Python, and are indicated in the table below for various gelled printing compositions 1000. Further indicated in the table is the first viscosity r|i of a printing composition 100 comprising 8 wt% alginate, 10 wt% first particulate material 110, first solvent 140, yet no first crosslinker 130. This printing composition 100 is labelled as “Comp, ink A”.

[0107]

[0108] The first viscosity r|i of Comp, ink A may be compared to the second viscosity r|2 of Ink A 8-10, as these inks may differ only in the presence of the first crosslinker 130 (and thus in the presence of (ionic) crosslinks in or between the first polymer material 120). Hence, as can be seen from the table above, the first viscosity r|i of the printing composition 100 may be increased by roughly a factor 6.8 to prepare the gelled printing composition 1000 having the second viscosity rp. Such an increase in viscosity may facilitate that the gelled printing composition 1000 may maintain its shape after 3D-printing, thereby allowing the 3D-printing of low-tolerance shapes and higher structures.Further, the table above indicates that, while increasing the wt% of alginate in ink A from 4 wt% to 8 wt% and 12 wt%, the second viscosity rp may increase from 1019 Pa-s to 3671 Pa-s and 6000 Pa-s, respectively. Yet, the wt% of the first particulate material 110 may have relatively little effect on the second viscosity rp. Especially, increasing the wt% of the first particulate material 110 from 1 wt% to 10% results in a decrease in the second viscosity rp of 929 Pa-s.

[0109] 3D-printing of gelled printing compositions 1000

[0110] The table below indicates the printing speeds Rp, printing pressures Pi, printing temperatures Tp, nozzle diameters Dn, and printing times tp used for 3D-printing Ink A 8-10, Ink B, and Ink C. Further, the layer heights HL obtained after 3D-printing are indicated.

[0111]

[0112] As can be seen from the table above, in all cases a layer height HL of 0.73*DnHL < 0.97*Dnis obtained after 3D-printing, indicating that the gelled printing composition 1000 maintains its shape after 3D-printing. As indicated for ink A, the gelled printing compositions 1000 could be printed at various printing speeds Rp and printing pressures Pi, and with various nozzle diameters Dn, with all parameters providing a suitable precursor structure 210.

[0113] Crosslinking of precursor structure 210

[0114] The precursor structures 210, obtained after the 3D-printing as indicated above, are crosslinked to provide the corresponding crosslinked structures 220. The precursor structures 210 based on ink A are crosslinked by submerging the precursor structures 210 in one of (i) a 200 mM solution of CaCh, (ii) a 200 mM solution of strontium acetate, and (iii) a 2% solution of glutaraldehyde. The precursor structures 210 were submerged for 15 min, 30 min, 60 min, 3 h, or overnight. For a Ixlxl cm3(solid) cube of precursor structure 210, full crosslinking of the interior of the precursor structure 210 is observed after submerging the precursor structure 210 in 200 mM CaCh for 6 h.

[0115] Precursor structures 210 based on ink B and / or ink C are crosslinked by exposing the respective precursor structure 210 to UV radiation. For a Ixlxl cm3(solid) cube of precursor structure 210, exposed to UV radiation with an intensity of 16 mW / cm2for 5 min, crosslinking is observed up to 1.57-2.6 mm into the sample. Optionally, precursor structures210 based on ink B are further crosslinked by submerging the precursor structure 210 in a 1 M CaCh solution.

[0116] Shrinkage of crosslinked structures 220

[0117] The crosslinked structures 220 are dried to provide 3D-printed structures 2000. Upon removing at least part of the first solvent 140, the dimensions and first volume Vi of the crosslinked structures 220 decrease to provide 3D-printed structures with a second volume V2. The shrinkage S of the crosslinked structures 220 is tested for both cylindrical shapes and cubic shapes, and the results are indicated in the tables below, along with the density of the 3D-printed structures 2000. The shrinkage S in percentages is determined using the formula S(%) = ((Di - D2) / Di)*100, with Di indicating a dimension in the crosslinked structure 220, and D2 indicating the corresponding dimension in the 3D-printed structure 2000. Hence, a larger value for the shrinkage S indicates that the relevant dimension D has decreased more upon removing the at least part of the first solvent 140. The shrinkage S in volume is based on the shrinkage S in height and (i) length and width, or (ii) diameter.

[0118] Shrinkage S for 3D-printed structures 2000 in cylindrical shapes

[0119]

[0120] As can be seen from the table above, an increase in the wt% of first particulate material 110 in the gelled printing composition 1000 results in a lower shrinkage S (i.e., a lower decrease in the relevant dimension D upon removing the at least part of the first solvent 140), a lower water evaporation, and a lower density of the 3D-printed structure 2000. Further, an increase in the wt% of the first polymer material 120 in the gelled printing composition 1000 results in a higher shrinkage S, a lower water evaporation (likely due to the hygroscopic properties of the first polymer material 120), and a higher density of the 3D-printed structure 2000.

[0121] Shrinkage S for 3D-printed structures 2000 in cubic shapes

[0122]

[0123] For ink A 8-10 (Sr acetate), strontium acetate is used as the second crosslinker 150, rather than CaCh. As can be seen by comparing this entry to the one for Ink A 8-10, the use of strontium acetate did not effect the shrinkage S or density. Further, for inks with the indication (heat), the crosslinked structure 220 is dried at 50 °C for 30, at 60 °C for 2.5 h, and subsequently airdried at room temperature. For ink A, this results in a slightly higher shrinkage S, while the water evaporation and density remain relatively unaffected. Further, for inks A with the indication (EtOH and heat), the crosslinked structure 220 is subsequently submerged in (i) 50% ethanol in demineralized water for 5 minutes, (ii) 70% ethanol in demineralized water for 5 minutes, and (iii) 90% ethanol in demineralized water for 5 minutes, followed by the drying procedure indicated above for samples labeled with (heat). The submerging in ethanol appears to lead to a slightly higher shrinkage and a slightly lower density.

[0124] Further, for several of the above cubic-shaped 3D-printed structures 2000, as well as additional 3D-printed structures, the volume shrinkage S, Young’s modulus, and density were determined. The Young’s modulus was determined at 10% strain.

[0125] Shrinkage S, Young’s modulus, and density for 3D-printed structures 2000 in cubic shapes

[0126]

[0127]

[0128] As can be seen by comparing the samples Ink A 4-10W, Ink A 8-10W, and Ink A 12- 10W, increasing the concentration of sodium alginate in the gelled printing composition 1000 leads to a higher shrinkage S upon drying of the 3D-printed structure 2000._It is hypothesized that the higher alginate content leads to denser hydrogel networks that contract more upon drying. Increasing the concentration of sodium alginate further leads to an increase in Young’s modulus and density. Comparing the samples Ink A 8-10W and Ink A 8-5W, it can be seen that decreasing the amount of first particulate material 110 leads to a significant increase in the shrinkage S, Young’s modulus, and density of the 3D-printed structure 2000. It is hypothesized that the incorporation of first particulate material 110 limits volume loss both by decreasing the fraction of matrix material subject to contraction and by physically impeding matrix collapse.

[0129] Comparing samples Ink A 8-10W, Ink A 8-100, and Ink A 8- 10S, it can be seen that 3D-printed structures 2000 prepared with ground olive pits exhibited the highest shrinkage, followed by ground walnut shell and ground hardwood. These differences may arise from the physical properties of the different first particulate materials 110. Ground olive pits contain finer particles with a relatively round morphology, which may enable tighter packing, leading to greater contraction during drying. Ground walnut shell, although similar in size to ground hardwood, is more compact, which may allow for more uniform packing compared to the fibrous, elongated particles in the ground hardwood. The ground hardwood may introduce internal voids and reduce packing efficiency, thereby mitigating volumetric shrinkage.

[0130] Comparing Ink A 8-10W and Ink B (both containing 10 wt% ground hardwood), it can be seen that the alginate-gelatin blend of Ink B exhibited significantly higher shrinkage than the alginate-only formulation of Ink 8-10W. This may be due to the increased water retention capacity of gelatin, which results in more substantial water loss and volume change upon drying. Further, the 3D-printed structure 2000 prepared from Ink B has a lower Young’s modulus than the 3D-printed structure 2000 prepared from Ink 8-10W.Swelling of 3D-printed structures 2000

[0131] The swelling behavior of the 3D-printed structures 2000 based on Ink A 8-10 is tested in both water at room temperature and PBS buffer at 37 °C. the 3D-printed structure 2000 is submerged in the solvent, and the percentual increase in weight and volume of the soaked 3D-printed structure 2000 relative to the “dry” 3D-printed structure 2000 is indicated in the table below.

[0132]

[0133] The 3D-printed structure 2000 exhibits rapid initial swelling, doubling its weight in both solvents within 2 hours. In water, a minimal additional increase is observed after 6 h and 24 h, indicating that the 3D-printed structure 2000 is nearing equilibrium. This may suggest a fast absorption phase driven by diffusion, with most of the swelling occurring early, followed by a slow plateau. In PBS buffer, swelling of the 3D-printed structure 2000 continues after 2 h, indicating no equilibrium has been reached. The higher weight swelling compared to volume swelling in the PBS buffer (also compared to the same ratio in water) may suggest significant water retention within the crosslinked polymer matrix 710 without excessive expansion, likely due to ionic crosslinking stability in PBS.

[0134] Recycling of the 3D-printed structure 2000

[0135] The 3D-printed structure 2000 is ground into a fine powder and weighed to determine the recovered material. To prepare the recycled bioink, 10% of the recycled powder is mixed with 90% of first polymer solution 610. The pre-crosslinking step is omitted, as it was observed to increase viscosity significantly, making the material less suitable for printing. The resulting recycled ink is 3D-printed as described above for the gelled printing composition 1000, and the recycled precursor structure is crosslinked to ensure structural stability. The shrinkage S, water evaporation, and density of a fresh 3D-printed structure 2000 and a recycled 3D-printed structure are compared.

[0136]

[0137] The properties of the recycled 3D-printed structure are similar to those of a fresh sample, with a slight increase in the density and the shrinkage S of the diameter of the recycled 3D-printed structure compared to the fresh 3D-printed structure 2000.

[0138] The term “plurality” refers to two or more. Furthermore, the terms “a plurality of’ and “a number of’ may be used interchangeably. The terms “substantially” or “essentially” herein, and similar terms, will be understood by the person skilled in the art. The terms “substantially” or “essentially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially or essentially may also be removed. Where applicable, the term “substantially” or the term “essentially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. Moreover, the terms ’’about” and “approximately” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. For numerical values it is to be understood that the terms “substantially”, “essentially”, “about”, and “approximately” may also relate to the range of 90% - 110%, such as 95%-105%, especially 99%- 101 % of the values(s) it refers to.

[0139] The term “comprise” also includes embodiments wherein the term “comprises” means “consists of’. The term “and / or” especially relates to one or more of the items mentioned before and after “and / or”. For instance, a phrase “item 1 and / or item 2” and similar phrases may relate to one or more of item 1 and item 2. The term "comprising" may in an embodiment refer to "consisting of but may in another embodiment also refer to "containing at least the defined species and optionally one or more other species". Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0140] The devices, apparatus, or systems may herein amongst others be described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation, or devices, apparatus, or systems in operation.

[0141] The term “further embodiment” and similar terms may refer to an embodiment comprising the features of the previously discussed embodiment, but may also refer to an alternative embodiment. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In theclaims, any reference signs placed between parentheses shall not be construed as limiting the claim.

[0142] Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, “include”, “including”, “contain”, “containing” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

[0143] The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim, or an apparatus claim, or a system claim, enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0144] The invention also provides a control system that may control the device, apparatus, or system, or that may execute the herein described method or process. Yet further, the invention also provides a computer program product, when running on a computer which is functionally coupled to or comprised by the device, apparatus, or system, controls one or more controllable elements of such device, apparatus, or system.

[0145] The invention further applies to a device, apparatus, or system comprising one or more of the characterizing features described in the description and / or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterizing features described in the description and / or shown in the attached drawings. Moreover, if a method or an embodiment of the method is described being executed in a device, apparatus, or system, it will be understood that the device, apparatus, or system is suitable for or configured for (executing) the method or the embodiment of the method, respectively.

[0146] The various aspects discussed in this patent can be combined in order to provide additional advantages. Further, the person skilled in the art will understand that embodiments can be combined, and that also more than two embodiments can be combined. Furthermore, some of the features can form the basis for one or more divisional applications.

Claims

CLAIMS:

1. A method for providing a 3D-printed structure (2000), wherein the method comprises:providing a gelled printing composition (1000); wherein the gelled printing composition (1000) comprises a first particulate material (110), a first polymer material (120), a first crosslinker (130), and a first solvent (140); wherein:(a) the first particulate material (110) comprises one or more of wood chips, ground pit material, ground seed material, and ground nut shell material; wherein at least 50% of the first particulate material (110) has a particle size (Dp) selected from the range of 6-500 pm; wherein the gelled printing composition (1000) comprises the first particulate material (110) in a particulate concentration (Cp) of 1-20 wt%;(b) the first polymer material (120) comprises one or more of an alginate-based polymer and a gelatin-based polymer; wherein the gelled printing composition (1000) comprises the first polymer material (120) in a polymer concentration (CM) of 2-15 wt%; and(c) the first crosslinker (130) is configured to partially crosslink the first polymer material (120); wherein the first polymer material (120) and the first crosslinker (130) are dissolved in the first solvent (140);3D-printing the gelled printing composition (1000) with a 3D-printer (400) to provide a precursor structure (210); wherein the 3D-printer (400) comprises a nozzle (410); wherein the nozzle (410) has a nozzle diameter (Dn) selected from the range of 0.4- 1.2 mm; wherein 3D-printing the gelled printing composition (1000) comprises extrading the gelled printing composition (1000) via the nozzle (410) at (i) a printing pressure (Pi) selected from the range of 40-250 kPa, (ii) a printing temperature (Tp) selected from the range of 18-30 °C, and (iii) a printing speed selected from the range of 4-12 mm / s; andcrosslinking the precursor structure (210) to provide a crosslinked structure (220); wherein crosslinking the precursor structure (210) comprises one or more of (i) exposing the precursor structure (210) to a second crosslinker (150), and (ii) exposing the precursor structure (210) to radiation (901); and removing at least part of the first solvent (140) from the crosslinked structure (220) to provide the 3D-printed structure (2000).

2. The method according to claim 1, wherein the method comprises preparing the gelled printing composition (1000) from a printing composition (100); wherein the printing composition (100) has a first viscosity (r|i), wherein the gelled printing composition (1000) has a second viscosity (12), wherein 4*r|i < rp < 12*rp ; and wherein preparing the gelled printing composition (1000) from the printing composition (100) comprises one or more of (i) adding the first crosslinker (130) to the printing composition (100), and allowing the first crosslinker (130) to crosslink the first polymer material (120), and (ii) reducing a temperature of the printing composition (100), wherein the first polymer material (120) comprises a gelatin-based polymer.

3. The method according to claim 2, wherein the second viscosity (rp) is selected from the range of 1000-7000 Pa-s.

4. The method according to any one of the preceding claims, wherein the first polymer material (120) further comprises one or more of a polyethylene-glycol-based polymer and a polyester-based polymer.

5. The method according to any one of the preceding claims, wherein the first crosslinker (130) comprises one or more of Ca2+, Mg2+, Ba2+, Sr2+, Al3+, Ru2+, and lithium phenyl-2,4,6-trimethylbenzoylphosphinate.

6. The method according to any one of the preceding claims, wherein one or more applies of: (i) the first solvent (140) comprises one or more of water and ethanol, and (ii) removing at least part of the first solvent (140) from the crosslinked structure (220) comprises removing > 60% of the first solvent (140) from the crosslinked structure (220).

7. The method according to any one of the preceding claims, wherein the gelled printing composition (1000) further comprises one or more of: (i) an antifungal material, (ii) a flame retardant material, (iii) an insect repellant, (iv) a mineral, (v) a fertilizer, and (vi) a second biological material, wherein the second biological material comprises one or more of a growthpromoting biological material, a fungal material, a plant material, a bacteria, and a cell material.

8. The method according to any one of the preceding claims, wherein one or more applies of: (i) the particulate concentration (Cp) is selected from the range of 2.5-10 wt%, and (ii) the polymer concentration (CM) is selected from the range of 3-13 wt%.

9. The method according to any one of the preceding claims, wherein the second crosslinker (150) comprises one or more of Ca2+, Mg2+, Ba2+, Sr2+, Al3+, strontium acetate, glutaraldehyde, epichlorohydrin, and genipin; and wherein crosslinking the precursor structure (210) comprises submerging the precursor structure (210) in a solution comprising the second crosslinker (150).

10. The method according to any one of the preceding claims, wherein one of the following applies: (i) the first crosslinker (130) comprises Ru2+, wherein the gelled printing composition (1000) further comprises sodium persulfate, and (ii) the first crosslinker (130) comprises lithium phenyl-2,4,6-trimethylbenzoylphosphinate; wherein crosslinking the precursor structure (210) comprises exposing the precursor structure (210) to UV radiation (901).

11. The method according to any one of the preceding claims, wherein the precursor structure (210) comprises a plurality of 3D-printed layers (500), wherein each layer (500) has a layer height (HL) selected from the range of 0.25-1 mm.

12. The method according to any one of the preceding claims, wherein providing the gelled printing composition (1000) comprises: (a) dissolving the first polymer material (120) in the first solvent (140) to provide a first polymer solution (610); (b) combining the first polymer solution (610) with the first particulate material (110) to form a first composition (620); and (c) combining the first composition (620) with the first crosslinker (130).

13. The method according to any one of the preceding claims, wherein the crosslinked structure (220) has a first volume (Vi), wherein the 3D-printed structure (2000) has a second volume (V2); wherein x*V2 = Vi, wherein x > 1.5; and wherein the 3D-printed structure (2000), scaled by a factor x, has > 90% overlap with a design of the 3D-printed structure (2000) used for the 3D printing.

14. The method according to any one of the preceding claims, wherein for at least 90% of the crosslinked structure (220) applies that a first distance di to a nearest surface (2201) is selected from the range of < 10 mm.

15. The method according to claim 14, wherein the crosslinked structure (220) comprises an outer surface (2210); wherein the crosslinked structure (220) comprises one or more channels (2250), wherein each channel (2250) is defined by a channel wall (2260); and wherein the nearest surface (2201) is selected from the group of the outer surface (2210) and the one or more channel walls (2260).

16. A 3D-printed structure (2000), wherein:the 3D-printed structure (2000) comprises a plurality of printed layers (700), wherein each printed layer (700) has a printed layer height (HLP) selected from the range of 0.08-0.8 mm;at least one of the plurality of printed layers (700) comprises a first particulate material (110) and a crosslinked polymer matrix (710); wherein the first particulate material (110) is configured embedded in the crosslinked polymer matrix (710);the first particulate material (110) comprises one or more of wood chips, ground pit material, ground seed material, and ground nut shell material; wherein at least 50% of the first particulate material (110) has a particle size (Dp) selected from the range of 6-500 pm; wherein the 3D-printed structure (2000) comprises the first particulate material (110) in a structure particulate concentration (Cps) of 4-95 wt%; andthe crosslinked polymer matrix (710) comprises one or more of a crosslinked alginate-based polymer and a crosslinked gelatin-based polymer; wherein the 3D-printed structure (2000) comprises the crosslinked polymer matrix (710) in a structure polymer concentration (CMS) of 5-80 wt%; and wherein Cps+ CMS > 30 wt%.

17. The 3D-printed structure (2000) according to claim 16, wherein the 3D-printed structure (2000) is obtainable using the method according to any one of claims 1-15.

18. The 3D-printed structure (2000) according to any one of claims 16-17, wherein the 3D-printed structure (2000) has a Young’s modulus at 10% strain selected from the range of 5-100 MPa.

19. The 3D-printed structure (2000) according to any one of claims 16-18, wherein the 3D-printed structure (2000) has a density selected from the range of 0.5- 1.0 g / cm3.

20. The 3D-printed structure (2000) according to any one of claims 16-19, wherein for at least 90% of the 3D-printed structure (2000) applies that a second distance d2 to a nearest structure surface (2001) is selected from the range of < 5 mm.

21. The 3D-printed structure (2000) according to claim 20, wherein the 3D-printed structure (2000) comprises a structure outer surface (2010); wherein the 3D-printed structure (2000) comprises one or more channels (2050), wherein each channel (2050) is defined by a structure channel wall (2060); and wherein the nearest structure surface (2001) is selected from the group of the structure outer surface (2010) and the one or more structure channel walls (2060).

22. An object (4000) comprising the 3D-printed structure (2000) according to any one of claims 16-21, wherein the object (4000) is selected from the group comprising an agricultural material element, a packaging material element, a culture medium element, a biomaterial element, a pharmaceutical material element, a cosmetic material element, and a construction material element.

23. Use of the 3D-printed structure (2000) according to any one of claims 16-21 in one or more of agriculture, packaging, pharmaceutics, cosmetics, recycling, and construction.

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