System and method for high throughput volumetric 3D printing

Abstract

A method of printing a three-dimensional object, comprising: providing a volume of a photopolymerizable liquid in an enclosed container comprising an inlet and an outlet connected by a passageway therebetween, the container comprising at least one printing zone, the printing zone comprising at least an optically transparent window to facilitate irradiation of an excitation light of a first wavelength through the at least optically transparent window into the printing zone, directing the excitation light through the at least optically transparent window into the printing zone to selectively photopolymerize the photopolymerizable liquid in the printing zone without a support structure to form a printed object, and applying a pressure to contents of the enclosed container and / or pumping additional photopolymerizable liquid into the enclosed container through the inlet to at least transport the printed object from the printing zone to the outlet.

Description

[0001] CLAIM OF PRIORITY

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 003,078, filed March 31, 2020, which is incorporated by reference in its entirety for all purposes. TECHNICAL FIELD

[0003] The present invention relates to the technical field of three-dimensional printing. SUMMARY

[0004] According to one aspect of the invention, there is provided a system for printing one or more three-dimensional objects, the system comprising:

[0005] a closed container comprising an inlet and an outlet, the inlet and the outlet being connected by a passageway therebetween, the closed container comprising a print zone, wherein the print zone comprises at least an optically transparent window to facilitate the directing of excitation light through the optically transparent window into the print zone to form a three-dimensional printed object within a volume of (some) photopolymerizable liquid in the print zone, and

[0006] a pump connected to the inlet of the closed container and adapted to be connected to a source of photopolymerizable liquid, the pump being capable of pumping an amount of (some) photopolymerizable liquid through the inlet into the closed container.

[0007] Preferably, the system is capable of being maintained in an inert atmosphere and wherein each connection and port (inlet and outlet) is gas-tight.

[0008] Preferably, the system is capable of being light-tight other than the print zone to reduce unwanted photopolymerization.

[0009] According to another aspect of the invention, there is provided a system for printing one or more three-dimensional objects, the system comprising:

[0010] a reservoir for containing a supply of photopolymerizable liquid, the reservoir having a reservoir outlet and a reservoir inlet,

[0011] a pump connected to the reservoir outlet for pumping an amount of photopolymerizable liquid from the reservoir through an inlet in the closed container into the closed container,

[0012] the closed container comprising an inlet and an outlet, the inlet and the outlet being connected by a passageway therebetween, the closed container comprising a print zone, the print zone comprising at least an optically transparent window to facilitate the directing of excitation light of a first wavelength through the optically transparent window into the print zone to form a three-dimensional printed object from the photopolymerizable liquid in the print zone, and

[0013] a separator unit connected to an outlet of the closed container for receiving the contents expelled from the closed container, the separator unit being capable of separating any printed object from un-polymerized photopolymerizable liquid contained in the expelled contents, the separator unit comprising a first discharge outlet for discharging any separated printed object from the separator unit and a second discharge outlet for discharging separated un-polymerized photopolymerizable liquid from the separator unit.

[0014] Preferably, the system is capable of being maintained in an inert atmosphere and each connection and port is gas-tight.

[0015] Preferably, the system is capable of being light-tight other than in the printing zone to reduce unwanted photopolymerization.

[0016] According to a further aspect of the application, there is provided a method of printing one or more three-dimensional objects, the method comprising:

[0017] providing a volume of photopolymerizable liquid in a closed container comprising an inlet and an outlet, the inlet and the outlet being connected by a passageway therebetween, the container comprising at least one printing zone, the printing zone comprising at least an optically transparent window to facilitate irradiation of excitation light of a first wavelength through the at least optically transparent window into the printing zone, wherein the photopolymerizable liquid preferably exhibits non-Newtonian rheological behavior such that an object formed within the photopolymerizable liquid in the printing zone remains in a fixed position during formation or is minimally displaced (minimal displacement) in the un-polymerized photopolymerizable liquid,

[0018] directing excitation light through the at least optically transparent window into the printing zone to selectively photopolymerize the photopolymerizable liquid in the printing zone without a support structure to form a printed object, wherein the printed object remains in a fixed position during formation or is minimally displaced in the un-polymerized photopolymerizable liquid, and

[0019] applying pressure to the contents of the closed container and / or pumping additional photopolymerizable liquid into the closed container through the inlet to at least transport the printed object from the printing zone to the outlet to expel at least a portion of the contents of the closed container from the closed container through the outlet.

[0020] The method can further comprise separating any printed object from un-polymerized photopolymerizable liquid contained in the expelled contents.

[0021] Optionally, the method further comprises recovering the separated un-polymerized photopolymerizable liquid from the expelled contents.

[0022] Preferably, the method is carried out in an inert atmosphere.

[0023] The systems and methods according to the present application are particularly useful for printing three-dimensional (3D) objects from a photopolymerizable liquid that exhibits non-Newtonian behavior and that can be cured at volume locations subjected to a stimulating light impact to form the printed object without the need for added support structures. Most 3D printing technologies involving photopolymerization technology generally require support structures to stabilize the part during printing or to allow for thin or fragile overhanging portions of the printed part; post-processing is required to remove the support structures after printing, which can damage or leave marks on the printed part. Avoiding the addition of support structures would advantageously simplify post-processing of the printed part.

[0024] The systems and methods according to the present application advantageously further do not require the printed object to be adhered to a fixed base (e.g., a build plate) at the beginning of the printing process, avoiding a post-processing step of detaching the printed object from the fixed base.

[0025] The foregoing and other aspects and implementations of the present disclosure described herein are all meant to be embodiments of the present application.

[0026] It will be appreciated by those skilled in the art that any feature described with respect to any particular aspect and / or implementation of the application described herein can be combined with one or more of any other feature(s) of any other aspect and / or implementation of the application described herein, and modified as appropriate to ensure compatibility of the combination. Such combinations are considered to be part of the disclosure and are contemplated by the present application.

[0027] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application as claimed.

[0028] Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 A diagram depicting an example of an implementation of a system according to one aspect of the present application.

[0030] Figure 2 A diagram depicting an example of an implementation of a system according to one aspect of the present application.

[0031] The accompanying drawings are presented to aid in the understanding of the application, and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and, together with the

[0032] For a better understanding of the present application, as well as other advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings. DETAILED DESCRIPTION

[0033] Various aspects and embodiments of the application will be further described in the following detailed description.

[0034] The present application relates to systems and methods for printing one or more three-dimensional objects.

[0035] According to an aspect of the application, there is provided a system for printing one or more three-dimensional objects, the system comprising:

[0036] a closed container comprising an inlet and an outlet, the inlet and the outlet being connected by a channel therebetween, the closed container comprising at least one print zone, wherein the print zone comprises at least an optically transparent window to facilitate directing excitation light through the optically transparent window into the print zone to form a three-dimensional printed object within a volume of photopolymerisable liquid in the print zone, and

[0037] a pump connected to the inlet of the closed container and adapted to be connected to a source of photopolymerisable liquid, the pump being capable of pumping an amount of photopolymerisable liquid into the closed container through the inlet.

[0038] Preferably, the system is capable of being maintained in an inert atmosphere and wherein each connection and port is airtight.

[0039] Preferably, the system is capable of being light-tight other than in the print zone to reduce unwanted photopolymerisation.

[0040] In use, the closed container is filled with photopolymerisable liquid to be selectively photopolymerised in the print zone to form a three-dimensional object.

[0041] Figure 1 An example of a diagram depicting an embodiment of a system according to an aspect of the application. The diagram depicts a system 1 comprising a pump 2 connected to an inlet 3 of a closed container 4. The pump is adapted to be connected to a source of photopolymerisable liquid (not shown). The closed container further comprises an outlet 5. The inlet 3 and the outlet 5 are connected by a channel 6 therebetween. As depicted, the channel comprises photopolymerisable liquid having a plurality of three-dimensional printed objects 8 therein, one of the plurality of three-dimensional printed objects 8 being in a print zone 9, the others being spaced apart due to successive displacement (displacement) from the print zone to the outlet by individual addition of a series of new amounts of photopolymerisable liquid pumped into the closed container by the pump. Figure 1 The arrows depicted in the centre of the diagram indicate the direction of flow of the photopolymerisable liquid in the channel from the point of introduction of the liquid into the inlet of the closed container to the outlet from which the contents of the closed container are expelled.

[0042] The system is preferably capable of being maintained in an inert atmosphere and each connection and port is airtight.

[0043] Preferably, the system can be light-tight except for the print zone to reduce unwanted photopolymerization.

[0044] For purposes of illustration, the channel portion of the enclosed container is depicted as optically transparent. While it can be desirable in some cases for the channel portion of the enclosed container or the entire enclosed container to be completely optically transparent, at least the window in the enclosed container is optically transparent to facilitate the transmission of excitation from the optical system into the photopolymerizable liquid in the print zone to print the object.

[0045] In some cases, it can be desirable for the portion of the enclosed container adjacent to the print zone to not be optically transparent to help prevent excitation light from spreading into areas of the enclosed container where photopolymerization is not desired beyond the print zone.

[0046] Additional information relating to the enclosed container and pump is provided below.

[0047] The system can also include an optical system 10 outside of the print zone of the enclosed container. The optical system can optionally be provided separately or can be included as part of the system in combination with the enclosed container and pump.

[0048] The optical system can be connected with an excitation light source. The optical system is positioned or can be positioned to illuminate excitation light through at least the optically transparent window of the print zone.

[0049] Figure 1 An optical system positioned above the print zone in the enclosed container is depicted.

[0050] Optionally, the optical system used with or included in the system can be movable relative to the print zone so that excitation light can be illuminated into the print zone from one or more sides of the print zone (e.g., from the top, one side, both sides, the bottom, or any combination including two or more sides). If a movable optical system is to be used, the print zone will include transparent portions to accommodate the illumination of excitation light into the print zone from one or more sides. For example, the sides or surfaces of the print zone through which excitation light will be illuminated will be optically transparent or at least include optically transparent windows through which excitation light can pass.

[0051] Optionally, the excitation light can be temporally and / or spatially modulated. Optionally, the intensity of the excitation light can be modulated.

[0052] Spatially modulating the excitation light can be produced by known spatial modulation techniques including, for example, liquid crystal displays (LCDs), digital micromirror displays (DMDs), or micro-LED arrays. Other known spatial modulation techniques can be readily identified by one of skill in the art.

[0053] The optical system can be selected to apply continuous excitation light. The optical system can also be selected to apply intermittent excitation light. Intermittent excitation may include random on-off application of light or periodic application of light. Examples of periodic application of light include pulses. The optical system can be selected to apply a combination of continuous and intermittent light, including, for example, an irradiation step that includes applying intermittent excitation light before or after irradiation with continuous light.

[0054] Preferably, the excitation light has a wavelength within the visible range.

[0055] The optical system may be movable relative to a given printing area in one or more of the x, y, and z directions.

[0056] Optionally, the printing area can be completely optically transparent.

[0057] The system may optionally include more than one printing zone. Each printing zone will include at least an optically transparent window to facilitate the irradiation of the excitation light into the photopolymerizable liquid within each printing zone. As discussed above, other portions or all of the printing zones may be optically transparent to accommodate the optical system to be used and its portability.

[0058] When the system includes more than one printing area, the system may include an optical system associated with each printing area. Alternatively, when the system includes more than one printing area, the system may include an optical system that is movable and repositionable, at least relative to the printing area within the enclosed container, to illuminate each printing area with excitation light, one at a time.

[0059] The system may optionally further include a separator unit connected to the outlet of the closed container. Figure 1 (Not shown in the image) It is used to receive contents discharged from a closed container. A separator unit is used to separate any printed object from the unpolymerized photopolymerizable liquid contained in the discharged contents. The separator unit includes a first outlet for discharging any separated printed object from the separator unit and a second outlet for discharging the separated unpolymerized photopolymerizable liquid from the separator unit. Optionally, the second outlet of the separator unit is adapted to be connected to a recirculation loop or return line for recycling the separated unpolymerized photopolymerizable liquid to a source of photopolymerizable liquid to be pumped into the closed container.

[0060] The separator unit is preferably sealed to prevent air or oxygen from being introduced into the unit during separation.

[0061] The separator unit preferably mechanically separates any printed object from the unpolymerized photopolymerizable liquid contained in the discharge contents from the enclosed vessel. Examples of techniques for mechanically separating a printed object from the unpolymerized photopolymerizable liquid in the discharge contents include, but are not limited to, sifting techniques, using a scoop or claw to grab any printed object from the discharge contents, cyclonic separators, spiral separators; and combinations of two or more techniques.

[0062] The separated unpolymerized photopolymerizable liquid can be subjected to treatment after separation from any printed object. Examples of such treatment include, but are not limited to, washing / purification, filtration, degassing or solvent, monomer addition.

[0063] The printed object collected from the separator unit can optionally be subjected to post-treatment.

[0064] Examples of post-treatment include, but are not limited to, washing, post-curing (e.g., by light, heat, non-ionizing radiation, ionizing radiation, pressure, or a combination of simultaneous or sequential techniques), metering, freeze-drying treatment, critical point drying, and packaging.

[0065] According to another aspect of the present application, there is provided a system for printing one or more three-dimensional objects, the system comprising: a reservoir for containing a supply of photopolymerizable liquid, the reservoir having a reservoir outlet and a reservoir inlet; a pump connected to the reservoir outlet for pumping an amount of photopolymerizable liquid from the reservoir through an inlet in the enclosed vessel into the enclosed vessel, the enclosed vessel comprising an inlet and an outlet connected by a passageway therebetween, the enclosed vessel comprising at least one printing zone comprising at least an optically transparent window to facilitate directing excitation light of a first wavelength through the optically transparent window into the printing zone to form a three-dimensional printed object from the photopolymerizable liquid in the printing zone; and a separator unit connected to the outlet of the enclosed vessel for receiving discharge contents from the enclosed vessel. The separator unit is capable of separating any printed object from the unpolymerized photopolymerizable liquid contained in the discharge contents. The separator unit further supplies any separated printed object from the separator unit through a first discharge outlet for collection and / or post-treatment. The separator unit further comprises a second discharge outlet for discharging separated unpolymerized photopolymerizable liquid from the separator unit. Optionally, the separator unit further comprises a return line or recirculation loop connected to the second discharge outlet for recirculating the separated unpolymerized photopolymerizable liquid to the reservoir.

[0066] Preferably, the system is capable of being maintained in an inert atmosphere and wherein each connection and port is gas-tight.

[0067] Preferably, the system is capable of being light-tight other than the printing zone to reduce unwanted photopolymerization.

[0068] In use, the enclosed container is filled with a photopolymerisable liquid to be selectively photopolymerised in a print zone to form a three-dimensional object.

[0069] Figure 2 A diagram depicting an example of an embodiment of a system according to an aspect of the application. The diagram depicts a system 20 comprising a pump 21 connected to an inlet 22 of an enclosed container 23. The pump is adapted to be connected to a reservoir (labelled "resin tank" in the diagram) 24 for containing a photopolymerisable liquid. The enclosed container further comprises an outlet 25. The inlet 22 and the outlet 25 are connected by a channel 26 therebetween. As depicted, the channel comprises a photopolymerisable liquid having a plurality of three-dimensional printed objects 28 therein, one of the plurality of three-dimensional printed objects 28 being in a print zone 27, the others being spaced apart due to successive displacement from the print zone to the outlet by individual additions of new quantities of the photopolymerisable liquid pumped into the enclosed container by the pump. Figure 2 The arrows depicted in the middle of the diagram represent the direction of flow of the photopolymerisable liquid in the channel from the point of introduction of the liquid into the enclosed container to the outlet from which the contents are expelled from the enclosed container. The moving contents comprise un-polymerised photopolymerisable liquid and any printed objects contained therein that have been moved from the print zone and are being transported along the length of the channel to the outlet by the addition of a series of new quantities of the photopolymerisable liquid into the enclosed container by the pump. The expelled contents exit the enclosed container through the outlet and are conveyed into a separator unit (labelled "separator" in the diagram) 29 connected to the outlet. The separator unit is capable of separating any printed objects from the un-polymerised photopolymerisable liquid contained in the expelled contents. The separator unit also feeds any separated printed objects from the separator unit through a first discharge outlet 30 for collection and / or post-processing. The separator unit also comprises a second discharge outlet 31 for expelling separated un-polymerised photopolymerisable liquid from the separator unit.

[0070] The separator unit is preferably sealed to prevent air or oxygen being introduced into the unit during separation.

[0071] The separator unit preferably mechanically separates any printed objects from the un-polymerised photopolymerisable liquid in the contents expelled from the enclosed container. Examples of techniques for mechanically separating printed objects from un-polymerised photopolymerisable liquid in the expelled contents include, but are not limited to, sifting techniques, using a shovel or claw to grab any printed objects from the expelled contents, cyclonic separators, spiral separators; and combinations of two or more techniques.

[0072] The separated un-polymerised photopolymerisable liquid can be treated after separation from any printed objects. Examples of such treatment include, but are not limited to, washing / purification, filtration, degassing or solvent, monomer addition.

[0073] Optionally, the system further comprises a recirculation line or recirculation loop (labeled "resin recirculation" in the figures) 32 for recirculating the separated unpolymerized photopolymerizable liquid to the reservoir 24, connected to the second discharge outlet 31 of the separator unit.

[0074] The printed object collected from the separator unit can optionally be post-processed. Examples of post-processing include, but are not limited to, washing, post-curing (e.g., by light, heat, non-ionizing radiation, ionizing radiation, pressure, or simultaneous or sequential combinations of techniques, metering, freeze-drying, critical point drying, and packaging.

[0075] For illustrative purposes, the passage portion of the enclosure is depicted as optically transparent.

[0076] While it can be desirable in some cases for the passage portion of the enclosure or the entire enclosure to be completely optically transparent, at least the window in the enclosure is optically transparent to facilitate the delivery of excitation from the optical system into the photopolymerizable liquid in the print zone to print the object.

[0077] In some cases, it can be desirable for the portion of the enclosure adjacent to the print zone to not be optically transparent to help prevent excitation light from spreading into areas of the enclosure where polymerization is not desired beyond the print zone.

[0078] Additional information relating to the enclosure and pump is provided below.

[0079] The system can also include an optical system 35 outside of the print zone of the enclosure. The optical system can optionally be provided separately or can be included as part of the system in combination with the enclosure and pump.

[0080] The optical system can be connected with an excitation light source. The optical system is positioned or can be positioned to illuminate excitation light through at least the optically transparent window of the print zone.

[0081] Figure 2 An optical system positioned above the print zone in the enclosure is depicted.

[0082] Optionally, the optical system used with or included in the system can be movable relative to the print zone such that excitation light can be illuminated into the print zone from one or more sides of the print zone (e.g., from the top, one side, both sides, the bottom, or any combination including two or more sides). If a movable optical system is to be used, the print zone will include transparent portions to accommodate the illumination of excitation light into the print zone from one or more sides. For example, the sides or surfaces of the print zone through which excitation light will be illuminated will be optically transparent or at least include optically transparent windows through which excitation light can pass.

[0083] Optionally, the excitation light can be temporally and / or spatially modulated. Optionally, the intensity of the excitation light can be modulated.

[0084] Spatially modulating the excitation light can be produced by known spatial modulation techniques, including, for example, liquid crystal displays (LCD), digital micromirror displays (DMD), or micro-LED arrays. Other known spatial modulation techniques can be readily identified by one skilled in the art.

[0085] The optical system can be selected to apply continuous excitation light. The optical system can be selected to apply intermittent excitation light. Intermittent excitation can include random on and off application of light or periodic application of light. Examples of periodic application of light include pulsing. The optical system can be selected to apply a combination of both continuous excitation light and intermittent light, including, for example, an irradiation step that includes applying intermittent excitation light before or after irradiation with continuous light.

[0086] Preferably, the excitation light has a wavelength in the visible range.

[0087] The optical system can be movable in one or more of the x, y, and z directions relative to a given print zone.

[0088] Optionally, the print zone can be entirely optically transparent.

[0089] The system can optionally include more than one print zone. Each print zone will include at least an optically transparent window to facilitate excitation light irradiation into the photopolymerizable liquid in each print zone. As discussed above, other portions or all of the print zone can be optically transparent to accommodate the optical system to be used and its mobility.

[0090] When the system includes more than one print zone, the system can include an optical system associated with each print zone. Alternatively, when the system includes more than one print zone, the system can include an optical system that is movable and repositionable relative to the location of the print zones in the enclosure and that can irradiate excitation light into each print zone, one at a time.

[0091] According to yet another aspect of the application, a method of printing one or more three-dimensional objects is provided. The method includes providing a volume of a photopolymerizable liquid in an enclosed vessel. The photopolymerizable liquid preferably exhibits non-Newtonian rheological behavior such that an object formed within the photopolymerizable liquid in a print zone remains in a fixed position or is minimally displaced in the un-polymerized photopolymerizable liquid during formation. The enclosed vessel includes an inlet and an outlet connected by a passageway therebetween. The enclosed vessel also includes at least one print zone in which an object is formed. Each print zone includes at least an optically transparent window through which excitation light of a first wavelength can be directed into the print zone. The method also includes directing excitation light through the at least optically transparent window into the print zone to selectively photopolymerize the photopolymerizable liquid in the print zone without the addition of support structures to form a printed object. The printed object remains in a fixed position or is minimally displaced in the un-polymerized photopolymerizable liquid during formation. The method also includes applying pressure to the contents of the enclosed vessel and / or pumping additional photopolymerizable liquid into the enclosed vessel through the inlet to at least transport the printed object from the print zone toward the outlet to expel at least a portion of the contents of the enclosed vessel from the enclosed vessel through the outlet.

[0092] The method can also include separating any printed objects from un-polymerized photopolymerizable liquid contained in the expelled contents.

[0093] Optionally, the method also includes recovering the separated un-polymerized photopolymerizable liquid from the expelled contents.

[0094] Preferably, the method is performed in an inert atmosphere.

[0095] In one example of the method, 1) the resin is photocured without support structures such that the part is suspended in the resin; 2) the resin is thixotropic (shear thinning) or has a yield stress such that the part remains fixed in space or experiences a minimal amount of displacement during the curing operation; 3) upon application of pressure, the cured part or parts are pumped out of the print zone and the print zone is refilled with new resin; 4) the part is separated from the resin; and 5) the resin is optionally recovered.

[0096] The method of the invention can utilize the photocuring of a photopolymerizable liquid comprising a photopolymerizable component exhibiting non-Newtonian rheological behavior to fabricate one or more printed objects. Examples of such non-Newtonian rheological behavior include pseudoplastic fluid, yield pseudoplastic or Bingham plastic. Such behavior can be inherent to the combination of reactive components (monomers and oligomers) in the resin or can be imparted by non-reactive additives (thixotropic agents, rheology modifiers). Formulations of photopolymerizable components exhibiting non-Newtonian behavior are within the skill of the relevant artisan. Examples include formulations of photopolymerizable liquids for use in the present method comprising 86 parts GENOMER 4259 (aliphatic urethane acrylate), 14 parts N,N-dimethylacrylamide, 13.3 parts of a 60 wt% dispersion of nanoparticles in N,N-dimethylacrylamide, 2 parts Rheobyk 410 thixotropic agent, 0.5 parts bis(2,6-difluoro-3-(1 -hydroxypyrrol-1 -yl)phenyl)titanocene photoinitiator, 0.0001 parts 2,2,6,6-tetramethyl-1 -piperidinyloxy free radical inhibitor.

[0097] By applying light in the printing zone, a printed object is formed in a volume of photopolymerizable liquid without the need to generate support structures and due to the rheological behavior (high zero shear viscosity or yield stress) the part displacement is minimal, which is acceptable for accurate reproduction of the predetermined part geometry during the time interval required to form the part. Once the part is formed, the part is moved from the printing zone by applying pressure and / or pumping additional photopolymerizable liquid into the closed container, which causes the photopolymerizable liquid to flow. While the object experiences little or no displacement during formation in the printing zone, when it is moved from the printing zone by pumping pressure and / or adding additional photopolymerizable liquid to the closed container, it can experience a positional shift in the contents as it moves towards the outlet.

[0098] For photopolymerizable liquids exhibiting non-Newtonian rheological behavior, the preferred steady shear viscosity is less than 10,000 cP and most preferably less than 1,000 cP. (Steady shear viscosity refers to the viscosity after breakdown of the thixotropic network).

[0099] The method according to the invention is additionally particularly useful for printing 3D objects from photopolymerizable liquids which exhibit non-Newtonian behavior and which can be cured by upconversion-induced photopolymerization at the volume location impacted by the excitation light of the first wavelength.

[0100] Preferably, the photopolymerizable liquid comprises (i) a photopolymerizable component; (ii) upconverting nanoparticles comprising a sensitizer and an annihilator, the sensitizer comprising a molecule selected to absorb light of a first wavelength and generate a triplet exciton, and the annihilator selected to emit light of a second wavelength after transfer of energy from the sensitizer to the annihilator, the second wavelength being shorter than the first wavelength; and (iii) a photoinitiator that initiates polymerization of the photopolymerizable component when excited by light of the second wavelength, wherein the photopolymerizable liquid exhibits non-Newtonian behavior.

[0101] As discussed herein, the photopolymerizable liquid can preferably comprise: a photopolymerizable component; upconverting nanoparticles comprising: a core portion comprising a sensitizer and an annihilator in a liquid (e.g., oleic acid) and an encapsulating coating or shell (e.g., silica) on at least a portion, preferably substantially all, of the outer surface of the core portion, wherein the sensitizer comprises a molecule selected to absorb light of a first wavelength and generate a triplet exciton, and the annihilator is selected to emit light of a second wavelength after transfer of energy from the sensitizer to the annihilator, the second wavelength being shorter than the first wavelength; and a photoinitiator that initiates polymerization of the photopolymerizable component when excited by light of the second wavelength. The upconverting nanoparticles can further comprise a ligand at their surface to facilitate distribution of the nanoparticles in the photopolymerizable component. Surfactants and other materials that can be used as ligands are commercially available. Examples of ligands include, but are not limited to, polyethylene glycol.

[0102] The annihilator can also be referred to as a triplet annihilator.

[0103] The upconverting nanoparticles preferably have an average particle size that is less than the wavelength of the excitation light. Examples of preferred average particle sizes are less than 100 nm, less than 80 nm, less than 50 nm, less than 30 nm, less than 20 nm, although still larger or smaller nanoparticles can also be used. Most preferably, the upconverting nanoparticles have an average particle size that does not produce significant (noticeable) light scattering.

[0104] Examples of materials used as sensitizers and annihilators are described in Congreve et al., International Application No. PCT / US2019 / 063629, filed November 27, 2019, S. Sanders et al., “Photon Upconversion in Aqueous Nanodroplets,” J. Amer. Chem. Soc. 2019, 141, 9180-9184, and Beauti, Sumar, Abstract titled “Search for New Chromophore Pairs for Triplet-Triplet Annihilation Upconversion” ISEF Projects Database, Finalist Abstract (2017) (available at https: / / abstracts.societyforscience.org), each of which is hereby incorporated by reference in its entirety herein. WO 2019 / 025717 to Baldeck et al. (published February 7, 2019), and International Application No. PCT / US2019 / 063629 to Congreve et al. (filed November 27, 2019) also provide potentially useful information regarding concentrations of upconversion nanoparticles and concentrations of sensitizers and annihilators in photopolymerizable liquids.

[0105] The annihilator can include a molecule capable of receiving a triplet exciton from a sensitizer molecule by triplet-triplet energy transfer, undergoing triplet fusion with another annihilator molecule triplet to produce a higher energy singlet that emits light at a second wavelength to excite the photosensitizer, thereby initiating polymerization of the photopolymerizable component. Examples of annihilators include, but are not limited to, polycyclic aromatic hydrocarbons such as anthracene, anthracene derivatives (e.g., diphenylanthracene (DPA), 9,10-dimethylanthracene (DMA), 9,10-dipolyanthracene (DTA), 2-chloro-9,10-diphtylanthracene (DTACI), 2-carbonitrile-9,10-diptetrylanthracene (DTACN), 2-carbonitrile-9,10-dinaphthylanthracene (DNACN), 2-methyl-9,10-dinaphthylanthracene (DNAMe), 2-chloro-9,10-dinaphthylanthracene (DNACI), 9,10-bis(phenylethynyl)anthracene (BPEA), 2-chloro-9,10-bis(phenylethynyl)anthracene (2CBPEA), 5,6,11,12-tetraphenylperiflanthene (rubrene), pyrene, and / or perylene (e.g., tetra-tert-butyl perylene (TTBP). The anthracene derivatives described above can also be functionalized with halogens. For example, DPA can be further functionalized with halogens (e.g., fluorine, chlorine, bromine, iodine). Fluorescent organic dyes can be preferred.

[0106] The sensitizer can include at least one molecule capable of transferring energy from a singlet state to a triplet state when the molecule absorbs an excited photon of a first wavelength. Examples of sensitizers include, but are not limited to, metalloporphyrins (e.g., tetraphenyltetrabutylporphyrin palladium (PdTPTBP), octaethylporphyrin platinum (PtOEP), octaethylporphyrin palladium (PdOEP), tetramethylphenylporphyrin palladium (PdTPP), meso-tetraphenyltetrabenzoporphyrin palladium 1 (PdPh4TBP), 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine (PdPc(OBu)), 2,3-butanedione (or biacetyl), or a combination of several of the above molecules).

[0107] The sensitizer preferably absorbs the first wavelength of excitation to maximize its energy.

[0108] Considerations in selecting a photosensitizer / annihilator pair can include compatibility of the pair with the photoinitiator used.

[0109] More preferably, at least a portion of the upconverting nanoparticle comprises a core portion comprising a sensitizing agent and an annihilator in a liquid (e.g., oleic acid) and an encapsulating coating or shell (e.g., silica) on at least a portion, preferably substantially all, of the outer surface of the core portion. The core can comprise a micelle comprising the sensitizing agent and the annihilator in the liquid. (Micelles are typically formed from one or more surfactants, e.g., having a relatively hydrophilic portion and a relatively hydrophobic portion.) An example of a preferred upconverting nanoparticle includes the nanocapsules described in Congreve et al. International Application No. PCT / US2019 / 063629 (filed November 27, 2019), which is hereby incorporated by reference herein in its entirety. Other information that can be useful regarding nanocapsules includes International Publication No. WO2015 / 059179 to Landfester et al. (published April 30, 2015) and S. Sanders et al., “Photon Upconversion in Aqueous Nanodroplets”, J. Amer. Chem. Soc. 2019, 141, 9180-9184, each of which is hereby incorporated by reference herein in its entirety.

[0110] The upconverting nanoparticle can further comprise a ligand at its surface to facilitate distribution of the nanoparticle in the photopolymerization component. Surfactants and other materials that can be used as ligands are commercially available. Examples of ligands include, but are not limited to, polyethylene glycol.

[0111] The photoinitiator can be readily selected by one of ordinary skill in the art with consideration to its suitability for the mechanism that will be used to initiate polymerization and its suitability and / or compatibility with the resin to be polymerized. Information that can be useful regarding photoinitiators can be found in WO2019 / 025717 to Baldeck et al. (published February 7, 2019) and International Application No. PCT / US2019 / 063629 to Congreve et al. (filed November 27, 2019), each of which is hereby incorporated by reference herein in its entirety.

[0112] The photopolymerizable liquid can further comprise additional additives. Examples of such additives include, but are not limited to, thixotropic agents, oxygen scavengers, and the like. WO2019 / 025717 to Baldeck et al. (published February 7, 2019) provides information that can be useful regarding additives.

[0113] Other information that can be useful for the present invention is U.S. Patent Application No. 62 / 911,125 to Congreve et al. filed October 4, 2019.

[0114] Examples of sources of excitation light sources for use in the methods described herein include laser diodes, such as those commercially available, light emitting diodes, DMD projection systems, micro-LED arrays, vertical cavity lasers (VCL). In some embodiments, the excitation radiation source (e.g., light source) is a light emitting diode (LED).

[0115] The systems and methods according to the present application are particularly useful for printing three-dimensional (3D) objects from a photopolymerizable liquid that exhibits non-Newtonian behavior and that can be cured at volume locations impinged by excitation light to form the printed object without the need for added support structures. Most 3D printing technologies involving photopolymerization technology generally require support structures to stabilize the part during printing or to allow for thin or fragile overhanging portions of the printed part; post-processing is required to remove the support structures after printing, which can damage or leave marks on the printed part. Avoiding the addition of support structures would advantageously simplify post-processing of the printed part.

[0116] The systems and methods according to the present application are advantageously further free of the need to adhere the object to be printed to a fixed base (e.g., a build plate) at the beginning of the printing process, avoiding a post-processing step of detaching the printed object from the fixed base.

[0117] The post-processing step of removing the support structures and / or removing the printed object from the fixed base adds labor (e.g., manual removal), waste (discarded support structures), and reduces throughput (production volume) (the build plate cannot be reused until the printed object is removed), all of which add cost to the process.

[0118] The systems and methods according to the present application are additionally particularly useful for printing 3D objects from a photopolymerizable liquid that exhibits non-Newtonian behavior and that can be cured at volume locations impinged by excitation light of a first wavelength by upconversion-induced photopolymerization. Preferably, the upconversion comprises triplet upconversion (or triplet-triplet annihilation, TTA), which can be used to generate light of higher energy relative to the light used to photoexcite the sensitizer or annihilator. Most preferably, the sensitizer absorbs low-energy light and upconverts it by transferring energy to the annihilator, where two triplet excitons can combine to produce a higher-energy singlet exciton, which can emit light of higher frequency or shorter wavelength, such as by annihilative upconversion.

[0119] Preferably, the photopolymerizable liquid comprises (i) a photopolymerizable component; (ii) upconversion nanoparticles comprising a sensitizer and an annihilator, the sensitizer comprising a molecule selected to absorb light of a first wavelength and generate a triplet exciton, and the annihilator selected to emit light of a second wavelength, shorter than the first wavelength, after transferring energy from the sensitizer to the annihilator; and (iii) a photoinitiator that initiates polymerization of the photopolymerizable component upon excitation by light of the second wavelength. More preferably, the photopolymerizable liquid exhibits non-Newtonian behavior.

[0120] The first and second wavelengths are within the visible range.

[0121] The enclosed container used in the systems and methods of the present invention may be a single-piece unit or may be constructed from two or more pieces.

[0122] The sealed container may be constructed from materials including, but not limited to, the following: glass, quartz, fluoropolymers (e.g., Teflon FEP, Teflon AF, Teflon PFA), cyclic olefin copolymers, polymethyl methacrylate (PMMA), polynorbornene, sapphire, or transparent ceramics.

[0123] Preferably, at least the optically transparent portion of the printed area is also optically flat.

[0124] Preferably, the photopolymerizable liquid is purged or sprayed with an inert gas before being introduced into a sealed container, and is maintained in an inert atmosphere while in the sealed container. It is also preferable that the photopolymerizable liquid contained in a reservoir for supplying the sealed container and the source of the photopolymerizable liquid are purged and maintained under inert conditions before use in the systems and methods of the present invention.

[0125] like Figure 1 and 2 As shown, the enclosed container is depicted in an elongated shape. This configuration facilitates the printing of multiple printed objects and their removal from the printing zone, one at a time, by pumping an additional amount of photopolymerizable liquid into the enclosed container to remove the printed object from the printing zone and introducing a new amount into the printing zone to print a new object, wherein the displaced contents are discharged from the outlet. After a series of printed parts and the addition of new photopolymerizable liquid to the printing zone, the printed object is ultimately contained within the discharged contents and is collected after separation from the discharged contents. The separated object can then be further post-processed.

[0126] For alternative designs, the length of the channel within the enclosed container can correspond to the size of the printing zone, where new photopolymerizable liquid is introduced to fill the printing zone and the printed object and unpolymerized photopolymerizable liquid are discharged from the printing zone and outlet for separation. Other enclosed container designs are desirable based on, for example, but not limited to, the number of printing zones and the type and number of optical systems selected.

[0127] The enclosed container channel can have a uniform cross-section over its length between the inlet and the outlet.

[0128] The enclosed container channel can alternatively have a non-uniform cross-section. A non-uniform cross-section can be used to control spacing between successively printed objects, e.g., if the cross-section becomes larger, the parts will move closer together; if the cross-section becomes smaller, the parts will move farther apart. Either can be potentially advantageous for object separation.

[0129] The channel can have a circular or elliptical cross-section. The channel can have a polygonal cross-section. The channel can have a rectangular or square cross-section.

[0130] The enclosed container can optionally further comprise a conveyor at the bottom of the channel to help transport the printed objects to the outlet. It can be beneficial for the conveyor to comprise an anti-reflective coating on the sides of the conveyor that can be impacted by the excitation light in the printing zone. One surface of the conveyor (e.g., the surface that transports the printed objects) or optionally both the surface that transports the printed objects and the opposite surface of the conveyor can comprise other coatings including an anti-corrosion or anti-damage coating. Other coating materials include polymers such as polyolefins and fluoropolymers.

[0131] The conveyor can be a belt conveyor, including by way of example but not limited to a solid belt, a mesh belt, a chain belt. A belt conveyor can likewise benefit from comprising an anti-reflective coating on the sides of the belt that can be impacted by the excitation light in the printing zone. The conveyor can be a cart or platform made of magnetizable metal that can be actuated from outside the container using a magnetic field.

[0132] The pump used in the systems and methods of the present invention preferably comprises a hydrostatic pump. Other suitable pumps can be used.

[0133] The pump is preferably capable of (i) pumping the photopolymerizable liquid from a source or reservoir into the enclosed container to fill the container with the photopolymerizable liquid and (ii) pumping an amount (which can be a metered amount) of the photopolymerizable liquid into the filled enclosed container to move a printed object from the printing zone in a direction toward the outlet, the outlet being adapted to expel from the enclosed container through the outlet a portion of the contents of the enclosed container displaced by the added amount of photopolymerizable liquid.

[0134] Optionally, the systems and methods of the present invention can comprise two pumps, where a first pump is used to move the photopolymerizable liquid to the printing zone and a second pump imparts other flow characteristics to the photopolymerizable liquid. Including a second pump can be beneficial to compensate for potential loss of effectiveness of a single pump with respect to distance.

[0135] Prior to printing, a digital file of the object to be printed is obtained. If the digital file is not in a format that can be used to print the object, the digital file is converted to a format that can be used to print the object. An example of a typical format that can be used for printing is an STL file. Typically, the STL file is then sliced into two-dimensional layers using three-dimensional slicer software and converted to G-code or a set of machine commands that facilitate building the object. See B. Redwood et al., “The 3D Printing Handbook - Technologies, designs applications”, 3D HUBS B.V. 2018.

[0136] When used as a property of a portion of a container or build chamber, “optically transparent” means having a high optical transmittance for the wavelength of light used, and “optically flat” means not distorting (e.g., the optical wavefront entering the portion of the container or build chamber is substantially left unaffected).

[0137] As used herein, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a material includes reference to one or more such materials.

[0138] Applicants specifically incorporate by reference in their entirety all cited references. In addition, where empirical values or other values or parameters are given, it is understood that the exact value spelt out is only one example of a value to be employed, and any value implicitly covering that specifically disclosed is equivalent in meaning. Numerical ranges are inclusive of their endpoints, unless otherwise indicated. It is intended that the application not be limited to the specific conditions, materials, or parameters described, since the circumstances and materials can vary.

[0139] Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the application being indicated by the following claims.

Claims

1. A system for printing one or more three-dimensional objects, the system comprising: a closed container comprising an inlet and an outlet, the inlet and the outlet being connected by a channel therebetween, the closed container comprising a print zone, wherein the print zone comprises at least an optically transparent window to facilitate directing excitation light through the optically transparent window into the print zone to form a three-dimensional printed object within a volume of photopolymerizable liquid in the print zone, and a pump connected to the inlet of the closed container and adapted to be connected to a source of photopolymerizable liquid, the pump being capable of pumping an amount of photopolymerizable liquid into the closed container through the inlet, wherein the system further comprises a separator unit connected to the outlet of the closed container for receiving contents discharged from the closed container, the separator unit being for separating any printed object from un-polymerized photopolymerizable liquid included in the discharged contents, the separator unit comprising a first discharge outlet for discharging any separated printed object from the separator unit and a second discharge outlet for discharging separated un-polymerized photopolymerizable liquid from the separator unit.

2. The system of claim 1, wherein the system is capable of being maintained in an inert atmosphere and wherein each connection and port is gas tight.

3. The system of claim 1, wherein the channel has a uniform cross-section over its length between the inlet and the outlet thereof.

4. The system of claim 1, wherein the channel is a cylinder having a circular or elliptical cross-section.

5. The system of claim 1, wherein the channel has a polygonal cross-section.

6. The system of claim 1, wherein the channel has a rectangular or square cross-section.

7. The system of claim 1 or 3, wherein the closed container is optically transparent.

8. The system of claim 1 or 3, wherein all sides of the print zone are optically transparent.

9. The system of claim 1 or 3, wherein one or more sides of the print zone are optically transparent top and sides.

10. The system of claim 1, wherein the closed container further comprises a conveyor at the bottom of the channel to assist in conveying a printed object to the outlet.

11. The system of claim 10, wherein the conveyor comprises an anti-reflective coating on the sides of the conveyor that can be impacted by excitation light in the print zone.

12. The system of claim 10, wherein the conveyor comprises a belt conveyor.

13. The system of claim 12, wherein the belt conveyor comprises a solid belt.

14. The system of claim 12, wherein the belt conveyor comprises a mesh belt.

15. The system of claim 12, wherein the belt conveyor comprises a chain conveyor.

16. The system of any one of claims 12-15, wherein the belt conveyor comprises an anti-reflective coating on the sides of the belt that can be impacted by excitation light in the print zone.

17. The system of claim 1, wherein the system further comprises a recirculation loop connected with the second discharge outlet for recirculating separated un-polymerized photopolymerizable liquid to the source.

18. The system of claim 1, wherein the pump comprises a hydrostatic pump.

19. The system of claim 1, wherein the system comprises two pumps, wherein a first pump is used to move the photopolymerizable liquid to the print zone and a second pump imparts other flow characteristics to the photopolymerizable liquid.

20. The system of claim 1, wherein the enclosed container is replaceable.

21. The system of claim 1, wherein the separator unit mechanically separates any printed object from un-polymerized photopolymerizable liquid.

22. The system of claim 1, wherein the pump is capable of (i) pumping photopolymerizable liquid from the source into the enclosed container to fill the container with the photopolymerizable liquid and (ii) pumping a metered amount of photopolymerizable liquid into the filled enclosed container to move a printed object out of the print zone in a direction toward the outlet, the outlet being adapted to discharge the contents of the enclosed container displaced by the metered amount from the enclosed container through the outlet.

23. The system of claim 1, further comprising an optical system positioned or positionable to irradiate excitation light through at least an optically transparent window of the print zone.

24. The system of claim 1, wherein the cross-section of the channel is non-uniform.

25. A system for printing one or more three-dimensional objects, the system comprising: a reservoir for containing a supply of photopolymerizable liquid, the reservoir having a reservoir outlet and a reservoir inlet, a pump connected with the reservoir outlet for pumping an amount of photopolymerizable liquid from the reservoir through an inlet in an enclosed container into the enclosed container, the enclosed container comprising an inlet and an outlet connected by a channel therebetween, the enclosed container comprising at least one print zone, the print zone comprising at least an optically transparent window to facilitate directing excitation light of a first wavelength through the optically transparent window into the print zone to form a three-dimensional printed object from the photopolymerizable liquid in the print zone, and a separator unit connected with the outlet of the enclosed container for receiving output discharged from the enclosed container, the separator unit for separating any printed object from un-polymerized photopolymerizable liquid contained in the discharged output, the separator unit comprising a first discharge outlet for discharging any separated printed object from the separator unit and a second discharge outlet for discharging separated un-polymerized photopolymerizable liquid from the separator unit.

26. The system of claim 25, wherein the system is capable of being maintained in an inert atmosphere and each connection and port is gas-tight.

27. The system of claim 25, wherein the system further comprises a recirculation loop connected with the second discharge outlet for recirculating separated un-polymerized photopolymerizable liquid to the reservoir.

28. The system of claim 1 or 25, further comprising one or more optical systems positioned or positionable to shine excitation light through an optically transparent window across the print zone.

29. The system of claim 25, wherein the channel has a uniform cross-section over its length between the inlet and the outlet.

30. The system of claim 25, wherein the channel is a cylinder having a circular or elliptical cross-section.

31. The system of claim 25, wherein the channel has a polygonal cross-section.

32. The system of claim 25, wherein the channel has a rectangular or square cross-section.

33. The system of claim 25 or 29, wherein the enclosed container is optically transparent.

34. The system of claim 25 or 29, wherein all sides of the print zone are optically transparent.

35. The system of claim 25 or 29, wherein one or more sides of the print zone are optically transparent top and sides.

36. The system of claim 25, wherein the enclosed container further comprises a conveyor at the bottom of the channel to aid in the transport of printed objects to the outlet.

37. The system of claim 36, wherein the conveyor comprises an anti-reflective coating on the side of the conveyor facing the point of entry of excitation light into the print zone.

38. The system of claim 36, wherein the conveyor comprises a belt conveyor.

39. The system of claim 38, wherein the belt conveyor comprises a solid belt.

40. The system of claim 38, wherein the belt conveyor comprises a mesh belt.

41. The system of claim 38, wherein the belt conveyor comprises a chain conveyor.

42. The system of any one of claims 38-41, wherein the belt conveyor comprises an anti-reflective coating on the side of the belt that can be impacted by excitation light in the print zone.

43. The system of claim 25, wherein the enclosed container is optically transparent.

44. The system of claim 25, wherein the enclosed container is replaceable.

45. The system of claim 25, wherein the separator unit mechanically separates one or more printed objects from unpolymerized photopolymerizable liquid.

46. The system of claim 25, wherein the pump comprises a hydrostatic pump.

47. The system of claim 25, wherein the system comprises two pumps, wherein a first pump is used to move the photopolymerizable liquid to the print zone and a second pump imparts other flow characteristics to the photopolymerizable liquid.

48. The system of claim 25, wherein the pump is capable of (i) pumping photo- polymerizable liquid from the reservoir into the enclosed container to fill the container with the photo-polymerizable liquid and (ii) pumping a metered amount of photo- polymerizable liquid into the filled enclosed container to move a printed object out of the printing zone in a direction toward the outlet, the outlet being adapted to expel the contents of the enclosed container displaced by the metered amount from the enclosed container through the outlet.

49. The system of claim 25, wherein the cross-section of the channel is non-uniform.

50. The system of claim 28, wherein an optical system is coupled to the excitation light source.

51. The system of claim 50, wherein the excitation light source comprises a DMD projection system.

52. A method of printing one or more three-dimensional objects, the method comprising: providing a volume of photo-polymerizable liquid in an enclosed container comprising an inlet and an outlet connected by a channel therebetween, the container comprising at least one printing zone, the printing zone comprising at least an optically transparent window to facilitate the irradiation of excitation light of a first wavelength through the at least optically transparent window into the printing zone, wherein the photo-polymerizable liquid exhibits non-Newtonian rheological behavior such that an object formed within the photo-polymerizable liquid in the printing zone remains in a fixed position or is minimally displaced in the un-polymerized photo-polymerizable liquid during formation, directing excitation light through the at least optically transparent window into the printing zone to selectively photo-polymerize the photo-polymerizable liquid in the printing zone without a support structure, thereby forming a printed object, wherein the printed object remains in a fixed position or is minimally displaced in the un-polymerized photo-polymerizable liquid during formation, and applying pressure to the contents of the enclosed container and / or pumping additional photo-polymerizable liquid into the enclosed container through the inlet to at least transport the printed object from the printing zone to the outlet, thereby expelling at least a portion of the contents of the enclosed container from the enclosed container through the outlet.

53. The method of claim 52, wherein the method is conducted in an inert atmosphere.

54. The method of claim 52, further comprising separating any printed object from un- polymerized photo-polymerizable liquid contained in the expelled contents.

55. The method of claim 52, further comprising recycling the un-polymerized photo- polymerizable liquid expelled after separating any printed object to the reservoir.

56. The method of claim 52, wherein minimally displaced comprises an amount of displacement of the printed object that is acceptable for accurately reproducing the geometry of the object to be printed over the time interval required to form the object.

57. The method of claim 52, wherein the printed object is formed by upconversion induced photopolymerization initiated by irradiation of the photo-polymerizable liquid in the printing zone with excitation light of a first wavelength.

58. The method according to claim 52 or 57, wherein the photopolymerizable liquid comprises: (i) a photo-polymerizable component; (ii) an upconverting nanoparticle comprising a sensitizer and an annihilator, the sensitizer comprising a molecule selected to absorb light of a first wavelength and generate a triplet exciton, and the annihilator selected to emit light of a second wavelength after transfer of energy from the sensitizer to the annihilator, the second wavelength being shorter than the first wavelength; and (iii) a photoinitiator which initiates polymerization of a photopolymerizable component when excited by light of the second wavelength.

59. The method of claim 58, wherein at least a portion of the upconverting nanoparticles comprise: a core portion comprising a sensitizer and an annihilator in a liquid, and an encapsulating shell on at least a portion, preferably substantially all, of the outer surface of the core portion.

Images