Programmable voxel interface behavior in extrusion-based 3D printing

Abstract

Voxel-Interface 3D Printing (VI3DP) enables comprehensive control over extruded voxel interfaces irrespective of the printhead diameter that conventionally dictates feature size. The present invention includes various optical, mechanical, and electrical functionalizations, attaining interface thicknesses up to three orders of magnitude smaller than the voxel size. Notable applications include encoding data in soft matter through fluorescent interfaces, creating tight fits and movable mechanisms through non-adhesive interfaces, fabricating bio-inspired composites with tailored failure modes, and developing a single filament capacitive touch sensor. VI3DP opens new avenues for enhanced functionality and efficiency across multiple fields, including biomedical technology, electronics, optics, and nanotechnology.

Description

PROGRAMMABLE VOXEL INTERFACE BEHAVIOR IN EXTRUSION-BASED 3D PRINTINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 703,604, filed October 4, 2024. The content of the aforementioned application is herein incorporated by reference, in its entirety.FIELD OF THE INVENTION

[0002] The present invention relates generally to three-dimensional (3D) printing. More particularly, the present invention relates to programmable voxel interface behavior in extrusion-based 3D printing.BACKGROUND OF THE INVENTION

[0003] Multimaterial 3D printing enables the monolithic fabrication of structurally complex objects with voxel-level precision, facilitating the creation of diverse multi-functional soft and rigid matter, such as soft actuators, wearable sensors, smart textiles, and tissue-like constructs. These applications have been enabled by various 3D printing processes, such as material jetting and vat photopolymerization, which offer high throughput and resolution but are typically restricted to UV-curable materials. In contrast, material extrusion offers the broadest materials palette in 3D printing, encompassing both organic and inorganic materials, making it particularly suitable for multifunctional structures.

[0004] However, like other 3D printing processes, material extrusion relies on a fixed voxel size, determined by printhead diameter and raw material properties. Although reducing voxel size increases the resolution, it also lengthens print time, limiting practicality. This power-law relationship becomes particularly challenging when approaching infinitesimally small length scales, such as those required in high aspect-ratio geometries and interfaces — the criticalcomponents in heterogeneous multimaterial structures that govern key mechanical, chemical, thermal, electrical, and optical properties in both nature and technology.

[0005] The dichotomy between small and large features requiring different resolutions is a major barrier to the efficacy of current 3D printing technologies, hindering widespread adoption. To address this challenge, printheads with different individual nozzle sizes can be integrated, but defects would be created in the transitions. Researchers have also proposed morphing nozzles capable of rapidly altering the outlet diameter during printing. Although these solutions partially mitigate the power-law scaling issue, they significantly constrict the design space, constrained by Cartesian motion controls, the interplay of gravitational forces with overhangs, and the adhesion of adjacent filaments with drastically different diameters.

[0006] Alternatively, a printhead can be subdivided internally to yield an effectively smaller voxel size without affecting the deposition rate. Direct ink writing (DIW) is well-suited for this method, given its broad materials palette and unique ability to extrude dissimilar materials with analogous pre-cured rheological behaviors and divergent solidified properties. For instance, DIW facilitates the extrusion of composite inks, by aligning fibers via shear forces and ferromagnetic particles via external fields. Incorporating microfluidic control enables co-extrusion of disparate inks, side-by-side or coaxially, through converging input channels. Although these methods achieve features of approximately 0.1 times the nozzle diameter, their resolution and geometric deposition control levels are targeted at fundamentally different applications. They remain impractical for interface-scale features, which continue to be economically or technologically infeasible.

[0007] Consequently, the capacity for precise control of voxel-to-voxel and voxel-to-air interfaces offers a distinctive and under-exploited avenue to embed (multi-)functionalmechanical, biological, chemical, and physical properties into various human-made objects, which are currently infeasible with extrusion-based 3D printing processes.

[0008] It would be advantageous to provide programmable voxel interface behavior in extrusion-based 3D printing.SUMMARY OF THE INVENTION

[0009] The foregoing needs are met, to a great extent, by the present invention, wherein one aspect is a device for three-dimensional (3D) printing having a nozzle. The nozzle has an input for a primary material and n inputs for a secondary material. The device includes a syringe barrel coupled to the nozzle. The device also includes a pressure system having an extruder for each of the primary material and the secondary material.

[0010] In accordance with an aspect of the present invention, the nozzle further includes a center channel for the primary material and outer channels that converge around an edge of the center channel in arc sections. Each one of the outer channel tapers into the center channel to allow for evenly distributed filament coverage by the secondary material. The extruder for the primary material applies pressure to the syringe barrel containing the primary material. The extruder for the secondary material applies pressure through solenoid valves with inputs connected to a single manifold and outputs connected to syringe barrels containing the secondary material. A number of solenoid valves corresponds to a number of syringe barrels, n number of inputs corresponds to the number of solenoid valves and the number of syringe barrels. The device includes a printhead tip. The printhead tip is configured for printing in a series of cubic voxels. The nozzle is configured to extrude a core filament of the primary material with an interface of the secondary material applied on top of the primary material.

[0011] In accordance with an aspect of the present invention, a system for three-dimensional (3D) printing including a nozzle. The nozzle has an input for a primary material and n inputs for a secondary material. The system includes a syringe barrel coupled to the nozzle. A pressure system includes an extruder for each of the primary material and the secondary7material. The system also includes a computer processing device configured to provide control of the nozzle, syringe barrel, and pressure system and consequently extrusion of the primary' and the secondary7material.

[0012] In accordance with an aspect of the present invention, the nozzle further includes a center channel for the primary7material and outer channels that converge around an edge of the center channel in arc sections. Each one of the outer channel tapers into the center channel to allow for evenly distributed filament coverage by the secondary material. The pressure control box for the primary7material applies pressure to the syringe barrel containing the primary material. The system includes an extruder for the secondary material applies pressure through solenoid valves with inputs connected to a single manifold and outputs connected to syringe barrels containing the secondary' material. A number of solenoid valves corresponds to a number of syringe barrels, n number of inputs corresponds to the number of solenoid valves and the number of syringe barrels. The system includes a printhead tip. The printhead tip is configured for printing in a series of cubic voxels. The computer processor is programmed for comprehensive control over primary and secondary material extrusion irrespective of a diameter of the printhead tip that dictates feature size.BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings provide visual representations, which will be used to more fully describe the representative embodiments disclosed herein and can be used bythose skilled in the art to better understand them and their inherent advantages. In these drawings, like reference numerals identify corresponding elements and:

[0014] FIGS. 1 A-1F illustrate schematic and image views of a voxel interface programming device and method via VI3DP.

[0015] FIGS. 2A-2I illustrate image, schematic, and graphical views of an impact of printhead design parameters on interface formation.

[0016] FIGS. 3A-3G illustrate graphical and image views of an effect of material flow rate, viscosity, and number of inputs on interface quality.

[0017] FIGS. 4A-4F illustrate image and schematic views of optical interfaces controlled via VI3DP.

[0018] FIGS. 5A-5F illustrate image, graphical, and schematic views of mechanical interfaces with programmable adhesion levels.

[0019] FIGS. 6A-6I illustrate schematic, image, and graphical views of a VI3DP of a single filament-thick capacitive touch sensor.DETAILED DESCRIPTION

[0020] The presently disclosed subject matter now will be described more fully hereinafter with reference to the accompanying Drawings, in which some, but not all embodiments of the inventions are shown. Like numbers refer to like elements throughout. The presently disclosed subject matter may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Indeed, many modifications and other embodiments of the presently disclosed subject matter set forth herein will come to mind to one skilled in the art to which the presently disclosed subject matter pertains having the benefit of the teachings presented in the foregoing descriptions and the associated Drawings. Therefore, it is to be understood that the presently disclosed subjectmater is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.

[0021] Interfaces are crucial in natural and engineered systems, dictating essential biological, ecological, and technological properties that augment performance, functionality, and user experience. Yet, achieving precise interfacial control poses significant challenges in both conventional and additive manufacturing, where scalability constraints impede the controlled deposition of quasi-two-dimensional layers within three-dimensional objects.

[0022] Voxel -Interface 3D Printing (VI3DP) enables comprehensive control over extruded voxel interfaces irrespective of the printhead diameter that conventionally dictates feature size. The present invention includes various optical, mechanical, and electrical functionalizations, ataining interface thicknesses up to three orders of magnitude smaller than the voxel size. Notable applications include encoding data in soft mater through fluorescent interfaces, creating tight fits and movable mechanisms through non-adhesive interfaces, fabricating bio-inspired composites with tailored failure modes, and developing a single filament capacitive touch sensor. VI3DP opens new avenues for enhanced functionality and efficiency across multiple fields, including biomedical technology, electronics, optics, and nanotechnology.

[0023] The 3D-printing printhead of the present invention enables the selective co-extrusion of multiple materials at once through concentric shell sections. The resultant filament (extrudate) has a primary core material or '‘ink” with the shell sections of a secondary material or “ink.” With current 3D printing techniques, printing multi-material parts requires separate nozzles sized for each feature size required. By having control of multiple materials on the fly integrated into a single nozzle, only one printhead is required. This saves time and requires less equipment. Also, using a single nozzle ensures that the effective size of the filament is consistent whether or not the secondary material is also being extruded. Thepresent invention is a ground-breaking improvement for specific applications. It is unique because it integrates different materials into different feature sizes within a single extruded filament while maintaining a relatively constant filament diameter.

[0024] The printhead system consists of several key components:- A pressure system with a pressure control box for each material.- One pressure control box applies pressure to a syringe barrel containing the primary material.- One pressure control box applies pressure through solenoid valves with inputs connected to a single manifold and outputs connected to syringe barrels containing the secondary material.- Each syringe barrel is attached to a custom-fabricated nozzle. The nozzle has one input for the primary' material and a vary ing number of inputs for the secondary' material depending on the quantity of desired shells around the filament.

[0025] The nozzle’s novel extrusion design consists of a center channel for the primary material core and outer channels that converge around the edge of the center channel in arc sections. Each outer channel tapers into the center channel to allow for evenly distributed filament coverage by the shell material.

[0026] The extruded filament can be modified by selectively turning on and off solenoid valves on the fly during printing, thus controlling which, if any, of the shells are on at a given time. Any amount of shell sections can be extruded at once, from 0-n, with n being the quantity of shell sections. Also, any combination of shell section can be extruded, with the number of possible combinations being equal to 2 n.

[0027] This system enables subvoxel control of interfacial filament behavior, where a voxel is defined as a small segment of filament and subvoxel features as individual components ofthe filament withing that small segment. Filament interfaces can be functionalized through the addition of a thin shell layer of a dissimilar material.

[0028] The arc or shell thickness can be programmatically controlled, where thin layers are preferred for some applications (e.g., breaking the bonding between two neighboring filaments) and thick layers for other applications (e.g., hinges).

[0029] Various behaviors can be modified depending on the material selection and the printed structure, including but not limited to:- Optical Properties: The visual appearance of the filament and resultant structure can be modified depending on where along the filament the shell is applied. For example, images can be printed, and data can be encoded (e.g., QR codes or barcodes).- Mechanical properties: Adhesion control. As a printed primary7material cures or hardens, adjacent filaments of the same material bond together. Adding a secondary7material to the filament interface that does not bond completely or at all to the primary7material enables adhesion to be controlled between neighboring filaments. This makes it possible to create multiple parts together that pull apart after print or mechanisms with moving components that don’t cure together.- Mechanical properties: Composite structures. Adding an interfacial material that does bond completely with the primary material but that has different mechanical properties will cause the resultant print to be a composite structure with combined properties of both materials. Crack propagation can be controlled for structures, as failure can be isolated to occur along specified interfaces.- Electrical Properties: Electrical features can be embedded in a filament by having the shell or the core be either a conductive material or an insulating material. Conductivity between adjacent filaments can be used to print circuits or electronic devices.

[0030] The present invention is an integrated 3D printing device and method capable of functionalizing and leveraging both intra- and inter-filament interfaces at single-voxel resolution. The Voxel-Interface 3D Printing (VI3DP) printhead controls all interfaces surrounding voxels through targeted interfacial material deposition, facilitated by rapidly cycling pneumatic solenoid valves, as illustrated in FIG. 1A. The filament cross-sections are approximately cubic, designated as positive or negative interfaces x+’ ', y+- ’, and z+’ ', as illustrated in FIG. IB. The cubic model is useful for printhead translations along the main Cartesian axes, with the printhead allowing further compartmentalization to achieve virtually any polygonal approximation and, thus, control of translational directions and interfacial overlaps. Intra-filament interfaces orthogonal to the print path are created via short extrusion pulses of all interface materials, temporarily halting the base material extrusion. In contrast, inter-filament interfaces are regulated by toggling the respective input channels, as illustrated in FIG. 1C. The integration of flow channels within the printhead permits the extrusion of interface layers significantly thinner (under extrusion) or thicker (over extrusion) than their fixed channel dimensions, decoupling the printhead design from part design requirements and creating a high level of versatility. Importantly, the programmable addressability7of each interface allows individual functionalization through material property modulation and continuous thickness control, ranging from below 20 pm to over 1 mm, corresponding to the core filament diameter. By way of example, differently colored functionalizable inks are used to illustrate both voxel-voxel and voxel-air interfaces, as illustrated in FIGS. ID, IE, and IF.

[0031] FIGS. 1 A-1F illustrate schematic and image views of a voxel interface programming device and method via VI3DP. FIG. 1A illustrates a system 10 showing the applied voltage scheme (up=open, down=closed) for five solenoid valves 12 (1-5) regulating extrusion via pneumatic pressure regulators 14 for the core (Pl, light gray) and functionalizable interfacial (P2, dark gray) materials over print distance or extrusion length. The material is extruded from input channels 18 through tip 16. FIG. IB illustrates schematics of extruded filament as a series of cubic voxels, their interfaces, and an example of programmatically functionalized interfaces. FIG. 1C illustrates a schematic view of the printhead tip 16 with cross-sectional depictions of various interface types on the extruded filament, following the control sequence of the waveforms in FIG. 1A, including inter-filament (i-v), and intra-filament (vi) interfaces. FIG. ID illustrates schematic views of functionalized interfaces patterned on a printed part, with voxel-voxel interfaces observable in the cross-sectional view (bottom). FIG. IE illustrates a side view of a demonstration of VI3DP with a 4-interface printhead. FIG. IF illustrates a perspective view of the print’s outer surface shows controllable external voxel -air interfaces and the cross-section internal voxel-voxel interfaces.

[0032] A salient feature of VI3DP is its capacity to geometrically program interface coverage, ranging from partial to full coverage and encompassing single to multiple interfaces with varying functionalities, contingent on the input materials. These combined complexities, defined by the user, dictate the printhead design, the number of required pressure controllers for pressure level adjustment, solenoid valves for activation / deactivation, material reservoirs, and the resulting part and print path design space. Voxel sides can be discretized into smaller sections by increasing the number of input channels. This also facilitates additional extrusion directions. For instance, to maintain a consistent interface on the top of a voxel (z+) during directional changes, four interface input channels are necessaryfor motion along the main Cartesian axes, while 8 are necessary' for motion at 45° between the x-y axes, and would be necessary' for motion at 22.5°. This would also necessitate increasing the number of channels that are extruded to maintain the same circumferential coverage and would permit additional control of partial voxel side coverage decoupled from the print path and timing. For instance, half coverage with an 8-input printhead, a quarter with a 16-input printhead, and so forth. The inputs can be symmetrically or asymmetrically patterned around the printhead; for example, design requirements may dictate complete fractional control on only one side of the filament while economizing on input channels for the remaining sides. When all materials exhibit similar characteristics, a single pressure regulator can control multiple solenoid valves, while diverse materials necessitate individual regulators. Solenoid valves can be combined for specific patterns, like consistently covering a voxel’s top and bottom surfaces. The current design employs pressure-driven extrusion for rapid switching, necessitating individual material reservoirs after each valve.

[0033] Depending on specific use cases, a target interface may uniformly cover a voxel with a defined, finite thickness, enabling the desired function while minimally impacting the base material or filament diameter. Using a two-sectioned, bipartite interface printhead, as illustrated in FIGS. 2A and 2B, interface quality' is assessed based on the thickness distribution along the filament length in the cross-sectional view and switching distance Is in the top view, where uniform thickness and short distance indicate higher interface quality. Is is measured by analyzing the white intensity along the filament length in photos taken under consistent conditions. Specifically, lsrepresents the distance required for the white intensity to switch between 90% and 10% of the print’s maximum white intensity, with the blue and white parts of the image corresponding to lower and higher brightness intensities, respectively. Several printhead design parameters influence these quality parameters.including the taper angle 0 at which the interface channels enter the core, the exit length h inside the printhead between the point where the interface material meets the core material and where the filament exits, and the diameter of the core material channel w relative to the final printhead.

[0034] Varying 0 (0°, 90°. 180°) does not significantly affect the switching distance Is with values of 0.52 mm, 0.63 mm, and 0.67 mm, respectively, as illustrated in FIGS. 2C, 2F, and 2G. Altering the ratio between the core channel input diameter w and the printhead outer diameter from 0.8: 1 to 1.2:1 does not impact interface coverage, but delays switching times, with Is values of 1.18 mm and 0.70 mm, respectively, as illustrated in FIGS. 2D, 2F, and 2H. Increasing the exit length (A) undesirably increases both the uneven distribution of the interface and the interface switching distance, with Is values of 1.64 mm and 1.24 mm for h values of 0.70 mm and 1.05 mm, respectively, which is greater than the L value of 0.67 mm for an h value of 0.35 mm, as illustrated in FIGS. 2E, 2F, and 21. As the current minimum exit length approaches the resolution limits of the 3D printer used to fabricate the printheads, shorter exit lengths could be investigated in the future to see if the observed trends continue. Currently, the printhead design exhibiting the best results features a channel taper angle 0 of 180°, a core channel diameter w of 1 mm, and a printhead exit length h of 0.35 mm.

[0035] FIGS. 2A-2I illustrate image, schematic, and graphical views of an impact of printhead design parameters on interface formation. FIG. 2A illustrates a perspective view of a visualization of a two 180° interface input VI3DP printhead used to investigate printhead design parameters, along with a representative fdament with a single interface applied on top. FIG. 2B illustrates a schematic of the target filament cross-section and top view of a printed sample, showing diameter <7, interface length s, and interface thickness t. Pneumatic valves control the blue interface “on” and “off’ at specified positions along the print path. FIG. 2Cillustrates an effect of interface channel taper angles 0 = 0°, 90°, and 180°. FIG. 2D illustrates an effect of exit diameter ratio between the core material channel w and the interface (shell) material channel, for ratios of 0.8: 1, 1: 1, and 1.2: 1, with G = 180° and h = 0:35 mm. FIG. 2E illustrates an influence of printhead exit length h after the interfacial material meets the core, with lengths of 0.35 mm, 0.70 mm, and 1.05 mm, keeping G = 180° and h = 0:35 mm. FIG. 2F illustrates an interface thicknesses t along the fdament circumference of the samples in FIG. 2C (top), FIG. 2D (center), and FIG. 2E (bottom). FIGS. 2G, 2H, and 21 illustrate graphical views of white intensity measured along the fdaments to determine the switching distance Is for each taper angle, core diameter, and exit length variation. lsis defined as the distance required to switch between 10% and 90% intensity. Scale bars = 0.2 mm and 1 mm for the filament cross-section and top views, respectively.

[0036] In addition, the interface flow rate and material viscosity' significantly impact the interfacial material’s distribution, thickness, and switching fidelity7, as illustrated in FIGS. 3A-3G. As DIW inks are ty pically shear-thinning, the viscoelastic properties of the interface material are crucial for both ink flow out of the printhead and its circumferential spread around a filament, as illustrated in FIG. 3A. To study these effects, high viscosity' (SE1700) and low viscosity' (Sylgardl82) polydimethylsiloxane (PDMS) are combined in varying ratios. Despite similar viscosities between the 50:50 and 75:25 mixes, their storage and loss moduli vary, allowing investigation of their independent viscoelastic response. Using the same two 180° input channel printhead, increased flow rates result in greater average interface thickness and standard deviation, due to more material concentrating in the channel centers, as illustrated in FIGS. 3B and 3D. Similarly, increasing interface material viscoelastic properties, while keeping flow rate constant, narrows material distribution, as illustrated in FIGS. 3C and 3D. Higher viscosity7also shortens the switching distance. Basedon these results, the 75:25 PDMS mix shows an effective balance between achieving a thin, even interface and maintaining high switching fidelity, while an interface flow rate of 7 / 6 mm3 / s also has a fairly even material distribution and high switching fidelity. This balance is further evidenced when analyzing intensity values indicative of switching performance, as illustrated in FIGS. 3E and 3F, where both interfacial material quantity7and switching distance are markedly influenced by flow rate and viscosity7.

[0037] To analyze interactions among multiple filaments, including interfaces, the interface arc length 5 is defined as a function of the number of interface inlet channels n and the printhead outlet diameter d of the VI3DP printhead, expressed as 5 = ditin. For instance, a printhead with two inlets (n = 2) extrudes filaments with interface arc lengths of di / l. FIG. 3G illustrates filament cross-sections for n = 2, 4, and 8, showing configurations with either a single interface or all interfaces active. The number of feasible interfaces contingent upon their physical accommodation within the VI3DP printhead, implying that printhead geometry7is primarily constrained by the resolution of the printhead fabrication technique. For enhanced interface quality7and reduced thicknesses, further refinement of design parameters and consideration of additional factors and levels is necessary7.

[0038] FIGS. 3A-3G illustrate graphical and image views of an effect of material flow rate, viscosity, and number of inputs on interface quality. FIG. 3 A illustrates a graphical view of rheological behavior of three representative PDMS ink mixtures with varying ratios of a viscous to less viscous PDMS ink (100:0, 75:25, and 50:50). FIG. 3B illustrates image views of an effect of interfacial material flow rate on filament cross-section and interface switching, employing a two 180° input channel VI3DP printhead with 100:0 PDMS core and a 75:25 PDMS mixture for the interfacial material. FIG. 3C illustrates an influence of interfacial material viscosity7on filament cross-section and interface switching using the same VI3DPprinthead, maintaining fixed flow rates of 7 mm3 / s for core and 1 mm3 / s for the interface at a 7 mm / s feed rate. FIG. 3D illustrates graphical views of a thickness t of interfacial material around filament edges in FIG. 3B and FIG. 3C at 30° increments following a unit circle. FIGS. 3E and 3F illustrate graphical views of switching fidelity for each flow' rate and viscosity variation, w ith intensity show n over the distance traveled. FIG. 3G illustrates image views of neighboring filament cross-sections with varying nozzle input numbers (2, 4, and 8) for VI3DP printheads with flow rates of 0.25 mL / s for the core and 0. 125, 0.062, and 0.031 rnL / s for individual interfaces (equalized interface flow' rates when all interfaces are active, regardless of printhead size). Scale bars = 0.2 mm and 1 mm for the filament cross-sections and switching samples, respectively.

[0039] The control of interfacial behavior of VI3DP prints is enabled by the difference in the optical, mechanical, and / or electrical properties of the core and interface materials.Depending on the desired performance and application, various material combinations can be integrated, provided their rheological properties are compatible, as seen in FIG. 3A. The specific differences in flow' properties, post-processing (curing), and final (solidified) performance vary depending on the materials and application.

[0040] To demonstrate spatial programming of functional interfaces in 3D-printed objects, samples are fabricated with outw ard-facing optical voxel-air interfaces in various colors. FIG. 4A illustrates the use of printheads with a single interface material (i), two interface materials (ii). and four interface materials (iii). The individual interface materials can be distributed across all interface channels (i) or allocated to discrete channels (ii, iii). each of w hich can be individually addressed. By embedding visual patterns with distinct colors, these interfaces can modify a printed part’s appearance or visually encode data in situ without altering its shape, topology', or mechanical properties. For instance, a purple interface material isintegrated in a bipartite interface VI3DP printhead with two 180° partitions to pattern two distinct designs on opposite sides of a single-layered extrusion part, as illustrated in FIG. 4D. A similar printhead was used to embed a 2D code on one side of an object, as illustrated in FIG. 4B, preserving its mechanical properties, as confirmed by tensile tests, as illustrated in FIG. 4C. This method allows for object individualization, tracking, or protection without additional post-processing. Information can also be invisibly encoded using fluorescent interfaces, for example, to deter counterfeiting. To demonstrate this, a quadripartite interface printhead is integrated with four 90° sections to wrap a continuous single-material interface around a solid 3D object, as FIG. 4E. Using a single material across different interface channels reduces the number of required pressure regulators, solenoid valves, and material reservoirs; however, allocating different materials to specific interface channels enables targeted coating of each side of a single-walled 3D object with unique patterns and colors, as FIG. 4E.

[0041] FIGS. 4A-4F illustrate image and schematic views of optical interfaces controlled via VI3DP. FIG. 4A illustrates an image view of exemplary printheads, (i) A single interface material distributed over n = 4 individually addressable 90° interface partitions (quadripartite circle), (ii) Two distinct interface materials activated in n = 2 180° interface sections (bipartite circle), suitable for parallel printing, (iii) Four interface materials distributed n = 4 90° interface sections. FIG. 4B illustrates image views of a soft matter featuring a QR code on one side and a logo on the opposite side of a one-fdament-thick print. FIG. 4C illustrates image and graphical views of morse code-embedded dogbone prints numbered 1-5, tested under tensile loading to verify unchanged mechanical properties with or without embedded code. FIG. 4D illustrates image views of a dual-sided print with distinct images extruded on each side of a single filament-thick print. FIG. 4E illustrates image views of a wraparoundimage extruded using single-material, UV-fluorescent PDMS interfaces. FIG. 4F illustrates image views of a four-sided printed soft matter with distinctly colored PDMS for each interface. Scale bars = 10 mm.

[0042] VI3DP also enables selective control of inter-voxel adhesion, facilitating functional mechanical behaviors, as illustrated in FIGS. 5A-5F. In extrusion-based 3D printing, the filaments solidify chemically to form a solid part. To prevent filament bonding, these methods typically require a significant air gap or a layer of sacrificial material, removed postprinting. VI3DP, however, allows for the integration of separating interfaces with varying adhesion levels through strategic materials design. This is evidenced by peel test samples exhibiting a broad range of peel strengths, from virtually none to that of the base material, as illustrated in FIG. 5B. This programmable control of inter-voxel adhesion permits various features, as illustrated in FIG. 5A. For instance, extruding objects with no-adhesion interfaces facilitates near-perfect fits, exemplified by a 2D puzzle, as illustrated in FIG. 5C. These parts were printed monolithically with a sacrificial support material thick enough to segregate the voxels but significantly thinner than a filament (~50 pm ). A partial separation feature is shown in FIGS. 5D and 5E, where a removable treasure (dark blue support structure) is printed inside a treasure chest as a single part in one print. These prints, requiring no gap between adjacent voxels, achieve nearly seamless fits.

[0043] Intermediate adhesion levels are uniquely suited for bio-inspired structures with hierarchical features and distinctive mechanical properties. For example, natural structures such as mollusk nacre shells provide both strength and toughness through a multimaterial and multi-scale “brick-and-mortar” configuration, formed from mineral-based bricks and organic mortar. A brittle epoxy, emulating the “bricks,” and a flexible elastomer, emulating the “mortar,” were printed. The core and interfacial materials cure and adhere weakly, leading toa failure mode markedly different from that of a single-material print with identical print path and material volume, as illustrated in FIG. 5F. Under three-point bending, composite failure occurs along the interfaces due to reduced epoxy-elastomer adhesion, causing brick separation in a predetermined fashion. This behavior is reflected in the stress-strain curves, where the composite exhibits significantly higher failure strains, albeit lower strength compared to the epoxy-only sample. Although not specifically optimized for certain mechanical properties, this example underscores the potential of in-process functionalization of interface material, thickness, and placement to achieve superior mechanical characteristics in 3D-printed objects.

[0044] FIGS. 5A-5F illustrate image, graphical, and schematic views of mechanical interfaces with programmable adhesion levels. FIG. 5A illustrates a schematic diagram of feature modes enabled by inter-filament adhesion switching, including complete and partial separation. FIG. 5B illustrates a graphical view of a peel test representative curves comparing epoxy / wax, epoxy / silicone rubber, and epoxy / epoxy interfaces of vary ing adhesion strengths. FIG. 5C illustrates image views of pull-apart puzzle pieces as intended and printed monolithically as a single entity with non-adhesive photopolymer resin interfaces and a silicone core. FIG. 5D illustrates perspective views of an illustration of a functional, movable printed part comprising a structural core and sacrificial interface materials. FIG. 5E illustrates perspective views of a functional treasure box printed monolithically with a PDMS and waxpetroleumjelly mix for the core and interfaces, respectively. FIG. 5F illustrates schematic, image, and graphical views of composite brick-mortar structures with full and intermediate adhesion levels, respectively, i, Uniform single “brittle” material structure, ii, Multimaterial brick-mortar composite, featuring a flexible “mortar” material interspersed among adjacent bricks. Close-up images exhibit a brick-mortar print pattern and an isometric view of thesamples post-fracture, iii, Stress-strain representative curves for samples under three-point bend loading highlight the altered toughness of the composite. Scale bars = 10 mm.

[0045] Voxel -interface behaviors extend to interactions between electrically conductive interfaces and insulating base material. In DIW. this concept has been used to fabricate various sensors, such as capacitive touch sensors, where material switching occurs via multiple printheads or, more recently, through different inlets wi thin the printhead. VI3DP facilitates the integration of multiple conductive traces within a single voxel. FIGS. 6A-6I showcase a VI3DP capacitive touch sensor, consisting of only a single filament-thick layer with an integrated three-dimensional circuit. The sensor includes capacitive surfaces on top (Cl and C2), resistive traces on the bottom (R1 and R2), and electrical contacts connecting the top and bottom to an Arduino microcontroller. The conductive top surface is segmented into two angled sections for a graded capacitive response, as illustrated in FIG. 6A. The Arduino sends a voltage to charge the sensor, then detects the increase of sensor capacitance as a finger comes into contact with the capacitive surfaces (Cl and C2), thereby triggering corresponding changes in an LED’s brightness depending on the finger’s placement along the gradient of the surfaces, as illustrated in FIG. 6B. A four-sectioned quadripartite interface printhead enables the deposition of electrical traces (colored) as conductive voxel-air continuous interfaces, as illustrated in FIGS. 6B and 6C, which are patterned along discrete sections of the filament circumference, allowing multiple separate electrical paths at subfilament resolution to coexist on a single filament, as illustrated in FIG. 6D. FIG. 6E illustrates the location of capacitive surfaces, resistive paths, and contacts on the printed sensor, fabricated with a transparent insulating PDMS core and a conductive PDMS / carbon black interface material. The brightness of the LED connected to the Arduino directly corresponds to the finger location on the sensor, confirming the circuit’s functionality, asillustrated in FIG. 61. This single-layer construction minimizes both volume and print duration compared to conventional extrusion additive fabrication methods, regardless of printhead size, since interfaces can always be incorporated as smaller features into such 3D circuits.

[0046] FIGS. 6A-6I illustrate schematic, image, and graphical views of a VI3DP of a single fdament-thick capacitive touch sensor. FIG. 6A illustrates a schematic diagram of the capacitive touch sensor, where LED brightness controlled by changes in capacitance caused by a finger. FIG. 6B illustrates a sectional, side-view of the sensor showing the working principle of self-capacitance, with capacitive surfaces (Cl and C2) and resistive paths (R1 and R2) necessary for proper function, as well as the electric field (E-Field) emitted by the capacitive surfaces. Inset filament cross-section shows conductive interfaces around an insulating core. FIG. 6C illustrates a schematic diagram of a fabrication of the sensor using an insulating core material and discrete conductive interfaces using a four-sectioned quadripartite interface VI3DP printhead. FIG. 6D illustrates a schematic diagram of an exploded view7of the sensor, detailing each interface integrated into the single filament-thick layer. FIGS. 6E illustrate image views of the printed sensor with false-colored overlays highlighting the functionalized interfaces of individual components, including capacitive surfaces (Cl, C2), and resistive paths (Rl, R2). FIG. 6F illustrates a side view of the single- filament-thick capacitive sensor. FIG. 6G illustrates a graphical view7comparing length versus resistance for a PDMS filament with a conductive ink interface of varying thicknesses. FIG. 6H illustrates a schematic view of an electrical diagram of the sensor connected to the Arduino microcontroller and an LED. FIG. 61 illustrates image views of the sensor’s interactive behavior: as a finger moves from one end to the other, LED brightness isprogrammed to vary. Scale bars = 10 mm and 5 mm for the sensor images and the LED images, respectively.

[0047] VI3DP is a versatile method for functionalizing voxel-voxel and voxel-air interfaces in extrusion 3D printing at sub-filament resolution. VI3DP employs a novel printhead design incorporating up to eight individually addressable compartments for interface materials, which are programmatically controlled for selective placement along voxel interfaces. It allows for partial coverage of voxel sides and diverse interface materials. With suitable material pairings, new functionalities in monolithically printed exemplars are realized, such as encoded data in soft matter, movable mechanisms from precise fits, bio-inspired composite materials, and single-filament-thin capacitive touch sensors with integrated conductive traces. VI3DP streamlines the fabrication of parts that would otherwise be economically infeasible or technologically challenging, eliminating the need for multiple printheads and superfluous gaps or features. The printhead is adaptable to nearly any extrusion-based multimaterial 3D printer, including DIW and fused filament fabrication. Future work could integrate multimaterial switching within the interface channels, refine interface transitions, and further investigate interfacial behavior across a wider range of engineering materials. This work paves the way for future developments, potentially enabling thinner interfaces, novel material combinations, and integrated functions like complex 3D circuits, electromechanical actuators, data-embedded composite structures, and print-in-place mechanisms with precise tolerances.

[0048] Materials: For the silicone inks, SE1700 (viscous) and Sylgardl82 (less viscous) (Dow Coming, Midland, MI, USA) were mixed at different ratios with catalyst and pigment (SilcPig, Smooth-On, Macungie, PA, USA) in a 10:1:0.1 weight ratio for 120 s at 2000 rpm using a DAC 300-100 Pro centrifugal speed mixer (Flaktek, Landrum, SC, USA). The photopolymer resin was prepared by mixing Aqua Gray 8K (Phrozen, Hsinchu, Taiwan) withfumed silica (CAB-O-SIL FAS-SIL 200, Fasco Epoxies, Inc.. Fort Pierce, FL) in a 10:0.8 weight ratio at 2000 rpm for 120 s in the speed mixer. For the wax ink, microcrystalline wax (SP-19. Strahl & Pitsch, West Babylon, NY, USA) was mixed with petroleum j elly (Unilever) at a 1:2 weight ratio. Both parts were combined and heated in a furnace to 100°C until melted, at which point they were mixed in a speed mixer for 120 s at 2000 rpm. The brittle epoxy was prepared by mixing a 10:3.5: 1.43 weight ratio of EPON828 (Hexion, Houston, TX, USA), Epikure3230 (Hexion), and fumed silica (CAB-O-SIL), with 1% w / w of dye (Let’s Resin, Hong Kong), in a sequence of 800 rpm for 90 s, 1600 rpm for 90 s, and 2000 rpm for 120 s in a speed mixer. The flexible elastomer ink was prepared by mixing a 10: 1 weight ratio of MoldMax20 part A (Smooth-On) with MoldMax20 part B (SmoothOn) at 2000 rpm for 120s in a speed mixer (FlackTek). Conductive inks were prepared by mixing a 10 g of Ecoflex30 part A (Smooth-On) with 1.4 g carbon black particles (AB100%03, Soltex, Houston, TX, USA) at 2000 rpm 18 minutes (divided up as 5 min, 5 min, 4 min, and 4 min, with 2 min breaks in between as per manufacturers guidelines) in the speed mixer (FlackTek). Separately, 10 g of Ecoflex30 part B (Smooth-On) was mixed with 1.4 g carbon black particles (Soltex) at 2000 rpm and in the same timing pattern as part A. 0.5 g of a cure retarder (SloJo, Smooth-On) was then mixed with mixture B for 120 s. at 2000 rpm. Parts A and B were then combined and mixed together for 12 min (divided into 4 min, 4 min, and 4 min) at 2000 rpm. Finally, 2.5 g more of retarder was added to the mixture and mixed for another 120 s at 2000 rpm in the speed mixer.

[0049] Printhead fabrication: The printheads were fabricated on a stereolithography 3D printer (Sonic mini 8K, Phrozen) using either Aqua Gray 8K Resin (Phrozen) for high resolution printing or Simple Clear Resin (Siraya Tech, Taiwan) for visualization. After printing, the printheads were washed with isopropyl alcohol and dried with an air stream.

[0050] The VI3DP printheads were connected with Luer locks to ink-filled syringe barrels (Nordson EFD, East Providence, RE EISA), and mounted onto a 6-axis motion gantry' stage controller (Aerotech, Inc., Pittsburgh, PA, USA). Air pressure was supplied to the syringes using Ultimus V pressure pumps (Nordson EFD) and gated by a system of pneumatic solenoid valves (VQD1151-5MO, SMC, Toky o, Japan). The solenoid valves were controlled by 0-24- V digital relays (WAGO, Minden, Germany) synchronized to the print motion with the A3200 motion controller (Aerotech).

[0051] Ink rheology: Rheological behavior of the ink was determined by viscoelastic testing with a rheometer (MCR 302, Anton Parr, Graz, Austria) using a 25 mm parallel plate for plateplate geometry'. Apparent viscosities were measured by shear-rate sweeps with shear rates from 0.1 s’1to 100 s’1. Storage and loss moduli were determined using oscillatory amplitude sweeps with shear rates from 0.01% to 100% with an angular frequency of 10 rad / s. Tests were performed within ~30 min. of mixing.

[0052] VI3DP printing process: Ink was filled into 3 cc, 10 cc, or 30 cc syringe barrels (Nordson EFD) and centrifuged at 5000 rpm for 60 sec in a ProcessMate 5000 (Nordson EFD) to remove air bubbles. For printhead geometry characterization, SE1700 was used for the core and 75:25 SE1700:Sylgardl82 was used for the interfacial material. The flow rate was fixed at 0.007 cm3 / s for the core and 0.001 cm3 / s for each interface with a 7 mm / s print speed. The material parameter characterization used the same print speed and core flow rate, but the materials and interface floyv rate varied. The wraparound serpent cube yvas printed with a print speed of 7 mm / s at pressures of 22 psi and 60 psi for the silicone and fluorescent inks, respectively. The multi-color cube was printed with a print speed of 7 mm / s at pressures of 22 psi and 26 psi for the white and colored silicone inks respectively. Data-encoded samples yvere printed with a print speed of 7 mm / s at pressures of 37 psi and 40 psi for thewhite and black silicone inks respectively. The puzzle pieces were printed with a print speed of 7 mm / s at pressures of 23 psi and 60 psi for the silicone and photopolymer inks, respectively. The treasure box was printed with a print speed of 10 mm / s at pressures of 21 psi and 38 psi for the silicone and wax inks, respectively. The full epoxy nacre-like structure was printed with a print speed of 14 mm / s at a pressure of 32 psi. The composite nacre-like structure was printed with a print speed of 14 mm / s at a pressure of 30 psi for the epoxy bricks and a range of 20-30 psi for several samples for the flexible mortar. The capacitive touch sensor was printed with a print speed of 7 mm / s at pressures of 21 and 60 psi for the silicone and conductive inks respectively.

[0053] After print, silicone samples were cured at a constant 100° C for about 24 h. Epoxy samples were cured at a constant 100° C for 24 h in a furnace (Heratherm, Thermo Fisher Scientific, Waltham, MA, USA). Wax / silicone structures were cured in a furnace at a constant 60° C for 20 h, which is hot enough to cure the silicone, but not hot enough to melt the wax.

[0054] Property characterization: Mechanical tests were done on an electromechanical testing system (MTS, Eden Prairie, MN, USA). Morse code-embedded dogbone samples were tested under a tensile load at a rate of 1 mm / s with a 5 kN load cell. Peel test samples were tested under a tensile load at a rate of 1 mm / s with a 5 N load cell. Composite samples were subjected to a compressive load in 3 -point bend configuration at a rate of 1 mm / s with a 5 kN load cell. The microscope used for imaging / measurement of filament samples was an optical microscope and attached digital camera (SZX12 and DP23, Olympus, Shinjuku, Japan). Electrical resistance was measured with a multimeter (MSR-A2000, ETEKCITY, Anaheim, CA, USA).

[0055] The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

[0056] It should be noted that the printing protocols described herein can be executed with a program(s) fixed on one or more non-transitory computer readable medium. The non- transitory computer readable medium can be loaded onto a computing device, server, imaging device processor, smartphone, tablet, phablet, or any other suitable device known to or conceivable by one of skill in the art.

[0057] It should also be noted that herein the steps of the method of printing described can be carried out using a computer, non-transitory computer readable medium, or alternately a computing device, microprocessor, or other computer type device independent of or incorporated with an imaging or signal collection device. An independent computing device can be networked together with the imaging device either with wires or wirelessly. The computing device for executing the present invention can be a completely unique computer designed especially for the implementation of this method. Indeed, any suitable method of analysis known to or conceivable by one of skill in the art could be used. It should also be noted that while specific equations are detailed herein, variations on these equations can also be derived, and this application includes any such equation known to or conceivable by one of skill in the art.

[0058] A non-transitory computer readable medium is understood to mean any article of manufacture that can be read by a computer. Such non-transitory computer readable mediaincludes, but is not limited to, magnetic media, such as a floppy disk, flexible disk, hard disk, reel-to-reel tape, cartridge tape, cassette tape or cards, optical media such as CD-ROM, writable compact disc, magneto-optical media in disc, tape or card form, and paper media, such as punched cards and paper tape.

[0059] It should be noted that the software associated with the present invention is programmed onto a non-transitory computer readable medium that can be read and executed by any of the computing devices mentioned in this application. The non-transitory computer readable medium can take any suitable form know n to one of skill in the art. The non- transitory computer readable medium is understood to be any article of manufacture readable by a computer. Such non-transitory computer readable media includes, but is not limited to, magnetic media, such as floppy disk, flexible disk, hard disk, reel-to-reel tape, cartridge tape, cassette tapes or cards, optical media such as CD-ROM, DVD, Blu-ray, writable compact discs, magneto-optical media in disc, tape, or card form, and paper media such as punch cards or paper tape. Alternately, the program for executing the method and algorithms of the present invention can reside on a remote server or other networked device. Any databases associated with the present invention can be housed on a central computing device, server(s), in cloud storage, or any other suitable means known to or conceivable by one of skill in the art. All of the information associated with the application is transmitted either wired or wirelessly over a network, via the internet, cellular telephone network, RFID, or any other suitable data transmission means known to or conceivable by one of skill in the art.

Claims

What is claimed is:

1. A device for three-dimensional (3D) printing comprising: a nozzle, wherein the nozzle has an input for a primary7material and n inputs for a secondarymaterial; a syringe barrel coupled to the nozzle; and a pressure system having an extruder for each of the primary material and the secondary7material.

2. The device of claim 1 wherein the nozzle further comprises a center channel for the primary7material and outer channels that converge around an edge of the center channel in arc sections.

3. The device of claim 2 wherein each one of the outer channel tapers into the center channel to allow for evenly distributed filament coverage by the secondary material.

4. The device of claim 1 wherein the pressure control box for the primary material applies pressure to the syringe barrel containing the primary7material.

5. The device of claim 1 wherein the extruder for the secondary material applies pressure through solenoid valves with inputs connected to a single manifold and outputs connected to syringe barrels containing the secondary material.

6. The device of claim 5 wherein a number of solenoid valves corresponds to a number of syringe barrels.

7. The device of claim 6 wherein n number of inputs corresponds to the number of solenoid valves and the number of syringe barrels.

8. The device of claim 1 further comprising a printhead tip.

9. The device of claim 8 wherein the printhead tip is configured for printing in a series of cubic voxels.

10. The device of claim 1 wherein the nozzle is configured to extrude a core filament of the primary’ material with an interface of the secondary material applied on top of the primary material.

11. A system for three-dimensional (3D) printing comprising: a nozzle, wherein the nozzle has an input for a primary material and n inputs for a secondary material; a syringe barrel coupled to the nozzle; a pressure system having an extruder for each of the primary material and the secondary material; and a computer processing device configured to provide control of the nozzle, syringe barrel, and pressure system and consequently extrusion of the primary and the secondary material.

12. The system of claim 11 wherein the nozzle further comprises a center channel for the primary’ material and outer channels that converge around an edge of the center channel in arc sections.

13. The system of claim 12 wherein each one of the outer channel tapers into the center channel to allow for evenly distributed filament coverage by the secondary material.

14. The system of claim 11 wherein the pressure control box for the primary material applies pressure to the syringe barrel containing the primary' material.

15. The system of claim 11 wherein the extruder for the secondary' material applies pressure through solenoid valves with inputs connected to a single manifold and outputs connected to syringe barrels containing the secondary' material.

16. The system of claim 15 wherein a number of solenoid valves corresponds to a number of syringe barrels.

17. The system of claim 16 wherein n number of inputs corresponds to the number of solenoid valves and the number of syringe barrels.

18. The system of claim 11 further comprising a printhead tip.

19. The system of claim 18 wherein the printhead tip is configured for printing in a series of cubic voxels.

20. The system of claim 18 wherein the computer processor is programmed for comprehensive control over primary and secondary material extrusion irrespective of a diameter of the printhead tip that dictates feature size.

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