A method and system for direct ink writing

Abstract

A method and system for direct ink writing. The method comprises the steps of providing a molten metallic material in a heated printhead such that the molten metallic material wets a nozzle of the printhead; disposing a tip of the nozzle adjacent to a substrate such that the molten metallic material flows onto the substrate; and moving the tip of the nozzle relative to the substrate such that a continuous line of the solidified metallic material is formed on the substrate; wherein, during the moving of the tip relative to the substrate, the molten metallic material is pulled from the nozzle under ambient pressure as a result of shear and / or tension between a leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

Description

[0001] A METHOD AND SYSTEM FOR DIRECT INK WRITING

[0002] FIELD OF INVENTION

[0003] The present invention relates broadly to a method and system for direct ink writing.

[0004] BACKGROUND

[0005] Any mention and / or discussion of prior art throughout the specification should not be considered, in any way, as an admission that this prior art is well known or forms part of common general knowledge in the field.

[0006] Most commercial 3D printers require support materials to build 3D structure, have low printing speed, need pressure suppliers such as pneumatic pump or mechanical actuator to drive printing, and need postprocessing such as sintering and annealing to improve their mechanical and electrical properties.

[0007] One existing technology in

[0001] uses shear-driven printing with eutectic gallium indium (EGain), which is liquid at room temperature, but is limited to creating 2D planar patterns.

[0008] Another existing technology in [2] uses an electrohydrodynamic (EHD) printing approach of Field’s metal and was reported to print short pillars with height of only ~2 mm. However, this technology is driven by electrical field force and requires a high voltage (> 1 kV) being applied between nozzle and substrate to drive the printing process, so the printer is complex and somewhat un-safe to operate.

[0009] Embodiments of the present invention seek to address at least one of the above problems.

[0010] SUMMARY

[0011] In accordance with a first aspect of the present invention, there is provided a method of direct ink writing comprising the steps of: providing a molten metallic material in a heated printhead such that the molten metallic material wets a nozzle of the printhead; disposing a tip of the nozzle adjacent to a substrate such that the molten metallic material flows onto the substrate; and moving the tip of the nozzle relative to the substrate such that a continuous line of the solidified metallic material is formed on the substrate; wherein, during the moving of the tip relative to the substrate, the molten metallic material is pulled from the nozzle under ambient pressure as a result of shear and / or tension between a leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0012] In accordance with a second aspect of the present invention, there is provided a direct ink writing system comprising: a heatable printhead configured for providing a molten metallic material such that the molten metallic material wets a nozzle of the printhead; and a stage for a substrate; wherein the system is configured for: positioning a tip of the nozzle adjacent to the substrate such that the molten metallic material can flow onto the substrate; moving the tip of the nozzle relative to the substrate such that a continuous line of the solidified metallic material can be formed on the substrate; and pulling, during the moving of the tip relative to the substrate, the molten metallic material from the nozzle under ambient pressure as a result of shear and / or tension between a leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0013] BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:

[0015] FIG. 1 A shows a schematic drawing illustrating the tension-driven printing for free-standing 3D metal architectures, according to an example embodiment.

[0016] FIG. IB shows a photograph of metal wires printed by pressure-driven ink writing. Scale bar 3 mm.

[0017] FIG. 1C shows a photograph of metal wires printed by tension-driven ink writing at the same condition, according to an example embodiment. Scale bar 3 mm.

[0018] FIG. ID shows a graph illustrating the line width of printed metal wires according to example embodiments versus the printing speed with different inner diameters of nozzles at 80 °C. FIG. IE shows a photograph of a free-standing 3D cubic framework printed according to an example embodiment.

[0019] FIG. IF shows photographs of metal patterns printed on different substrates, including glass, polyimide (PI), polyethylene terephthalate (PET), office paper, PDMS, and Ecoflex from left to right, respectively, according to example embodiments. Scale bars 5 mm.

[0020] Fig. 2A shows a photograph of an out-of-plane 3D architecture arrays printed by the layer-by- layer deposition of Field’s metal, according to an example embodiment.

[0021] FIG. 2B shows a photograph of printed free-standing metal filaments according to example embodiments, with slope angles of 90 ° (vertical), 60 °, and 45 °. Scale bar 5 mm

[0022] FIG. 2C shows a photograph of a printed free-standing metal filament according to example embodiments, with a slope angle of 30 °. Scale bar 5 mm

[0023] FIG. 2D shows a photograph of a printed free-standing metal filament according to example embodiments, with a slope angle of 10 °. Scale bar 5 mm

[0024] FIG. 2E shows a photograph of a printed free-standing metal filament according to example embodiments, with a slope angle of 00(horizontal). Scale bar 5 mm.

[0025] FIG. 2F shows a graph illustrating the width of printed free-standing metal filaments versus the slope angles with different inner diameters of nozzles at 80 °C, according to example embodiments.

[0026] FIG. 2G shows a photograph of the printed free-standing letters of “NUS”. Scale bars 3 mm

[0027] FIG. 2H shows a photograph of the printed free-standing helical metal structures with diameters of 1 , 3, 5, and 7 mm from left to right, respectively, according to example embodiments.

[0028] FIG. 21 shows a photograph of a printed free-standing hemispherical helix array with diameters of 4, 5, 6, 5, and 4 mm from left to right, respectively, according to example embodiments.

[0029] FIG. 2.1 shows a graph illustrating a comparison of the conductivity and aspect ratio of reported 3D printed conductors built by direct ink writing and an example embodiment of the present invention.

[0030] FIG. 3 A shows a photograph of a printed circuit according to an example embodiment, initially.

[0031] FIG. 3B shows a photograph of the printed circuit according to an example embodiment, when broken. The insert is a photograph of the broken point from an optical microscope. Scale bar 200 pm.

[0032] FIG. 3C shows a photograph of the printed circuit according to an example embodiment, after self-healing with a gentle heat treatment. The insert is a photograph of the self-healed point from optical microscope. Scale bar 200 pm. FIG. 3D shows a photograph of printed arc bridges with different radii (r) from top view, according to example embodiments.

[0033] FIG. 3E shows a photograph of printed arc bridges with different radii (r) front view, according to example embodiments.

[0034] FIG. 3F shows a photograph of printed multiple arc bridges with diverse radii (r) crossing over the same point from top view, according to example embodiments.

[0035] FIG. 3G shows a photograph of printed multiple arc bridges with diverse radii (r) crossing over the same point from front view, according to example embodiments.

[0036] FIG. 3H shows a photograph of a printed 3D circuit with VIAs connecting four layers at varying heights from side view, according to example embodiments.

[0037] FIG. 31 shows a photograph of a printed 3D circuit with VIAs connecting four layers at varying heights from top view, according to example embodiments.

[0038] FIG. 3J shows a schematic drawing illustrating a 3D printed multilayer circuit for a battery- free temperature sensor, according to an example embodiment.

[0039] FIG. 3K shows a photograph of a 3D printed multilayer circuit for a battery-free temperature sensor, according to an example embodiment. Scale bar 5 mm.

[0040] FIG. 3Lshows a photograph of a side view of the printed VIAs with height (h) of 2 mm on the top layer circuit, according to an example embodiment.

[0041] FIG. 3M shows a photograph of the printed circuit powered by a phone via NFC at 25 °C, according to an example embodiment.

[0042] FIG. 3N shows a photograph of the printed circuit worn on human hand at 35 °C, according to an example embodiment.

[0043] FIG. 4A shows a photograph of the electromagnetic band gap (EBG) metamaterial created by tension-driven 3D printing according to an example embodiment.

[0044] FIG, 4B is a schematic drawing illustrating the EBG metamaterial structure and geometrical design parameters (h = 4 mm, g = 1 .275 mm, w = 8 mm), according to an example embodiment.

[0045] FIG. 4C shows a dispersion diagram of the EBG metamaterial according to an example embodiment, using CST Microwave Studio. The vertical axis shows the frequency, and the horizontal axis represents the values of the wavenumbers in Brillouin zone.

[0046] Figure 5 shows a flowchart illustrating a method of direct ink writing according to an example embodiment. DETAILED DESCRIPTION

[0047] Example embodiments of the present invention can provide a high-speed tension-driven 3D printing of free-standing metal structures, which employs tension between molten metal in a nozzle and the leading edge of the printed part to direct- write 3D structures with, for example, Field’s metal. The use of tension advantageously obviates the need of external pressure for extrusion. The method according to an example embodiment creates highly uniform and smooth metal microwirc structures on various substrates with high printing speed of up to 100 mm s’1. Free-standing metal filaments with high aspect ratio of up to 750 can be printed at sloping angles ranging from even 0° to 90° without support materials. Various free-standing 3D metal structures, including vertical letters, a cubic framework, and scalable helixes, have been directly printed without post treatment according to exemplary embodiments, exhibiting high precision and superior structural retention. The printed metal structures advantageously possess high electrical conductivity of 2 x 104S cm’1, self-healing capability and can be readily recycled. In various embodiments, the printing method can be generalized to a broad range of metallic materials, including ordinary metals, such as aluminum-based alloys and steels, and low-melting-temperature alloys, for example, gallium, Field’s metal, Wood’s metal, and Rose’s metal. In example embodiments described herein, the present invention has been applied to print a 3D circuit for wearable battery-free temperature sensing, hemispherical helical antennas for highly sensitive and wireless vital sign monitoring, and 3D metamaterials for electromagnetic wave manipulation, by way of example, not limitation.

[0048] Conductive high-aspcct-ratio metal 3D printing method driven by tension according to example embodiments is described herein in detail (also referred to as “CHARM3D” herein). As shown in FIG. 1A, the printing system 100 used for CHARM3D comprises a commercial printhead 102 with a heater 104 integrated and a 4-axis micropositioning stage 106, which is able to move in x, y, and z axes and rotate in xy plane according to a 3D predesign.

[0049] In existing techniques, pressure sources such as pneumatic pressure controllers or mechanical pumps arc typically one of the essential components for 3D printers to drive direct-ink- writing (DIW). In contrast, in the CHARMD3D system 100, the end 107 of the ink barrel 108 is open to air instead of connecting with a pressure supplier. Advantageously, there is no external pressure needed during the whole 3D printing process. Such pressure-free technique advantageously avoids bulky pumps and pressure controllers and thereby significantly simplifies the 3D printer according to example embodiments.

[0050] Molten Field’s metal has a large surface tension of around 410 mN m-1 at 108 °C and a low viscosity of about 27 rnPa s at 80 °C. Due to its large surface tension and low viscosity, pressure-driven printing cannot produce uniform metal wires on a substrate. Indeed, it was observed that a scries of small beads c.g. 110 arose for printed Field’s metal wires before solidifying (FIG. IB). In stark contrast, the CHARM3D system 100 is driven by shear for the planar 2D ink writing and by tension for the out-of-planc 3D printing. The tension arises from the liquid bridge that spans from the molten metal in the nozzle 112 and the leading edge 114 of the solid, printed part. Compared to pres sure -driven printing (FIG. IB), tension-driven CHARM3D can print continuous, uniform, and smooth metal wires 111 at the same condition, i.c. same printhead, same heating temperature, same substrates, same printing speed, but shear- driven instead of pressure-driven (FIG. 1C).

[0051] To initiate the printhead 102, the entire steel nozzle 112 tube is fully filled and wetted with molten metal before the first printing. Once initiated, the nozzle 112 tube will be wetted all the time even when the metal ink runs out in the barrel, and the nozzle 112 was found to be reusable for more than one month without a sign of corrosion or clogging. Specifically, it was found that there is always residual Field's metal inside the nozzle even when the ink runs out, so it is wetted still once melted on re-start. To print metal patterns, the nozzle 112 tip is disposed in close proximity to the substrate 115. Then, the relative parallel motion between nozzle 112 and the substrate 115 induces shear to pull out molten metal from nozzle 112 continuously owing to its low viscosity, for 2-D printing. During printing, the stage 106 that supports the substrate 115 preferably disperses heat efficiently, c.g. by being made from steel or other thermally conductive material, such that the printed metal patterns solidify immediately. To terminate the printed structure, a high speed of c.g. over 30 mm s-1 can be applied in z axis to break the ink flow by lifting the printhead 102. As mentioned above, it was found that if the metal ink runs out, printing can be carried on after re-adding metal into ink barrel 108 of the printhead 102, without initiation step. Moreover, in an example embodiment, the printed metal 116 can be recycled as e.g. Field’s metal does not react with the substrates used and hence the printed metal structures can be peeled off from the substrates without residue readily.

[0052] For out-of-planc 3D printing, in the CHARM3D system 100 the tension generated when the nozzle 112 is moving away from substrate 115 (moving in z axis) was found to be able to pull out the molten metal smoothly to create various free-standing 3D metal structures. Once leaving the nozzle 112 tip, printed metal filaments were found to cool down and solidify immediately and sufficiently to support subsequent ink, and thus, advantageously, no external support is required during the whole 3D printing process, according to an example embodiment. To print free-standing 3D structures without support is in high demand but currently a bottleneck for conventional 3D printing, since the inks used in conventional 3D printing cannot solidify immediately after printing. Furthermore, it has been recognized by the present inventors that the substantially immediate solidifying of metal inks such as Field’s metal enables tension-driven 3D printing, i.e. without the use of additional driving mechanisms such as pressure or electric field force. Therefore, the CHARM3D system 100 can advantageously create such challenging free-standing 3D structures and even over-hanging structures readily.

[0053] To demonstrate such superior ability, as one example, a free-standing 3D metal cubic framework 118 was printed by the CHARM3D system 100 (FIG. IE), and noting that printing the over-hanging top square of the framework 118 without support has not been possible in existing printing methods, to the best of the inventors’ knowledge. Upon printing, the 3D structures arc rigid, stable, and highly conductive, so that they can advantageously be applied in various electronic devices without any post treatments. To investigate the planar 2D printability of CHARM3D, a series of parameters such as printing resolution, nozzle inner diameter (ID), printhead temperature, and travelling speed (printing speed) were systematically studied (FIG. ID). Polydimcthylsiloxanc (PDMS) film was chosen as the substrate and the distance between the nozzle tip and substrate was fixed at 100 pm during every printing. As shown in FIG. ID, the line width of printed metal wires is tunable between 100 pm and 300 pm in example embodiments. At low printing speed range (10-20 mm s-1), line widths arc close to the nozzle inner dimensions (IDs). Line widths reduce as printing speed increases for a certain nozzle ID and temperature, then, remain constant after printing speed exceeds 100 mm s-1 in example embodiment. When the printing speed is higher than 100 mm s-1, the printer vibrates in the current experimental set-up, which may be detrimental to the printing system. Thus, the speed range for stable and reliable printing using the experimental set-up was between 10 mm s-1 and 100 mm s-1. Moreover, line widths increase for a given printing speed as nozzle ID increases from 210 pm to 260 pm. On the other hand, the printhead temperature also has an impact on the printing resolution because it determines the viscosity of the molten metal ink. For a given nozzle ID and printing speed, line width may increase as the printhead temperature increases from 80 °C to 100 °C owing to reduced ink viscosity. It is worth mentioning that the printing speed in the experimental set-up is one order of magnitude higher than that of previously reported printing methods. Usually, the printing speed of existing DIWs is smaller than 10 mm s-1, as typical conductive inks such as silver pastes have a high viscosity, which restricts the flow rate even under a high pressure. However, the low viscosity of molten Field’s metal according to an example embodiment advantageously enables fast printing, resulting in a high manufacturing efficiency, especially for large-scale structures.

[0054] Using the CHARM3D, various planar metal patterns can be created with high quality and high efficiency, such as a 2D inductor. The metal patterns can be printed reliably on diverse substrates with a wide range of Young's modulus, from rigid glass to soft PDMS and Ecoflcx (FIG. IF). Moreover, surface roughness of substrates seems to have no significant impact on the performance of CHARM3D, for example, metal patterns could be printed on rough surfaces like office paper. Such great substrate compatibility enables extensive potential applications in both soft and rigid devices fabrication. Furthermore, there is no surface pretreatment required for all the substrates utilized in example embodiments described herein except basic cleaning. In contrast, existing techniques may require surface treatments such as UV ozone treatment and oxygen plasma to modify surface chemistry and enhance interfacial adhesion between substrates and printed metal features.

[0055] Creating complex out-of-planc 3D structures is the most attractive capability for 3D printing. Layer-by-layer depositing metal filament was used to demonstrate 3D constructing ability for CHARM3D according to example embodiments. A series of out-of-planc metal structures with 30 layers were, for example, printed readily (FIG. 2A). For each structure, all the 30 layers advantageously bond intimately as a single entity rather than separate filaments, because hot metal ink re-melts the top volume of the underlying layer and fuses together during stacking printing. This feature enables printed layers to become a solid and stable foundation for subsequent layers, thus making it possible to create tall and complex structures without deviation or collapse. To illustrate this superiority of an example embodiment of the present invention for layer-by-layer 3D printing, two hollow cylinders 200, 202 with variational radii were printed (FIG. 2A). The variation of radius shrinkage and expansion is 90 pm for every layer, where the line width is 200 pm, showing the whole structures are still printable and stable even if nearly half of every metal filament is over-hanging. In addition, two twisted cubes 204, 206 were printed successfully by rotating the xy plane after each layer printing with a magnitude of 1 ° and 2 °, respectively (FIG. 2A).

[0056] Layer-by-layer deposition, which can only construct a limited amount of 3D structures, is the only strategy for 3D printing using existing DIW. In stark contrast, to further demonstrate the potential to create sophisticated and functional out-of-plane structures using CHARM3D according to example embodiments, a series of metal filaments with various slope angles (FIG. 2B-E) arc pulled out from the nozzle readily by tension when the nozzle is retracting at corresponding slope angles. All of the sloped filaments were able to stand steadily without any supports, even at slope angles of lower than 45 °, while support structures arc always required when printing at slope angles of lower than 450for conventional 3D printing. Those filaments arc advantageously uniform and smooth with excellent structural retention irrespective of slope angle. The line widths (diameters) of free-standing filaments were found to be determined by the nozzle size mostly when slope angles are higher than 60 °, and they were found to reduce slightly as slope angles diminish (FIG. 2F). The length of the free-standing filaments was sealable from millimeters to decimeters and the longest single free-standing filament printed using the experimental set-up was around 150 mm due to the limited moving range of the stage and the printhead. It is plausible that further elongated free-standing filaments can be printed by CHARM3D according to example embodiments, as solidified metal is rigid. It is noted that the tension required to pull out the metal ink according to example embodiments is much smaller than the force required to overcome the adhesion between the printed metal and the substrate, because melted metal has low viscosity. Accordingly, some free-standing structures were found to not need, e.g. a planar line as a support structure, like the cube framework 118 (FIG. IE) and “NUS” letters (FIG. 2G). For complex structures according to various example embodiments, a planar stand structure can improve success rate of free-standing printing.

[0057] Such effective and precise free-standing printing capability advantageously enables freeform 3D creation of sophisticated metal structures. For example, three free-standing letters of “NUS” with a height of around 6 mm were printed out of plane (FIG. 2G). Although the letters were printed at 75 °C by two or three steps, each letter is continuous mechanically, and can be lifted by holding one point. Moreover, a free-standing cubic framework 118 with a side length of 1 cm was printed (FIG. IE). Horizontally printing over-hanging structures such as the square on the top of the cuic framework 118 is always an enormous challenge for conventional DIWs. However, the tension-driven CHARM3D according to example embodiments can create such challenging structures effectively. A smooth and uniform metal filament square has been successfully printed at the top of the cubic framework 118 with high precision. At each vertex, the printhead was programmed to deposit metal ink exactly on the top tip of every vertically standing filament. Solid adhesion is achieved at every vertex to support and stabilize the whole architecture as the just-printed metal ink is hot enough to re-melt a small volume of solidified metal on the vertical filament top.

[0058] Furthermore, the capability of rotating the stage in the xy plane advantageously enables printing circular- and spiral-based 2D and 3D structures with high precision. To illustrate, a series of free-standing helixes with various dimensions were printed by coordinating the stage rotation in the xy plane while the printhead retracts along the z axis (FIG. 2H). The diameters of these printed helical structures were sealable from micrometers to millimeters with the smallest pitch of around 600 pm, using the experimental set-up. If the diameter is higher than 5mm, the bottom parts may deform due to gravity, using the experimental set-up. During tensile testing, the printed helical structures exhibited elastic and plastic behavior. Their spring mechanical properties and high electrical conductivity advantageously enable printed helixes to be a candidate for stretchable conductors, for example. In addition, a series of free-standing hemispherical helical structures with variable diameters from 4 mm to 6 mm were printed using CHARM3D (FIG. 21). The whole hemispherical helix c.g. 210 is supported by a single short vertical filament c.g. 212, a structure which cannot be printed by conventional methods. Such hemispherical helixes could, for example, be applied as 3D antennas. All the complex 3D architectures printed by CHARM3D according to example embodiments described herein present excellent structural retention, and no post treatment is required, hi contrast, post treatments such as annealing or laser sintering arc indispensable for conventional DIWs to realize desired electrical functions, which always lead to unavoidable volume shrinkage in printed 3D structures.

[0059] Highly conductive and high-aspect-ratio printed 3D structures are useful for creating complex components integrated in a decreasing area or volume, for example. However, conventional printing methods may not be able to meet these requirements easily. Among the well-developed inks for conventional, pressure-driven printing, conductive polymer-based inks and graphenebased inks exhibit relatively low electrical conductivity (FIG. 2 J). Liquid metal-based inks and silver nanoparticlcs / flakc -based inks can realize sufficient conductivity, but post treatments such as annealing or laser sintering arc required for silver paste, for example.

[0060] Also, to achieve high aspect ratio geometries, the ratio of height or length to width for freestanding filaments, is a big challenge for conventional conductive inks, because they arc unable to solidify immediately once printed and post treatments arc always needed to accelerate their solidification, while liquid metal is liquid and soft intrinsically at room temperature. Thus, it is difficult for those inks to become self- supported high- aspect-ratio 3D structures by printing.

[0061] CHARM3D of Field’s metal according to example embodiments overcomes such challenge successfully. All the printed free-standing 3D metal structures described herein exhibited high electrical conductivity and high aspect ratio. The highest aspect ratio of CHARM3D using the experimental set-up was 750, achieved by a 150 mm-long free-standing filament. Such outstanding features of CHARM3D according to example embodiments pave the way to extensive applications in various 3D electronics. In various example embodiments, it is plausible that CHARM3D can be generalized to a broad range of metallic materials, including ordinary metals, such as aluminum based alloys and steels, and low-melting-temperature alloys, for example, gallium, Field’s metal, Wood’s metal, and Rose’s metal, preferably exhibiting the following characteristics (1) low viscosity in the molten state, so that they can be pulled out through the nozzle by the small tension that is the driving force for 3D printing. (2) Oxidation occurs immediately on the surface of the just- printed metals to prevent forming beads before solidifying and thus, they can retain the uniform and smooth filament shape; (3) be solid at room temperature so that molten metals can solidify fast after printing to sustain the designed free-standing 3D structures.

[0062] Printed metal circuits with a self-healing capability owing to the low melting point of Field’s metal were demonstrated according to an example embodiment. For example, a circuit was fabricated by printing serpentine wires 300 to connect a red LED 302 and encapsulating them with PDMS (FIG. 3A). The LED 302 was lighting initially, but it lost power once a break occurred (FIG. 3B). When heated over 62 °C by a mild hot air gun, the Field’s metal re-melted and the two ends on either side of the break point fused and re-joined together without an obvious scar (FIG. 3C). Such effective self-healing process enabled the LED 302 to light again. This ability could extend the lifetime and reduce the cost for wearable electronics as they arc easy to wcar / tcar or disrupt during daily use.

[0063] Moreover, a series of are-shaped metal bridges were created by CHARM3D to cross over 2D wires and 3D structures without physical contact, according to example embodiments. The radius of arc bridges c.g. 310 was tunable from submillimctcr to millimeter scales using the experimental set-up, to meet the different requirements of crossing span (FIG. 3D, E). To demonstrate its superior reliability and precision, a multiple-cross architecture 312 was printed by CHARM3D according to an example embodiment (FIG. 3F, G), consisting of 3 arc bridges e.g. 314 with various radii of 1 mm, 1.5 mm, and 2 mm, respectively, crossing over the same point.

[0064] As another example application, vertical interconnect accesses (VIAs) become difficult to form when fabricating 3D circuits. To make VIAs, previous strategics such as laser ablating holes followed by filling with conductive materials arc complicated and time-consuming. The excellent free-standing 3D printing capability enables CHARM3D according to example embodiments to address this difficulty efficiently. To illustrate, a series of VIAs c.g. 320 with various heights and following over-hanging wires were printed to connect 4 layers into one 3D circuit (FIG. 3H, I). The height and slope angle of VIAs created by CHARM3D were tunable according to requirements within 0-40 mm and 0-90°, respectively, using the experimental setup, hence increasing the degree of freedom for circuit design tremendously.

[0065] As another example, to present the unique capability of fabricating complex circuits, a 3D circuit with two layers 326, 328 of a battery-free temperature sensor systcm / dcvicc 330 was designed and printed by CHARM3D (FIG. 3 J, K). All the sensor components such as chip 332 in layer 326 were embedded in PDMS 334 but those pads c.g. 336 required for connection were exposed. Then, in-plane metal wires e.g. 338 and out-of-plane VIAs e.g. 340 of the layer 328 were printed to create coils c.g. 342 and to connect those exposed pads c.g. 336 precisely and efficiently in accordance with the circuit design (FIG. 3L). Next, after also encapsulating the printed in-plane wires, the second layer 328 of coils c.g. 350 and wires c.g. 352 (FIG. 3 J) were printed to connect all the VlAs e.g. 340 and to complete the circuit, and finally, the second layer 328 was encapsulated with PDMS. Owing to the low melting point of Field’s metal, traditional soldering was replaced with a 10-sccond heat treatment after fabrication, where printed wires re-melted to further enhance the adhesion on component pads c.g. 336 and joined with VIAs c.g. 340 to form solid connection. This printed multilayer device was powered wirelessly by a smart phone through ncar-ficld communication (NFC) (Fig. 3m). The temperature sensor could sense temperature change and determine the on / off status of each LED accordingly. As a result, temperature variation was displayed by shifting lighting LEDs from green, yellow, orange, to red while the temperature is increasing. The device 330 can, for example, be worn on a human hand 360 to detect environment temperature with great comfort (FIG. 3N). This demonstrates an important advance towards complex 3D electronics by direct printing VIAs, according to example embodiments.

[0066] As another example application, CHARM3D paves a new way to rapidly manufacture mctamatcrials. As an illustrative example, tension-driven 3D printing according to an example embodiment was employed to print an electromagnetic band gap (EBG) mctamatcrial structure 400 (FIG. 4A). This EBG metamaterial structure comprises periodic 3D unit cells e.g. 402 (FIG. 4B). FIG. 4c shows that the EBG mctamatcrial structure 400 can create a stopband within a specified frequency range, wherein wave propagation through the structure 400 is inhibited.

[0067] Figure 5 shows a flowchart 500 illustrating a method of direct ink writing according to an example embodiment. At step 502, a molten metallic material is provided in a heated printhead such that the molten metallic material wets a nozzle of the printhead. At step 504, a tip of the nozzle is disposed adjacent to a substrate such that the molten metallic material flows onto the substrate. At step 506, the tip of the nozzle is moved relative to the substrate such that a continuous line of the solidified metallic material is formed on the substrate; wherein, during the moving of the tip relative to the substrate, the molten metallic material is pulled from the nozzle under ambient pressure as a result of shear and / or tension between a leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0068] The method may comprise printing a planar 2-D structure on the substrate. The method may comprise pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0069] The method may comprise printing a 3-D structure in a layer by layer manner. The method may comprise pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle. The method may comprise melting a surface of the solidified metallic material in one layer during printing of a next layer as a result of the interaction between the molten metallic material in the next layer with the surface of the solidified metallic material in said one layer.

[0070] The method may comprise printing a free standing 3-D structure. The method may comprise pulling out the molten metallic material from the nozzle as the result of the tension between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0071] The leading edge of the solidified metallic material may be suspended above a surface of the substrate.

[0072] The method may comprise rotating the substrate relative to the tip of the nozzle for printing a curved line of the solidified metallic material. A leading edge of the curved line may be suspended above a surface of the substrate.

[0073] In one embodiment, a method of printing a circuit is provided, comprising performing the method of direct ink writing according to an example embodiment described herein.

[0074] The method may comprise embedding a first portion of the circuit and printing a second portion of the circuit on the embedded first portion, wherein the first and second portions are electrically connected.

[0075] The method may comprise interconnecting a device element in the circuit.

[0076] The method may comprise self-healing a broken portion of the line of solidified metallic material by applying a heat treatment.

[0077] The method may comprise printing vertical interconnects between different parts of the circuit.

[0078] The circuit may comprise an electromagnetic band gap, EBG, metamaterial.

[0079] In one embodiment, a direct ink writing system is provided, comprising a heatable printhead configured for providing a molten metallic material such that the molten metallic material wets a nozzle of the printhead; and a stage for a substrate; wherein the system is configured for: positioning a tip of the nozzle adjacent to the substrate such that the molten metallic material can flow onto the substrate; moving the tip of the nozzle relative to the substrate such that a continuous line of the solidified metallic material can be formed on the substrate; and pulling, during the moving of the tip relative to the substrate, the molten metallic material from the nozzle under ambient pressure as a result of shear and / or tension between a leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0080] The system may be configured for printing a planar 2-D structure on the substrate. The system may be configured for pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle. The system may be configured for printing a 3-D structure in a layer by layer manner. The system may be configured for pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0081] The system may be configured for melting a surface of the solidified metallic material in one layer during printing of a next layer as a result of the interaction between the molten metallic material in the next layer with the surface of the solidified metallic material in said one layer.

[0082] The system may be configured for printing a free standing 3-D structure. The system may be configured for pulling out the molten metallic material from the nozzle as the result of the tension between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

[0083] The system may be configured such that the leading edge of the solidified metallic material is suspended above a surface of the substrate.

[0084] The system may be configured for rotating the substrate relative to the tip of the nozzle for printing a curved line of the solidified metallic material. A leading edge of the curved line may be suspended above a surface of the substrate.

[0085] The system may be configured for printing a circuit. The system may be configured for providing an embedded first portion of the circuit and for printing a second portion of the circuit on the embedded first portion, wherein the first and second portions are electrically connected.

[0086] The system may be configured for interconnecting a device element in the circuit.

[0087] The system may be configured for self-healing a broken portion of the line of solidified metallic material by applying a heat treatment.

[0088] The system may be configured for printing vertical interconnects between different parts of the circuit.

[0089] The circuit may comprise an electromagnetic band gap, EBG, metamaterial.

[0090] Industrial applications of example embodiments

[0091] CHARM3D according to example embodiments can significantly benefit in applications such, but not limited to, as wearable electronics, 5G technologies, semiconductor industry, and internet of things (loT) applications.

[0092] Embodiments of the present invention can have one or more of the following features and associated advantages:

[0093] Aspects of the systems and methods described herein such as the control of the printing head and stage may be implemented on computing device(s), including cloud-based computing device(s) and / or Internet-of-Things computing device(s), for example as functionality programmed into any of a variety of circuitry, including programmable logic devices (PLDs), such as field programmable gate arrays (FPGAs), programmable array logic (PAL) devices, electrically programmable logic and memory devices and standard cell-based devices, as well as application specific integrated circuits (ASICs). Some other possibilities for implementing aspects of the system include: microcontrollers with memory (such as electronically erasable programmable read only memory (EEPROM)), embedded microprocessors, firmware, software, etc. Furthermore, aspects of the system may be embodied in microprocessors having software-based circuit emulation, discrete logic (sequential and combinatorial), custom devices, fuzzy (neural) logic, quantum devices, and hybrids of any of the above device types. Of course the underlying device technologies may be provided in a variety of component types, e.g., metal-oxide semiconductor field-effect transistor (MOSFET) technologies like complementary metal-oxide semiconductor (CMOS) ), fin field-effect transistor (FinFET), gate-all-around field-effect transistor GAAFET, bipolar' technologies like emitter-coupled logic (ECL), polymer technologies (e.g., silicon-conjugated polymer and metal-conjugated polymer-metal structures), mixed analog and digital, etc.

[0094] It will be appreciated by a person skilled in the art that numerous variations and / or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive. Also, the invention includes any combination of features described for different embodiments, including in the summary section, even if the feature or combination of features is not explicitly specified in the claims or the detailed description of the present embodiments.

[0095] In general, in the following claims, the terms used should not be construed to limit the systems and methods to the specific embodiments disclosed in the specification and the claims, but should be construed to include all processing systems that operate under the claims. Accordingly, the systems and methods arc not limited by the disclosure, but instead the scope of the systems and methods is to be determined entirely by the claims.

[0096] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in a sense of "including, but not limited to." Words using the singular or plural number also include the plural or singular number respectively. Additionally, the words "herein," "hereunder," "above," "below," and words of similar import refer to this application as a whole and not to any particular portions of this application. When the word "or" is used in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list and any combination of the items in the list.

[0097] References

[0098] [1] Cook A, Parekh DP, Ladd C, Kotwal G, Panich L, Durstock M, et al. Shear-Driven Direct- Write Printing of Room - Temperature Gallium - Based Liquid Metal Alloys. Advanced Engineering Materials 2019, 21(11).

[0099] [2] Ren P, Dong J. Direct Fabrication of VIA Interconnects by Elcctrohydrodynamic Printing for Multi-Layer 3D Flexible and Stretchable Electronics. Advanced Materials Technologies 2021, 6(9).

Claims

CLAIMS1. A method of direct ink writing comprising the steps of: providing a molten metallic material in a heated printhead such that the molten metallic material wets a nozzle of the printhead; disposing a tip of the nozzle adjacent to a substrate such that the molten metallic material flows onto the substrate; and moving the tip of the nozzle relative to the substrate such that a continuous line of the solidified metallic material is formed on the substrate; wherein, during the moving of the tip relative to the substrate, the molten metallic material is pulled from the nozzle under ambient pressure as a result of shear and / or tension between a leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

2. The method of claim 1, comprising printing a planar 2-D structure on the substrate.

3. The method of claim 2, comprising pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

4. The method of any one of the preceding claims, comprising printing a 3-D structure in a layer by layer manner.

5. The method of claim 4, comprising pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

6. The method of claims 4 or 5, comprising melting a surface of the solidified metallic material in one layer during printing of a next layer as a result of the interaction between the molten metallic material in the next layer with the surface of the solidified metallic material in said one layer.

7. The method of any one of the preceding claims, comprising printing a free standing 3-D structure.

8. The method of claim 7, comprising pulling out the molten metallic material from the nozzle as the result of the tension between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

9. The method of claims 7 or 8, wherein the leading edge of the solidified metallic material is suspended above a surface of the substrate.

10. The method of any one of the preceding claims, comprising rotating the substrate relative to the tip of the nozzle for printing a curved line of the solidified metallic material.

11. The method of claim 10, wherein a leading edge of the curved line is suspended above a surface of the substrate.

12. A method of printing a circuit, comprising performing the method of any one of the preceding claims.

13. The method of claim 12, comprising embedding a first portion of the circuit and printing a second portion of the circuit on the embedded first portion, wherein the first and second portions are electrically connected.

14. The method of claims 12 or 13, comprising interconnecting a device element in the circuit.

15. The method of any one of claims 12 to 14, comprising self-healing a broken portion of the line of solidified metallic material by applying a heat treatment.16.The method of any one of claims 12 to 15, comprising printing vertical interconnects between different parts of the circuit.

17. The method of any one of claims 12 to 16, wherein the circuit comprises an electromagnetic band gap, EBG, metamaterial.

18. A direct ink writing system comprising: a heatable printhead configured for providing a molten metallic material such that the molten metallic material wets a nozzle of the printhead; and a stage for a substrate; wherein the system is configured for: positioning a tip of the nozzle adjacent to the substrate such that the molten metallic material can flow onto the substrate; moving the tip of the nozzle relative to the substrate such that a continuous line of the solidified metallic material can be formed on the substrate; and pulling, during the moving of the tip relative to the substrate, the molten metallic material from the nozzle under ambient pressure as a result of shear and / or tension between a leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

19. The system of claim 18, configured for printing a planar 2-D structure on the substrate.

20. The system of claim 19. configured for pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

21. The system of any one of claims 18 to 20, configured for printing a 3-D structure in a layer by layer manner.

22. The system of claim 21, configured for pulling out the molten metallic material from the nozzle as the result of the shear between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

23. The system of claims 21 or 22, configured for melting a surface of the solidified metallic material in one layer during printing of a next layer as a result of the interaction between the molten metallic material in the next layer with the surface of the solidified metallic material in said one layer.

24. The system of any one of the claims 18 to 23, configured for printing a free standing 3-D structure.

25. The system of claim 24, configured for pulling out the molten metallic material from the nozzle as the result of the tension between the leading edge of the solidified metallic material on the substrate and the molten metallic material in the nozzle.

26. The system of claims 24 or 25, configured such that the leading edge of the solidified metallic material is suspended above a surface of the substrate.

27. The system of any one of claims 18 to 26, configured for rotating the substrate relative to the tip of the nozzle for printing a curved line of the solidified metallic material.

28. The system of claim 27, wherein a leading edge of the curved line is suspended above a surface of the substrate.

29. The system of any one of claims 18 to 28, configured for printing a circuit.

30. The system of claim 29, configured for providing an embedded first portion of the circuit and for printing a second portion of the circuit on the embedded first portion, wherein the first and second portions are electrically connected.

31. The system of claims 29 or 30, configured for interconnecting a device element in the circuit.

32. The system of any one of claims 29 to 31 , configured for self-healing a broken portion of the line of solidified metallic material by applying a heat treatment.33.The system of any one of claims 29 to 32, configured for printing vertical interconnects between different parts of the circuit.

34. The system of any one of claims 29 to 33, wherein the circuit comprises an electromagnetic band gap, EBG, metamaterial.

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