PROCESS FOR FABRICATING CONDUCTIVE PATTERNS ON 3-DIMENSIONAL SURFACES BY HYDRO-PRINTING

Abstract

Provided is a process for fabricating a conductive pattern on a three-dimensional (3D) object, involving hydroprinting a 2-dimensional (2D) conductive planar pattern on a 2D sacrificial substrate, and transferring the pattern to the 3D object.

Claims

1. A process for fabricating a conductive pattern on a three-dimensional (3D) object, the process comprising printing a 2-dimensional (2D) conductive planar pattern on a surface region of a 2D sacrificial substrate, causing said sacrificial substrate to decompose on a surface of a liquid, and contact-transferring said conductive pattern to a surface region of a 3D object such that the 2D conductive pattern aligns with features on the surface region of the 3D object.

2. The process according to claim 1, wherein the process is repeated two or more times to thereby contact-transfer a further pattern, optionally conductive, onto a surface region of the 3D object.

3. The process according to claim 1, wherein the printing of a 2D conductive planar pattern comprises printing a 2D non-conductive planar pattern and subsequently rendering it conductive.

4. The process according to claim 1, the process comprising: printing on a 2D sacrificial substrate a conductive pattern having a layout alignment of surface features to a surface region of a 3D object to be associated with said pattern; said conductive pattern comprising at least one conductive material; placing the printed sacrificial substrate onto a surface of a liquid, the liquid being selected to interact with the sacrificial substrate and cause its dissolution or decomposition, such that the conductive pattern remains intact on the surface of the liquid; and contacting said conductive pattern with the 3D object permitting the conductive pattern to three-dimensionally align and associate with its surface.

5. The process according to claim 1, the process comprising obtaining a sacrificial substrate and printing thereon a pattern, the pattern having a layout enabling alignment of surface features to a surface region of a 3D object to be associated with said pattern.

6. The process according to claim 5, wherein the pattern is a non-conductive pattern and the process further comprises sintering the non-conductive pattern under conditions permitting coalescence of the non-conductive material, rendering the pattern conductive.

7. The process according to claim 1, wherein the pattern is formed of a material selected from the group consisting of carbon nanotubes (CNT), graphene, conductive polymers and quantum dots (QDs), and wherein the process is optionally absent of a sintering step.

8. The process according to claim 1, wherein the pattern is a transparent electrode(s) formed of a material selected from the group consisting of carbon nanotubes (CNT), sintered metal nanoparticles, conductive polymers and quantum dots (QDs).

9.-11. (canceled)

12. The process according to claim 1, the process comprising: printing on a 2D sacrificial substrate a non-conductive pattern having a layout alignment of surface features to a surface region of a 3D object to be associated with said pattern; causing said non-conductive pattern to be conductive; placing the printed sacrificial substrate onto a surface of a liquid, the liquid being selected to interact with the sacrificial substrate and cause its dissolution or decomposition, such that the conductive pattern remains intact on the surface of the liquid; and contacting said conductive pattern with the 3D object permitting the conductive pattern to three-dimensionally align and associate with its surface.

13. The process according to claim 1, the process comprising: printing on a 2D sacrificial substrate a non-conductive pattern having a layout alignment of surface features to a surface region of a 3D object to be associated with said pattern; placing the printed sacrificial substrate onto a surface of a liquid, the liquid being selected to interact with the sacrificial substrate and cause its dissolution or decomposition, such that the conductive pattern remains intact on the surface of the liquid; prior to complete dissolution or decomposition, causing said non-conductive pattern to be conductive; and contacting said conductive pattern with the 3D object permitting the conductive pattern to three-dimensionally align and associate with its surface.

14. The process according to claim 1, wherein conductivity is achieved by sintering a non-conductive pattern by treating the non-conductive pattern with a sintering agent or under sintering conditions when the patterned substrate is not floating on the liquid surface, or when the patterned substrate is on the liquid surface.

15. The process according to claim 1, wherein the sacrificial substrate is of a material selected from polymers, water-soluble materials, organic liquid soluble solids and ionic materials, or wherein the substrate is optionally selected amongst heat-sensitive plastic substrates.

16.-18. (canceled)

19. The process according to claim 15, wherein the sacrificial substrate is composed of a water-soluble material, optionally polymeric.

20.-22. (canceled)

23. The process according to claim 1, comprising printing on a sacrificial substrate a non-conductive pattern having a layout alignment of surface features to a surface region of a 3D object to be associated with said pattern; the printing is performed while the substrate is optionally on a surface of a liquid; and subsequently rendering the nonconductive pattern conductive.

24. The process according to claim 23, wherein the sacrificial substrate is placed on a surface of a liquid and the pattern is thereafter formed.

25. The process according to claim 1, wherein the pattern is formed by nonimpact printing.

26. (canceled)

27. The process according to claim 25, wherein printing comprises jetting an ink formulation comprising at least one metal nanoparticle or at least one conductive material.

28.-30. (canceled)

31. The process according to claim 1, wherein the conductive pattern is formed by patterning the sacrificial surface with a non-conductive metallic pattern and rendering the metallic pattern continuous and electrically conductive.

32. The process according to claim 31, wherein conductivity is rendered by sintering the metallic pattern by exposing the pattern to a sintering agent or to sintering conditions.

33.-45. (canceled)

46. A 3D object formed according to the process of claim 1.

47.-48. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0087] FIGS. 1A-E schematically demonstrates electronic printing using the hydro-printing method. In an exemplary embodiment of the invention: (A) PVA film with conductive pattern on water (top view), (B) After PVA elimination, the conductive pattern floats on the water (top view), (C) 3D object is immersed into the water (side view), (D) Conductive pattern is fully adhered to the 3D object (side view), (E) hydroprinted object after washing from PVA residues.

[0088] FIGS. 2A-D schematically depicts a water level lowering method, in which (A) Printed pattern on PVA is fixed on the water surface, (B) After PVA dissolution, the water is pump out of the tank, (C) Water level is lowered until the Ag pattern makes contact with the object. The pattern mimics the object geometry (D).

[0089] FIGS. 3A-G provide images of conductive silver lines transferred onto: (A) Different-sized domes (B) 90° angled cubes (C) Spheres made of (left to right): epoxy, acrylate (half matte and half glossy), gypsum and glass (not printed), (D) Electric circuit transferred onto a wave like object (E) Electric circuit shaped as HUH transferred onto a dome structure (F) Heater device transferred onto a glass sphere (G) Temperature of 87.4° C. was achieved by providing 50 volt to the spiral pattern. For samples with several un-connected lines, additional coating with PVB may be required.

[0090] FIGS. 4A-B show three lines, conductive on both sides, hydroprinted separately in three different steps. The silver lines overlap at the square shaped edges to ensure conductivity from end to end.

[0091] FIGS. 5A-D depict: (A) NFC tag hydroprinted onto a dome. To prevent short-circuit, a few drops of isolating polymer solution were casted on the internal loops leaving the coil's terminals uncovered, (B) Coil's terminals connected by secondary hydroprinting of a conductive bridge, (C) Illustration of bridge hydroprinting (D) A 250 μm line width forming a 12 coils NFC 13.56 MHz antenna hydroprinted onto a dome, connected to a commercial 144 bytes Ntag203 chip using silver paste.

[0092] FIGS. 6A-D present: (A) The correlation between line resistance and number of printed layers at various printing resolution (400, 500, and 600 DPI, identified by a square, rhombus, and triangle, respectively). Each resistance value was measured for three hydroprinted samples at a constant length of 1 cm. (B) Electrical resistance in hydroprinted samples while the process is conducted at various concentrations of NaCl. Each measurement was conducted for 3 samples at a constant length of 1 cm. (C) Ag NPs before sintering, (D) Ag NPs sintered in 1.5 wt % NaCl solution; necking between the particles and showing possible presence of NaCl particles.

[0093] FIG. 7 depicts Ag NPs thickness analysis by FIB. The sample (five layers at 600 DPI) was coated with 2 layers of platinum. It was found that the sample average height was 864 nm (±12.8).

[0094] FIGS. 8A-B show (A) Silver nano wires conductive line hydroprinted onto a finger of nitrile glove, (B) CNT conductive line hydroprinted onto ABS dome.

DETAILED DESCRIPTION OF EMBODIMENTS

[0095] A conductive pattern according to the invention is printed on a flat substrate by conventional printers. Typically while fabricating 2D conductive patterns composed of metal nanoparticles (NPs), a sintering process is required, typically performed at high temperatures. However, with heat sensitive substrates, such as PVA exemplified by the present approach, elevated temperatures may cause deformation of the substrate, and only low-temperature processes should be performed. Such may include photonic, plasma, laser and chemical sintering.

[0096] For example, silver NP-based inks undergo sintering at room temperature by exposure to HCl vapor, yielding continuous conductive patterns. When the silver ink is exposed to negatively charged chloride ions, the latter replace the physically bonded stabilizer on the surface of the silver NPs, thus allowing them to form necks that later overlap and sinter. This unique property enables inkjet printing of functional conductive patterns on heat-sensitive plastic substrates, such as polyvinyl alcohol (PVA). For the first step of the process, a conductive pattern was printed on a PVA substrate. PVA is water soluble. Subsequent to printing, a chemical sintering process was performed at room temperature. An exemplary process scheme is presented in FIG. 1. In the next step, the PVA film with the printed pattern facing upwards is placed at the air-water interface of a hot (50° C.) deionized water bath (FIG. 1A), leading to dissolution of the PVA film, while the printed conductor remains intact. The film dissolution time can vary between a few seconds to many minutes, and depends on various parameters such as film thickness, type and concentration of plasticizers, polymer molecular weight, and immersion bath temperature. The film properties were optimized to enable dissolution within less than 2 minutes, for films with an average thickness of 35 μm.

[0097] As schematically shown in FIG. 1B, following placement of the film on the water surface and its dissolution, the object is immersed into the water while passing through the pattern which lies on the water surface. It was observed that once an initial contact is made between the printed pattern and the object, the entire pattern sticks to the surface of the object, following its topography, no matter how complex it was. After the whole printed pattern is placed on the object, in a process which takes only a few seconds, the object is removed from the water and left to dry. In the final step, all PVA residues are washed off.

[0098] An alternative sequence is depicted in FIGS. 2A-D.

[0099] To show the versatility of the method, objects were fabricated composed of various materials, by 3D-printing (by plastics and ceramic printers, excluding the glass sphere), some with 90-degree angles and some with round shapes. FIGS. 3A-C show variously sized and shaped objects: domes, cube-shaped steps and spheres. Silver conductive lines were hydroprinted on all these shapes, yielding continues lines which were conductive in their entirely, without damaging the original dimensions of the printed patterns. Overall, it was found that the process was suitable for object structures made of epoxy, acrylate (both smooth and rough), gypsum and glass (FIG. 3C). A remarkable finding was the printing over the 90-degree angles, which is impossible to achieve with simple direct printers and so rapidly. The hydro-printing onto the 90°-angle steps which is shown in FIG. 3B was performed by lowering the water with the floating PVA film rather than by immersing the object into the water as in all the other demonstrations. The resolution of the hydroprinting is mainly defined by the resolution of the printing process. A significant change was not noticed in the dimensions of the printed lines, after the hydroprinting process, probably since the lines were sintered. Having said that, it could be that with very complex topologies some mis-alignment may occur. The resolution of hydroprinted patterns should be defined mainly by the printing technology of the conductive pattern on the film. With the 10 picolitter Dimatix printhead a line width of 133 μm was obtained. It should be noted that for these printed samples, some deterioration in conductivity may occur, for example printed lines (167±10 μm) having an average resistance of 0.66±0.26Ω, at first stage and 1.32±0.16Ω after hydroprinting. This range of line widths is relevant to a variety of applications including near field and Bluetooth antennas.

[0100] The possibility to hydroprint an electrical circuit, which consist of several free-standing lines, was investigated as well, by a single step as shown in FIGS. 3D-E. In FIGS. 3D-E, the hydroprinted electrical circuits were provided on curved surfaces, which were assembled with an LED and a resistor. Further performed was hydro-printing of an electrical heater onto a glass sphere (FIGS. 3F-G), which shows that hydroprinted patterns can withstand high temperatures.

[0101] It was found that the hydro-printing method is also suitable for fabrication of multilayer circuits, simply by repeating the process as many layers as required, as shown in FIG. 4. This result emphasizes the novelty of using hydroprinting method, which enables fabricating of overlapping circuits, since there is no insulating layer. The resulting circuit is constructed from three separate conductive lines (12.4, 41.6 and 6.0Ω respectively) and resulted in an overlapping circuit having a resistance from edge-to-edge. This result shows that there are no insulating PVA residues, which remain after the hydroprinting.

[0102] In order to further show the applicability of multilayer hydroprinting, a near field communication (NFC) antenna was fabricated which was hydroprinted onto a dome structure (FIG. 5). This antenna was hydroprinted in two stages as shown in FIG. 5C. At first, the circular coil was hydroprinted on the dome, followed by hydroprinting of a conductive bridge, which connected the two coils edges. In order to prevent short-circuit of the coil due to the bridge connector, a PVB insulator was placed onto the inner coil circles prior to second hydroprinting. Due to the complete dissolution of the sacrificial layer during the hydroprinting process, the conductive bridge-line was free from any isolating layer, thus, enabling the connectivity of the two terminals, which were located on an uneven topography (FIGS. 5A, B and D). The hydroprinting of the NFC antenna opens the door for fabrication of sensors and electronic devices directly onto 3D structures, that is essential for communication between objects, in view of the emerging field of Internet of Things (IoT). Based on the measured resistance of the antenna in FIG. 5D, the induction was calculated to be 2.34 μH.

[0103] It was observed that during dissolution of the PVA film, cracks in the printed patterns may occur, causing an increase of the electrical resistance. The cracking can be prevented by modifying either the parameters (mainly the dot-per-inch, DPI) or by increasing the amount of silver NP through printing more layers on the PVA film. FIG. 6A shows the resistance change in hydroprinted patterns as a function of number of layers printed on the PVA and the DPI. As shown, the resistance decreases with the number of layers while the effect is most pronounced for the first layers printed with 600 DPI mode. The resistivity could be expected to be linearly proportional to the number of printed layers, however the results indicate that there are other parameters than the amount of silver, such as dissolution and removal of nonconductive dispersants present in the inks. It should be noted that printing of multiple layers can also improve maintaining the integrity of the printed pattern during the PVA dissolution.

[0104] In order to evaluate the resistivity of the hydroprinted lines, the height was measured from a cross section of the lines (FIG. 7), by using a focused ion beam (FIB). It was found that the average height was 864 nm (±12.8), and the calculated resistivity of 27.16 μΩcm. This is times the bulk resistivity of Ag, 1.59 μΩcm, and considered suitable for many applications. Further improvement in resistivity can be probably obtained by additional post printing processes that are suitable for 3D structures such as plasma treatment

[0105] It should be noted that the hydro-printing process has to be performed with 2D printed patterns that are sintered prior to the PVA dissolution step, otherwise the silver NPs in the pattern start to re-disperse within the aqueous bath. To overcome this, the sintering and dissolution of the PVA may be performed simultaneously, by immersing the object in an NaCl solution instead of just water (the NaCl causes the chemical sintering). In order to evaluate this possibility, the hydro-printing process was performed in aqueous solutions of NaCl at various concentrations (FIG. 6B).

[0106] As shown, conductive lines were obtained already with 0.05 wt % NaCl solution. The resistance was further decreased when dipping in a solution of 1 wt % NaCl, but above that concentration the resistance started to slightly increase. This small increase in resistance could be attributed to the presence of NaCl particles on top of the surface of the metallic pattern and in between the sintered particles. The presence of salt particles was confirmed by energy dispersive x-ray spectroscopy (EDX) and can be clearly seen in FIG. 6D.

[0107] Overall, the results show that it is indeed possible to combine sintering and immersion processes. However, it seems that as soon as the printed PVA film is immersed in the solution, some of the silver NPs, which are not sintered yet by the chloride ions in the solution, begin to re-disperse in the immersion bath. This causes the pattern to start to break up, making the process less environmentally friendly. In this context, it is worth noting that a quantitative adhesion test was performed according to ISO 2409 standard tape test with 6 parallel cross cuts. A square of two by two cm sintered pattern was hydroprinted onto Vero-matte substrate. The resulting adhesion was classified as class 2, which is considered good adhesion. It is further worth noting that the versatility of the process was demonstrated by successful hydroprinting of other materials, namely CNTs and silver nanowires onto plastic objects (as shown in FIG. 8).

Experimental Section

[0108] Water-soluble PVA film preparation: 13,000-23,000 MW PVA (Sigma-Aldrich), glycerol (Sigma-Aldrich), and a wetting agent BYK 348 (BYK Chemie) were mixed at a ratio of 15:3:0.1 respectively in deionized water at 85° C. for 2 hours, until the solution was homogeneous. A thin wet film of-120 jxm was formed by draw-down coating (RK Print-Coat Instruments, 120 μm bar) of 10 ml solution on a 125-jxm-thick polyethylenephtalate (PET) (Jolybar ltd., Israel) substrate. After overnight drying at ambient humidity and temperature, a PVA film of 35-μm average thickness was obtained, capable of dissolving in water within 2 minutes at 50° C.

[0109] Inkjet printing, sintering and sample preparation: 2D pattern printing was performed with a Dimatix inkjet printer, with a 10 pL print head (Dimatix, Fuji-Film). The ink used was Ag NP conductive ink that contained 20 wt % silver NPs (Xjet ltd., Israel). The substrate temperature was 60° C. and the substrate-printhead gap, was 1.2 mm. After printing, sintering was obtained by exposing the printed pattern to 37% hydrochloric acid vapors (Sigma-Aldrich) for 20 seconds. When the printed pattern comprises more than one free-standing line, after the 2D printing, a very thin transparent film was formed by spraying a 10 wt % PVB (Piolorfom BL 18)-ethanol solution over the pattern. This aided in keeping the gap between the lines after PVA elimination. Other polymers can be used as well, including such that can be removed after the hydroprinting.

[0110] Hydroprinting: Two methods were used for hydroprinting of conductive patterns: the object was placed above the floating pattern and immersed in it from above (FIG. 1), or the object was placed at the bottom of the water bath and the water level was lowered until the bottom side of the pattern makes contact with the object's top surface.

[0111] Using the method shown in FIG. 1, the PVA's hydrophilic polymer was placed on the water interface, and after about 2 minutes the object was carefully immersed through the printed pattern which was floating on top of the water bath. The pattern precisely adheres to the angles and shape of the object. Next, the object was pulled out of the water bath and left to fully dry in a 60° C. oven. Finally, the object was re-immersed for two minutes in a water bath to remove all PVA residues. During the immersion step, the object was oriented such that air bubbles could not become trapped between the printed pattern and the object's surface. A typical PVA film required approximately 2 minutes to fully dissolve. When hydroprinting a single line, an object could remain immersed until the PVA was fully dissolved and dried; no washing step was necessary. For hydroprinting complex patterns with multiple lines, after the object was placed at the bottom of the water bath, the PVA film was carefully placed on the water-air interface. The water was then slowly drained out of the system with a vacuum pump, until the water level with the floating PVA film makes contact with the object, and the film gently wraps itself around the object.

[0112] Demonstration of sequential overlapping transfer: Scheme of the process shown in FIG. 4B. The first line was hydroprinted and dried at 120° C. for 35 minutes, and the process is repeated twice more such that one edge of the pattern overlapped with the previous hydroprinted line (FIG. 4A).

[0113] Conductivity measurement: For measuring the effect of line conductivity on the number of layers (FIG. 6A), line patterns measuring forty by one mm were sintered and hydroprinted.

[0114] Three samples with the same number of layers were measured for resistance, and the process was repeated for lines printed with 3, 5, 7, and 9 layers. All measurements were repeated three times with resolutions of 400, 500, and 600 DPI. After the substrate was fully dried, a conductive silver paste was placed on the line, and the resistance was measured for 1 cm line, by a two-point milliohm meter (Lutron Electronic M0-2002).

[0115] NaCl sintering: Sodium chloride (Sigma Aldrich) water solution was used as the immersion liquid, at 50° C. Initial sintering was performed by immersing the printed pattern in the solution, followed by a 2.5-minute wash in the same bath after drying. This process was performed with hydroprinted patterns in NaCl at concentrations of 0.05, 0.1, 0.5, 1, 1.5, and 2 wt %. The resistance measurement was performed as described previously.

[0116] 3D printing of objects: The objects used for the hydro-printing demonstration were printed with three different printers. The white dome and the wave structure (FIGS. 3D and 3E) were printed by an FDM Makerbot printer loaded with 1.75 mm of ABS filament. The spheres were printed by an Objet 30 printer loaded with Vero blue ink. The squares and other shapes in FIG. 3 were printed using the same printer, loaded with Vero white ink, all with a glossy finish. The white spheres were printed using a powder binder printer (Projet 160,3D system USA). In order to show the adhesion to two types of surfaces, one sphere was fully glued using epoxy resin and the other sphere was left with the powder-like surface.

[0117] Resistivity measurement: In order to estimate resistivity, it was necessary to measure the hydroprinted pattern thickness. An FIB was used to cut a cross section (FIG. 7). The sample was coated with two layers of platinum in order to protect the Ag pattern surface from amorphization by the intense ion beam. A low energetic electron beam was used to fabricate the first titanium layer −300 nm thick. Next, a second micron thick titanium layer was fabricated using a high energy ion beam. Last, an intense penetrate ion beam was used to cut a cross section through the Ag pattern. Height was measured with the FIB camera, which was tilted at a 53° angle. Cross-section height measurements were repeated in three randomly selected places.

[0118] Resistivity was calculated by p=Rsh, where p is resistivity, Rs is sheet resistance, and h is height.

[0119] Adhesion test: Adhesion rating based on ISO 2409 standard tape test was done according to the peeled fraction from the substrate, and classified by zero to five scale. Zero value indicates excellent adhesion (no detachments) and 5 value indicates a poor adhesion (more than 65% detachments).