Fabrication of high-resolution graphene-based flexible electronics via polymer casting

Abstract

An economical, efficient, and effective formation of a high resolution pattern of conductive material on a variety of films by polymer casting. This allows, for example, quite small-scale patterns with sufficient resolution for such things as effective microelectronics without complex systems or steps and with substantial control over the characteristics of the film. A final end product that includes that high resolution functional pattern on any of a variety of substrates, including flexible, stretchable, porous, biodegradable, and/or biocompatible. This allows, for example, highly beneficial options in design of high resolution conductive patterns for a wide variety of applications.

Claims

1. A method of transfer with high transfer efficiency of a high resolution patterned conductive material formed on a first substrate to a target polymeric film having a target polymeric film surface-comprising: (a) selecting the first substrate based on predetermined material characteristics; (b) selecting the conductive material based on predetermined material characteristics, including an estimated first work of adhesion at a first interface between the first substrate and the conductive material; (c) selecting the target polymeric film based on predetermined material characteristics including an estimated second work of adhesion at a second interface between between the conductive material and the target polymeric film, wherein the estimated second work of adhesion is greater than the estimated first work of adhesion; (d) preparing a polymeric solution that turns into the target polymeric film when dried in ambient conditions; (e) forming the conductive material into the high resolution patterned conductive material on the first substrate; (f) directly covering the target polymeric solution over the first substrate and the high resolution patterned conductive material to cover and contact the patterned conductive material; (g) forming in situ the directly covering target polymeric solution into the target polymeric film by drying, including in ambient conditions; and (h) after target polymeric film formation, removing for use the target polymeric film from the first substrate with the conductive material transferred at high transfer efficiency from on the first substrate because of difference between the first and second works of adhesion, without the need for post-processing steps involving high temperatures.

2. The method of claim 1 wherein the material characteristics of the first substrate include one or rigid or flexible.

3. The method of claim 1 wherein the first substrate comprises: (a) Teflon; (b) Delrin; or (c) silicon.

4. The method of claim 1 wherein the conductive material comprises graphene or graphene-based or graphene derivative conductive material.

5. The method of claim 4 wherein the conductive material is pre-annealed before casting of the target polymeric solution.

6. The method of claim 1 wherein the conductive material is: (a) patterned on the first substrate in high resolution with at least one feature dimension as small as on the order of approximately 5 μm width and depth; (b) deposited on the first substrate; (c) printed on the first substrate; or (d) grown on the first substrate.

7. The method of claim 1 wherein the material characteristics of the target polymeric film after film formation include pre-selected: (a) elasticity; (b) porosity; (c) pore size; (d) mechanical properties; and (e) microstructural properties.

8. The method of claim 1 wherein the target polymeric film after film formation and the conductive material are selected to result in different surface energies at the second interface effective to deter mechanical deformation or destruction of the conductive material during transfer and/or the transferred pattern in use.

9. The method of claim 1 wherein the target polymeric film comprises: (a) a natural or synthetic polymer; or (b) a biodegradable or non-biodegradable polymer.

10. The method of claim 1 wherein the target polymeric film comprises: (a) poly-L-lactic acid (PLLA); (b) cellulose acetate (CA); (c) gelatin (GEL); (d) poly lactic-co-glycolic acid (PLGA); or (e) whey protein isolate (WPI).

11. A system for fabrication of high resolution conductive patterns on polymeric substrates comprising: a subsystem for forming a high resolution pattern with 3D features on a starting substrate; a subsystem for forming an end product comprising a combination of a high-resolution conductive pattern based on the high resolution pattern on a polymeric substrate by the method of claim.

12. A product made by the process of claim 1.

13. A product made by the system of claim 11.

14. The method of claim 1 wherein the material characteristics of the first substrate include hydrophobic.

15. The method of claim 1 wherein polymers of the target polymeric solution dissolve in polar or non-polar solvents so that the removing does not depend on polarity or viscosity of the target polymeric solution.

Description

III. BRIEF DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. Frequent reference will be taken to the appended drawings, which are summarized as follows:

(2) FIGS. 1a-c are graphene circuits fabricated via polymer casting according to one exemplary embodiment of the invention. FIGS. 1(a-c) are highly magnified images showing the steps of graphene transfer from rigid Delrin mold via Poly-L-Lactic Acid (PLLA) casting. FIG. 1(a): Casting of PLLA solution on graphene patterns formed on Delrin surface via microfluidic filling. FIG. 1 (b): Peeling off the PLLA film from the rigid Delrin surface and transfer of graphene upon drying of polymer solution and film formation. FIG. 1 (c): PLLA film with graphene patterns on the surface and Delrin substrate after the process. FIGS. 1(d-f): Images showing the steps of ink-jet printed graphene transfer from flexible polyimide substrate via Cellulose Acetate (CA) casting. FIG. 1(d): Casting of CA solution on graphene patterns printed on flexible polyimide substrate. FIG. 1 (e): Peeling off the CA film from the polyimide substrate and transfer of graphene upon drying of polymer solution and film formation. FIG. 1 (0: CA film with graphene patterns on the surface and polyimide substrate after the process. Graphene pattern of FIG. 1 (g) 400 μm and 200 μm width, 150 μm depth on Gelatin (GEL) film; FIG. 1 (h) 300 μm width, 150 μm depth on Poly Lactic-co-Glycolic Acid (PLGA) film; FIG. 1 (i) 400 μm width, 1.50 μm depth on Whey Protein Isolate (WPI) film; FIG. 1 (j) 15 μm width, 50 μm depth on PLLA film (image take by light microscope).

(3) FIGS. 2a-c are highly magnified images of CVD grown graphene film transferred to a PLLA surface from FIG. 2(a) copper foil substrate and FIG. 2(b) quartz substrate according to an exemplary embodiment of the invention. FIG. 2(c) is CVD grown graphene-silver nanowire film transferred to PLLA surface from quartz substrate according to an exemplary embodiment of the invention.

(4) FIGS. 3a-d are illustrations, according to exemplary embodiments of the invention. FIG. 3(a): XPS analysis of PLLA films with graphene surface patterns pre-annealed at 25, 50 and 75° C. and their corresponding structures as SEM images. TEM images of GNPs FIG. 3(b) non-annealed and FIG. 3(c) pre-annealed at 75° C. FIG. 3(d): The change in sheet resistance with respect to pre-annealing temperature. According to the ANOVA analysis by Tukey's method with a 95% confidence interval the p value was found to be smaller than 0.05 showing significant difference between the groups (p<0.05). (n=3, and error bar represents one standard deviation from the average).

(5) FIGS. 4a-d are SEM images of PLLA films and graphene micropatterns and an illustrative graph according to aspects of the present invention. FIG. 4(a): Cross section image of 3D porous microstructure of PLLA film. FIG. 4(b) Graphene micropatterns (500 μm width and 200 μm depth) on PLLA film surface. FIG. 4(c) Graphene micropatterns (15 μm width and 5 μm depth) on PLLA film surface. FIG. 4(d) Raman spectra of the transferred graphene on PLLA films.

(6) FIGS. 5a-e are images and an illustrative graph of stability and conductivity of the graphene patterns according to exemplary embodiments of the present invention. Graphene patterns are flexible and bendable and keep their mechanical stability after FIG. 5(a) multiple bending (100 times), FIG. 5(b) washing cycles (24 h incubation) and FIG. 5(c) mechanical peeling applied via adhesive tape. FIG. 5(d): Graphene patterns have good conductivity and low resistance to build an active circuit. FIG. 5(e): Sheet resistance did not change after multiple bending and washing cycles. According to the ANOVA analysis by Tukey's method with a 95% confidence interval the p value was found to be higher than 0.05 showing insignificant difference (p>0.05). (n=3, and error bar represents one standard deviation from the average).

(7) FIGS. 6a-e a r e SEM images of graphene patterns transferred on WPI films according to exemplary embodiments of the invention; FIG. 6(a) before (5 cm initial length) and FIG. 6(b) after 5 cycles of stretching test (3 times of 2 cm stretching and retraction was applied). FIG. 6(c): Change of the resistance after cyclic stretching test. FIG. 6(d): SEM images of graphene patterns transferred on WPI films after 10 cm of (the final stretched length) stretching. Stretching distance is 5 cm. FIG. 6(e): Change in the resistance as the stretching distance increases to the final distance of 10 cm. For FIGS. 6(c,e) ANOVA analysis by Tukey's method with a 95% confidence interval was conducted and the p value was found to be smaller than 0.05 for indicated letter “a” showing significant difference. (n=3, and error bar represents one standard deviation from the average).

(8) FIGS. 7a-e are highly magnified images of demonstration of using fabricated devices for controlling the transdifferentiation of MSCs into SCs using electrical stimuli according to aspects of the invention. FIG. 7(a): MSCs attaching and growing on the PLLA film and graphene patterns. FIGS. 7(b,c): Directed growth of MSCs along the graphene patterns. FIG. 7(d): Graphene and PLLA-based device fabricated using microfluidic approach. FIG. 7(e): Immunocytochemistry staining of transdifferentiated M:SCs with selected SCs markers.

(9) FIGS. 8a-e are diagrammatic illustrations of graphene transfer via polymer casting according to exemplary embodiments of the present invention. FIGS. 8(a-e): Schematic representation of graphene transfer via polymer casting. FIG. 8(a): Substrates with graphene micropatterns. FIG. 8(b): Casting of polymeric film formulation on substrates with graphene-based micro-circuit patterns of various feature sizes. FIG. 8(c): Drying of casting solution and film formation. FIG. 8(d): Peeling off the polymeric films and transfer of graphene-based micropatterns from the rigid or flexible substrate to the film surface. FIG. 8(e): Polymeric film with graphene micropatterns on the surface.

(10) FIGS. 9a-9d are images that illustrate we are able to transfer any negative or positive (graphene) pattern from a rigid mold to the polymer substrate according to aspects of the present invention.

(11) FIGS. 10a-i are images that further illustrate an exemplary method according to aspects of the present invention.

IV. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

(12) A. Overview

(13) For a better understanding of the invention, several specific examples of forms or embodiments it can take will now be set forth in detail. It is to be understood that these examples are neither inclusive nor exclusive of all forms and embodiment. For example, variations obvious to those skilled in the art will be included within the invention.

(14) Additionally, some of the techniques for individual steps of the examples of the invention are based on techniques known to those skilled in the art. Additional information about them can be found in some of the references listed in the Background of the Invention supra.

(15) Also, some of the method steps according to the invention can be practiced with reference to steps detailed in co-pending U.S. Provisional Ser. No. 62/720,693 and its non-provisional U.S. Ser. No. 16/547,239 both to inventors Uz and Mallapragada, and both of which are incorporated by reference herein in their entireties.

B. Generalized Embodiment

(16) The methods, systems, and end products according to aspects of the invention represent a substantive departure from the state-of-the-art by developing polymeric flexible electronics with precise control on 3D microstructural and mechanical properties (such as film porosity, pore size, elasticity etc.) and with high resolutions of graphene patterns (feature dimensions of ˜10 μm width/depth). The aspects of the invention can also be broadly applied to various natural or synthetic biodegradable materials with well-defined characteristics including but not limited to gelatin, collagen, chitosan, alginate, whey protein isolate, PLLA, PLGA, PVA etc.

(17) Conventional state-of-the-art flexible electronic films are mostly non-porous, non-biodegradable and inherently planar (2D). Thus, the 3D porous microstructure, mechanical properties, flexibility and biodegradation rate of the films according to the present invention are controlled by using established phase inversion techniques, pore forming agents, plasticizers or cross-linking strategies. Development of such electrically conductive and flexible films via the methods of the invention eliminates the need for any type of expensive equipment, photolithography patterning, post-processing, transferring or stamping process and conventionally used PDMS molding or Cu foil-based transfer. This is a significant advancement over other flexible electrodes, including the metals, that require expensive pre- or post-processing and are inherently planar. The processes according to the invention also enable circuit design on biodegradable polymeric films which is not possible with chemically degrading, lithographic patterning techniques. Therefore, these environmentally-friendly (green) production methods reduce cost, energy and time spent as well as eliminate the problem of mounting electronic waste.

(18) Advantages include at least one or more of: 1) Green production methods reducing cost, energy and time spent as well as eliminating problem of mounting electronic waste. 2) Applicable to various synthetic or natural biodegradable polymers. 3) Allows precise control on film microstructure. 4) Easy control on micropattern design and dimensions. 5) No post-processing, PDMS mold or Cu foil-based transfer, etching or other chemical, temperature or laser-based methods are needed. 6) Provides available microenvironment for such things as material-cell interfaces. 7) Available for surgical implantation. 8) Potentially can be used as microfluidic device for various applications.

(19) This technology enables the development of biocompatible, biodegradable, flexible films with electrically-conductive graphene circuits that can be used in various applications. On a broader scale, this technology has a potential to pave the way for implantable heart sensors/stimulators, implantable brain-computer interfaces, robotics, or for low-cost wearable biosensors, integrated circuits, batteries, displays, thin film transistors and so on. Overall, this technology will open a new route for low-cost, scalable, and green production of graphene-based flexible electronics or sensors on different biodegradable polymeric substrates.

(20) As will be appreciated, both techniques provide at least the following features: (1) An economical, efficient, and effective formation of a high resolution pattern of conductive material. This allows, for example, quite small-scale patterns (down to 1 μm) with sufficient resolution for such things as effective microelectronics. (2) A final end product that includes that high resolution functional pattern on any of a variety of substrates, including flexible, biodegradable, and/or biocompatible. This allows, for example, highly beneficial options in design of high resolution conductive patterns for a wide variety of applications.

(21) Some individual steps can be practiced with reference to disclosures in co-pending U.S. Provisional Ser. No. 62/720,693 and its non-provisional U.S. Ser. No. 16/547,239 both to inventors Uz and Mallapragada, and both of which are incorporated by reference herein in the r entireties.

C. Specific Embodiment(s) and Examples

1. Specific Example 1

(22) The following specific example of practicing aspects according to the invention is taken from Metin U z, Kyle Jackson, Maxsam S. Donta, Juhyung Jung, Matthew T. Lentner, John A. Hondred, Jonathan C. Claussen, and Surya K. Mallapragada Scientific Reports (2019) 9:10595 https://doi:org/10.1038/s41598-019-46978-z, entitled “Fabrication of High-resolution Graphene-based Flexible electronics via Polymer Casting” and which is incorporated by reference herein in its entirety.

(23) As will be appreciated by those skilled in this technical art, these examples show proof of concept with the specific examples. Variations obvious to those skilled in the art are included in the invention which is defined by the appended claims and not necessarily limited to the specific examples set forth herein.

(24) In these examples, a methods based on the transfer of graphene patterns from a rigid or flexible substrate onto a polymeric film surface via solvent casting was developed. The method involves the creation of predetermined graphene patterns on the substrate, casting a polymer solution, and directly transferring the graphene patterns from the substrate to the surface of the target polymer film via a peeling-off method. The feature sizes of the graphene patterns on the final film can vary from a few micrometers (as low as 5 μm) to few millimeters range. this process, applied at room temperature, eliminates the need for harsh post-processing techniques and enables creation of conductive graphene circuits (sheet resistance: ˜0.2 kΩ/sq) with high stability (stable after 100 bending and 24 h washing cycles) on various polymeric flexible substrates. Moreover, this approach allows precise control of the substrate properties such as composition, biodegradability, 3D microstructure, pore size, porosity and mechanical properties using different film formation techniques. This approach can also be used to fabricate flexible biointerfaces to control stem cell behavior, such as differentiation and alignment. Overall, this promising approach provides a facile and low-cost method for the fabrication of flexible and stretchable electronic circuits.

(25) With reference to appended FIGS. 1-8, details regarding non-limiting ways to make, use, and practice aspects of the present invention are set forth, including proof of concept data. Reference numbers will be used in the drawings to point to certain parts or locations of the drawings. In general, the reference numbers refer to the following:

(26) TABLE-US-00001 10 collectively the combination of substrate 12, pattern 14 on substrate 12 (if used), conductive material 16, and polymeric film 22 before transfer of conductive material 17 first substrate (rigid or flexible) 14 if used, 3D pattern in substrate 12 15 any remnant conductive material on substrate 12 after transfer to film 22 16 conductive material (whether patterned, deposited, or grown on substrate 12) 17 if used, individual nanoparticles in conductive material (e.g. nanotubes) 20 collectively the end product of polymeric film 22 with transferred conductive material 16. 22 Polymeric film (after forming in place on substrate 12 from solution) 23 Polymeric solution 30 if applicable, electrical or electronic load that can be serviced by end product 20
Results and Discussion

(27) The developed method is focused on direct transfer of graphene-based patterns from rigid or flexible substrates to the polymeric flexible films via polymer casting. The method consists of three main steps; (i) Preparation of graphene-based patterns/film 16 via channel filling, ink-jet printing or CVD on rigid or flexible substrates/molds 12; (ii) Casting of the target substrate polymer solution 23 on the graphene-based patterns/films 16 formed on substrates/molds 12; (iii) Drying of the solvent in solution 23 and formation of films 22 followed by peeling off the films 22 from the substrate mold 12, transferring the graphene pattern 16 from substrate/mold 12 surface to the target polymeric film 22 surface. The application steps of the graphene transfer via direct polymer casting on rigid Delrin and ink-jet printed flexible polyimide substrate 12 was shown in FIG. 1a-f, respectively. As seen in the figures, a complete transfer of graphene (negligible amount of residue) 16 was obtained for both cases. In addition, this process works for different polymers and their respective solvents (FIG. 1a-f).

(28) The remaining amount after transfer was negligible in terms of efficacy of the transfer for at least most purposes. As will be appreciated by those skilled in this technical field, the remaining amount can change depending on the type of the mold or film material since the adhesive forces between them affects the transfer. For instance, a transfer from Teflon to PLLA resulted in a complete (100%) transfer, while some residues in graphene pattern transferred from an inkjet printed substrate to CA but they are small or negligible in terms of efficacy of the transfer.

(29) FIG. 8 provides a diagrammatic view of steps of one example of a method of fabrication according to the invention. A starting substrate 12 can include a pattern 14 with 3D features, including at resolutions at least down to the scale of approximately 5 μm in width and depth by techniques known to those of skill in this technical art. A conductive material 16 (e.g. graphene or graphene derivative) can be applied in high resolution to pattern 14 by any number of techniques. Thus, a high resolution patterned conductive material on substrate (12/14/16) is formed. A target polymeric solution 23 is pre-engineered and pre-selected to have desired characteristics or properties conducive to a transfer of the patterned conductive material 16 on substrate 12 to a target polymeric film 22 that will be formed in situ on the patterned substrate/conductive material combination 12/14/16. Target solution 23 is cast over combination 12/14/16. This forms combination 10, namely, the collective combination of substrate 12 with pattern 14, conductive material 16 following pattern 14, and cast target polymeric solution 23. Solution 23 forms in situ into a film 22 with adhesion to patterned conductive material 16. Removal (e.g. peel off) of film 22 transfers patterned conductive material 16 from substrate 12 to film 22 retaining resolution of pattern 14 to form end product 20, namely film 22 with transferred conductive patterned material 16.

(30) This method is also versatile and can be applied to many different polymeric materials including but not limited to PLLA, PLGA, CA, GEL and WPI films (FIG. 1g-j). It was noted that almost 100% of the graphene patterns present on the substrates were successfully transferred to the polymeric film surface (FIG. 1). This high transfer efficacy could be mostly attributed to the surface properties, particularly the hydrophobicity of the substrate material 12. The hydrophobic substrates 12, such as Delrin, Teflon or polyimide, make it easy to remove the formed films 22 along with the graphene pattern transfer 16. It was observed that this process does not depend on the polarity or viscosity of the polymer solution 23 since the natural or synthetic polymers dissolved in polar or non-polar solvents. For instance, 10% PLLA dissolves in chloroform, which is a non-polar solvent, while 5% GEL or WPI dissolve in water, which is a polar solvent. For both cases we observed 100% of graphene transfer. Practically, the polymer casting-based graphene patterning and transfer technique utilizes adhesion forces of two contacting materials at the interface and their respective surface energies.sup.22. The difference in the surface energies between the polymer solution 23 and graphene patterns 16 upon the formation of polymeric films 22 makes it possible to remove the graphene pattern 16 precisely from the solid mold surface 12 and transfer the patterned graphene 16 onto the polymeric film surface 22. The work of adhesion at the graphene-polymer interface is higher than the work of adhesion at the graphene-mold interface, which makes the complete transfer of graphene possible as described in Equation 1.sup.22

(31) $\begin{array}{cc}{W}_{A-B}=4\left(\frac{{\gamma }_A^d{\gamma }_B^d}{{\gamma }_B^d+{\gamma }_B^d}+\frac{{\gamma }_A^p{\gamma }_B^p}{{\gamma }_B^p+{\gamma }_B^p}\right)& \left(1\right)\end{array}$
where γd and γP correspond to the dispersion and polar components of surface energy (γ=γ.sup.d+γp). The theoretically calculated work of adhesion between each material interface support the proposed hypothesis. For instance, .sup.WGraphene-Delrin (77 mJ/m.sup.2)<.sup.WGraphene-PULA (90 mJ/m.sup.2); .sup.WGraphene-Delrin (77 mJ/m.sup.2)<.sup.WGraphene-PLEA (88 mJ/m.sup.2); and .sup.WGraphene-Delrin (77 mJ/m.sup.2)<.sup.WGraphene-GEL (88 mJ/m.sup.2).

(32) This graphene transfer approach can also be used to transfer the graphene patterns 16 from one flexible substrate 12′ to another. For instance, we already demonstrated that the graphene patterns 16′, ink jet printed (no pattern 14 in flexible substrate 12′ is used) and post-processed (laser or thermal annealed), can easily be transferred to 3D microstructured and porous CA-based films 22′ using polymer casting approach (FIG. 1d-f). The successful transfer of graphene 16′ can also be supported by the calculated work of adhesion at the interface of graphene-polyimide 16′/12′ and graphene-CA 16′/22′, W.sub.Graphene-Polyimide (86 mJ/m.sup.2)<W.sub.Graphene-CA (94 mJ/m.sup.2). Therefore, this fabrication approach allows formation of high-resolution patterns on the surfaces of versatile polymeric films as long as their free surface energies are sufficiently different to enable strong adhesion to one another. Similarly, we also demonstrated the potential of the direct polymer casting and peeling approach for the transfer of CVD grown graphene 16 as illustrated in FIG. 2. The graphene films 16 grown on copper foil (W.sub.Graphene-Cu=46 mJ/m.sup.2) (FIG. 2a) and quartz (FIG. 2b) as well as graphene-silver nanowire 17′ grown on quartz (W.sub.Graphene-Quartz=66 mJ/m.sup.2) (FIG. 2c) substrates 12 via CVD approach were successfully transferred to the polymer surface 22 (W.sub.Graphene-PLLA=90 mJ/m.sup.2) using our direct polymer casting approach. After the transfer, we did not observe a significant change in the resistance of graphene films 20, which indicates the efficiency of the transfer.

(33) Note just a few non-limiting examples of pattern form factors possible (e.g. compare FIGS. 1a-g with the transferred patterns 16A′, 1613′, 16C′ in FIG. 1h.

(34) The conductivity of graphene was enhanced by applying thermal annealing (pre-annealing temperature of 75° C. for 3 h for 60 mg/mL concentration) prior to the application of graphene solution 23 to create the substrate 12 patterns 14 via the channel filling approach. The transferred graphene 16 via polymer casting has the sheet resistance of ˜0.2 kΩ/sq. Similarly, the ink-jet printed and laser annealed graphene 16 was also transferred from flexible polyimide substrate 12 to CA substrate 22, which has the sheet resistance of ˜0.7 kΩ/sq.sup.59.

(35) It is possible to control the conductivity by changing the pre-annealing temperature along with the amount of graphene used for the filling approach. The effect of pre-annealing temperature on the graphene structure can also be observed via XPS analysis (FIG. 3a). The XPS analysis of graphene pattern on the MIA film surface revealed the presence of classical C-C(284.5 eV), C═O(˜287.8 eV) and O—C═O (˜288.9 eV) graphene peaks.sup.60,61 along with additional peaks around 282.7 eV, which potentially stems from the existence of graphene layers on the sample (FIG. 3a). With the increase in pre-annealing temperature from 25° C. to 75° C., we did not notice a significant change in C—C(˜284.5 V), C═O (˜287.8 eV) and O—C═O (˜288.9 eV) graphene peaks, whereas a significant decrease in 282.7 eV peak, accompanied by a slight shift toward ˜281.5 eV, was also observed, indicating the structural change upon pre-annealing. These peaks formed at binding energies around 282 eV are not classical graphene peaks, which generally can be observed in the range of 284-288 eV.sup.60, but could stem from the car-bides in the structure of graphene.sup.62. Carbides are also known as excellent semiconductors and simple thermal annealing induces an in-situ transformation of silicon carbide films into the graphene matrix.sup.62-65. Therefore, the decrease in the carbide peak upon temperature annealing could be another reason for enhanced conductivity. In addition, we also observed formation of a dense and compact graphene structure as the pre-annealing temperature increases to 75° C. (FIG. 3a). The reduction in the GNP size and increase in surface area after pre-annealing and probe sonication, observed in TEM images (FIG. 3b,c), could also be another reason for enhanced electrical conductivity due to the continuous and densely packed graphene platelets within the film microchannels.

(36) In FIG. 3b, the GNP structure can be observed (non-annealed), while the structure of graphene was changed from nanoplatelet form to small particulate form upon pre-annealing and sonication (FIG. 3c). The change in the GNP size upon annealing and sonication was also confirmed by dynamic light scattering measurements. The non-annealed GNP size was around ˜600 nm while upon annealing and sonication it became ˜250 nm. The result of these structural changes is also reflected in the conductivity of the formed patterns. The increase in pre-annealing temperature resulted in a significant decrease in the sheet resistance of graphene patterns (FIG. 3d).

(37) The dense, compact and continuous filling of graphene patterns on the PLLA film surface are shown in FIG. 4. The 3D porous microstructure of substrate material was observed using dry-phase inversion technique on PLLA film (FIG. 4a). Using polymer casting method, it is possible to precisely control the microstructure of variety of substrate material, including but not limited with natural/synthetic or biodegradable/non-biodegradable polymers, which is not possible to obtain with currently available flexible electronic fabrication techniques. This demonstrate the versatility of our approach. FIG. 4b demonstrates the graphene pattern with 500 μm of width and 200 of μm depth along with the structure of graphene on the PLLA film surface. In addition to this, it is also possible to obtain graphene patterns with small feature size. As shown in FIG. 4c, we were able to obtain graphene patterns on PLLA film surface with as low as 15 μm of width and 5 μm of depth. Although we have demonstrated examples mostly based on PLLA, it should be noted that this method is universal and can be applied to any type of polymer. FIG. 4d displays Raman spectra for the transferred graphene on the PLLA film surface. The distinct and classical D, G, and 2D peaks (˜1350, 1580, and 2700 cm.sup.−1, respectively) was observed.sup.59,66. Samples show a small D peak associated with lattice structure imperfections and edge plane defects in the graphene, as well as large G/2D peaks characteristic of sp2-hybridizated carbon (graphite/graphene structure).sup.59. Graphene patterns displayed low (I.sub.G/I.sub.D) ratio (0.33±0.01), which demonstrates that the transferred material is most adequately characterized as a multi-layer graphene structure.sup.67-69.

(38) The obtained graphene patterns showed significant stability after multiple bending and washing cycles (FIG. 50). In addition, their assembly on the film surface is mechanically strong, and even after multiple stick and peel cycles using a commercially available adhesive tape, the graphene patterns were stable (FIG. 5c). Therefore, the graphene patterns obtained with polymer casting approach exhibited good stability and conductivity to obtain active circuits (FIG. 5d). These circuits boards were made of graphene patterns of 300 μm width and 100 μm depth connected to a 9V battery through copper wiring to light up an LED 30 (with electrical leads 31L and R in electrical communication with the transferred graphene pattern 16). In addition, they maintained their conductivity as the sheet resistance did not change after multiple washing and bending cycles (FIG. 5e).

(39) As an alternative to the flexible electronics, this novel method can also be used to fabricate stretchable electronics. For this purpose, we fabricated flexible and stretchable WPI films with conductive graphene patterns using polymer casting method. The initial length of WPI film with graphene patters was 5 cm (FIG. 6a). A cyclic stretching test (5 cycles: 3 times of 2 cm stretching and retraction) was applied to detect the changes in the graphene pattern structure and resistance (FIG. 6b,c). FIG. 6b indicated that after cyclic stretching test there was no significant change in the graphene structure, which also resulted in the observation of stable resistance in the graphene patterns as illustrated in FIG. 6c. Then, the WPI film was stretched gradually (1 cm each time) up to 10 cm (FIG. 6d). FIG. 6d,e show that after 5 cm of stretching, the graphene structure started to break its continuity due to the effect of extension, which in turn resulted in increased resistance and decreased conductivity (FIG. 6e). The patterns were able to maintain their resistance around 2 kΩ up to 2 cm stretching (final extension till 7 cm) after which the resistance increased up to 9 kΩ when the stretching reached to 5 cm (final extension till 10 cm). This is clearly due to the discontinuity of the graphene patterns upon stretching as indicated in SEM images. Nevertheless, this experiment indicated that it is possible to fabricate stretchable electronic circuits with polymer casting method.

(40) These results clearly demonstrate that it is easy, fast, green and cost effective to fabricate graphene-based flexible and stretchable electronic devices on various biodegradable and biocompatible polymeric flexible substrates with proper 3D microstructural properties, small feature sizes and high-resolutions via the mentioned method. Therefore, the fabricated devices can be used in various biomedical and healthcare applications. Considering this, we conducted additional experiments to demonstrate the potential use of the developed devices with polymer casting method for controlling the stem cell differentiation.

(41) In our previous studies, we showed precise control of the transdifferentiation of stem cells using mechanical, physical, topographical, chemical and electrical cues for peripheral nerve regeneration purposes.sup.70-73. Very recently, we have demonstrated the successful transdifferentiation of mesenchymal stem cells (MSCs) into Schwann cells (SCs) using electrical stimuli through ink-jet printed and laser annealed graphene circuits on flexible polyimide substrates.sup.70. Although the transdifferentiation was successful, the non-biodegradable and non-porous nature of polyimide substrate limits the potential translation of this strategy for surgical implantation and clinical applications related to nerve regeneration. Therefore, the fabrication of such graphene-based circuit on biodegradable, 3D microstructured and porous substrates could make the surgical and clinical application of this strategy possible.sup.74. Therefore, the in situ-in vivo precise control on MSCs differentiation, migration and fate commitment upon the surgical implantation could be possible.sup.75. Considering this, we developed the same device (graphene circuit ink jet printed and laser annealed on polyimide substrate) on biodegradable, 3D microstructured and porous PLLA films using our graphene transfer via polymer casting approach as described above.

(42) We first investigated the attachment, growth and alignment of MSCs on the graphene patterns on the PLLA films. As demonstrated in FIG. 7a, MSCs grew both on the PLLA surface and graphene micropatterns. In addition, FIG. 7b,c also demonstrated the directed alignment of the MSCs along with the graphene patterns suggesting potential control on directional growth. FIG. 7d shows the graphene circuit design on PLLA films fabricated using graphene transfer method. FIG. 7d also illustrates that these circuits can be rolled into conduits as implants for specific peripheral nerve regeneration surgeries implying that they can potentially be used for other surgical or clinical applications. Following the attachment and growth of the MSCs on the devices, a specific electrical stimulus (100 mV at 50 Hz for 10 min per day for 10 days) was applied to the MSCs based on our previous study in order to differentiate them into SC-like phenotypes.sup.70. Our results showed that almost ˜90% of the cells got immunolabelled with the selected SC markers suggesting the successful transdifferentiation (FIG. 7e). These results are in accordance with our previously published findings.sup.70 demonstrating the potential of the device and our new fabrication method. These results also confirm the use of this technology to control MSCs differentiation and further enables its potential application for surgical and clinical translation by developing biodegradable and biocompatible devices with our new polymer casting approach.

CONCLUSIONS

(43) This novel method enables the fabrication of flexible electronics based on graphene and various polymeric substrates with precise control of 3D microstructural and mechanical properties (such as film porosity, pore size, elasticity etc.). The flexibility in the polymeric material selection along with precise control on substrate proper-ties, enables the potential use of the developed devices in biomedical applications or implantations. In addition, with this approach it is possible to obtain graphene patterns with high resolution, low feature size (as low as 5 μm; 15 μm width and 5 μm depth), high conductivity (sheet resistance of ˜0.2 kΩ/sq) and high stability (maintains sheet resistance after 100 bending and 24 h washing cycles). Moreover, this is a novel, facile, versatile, scalable and cost-effective manufacturing approach that eliminates the need for expensive equipment (except for initial lithography to create small microchannel patterning for the molds), physical or chemical post-processing or complex transferring/stamping processes. Furthermore, this is an alternative approach to the conventionally used PDMS molding or Cu foil-based transfer methods. Overall, this promising method has the potential to pave the way for flexible and stretchable electronics fabrication, particularly for the biomedical applications including but not limited to brain-computer interfaces, robotics or lab/organ-on-a-chip.

(44) Methods

(45) Preparation of substrates with graphene patterns/films. The Teflon or Delrin substrates with small feature size micropatterns were prepared using computer numerical control (CNC) machine while the silicon After molds with small pattern features were prepared using photolithography only once. The pattern dimensions were varied between 5 to 400 μm in width and dept. Graphene nanoplatelet (Sigma Aldrich) solution, sonicated and thermally annealed (at 75° C.) prior to application (pre-annealing), was used to fill the micropatterns. The excess graphene on the substrate was removed and cleaned by sticking and peeling a commercially available scotch tape, which leaves the graphene in the patterns.sup.46. In another approach, a graphene pattern was ink-jet printed on a rigid or flexible polyimide substrate and thermally or laser annealed (to enhance the conductivity) to create graphene substrates.sup.59,70,76. In addition, graphene and silver nanowire films were grown on quartz and Cu foil substrates via conventional CVD method.sup.23,77.

(46) Polymer casting, film formation and transfer of graphene. Following formation of graphene patterns on the substrates using the techniques mentioned above, the polymer casting solutions were prepared at desired formulations and concentrations. The polymer solution (could be Poly-L-Lactic Acid (PLLA), Cellulose Acetate (CA), Gelatin (GEL), Poly Lactic-co-Glycolic Acid (PLGA) or Whey Protein Isolate (WPI)) was then cast on the substrate with graphene patterns/films and left for drying. The 3D microstructure, mechanical properties, porosity and pore size of the films can be adjusted via well-established phase inversion techniques or use of pore forming or plasticizer agents. Upon the film formation, the film was peeled off and graphene patterns were transferred from the substrates to flexible film surface (FIG. 8a-e). This polymer casting approach can be applied for both rigid and flexible graphene substrates.

(47) Characterization of the prepared devices. The stability of the graphene patterns on the polymer devices was tested through multiple washing, bending and peeling-off cycles. The conductivity of the devices was determined by building up a circuit and measuring resistance. The microstructure of graphene patterns and devices were characterized through scanning electron microscopy (SEM) (FEI Quanta 250 FE-SEM), x-ray photoelectron spectroscopy (XPS) (Amicus XPS) and Raman spectroscopy (Bruker FT-Raman Spectrometer) analysis. SEM samples were sputter coated with 2 nm iridium before the analysis and images were taken using secondary electron mode. Monochromatic Al Kα X-ray source (1486.6 eV) was used in XI'S analysis with an electron take-off angle 45° from a normal sampling surface. Survey scans were collected from 10 eV to 1100 eV with a pass energy of 187.85 eV. Raman spectra were collected with a backscattering geometry, 1064 nm Nd:YAG laser and a spot size of about 1 mm.

(48) Use of graphene devices as biointerfaces. We investigated the potential use of the fabricated devices as bio-interfaces to control the stem cell behavior. We used Brown Norway rat mesenchymal stem cell (MSCs), which were provided by Dr. Donald S. Sakaguchi in Genetics, Development and Cell Biology Department at Iowa State university.

(49) The MSCs isolation procedure from Brown Norway rats was conducted in accordance with the NIH Guide for the Care and Use of Laboratory Animals guidelines and the principles in the “Guidelines for the Use of Animals in Neuroscience Research” presented by the Society for Neuroscience. All animal procedures had the approval of the Iowa State University Institutional Animal Care and Use Committee and were performed in accordance with committee guidelines.

(50) MSCs were plated in T75 flasks in maintenance media (MM), consisting of a minimum essential medium (αMEM, Gibco BRL), 20% fetal bovine serum (FBS; Atlanta Biologicals), 4 mM1-glutamine (Gibco), and antibiotic-antimycotic (Invitrogen) and incubated at 37° C. and 5% CO.sub.2. MSCs were sub-cultured when they reached 80% confluency approximately every 2-3 days.

(51) The ink-jet printed graphene patterns with finger dimensions of 400 μm finger width and 250 μm finger-to-finger spacing were transferred to the surface of PLLA film by applying polymer casting method and the obtained devices were used for electrical transdifferentiation of MSCs. 2×10.sup.5 cells were seeded on devices and electrical transdifferentiation into Schwan cell-like phenotypes (SCs) was conducted exactly as described in our previous work.sup.70. At the end of transdifferentiation, immunocytochemical analysis was performed on MSCs possessing SC-like phenotypes seeded on the device using selected SC markers, s100, s100β and p75 as described previously.sup.70.

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D. Specific Example 2

(53) Additional examples of ways to make, use, and practice aspects of the invention are set forth below with particular reference to appended FIGS. 9a-d.

1. Method of Creating High Resolution Starting Pattern

(54) As will be appreciated by those skilled in this technical field, a simple, versatile, high-resolution, and scalable patterning of a starting substrate can be achieved in a variety of ways. One example is on polymeric films (including biodegradable polymer films) by the formation of polymeric films with different surface micropatterns via solvent casting on micropatterned molds.

(55) Procedure: The polymer solutions are cast on Teflon molds (5×5 cm), with different patterns and dimensions (width: from 10 to 500 μm and depth: from 10 to 300 μm) and left for overnight drying. Preparation of super-hydrophobic Teflon molds with different patterns is easy, fast, cheap and they can be reused multiple times. Therefore, they can be a promising alternative to PDMS based molds. Following the overnight drying, the films are peeled off, where the patterns on the molds are transferred to the films' surface.

(56) Details about specific formulations of polymeric solutions or graphene-based solutions for use in this embodiment are set forth above.

(57) The illustrations of FIGS. 9a-d assist in an understanding of this example: FIG. 9a: Shows (see scale ruler) a starting substrate (e.g. Teflon) with formed micropatterns (e.g. high resolution parallel channels of 3D controlled structure). Note how this example shows different 3D structure of different sections of the mold (thinner versus thicker parallel ridges in the mold). It is possible to have different depths and between ridges and even along ridges. FIG. 9b: Shows a polymer-based solution coated over the micropatterned starting substrate. FIG. 9c: Shows how the micropattern in starting substrate is transferred to the polymer-based film by drying the polymer-based solution in place on the starting substrate and then removing or peeling the dried polymer film away. Importantly, by control of the solution and the drying, the original high resolution micropattern is effectively transferred to the polymer film keeping on the order of resolution of the starting micropattern but now in the polymer film. FIG. 9d: Shows several examples of the final end product of polymer film with graphene in the micropatterns.

(58) As described earlier, the polymer solutions and the conductive-particle-based solutions can be pre-configured to effectuate the end product, including some pre-processing of the conductive-particle-based solution, while in solution (e.g. annealing), to control physical, electrical, and other characteristics. As can be appreciated by the foregoing, the original high resolution mold can be produced in a number of ways on the first substrate. The structural features of the mold can be made to be 2D or 3D with such high resolution. Because the mold does not have to be the final polymer-based thin film, a variety of techniques that are relatively economical can produce such 2D or 3D high resolution structures. Then, the high resolution mold can be relatively economically, quickly, effectively used to transfer the micro-pattern to a thin film polymer-based layer. As is appreciated by those skilled in the art, such thin polymer-based films have mechanical, temperature, and other characteristics that could negatively influence any attempt to create such high resolution 2D or 3D patterns into such material. As discussed above, the transferred micropattern in polymer-based film is then at least partially filled with material such as conductive-particle-based (e.g. graphene). It assumes the 2D or 3D structural features of the micropattern, adheres to the polymer-based film, and, thus, is an economical, effective combination of high resolution conductive micropattern on/in thin polymer-based film. Also, pre-processing of the conductive-particle-based material (e.g. heating) can be done apart from the polymer-based thin film, again protecting the polymer-based thin film from damage, deformation, or other changes that would materially affect the high resolution.

2. Alternative Example of Starting Pattern and Solvent Casting Method

(59) With reference to FIGS. 10a-i, an alternative similar to Specific Example 1 is set forth.

(60) Method: This method is focused on direct transfer of graphene-based patterns from different solid or flexible substrates to the polymeric flexible films via solvent casting. The method consists of three main steps. (i) preparation of graphene-based patterns via microfluidic approach on patterned solid substrates (Teflon, glass or Silicon wafer) (ii) casting of the polymer solution on the graphene-based patterns formed on solid substrates (iii) drying of the solvent and formation of films followed by peeling off the films from the substrate, transferring the graphene pattern to the polymeric film surface.

(61) Procedure: Teflon, glass or silicon wafer-based substrates with different patterns and dimensions (width: from 10 to 500 μm and depth: from 10 to 300 μm) are prepared using CNC machine or lithography. Then, the graphene nanoflake solution (concentrations varying from 20 to 100 mg/mL in 70% ethanol and thermally annealed at ˜75° C. or higher for more than 3 hours) is pumped through the microchannels at certain volume and flow rate using a syringe pump set up. Alternatively, inkjet printing of graphene patterns on flexible polyimide substrates or any other substrate can also be used for graphene-based pattern formation. After the formation of graphene-based patterns, the polymer solution is cast on the patterned substrates at certain volume via direct pouring or spin casting. After the solvent drying, the formed films are peeled off from the substrate surface and the graphene patters are transferred to the biodegradable polymer-based flexible film surface.

(62) Details about specific formulations of polymeric solutions or graphene-based solutions for use in this embodiment are set forth above.

(63) The illustrations of FIGS. 10a-h assist in an understanding of this example: FIG. 10a: Shows (on small general scale as FIG. 9a) a starting substrate (e.g. Teflon) with either (a) formed micropatterns (e.g. high resolution parallel channels of 3D controlled structure) with those channels filled with a conductive-particle-based material (e.g. graphene) to controllable height, or (b) a graphene pattern of 2D or 3D form factors along each of its portions on the substrate (e.g. by inkjet printing directly on a substrate surface). Thus, a high resolution graphene micropattern/Teflon substrate combination is presented. FIG. 10b: Shows polymer-based solution coated over the graphene micropatterned starting substrate. Note how the polymer-based solution in this example is transparent enough that the underlying graphene-filled micropatterned starting substrate can be seen in this image. Once the polymer-based solution is dried in place, it is peeled off. FIG. 10c: Shows how micropattern in starting Teflon substrate is now devoid of most of the graphene after the polymer layer is peeled off, because the graphene adheres more strongly to the dried polymer than the Teflon. FIGS. 10d-h (including multiple frames): Shows steps of the process, here an ink-jet printed graphene micropattern on polyimide substrate transferred to cellulose acetate film by selected frames from movie. Frame at T1 shows an inkjet printed graphene micropattern on polyimide starting substrate (yellow or top film) to which a cellulose acetate (clear layer) has been coated. Frame at T1 shows combination inverted so that cellulose acetate (clear) layer is on bottom and polyimide starting substrate (yellow) is on top so that tweezers grips one edge of polyimide and begins peel off of the polyimide starting substrate (yellow). Not how a substantial replica, with on the order of the same high resolution, of the original inkjet printed graphene pattern on the yellow polyimide adheres to, and thus is transferred to, the clear cellulose acetate layer. Frame at T2 shows further peeling away of the yellow polyimide layer, the starting substrate, from the clear end product cellulose acetate layer, including how replica of micropatterned graphene of starting substrate is transferred to the cellulose acetate film. Frame at T3 shows the yellow polyimide film completely separated by peeling away from the clear cellulose acetate layer and the transferred graphene micropattern. Frame at T4 shows how the high resolution of the micropattern of the starting yellow substrate is effectively transferred to the second substrate (clear). Noe how a slight residual remnant of graphene stays with the yellow starting substrate but that the bulk of graphene is transferred, with resolution intact, to the clear cellulose acetate second substrate. Frame at T5 shows flexible nature of both the thin polyimide starting substrate and the thin film cellulose acetate with transferred micropattern of graphene. FIG. 10i: Shows how a transferred micropattern of graphene on polymer-based film (such as with the technique of FIGS. 10a-h, has sufficient high resolution, continuity, and 3D structure to function as high resolution electrical circuit to both connect to electrical power and conduct electrical power for electrical/electronic circuit applications (here showing power to two spaced apart graphene traces (two right side wires) to power up a blue LED (two left side wires). The inset in upper left corner illustrates the integrity of the transferred graphene at high magnification.

(64) As described earlier, the polymer solutions and the conductive-particle-based solutions can be pre-configured to effectuate the end product, including some pre-processing of the conductive-particle-based solution, while in solution (e.g. annealing), to control physical, electrical, and other characteristics.

(65) As can be appreciated by the foregoing, the original high resolution micropattern of material (e.g. graphene or other conductive particles or material) can be produced in a number of ways directly on the first substrate. One example is by direct inkjet printing to that first substrate. The structural features of that pattern can be made to be 2D or 3D with high resolution by control of the printing. Note that, in this example, the starting substrate is itself a thin film and a polymer (e.g. polyimide). But a variety of techniques that are relatively economical can produce such 2D or 3D high resolution structures. Then, the high resolution micropattern on the starting substrate can be relatively economically, quickly, effectively transferred to another thin film polymer-based layer (e.g. cellulose acetate). As is appreciated by those skilled in the art, polyimide has mechanical, temperature, and other characteristics that could negatively influence any attempt to create such high resolution 2D or 3D patterns by some techniques but is sufficient to effectively receive inkjet printed material (e.g. can have relatively high heat-resistance and is relatively rugged) even in film form. As discussed above, the micropattern of material on the starting polyimide substrate is then at least substantially transferred, including its 2D or 3D structural features, from the starting substrate by peeling away from the second polymer-based thin film substrate, and, thus, is an economical, effective final combination of high resolution conductive micropattern on/in thin polymer-based film.

E. Options and Alternatives

(66) As indicated above, the examples in this detailed description are for purposes of better understanding of the invention but not as limitations to the invention. Some examples of options and alternatives are set forth above. Variations obvious to those skilled in this technical art are also included.