Stretchable and flexible electronic applications require mechanically suitable electrical wiring. This article presents, for the first time, the microfabrication of photolithographically patterned microwiring on an electrospun nanofiber mat. The use of a photoresist-based technology allows for better than 10 μm pattern resolution on a good quality nanofiber mat, providing an order of magnitude smaller feature sizes than what has been published before. We demonstrate metallic wiring patterned from a 75 nm thick coating on top of the nanofiber mat. A silicone elastomer was incorporated to serve as a matrix material and form a composite substrate and an encapsulation layer on top of the microwiring. We demonstrate clean and anisotropic dry etching of the elastomer to open electrode sites that can be smaller than 10 μm in size. We speculate that these structures will be mechanically robust while being soft at the same time and provide the properties necessary for potential use in stretchable and flexible electronics.

Soft and stretchable electronics are useful and have been the focus of myriad academic and commercial efforts owing to their combination of conformable, flexible, and stretchable properties. They have been used extensively for applications in dynamically mobile environments and for integration with nontraditionally shaped surfaces.1,2 Examples include areas pertaining to wearable healthcare,3–5 e-skins,4,6,7 and implantable electronics. Recent reviews by Feiner and Dvir8 and Bettinger et al.9 discuss research and developments in flexible and stretchable materials as tissue–electronics interfaces. Stretchability in devices can be achieved by incorporating intrinsically stretchable materials as substrates and as electrical components or by innovative structural designs. As substrate materials, thin (tens of nanometer-scale) silicon nanomembranes,10 polydimethylsiloxane,11,12 Parylene C,13–15 polyimides,16 polyethylene naphthalate and polyethylene terephthalate,17–19 acrylates,20 thiol-ene/acrylates,21 and hydrogels22–24 have been explored in the past. As traditional electrically conductive materials, metals have been shaped into physical structures like waves, wrinkles,25–27 serpentine,28–30 and out of plane structures like arc bridges based on prestretched deposition31 and folds and cuts based on origami/kirigami32,33 to help accommodate strain in otherwise rigid metals. As alternative electrically conductive materials, incorporation of deformable electrical elements like liquid metals28,34,35 and conductive polymers like poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS),36 polyaniline,37,38 and nanomaterial fillers39 has been studied.

Here, we employ electrospun nanofibers as the principal structural component of our stretchable electronic wiring technology. The otherwise electrically insulating interlaced nanofibers can be metallized to provide electrical conduction on their surface while allowing for mechanical deformations in virtue small diameter and intricately layered network, as has been reported earlier by the Cui group40 and the Someya group7,41 for fiber diameters ranging from 300 to 900 nm. In comparison, we employ an average fiber diameter below 50 nm in this work. We use the same nanofiber mats as reinforcement for elastomeric silicone and introduce a microfabrication process flow that is facile in creating a nanocomposite substrate and an encapsulation for the wiring. The elastomer material is chosen to have an elastic modulus smaller than that of the nanofiber network, an idea explored by Hanif et al. earlier.42,43 The resulting nanocomposite is expected to possess a tensile strength determined by the network structure and strength of the nanofibers, and the elastomeric matrix is expected to comply with deformations while maintaining its function as an electrical insulator.

Electrospinning is a facile, economical, and large-scale manufacturing method of producing nanofibers. The choice of a relatively densely electrospun nanofiber mat (as compared to microfibers) provides for the capability of high spatial resolution patterns. Electrospun nanofibers have considerable tensile strength and modulus due to higher crystallinity and significant molecular orientation induced by electric forces during electrospinning as covered in detail in a critical review by Rashid et al.44 Some studies have used conductive fillers or semiconducting materials in the polymer dispersion to produce electrospun nanofiber-based conductors.45–50 Others have immersed insulating nanofibers in conductive dispersions after electrospinning.51–53 Yet other strategies reported by Miyamoto et al.7 and Wu et al.40 employ electrospun nanofibers as templates for duplicating their physical structure in a metal layer and creating a conductive nano-trough network. A study by Lee et al.41 has demonstrated nanomesh gold conductors on supporting nanofiber-based substrates. In these works, shadow masking techniques have been used to create about 500 μm wide conducting traces with conductivity in the range from 0.9 × 106 to 22 × 106 S/m.

We demonstrate the microfabrication of lithographically patterned stretchable gold microwiring on a Nylon 6 electrospun nanofiber mat-based silicone composite substrate as depicted in Fig. 1. We report the use of physical vapor deposition, photolithography, and wet etching processes to deposit and pattern the metal layer. In addition, we present the application of a medical-grade silicone dispersion to form the elastomer-nanofiber composite and a top encapsulation layer. We patterned the encapsulation and the nanocomposite layers using inductively coupled plasma etching. Electrical characterizations confirmed the functionality of the wiring.

FIG. 1.

Graphical rendering of (a) lithographically patterned metallic microwiring on a flexible nanofiber-based substrate. The nanofiber mesh is employed to mechanically support a thin layer of metal coating, (b) schematic of structures released to measure electrical isolation between adjacent wires, and (c) cross section view of the architecture of structures built on a nanofiber-reinforced elastomeric composite, with gold metallized nanofibers embedded within encapsulation layers. The encapsulation layer has been etched to open access to gold sites.

FIG. 1.

Graphical rendering of (a) lithographically patterned metallic microwiring on a flexible nanofiber-based substrate. The nanofiber mesh is employed to mechanically support a thin layer of metal coating, (b) schematic of structures released to measure electrical isolation between adjacent wires, and (c) cross section view of the architecture of structures built on a nanofiber-reinforced elastomeric composite, with gold metallized nanofibers embedded within encapsulation layers. The encapsulation layer has been etched to open access to gold sites.

Close modal

While this technique is prohibitively expensive for smart textiles and large-area flexible consumer electronics, our method can provide patterns that are more than an order of magnitude smaller than what is achievable with previously demonstrated techniques. Many applications of wearable flexible electronics (e.g., skin-mounted sensors) and implantable flexible electronics (e.g., nerve cuffs and intraneural probes) would benefit from the scaling down and can accommodate this cost structure in commercial devices.

1. Electrospun nanofiber mat production

The nanofiber mat was electrospun using standard needle-free electrospinning technique on an Elmarco Nanospider NSLab at Luna Innovations, Inc. A solution of Nylon 6 (Sigma Aldrich) was dissolved at 12% w/v in a 2:1 solution of formic acid:acetic acid. This polymer solution was then electrospun on the NanoSpider using a 0.7 mm diameter orifice at 70 kV with a working distance of 150 mm, a carriage speed of 150 mm/s, and a substrate speed of 3 cm/min. The resultant fibers were collected on a 4-in. diameter chromium-coated silicon wafer. The silicon wafers were precoated with thermally evaporated Cr to serve as a sacrificial layer for releasing the final structures.

2. Microfabrication process flow

The nanofiber mats as represented in Fig. 2(a) were prepared for metal deposition by spin-coating photoresist AZ 1512 [Fig. 2(b)]. This step fills in the porosities of the mat and creates a layer of photoresist on top of the mat surface. The photoresist is then soft-baked at 85 °C for between 6 and 7 min and exposed to UV-light on a Karl Suss MA6/BA6 contact mask aligner for flood exposure. The top photoresist layer is then removed using AZ 917 MIF developer solution, exposing the fiber mat structure [Fig. 2(c)] for subsequent metal deposition.

FIG. 2.

Process flow of the fabrication steps starting with (a) a pristine nanofiber mat that is (b) coated and filled in with a photoresist. (c) The top layer of the nanofiber is exposed by developing the resist and (d) metal (Ti/Au/Ti) coating is deposited on the nanofibers. (e) The metallized fibers are patterned using photolithography and wet metal etching, after which the photoresist is removed. (f) A silicone dispersion is spin-coated on the wafer to fill in the nanofiber mat and encapsulate the gold microwiring. A hard mask of SiNX and Al is deposited after curing and is used to (g) pattern the encapsulation and expose electrode sites that access the gold-coated nanofibers in a dry etching process. (h) The hard mask is then recoated and patterned to etch the structure outline. Finally, the structures are released from the carrier wafer.

FIG. 2.

Process flow of the fabrication steps starting with (a) a pristine nanofiber mat that is (b) coated and filled in with a photoresist. (c) The top layer of the nanofiber is exposed by developing the resist and (d) metal (Ti/Au/Ti) coating is deposited on the nanofibers. (e) The metallized fibers are patterned using photolithography and wet metal etching, after which the photoresist is removed. (f) A silicone dispersion is spin-coated on the wafer to fill in the nanofiber mat and encapsulate the gold microwiring. A hard mask of SiNX and Al is deposited after curing and is used to (g) pattern the encapsulation and expose electrode sites that access the gold-coated nanofibers in a dry etching process. (h) The hard mask is then recoated and patterned to etch the structure outline. Finally, the structures are released from the carrier wafer.

Close modal

The trilayer of titanium-gold-titanium was sputter deposited [Fig. 2(d)] on the exposed fibers using an AJA Orion Magnetron Sputter Deposition system with 2-in. diameter targets and using an argon plasma at 4 mTorr pressure. The titanium layers were deposited at a rate of 4 nm/min for 6 min at 200 W RF power. The target was allowed to presputter for 5 min before the first layer deposition. The gold layer was deposited at a rate of 20 nm/min for 1 min 15 s, 2 min 30 s, 5 min, and 10 min on four separate wafers (to yield 25, 50, 100, and 200 nm layer thicknesses, respectively) at 100 W DC power. Deposition rates were characterized on bare silicon wafers ahead of time.

The metallized nanofibers were photolithographically patterned using an AZ 1512 photoresist on a Karl Suss MA6/BA6 contact mask aligner [Fig. 2(e)]. 10 mm long traces of varying nominal widths (10 μm, 100 μm, and 1 mm) and varying spacing were patterned as microwiring. Wet etching was performed at room temperature, using buffered hydrofluoric acid (BOE 7:1 by J. T. Baker) to etch the titanium and a potassium iodide solution (Gold Etch by Transene) to etch the gold. The photoresist was then removed from the mat [Fig. 2(e)] by soaking in acetone and thoroughly washed off with plenty of de-ionized water and dried with nitrogen, followed by overnight vacuum desiccation.

Nusil MED6-6606 RTV silicone dispersion was spin-coated onto the nanofiber mat using manual dispensing. The solution was poured slowly in the center of the wafer, and it was spread by tilting and rotating the wafer until the solution visibly penetrated the nanofiber mat across the whole surface (the fiber mat turns from white to translucent). After the dispersion wetted the fibers completely, it was left untouched for 30 s and then spun at 4000 rpm for 4 min. The wafers were left partially covered in a fume hood to cure for 3 days [Fig. 2(f)].

Low-stress silicon nitride (SiNx)was deposited on a PlasmaTherm 790 PECVD system at 150 °C, 900 mTorr processing pressure at 100 W for 75 min at a deposition rate of about 14 nm/min. Aluminum was deposited using an AJA 1500 Magnetron Sputter Deposition System using a DC gun powered at 200 W, 4 mTorr argon pressure, and 40 min deposition time at a deposition rate of about 5 nm/min. Aluminum was patterned using photolithography with an AZ 1512 photoresist, soft baking at a lower temperature at 85 °C for 6 min and UV exposure on a Karl Suss MA6/BA6 contact aligner. The resist was developed using AZ 917 MIF and the Al was etched using Transene aluminum etchant type A at room temperature [Fig. 2(g)]. The pattern was transferred to the silicon nitride using the same dry etching process that was employed for the patterning of the top silicone layer.

An inductively coupled plasma-based reactive-ion etching process was used to pattern silicon nitride, silicone, and silicone-nanofiber composite on a PlamsaTherm Versaline ICP etcher. Using a mixture of SF6 and O2 gases at 1:1 ratio and 10 mTorr pressure, 1200 W RF ICP power, and an RF bias ranging from 11 to 44 W, the wafers were etched at 50 °C substrate temperature [Fig. 2(g)]. After etching and opening of the electrode sites, the remaining SiNX/Al hardmask was removed by wet etching in BOE 7:1. The same hardmask stack was then recoated for another photolithography step aimed at defining the outline of the structures. The outline etch was performed using the same ICP RIE process recipe as for the top layer silicone earlier [Fig. 2(h)]. The remaining SiNX/Al hardmask was again removed by wet etching. Finally, the structures were released from the silicon wafer by soaking in a chromium etchant KMG CR-7S for 2–3 days. The structures were washed with de-ionized water and dried on a cleanroom wipe.

1. Morphological characterization

Scanning electron microscope (SEM) imaging was performed by a Zeiss Supra 40 SEM on the wafers to obtain high-resolution images of the fibers: pristine and metallized. SEM was also used to check the progress of the fabrication flow and to determine the etch quality.

Optical microscopy was carried out on a Leica INM200 and a Zeiss SteREO Discovery v12 stereoscope to check pattern quality and confirm continuity of the patterned wiring.

A stylus profiler Bruker DektakXT was used with a 3 mg force to measure the thickness of silicone encapsulation and the nanofiber-reinforced silicone composite.

2. Electrical measurements

Current-voltage (IV) measurements were performed on the patterned microwiring before and after encapsulation using a Keithley Source Meter 2604B that provided 0–5 V sweeps in 50 mV steps and recorded the corresponding current. Micropositioners and probes were placed on the two electrode pads of straight wiring traces to measure resistance. The probes were placed on the pads of the adjacent bent traces with varying spacings to check for electrical isolation between traces. The resistance values were recorded, and sheet resistance, resistivity, and conductivity were calculated and averaged. The data were checked for normality using the Kolmogorov Smirnov test. All data passed the normality test. Each width of wiring (10 μm, 100 μm, and 1 mm) was fabricated and measured in 15 specimens from the same sample; specific specimens were rejected and excluded from the results due to lithographic defects and wire discontinuity.

The produced nanofiber mats were comprised of nanofibers with 45 nm average diameter and some additional larger fibrous deposits and bead formations. A SEM image of a mat with relatively large beads embedded and larger fibers visible on its surface is shown in Fig. 3(a) as an example. These electrospinning artifacts were addressed during the preparation of the mat. The edges of the mat were trimmed to be able to accommodate subsequent cleanroom process equipment standards. Any large fibrous artifacts or beads were melted down and the edges were sealed using a soldering iron (JBC Nano soldering station) set at 215 °C. Figure 3(b) is an optical image of a mat prepared in such a manner. While the molten beads still present defects in the mat, they provide the ability to contact printing patterns without a significant airgap between the mask plate and the surface of the nanofiber mat. An ideal quality mat is one without large bead formations as these beads introduce roughness that dictates the resolution of the photolithography for patterning narrow wires. An acceptable size bead that we can accommodate in our process is shown in Fig. 3(c). Luna Innovations Inc. has implemented cleaning, filtration, and air control processes to improve nanofiber quality and homogeneity.

FIG. 3.

(a) Bird's eye view SEM image of the surface of a nanofiber mat with fibrous deposits and large bead formations, taken at 1 kV acceleration voltage and 390× magnification at 4.2 mm working distance using an Everhart–Thornley secondary electron detector. (b) Optical image of an electrospun nanofiber mat on a silicon wafer. The fiber mat was cleared from the edges of the wafer using a razor blade and the edges of the mat were melted with a soldering iron. Larger nylon beads were also melted down, creating visible speckles on the surface. (c) SEM image of a relatively small nylon bead taken at 1 kV acceleration voltage and 7.83 K× magnification at 4.2 mm working distance using an Everhart–Thornley secondary electron detector.

FIG. 3.

(a) Bird's eye view SEM image of the surface of a nanofiber mat with fibrous deposits and large bead formations, taken at 1 kV acceleration voltage and 390× magnification at 4.2 mm working distance using an Everhart–Thornley secondary electron detector. (b) Optical image of an electrospun nanofiber mat on a silicon wafer. The fiber mat was cleared from the edges of the wafer using a razor blade and the edges of the mat were melted with a soldering iron. Larger nylon beads were also melted down, creating visible speckles on the surface. (c) SEM image of a relatively small nylon bead taken at 1 kV acceleration voltage and 7.83 K× magnification at 4.2 mm working distance using an Everhart–Thornley secondary electron detector.

Close modal

For the metallization of the top nanofibers, we developed a trilayer sputter-deposition process that involves the filling of the fiber mat with a photoresist underneath. Four wafers were prepared with varying gold thicknesses (25, 50, 100, and 200 nm) and constant titanium thickness (25 nm) both below and above the gold layer. Going forward, we refer to the samples by the nominal thickness of the deposited gold. Figures 4(a) and 4(b) show the SEM images of metallized nanofibers with a nominal gold thickness of 25 and 200 nm, respectively. The conformal coating of the metal on the nanofibers is evident from these images. Also is evident that the metal had coated recessed and underlying fibers that are in the line-of-sight of deposition, thereby creating a complex structure for electrical wiring. The difference between the two coatings in terms of metal thickness and porosity is apparent. The 200 nm coating creates a well-connected and almost continuous metal layer, while the 25 nm coating better maintains the nanofiber structure. The retained porosities and mesh-like architecture of the coated nanofibers should allow for mechanical deformations and stretchability.54 

FIG. 4.

SEM images of as-deposited metal-coated nanofibers taken using an in-lens secondary electron detector at 1 kV acceleration voltage and 50 K× magnification at a working distance of 3 mm of (a) 25 nm gold metallization (75 nm Ti/Au/Ti) and (b) 200 nm gold metallization (250 nm Ti/Au/Ti). Thinner coatings retain the fiber structure better, while thicker coatings form a more continuous and filled-in layer of metallization.

FIG. 4.

SEM images of as-deposited metal-coated nanofibers taken using an in-lens secondary electron detector at 1 kV acceleration voltage and 50 K× magnification at a working distance of 3 mm of (a) 25 nm gold metallization (75 nm Ti/Au/Ti) and (b) 200 nm gold metallization (250 nm Ti/Au/Ti). Thinner coatings retain the fiber structure better, while thicker coatings form a more continuous and filled-in layer of metallization.

Close modal

Photolithography and wet etching were used to pattern microwiring on top of the nanofiber mat. Etching the bottom layer of titanium was challenging because an extended etch time was needed to remove titanium from the pores, while the top-most layer of titanium was slowly receding from the edges at the same time. Figures 5(a) and 5(b) show optical micrographs of the wiring that was lithographically patterned on the metallized nanofibers. Figure 5(a) shows 10 mm long traces of varying nominal widths (10 μm, 100 μm, and 1 mm) that were measured for conductance, and Fig. 5(b) shows 10 mm long traces with varying spacing (20, 50, and 100 μm) that were measured for leakage currents between adjacent traces. Figures 5(c) and 5(d) depict the structural details of a 10 μm wide wiring. The boundary between the metallized and the metal-stripped nanofibers is clearly visible. The metal stripped fibers do not show metal residue and appear to have retained their structure and integrity. Electrical measurements confirmed electrical isolation between adjacent traces of wiring, and leakage currents were measured to be below our detection limit of 1 pA in all measurements.

FIG. 5.

Optical microscope images of the 10 mm long metal wires patterned on the fiber mat (a) with varying widths and (b) with varying spacing show large-scale pattern quality, along with typical defect size and density. SEM images taken at 1 kV acceleration voltage and 2.8 mm working distance using an Everhart–Thornley secondary electron detector show (c) close-up of a 10 μm wide wire and (d) details of the boundary between metalized and stripped nanofibers.

FIG. 5.

Optical microscope images of the 10 mm long metal wires patterned on the fiber mat (a) with varying widths and (b) with varying spacing show large-scale pattern quality, along with typical defect size and density. SEM images taken at 1 kV acceleration voltage and 2.8 mm working distance using an Everhart–Thornley secondary electron detector show (c) close-up of a 10 μm wide wire and (d) details of the boundary between metalized and stripped nanofibers.

Close modal

Figure 6 shows the results of IV measurements on wires of varying widths and gold thicknesses. All IV curves were purely resistive, suggesting metallic electrical conduction along the traces. Conductivity was calculated from the measured conductance by normalizing with the nominal length, width, and gold thickness. Fifteen specimens each were measured for all wire widths on all samples. Only the narrowest, 10 μm wide, traces were found to have discontinuities. Fabrication yield was related to mat quality and roughness arising from embedded nylon beads and was found to be unrelated to the thickness of metallization in our samples. On the sample with 25 nm gold thickness, all 15 structures were continuous. One structure was continuous on the 50 nm thickness sample, 10 structures on the 100 nm thickness, and 4 structures on the 200 nm thickness. The discontinuous wires were excluded from our calculations. As a trend, thinner coatings showed less conductivity. The 25 nm thick and 100 μm wide gold traces showed an average conductivity of 2.64 × 106 S/m, while the 200 nm thick and 100 μm wide gold traces showed an average conductivity of 14.6 × 106 S/m. The conductivity of pure gold in bulk is about 42.5 × 106 S/m.55 Conductivity is an intrinsic material property independent of sample dimensions in general. However, when sample dimensions are reduced to the nanoscale, such as in the case of thin-film coatings, increased surface scattering and grain boundary collisions reduce the mean free path of charge carriers and increase resistivity.56–58 These effects are prevalent in planar thin films and are expected to affect conductivity in our samples. In addition to the small metal thickness, our metal-coated nanofiber wires have a more complex physical structure with larger surface area and increased conduction path as compared to a planar wire of the same nominal dimensions. Nevertheless, our data demonstrate that a 25 nm thick gold coating (75 nm Ti/Au/Ti) can provide sufficient conductivity for most applications. The electrical performance of these coatings under mechanical deformations will be tested in the future. Our demonstration of 10 μm wide metal wiring on top of a nanofiber mat represents an order of magnitude increase in pattern resolution as compared to other work using shadow masking or printing.7,41 The photolithography resolution is not expected to be affected by the thickness of the nanofiber mat as long as the mat quality is maintained.

FIG. 6.

Bar chart of average conductivity of a metal-coated 10 mm long microwiring for 10, 100, and 1000 μm widths and various nominal deposition thicknesses. The error bars represent standard deviation. Note that 15 specimens were measured for all wire widths on all samples, but for 10 μm width some samples were found to be discontinuous and were excluded (n = 15 for 25 nm thickness, n = 1 for 50 nm thickness, n = 10 for 100 nm thickness, and n = 4 for 200 nm thickness). The data were checked for normality using the Kolmogorov–Smirnov test and all passed.

FIG. 6.

Bar chart of average conductivity of a metal-coated 10 mm long microwiring for 10, 100, and 1000 μm widths and various nominal deposition thicknesses. The error bars represent standard deviation. Note that 15 specimens were measured for all wire widths on all samples, but for 10 μm width some samples were found to be discontinuous and were excluded (n = 15 for 25 nm thickness, n = 1 for 50 nm thickness, n = 10 for 100 nm thickness, and n = 4 for 200 nm thickness). The data were checked for normality using the Kolmogorov–Smirnov test and all passed.

Close modal

Figure 7(a) shows a SEM cross section view of a nanofiber-silicone composite on a Si wafer, prepared with spin-coating silicone dispersion as described earlier. The silicon wafer was cleaved, and the composite was cut with a razor blade to create the cross section view. While such a preparation can cause artifacts, we were able to confirm that the nanofiber mat was filled with silicone across its whole thickness and that there were no large voids present. In addition to penetrating and filling the nanofiber mat, the spin-coating process also yielded a pure silicone layer on top of the nanofiber mat. We refer to this additional top layer as “silicone encapsulation,” pointing to its function of electrically encapsulating and isolating the metal patterns. Figure 7(b) shows a similar cross section view but with an embedded gold microwiring in the picture. There is no apparent difference in the composite structure or the silicone encapsulation thickness around the wiring as compared to metal-free regions of the nanofiber mat. The final thickness of the silicone-nanofiber composite was found to be about 14–15 μm (dependent on the original thickness of the nanofiber mat) and the thickness of the top silicone encapsulant was 4–5 μm.

FIG. 7.

SEM images taken with an in-lens secondary electron detector at 1 kV acceleration voltage show a cross section view of a Nylon 6 nanofiber mat filled with silicone dispersion and reveal (a) about a 5 μm thick silicone layer on top of the composite and (b) an embedded microwire at the interface of composite and silicone layers.

FIG. 7.

SEM images taken with an in-lens secondary electron detector at 1 kV acceleration voltage show a cross section view of a Nylon 6 nanofiber mat filled with silicone dispersion and reveal (a) about a 5 μm thick silicone layer on top of the composite and (b) an embedded microwire at the interface of composite and silicone layers.

Close modal

Both the silicone encapsulation and the nanofiber-silicone composite were patterned using ICP-RIE etching at 1200 W RF ICP power and 44–11 W RF bias power, 10 mTorr pressure, SF6:O2 gas ratio of 1:1, and 50 °C substrate temperature. We report an approximate etch rate of 10 nm/s (0.6 μm/min) for the top layer silicone encapsulation and of 18 nm/s (1.1 μm/min) for the nanocomposite at 44 W RF bias. These numbers fit into the range of etch rates reported by Hill et al.59 and Vlachopoulou et al.60 for similar etch conditions. We adopted a relatively low RF bias and a relatively high substrate temperature at 50 °C that effectively reduced the occurrence of residues (also referred to as “grass”). The benefit of elevated substrate temperature was reported earlier by Szmigiel et al.61 The low RF bias was also selected given the unconventional etch stop layer when opening windows in the silicone encapsulation on the metal-coated nanofiber mat. RF bias powers of 44, 22, and 11 W were tested for appropriate etch control. Our goal was to maintain the integrity of the delicate metallized fibers at the bottom of an etch site and to leave little to no etch residue. Figure 8(a) shows elongated, channel-like patterns of 13 μm pitch, etched in silicone encapsulation on top of 25 nm gold-coated fibers employing 44 W RF bias. Figure 8(b) depicts a SEM image of 25 nm gold-coated fibers exposed at a larger-sized site without damage or residue after etching at 11 W RF bias. Note that our SF6-O2 plasma etching process attacks and removes the top layer titanium of the metal-coated nanofibers, exposing the gold. We have found that lowering the RF bias to 11 W allows for etching a wider range of sizes of patterns. Larger RF bias, like the 44 W in this case, has the benefit of increased anisotropy in the etch profile but has the disadvantage of increased gold erosion. We have found that while the 200 nm gold-coated nanofiber mat was able to withstand and complete the 44 W biased etch process across all pattern sizes we tested between 40 000 and 50 μm2, the 25 nm gold-coated nanofiber mat was over-etched and destroyed at larger sized patterns while the small patterns still had not completed the etch. At 11 W RF bias, however, a wider range of pattern sizes could be completed without over-etching the 25 nm gold-coated nanofiber mat. We achieved the best resolution etch using 44 W bias on the 200 nm gold-coated nanofiber mat: openings in 5 μm silicone encapsulation as small as 6.8 μm [bottom diameter of the pattern in Figs. 8(c) and 8(d)]. Similarly high spatial resolution patterning of a thin silicone layer by ICP-RIE was previously demonstrated by Quirod-Solano et al.62 

FIG. 8.

SEM images taken at a 25° tilt angle and at 500 V acceleration voltage (a), (c), and (d) or at 2 kV acceleration voltage (b) show patterns in a 5 μm thick silicone layer. Openings to access the metal-coated nanofibers can be made of various shapes and residue-free. Before imaging, the ICP RIE hard mask has been removed. (c) and (d) are secondary electron images of the same site using the in-lens detector and the Everhart–Thornley detector, respectively.

FIG. 8.

SEM images taken at a 25° tilt angle and at 500 V acceleration voltage (a), (c), and (d) or at 2 kV acceleration voltage (b) show patterns in a 5 μm thick silicone layer. Openings to access the metal-coated nanofibers can be made of various shapes and residue-free. Before imaging, the ICP RIE hard mask has been removed. (c) and (d) are secondary electron images of the same site using the in-lens detector and the Everhart–Thornley detector, respectively.

Close modal

After repeated hard mask coating, photolithography, and RIE (using the same recipes as for the silicone encapsulation layer patterning), both silicone encapsulation and the nanocomposite were etched (in about 900 s total etch time) to form the outline of desired structures. The Cr layer underneath the nanofiber mat acts as an etch stop in this process, and in addition, it facilitates the release of the structures. Soaking the carrier wafer in a Cr etchant at room temperature for 2–3 days allows for release, floating the structures on the etchant. Upon their release, the structures were carefully fished out of the etchant using tweezers, and they were washed thoroughly in de-ionized water. Handling thin silicone film structures is a reportedly challenging task,63,64 but we have found that our structures handle well. Figure 9 shows approximately 15 μm thick structures (10 μm nanofiber-silicone composite and 5 μm silicone encapsulation).

FIG. 9.

Photographs of released structures: (a) microwiring isolation study pattern and (b) electrical conductivity measurement pattern to be further used for mechanical testing to check for functionality under strain.

FIG. 9.

Photographs of released structures: (a) microwiring isolation study pattern and (b) electrical conductivity measurement pattern to be further used for mechanical testing to check for functionality under strain.

Close modal

This article reports lithographically patterned stretchable microwiring on a nanofiber mesh substrate. The patterned fiber microwires show metallic-type conduction and a complex architected physical structure. A 25 nm gold coating showed a conductivity of 2.64 × 106 and 14.6 × 106 S/m for 200 nm gold coating. While thinner coatings are less conductive, they are expected to allow for more deformations. Coating the nanofiber mat with silicone dispersion creates a composite matrix and an encapsulation for microwiring. ICP-RIE patterning at a resolution below 10 μm is sufficient to match the resolution of the wire patterns. This architecture will find applications in areas for flexible and stretchable electronics like wearables, e-skins, and bioimplants.

We anticipate that the nanofiber mat thickness can be scaled and customized for desired mechanical performance. Photolithography resolution is not expected to be affected by the thickness of the nanofiber mat if mat quality is maintained. However, for thicker fiber mats, the outline etch of structures may get demanding in terms of time and steps involved. Future work will involve mechanical behavior investigations and studies on functionality retention under axial strains, bending, and torsion.

This work was supported by DARPA (Contract No. D17PC00125). The views, opinions, and findings contained are those of the author(s) and should not be construed as official DARPA position, policy, or decision unless designated by other documentation. The authors thank Ken Yoshida at the Purdue School of Engineering and Technology, Indiana University—Purdue University Indianapolis for insightful discussions over the use of nanofiber reinforcements in elastomers. The authors would also like to thank Kelsey Broderick, LUNA Inc. for her help in electrospinning the Nylon nanofiber mats. The authors extend their thanks to Aldo Garcia-Sandoval for reviewing the article and providing helpful comments and corrections. The authors also express their gratitude toward the Cleanroom Research Lab at The University of Texas at Dallas for providing microfabrication equipment and characterization support and the Advanced Polymer Research Lab at The University of Texas at Dallas for characterization tools and equipment.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

1.
C.
Wang
,
C.
Wang
,
Z.
Huang
, and
S.
Xu
,
Adv. Mater.
30
,
1801368
(
2018
).
2.
J. A.
Rogers
,
T.
Someya
, and
Y.
Huang
,
Science
327
,
1603
(
2010
).
3.
C.
Dagdeviren
 et al,
Nat. Mater.
14
,
728
(
2015
).
4.
W.
Yeo
 et al,
Adv. Mater.
25
,
2773
(
2013
).
5.
W.
Gao
 et al,
Nature
529
,
509
(
2016
).
6.
G.
Schwartz
,
B.
Tee
,
J.
Mei
,
A.
Appleton
,
D.
Kim
,
H.
Wang
, and
Z.
Bao
,
Nat. Commun.
4
,
1859
(
2013
).
7.
A.
Miyamoto
 et al,
Nat. Nanotechnol.
12
,
907
(
2017
).
8.
R.
Feiner
and
T.
Dvir
,
Nat. Rev. Mater.
3
,
17076
(
2017
).
9.
C.
Bettinger
,
M.
Ecker
,
T.
Kozai
,
G.
Malliaras
,
E.
Meng
, and
W.
Voit
,
MRS Bull.
45
,
655
(
2020
).
10.
J.
Rogers
,
M.
Lagally
, and
R.
Nuzzo
,
Nature
477
,
45
(
2011
).
11.
Y.
Poojari
,
Silicon
9
,
645
(
2017
).
12.
N.
Lu
,
C.
Lu
,
S.
Yang
, and
J.
Rogers
,
Adv. Func. Mater.
22
,
4044
(
2012
).
13.
B.
Ji
,
Z.
Xie
,
W.
Hong
,
C.
Jiang
,
Z.
Guo
,
L.
Wang
,
X.
Wang
,
B.
Yang
, and
J.
Liu
,
J. Materiomics
6
,
330
(
2020
).
14.
D.
Park
 et al,
Nat. Commun.
5
,
5258
(
2014
).
15.
T.
Matsuo
 et al,
Front. Syst. Neurosci.
5
,
34
(
2011
).
16.
J.
Kim
,
I.
Song
,
S.
Lee
,
H.
Choi
,
H.
Byeon
,
I.
Kim
, and
S.
Lee
,
IEEE Trans. Biomed. Eng.
60
,
3425
(
2013
).
17.
C.
Mills
,
J.
Escarŕe
,
E.
Engel
,
E.
Martinez
,
A.
Errachid
,
J.
Bertomeu
,
J.
Andreu
,
J.
Planell
, and
J.
Samitier
,
Nanotechnology
16
,
369
(
2005
).
18.
F.
Fan
,
Z.
Tian
, and
Z.
Wang
,
Nano Energy
1
,
328
(
2012
).
19.
L.
Kinner
,
M.
Bauch
,
R.
Wibowo
,
G.
Ligorio
,
E.
List-Kratochvil
, and
T.
Dimopoulos
,
Mater. Des.
168
,
107663
(
2019
).
20.
M.
Klatenbrunner
,
G.
kettlgruber
,
C.
Siket
,
R.
Schondiauer
, and
S.
Bauer
,
Adv. Mater.
22
,
2065
(
2010
).
21.
T.
Ware
,
D.
Simon
,
C.
Liu
,
T.
Musa
,
S.
Vasudevan
,
A.
Sloan
,
E.
Keefer
,
R.
Rennaker
, and
W.
Voit
,
J. Biomed. Mater. Res. Part B Appl. Biomater.
102
,
1
(
2014
).
22.
H.
Yuk
,
T.
Zhang
,
G.
Parada
,
X.
Liu
, and
X.
Zhao
,
Nat. Commun.
7
,
12028
(
2016
).
23.
H.
Yuk
,
S.
Lin
,
C.
Ma
,
M.
Takaffoli
,
N.
Fang
, and
X.
Zhao
,
Nat. Commun.
8
,
14230
(
2017
).
24.
C.
Kim
,
H.
Lee
,
K.
Oh
, and
J.
Sun
,
Science
353
,
682
(
2016
).
25.
N.
Bowden
,
S.
Brittain
,
A.
Evans
,
J.
Hutchinson
, and
G.
Whitesides
,
Nature
393
,
146
(
1998
).
26.
S.
Wang
,
J.
Song
,
D.
Kim
,
Y.
Huang
, and
J.
Rogers
,
Appl. Phys. Lett.
93
,
023126
(
2008
).
27.
D.
Kim
and
J.
Rogers
,
Adv. Mater.
20
,
4887
(
2008
).
28.
S.
Zhu
,
J.
So
,
R.
Mays
,
S.
Desai
,
W.
Barnes
,
B.
Pourdeyhimi
, and
M.
Dickey
,
Adv. Funct. Mater.
23
,
2308
(
2013
).
29.
T.
Widlund
,
S.
Yang
,
Y.
Hsu
, and
N.
Lu
,
Int. J. Solids Struct.
51
,
4026
(
2014
).
30.
M.
Rehman
and
J.
Rojas
,
Extreme Mech. Lett.
15
,
44
(
2017
).
31.
H.
Ko
 et al,
Small
5
,
2703
(
2009
).
32.
S.
Callens
and
A.
Zadpoor
,
Mater. Today
21
,
241
(
2018
).
33.
J.
Cho
,
M.
Keung
,
N.
Verellen
,
L.
Lagae
,
V.
Moshchalkov
,
P.
Van Dorpe
, and
D.
Gracias
,
Small
7
,
1943
(
2011
).
34.
R.
Kramer
,
C.
Majidi
, and
R. J.
Wood
,
Adv. Funct. Mater.
23
,
5292
(
2013
).
35.
W.
Shan
,
T.
Lu
, and
C.
Majidi
,
Smart Mater. Struct.
22
,
085005
(
2013
).
36.
J.
Oh
,
S.
Kim
,
H.
Baik
, and
U.
Jeong
,
Adv. Mater.
28
,
4455
(
2016
).
37.
S.
Hong
,
Y.
Lee
,
H.
Park
,
S.
Jin
,
Y.
Jeong
,
J.
Yun
,
I.
You
,
G.
Zi
, and
J.
Ha
,
Adv. Mater.
28
,
930
(
2016
).
38.
G.
Hao
,
F.
Hippauf
,
M.
Oschatz
,
F.
Wisser
,
A.
Leifert
,
W.
Nickel
,
N.
Mohamed-Noriega
,
Z.
Zheng
, and
S.
Kaskel
,
ACS Nano
8
,
7138
(
2014
).
39.
J.
Lee
,
H.
Kim
,
J.
Kim
,
I.
Kim
, and
S.
Lee
, in
Stretchable Bioelectronics for Medical Devices and Systems
, edited by
J. A.
Rogers
,
R.
Ghaffari
, and
D.
Kim
(
Springer International Publishing
,
Cham
,
2016
), pp.
227
254
.
40.
H.
Wu
 et al,
Nat. Nanotechnol.
8
,
421
(
2013
).
41.
S.
Lee
 et al,
Nat. Nanotechnol.
14
,
156
(
2019
).
42.
A.
Hanif
,
A.
Bag
,
A.
Zabeeb
,
D.
Moon
,
S.
Kumar
,
S.
Shrivastava
, and
N.
Lee
,
Adv. Funct. Mater.
30
,
2003540
(
2020
).
43.
A.
Hanif
,
Q.
Trung
,
S.
Siddiqui
,
P.
Toi
, and
N.
Lee
,
ACS Appl. Mater. Interfaces
10
,
27297
(
2018
).
44.
T.
Rashid
,
R.
Gorga
, and
W.
Krause
,
Adv. Eng. Mater.
23
,
2100153
(
2021
).
45.
Y.
Wang
,
T.
Yokota
, and
T.
Someya
,
NPG Asia Mater.
13
,
22
(
2021
).
46.
B.
Sun
,
Y.
Long
,
Z.
Chen
,
S.
Liu
,
H.
Zhang
,
J.
Zhang
, and
W.
Han
,
J. Mater. Chem. C
2
,
1209
(
2014
).
47.
Z.
Tai
,
X.
Yan
,
J.
Lang
, and
Q.
Xue
,
J. Power Sources
199
,
373
(
2012
).
48.
T.
Yan
,
Z.
Wang
,
Y.
Wang
, and
Z.
Pan
,
Mater. Des.
143
,
214
(
2018
).
49.
M.
Yu
,
Z.
Wang
,
Y.
Wang
,
Y.
Dong
, and
J.
Qiu
,
Adv. Energy Mater.
7
,
1700018
(
2017
).
50.
X.
Xiao
 et al,
Adv. Mater.
23
,
5440
(
2011
).
51.
P.
Hsu
,
H.
Wu
,
T.
Carney
,
M.
McDowell
,
Y.
Yang
,
E.
Garnett
,
M.
Li
,
L.
Hu
, and
Y.
Cui
,
ACS Nano
6
,
5150
(
2012
).
52.
H.
Li
,
W.
Pan
,
W.
Zhang
,
S.
Huang
, and
H.
Wu
,
Adv. Funct. Mater.
23
,
209
(
2013
).
53.
M.
Xue
,
Z.
Xie
,
L.
Zhang
,
X.
Ma
,
X.
Wu
,
Y.
Guo
,
W.
Song
,
Z.
Li
, and
T.
Cao
,
Nanoscale
3
,
2703
(
2011
).
54.
P.
Chavoshnejad
and
M.
Razavi
,
Sci. Rep.
10
,
7709
(
2020
).
55.
K.
Ralls
,
T.
Courtney
, and
J.
Wulff
,
Introduction to Materials Science and Engineering
(
John Wiley & Sons
, New York, Chichester, Brisbane, Toronto, Singapore,
1976
), p.
501
.
56.
K.
Fuchs
,
Math. Proc. Camb. Philos. Soc.
34(1), 100–108 (
1938
).
57.
F.
Lacy
,
Nanoscale Res. Lett.
6
,
636
(
2011
).
58.
T.
Gilani
and
D.
Rabchuk
,
Can. J. Phys.
96
,
272
(
2017
).
59.
S.
Hill
,
W.
Qian
,
W.
Chen
, and
J.
Fu
,
Biomicrofluidics
10
,
054114
(
2016
).
60.
M.
Vlachopoulou
,
G.
Kokkoris
,
C.
Cardinaud
,
E.
Gogolides
, and
A.
Tserepi
,
Plasma Process. Polym.
10
,
29
(
2013
).
61.
D.
Szmigiel
,
C.
Hibert
,
A.
Bertsch
,
E.
Pamula
,
K.
Domanski
,
P.
Grabiec
,
P.
Prokaryn
,
A.
Scislowska-Czarnecka
, and
B.
Plytycz
,
Plasma Process. Polym.
5
,
246
(
2008
).
62.
W.
Quiros-Solano
 et al,
Sci. Rep.
8
,
13524
(
2018
).
63.
T.
Maleki
,
G.
Chitnis
,
A.
Kim
, and
B.
Ziaie
,
J. Microelectromech. Syst.
21
, 859 (
2012
).
64.
Y.
Liu
,
Q.
Liu
,
Y.
Li
,
P.
Huang
,
J.
Yao
,
N.
Hu
, and
S.
Fu
,
App. Mater. Interfaces
12
,
30871
(
2020
).