Three-Dimensional Stretchable and Transparent Conductors with

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Three-Dimensional Stretchable and Transparent Conductors with Controllable Strain-Distribution Based on Template-Assisted Transfer Printing Wanli Li,*,†,‡ Yang Yang,‡,§ Bowen Zhang,†,‡ Lingying Li,†,‡ Guiming Liu,‡,∥ Cai-Fu Li,*,‡ Jinting Jiu,*,‡ and Katsuaki Suganuma‡

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/03/19. For personal use only.

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Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan ‡ The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan § Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States ∥ State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China S Supporting Information *

ABSTRACT: Although stretchable transparent conductors, stemmed from the strategies of both conductive composite and structural design of nonstretchable conductors, have been extensively studied, these conductors either suffer from low stretchability or require a complex fabrication process, which drastically limits their practical applications. Here, we propose a novel strategy combining the design of substrates and a simple template-assisted transfer printing process to fabricate threedimensional (3D) transparent conductors. The strategy not only eliminates the complex and costly fabrication processes but it also endows conductors with high stretchability and long-term stability, thanks to the controllable strain distribution as well as the seamless connection between the conductor layer and the substrate. These newly designed 3D conductors achieve a low sheet resistance of 1.0 Ω/sq with a high transmittance of above 85% and remain stable without obvious resistance change during 1000 stretching-relaxation cycles until 60% strain, which are superior to most reported conductors. A large-area stretchable heater based on the 3D conductor realizes the temperature fluctuation below 10% even under a large strain, thus showing huge application prospects in the field of wearable healthcare electronics. The simple solution-processed fabrication method and high performance such as stretchability and low resistance change over a large strain range promote the practical applications of these newly designed 3D conductors. KEYWORDS: 3D substrates, template-assisted transfer printing, solution-processed, stretchable transparent conductors, wearable electronics

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INTRODUCTION

under harsh conditions, such as bending, twisting, or even stretching deformation. Two strategies have been thoroughly explored and researched to address the above challenge.11 One is to develop novel conductors by embedding or surface-embedding

Stretchable electronic devices are defined as those devices that can maintain their functionality even under large mechanical deformations.1,2 It is expected that these devices will be widely used in next-generation smart electronic devices such as conformable displays,3,4 wearable solar cells,5 wearable sensors,6 and smart artificial skins.7−10 However, a critical challenge for the practical applications of these devices is how to keep their inner conductors highly conductive and stable © XXXX American Chemical Society

Received: October 24, 2018 Accepted: December 20, 2018 Published: December 20, 2018 A

DOI: 10.1021/acsami.8b18670 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. 3D stretchable conductors enabled by the simple template-assisted transfer printing process. (a,b) Concept of the 3D stretchable substrate with controllable strain distribution. (c−f) Template-assisted transfer printing process for the fabrication of 3D conductors. (g) Optical image of a large-area 3D conductor (13 cm × 13 cm × 0.024 cm), showing a mass production feasibility; the inset in the upper right corner is the cross-sectional view of the 3D conductor. (h) HR-SEM image showing the surface-embedded structure of AgNWs in the PDMS elastomer. (i) Typical stress−strain curves of the pure 3D substrates and 3D conductors.

flat surfaces, have been proposed to improve the stretchability of conductors.40−42 Compared with the modification of the conductor structure themselves, the design of stretchable substrates is potentially simpler, more cost-effective, and more promising for large-scale applications. For example, Zheng et al. utilized natural rose petals as templates to fabricate stretchable substrates with continuous 3D microscale craterlike topographies, in which the sharp ridges would prohibit the propagation of microcracks in the conductor layer;40 Lee et al. optimized 3D morphologies by using an artificial template and further improved the stretchability of conductors.41 However, high strain concentration occurring at the inflection points in these conductor layers still was not avoided, which heavily decreased the electrical stability, especially under a large strain. Moreover, the adhesion between the substrate and the conductor layer deposited by a thermal evaporator requires additional attention because of the vulnerable interface, which further deteriorates long-term stability (repeated stretchability). Nevertheless, this new design of the substrate provides an exciting approach toward enhanced development of highly stretchable and stable conductors. In this study, we design a 3D stretchable substrate with a netlike convex platform structure followed by a selective deposition of conductive materials through a template-assisted transfer printing process to obtain 3D stretchable transparent conductors. The convex platform structure effectively controls the strain distribution of substrate and minimizes the strain and especially strain concentration occurring in the upper conductor layer. The template-assisted transfer printing process realizes the fabrication and integration of a 3D substrate with surface embedded conductive materials in one step, and simultaneously improves the adhesion between the substrate and the conductive materials. The resulting 3D conductors possess a high conductivity (1.0 Ω/sq) with a high

conductive fillers such as metal particles,12,13 metal nanowires (NWs),14−17 metal nanotroughs,18,19 metal nanofibers,20 carbon nanotubes,21,22 and/or graphene23 in insulating elastomers. The conductive fillers embedded in the elastomer matrix are likely to self-adjust their positions or shapes for absorbing strain and retaining the electrical properties of the whole conductors. The second strategy is to design the geometry of conductors into wavy, 1,24 serpentine,25,26 mesh,27,28 or space network structures,29 which tend to accommodate in-plane strain by out-of-plane displacements or transform tensile deformations to in-plane rotations.30 This structural design enables nonstretchable materials to become stretchable while maintaining their electrical properties. However, some inevitable issues that exist in either of the abovementioned strategies jeopardize the practical applications of stretchable electronic devices. The conductors prepared using the first approach either have a very limited stretchability of below 30% strain (the resistance drastically increases to over 2 times once the strain reaches above 40%) or have a very low conductivity due to the large amount of insulating elastomers.31−37 On the other hand, the second approach often requires a cumbersome or high-cost fabrication process, such as prestretching or photolithography/etching,24,26,28,38 to realize the special geometry of each conductor. However, conductors fabricated with this process are unable to mitigate high strain concentration in their inflection points and junction points.38,39 These points are vulnerable to fractures under large strain and are the weakest areas in the conductor. When we focus only on the conductors, alternative approaches can be overlooked. As these conductors must be packaged on diverse substrates to create devices, to render conductors stretchable, the design of the substrate should also be considered. Very recently, stretchable substrates with threedimensional (3D) morphologies, instead of the conventional B


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Figure 2. Electrical properties and optical transmittance of the 3D conductors. (a) R/R0 of conductors with different H values under a strain of up to 100% in the first stretching. W of the convex platform is 300 μm. (b) Comparison between our 3D conductors and representative conductors reported in literature. (c) Strain distribution of the 3D conductor at 60% stretching in a single direction through computer simulation. (d) Effect of H on the strain distribution of the 3D conductors. (e) Transmittance of the 3D conductors in relation to the aspect ratio of netlike patterns. (f) Relationship between the usage amount of AgNW solution and achieved 3D conductors’ sheet resistance and transmittance.

the convex platform, a novel template-assisted transfer printing process has been developed. First, a netlike groove was manufactured in a Si wafer through an etching method and used as the template repeatedly for generating the convex platform (Figures 1c and S1). Next, a conductive material such as silver NW (AgNW) solution was microdropped into the groove, thus forming a conductive network after the evaporation of the solvent (Figure 1d, this step can be omitted if only 3D substrates are fabricated). After that, liquid polydimethylsiloxane (PDMS) was cast on the surface of the Si template and filled into the grooves, followed by curing at 70 °C for 100 min (Figure 1e). After it was peeled off from the Si template, a 3D substrate was obtained and the conductive AgNW network was successfully transferred only onto the surface of the platform, thanks to the precise quantitative printing of AgNWs and PDMS (Figure 1f). The obtained samples are called as 3D conductors which are bendable, foldable, and easily attached to the skin (Figure S2). Because the entire fabrication process is solution-processed at room temperature, it is readily adapted to large-area 3D conductors (13 cm × 13 cm) as shown in Figure 1g and promising for large-scale applications. The enlarged cross-sectional view in the upper right corner clearly shows the convex platform

transmittance (above 85%), and an especially high stability even at 80% strain, providing a high-performance and reliable platform for various wearable electronics.

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RESULTS AND DISCUSSION When a homogeneous object is stretched, the thicker portion always experiences a smaller stress/deformation than other regions. As shown in Figure 1a, section A and section C of the substrate have an equal area which is smaller than that of section B of the convex platform. When a stretching force is applied to this substrate, the stress generated in section A and section C is the same; yet, it is larger than that occurring in section B based on the mechanics of materials.43 The larger the amount of local stress, the greater the amount of local deformation and strain will occur (Figure 1b). Therefore, in the 3D substrate, the surface of the convex platform will always have a much smaller strain compared with other regions. In other words, if the conductive materials are printed only on the surface of the convex platform, its deformation can be greatly minimized, thus enhancing the electrical stability even under a large stress. For fabricating these types of 3D substrates and realizing accurate deposition of conductive materials on the surface of C


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Figure 3. Long-term electrical stability of the 3D conductors. (a) Evolution of the R/R0 of conductors with different height ratios during the cyclic test at 40% strain. (b) Enlarged 1st, 250th, 500th, 750th, and 1000th cycles in (a). (c) Evolution of the R/R0 of conductors with H = 1.00 during the cyclic tests at 20, 40, 60, and 80% strain, respectively. (d) Evolution of the R/R0 of conductors with H = 1.00 under different constantly applied strains over 300 min. (e) Evolution of the R/R0 of conductors with H = 1.00 under various cyclic mechanical deformations. The insets show the 3D conductor under outer bending, inner bending, and folding. The bending radius is 4 mm.

substrate (hb). It is found that increasing the H improves the resistance stability of conductors drastically. For example, with the increase of H from 0.00 (traditional 2D conductors) to 1.00 (3D conductors), the R/R0 decreases from 1.90 to 1.05 at 30% and from 18.0 to 1.50 at 70% strain, respectively. Even at 100% strain, the R/R0 is below 2.50, which is better than most reported conductors. With further increase of H to 2.00, the conductors show a similar resistance stability to those with H = 1.00. However, when the tensile strain increases to above 80%, the 3D conductors with H = 2.00 have a rapid increase in R/R0 and break first. It is likely that a large strain occurring at the base substrate or a large strain concentration occurring at the inflection points results in the crack and, therefore, the fracture of 3D conductors. Therefore, a suitable H is important for achieving high stability. The usage amount of AgNW solution also affects the R/R0 values (Figure S5). It is found that a high amount of AgNW solution value above 1.5 mL is enough to create a strongly conductive network in the fabricated 3D conductors. Moreover, changing the line width (W) of the convex platform from 350 to 200 μm has little effect on the resistance stability (Figure S6), which suggests that various patterns can be designed. Compared with reported stretchable

structure. The height of the platform is easily controlled by the corresponding depth of the groove in Si templates. Highresolution scanning electron microscopy (HR-SEM) observation indicates that the AgNWs are successfully printed into the surface layer of the convex platform without contamination of the base substrate (Figure S3). Also, it is noteworthy that the majority of the AgNWs are surface-embedded in the PDMS elastomer, having a seamless connection with the substrate (Figure 1h). The mechanical property of the 3D conductor is similar to that of the stretchable 3D substrate as illustrated in Figure 1i, indicating that the integrated conductive material (AgNWs) has not deteriorated the rheological characteristic of the substrate, which is important for stretchable/wearable electronics.44 To evaluate the performance of the 3D stretchable conductors, they were stretched using a tensile testing machine (Figure S4) while the electrical resistance was in situ measured. Figure 2a shows the changes of relative resistance (R/R0, R0 and R are the resistance of conductors before and during testing) of the 3D conductors with different height ratios (H) under various strains. The H is defined as the ratio of the height of the convex platform (hc) to the thickness of the base D


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Figure 4. Joule heating performance of stretchable heaters fabricated from the 3D conductors. (a) Surface temperature profiles of the heater under different applied voltages. (b) Surface temperature profiles of the heaters with different sheet resistances. (c) Thermal stability of the heater by switching the voltage between 0 and 5 V every 30 s. (d) Thermal stability of the heater during the cyclic test under different strains with an applied voltage of 4 V. The transmittance and the sheet resistance of the heater are 72% and 6.2 Ω/sq, respectively. (e) Application of the large-area stretchable heater on the knee under different bending angles from 0° to 135° with an applied voltage of 3 V.

conductor layer from failure. In addition, the netlike pattern design accommodates the tensile strain through the in-plane rotation, decreasing local strain occurring in the conductor layer.30 Figure 2d shows the quantitative comparison of the average equivalent elastic strain, εave, of area A and area B as a function of H. With the increase of H from 0.00 to 1.00, the εave of area B maintains a high value of 50% while that of area A has a rapid decrease from 50% to only 10%. This confirms that the convex platform effectively mitigates the accumulation of the strain, and therefore improves the resistance stability of the conductor (Figure 2a). On the other hand, the tunable parameters of the convex platform give the 3D conductor great potential to be used as high-transmittance, stretchable conductors. Figure 2e depicts the relationship between the transmittance and the aspect ratio of netlike patterns (inset defines the aspect ratio). By increasing the aspect ratio from 4.0 to 14.0, the transmittance of the 3D conductors can be adjusted from 60 to 85% (with the PDMS substrate), which meets the requirements of most transparent electronic devices. In addition, the groove structure in the template helps to assemble AgNWs efficiently, which is beneficial for controlling conductivity. Our highly customizable 3D conductors can show various sheet resistances from 160 to 1.0 Ω/sq without heavily sacrificing the transmittance by increasing the AgNW content (Figure 2f). Normally, in order

conductors except the CuNW mesh (Figure 2b),14−16,21,28,29,34,40,41 the newly designed 3D conductors have the best performance with a slower increase in resistance as well as a wider strain-enduring range from 0 to 100%, showing a huge potential for applications in next-generation stretchable electronics. To further understand the results, we performed a finite element simulation of the deformation in the 3D conductor at 60% strain. During the simulation, a homogeneous-type geometry was utilized because the thickness of the conductive part embedded in the surface layer of the convex platform (from 2 to 5 μm, Figure S7) is negligible compared with the thickness of the PDMS substrate (240 μm, Figure S7). From the 3D mapping of the equivalent elastic strain (Figure 2c), it is evident that the convex platform (area A) experiences a much smaller strain than the base substrate (area B). Also, there are no obvious strain concentration sites appearing on the surface of the convex platform. The reason for this is that during the stretching, the base substrate with a smaller effective sectional area tends to undergo most of the applied strain while the convex platform with a larger effective sectional area largely disperses the stress (Figure S8). The deformation in convex platform from the bottle part to the top part shows a declining trend, which relieves the strain concentration occurring at the inflection points in the conductor layer and protects the E


Research Article


(V), which agrees with the Joule heat theory, that is, P = V2/ R.46 Here, P is the power supplied to the heaters and R is the total resistance of the heater. The Joule heating performance can be adjusted by increasing the voltage or decreasing the resistance. For example, the steady-state temperature is about 48 °C under an applied voltage of 2 V while it increases to 145 °C with an applied voltage of 6 V. On the other hand, a very high temperature of 240 °C can be easily achieved under the same voltage as the Rs decreases from 6.7 to 2.4 Ω/sq (Figure 4b). The thermal stability of the heater was evaluated by repeatedly switching on/off the applied voltage of 5 V every 30 s. The surface temperature variation is in the range of 25−118 °C and keeps stable even after 40 cycles (Figure 4c). It should be noted that although AgNWs themselves are very sensitive to the Joule heat,47 they become very stable due to the embedded structure in the PDMS substrate. Figure 4d shows the surface temperature variation of the heater during the cyclic tests at 20, 40, and 60% strain. As seen, the temperature of 104 °C remains almost unchanged during the stretching−releasing cycle at 20% strain. A very small decrease from 104 to 99 °C is seen at the 40% strain-stretching state, however, it can be rapidly recovered when the strain is released. Further increasing the strain to 60%, the temperature fluctuation still remains below 10% with excellent recovery property, which is very important for wearable thermal therapy devices with a stable heat supply. The insets in the Figure 4d show the corresponding temperature distribution images before stretching, after stretching at 60% strain, and releasing, respectively. Moreover, a large-area stretchable heater was attached on a volunteer’s knee (Figure 4e). Compared with the stretchable heater fabricated form pure metal mesh,48 it is found that the heater fabricated from the newly designed 3D conductor keeps more stable and the temperature distributions are also more uniform and invariable when the knee bends from 0° to 135°. This demonstrates that the heater could work as a wearable thermal therapy device and maintain stable heating performance even when the patient is exercising. To sum up, both the outstanding Joule heating performance and thermal stability of the heater in static and dynamic occasions promises great potential as wearable heaters.

to decrease the sheet resistance of transparent conductors, a high coverage of conductive material on the substrate is required.45 This inevitably damages the transmittance of conductors. However, in the 3D conductors, the conductor materials are accumulated in the vertical direction to decrease their resistance, which eliminates the transmittance loss of whole conductors. Therefore, highly stretchable conductors with a high conductivity of 1.0 Ω/sq and a high transmittance of 85% can easily be achieved. Besides the tensile test, the cyclic stretching-relaxation test was also conducted to evaluate the long-term stability of the developed stretchable conductors. Figure 3a shows the change to the R/R0 of the 3D conductors with different H values during the cyclic test at 40% strain. It can be seen that the increase in R/R0 of all 3D conductors (H > 0.00) is much smaller than the increase in 2D conductors (H = 0.00), and the higher the H, the more stable the conductor. For example, after 1000 cycles, R/R0 increases to 38.0 for H = 0.00 while they increase to only 1.20 and 1.08 for H = 0.50 and 1.00, respectively. Besides, it should be noted that the R/R0 fluctuation of the 2D conductor (H = 0.00) in each cycle increases rapidly as the number of cycles increases, whereas that of the 3D conductor hardly changes, especially with H = 1.00 (Figure 3b). This further confirms that the newly designed 3D conductors possess a higher stability than traditional 2D conductors. Moreover, the changes of R/R0 for 3D conductors with H = 1.00 under different strain cyclings are shown in Figure 3c. It is found that when the strain is below 40%, R/R0 remains almost unchanged after 1000 cycles. At 60% strain, the R/R0 shows a slight increase with a weak fluctuation. When the strain is 80%, an increased R/R0 fluctuation was observed, however the increase is still below 3-fold after testing. Compared with those conductor layers formed on the 3D petal or mogul-patterned substrates,40,41 the R/R0 in this work is much smaller under the same strain. The improved electrical stability is attributed to the controllable strain distribution on the conductor layer as well as the seamless connection between the conductor layer and the substrate. The operational stability of the conductors under deformation conditions is also necessary. Although the increased R/R0 values are different under various strains, it remains stable for more than 300 min without any vibration even at 80% strain (Figure 3d). This implies that these 3D conductors can withstand not only sudden strain conditions but also long-term strain conditions. In addition, a comprehensive cyclic test, including outer bending, inner bending, and folding, was also conducted to further confirm the flexibility and stretchability of the 3D conductors (Figure 3e). The R/R0 experiences a negligible fluctuation even after 3000 cycles with concurrent outer bending, inner bending, and folding, exhibiting excellent stability. With their clearly superior electrical performance and their mechanical endurance, these 3D conductors are most promising for use in next-generation stretchable electronic devices. As a proof-of-concept application of these highly stretchable and stable 3D conductors, a stretchable heater for thermal therapy was fabricated. Figure 4a shows the temperature profiles of the heater under different applied voltages. The temperature immediately increases once the voltage is applied and then rapidly reaches a steady-state temperature. It needs only below 20 s, indicating its rapid responsiveness. The steady-state temperature increases with the elevated voltage

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CONCLUSIONS In summary, we report a novel 3D transparent conductor which avoids a complicated and costly fabrication process while possessing high stretchability and long-term stability. Two key points are especially worthy of attention: (1) the design of 3D substrates with a netlike convex platform controls the strain distribution and minimizes the strain and strain concentration occurring in the conductor, (2) the developed template-assisted transfer printing process not only enables precise integrated fabrication of the 3D substrate and conductor layer deposition, but also ensures good adhesion between the substrate and the conductor layer. These 3D conductors remain stable without resistance change during the cyclic test at strains below 60%, and show only small increases in relative resistance even at 80% strain. In addition, the 3D conductors with customizable transmittance and sheet resistance can be readily designed by adjusting the parameters of the netlike convex platform along with the amount of printed conductive material. We believe that this type of stretchable transparent conductor will be extremely useful for next-generation stretchable electronics such as wearable F


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were recorded by using an IR camera (R500EX, Nippon Avionics , sensitivity: 0.025 °C).

heaters, stretchable displays/solar cells, robotic skins, and so on.

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EXPERIMENTAL SECTION

ASSOCIATED CONTENT

S Supporting Information *

Fabrication of the Si Template. The Si template was fabricated by the photolithography and etching method. First, SU-8 photoresist (MicroChem Corp.) was spin-coated on a Si wafer followed by a prebake at 65 °C for 25 min and a soft-bake at 95 °C for 50 min. Then, it was exposed to UV light (λ = 365 nm) by using a maskless exposure equipment (PLS-1000, PMT Corp., Fukuoka, Japan). After exposure, a postbake was performed to selectively cross-link the exposed portions of the SU-8 photoresist. And then, the SU-8 developer (MicroChem Corp.) was used to dissolve away the SU-8 photoresist that was not exposed to light, leaving behind the designed pattern. Finally, the bare portions of the Si wafer were etched to form a netlike groove by using a reactive-ion etching setup (RIE 10NR, SAMCO Inc.). The etching condition was as follows: gas flow rate: SF6 40 sccm, CHF3 10 sccm, O2 14 sccm; pressure: 4 Pa; radio frequency power: 60 W. The etching time depended on the required depth of the netlike groove (Figure S9). In the case of 120 μm in depth, etching time was about 8 h. The SU-8 photoresist residue was removed by O2 plasma. Fabrication of 3D Stretchable Conductors. AgNWs were used as the conductive materials in 3D stretchable conductors, which were synthesized by using a previously reported one-step polyol method. For the fabrication of the 3D stretchable conductor, a dispersion of AgNWs in ethanol (∼0.5 wt %, Figure S10) was first microdropped into the groove of the Si template by using a syringe. The viscosity of AgNW solution at 25 °C is similar to ethanol, about 1.08 Pa·s. After drying for a few seconds in air, the liquid PDMS, prepared by mixing the base and curing agent (SYLGARD 184, Dow Corning) with a ratio of 10:1, was cast on the surface of the Si template and filled in the inside groove, followed by a low temperature curing at 70 °C for 100 min. The thickness of the casted PDMS substrate controlled by tape mask is fixed at 200 μm and it will decrease to about 120 μm after curing. Finally, the 3D stretchable conductor was obtained by peeling off the cured PDMS film from the Si template. The height of the convex platform on the 3D conductor is determined by the corresponding depth of the groove in the Si template. In this study, Si templates with the netlike groove of 30, 60, 90, and 120 μm in depth are used to fabricate 3D conductors with the convex platform of 30, 60, 90, and 120 μm in height, respectively. Characterization of the 3D Stretchable Conductor. The microstructures of the 3D conductor were observed using a field emission scanning electron microscope (Hitachi SU8020). The resistance (R0) of the 3D conductors was measured using a fourpoint probe and the corresponding sheet resistance (Rs) was calculated by the equation: Rs = R0 × W/L, where W and L are the width and length of the 3D stretchable conductors. To evaluate the stretchability of the 3D conductors, tensile strain (1 mm/min), cyclic tensile strain (40 mm/min), and creep strain tests were carried out on a desk-top universal testing machine (EZ Test, Shimadzu). The width and length of the test samples are 20 and 30 mm, respectively. The change of electrical resistance of the samples during the tensile and cyclic tensile tests was recorded with the digital multimeter (Keithley 2110 5 1/2). Finite element analysis for simulating the strain distributions in the 3D conductor under tensile conditions was carried out by using the software ANSYS 14.0. The optical transmittance spectra of 3D conductors over the wavelength range of 300−800 nm were measured by a UV−visible near infrared (IR) spectrophotometer (V670, JASCO Corp.) using air as a reference. Characterization of the Stretchable Heater. The 3D conductors were used as stretchable heaters without further posttreatment. To evaluate the thermal stability both in static and dynamic occasions, two ends of the stretchable heaters were fixed tightly on a desk-top universal testing machine, and two electrically conductive clips were connected to the stretchable heaters to supply a constant voltage. The temperature evolutions on the surface of the heater under different applied voltages and different strain conditions

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b18670. Si template with netlike groove; photographs of 3D conductors; surface morphology of the 3D conductor; optical photographs of the 3D conductor under strain of 0, 50, 80, and 100%,; R/R0 of 3D conductors with different usage amount of AgNW solution; effect of the line width (W) of the convex platform on the change of relative resistance of 3D conductors; thickness of the conductive part embedded in the surface layer of convex platform; strain distribution in the cross-section of convex platform; and relationship between etching time and the achieved depth of netlike groove; AgNW solution (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.L.). *E-mail: [email protected] (C.-F.L.). *E-mail: [email protected] (J.J.). ORCID

Wanli Li: 0000-0003-0271-5782 Yang Yang: 0000-0003-0298-9118 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from Huawei Innovation Research Program and “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The authors are thankful to Shouichi Sakakihara in the Nanofabrication Shop, ISIR, Osaka University for assistance with the fabrication of Si templates.

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REFERENCES

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