Three-Dimensional Continuous Conductive Nanostructure for Highly

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Three-dimensionally Continuous Conductive Nanostructure for Highly Sensitive and Stretchable Strain Sensor Donghwi Cho, Junyong Park, Jin Kim, Taehoon Kim, Jungmo Kim, Inkyu Park, and Seokwoo Jeon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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ACS Applied Materials & Interfaces

Three-dimensionally Continuous Conductive Nanostructure for Highly Sensitive and Stretchable Strain Sensor Donghwi Cho, † Junyong Park, † Jin Kim, † Taehoon Kim, † Jungmo Kim, † Inkyu Park, ‡ and Seokwoo Jeon*,† †

Department of Materials Science and Engineering, KAIST Institute for The Nanocentury,

Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡

Department of Mechanical Engineering, KAIST Institute for the Nanocentury, Mobile

Sensor and IT Convergence (MOSAIC) Center, KAIST, Daejeon 34141, Republic of Korea *

Correspondence should be addressed to S.J. (email: [email protected])

Keywords: three-dimensional nanopatterning, strain sensor, stretchable electrode, carbon nanotube, 3D nanostructure

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ABSTRACT: The demand for wearable strain gauges that can detect dynamic human motions is growing in the area of healthcare technology. However, the realization of efficient sensing materials for effective detection of human motions in daily life is technically challenging due to absence of the optimally designed electrode. Here, we propose a novel concept for overcoming the intrinsic limits of conventional strain sensors based on planar electrodes by developing highly periodic and three-dimensionally (3D) bicontinuous nanoporous electrodes. We create a 3D bicontinuous nanoporous electrode by constructing conductive percolation networks along the surface of porous 3D nanostructured poly(dimethylsiloxane) with single-walled carbon nanotubes. The 3D structural platform allows fabrication of a strain sensor with robust properties such as gauge factor of up to 134 at tensile strain of 40%, widened detection range of up to 160%, and cyclic property of over 1,000 cycles. Collectively, this study provides new design opportunities for a highly efficient sensing system that finely captures human motions, including phonations and joint movements.


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INTRODUCTION Real-time and sensitive detection of human motion (i.e., phonation and joint movement) is of great importance to ubiquitous healthcare technology. In the past decade, there have been numerous efforts to develop body-attachable human-machine interfaces based on flexible and stretchable conductors.1-7 Among those, a resistance-mode wearable strain gauge has been showing great potential for commercialization due to the reliability and simplicity of the product.8-13 In order to achieve the high quality resistance-mode strain gauge, three major directions of technical development should be satisfied: 1) improving sensitivity (i.e., gauge factor over 200%), 2) widening the working strain range, and 3) retaining intrinsic piezoresistive properties without severe performance degradation after repetitive usage. Several approaches have been suggested to fabricate strain gauges by utilizing electrically conductive polymer composites14-18 due to their exceptional response to external stimuli such as stretching, bending, and twisting, as well as outstanding processability. Among them, application of carbon nanomaterials on a flexible planar substrate for conductive bilayer polymer thin film has shown great potential because of robust mechanical and electrical properties.19 In particular, carbon nanotubes (CNTs), which possess a one-dimensional morphology with high aspect ratio, are especially promising for a highly stretchable strainsensing system with enhanced working range due to the formation of a highly interconnected percolation network. For example, Lipomi et. al. investigated the effects of applied strain on spray-coated CNT film on poly(dimethylsiloxane) (PDMS) substrates.16 Since the CNTs are mechanically compliant to severe deformations of the planar substrate, the fabricated sensor showed electrical stability with maximum stretchability of 50% in its working range. However, the sensitivity of such bilayer sensors is often insufficient in terms of the stretchability in their working range for practical usage because of the excessively interconnected percolation network which can diminish sensitivity even under a large strain.



In addition, they generally suffer from electrical percolation network breakdown under repetitive usage and severe deformations, making them unsuitable for wearable electronic applications.20-22 Therefore, simultaneously achieving high sensitivity and cyclability coupled with high stretchability remains a challenge when fabricating a strain gauge. As an alternative, template-assisted assembly,23 preconstruction,24-26 and self-assembly of percolation networks27,28 have been developed to overcome the forementioned technical challenge by constructing a continuous percolation network in a three-dimensional (3D) structured substrate. Since the pre-structured substrate provides capability of efficient construction of percolation network even with low carbon filler contents,28 the less densely percolated network could easily respond to applied strain, leading to enhanced sensitivity. Furthermore, 3D structured substrate, especially with high periodicity, has been proposed to enhance the mechanical properties beyond the intrinsic properties of the 2D bulk substrate. For example, the 3D net-shaped structure can significantly enhance not only the stretchability and fracture strain by ~ 62% and ~ 225%, respectively, of the structured polymeric substrate, but also the cyclability compared to the bulk material.29 Therefore, the introduction of the uniform and periodic 3D structure in the electrode system could be a promising and effective route to simultaneously achieve high properties of the sensory device, including sensitivity, stretchability, and cyclability. Herein, we demonstrate a highly sensitive and stretchable strain gauge consisting of a 3D continuous percolation network of single-walled carbon nanotubes (SWCNTs) formed along a 3D nanostructured porous elastomer PDMS. The uniformly interconnected 3D nanostructure can greatly improve the stretchability (>200%) of the sensor. Moreover, it can provide a selective response to tensile strain on a non-planar surface where the bending and tensile strain co-exist by 3D rotation and elongation of the elastic bridge elements in the 3D PDMS under bending to accommodate applied strain.29 In addition, the structure enables


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efficient electrical percolation of conductive networks even at an extremely low concentration of SWCNTs (0.9 vol%), leading to dramatically increased sensitivity. Based on the geometric advances of the 3D bicontinuous electrode system, we achieve a highest gauge factor of 134 at the tensile strain of 40% and a wide working range of over 160% with good cyclic property (>1,000 cycles). To the best of our knowledge, these performances greatly exceed those of other recently reported stretchable, CNT-based resistive strain sensors.4,27,3035

EXPERIMENTAL SECTION Preparation of a photoresist-coated substrate. A thin layer (~100 nm) of photoresist (NR760p, Futurrex) was firstly spincoated on a glass substrate as a releasing layer. After hardbaking of the sample on a hot plate at 180 C ̊ for 3 min, a relatively thick layer (~10 µm) of photoresist (NR5, Futurrex) was spincoated on the releasing layer. To suppress the bubble generation in the coated photoresist, the three-step softbaking was carefully conducted on a hotplate at 55 ̊C for 10 min, 90 ̊C for 8 min, and 130 ̊C for 2 min.

Fabrication of a 3D nanostructured template. A conformal phase mask consisting of square arrays of hole patterns with a diameter of ~480 nm, a depth of ~400 nm, and a periodicity of ~600 nm was replicated from a Si master.36,37 After conformal contact the mask on the photoresist-coated substrate, an expanded (~1 in) and collimated laser source (~355 nm) exposed to the mask, producing 3D interference in the photoresist film.38 An exposure dose was optimized at ~150 mJ/cm2. Then, the postbaking was conducted on a hotplate at 60 ̊C for 9 min. A non-crosslinked region in the photoresist was gently removed by developer (RD 6, Futurrex). Finally, the sample was rinsed by DI water several times.



Fabrication of a 3D PDMS film. A poly(dimethylsiloxane) (PDMS) (Sylgard 184, Dow Chemical) prepolymer was prepared by mixing a monomer and a crosslinking agent with a weight ratio of 10 to 1. Then, the PDMS prepolymer was poured onto the 3D nanostructured template. A thickness of an overlayer on the 3D nanostructured part was precisely controlled by varying a spin-speed from 1,000 to 2,500 rpm. To guarantee perfect filling of the PDMS prepolymer into interstitial pores in the 3D nanostructured template, the sample was degassed in a desiccator for 2 hrs. After curing the PDMS prepolymer in an oven, the 3D nanostructured template and the releasing layer were simultaneously removed by a DMSObased solution (RR 41, Futurrex) at 50 ̊C for 3 min. A free-standing 3D PDMS film with the inverse 3D nanostructure of the original template spontaneously floated on a remover bath, and could be handled with an aid of a supporting PDMS block.29 Finally, the sample was rinsed by DI water several times.

Fabrication of a conductive 3D PDMS film. The 3D PDMS film exposed to the UV/Ozone for ~7 min to make a hydrophilic surface. The bottom and top surfaces of the 3D PDMS were sealed by thick PDMS blocks. Then, a small drop of SWCNT-dispersed water (0.1 wt%) (KH WS, KH Chemicals) was dropped at an open side of the sandwiched 3D PDMS film and spontaneously infused into a porous network by a capillary force within a minute. The infused water in the 3D PDMS film was slowly dried at room temperature for 2 hrs, yielding a conductive 3D PDMS film with conformally coated SWCNTs. An infiltration step was repeated several times to control the electrical and mechanical properties.

Detection of resistance change under strain. The Cu strips with adhesives (1181-5, 3M) were attached to both ends of the conductive 3D PDMS film and were connected to a resistance meter (VersaSTAT, Princeton Applied Research). A gauge length between Cu


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strips was ~13 mm. The sample was then fixed at a homemade stretching/bending tester. A resistance of the sample was measured under the repeated stretching/releasing in real time. To evaluate an effect of the 3D nanostructure on bending sensitivity, the samples with different thicknesses (60 µm, 90 µm, 160 µm, 1 mm, and 15 mm) were prepared by controlling the thickness of the overlayer on the 3D nanostructure. To detect practical human motions by resistance change, the sample was attached to a neck, a joint of a finger, a wrist, and a knee with an aid of an adhesive tape.

Other characterizations. The structural details of the 3D nanostructured template and PDMS were visualized by field-emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi), operated at an accelerating voltage of 5 to 10 kV. The optical properties such as absorbance and transmittance were measured by a UV-VIS Spectrometer (UV-310PC, Shimadzu). The wetting energy was evaluated by a contact angle analyzer (Phoenix 300, SEO). The stress-strain curves were measured by a micro-tensile tester (INSTRON 8848). Over 10 samples for each infiltration cycle of SWCNTs into the 3D PDMS were used to obtain reliable stress-strain curves.

RESULTS AND DISCUSSION Concept and Strategy for Realizing 3D Continuous Conductive Nanostructure. Figure 1 schematically illustrates the overall concept of this study. The process begins with the fabrication of a 3D nanostructured PDMS, which acts as a scaffold for the construction of the 3D continuous percolation network. The 3D PDMS is inversely replicated from a porous template produced by proximity-field nanopatterning (PnP).29,36-38 The transparent (transmittance 70% at patterning wavelength 355 nm) and thick, negative-tone photoresist, NR5, composed of phenolic components, is newly employed to fabricate the easily



removable polymer template with well-defined 3D nanostructures (Figure S1 and Figure S2). The NR5 is more advantageous in replicating such 3D nanostructure due to it having good structural stability and processibilty,39 both of which are common problems of the conventional photoresists such as AZ9260, 29 and SU-8.40-42 Without harsh and complicated removal process, NR5 can be easily removed by a single drop of a mild organic solvent such as acetone, ethanol, and dimethyl sulfoxide (DMSO). Among those solvents, a DMSO-based remover is chosen for the 3D template removal after infiltration of PDMS because DMSO has the lowest swelling ratio of PDMS (D/D0 = 1.00, where D is the length of PDMS in the solvent and D0 is the length of the dry PDMS).43 The successfully fabricated 3D PDMS (Figure S3) shows little structural degradation, with periodically arranged and interconnected small pores (~200 nm) in 3D axes over a large-area (~1 inch).29 The change in optical transmittance at visible light from the intrinsic transparency (~ 90%) of the bulk PDMS to opaqueness (~ 0%) of the 3D PDMS proves the existence of 3D nanostructures which act as scattering sites (Figure S4). Considering that 50% of optical transmittance change could be resulted from surface scattering, such as from wrinkles,44 pillars,45 and cracks,46 the significant drop of transmittance evidently proves the porous 3D nanostructure existing inside the PDMS. Moreover, the uniformly ordered net-shaped 3D PDMS contributes to an extraordinary enhancement of stretchability up to 220% which surpasses that of the intrinsic limits of the bulk counterpart (Figure 1b). Subsequently, a 3D continuous percolation network composed of SWCNTs is formed along the 3D PDMS by infiltrating a uniformly dispersed aqueous SWCNT solution (Figure S5) with a low concentration of 0.1 wt% into the porous substrate by capillary action. The absolute amount of the conformally deposited SWCNTs in the porous substrate is varied to control the resistance and stretchability of the final product (Figure 1c). The capability of the 3D bicontinuous nanoporous electrode as a strain gauge is carefully evaluated by measuring the change in resistance (∆R/R0) during


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stretching over a wide range up to 160%. When a tensile strain is applied, since the percolation network forms along the surface of the 3D PDMS, the network follows any changes in the shape of the stretched 3D PDMS, resulting in outstanding stretchability without severe percolation breakdown during mechanical deformation (Figure S6). For further demonstration of practical applications of the strain gauge, the strain sensor is attached to various non-planar surfaces of the human body to measure delicate human motion (i.e., phonation and joint movement) (Figure 1d). Characterization of the 3D Continuous Conductive Nanostructure. Figure 2 shows that the highly stretchable 3D PDMS provides an open porous substrate for solid state percolation networks. In order to uniformly construct the percolation network on the surface of the 3D PDMS through infiltration of an SWCNT solution, the hydrophobic nature of the 3D PDMS should be converted to be hydrophilic (Figure 2a). It is well-known that using O2 plasma,47 and UV/Ozone48 can be appropriate approaches for surface modification of PDMS from hydrophobic to hydrophilic by creating a silica-like layer. The UV/Ozone treatment is more suitable for our system because it has been generally adopted for surface modification of structured polymers owing to its ability to cover not only the top surface, but also the inner complex surface of the 3D structure. The slightly increased surface roughness from the silicalike layer can provide a suitable site for coiling SWCNTs along the 3D nanostructure.49 When the solution is infiltrated without surface modification, SWCNTs agglomerate on top of the surface and clog the outer pores, severely restricting penetration of the SWCNT solution into the inner structure (Figure 2b). Moreover, the SWCNTs percolation network is formed in the shape of bridge, rather than conformally attaching to the surface of the substrate because of the poor wetting (Figure S7). On the other hand, after the surface modification, the SWCNT solution can be successfully infiltrated into the porous substrate by capillary action50 with increased wetting area, and form the percolation network on the



surface of the substrate (Figure 3). The open porous 3D nanostructure contributes to the effective construction of percolation networks even at an extremely low concentration of SWCNTs (0.9 vol%) while planar composite electrode systems normally require higher concentraion (3.6 vol%).28,32,51 Accordingly, the percolation network along the 3D PDMS is uniformly formed, and the amount of the SWCNTs can be simply controlled by varying the number of infiltrating cycles of the SWCNT solution. As the infiltration cycle increases from 3 to 5, the fraction of SWCNTs in the 3D PDMS also increases from 0.9 to 1.5 vol% (Figure 3a and Figure 3b). Thus, the electrical and mechanical properties of the 3D continuous conductive nanostructure can be controlled by varying the infiltrating cycles for individually designed strain sensor application. For example, the initial resistance of the composite can be controlled from 5*103 to 2.0*106 Ω·m by controlling the infiltration cycles (Figure 3c). In addition to the tunability of electrical conductance, the mechanical strength can also be modified by increasing the density of the percolation network (Figure 3d). The constructed percolation network composed of the SWCNTs which have a long length (< 50 µm) significantly restricts mechanical deformation of the substrate by coiling aroud the substrate and minimizing interfacial interaction between the substrate and the fillers.15 For example, the maximum stretchability of the 3D PDMS decreases from 220% to 194% after the UV/Ozone treatment because of the generated silica-like layer (elastic modulus = 1.5 GPa). As the density of the deposited SWCNT networks on the substrate increases, the conductive 3D nanostructure shows relatively decreased its stretching limit, but rather shows improved mechanical strength, stiffness, and toughness,52 as demonstrated in Figure 3d. According to the repeated measurement of stress-strain curves, the average and maximum stretching limits of conductive 3D PDMS with 5 infiltration cycles are 150% and 160%, respectively (Figure S8). Therefore, the 3D bicontinuous nanoporous electrodes can be individually designed to


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respond to different external stimuli such as stretching and bending for various applications with optimized strain sensing performance. 3D Bicontinuous Nanoporous Electrode for a Strain Sensor. Figure 4 summarizes the electrical properties of the 3D strain sensor, and Figure S9 and Figure S10 exhibit resistance change under applied tensile strain by connecting the sensor to LEDs. In order to successfully utilize the sensor for human motion detecting system which requires application on a nonplanar surface, the initial resistance of the sensor should be maintained whether in bent or flat state. We therefore measured the response to applied strain by varying bending radii with the strain sensor which has total thickness of 60 µm and the nanostructured part thickness of 10 µm. The inset of Figure 4a demonstrates how the measurement was done. The sensor performance is evaluated at bending radii of 3 cm, 10 mm, 1 mm, and 800 µm. 10% of tensile strain is repeatedly applied to the non-patterned area at the edge of the sensor. The relative change in current remains unchanged for all four bending radii, while change occurs only under tensile strain. The current is 8 µA with no applied tensile strain, while the current decreases to 2 µA with applied tensile strain of 10%. The relatively thin 3D strain sensor with a thickness of ~60 µm is insensitive to the bending radius below 1mm corresponding to the bending strain of 3.75%.53 However, thicker 3D strain sensors above ~90 µm prepared by increasing the overlayer on the 3D nanostructured part, show bending-sensitive characteristics due to the increased portion of the unstructured part (Figure S11). The response behaviors of the strain sensors with different infiltration cycles; from 3 - 5 times; until their maximum working range are demonstrated in Figure 4b. It is noted that the strain gauge factor ((∆R/R0)/ε, where ∆R is the resistance change while straining, R0 is the initial resistance, and ε is the applied strain) is employed to calculate the sensitivity of the strain sensors. The gauge factor of 3 times infiltrated composite is computed to be 134 with a maximum working range of 40% strain. As the number of infiltration cycles is increased to 4



and 5 times, the gauge factors of the sensors become 61 with a maximum working range of 80%, and 24 with a maximum working range of 160%, respectively. The 3 times infiltrated 3D strain sensor also features the maximum gauge factor of 23 even under a very small strain in the range of 0-1%, which is almost 11 times to that of the conventional metal gauge (2.0).54 However, the response and recovery time, repesenting the time intervals between 10% and 90% of the steady state values,55 are measured as 1.05 and 0.95 s, respectively (Figure S12). The resulting time delay could be explained by the bicontinuous structure in the sensor system (Figure S6). On the other hand, compared to other recently reported CNT-based elastomeric sensors,4,27,30-35 our sensors show significantly enhanced gauge factors in the strain range suitable for human motions in daily life,15,56 as illustrated in Figure 4c. The high sensitivity with the widened working strain range of the sensor enable the 3D strain sensor to monitor various human activities. The responses of fabricated strain sensors with different infiltration cycles commonly show non-linear behaviors to tensile strain; a relatively flat response to the applied strain, followed by a steep response. Since the effective elastic modulus of structural elements of the 3D PDMS is much smaller than that of the bulk PDMS under the same strain,29 the sensor shows a flat response to a small amount of tensile strain. However, under a larger tensile strain, which causes 3D rotation and elongation of the bridge elements in 3D PDMS, the contact junctions within the percolation network significantly decrease by creeping and disconnecting, leading to improved sensitivity with a steep response. On the contrary, when the applied strain is released, the 3D nanostructured substrate flexibly recovers to the original state without intense structural damage due to the robust recoverability of the 3D PDMS. Subsequently, the durability of the sensor is evaluated by a repetitive stretching/releasing process of 1,000 cycles, as shown in Figure 4d. All three types of fabricated sensors retain their initial performance after a few cycles during 1,000 cyclic tests with 20%, 50%, and 90%


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of tensile strain, respectively. After a stabilization step, the 3D strain sensor with 4 infiltration cycles exhibits the periodic and constant resistance change under the repeated stretching and releasing (Figure 4e and Figure S13). Figure 4f illustrates the relative resistance change versus strain at the first cycle. The hysteresis under tensile strain of 40% is negligible for all 3D strain sensors with different infiltration cycle. Considering that the resistive strain sensor often exibit hysteresis during loading and unloading cycles unlike capacitive strain sensor,14,55 the open porous 3D scaffold in resistive strain sensor could provide a new design opportunity for decreasing hysteresis due to its geometric advances (Figure S6). Since the conductive fillers are not dispersed in the PDMS matrix, but rather coated on the surface of the 3D PDMS substrate, resulting in reduced filler-to-substrate interfacial area under a large strain, friction that causes significant creep, and percolation failure could be minimized, leading to improved durability.15 High Performances of 3D Strain Sensor for Human Motion Detections. Due to high sensitivity, coupled with stretchability and flexibility, the fabricated 3D bicontinuous nanoporous electrodes could be effectively utilized as a human motion detecting strain sensor which operates even under large deformation on a non-planar surface. Figure 5 and Movie S1 represent the measurement of various human motions with the 3D strain sensors. For practical demonstrations of the strain sensors, from small motions such as phonation to large movements such as joint movement in daily life which produce at least 30% of deformation,55,56 the sensors are individually designed by varying the SWCNTs infiltration cycles for each measuring condition. For example, the 3D strain sensor with 3 infiltration cycles which is attached to the throat (Figure 5a), can sensitively capture phonation by detecting motion of the laryngeal prominence.15 The same patterns are reproduced when the same words are repeated, confirming reliability. The 4 cycles infiltrated 3D strain sensors are attached to the skin of an index finger, a wrist, and a knee to capture their motion by



measuring the resistance change. Time-dependent ∆R/R0 responses of the sensor attached to the index finger are shown in Figure 5b. As the bending angle of the index finger increases from 45 degrees to 90 degrees, the strain sensor produces signals with different intensities of resistance change (∆R/R0 from 2.6 to 3.4 that corresponds to 28 to 36% tensile strain, respectively), which is much higher than that of previously reported strain sensors.14,15,21 Figure 5c shows the ∆R/R0 responses of the 3D strain sensor attached to the wrist, when the wrist is bent upward and downward. As the wrist moves upward from the center, the resistance change is nearly negligible, since the small amount of compressive strain at the patterned part induced from the relatively small angle is not enough to change the resistance due to the 3D rotation of the bridge elements in the 3D PDMS. On the other hand, when the wrist moves downward, the resistance increases approximately 1.5 times corresponding to relatively large tensile strain of 20% applied at the patterned region. Moreover, the movement of the knee joint can be easily detected with an enhanced signal (Figure 5d). When the leg is bent with angles of 30, 90, and 135, the 3D strain sensor shows different intensities of resistance change due to differently applied strain upto 45% on the sensor.55 The capabilities of proportional response according to the intensity of the applied external stimuli prove the sensor’s strong abilities of detection and quantification of the strain. Therefore, the high performance of the 3D strain sensor could be potentially used as a wearable strain gauge which can act as a part of a human-machine interface. Additionally, the 3D sensor can be optically transparent by infilling the identical PDMS as an index-matching material into the 3D networked interstitial nanopores (Figure 6). The numerous optical boundaries inside 3D PDMS generally occur significant drop in transmittance down to ~ 0% over an entire visible range.37,57 However, the infiltrated material with a similar refractive-index (n=1.4) to PDMS simply recover the optical transparency (~ 88%) of 3D PDMS as a result of the removed nanopores inside. Despite the slight decline in


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enhanced stretchability from porous nature,29 the infilled 3D continuous conductive nanostructure with the index-matching material still exhibits the high strain sensing performance with the working range and the gauge factor of 0% to 150% and 4 to 14, respectively, according to the infiltration cycle of SWCNTs (Figure S14). This kind of highly transparent strain gauges could expand its application field to emerging transparent electronics.58,59 CONCLUSIONS In this work, the method and the collective set of results represent a strategy for improving the performances of the strain sensor. The biggest novelty of this work is the first development of the solid-state, conductive 3D nanonetwork with optimal percolations that can maximize the performance of the resistive strain sensor, as demonstrated by the highest gauge factor of ~134 among CNT-based sensors reported to date. To achieve this, we have solved the dispersion problem of CNTs in the 3D nanostructured elastomeric matrix by surface treatment even at extremely low concentration of CNTs (< 1 vol%). Therefore, the solid-state, conductive 3D nanonetwork suggests a possible strategy toward applications that require high sensitivity, such as capturing delicate phonation and dynamic detection of human joint motion. One of the promising research directions is the extension of detection capability of 3D sensors, for example distinguishing different words, by further optimization of the structure and material. Therefore, coupled with high stretchability, durability, and optical transparency, a versatile use in the fields of robotics, health care systems, virtual reality, and many other applications will be realized as promising directions for future work.



Figure 1. Concept and strategy for achieving a 3D bicontinuous nanoporous electrode. (a) A schematic illustration of the fabrication procedure for the 3D continuous conductive nanostructure and its demonstration as a 3D electrode for strain sensor. (b) Photographs of the highly stretchable 3D PDMS before and after stretching of ε=220%. (c) A top view SEM image of a 20% stretched 3D continuous conductive nanostructure after removing a part of the first layer (scale bar, 1 µm). (d) Photographs of 3D strain sensors attached to finger, neck, and wrist to detect dynamic human motion such as phonation and joint movement.


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Figure 2. Characteristics of nanostructures with UV/O3 treatment and infiltration of SWCNT solution for a 3D continuous conductive nanostructure. (a) Cross-sectional images of a deionized water droplet on the surfaces of the 3D PDMS, and analysis of the contact angle and wetting energy before and after UV/O3 treatment. (b) Top and cross-sectional view SEM images of top part of the 3D PDMS without UV/O3 treatment after SWCNT solution infiltration.



Figure 3. Tunability of electrical and mechanical properties of the 3D continuous conductive nanostructure with different SWCNT solution infiltration cycles. (a) SEM images of 3D PDMS with the SWCNT solution infiltrated in a different number of cycles. Scale bar, 200 nm. (b) AFM images of the 3D continuous conductive nanostructure matched with (a). A scale bar in XY-plane is 200 nm and a scale bar in Z-axis ranges from 1.9 µm to 2.6 µm. (c) Relationship between the electrical resistance and infiltration cycles of the SWCNT solution. Inset : close-up of the infiltration cycles of 2 to 5. (d) Stress-strain curves of the films with different SWCNT solution infiltrating cycles.


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Figure 4. Electrical properties of the strain-sensitive 3D continuous conductive nanostructure. (a) Strain response of the sensor from a bending radius from 800 µm to 3 cm with 10% tensile strain. (b) Plot of the ∆R/R0 of the 3D continuous conductive nanostructures as a function of the applied strain. (c) The comparison of the gauge factors of recently reported CNT/elastomer-based strain sensors. (d) Stretching-releasing cycles of the sensors at strains of 20%, 50%, and 90%. (e) The relative resistance change of the 3D strain sensor with 4 infiltration cycles during 10 cycles of stretching (~ 40%) and releasing. (f) The hysteresis below tensile strain of 40% for the 3D strain sensors with different infiltration cycle.



Figure 5. Demonstration of the 3D bicontinuous nanoporous electrode for a human motion detecting strain sensor by measuring strain from various movements. Photographs of the 3D strain sensor attached to the (a) throat, (b) index finger, (c) wrist, and (d) knee, and their relative changes in resistance versus time.


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Figure 6. Optical transparency in visible wavelength range of the 3D strain sensor with refractive index-matching material infiltrated. (a) Schematics of the unit-cell of 3D porous platform showing low transmittance. (b) Schematics of the unit-cell of 3D nanostructure after infiltrating index-matching material. (c) Transmittance spectra comparison.



ASSOCIATED CONTENT Supporting Information. Optical absorption of previously used commercial photoresists for PnP technique, SEM images of 3D polymeric template and the 3D PDMS, transmittance spectra comparison of 3D nanostructures, fundamental analysis of SWCNTs, AFM data of 3D continuous conductive nanostructure, cross-sectional SEM image of SWCNT solution infiltrated 3D polymer template, experimental details of LED system, relationship between applied tensile strain and LED output, and bending response of the strain sensor used in detail. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This work was supported by the Global Frontier Program (2014M3A6B3063709) and BK21 Plus (21A20131912234) through a National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT & Future Planning. This work was also supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2016R1E1A1A01943131).


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Insert Table of Contents Graphic and Synopsis:



Figure 1. Concept and strategy for achieving a 3D bicontinuous nanoporous electrode. (a) A schematic illustration of the fabrication procedure for the 3D continuous conductive nanostructure and its demonstration as a 3D electrode for strain sensor. (b) Photographs of the highly stretchable 3D PDMS before and after stretching of ε=220%. (c) A top view SEM image of a 20% stretched 3D continuous conductive nanostructure after removing a part of the first layer (scale bar, 1 µm). (d) Photographs of 3D strain sensors attached to finger, neck, and wrist to detect dynamic human motion such as phonation and joint movement. 793x382mm (96 x 96 DPI)


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Figure 2. Characteristics of nanostructures with UV/O3 treatment and infiltration of SWCNT solution for a 3D continuous conductive nanostructure. (a) Cross-sectional images of a de-ionized water droplet on the surfaces of the 3D PDMS, and analysis of the contact angle and wetting energy before and after UV/O3 treatment. (b) Top and cross-sectional view SEM images of top part of the 3D PDMS without UV/O3 treatment after SWCNT solution infiltration. 161x210mm (96 x 96 DPI)



Figure 3. Tunability of electrical and mechanical properties of the 3D continuous conductive nanostructure with different SWCNT solution infiltration cycles. (a) SEM images of 3D PDMS with the SWCNT solution infiltrated in a different number of cycles. Scale bar, 200 nm. (b) AFM images of the 3D continuous conductive nanostructure matched with (a). A scale bar in XY-plane is 200 nm and a scale bar in Z-axis ranges from 1.9 µm to 2.6 µm. (c) Relationship between the electrical resistance and infiltration cycles of the SWCNT solution. Inset : close-up of the infiltration cycles of 2 to 5. (d) Stress-strain curves of the films with different SWCNT solution infiltrating cycles. 358x286mm (96 x 96 DPI)


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Figure 4. Electrical properties of the strain-sensitive 3D continuous conductive nanostructure. (a) Strain response of the sensor from a bending radius from 800 µm to 3 cm with 10% tensile strain. (b) Plot of the ∆R/R0 of the 3D continuous conductive nanostructures as a function of the applied strain. (c) The comparison of the gauge factors of recently reported CNT/elastomer-based strain sensors. (d) Stretchingreleasing cycles of the sensors at strains of 20%, 50%, and 90%. (e) The relative resistance change of the 3D strain sensor with 4 infiltration cycles during 10 cycles of stretching (~ 40%) and releasing. (f) The hysteresis below tensile strain of 40% for the 3D strain sensors with different infiltration cycle. 391x401mm (96 x 96 DPI)



Figure 5. Demonstration of the 3D bicontinuous nanoporous electrode for a human motion detecting strain sensor by measuring strain from various movements. Photographs of the 3D strain sensor attached to the (a) throat, (b) index finger, (c) wrist, and (d) knee, and their relative changes in resistance versus time. 395x275mm (96 x 96 DPI)


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Figure 6. Optical transparency in visible wavelength range of the 3D strain sensor with refractive indexmatching material infiltrated. (a) Schematics of the unit-cell of 3D porous platform showing low transmittance. (b) Schematics of the unit-cell of 3D nanostructure after infiltrating index-matching material. (c) Transmittance spectra comparison. 264x291mm (96 x 96 DPI)


Three-Dimensional Continuous Conductive Nanostructure for Highly

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