Highly Durable Nanofiber-Reinforced Elastic Conductors for Skin

Jun 17, 2019 - (b) EMG, motion, and ECG signals in the real-time monitoring while lifting the 3 kg weight and subsequent release. (c) One hour continu...
0 downloads 0 Views 3MB Size
Download PDF
www.acsnano.org

Highly Durable Nanofiber-Reinforced Elastic Conductors for Skin-Tight Electronic Textiles Hanbit Jin,†,§ Md Osman Goni Nayeem,† Sunghoon Lee,† Naoji Matsuhisa,† Daishi Inoue,‡ Tomoyuki Yokota,† Daisuke Hashizume,‡ and Takao Someya*,†,‡

Downloaded via BUFFALO STATE on July 17, 2019 at 15:13:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Electrical Engineering and Information Systems, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Center for Emergent Matter Science (CEMS), RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan S Supporting Information *

ABSTRACT: Soft and stretchable electrodes are essential components for skin-tight wearable devices, which can provide comfortable, unobtrusive, and accurate physiological monitoring and physical sensing for applications such as healthcare, medical treatment, and human-machine interfaces. Metal−elastomer nanocomposites are a promising approach, enabling high conductivity and stretchability derived from metallic conduction and percolation networks of metal nano/micro fillers. However, their practical application is still limited by their inferior cyclic stability and long-term durability. Here, we report on a highly durable nanofiber-reinforced metal−elastomer composite consisting of (i) metal fillers, (ii) an elastomeric binder matrix, and (iii) electrospun polyvinylidene fluoride nanofibers for enhancing both cyclic stability and conductivity. Embedded polyvinylidene fluoride (PVDF) nanofibers enhance the toughness and suppress the crack growth by providing a fiber reinforcing effect. Furthermore, the conductivity of nanofiber-reinforced elastic conductor is four times greater than the pristine material because the silver-rich layer is selfassembled on the top surface by a filtering effect. As a result, a stretchable electrode made from nanofiber-reinforced elastic conductors and wrinkled structures has both excellent cyclic durability and high conductivity and is stretchable up to 800%. The cyclic degradation (ΔR/R0) remains at 0.56 after 5000 stretching cycles (50% strain), whereas initial conductivity and sheet resistance are 9903 S cm−1 and 0.047 Ω sq−1, respectively. By utilizing a highly conductive and durable elastic conductor as sensor electrodes and wirings, a skin-tight multimodal physiological sensing suit is demonstrated. Continuous long-term monitoring of electrocardiogram, electromyogram, and motions during weightlifting exercises are successfully demonstrated without significant degradation of signal quality. KEYWORDS: stretchable electrode, electrospun nanofibers, fiber-reinforced composite, strain sensor, wearable e-textiles

S

repeated strain, a conductive material that has robust electrical conductivity under large and repeated deformations is essential. Recently, significant progress in stretchable electrodes has been achieved. Various types of stretchable electrodes have been demonstrated by utilizing microstructures,5,8,9 nanocomposites,10−12 liquid metals,13−15 and conductive polymers.16−18 Among the various solutions for stretchable electrodes, nanocomposites are one of the most promising approaches for electronic textile applications. They can provide a high degree of stretchability and low sheet resistance with a simple fabrication process, owing to a wide choice of fillers and binder materials, and the ability to form a thick layer with a single printing. Sheet resistance should be considered as much as conductivity for practical circuit designs. Moreover, nanocomposites are capable of textile-compatible fabrication

kin-tight wearable devices are required for comfortable, unobtrusive, and accurate physiological monitoring and physical sensing for healthcare, medical treatment, and human computer interfaces.1−6 Textiles have beneficial characteristics for wearable applications, such as lightweight, softness, stretchability, and the ability to sustain the warmth of the human body.7 In particular, soft and skin-tight textiles that can make intimate contact with the human body, such as underwear and compression sportwear, are a promising platform for providing a conformal contact with skin while accommodating body movements. Integration of stretchable electronic devices into skin-tight textiles is desirable to realize highly accessible wearable electronic systems with maximum comfort. Skin-tight wearable electronic textiles are subjected to large and repeated strains over extended daily wear. Therefore, longterm durability is a critical issue for electronic textiles, unlike disposable devices. In order to secure long-term durability and maintain reliable operation of electronic devices under © XXXX American Chemical Society

Received: March 24, 2019 Accepted: June 17, 2019 Published: June 17, 2019 A

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX

Article

Cite This: ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano processes such as screen/stencil printing19,20 and the hot-melt process,12,20,21 which are advantageous for large-area patterning and textile-integration. Conductivity and stretchability have been considered as performance indicators of nanocomposites. These performance indicators have been significantly improved through studies of nanofillers and their fabrication techniques. Conductive pathways have been efficiently constructed by 1D or 2D high-aspect-ratio nanofillers that have a low percolation threshold, such as graphene, carbon nanotubes,22−24 metal flakes, and nanowires.25,26 Self-assembly of conductive pathways have also been introduced for high conductivity and stretchability, such as phase separation11,27 and in situ formation of nanoparticles.12 For wearable applications, cyclic durability is a crucial property that determines the reliability and long-term durability of the device. Although many factors contribute to long-term durability, mechanical fatigue and the nucleation and growth of cracks in the rubber are often the primary considerations.28 Recently, conductive composites with high cyclic durability have been demonstrated by using nonmetallic fillers such as carbon-based fillers29,30 and conductive polymers,18,31 which have a Young’s modulus that is approximately one-order of magnitude lower than metallic fillers. The best cyclic durability is reported for the composite material of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) with an ionic compound additive.18 Cyclic degradation after 1000 cycles of stretching (strain, 50%) is ΔR/R0 = 0.087, whereas the initial conductivity and sheet resistance are 3100 S cm−1 and 10 Ω sq−1, respectively. Despite the high cyclic durability that has been achieved by these materials, further improvement of conductivity and reduction of sheet resistance has remained difficult, because nonmetallic fillers are inferior to metallic fillers in terms of intrinsic conductivity. To realize simultaneous achievement of cyclic durability and high conductivity, a large fraction of metallic fillers needs to be introduced without causing severe cracking when stretched. However, large fractions of metallic fillers makes the composite brittle, because the metallic fillers in the elastomer matrix act as defects that initiate crack growth at the filler/elastomer interface.32,33 Hence, a material design concept that can suppress crack growth in metal-filled elastomer is required to realize a highly durable and highly conductive elastic conductor, which can be integrated into skin-tight wearable electronic textiles. In this study, we developed a metal−elastomer composite with a nanofiber reinforcement composing a three-phase composite material. By embedding randomly aligned polymer nanofibers into a silver−fluoroelastomer composite, the elastomer matrix was reinforced (325 times tougher) and the stress was dissipated by the nanofiber scaffolds (which suppressed the crack propagation) when 75 wt % of metal fillers were introduced. Moreover, combined with the buckled structure, high cyclic durability against repeated stretching was achieved. Consequently, the stretchable electrode exhibited a minimal cyclic degradation against repeated strain (ΔR/R0 = 0.56) after 5000 cycles (strain, 50%) while initial conductivity and sheet resistance were 9903 S cm−1 and 0.047 Ω sq−1, respectively. Using the highly durable and soft metal− elastomer composites, we successfully fabricated a highly reproducible strain sensor and multimodal sensing skin-tight electronic shirt.

RESULTS AND DISCUSSION Design and Fabrication of Nanofiber-Reinforced Elastic Conductor. The designs and components of the nanofiber-reinforced elastic conductor are shown in Figure 1.

Figure 1. Printable tough elastic conductor reinforced by PVDF nanofibers. (a) Schematic of the tough elastic conductor; PVDF nanofibers are embedded in the composite (top); schematic of the buckled tough elastic conductor (bottom). (b−e) SEM images of the elastic conductor without PVDF nanofibers before stretching (b), elastic conductor without nanofibers after the stretching to 100% (c), elastic conductor with PVDF nanofibers (d), and nanofiber-reinforced elastic conductor during the stretching to 100% (e). The scale bars correspond to 10 μm.

To reinforce the elastomer matrix and suppress crack growth we used continuously long, randomly aligned nanofiber networks for a fiber reinforcing effect, followed by the construction of a wrinkle structure for further improvement of cyclic durability. In microscale, the strain is dissipated and stress is supported by embedded nanofibers, so that the composite can exhibit enhanced toughness29,34 and a good resistance to fatigue.35 In macroscale, wrinkle structures can relieve the applied tensile stress providing further improvement of stretchability and cyclic durability. The material design concept is inspired by skin tissue. Human and animal skin has very high tear resistances and elasticity, owing to the collagen nanofiber networks in the dermis tissue.36 The nanofiber-reinforced elastic conductor was fabricated by direct stencil printing of a silver−fluoroelastomer composite ink on a polyvinylidene fluoride (PVDF) nanofiber sheet. The PVDF nanofiber was fabricated by electrospinning. The fiber was continuously drawn by applying a high voltage between the collector stage and the needle. The electrospun PVDF nanofibers had diameters ranging from 300 to 500 nm and an aspect ratio greater than 103, as the fiber was continuously drawn. Small diameters and high aspect ratios are favorable for the fiber-reinforcing effect.29 The electrospun nanofibers had random orientations, which provided stretchability of the PVDF nanofiber membrane by realignment to the stretched direction when its density was sufficiently low, are shown in Figure S1. The PVDF nanofiber was chosen because it formed a strong interface adhesion with the vinylidene fluoride− hexafluoropropylene (VDF−HFP) copolymer, which is a binder matrix. Both fiber and matrix are soluble in the same solvent with different dissolution rates, yielding an entangled polymer chain with strong adhesion37,38 at the interface. A silicon-coated paper and/or polyurethane substrate was used, depending on the final form and usage of the printed film. The silicon-coated paper was used for a free-standing film, which B

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano was necessary for the following buckling process and the evaluation of intrinsic properties of the composite material. The polyurethane substrate was used for a substrate-supported printed film. The fraction of PVDF nanofibers in the composite was controlled by the electrospinning time (10− 80 min) to investigate the proper weight percentage of nanofibers in the composite (Figure S2). After the preparation of the nanofiber sheet, the silver− fluoroelastomer ink was formulated by mixing silver flakes, fluoroelastomer, and 2-(2-butoxyethoxy) ethyl acetate at a weight ratio of 3:1:2.45. A fluoroelastomer with a lower molecular weight (2.5 times lower) than that of a previously developed silver−fluoroelastomer composite20 was used to provide a smaller resistance force to the fiber alignment. The silver−fluoroelastomer ink was stencil-printed directly onto the nanofiber sheet (Figure S3), followed by annealing at 80 °C for 2 h to remove the solvent. After printing, the composite ink permeated the nanofiber sheet and embedded the nanofibers. The randomly aligned nanofiber networks were introduced to the binder matrix by the simple stencil printing process. The fiber networks minimized the crack growth rate as the elastic conductors were stretched. A scanning electron microscope (SEM) surface observation revealed crack generation by comparing the images before and after the stretching of the sample at the strain of 100%. The SEM images of the elastic conductor without nanofiber showed propagated cracks after stretching at 100% (Figure 1b,c). On the other hand, the SEM images of the elastic conductor reinforced with nanofibers revealed that the crack propagation was suppressed by the fiber networks (Figure 1d,e). Surface images for different nanofiber fractions at low magnifications in the stretched states are shown in Figure S4. The crack generation was significantly reduced in the overall area, and the prevention of cracking was enhanced with an increased nanofiber fraction. Electromechanical Performance of Nanofiber-Reinforced Elastic Conductor. The electrical performance was characterized while the sample was stretched by a universal electromechanical testing machine. Figure 2a shows the strain−conductivity curves of the elastic conductors, both with and without nanofiber reinforcement. The silver− fluoroelastomer composite ink was printed onto the polyurethane substrates both with and without PVDF nanofibers to investigate the electromechanical properties. The silver− fluoroelastomer composite with nanofibers exhibited higher conductivity and stretchability compared to the pristine material. The initial conductivity of the nanofiber-reinforced composite was 3667 S cm−1, which was four times higher than the pristine composite. The conductivity decreased to 1018 S cm−1 at a strain of 450%. Further, the initial conductivity of the pristine composite material was 923 S cm−1, which decreased to 153 S cm−1 at a strain of 230%. The electrical conductivity of the nanofiber-reinforced composite material was higher than that of the pristine composite material. This conductivity enhancement is due to the filtering effect of the nanofiber membrane. The nanofiber membrane has an average pore size of 2.9 ± 1.0 μm (Figures S5 and S12). Silver flakes with sizes larger than 3.0 μm (Figure S5d) hardly penetrated and filtered out, causing the ink to form two separated regions on the top and bottom of the printed film during the printing process. The top layer is a silver-rich conducting layer, whereas the bottom layer is a tough fluoroelastomer- and nanofiber-rich layer, as shown in Figures S5 and S13.

Figure 2. Electrical and mechanical characteristics of the nanofiber-reinforced elastic conductors. (a) Conductivity change during the stretching. The black curve represents the resistance of the pristine material, whereas the red curve represents that of the nanofiber-reinforced material. (b) Stretchability and conductivity dependences on the nanofiber fraction for the free-standing film. (c) Strain−stress curves of the nanofiber-reinforced composites. (d) Toughness of the nanofiber-reinforced composites. (e) Eight hundrd percent stretchability of the strain-dissipative wrinkled structure. The resistance change is almost negligible during the stretching to 800%. (f) Cyclic stabilities of the buckled structures of the tough elastic conductors with and without nanofiber reinforcement, evaluated during 1000 cycles of strain application (50%).

To investigate the effect of the nanofiber reinforcement, we systematically changed the fraction of nanofiber content in the composite material (from 0 to 4.68 wt %) by changing the electrospinning time (Figure S2). With the increase in the nanofiber fraction, the stretchability and conductivity simultaneously improved, as shown in Figure 2b. The initial conductivity of the nanofiber-reinforced Ag−fluoroelastomer composite was 9903 S cm−1 on average (highest value: 11744 S cm−1) at a nanofiber fraction of 3.11 wt %. The conductivity started to decrease above 4.68 wt %. This occurs because when the nanofiber film is too thick, silver cannot penetrate the thick nanofiber film and thus remains on the surface, while the rubber solution permeates and reaches the bottom, so that the portion of the nonconductive region becomes thicker. The mechanical properties were investigated by measuring the stresses during the elongations of the nanofiber-reinforced composites for various nanofiber fractions, as shown in Figure 2c. The toughness was obtained from the strain−stress curve by integrating the curve area. The toughness of the nanofiberreinforced composite was improved by a factor of 188 at a nanofiber fraction of 3.11 wt %, and by a factor of 325 at 4.68 wt % compared to that of the pristine material. This implies that increases of 188 and 325 times greater mechanical energy, C

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

durable to realize reliable sensing systems. To fabricate the capacitive-type strain sensor, the nanofiber-reinforced elastic conductor film and stretchable dielectric layer were stacked together as a sandwich structure on a prestretched elastomer and released. This pair of electrodes with a wrinkled accordionlike structure formed a capacitor, as shown in Figure 3a. Upon

respectively, are needed for tearing (Figure 2d). The toughness of the silver−fluoroelastomer composite was 0.035 MJ m−3. After the introduction of 3.11 wt % of PVDF nanofibers (optimized for a high conductivity), the toughness increased to 6.58 MJ m−3, which further increased to 11.4 MJ m−3 at 4.68 wt % of nanofibers (maximum). The nanofiber-reinforced composite exhibited a greater toughness than each of the single elements (fiber and elastomer), as shown in Figure S6. Moreover, this tough elastic conductor can be formed into wavy microscale structures to dissipate the strain to achieve very high cyclic stability. The combination of nanofiber reinforcement and a buckling structure provided the high durability of the elastic conductor. The nanofiber-reinforced elastic conductor film printed on the silicon-coated paper can be easily transferred to the prestretched elastomer substrate without tearing and cracking. A surface SEM image of the buckled film revealed that the nanofiber-reinforced composite formed wrinkles without cracking at a small bending radius of ∼35 μm (Figure S7). Buckling behavior of nanofiberreinforced elastic conductor strongly affected by the thickness of film (Figure S15). A biaxial prestrain can also be applied to demonstrate biaxial stretchability, where even harsh mechanical deformations such as poking (Figure S8) can be applied. The buckled elastic conductor exhibited a very high stretchability (up to 800%) and negligible cyclic degradation (ΔR/R0 = 0.56) after 5000 cycles of strain application (50%), which is a very desirable cyclic stability for applications in stretchable and wearable electronics. Figure 2e shows the resistance change of the buckled elastic conductor with nanofiber reinforcement during stretching up to 800%. The resistance of the nanofiber-reinforced composite with wrinkles changed from 0.5 to only 0.8 Ω under the strain of up to 800%, which reflects the highly stable electrical performance maintaining the low resistance. The cyclic stabilities of the buckled elastic conductors both with and without nanofiber reinforcement were evaluated. The resistance of the electrodes was continuously monitored during the repeated application of strain of 50% for up to 1000 cycles and 5000 cycles (Figure S14). Figure 2f shows the significant improvement in cyclic stability upon the introduction of the nanofiber reinforcing agent. The resistance change (Rmax/R0) after the 1000 cycles can be defined as cyclic degradation. The cyclic degradation was suppressed from 105 to 1.64 by introducing the nanofiber network. The cyclic degradation after the 1000 cycles of strain application (50%) and initial conductance of the nanofiberreinforced elastic conductor are compared with those in related reports on printable elastic conductors in Table S1 and Figure S9. The nanofiber-reinforced elastic conductor exhibited a small cyclic degradation and the lowest sheet resistance (highest sheet conductance). Statistics of cyclic test (N = 5, batch to batch) are shown in Figure S16. Initial resistance is 1.5 ± 0.5 Ω on average. ΔR/R0 is 0.43 ± 0.40 on average after 1000 cycles. Demonstration of Highly Durable Strain Sensor and Multimodal Sensing Suit. Highly stretchable and reliable elastic conductors can be utilized for repeatedly deformable textile sensor applications. Using the tough and stretchable electrode, we demonstrated a highly reliable capacitive-type strain sensor. The strain sensor is constantly exposed to repeated strain so that the cyclic degradation and hysteresis are crucial for sensor performance. Therefore, the stretchable electrodes and wiring, which are basic components of both resistive- and capacitive-type strain sensors, should be highly

Figure 3. Strain-dissipative buckling structure of the tough elastic conductor and capacitive strain sensor applications. (a) SEM cross-sectional image (top) and schematic of the capacitive strain sensor (bottom). The scale bar corresponds to 100 μm. (b) Relative capacitance changes of the strain sensor during the stretching and releasing. The elastomer is not fully recovered after the release. (c) The 50%-stretching cycle test. Initial 5 (top) and 1000 (bottom) cycles of strain response.

stretching, the thickness of the dielectric layer changes so that the capacitance increases. The relationship between the capacitance and strain is highly linear (R2 = 0.995) with a gauge factor (GF) of 1.52, which is larger than the theoretical limit (GF = 1) of a parallel-plate capacitive strain sensor. This is because the buckled structures introduce an additional degree of freedom to the parallel-plate capacitor structure through an out-of-plane deformation.39 Excellent cyclic stability and reproducibility were obtained during the cyclic D

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 4. Stretchable multimodal sensing suit with the wireless transmission module for a long-term continuous monitoring of physiological activities. (a) Multimodal sensing suit composed of EMG, ECG, and strain sensors worn by the volunteer (left). View from the back (middle). Strain sensor located on the elbow during the arm bending (right). (b) EMG, motion, and ECG signals in the real-time monitoring while lifting the 3 kg weight and subsequent release. (c) One hour continuous monitoring. BPM obtained from the ECG signal (top), EMG (middle), and capacitance change of the strain sensor (bottom).

introduced on the moving part of the bicep muscles, instead of directly attaching the obstructing rigid components. A strain sensor (30 cm long) was located on the elbow to monitor joint movements. As the length of the strain sensor was sufficiently large, the sensor could cover the region from the elbow to the rear of the upper arm. This provided a seamlessly soft structure and avoided failure at interconnections. The ECG electrode was positioned on the chest and connected to the wireless module to monitor the ECG signals and heart rate (beats per minute [BPM]). A volunteer wore the compression suit and performed weight lifting with the left arm. The EMG, ECG, and joint movements were continuously monitored in real time during a 1 h exercise. Figure 4b shows the EMG, ECG, and elbow movement (strain) during the weight lifting and releasing. The EMG signal and movement of the elbow are synchronized with the arm lifting and releasing. In the magnified time scale in Figure 4c, the signals were measured

50% stretching test (Figure 3b,c). Owing to the slow recovery of the viscoelastic substrate at the small-strain range (ε < 20%), the strain was repeatedly applied in the range of 20% to 50%. Finally, a multimodal sensing suit was fabricated by transferring the highly durable and large-area elastic conductor onto a textile, which enabled continuous long-term monitoring of electrocardiogram (ECG), electromyograph (EMG), and motions in real-time. Using the nanofiber-reinforced elastic conductor, strain sensors, EMG sensors, and transmission lines were fabricated and successfully integrated to the compression suit by a T-shirt heat press process, as shown in Figure 4a. For the transmission lines of the EMG sensor, a pattern (length 300 mm) was printed and reduced to 150 mm by a buckling process. EMG gel electrodes (hydrogel sheets) were positioned on the left upper arm and connected with a wireless module through the stretchable transmission lines to monitor bicep muscle activity. Only the soft and stretchable part was E

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

translation stage. For the 1000% prestretch, three layers of acrylic tape with a total thickness of 3 mm were used as a prestrained substrate. To enhance the adhesion between the Ag−fluoroelastomer and elastomer substrate, a 300 nm thick parylene layer was deposited onto the printed film. After the transfer of the film, the prestrain was released from 200% or 1000% to the relaxed state. The acrylic tape was not perfectly recovered to 0%. Electrical and Mechanical Characterizations of the Nanofiber-reinforced Composites. The surface and cross section of the printed traces were observed by a laser microscope (VK-9710, KEYENCE Corp., Japan) and an SEM (S4800, Hitachi HighTechnologies with an accelerating voltage of 0.6-1 kV). The sheet resistance was measured by a resistivity process (MODEL Sigma-5+, NPS, INC., Japan). The printed samples were stretched with a universal electromechanical testing machine (AG-X, SHIMADZU Corporation) with a stretching speed of 3 mm min−1; the resistance was simultaneously monitored using the four-probe method with a digital multisource meter at a sampling rate of 5 Hz (2400, Keithley Instruments Inc.). For the cyclic test, the stretching and releasing speed were 30 mm min−1. The distance between the electrodes was 30 mm. Fabrication of the Capacitive-type Buckled Strain Sensor and Characterization. The strain sensor was fabricated using the nanofiber-reinforced elastic conductor as the electrode, acrylic tape (3 M VHB 4905) as the substrate, and 100 μm thick stretchable adhesive film (Free Crystal, Bando Chemical Industries, Ltd., Japan) as the dielectric layer. Electrodes with widths of 5 mm and lengths of 90 cm were stencil-printed and annealed at 80 °C for 2 h. The printed film was coated with a 300 nm thick parylene and transferred on a 200% prestretched acrylic tape using the XYZ linear translation stage. The stretchable adhesive film was stretched to 200% and attached onto the electrode; a counter electrode was attached onto the adhesive film. Finally, the acrylic tape was stretched to 200% and attached onto the electrode−dielectric−electrode stack. The interconnection was fabricated by an anisotropic conductive film (ACF) tape and 100 nm gold-coated 12.5 μm thick polyimide film. For the basic characterization including the measurement of the strain−capacitance curve and cyclic test, a high-precision mechanical system (AG-X, SHIMADZU Corporation) was used to apply strain. The capacitance of the sensor was measured by an Agilent 4284A Precision LCR meter and wireless module (C-STRETCH kit BT01, Bando Chemical Industries, Ltd., Japan). The study protocol was thoroughly reviewed and approved by the ethical committee of the University of Tokyo (approval number KE18−57). Fabrication of Multimodal Sensors and Textile Integration. The stretchable transmission line, EMG sensor, and strain sensor were integrated on a compression-type sport wear (MUD01-KLB, TESLA) to obtain a multimodal sensing suit. For EMG sensor, the stretchable transmission line was fabricated by the nanofiber-reinforced composite with the buckled structure. Both ends of the stretchable transmission line were opened for a contact pad. One end was connected with a snap fastener using a conductive epoxy. The other end was terminated with hydrogel sheets cut to dimensions of 1 cm × 3 cm (top-touch gel sheet, Setsu Planning, Japan) for a stable contact with the skin. Electrode access to the skin was provided via a hole in the textile. The stretchable transmission line and strain sensor were laminated on the sportswear using a 50 μm thick thermoplastic polyurethane film (NSK Echomark, Japan) and heat-pressed at 120 °C for 1 min by a heat-press machine (HP-4536A-12). The stretchable transmission line was connected to the wireless module (RF-ECG2, Micro medical device, Inc.) for the EMG monitoring. For ECG measurement, the wireless module (RF-ECG2, Micro medical device, Inc.) was directly attached onto the skin with a gel electrode (Ambu BlueSensor R, Ambu) separated from the shirt. The strain sensor was connected to the wireless module (C-STRETCH kit BT01, Bando Chemical Industries, Ltd., Japan).

without significant degradation. The initial noise level of EMG monitoring was 75 μV (peak-to-peak), increasing by 4% (78 μV) after a 1 h exercise (Figure S11). Using the multimodal sensor suit, a better understanding of the physiology can be provided, such as the physical load. An obvious change in BPM between the exercise and rest states was observed. During the exercise, the BPM increased; the EMG signal revealed the exercise intensity through the frequency of the signal and its amplitude. The multimodal sensing suit provides more information than a single sensor for physiological phenomena and provides indications that help to filter out the motion artifacts or other external noises.

CONCLUSIONS In our nanofiber-reinforced elastic conductor, low sheet resistance and cyclic stability were simultaneously achieved by introducing PVDF nanofibers into the composite. The nanofiber-reinforced elastic conductor exhibited improved conductivity and cyclic stability through the following mechanisms. First, the nanofiber network reinforced the elastomer matrix and suppressed crack formation by dissipating the stress. Second, the nanofiber membrane acted as a filter and created highly conductive silver-rich regions on the top surface of the composite. Low sheet resistance is required to reduce the energy loss in the transmission line, particularly in large-area applications. Cyclic stability is a crucial requirement for long-term reliable operation of wearable sensor networks. Finally, we demonstrated wearable electronic textile applications, which required a large-area fabrication process and extended durability. The highly conductive and repeatedly deformable electrodes present reliable designs for wearable electronics and textile-based sensors. By distributing the soft and conformable sensor/actuators on a large area (such as the entire body surface), it is expected that the accessibility of computing technologies can be significantly improved, which can contribute to personalized healthcare and robotic assistance in the future. MATERIALS AND METHODS Fabrication of the PVDF Nanofibers. A 19 wt % PVDF solution was prepared by dissolving PVDF (Mw ∼ 275 000, Sigma-Aldrich, product number: 427144−100G) into an acetone and dimethylformamide (DMF) mixed solvent at a weight ratio of 4:6. The solution was stirred at 70 °C for 30 min. The PVDF solution was then electrospun at 20 kV, pump rate of 10 μL min−1, and stage distance of 15 cm. The spinning time was varied in the range of 10−80 min. Formulation of the Elastic Conductor Ink and Stencil Printing Process. The fluoroelastomer (DAI-EL G8001, Daikin Industries) and organic solvent (2-(2-butoxyetoxy) ethyl acetate) were mixed at a weight ratio of 1:2.45. After stirring with a magnetic stirrer for 12 h, silver flakes (product number: 327077, Aldrich, 10 μm, trace metal basis >99.9%) were added to the solution at a weight ratio of 3:1 (silver/fluoroelastomer) and mixed with the magnetic stirrer for 3 h before use. After the formulation of the ink, conductive traces were stencil-printed with a width of 5 mm and length of 40 mm on the PVDF nanofiber sheet attached on the weakly adhesive supporting film. The stencil mask consisted of a 125 μm thick polyimide film cut by a green laser (MD-T1010, Keyence). A glass slide was used to spread the ink. Three passes of printing were carried out to deliver a sufficient amount of ink. Subsequently, the printed film was dried in an oven at 80 °C for 2 h. Buckling Process of the Printed Nanofiber-reinforced Elastic Conductor Film. For the buckling process, a 1 mm thick acrylic tape (3 M VHB 4905) was prestretched to 200% (for sensor applications) and 1000% (highest stretchability) using an XYZ linear F

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

(5) Kim, D.-H.; Lu, N.; Ma, R.; Kim, Y.-S.; Kim, R.-H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K.; Kim, T.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H.; Keum, H.; McCormick, M.; Rogers, J.; et al. Epidermal Electronics. Science 2011, 333, 838−843. (6) Wang, S.; Xu, J.; Wang, W.; Wang, G.-J. N.; Rastak, R.; MolinaLopez, F.; Chung, J. W.; Niu, S.; Feig, V. R.; Lopez, J.; Lei, T.; Kwon, S. K.; Kim, Y.; Foudeh, A. M.; Ehrlich, A.; Gasperini, A.; Yun, Y.; Murmann, B.; Tok, J. B.; Bao, Z. Skin Electronics from Scalable Fabrication of an Intrinsically Stretchable Transistor Array. Nature 2018, 555, 83−88. (7) Heo, J. S.; Eom, J.; Kim, Y.-H.; Park, S. K. Recent Progress of Textile-Based Wearable Electronics: A Comprehensive Review of Materials, Devices, and Applications. Small 2018, 14, 1703034. (8) Khang, D.-Y. A Stretchable Form of Single-Crystal Silicon for High-Performance Electronics on Rubber Substrates. Science 2006, 311, 208−212. (9) Hua, Q.; Sun, J.; Liu, H.; Bao, R.; Yu, R.; Zhai, J.; Pan, C.; Wang, Z. L. Skin-Inspired Highly Stretchable and Conformable Matrix Networks for Multifunctional Sensing. Nat. Commun. 2018, 9, 244. (10) Sekitani, T.; Noguchi, Y.; Hata, K.; Fukushima, T.; Aida, T.; Someya, T. A Rubberlike Stretchable Active Matrix Using Elastic Conductors. Science 2008, 321, 1468−1472. (11) Choi, S.; Han, S. I.; Jung, D.; Hwang, H. J.; Lim, C.; Bae, S.; Park, O. K.; Tschabrunn, C. M.; Lee, M.; Bae, S. Y.; Yu, J. W.; Ryu, J. H.; Lee, S. W.; Park, K.; Kang, P. M.; Lee, W. B.; Nezafat, R.; Hyeon, T.; Kim, D. H. Highly Conductive, Stretchable and Biocompatible Ag−Au Core−Sheath Nanowire Composite for Wearable and Implantable Bioelectronics. Nat. Nanotechnol. 2018, 13, 1048−1056. (12) Matsuhisa, N.; Inoue, D.; Zalar, P.; Jin, H.; Matsuba, Y.; Itoh, A.; Yokota, T.; Hashizume, D.; Someya, T. Printable Elastic Conductors by In Situ Formation of Silver Nanoparticles from Silver Flakes. Nat. Mater. 2017, 16, 834−840. (13) Park, J.; Wang, S.; Li, M.; Ahn, C.; Hyun, J. K.; Kim, D. S.; Kim, D. K.; Rogers, J. A.; Huang, Y.; Jeon, S. Three-Dimensional Nanonetworks for Giant Stretchability in Dielectrics and Conductors. Nat. Commun. 2012, 3, 916. (14) Dickey, M. D. Stretchable and Soft Electronics Using Liquid Metals. Adv. Mater. 2017, 29, 1606425. (15) Zhu, S.; So, J.-H.; Mays, R.; Desai, S.; Barnes, W. R.; Pourdeyhimi, B.; Dickey, M. D. Ultrastretchable Fibers with Metallic Conductivity Using a Liquid Metal Alloy Core. Adv. Funct. Mater. 2013, 23, 2308−2314. (16) Lee, Y.-Y.; Kang, H.-Y.; Gwon, S. H.; Choi, G. M.; Lim, S.-M.; Sun, J.-Y.; Joo, Y.-C. A Strain-Insensitive Stretchable Electronic Conductor: PEDOT:PSS/Acrylamide Organogels. Adv. Mater. 2016, 28, 1636−1643. (17) Lipomi, D. J.; Lee, J. A.; Vosgueritchian, M.; Tee, B. C.-K.; Bolander, J. A.; Bao, Z. Electronic Properties of Transparent Conductive Films of PEDOT:PSS on Stretchable Substrates. Chem. Mater. 2012, 24, 373−382. (18) Wang, Y.; Zhu, C.; Pfattner, R.; Yan, H.; Jin, L.; Chen, S.; Molina-Lopez, F.; Lissel, F.; Liu, J.; Rabiah, N. I.; Chen, Z.; Chung, J. W.; Linder, C.; Toney, M. F.; Murmann, B.; Bao, Z. A Highly Stretchable, Transparent, and Conductive Polymer. Sci. Adv. 2017, 3, e1602076. (19) Khan, S.; Lorenzelli, L.; Dahiya, R. S. Technologies for Printing Sensors and Electronics over Large Flexible Substrates: A Review. IEEE Sens. J. 2015, 15, 3164−3185. (20) Jin, H.; Matsuhisa, N.; Lee, S.; Abbas, M.; Yokota, T.; Someya, T. Enhancing the Performance of Stretchable Conductors for ETextiles by Controlled Ink Permeation. Adv. Mater. 2017, 29, 1605848. (21) Xu, X.; Fukuda, K.; Karki, A.; Park, S.; Kimura, H.; Jinno, H.; Watanabe, N.; Yamamoto, S.; Shimomura, S.; Kitazawa, D.; Yokota, T.; Umezu, S.; Nguyen, T. Q.; Someya, T. Thermally Stable, Highly Efficient, Ultraflexible Organic Photovoltaics. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 4589−4594. (22) Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Stretchable Active-Matrix Organic Light-

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02297. Supplementary table, and Figures S1−S16. Fabrication process of the nanofiber-reinforced elastic conductor, crack observation in various nanofiber contents, filler localization by the filtering effect, strain−stress curves of the composite materials, comparison of buckling behavior between 100 nm gold on 1.4 μm PET film and the tough elastic conductor made by 1000% prestrain, biaxial wrinkle structure, and images of the poking test; comparison of the obtained cyclic durability with those of previously reported stretchable electrodes (strain, 50%, 1000 cycles), noise level of EMG sensor in multimodal sensing suit; estimation of pore size by image processing; cyclic stabilities of the tough elastic conductors at 5000 cycles (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Sunghoon Lee: 0000-0002-3592-388X Tomoyuki Yokota: 0000-0003-1546-8864 Daisuke Hashizume: 0000-0001-7152-4408 Takao Someya: 0000-0003-3051-1138 Present Address §

(H.J.) ICT Materials & Components Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), Daejeon 34129, Republic of Korea. Author Contributions

H.J. and T.So. conceived and designed the experiments. H.J., M.O.G.N., S.L, and T.Y, fabricated the materials and devices. D.I. and D.H. conducted FIB-SEM observation. H.J. and M.O.G.N. characterized the multimodal sensor system. H.J., D.I., D.H., N.M., S.L., T.Y., and T.So. analyzed the data. H.J. and T.So. wrote the manuscript. T.So. supervised this project. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was financially supported by the Japan Science and Technology Agency ACCEL (Grant JPMJMI17F1). H.J. was supported by Graduate Program for Leaders in Life Innovation. The authors thank BANDO Chemistry and Daikin Industries Co. Ltd. for the materials used in this study. REFERENCES (1) Stoppa, M.; Chiolerio, A. Wearable Electronics and Smart Textiles: A Critical Review. Sensors 2014, 14, 11957−11992. (2) Castano, L. M.; Flatau, A. B. Smart Fabric Sensors and E-Textile Technologies: A Review. Smart Mater. Struct. 2014, 23, 053001. (3) Zhang, Z.; Cui, L.; Shi, X.; Tian, X.; Wang, D.; Gu, C.; Chen, E.; Cheng, X.; Xu, Y.; Hu, Y.; Zhang, J.; Zhou, L.; Fong, H. H.; Ma, P.; Jiang, G.; Sun, X.; Zhang, B.; Peng, H. Textile Display for Electronic and Brain-Interfaced Communications. Adv. Mater. 2018, 30, 1800323. (4) Kaltenbrunner, M.; Sekitani, T.; Reeder, J.; Yokota, T.; Kuribara, K.; Tokuhara, T.; Drack, M.; Schwödiauer, R.; Graz, I.; BauerGogonea, S.; Bauer, S.; Someya, T. An Ultra-Lightweight Design for Imperceptible Plastic Electronics. Nature 2013, 499, 458−463. G

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano Emitting Diode Display Using Printable Elastic Conductors. Nat. Mater. 2009, 8, 494−499. (23) Chun, K.-Y.; Oh, Y.; Rho, J.; Ahn, J.-H.; Kim, Y.-J.; Choi, H. R.; Baik, S. Highly Conductive, Printable and Stretchable Composite Films of Carbon Nanotubes and Silver. Nat. Nanotechnol. 2010, 5, 853−857. (24) Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C.-K.; Hellstrom, S. L.; Lee, J. a.; Fox, C. H.; Bao, Z. Skin-like Pressure and Strain Sensors Based on Transparent Elastic Films of Carbon Nanotubes. Nat. Nanotechnol. 2011, 6, 788−792. (25) Xu, F.; Zhu, Y. Highly Conductive and Stretchable Silver Nanowire Conductors. Adv. Mater. 2012, 24, 5117−5122. (26) Liang, J.; Tong, K.; Pei, Q. A Water-Based Silver-Nanowire Screen-Print Ink for the Fabrication of Stretchable Conductors and Wearable Thin-Film Transistors. Adv. Mater. 2016, 28, 5986−5996. (27) Matsuhisa, N.; Kaltenbrunner, M.; Yokota, T.; Jinno, H.; Kuribara, K.; Sekitani, T.; Someya, T. Printable Elastic Conductors with a High Conductivity for Electronic Textile Applications. Nat. Commun. 2015, 6, 7461. (28) Mars, W. V.; Fatemi, A. A Literature Survey on Fatigue Analysis Approaches for Rubber. Int. J. Fatigue 2002, 24, 949−961. (29) Shi, G.; Zhao, Z.; Pai, J.-H.; Lee, I.; Zhang, L.; Stevenson, C.; Ishara, K.; Zhang, R.; Zhu, H.; Ma, J. Highly Sensitive, Wearable, Durable Strain Sensors and Stretchable Conductors Using Graphene/ Silicon Rubber Composites. Adv. Funct. Mater. 2016, 26, 7614−7625. (30) Wu, S.; Zhang, J.; Ladani, R. B.; Ravindran, A. R.; Mouritz, A. P.; Kinloch, A. J.; Wang, C. H. Novel Electrically Conductive Porous PDMS/Carbon Nanofiber Composites for Deformable Strain Sensors and Conductors. ACS Appl. Mater. Interfaces 2017, 9, 14207−14215. (31) Choong, C.-L.; Shim, M.-B.; Lee, B.-S.; Jeon, S.; Ko, D.-S.; Kang, T.-H.; Bae, J.; Hoon Lee, S.; Byun, K.-E.; Im, J.; Jeong, Y. J.; Park, C. E.; Park, J. J.; Chung, U. I. Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array. Adv. Mater. 2014, 26, 3451−3458. (32) Lee, B. L.; Liu, D. S. Cumulative Damage of Fiber-Reinforced Elastomer Composites under Fatigue Loading. J. Compos. Mater. 1994, 28, 1261−1286. (33) Le Cam, J.-B.; Huneau, B.; Verron, E.; Gornet, L. Mechanism of Fatigue Crack Growth in Carbon Black Filled Natural Rubber. Macromolecules 2004, 37, 5011−5017. (34) Cooper, C. B.; Joshipura, I. D.; Parekh, D. P.; Norkett, J.; Mailen, R.; Miller, V. M.; Genzer, J.; Dickey, M. D. Toughening Stretchable Fibers via Serial Fracturing of a Metallic Core. Sci. Adv. 2019, 5, eaat4600. (35) Jiang, S.; Chen, Y.; Duan, G.; Mei, C.; Greiner, A.; Agarwal, S. Electrospun Nanofiber Reinforced Composites: A Review. Polym. Chem. 2018, 9, 2685−2720. (36) Yang, W.; Sherman, V. R.; Gludovatz, B.; Schaible, E.; Stewart, P.; Ritchie, R. O.; Meyers, M. A. On the Tear Resistance of Skin. Nat. Commun. 2015, 6, 6649. (37) Brown, H. R. The Adhesion Between Polymers. Annu. Rev. Mater. Sci. 1991, 21, 463−489. (38) Maeda, N.; Chen, N.; Tirrell, M.; Israelachvili, J. N. Adhesion and Friction Mechanisms of Polymer-on-Polymer Surfaces. Science 2002, 297, 379−382. (39) Nur, R.; Matsuhisa, N.; Jiang, Z.; Nayeem, M. O. G.; Yokota, T.; Someya, T. A Highly Sensitive Capacitive-Type Strain Sensor Using Wrinkled Ultrathin Gold Films. Nano Lett. 2018, 18, 5610− 5617.

H

DOI: 10.1021/acsnano.9b02297 ACS Nano XXXX, XXX, XXX−XXX