Liquid-Wetting-Solid Strategy To Fabricate Stretchable Sensors for

Jan 19, 2016 - To address this challenge, we report a simple yet general liquid wetting solid strategy to fabricate stretchable conductors, which can ...
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Liquid-Wetting-Solid Strategy To Fabricate Stretchable Sensors for Human-Motion Detection Zheng Ma,† Bin Su,*,† Shu Gong,† Yan Wang,† Lim Wei Yap,† George. P. Simon,‡ and Wenlong Cheng*,†,§ †

Department of Chemical Engineering and ‡Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia § The Melbourne Centre for Nanofabrication, Clayton, Victoria 3800, Australia S Supporting Information *

ABSTRACT: The inherent limitation of stretchable conductor design is mechanical mismatch, because typical Young’s moduli of inorganic conductors are 5−6 orders of magnitude larger than that of the soft elastomersleading to material delamination and/or local fracturing under large strains. To address this challenge, we report a simple yet general liquid wetting solid strategy to fabricate stretchable conductors, which can overcome the aforementioned challenges. Our approachutilizing ionic liquids (ILs) as the conductive componentsis conceptually different from traditional metals/polymers (briefly rigid-on-soft type) construction, since we employ a lower Young’s modulus conductive liquid to integrate with elastomers (briefly soft-on-soft type). It is also different from previously reported liquid metal strategy, in which high surface tension limits the scope of applications. Our IL-based strategy is universal and applicable to different hydrophilic/hydrophobic IL species, and able to turn diverse soft elastomeric supports into stretchable conductors in a simple and rapid manner. The ILbased conductors exhibit exceptional performancefunctioning at ultralarge strains (ε > 600%); high sensitivity down to a low-strain of 0.05%; high durability with negligible loading−unloading signal changes over 10 000 cycles. In addition, skin-attachable and cloth-integratable features allow a wide range of human-motion detections. We envision that this liquid-wetting-solid strategy will be promising on the large-scale fabrication of stretchable electronics, personal health monitoring, and “smart” electrical skins for soft robots and prosthetics. KEYWORDS: stretchable conductor, ionic liquids, strain sensor, wetting, health monitoring

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the elastic scaffolds have been used to construct continuous, percolative conductive networks. Although significant efforts have been invested in increasing the composite material strength, the intrinsic difference in Young’s modulus between the rigid conductors and soft support material remains, leading to poor long-term durability due to material delamination and/ or local fracture in rigid electronic components. Compared with solid conductive materials, liquid conductors possess a much smaller Young’s modulus, e.g., several Pascals,10 and can spread upon the elastic supports, and remain so even under a large strain. The strategy to use softer liquid materials to generate stretchable conductors is thus a promising strategy to produce high-performance stretchable sensors. In this paper we demonstrate a general and robust strategy for the fabrication of reliable and long-term stretchable sensors using ionic liquids (ILs) which can overcome the mechanicalmismatch problem due to their low Young’s moduli. Our ILbased strategy is universal and can be used for a diverse range of

tretchable conductors have broad technological application for areas ranging from autonomous artificial intelligence, for example, electronic skins on robots,1−3 to wearable health monitors.4−8 The simultaneous combination of mechanical robustness and electronic performance is the key for reliable stretchable conductors.9 Flexible polymers, with Young’s moduli of 105−107 Pa, are commonly used as the supports due to their reversible, stretchable properties.10 To render these soft elastomers conductive, rigid metal/semiconductor building blocks, such as gold,2a,c copper,2b silicon,3b carbon,3c or others (high Young’s modulus of 1011−1012 Pa) have been filled, deposited, or grown on (or in) the polymeric support materials.11,12 Note that there is a mechanical mismatch between the elastomers and the conductive fillers; the intrinsic moduli of the former are 5−6 orders of magnitude less than of the latter, which is the reason the rigid conductive components cannot conformally follow the deformation of the soft elastic supports, especially under large strains. To solve this mechanical mismatch issue, waves,13−15 meanders,16 helices,17 spirals, or net structured18 conductive components are designed to improve their apparent stretchability. On the other hand, nanomaterial-based methods utilizing nanoparticles,19,20 nanowires,21 or nanosheets22 to fill © XXXX American Chemical Society

Received: October 30, 2015 Accepted: January 19, 2016

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Figure 1. Facile fabrication and durable stretchable conductors by the IL wetting strategy. (a) Schematic showing the fabrication of IL-based stretchable conductors created by dropping ILs upon the fabric. Due to their lower Young’s modulus (5−6 orders of magnitude smaller than that of elastomers), the ILs remain as a thin yet continuous liquid layer, even if the fabric is stretched under a large strain, effectively yielding a large-strain stretchable conductors. (b) Current−time characteristics of the fabrication process of ILs infused polymeric fibers. The ILs could be directly dropped onto the fiber mats by a pipet. After several cycles of stretching, the ILs uniformly spread out upon the fiber surfaces, prducing stable and continuous electrical responses after 23 s. It should be noted that the whole time for fabrication and adjustment was just 10 s. These inset pictures are timesequence images of fabricating the IL-based stretchable conductors (more details can be found in Supplemental Movie S1). After storing the ILbased conductors for two months, their force-responsive ability remained without change. The applied voltage in all the electrical tests was 5 V, the relative humidity was 45% and the temperature was 25 °C. 3D laser scanning confocal microscope observations of IL wetting POE fiber mats under (c) 0% and (d) 100% strains are shown. These pictures are top-view images collected from different slices in (c, d) using built-in software. The Rhodamine B was dissolved inside ILs to enable it to show red fluorescence when exposed to the laser irradiation. Regardless of whether they were imaged before or after stretching, continuous red fluorescence could be observed along the fibers, indicating the existence of IL layer upon fiber surfaces. In this case, the fiber conductors could remain conductive, yielding a large-strain stretchable conductor. methylimidazolium trifluoromethanesulfonate [bmim][TfO], tetrabutylphosphonium methanesulfonate, 1-methyl-3-octylimidazolium chloride, 1-butyl-3-methylimidazolium bromide, and 1-(3-cyanopropyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)amide, were purchased from Sigma-Aldrich. Water was added in some cases to make the materials liquid. These chemicals remain in the liquid state and can be used in further study. Chloroform (∼99.9%) and ethanol (∼99.0%) were purchased from Merck KGaA. Porous polyurethane sponges, cotton T-shirts, and sport clothes were common commercial products that were infiltyrated and were purchased. The stainless, thin conductive threads for making electrical connection were purchased from Adafruit Industries. Preparation IL-Based Stretchable Conductors. For POE fiberbased samples, 6 g POE pallets were dissolved in 60 mL chloroform and stirred for 24 h. Three milliliters of ethanol was added into the solution and stirred for another 24 h. The well dissolved polymer solution has been force-spun on the Force Spinner Cyclone L-1000 M (FibeRio) at 5000 rpm to generate the POE fiber mats, which have been dried at room temperature. This device is a rotary spinner in which a fine, micron-sized solution jet is sprayed out due to centrifugal

hydrophilic/hydrophobic IL species, which is able to turn different soft elastomeric supports (force- or electro-spun fiber mats, rubbers, clothes, sponges, etc.) into stretchable conductors in a simple and rapid manner. As-prepared stretchable conductors exhibit exceptional performance functioning up to ultralarge strains (ε > 600%); possessing high sensitivity down to a low strain of 0.05%; show a high durability with negligible loading−unloading signal changes over 10 000 cycles. In addition, skin-attachable and clothintegratable features allow for a wide range of human-motion detection. Our methodology opens up a new powerful route to fabricate stretchable conductors for a myriad of applications in future flexible electronics.

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EXPERIMENTAL SECTION

Materials. Polyolefin elastomer (POE, Engage 7467) was purchased from Dow while Ecoflex (00−30) was purchased from Smooth-On, Inc. Rhodamine B (>95%), ILs, such as 1-butyl-3B

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ACS Sensors forces generated by the rapid rotation of a cylindrical reservoir with a 20-μm-diameter hole. The POE fiber mat was cut into ∼1 × 2 cm2 fiber sheets and ILs drop-cast onto it (the weight ratio of liquid/solid is 2/1). In order to generate a uniform liquid conductive layer along the fiber surfaces, a plastic plate was utilized to induce the spreading of ILs for at least 5 times, yielding stable, stretchable sensors. For the generation of cotton fabric, or sponge type stretchable sensors, ∼1 × 2 cm2 sample pieces have been cut from a commercial cotton T-shirt or polyurethane washing sponges, respectively. The rubber band thread consisted of three rubber bands interlocked with each other. The following processes were similar to that of POE mat based sensors. Preparation of IL Patterned Sensors. For the IL patterned rubber film, a “Kangaroo” type plastic stamp was fabricated by an Objet 3D printer (Eden 260 V). The stamp was immersed in Rhodamine B dyed ILs and pressed on to a rubber film for at least 5 s and removed. As a result, an IL-patterned rubber film was generated. Preparation of IL/Rubber-Film Sensors toward Wrist Pulse. A 10 μL droplet of ILs has been connected with two conductive threads at two ends, and sandwiched between two layers of Ecoflex films with thickness of ca. 500 μm. A few Ecoflex liquid prepolymers (a mixture of component A with component B at weight ratio of 1/1) were used to seal these two layers by heating at 80 °C. This IL/rubberfilm sensor was able to be directly placed onto the artery of the volunteer’s wrist. Characterization. The electrical characteristics for all the stretchable conductors were recorded by the Parstat 2273 Electronchemical System (Princeton Applied Research) with the assistance of a translation stage (Model LTS150/M, Thorlabs). For proof-ofprinciple demonstrations to monitor human behavior, all the fabric based sensors have been cut into suitable sizes that closely attached to the human body. To connect the electronic system, as well as physical integration to cloth, commercial conductive threads were directly sewn to the two ends of sensors, as the source-drain electrodes. Confocal images were obtained using a Nikon A1Rsi laser scanning confocal microscope with the excitation wavelength at 561.3 nm. Optical images and demonstration videos were taken by a Nikon Digital Sight DS-Fil camera. SEM images were carried out using a FEI Nova FEGSEM operated at 5 kV beam voltage. The contact angle was measured on a contact angle goniometer (OCA 15EC).

support (Figure S1a,c). In comparison, ILs were able to spread uniformly across the elastic support, regardless of strain, for example, 0% or 40% strain (Figure S1b,d), indicating that their wettability meets our Criteria 2. In addition to the easy-pinning advantage, the fabrication of IL-based stretchable conductors is simple, equipment-free and scalable (Criteria 3). In the following sections, [bmim][TfO] has been used as the conductive component due to its stable electrical performance and low cost. Polyolefin elastomer (POE) was utilized as the elastic support since this new class of copolymers can exhibit excellent mechanical robustness up to 600% strain.30 Using this simple, yet efficient IL-wetting approach, any elastomeric objects with dimensions from microscale to macroscale can be converted into highly deformable stretchable conductors in a short time frame. A POE fiber mat was fabricated by the force-spinning technique (see Experimental Section). The POE mat without ILs exhibited a constant resistance value, even under diverse stains. ILs were able to uniformly wet the POE mat by simple drop casting, leading to a conductive composite after a few cycles of stretching (Figure 1a). As a result, a highly stretchable conductor could be fabricated within some 20 s (Figure 1b and Supplemental Movie S1). The resistance switching between State A (native state) and State B (stretched state) was fully reversible. Remarkably, such stretching-induced responses were highly stable and could be reproduced after 2 months of sample aging under ambient conditions (Figure S2). It is worth noting that two months is not the final presumed lifetime limitation of this liquid-wettingsolid type conductor, since the ILs have the potential to remain liquid state at ambient for years.29 As well as being stored indoors, as-prepared IL-based conductors have been placed in a much harsher environment (Figure S3). It is noteworthy that the resistance of the samples showed negligible change, even after being exposed to a wide range of changes of temperature or humidity. Furthermore, the influence of gravity of the liquid showed a negligible influence on their electrical responses due to the low quantity of ILs used and strong capillary force within fiber gaps. No obvious change in sensitivity was found, even after the fiber mats were held horizontally and exposed to ambient conditions for 24 h (Figure S4). High reproducibility and durability could be attributed to the strong adhesion of ILs onto POE fibers, which has been observed by laser scanning confocal microscope and the contact angle measurement. In confocal testing (Figure 1c), Rhodamine B was used to stain ILs to facilitate observation. Before stretching, red fluorescence appeared inside fiber gaps (the fibers have no fluorescence; see the black wires), indicating that the ILs have fully permeated the fiber network. Under 100% strain, several aligned fiber structures could be found parallel to the stretching direction. Notably, red fluorescent coloring covered the whole fiber structures, indicative of a continuous IL layer associated with deformed fiber surfaces. In the side-view contact angle observation (Figure S5), the three-phase contact line (TCL) of liquid firmly pinned upon the fibers before/after the stretch, indicating considerable adhesion existed between the liquid and polymer. Since the Young’s modulus of IL is 5−6 orders of magnitude smaller than that of elastomers,10 the strain limitation of this ILbased stretchable conductor depends on the elastic supports themselves, rather than conductive components. Hence, such stretchable conductors are able to function until the mechanical breakdown of elastomers following severe deformation. The

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RESULTS AND DISCUSSION Advances of Ionic Liquids for Building Stretchable Conductors. We designed the liquid-wetting-solid type stretchable conductors based on three criteria: (1) The conductive liquid should have extremely low vapor pressure under ambient conditions. (2) The conductive liquid should adhere firmly to the flexible polymers. Previously, metallic liquids, such as eutectic gallium−indium (EGaIn), have been used in the fabrication of stretchable conductors.23−26 However, liquid metals show poor adhesion to the elastic supports due to their large surface tension (hundreds of mN· m−1).27 Hence, they have to be sealed inside pregenerated microchannels, rendering these devices neither facile in construction nor universal. (3) The fabrication of conductive liquids incorporated with elastic supports should be simple, equipment-free, and scalable. Room-temperature ILs are salts in the liquid state under ambient temperature of ∼25 °C.28 They behave like solid exhibiting ultralow evaporation rate, demonstrating a long lifetime in conductive liquid state (Criteria 1). In addition, ILs usually possess high viscosity and low surface tension (e.g., ∼20−40 mN·m−1),29 which are attributes for their easy integration with elastomeric supports. To prove this, wetting properties of liquid metals and ILs have been compared on the same elastic supports. Owing to its considerably high surface tension, the liquid metal remained in a spherical shape and is difficult to deform even under a 40% strain of the bottom C

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Figure 2. Stable electrical performance of IL/POE type stretchable conductors. The dependence of the resistance change of IL/POE samples on (a) temperature and (b) relative humidity. (c) Strain-response plots for the IL/POE conductor. The conductor exhibit a gauge factor (GF) of 1.11 during 0% and 600% strain. The inset image is a strain-response plot for the conductor under small strain from 0% to 4%. (d) Plots of resistance change of the conductor as a function of time (input frequency: 0.5 Hz) for the applied strain in the range of 1−200%. (e) Lifetime test under a strain of 10% at a frequency of 1 Hz. Left part: The resistance change curves were recorded after each 2000 cycles and 200 cycles of data are shown in each recording. Right part: The magnified view of the part of the ΔR/R0 − t curve after 10 000 loading−unloading cycles. (f) Plots of resistance change of the conductor as a function of time (input strain: 1%) for diverse frequencies including 9, 4, and 1.4 Hz. The samples have been fixed at IL/fiber weight ratio around 200%, the applied voltage in all the electrical tests was 5 V, the relative humidity was 45%, and the temperature was 25 °C.

1 cm2, the resistance gradually decreased from 8.15 MΩ (ratio of 31 wt %) to 0.15 MΩ (ratio of 294 wt %). Typically, the IL/ fiber weight ratio was maintained at around 200% to obtain a stable resistance of ca. 0.20 MΩ. It is worth noting that the cost for this stretchable conductor is about 14 cents/cm2 (the amount of ILs upon 1 × 1 cm2 fiber mat is just ∼6.7 mg and the price of this commercial IL is 3.14 dollars/gram), indicating this IL-based fabrication strategy is both a promising and af fordable route to produce stretchable conductors. Temperature increase will facilitate the movement of ions in the ILs, leading to a decreased overall resistance.31 We measured the conductivity of the ILs as a function of temperature. Figure 2a shows the temperature dependence of resistance, normalized to their values at 25 °C. Specifically, as

IL/POE conductors fabricated in this study could be deformed to strains of more than 600% (see Figure S6), yet still returned to original conductivity after strain release. The dependence of the mat pore size on the electrical responding has been investigated (Figure S7). Three kinds of fiber mats with average pore sizes of 41, 63, 112 μm have been fabricated and wetted by the same amount of ionic liquids, and their electrical performances determined. As shown in Figure S7, the fiber mat with the smaller pore size showed a higher resistance change than those with larger pores. Electrical Performance of IL-Based Conductors. The weight ratio of ILs/fibers played an important role in determining their overall resistance (Figure S8). By gradually dropping additional amounts of ILs on to the fiber mats of 1 × D

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Figure 3. Any topologically complex structures at the multiscale can be wetted by ILs, yielding stretchable conductors. Digital images of (a) a commercial cotton T-shirt, (b) a rubber band, and (c) a commercial polyurethane washing sponge. These inset images are scanning electronic microscopy (SEM) observations of (a−c) showing fiber or porous structures of elastic supports. The side images are 3D laser scanning confocal microscope observations of IL wetting (a) a commercial cotton T-shirt, (b) a rubber band, and (c) a commercial polyurethane washing sponge under 50% strain. Rhodamine B was used to stain ILs. Continuous red fluorescence could be observed along these microstructures, indicating that the ILs could wet any elastic surfaces, as well as the microstructure. Therefore, these IL infused elastic scaffolds can all serve as the stretchable conductors under diverse strains (more details can be found in SI Figure S9 and Movie S2). (d), Digital images of a “kangaroo” type IL pattern upon a rubber film under 0% strain (top) and 50% strains (bottom). The resistance of IL pattern changed according to the strain of the rubber film, yielding a stable stretchable conductor.

the temperature increased from 25 to 75 °C, conductance values decreased by ∼75%. Previous studies have also shown diverse sensitivities of ILs to humidity depending on their chemical structure. Such a sensitivity is often attributed to enhanced ion conductivity and/or concentration.32,33 We therefore investigated the relationship between the conductivity of ILs and surrounding humidity. [bmim][TfO] exhibited a high sensitivity to humidity, with a 1.4% change in conductance per 1% increase in humidity (Figure 2b). The balance of humidity and temperature on affecting the sensor performance has been determined, as shown in Figure S2. The humidity plays a dominating role in deciding the final electrical changes. Thus, the following testing temperature was maintained at 25 °C while the humidity remained at 45% for further studies. The dependence of resistance change on a range of physical strains (from 0% to 600%) was investigated (Figure 2c). The fitting of these points showed good linear behavior (R2 = 0.995) with a gauge factor (GF) value of 1.11 (the calculation of GF can be found in SI Note 1), indicating the device can serve as a reliable, stretchable conductor. This linear dependence became more obvious in the small strain range (0% to 4%, see the inset image in Figure 2c). The responses of IL/POE conductors to both static and dynamic mechanical stretching were determined. The conductor exhibited a steady response to

static stretching and the resistance under each strain was constant (Figure S9). To investigate the strain range of this conductor toward dynamic forces, a computer-controlled stretch with a minimum displacement of 1 μm was applied to the conductors, using a tensile device. As shown in Figure 2d, the stable and continuous responses could be observed at the strain ranging from 1% to 200%. Indeed, a strain of 0.05% could be detected (Figure S10), which indicates a 5 μm length increase for a 1 × 1 cm2 sample. The cycling stability of asprepared conductor was tested under a 10% strain, at a frequency of 1 Hz (Figure 2e). The resistance change with strain applied to the conductor is very consistent, and could be maintained after 10 000 loading−unloading cycles, implying a long working life and reliability of this IL-based conductor. The response of this conductor toward different frequencies was also investigated (Figure 2f). Note that the output electrical signals remained stable without obvious change in amplitude at typical frequencies of 1.4, 4.0, and 9.0 Hz. Reliable responses to other diverse frequencies from 0.1 to 8.5 Hz were also observed (Figure S11). Universal to Different IL Species and Diverse Soft Elastomeric Supports. Our simple yet efficient IL-based conductor fabrication strategy is general and universal for different hydrophilic/hydrophobic IL species, and is able to E

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Figure 4. Proof-of-principle demonstrations to monitor human skin, muscles, or wrist pulse behaviors by the IL-based conductor. The IL/POE mats have been cut into suitable sizes that were intimately and conformally attached to the human body. The fabric conductors have been fixed at ILs/ fibers weight ratio around 200%. To connect the electronic system, as well as physical integration to cloth, commercial conductive threads were directly sewn on the two ends of conductors as the source-drain electrodes. Due to the long-range sensitivity features (strain from 0.05% to 600%) of this IL-based conductor, human behaviors, such as (a) touching, (b) swiping, (c), swinging the arm, (d) kicking the leg, (e) making a fist, or (f) wrist pulse, could all be monitored by the upward and downward slopes of the relative resistance associated with external-force-induced deformation of conductors. The inset images in (a−f) are representative digital images to show the human behaviors during the test (more details can be found in SI Movie S3). The applied voltage in all the electrical tests was 5 V, the relative humidity was 45% and the temperature was 25 °C.

turn a diverse range of soft elastomeric supports into stretchable conductors in a simple yet efficient manner. Four other kinds of ILs were incorporated into the POE fiber mats, leading to highly stretchable strain conductors (Figure S12). The sensitivities of as-prepared IL-based conductors were similar to each other, with GF values of 1.1 ± 0.1. ILs with anion/cation groups with similar volume showed a slightly higher GF value compared with those consisting of small anions and large cations. It is known that ionic conductivity is determined by the migration rate of anions/cations.32,33 Larger

anions resulted in a reduced rate of diffusion, resulting in a larger resistance change when stretched under a certain strain and the GF values of these ILs are thus larger than those involving smaller anions. Nevertheless, the difference was not significant, approximately up to 20% in GF values. More importantly, we demonstrate that this IL-based wetting strategy could be applied to virtually a series of elastomeric scaffolds (Figure 3). Besides POE fiber mats, other kinds of soft supports, including 1D rubber band (Figure 3b), 2D commercial cotton fabric (Figure 3a), or 3D porous sponges F

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from the lower body.35 The radial augmentation index AIr = P2/ P1 is a characteristic value for arterial stiffness, which is closely related to the age of people. From a series of waveforms measured with our IL/rubber-film sensor, we estimated an average AIr of 0.43, agreeing quite well with the literature data35 for a healthy 25-year-old female. This result demonstrates that the subtle differences in blood pulses could be detected by our IL/rubber-film sensors, indicating its potential to serve as a wearable diagnostic device to monitor human health in real time.

(Figure 3c), could also be wetted by the ILs to generate stretchable conductors very rapidly, within half a minute. Owing to the existence of continuous IL layers upon the fabric or porous networks (see the confocal results in Figure S13), these stretchable conductors showed stable electrical responses to large strains (ε of 100% for cotton fabric, ε of 200% for rubber band, and ε of 120% for the sponge, see Figure S14 and SI Movie S2). Notably, the IL could also be patterned upon a rubber film simply by stamping it in a particular shape/image, including aesthetic patterns such as kangaroo (Figure 3d), simultaneously serving as highly deformable and wearable piezo-resistive materials. Human-Motion Detection. Our IL-based stretchable conductors are skin-attachable and fabric cloth-integratable, enabling comprehensive body motion and health monitoring. By attaching POE fiber mat patches directly onto the skin or sewing them into standard clothing, stretching of skins and muscles associated with routine human motions can be monitored in real time and in situ (Figure 4). In particular, the sensor on the human shoulder is able to detect finger clicking/scanning and hand touching (Figure 4a). These different activities provide the body skin with different pressures and strains, which can be identified by this IL-based conductor. Other human behaviors, such as swiping (sensor on shoulder, see Figure 4b), swinging arm (sensor on elbow, see Figure 4c), diverse leg movements (sensor on knee, see Figure 4d and SI Movie S3), shaking or nodding head (sensor on trapezius, see Figure S11), could also be detected by these IL-patch conductors. Because our stretchable conductors could detect strains over a very broad range (strains from 0.05% to 600%, see Figure 2d and Figure S10), a small muscle traction generated by shaking head (resistance change up to 1%, see Figure S15), or kicking the leg by a large margin (resistance change from −10% to 10%, see Figure 4d), could all be detected. Such sensors might be useful to monitor athletes for the early detection of overexercise induced muscle bruising, alerting coaches to any potential problems.34 The reproducibility of the sensors has been tested. With the swiping arm motion as an example, the electrical signals remained stable even when we repeated the same behavior three times, indicating the sensor is reliable in monitoring the human motion (Figure S16). In addition to its ready integration into a fabric, the ILsensing patch could also be attached onto the human skin to monitor the muscle movement (Both ILs and POE polymers are nontoxic to human skins). A ∼100-μm-thick polymeric tape has been sandwiched between the IL-based conductor and the human skin, so that for safety considerations the ILs do not permeate onto the skins. The functionality of the tape has been tested, shown in Figure S17. None of dyed liquids were absorbed by the tissue paper, indicating the ability of this tape to prevent the liquid permeating onto the human skins. Figure 4e and SI Movie S3 showed real-time monitoring of muscle tension/stretching associated with making fists. Some 5% of resistance changes occurred following such rapid tension/stretching cycles. Our skin-attachable IL-patch sensor was also sensitive enough to monitor a human radial artery pulse in real time (Figure 4f). The wrist pulses could be read out accurately with two clearly distinguishable peaks (P1 and P3) and a late systolic augmentation shoulder (P2). This line shape is commonly caused by constitution of the blood pressure from the left ventricle contracts and reflective wave

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CONCLUSION We report on a simple yet powerful and general strategy to fabricate piezo-resistive sensors using ionic liquids, which is advantageously different from previous reports which used liquid metals. First, the surface tension of liquid metals (hundreds of N·m−1) are much larger than that of ILs (tens of N·m−1). As a result, the liquid metals retain a spherical shape, do not spread well on the elastomers, and will not conform to the elastomer deformation without appropriate sealing (Figure S1a). This limitation could be overcome with ionic liquids, as demonstrated in this work. Also, current successes with liquid metals are limited to narrow channels, mostly using PDMS materials. In contrast, the choices of ILs are wide, and can be easily included with a diverse range of soft elastomeric materials, from macroscopic scale to microscale (force-spun fiber mats, rubbers, clothes, sponges, etc.), as seen in Figure 3. These proof-of-principle experiments demonstrate the versatility of this liquid-wetting-solid strategy to fabricate stretchable conductors. Furthermore, the ion conductivity is tunable by tailoring the molecular structures, IL concentrations, or incorporation of other conductive materials.31−33 Compared to most currently used inorganic nanomaterialsbased approaches,11−22 ILs possess two unique features: a low surface tension and a low Young’s moduli. The former allows for rapid fabrication on diverse elastomeric supports simply by wetting; the latter circumvents the issues of materials delamination and cracking for the nanomaterials-based strategy. Our methodology represents a powerful enabling route to make stretchable conductors attached to curved, textured surfaces of human body and in clothes for strains of muscles and skins. Thus, our IL-based biomedical conductors are promising components in future stretchable electronics for remote health monitoring and sport performance tracking.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.5b00195. Different adhesion behaviors of liquid metals and ionic liquids; stable and reliable stretchable conductors even stored after two months; stable conductivity of IL-based conductors in a much harsher environment; the gravity of the liquid showed negligible influence on their electrically responding; stable three-phase contact line (TCL) of liquid firmly pinned upon the fibers before/ after the strain; large-strain-available stretchable IL/POE conductors; fiber mats with smaller pore size can show a higher resistance change than the counterparts with larger pores; more ILs yielded decreased resistance of the conductors; steady response to static stretching of IL/ G

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POE conductors; a 0.05% strain could be detected by IL/ POE conductors; the IL/POE conductor was able to respond to diverse frequencies from 0.1 to 9 Hz; diverse kinds of IL species can be used in this IL-based strategy; a continuous IL layer existed upon the cotton fabric, the polyurethane sponge or the rubber band under a certain strain; stable stretchable conductors contributed by ILs wetting cotton fabric, polyurethane sponge or rubber band; shaking/nodding head behaviors can be monitored by the IL wetting POE fiber conductor; reliable reproducibility of the sensors on monitoring the human motion; the functionality of the tape to prevent the liquid permeating onto the human skins. (PDF) Time-sequence images of fabricating the IL-based stretchable conductors (AVI) Stretchable conductors under diverse strains (AVI) Real-time monitoring of muscle tension/stretching (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding: This research was financially supported under Australian Research Council’s Discovery Early Career Researcher Award (DECRA) funding scheme (DE140100541) and Discovery projects funding scheme (DP140100052 and DP150103750). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).

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DOI: 10.1021/acssensors.5b00195 ACS Sens. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssensors.5b00195 ACS Sens. XXXX, XXX, XXX−XXX