Transparent and Waterproof Ionic Liquid-Based Fibers for Highly

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Transparent and Waterproof Ionic Liquids-Based Fibers for Highly Durable Multifunctional Sensors and Strain-Insensitive Stretchable Conductors Song Chen, Haizhou Liu, Shuqi Liu, Pingping Wang, Songshan Zeng, Luyi Sun, and Lan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17790 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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ACS Applied Materials & Interfaces

Transparent and Waterproof Ionic Liquids-Based Fibers for Highly Durable Multifunctional Sensors and Strain-Insensitive Stretchable Conductors Song Chen,ab Haizhou Liu,a Shuqi Liu,a Pingping Wang,a Songshan Zeng,b Luyi Sun*b and Lan Liu*a a

College of Materials Science and Engineering, Key Lab of Guangdong Province for High

Property and Functional Macromolecular Materials, South China University of Technology, Guangzhou, 510640, P. R. China. b

Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of

Materials Science, University of Connecticut, Storrs,Connecticut, 06269, USA.

ABSTRACT Ionic liquids (ILs) are regarded as ideal components in the next generation of strain sensors because their ultralow modulus can commendably circumvent or manage the mechanical mismatch in traditional strain sensors. In addition to strain sensors, stretchable conductors with strain-insensitive conductance are also indispensable in artificial systems for connecting and transporting electrons, similar to the function of blood vessels in the human body. In this work, two types of ILs-based conductive fibers were fabricated by developing hollow fibers with specific micro-scale channels which were then filled with ILs. Typically, the ILs-based fiber with straight micro-channels exhibited a high strain sensitivity and simultaneously rapid responses to strain, pressure, and temperature. The other ILs-based fiber with helical micro-channels exhibited good strain-isolate conductance under strain. Due to the high transparency of ILs along with the sealing process, the as-prepared ILs-based fibers are both highly transparent and waterproof. More importantly, owing to the low modulus of ILs and the core-shell structure, both of the conductive fiber prototypes demonstrated high durability (>10000 times) and long term stability (>four months). Ultimately, the ILs-based fibrous sensors were successfully woven into gloves flaunting the ability to detect human breathing patterns, sign language, hand gestures, and arm motions. The ILs-based strain-insensitive fibers were successfully applied in stretchable wires as well. Keywords: ionic liquids; helical channel; high durability; multifunctional sensor; strain-insensitive conductor INTRODUCTION Stretchable sensory systems and strain-insensitive stretchable interconnectors are two essential

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components in advanced wearable devices; one for perceiving stimulation in a way much like human skin;1–9 the other for connecting and transporting electrons similar to the network of blood vessels which transport blood throughout the body.10–14 Although multifarious stretchable sensors or strain-insensitive conductors based on buckle structures,15–19 micro-cracks,20,21 micro-porous structures,22,23 and percolation networks24,25 have been widely reported, currently almost all were fabricated by integrating rigid metal, semiconductor, and carbonaceous materials into supporting elastic materials. These strategies have achieved some encouraging progress to date, but the most significant issue of the mechanical mismatch between the filler and the supporting material remains unresolved.26–29 Typically, most rigid fillers have a high Young’s modulus of 1011−1012 Pa, about 6 orders of magnitude higher than that of the elastic substrate (Young’s modulus of 105−107 Pa).30 The large difference in Young’s modulus can easily lead to the delamination and local fracturing of the material when subject to strain, thus limiting the durability and stability of the final products. For this reason, liquid conductors such as liquid metal (LM) and ILs, with low Young’s modulus of 0) is the deformation factor accounting for intrinsic properties of PDMS (i.e., Young’s modulus). Apparently, after loading force, the numerator increases while the denominator decreases. Thus even a tiny deformation may translate to a large resistance change, resulting in the high sensitivity of the SCF. The response time of the SCF to pressure was also further recorded by


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applying an instantaneous pressure of 1 N. As exhibited in Figure S14 (Supporting Information), the SCF presented a fast response time of 80 ms to pressure, showing fast response to pressure. In addition, the SCF was compressed under three different forces (1N, 5N, and 7N) with each loading/unloading cycle repeated 7 times. As shown in Figure 4b, the SCF exhibits favorable responses and high reproducibility to different pressures, demonstrating its potential for pressure sensor applications. For temperature sensors, it is well known that the ILs, being a kind of salts in liquid, possess the property of ultra-thermal-sensitivity. Figure 4c shows the relative resistance change of the SCF versus temperature. The SCF curve exhibits a strong linearity from room temperature (25 °C) up to 100°C, with a high thermal sensitivity of 0.0092 °C-1. This presents the SCF as a great candidate for applications in humanoid robots and artificial electronic skins, where the ability to sense temperature can help them to avoid potential harm from high temperature, effectively making them more humanoid. For instance, Figure 4d and Video S2 (Supporting Information) display the response of the SCF to hot water at about 80 °C. When hot water was added into a beaker where the SCF was fixed (inset in Figure 4d), the relative resistance dropped rapidly (response time of 17.37 s), demonstrating its ability to avoid being scalded. In another instance, the SCF was fixed on a glass slide to detect human breathing. As shown in Figure 4e, when the SCF was exposed to a mouth exhalation, an immediate resistance jump was captured (Video S3, Supporting Information). Multiple tests further proved that the SCF had a rapid and stable response to mouth exhalation. Remarkably, a similar but weaker response was also measured for noise exhalation (Figure 4f and Video S4, Supporting Information). These results show that the SCF has major potential in thermal controlling applications, such as the ability to trigger an operating behavior when a human yawns or blows. Due to the ILs being sealed in the micro-channel, the as-prepared SCF is waterproof. As documented in Video S5 (Supporting Information), a SCF was submerged in water for strain sensing tests. Interestingly, as light decrease in resistance could be observed as the SCF was immersed in water. The minor change was due to the water temperature being slightly higher than room temperature as well as the pressure under water being higher than air pressure. As described in Figure 4g, the SCF in water provided rapid and accurate responses to stretching, holding, and releasing. Furthermore, the SCF also exhibited high stability and durability when used in water, evidenced in that all of the stretching/releasing tests were repeated multiple times. The findings reveal that the SCF is promising for applications in monitoring human motions underwater such as swimming or diving.



Figure 5. Sign language and hand gesture recognition of the SCF woven into a smart glove. (a, b) Digital photograph of the as-prepared smart glove to show that the SCFs are successfully woven into the fingers. (c) Predefined hand configuration and the corresponding smart glove photographs for sign language letters “S”, “C”, “U”, and “T”. (d) The output relative resistance curves measured from each SCF when expressing the letters “S”, “C”, “U”, and “T”. (e) Predefined hand configuration and the corresponding smart glove photographs for hand gestures “Phone”, “OK”, “Right”, and “Love”. (f) Resistance change of the SCFs on the smart glove for several hand gestures. Next, a smart glove for the right hand was fabricated for the recognition of human sign language and hand gestures by integrating the SCFs into each finger of a fabric glove (Figure 5a). As shown in Figure 5b, the fiber-shaped SCF was successfully woven into the index finger of the glove. The smart glove was then worn by a volunteer. When the volunteer bent and straightened his fingers, the resistance corresponding to each SCF rapidly increased and decreased, meaning that the smart glove was able to monitor various digit motions in real time with no noticeable crosstalk between each SCF (Figure S15, Supporting Information). Consequently, the smart glove was able to successfully interpret the overall hand configuration (i.e. American Sign Language (ASL) letters, complicated human gestures) during spatial deformations. For instance, in sign language, the bent state of each finger and their positions are varied to express different letters. Figure 5c displays the four hand gestures for the ASL letters: “S”, “C”, “U”, and “T” (denoted by “South China University of Technology”), and the successful corresponding output resistance change for each finger are detailed in Figure 5d. It can be observed that no significant crosstalk appears between the integrated HCFs, denoting their important academic value in wearable human-machine interaction, prosthetic limbs, and communication with deaf-mute people. To characterize their application in complicated human gestures, four familiar hand gestures (“phone”, “OK”, “right”, and “love”) were investigated (Figure 5e). As shown in Figure 5f, different data shapes were


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gathered in real-time, showing the HCF’s accurate and rapid response to complicated hand gestures.

Figure 6. (a) Monitoring of various human motions in real time through the physical movements of various body parts (hands, wrist, and elbows). (b) The relative resistance change of the SCF in response to the wrist motions. Insets are photographs showing a wrister used for the SCF integration and motion detection. (c) Digital photographs showing the SCF integrated in an elbow wrap to monitor the bending motion of the elbow at different angles of 45°, 90°, and 135°. (d) The corresponding resistance change in response to the elbow motions at different angles. In addition to the signal sensing of hand gestures, the SCF also has excellent capacity in the detection of wrist and elbow motions (Figure 6a). As shown in Figure 6b, a wrister functionalized by the SCF could clearly capture the wrist motions in real time. To monitor elbow bending, the SCF was integrated in an elbow wrap which was then worn on a human’s arm such that the SCF was oriented at the outside of the elbow. With the bending of the human arm at angles of 45°, 90°, and 135° (Figure 6c), the curve of the response resistance change was generated for Figure 6d, showing that the relative resistance increases proportionally to the applied angles and demonstrating the SCF’s ability in both detecting and quantifying the elbow motions. The incorporation of the detection of finger, wrist, and elbow motions establishes our sensor as a powerful tool in wearable devices, human-computer interactions, gesture-based identification, and humanoid limbs. SUMMARY In summary, two types of transparent and waterproof ILs-based conductive fibers with micro-scale



channels (100 µm) were fabricated for multifunctional sensors and stretchable strain-insensitive wires. For ILs-based sensors, the conductive mechanism and the influence of channel sizes on sensitivity were investigated. The SCF demonstrated superb responses to strain, pressure, and temperature, and was further used in measuring breathing, capturing signal language, detecting hand gestures and human motion. For ILs-based strain-insensitive wires, the helical micro-channel model was originally designed. The HCF was shown to exhibit an insensitive conductivity under 100% stretching, and was successfully used to replace commercial wire in an LED circuit. More importantly, both the SCF and HCF exhibit excellent durability for practical applications due to the low modulus of ILs and the core-shell structures, endowing them with huge potentials in wearable devices, human-machine interaction, and artificial skin in the future. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. The molecular structure of the ionic liquids, digital photograph of as-prepared hollow silicone fiber, schematics of electromechanical measurement setup, relevant parameters of the four different hollow silicone fibers, digital photograph of the four different hollow silicone fibers, ILs and channel deformation during stretching, the summary of channel size, channel shape, and gauge factor for typical ILs-based sensors reported in recent three years, schematic illustration of the fabrication of helical Cu template, SEM image of helical Cu template, cross sectional SEM images of hollow PDMS fibers with straight channel and helical channel, stress-strain curves of as-prepared PDMS fibers with straight and helical channel before and after ILs filling, electrical responses at the dynamic strain of 5%: strain input frequency of (a) 1 Hz, (b) 2 Hz, digital photograph of the industrial wires and HCF, schematic illustration of the industrial wire and HCF, LED circuit carrying out by replacing a section of Cu wire with HCF, digital photograph of the SCF under pressure, schematic illustration of the SCF deformation under pressure, response property of the SCF to pressure, relative resistance change of the SCF on each finger of the smart glove, and the summary of stretchability for typical PDMS-based stretchable electronics reported in recent years (PDF) CORRESPONDING AUTHOR INFORMATION *Lan Liu, E-mail: [email protected]; *Luyi Sun, E-mail: [email protected]. ORCID Lan Liu: 0000-0001-9514-4602


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Luyi Sun: 0000-0002-5419-1063 ACKNOWLEDGEMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 51573053), the Science and Technology Planning Project of Guangdong Province (Grant No. 2014A010105022), and the Special Funds for Applied Science and Technology Research and Development of Guangdong Province (Grant No. 2015B020237004). REFERENCES (1)

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Transparent and Waterproof Ionic Liquid-Based Fibers for Highly

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