Highly Sensitive Textile Strain Sensors and Wireless User-Interface

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Highly Sensitive Textile Strain Sensors and Wireless UserInterface Devices using All-Polymeric Conducting Fibers Jimi Eom, Rawat Jaisutti, Hyungseok Lee, Woobin Lee, Jae Sang Heo, Jun Young Lee, Sung Kyu Park, and Yong-Hoon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01771 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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ACS Applied Materials & Interfaces

Highly Sensitive Textile Strain Sensors and Wireless User-Interface Devices using All-Polymeric Conducting Fibers

Jimi Eom,1 Rawat Jaisutti,2,3 Hyungseok Lee,4 Woobin Lee,1 Jae-Sang Heo,5 Jun-Young Lee,4 Sung Kyu Park,5* and Yong-Hoon Kim1,2*

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SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, Korea

2

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Korea

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Department of Physics, Faculty of Science and Technology, Thammasat University, Pathumthani,

Thailand 4

School of Chemical Engineering, Sungkyunkwan University, Suwon, Korea

5

School of Electrical and Electronic Engineering, Chung-Ang University, Seoul, Korea

ABSTRACT: Emulation of diverse electronic devices on textile platform is considered as a promising approach for implementing wearable smart electronics. Of particular, the development of multi-functional polymeric fibers and their integration in common fabrics have been extensively researched for human friendly wearable platforms. Here we report a successful emulation of multi-functional body-motion sensors and user-interface (UI) devices in textile platform by using in-situ polymerized poly(3,4-ethylenedioxythiophene) (PEDOT)-coated fibers.

ACS Paragon Plus Environment

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With the integration of PEDOT fibers in a fabric, via an optimization of the fiber pattern design, multifunctional textile sensors such as highly sensitive and reliable strain sensors (with maximum gauge factor of ~1), body-motion monitoring sensors, touch sensors, and multi-level strain recognition UI devices were successfully emulated. We demonstrate the facile utilization of the textile-based multifunctional sensors and UI devices by implementing in a wireless system that is capable of expressing the American Sign Language through pre-defined hand gestures.

KEYWORDS: Textile sensor, conducting fiber, PEDOT, in situ polymerization, Wireless


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1. INTRODUCTION Recent growing demand for wearable devices has created huge interest in patchable electronics and electronic textiles (e-textiles).1-11 For example, patchable electronics are ultra-thin, lightweight, and possess good mechanical reliability, which allow them to be utilized in electronic skin.12-14 Although patchable electronics are suitable for wearable and imperceptible electronics, providing sufficient electric power to the devices remains problematic, primarily because of the absence of ultra-thin/high-density wearable batteries15 or highly efficient energy harvesting devices. To overcome these issues, a remote-type wireless charging system has been suggested;16 however, the power transmission rate remains somewhat limited. Furthermore, there exists a spatial restriction of the transmission domains. On the other hand, in the case of e-textiles, electric power can be applied in a rather simple way, i.e., using a fiber network that is interconnected with an embedded battery or an energy harvesting device.17,18 Moreover, etextiles can be made on common fabrics or clothes, and thus, have the advantages of comfort and imperceptibility. One of the most widely studied applications of e-textiles is creating sensory systems for healthcare- and body-monitoring purposes. Specifically, various strain sensing and pressure sensing devices have been demonstrated on textile platforms. Lee et al. demonstrated a fiberbased pressure sensor where conductive silver nanoparticles were directly synthesized on the surface of poly(styrene-block-butadienstyrene)-coated Kevlar fibers.19 Additionally, BautistaQuijano et al. reported fiber strain sensors based on melt-spun polycarbonate/multiwall carbon nanotube (CNT) mono-filament fibers.20 Takamatsu et al. demonstrated a wearable keyboard based on capacitive sensors made of a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-coated knitted textile21, and Zhou et al. reported a wet-spun PEDOT:PSS


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microfibers for electromechanical devices.22,23 Although these previous investigations have successfully proven the possibility of using textiles as a platform for wearable devices and also the possibility of using a composite of silver nanoparticles and elastomeric fibers for a highly stretchable electric circuit, the use of a considerable amount of metallic/rigid components or high Young’s modulus PEDOT:PSS fibers (5.9±0.7 GPa)22, may lead to the loss of certain textile characteristics such as draping properties. Additionally, knitted-type sensors have somewhat limited application to other varieties of fabrics, because of their unique knitting patterns and fabrication processes. These drawbacks have driven efforts to realize wearable textile sensors that are fully organic and, at the same time, are applicable to a wide variety of fabrics. In addition, considering the strain sensing requirements for hand gesture or body posture sensors24,25, strain sensing range of ~20% is required. In this paper, we propose a new technique for fabricating textile-based strain/touch/pressure sensors, and user-interface (UI) devices using fully polymeric conducting fibers. In the structure, a conducting PEDOT layer is directly polymerized on the surface of polyester (PS) fiber, retaining the original mechanical properties of the PS fiber such as low Young’s modulus. This in situ polymerized PEDOT/PS fiber exhibited an initial electrical resistance of ~600 Ω cm-1 and can be utilized as a strain-sensing element. After integration with fabric, the textile strain sensors exhibited a strain gauge factor higher than 0.76, which is comparable to that of polydimethylsiloxane/CNT-based strain sensors.26 Furthermore, as a possible extension of the utilization of PEDOT/PS fibers, UI devices such as gesture sensors were fabricated; these devices could wirelessly interpret American Sign Language (ASL) letters using simple hand gestures.


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2. EXPERIMENTAL SECTION 2.1 Fabrication and characterization of PEDOT/PS conducting fibers. The PEDOT/PS fiber was prepared by direct polymerization of PEDOT on PS fibers based on methods reported in our previous studies (Figure 1a).27,28 First, a monomer solution was prepared by dissolving the following in 1-butanol: EDOT as a monomer, PVP as a binder, and pyridine as a polymerization retarder. An oxidant solution was also prepared by dissolving ferric p-toluene sulfonate in 1butanol. The monomer and oxidant solutions were mixed immediately before coating. Then, the PS fiber was immersed in the mixed solution and polymerization was carried out at 70 °C for 20 min. Next, the PEDOT-coated PS fiber was washed with methanol to remove any residual monomer and oxidant, and dried in a convection oven at 70 °C for 30 min. The coating process was repeated until the required resistance was obtained (3~4 times). In addition, for the PMMA coating on a PEDOT/PS fiber, the fiber was immersed in a PMMA solution dissolved in acetone, having a concentration of 23 µM. After immersion for about 1 min, the fiber was collected and dried in a convection oven at 70 °C for 5 min. To analyze the surface morphology and microstructure of the conductive fibers, a fieldemission scanning electron microscope (FESEM, JEOL JSM-7600F) system equipped with energy-dispersive X-ray spectroscope (EDS) was used. The stress-strain curves of the PEDOT/PS and pristine PS fibers were obtained using a material testing machine (LR30K, AMETEK Inc.). The load cell was 205 N and the pull speed was 10 mm/min (sample size of 2 cm). The electrical characteristics of the fibers were analyzed using a DC source-meter (Keithley 2400 SourceMeter). 2.2 Fabrication and characterization of textile sensors and UI devices. The PEDOT/PS fiber-based strain sensor was fabricated by sewing the fibers onto fabrics using a mechanical


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sewing machine (Brother, Innovis 55p). For the interlocking of PEDOT/PS fibers on the fabric, an electrically insulating interlocking fiber was used (PS 100%). For the fabrication of the strainsensor-based UI devices, the textile strain sensors were sewn to the finger sections of a glove. The end of each strain sensor was bonded to a stainless steel fiber using a silver epoxy and connected to a voltage divider circuit. The five strain sensors were connected to an analog input port of a microcontroller unit (MCU, PIC18f1320, Microchip Technology Inc.). The voltage variation from each strain sensor was measured and analyzed using in-house software. The data were simultaneously transferred to an external device via a wireless module (XBee Pro, Digi International Inc.). Also, an in-house software was used to send the acquired data commands to the sensor, save received signals, analyze the data, and display the letters corresponding to each finger gesture pattern.

3. RESULTS AND DISCUSSION 3.1 Characterization of PEDOT/PS conducting fibers. Aforementioned, conducting fibers with high metallic or carbon components may lead to the loss of certain textile characteristics. In contrast, fully polymeric conducting fibers can retain the original mechanical properties of polymer while having high electrical conductivity. PEDOT is a well-studied conducting polymer of which its electrical conductivity can reach up to 1000 S cm-1 by applying a simple posttreatment such as dilute sulfuric acids29 or with a methanol solvent.30 Moreover, due to its low Young’s modulus (~2 GPa)31, PEDOT can be a suitable material for conductive coating on polymeric fibers. As described in the experimental section, we used direct polymerization of PEDOT on the surface of PS fibers. This in-situ type polymerization of PEDOT could lead to conformal coating


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and higher adhesion of PEDOT on the core PS fiber, which might increase the stability and stretchability of the conductive fibers. The conformal coating of PEDOT on a PS fiber was confirmed by FESEM and EDS analyses. As displayed in Figure 1b and Figure S1, the PEDOT was successfully formed on the outer region of the multi-filament fiber as well as on the monofilament surfaces (thickness of 100~300 nm). Also, from a spatial mapping of sulfur (S) Kα1 in the thiophene groups of PEDOT (Figure 1b, bottom), a uniform distribution of sulfur and good coverage of PEDOT on the surface of the fiber was confirmed. The electrical resistance of the PEDOT/PS fiber was approximately 600 Ω with probes spaced at ~1 cm. Also, Figure 1c and Figure S2 show the stress-strain curves for the pristine PS and PEDOT/PS fibers. From these stress-strain curves, particularly in the initial linear regions within ~3% of strain, the elastic modulus values were extracted. The extracted elastic modulus values of the pristine PS and PEDOT/PS fibers were 0.682±0.101 GPa and 0.813±0.057 GPa, respectively. Although a slight increase in the elastic modulus was observed due to the relatively high Young’s modulus of PEDOT (~2 GPa)31 or wet-spun PEDOT:PSS fibers (5.9±0.7 GPa)22, the stress-strain behaviors of both fibers were almost identical, suggesting that the mechanical properties of PEDOT/PS fibers are similar to those of pristine PS fibers. 3.2 Characterization of PEDOT/PS strain sensors. To create a textile strain sensor using the conducting PEDOT/PS fiber, the fiber was embedded in a commercially available fabric. Here, we used a typical sewing machine to embed the PEDOT/PS fiber in a fabric, which makes it simple and easy to realize various patterns, regardless of the fabric type. Figure 1d shows the mechanically sewed PEDOT/PS fibers with linear- and zigzag-type patterns. To maintain their patterns, additional PS fibers were used as interlocking fibers (Figure 1e, blue-colored fiber), which fixed the PEDOT/PS fibers firmly to the fabric. Figure 2a shows the strain vs. resistance


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change (∆R/R0) plots for PEDOT/PS strain sensors with linear and zigzag patterns. Here, the maximum strain was set at 20% and the average values were acquired from four consecutive stretching and releasing tests. Also, the length of the stretched or released region was fixed at ~2 cm. Regardless of the sensor type (i.e., linear or zigzag), as well as the standalone multifilament PEDOT/PS fibers (Figure S3), the resistance tends to decrease with increasing strain, which is attributed to the multi-filament structure of the PEDOT/PS fiber. In fact, when the PEDOT/PS fiber is stretched, the inter-connectivity between the PEDOT/PS mono-filaments is enhanced, creating more electrical junctions between the mono-filaments which may increase overall conductivity of the fiber.32,33 On the other hand, it should be noted that a single wet-spun PEDOT:PSS microfiber shows opposite behavior when it is mechanically strained.22 Figure S4 shows the change in the apparent diameter of a PEDOT/PS fiber when it is stretched with strains of 0%, 10% and 20%. In this case, the apparent diameter of the fiber was reduced from 296.2 µm to 248.6 µm (10%) and 230.6 µm (20%), respectively, which confirms the enhanced interconnectivity within the fibers when they are mechanically stretched. As shown in Figure 2a, the strain-dependent resistance change was significantly different depending on the sensor patterns. Particularly, the ∆R/R0 value for the linear-type strain sensor was typically higher (∆R/R0 ≈ –0.17) than that of the zigzag-type strain sensor (∆R/R0 ≈ –0.02), when a strain of 20% was applied. Correspondingly, the gauge factor (G), which is defined as G = (|∆R|/R0)/(∆L/L0), was higher for the linear-type strain sensor (G = 0.76) than for the zigzagtype strain sensor (G = 0.10) (Figure 2b). The low gauge factor found in the zigzag-type sensor is thought to be caused by the angle (θ) between the direction of the applied force (F) and the embedded PEDOT/PS fiber. As shown in the inset of Figure 2a, in the case of a zigzag-type sensor, the stress applied to the PEDOT/PS fiber is σzigzag = F||/A or F┴/A, where the F|| is the


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force acting parallel to the fiber, F┴ the force acting perpendicular to the fiber direction and A the cross-sectional area. However, in the case of a linear-type sensor, under identical applied force, the stress applied to the PEDOT/PS fiber is σlinear = F/A, which leads to higher inter-connectivity between the PEDOT/PS mono-filaments. At 20% strain, the applied stress to the fiber in the linear-type sensor is σlinear = 11.49 N/m2. However, for the zigzag-type sensor, σzigzag = 9.71 N/m2 (F||) or 6.14 N/m2 (F┴), showing lower applied stress compared to a linear-type sensor (θ = 32.33o, Young’s modulus = 0.813 GPa, diameter = 300 µm). Therefore, in this strain range, the linear-type sensor is more favorable in terms of obtaining high strain sensitivity. Furthermore, upon increasing the maximum strain range up to 50% and 70%, the gauge factor of the lineartype strain sensor was decreased to 0.665 and 0.244, respectively (Figure 2b). Also, the gauge factor of the zigzag-type strain sensor was increased to 0.171 and 0.253, respectively. These results suggest that there exists a limitation in the strain sensing range using the linear-type PEDOT/PS sensor. Therefore, at a higher strain sensing range, zigzag-type or other pattern type sensors can be more favorable. Furthermore, using a knitted structure, higher strain gauge factor could be obtained as reported by Atalay et al. showing a maximum gauge factor of 3.75~4.2.34 Therefore, for further improvement of the gauge factor using the PEDOT/PS fibers, adopting various knitted structures could be considered. In addition, the operational reliability of the PEDOT/PS strain sensor was determined. In Figure 2c, the stretch/release cyclic test data for the linear-type textile strain sensor is shown. A total of 1000 cycles at a strain of 20% were performed. We observed that, at the initial stage, a rather abrupt increase in resistance was occurred for both stretched and released states; this phenomenon can be attributed to partial cracking of the PEDOT coating the PS mono-filaments (Figure S5). Thereafter, the rate of resistance increase slowly decreases. However, despite the


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resistance increase, we found that the gauge factor of the sensor became relatively stable over repeated cycles (Figure 2c and Figure S6), which suggests that the PEDOT/PS strain sensor can be stably operated when embedded in fabric. Also, it has been reported that the PEDOT has good adhesion on PS yarns35 and poly(ethylene terephtalate).36,37 To evaluate the adhesion of PEDOT on PS fibers, the change of resistance after a repeated adhesion tape test was analyzed. As displayed in Figure S7, the increase of resistance was ~1.07% after three times of adhesion tape test showing its reasonable adhesion characteristics. Furthermore, an additional poly(methyl methacrylate) (PMMA) protecting layer can be used to passivate the PEDOT/PS fiber to prevent any mechanical damages or block current leakage through from physical contacts. Supporting Information, Figure S8 shows the stretch/release cyclic test data for a PMMA-passivated PEDOT/PS strain sensor (linear-type) repeated for 1000 cycles. Note that the sensor also exhibited stable operation similar to the PEDOT/PS strain sensor and a high gauge factor (G ≈ 0.9). 3.3 Emulation of wearable body-motion monitoring sensors. Using PEDOT/PS strain sensors, a textile-based body-motion monitoring sensor was fabricated by integrating a lineartype strain sensor in an article of clothing. Specifically, the PEDOT/PS strain sensor was embedded at the knee joint part of a clothing, as shown in Figure 3a. Under the bent-knee condition, an in-axis strain is applied to the PEDOT/PS strain sensor, causing a decrease in resistance. Likewise, under the straight-knee condition, the resistance returns to its original value. Therefore, using real-time monitoring of the resistance change of the strain sensor, the physical state of the knee can be identified. Specifically, ∆R/R0 was ~20% with a knee angle difference of approximately 90o (bent to straight). Figure 3a shows that the sensor had good repeatability under repeated bending and straightening cycles (approximately 100 cycles).


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Additionally, we found that the rate of resistance change (∂R/∂t) differs when the body is in walking and running conditions. Under a steady walking condition, the ∂R/∂t value was small, typically less than 50 kΩ s-1 (Figure 3b). However, under the running condition, ∂R/∂t increased to 100 kΩ s-1 to 400 kΩ s-1. Therefore, by tracing the rate of resistance change of the textile strain sensor, the body motion status (such as walking and running) can be identified. Moreover, because the linear-type textile strain sensors mainly respond to in-axis strain, by integrating two linear-type strain sensors in orthogonal directions (x- and y-axis) as shown in Figure 3c, a biaxial strain sensor can be realized. In particular, when a strain was applied parallel to the x-axis, only the resistance of the x-axis strain sensor was decreased (Figure 3c); there was no significant change in the resistance of the y-axis strain sensor. 3.4 Wearable UI devices. To provide a system-level demonstration using the developed textile strain sensors, a wearable human-machine interface device that is capable of expressing sign languages by recognizing predefined hand gestures was fabricated. As shown in Figure 4a, PEDOT/PS strain sensors were embedded in each finger of the glove (thumb, index, middle, ring, and little fingers) and wired to a controller attached to the glove. Here, to read the change of the resistance, a voltage divider was created in the glove, as shown at the bottom of Figure 4a. Therefore, when the finger is bent or straightened the resistance of the sensor changes, and consequently, the output voltage changes according to the equation, V0n = (Rn/(RSn + Rn)) × VCC, where V0n is the output voltage (n = 1 to 5), Rn is the fixed resistance, RSn is the resistance of the strain sensor, and VCC is the supplied voltage. After sensing, the voltage signals acquired from each strain sensor are sent to a receiving unit by a short-range wireless communication unit (XBee®). The detailed system layout of the wearable UI device is shown in Figure 4a and Supporting Information, Figure S9. Figure 4b shows the output voltage signals measured from


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the textile strain sensors embedded in thumb, index, middle, ring, and little fingers. The fingers were bent and then straightened in a sequence and the output voltage data show the successful operation of the textile strain sensor and the isolation of each sensor (i.e., there is no noticeable crosstalk). Under the straightened state of the finger, V0 was approximately 1.0~1.1 V; under the bent state, V0 increased to 1.5 to 1.7 V. Supporting Information, Figure S10 shows the sequential responses of the textile strain sensors embedded in each finger so as to identify any crosstalk between the strain sensors. Here, from the thumb to the little finger, the fingers were bent and then straightened in a sequence, one at a time. As shown in the output voltage data (displayed on the monitor), only the strain sensors that were bent or straightened responded (indicated by blue arrows), while the other strain sensors remained at their low output voltage state. This series of experiments clearly demonstrates that the PEDOT/PS textile-based strain sensors can be integrated with a wearable device and used as wearable-type human-machine interface devices. For further application of the textile-based strain sensors in human-machine interface devices, we demonstrated a multi-level strain recognition UI device that can interpret sign language, specifically, ASL letters,38 using hand gestures. Generally, in sign languages, the shape of fingers (specifically, the bent state of each finger and their positions) are varied to express different letters. Moreover, several letters require a complex representation of finger bending states, for example, the half-bent state of the fingers (c.f., the letter ‘C’). Therefore, the wearable UI device should be capable of generating multi-level output voltages for different bending states of the fingers (straightened, half-bent, and fully bent states). In Figure 5a, several pre-defined hand gestures for the ASL letters are shown (characters ‘S’, ‘K’, and ‘U’), and interpreted by a wearable UI device. By performing the pre-defined hand gestures, the corresponding characters can be conveyed via wireless communication. In Figure 5b, the output voltage waveforms from


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each strain sensor are displayed; we observe no significant crosstalk between the integrated strain sensors. Figure 5c presents a similar demonstration for the character sets of ‘C’, ‘A’, and ‘U’. Note that the character ‘C’ requires a half-bent state identification for index, middle, ring, and little fingers. In the output voltage waveforms (Figure 5d), the half-bent state can be distinguished; it results in an output voltage level between 1.0 and 1.35 V, lower than that of the fully bent state (1.5 to 1.7 V) (Figure S11). Nevertheless, using the fully polymeric textile-based strain sensors, realization of a wearable human-machine interface system was possible. Additionally, we observed slight variation in the output voltages from the integrated sensors. We assume that this variation is primarily due to differences in the resistance values of the stainless steel interconnection fibers. Although we could tune the voltage levels for each sensor from an in-house software, it is more appropriate to design the circuit components more accurately so as to minimize the output voltage variation. Furthermore, several ASL characters have finger motions in their expressions, which would require a motion detection sensor in addition to the (static) gesture sensor. 3.5 Textile touch sensors. Another useful application of the textile-based UI device is as a touch/pressure sensor because of its versatile uses in electronics. Previously, a flexible pressuresensing device utilizing carbon nanotube-coated double-twisted smart threads was reported, where the variation of resistance is used to identify the magnitude and position of the pressure applied.39 Similarly, in this study, a textile touch sensor that recognizes contact with conducting surfaces such as human skin is demonstrated using a PEDOT-coated conductive fiber. If the PEDOT-coated fibers are sewn in a specific pattern in which a small portion is physically disconnected, as shown in Figure S12a, the electrical conductivity between the two conductive fibers is almost zero (i.e., an open circuit). However, when a conductive surface such as a human


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finger touches the disconnected points, electrical contact is made (creating a closed circuit), thereby providing a measurable range of resistance. As shown in Figure S12b, when contact is made, the resistance value decreases to approximately 5 to 10 MΩ, which is significantly lower than that of an open circuit (>200 MΩ). Although the demonstration only shows a one-point identification of the contact, an array structure would further improve the operational area of the sensor and its detection resolution. Furthermore, as shown in Figure S12c, the resistance of the touch sensor circuit is modulated by the pressure of the contact material (a finger, in this case) because of the enlarged contact area between the contact material and the PEDOT-coated fibers. Therefore, simultaneous sensing of touch and pressure is possible by monitoring the resistance value of the touch sensor circuit.

4. CONCLUSIONS We successfully emulated multi-functional sensors and UI devices using in situ polymerized PEDOT-coated fibers on a textile platform. Using the integrated PEDOT fibers, highly sensitive uniaxial and biaxial strain sensors, body-motion monitoring sensors, touch sensors, and multilevel strain recognition UI devices can be realized. Furthermore, we demonstrate the feasibility of the textile-based UI devices by implementing them in a wireless wearable system capable of interpreting sign language via various hand gestures.


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Supporting Information FESEM images of PEDOT/PS fibers, resistance variation of a standalone PEDOT/PS fiber, strain gauge factor variation for a linear-type PEDOT/PS strain sensor, resistance variation of a PEDOT/PS fiber upon a repeated adhesion tape test, cyclic stretch/release test data for a PMMApassivated PEDOT/PS textile strain sensor, a layout of a wearable strain sensor system, sequential responses of a textile-based wearable strain sensor system, representation of ASL alphabet letters by hand gestures, and PEDOT/PS fiber-based textile touch/pressure sensors. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors *Yong-Hoon Kim ([email protected]) *Sung Kyu Park ([email protected]) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Ministry of Trade, Industry & Energy, Republic of Korea (10048884), by the “Human Resources Program in Energy Technology” of the Korea Institute of


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Energy Technology Evaluation and Planning (KETEP), granted financial support from the Ministry of Trade, Industry & Energy, Republic of Korea (no. 20154030200870), and by the Basic Research Lab program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A4A1008474).


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(36) Kim, J. –Y.; Kwon, M. –H.; Min, Y. –K.; Kwon, S.; Ihm, D. –W. Self-Assembly and Crystalline Growth of Poly(3,4-ethylenedioxythiophene) Nanofilms. Adv. Mater. 2007, 19, 3501-3506. (37) Im, S. G.; Yoo, P. J.; Hammond, P. T.; Gleason, K. K. Grafted Conducting Polymer Films for Nano-Patterning onto Various Organic and Inorganic Substrates by Oxidative Chemical Vapor Deposition. Adv. Mater. 2007, 19, 2863-2867. (38) Valli, C.; Lucas, C.; Linguistics of American Sign Language: An Introduction, Clerc Books, Gallaudet University Press, Washington, D. C. 2000. (39) Tai Y.; Lubineau, G. Double-Twisted Conductive Smart Threads Comprising a Homogeneously and a Gradient-Coated Thread for Multidimensional Flexible PressureSensing Devices. Adv. Funct. Mater. 2016, 26, 4078-4084.


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Figure 1. (a) Schematic illustration of polymerization of PEDOT on a polyester (PS) fiber. (b) Field-emission scanning electron microscope (FESEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping of sulfur (S) Kα1 of a PEDOT/PS fiber. (c) Stress-strain curves for pristine PS and PEDOT/PS fibers. (d) Optical images of linear- and zigzag-type textile strain sensors embedded fabrics. To interlock the PEDOT/PS fibers (black-colored fiber) on the fabric, electrically insulating PS fibers were simultaneously embedded on the fabric (blue-colored fiber). (e) Optical microscope images of PEDOT/PS textile strain sensors showing the immobilized PEDOT/PS fibers after interlocking with the PS fibers.


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Figure 2. (a) Strain-dependent resistance change (∆R/R0) characteristics of PEDOT/PS strain sensors with linear and zigzag patterns (maximum strain was set at 20%). (b) Gauge factors of PEDOT/PS strain sensors (linear and zigzag patterns) with different maximum strain ranges. (c) Cyclic stretch/release test data for a linear-type PEDOT/PS textile strain sensor (1000 cycles, 20% strain).


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Figure 3. (a) Real-time monitoring of knee motion using a PEDOT/PS textile strain sensor (linear-type, ~100 repeated cycles). (b) Rate of resistance change (∂R/∂t) of a body-motion sensor for monitoring of body motion in walking or running states. (c) Sensing characteristics of a textile-based biaxial strain sensor with two embedded linear-type strain sensors in orthogonal directions (x- and y-axis) in a fabric.


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Figure 4. (a) Images of the textile-based wearable UI device which includes five PEDOT/PS strain sensors, stainless steel fiber interconnections, a voltage divider configuration, a battery, and a wireless transmission unit. (b) Output voltage signals from the textile strain sensors embedded in thumb, index, middle, ring, and little fingers. Each finger was bent and then straightened in a sequence. Under a straightened condition, the output voltage was approximately 1.0~1.1 V, which increased to 1.5~1.7 V under a bent condition.


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Figure 5. (a) Pre-defined hand gestures for ASL letters ‘S’, ‘K’, and ‘U’, and interpretation by the textile-based wearable UI device. (b) The output voltage waveforms measured from each strain sensor when expressing the pseudoword ‘S’, ‘K’, ‘K’, ‘U’. (c) Pre-defined hand gestures for ASL letters ‘C’, ‘A’, and ‘U’, and interpretation by a textile-based wearable UI device. (d) Output voltage waveforms measured from each strain sensor for pseudoword ‘C’, ‘A’, ‘U’.


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Table of Contents (TOC) graphic


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Highly Sensitive Textile Strain Sensors and Wireless User-Interface

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