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Sheath-Core Graphite/Silk Fiber Made by DryMeyer-Rod-Coating for Wearable Strain Sensors Mingchao Zhang, Chunya Wang, Qi Wang, Muqiang Jian, and Yingying Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06984 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on August 4, 2016

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ACS Applied Materials & Interfaces

Sheath-Core Graphite/Silk Fiber Made by DryMeyer-Rod-Coating for Wearable Strain Sensors Mingchao Zhang, Chunya Wang, Qi Wang, Muqiang Jian, Yingying Zhang* Department of Chemistry and Center for Nano and Micro Mechanics (CNMM), Tsinghua University, Beijing 100084, PR China KEYWORDS: dry-Meyer-rod-coating, strain sensor, silk fiber, graphite flakes, sheath-core structure, flexible electronics ABSTRACT: Recent years have witnessed the explosive development of flexible strain sensors. Nanomaterials have been widely utilized to fabricate flexible strain sensors owing to their high flexibility and electrical conductivity. However, the fabrication processes for nanomaterials and the subsequent strain sensors are generally complicated and at high cost. In this work, we developed a facile dry-Meyer-rod-coating process to fabricate sheath-core structured single fiber strain sensors using ultrafine graphite flakes as the sheath and silk fibers as the core by virtue of their flexibility, high production and low cost. The fabricated strain sensor exhibits a high sensitivity with a gauge factor of 14.5 within wide workable strain range up to 15%, and outstanding stability (up to 3000 cycles). The single fiber based strain sensors could be attached to a human body to detect joint motions or easily integrated into the multi-directional strain sensor for monitoring multi-axial strain, showing great potential applications as wearable strain sensors. 1. Introduction

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Recent decades have witnessed the booming development of flexible and wearable electronics for their great potential applications as human motion and health monitor, fitness trackers, and wearable communication devices.1-8 In particular, flexible and wearable strain sensors with good flexibility, high sensitivity and large workable strain range, which can be integrated into the clothing or mounted on the human skin to monitor human activities and physiological signals, have attracted extensive concern and efforts.9-14 However, conventional strain sensors based on semiconductor flakes and metallic foils are generally rigid and have limited tolerable strain (< 5%), disabling them for applications in flexible and wearable devices.15-17 Recently, nanomaterials, such as metal nanowires,18,19 metal nanoparticles,20 metal thin films,21 synthetic polymer nanocomposites,22-24 and carbon nanomaterials25-33 have been widely investigated for their applications in wearable strain sensors considering their potential superiority of good electrical conductivity, high flexibility and high sensitivity. These flexible strain sensors pave the way to the wearable and skin-mountable devices with satisfactory sensitivity for detection of human motions. Nonetheless, relatively complicated and costly producing processes are generally needed to fabricate nanomaterials and subsequent nanomaterial-based strain sensors, which may impose restrictions on their scalable applications. Therefore, wearable strain sensors which could be produced with eco-friendly, highly efficient, low-cost and scalable fabrication schemes are competitive and attractive candidates for promoting the development of wearable electronics. Silkworm silk is a kind of well-known natural fiber which has been used for several thousand years in textile industry for its lustrous appearance, softness, toughness, biocompatibility and high yield,34 making it an attractive material for fabrication of wearable accessories. On the other side, graphite, as an abundant natural mineral powder with sp2 carbon structures, owns good


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electrical conductivity,35 Low-cost and high-performance flexible electronics may be fabricated if the merits of silk fibers and graphite flakes could be combined together through a facile process. In this work, we report the fabrication of conductive sheath/core-structured graphite/silk fibers (GSF) by a facile dry-Meyer-rod-coating process and demonstrate their applications as wearable strain sensors. The GSF strain sensors show high sensitivity with a gauge factor (GF) of 14.5 in a wide workable strain range of 0-15%, fast response and excellent stability (>3000 cycles), endowing the individual GSF strain sensors and the integrated rosette-shaped strain sensors with the capabilities in human motion detection. Besides, the dry-Meyer-rod-coating process could also be used for coating graphite on the surfaces of other fibers, such as human hair, polypropylene (PP) fiber, and Spandex fiber, enabling the fabrication of a variety of fiber strain sensors with different performance. The sheath/core-structured graphite/fiber strain sensors fabricated through the facile dry-Meyer-rod-coating process may have great potential application in the development of wearable electronics and intelligent robotics. 2. Materials and Methods Fabrication process of the GSF strain sensor. The silk fibers were obtained from silkworm cocoons which were treated with deionized water at 80 oC for an hour. A bundle of silk fibers were aligned on a flat plate with one of their ends fixed by a clip. Graphite flakes with average granularity of ~5 um were put on the fixed end of the silk fibers. A Meyer rod (K hand coater, PK printcoat Instruments Ltd.) was pressed on the graphite flakes and moved along the direction of the silk fibers. This process were repeated for ten times in order to get reliable products. Individual GSF were put on an Ecoflex (Ecoflex Supersoft 0050, smooth-on, Inc.) substrate and then copper wires were connected to each end of the fiber using silver paste. Subsequently, liquid Ecoflex was uniformly coated onto the samples and then dried in ambient environment for


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several hours for encapsulation. Other fibers, such as human hair, PP fiber and Spandex fiber, were rinsed in acetone three times to remove impurities. Then the same above fabrication procedure was carried out to fabricate strain sensors based on different single fibers. Characterization and electrical signal measurement. The morphologies of the pristine fibers and graphite coated fibers were characterized by a field emission SEM (FE-SEM, (Quanta FEG 450) The Raman spectrum was performed by a Raman spectroscope (HORIBA, HR800) with a laser excitation wavelength of 532 nm. The loading of the tensile strain and the measurement of the stress-strain curve were carried out with a universal testing machine (Shimadzu AGS-X). The electrical signals were recorded by a digital source meter (Keithley 2400) at a constant bias voltage of 30 V. The strain sensors were trained with 10 cycles of loading-unloading process to get a stable performance before the measurement of the responsive performance. 3. Results and discussion

Figure 1. Fabrication of sheath-core structured graphite/silk strain sensors through a dry-Meyerrod-coating process. (a) Schematic illustration showing the fabrication process. (b) Photograph


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of the flexible strain sensor. (c) Photograph of the strain sensor subjected to strain of 0% and 15%. (d, e) SEM images of a pristine silk fiber (d) and a graphite/silk fiber (e). (f) Raman spectrum of the graphite flakes. Figure 1a illustrates the fabrication of graphite/silk sheath-core fibers through the dry-Meyerrod-coating process. Silkworm silk fibers were aligned on a flat plate with one of their ends fixed. Ultrafine graphite flakes were placed on the fixed end. A Meyer rod was pressed on the graphite flakes and rolled from one end of the silk fibers to the other end, leading to the coating of graphite flakes onto the silk fibers. Excess graphite flakes could be removed by shaking the plate and many GSF were thus obtained. Subsequently, GSF strain sensors could be fabricated by encapsulating the individual GSF in elastic polymers. Ecoflex was used in this work as a concept of proof. As shown in Figure 1b and 1c, the GSF strain sensor exhibited good flexibility, stretchability and robustness. Since the graphite layer is very thin, the GSF have similar diameters with the pristine silk fibers (Figure 1d and 1e). The graphite flakes were conformably coated on the silk fibers, which could be attributed to van der Waals forces and electrostatic force (Figure 1e). It is noteworthy that, using the same process, graphite flakes could also be uniformly coated on other fibers, such as human hair, PP fiber and Spandex fiber (Figure S1), enabling the fabrication of a series of fiber strain sensors with different characteristics. Figure 1f shows a typical Raman spectrum of the graphite flakes used in the work, which presents three well defined peaks of 1357 cm-1 (D-band), 1579 cm-1 (G-band,) and 2717 cm-1 (2D-band) and confirms the good graphitic crystalline carbon structures.37


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Figure 2. Basic performance of the GSF strain sensor. (a) Relative change in resistance versus strain of a GSF strain sensor when being stretched until broken, where R0 and R represent the pristine resistance of the strain sensor and the real-time resistance when being stretched, respectively. (b) Comparison of the sensitivity and elongation of sheath-core graphite/fiber strain sensors with different core fibers, including hair, silk, PP, and Spandex fibers. (c) Relative resistance variation tensile strain of 1%, 3% and 5% at a frequency of 0.75 Hz. (d) Relative resistance variation with tensile strain of 5% at frequency of 0.25 Hz, 1 Hz, 3 Hz and 6 Hz. The flexible GSF strain sensors showed high sensitivity and good reliability. Figure 2a shows a typical plot of relative change in resistance of the GSF strain sensor versus strain, where the fabricated GSF strain sensor displays a linearly monotonic increase in resistance with the corresponding GF of 14.5 and a maximal strain over 15%. It is worth noted that the GF of our


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strain sensor is more than 10-fold higher than that of the carbon nanotube coated spider fiber based strain sensor36. As shown in Figure 2b, the sheath-core structured graphite/fiber strain sensors based on other fibers, including human hair (diameter of 80 µm), PP fiber (diameter of 27 µm), and Spandex fiber (diameter of 26 µm) exhibited GF of 71.1, 14.2 and 14.0, respectively. Besides, due to the different elongation at break of the core fibers (see Figure S2), the Spandex fiber based strain sensor has a much wider workable strain range (~30%). But this work still takes silk as an example to emphasize its facile and green fabrication process. The strain sensors shown in Figure 2b exhibited different workable strain ranges, providing different options for certain applications. In addition, the relative resistance change of the GSF strain sensor depends only on the strain level instead of the frequency (Figure 2c and 2d), indicating that the applying rate of the strain does not affect the output signals, which is of great importance for practical applications to get reliable response.


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Figure 3. Drift, temperature dependence, long-term stability and sensing mechanism of the GSF strain sensor. (a) Drift of the strain sensor when it was held under certain strains. (b) The dependence of current on temperature. (c) Relative change in resistance of the strain sensor under repeated loading-unloading cycles of 10% strain with a strain rate of 25%/s for 3000 cycles. (d) Schematic illustration of sensing mechanism. The GSF stain sensors showed low drift, low hysteresis, high stability and excellent durability. When the GSF strain sensor was held under certain strains, the electrical resistance remained almost constant at each strain level (Figure 3a), indicating its low drift. It is noted that the GSF strain sensor exhibited a small overshoot of the resistance if the strain was abruptly loaded, which could be ascribed to the viscoelastic nature of the Ecoflex matrix and is in consistence with reported results.14 The linear I-V curves under loading of certain strains further demonstrates the low drift of the GSF sensors (Figure S3).Besides, the GSF strain sensor exhibited negligible hysteresis of during the loading and unloading of 5% strain (Figure S4) and


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fast response with response time of about 135 ms (Figure S5). It is well-known that temperature will influence the conductivity of metals or semiconductors. The current of a GSF strain sensor at different temperature was measured under a constant bias voltage. As shown in Figure 3b, the current shows a positive linear dependence on the temperature with a slope of 0.17 µA/ºC and good linearity (R2=0.997), which is desirable for practical applications since the influence of temperature could be easily remedied through offset calibration. In addition, the conducting mechanism can be understood by the temperature-dependent conductivity. A good linear relationship (R2=0.991) between ln R and T1/2 indicates the dominant electrical conduction is mainly controlled by the tunneling effect (see Figure S6), according to the tunneling conduction

mechanism, which can be described as = (− ), where R is the resistance, T is the temperature, R0 and A are two positive constants.32,40. Furthermore, for practical applications of strain sensors, long-term stability and durability is of great importance. The GSF strain sensor exhibited remarkably long-term stability during 3000 loading and unloading cycles of 10% strain (Figure 3d), demonstrating that the sheath-core graphite/silk fiber structure is very stable without exfoliation of graphite flakes under cyclic loading of tensile strain. The working mechanism of the GSF strain sensor was illustrated in Figure 3g. When the sheathcore graphite/fiber was subjected to a tensile strain, the overlapping area between the coated graphite flakes decreased, leading to increased electrical resistance. After release of the tensile strain, the contact between the graphite flakes could be recovered, enabling the repeatability of this process. To understand the working mechanism, a simple model based on Holm’s theory41 was proposed, which is in good consistence with experimental results (see more details in Supplementary information).


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Figure 4. Application of a single GSF strain sensor in joint motion detection. (a) Response of the wearable sensor to step-wise bending of an index finger. Insets: photographs of a strain sensor attached along a finger using adhesive tape. (b) Responsive of the wearable sensor to cyclic motion of a wrist. Insets: photographs of a strain sensor attached on a wrist. The GSF strain sensor, due to its high sensitivity and large workable strain range, could be directly assembled on human bodies to monitor motion of joints. For demonstration, a GSF strain sensor was attached to a finger (insets of Figure 4a) and a wrist (insets of Figure 4b) by medical adhesive tape to monitor the motion of the finger and wrist joints. As shown in Figure 4a, the current dropped abruptly to a lower value when the finger bent to a certain angle, and further bending leads to further decrease, demonstrating its rapid response and high sensitivity. Similarly, a repeatable response of the GSF strain sensor to the motion of a wrist joint was observed (Figure 4b). The prompt and reversible response of the GSF strain sensor to the cyclic motion of joints promises its applications in human motion detection.35, 36


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Figure 5. Integrated rosette-shaped GSF strain senor for multi-directional motion detection. (a) Illustration of the configuration of the strain sensor composed of three individual fiber strain sensors (GSF-A/B/C). (b, c) Photographs of the strain sensor attached on a wrist. (d-f) Responsive signals of GSF-A (d), GSF-B (e) and GSF-C (f) during cyclic bending-unbending motion of the wrist. Complicated deformation of human body, which is generally multi-directional, could not be monitored by a single strain sensor. To measure strains along multiple axes, several single fiber strain sensors could be integrated in multi-directions to obtain a multi-axial strain sensor. As a proof of the concept, a rosette-shaped strain sensor was fabricated by connecting three GSF strain sensors with one end of them in the center and an intersecting angle of 120° (Figure 5a). The three individual GSF strain sensors are named as GSF-A, GSF-B and GSF-C, and the


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corresponding strain values of the sensors are defined as εa, εb and εc, respectively. As shown in Figure 5b, the fabricated rosette-shaped strain sensor was mounted on a wrist with GSF-A paralleled to the arm, and the two others located alongside. Figure 5c shows the relaxed and bending states of the wrist joint. Figures 5d-f show the signals of each individual fiber strain sensors recorded during cyclic bending-unbending motions of the wrist joint. During the bending process, GSF-A presented a relative resistance increasing of ~65% (Figure 5d), while GSF-B and GSF-C exhibited a comparable resistance increasing of about 20% (Figure 5e and 5f), indicating the strain sensor along the direction of bending bore larger strain than others. According to the defined coordinated system (Figure 5a), the strain in an arbitrary direction could be calculated according the values of εa, εb and εc (see supplementary information for details of calculation), enabling the monitoring of human motions in multi-directions. 4. Conclusion In summary, we reported a graphite-sheath/silk-core fiber strain sensor fabricated by a facile dryMeyer-rod-coating process. Graphite flakes could be conformally coated on silk fibers with the assistance of a Meyer rod, enabling the low cost and high efficient fabrication of electrical conductive GSF fibers. The overlapping area between the graphite flakes varies with the loading and unloading of external tensile strain, endowing the GSF with great potential in application as strain sensors. The flexible GSF strain sensor exhibited a GF of 14.5 within a strain range of 015%, which is obviously superior as compared to conventional strain sensors. Furthermore, the GSF strain sensor showed outstanding performance with fast response, long-term stability, small hysteresis and low drift. We demonstrated that the GSF strain sensor could be used individually in detection of unidirectional joint motions, such as bending of fingers and wrists, or further integrated into multi-directional strain sensor to monitor multi-axial deformation of human


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motion. Besides, the dry-Meyer-rod-coating process could be used for fabrication of sheath-core fibers with other core fibers, such as human hair, PP fiber, and Spandex fiber, demonstrating the versatility of this approach and providing optional fiber sensors for practical applications. The facial fabrication of this high-performance sheath-core structured strain sensors, combining the attractive properties and abundant supply of graphite and silk, paves a new way for the fabrication of low-cost and eco-friendly wearable strain sensors. Corresponding Author [email protected] Author Contributions M. C. Zhang and C. Y. Wang contributed equally to this work. The author declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the NSF of China (51422204, 51372132), the National Key Basic Research and Development Program (No. 2016YFA0200103, 2013CB228506) and the Cyrus Tang Foundation (Grant Number 202003). ABBREVIATIONS GSF, graphite/silk fibers; SEM, scanning electron microscope REFERENCES (1) Amjadi, M.; Kyung, K. U.; Park, I.; Sitti, M. Stretchable, Skin-Mountable, and Wearable Strain Sensors and Their Potential Applications: A Review. Adv. Funct. Mater. 2016. 26,16781698.


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Table of Contents A flexible graphite/silk sheath-core fiber based strain sensor is fabricated by a facile dryMeyer-rod-coating approach. The sensor shows high sensentivity, wide workable strain range, low drift, small hysteresis and excellent stability, promising its wide applications in monitoring various human motions.


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Silk Fiber Made by Dry ... - ACS Publications

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