Wearable and Transparent Capacitive Strain Sensor with High

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Wearable and Transparent Capacitive Strain Sensor with High Sensitivity Based on Patterned Ag Nanowire Networks Seung-Rok Kim, Jin-Hoon Kim, and Jin-Woo Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06474 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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ACS Applied Materials & Interfaces

Wearable and Transparent Capacitive Strain Sensor with High Sensitivity Based on Patterned Ag Nanowire Networks Seung-Rok Kim, Jin-Hoon Kim, and Jin-Woo Park* Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea *Corresponding author’s contact: E-mail: [email protected], Phone: +82-221235834, Fax: +82-221235834 Keywords: wearable device, interdigitated capacitive strain sensor, silver nanowire, capillary force lithography, body motion detection

ABSTRACT

In this study, a transparent and stretchable thin-film capacitive strain sensor based on patterned Ag nanowire networks (AgNWs) was successfully fabricated. The AgNWs were patterned using a capillary force lithography (CFL) method and were embedded onto the surface of the polydimethylsiloxane (PDMS) substrate. The strain (ε) sensitivity of the capacitive strain sensor was controlled and enhanced by patterning the AgNWs into electrodes with an interdigitated shape. Interdigitated capacitive strain sensor (ICSS) is 1

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expected to have -1.57 gauge factor (GF) at 30% ε by calculation, which is much higher than the sensitivity of typical parallel-plate-type capacitive strain sensors. Due to the interdigitated pattern of the electrodes, the GF of the ICSS was increased up to -2.0. The ICSS had no hysteresis behavior up to ε values of 15% and showed stable ε sensing performance during the repeated stretching test at ε values of 10% for 1000 cycles. Furthermore, there was no crosstalk between the ε and pressure sensing in the AgNW-based ICSS, which was found to be insensitive to externally applied pressure. The ICSS was then used to detect the finger and wrist muscle motions of the human body in order to simulate its application to large and small ε sensing.

INTRODUCTION

Strain sensors are used to measure the deformation experienced by objects.1 Interest has recently increased in the development of flexible and stretchable strain sensors2 that can be attached on clothing3-4 and the human body or that can act as the electronic skin5-6 of robots. These strain sensors are used to monitor the kinetic motions that help in disease diagnosis, rehabilitation assistance or physical activity in general.7 These strain sensors are required to have high flexibility or stretchability, long durability, and fast response and recovery speeds in addition to being highly sensitive to strain (ε).7 Furthermore, strain sensors are required to be optically transparent and unperceivable when incorporated into the multi-component systems of wearable devices.8-9 Wearable strain sensors should not obscure the transmission of light, which is commonly used by display screens to relay information or by photovoltaic cells of wearable devices to harvest energy and easily obtain the applied tensile strain when it 2


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is mounted on the skin.10 These strain-sensor requirements are dependent on their sensing mechanisms and component materials.11 The sensing mechanisms of strain sensors rely on the resistive,6, 12-13 capacitive,14-16 and piezoelectric17-18 properties of their component materials. Resistive-type strain sensors detect ε variations based on changes in the electrical resistance of the component materials in relation to deformation.1 However, changes in the electrical resistance of the materials not only depend on changes in their geometric structure but are also a function of electrical resistivity in relation to the applied strain.19 Hence, resistive strain sensors have non-linear relationship with the applied ε and also exhibit severe hysteresis behavior.6 In contrast, capacitive strain sensors typically feature a three-dimensional multi-layered structure consisting of a dielectric layer sandwiched between two parallel-plate electrodes as schematically shown in Figure 1a. Although capacitive-type strain sensors also feature some changes in the electrical resistance of their component materials, the capacitance (C) change in the sensor is largely dependent on variation in the geometric structure of the two parallelplate electrodes and the dielectric layer.14 Hence, capacitive strain sensors not only facilitate a linear relationship between sensitivity and ε with the theoretically maximum gauge factor (GF) value of 1 but also have no hysteresis behavior.15 However, due to the parallel-plate structure of the typical parallel-plate-type capacitive strain sensor, both ε and pressure can induce changes in the C of sensor, and the stackedlayered structure also hinders the minimization of the sensor thickness for conformal contact of the wearable devices.8, 14 The typical parallel-plate-type capacitive strain sensor usually exhibits a crosstalk problem between ε and pressure sensing, and are difficult to conform to surfaces of irregular shapes such the human skin.8, 14 Consequently, interdigitated capacitive strain sensors (ICSS) with in plane electrodes parallel to their substrates showed better ε 3



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sensitivity, sensing linearity, and very low hysteresis than typical parallel-plate-type capacitive strain sensors.16 As a key element of the capacitive strain sensor, a proper electrode material should be chosen to impart excellent ε sensing performance.11 The electrode should maintain its electrical conductivity and electrical charge retention properties even under very large deformations. Among the numerous stretchable thin films of continuous and nanostructured materials,3-6, 12-16, 20-29 Ag nanowire networks (AgNWs) are known to be one of the most promising materials for deformable electrodes.6 AgNWs maintain their excellent electrical properties even under very large mechanical deformations due to their innate metallic properties. Furthermore, the nanometer dimensions of AgNWs allow their percolation networks to be optically transparent.12 However, AgNWs must be precisely patterned into conductive paths to be effectively applied as electrodes in capacitive strain sensors. AgNWs should be formed into micrometer-size patterned electrodes such that capacitive strain sensors have a large C for easy detection and small noise. Gravure printing,30 lift-off lithography,31 and laser patterning32 are the commonly used methods for the precise patterning of AgNWs. Even though these techniques are effective in patterning AgNWs, they face several drawbacks, such as multiple-step processes, limited patterning shapes, postprocess chemical residues and expensive equipment.33 However, capillary force lithography (CFL) has been shown to be an excellent alternative patterning technique.34 CFL is a simple patterning method that includes the stamping and polymerization of hydrogels patterned into the negative image of any desired patterns. Furthermore, CFL does not require expensive high resolution UV-light curing equipment. Soft stamps made of cheap elastomeric polydimethylsiloxane (PDMS) can be easily prepared from a single master mold

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multiple times. The desired chemical residue-free AgNWs patterns are then obtained after washing off the polymerized hydrogel and nonessential AgNWs.33

Figure 1. The schematic images of the (a) conventional, parallel-plate-type strain sensor and (b) ICSS. In contrast to the typical parallel-plate-type capacitive strain sensor which is largely affected by the thickness of its insulator layer, the thin configuration of the ICSS minimizes the effect of the normal stress on the measured strain values assuming that only plane stress condition is experienced by its thin membrane, and due to the electrodes placed perpendicular to the direction of the normal stress.

In this study, we designed the ICSS with thin-film electrodes based on the patterned AgNWs. It is expected that the alignment of the electrodes in the same plane perpendicular to the normal stress will enable the ICSS to be insensitive to external pressure due to the plane 5



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stress condition experience by the very thin ICSS. Furthermore, as illustrated in the Figure 1b, the ICSS is at least one thirds thinner than the conventional parallel-plate-type capacitive strain sensor (Figure 1a), making it easier to stretch the ICSS using a smaller tensile force for a fixed applied ε and easy to conform to the human skin.8 The AgNWs were patterned into interdigitated electrodes with micrometer dimensions and spacing using CFL. The ICSS was then prepared by embedding the patterned AgNWs electrode onto the surface of the transparent PDMS substrate. The ε sensing performance of the ICSS was evaluated through its GF. Furthermore, the sensing stability of the ICSS was determined using its hysteresis behavior and stability from a cyclic stretching test. The pressure insensitivity of ICSS was compared with ε sensitivity. The ICSS was then used to measure the finger and wrist muscle motions of the human body in order to simulate actual large and small ε sensing.

EXPERIMENTAL SECTION

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Figure 2. Schematic descriptions of the fabrication process of the (a) patterned AgNWs by CFL process and (b) capacitive strain sensor embedded in PDMS. 7



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A schematic diagram of the CFL process is shown in Figure 2a. The AgNWs on glass substrates were prepared by spin-coating a 0.25 wt.% AgNW solution (the AgNWs had an average diameter and length of 27 nm and 22 µm, respectively, and (111) crystalline texture from Nanopyxis Co., Ltd., Korea) in ethanol on the glass substrates three times at a spin rate of 1000 rpm for 60 s. The AgNW-coated glass substrate was then annealed at 150 °C for 2 min to completely evaporate the solvent residue. After annealing and cooling the AgNWcoated glass substrate, the surface of the PDMS stamp (made using the Sylgard 184 elastomer kit from Dow Corning, USA, mixed in a base-to-curing agent weight ratio of 10 to 1 and cured at 80 °C, for 2 h) with grooves of interdigitated rectangular line patterns was made to contact the surface of the AgNWs. The UV-curable hydrogel solution composed of poly(ethyleneglycol)-diacrylate

(PEG-DA,

Mw

=

575)

and

2-hydroxy-2-

methylpropiophenone (HOMPP) in a weight ratio of 1:1 was then dropped at the edge of the PDMS stamp and allowed to infiltrate the grooves on the PDMS stamp through capillary force interaction with the AgNW film and PDMS stamp. The uniform contact between the surfaces of the AgNW-coated glass and PDMS stamp was ensured by clipping two opposite edges of a glass/AgNW/PDMS stamp/glass configuration. The surface-to-surface van der Waals interaction of the PDMS stamp and the hydrogel resulted in the formation of the uniform interdigitated line patterns of the hydrogel. The hydrogel was then cured via polymerization using UV-light irradiation for 10 min with a UV lamp (at 365 nm and an intensity of 53 mW/cm2) placed 2 cm from the surface of the AgNWs. After detaching the PDMS stamp, the cured hydrogel patterns on the AgNWs were washed off by shaking the AgNW-coated glass substrate inside an ethanol bath for 10 s. The nonessential AgNWs covered with the cured hydrogel were removed, together with the cured

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hydrogel, during the ethanol bath wash, resulting in the formation of an interdigitated pattern of the remaining hydrogel-free AgNWs. A schematic diagram of the embedding process35 and the preparation of the ICSS is shown in Figure 2b. The strain sensor was made by embedding the AgNWs with an interdigitated pattern onto the surface of the PDMS film substrate using silica aerogel nanoparticles (JIOS Aerogel Corporation, Korea). The 4 wt.% silica aerogel solution in ethanol was spin-coated atop the patterned AgNWs at 1000 rpm for 60 s and annealed at 100 °C for 20 min. The silica aerogel nanoparticles were used to improve the adhesion between the AgNWs and PDMS film substrates through enhanced van der Waals interactions.35 The liquid PDMS mixture with a 10:1 weight ratio of the base and curing agent was then spin-coated at 400 rpm for 60 s and cured at 120 °C for 12 h atop the silica aerogel nanoparticles. Subsequently, the AgNWs, silica aerogel nanoparticles and PDMS composite film were dipped into a deionized water bath at 50 °C for 60 min. The composite film was peeled from the glass substrate while it was submerged in the deionized water bath. The composite film was dried on a hot plate at 80 °C for 30 min to remove the residual deionized water. The morphologies of the AgNWs, before and after patterning, and the hydrogel coating were analyzed using surface profilometry (SP), optical microscopy (OM) and field-emission scanning electron microscopy (FE-SEM). The optical and electrical properties of the patterned AgNWs were determined using a UV-visible spectrophotometry and a two-point probe electrical multi-meter. The ε was applied on the ICSS using a stretching stage. The variation of C with ε was measured using a probe (model E4980AL) from Agilent.

RESULTS AND DISCUSSION

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Figure 3. (a) Design of the interdigitated electrode patterns and its magnified images while charging, with deformations when stretched in the (b) x-direction and with (c) compression along the z-direction. (d) GF as a function of ε, from 0% to 100% ε (left graph) and 0% to 30% ε (right graph).

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A schematic design of the interdigitated pattern of the AgNW-based ICSS is shown in Figure 3a. This interdigitated pattern was composed of a number of electrode fingers with an initial length (l0), width (w0) and spacing (d0) of 1 cm, 100 µm and 50 µm, respectively. The initial thickness (t0) of the interdigitated electrodes was approximately 60 nm, with the patterned AgNWs embedded onto the PDMS substrate surface. In the interdigitated electrode design, two neighboring electrode fingers act as the charging electrodes, while the PDMS in between play the role of the dielectric layer of a capacitor. Hence, multiple capacitors are connected in parallel in the ICSS. The interdigitated pattern design of the ICSS allows it to have high ε sensitivity in the x-direction (Figure 3b) because small variations in spacing (d) will result in detectable changes in the large C of the parallel connected capacitors. Furthermore, due to the structure of its component capacitor elements, the ICSS was predicted to be less sensitive to any applied pressure on its surface (Figure 3c), in comparison with conventional strain sensors with parallel-plate capacitors. The C of a single capacitor component of the ICSS is very small due to the small charging surface area (l0 ⋅ t0 in Figure 3a) of each capacitor element. In contrast, the configuration of multiple capacitor elements electrically connected in parallel enables ICSS to have a large initial C (C0). The C0 of the ICSS is expressed as

C0 = κ

l0 ⋅ t 0 ( n − 1) d0

(1)

where κ is the permittivity of the PDMS substrate, and n is the number of the finger electrodes. Because the electron charging will be between two opposite electrodes, there will be n-1 electrically charging fields with n finger electrodes. The relative change of the C (∆C/C0) and sensitivity of the sensor to ε, expressed as GF, are defined as equations (2) and (3), respectively: 11



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∆C C − C0 = C0 C0

(2)

∆C ε C0

GF =

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(3)

Figure 3b shows a schematic image of the sensor stretched in the x-direction. When the strain sensor is stretched in the x-direction, d0 would increase to (1+ε)d0. However, l0 and t0, which are perpendicular to the stretching direction, would be decreased to (1-νε)l0 and (1νε)t0, respectively, due to Poisson’s contraction ratio (ν) along the y- and z-directions.36 Hence, the C and its variation, expressed in ∆C/Co as a function of the ε while the ICSS is being stretched along x-direction, are defined as:

C=κ

(1 − νε) l ⋅ (1 − νε) t (1 + ε) d 0

0

(n − 1)

(4)

0

∆C (1 − νε ) ⋅ (1 − νε ) = −1 (1 + ε ) C0

(5)

As shown in equation (5), the ε sensing performance of the ICSS depends only on ε and not on its initial dimensions (d0, l0 and t0) and n. The PDMS substrate is assumed to undergo elastic deformation when stretched up to 30% ε.37 Furthermore, it is assumed that the physical properties of the PDMS substrate remain unaffected by the embedded AgNWs because the thickness (t) of the AgNWs (60 nm) is significantly smaller than the t of the PDMS substrate (250 µm) in the ICSS. The ν of the PDMS is taken as 0.5.38

However, the thin film geometry of the ICSS might be a potential

source of inaccuracy in the Poisson’s ratio value of 0.5 measured for bulk PDMS. If we focus on the ε range from 0% to 30% as shown in the graph from equation (5) on the right of Figure 3d, it is expected that C varies approximately linearly with ε. When a line that crosses the 0% ε is fitted into equation (5), the GF of the ICSS is expected to be -1.57, in the range of 0 to 12


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30% ε along the x-direction. The GF of the ICSS along the y-direction can be determined as equivalent to -1.21 In a conventional parallel-plate capacitor, the C value is significantly affected by the pressure applied on its surface, because the t of the dielectric layer is decreased by the applied pressure.14 However, if the two electrodes and the dielectric layer are placed in the same plane, as for the configuration of the ICSS, the C value would be less affected by the applied pressure. As shown in Figure 3c, the t0 of the sample decreases to (1-ε)t0, while d0 and l0 would increase to (1+νε)d0 and (1+νε)l0, respectively, when the ICSS is compressed along the z-direction. Hence, the C and ∆C/Co will have the respective forms of

C =κ

(1 + νε) l ⋅ (1 − ε ) t (1 + νε) d

(6)

and

∆C (1 + νε ) ⋅ (1 − ε ) = − 1 = −ε (1 + νε ) C0

(7)

Consequently, the GF of the ICSS, in terms of ε along the z-direction, is equal to -1. However, the absolute value of the GF of ICSS will be much less than 1 when the GF is expressed in terms of the applied pressure along the z-direction.39 Therefore, here, the performance of the AgNW-based ICSS was evaluated by stretching along the x-direction, for which it has the highest sensitivity (expected GF of approximately -1.57).

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Figure 4. Three-dimensional surface profile (SP) images (left) and height profiles (right) of patterned (a) hydrogel and (b) AgNWs on glass.

The CFL method was used to successfully prepare the interdigitated pattern of the AgNWs. The three-dimensional height profiles of the cured hydrogel patterns (Region 1) on the AgNWs are shown in Figure 4a. The average height of the hydrogel patterns was approximately 17 µm, which is high enough for effective patterning of the AgNWs.33 After washing off the cured hydrogel pattern with ethanol, the excess AgNWs in Region 1 (Figure 4a) were peeled off, together with the cured hydrogel. The AgNWs in Region 2 (Figure 4a and 4b) that were not covered by the hydrogel patterns remained on the glass substrates and were formed into an interdigitated pattern. As suggested by the dimensions of Region 1 in 14


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Figure 4a and 4b, the d of 50 µm between the fingers of the interdigitated pattern of the AgNWs was very similar to that of the w of the cured hydrogel patterns. Furthermore, the t and w of the AgNW patterns (Region 2 of Figure 4b) were approximately 60 nm and 100 µm, respectively.

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Figure 5. (a) OM and (b) FE-SEM images of the interdigitated pattern of AgNWs on glass, (c) FE-SEM image of patterned AgNWs embedded onto the surface of PDMS. The (d) optical transparencies of the bare PDMS substrate, fully-coated AgNWs embedded in the PDMS and patterned AgNWs embedded in the PDMS for ICSS together with (e) actual 16


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images of ICSS. The variations of the (f) electrical resistance (R) and the conductivity (σ) of the patterned AgNWs with respect to the applied ε.

Figure 5a to 5c are the OM and FE-SEM images, respectively, showing the uniformity of the interdigitated pattern of AgNWs. As shown in Figure 5a, the AgNWs were uniformly patterned into the interdigitated configuration on the glass substrate, with the width (w) and d of the fingers of the interdigitated pattern measured as approximately 100 and 50 µm, respectively. Furthermore, the FE-SEM image in Figure 5b shows that the fingers of the interdigitated pattern have well-defined edges. Figure 5c shows the surface of the PDMS substrate with the embedded, interdigitated, patterned AgNWs. According to Figure 5c, the embedding process allowed all the AgNWs to be transferred while maintaining the well-defined edges of the interdigitated electrode fingers. In Figure 5c, there were blurred stains similar to that of the PDMS substrate in some spots on the surface of the embedded AgNW electrode. These stains indicate that the PDMS infiltrated through the aperture between the glass substrate and AgNWs that did not directly contact the substrate. Figure 5d shows the optical transmittance of the bare PDMS and the PDMS with as-coated and patterned AgNWs. For accurate comparison, the density of the AgNWs between ascoated and patterned AgNWs, and the thickness of the PDMS were same as each other. The AgNWs with an interdigitated pattern showed a relatively higher optical transmittance of approximately 85% at 550 nm, compared to 79.5% for the as-coated AgNWs. As a reference, the bare PDMS substrate showed 92.2% optical transmittance at 550 nm. The transmittance difference at 550 nm between the bare PDMS substrate and PDMS with the as-coated AgNWs was 12.7%. The variation in optical transmittance between the bare PDMS substrate 17



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and the PDMS with patterned AgNWs was approximately 7.2%, near the expected value of 8.5%, because only 66% of the AgNWs remained after the interdigitated patterning process. AgNWs with an interdigitated pattern embedded onto the surface of the PDMS substrate had a similar optical transparency as that prior to the embedding of the electrode. The ICSS showed better optical transmittance than other transparent strain sensors reported in the related literature, with transmittance between 62% and 80% at 550 nm.8, 14, 24, 28, 40-41 As shown in Figure 5e, the university logo is clearly visible behind the ICSS. The line resistance (R) of the patterned AgNWs was measured to show that the patterning process does not deteriorate the electrical performance of the electrodes. The R of the AgNWs was measured over an area with length and width values of 8 mm and 15 mm, respectively. The R of the AgNWs with an interdigitated pattern was approximately 60 Ω, which is larger than the 45 Ω of the fully coated or unpatterned AgNWs. The increase in R was expected, because there was a 34% reduction in the total w or 34% reduction in the overall coverage of the AgNWs after patterning into the interdigitated shape. Hence, the R of the AgNWs was not significantly altered by the CFL patterning process. Furthermore, as shown in Figure 5f, the ICSS had an electrical conductivity (σ) of about 42.2 S/cm at 30% ε, which suggest that the electrodes of the ICSS does not have severe cracks, and the ICSS can still effectively perform its capacitor function with the ε range up to 30% (the ε range being studied here).14

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Figure 6. (a) The GF of ICSS as a function of applied tensile ε. (b) The optimization of the AgNWs spin-coating cycles with respect to the optical transmittance, and variation of the C after stretching up to 30% and subsequent relaxation. Hysteresis curves of the strain sensor at (c) 5%, 10% and 15% ε, together with (d) 20% and 30% ε conditions. (e, f) Dynamic durability of the strain sensor.

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The change in C relative to the applied ε of the ICSS is shown in Figure 6a. As expected from equation (5), the ICSS showed an almost linear behavior in terms of the variation in relative C with applied ε. The C of the ICSS decreased with the increase in the applied tensile ε along the x-direction. Consequently, the GF of the ICSS was determined to be approximately -2.0, which was larger in magnitude than the predicted value of -1.57 in Figure 3d and still higher GF than approximately 1 for typical parallel-plate-type capacitive strain sensors.14-15, 27, 41 Figure 6b shows the results for the optimization of the AgNW solution spin-coating cycles with respect to the optical transmittance, and variation of the C after stretching up to 30% and subsequent relaxation. The spin-coating cycles of the AgNWs (0.25 wt%) solution was varied from 1 to 3 and 5 cycles. 1 coating-cycle sample showed the best optical transmittance (about 88% at 550 nm light wavelength) but had the worst hysteresis among the three samples mentioned. Consequently, 5 coating-cycle sample had the worst optical transmittance (about 69% at 550 nm light wavelength) even though it had an excellent hysteresis performance together with the 3 coating-cycle sample (optical transmittance at about 83% at 550 nm light wavelength). Hence, the 3 coating cycles sample was used for further analysis in this study. Figure 6c shows the hysteresis behavior of the ICSS from ε values of 0% to 5%, 10% and 15%, while Figure 6d shows similar results for ε values from 0% to 20% and 30%. The ICSS showed negligible hysteresis behavior up to ε of 15%; the C of the strain sensor only varied within 1% of its initial value after the release of the applied ε. However, as shown in Figure 6d, the hysteresis behavior of ICSS was more pronounced over 20% ε. The C of the strain sensor decreased by 5% and 10%, respectively, after the release of 20% and 30% ε. The severe hysteresis behavior of the ICSS over 15% ε was found to be due to the fracture of some of AgNWs at this ε.6 20


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The ICSS was repeatedly stretched from ε values of 0% up to 10% for 1000 cycles to determine its potential ability to measure the motion of the fingers of the human body. As shown in Figure 6e, after the cyclic stretching and relaxing up to 10% ε for 1000 cycles, the C of the ICSS only changed within 1% of its initial value. Hence, the ICSS developed here is highly stable under repeated stretching.

Figure 7. (a) Various weights to test the pressure sensitivity of the ICSS together with (b) the proximity data which is the capacitance change as the pressure is applied and (c) pressure sensitivities of the strain sensor varying weights.

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It was predicted that ICSS would be insensitive to applied pressure along the z-direction, as shown in Figure 3c. To prove this prediction, pressure was applied on the ICSS using various weights. As shown in Figure 7a, the applied pressure on the sensor was varied from 44, 66 and up to 79 kPa by exerting weights of 1, 1.5, and 1.8 kg, respectively. However, as shown in Figure 6b, the C of the strain sensor abruptly increased by 60% when the 1 kg weight, in the form of a brass block, was initially placed under the strain sensor. The increase in C value is not due to pressure; it is due to the piezocapacitive effect because there was no pressure applied on the ICSS.16 The proximity of the brass block to the ICSS resulted in variation in the permittivity of the AgNW electrodes in the ICSS. Hence, by calibrating the induced piezocapacitive effect using the brass blocks, the degree to which applied pressure of the ICSS affected the system was properly determined. As shown in Figure 7c, C decreased with additional applied pressure of 22 kPa after the initially applied pressure of 44 kPa. Hence, C actually decreased with the increase in applied pressure when the initial C variation, due to the inclusion of proximity or piezocapacitive effect. C decreased by only approximately 1.2% after the addition of 22 kPa of applied pressure. Furthermore, C decreased by approximately 0.2% with the application of 13 kPa of pressure. Consequently, the ICSS based on patterned AgNWs was insensitive to applied pressure on its surface because its ε sensitivity was much larger than its pressure sensitivity.

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Figure 8. Sensing performance of ICSS applied as a motion sensor to the cyclic movements of the human (a) finger and (b) wrist muscle.

The ICSS based on patterned AgNWs was applied to body motion sensing, because it exhibited linear ε sensing behavior coupled with very low hysteresis and stable performance under a cyclic stretching test. The motions of the finger and wrist muscles were measured to simulate large and small ε sensing, respectively. When ICSS was applied to body motion detection, the change in C due to the piezocapacitive effect was also expected. However, 23



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based on our measurement, the piezocapacitive effect did not affect the GF of the ε because the increase in C was proportional to ε. As shown in Figure 8a and 8b, ICSS was placed and anchored on the skin using polyimide tape. There was a stable and predictable variation in C of the ICSS when the finger and wrist muscles were set into repeated motion. In Figure 8a, folding of the finger resulted in a decrease in C of approximately 10%, which corresponds to the ε of approximately 5% according to the graph presented in Figure 6a. The wrist musclemotion measurement presented in Figure 8b showed a 6% decrease in C, corresponding to the predicted 3% ε. As shown in Figure 8a and 8b, ICSS showed no significant variation in C, even after several finger and wrist motions. Hence, these results indicate that ICSS can be used effectively in sensing body motion.

CONCLUSION

We successfully fabricated a strain sensor based on AgNWs with an interdigitated pattern embedded onto the surface of the PDMS substrate. To the best of our knowledge, this is the first strain sensor that uses a system of multiple in-plane capacitors to detect ε from 0% to 30%. The CFL method was used to prepare the desired interdigitated AgNW pattern without any effect on the electrical performance of the AgNWs. Furthermore, the use of AgNWs and formation of an interdigitated pattern imparted the ICSS with excellent optical transmittance. The ICSS showed a GF of -2.0, which is larger than the GF limit of 1 for the typical parallelplate capacitive strain sensors. Moreover, the ICSS did not have any hysteresis behavior below ε values of 15%. Consequently, the ICSS showed stable cyclic sensing performance at 10% ε when repeatedly applied for 1000 cycles. The typical strain-pressure crosstalk problem of capacitive strain sensors was solved by placing the electrodes of the capacitor in the same 24


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plane, and thus the ICSS showed very low pressure-sensitivity. The ICSS was also shown to effectively and stably measure the repeated motion of the finger and wrist muscles of the human body. As a result, the ICSS could be used for body motion sensing applications with high sensitivity and mechanical stability. The study of the application of ICSS in large-area strain mapping and improvement of the ε dynamic range beyond 30% strain are the subjects of our future work. This work will include the development of the elastomeric substrate with elastic deformation range beyond the current 50% ε of the PDMS used in here, the improvement of the electrical conductivity of the AgNW anodes while maintaining a high optical transparency, and also a circuit design of multiple ICSS units.

ACKNOWLEDGEMENT This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (grant number 2015R1D1A1A01061340) and by the Joint Program for Samsung Electronics-Yonsei University.

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Graphical Abstract

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