Nanoscale. A skin-integrated transparent and stretchable strain sensor with interactive color-changing electrochromic displays PAPER.

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1 PAPER View Article Online View Journal View Issue Cite this:, 2017, 9, 7631 Received 27th March 2017, Accepted 13th May 2017 DOI: /c7nr02147j rsc.li/nanoscale Introduction A skin-integrated transparent and stretchable strain sensor with interactive color-changing electrochromic displays Heun Park, a Dong Sik Kim, a Soo Yeong Hong, a Chulmin Kim, b Jun Yeong Yun, a Seung Yun Oh, c Sang Woo Jin, c Yu Ra Jeong, a Gyu Tae Kim b and Jeong Sook Ha * a,c Along with the increasing demand for the next-generation technologies, wearable electronic devices have been rapidly developed in recent years. Electronic skin (e-skin), in particular, has been extensively investigated for advanced humaninteractive devices. Such devices are required to display information by sensing external stimuli and simultaneously converting that signal into a human-readable form. Recently, there have been significant efforts in the development of skinattachable flexible/stretchable e-skin inspired by animal/insect skin, in which various sensing signals can be visually detected. The Rogers research group reported an optoelectronic camouflage system using a leucodye composite from cephalopod skin, where the color pattern changed from black to white in response to light. The Whitesides research group developed a Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea b Department of Electrical Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea c KU-KIST Graduate School of Converging Science and Technology, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea. Jeong Sook Ha, jeongsha@korea.ac.kr Electronic supplementary information (ESI) available. See DOI: / c7nr02147j In this study, we report on the development of a stretchable, transparent, and skin-attachable strain sensor integrated with a flexible electrochromic device as a human skin-inspired interactive color-changing system. The strain sensor consists of a spin-coated conductive nanocomposite film of poly(vinyl alcohol)/multi-walled carbon nanotube/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) on a polydimethylsiloxane substrate. The sensor exhibits excellent performance of high sensitivity, high durability, fast response, and high transparency. An electrochromic device (ECD) made of electrochemically synthesized polyaniline nanofibers and V 2 O 5 on an indium tin-oxide-coated polyethylene terephthalate film experiences a change in color from yellow to dark blue on application of voltage. The strain sensor and ECD are integrated on skin via an Arduino circuit for an interactive color change with the variation of the applied strain, which enables a real-time visual display of body motion. This integrated system demonstrates high potential for use in interactive wearable devices, military applications, and smart robots. a cephalopod-mimicking soft robot with microfluidic channels that can be filled with a colored liquid or flushed by pumping. E-skin with integrated pressure sensors and the corresponding active-matrix-based organic light-emitting diodes (OLEDs) was demonstrated by the Javey research group. The Bao research group fabricated an e-skin mimicking the chameleon s skin that changes the color of the device depending on the applied pressure by using an integrated tactile sensor and visual display. Even with these pioneering advances in e-skin, there still remains a room for further improvement such as integrating skin-attachable deformable devices with low-power consuming displays of enhanced sensor-driven color-changes. 1 5 In this work, we demonstrate an e-skin, imitating the color change in human skin at the finger joint upon bending. For the first time, we could monitor the human motion of finger bending detected by the fabricated transparent stretchable strain sensor via the visual display of the corresponding color change of the flexible electrochromic device (ECD), which could be done with the use of the Arduino circuit. A skin-attached stretchable strain sensor detecting various body movements can be operated as a transducer to convert an applied force into an electrical signal. To date, various flexible/ stretchable strain sensors have been developed with piezoresistive materials such as metals, 6 metal nanowires, 7 metal nanoparticles, 8 graphene, 9 and carbon nanotubes, 10 owing to their simple design, low cost, ease of fabrication, and high This journal is The Royal Society of Chemistry 2017,2017,9,

2 sensitivity (i.e., gauge factor). The gauge factor, GF, is defined as GF = (ΔR/R 0 )/ε, which is the ratio of the relative resistance change (ΔR/R 0 ) to the applied strain (ε). 11 A strain sensor for e-skin should be conformable to non-planar substrates and provide high sensitivity, GF. It is also important for the sensor to be optically transparent because it is attached to the body during daily activities. 12,13 However, most reported conformable stretchable strain sensors are nontransparent since they are made of composite materials with a high nanofiller content. As an active material for detecting strain, we used a poly(vinyl alcohol) (PVA)/multi-walled carbon nanotube (MWCNT)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) nanocomposite which has not been reported yet but can facilely form uniform films on a stretchable polydimethylsiloxane (PDMS) substrate. Via the variation of the content of carbon nanotubes (CNTs), the electrical conductivity, sensitivity, and optical properties of the sensor could be controlled. This was because the network of CNTs enabled percolation transport between adjacent CNTs under mechanical deformation. Conformal adhesion of the fabricated strain sensor onto skin without using glue could be done via optimizing the properties of PDMS depending on the mixing ratio of the component polymers, which could endure the large deformation without delamination-induced degradation of the sensor. ECDs exhibit reversible and visible changes in transmittance or reflectance owing to the electrochemical oxidation reduction reaction of the electrochromic film. Although ECDs have a slow response compared to conventional LCDs and LEDs, they can operate over a large area with low cost and low power consumption. 14 Thus, they can be applied to smart windows for energy-efficient buildings, rear mirrors in automobiles, and electronic papers, for example. For the fabrication of ECDs, various organic conducting polymers and inorganic materials have been used. In particular, electrochromic conductive polymers have the advantages of high coloration efficiency, fast response, and a variety of color changes within the same material. 15 On the other hand, inorganic electrochromic materials have high chemical and thermal stability and excellent mechanical strength. 16 Therefore, inorganic organic hybrid combinations can enhance ECD performance over those made of either inorganic or organic materials. In our ECD, we could obtain the high performance via combining the electrochromic materials of inorganic vanadium pentoxide (V 2 O 5 ) and organic polyaniline nanofibers together, which has not been reported yet. Via the combined use of polyaniline nanofibers and V 2 O 5, the fabricated ECD could exhibit a much wider range of color displays from yellow to dark-blue compared to the use of a single component. Results and discussion In Fig. 1a, a schematic illustration of the interactive colorchangeable integrated system consisting of a strain sensor and ECD is given. The skin color at the joints changes with the applied strain due to finger bending, as shown in Fig. S1. Imitating such a phenomenon, the strain sensor and ECD are integrated on skin via an Arduino circuit for an interactive color change with the variation of the applied strain, which enables a real-time visual display of body motion. Thus, simple monitoring of the color change can give information on the extent of finger bending. For measuring the strain corresponding to such bending motions, a resistor-type strain sensor based on a nanocomposite of PVA/MWCNT/PEDOT:PSS on a PDMS substrate was fabricated as shown in Fig. 1b. The inset clearly shows its high transparency. In addition, nontoxic, environmentally friendly, and water-soluble PVA was added to the nanocomposite since it has excellent film-forming adhesive properties as well as high tensile strength and good flexibility. 17 Without PVA, such an aqueous composite film cannot form on the hydrophobic PDMS, as clearly shown in Fig. S2. Addition of PVA increases the viscosity of the composite to be easily spin-coated on PDMS. The contact angle of the nanocomposite solution on the PDMS film decreased from 120 to 99 after addition of PVA into the MWCNT and PEDOT:PSS solution. Thus, the composite film firmly adhered to the PDMS substrate without delamination during cycles of stretching and releasing. Scanning electron microscopy (SEM) images are shown in Fig. S3a, from which the composite film thickness is estimated to be 830 nm. Fig. S3b shows the Raman spectrum of the nanocomposite film compared to those of the component materials. G and D modes of the MWCNT at 1580 and 1344 cm 1, respectively, and shifting of the G mode to 1589 cm 1 after the incorporation of PEDOT:PSS are observed. The blue shift of the G mode indicates that the intermolecular interaction between the MWCNT and PEDOT:PSS enhances the phonon excitation with the increase in coherence length and electron mobility under the electron phonon scattering and binding. 18,19 Meanwhile, the conjugated aromatic ring mode of PEDOT:PSS at 1432 cm 1 shifts to 1439 cm 1 after the formation of composite films. This observation verifies the strong interactions between the MWCNT and the conjugated thiophene chain. 20 The Raman spectrum of the composite film confirms the existence of the component materials. By controlling the mixing ratio of the silicon base and the curing agent (cross-linker), the adhesion of the PDMS onto human skin can be enhanced. A mixing ratio of 30 : 1 enables the conformal adhesion of PDMS onto skin, allowing a large deformation without delamination-induced degradation of the sensor. 21 Fig. S4a are optical images showing the conformal attachment of PDMS to the fingertip and its strength of adhesion to a coin. The strong adhesion of this fabricated PDMS to human skin is also demonstrated in Fig. S4b. To analyze the electrical properties of the sensor, a few drops of liquid metal, Galinstan (eutectic alloy of 68.5% Ga, 21.5% In, and 10% Sn; m.p. 19 C), were coated on both ends of the sensor. 22 The reason for using liquid metal rather than silver paste was that rigid silver paste could not endure a strain over 30%, and thus it interfered with the electrical signal of the sensor. In contrast, Galinstan retains high conductivity under 7632,2017,9, This journal is The Royal Society of Chemistry 2017

3 Fig. 1 (a) (Top) Schematic illustration of the interactive color-changeable system of a strain sensor and an electrochromic device on hand skin. (Bottom) Circuit diagram of the integrated system. (b) (Top) Schematic illustration of the strain sensor with the nanocomposite of PVA/MWCNT/ PEDOT:PSS on a PDMS substrate. (Right) Transmittance spectrum of the strain sensor in the visible wavelength range from 380 to 780 nm. The inset shows the transparency of the sensor. (Bottom left) Photograph of the skin-attached sensor on a finger joint. (c) (Top) Schematic illustration of the ECD consisting of a polyaniline nanofiber/electrolyte/v 2 O 5 with an ITO-coated PET film as an electrode. (Bottom) Color gradient and photograph of the device with color change from yellow to dark blue upon applied voltage. various deformations and also generates low friction with the polymer surfaces when subjected to external strain. 23 Fig. 1c shows the scheme of the flexible ECD on the indium tin-oxide (ITO)-coated polyethylene terephthalate (PET) film, functioning as a visual indicator of the applied strain. The ECD was fabricated in a simple two-electrode configuration with conductive polyaniline nanofibers and V 2 O 5. Polyaniline has been extensively studied owing to its good environmental stability, tunable conductivity switching between insulating and semiconducting materials, ease of synthesis, and a wide range of potential applications. 24 V 2 O 5 has also been extensively investigated for its excellent properties such as its layered structure with high charge capacity, good Li + ion intercalation ability, multicolor displays, and lowvalence vanadium state, which leads to easy color changes. 25 The detailed synthesis results of polyaniline nanofibers and the V 2 O 5 film are shown in Fig. S5. Polyaniline nanofibers were grown on the ITO-coated PET film via the potentiodynamic method. After 10 cycles of charging and discharging, a 4.7 µm-thick film of polyaniline nanofibers was obtained (Fig. S5a and b ). The V 2 O 5 film synthesized on the ITO-coated PET film using the potentiostatic method at a constant voltage of 1 V for 1200 s gives the thickness of 6.9 µm as given in the cross-sectional SEM image (Fig. S5c and d ). The synthesis of polycrystalline polyaniline nanofibers and V 2 O 5 is observed in the X-ray diffraction (XRD) pattern and X-ray photoelectron spectrum (XPS), respectively (Fig. S6 ). Fig. S6a shows that the polyaniline appears in polycrystalline form. The XRD pattern reveals three intense and sharp peaks. The characteristic peaks for polyaniline occur at 23.2 and 26.2, corresponding to the (020) and (200) Millar planes of polyaniline, respectively. The other peak, at 30.5, matches well with the diffraction pattern for the (222) orientation of cubic crystalline ITO. The (020) reflection at 2θ = 23.2 is caused by the layers of the polymer chains at alternating distances. The (200) reflection at 2θ = 26.2 is attributed to the periodicity parallel to the polymer chain Fig. S6b shows the curve fit for the cleaved V 2 O 5 (001) single crystal. The difference in binding energy (Δ) between the O 1s and V 2p 3/2 levels was used to determine the oxidation state of the vanadium oxide. The O 1s and V 2p 3/2 of V 2 O 5 (001) are found at and ev, respectively, and the Δ value for V 5+ is 12.9 ev As an electrolyte, gel-type ACN-PC-LiClO 4 -PMMA was used because of its advantages of easy processing, good mechanical strength, a wide range of working temperatures, and enhanced stability. 33 Above all, it does not need encapsulation. Finally, the two electrodes with the electrolyte sandwiched between them were assembled to make a traditional and complementary device. This journal is The Royal Society of Chemistry 2017,2017,9,

4 Fig. 2 Electromechanical properties of the strain sensor. (a) Current voltage curves of the strain sensor under various strain loadings. (b) Relative resistance changes vs. strain curves. (c) Repetitive measurements of change in the relative resistance of the sensor with the variation of strain in a sequence of 10%, 15%, 20%, and 30%. (d) Response curve of the strain sensor with an applied strain of 5% showing a response time of 20 ms. (e) Characteristic of the sensor under a constant strain of ε = 10% for 10 min (f) Change in the relative resistance of the sensor with a repetition of elongation/relaxation cycles by ε = 10%. Detection of bio-signals with the strain sensor attached to the human body. Resistance change with (g) finger bending, (h) wrist bending, and (i) saliva swallowing. ( j) Changes in the relative resistance of the sensor in response to the pulse of the radial artery at wrist. The inset shows the region in a dashed box. The electrical characteristics of the strain sensor are presented in Fig. 2. The strain sensor demonstrates linear current voltage curves under various strains (Fig. 2a), indicating the ohmic behavior of the sensor. As the applied strain was increased, the resistance increased accordingly. Applied strain is defined as ε =(l l 0 )/l 0 100, where l and l 0 are the distances between the fixed edges before and after being stretched, respectively. Fig. 2b shows the change in the relative resistance (ΔR/R 0 ) with applied strain with error bars, taken from five different sensors. ΔR/R 0 increases with applied strain up to 50%. The sensor shows a slight hysteresis due to the mechanical properties of PDMS. 34 The slope of the ΔR/R 0 vs. strain (ε) curve, i.e., the GF, is estimated to be 5.2 up to 50% strain from the linear least-squares fit, with R 2 = In Fig. 2c, the reliable performance of the strain sensor is confirmed with constant ΔR/R 0 values under repetitive application of strain from 10% to 30%. Very fast response to the applied strain was observed, as seen in Fig. 2d, where the response time is estimated to be 20 ms under an applied strain of 5%. Stable strain sensing without any electrical drift over 10 min under a constant applied strain of 10% (Fig. 2e) and reversible sensing performance over repetitive cycles of 10% stretching/releasing (Fig. 2f) are observed. The increase in resistance (R 0 ) after the first cycle of stretching/releasing is attributed to the irreversible deformation of the nanocomposite film and the resultant longer electrical current path. 11 The absolute ΔR/R 0 values shifted slightly within 4% over repetitive stretching/releasing cycles because of creep deformation of the PDMS substrate after cyclic loading. 35 The performance of our sensor in terms of GF, stretchability, transparency, and durability is shown in Table S1 compared with that reported previously. The sensor is as thin as 500 µm so that it can be simply attached to the human skin without any adhesives, for detecting various biosignals. Fig. 2g and h illustrate the detection of finger and wrist bending, respectively. We compared the actual strain applied to the skin with the response of the strain sensor under finger movement (Fig. S7 ). The actual strain is obtained by measuring the change in the length between two fixed points around the joint of the index finger while per- 7634,2017,9, This journal is The Royal Society of Chemistry 2017

5 forming flexion movements depending on the extent of bending. When the finger is bent, corresponding to the flexion angle of 60, the actual skin strain is found to be 30%, while the sensor exhibits the resistance change corresponding to the applied strain of 25%. Such a slightly reduced strain by 5% is attributed to the difference in the Young s modulus between skin ( 130 kpa) and PDMS (30 : 1) elastomer ( kpa) since the strain generated on the skin is not fully transferred to the strain sensor The overshooting observed in the response of the strain sensor is attributed to the acceleration and viscoelastic properties of the polymer substrate. 39,40 The signals from saliva swallowing and the wrist pulse are shown in Fig. 2i and j, respectively. Time-dependent relative resistance changes show waves with a periodicity of 65 beats per min. The measured radial artery pulse exhibits two clearly distinguishable peaks, the systolic and diastolic peaks, in Fig. 2j (inset). 41 With the variation of the MWCNT content, the optical transmittance and electrical conductivity of the composite film changed. Fig. S8a shows the decrease in sheet resistance with the increase in MWCNT content owing to its high conductivity. Increasing the concentration of MWCNTs provides better contact with the PEDOT:PSS phase, enhancing the current flow. 42 The optical transmittance in the visible range from 380 to 780 nm was found to be 86, 77, and 48% with the MWCNT content of 0, 10, and 20 mg, respectively (Fig. S8b ). The increase in GF with the increasing MWCNT content was measured (Fig. S8c ). Addition of rigid MWCNTs with an elastic modulus (E n = TPa) higher than that of PEDOT:PSS (E n = 1 GPa) increases the stiffness of the nanocomposite film. 43,44 Consequently, under application of the same strain, the film with a higher MWCNT content will experience higher stress than that with a lower MWCNT content. Therefore, the strain sensor with a higher MWCNT content exhibits higher resistance-strain sensitivity. 45 After encapsulation with PDMS, the sensitivity decreases (Fig. S8d ) because of the reduced formation of microcracks under applied strain. 46 Through such investigation, a transparent and highly sensitive strain sensor was obtained with a nanocomposite film of 10 mg MWCNTs. These results suggest the high potential of the fabricated strain sensor for use as a skinattached health-monitoring device. As clearly demonstrated in Fig. 2, stretching of the nanocomposite film on an elastomeric polymer causes an increase in resistance. Such behavior can be explained by the scenario given in Fig. 3. After the first cycle of stretching and releasing, irreversible formation of microcracks on the nanocomposite film can be expected as shown in the schematic illustration of Fig. 3a (top). Upon the elongation of the strain sensor, the difference in Young s modulus between the stiff nanocomposite film and the underlying soft PDMS film induces cracking in the stiff film, and these microcracks will remain after releasing even though the width of the microcracks and the space between the cracks are reduced. This corresponds to Fig. 3 (a) Schematic illustration of the PVA/MWCNT/PEDOT:PSS composite film on the PDMS substrate during the elongation/relaxation cycle and the corresponding SEM images showing cracks formed on the composite film at ε = 0%, 50%, and 0%, respectively. (b) Schematic illustration of the current path during an elongation/relaxation cycle. This journal is The Royal Society of Chemistry 2017,2017,9,

6 the increase in initial resistance (R 0 ) after the first cycle as observed in Fig. 2f. SEM images taken along with the first stretching/releasing cycle of 50% are shown in Fig. 3a (bottom). Upon the initial stretching by 50%, the film also exhibited buckles along the direction of stretching in addition to the microcracks in the direction perpendicular to stretching. These buckles are attributed to the positive Poisson ratio for both PDMS and nanocomposite films. 10,47 Highly zoomed SEM images are shown in Fig. S9. After the first stretching/ releasing cycle, reversible widening/narrowing of the microcracks is observed under repeated stretching/releasing cycles. Such formation of microcracks under stretching can change the transport of charge carriers through the nanocomposite film. As schematically drawn in Fig. 3b, the path length of the charge carrier between the electrodes, made of liquid metal Galinstan, would be longer in the stretched state than in the released state. As a result, the resistance of a strain sensor is expected to increase in accordance with the experimentally measured phenomenon. After release of the strain, the resistance will be the same as that of the sensor after the first cycle. Fig. 4 shows the electrical and optical performance of the flexible ECD. Polyaniline undergoes two redox processes (i.e., leucoemeraldine emeraldine and emeraldine pernigraniline) depending on the potential region. The redox process involving anion insertion and extraction is associated with different structures resulting from the different forms of polyaniline, such as reduced insulating pale yellow leucoemeraldine, oxidized conducting green emeraldine, and insulating overoxidized dark blue pernigraniline. 48 The redox process can be represented by the following simplified equations. 49 The change between the leucoemeraldine base (LB) form and the emeraldine salt (ES) form with an anion can be expressed in terms of doping upon oxidation and dedoping upon reduction, respectively, according to eqn (1): Polyaniline þ nclo 4 ðlb; Pale yellowþ $ðpolyaniline nþ ÞðClO 4 Þ n þ ne ðes; GreenÞ: The change between the emeraldine salt form (ES) and the emeraldine base form (EB) by the protonation/deprotonation process can be represented by eqn (2): ES $ EB þ nclo 4 þ nh þ : The change between EB and the pernigraniline salt (PS) is induced by doping/dedoping with an anion, which can be represented by eqn (3): EB þ mclo 4 ðgreenþ $ðeb mþ ÞðClO 4 Þ m þ me ðps; Dark blueþ: Fig. S10 gives the molecular structure of polyaniline nanofibers depending on the redox state. On the other hand, the injection/extraction of Li + ions into and from the V 2 O 5 matrix changes the color of the V 2 O 5 film from yellow to green. This can be explained by the following eqn (4): 50,51 V 2 O 5 ðyellowþþxli þ þ xe $ Li x V 2 O 5 ðgreenþ: Cyclic voltammetry (CV) curves (Fig. S11 ) were taken for polyaniline nanofibers/v 2 O 5, polyaniline nanofibers, and V 2 O 5 ECDs. Polyaniline nanofibers ECD can change its color from pale yellow to dark blue, whereas the V 2 O 5 ECD changes its ð1þ ð2þ ð3þ ð4þ Fig. 4 (a) Optical transmittance spectra and photographs of the ECD under potentials of 2.5 V, 0 V, and +2.5 V. (b) Transmittance variation and (c) switching time characteristics measured at ±2.5 V for 10 s with a wavelength of 600 nm. (d) Cyclic switching for 1 h. Optical images of the (e) patterned ECD and (f) ECD attached to curved surfaces. 7636,2017,9, This journal is The Royal Society of Chemistry 2017

7 color from yellow to green according to the applied voltage (Fig. S12 ). The fabricated ECD exhibits a wide color range, from yellow at 2.5 V to dark blue at +2.5 V. Fig. 4a shows the optical transmittance spectra of the flexible ECD taken in the visible range from 380 to 780 nm. A two-dimensional x y representation known as the chromaticity diagram utilized to identify the color of the ECD is shown in Fig. S13. The shift in x y co-ordinates occurs once the potential is switched depending on applied potentials. It shows the color change from yellow to dark blue with the change in voltage from 2.5 to +2.5 V. The optical transmittance was measured to investigate the switching behavior and stability of the ECD at a wavelength of 600 nm, with a cyclic change of the bias voltage between 2.5 V and +2.5 V with a 10 s step under low power consumption (1.14 mw cm 2 ), as shown in Fig. 4b. 52 Coloration efficiency (CE) is an important element often used to characterize an electrochromic material; it is defined as the ratio of the optical density change (ΔOD) of the film at a certain wavelength to the injected (or ejected) charge per unit area (Q d ). It can be calculated according to eqn (5) and (6): 53,54 CEðλÞ ¼ ΔODðλÞ Q d ΔODðλÞ ¼log T b ð6þ T c where T b and T c denote the transmittance of the film in the bleached and colored states, respectively. Fig. S14a shows the charge/discharge amount for the ECD. From Fig. S14b, the charge amount (Q d ) for obtaining dark blue color after switching the applied voltage from 2.5 V to +2.5 V is estimated to ð5þ be mc cm 2. Similarly, the amount of discharge needed for a yellow ECD is estimated to be 9.30 mc cm 2 (Fig. S14c ). Using eqn (5), the coloration efficiency is calculated to be cm 2 C 1 for dark blue and cm 2 C 1 for yellow. It took 2.6 and 1.5 s for the increase and decrease, respectively, of the transmittance at 600 nm by 28% (Fig. 4c), which corresponds to the color changes to yellow and dark blue, respectively. The switching response is faster than that of the previously reported ECDs using WO 3, PEDOT:PSS, and Prussian blue because of the large surface area between the nanostructured polyaniline nanofibers and electrolyte The fabricated ECD showed stable cyclic switching, whereas the color contrast degraded slightly (5.2%) during 1 h of operation (Fig. 4d). We fabricated a display using a logo-patterned substrate via a photolithography process (Fig. 4e). The pattern is not clearly resolved at 2.5 V because V 2 O 5 is oxidized to yellow, and polyaniline nanofibers are reduced to pale yellow. On the other hand, the logo patterns KU and KOREA UNIVERSITY S.N.F.L. are clearly displayed at +2.5 V through simultaneous reduction of V 2 O 5 and oxidation of polyaniline nanofibers. As a result, the color pattern is clearly resolved with dark blue characters of polyaniline nanofibers. The detailed fabrication process of the character display is described in Fig. S15. Fig. 4f shows a photograph of the fabricated device wrapped around a vial having a radius of 7 mm. The transmittance spectra shown in Fig. S16 also confirm the flexibility of our ECD. There is just a slight change ( 2.3%) in the resistance of the electrode under bending (Fig. S17a ) which can be explained with the formation of cracks with a narrow surface gap in the SEM images (Fig. S17b and c ). This result suggests that our flexible ECD is a promising candidate for applications in various optoelectronic devices. 59 Fig. 5 (a) Change in transmittance with the variation of applied strain. (b) Real-time transmittance changes corresponding to various applied strains, each maintained for 10 s. (c) Photograph of the integrated strain sensor with the ECD under applied strain. (d) Sequential images of a hand with color changes together with finger motions. This journal is The Royal Society of Chemistry 2017,2017,9,

8 The stretchable strain sensor and flexible ECD are integrated on skin via the associated electronic circuit with the Arduino system. A detailed explanation of the interactive display of strain via the Arduino circuit is given in Fig. S18. Fig. 5a shows the change in optical transmittance due to the application of strain from 0% to 30%. With the increase in strain, transmittance decreases, and the maximum peak shows a blue shift, which corresponds to the color change from yellow to dark blue. The color of each spectrum distinguishes the magnitude of the applied strain. Under repeated stretching cycles, very reproducible transmittance values are observed, indicating the visual display of the applied strain in a quantitative manner (Fig. 5b). As clearly seen in the optical images of Fig. 5c, it is possible to estimate the applied strain via comparison with the color change of the ECD without any electrical measurement of the strain sensor. Lastly,asademonstrationofane-skinwithmotionsensing and interactive color-changing properties, a stretchable strain sensor and flexible ECD are attached at the finger joint and back of the hand, respectively (Fig. 5d). When the finger is straight (0% strain), the ECD display becomes yellow. As the finger is bent, strain is applied to reach 25%, and the ECD display turns into dark green. With straightening the finger, the ECD color returns to yellow indicating the release of the strain back to 0% (ESI Movie S1 ). Here, a slight darkening of the ECD color is observed when it is attached to skin compared to that shown in Fig. 5c under the same strain, due to the effect of skin color (Fig. S19 ). This clearly demonstrates the feasibility of expressing information from motion sensing as visible color changes. However, there still remain further improvements such as making a miniaturized stretchable Arduino circuit to be integrated onto the skin-attachable visual sensing system, consisting of the strain sensor, ECD, and Arduino circuit on a single stretchable substrate. Conclusion We have successfully demonstrated a skin-integrated transparent and stretchable strain sensor with an interactive color-changing ECD. The fabricated strain sensor exhibited a good GF of 5.2 up to 50% strain, high transparency of 77%, fast response time of 20 ms, and high durability over stretching/releasing cycles. The skin-attached strain sensor showed successful detection of bio-signals such as finger and wrist bending, swallowing, and wrist pulse. The flexible ECD showed a color change from yellow to green, or to dark blue depending on the applied voltage, with fast switching time under low power consumption. With the integrated system of the strain sensor, ECD, and Arduino circuit, applied strain due to finger motion can be detected simultaneously as a color change without electrical measurement of the strain. Experimental section Preparation of functionalized MWCNTs Multi-walled carbon nanotubes (>90% carbon basis, length 5 9 μm, outer diameter nm; Aldrich) were refluxed in concentrated sulfuric acid and nitric acid (3 : 1 v/v; Sigma- Aldrich) at 70 C for 3 h to prepare carboxylic acid functionalized MWCNTs (MWCNT COOH). These functionalized MWCNTs were rinsed with deionized (DI) water several times using a cellulose ester membrane filter (pore size 0.2 μm, diameter 47 mm; Advantec MFS, Inc.). After osmosis filtration with a tube cellulose membrane (average flat width 33 mm, average diameter 21 mm; Sigma), MWCNT COOH was finally obtained. Preparation of the nanocomposite for the strain sensor PEDOT:PSS solution (Clevios PH 1000 from Heraeus) was filtered using a Minisart filter ( pore size 0.45 µm). Functionalized MWCNTs (10 mg) were dispersed in filtered PEDOT:PSS solution (5 g). For the preparation of the gel-state nanocomposite, 0.2 g of poly(vinyl alcohol) (PVA) (Aldrich MW ) was added. Then, the solution was mixed at 150 C with vigorous stirring for 5 h. Fabrication of stretchable, transparent, skin-attachable strain sensors Liquid PDMS (Sylgard 184, Dow Corning) was made with a weight-mixing ratio of 30 : 1 (base : curing agent) for a stretchable substrate. PDMS was poured on a Petri dish and spincoated for 30 s at 800 rpm. Next, it was cured in an oven at 65 C for 2 h. The strain sensor was fabricated by a spincoating process: the nanocomposite was spin-coated on a stretchable substrate by a two-step process, at 500 rpm for 10 s and at 4000 rpm for 40 s. Galinstan (68.5% Ga, 21.5% In, and 10% Sn; RotoMetals) was cast onto both ends of the strain sensor electrode for electrical characterization. Synthesis of polyaniline nanofibers on an ITO-coated PET film for ECDs The polyaniline nanofiber was deposited on an ITO-coated PET film (1 cm 2 cm, thickness 127 µm) using electropolymerization. The film was potentiodynamically grown at a scan rate of 50 mv s 1 between 0.20 and V (vs. Ag/AgCl) for 10 cycles in a solution of 0.1 M aniline monomer (99 wt%; Sigma-Aldrich) and 0.5 M sulfuric acid (H 2 SO 4 ) (95.0%; Samchun Chemical) at room temperature. Synthesis of V 2 O 5 on an ITO-coated PET film for ECDs The V 2 O 5 film was deposited on an ITO-coated PET film (1 cm 2 cm, thickness 127 µm) using electropolymerization. The electrodeposition solution was prepared with a 1 : 1 (v/v) mixture of DI water and ethyl alcohol ( 99.5%; Sigma-Aldrich), to which vanadium(iv) oxide sulfate hydrate (VOSO 4 xh 2 O) was added to make a 0.1 m solution. The electrodeposition was carried out by applying a constant electrical voltage of 1 V (vs. Ag/AgCl) for 1200 s. Preparation of a gel polymer electrolyte An electrolyte casting solution was prepared by adding acetonitrile (ACN) and propylene carbonate (PC) using a syringe into a vial containing lithium perchlorate (LiClO 4 ). Then, poly 7638,2017,9, This journal is The Royal Society of Chemistry 2017

9 (methyl methacrylate) (PMMA) was slowly added into the solution, followed by stirring at 65 C for 12 h. The gel electrolyte formed with a weight percent composition of 61 : 17 : 7 : 15 (ACN : PC : LiClO 4 : PMMA). The gel electrolyte was spread onto the electrode. Fabrication of the ECD The gel polymer electrolyte was cast on the synthesized polyaniline nanofiber film. The polyaniline nanofiber film as the working electrode and the V 2 O 5 film as the counter electrode were assembled together. After assembling the electrodes, the gel-state electrolyte changes to a solid-state electrolyte after drying naturally in air for 2 h. Arduino circuit The analog input of the Arduino is the A0 pin, and the digital outputs using the Pulse Width Modulation (PWM) are 5 and 9 pins. The voltage will change between pins 5 and 9 depending on the voltage input to A0. 9 V power supply is connected in parallel to the power supply of the Arduino and sensing part. In the sensing part, R r and R s are connected in series, and the voltage V r applied to R r is applied directly to A0. The fixed resistor, R r, is selected to be 1.95 MΩ. V r is normally 4.5 V. The equation for obtaining V r is given as eqn (7): R r V r ¼ 9V : ð7þ R r þ R s When the strain sensor increases to 30%, the resistance of the sensor increases to 2.45 MΩ, resulting in a V r of V. As a result of internal coding, V r decreases from 4.5 to V, and the ECD voltage V5 increases linearly from 2.5 V to +2.5 V based on V9. Characterization The surface morphology and the cross-sectional view of the fabricated strain sensor and ECD were investigated by SEM (Hitachi S-4800). X-ray diffraction (XRD) (SmartLab, Rigaku) and X-ray photoelectron spectroscopy (XPS) (X-tool, ULVAC-PHI) were used to investigate the chemical composition of the electrochromic materials grown. Raman spectral images of the strain sensor nanocomposite film were obtained (HORIBA LabRAM ARAMIS IR2), for which the laser wavelength and power were 532 nm and 0.5 mw, respectively. Cyclic voltammetry (CV) curves were obtained using an electrochemical analyzer (Ivium Technologies, CompactStat). A Canon EOS 7D was used to capture the optical images. The sheet resistance of the coated nanocomposite film was measured using a four-point probe (Desk 205, MS Tech) with a source measurement unit (Keithley 2400 SourceMeter). The electromechanical characterization of the strain sensor was performed by mounting it onto a custom-built stretching stage and controlling the position with computer software (PMC-1HS, Autonics Corp., Korea). The electrical signal was measured using an Agilent Technologies B1500A. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (Grant No. NRF-2016R1A2A1A ). It was also supported by the KU-KIST graduate school program of Korea University. References 1 C. Yu, Y. Li, X. Zhang, X. Huang, V. Malyarchuk, S. Wang, Y. Shi, L. Gao, Y. Su, Y. Zhang, H. Xu, R. T. Hanlon, Y. Huang and J. A. Rogers, Proc. Natl. Acad. Sci. U. S. A., 2014, 111, S. A. Morin, R. F. Shepherd, S. W. Kwok, A. A. Stokes, A. Nemiroski and G. M. Whitesides, Science, 2012, 337, C. Wang, D. Hwang, Z. Yu, K. Takei, J. Park, T. Chen, B. Ma and A. Javey, Nat. Mater., 2013, 12, H.-H. Chou, A. Nguyen, A. Chortos, J. W. F. To, C. Lu, J. Mei, T. Kurosawa, W.-G. Bae, J. B. H. Tok and Z. Bao, Nat. Commun., 2015, 6, Y. Chen, B. Lu, Y. Chen and X. Feng, Sci. Rep., 2015, 5, Y. Chen, B. Lu, Y. Chen and X. Feng, IEEE Electron Device Lett., 2016, 37, M. Amjadi, A. Pichitpajongkit, S. Lee, S. Ryu and I. Park, ACS Nano, 2014, 8, J. Lee, S. Kim, J. Lee, D. Yang, B. C. Park, S. Ryu and I. Park,, 2014, 6, X. Li, R. Zhang, W. Yu, K. Wang, J. Wei, D. Wu, A. Cao, Z. Li, Y. Cheng, Q. Zheng, R. S. Ruoff and H. Zhu, Sci. Rep., 2012, 2, T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. Izadi-Najafabadi, D. N. Futaba and K. Hata, Nat. Nanotechnol., 2011, 6, Y. R. Jeong, H. Park, S. W. Jin, S. Y. Hong, S.-S. Lee and J. S. Ha, Adv. Funct. Mater., 2015, 25, B.-U. Hwang, J.-H. Lee, T. Q. Trung, E. Roh, D.-I. Kim, S.-W. Kim and N.-E. Lee, ACS Nano, 2015, 9, S. Lim, D. Son, J. Kim, Y. B. Lee, J.-K. Song, S. Choi, D. J. Lee, J. H. Kim, M. Lee, T. Hyeon and D.-H. Kim, Adv. Funct. Mater., 2015, 25, J. Sun, Y. Chen and Z. Liang, Adv. Funct. Mater., 2016, 26, J. Jensen, M. Hösel, A. L. Dyer and F. C. Krebs, Adv. Funct. Mater., 2015, 25, S. S. Kalagi, S. S. Mali, D. S. Dalavi, A. I. Inamdar, H. Im and P. S. Patil, Synth. Met., 2011, 161, N. Liu, G. Fang, J. Wan, H. Zhou, H. Long and X. Zhao, J. Mater. Chem., 2011, 21, G. Guan, Z. Yang, L. Qiu, X. Sun, Z. Zhang, J. Ren and H. Peng, J. Mater. Chem. A, 2013, 1, G. R. Perumallapelli, S. R. Vasa, A. Kanwat and J. Jang, Mater. Res. Bull., 2016, 74, This journal is The Royal Society of Chemistry 2017,2017,9,

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