Energetically Autonomous, Wearable, and Multifunctional Sensor

Jan 4, 2018 - Self-powered tactile sensing is the upcoming technological orientation for developing compact, robust, and energy-saving devices in huma...
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Article Cite This: ACS Sens. XXXX, XXX, XXX−XXX

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Energetically Autonomous, Wearable, and Multifunctional Sensor Hsing-Hua Hsieh,† Fang-Chi Hsu,*,‡ and Yang-Fang Chen*,† †

Department of Physics, National Taiwan University, Taipei 106, Taiwan Department of Materials Science and Engineering, National United University, Miaoli 360, Taiwan

‡

S Supporting Information *

ABSTRACT: Self-powered tactile sensing is the upcoming technological orientation for developing compact, robust, and energy-saving devices in human-machine interfacing and electronic skin. Here, we report an intriguing type of sensing device composed of a Pt crack-based sensor in series with a polymer solar cell as a building block for energetically autonomous, wearable, and tactile sensor. This coplanar device enables human activity and physiological monitoring under indoor light illumination (2 mW/cm2) with acceptable and readible output signals. Additionally, the device can also function as a photodetector and a thermometer owing to the rapid response of the solar cell made from polymers. Consequently, the proposed device is multifuntional, mechanically robust, flexible, stretchable, and eco-friendly, which makes it suitable for long-term medical healthcare and wearable technology as well as environmental indication. Our designed green energy powered device therefore opens up a new route of developing renewable energy based portable and wearable systems. KEYWORDS: self-powered electronics, stretchable optoelectronics, strain sensor, tactile sensor, photodetector, temperature sensor ecently, flexible and stretchable sensing devices with high tactile sensitivity have gained much attention in the research community for their potential for enabling a plethora of new applications in personal healthcare,1 human activity monitoring,2 and human−machine interfacing.3 Generally, those devices work based on piezoelectricity,4−7 piezoresistivity,8−10 pyroelectricity,11 capacitance,12,13 chemo-mechanics,14,15 and triboelectrification16−18 mechanisms to explore their tactile sensing ability. Unfortunately, for normal operation, all those sensors require an external power supply to generate an electrical signal to characterize the output of the devices in response to exterior stimuli, which is relatively bulky and incompact for technology orientation nowadays. As size and power consumption shrink day by day, selfpowered sensors, in which the active devices gain no power from external bulky electricity sources, would be the next generation of body electronics to have good integration with humans by either working alongside or attaching on the skin surface. There are a few reports on powering piezoelectric and electrochemical sensors via harvesting mechanical energy in detection of body glucose level19,20 and distinguishing humantouch events,21 respectively. Those self-powered sensors work by the bifunctional effects of mechanical deformation, i.e., to trigger energy generation and to induce subsequent chemical reactions for sensing. Several groups have developed triboelectric sensors, which harvest biomechanical energy from human motion, to provide an approach for human−machine interfacing such as touch pad, touch-enabled switch, etc.16−18 For those that do not harvest mechanical energy, e.g., strain

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© XXXX American Chemical Society

sensors, they are required to connect to an electricity supply for operation. To develop mechanically robust electronics, it is essential to integrate a suitable power supply preferentially in a soft form with the functioning device for full mechanical compliance. Few groups have tried to develop stretchable/ flexible power sources. Bauer and co-workers22 reported an electrochemical dry gel cell battery by dispersing the components in an elastomeric matrix; Yu et al.,23 Lee et al.,24 and Moon et al.25 demonstrated a stretchable and stable supercapacitor based on nanowire network; Bao’s group26 and Lipomi’s group27 developed stretchable and wearable polymer solar cells, respectively; Cha’s group28−30 presented flexible fuel cells using a highly conductive Ag nanowire network as current collectors. Among them, renewable sources of power, e.g., solar cells, work based on eco-friendly power conversion principles and have remarkable advantages over traditional power sources like batteries. The integration of renewable sources of power into stretchable devices can advance the field of conformable electronics further. So far, the adoption of soft power sources in connection with tactile sensors has rarely been reported in the literature. Herein, we report a multifunctional and energetically autonomous device based on a stretchable solar cell in series with a cracked Pt metal film on an elastic substrate. This Received: September 14, 2017 Accepted: December 20, 2017

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DOI: 10.1021/acssensors.7b00690 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors coplanar device possesses flexibility and stretchability with potential to operate at indoor conditions in response to multiple stimuli such as pressure, shear stress, vibration, light, and temperature with high sensitivity, high resolution, and no time lag. Specifically, the device enables human activity and physiological monitoring at a consumption of 2% of sun power. In addition, the device by itself can also function as a photodetector and a thermometer triggered by the surrounding light and temperature, respectively. Our designed device, the integration of both technologies, not only diversifies the working function of a crack sensor but also has a critical feature that would enable better portability and longer operation times; therefore it opens up a new paradigm for the development of renewable energy based multifunctional and wearable sensors toward the next generation of robust systems.

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coplanar device was illuminated by a white light LED (LCS-6500, Mightex) with tunable light intensity varying from 0.02 to 1.02 sun driven by LED drivers (SLC-AA02-US, Mightex). The intensity of illuminated white light was measured by a calibrated Si photodector (SRC-1000-TC-QZ-N, Oriel). The temperature response of the coplanar device was performed in the temperature range of 24 to 150 °C with intervals of 10 °C by placing the device on a hot plate in the laboratory environment. In this measurement, the solar cell component was encapsulated by PDMS to prevent damage at high temperature. An infrared thermometer (TES 1326S) was used to monitor the temperature of the device and each measurement was started 5 min after reaching the set point temperature. The current outputs of the coplanar device for detecting hand motion and heartbeat rate were measured by an ammeter (Agilent 4651c), while those for sound, temperature, and light intensity were recorded by an oscilloscope (Agilent DSO 5052A). The thickness of the film was measured by a Veeco dektak 6 M surface profiler.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION Figure 1 shows the schematics of a finished flexible and stretchable coplanar device along with its equivalent circuit.

Materials Preparation. PDMS solution (Sylgard 184) was purchased from Dow Corning Co. Ltd. and contained the base and the cross-linker in a weight ratio of 10:1. A PET film (∼1.4 μm) was purchased from Ruilong Co. Ltd. (Taiwan). PEDOT:PSS (Clevios PH 1000), P3HT, and PCBM were obtained commercially and used as received. The received PEDOT:PSS solution was intermixed with 4 wt % dimethyl sulfoxide and 1 wt % Zonyl FS-300 fluorosurfactant (Fluka) to enhance its conductivity for electrode deposition. The polymer blend solution was prepared by mixing P3HT (25 mg, Mw = 40 KD, PDI = 1.3, RR = 99.5%) and PCBM (20 mg) in dichlorobenzene (1 mL) solvent. Device Fabrication. Initially, glass substrates were cleaned by successively ultrasonicating in detergent, deionized water, acetone, and isopropyl alcohol and then dried in nitrogen gas flow. The PDMS solution was then spin-coated onto those 2 × 2 cm2 clean glass substrates at a spin rate of 2000 rpm for 60 s. After drying those PDMS films (∼12 μm) at 60 °C for 1 h, the purchased thin PET film (∼1.4 μm) was attached to each deposited PDMS film mechanically. Another layer of PDMS film (∼12 μm) with dimensions 2 × 0.5 cm2 was deposited on half of the PET surface by a spin-coating method using the same processing parameters as previously used. After drying, this PDMS layer was partially masked followed by spin-coating PEDOT:PSS solution onto the remaining area. The resulting PEDOT:PSS layer (∼40 nm) was dried in vacuum for 1 day. Then, an ∼150-nm-thick photoactive material P3HT:PCBM was spin-coated on top of PEDOT:PSS at spin procedures of 1000 rpm for 30 s followed by 2000 rpm for 30 s, and then dried in vacuum. Finally, a layer of EGaIn (Alfa Aesar) liquid metal was applied on top of the P3HT:PCBM film by cotton swabs as the second electrode for the solar cell, as well as to define the active device area of 2 × 0.5 cm2. Later on, a 30-nm-thick Pt was sputtered onto the masked PDMS film after uncovering the mask to complete the Pt sensor fabrication. Finally, a small amount of EGaIn was applied on the edge to link the Pt film and the PEDOT:PSS film. This integrated structure (selfpowered coplanar device) was then peeled off from the glass/PDMS substrate, leaving the thin PET film as the new substrate. For temperature measurements, the finished solar cell device was encapsulated with one more layer of PDMS (12 μm) before peeling off from the glass/PDMS substrate. Characterization. The elongation and bending measurements of the Pt film were carried out by positioning the film on a translational stage and the change of the film length was set by the micrometer. The elongation strain was defined as the ratio of the variation of the film length to its original length at rest. The bending radius was determined by taking a photograph of the bent state laterally followed by analyzed the bending curve with a circular function to obtain its radius. The current−voltage characteristics of Pt film and solar cells were recorded by using a Keithley Model 2400 source meter. For solar cell performance, the cell was evaluated under illumination intensity of 2 and 100 mW/cm2 from a solar simulator (Newport Inc.) with AM 1.5G filter. For the photoresponse measurements, the unstretched

Figure 1. Schematics of a finished coplanar device composed of a Pt strain sensor in series with a solar cell on the same flexible and stretchable substrate as well as the corresponding equivalent circuit shown alongside.

The coplanar device consists of a Pt strain sensor in series with an organic solar cell on the same substrate. The layered structures for the Pt strain sensor and the organic solar cell were polyethylene terephthalate/polydimethylsiloxane/Pt (PET/PDMS/Pt) and PET/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/poly(3-hexylthiophene):phenyl-C61butyric acid methyl ester/eutectic gallium indium (PET/ PEDOT:PSS/P3HT:PCBM/EGaIn), respectively. These two separate units were jointed through the application of EGaIn to bridge the Pt film and the PEDOT:PSS electrode. The whole device, including the substrate, was fabricated with soft polymeric materials and metals, which are of inherent good stretching and bending properties. The detailed fabrication procedures for the Pt strain sensor and the solar cell are described in the experimental section and shown schematically in Figure S1. In this coplanar device, the solar cell absorbs photons and generates power to drive the Pt film of varying status, a setup which can be viewed as a battery in series with a variable resistor. Since the operation of an organic solar cell is equivalent to a circuit composed of a capacitor (C) in parallel with a shunt resistance (Rsh) connected to a series resistance (Rs), the resistance of the PEDOT:PSS electrode can be combined into Rs. Before demonstrating the applications of the coplanar device, we characterize the fundamental properties of the Pt strain sensor and the solar cell individually. The morphology and the schematic of the Pt film after prestraining can be found in Figure S2. The relative resistance variation (ΔR/R0) of the PET/PDMS/Pt strain sensor after prestraining was measured under various elongation and bending motions (see Figure S3). The gauge factor (GF) determined from ΔR/(R0ε), where ε B

DOI: 10.1021/acssensors.7b00690 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors stands for strain, exceeds 110 at elongation strains of 0−2%; for instance, GF = 1551 at 2% strain (see Figure S3b). This Pt strain sensor failed when ε increased past 3%. Under bending, the outward bending (extension) of the stretchable sensor results in an increase in resistance with a decrease in bending radius whereas the reverse motion (compression) is the other way (see Figure S3c). For both cases, the variation of resistance is relatively rapid when the bending radius is smaller than 10 mm. The fabricated PET/PEDOT:PSS/P3HT:PCBM/EGaIn solar cell before peeling off from a glass substrate was characterized under 1 and 0.02 (indoor light intensity) sun conditions. The corresponding schematic diagram of the cell configuration is shown in the inset in Figure S4. At 1 sun, a power conversion efficiency (PCE) of 1.94% was obtained from a short circuit density (Jsc), an open circuit voltage (Voc), and fill factor (FF) of 8.05 mA/cm2, 0.64 V, and 38.0%, respectively. At 0.02 sun (2 mW/cm2), a PCE of 0.0091% can be achieved from Jsc, Voc, and FF of 0.6054 mA/cm2, 0.0614 V, and 24.6%, respectively. In order to ensure that the solar cell provides enough power to drive the Pt strain sensor for acceptable and readable output signals under exterior stimuli at indoor light intensity of 0.02 sun (2 mW/cm2), the dimensions of the Pt film were carefully designed to be 2 × 0.5 cm2. Applications for the Coplanar Device at Indoor Light Intensity of 2 mW/cm2. In the following dynamic measurements, we monitored the output signals of the coplanar device by an ammeter and the obtained results were plotted as the ratio of current (I/I0) vs time, where I0 is the current level flowing through the circuit before starting any exterior stimuli. The role of the solar cell here is to supply a constant voltage to the Pt sensor. Detection of Hand Motion. Figure 2 depicts two cycles of repeated hand motion detected by the coplanar device powered by indoor light. When the device was attached on the middle finger, the Pt-strain sensor was aligned on top of the knuckle leaving the solar cell on the side. The whole structure was positioned by the underneath oversized PET substrate wrapped around the finger (see inset in Figure 2a). By repeatedly bending and unbending the finger, the attached circuit underwent stretching and recovery cycles. As shown in Figure 2a, the response of the device was immediate, and dynamic variation of the current (∼2%) for the bending−unbending motion was detectable. Since the Pt-cracked film was bent inward, the output current level is expected to be higher than the flat case due to the shrinkage of the crack gap, which is beneficial for electron conduction through tunneling. However, the obtained signal shows a lower output current under bending. This is because, during bending the finger, the skin at the joint is also elongated, which introduces a sheer stress on the film. Therefore, the measured signal is a combination effect of bending and stretching of the Pt film. The lower current level at bend suggests that the stretching effect dominates the output. Figure 2b depicts the output waveform when the device was on the back of the hand to detect the extending−clenching action of all fingers. In the clenching state, the device was stretched with Pt strain sensor primarily elongated leading to an enhancement of resistance and, hence, a lower current flow. The current flow can be recovered when the device returns to its unstretched state, the extending state of fingers. The small recovery of current level during clenching and the small initial drop of current flow when extending can be attributed to the detailed motion of metacarpal bones, which affect the

Figure 2. Applications of the coplanar device to detect hand motion. The device is attached on the (a) knuckle of the middle finger and (b) back of the hand. Insets: images of device attached on the measuring locations. The schematics in (a) showing the deformation of the Pt strain sensor with the finger motion.

mechanical deformation mode of the Pt strain sensor. In this detection, the current variation level was 0.8% and this discernible small difference is with the integrated device primarily operated at the elongation mode. Detection of Heart Beating and Sound. Here we demonstrate the cases when the external stimuli is applied perpendicular to the Pt-cracked film surface. Figure 3a presents heartbeat waveform with the integrated device attached to a person’s wrist. The signals were successfully detected in situ and offered crucial heart physiological information, including the systotlic and diastolic movements of the heart. In another application for detecting sound, the whole device was attached to human neck to detect the vibration of vocal folds during speech. As shown in Figure 3b, by repeatedly speaking simple words such as “hi”, “go”, “hello”, and “thank you” twice, the device can respond immediately with abrupt changes up to 70% in the output current of the circuit. Those vibration patterns are reproducible and carry distinct features for each word. It is noted that when measuring heartbeat and sound, the vibration impulses strike the cracked film normal to its surface, and this causes the two-dimensional deformation of the film surface curving inward, resulting in a decrease of crack gap beneficial for electron conduction. However, the output current of the sound detection shows an enhanced resistance during speech. We believe that when detecting sound, in addition to the vibration of vocal folds, additional stress can be introduced on the film due to the movement of muscle arising from speaking. Accordingly, the measured signal of the sound is also a cooperative effect of bending and stretching of the cracked film, as that of finger bending and the stretching mode dominates in the measurement resulting in lowering the output current. C

DOI: 10.1021/acssensors.7b00690 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors

Figure 3. Coplanar device application to the detection of heartbeat and sound. (a) Human physiological monitoring showing the typical characteristics of the wrist pulse including percussion, tidal, and diastolic waves. Inset: image of device attached on wrist and the schematic of Pt strain sensor deformation under heart beating. (b) Application for speech by repetition saying words such as “Hi”, “Go”, “Hello”, and “Thank you”. The device is attached to the human neck.

Comparing the detection of mechanical motion and vibration, the greater output I/I0 value of human physiological detection than finger bending can be explained as follows. As mentioned previously, the motion of bending fingers involves both inward curving and linear stretching processes of the crack film. These two processes produce the opposite effect on the film conductance. Though the actual motion of bending fingers is large, the overall output current change is smaller than that obtained from the vibration of the beating heart. Light Intensity Effect. In addition to detecting mechanical stretching and external impulse with indoor light, the coplanar device by itself can also function as a photodetector. Figure 4a displays the photoresponse of the device with respect to light intensity. The intensity of incident light was varied from 0.02 (indoor) to 1.02 sun with periodic on−off cycles. In the off-

state, no light shone on the device such that the current reading was zero. As soon as the light was on, the device responded immediately with current flowing through the circuit and the magnitude of current increased with the increase in light intensity as shown in Figure 4b. When the light was turned off, the current level returned to zero rapidly. The input light power sensitivity (responsivity) of the coplanar device can be defined as SP = δ(ΔI)/δP, where ΔI is the difference of current flow under illumination and in the dark. Fitting the data in Figure 4b produced SP = 5.28 mA/sun, which is equivalent to 0.0528 A cm2/W. Due to the photoactive material, P3HT:PCBM, absorbing visible photons in the range of 400−650 nm,31 this coplanar device can be used as a visible light detector to detect the light intensity of environment. In a sense, the device can support itself electric power depending on the external light D

DOI: 10.1021/acssensors.7b00690 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors

power. As demonstrated above, ∼2% light intensity of one sun with supply voltage of 0.06 V is sufficient to drive the strain sensor with acceptable and readable output signals for human body motion and heartbeat. Temperature Effect. Figure 5a depicts the temperature effect of the coplanar device in the dark and at indoor light intensity. At each temperature (T), in the dark, there was no current flowing in the device circuits. When the indoor light was on, the solar cell generated power within 1 ms to provide current flowing through the Pt film (I0). By plotting the relative current variation (ΔI/I0) as a function of T (Figure 5b), current increases with an increase in T from 24 to 150 °C. This negative temperature coefficient of resistance is not common in metal and the obtained result is reproducible for the same integrated device in the studied temperature range. Though high temperature could cause the degradation of solar cell performance, the degradation effect is not apparent in the measured temperature range. We would expect that the degradation effect may become pronounced if the temperature goes beyond 150 °C. It is known that the resistivity of metals increases with increasing T due to stronger electron−phonon interaction, which reduces the electron mean free path. In contrast, high temperature facilitates the hopping motion of charge transport in organic materials leading to an improvement of charge mobility.32 Based on the equivalent circuit of an organic solar shown in Figure 1, we evaluated the performance of the solar cell under indoor light illumination at T = 24, 60, and 100 °C (see Figure 5c) and plotted the corresponding parameters including Voc, Rs, and Rsh as a function of T as the result shown in Figure 5d. All three parameters decrease with an increase in T with rates of 0.37 V/°C, 0.65 Ω cm2/°C, and

Figure 4. Response of the coplanar device to light intensity. (a) Photoresponse of the device in response to repeatedly turned on and off light at intensities from 0.02 to 1.02 sun. (b) Current variation (ΔI) as a function of light intensity. The solid line is the linear fit.

Figure 5. Temperature response of the coplanar device. (a) Current response of the device in the dark and powered by indoor light for each temperature. The temperature ranges from 24 to 150 °C. (b) Relative current variation (ΔI/Io) as a function of temperature fitted by linear dashed line, where I0 stands for the current level at 24 °C under illumination. (c) Temperature dependence of J−V characteristics for solar cell powered by indoor light and the associated performance parameters such as Voc, Rs, and Rsh in response to temperature are summarized in (d). The linear lines are the fits. E

DOI: 10.1021/acssensors.7b00690 ACS Sens. XXXX, XXX, XXX−XXX

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ACS Sensors 0.74 Ω cm2/°C for Voc, Rs, and Rsh, respectively. The change of Voc is less pronounced than that of Rs with respect to T. Accordingly, the output current of the solar cell itself is dominated by Rs, suggesting a larger current flow at high T. Thus, the obtained enhanced device current at higher T is presumably dominated by the properties of the solar cell. Based on this result, the coplanar device can also function as a temperature sensor powered by indoor light. As a temperature sensor, the temperature sensitivity (ST) defined by ST = δ(ΔI/ I0)/δT yields 3.6%/°C. According to Park et al.,11 temperature sensors made from ferroelectric rGO/PVDF composite film (1.58%/°C) are less sensitive than those made in the form of interlocked microdome arrays (3.3%/°C). Compared with the latter case, the sensitivity of our device is slightly higher; the structure of our device is relatively simpler; furthermore, our device is selfpowered. Based on the study above, the fabricated coplanar device can respond to mechanical deformation, light, and temperature individually with discernible output signals even triggered by indoor light level if necessary. This multifunctional characteristic can be attributed to the integration of an electrical and an opto-electrical components, which offers an additional degree of freedom for operation. The effective integration of both photovoltaics and sensor technologies could, in the future, power actuators or power-up integrated circuits on the large area of the e-skin, leading to self-powered technology. In real applications, those stimuli may act on the device simultaneously and the output current contains the effects of temperature, light intensity, and mechanical strain. For the validity of the device performance, it is necessary to decouple those effects for an actual functioning mode. We would suggest that it might be possible to design an external calibration circuit containing the basic information on the device in response to light intensity, temperature, and mechanical strain individually. When the device functions, the output current can be calibrated to separate each contribution and identify the functioning mode of the device. In addition to the operational characteristics, the fabrication process of the whole circuit is facile, energy saving, and eco-friendly, while the operation of the device is also based on the green energy concept. Consequently, the coplanar device is a green energy device and can show diverse applications either directly or indirectly interacting with humans. Bao’s group26 has demonstrated that the solar cell performance is nearly independent of elongation up to 18.5% strain for cells having the same structure as ours. Meanwhile, Yang et al.33 and Zhou et al.34 also reported that bending of solar cells can reduce the cell performance due to microcracks generated by the mechanical stress in ITO electrode contacting the substrate. Fortunately, we adopted polymeric material, PEDOT:PSS, and metallic EGaIn as electrodes, and there should be no cracks created after bending. Additionally, Kim et al.35 have demonstrated that the conductance of polymer:fullerene film is unchanged up to a bending radius of 3 mm. For detecting the nonjoint area tiny strain, the cell is primary stretched, whereas it is also bent when positioned on the joint. When bending fingers, the polymer solar cell can only be positioned on the nearby flat skin due to the limited space available and will not be involved in the large bending motion. Therefore, for the detection shown in this study, the mechanical deformation of the polymer solar cell is within the tolerable range of bending and stretching, and we would expect that the performance of

the polymer solar cell should not have significant impact on the I/I0. Accordingly, the current output of our integrated device under strain in this study is dominated by the resistance of the Pt strain sensor. Wang et al.36 also show that a metal-cracked Ti/Au sensor with cracks generated by prestraining on PDMS substrate and the device has shown good stability over 700 bending cycles. Since our Pt-cracked sensor is also prepared on PDMS with prestraining, it possesses similar structure and materials. Therefore, our device also shows good stability over 700 bending cycles. Kang et al.37 fabricated the Pt crack-based sensor using polyurethane acrylate (PUA) substrate and they generated cracks through bending the sensor at various radii. The generated cracks are parallel to each other with deep depth (≥40 nm) and they can penetrate throughout the Pt film and extend into PUA. According to Park et al.,38 the sensitivity of this kind of Pt crack sensor is crack depth dependent; i.e., higher GF value for deeper depth. In our case, we generated the cracks by elongating the Pt film instead of bending and those cracks resided in the film with depth of around 20 nm. Accordingly, the sensitivity of our Pt sensor is less sensitive than those with crack depths greater than 40 nm generated by bending. Though those deep crack sensors fabricated on PUA substrates show higher sensitivity than ours, they respond more slowly to external stimuli due to the inherent property of a viscoelastic polymer PUA37,38 in contrast to the rapid action of an elastomer PDMS used in our devices. Wang et al.39 has also demonstrated that microcracked Ti/Au film on PDMS produced by prestretching with crack depth of 60 nm shows GF value of about 5000 at the strain range of 0−1%, which is higher than our Pt sensor. By comparison of our sensor with theirs, the sensitivity of our Pt sensor is sufficient to detect the motion of nonjoint areas of human body, and most important of all, our sensor is self-powered and multifunctional. On the other hand, we also observe the waving feature of our Pt strain sensor due to the mechanical response of PDMS to strain. The effect of this wave feature on sensor sensitivity is still under study. Further improvement of the sensitivity of our devices can be carried out by changing the parameters of cracks such as depth, density, and so on through applying various elongation stress.

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CONCLUSION In conclusion, we have demonstrated a multifunctional, flexible, stretchable, and eco-friendly sensing device composed of a stretchable solar cell combined with a Pt cracked sensor. This coplanar device can be driven by indoor light for human activity and physiological monitoring with acceptable and readable output signals. In addition to mechanical deformation, the coplanar device also responds to various degrees of light intensity and temperature, a feature which suggests diverse applications of the device. This solar cell-based coplanar device provides advantages over traditional batteries with longer lifetime, compact circuitry, environmental friendliness, better portability, and so forth. Therefore, our study as shown here can provide a key step for the development of future smart, robust systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssensors.7b00690. F

DOI: 10.1021/acssensors.7b00690 ACS Sens. XXXX, XXX, XXX−XXX

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Fabrication procedure for the coplanar device, morphology and performance of the prestrained PET/PDMS/Pt sensor, and performance of solar cell under 1 and 0.2 sun (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fang-Chi Hsu: 0000-0003-1961-5569 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Ministry of Science and Technology, Taiwan (Project Nos. MOST 102-2112-M-239-001-MY3 and MOST 105-2112-M-239-001-MY3).

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DOI: 10.1021/acssensors.7b00690 ACS Sens. XXXX, XXX, XXX−XXX