A Stretchable and Transparent Nanocomposite Nanogenerator for Self

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A Stretchable and Transparent Nanocomposite Nanogenerator for Self-powered Physiological Monitoring Xiaoliang Chen, Kaushik Parida, Jiangxin Wang, Jiaqing Xiong, Meng-Fang Lin, Jinyou Shao, and Pooi-See Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13767 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Applied Materials & Interfaces

A Stretchable and Transparent Nanocomposite Nanogenerator for Self-powered Physiological Monitoring Xiaoliang Chena,b, Kaushik Paridaa, Jiangxin Wanga, Jiaqing Xionga, Meng-Fang Lina, Jinyou Shao*b and Pooi See Lee*a a

School of Materials Science and Engineering, Nanyang Technological University,

50 Nanyang Avenue, 639798, Singapore. b

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University,

Xi’an, Shaanxi 710049, China. Keywords: stretchable, transparent, piezoelectric nanocomposite, triboelectric, strain sensor, Self-powered, physiological monitoring

Abstract Smart sensing electronic device with good transparency, high stretchability and self-powered sensing characteristics are essential in wearable health monitoring systems. This paper innovatively proposes a stretchable nanocomposite nanogenerator with good transparency that can be conformally attached on human body to harvest biomechanical energy and monitor physiological signals. The work reports an innovative device that uses sprayed silver nanowires

ACS Paragon Plus Environment

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as transparent electrodes and sandwiches a nanocomposite of piezoelectric BaTiO3 and PDMS as the sensing layer, which exhibits good transparency and mechanical transformability with stretchable, foldable, and twistable properties. The highly flexible nanogenerator affords good input-output linearity under the vertical force and the sensing ability to detect lateral stretching deformation up to 60% strain under piezoelectric mechanisms. Furthermore, the proposed device can effectively harvest touch energies from human body as a single-electrode triboelectric nanogenerator. Under periodic contact and separation, a maximum output voltage of 105 V, a current density of 6.5 µA/cm2 and a power density of 102 µW/cm2 can be achieved, exhibiting a good power generation performance. Owing to the high conformability and excellent sensitivity of the nanogenerator, it can also act as a self-powered wearable sensor attached onto different parts of human body for real-time monitoring of the human physiological signals such as eye blinking, pronunciation, arm movement and radial artery pulse. The designed nanocomposite nanogenerator shows great potential for use in self-powered e-skins and healthcare monitoring systems. Introduction Flexible and wearable electronics have recently attracted much attention in the research and industrial community for its huge potential applications such as artificial electronic skins, personal portable devices, robotics with human-like functionalities, and human body health monitoring.1-2 Fundamentally, practical wearable electronics should be highly flexible, conformally attached to human body and operational under various mechanical strain conditions, such as bending, twisting and stretching deformation.3-6 In addition, highly sensitive sensor networks that are able to perceive mechanical stimuli should be integrated for recognition of human activity.7-9 In achieving wearable electronic devices with high sensitivity, full flexibility


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and robust mechanical durability, there has been enormous progress in the development of new materials, novel structure design and effective device configurations.10-13 More recently, various flexible sensing electronic devices based on different sensing principle such as piezoresistive14-15, capacitive16-17 and field effect transistor18-19 were demonstrated and exhibited promising performance on monitoring the human physiological signals such as heart rate20, respiration21, and body temperature.22 Although significant progress has been made in developing practical wearable electronics, additional bias voltages must be applied to most of the above-mentioned wearable devices, which will cause issues such as power consumption and structural complexity considering the external power supply.23 Therefore, incorporating self-powered characteristics capable of harvesting mechanical energy from human movement will be very promising in portable and wearable devices as wearable electronics are typically subjected to various kinds of mechanical strain during its operation.24-25 In this regard, nanogenerators are the most promising candidates due to the capability of harvesting energy from ambient surroundings and biological bodies.26-28 Since the first piezoelectric nanogenerator based on Zinc oxide nanowire arrays has been proposed,29 piezoelectric nanogenerators and triboelectric nanogenerators are being widely investigated to convert low-level mechanical energy into electricity and perform as self-powered sensors.30-35 Triboelectric nanogenerators (TENGs) have the advantages of higher output performance, low cost, easy fabrication, high efficiency and a wide choice of materials, which can be easily integrated with any surfaces to harvest the contact-separation energy.36-39 On the other hand, piezoelectric nanogenerators (PENGs) featured simple structure and long-term stability making them favorable for use in powering micro/nanosystems and various small power consumer devices.40-42 So far, piezoelectric device using materials like ZnO nanowires, PVDF nanostructures, and various ceramic nanomaterials have been studied extensively to implement


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in self-powered devices.43-47 The output from these piezoelectric nanogenerators are not just limited to power generation purpose, the ability of piezoelectric materials to deliver different output waveform of electric signals under the stimulus of many kinds of mechanical strain enabled the realization of self-powered sensing systems.48-50 However, most of these devices have limited lateral stretchability and could not be used to detect lateral strain, which greatly limit their applications on large strain deformation.51-53 Alternatively, various piezoelectric fillers are mixed with highly flexible polymer matrix to construct nanocomposite nanogenerator (NCNG) for achieving flexibility and robustness with good performance.54-56 Although NCNG shows great possibility to develop stretchable wearable system, a fully flexible and stretchable NCNG has not fully realized yet due to the absence of suitable composite materials, electrodes and substrate with stretchability and mechanical stability.57-60 Therefore, further investigations are still required to realize practical stretchable NCNG with self-powered sensing ability. In particular, integration of optical transparency in wearable electronics is of significant importance for expanding the realm of current flexible electronics, especially in the applications of selfpowered touch screen devices, transparent electronic skins, wearable power sources, displays and electronic papers.61 Fabricating wearable devices with good transparency, high sensitivity, full flexibility, stretchability, and with the capability of energy harvesting are extremely appealing but still remains a challenge. In this paper, we demonstrate a transparent and stretchable nanocomposite nanogenerator that can be conformally attached on human body to harvest biomechanical energy and monitor physiological signals. The novel device integrates spray-coated AgNW as the stretchable electrodes, BaTiO3 embedded into polydimethylsiloxane (PDMS) as the piezoelectric sensing layer and a thin layer of PDMS as the protective cover and the friction layer. By integrating


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single-electrode TENG and nanocomposite-based PENGs together, the device not only can effectively generate electricity during the contact-separation process but also provides sensing ability under the compressive and stretching deformation. Furthermore, by virtue of the high conformability and excellent sensitivity of the piezoelectric nanocomposite, the proposed device was demonstrated as a self-powered wearable sensor to quantitatively monitor human physiological signals, such as eye blinking, pronunciation, arm movement and radial artery pulse, indicating its promising applications in smart healthcare monitoring systems and artificial electronic skins. Experimental Methods Materials and preparation. BaTiO3 NPs with an average diameter of 200 nm were purchased from Nanostructured & Amorphous Materials, Inc. The sprayed AgNWs (XFNANO Technology) have an average diameter of 50-100 nm and ultra-length of 150-200 µm. The AgNW/ isopropyl alcohol solution with a concentration of 0.5 mg/ml was sprayed onto PDMS substrates by a spray gun to achieve a good transparent and uniform conductive layer. The sprayed solution can be dried immediately after being deposited onto the substrates for the fast evaporation rate of isopropyl alcohol solvent. The PDMS film was prepared by spin-coating the mixture of Sylgard 184 base and curing agents (10:1 by weight) followed with a 90°C treatment for whole night. PDMS was selected as the substrate, polymer matrix of nanocomposite and the encapsulation layer of the device, it is known as a high stretchable and good transparent elastomer. The weight composition of the piezoelectric mixture of BaTiO3/PDMS was optimized to be 5% to achieve a good transparency and high elastic properties. Measurement and Characterizations. The morphologies of the nanowires and devices were characterized using a field-emission SEM (FE-SEM, 6340). The optical transmittance was


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measured by a Shimadzu UV-3600 spectrophotometer equipped with an optical integration sphere. The crystalline phase of the BaTiO3 nanoparticles is characterized by X-ray diffractometer (Shimadzu). The Raman spectra were obtained by the Raman spectroscopy (InVia-Reflex) with an Ar+ laser source. The voltage used in the poling process was provided by an amplifier/controller (TREK 610E H.V.). The ferroelectric effects of BaTiO3/PDMS nanocomposite films were measured by polarization hysteresis loops (P-E loops) using TF Analyzer 2000 ferroelectric test system applying triangular pulses from -40 to 40 MV/m between the top and bottom electrodes of the samples. The performance of the nanogenerator was evaluated by analyzing the output signals measured by a customized system, which was mainly composed of a function generator, shaker, and signal amplifier. Instron 8516 is used to apply the periodic compressive forces onto the devices. The applied force is measured using the force sensor located on the top platen. Oscilloscope (Yokogawa DL1620) with a probe of 100 MΩ is used to measure the output voltage. Short circuit current from devices are measured using lownoise current preamplifier (Model No. SR570, Stanford Research Systems). Results and discussion The schematic illustrations of the detail fabrication process of the stretchable and transparent nanocomposite generator (STNG) are shown in Figure 1a. First, the silver nanowires networks were spray coated onto a thin PDMS substrate which has been oxygen plasma treated and serves as the bottom electrode. Piezoelectric BaTiO3 nanoparticles mixed with liquid PDMS were then spun onto the bottom electrode and degassed for half an hour under vacuum to make sure the piezoelectric nanocomposite fully embeds and wraps around the AgNW network. After curing the nanocomposite overnight at 90 °C, another layer of AgNW was spray coated on the top of the nanocomposite layer as the upper electrode. Afterwards, a thin PDMS encapsulation layer was


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spin-coated onto the upper electrode to embed the AgNW networks and cap the whole device. The encapsulation layer not only protects the AgNWs from oxidation but also constrains the AgNW network in the PDMS layer to maintain a good conductivity under deformation. It is highly advantageous to utilize PDMS for the structural components of the device given its shape variability, stretchability, biocompatibility and good transparency. Besides, the high resistance of the PDMS suppresses the leakage current of the device and improves the polarization of the nanocomposite film during the poling process, which benefits the device performance. To align ferroelectric dipoles in the BaTiO3 nanoparticles, an electrical poling process was conducted with an external voltage of 400 kV/cm at 100 °C. Figure 1b shows the photographs of the resultant STNG device in its original state, stretchable, twistable and foldable state which shows good transformability. Furthermore, the STNG device also shows good transparency as shown in the photograph in Figure 1c. We can see from the transmittance spectrum, the transparency of the PDMS substrate, BaTiO3/PDMS nanocomposite film and resultant STNG device are about 93%, 72%, and 66% at 550nm respectively. The flexibility, stretchability, and transformability of the inherent polymeric nanocomposite properties could provide multifunctionality to the integrated devices.


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Figure 1. (a) Fabrication process of a stretchable and transparent nanocomposite nanogenerator (STNG). (b) The photographs of the fabricated device that is stretchable, twisted and foldable. (c) The digital images and transmittance spectrums of PDMS substrate, BaTiO3/PDMS nanocomposite film (NC film) and the resultant STNG device. The SEM image of the piezoelectric BaTiO3 nanoparticles is shown in Figure 2a, which exhibits a rounded shape with an average size of 200 nm. In order to characterize the phase of the BaTiO3 NPs, Raman spectrum of the BaTiO3 NPs was measured (the inset of Figure 2a). The sharp peaks in the spectrum range of 305-720 cm−1 are ascribed to the A1 and E (longitudinal optical) modes, which are specific to a tetragonal phase of BaTiO3.62 X-ray diffraction (XRD) is conducted to provide a more comprehensive phase characterization of the BaTiO3 NPs, as shown in Figure 2b. The results show that the BaTiO3 NPs have six diffraction peaks ((1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), (2 1 1)). Furthermore, the double-peak profile at 45°indicates that BaTiO3 NPs have high crystallinity with an excellent tetragonal phase.63 Figure 2c shows the cross-


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section image of the STNG device, which is a freestanding sandwich structure with a total thickness of 200 µm. This thin freestanding structure was purposely designed to ensure it can be conformally attached to the skin and compliable with the tiny skin deformation. The sandwiched nanocomposite sensing layer has a thickness of about 100 µm, which can be effectively polarized under the electric field. The AgNW layer coated on the both sides of the nanocomposite was examined and is shown in Figure 2d. The sprayed AgNWs were well-embedded into the polymers so that they could harvest the restoring force from the polymer matrix and maintain their locations in the host polymers under deformations.64 Figure 2e presents a SEM image of the BaTiO3 nanoparticles well dispersed in the PDMS matrix. To further ensure the ferroelectric properties of the nanocomposite film, the hysteresis in spontaneous polarization (P) with electric field (E) was measured, as shown in Figure 2f. Due to the nonferroelectric characteristics of PDMS polymer, a ferroelectric property with comparatively smaller remnant polarization (Pr) of 0.23 µC/cm2 and slightly higher coercive field (Ec) of 17.7 kVmm-1 than Pr and Ec of pure BaTiO3 is measured. This result confirmed that electric dipoles alignment in the direction of applied field is possible under application of suitable electric poling.


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Figure 2. (a) SEM image of BaTiO3 NPs. Inset is the Raman spectrum pattern of the BaTiO3 NPs. (b) X-ray diffraction pattern of BaTiO3 NPs. (c) The cross-section SEM image of the STNG device. (d) The magnified SEM image of AgNW conducting layer. (e) The SEM image of the BaTiO3/PDMS nanocomposite layer. (f) The P–E loops of BaTiO3/PDMS nanocomposite film. The conductivity of the electrodes under stretching deformation is important for the stretchable nanogenerator. For characterizing the performance of the AgNW/PDMS electrodes, the resistance changes under different stretching ratio have been measured. Figure 3a shows the photographs of AgNW coated transparent electrodes under different stretch ratio up to 100%.


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During the measurement, we find that the electrical connection between the sprayed AgNW network and the conductive tape is crucial for achieving good electrical conductivity under stretching deformation. Figure 3b shows the measured resistances with two different connections, one is the common-used commercial silver paste, another is our developed filtered AgNWs layer connection layer. We can see that the resistance of electrodes with the connected silver paste rapidly increases when the stretch ratio is larger than 20%. Under the same experimental condition, the resistances of electrodes with the connected filtered AgNWs layer can maintain good conductivity even when the stretch ratio reaches 80%. Being different in the fracture nature of the silver paste under stretching, the highly cross-linked AgNW networks can maintain its conducting paths during deformation. Therefore, in our experiment, a thick AgNWs connection layer was filtered by a suction pressure process, and stably transferred onto the joint contacts to maintain the good conductivity of the electrodes, as shown in Figure 3c. Figure 3d shows the sprayed coated AgNW network on the PDMS substrate, which exhibits a homogeneous and highly interconnected nanowire network. As the sprayed AgNWs have a long length of 150-200 µm, they are easily confined in the micro-droplets solution with entangled structures during the spray coating process, leading to the rolled AgNWs after drying on the substrates. The AgNW networks could accommodate the mechanical strains by untangling and sliding during stretching. Consequently, the stretchability and mechanical stability of the electrode can be improved. The resistance changes of electrode under stretch-release cycles were measured, as shown in Supporting Figure S1. Despite some increase in resistance under stretching, the resistance under 60% stretching state after 100 cycles is about 487.5 ohm at the two ends of the electrode (approximately 32mm distance), which indicates good electrical conductivity. Moreover, the resistance of electrodes at the original state only increases from 44.5


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ohm to 87.3 ohm after 100 stretch-release cycles, which shows sustained stability. Since the inherent impedances of piezoelectric and triboelectric devices are in the range of hundreds of MΩ, the slightly increased resistance of our electrodes is much smaller than the inherent impedance and has almost no negative impact on the performance of the device.65-68

Figure 3. (a) The photographs of AgNW coated transparent electrodes under different stretch ratio up to 100%. (b) The comparison of the resistance between silver paste connected and AgNWs layer connected transparent electrodes under stretching. (c) The SEM image of the transferred filtered Ag NWs connected layer. (d) The SEM image of the sprayed Ag NW network. For characterizing the performance of the STNG device, a mechanical vibrator was used to apply a periodic vertical force to compress the device. Figure 4b shows the typical measured piezoelectric voltage and current under the compressive force of 50N at a frequency of 5Hz. An output voltage of 2.8V and current density of 130nA/cm2 were obtained. To confirm the outputs were truly generated from the BaTiO3/PDMS piezoelectric device, the widely accepted polarity


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switching tests were performed. As shown in Figure S2 in the Supporting Information, a switching in the polarity of the measured signal was obtained when the electrode connections were reversed. Furthermore, we carefully measured the device without electrical poling under the same condition and no obvious output signals can be obtained, as shown in Figure S3. All these measurements demonstrate that the obtained signals are indeed originated from the piezoelectric effect of the nanocomposite film. Furthermore, we measured the output voltage of the device under the compressive force ranges from 5 to 50N, as shown in Figure 4c,d. The output voltage linear increases with the input force with a sensitivity of 50.8 mV/N, which shows good sensing performance under compressive forces. Owing to the good conductivity of the AgNW electrodes under stretching deformation, the STNG device can also work in the lateral stretching mode, as shown in Figure 4e. Figure 4f shows the pictures of the tested device under different stretching ratio. Figure 4g and 4h show the output voltage under stretch ratio ranging from 5% to 60%. We can see the output gradually increase with the stretch ratio as expected. The device can work normally under a high stretch deformation up to 60%, which is larger than most of the reported transparent and stretchable piezoelectric devices.43,

52, 57, 69-70

The long-term stability test is

conducted to confirm the mechanical endurance of the device. As shown in Figure S4, the stretchable device exhibits good mechanical stability under period stretching test, and shows a small variation of the output during the cycling test period of 2000s under a stretch ratio of 60%. The stable performance can be attributed to the robust mechanical property of the nanocomposite under significant deformation and the full flexibility design of the entire structure.


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Figure 4. The piezoelectric output of the STNG device under compressive force (a-d) and stretching deformation (e-h). Since PDMS has a strong triboelectric characteristic of gaining electrons (triboelectric negative), once an object (typically dielectric materials) or human skin contacts with the PDMS surface of the STNG, electrification will occur at the interface and generates the same amount of charges with opposite polarities. Therefore, the developed stretchable STNG device can also act as a triboelectric nanogenerator in single electrode mode under periodic contact and separation, as shown in Figure 5a. The detail working principle of single electrode mode was illustrated in Figure S5. Figure 5b shows the typical output voltage and current of the device, in which the output voltage reaches 105V and the current density reaches 6.5 µA/cm2. There are some slight differences between the measured wave shapes of piezoelectric and triboelectric output, which are respectively operated under the compress-recover and contact-separate process, as shown in Figure S6. For the measured triboelectric output, when the PET film gets contact with the device and the external force persists, the bottom BaTiO3/PDMS composite layer is compressed and induced a piezoelectric potential. The positive piezoelectric potential on the top surface of the BaTiO3/PDMS film can also attract corresponding electrons from the ground to the top electrode,


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which will enhance the output, as shown in Figure S7a. We compare the output voltages of the device before and after electrical poling. The compared output waves are shown in Figure S7b. Because the electrical output of the triboelectric effect is substantially greater than that of the piezoelectric effect, so the enhancement is not obvious in practical measurement. Therefore, during the contact-separate process, the electrical output originates mostly from the triboelectric effect, whereas the piezoelectric effect plays a small role. The effective instantaneous power of the device was calculated by measuring the output voltages across various loading resistors increasing from 1 to 100 MΩ, as shown in Figure 5c. With a load resistance of 16.6 MΩ, the maximum output power reaches up to 102 µW/cm2. To investigate the effect of loading pressure on the performance of the device, the output voltages under different forces ranging from 5 N to 40N are measured, as shown in Figure 5d. The output voltage increases to a saturated value when the applied force increases to 25N. Because the PDMS layer and the STNG device are soft, so they will deform under higher pressure, the occurred deformation will lead to higher effective contacting area and electrostatic charges at the interfaces. When the applied force was large enough to achieve a full contact, the output maintains the maximum value, as shown in Figure 5e. Figure 5f shows the generated voltage of the device under the same force of 20N with different frequency. The output voltage was not affected by increasing the driving frequency from 5Hz to 14Hz, which means the STNG device can also harvest energy with variable frequency and amplitude in our living environment. In order to demonstrate the ability to harvest the energy from the human, the device was tested under the finger tapping on the original state and stretch ratio of 60%, as shown in Figure 5g. Although the voltage decreases slightly after stretching up to 60%, but there is still a high output of 70V, which demonstrates the device can still harvest energy under harsh conditions of large stretching deformation. The slightly


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decreased voltage may be caused by the decreased contact area between the finger and the highly tensed device under large stretching deformation. Besides, the number of conductive AgNWs under the tapping area was decreased slightly due to the decreased density of the AgNWs per unit area under the stretching state, which also may cause slight degeneration of the performance. For potential utilization of the electricity generated from the nanogenerator, a full-wave bridge was used to convert the alternating voltage generated by the nanogenerator to direct voltage, and then used to charge the capacitor or directly drive the electronic device (Figure 5h). The entire charging processes were recorded by monitoring the voltages across the capacitor. Figure 5i shows the charging curves of a capacitor of 10 µF. The voltage of the capacitor reaches up to 3.2 V within 70 s. Besides, the high output of the device can directly lit up a series of 25 blue LEDs without any storage units, as shown in the inset of Figure 5i, which proves that the efficiency and utility of the device that is valuable in powering commercial electronics. In practical application, the proposed device can generate energy from moving body parts by triboelectric mechanism and also can detect the vertical and lateral strain signals by piezoelectric effect. What's more, the device presents the good transparency, which will make it more easily integrated with exciting novel applications, such as integrating with foldable screens and other transparent electronics.


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Figure 5. (a) The illustration of STNG device that works under single electrode mode of triboelectricity. (b) The typical output of the triboelectric nanogenerator under periodic contact and separation. (c) The output voltage and instantaneous power with various load resistors. (d,e) The output voltages under different impact forces ranging from 5 N to 40N. (f) The output of the device under a same force of 20N with different frequency. (g) The output of the device in the original state and under the stretch ratio of 60%. (h) The equivalent circuit for rectifier, charging capacitors, or lighting electronic devices. (i) Voltage-charging time relationship of a 10 µF capacitor by using the device under single electrode mode. The inset shows the electricity generated from the device was directly used to light 25 LEDs without any storage units.


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Taking advantage of the good flexibility, high conformability and high sensitivity, the STNG device can act as a self-powered sensor used for physiological monitoring. The device was first attached to human face to detect the blinking of eyes, as shown in Figure 6a. The device is subjected to deformation following the dynamic wrinkling of the skin upon eyes blinking, so the output signals gradually increase with the blink force. Besides, the sensor can also monitor blink rate according to the number of the output peaks. When the tester blinks continuously, the corresponding peaks of output can be detected. Figure 6b shows the sensor was attached to the throat of a speaker to identify different pronunciation. When the speaker pronounces the words Hi, Hello and Logical, three different resulting waveforms of output signals were obtained. In the case of Hi, only one positive peak was detected. For Hello, there are two positive peaks. The numbers of positive peaks are apparently in accordance with the number of pronunciation. When the speaker pronounces Logical, three positive peaks are generated, which proved our assumption. These results demonstrated the STNG device can be used to preliminarily identify different pronunciation, which may have potential application in human-machine interactive device or for security purposes. Figure 6c shows the application of the STNG device attached on the joints of the arm which will produce a larger deformation during movement. During the test, the arm was bended to different degree and the according outputs were measured. We can see a larger bending degree produces a larger strain on the device, which will lead to a higher output. Furthermore, the sensor is attached on the wrist to monitor the radial artery pulse in real-time, as shown in the inset of Figure 6d. The signals of heartbeat during 10 s of a tester before and after exercise are measured. As expected, the pulse frequency of 60 beats per min increases to 84 beats after exercise. Figure 6e shows an enlarged view of the multipeak waveform, which appeared to consist of small features, such as percussion, tidal, and dicrotic waves.71 Owing to


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the high sensitivity and rapid response, the measured waveform could provide more information for personal health assessment and medical diagnostics based on the physiology of the pulse waveform. Besides, the sensor could be attached on the wrist to simultaneously monitor the radial artery pulse and the wrist bending action, in which the heart beating pressure and the stretching deformation from the wrist bending were simultaneously applied to the device. The measured waveforms of the output signals were shown in Figure S8. In the first 8s, the wrist was motionless, so the output signals were steadily generated by the heart beating pressure. After that, the wrist was periodically bent to cause the stretching deformation of the attached sensor. We can see there were obvious peaks with longer output widths corresponding to the bending action of the wrist. Besides, during the bending action of the wrist, the pressure from the heart beating could also generate pulse signals, which cause a multi-peak output. Owing to the high sensitivity and rapid response of the proposed sensor, we can distinguish the two stimuli according to the output waves and the characteristics of the stimuli. All these applications demonstrate the developed stretchable and transparent nanogenerator can be used as an active sensor capable of detecting a small amount of skin movement. For the above self-powered physiological monitoring, the device was attached on the skin without contacting other objects and there was not contact-separate process, so the measured signals were originated from the piezoelectric signals generated by the mechanical strain induced piezoelectric potential along the nanocomposite, and did not depend on the charges from triboelectric effect. For the blinking and pronunciation applications, the sensor was subjected to bend/stretch deformation following the wrinkling of the skin upon eyes blinking and pronunciation, so the piezoelectric sensor mainly works at d31 mode. Owing to the self-powered sensing property, the piezoelectric signals can be


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directly measured and no additional bias voltages need be applied to the devices to deliver the signals, which can be potentially used for the development of self-powering system.

Figure 6. The STNG device used for self-powered physiological monitoring. (a) eye blinking (b) pronunciation (c) arm movement (d,e) heart beating.

Conclusion In summary, we have demonstrated a novel transparent, stretchable and biocompatible nanogenerator that can be mounted onto human skin for energy harvesting and physiological monitoring. The newly designed device can effectively generate energy from moving body parts by triboelectric mechanism and also can detect the vertical and lateral strain signals by piezoelectric effect. The highly flexible nanogenerator exhibits good input-output linearity under the compressive force and stable sensing ability to detect lateral stretching deformation up to 60%. Owing to the highly flexible and resilient material properties that were given by the component characteristics of all flexible materials, the device also showed good durability


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against large stretching deformation (60% strain) under a duration of 2000s cycling test in terms of low signal variation. For energy harvesting performance, the device can act as a singleelectrode triboelectric nanogenerator, which can provide a maximum voltage of 105 V and a power density up to 102 µW/cm2 under periodic contact and separation. Moreover, by virtue of the high sensitivity of the nanocomposite material, the device can also be used as a self-powered sensor for real-time monitoring human physiological signals such as eye blinking, pronunciation, arm movement and radial artery pulse. Taken together, our fabricated device is simple, low-cost and has excellent advantages including good transmittance, high stretchability, excellent sensitivity and energy harvesting ability, the developed devices are expected to contribute to new applications in the field of flexible and wearable electronics, particularly for health-monitoring medical devices. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. The resistance change of sprayed electrode under stretch-release cycles; upon the repeatedly vertical compressive force, the corresponding output voltage-time curves when reverseconnected to the measurement system; the output of the device without electrical poling; the cycling stability test of the STNG device; energy harvesting mechanism of the triboelectric nanogenerator under single electrode mode; the comparison between output wave shapes of piezoelectric effect

and triboelectric effect; The compare output waves before and after

electrical poling; and the output waves of the sensor when it attached on the wrist to simultaneously monitor the radial artery pulse and the wrist bending action. AUTHOR INFORMATION


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Corresponding Author [email protected] (Jinyou Shao), [email protected] (Pooi See Lee) Present Addresses a

School of Materials Science and Engineering, Nanyang Technological University,

50 Nanyang Avenue, 639798, Singapore. b

State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University,

Xi’an, Shaanxi 710049, China. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest ACKNOWLEDGMENT This work was supported by the NSFC Major Research Plan on Nanomanufacturing under Grant 91323303, NSFC Funds under Grants 51421004 and 51522508. This work was also supported by the NRF Investigatorship Award No. NRF-NRFI2016-05, funded by the National Research Foundation, Singapore, under the Prime Minister Office. REFERENCES (1) Sekitani, T.; Someya, T. Stretchable, large-area organic electronics. Adv. Mater. 2010, 22, 2228-2246. (2) Wang, X.; Liu, Z.; Zhang, T. Flexible Sensing Electronics for Wearable/Attachable Health


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A Stretchable and Transparent Nanocomposite Nanogenerator for Self

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