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Non-contact heartbeat and respiration monitoring based on hollow-microstructure self-powered pressure sensor Shuwen Chen, Nan Wu, Long Ma, Shizhe Lin, Fang Yuan, Zisheng Xu, Wenbo Li, Bo Wang, and Jun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17723 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018
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ACS Applied Materials & Interfaces
Non-contact heartbeat and respiration monitoring based on hollow-microstructure self-powered pressure sensor Shuwen Chen,† Nan Wu,† Long Ma,‡ Shizhe Lin,† Fang Yuan,† Zisheng Xu,† Wenbo Li,† Bo Wang,§ Jun Zhou*,† †
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and
Technology, Wuhan, 430074, P. R. China ‡
Wuhan Mechanical Technology College, Wuhan, 430075, China
§
School of Electrical Engineering and Automation, Luoyang Institute of Science and
Technology, Luoyang 471023, Henan, P. R. China
ABSTRACT: Advances in mobile networks and low-power electronics have driven smart mobile medical devices at a tremendous pace, evoking increased interest in household healthcare, especially for those with cardiovascular or respiratory disease. Thus, flexible, battery-free pressure sensors, with great potential for monitoring respiration and heartbeat in a smart way, are urgently demanded. However, traditional flexible battery-free pressure sensors for subtle physiological signals detecting, are mostly tightly adhered onto skin instead of working under the pressure of body weight in a non-contact mode, as the low sensitivity in high-pressure region can hardly meet the demands. Moreover, hollow-microstructure (HM), with higher deformation than solid microstructures and great potential for improving 1 ACS Paragon Plus Environment
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pressure-sensitivity of self-powered sensors, has never been investigated. Here, for the first time, we demonstrated a non-contact heartbeat and respiration monitoring system based on flexible hollow-microstructure (HM) enhanced self-powered pressure sensor, which possesses advantages of low-cost, high dynamic-pressure sensitivity of 18.98 V·kPa-1 and wide working range of 40 kPa simultaneously. Specific superiority of physiological detection under high pressure is also observed. Continuous reliable heartbeat and respiration information is successfully detected in a non-contact mode and transmitted to a mobile phone.
KEYWORDS: Self-powered, nanogenerator, energy harvesting, flexible, heartbeat, respiration
INTRODUCTION Booming development of mobile networks and increasing awareness of healthcare, have greatly propelled the blossom of smart mobile medical electronics. As heart rate and respiratory rhythm could provide critical information about the status of health and have tremendous diagnostic value, smart mobile or household systems for heartbeat and respiration monitoring are ubiquitously demanded.1-4 At present, various traditional sensors can be utilized for monitoring heartbeat or respiration. Despite the applicability, cumbersome structure, bad portability, reliance on external power sources and uncomfortable user experience, possibly restricted their widespread use in smart mobile medical electronics. Flexible pressure sensors, with merits of light-weight and mobility, possessing wide applications in speech recognition,5 wearable electronics,6-7 electronic skin,8-14 health monitoring,4, 6, 15 have attracted particular interests for smart mobile medical electronics. To 2 ACS Paragon Plus Environment
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achieve high pressure sensitivity and wide range, significant breakthroughs have been made on flexible pressure sensors using mechanisms of capacitance12, 16-17 and piezoresistivity18. However, these flexible pressure sensors are mainly powered by external source and can hardly achieve traits of high sensitivity and wide working range simultaneously. Moreover, capacitance and piezoresistivity based pressure sensors generally use sophisticated lithography-based method to fabricate solid micro/nano structures such as interlock,19-21 pyramid,4, 9, 22-24 micro-dome20, 25-26 and bioinspired arrays19, 27-31 for improving sensitivity, making them expensive and complicated. Even though flexible piezoelectric pressure sensors can exempt rigid battery, they generally have low pressure sensitivity. Fortunately, flexible nanogenerators (NGs) have been recently proposed as a new model for self-powered pressure sensors.2, 5, 18, 23, 25, 32 However, how to achieve low cost, high sensitivity and wide working range simultaneously is still a challenge. Moreover, as pressure sensing is mainly based on contact area change of sensing material when pressure is applied, highly deformable hollow microstructure NG with great potential of high sensitivity has never been studied. In addition, most reported flexible pressure sensors for physiological detecting are uncomfortably adhered onto skin. For the sake of comfortability, self-powered pressure sensors with capability of detecting physiological signals in a non-contact mode under pressure of body weight are also desired. Here, based on flexible hollow-microstructure (HM) enhanced self-powered NG, we firstly present a non-contact heartbeat and respiration monitoring system with capability of working under high pressure of body weight without direct skin contact. High dynamic-pressure sensitivity of 18.98 V·kPa-1 and wide working range of 40 kPa are 3 ACS Paragon Plus Environment
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simultaneously achieved only using commercial polymeric films. Specific superiority of physiological detection under high pressure is also observed. Continuous reliable heartbeat and respiration information is successfully detected in a non-contact mode and transmitted to a mobile phone.
RESULTS AND DISCUSSION Figure 1a schematically depicts the proposed non-contact heartbeat and respiration monitoring system based on a flexible hollow microstructure self-powered pressure sensor (HM-SPS) strip (2 cm × 35 cm), which actually is a pressure-sensitive electret NG strip, featuring a packaged sheath-core structure (Figure 1b (i) and (ii)). As can be seen, the outer sheath is made of a folded Fluorinated Ethylene Propylene (FEP)/Ag sheet and the inner core is made of a folded EVA/Ag sheet with HMs. To get a better understanding of the structure, we sketched the expanded view of both components in Figure 1b (iii), where both folded components are flattened. The morphology of EVA film with HMs is shown in Figure 1c. Detailed fabrication process of HMs and the sheath-core HM-SPS are explained in the Experimental Section, Supplementary Note 1 and Supplementary Figure S1. The as-fabricated sheath-core HM-SPS strip is shown in Figure 1d. Before assembling the device, the FEP layer of FEP/Ag component was firstly negatively charged and the surface potential of charged FEP film (Figure 1e) finally plateaued at~1.2 kV after decaying for 7 days. Corresponding surface charge density is about 0.648 mC·m-2 (Figure S2). In fact, after assembling into device, the surface potential and charge density of FEP decreases sharply during previous contact-separation cycles of working process, and 4 ACS Paragon Plus Environment
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then stabilized at about -800 V and -0.47 mC·m-2 respectively (Supplementary Note 2 and Figure S3). Even though the charge losses a lot at the initial stage, the output performance of the charged device is still much higher than the device without charging (Figure S4) The working mechanism of the HM-SPS is mainly based on electrostatic effect, and resembles capacitor in some way.33-35 The perpetual negative electrostatic charges on FEP surface induce variable positive charges on opposite Ag layer as the FEP/Ag component approaches and departs the EVA/Ag component, resulting in charges flowing and current output in external circuit (Figure S5). As pressure increases, the HMs deform and compress; at the same time, the air gap between the two components decreases and the effective overlap area increases, inducing larger voltage output (Supplementary Note 3 and Figure S6). To characterize the electric output and dynamic pressure sensing performance of the HM-SPS, a machine-controlled vibrator and a force sensor were employed (Figure 2a, Supplementary Note 4). The electric output of HM-SPS (Figure S7, effective area, 4.5 cm × 4.5 cm) was measured under a pressure of 5 N and the vibrator oscillated periodically under a frequency of 5Hz and amplitude of ~1.5 mm. The open circuit voltage output of the pressure sensor is about 110 V (Figure 2b) and keeps stable even after 5000 cycles (Figure 2c) under the above condition. The open circuit voltage remains steady as the vibration frequency increases; while the short circuit current increases with frequency (Figure S8). When the HM-SPS is connected with load resistance, the load voltage increases with the rising of external load resistance and reaches a plateau of 120 V at 120 MΩ (Figure 2d). The peak and average power output are about 219.8 µW, 14.1 µW respectively (Figure S9).
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The dynamic pressure sensing performance of HM-SPS was tested and compared with commercial PVDF pressure sensor with the same size at the same condition (effective area, 1 cm × 2 cm; oscillation frequency, 2 Hz; amplitude, ~1.5 mm). As shown in Figure 2e, under a dynamic pressure of 1 kPa, a pulse output voltage of 11.87 V for HM-SPS is observed, which is almost 22 times of the voltage output of commercial PVDF pressure sensor (0.53 V). As we can see, for all ranges, the pressure response and sensitivity of HM-SPS is much larger than that of commercial PVDF pressure sensor (Figure 2f and Supplementary Figure S10). Thus, even for a gentle blow, the HM-SPS could exhibit a high voltage response (Figure S11). It is also worth noting that the pressure response curves of HM-SPS exhibit three distinct regions with different slopes (Figure 2g). In low-pressure region (10 kPa) the sensitivity further decreases to 0.25 V·kPa−1. The sensitivity differences are attributed to different deformability in three regions. In comparison, sensitivity of commercial PVDF pressure sensor is only 0.34 V·kPa−1 in low-pressure region and almost decreases to zero in the medium and high-pressure regions. Even compared with recently reported flexible piezoelectric pressure sensor (highest sensitivity is 0.79 V·kPa−1),36 the sensitivity of HM-SPS is also much higher. And the pressure range (0.5-40 kPa) is also much wider than most reported dynamic pressure range (within 0-20 kPa).5, 21, 36 Thus, HM-SPS owns peculiarity in terms of high dynamic pressure sensitively and wide pressure range, making it much specific under high pressure situation. Furthermore, the pressure response of HM-SPS still behaves stably after several pressure sweeps (Figure 2h). To validate the 6 ACS Paragon Plus Environment
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preponderance under high pressure, we compared the response of HM-SPS and commercial PVDF pressure sensor through changing a small pressure that exerted on both sensor, that is we put a 10 g weight onto or take the weight away from a 200 g weight that is exerted on both pressure sensors (～5% pressure on or off). As shown in Figure 2i, there is almost no response for PVDF pressure sensor, while the output could high up to 0.45 V for HM-SPS, which verified that HM-SPS possesses specific distinguished pressure-sensitivity under high pressure. As the deformability of sensing element influences pressure response a lot, thus it is of great importance to investigate the deformability of EVA film with different microstructures. Through comparing the deformation of EVA films with different micro-hemispheres (Figure 3a), we found that EVA film with open HMs possesses the largest deformability, while EVA films with solid micro-hemispheres and closed HMs almost have lower equivalent deformation; and EVA film without micro-hemispheres exhibits least deformation under the same pressure. As larger deformability of these hemispheres corresponds to bigger voltage output of NG, EVA film with open HMs is supposed to have the best performance and is thus chosen for our pressure sensor. Additionally, since the resilience of sensing element influences pressure sensing performance a lot, it is also necessary to test the resilience of the open HMs. Through Figure 3b and Supplementary Video 1 we can see that the open HMs could recover to its original state after repeated compress, suggesting HMs endowed with supreme resilience and long-term reliability. We also investigated the open circuit output of the self-powered pressure sensor (SPS) with and without HMs. As shown in Figure 3c, the output of SPS with HMs under high 7 ACS Paragon Plus Environment
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pressure (>10 kPa) is higher than the output of SPS without HMs, while the output of SPS with HMs is only slightly higher or equivalent to the output of SPS without HMs under low and medium pressure (
