A Portable Triboelectric Nanogenerator for Real-Time Respiration Monitoring
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As a reliable indicator of human physiological health, respiratory rate has been utilized in more and more cases for prediction and diagnosis of potential respiratory diseases and the respiratory dysfunction caused by cystic fibrosis. However, compared with smart mobile electronics, traditional clinical respiration monitoring systems is not convenient to work as a household wearable device for real-time respiration monitoring in daily life due to its cumbersome structure, complex operability, and reliance on external power sources. Thus, we propose a wearable wireless respiration sensor based on lateral sliding mode triboelectric nanogenerator (TENG) to monitor respiratory rates by sensing the variation of the abdominal circumference. In this paper, we validate the possibility of the device as a respiration monitoring sensor via an established theoretical model and investigate the output performance of the sensor via a series of mechanical tests. Furtherly, the applications of the respiration sensor in different individuals, different breathing rhythms, different active states, and wireless transmission have been verified by a lot of volunteer tests. All the results demonstrate the potential of the proposed wearable sensor as a new alternative for detecting and monitoring real-time respiratory rates with general applicability and sensitivity.
KeywordsWaist-wearable respiration sensor Triboelectric nanogenerator Wireless transmission
Analog digital converter
Fast Fourier transform
Obstructive sleep apnea syndrome
Accompanied with the global climate deterioration, increasing serious air pollution and the aggravation tendency of aged population, the human health, especially the health of the respiratory system, is exposed to more and more threats [1, 2, 3]. Meanwhile, the monitoring for human’s physical health becomes the focus of attention for preventing latent diseases [4, 5, 6, 7]. Respiratory rate, as one of the most important and reliable indicators directly reflecting human physiological health, may provide key information for the prediction and diagnosis of potential respiratory diseases like obstructive sleep apnea syndrome (OSAS) and the respiratory dysfunction caused by cystic fibrosis [8, 9, 10, 11]. There has been various traditional medical equipment utilized for monitoring respiration status, and extraordinary efforts have also been committed to develop technologies for innovative respiration monitoring. Despite the great clinical applicability and monitoring accuracy, the cumbersome structure, complex operability, reliance on external power sources, and bad portability restrict their further development as smart mobile medical electronics. In recent years, the advances in mobile network and low-power electronics have driven the intelligent mobile medical devices at a tremendous pace and have evoked increasing interest in household healthcare and flexible wearable electronics [6, 12, 13, 14, 15, 16, 17, 18]. Therefore, the battery-free wearable healthcare sensors with great potential for respiration monitoring, in a smart way, are ubiquitously demanded.
Compared to some relatively mature bioenergy scavenging technologies like electromagnetic [19, 20] and piezoelectric [21, 22, 23, 24, 25], triboelectric nanogenerators (TENGs) [26, 27, 28, 29, 30], with the merits of light weight, high density of energy, and high sensing sensitivity, possess better potential in applications as bioenergy harvesters, wearable electronics, and self-powered health monitoring devices. Furtherly, the TENG-based energy harvesters are more capable in scavenging bioenergy in working environment with the bandwidth of frequency below 10 Hz like human breath [31, 32], and the materials used for TENGs are lead free which are safe to be used for healthcare sensors. Therefore, TENG is of no doubt one of the best choices for wearable and self-powered breathing monitoring devices. To meet the increasing demands for wearable and self-powered health monitoring technology, many novel TENG-based sensors have been developed to monitor human physiological status. Lin et al. proposed a self-powered wireless body sensor network (BSN) system for heart rate monitoring via integration of a downy structure-based TENG (D-TENG), a power management circuit, a TENG-based heart rate sensor, a signal processing unit, and Bluetooth module for wireless data transmission in 2018 . P. Maharjan et al. designed a novel curve-shaped wearable hybridized electromagnetic-TENG (WHEM-TENG) in 2018, working as an electronic wrist watch powered by biomechanical energy harvested from a swing arm, which was also demonstrated to power for a pulse signal and heart rate monitoring . Chen et al. reported a flexible hybrid nanogenerator of piezoelectric and triboelectric properties in 2017 that can be conformally attached on soft surfaces like human skin to harvest diversity touch energies based on electrospun nanofiber mat and monitor the real-time physiological signals such as respiratory information and radial artery pulse . Cu et al. reported a pulse sensor based on a single-electrode TENG with high flexibility and comfortability to human skin in 2018, with which a typical human pulse waveform that represents the radial artery pressure wave can be successfully obtained . The abovementioned works have greatly propelled the development of TENG-based wearable and self-powered intelligent devices in human physical monitoring.
The variation of abdominal circumference is a natural physical behavior of human during breathing process so that capturing information from abdominal deformations is a sensing approach and has no negative effect on normal activities of human beings, which may also be a possible energy source by scavenging biokinetic energy. In this paper, we propose an integrated waist-wearable wireless respiration sensor based on sliding mode TENG, with the merits of portability, mobility, and intelligence, simultaneously. It may be applied in different daily activities for continuous real-time respiration monitoring and OSAS detecting with neither adverse effect on the normal function of the device nor adverse influence on daily activities of the user. A smart belt is built with the TENG sensor to sense the variation of the user’s abdominal circumference during breathing and transfer the periodic variation to the reciprocating oscillation of the tribo-pair of the TENG, so that the electric signals containing respiration information can be output by the TENG. The whole sensing process needs no external power source. The device is also equipped with a wireless transmission chip powered by an external source to realize respiration signal transmission. The information for the breath status will be finally displayed on a mobile phone. Here, we report the research work on the TENG-based respiration sensor to show its excellent potential as a possible intelligent wearable and self-powered device for real-time respiration monitoring.
Architecture of the Respiration Sensor
The structure of the device is designed with a series of obvious merits. First of all, the deformable parts of the belt are utilized here to accommodate the expansion of the abdomen during respiration and offer the restoring force in the contracting procedure of the abdomen during inhalation process, so that the real-time detection with continuous signal will be realized via the smart belt with no uncomfortable feelings and negative influence on normal activities of the user. Secondly, the inextensible parts of the belt are used to restrict the deformation of the belt to make sure part of the abdominal circumference variation is used to drive the sliding behavior of the tribo-pair. Also, the simple structure and the commercial materials adopted in the device make it low cost and easy to fabricate, which may facilitate its marketable promotion prospect.
Furthermore, a set of hardware and software modules are applied to form a wireless transmission system for signal transmission, and the information of the real-time respiration is assumed to be displayed on a mobile phone (Fig. 1a (v)). As shown in Fig. 1b, the hardware module, consisting of a voltage follower, a voltage rising circuit, and a wireless transmission chip, are integrated into a circuit board. It is noticed that the TENG outputs high voltage but relatively low current, resulting in a high output impedance and affecting its applicability in the wireless transmission system. In this regard, the voltage follower is integrated in the circuit to lower the output impedance of the TENG so that it can roughly match that of the wireless transmission unit. Also, as a concern for practical applicability, the electric output of the TENG is characterized as alternating current, of which the negative signal values cannot be used as the input signal for the Analog Digital Converter (ADC). Therefore, the electrical level-rising circuit is used to elevate the whole signal curve of the output voltage of the TENG to positive level for the ADC to acquire the whole signals. The wireless transmission chip consists of an ADC, a microprocessor, an antenna, and a battery to provide power for the unit. The software module includes signal sampling, signal processing, signal storing, and signal displaying units. Through the signal sampling and processing units, the signals transmitted to the mobile phone are converted back to the oscillation with positive and negative components, but the signal waveforms and amplitudes are not converted back proportionally to the original values of the TENG output; thus, it is only indicative of respiration rates. And through the signal displaying and signal storing units, the transmitted signals of the real-time respiration rates are systematically stored and displayed on a mobile phone.
Sensing Principle and Working Mechanism
Human breathing is usually categorized into thoracic and abdominal breathing, and most of us use the first type in our daily life. During the thoracic breathing process, the abdomen cavity periodically expands and contracts as the exhalation and the inhalation processes occur, respectively, which may induce stretching and contraction of the wearable belt attached around the waist. Meanwhile, the tribo-pair is forced to slide outward and inward via the deformation of the abdomen circumference. During the reciprocating sliding process, the respiration status will be obtained via the smart belt with the TENG device.
The electrical output performances of the respiration sensor were recorded by a Keysight B2983A system electrometer.
Results and Discussion
For clinical applications, respiratory rates may provide vital information for early warning and prompt diagnosis of the respiratory diseases like OSAS. The waist-wearable wireless respiration sensor is proposed in this paper to offer an alternative strategy for monitoring real-time respiration by sensing the variation of the abdominal circumference in the breathing process and displaying the wireless signal on a mobile phone. The configuration of the device contains a wearable bilayer belt, a sliding mode TENG sensor built in the belt and a wireless transmission system. And the applicability, portability, and accuracy of the device have been validated through theoretical analyses, mechanical tests and real-time tests by volunteers.
Device parameters used in the experiment and simulation
Thickness of nylon plate d1 (μm)
Relative permittivity of nylon εr1
Thickness of PTFE plate d2 (μm)
Relative permittivity of PTFE εr2
Permittivity of vacuum ε0 (pF/m)
Load resistance R (MΩ)
Area of the dielectrics S (cm2)
Surface charge density σ (μCm−2)
Maximum separate distance A (mm)
Cycle length of the excitation pattern T (s)
Speed of sliding outwards / inwards v1 / v2 (m/s)
Ratio of the exhalation time to the whole breathing cycle η
Furtherly, two volunteers, one aged 22 years old with a waistline of 72.8 cm and another aged 24 years old with a waistline of 98.6 cm, were invited to test the ability of the smart belt in reflecting specific breath behaviors of different individuals. To test the sensitivity of the device to different respiratory rates, the breathing processes offered by the volunteers involve three different breathing rhythms, i.e., normal, rapid, and deep breaths. During the breathing process with different rhythms, the electrical signals generated by the TENG sensor are successfully detected and shown in Fig 5c and d for the two volunteers, respectively. The voltage signals are repeatable and reliable for each rhythm, that presents obvious difference of respiratory rates in the breathing process. The time histories of the output voltage (Fig. 5c and d) for the two volunteers respectively exhibit steady variation (constant frequency and peak-valley value) in the processes of three breathing rhythms. Reflected by the results of fast Fourier transform (FFT) in Fig. 5c and d, the extracted frequency of the normal, rapid, and deep breaths are 0.68, 1.10, and 0.40 Hz, respectively for the 22-year-old volunteer and 0.60, 1.40, and 0.47 Hz for the 24-year-old one; those are reasonable respiratory rates for healthy adults . It means that the key information of the respiratory rates can be collected via the electrical signals. On the other hand, the two volunteers in the tests are asked to hold breath to simulate the breathing pause caused by the symptom of apnea. Correspondingly, it is presented in Fig. 5c and d that the signals with value of zero volt last for about 10 s between two different breathing rhythms. It may be utilized as a judgment basis for OSAS and a further accordance for its diagnosis and warning. These results demonstrate that this TENG sensor can detect not only the respiratory rates but also the symptoms of the apnea.
In summary, we have designed and fabricated a waist-wearable wireless respiration sensor to monitor real-time respiratory status of humans in daily life and to transmit the breathing information to a mobile cell via a wireless transmission system. We furtherly illustrated its working mechanism in detail that it senses the variation of the abdominal circumference while breathing and output electrical signals containing rhythm information of the respiratory processes. In this study, theoretical analyses were performed to predict the output signals of the TENG and validate the possibility of the TENG to work as a respiration sensor. It was also demonstrated by a mechanical test that the sensor can be easily driven by a sliding displacement with an amplitude of 2.5 mm, which makes it feasible for use as a wearable sensor. To validate the applicability in reality, we carried out a series of tests by two volunteers to investigate the feasibility, accuracy, and sensitivity of the device to different individuals, different breathing rhythms, and different active states. The device was demonstrated applicable for not only the detection of apnea symptom but also the real-time monitoring of breath. Lastly, the wireless transmission system of the sensor was also proved to be efficient in wireless electrical signal transmission. Results stated above have shown the potential of the proposed sensor as a smart wearable respiration sensor and the household healthcare monitoring system comprehensively.
HZ, JKL and ZCZ developed the research ideas. JWZ and HZ designed the sensor device and the experiments. JWZ, ZWH and WPX conducted the experiments. JWZ and HGW analyzed the data. HZ, JKL and JWZ finished the manuscript. All authors discussed the results. All authors read and approved the final manuscript.
This study is funded by Key Research Project of Southern Xinjiang (grant no. 2019 DB013), National Natural Science Foundation of China (nos. 51978609 and 11472244), and Fundamental Research Funds for the Central Universities (no. 2019QNA4040).
The authors declare that they have no competing interests.
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