All-in-One Wearable Self-Powered Respiratory Sensor Enabled By Contact Electrification
Authors: Su Yuanjie , Wang Si , Yao Mingliang , Xie Guangzhong , Tai Huiling , Du Xiaosong , Jiang Yadong 
Published: May 2020

Introduction

Human respiration is rich in physiological and pathological information as biomarker for health assessment and illness prediction. Respiratory diagnosis method carries various advantages, such as continuity, non-invasiveness, comfort and user-friendly [6]. Typically, ammonia (NH3) in exhaled breath can be served as biomarker for some diseases like end-stage renal disease (ESRD), ulcers caused by Helicobacter pylori or bacterial in oral cavity [12-15]. NH3 concentration in healthy people’ exhaled breath is about 0.425-1.8 ppm (mean 0.96 ppm), while the one in ESRD patients’ breathing ranges from 0.82 to14.7 ppm (mean 4.88 ppm) [12].
Even though various traditional sensors have been applied to monitor human physiological information like heartbeat or respiration, some limitations, such as bulky structure, poor portability and requirement of external power sources remarkably restricted their widespread application in smart mobile medical electronics [16]. Therefore, a self-powered ammonia sensor that can harvest energy from human body is desperately needed. In addition, it is underexplored but will be a novel and superior solution if a wearable sensor that could simultaneously chemically detect the inhalation and measure the breath rate for respiration analysis without a power supply.

Device fabrication

The as-developed triboelectric self-powered respiration sensor (TSRS) is composed of Ce-doped ZnO, Polydimethylsiloxane (PDMS), Au foils and PET films, as revealed in Fig. 1a. Ce-doped ZnO plays dual roles of gas sensing material and triboelectrification layer (Fig. 1b). PDMS functions as the triboelectric layer and Au foils were coated on the back of both triboelectric layers as electrodes. To improve the flexibility and air tightness, the whole device was packed with soft silica gel films. The patterned PDMS film exhibits good transparency and flexibility, as shown in Fig. 1c and 1d. Figure 1e sketches the fabrication flow of the TSRS. Driven by the continuous expansion and contraction of the chest during respiration (Fig. 1g), the deformation of TSRS inflated can be converted into electric signals and human exhaled gas can flow into the TSRS through a soft tube with the breathing process, enabling the real-time spontaneous monitoring of both respiratory behaviors and exhaled gas.

Method

To explore the sensing behavior of as-prepared TSRS toward NH3 from 0.1 to 10 ppm, including the trace level region (region Ⅰ, 0.1 to 1 ppm) and micro level region (region Ⅱ, 2 to 10 ppm), the NH3-sensing response-concentration fitting curves are plotted in Fig. 2a for five samples. It is found that without moisture atmosphere, the TSRS exhibits an opposite sensing behavior and the response declines with increasing ammonia concentration in both region Ⅰ (Fig. 2e) and region Ⅱ (Fig. 2f). A steeper slope is observed under humid NH3 atmosphere in comparison with the dry NH3 atmosphere, indicating that the existence of water molecular will raise the sensing performance. As a consequence, the moisture-resistant feature of the as-developed TSRS favours NH3 detection in human exhaled gas. Figure 2g elucidates dynamic response profile of TSRS when exposed to 0.01 ppm NH3 with 97.5%RH, which indicates that the TSRS is sensitive to NH3 as low as 0.01 ppm and the response time is nearly 155 s.

Results and Conclusions

Attached on the human chest, the as-prepared TSRS delivers a stable output voltage signal within 5 min normal breathing (Fig. 3a). The wearable sensor can be applied to distinguish different types of breathing behaviors and human respiratory states after physical exercise. Figure 3b presents the real-time output recording of diverse breathing behaviors, including normal breathing, deep breathing, shallow breathing and rapid breathing. Human respiration rate and depth can be clearly recognized in terms of frequency and amplitude of the output voltage signals. The deep breathing pattern has a bigger peak-to-peak amplitude and wider interval than normal breathing pattern due to the larger and slower change in chest circumference. In addition, the physiological process of human body after exercise can also be accurately reflected by breathing intensity and frequence. The breathing amplitude after 100 squats is more intense than that after 50 squats, as displayed in Fig. 3c. Meanwhile, the injection of exhaled NH3 gas into the TSRS apparently enlarges the amplitude of the output voltage (Fig. 3d), revealing the capability of simultaneously detecting breathing behaviors and exhaled gases.

References

[1] M. Righettoni, A. Amann, S.E. Pratsinis, Breath analysis by nanostructured metal oxides as chemo-resistive gas sensors, Materials Today. 18 (2015) 163–171. doi:10.1016/j.mattod.2014.08.017
[2] P. Le Maout, J.L. Wojkiewicz, N. Redon, C. Lahuec, F. Seguin, L. Dupont, , Polyaniline nanocomposites based sensor array for breath ammonia analysis. Portable e-nose approach to non-invasive diagnosis of chronic kidney disease, Sens. Actuators, B Chem. 274 (2018) 616 –626. doi:10.1016/j.snb.2018.07.178
[3] S. Wang, H. Tai, B. Liu, Z. Duan, Z. Yuan, H. Pan, Y. Su, G. Xie, X. Du, Y. Jiang, A Facile Respiration-Driven Triboelectric Nanogenerator for Multifunctional Respiratory Monitoring, Nano Energy. 58 (2019) 312–321. doi:10.1016/j.nanoen.2019.01.042
[4] Y. Jung, H.G. Moon, C. Lim, K. Choi, H.S. Song, S. Bae, S.M. Kim, M. Seo, T. Lee, S. Lee, H.H. Park, S.C. Jun, C.Y. Kang, C. Kim, Humidity-Tolerant Single-Stranded DNA-Functionalized Graphene Probe for Medical Applications of Exhaled Breath Analysis, Advanced Functional Materials. 27 (2017) 1–9. doi:10.1002/adfm.201700068

Figure 1