An ultrafast-response and flexible humidity sensor for human respiration monitoring and noncontact safety warning

Xiaoyi Wang,Yik Kin Cheung,Peng Jiang,Yang Deng,Hongyu Yu,Xingru Chen

doi:10.1038/s41378-021-00324-4

Xiaoyi Wang, Yik Kin Cheung + Show 4 more

Open Access

https://doi.org/10.1038/s41378-021-00324-4

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Abstract

The humidity sensor is an essential sensing node in medical diagnosis and industrial processing control. To date, most of the reported relative humidity sensors have a long response time of several seconds or even hundreds of seconds, which would limit their real application for certain critical areas with fast-varying signals. In this paper, we propose a flexible and low-cost humidity sensor using vertically aligned carbon nanotubes (VACNTs) as electrodes, a PDMS-Parylene C double layer as the flexible substrate, and graphene oxide as the sensing material. The humidity sensor has an ultrafast response of ~20 ms, which is more than two orders faster than most of the previously reported flexible humidity sensors. Moreover, the sensor has a high sensitivity (16.7 pF/% RH), low hysteresis (<0.44%), high repeatability (2.7%), good long-term stability, and outstanding flexibility. Benefiting from these advantages, especially the fast response, the device has been demonstrated in precise human respiration monitoring (fast breathing, normal breathing, deep breathing, asthma, choking, and apnea), noncontact electrical safety warning for bare hand and wet gloves, and noncontact pipe leakage detection. In addition, the facile fabrication of the flexible platform with the PDMS-Parylene C double layer can be easily integrated with multisensing functions such as pH sensing, ammonium ion sensing, and temperature sensing, all of which are useful for more pattern recognition of human activity.

Highlights

Humidity sensing is essential in broad applications, including process control in industry, medical facilities, environmental monitoring of greenhouses, cleanrooms, and heating, ventilation, air conditioning (HVAC) systems[1,2,3,4,5,6,7,8]
Preparation of the flexible substrate Polydimethylsiloxane (PDMS) and Parylene C were used to fabricate the double-layer flexible substrate, which was used to peel off the vertically aligned carbon nanotubes (VACNTs) electrodes from the silicon wafer
Performance characterization Before investigating the various potential applications of the flexible relative humidity sensor, the device was tested with a homemade testing setup (Fig. 3a) to characterize the sensing performance

Summary

Introduction

Humidity sensing is essential in broad applications, including process control in industry, medical facilities, environmental monitoring of greenhouses, cleanrooms, and heating, ventilation, air conditioning (HVAC) systems[1,2,3,4,5,6,7,8]. Since the humidity level is nearly the same for different breathing conditions (normal, fast, and deep breathing conditions), sensors with larger response times can sense changes in respiration, but the sensor signal variation amplitudes between fast breathing, normal breathing, and deep breathing would be quite different[29,49]. For the first time, we present a flexible relative humidity sensor integrating a PDMS-Parylene C doublelayer thin film as the flexible substrate, vertically aligned carbon nanotubes as the electrodes, and graphene oxide as the sensing material. A high-performance relative humidity sensor is demonstrated in real applications for human respiration monitoring, pipe leakage detection, and noncontact electric safety warning, showing its potential as a flexible or curved humidity-sensing node in wearable electronics or industry monitoring

Materials and methods

Results and discussion

Pm O1s

Conclusions