Passive wireless respiratory sensor

Bin Feng,1 Kai Zhao,2 Qiucheng Su,1 Zhentao Yu,1 and Hao Jin1,3,✉ 1Key Laboratory of Advanced Micro/Nano Electronic Devices & Smart Systems of Zhejiang, College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou 310027, China 2Shaoxing Customs of the People’s Republic of China, Shaoxing 312099, China 3ZJU-Hangzhou Global Scientific and Technological Innovation Center, Hangzhou 311200, China ✉Email: hjin@zju.edu.cn

✉ Email: hjin@zju.edu.cn Monitoring respiratory characteristics is an important method to monitor sleep respiratory diseases such as obstructive sleep apnea syndrome (OSAS). In this letter, a passive wireless respiratory sensor is developed, which is based on a surface acoustic wave (SAW) microsensor using graphene oxide as a sensitive layer. The developed sensor is placed on the upper lip below the nose, which monitors human's breathing by detecting the humidity changes while inhale and exhale. To meet the fast response requirement of human respiratory, the effect of the thicknesses of graphene oxide (GO) films is investigated. Results show that the response and recovery time of the sensor can achieve 0.4 and 1.4 s, respectively, with the optimized 70-nm thickness of GO. The respiratory rate that the sensing system can measure is greater than 33 breaths per minute, outcompeting the average number of breaths per minute from an adult (i.e. 16-20 breaths per minute). The new sensor proposed here paves the way for the fabrication of high-performance and low-cost respiratory sensors in human breathing monitoring in real life.
Introduction: Sensors in health care field have received much attention in recent years. In the prevention and treatment of sleep respiratory diseases such as obstructive sleep apnea syndrome (OSAS), a respiratory monitoring system is highly demanded [1]. However, the existing respiratory monitoring system used in practice is often complex and inconvenient. Our group previously developed a respiratory sensor based on surface acoustic wave (SAW) humidity microsensor, which monitors breathing by detecting the humidity changes caused by respiration [2]. Compared with other respiratory sensing methods, such as respiratory inductive plethysmograph technology [3], impedance pneumography technology [4], induced eddy currents of inductor-capacitor tank technology [5], and estimation by electrocardiogram signals [6], the respiratory sensor based on SAW device is simpler, more convenient and more intuitive. However, LiNbO 3 and ZnO, which are widely used piezoelectric substrates for SAW device, are hydrophilic materials; therefore water molecules can permeate into the materials and affect the life of devices.
Here, by using a commercial quartz-based SAW, we developed a redesigned SAW-based respiratory sensor with graphene oxide (GO) as a sensitive layer. GO is an ideal humidity-sensitive material, owing to the characteristics of large surface-to-volume ratio and high hydrophilicity [7]. We have proposed a QCM humidity sensor with GO sensitive film prepared by simple dropping coating, which had good sensitivity, repeatability and response/recovery speed, and the response and recovery time was about 20 and 3 s, respectively [8]. However, the response and recovery time of the QCM sensors cannot meet the response speed requirements of respiratory monitoring, which should be less than 3 s (i.e. the normal respiratory rate of an adult is 16-20 breaths per minute) [9]. To reduce the response and recovery time of the humidity sensor to meet the criteria of respiratory sensing, we investigated the effect of thickness of GO sensitive films. Results show the response and recovery time of this sensor is 0.4 and 1.4 s, respectively, which is suitable for OSAS monitoring.
Fundamentals and experiments: When the human breathes, the humidity of the exhaled air is always higher than that of the surrounding environment, which periodically changes air humidity between the upper lip and the nose. The developed humidity SAW sensor was placed on the upper lip beneath the nose where a variation of the humidity in the air can be detected by measuring the shift of resonant frequency of the SAW sensor. By monitoring the breathing patterns through humidity changes, we can detect the OSAS of the person in real-time. Since SAW devices are a type of passive wireless sensor, the proposed SAW respiratory sensor could be a single SAW microsensor with an integrated microstrip antenna attached to the wearer, whereas the electronics of the other transmitters/receivers are detached from the wearer. This greatly increases the mobility of the patients as the wireless communication ranges could be a few meters to tens of meters for SAW sensors.
The Quartz-based SAW with a frequency of 433.92 MHz is provided by Shenzhen Xingdejing Electronic Co., Ltd (Guangdong, China). The frequency temperature coefficient (TCF) is about 0.032 ppm/ • C. The GO is from Suzhou Carboniferous Graphene Technology Co., Ltd (Jiangsu, China). The concentration of GO dispersions is 0.2 mg/ml for maintaining sensitive humidity sensing ability. The GO solution was evenly distributed on SAW by spin-coating and dried on a hotplate at ∼80°C. GO sensitive films of different thickness were obtained by controlling the amount of GO dispersion, rotation speed and time. Four kinds of GO sensitive films with different thickness were prepared, which are 70, 120, 200, and 280 nm, respectively. SAW sensors were glued onto small flexible PCBs, and then placed on the upper lip of the volunteer. The electronic reader (commercially available) sends an interrogation signal with an operation frequency of f 0 to the SAW sensor, and returns a response signal with a frequency of f 1 containing respiration information. The principle of a wireless SAW sensor is well known [10], and herein will not be discussed. The  Figure 1. The temperature and relative humidity of the surrounding environment were checked to be ∼22°C and ∼60% RH, respectively.
Results and discussion: Figure 2 shows the resonance frequency shift of the SAW sensors with different thicknesses of GO sensitive films when the volunteer breathed deeply.
The SAW sensor with the thinnest GO film (70 nm) has a minimum response and recovery time of 0.4 and 1.4 s, respectively. When the thickness of GO film is greater than 120 nm, the total period time of the response and recovery is more than 3 s. As a result, the SAW sensor could not recover to the initial frequency, and the frequency shift will fluctuate slightly near the maximum value. It is believed that the time of water molecules leaving GO film can be shortened by reducing the thickness of the GO film, thus the recovery time of the sensor can be reduced.
During breathing, the expiratory air will change the temperature, humidity and pressure on the surface of the SAW sensor. Frequency shift of SAW sensor caused by a variation of pressure can be alleviated by placing the sensor in parallel with the direction of breathing. The effect of temperature on frequency shift was minimized by choosing SAW sensors with low TCF. However, the analysis of the effect of temperature and pressure on SAW humidity sensor is investigated in this work. SAW sensors without or with a 70 nm GO film were investigated. We asked the volunteer to breathe discontinuously to imitate the conditions of someone suffering from OSAS and the respiration effect on the resonant frequency of SAW sensors is shown in Figure 3. The resonate frequency of the SAW sensor without the GO film modified changes is very small, while the smallest frequency shift of the SAW sensor with a 70 nm GO film is ∼350 kHz. Thus, the influence of temperature and air pressure on resonate frequency of SAW respiratory sensor is ignorable.
Conclusion: A passive wireless respiratory sensor with an optimal GO sensitive film and a complete system is proposed for monitoring human's respiratory characteristics in real-time by detecting the humidity change during breathing. With the optimized 70-nm thickness of GO film, the response and recovery time of the sensor are reduced to 0.4 and 1.4 s, respectively, which meets the requirement of response speed for continuous respiratory monitoring. The system is simple in its structure, user-friendly and promising for wearable and home-use healthcare applications. The cost of our proposed SAW-based passive wireless OSAS monitoring system is less than $71, which is cost-efficient and highly accessible.