Development of highly sensitive NiO thin film humiditysensors for medical applications

Sensors in the medical field have contributed greatly to patient care in clinics in terms of monitoring human vital signs such as heartbeat, pressure, body temperature, and breathing. Respiratory sensors have received great attention, especially after Covid-19 pandemic attacked the world, due to their bad and strong impact on the patient's natural breathing process. Breathing sensors can be classified into two main categories. First, sensors detect certain concentrations of certain gases in our exhaled breath, which may indicate a health problem (such as increased ammonia gas concentration, which indicates a health problem related to kidney function). The other type of sensors, which is the focus of this study, are "breathing sensors" that monitor breathing dynamics, or so-called breathing patterns. Respiratory sensors help in early diagnosis of the disease, which greatly helps in developing an appropriate treatment plan that leads to a speedy recovery of the patient. However, the breathing sensors involved in this study suffer from several problems, including a relatively long response or recovery time, which is not suitable for real-time monitoring of the patient, especially in critical cases that require timely intervention. Another problem with this type of sensor is the high cost of production, such as sensors produced using graphene technology. In this study, an ultrafast and highly sensitive low powered humidity sensor was produced and developed based on a thin film of nickel oxide deposited on silver interdigitated electrodes. The fabricated NiO sensor can detect even the slightest changes in humidity levels in the range of 35-95% RH, within 0.28 and 0.40 s response and recovery time, respectively. The sensor can be operated at very low voltage (less than 0.1 V) and room temperature. The fabricated NiO sensor was produced by an easy and cost-effective spray method (airbrush) that could be manufactured under atmospheric conditions with a relatively low deposition temperature (270°C). Simulation tests and real tests on healthy volunteers showed that the fabricated NiO humidity sensor is capable of monitoring mouth and nose breathing at different speed rates. The sensor also has a high ability to distinguish between wet and dry human skin within 2 cm and without contact. The sensor is undoubtedly expected to monitor a variety of fast / slow, shallow / deep breathing patterns including Eupnea, Apnea, Bradypnea, Tachypnea etc. Keywords: NiO Thin film, Medical sensors, Monitoring breathing, Relative humidity