All-range flexible and biocompatible humidity sensor based on poly lactic glycolic acid (PLGA) and its application in human breathing for wearable health monitoring

This paper reports a fast, linear, all-range sensitive and biocompatible humidity sensor using poly lactic glycolic acid (PLGA) as a sensing layer for the first time. The device is fabricated using all printing methods. The high definition interdigitated electrode structure was printed on flexible PET substrate by reverse offset printing setup using silver ink, while PLGA ink was deposited using spin coating. IDEs with two different dimensions were printed with three samples of each. The output of sensor is impedance which decreases with increase in relative humidity in a controlled chamber. The results show excellent stability, obvious response to RH for full range from 0 to 100 %RH with fast response time of 3 s and recovery time of 6 s. Sensor data can be received continuously using smart phone with a Wi-Fi connection with ESP8266 MCUNODE. Sensor’s flexibility test was also performed for 5 mm mechanical bending radius which showed reasonable consistency with results of flat sensor. The biocompatibility of sensor’s active layer was analyzed and confirmed by culturing human small air way epithelial cells for 7 days. To utilize sensor’s feature of being biocompatible, it was then used for human respiration monitoring which enabled the proposed device to be strong candidate for wearable electronics and health monitoring applications.