Flexible humidity sensor for smart agricultural applications

In order to meet the increasing food demand, sensors that measure the ambient temperature and humidity in greenhouses are needed for more efficient vegetable and fruit production. For this purpose, two different flexible and resistant textile-based humidity sensors that can measure the humidity at higher levels (80%, 90%, and 100% relative humidity (RH)) were designed and printed directly on the four different greenhouse fabrics using silver and carbon conductive inks. Depending on the humidity value in the environment, the sensor performance was tested based on sensor electrical resistance measurements with respect to repeated bending/cyclic tests, rubbing, ultraviolet (UV) exposure/ weatherability (against UV and raining) tests in order to simulate greenhouse conditions for smart agriculture. Despite applying 1024 bending cycles, up to 20 rubbing cycles, and 10 times UV and rain exposure to the humidity sensors at high RH, no significant change was detected in the resistance values of the humidity sensors. Moreover, some important features of the sensors such as hysteresis, repeatability, response time have been also examined. According to hysteresis test results, humidity sensors show acceptable dynamic response and response time of the sensors are 15.8, 17.3, 24.8, and 25 s at 100% RH for G2S, W1S, B2C, and W1C, respectively. Statistical analyses showed that the sensor designs and type of conductive inks had significant effects on the performance of the humidity sensors and the best sensor performance was obtained with the polypropylene coated fabric using design II and silver based conductive ink. The fabricated textile based flexible humidity sensors detect the change in RH levels from 80% RH to 100% RH and achieve good durability, and repeatability even after prolonged UV exposure and raining. Thus, the developed textile-based flexible humidity sensor might be useful for future smart agricultural applications.