Design and test of a nanosilver flexible sensing chip for agro-enviroment temperature monitoring

Abstract: Flexible sensing technology can greatly expand the physical energy conversion carrier morphology and application scenarios of "machine object" perception and information interaction in an ecological unmanned farm. In this study, a flexible temperature sensor chip was fabricated using 3D printing (additive manufacturing). A sandwich structure of sensor was adopted with four layers, including the substrate, temperature-sensitive layer with nanosilver ink, an electrode layer, and a PDMS protective layer. Among them, the temperature-sensitive layer was fabricated as a "turn-back track" shape to increase the temperature-sensitive area of nanosilver ink. A systematic analysis was made to explore the effects of the PDMS protective layer and chip structure parameters on the sensor performances, including the sensitivity, accuracy, and stability. The feasibility of the sensor for agricultural temperature measurement was verified using the dynamic thermo-monitoring on the bodies of agro-living objects, plant rhizosphere, soil, and agro-equipment. The results showed that the PDMS protective layer realized the waterproof protection in the nanosilver temperature-sensitive layer, thereby improving the environmental adaptability and service life of sensors. Optimal line width and spacing in 3D printing were achieved in the range of 450/300, 350/250, and 250/200 μm, particularly for the temperature-sensitive layer of self-developed flexible sensor chips. The experimental results show that when "line width/line spacing" is 250/200 μm, the sensitivity of the flexible temperature sensor chip can reach 0.317 ℃-1, which is the highest sensitivity. In addition, the resistance change rate of temperature-sensitive wire per unit substrate area increased, with the decreasing of line width and spacing. An optimized fabrication structure was chosen as the line width of 250 μm and the line space of 200 μm. Correspondingly, the optimal performance was achieved, where the sensitivity of the temperature sensor was 0.330 ℃-1, while the measurement error was less than 0.5 ℃, and the stability was 0.02 ℃/min. The sensor was bent along the rounded edge of circles with diameters of 4 and 6 cm, respectively. The resistance variation was measured at different temperatures. The data showed that the bending angle could not affect the performance of the temperature sensor. The flexible temperature sensor was pasted on the human forehead, arm, and armpit to measure the body temperature before and after exercise. The measurement demonstrated that the flexible temperature sensor accurately presented the changes in body temperature, where the maximum error was less than 0.5 ℃. A 7-day continuous temperature monitoring test was performed on the plant body and the nutrient solution in hydroponic lettuce cultivation. One flexible sensor was stuck onto the stem and leaf of lettuce. Another sensor was installed under the nutrient solution, close to the lettuce root. The temperature sensor accurately reflected the change of the daily average temperature of lettuce, where the maximum error was less than 0.6 ℃. The fluctuation trend of temperature in different parts of the plant was consistent with the room temperature during the testing duration. Additionally, the sensor tracked the process of soil frozen and water boiling, where the maximum error was less than 0.4 ℃. A self-made flexible temperature sensor chip was designed and subsequently tested in the typical agricultural temperature measurement. An excellent agreement was achieved in the flexible sensor with the high-precision platinum resistance sensor, where the measurement error was less than 0.6 ℃, indicating better performance than that of the non-contacted temperature measurements conducted by the infrared sensor. Flexible nanosilver temperature sensor chip can quickly and accurately capture the temperature change of measured target, indicating a promising agricultural application prospect.