Multimodal Flexible Sensors for Minimally-Invasive and Real-Time Plant Monitoring

In recent years, global population growth has accelerated the food crisis, especially in developing countries. Smart agriculture, which integrates agriculture and Internet of Things (IoT), is attracting attention as one way to overcome this problem. Efficient management of crops at all times and remotely leads to improve crop productivity and quality. Many soil sensors and plant wearable sensors have been developed to promote smart agriculture. In particular, growth rate monitoring of plant is focused by using sensors, which is an important indicator of plant healthcare. However, to realize the smart agriculture, most likely more plant information needs to be monitored simultaneously, which have yet to be reported to date. If the growth rate and soil condition can be measured simultaneously, it should be possible to analyze the correlation between them for more efficient and healthier crop cultivation. To conduct this challenge, this study aims to develop a multimodal integrated flexible sensor sheet consisted of a strain sensor to measure plant stem growth rate, an impedance sensor to measure soil moisture, and a pH sensor to measure soil pH.<br/>The flexible strain sensor was fabricated by mixing carbon black (CB), polydimethylsiloxane (PDMS), and Ecoflex. Kirigami structure was applied to the sensor film to dramatically reduce the load on the plant due to sensor expansion and contraction, as well as to allow minimal-invasive measurements that do not interfere with respiration and photosynthesis at the stem surface. While Young's modulus without kirigami structure was ~5 MPa, the one for a sensor with kirigami structure was ~41 kPa, which 120 times softer sensor was realized. The maximum strain measurement range of the sensor was 259 % with a gauge factor (GF) 1.15 in the strain up to 40 %, and GF = 1.75 in the strain range between 40 %-55 %. In the strain more than 55 %, GF increased exponentially. Since PDMS and Ecoflex have a high coefficient of thermal expansion, their temperature characteristics were examined to determine the effect of changes in environmental temperature. The resistance change ratio of the strain sensor was 5.8% when the ambient temperature was varied from 15°C to 40°C, suggesting that a temperature sensor is required to integrate with this sensor sheet to measure growth rate of plant accurately in the future.<br/>Impedance sensor was fabricated by printing Ag and CB electrodes on PET film. Soils with different mass moisture contents were prepared, and the impedance was measured by inserting the sensors into each soil. The impedance at 0 % moisture content of the soil was about 800 kΩ, and the impedance decreased with a sensitivity of 1.95 kΩ/% between 0 %-160 %. The impedance reached about 20 kΩ at 340 % moisture content, and the value did not change at higher moisture contents.<br/>The pH sensor was fabricated by printing a CB electrode and an Ag/AgCl reference electrode on PET film and forming a pH detection film, polyaniline (PANI), on the CB electrode by a cyclic voltammetry method. When the potential difference was measured by placing it on solutions of different pH, a strong negative correlation was obtained, with a sensitivity of 14.9 mV/pH.<br/>It is worth to note that all three sensors were characterized with commercially available sensors as control experiments. All results were well matched between these flexible sensors and commercial sensors. After confirming the consistent results using the developed flexible sensors, as a proof-of-concept for multimodal plant sensor sheet, three sensors were integrated on a PET film. Demonstration experiments were conducted using soybean seedlings and succeeded in real-time measurements of these information.<br/>In conclusion, this study developed multimodal flexible sensor sheet to monitor plant healthcare. Although further studies are required to realize smart agriculture, this sensor sheet may help to open a way to monitor real-time plant information.