(Invited) A Stretchable Leaf Sensor for Plant Monitoring
Authors: Zhao Yicong , Sun Zhenci , Huang Xian 
Published: February 2020

Ecological deterioration, increasing population, and nature resource shortage post huge pressure on the conventional agriculture mode. More advanced modes based on precision agriculture apply geospatial techniques and sensors (e.g., geographic information systems, remote sensing, GPS) to obtain quantified field and environmental data, and hold the promise to improve efficiency of resource distribution and production yield. Typical approaches in precision agriculture rely on aerial sensing information or non-contact measurement. However, such measurement techniques are difficult to provide continuous information, making prompt response to disastrous environmental conditions and precision guidance of agricultural activities extremely difficult. Stretchable and flexible electronic sensors that have been applied to human skin for monitoring the health conditions of the wearers can be potentially applied to the surface of plants, and grow with the plants over their entire life spans. Here, we present a highly stretchable, ultrathin sensor that can conformably attached on leaf and monitor both the physiological and environmental conditions of plants (Figure 1). The sensor possesses excellent stretchability that is compatible to the growth of the leaf with minimized influence to plant biology.
The sensor contains stacked layers of polymeric and metallic meshes and four sensing elements to measure hydration, stain, temperature, and light intensity on leaf, respectively (Figure 2). The serpentine interconnects within the sensor is made of two levels of stretchable structures, allowing extremely large stretchability to adapt to the growth of leaf. The hydration sensing element contains a pair of electrodes in the form of an inner circular disk (200 μm in radius) and an outer circular annulus (400 μm in inner radius and 600 mm in outer radius). The strain sensing element consists of two perpendicular SWCNTs strips (500×30 μm2) in response to strain changes both in vertical and horizontal directions. The temperature sensor exploits ultrathin copper film (thickness ~50nm) that designed to serpentine shape, in order to increase the resistance compared to the serpentine interconnections. In addition, a phototransistor is used to measure the ambient light intensity.
The sensor can be connected with a wireless measurement circuits to conduct remote data transmission. The circuit, which is 3×3×2 cm2 in its dimension, consists of 4 stacked PCB layers, and uses Zigbee protocol to enable longer data transmission range (Figure 3). The power of the circuit is both supplied by a polymer rechargeable battery and solar cells to accommodate the continuous requirement of data monitoring and transmission on field.
We have conducted preliminary experiments of this sensor under controlled environment. The impedance measurement results obtained by injecting a piece of filter paper with different amount of water demonstrate that the sensor can response to a hydration changes from wet to dry, with more than 11% changes in its impedance at a frequency of 70 kHz (Figure 4). In addition, the temperature sensing element has been calibrated on a hot plate with varied temperature from 40 to 100 °C. The sensing element exhibits linear resistance changes from 4100 to 4250 Ω (Figure 5). Furthermore, the phototransistor can response to light intensity changes from 0 to 100000 lux (figure 6), which is equivalent to the light intensity during the night and the day. More experiments related to other sensing elements as well as the wireless circuit will be reported during the meeting.
In conclusion, the stretchable leaf sensor offers high stretchability to match the mechanical properties of leaf. The leaf sensor can be conformably attached on the surface of leaf and accommodate to the growth of leaf. The experimental results have demonstrated that the sensor can be used to monitor the biophysiological and environmental conditions on leaf to facilitate precision agriculture.

Figure 1