Research progress and prospects of the plant flexible sensors

Abstract: Sensing technology can be greatly contributed to smart production in modern agriculture in recent years. It is a high demand for the accuracy and biosafety requirements of agricultural sensors. However, most traditional agricultural sensors cannot be deformed, due to their rigid properties. As a result, such sensors often hinder and even damage the normal growth of plants, due to misalignment. In addition, the intrusive detection behavior of rigid sensors can also lead to data distortion, due to the activation of the plant's self-healing mechanism. The new materials and manufacturing preparation have produced the plant's flexible sensors. Different from traditional rigid sensors, plant flexible sensors have attracted widespread attention and research interest in the agricultural field, due to their excellent mechanical properties and biocompatibility. In this review, the materials and preparation were firstly outlined required to fabricate the plant's flexible sensors. Three categories were divided into substrate, functional, and packaging materials, according to their function. At the same time, the preparation materials should meet the requirements of the application, in terms of biocompatibility, air permeability, and light transmission, in addition to the corresponding functions. A summary was proposed to compare the preparation materials and properties of existing plant flexible sensors. The preparation of plant flexible sensors was presented in two aspects of film preparation and micro-nano patterning. And then the review was focused on the implementation of flexible plant sensors to track the growth process of crops. The monitoring reviewed the physiological information of the crop, such as the plant's electrical signals, volatile chemicals, water content, and growth rate. The flexible plant sensors were used to monitor the environment of the growing plant, including the plant surface temperature, humidity, and illumination. The real-time monitoring data of plant growth status was recorded to propose timely and reliable response strategies. At the same time, the state-of-the-art flexible electronic self-powering was presented to introduce the existing flexible power supply system. The most promising plant-flexible electronic power supply system was then set as the emerging nano-friction power generation, due to its excellent biocompatibility and highly flexible. As a result, the improved system was better adapted to the properties of plants. Such power supplies were still at the laboratory stage. The bottleneck and development trend of plant flexible electronics in the field of smart agriculture were: 1) How to achieve multi-functional monitoring under the premise of data reliability. The medical flexible electronics and bio-machine interface were established using signal conditioning pattern recognition. 2) A high demand was to avoid the weight of the sensor and the damage caused by chemical leaching to plant growth and life, in addition to the potential negative effects on plant photosynthesis and respiration. A reliable strategy was to select biodegradable materials with excellent biocompatibility for the manufacture of sensors. 3) The mismatch can be one of the most challenges of flexible sensors that are applied to plants, due to the rapid growth of plants. The solutions were proposed from two aspects: Material and structure; 4) It is necessary to develop the nano-friction power generation, and some functional systems, such as flexible lithium batteries, photovoltaic cells, and supercapacitors.