Flexible sensors for in situ monitoring of plant physiology

• Main Author: Zhang, Kaiyi
• Other Authors: Chen Xiaodong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2025
• Subjects: Engineering; Plant wearable sensors
• Online Access: https://hdl.handle.net/10356/182722
• Institution: Nanyang Technological University

Plants serve as the cornerstone of all ecosystems and the food chain, providing sustenance for both human and animal populations while maintaining ecological balance through photosynthesis. With the rapid growth of human population, the demand for plant productivity is ever growing. However, the occurrence of yield losses due to biotic and abiotic stresses poses a significant challenge to achieving optimal productivity. Monitoring plant health, optimizing growth conditions, and intervening at the early detection of physiological anomalies are critical steps towards enhancing yield. Plant wearable sensors offer a promising solution to address these challenges, with emerging wearable technologies driving advancements in this field. The monitoring of plant health can be achieved through the tracking of various plant phenotypes and environmental factors. Several plant wearable sensors have been developed to measure plant physical growth, physiology, and emitted chemicals. However, the lack of wearable or flexible sensors for chlorophyll content measurement and sap flow measurement, which are two critical aspects of plant physiology monitoring, hinders effective plant health monitoring. The current thesis discusses the importance of leaf chlorophyll content and sap flow rate as physiological signals of plants and current-existing methods for their measurements. Plant wearable sensors for leaf chlorophyll measurement and sap flow measurement are developed, and in-situ, long-term monitoring of plants is realized with the developed sensors. In this thesis, a miniaturized, flexible, and leaf-patchable chlorophyll meter is developed, which is designed for non-invasive, in situ, and long-term monitoring of leaf chlorophyll content. Inspired by reflectance-based vegetation indices, the leaf-patchable meter employs a monochromatic LED and a pair of photodetectors for incident radiation and intensity measurement of reflected light respectively. Compared to conventional portable chlorophyll meters, leaf-patchable chlorophyll meter demonstrates enhanced accuracy (r2 > 0.9) and sensitivity in chlorophyll content assessment. Furthermore, its long-term monitoring capabilities (over 2 weeks) allow for earlier detection of chlorophyll loss due to environmental stress comparing with conventional methods, which facilitates prompt interventions. This thesis also presents the development of an ultrathin polymeric sap flow sensor tailored for herbaceous plants. The adoption of thin film substrate and polymeric materials significantly reduces the total thickness to less than 10 μm, enabling the conformal lamination on the thin stems of herbaceous plants. Through the establishment of a solid linear correlation between flow rate and reciprocal response time which is validated by experiments, simulations and theoretical calculations, the sensor generates a sap flow index which can indicate the flow rates in the range of 10-250 μL/min accurately. Short-term and long-term monitoring have been realized with the sensor to record the sap flow rate in 24 hours and explore the relationship between sap flow rate and plant growth. In conclusion, this thesis addresses the gap of existing plant wearable sensors by developing a leaf-patchable chlorophyll meter and an ultrathin polymeric sap flow sensor. The two innovative wearable sensors hold immense potential for revolutionizing agriculture by enabling real-time monitoring, early detection of stresses and data-driven interventions to enhance crop productivity and sustainability, paving the way for future advancements in smart and precision agriculture.