An FBG-Based Adhesive Wearable Sensor for Bidirectional Fruit Growth Monitoring in Real Outdoor Scenario

The growing demand for sustainable agriculture requires robust and innovative solutions for continuous plant health monitoring. The development of smart, minimally invasive, and highly adhesive sensing systems is crucial to provide reliable data for effective crop management. In this context, Fiber Bragg gratings (FBGs) prove to be a valid solution due to their high sensitivity, multiplexing capabilities, flexibility, lightness, and transparency, making them ideal for long-term monitoring in outdoor conditions. This study presents a bidirectional wearable sensor based on FBG technology for fruit monitoring in an open-field scenario. The sensor, consisting of two orthogonally arranged FBGs embedded in a flower-shaped, transparent, biocompatible, self-adhesive hydrogel matrix, was fabricated through a two-step process. The influence of strain and environmental quantities [i.e., temperature and relative humidity (RH)] was investigated, yielding Sϵ =0.08 nm/mϵ, ST =0.01 nm/°C, and negligible response to RH. Finally, the sensor was applied to the surface of Solanum melongena (eggplant) over a five-day period. Despite fluctuating weather conditions, the sensor remained securely adhered throughout the entire monitoring duration and provided continuous real-time data, capturing periodic trends linked to the day-night cycle and revealing slight growth-related signal variations. This work demonstrates the potential of FBG-based sensors for long-term, multidirectional fruit monitoring, contributing to the advancement of precision agriculture tools for improved crop management and early detection of plant stress or disease.