Liquid Metal-Based Stretchable Strain Sensor for Fruit Growth Monitoring

Abstract

Precision agriculture (PA) increasingly relies on advanced sensing technologies to optimize crop yield, quality, and management practices. In this study, we present a stretchable strain sensor specifically designed for monitoring fruit growth ranging from 8 mm to 20 mm in diameter, a critical window for determining the timing for fruit thinning. The sensor employs a silicone elastomer (Ecoflex 00-30) as both substrate and encapsulant, and a liquid metal (LM) eutectic alloy (Galinstan) as a sensing/conductive channel. Uniform spray coating was used to deposit LM as a sensing channel. The sensor was reshaped into a cylindrical structure, which enabled conformity to the fruit surfaces. The sensor demonstrates stable electromechanical performance with resistance increasing from 2.4 +/- 0.5 to 12.2 +/- 0.5 Omega under 150% strain (axial). Environmental testing confirmed durability under UV, moisture, and temperature fluctuations. A field validation test was carried out using a manual, self-built readout system. The sensor exhibits a promising response during the growth period from 8 to 20 mm. Results from the Lab and field indicate the sensor's durability and reliability, making it suitable for long-term deployment in fields/orchards. The proposed sensor offers growers actionable data to optimize thinning, irrigation, and fertilization practices by providing periodic monitoring of the fruit growth.