Highly Stretchable, Fully Screen-Printed Resistive Strain Sensor for Apple Fruit Growth Monitoring

Abstract

Fruit size at harvest is a critical quality attribute in the global apple market, directly influencing both market value and growers’ income. Traditional monitoring tools such as vernier calipers and fruit gauges, while widely used, are labor-intensive, costly, and require regular maintenance, making them unsuitable for large-scale use in commercial orchards. This paper introduces a novel, fully screen-printed, stretchable resistive strain sensor designed for continuous, non-invasive monitoring of apple fruit growth under field conditions. The sensor is fabricated by printing a stretchable conductive silver paste onto a thermoplastic polyurethane substrate, followed by encapsulation, laser-cut, and crimping onto terminal pads. It is secured around the fruit using a mechanically interlocking clamp that functions as a buckle, ensuring stable placement at the fruit’s maximum equatorial diameter. Experimental results demonstrate that the sensor performs reliably across a fruit diameter range of 40–80 mm, corresponding to 100% strain. The best-performing sample exhibited a 247-fold increase in resistance across the full strain range (0–100%). These findings confirm the feasibility of the proposed approach under laboratory-simulated field conditions.