A Novel Sensor for Fruit Ripeness Estimation Using Lithography Free Approach

A flexible tactile sensor has the ability to sense and characterize the firmness and mechanical stiffness of the object. In this work, we demonstrate a low-cost, highly sensitive, flexible capacitive tactile sensor using a nanoneedle-patterned polydimethylsiloxane (PDMS) as a dielectric layer to detect the ripeness of the fruits. The nanoneedle-patterned PDMS was formed using an anodic oxidized porous alumina film as a mold. The developed tactile sensor exhibited high sensitivity due to the high deformity and elasticity of the nanoneedles on the PDMS layer. The higher sensitivity of 1.042-0.67 kPa-1 for a low-pressure regime below 0.2 kPa and the lower sensitivity of 0.35 kPa-1 for a pressure range above 0.6 kPa were demonstrated. This corresponds to a 12% change in capacitance at 0.1-kPa pressure. Loading unloading tests for over 500 cycles were performed to confirm the absence of hysteresis. The mechanical elastic modulus of different tomato samples was tested using the tactile sensor and a universal test machine. The device could easily discriminate tomatoes of different stiffness. The tomato ripeness detection experiments demonstrate that the sensor can be used in robotic sensors, to estimate the qualitative ripeness of the fruits, during harvesting and transportation. This concept can be utilized in assessing ripeness levels of fruits like mangoes, kiwis, pears, avocados, sapodilla, and so on, after adequate recalibration of the sensor, leading to different sensing ranges.