Effects of Hardness on the Sensitivity and Load Capacity of 3D Printed Sensors

One recent success of additive manufacturing (AM; also known as 3D printing) technologies is a 3D printed pressure-sensitive sensor (i.e. tactile sensor) with a greater degree of design complexity and multi-material components. Although 3D printed pressure sensors have been realized, there still exists a topic of extensive ongoing research. This study aimed to investigate the effects of hardness of the 3D printed sensors on characteristics such as the sensitivity and load capacity of the sensors. The ultimate goal of this work is to provide guidelines for selecting hardness for 3D printed sensors used in different sensor applications (i.e., soft and highly sensitive humanoid hands vs. less soft and less sensitive industrial robotic hands). A multi-material direct-print photopolymerization (DPP) process was used to produce an entire sensor that consists of insulating layers, electrode layers, and a pressure-sensitive layer. Soft and rigid photopolymers were blended to achieve six different hardness levels such as Shore A of 50, 60, 70, 85, 95 and 98. A carbon nanotube/polymer composite was used to create the electrodes, and 1-ethyl-3-methylimidazolium tetrafluoroborate as an ionic liquid was used with a photopolymer for the pressure-sensitive layer. Sensors of different hardness were tested by applying varying loads using a force gauge, and sensor signals were collected. Soft sensors with Shore A hardness of 50, 60 and 70 showed reliable outputs, where the softer sensor provided better sensitivity and smaller errors but lower load capacity. Sensors with Shore A of 85, 95 and 98 did not show reliable outputs, where the harder insulating layer did not allow the force gauge to press into the sensor, instead causing the gauge to slip over the surface. These findings could be useful for designing customized sensors for applications with different load conditions.