Characterization of Elastomers and Elastomer-Based Sensors in Extreme Environments

Soft devices for assistive action or health monitoring can improve wearer comfort compared to their more rigid counterparts. These devices may be used in extreme temperature and humidity conditions around the world or in extraterrestrial settings. Extreme low humidity and temperature conditions can be experienced towards the poles and extreme high humidity and temperature in hot climates towards the equator. Robots in these extreme environments are exposed to very different conditions than to the lab environments in which they are developed and tested. However, limited data is available on the material characteristics of commonly used elastomers in soft robots at different temperature and humidity conditions. This lack of knowledge can lead to unexpected behavior of the system and ultimately failure.<br/>In this study, we characterize four commonly used elastomers in soft robotics (Ecoflex, Dragon Skin, Smooth-Sil, and Sylgard) in a temperature range of -40°C to 80°C and humidity range of 5% to 95% RH. We performed pull-to-failure, stiffness, and stress-relaxation tests, using image tracking techniques to determine the true stress and strain of the soft materials. We manufactured capacitive sensors with Dragon Skin and Smooth-Sil as base materials and carried out electromechanical characterization, measuring the capacitance response to tensile strain in the different environmental conditions.<br/>We found that both the mechanical behavior of the elastomers and the electromechanical behavior of the sensors depend on the environmental conditions of operation. All elastomers show temperature dependent behavior, with typical stiffening of the material and a lower strain at failure with increasing temperature. The stress-relaxation behavior was most sensitive to sub-zero temperatures, with both reduced and increased relaxation observed. No clear trends were observed between the mechanical behavior of the elastomers and humidity conditions. The capacitive sensors exhibited changes in performance that related to both humidity and temperature conditions. A nonlinear relationship was found between the capacitance of the sensors and the temperature conditions. The capacitance was found to be decreasing with temperature between 0°C and 80°C and to be increasing with temperature between -40°C and 0°C. The humidity response of the sensors resulted in a slightly lower capacitance at low humidity and a slightly higher capacitance at high humidity.<br/>The results from this study show that the environmental conditions of operation need to be considered in the design of a robotic system. The potential change in stiffness, failure point, and relaxation of the materials can affect design choices such as the used materials, range of motion, and power requirements. The temperature and humidity dependent sensor performance means that sensor output can only be accurately read with knowledge of the environmental conditions in which they operate. In conclusion, this study provides essential insights into elastomer behavior for the design and successful operation of soft robotic systems for wearable applications outside of a lab setting.