Session: 12-29-01: Mechanics of Soft Materials

Paper Number: 144111

144111 - Fatigue-Resistant Photoelastic Soft Materials for Vision-Based Tactile Sensor 

Soft materials that can exhibit color changes in response to mechanical deformations have significant values due to their ability to provide real-time and dynamic feedback on stress visualization. For example, these materials can be integrated into smart textiles to monitor sports performance or be incorporated into healthcare materials to visualize pressure in affected areas. Moreover, they can be designed as tactile sensors to gather interaction information to help robots adapt movement and avoid obstacles and collisions. These advances enhance modern technologies and create new opportunities in diverse fields, such as healthcare, robotics, and entertainment.

Present strategies to design color-changing soft materials primarily depend on synthesis of mechanophores and photoelasticity of polymers. However, these methods exhibit inherent limitations for enduring dynamic stress sensing. Although certain mechanophores have recently shown reversible changes, they still face exhaustion and become depleted after undergoing 10 to 50 cycles of loading. In contrast, the color observed in photoelasticity depends on the elasticity of polymer chains rather than the scission of chains. When a mechanical force is applied, the randomly oriented polymer chains align and orient along specific directions. This causes a refractive index difference, known as birefringence, on the principal stress direction, leading to a stress distribution pattern through changes in light polarization. While photoelasticity offers a reliable tool to visualize stress distribution, most materials suffer from fatigue, meaning they undergo the scission of polymer chains when subjected to repeated force. The broken chains affect the structure of chain alignments with each occurrence, leading to varying stress states and different photoelastic colors under identical mechanical conditions. The limitations of these materials significantly constrain the long-term applicability. Therefore, the design of fatigue-resistant color-changing soft material is needed to offer greater versatility and practicality for long-term dynamic sensing areas.

In this work, we proposed a fatigue-resistance photoelastic soft material (FPSM) which has high stretchability, low hysteresis, and high fatigue resistance in both mechanical and photoelastic properties. Unlike common hydrogels, FPSM is composed of very long polymer chains, which are intertwined by molecular entanglements. The dense entanglements work as slip links and transmit mechanical force along polymer chains without the scission of polymer networks. Moreover, we introduce hygroscopic salts into the material to prevent dehydration when exposed to air. The retained water ensures the smooth sliding of polymer chains. We examined the impact of these two designed parameters, molecular entanglements and water content, on the mechanical and photoelastic properties: the shear modulus can be adjusted from 10 kPa to 50 kPa by altering the number of entanglements, achieved by varying the crosslinker density; the stress-optical coefficient can be tuned between 3×10^-30 and 7×10^-30 through the regulation of the water content, achieved by altering hygroscopic salt concentration. Moreover, this molecular design enables FPSM to exhibit a high fracture toughness ~ 3000 J/m² and a fatigue threshold above 400 J/m². Its mechanical and photoelastic performance remain constant throughout a long-term test of 10,000 dynamic tensile loading cycles, and maintain stability during a short-term test with loading rates ranging from 0.5 mm/s to 10 mm/s. These fatigue-resistant attributes of FPSM make it highly suitable for long-term stress sensing. Finally, we present a low-cost, long-lasting, vision-based tactile sensor that leverages the reflective photoelastic principle, designed to deliver tactile sensation to robotic fingers. It is capable of distinguishing material modulus, object shape, spatial positioning, and applied pressure, by directly visualizing the photoelastic stress distribution. As the captured stress map closely aligns with the simulated stress pattern, the sensor constructs a framework for precise interpreting mechanical interactions between robotic systems and encountered objects. This design shows great promise for enhancing the capabilities of future tactile robotics.

Presenting Author: Jiabin Liu Michigan State University

Presenting Author Biography: Jiabin is currently a PhD student in Mechanical Engineering at MSU, supervised by Dr. Shaoting Lin. She joined Lin Research Group in the Fall of 2022 after receiving two Master's degrees in Electrical Engineering at Tsinghua University and Northwestern University. Her research interests include high-performing color-changing soft materials for tactile sensing and particle diffusion in deformed polymer networks.

Authors:

Jiabin Liu Michigan State University
Shaoting Lin Michigan State University
Wei Li Stony Brook University
Yu She Purdue University