Electrically Activated Stiffness-Switching with Low-Melting-Point Conductive Thermoplastics

As technology becomes more integrated into our daily lives, the need for machines that can safely and comfortably interact with the human body has grown. While the rigidity of traditional robotic materials, such as metals and plastics, can provide mechanical and electrical stability to these devices, it can also reduce safety and comfort when placed in contact with soft human tissue. In recent years, these issues have been addressed by incorporating compliant materials, like liquids or soft polymers, into wearable or biomedical devices. However, these materials, by virtue of their softness, cannot support the high loads required for operations like stabilization or gripping. To address this apparent trade-off between load-bearing stiffness and conformable softness, several groups have constructed stiffness-tuning devices, capable of alternating between a high-stiffness state and a low-stiffness state. Although there exist a wide variety of mechanisms by which we can achieve this switching behavior, thermally activated phase change provides the highest stiffness ratio between the soft and stiff states. In this work, use low-melting point conductive thermoplastics to create electrically activated stiffness-switching devices. When a voltage is applied across this thermoplastic, the resulting electric current causes the polymer to heat and melt. This phase change corresponds to an effective stiffness change.In the first study, we introduce a novel stiffness switch layout that employs liquid metal as compliant electrodes oriented across the face of a conductive thermoplastic. This new layout results in an 80% decrease in required voltage, a 60% decrease in activation time, and the ability to switch the stiffness of arbitrary geometries.In the second study, we examine the effects of the composition of a conductive thermoplastic composite on its stiffness-switching properties, and use these findings can help guide the design of stiffness-switching composites for a three soft robotic applications