2, Iowa State University, Ames, Iowa, United States
Soft and stretchable electronics have paved the way for sensing and actuation that is intrinsically soft and exhibits properties similar to that of natural biological tissue. While these technologies are capable of undergoing extreme deformations, these soft circuits lose electronic functionality when damaged and repair is only possible in certain cases through manual intervention, the use of redundant electronics, or the application of heat. The ability for these devices to operate in complex environments is critical for progress in wearable computing, soft machines and robotics, and inflatable structures and deployables. Here, we use a liquid metal embedded elastomer (LMEE) that is composed of micron-scale droplets embedded within a soft silicone elastomer matrix. The composite is intrinsically insulating. Application of local pressure causes a local change in electrical conductivity through the formation of a percolating conductive network of liquid metal droplets with high electrical conductivity (σ = 1370 S-cm-1). This enables circuits to both be created and subsequently reconfigured when damaged, through the autonomous, in-situ formation of new electrical pathways. The mechanically-sintered lines are experimentally characterized under uniaxial strain for different volume loadings of liquid metal. The LMEE composite is soft (E = 1.5MPa) and exhibits negligible changes in trace resistance (< 10% increase) when loaded to 50% strain. To demonstrate the use of these material as damage-resistant wiring for soft robotics, we integrate it into an electrically-powered soft quadruped. The LMEE composite is used to transmit power to shape memory alloy actuators embedded in the soft robot limbs. After damage, the power trace autonomously self-heals and there is no visual change in the gait of the soft robot. This materials architecture demonstrates an unprecedented level of robust functionality that has the potential to enable soft-matter electronics and machines to exhibit the extraordinary resilience of soft biological tissue and organisms.