Reconfigurable Electrical Networks within a Conductive Hydrogel Composite.

Soft materials that exhibit compliance, programmability, and reconfigurability can have a transformative impact as electronic skin for applications in wearable electronics/soft robotics. There has been significant progress in soft conductive materials; however, achieving electrically controlled and reversible changes in conductivity and circuit connectivity remains challenging. To overcome this limitation, a soft material architecture with reconfigurable conductive networks of silver flakes embedded within a hydrogel matrix is presented. The conductive networks can be reversibly created/disconnected through various stimuli, including current, humidity, or temperature. Such stimuli affect electrical connectivity of the hydrogel by controlling its water content, which can be modulated by evaporation under ambient conditions (passive dehydration), evaporation through electrical Joule heating (active dehydration), or absorption of additional water (rehydration). The resulting change in electrical conductivity is reversible and repeatable, endowing the composite with on-demand reconfigurable conductivity. To highlight this material's unique properties, it is shown that conductive traces can be reconfigured after severe damage and revert to lower conductivity after rehydration. Additionally, a quadruped robot is demonstrated that can respond to stimuli by changing direction following exposure to excess water, thereby achieving reprogrammable locomotion behaviors.

Silver-Coated Poly(dimethylsiloxane) Beads for Soft, Stretchable, and Thermally Stable Conductive Elastomer Composites.

We introduce an elastomer composite filled with silver (Ag) flakes and Ag-coated poly(dimethylsiloxane) (PDMS) beads that exhibits electrical conductivity that is 2 orders of magnitude greater than that of elastomers in which the same concentration of Ag filler is uniformly dispersed. In addition to the dramatic enhancement in conductivity, these composites exhibit high mechanical compliance (strain limit, >100%) and robust thermal stability (conductivity change, <10% at 150 °C). The incorporation of Ag-coated PDMS beads introduces an effective phase segregation in which Ag flakes are confined to the "grain boundaries" between the embedded beads. This morphological control aids in the percolation of the Ag flakes and the formation of conductive bridges between neighboring Ag shells. The confinement of Ag flakes also suppresses thermal expansion and changes in electrical conductivity of the percolating networks when the composite is heated. We demonstrate potential applications of thermally stable elastic conductors in wearable devices and soft robotics by fabricating a highly stretchable antenna for a "smart" furnace glove and a strain sensor for soft gripper operation in hot water.

EGaIn-Assisted Room-Temperature Sintering of Silver Nanoparticles for Stretchable, Inkjet-Printed, Thin-Film Electronics.

Coating inkjet-printed traces of silver nanoparticle (AgNP) ink with a thin layer of eutectic gallium indium (EGaIn) increases the electrical conductivity by six-orders of magnitude and significantly improves tolerance to tensile strain. This enhancement is achieved through a room-temperature "sintering" process in which the liquid-phase EGaIn alloy binds the AgNP particles (≈100 nm diameter) to form a continuous conductive trace. Ultrathin and hydrographically transferrable electronics are produced by printing traces with a composition of AgNP-Ga-In on a 5 µm-thick temporary tattoo paper. The printed circuit is flexible enough to remain functional when deformed and can support strains above 80% with modest electromechanical coupling (gauge factor ≈1). These mechanically robust thin-film circuits are well suited for transfer to highly curved and nondevelopable 3D surfaces as well as skin and other soft deformable substrates. In contrast to other stretchable tattoo-like electronics, the low-cost processing steps introduced here eliminate the need for cleanroom fabrication and instead requires only a commercial desktop printer. Most significantly, it enables functionalities like "electronic tattoos" and 3D hydrographic transfer that have not been previously reported with EGaIn or EGaIn-based biphasic electronics.