Yunsik Ohm

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.

Printed soft conductive materials for stretchable electronics should have low electrical resistivity, high strain limit, and stable electrical properties when stretched. Previously, it has been shown that a bi‐phasic ink composed of silver (Ag) microflakes, eutectic gallium−indium (EGaIn) alloy, and styrene isoprene (SIS) block copolymer is a promising formulation for printed soft electronics and has the potential to satisfy the necessary criteria. In this study, further improvements to the ink formulation are explored, with a focus on how the choice of Ag microflakes affects the electrical and electromechanical properties of the composite. By using specific Ag microflakes, AgInGa‐SIS inks that have conductivity as high as 6.38 × 10⁵ S m⁻¹ and a strain limit of over 1000%, with low electromechanical coupling can be synthesized. More broadly, when comparing the composite with different silver flakes, there is a 176% relative difference in conductivity, >600% difference in strain limit, and 277% relative difference in electromechanical coupling. To demonstrate the applicability of these inks for various use cases such as wearable bioelectronics, interconnects are printed for connecting electronic breakout boards with microcontrollers that provide a stable electrical connection when stretched, and the interconnects and electrodes of a wearable electrocardiography system that monitors the heart pulses in real‐time.

Soft polymers embedded with liquid metals like eutectic gallium-indium (EGaIn) exhibit unique combinations of mechanical, electrical, and thermal properties that are not possible with other material systems. For example, a soft silicone elastomer embedded with a percolating network of EGaIn microdroplets can function as a highly soft and elastic conductor that can be stretched to 600% strain without significant change in electrical resistance. Depending on the choice of polymer matrix and EGaIn microstructure, these soft material composites can be engineered to exhibit mechanical and electrical self-healing properties as well as high fracture toughness and resistance to tearing. Moreover, when solid filler particles like silver flakes are added to EGaIn-polymer composites, they can function as printable conductive inks that are fully elastic, non-marking, and non-smearing when cured. In this short review, we present different classes of EGaIn-polymer composites, discuss approaches to materials synthesis and patterning, and compare their properties with other material systems. Additionally, we will review applications of this emerging class of materials in domains ranging from wearable bioelectronics to soft robotics, shape programmable smart materials, as well as energy storage and harvesting devices.

Materials that can assist with perception and responsivity of an engineered machine are said to promote physical intelligence. Physical intelligence may be important for flexible and soft materials that will be used in applications like soft robotics, wearable computers, and healthcare. These applications require stimuli responsivity, sensing, and actuation that allow a machine to perceive and react to its environment. The development of materials that exhibit some form of physical intelligence has relied on functional polymers and composites that contain these polymers. This review will focus on composites of functional polymers that display physical intelligence by assisting with perception, responsivity, or by off-loading computation. Composites of liquid crystal elastomers, shape-memory polymers, hydrogels, self-healing materials, and transient materials and their functionalities are examined with a viewpoint that considers physical intelligence. Graphic Abstract