Due to the growing interest in soft robotics, stretchable electronics, and electronic skins, there is demand for compliant electrodes and interconnects that are soft, stretchable and conductive.Here, we use dielectrophoresis (DEP) to assemble, align, and sinter microdroplets of liquid metal-eutectic Ga-In (EGaIn)-in uncured polydimethylsiloxane (PDMS) to form electrically conducting microwires. There are several noteworthy aspects of this approach: (1) Generally, liquid metal droplets in silicone at loadings approaching 90 wt% remain insulating and form conductive network only when subjected to sintering. Here the use of DEP facilitates assembly of the filler droplets into conductive microwires at loadings as low as 10 wt% EGaIn. (2) Dielectrophoresis is done in silicone for the first time, enabling the microwires to be cured in a stretchable matrix. (3) Because the droplets are liquid, they sinter during dielectrophoresis to form a stretchable metallic microwire that retains its shape after curing the silicone and does not change resistance during mechanical strain. (4) The use of liquid metal eliminates the issue of compliance mismatch observed in soft polymers with solid fillers. (5) The silicone-EGaIn "ink" can be placed in holes created by severely damaged regions of stretchable wires to create stretchable interconnects that heal the damage both mechanically and electrically. We characterize the DEP process of this unique set of materials and demonstrate the interesting attributes enabled by such liquid microwires.

This review highlights the unique techniques for patterning liquid metals containing gallium (e.g., eutectic gallium indium, EGaIn). These techniques are enabled by two unique attributes of these liquids relative to solid metals: 1) The fluidity of the metal allows it to be injected, sprayed, and generally dispensed. 2) The solid native oxide shell allows the metal to adhere to surfaces and be shaped in ways that would normally be prohibited due to surface tension. The ability to shape liquid metals into non‐spherical structures such as wires, antennas, and electrodes can enable fluidic metallic conductors for stretchable electronics, soft robotics, e‐skins, and wearables. The key properties of these metals with a focus on methods to pattern liquid metals into soft or stretchable devices are summari.

Soft robotics focuses on mimicking natural systems to produce dexterous motion. Dielectric elastomer actuators (DEAs) are an attractive option due to their large strains, high efficiencies, lightweight design, and integrability, but require high electric fields. Conventional approaches to improve DEA performance by incorporating solid fillers in the polymer matrices can increase the dielectric constant but to the detriment of mechanical properties. In the present work, we draw inspiration from soft and deformable human skin, enabled by its unique structure, which consists of a fluid-filled membrane, to create self-enclosed liquid filler (SELF)–polymer composites by mixing an ionic liquid into the elastomeric matrix. Unlike hydrogels and ionogels, the SELF–polymer composites are made from immiscible liquid fillers, selected based on interfacial interaction with the elastomer matrix, and exist as dispersed globular phases. This combination of structure and filler selection unlocks synergetic improvements in electromechanical propertiesdoubling of dielectric constant, 100 times decrease in Young’s modulus, and ∼5 times increase in stretchability. These composites show superior thermal stability to volatile losses, combined with excellent transparency. These ultrasoft high-k composites enable a significant improvement in the actuation performance of DEAslongitudinal strain (5 times) and areal strain (8 times)at low applied nominal electric fields (4 V/μm). They also enable high-sensitivity capacitive pressure sensors without the need of miniaturization and microstructuring. This class of self-enclosed ionic liquid polymer composites could impact the areas of soft robotics, shape morphing, flexible electronics, and optoelectronics.

Soft materials tend to be highly permeable to gases, making it difficult to create stretchable hermetic seals. With the integration of spacers, we demonstrate the use of liquid metals, which show both metallic and fluidic properties, as stretchable hermetic seals. Such soft seals are used in both a stretchable battery and a stretchable heat transfer system that involve volatile fluids, including water and organic fluids. The capacity retention of the battery was ~72.5% after 500 cycles, and the sealed heat transfer system showed an increased thermal conductivity of approximately 309 watts per meter-kelvin while strained and heated. Furthermore, with the incorporation of a signal transmission window, we demonstrated wireless communication through such seals. This work provides a route to create stretchable yet hermetic packaging design solutions for soft devices.

This review focuses on surface modifications of liquid metal (LM). Gallium (Ga) and Ga-based LMs show the promising ability to maintain metallic properties under large mechanical strains when encapsulated inside an elastomer matrix. [1-7] This property is useful in a variety of applications, including wearable/deformable devices, sensors, and soft actuators-the focus of this special issue. There are several motives for creating soft and stretchable devices. For example, devices that are soft and flexible can make conformal contact to human skin for continuous and long-term monitoring. [8-11] Likewise, devices assembled on or within soft robots can maintain function during actuation (bending or stretching) while sensing dynamic motion. Conventional conductors in electronic systems are typically rigid (e.g., copper) and thus poorly suited for soft and stretchable devices. To address this issue, efforts have been taken to create stretchable and deformable conductors using thin metal films with special geometries [12-14] (e.g., pop-up and serpentine electrodes) on a deformable substrate or polymer composites filled with conductive nanomaterials in a stretchable matrix. [15,16] These approaches effectively render rigid materials stretchable through clever engineering of their geometry. However, due to the inherently nonstretchable nature of solid metal films or solid conductive particles, these components/inclusions would add to the overall rigidity of the composite and limit their deformability. LMs are interesting as stretchable conductors because they have metallic conductivity and deformability defined primarily by the encasing material (e.g., elastomer).