A method to produce soft and stretchable microelectronics composed of a liquid-phase Gallium-Indium alloy with micron-scale circuit features is introduced. Microchannels are molded onto the surface of a poly(dimethylsiloxane) (PDMS) elastomer and filled with EGaIn using a micro-transfer deposition step that exploits the unique wetting properties of EGaIn in air. The liquid-filled channels function as stretchable circuit wires or capacitor electrodes with a 2 μm linewidth and 1 μm spacing.

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liquid metal alloys including injection of liquid metals into elastomeric channel networks, [10][11][12][13][14] freeze-casting, [15] surface patterning using lithographic techniques, [16][17][18][19] laser engraving, [20] or additive approaches including flow-based direct-writing, [21,22] droplet-by-droplet deposition, [23] heated roller-pen printing, [24,25] ink-jetting, [26] deposition at microscale in the nanoparticle ink form, followed by sintering, [27] or combination of direct-writing and transfer printing. [28] These methods have been utilized to realize various flexible electronic components including interconnects, [10,29] passive circuitry, [11] diodes and memristors, [30,31] antennas for wireless communication, [21,32,33] and wearable force and pressure sensors. [12,34] Despite their demonstrated success, the 3D geometric capability of these methods is still limited. The geometric capability of the injection and lithography is determined by that of the 2D methods used to fabricate the elastomeric channel networks, stencils or stamps. The additive manufacturing methods provide a higher level of geometric capability, however, still are mostly limited to 2D planar designs since the liquid metal cannot maintain its mechanical stability as complex 3D structures. Using these methods, obtaining a limited range of 3D designs still require a number of rigorous process steps including molding, metal injection and vacuuming, as well as layer bonding. In a recent study, Ladd et al. showed that the moldability of liquid metal alloys can be utilized to form vertically extended structures in mm-level sizes, [22] demonstrating the promise for true 3D-printing using liquid metals. However, as also demonstrated in this work, the structural stability of the oxide skin is not sufficient to form complex 3D networks of liquid metal alloys in practical size scales. A number of recent studies combined liquid metal and elastomer printing to build multilayer planar networks of liquid metals where the printed elastomer essentially used as the support material. [35][36][37] Even though these studies improved the geometric capability of additive manufacturing of EGaIn networks, printing of EGaIn along truly 3D paths in a continuous fashion has not been realized. Realization of such a printing method will enable high-density 3D interconnect networks, antennae, and passive circuitry for flexible electronics with substantially increased design complexity.In this paper, we present the freeze-printing method to address this need. This method is schematically described in Figure 1a-d. Here, the liquid metal alloy, EGaIn is dispensed from a nozzle on a substrate, the temperature of which is kept below the melting point of the alloy. As the nozzle is translated in 3D, more liquid is added to the printed structure along the translation path and maintains its cylindrical shape owing to the spontaneous formation and deformation of the oxide skin. In the meantime, the printed structure starts freezing from the substrate generating a moving f...

The electrodes in energy storage devices, such as lithium/sodium ion batteries, are typical multicomponent system consisting of inorganic electrode particles, polymer binders, conductive fillers, current collectors, and other components. These components are usually porously combined by a polymeric binder to accomplish the required electrochemical functions. In spite of the great success, this classic porous configuration faces serious issues in mechanical stability and flexibility due to weak and instable structures/interfaces. Here, by learning from polymeric nanocomposites, a concept of electrode matrix is proposed based on a gum‐like nanocomposite, a dual‐conductive adhesive. As an electrode matrix, the gum‐like nanocomposite integrates the functions of binder, electrolyte, and conductive fillers. In particular, it shows strong adhesion, high electrical/ionic conductivities, and appropriate mechanical and self‐healing properties. Finally, it is demonstrated that, with the electrode matrix, battery electrodes can be fabricated into nonporous composite showing not only excellent mechanical flexibility/stability but also improved electrochemical performance when working with a gum‐like electrolyte.

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