A series of rapid and inexpensive methods to produce elastically soft sensors and circuits in minutes using a CO2 laser (10.6 μm wavelength) are introduced. These soft‐matter electronics are composed of laser‐patterned films of conductive poly(dimethylsiloxane) (cPDMS) and liquid‐phase gallium–indium (GaIn) alloy embedded in a thin sheet of soft silicone elastomer. Direct laser patterning eliminates the need for photolithography, replica molding, and customized inkjet or microcontact (μCP) printing, and allows conductive traces of cPDMS and liquid GaIn to be rapidly integrated into a single soft‐matter circuit. The versatility of this fabrication method is demonstrated by the production of a variety of electrically functional soft‐matter sensors and circuit elements that contain features with >150 μm planar dimensions. It is postulate that in the case of GaIn alloy patterning occurs when the recoil force of the escaping vapor exceeds the liquid's surface tension. This mechanism exploits the unique “moldability” of liquid GaIn alloy, which forms a surface oxide of Ga2O3 that allows the patterned film to maintain its shape.

provide increased robustness and better mechanical impedance matching with the host material or structure. For instance, they can be integrated into clothing or mounted on the skin without constraining natural body motion or causing discomfort. A promising approach for realizing stretchable electronics is to create microfluidic traces of liquid-metal (LM) embedded in a soft elastomer. [1][2][3] Ga-based LM circuits offer attractive advantages over alternative approaches. Stretchable electronics based on soft-elastomers embedded with percolating networks of rigid metallic particles, [4,5] carbon allotropes, [6,7] or conductive polymers [8,9] typically suffer from low conductivity (three orders of magnitude lower than metals) or poor electromechanical properties. Micro/nanoscale geometries of thin conductive elements (e.g., serpentine and "wavy" electronics) represent a promising alternative that achieves stretchable functionality through flexure or twisting on a prestrained elastomer substrates. [10][11][12][13][14] However, obtaining stretchability with deterministic architectures requires conductive traces to be patterned into specific geometries (e.g., prebuckled waves, planar serpentines) that are only deformable in prescribed directions. By contrast, Ga-based LM alloys, such as eutectic Ga-In (EGaIn; 75% Ga and 25% In, by weight) and Ga-In-Sn (Galinstan; 68% Ga, 22% In, 10% Sn), can be incorporated into elastomers and preserve their elastic properties at all length scales and in all loading conditions without requiring specialized geometries. [15] These alloys provide high electrical conductivity (3.4 × 10 6 S m −1 ), low melting point (−19 °C for Galinstan, 15 °C for EGaIn), low viscosity (2 mPa s), low toxicity, [16] and negligible vapor pressure. [3,15] Since they are liquid at room temperature and have metallic conductivity, EGaIn and Galinstan can function as intrinsically stretchable and deformable conductors that are not subject to the limitations of conductive polymers or deterministic architectures. As such, LM-based electronics can provide a unique combination of metallic conductivity and elastomeric deformability. Although this promise of LM-based electronics has been well recognized in recent literature, [17] advances in scalable fabrication approaches and effective electrical interfaces between liquid metal traces and microelectronics are still needed to create functional and practical soft and stretchable electronics.Eutectic gallium-indium (EGaIn) has attracted significant attention in recent years for its use in soft and stretchable electronics. However, advances in scalable fabrication approaches and effective electromechanical interfaces between liquid metal (LM) traces and microelectronics are still needed to create functional soft and stretchable electronics. In this study, EGaIn-metal interfacing for the effective integration of surface-mount microelectronics with LM interconnects is investigated. The electrical interconnects are produced by creating copper patterns on a soft-elastomer su...

When immersed in an electrolyte, droplets of Ga‐based liquid metal (LM) alloy can be manipulated in ways not possible with conventional electrocapillarity or electrowetting. This study demonstrates how LM electrochemistry can be exploited to coalesce and separate droplets under moderate voltages of ~1–10 V. This novel approach to droplet interaction can be explained with a theory that accounts for oxidation and reduction as well as fluidic instabilities. Based on simulations and experimental analysis, this study finds that droplet separation is governed by a unique limit‐point instability that arises from gradients in bipolar electrochemical reactions that lead to gradients in interfacial tension. The LM coalescence and separation are used to create a field‐programmable electrical switch. As with conventional relays or flip‐flop latch circuits, the system can transition between bistable (separated or coalesced) states, making it useful for memory storage, logic, and shape‐programmable circuitry using entirely liquids instead of solid‐state materials.

We introduce a method for sealing liquid metal (LM) circuits with soft anisotropic conductors that prevent leaking, while simultaneously allowing for electrical contact with skin and surface mounted electronics. These films are composed of polydimethylsiloxane (PDMS) embedded with vertically aligned columns of ferromagnetic Ag-Ni microparticles. The microparticles are magnetically aligned and support electrical conductivity only through the thickness (z-axis) of the elastomer film. Measurements on 10-40% (by wt) composites show moderate volumetric resistivity (as low as ρ = 0.03 Ω/m) through the thickness and no conductivity between adjacent traces. Functionality is demonstrated with several illustrative applications related to tactile sensing and electronics hardware integration.

We examine the electrocapillary properties of a fluidic actuator composed of a liquid metal droplet that is submerged in electrolytic solution and attached to an elastic beam. The beam deflection is controlled by electrochemically driven changes in the surface energy of the droplet. The metal is a eutectic gallium-indium alloy that is liquid at room temperature and forms an nm-thin Ga 2 O 3 skin when oxidized. The effective surface tension of the droplet changes dramatically with oxidation and reduction, which are reversibly controlled by applying low voltage to the electrolytic bath. Wetting the droplet to two copper pads allows for a controllable tensile force to be developed between the opposing surfaces. We demonstrate the ability to reliably control force by changing the applied oxidizing voltage. Actuator forces and droplet geometries are also examined by performing a computational fluid mechanics simulation using Surface Evolver. The theoretical predictions are in qualitative agreement with the experimental measurements and provide additional confirmation that actuation is driven by surface tension.