Towards sub-microscale liquid metal patterns: Cascade phase change mediated pick-n-place transfer of liquid metals printed and stretched over a flexible substrate

Abstract

Recently, eutectic gallium indium (EGaIn) has been actively investigated towards stretchable and wearable electronic devices with the aid of high fluidity, high electrical conductivity and low toxicity. However, high surface tension along with spontaneous oxidation makes it difficult to realize fine patterning below ∼10 μm, thus physical molding into an elastomeric mold is thought to be a unique solution. Here, we present a novel manufacturing technique that enables EGaIn patterns of single-digit micrometer width without using a guide mold for the first time. First, a custom direct printing setup is constructed with a laser displacement sensor, a 3-axis motorized stage, and electronic pressure regulators to enable continuous and uniform printing of EGaIn by feedback control of the distance between the dispensing needle and the substrate. With the custom direct printing setup, a 120μm wide linear pattern is printed on an Ecoflex, platinum catalyzed silicone elastomer. To enable a single-digit micrometer pattern, the initial printed line is stretched, frozen with deionized (DI) water, and then transferred to an unstretched Ecoflex substrate. Upon gentle heating after the pick-and-place of the EGaIn line frozen with DI water, only the stretched EGaIn line is left on the new Ecoflex substrate. Aforementioned pick-and-place transfer of the stretched EGaIn frozen with DI water is cascaded multiple times until a target width is obtained. With the proposed idea, a 2μm wide linear pattern, 60-fold reduction with respect to the initial dimension, is acquired. For practical applications, strain and tactile sensors are demonstrated with width-reduced EGaIn patterns.