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

Eutectic gallium indium (EGaIn) is actively investigated toward wearable and stretchable electronic devices due to the high fluidity, high electrical conductivity, and low toxicity. However, high surface tension along with spontaneous oxidation makes fine patterning below approximate to 10 mu m challenging. In this paper, a novel manufacturing technique that enables EGaIn patterns of single-digit micrometer widths on planar elastomeric substrates is presented. First, a custom direct printing setup is constructed for continuous and uniform printing of EGaIn by feedback control of the distance between the dispensing needle and the substrate. With this setup, a 120 mu m wide linear pattern is printed on the Ecoflex, a stretchable elastomer. Then, the initial printed line is stretched, frozen with deionized water, and transferred to an unstretched Ecoflex substrate. Upon gentle heating after the pick-n-place of the EGaIn line frozen with deionized water, only the stretched EGaIn line is left on the new Ecoflex substrate. The aforementioned pick-n-place transfer of the stretched EGaIn frozen with water is cascaded multiple times until a target width is obtained. Finally, a 2 mu m wide linear pattern, 60-fold reduction with respect to the initial dimension, is acquired. For practical applications, strain and tactile sensors are demonstrated by width-reduced EGaIn patterns.