Soft Electronics Manufacturing Using Microcontact Printing

Abstract

This work describes a microcontact printing (µCP) process for reproducible manufacturing of liquid gallium alloy-based soft and stretchable electronics. One of the leading approaches to create soft and stretchable electronics involves embedding liquid metals (LM) into an elastomer matrix. Although the advantages of liquid metal-based electronics have been well established, their mainstream adoption and commercialization necessitates development of precise and scalable manufacturing methods. To address this need, a scalable µCP process is presented that uses surface-functionalized, reusable rigid, or deformable stamps to transfer eutectic gallium-indium (EGaIn) patterns onto elastomer substrates. A novel approach is developed to create the surfacefunctionalized stamps, enabling selective transfer of LM to desired locations on a substrate without residues or electrical shorts. To address the critical needs of precise and reproducible positioning, alignment, and stamping force application, a high-precision automated µCP system is designed. After describing the approach, the precision of stamps is evaluated and EGaIn features (as small as 15 µm line width), as well as electrical functionality of printed circuits with and without deformation, are fabricated. The presented process addresses many of the limitations associated with the alternative fabrication processes, and thus provides an effective approach for scalable fabrication of LM-based soft and stretchable microelectronics.tissue damage, and impairment of natural motion, thereby providing increased robustness, safety, mechanical conformability, and compliance matching with biological tissues. As such, soft and stretchable electronics could have a transformative impact in wearable technologies for personal computing and healthcare, [1] implantable electronics, [2] and soft robotics. [3,4] In recent years, many different types of soft and stretchable electronics have been developed, as outlined in two recent review papers. [1,2] One of the leading approaches to realize soft and stretchable electronic circuits involves embedding gallium-based liquid metal (LM) alloy traces at desired spatial locations within an elastomer body. [5,6] Specifically, the binary alloy of eutectic gallium-indium (EGaIn: 75 wt% Ga and 25 wt% of In) and the ternary alloy of gallium-indium-tin (Galinstan) are both liquid at room temperature: they flow freely with negligible resistance (viscosity of EGaIn is about 1.9 mPa s [7] ) and accommodate changes in channel shapes, thereby maintaining electrical circuit functionality without failure even at large strains (>100%). The electrical conductivity of gallium-based liquid metal alloys (3.4 × 10 6 S m −1 for EGaIn, [8] which is about 1/16th of copper) is orders of magnitude higher than ionic liquids and conductive elastomers. These alloys are of low toxicity [9] and have very low vapor pressure, [8] making them ideal for wearable computing and health care applications.Uniquely, when exposed to even low levels of oxygen, a few nanometer thick,...

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