Abstract
We report a spray deposition technique for patterning liquid metal alloys to form stretchable conductors, which can then be encapsulated in silicone elastomers via the same spraying procedure. While spraying has been used previously to deposit many materials, including liquid metals, this work focuses on quantifying the spraying process and combining it with silicones. Spraying generates liquid metal microparticles (~5 μm diameter) that pass through openings in a stencil to produce traces with high resolution (~300 µm resolution using stencils from a craft cutter) on a substrate. The spraying produces sufficient kinetic energy (~14 m/s) to distort the particles on impact, which allows them to merge together. This merging process depends on both particle size and velocity. Particles of similar size do not merge when cast as a film. Likewise, smaller particles (<1 µm) moving at the same speed do not rupture on impact either, though calculations suggest that such particles could rupture at higher velocities. The liquid metal features can be encased by spraying uncured silicone elastomer from a volatile solvent to form a conformal coating that does not disrupt the liquid metal features during spraying. Alternating layers of liquid metal and elastomer may be patterned sequentially to build multilayer devices, such as soft and stretchable sensors.
Highlights
Flexible, soft, and stretchable electronics find applications in sensors, actuators, energy harvesters, and antennas
The compressed region is clearly metallic and highly reflective, while the uncompressed region is still primarily a collection of individual particles, approximately 1 μm in average diameter. This experiment confirms that particles of larger size require less force to rupture than smaller particles, yet size alone cannot account for the behavior seen with spray coating liquid metal
This sample was prepared on a glass slide with a thin layer of polyvinyl alcohol (PVA) spin-coated on the surface to allow for easy detachment of the final device
Summary
Soft, and stretchable electronics find applications in sensors, actuators, energy harvesters, and antennas. This issue has been partly addressed by applying electric fields between the metal and substrate to promote adhesion [35,36] Another approach is to disperse liquid metal particles in a polymer matrix (e.g., silicone) to make a printable ink [37,38] though these materials requires some post-processing such as mechanical force or freezing to “sinter” the liquid metal particles and form a conductive path [39,40]. Since not all the metal passes through the stencil, there is some waste, yet most of the metal blocked by the stencil can be recovered This technique has been used successfully to fabricate soft electrodes [51,52,53], sensors [54,55], energy harvesters [56,57,58] and antennas [59] Motivated by these applications, this study examines how it forms robust electrical contacts, especially in comparison to inkjet processing.
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