Liquid metal microscale deposition for soft and stretchable skin-like electronics : Providing a soft and gentle contact to living beings

Abstract: Skin-like electronics could provide a soft and gentle contact with living beings for perceiving and delivering the information of pressure, strain, temperature with higher spatial resolution and sensitivity than our skin, without disturbing the user. Gallium-based liquid metal (LM) is an excellent material for soft and stretchable skin-like devices, since it has high electrical and thermal conductivity, flowability, and self-healable capability. However, the LM patterning technique is not well developed and has challenges due to its high surface tension. This thesis presents several methods to advance LM patterning capability, targeting thin, homogenous, high-resolution, and high-density skin-like electronics. Especially, masked deposition is a versatile technique in microsystem fabrication but has been limited by the masking and deposition. Firstly, a mask material that is well suited for lift-off processing at high resolution has been sought for. In this thesis, we introduced the use of a water dissolvable sacrificial thin polyvinyl alcohol (PVA) film for LM masked deposition. The dyed PVA film could be cut by laser and the fabricated mask had a resolution down to 20 µm in width. Secondly, our previously demonstrated LM atomization has a large variation of the droplets size generated by the airbrush, which severely hinder the depositing uniformity at microscale. Sonication produced LM particles has a narrow size distribution, but the particles are insulating. We proposed a simple and effective technique to merge these particles by precise spraying, depositing a LM layer that is microscale thin (5 µm), smooth (area roughness, Sa=0.8 µm), and gas permeable. More importantly, it has conductivity similar to pure LM. With this technique, a skin-like high resolution sensor was fabricated, which is able to monitor the local wrinkle skin movement. Moreover, a high-resolution skin-like micro-heater was made and placed on the fragile and oval abdomen of Drosophila for localized ectopic genetic expression in its gut. In addition, printing LM based skin-like electronics on a 3D surface is also challenging. Hence, we developed two complementary techniques that could gently and conformally transfer a planar pattern to a 3-D surface: A PDMS-based flexible stamp that could transfer a 2-D pattern to a 3-D surface and hydro printing with PVA film as a medium. Using hydro printing, the circuit that was transferred on delicate plants could monitor their physiological information and control their growth.