Investigation of the role of sweat and strain on liquid metals as stretchable conductors

• Main Author: Goh, Eugene Chin Wei
• Other Authors: Lee Pooi See
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/148435
• Institution: Nanyang Technological University

The use of room-temperature liquid metals such as the Gallium-based alloys have been a key area of focus for many researchers given the promising capabilities in its usage for a multitude of industries ranging from flexible electronics, soft robotics, to healthcare devices. As such liquid metals provide low toxicity and high conductivity in the liquid phase, it makes it appropriate for applications even in the wearables industry. However, the role of human sweat combined with strain is a relatively new area and the corresponding effect on liquid metals have yet to be established. Here in this study, we fabricate a liquid-metal embedded hydrogel through the sonication of 50 wt% Galinstan (GaInSn) in an aqueous solution of polar 2-hydroxyethyl methacrylate (HEMA) monomers in the presence of a cross-linker. The hydrogel was subsequently UV-cured, leading to rapid free radical polymerization (FRP). Due to the passivating native oxide layer that forms in ambient conditions, the hydrogel remains effectively inert. Results show that even at maximum swelling of the elastomeric mould at approximately 16 hours soak time, the acidic condition of sweat is insufficient to rupture the oxide shell and the hydrogel has no significant change in resistivity. Interestingly, even when coupled with tensile force of up to 300% strain, the liquid metal remains intact in its spherical shape. This is contrary to existing literature whereby the oxide layer ruptures at strain rates of less than 100%. While friction-induced electrical conductivity was observed under certain conditions, the lack of conductivity for the majority of experiments could be explained by a layer of polymeric HEMA coating that has well-embedded the liquid metal particles. This hypothesis is supported by EDS mapping analysis indicating the presence of the polymer on the surface of the spherical liquid metal particles, thereby conferring strong mechanical protection in addition to the oxide layer shell.