This talk will discuss the use of liquid metals for wearable devices. Gallium-based liquid metal alloys are noted for their low viscosity, low toxicity, and negligible volatility. Despite the large surface tension of the metal, the alloy can be patterned into non-spherical 2D and 3D shapes due to the presence of an ultra-thin oxide skin that forms on its surface. The metal can be patterned by injection into microchannels or by direct-write techniques including 3D printing. Because it is a liquid, the metal is extremely soft and flows in response to stress to retain electrical continuity under extreme deformation. The ability of the oxide to reform instantaneously also allows the metal to self-heal in response to damage. Recently, we have explored the possibility of harnessing these properties to make soft electrodes for interfacing the skin.  EGaIn provides electrically conductive pathways and the lack of rigid components creates an extremely soft and deformable electrode, but unfortunately EGaIn has a resistive native oxide skin that can be problematic for sensing devices .  We control this oxide skin the through modification of the interfaced hydrogel.  As a case study, we created and tested electrodes for an electrocardiogram (ECG).  ECG electrodes require low impedance at biomedically relevant frequencies (1-50 Hz). Potentiostatic electrochemical impedance spectroscopy measurements show that capacitive effects at the hydrogel-EGaIn interface dominate the impedance at these low frequencies, yet can be reduced by utilizing embedded acidic and basic hydrogels that remove the native oxide skin.  The resulting devices fabricated have signal-to-noise ratios that exceed commercial ECG electrodes. The softness of these hydrogels can be modified without compromising the electrical properties to create truly soft electrodes.  Using liquid metal as an electrode for hydrogels represents a potential strategy of creating soft ‘bio-electrodes’ for various bioelectronic applications, e-skins, and next-generation soft robotics.

Liquid metals for wearable sensors

255th National Meeting (2018)

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