An e-Body Painting by Printable Liquid Metal for Biometric Measurement

In non-invasive electromyogram (EMG) measurement devices, hydrogel electrodes attached to the target muscle are wired to a wireless transmitter thorough cables. However, shaking of the cables and the transmitter is picked up as noise, making it difficult to obtain EMG when the body movement is intense. In this study, we developed an e-body painting, a direct body painting of liquid metal, to enable EMG measurement under large deformation of muscles or intense physical activity. The liquid metals painted on bodies work as both low-skin-impedance electrodes and wirings. The wirings formed on a body surface can place the transmitter at a location where it is less likely to be shaken by body movements. We used oxidized Gain (O-GaIn) as the body painting.[cite] Although normal GaIn is a biocompatible room temperature liquid metal, it is difficult to apply to the skin due to its high surface tension. On the other hand, O-GaIn can be applied to the skin because its wettability and viscosity are improved by the inclusion of solid GaOx in the material. O-GaIn electrodes were formed on the biceps brachii muscle, and the O-GaIn wiring was extended to the back of the shoulder, which was less affected by arm movement. In the wiring area, an elastic insulating sheet that adhered to the skin to avoid contact of O-GaIn with the skin. The very high conductance of O-GaIn enabled stable skin impedance measurements in the following three conditions: (1) With the arm extended; (2) With the arm bent; (3) With the arm actively bent and extended.<br/>Remarkably, the skin-impedance of O-GaIn electrodes was lower than that of the hydrogel electrode. This low impedance of O-GaIn resulted in lower noise level in EMG measurement using e-body painting than commercial hydrogel electrodes.<br/>This work was partially supported by JST, PRESTO Grant Number JPMJPR20B7, Japan.