Bio-Electrical Impedance Analysis for Wrist-Wearable Devices

In this work we described results of the bio-electrical impedance analysis (BIA) algorithm implementation that does not require information about parasitic impedances values in a smartwatch structure, and skin contact impedances values. Only voltages and currents directly measured by BIA device are taken into consideration. It makes BIA process independent of complex hardware of smartwatches (including small size of the electrodes) and avoids additional factory mode calibrations in case of the minor structural changes. The applicability and accuracy of the method has been verified at circuit simulation for pre-commercial smartwatch prototype which has two electrodes embedded in control buttons with an ~0.3 cm2 area of each and two electrodes embedded into the bottom side with an ~1.5 cm2 area of each. The bio-electrical impedance errors were analyzed at variation of the parasitic capacitance between contact electrodes and BIA analog-front-end circuit and at variation of skin contact impedance magnitude up to 15 kOhm per 1 cm2 of the electrode area at 50 kHz of signal frequency. Such high magnitude of skin contact impedance covers the most extreme cases at low humidity, very dry or damaged skin, too weak or too hard touches by user.