Skin safety test in electrowetting application: a preliminary study

In the last ten years, the market for wearable technologies has increased significantly. Existing commercially available multisensor systems, such as intelligent devices and activity trackers, are able to acquire physiological signals like heart rate and oxygen saturation percent level. Nowadays, the integration of chemical biomarkers into wearables, particularly in fitness applications, represents a new market share. However, the development of sweat-sensing devices for monitoring hospitalized patients is still an open challenge. The European project ‘Sentinel’ aims to develop a wearable sweat sensor patch for e-health monitoring of hospitalized patients based on the electrowetting principle. This paper presents a preliminary study regarding skin safety testing for this electrowetting application. The tests were conducted on ten skin samples from five Wild-Type mice. An ad-hoc gold-based electrode representing the electrowetting configuration was realized. The skin impedance and current flow were assessed with a signal generator, a milli-amperemeter and an oscilloscope. The results show that the skin impedance and current depend on the voltage applied to the electrodes, as well as on the amount of liquid inside the sample; on the other hand, the results do not depend on the applied external force, representing the clamping force on the sweat sensor patch. In particular, the impedance values are in the range of 89 kΩ-350kΩ, while the current is always lower than 25 μA. Linear regression shows how for 60, 80 and 100 V, we obtain a max current of 75 μA, which is lower than the safety level required by international standards and recommendations.