Electrochemical Analysis and Numerical Simulations for Geometry Improvement of Wearable Electrodes for Iontophoretic Drug Delivery

Abstract This study focuses on conformable screen-printed Ag/AgCl electrodes for transdermal applications, such as drug delivery. More specifically, we are interested in avoiding the major side effects of iontophoresis (ITP) electrodes commonly reported: skin burns and irritation due to heterogeneous current distribution (hotspots) and parasitic electrochemical reactions (water electrolysis). Chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) help follow electrode degradation overtime using a three-electrode assembly in physiological conditions. CP curve profiles allow to identify a maximum electrode lifetime τ beyond which ITP becomes unsafe to use on skin. EIS highlights the formation of a passivation layer responsible for stopping the desired reactions and, therefore, the safe use of ITP electrodes. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy shows that the working electrode is fully passivated with AgCl and initiates water electrolysis. Additionally, a numerical tool is developed to visualize the spatial distribution of the current density on electrodes using the Finite Element program Comsol Multiphysics. The same setup as for electrochemical characterization is used to validate the numerical model by comparing computed and measured EIS spectra. Finally, two-electrode designs are compared, with either a plain or grid-shaped conductive layer to improve current distribution at the surface. CP results show no differences in the lifetime. Numerical simulations suggest that the 4-layer stack inhibits the effect of the grid-shaped layer. For both geometries, results reveal dangerous hotspots at the perimeter of the electrodes, intensified by the presence of a protective insulating layer commonly used in screen-printing.