Optical Monitoring of Water Side Permeation in Thin Film Encapsulation

Abstract The stability of long‐term microfabricated implants is hindered by the presence of multiple water diffusion paths within artificially patterned thin‐film encapsulations. Side permeation, defined as infiltration of molecules through the lateral surface of the thin structure, becomes increasingly critical with the trend of developing high‐density and miniaturized neural electrodes. However, current permeability measurement methods do not account for side permeation accurately nor quantitatively. Here, a novel optical, magnesium (Mg)‐based method is proposed to quantify the side water transmission rate (SWTR) through thin film encapsulation and validate the approach using micrometric polyimide (PI) and polyimide‐silicon carbide (PI‐SiC) multilayers. Through computed digital grayscale images collected with corroding Mg film microcells coated with the thin encapsulation, side and surface WTRs are quantified. A 4.5‐fold ratio between side and surface permeation is observed, highlighting the crucial role of the PI–PI interface in lateral diffusion. Universal guidelines for the design of flexible, hermetic neural interfaces are proposed. Increasing encapsulation’s width (interelectrode spacing), creating stronger interfacial interactions, and integrating high‐barrier interlayers such as SiC significantly enhance the lateral hermeticity.