Thin-Film Encapsulations for Compliant Implantable Bioelectronics: Advanced Materials and Characterisation Methods Based on Bioresorbable Magnesium Permeability-Sensing Structures

The next-generation bioelectronic implants will be microfabricated, soft and compliant; they need to be protected from the body environment through encapsulations that must guarantee simultaneously hermeticity, biocompatibility, mechanical compliance, compatibility with microfabrication processes, and long-term reliability. This still represents an unsolved challenge: standard biocompatible barrier technologies based on rigid Ti/silicone capsules do not answer such needs. Advanced hermetic barriers should instead rely on thin-film encapsulations (TFE), based on organic or inorganic films. However, water permeation and mechanical defects are prime drivers for TFE failure, especially for <i>in vivo</i> and flexible implantable systems, so a universal solution is still missing. Conventional strategies rely on multilayer architectures, consisting of alternating dyads made of organic and inorganic layers. The accurate evaluation of their barrier performances is challenging because standard measuring systems display too high sensitivity limits (~ 10-5 g/m2/day) which cannot detect the ultra-low permeation capabilities of the coatings, and are not suitable for flexible microsystem formfactors. We present here for the first time a comprehensive accelerated-aging method to quantify ultra-low permeability of TFE engineered for bioelectronic micro-devices. The method relies on bioresorbable magnesium (Mg) thin films. Corrosion of Mg induced by water diffusion through the barrier coating is real-time monitored in several testing conditions, including exposure to wet air, soaking in a phosphate buffer saline (PBS) solution that mimics the body biofluids (in vitro), and in biological tissues (in vivo). The electrical properties of Mg are then monitored and analysed. High temperatures for in vitro tests are chosen to perform accelerated aging whereas specific novel analytical/computational models are used to extract from the measurements the water transmission rates (WTR) of the barriers. The advantages of this approach are that Mg does not need a fully inert atmosphere and it can be easily integrated in the microfabrication processes of implantable bioelectronic devices. Therefore, the proposed characterization strategy answers a long-term need to assess reliably and universally thin-film hermeticity in situ, and for any types of conformal TFE for bioelectronic systems.