Reactive-accelerated-aging soak testing of thin-film polyimide layers treated with an oxygen plasma and chemical soaking.

Reliable bonding between polyimide layers is critical for the long-term performance of thin-film neural interfaces, yet the long-term durability of common adhesion methods under physiologically relevant conditions remains unclear. This study uses reactive-accelerated-aging (RAA) soak testing at 87°C to compare the ability of surface treatments based on oxygen plasma and KOH/HCl soaks to enhance polyimide-to-polyimide adhesion. Peel testing was performed as a function of soak time and strip width to quantify degradation behavior. Oxygen plasma-treated samples exhibited high initial adhesion (~262 mN/mm) but degraded rapidly in RAA, with bond strength decreasing linearly and failing within 3 to 6 days depending on width. The width dependence indicates edge-driven degradation, for which a predictive model of adhesion loss was developed. In contrast, KOH/HCl-treated samples showed lower initial adhesion strength (~150 mN/mm) but minimal degradation over the test duration and corresponded to an estimated minimum 40-fold improvement in bond lifetime. These overall results from this study demonstrate that plasma-based bonding of polyimide films is highly susceptible to oxidative degradation, while chemically reconstituted interfaces provide significantly improved stability that could be beneficial for chronic neural-implant applications.