On the stability of CMOS ICs with plasma-enhanced silicone encapsulation for active implantable neurotechnology: 4.3 years of accelerated life testing

The long-term stability of polymer-encapsulated CMOS integrated circuits (ICs) is essential for mm-size active implantable medical devices (AIMDs), where hermetic packaging is impractical. Validating robust, biocompatible, implantable IC encapsulation is a prerequisite for chip-scale polymeric bioelectronic implants. This study presents the first long-term validation of a design strategy combining medical-grade silicone, plasma surface treatment for adhesion, and foundry-manufactured CMOS ICs. These ICs feature a perforated shield layer (top metal layer) and a double-layer wall-of-vias to reduce moisture ingress and mechanical delamination. Test structures with silicon oxide/nitride passivation were encapsulated using implant-compatible processes and immersed in saline under accelerated ageing conditions (47 °C, 67 °C, and 87 °C) for up to 4.3 years, under DC and biphasic biases. Throughout the study, electrochemical impedance spectroscopy (EIS) showed no insulation failures. Minor visual corrosion was confined to wire bonds and solder pads, with no correlation to electrical degradation. These results demonstrate the robustness to biofluid exposure of modern IC passivation when combined with well-adhered silicone encapsulation. To our knowledge, this is the longest and most comprehensive accelerated ageing study of its kind, and the first to establish a scalable, industry-compatible encapsulation method for implanted ICs. Our findings provide critical evidence supporting the integration of CMOS ICs into next-generation bioelectronic implants.