Shah Idil, Ahmad; (2025) Multi-Anchored Membranes: A Theory of Silicone Encapsulation of ICs for Implantable Neuroprostheses. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The next generation of neural prostheses demands devices that are smaller, possess higher channel counts, and can interface directly with neural tissue. Traditional hermetic packaging approaches are increasingly unsuitable for such systems. Silicone encapsulation of integrated circuits (ICs) offers a promising alternative, enabling chip-scale implants to operate in direct contact with target tissues. This thesis presents experimental validation of this approach, demonstrating that silicone (PDMS) protects implanted electronics not by acting as an impermeable moisture barrier but through the formation of a robust adhesive interface. This interface prevents the condensation of liquid water at the device surface, thereby preserving passivation integrity and maintaining a high leakage impedance between conductors. Aluminium interdigitated comb (IDC) test structures were fabricated on top of three common IC passivation materials: silicon oxide, silicon oxynitride, and silicon nitride. The test structures were then encapsulated in medical-grade silicone rubber (Nusil MED-6015) to form test samples. The samples were aged in phosphate-buffered saline at 67 °C for one year in a custom-built accelerated ageing system: the ALTA (“Accelerated Life-Testing Apparatus”). Electrochemical impedance spectroscopy (EIS) revealed a characteristic initial drop in low-frequency (f = 10 mHz) impedance magnitude from ~10¹² Ω (dry state) to ~10⁹ Ω (wet state); this was attributed to hydration of the polymer/IDC interface. Unexpectedly, a recovery of impedance was observed beyond one month of ageing, with values trending upwards log-linearly over time. The mechanism of this “impedance recovery” remains incompletely understood; possible explanations include osmotic pumping effects across the PDMS membrane, dynamic equilibrium processes at the adhesive interface involving bond breakage and reformation, or thickening of the aluminium passivation. Nevertheless, over the full ageing period, the system stabilised to a sheet resistance of ~10¹⁴ Ω/□. No statistically significant performance difference was observed between the passivation batches in terms of impedance magnitude, though there were differences in the kinetics, with silicon nitride reaching equilibrium the fastest. Finally, the chemical nature of the adhesive interface was investigated. ¹H-NMR analysis of the MED-6015 silicone indicated an excess of unreacted vinyl groups, supporting a structure of free chains. Surface radicals or silanols generated on the passivation layer during air-plasma treatment may form covalent bonds directly with dangling hydrides or with siloxane chains. The density and nature of these reactive sites are likely influenced by the plasma conditions and the PECVD process parameters used to deposit the passivation layers. We propose that PDMS encapsulation forms an extension of the device surface itself — a “multi-anchored membrane” — covalently tethered via siloxane bonds to plasma-activated sites on the IC surface, a protective structure stable in the implant environment. Thus, this work establishes silicone encapsulation as a viable, stable, and adaptable strategy for future chip-scale neural implants.

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