On the reliability of non-hermetically encapsulated ceramic printed circuit boards as bridging elements in neural implants.

INTRODUCTION

Robust, biocompatible ceramic electrical interconnects are essential for bridging the gap between macro-scale components, such as helically wound leads and connectors, and thin-film polyimide electrodes in neural interfaces. While screen-printed thick-film structures have been utilized for decades, there is a need to broaden the design space and improve reliability through thin-film techniques. This study evaluates various surface configurations to identify the optimal combination of mechanical adhesion, electrical insulation, and functional longevity.

METHODS

All samples were fabricated on 96% pure alumina (Al2O3) substrates. Mechanical stability was evaluated via tensile shear tests, comparing thin-film platinum (Pt) with different adhesion promoters (Si x N y , SiO2), laser-patterned Pt thin-films, and bare Pt thin-film against reference Al2O3 substrates. Electrical insulation was assessed by performing electrochemical impedance spectroscopy and DC resistance measurements for degradation monitoring. The evaluated layer stacks with PDMS encapsulation included bare thin-film Pt, thick-film Pt/Au with Overglaze, a SiO2 layer, a Si x N y layer, and a layer of pulsed laser deposition (PLD) Al2O3. All samples underwent accelerated aging in phosphate buffered saline (PBS) at 60°C (acceleration factor ∼ 4.92 vs. 37°C).

RESULTS

Tensile shear strength decreased over the aging period across all groups. However, the integration of adhesion promoters increased mechanical stability compared to bare sputtered Pt, with Si x N y coating bringing adhesion levels close to the Al2O3 reference samples. Electrically, Si x N y -, PLD-Al2O3, and laser-patterned samples maintained higher insulation impedances over time than PDMS on bare Pt and SiO2. DC resistance measurements indicate good capabilities in protecting the conductors from degradation.

DISCUSSION: The findings demonstrate that the addition of adhesion promoters enhances both the longevity of the mechanical bond and stability of the electrical insulation in ion-rich environments. These results provide guidance for selecting optimal ceramic interconnects for chronic peripheral-nerve implants, balancing reliability, manufacturability, and cost.