Silicone rubber and thin-film polyimide for hybrid neural interfaces — A MEMS-based adhesion promotion technique

Strong permanent adhesion between thin-film polyimide (BPDA-PPD) and silicone rubber (MED-1000) was achieved through deposition of a chemically-transitive intermediate adhesion promoting layer. Plasma-enhanced chemical vapor deposition (PECVD) of SiC and SiO₂ was used to grow a thin 50 nm layer directly on a 5 μm thin polyimide substrate. The deposition at low pressures permitted the fabrication of an adaptive covalent bond transition from sp2-hybridized carbon (in polyimide) towards the sp3 bonding in SiC, continuing to SiO₂ which provides a good bonding partner for one-component poly-dimethyl siloxane (PDMS). The fabricated laminates together with reference probes containing no adhesion promoting layer were subjected to intense accelerated aging at 125°C and 130 kPa (pressure cooker) over 96 hrs in phosphate buffered saline solution. While the reference polyimide-PDMS laminates failed just after 30 min in the pressure cooker, no failure was detected on samples using the proposed adhesion promoter technique. Mechanical loading of the samples resulted in cohesive crack formation at the polyimide, propagating across the bulk with no evidence of adhesive failure between any of the materials. The strong permanent adhesion brings the fabrication of hybrid neural interfaces one step forward, achieving the combination of thin-film manufacturing and PDMS.