Analysis of gold-silicone films for the electrical interface of soft neural implants

Neurodegenerative diseases such as Alzheimer's disease a ect an increasing number of people in our ageing population exceeding annual costs of 220 billion US$. Neuromodulation of glial cells is a promising approach to slow down or stop the progress of this disease. However, the treatment is limited by the lack of appropriate tools to probe the nervous system. The mechanical properties of platinum-iridium or titanium electrodes, which represent the actual neural interface, di er by orders of magnitude from that of soft neural tissue leading to brosis at the tissue-implant interface. Recent research has shown that soft gold/silicone electrodes remain conductive beyond 60% strain, which is not understood so far. For this purpose, we fabricated gold layers on crosslinked nanometre-thin silicone lms using molecular beam deposition. We applied electron microscopy, contactangle measurements and nanoindentation tests to analyze the gold/silicone electrodes. Our analysis manifested linked gold clusters of nanometre size and a network of cracks evolving with increased gold deposition. Four-point conductivity measurement exhibit a percolation threshold at a nominal gold thickness of 60 nm. Nano-indentation studies revealed an inverse relation between the adhesion force and nominal gold thickness. Pull-o forces of (75 ± 8) and (70 ± 7) nN were found for 30 and 60 nm-thin gold, respectively. Contact-angle measurements revealed a hydrophobic behavior characterized by a Young's angle of (103 ± 3) degrees, as known from bare silicone. Therefore, we could conclude that the surface contained mostly silicone and gold migrated into the elastomer and, therefore forms a soft electrode/neural tissue interface