A comparative study assessing neural recording quality and inflammatory tissue response between stiff and flexible microelectrode arrays

Implantable microelectrode arrays (MEAs) are powerful tools for neurophysiological studies and hold tremendous potential as a key component of clinical neuroprostheses. However, their long-term efficacy is hampered by inflammatory responses to implantation. This study examines the impact of MEA substrate rigidity on neural recording quality and inflammatory tissue responses. We implanted stiff silicon-based and flexible polyimide-based MEAs with similar geometries in the deep brain region of the mouse striatum and performed weekly impedance and electrophysiological measurements over 4 weeks. The flexible MEAs demonstrated more reliable recording quality in terms of stable signal-to-noise ratio, single-unit yield, peak-to-peak amplitude, noise, and impedance, compared to the stiff probes, whose performance deteriorated over time. Endpoint histological analyses were performed to assess the glial scarring (Iba-1, GFAP), blood-brain barrier integrity (IgG), mechanosensitive ion channel expression (Piezo1), and neuronal population (NeuN and NF200) around the implant site. We found reduced microglial activation and IgG fouling within a 50-μm radius around the flexible MEAs compared to the stiff group. Additionally, a higher expression of Piezo1 was observed localized around the stiff MEA, correlating positively with increased Iba-1 and GFAP intensity. Our results show that flexible MEAs have improved device-tissue integration, indicated by lower inflammation levels and better recording stability. This research underscores the critical role of MEA substrate rigidity in influencing adverse tissue reactions, especially for deep brain regions. This understanding will improve our ability to design stable and reliable devices for chronic neural interface devices and beyond.