Towards Bio-Integrating Interfaces in Organic Neurotechnology Development.

Abstract

Penetrating electrodes allow investigators and clinicians direct access to the underlying neural pathway via stimulation and recording. While demonstrating acute reliability, chronic implants exhibit variability and limited reliability in recording performances and inflammatory tissue responses. Although flexible probe technology has been developed for many years, penetrating microscale microelectrodes made from flexible polymers tend to bend or deflect and may fail to reach their target location. In the first study, we investigate the use of an electronegative self-assembled monolayer (SAM) as a coating on a stiff insertion shuttle to carry a polymer probe into the cerebral cortex without deflection, and then detach the shuttle from the probe by altering the shuttle's hydrophobicity. The average relative displacement of PDMS and polyimide probes was (2.1±1.1)% and (1.0±0.66)% with the SAM and 100% and (26.5±3.7)% without the SAM, respectively.

While advances in technology are pointing to incremental improvements for solving this longstanding reactive tissue response, understanding the role of vascular disruption on implants have been limited by challenges in vivo. During insertion, the highly-regulated blood brain barrier is compromised leading to plasma release into the surrounding parenchyma and adsorbtion onto the electrode surface. In the second study, we investigate localized bleeding resulting from inserting microscale neural probes into the cortex using in vivo 3D multi-photon vascular mapping to explore an approach to minimize blood vessel disruption. An 82.8±14.3% reduction in neurovascular damage was observed when probes were inserted in regions devoid of major sub-surface vessels.

These findings combined with the growing literature emphasize novel penetrating electrode designs for improving chronic recordings. Here, we report an integrated composite electrode consisting of a carbon fiber core, poly(p-xylylene)-based thin-film coating that acts as dielectric barrier and is functionalized to control intrinsic biological processes, and a poly(thiophene)-based recording pad. The resulting implants are an order of magnitude smaller and more mechanically compliant with brain tissue than traditional recording electrodes, were found to elicit negligible chronic reactive tissue responses, and have excellent recording characteristics in acute and chronic experiments in rats. This technology establishes a new development path for highly selective and long-lasting stealthy neural interface materials and devices.