Silicon-Based Microfabrication of Free-Floating Neural Probes and Insertion Tool for Chronic Applications

Abstract

Bidirectional neural interfaces for multi-channel, high-density recording and electrical stimulation of neural activity in the central nervous system are fundamental tools for neuroscience and medical applications. Especially for clinical use, these electrical interfaces must be stable over several years, which is still a major challenge due to the foreign body response of neural tissue. A feasible solution to reduce this inflammatory response is to enable a free-floating implantation of high-density, silicon-based neural probes to avoid mechanical coupling between the skull and the cortex during brain micromotion. This paper presents our latest development of a reproducible microfabrication process, which allows a monolithic integration of a highly-flexible, polyimide-based cable with a silicon-stiffened neural probe at a high resolution of 1 µm. For a precise and complete insertion of the free-floating probes into the cortex, a new silicon-based, vacuum-actuated insertion tool is presented, which can be attached to commercially available electrode drives. To reduce the electrode impedance and enable safe and stable microstimulation an additional coating with the electrical conductive polymer PEDOT:PSS is used. The long-term stability of the presented free-floating neural probes is demonstrated in vitro and in vivo. The promising results suggest the feasibility of these neural probes for chronic applications.

Hybrid multisite silicon neural probe with integrated flexible connector for interchangeable packaging

“…Additionally, alumina is biocompatible and chemically stable for chronic implantation [38]. Parylene or PI have also been previously used as insulators for other Si neural probes with flexible integrated interconnect cables [24], [26], but it significantly increased the thickness of the implanted portion of those probes when compared with ours. Although using PI as a passivation layer could facilitate the integration of our PI cable, the 500 nm thick alumina passivation layer used ensured that probe thickness was kept to the minimum (which would have otherwise increased if a 3-5 µm thick polyimide passivation layer was used).…”

“…The monolithic fabrication process described permitted the integration of a multi-shank multisite 64channel silicon (Si) neural probe with an 8 µm-thick highly flexible polyimide (PI) cable. When compared with previously published hybrid neural probes with flexible cables [23]- [26], [29], the one described here presents a significantly higher number of electrode sites and a thinner interconnect cable. Additionally, an open-ended connector pad was designed at the end of the flexible cable to permit the use of any desired printed circuit board (PCB) for probe interfacing, as long as a ZIF connector is present on the PCB side.…”

Hybrid Multisite Silicon Neural Probe with Integrated Flexible Connector for Interchangeable Packaging

“…Additionally, alumina is biocompatible and chemically stable for chronic implantation [43]. Parylene or PI have also been previously used as insulators for other Si neural probes with flexible integrated interconnect cables [33,34], but they significantly increased the thickness of the implanted portion of those probes when compared with ours. Although using PI as a passivation layer could facilitate the monolithic integration of our PI cable, the use of a 500 nm thick alumina passivation layer ensured that probe thickness was kept to the minimum (which would have otherwise increased if a 3-7.5 µm thick polyimide passivation layer was used as in [24,28,44]).…”

“…To facilitate wider dissemination and use, we propose here a new fabrication process for hybrid multisite silicon probes with monolithically integrated polyimide flexible interconnect cabling that allows interchangeable packaging options. By combining optimized fabrication processes for Si and PI, we fabricated several small-footprint multishank probes with a higher channel count (64 electrode sites) and thinner flexible polymer interconnect cabling (8 µm thick) than previously reported hybrid silicon/polymer probes [23][24][25]28,33,34]. By designing an integrated open-ended flexible connector pad that can mate with commercial zero-insertion force (ZIF) connectors on the PCB side, definitive packaging for these probes is avoided allowing great experimental flexibility.…”

Towards Long-Term Stable Polyimide-Based Flexible Electrical Insulation for Chronically Implanted Neural Electrodes

“…To investigate the long-term stability performance, test samples were designed and fabricated using microtechnology processes, which are also used for the fabrication of flexible neural implants [15]. The design of the test sample is shown in Figure 2.…”