The Interface Technology Thrust is focused on two different electrode technologies; one flexible to match the curved contour of the retina and the second penetrating to reach deep within the brain to the hippocampus. The use of parylene as a flexible material to serve as the interface between electronic and neural circuits in retinal implants is driven in part by the material’s proven biocompatibility (it is one of few polymers licensed as USP Class VI), its strength and flexibility (Young’s modulus ≈ 4 GPa), its conformal pinhole-free room-temperature deposition, its low dielectric constant (≈ 3), and its high volume resistivity (> 1016 Ω-cm). Parylene can also be manipulated using standard microfabrication techniques such as reactive-ion etching. A monolithic parylene-based system is under development for the retinal testbed, comprising novel high lead count and high density dual layer flexible electrode arrays, a chip-level integrated interconnect (CL-I2) drop-chip technology for chip integration and packaging and flexible radiofrequency (RF), inductively-coupled coils for power and data transmission. Implantation studies proving that these technologies are robust under typical surgical conditions are underway as well as extensive electrochemical and electroplating tests on electrode surfaces that show that the physical constraints of electrode size do not necessarily place limits on their overall surface area.
)-based microelectrode arrays can be used for conformal recordings of neural activity, stimulation of neurons and neurochemical recordings in the rat and monkey brain. Over the last 3 years, new conformal microelectrode arrays have been fabricated using photolithographic methods coupled to the use of ceramic substrates. The thin, 37 – 125 micron ceramic substrates are 1) inexpensive, 2) inert to the CNS, 3) stronger than more standard glass or silicon substrates and 4) easy to pattern and mass fabricate. We have fabricated 4 new conformal designs with 4 or 8 recording sites per microelectrode, which are capable of recording 1-2 single units per recording site. In addition, we have recently fabricated “dual-sided” electrodes that have double recording density for up to 16 electrode recording sites in the same recoding area as the 8 site conformal electrodes. We have 1) recorded single unit neuronal activity for up to 6 months in the hippocampus of freely moving rats, 2) carried out stimulation and recording of cells in the hippocampus of awake rats and 3) recorded single units from the nonhuman primate hippocampus, using a specially designed deep recording electrode for the primate brain. We have also developed a flexible shank electrode in conjunction with Ad-Tech Medical Instrument Corporation for future Phase I studies in patients with temporal lobe epilepsy. Moreover, we will continue the development of a conformal array using multiple microelectrode tips that are interconnected with Parylene or polyimide flexible interfaces to achieve the necessary recording density for the hippocampal recordings in the Cortical Prosthesis Testbed.