Introduction

Before electrode arrays can be used as permanent cortical implants with the purpose of restoring useful vision, it must first be determined whether they are biocompatible with neural tissue. Determination of an optimal implantation technique is also essential. Recent studies have been performed in order to meet these goals.

Implantation Technique and Overall Biocompatibility in Rabbits

David J. Edell, Vo Van Toi, Vincent M. McNeil, and Lloyd D. Clark performed a study in which they studied the basic biocompatibility of various implant techniques in rabbit cortex. They determined that a chisel-point design was the best compromise in electrode shape since it caused minimal tissue damage, yet could still be machined on multishaft arrays. Since a sharp tip is essential in reducing tissue damage, it was necessary to find a sharpening agent that could be completely removed since even a monolayer of residue could lead to an inflammatory reaction. Deionized water was used since it functioned as an adequate polishing device without leaving any residue. Electrode tips must also be free of defects in order to slip through the dura without causing the cortex to dimple. Finally, axial alignment is extraordinarily important since if the electrodes are not inserted perpendicular to the tissue then a large swath of slashed tissue can be generated.

The silicon arrays were left in place within the rabbit cortex for six months. Afterwards, the neuron density as a function of radius from the center of the shafts was used as the primary means of assessing damage. The above researchers concluded that the electrode array showed long term biocompatibility since normal brain tissue was observed within microns of the surface of the shafts. Occasionally, healthy neurons were even in contact with the encapsulation layer of the shaft.

Further research needs to be completed in order to gain a more complete understanding of different implantation and electrode variables. The optimal velocity of insertion needs to be studied since although rapid insertion can compensate for poor tip design thereby limiting tissue damage, slow insertion generates minimal vibration and mechanical shock. The design of the electrodes also needs further investigation into areas such as tip radius, tip angle, shaft width and thickness, shaft roughness, and surface chemistry.

Biocompatibility of Electrode Arrays in Felines

Susan Schmidt, Kenneth Horch and Richard Norman tested arrays in three configurations for both short and long term implants. Arrays were either uncoated, coated with polyimide placed directly on the silicon, or coated with polyimide after being treated with aluminum chelate primer. Each variety of array was implanted in trials of 6 months and 24 hours. The limited appearance of edema and hemorrhage in the 24 hour trials indicates that the trauma of insertion was tolerated fairly well by the neural tissue. No statistical differences were found between the different types of arrays in the short term trials. The restricted tissue reaction present in the 6 month implants implies that the tissue was able to recover from any initial damage due to implantation. Also notable is the fact that the primed polyimide coated array produced significantly more tissue reaction in terms of macrophage reaction, gliosis, and capsule formation. This is most likely due to a tissue reaction to the aluminum oxide in the primer. The researchers concluded that the overall tissue reaction to the implants was modest and the structure of the array would be potentially acceptable for use in long-term cortical implants.

Horch, K., Normann, R.A., and Schmidt, S. (1993) Biocompatibility of silicon-based electrode arrays implanted in feline cortical tissue. J Biomed Mater Res, 27(11), pp. 1393-1399.