Factors influencing the biocompatibility of insertable silicon microshafts in cerebral cortex

Abstract

Insertable microelectrode arrays can be used to activate neurons or to sense neural signals for use in prosthetics. The relationship of the microelectrodes to the neurons is determined by random alignment and by biocompatibility. Issues that determine the biocompatibility of insertable microelectrode arrays were investigated. Arrays were implanted into the cortex of rabbit brain and fixed to the skull. Following six-month survival, neuron density as a function of distance from the shafts of the arrays was measured to assess destruction of neurons. Results from a limited number of tests indicated that there was minimal tissue response along the sides of the shafts when shafts were well sharpened, had sufficiently small tip angles, and were clean. Tissue was usually more reactive at the tips of the shafts. It was concluded that silicon microshafts of appropriate shaft and tip design were biocompatible along the sides of the shaft, but that relatively severe reactions could be anticipated at the tips. Recording or stimulation sites should be located away from the tips on the sides of the shafts for better coupling with individual neurons. Measurement of neuron density as a function of distance from the shafts was a sensitive and quantitative technique for assessing biocompatibility. Additional measures such as glial density as a function of distance from the shafts, and incidence of microhematoma formation were proposed.

Electrochemically controlled release of dexamethasone from conducting polymer polypyrrole coated electrode

“…The chronic application of these devices has been limited by their unpredictable failure over extended periods. Although the precise mechanism for this failure is not completely known, it is suggested that an undesirable brain tissue reaction to the device leads to the formation of a glial scar which then serves as a barrier to neural signal transduction [1][2][3][4][5][6][7][8].…”

A lithographically-patterned, elastic multi-electrode array for surface stimulation of the spinal cord

“…A subset of these prostheses communicates with neural tissue via penetrating multiple-electrode arrays (MEAs), which can provide highly specific and robust activation of the targeted neurons (Branner et al, 2001;Hillman et al, 2003;McDonnall et al, 2004;Tyler and Durand, 2002). Multi-electrode NPs that penetrate neural tissue, however, have been shown to incur acute and chronic damage, which in turn can result in degradation of both the interfaced tissue and the implanted device (Edell et al, 1992;Polikov et al, 2005;Schmidt et al, 1993). There has been a variety of successful efforts to minimize the adverse effects of MEA invasiveness, including those using specialized device geometries and protective surface coatings (He and Bellamkonda, 2005;Holman G., 2002;Naples et al, 1988;Zhong et al, 2001).…”

Implanted intracortical electrodes as chronic neural interfaces to the central nervous system

“…The shape and size of the probe can be customized by the boron doping step for different uses as well. For example, to reduce risk of more injury during insertion, the tip of the shanks can be made sharp by adjusting the boron diffusion so that it is extremely shallow at the tip and then tapers at an angle less than 10 degrees [48,18]. Figure 6: (Left) A scanning electron micrograph image of the Utah Electrode Array.…”

“…Some studies suggest that the signal loss is attributed to glial scarring (seen in Figure 8) [83,48,84], yet others suggest that the prolonged immune response results in loss of neurons in a so called "kill zone" [85,48]. While it is still an open question as to what the actual cause is, both of these theories seem promising, yet both are caused by the immune response of the body.…”