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

Examples of microwire arrays and bundles. Left, microwire arrays (1) were used for cortical and thalamic implants and bundles (2) used for brain stem implants. Image reproduced from [33]. Middle, examples of two 4x4 electrode arrays sharing a 32-channel connector. Image reproduced from [34]. Right, an example of a bundle of “eight Ni-Chrome wires spun around one Pt/Irwire in a braiding type manual process” used for deep brain recording [35].
Recent developments in neural interfaces show that it is possible to have fine control of a robotic prosthetic by interfacing with the motor cortex of the human brain. Development of long term systems for this purpose is a challenging task with many different possibilities. Intracortical implants have shown the most promise in providing enough signal selectivity and throughput for complex control systems with many degrees of freedom. Intracortical systems generally fall into two categories: MEMS devices and bundle of wire systems. While both show promise, MEMS systems have been greatly popularized due to their reproducibility. In particular, the Michigan probe and Utah microarray are often used as a base for construction of more complex intracortical systems. However, these systems still carry many downsides. Their long-term viability is questionable, with mixed results. The effects of damage from implantation are still inconclusive and immune responses remain a problem for long-term use. While there is some promising research in the use of bioactive molecules and biocompatible materials to prevent immune responses, more controlled study is needed before intracortical systems become widespread. Here we review the literature surrounding these methods and their reported effectiveness.