While clinical applications are close to being ready for market, significant challenges still exist. For example, electrode arrays currently lose their efficacy after a period of several months due to gliosis, whereby cells encapsulate the electrode arrays in an attempt to reject them from the brain. Neural prosthetic devices will need to retain their effectiveness over a period of years, if not decades, in order to ethically justify the significant risk associated with the implantation surgery. To address this serious problem, some researcher groups are developing glial inhibitors that can be delivered into the brain via specialized electrode arrays. Other labs are exploring the possibility of using the summed activity of large populations of neurons, which is available much longer than single-unit activity.
Additionally, extant electrode arrays are fixed to the skull, a practice that results in small movements of the electrode with respect to the neurons being recorded. To address this, one research group is currently developing a moveable probe technology that will automate the process of finding and tracking optimal recording sites over time.
In addition, the surgical risk also requires that such a device will contribute to a substantial increase in the subject's quality of life. For this to occur, a neural prosthetic device will need to provide highly accurate control of either a robotic arm or the subject's own limbs. There are disagreements over how many neural recordings must be made to derive this level of control, with estimates ranging from a few dozen to several hundred neurons. Of course, this question will depend crucially on the specific design characteristics of the prosthetic systems that will be controlled via the neural interface.
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