Wireless implants for interaction with the cortex have developed rapidly over the last decade and increasingly meet demands of clinical brain-computer interfaces. For such applications, well-established technologies are available, suitable for recording of neural activity at different spatial scales and adequate for modulating brain activity by cortical electrical stimulation. The incorporation of recording and stimulation into closed-loop systems is a major aim in active, fully implantable medical device design. To reduce clinical long-term implantation risk and to increase the spatial specificity of epicortical recordings and stimulation, micro-electrocorticography is a promising technology. However, currently there is a lack of implants suitable for chronic human clinical applications that utilize micro-electrocorticography and possess closed-loop functionality. Here, we describe the clinical importance of cortical stimulation, give an overview of existing implants that use mainly epicortical recording methods, and present results of a closed-loop microelectrocorticography system developed for clinical application within a collaborative framework. Finally, we conclude with our vision of future design options in the field of neuroprosthetic devices.
Thin-film-based electrodes used to interact with nervous tissue often fail quickly if used for electrical stimulation, impairing their translation into long-term clinical applications. We initiated investigations about the mechanical load on thin-film electrodes caused by the fact of electrical stimulation. Platinum electrodes of Ø 300μm on a polyimide carrier were subjected to approximately 50 000 asymmetrical, biphasic stimulation pulses in vitro. The electrode's surface was investigated optically by means of white-light interferometry. The structural expansion for the metallic surface subjected to stimulation was measured to reach roughly 30%. The study points towards a failure mechanism of thin-films being of mechanical nature, inherent to the unavoidable electrochemical processes involved (change in lattice constants) during electrical stimulation at the electrode's surface. Based on further scientific facts, we set 3 hypotheses for the exact mechanisms involved in the failure of thin-films used for electrical stimulation, opening a new door for research and improvement of novel neuroprosthetic devices.
The variety of "`ready-to-use'" implantable recording and stimulation systems commercially available for neuroscience is very limited and fabrication of custom made implants is commonly considered expensive and time consuming. We present a circuit design that allows cost efficient and fast translation of available components into fully wireless implants. As demonstration fully wireless implantable bidirectional neural interfaces are presented which are made of commercial off-the-shelf components (COTS) only. It is demonstrated that they are competitive to currently available state-of-the-art systems regarding size and performance.
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