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| The FMA above ha 3 mm long platinum-iridium microelectrodes and helical for flexibility that is 5 cm long.
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Neuroscientists have routinely used metal microelectrdoes inserted into the cortex and spinal cord to record and electrically stimulate neural elements for over fifty years. During this time, many electrode designs ranging from single and bundled mirco-wires to sophisticated silicon probes have seen various successes in acute and chronic applications. For acute experiments many neuroscientists typically fabricate bundles of micro-wires and insert them into the cortex using microdrives. As neuroscience research moves towards studying large populations of cells in chronic rather than acute experiments, more sophisticated technologies must be employed to povide multi-electrode systems that can satisfy a diverse scope of experimental paradigms. Chronic experiments, which are carried out over months or even years, will require the use of intracortical microelectrodes for reliable neural inerfaces for both stimulation and recording paradigms. The need for arrays to have flexible design characteristics will be necessary to accommodate the varied experimental paradigms and animal models used among neuroscience researchers. Multi-electrode arrays that have regular and irregular electrode-to-electrode geometric spacing, with electrodes set at multiple depths that can record from and stimulate neurons without causing tissue damage or deterioration of the electrodes, are becoming essential tools for many neuroscience investigators. Even in the periphery, emerging studies are investigating arrays of electrodes, inserted into the spinal cord or nerve branches, with irregular electrode spacing, depth, and metal type, as a means of providing a more sophisticated artificial neural interface. Current research on neural prosthesis applications including cochlear nucleus stimulation for an auditory prosthesis, cortical stimulation for a visual prosthesis, and cortical recording for brain-machine interfaces all require using arrays of electrodes maintained in a stable mechanical position relative to the associated neuronal structures.
We have developed, tested, and are now offering the Floating-Micro-Electrode-Array(FMA) whose design permits the mixing of electrode types, impedance values, irregular electrode spacing, arbitrary electrode lengths, and electrode materials such as platinum-iridium and activated-iridium-oxide, within the same microelectrode array. There are many investigative paradigms that require electrodes contained within a single array to have a range of tips exposures, as often characterized by their impedance, or a variety of electrode shaft lengths. Sometimes recordings are done in a differentail mode requiring a reference electrode, which typically has an impedance value that is required to be an order of magnitude less than the recording electrdoes. "Ground" or "common" electrodes are also required to be withing arrays of both recording and stimulating multi-electrode arrays. Often it is desireable to implant an array along a sulcus, where some of the electrodes need to be much longer along the sulcus and shorter away from the sulcus.
Our arrays are fabricated from biocompatible materials: alumina ceramic, Parylene-C, noble metals (gold, and platinum/iridium(70/30%) or pure iridium), and medical implant grade silicone elastomers. Rigid microelectrode designs using the same materials also offered by Micro Probe have been implanted in several studies for periods of up to three years and exhibited single unit activity. Our FMA design is based on using solid core conductors instead of silicone technology for several reasons. First, as a result of our initial work with the Visual Prosthesis Program at the Illinois Institute of Technology, directed by Dr. Phil Troyk, an electrode design was requried that would stand up to indefinite stimulation without compromising either the metal conductor or the intergrity of the insulation interface. To date metalized silicone probes have not demonstrated sufficiently robust behavior to warrant long-term stimulation. Secondly, our work with Dr. Richard Andersen's laboratory at Cal Tach required floating microelectrode array desings that would accommodate electrode lengths up to 8 mm. They also expressed the desire to have electrodes with different lengths within the same array. We have worked with these groups and others to develop a very flexible array design that is also very affordable for most laboratories. |
