MicroProbes for Life Science

This FMA has 3 mm long platinum-iridium microelectrodes and a 5 cm long helical cable for flexibility.

For over 50 years, Neuroscientists have routinely used metal microelectrodes inserted into the cortex and spinal cord to record and electrically stimulate neural elements. During this time, many electrode designs, ranging from single or bundled micro-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 micro-drives. As neuroscience research evolves to the study of large populations of cells in chronic rather than acute experiments, more sophisticated technologies must be employed to provide multi-electrode systems that can satisfy a diverse scope of experimental paradigms. Chronic experiments, which are conducted over months or even years, will require the use of intracortical microelectrodes for reliable neural interfaces 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 may have regular or irregular electrode-to-electrode geometric spacing, with multiple electrode depths that can stimulate or record from neurons without causing tissue damage or deterioration of the electrodes, are becoming essential tools. Even in the peripheral nervous system, 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 use of electrode arrays maintained in a stable mechanical position relative to the associated neuronal structures.

We have commercialized this innovated Floating Microelectrode Array (FMA) whose design permits the mixing of electrode types, impedance values, irregular electrode spacing, arbitrary electrode lengths, and electrode metals 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 performed in a differential mode, requiring a reference electrode, which typically has an impedance value that is required to be an order of magnitude less than the recording electrodes. “Ground” or “common” electrodes are also required to be included in both recording and stimulation multi-electrode arrays. Also, it is often desirable 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 MicroProbes for Life Science have been implanted in animals for periods of up to 3 years and exhibited single unit activity. Our FMA design incorporates solid core conductors instead of silicone technology for several reasons. First, as a result of our initial research with the Visual Prosthesis Program at the Illinois Institute of Technology, (directed by Dr. Phil Troyk), an electrode design was required that could withstand indefinite stimulation without compromising the metal conductor or the integrity 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 Tech required floating microelectrode array designs that would accommodate electrode lengths up to 8 mm. Researchers there 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.