The emerging field of neuroprosthetics is focused on design and implementation of neural prostheses to restore some of the lost neural functions. Remarkable progress has been reported at most bioelectronic levels—particularly the various brain-machine interfaces (BMIs)—but the electrode-tissue contacts (ETCs) remain one of the major obstacles. The success of these BMIs relies on electrodes which are in contact with the neural tissue. Biological response to chronic implantation of Microelectrode arrays (MEAs) is an essential factor in determining a successful electrode design. By altering the material compositions and geometries of the arrays, fabrication techniques of MEAs insuring these ETCs try to obtain consistent recording signals from small groups of neurons without losing microstimulation capabilities, while maintaining low-impedance pathways for charge injection, high-charge transfer, and high-spatial resolution in recent years. So far, none of these attempts have led to a major breakthrough. Clearly, much work still needs to be done to accept a standard model of MEAs for clinical purposes. In this paper, we review different microfabrication techniques of MEAs with their advantages and drawbacks, and comment on various coating materials to enhance electrode performance. Then, we propose high-density, three-dimensional (3D), silicon-based MEAs using micromachining methods. The geometries that will be used include arrays of penetrating variable-height probes.
Microelectrode arrays (MEAs) act as an interface between electronic circuits and neural tissues of implantable devices. Biological response to chronic implantation of MEAs is an essential factor in determining a successful electrode design. Finding appropriate coating materials which are biocompatible and improve electrical properties of MEAs are among the main challenges. In this paper, we propose a novel, three-dimensional (3D), high-density, silicon-based MEAs for both neural recording and stimulation. Electrodes were fabricated using micromachining techniques. Geometrical features of these electrodes not only cause less tissue damage during insertion but also provide more contacts between the electrodes and targeted neural tissues. In order to achieve the proposed geometry, we introduce a novel masking method to coat variable-height electrodes with uniform and small tip-exposure. More importantly, compared to conventional techniques, the new masking method significantly improves process time and costs. This technique needs only one step masking and reduces the conventional masking steps from ten to three. In the next step, the active sites of the electrodes were coated with thin-films of molybdenum (Mo) and platinum (Pt) due to their ability to transfer between ionic and electronic current and to resist corrosion. Electrodes were characterized by scanning electron microscopy and impedance measurements. The average impedance of Mo and Pt electrodes at 1 kHz was 350 ± 50 kΩ and 150 ± 10 kΩ, respectively.
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