Finite element analysis of the current-density and electric field generated by metal microelectrodes

Electrical stimulation via implanted microelectrodes permits excitation of small, highly localized populations of neurons, and allows access to featur es of neuronal organization that are not accessible with larger electrodes implanted on the surface of the brain or spinal cord. As a result there are a wide range of potential applications for the use of microelectrodes in n eural engineering. However, little is known about the current-density and e lectric field generated by microelectrodes. The objectives of this project were to answer three fundamental questions regarding electrical stimulation with metal microelectrodes using geometrically and electrically accurate f inite elements models. First, what is the spatial distribution of the curre nt density over the surface of the electrode? Second, how do alterations in the electrode geometry effect neural excitation? Third, under what conditi ons can an electrode of finite size be modeled as a point source? Analysis of the models showed that the current density was concentrated at the tip o f the microelectrode and at the electrode-insulation interface. Changing th e surface area of the electrode, radius of curvature of the electrode tip, or applying a resistive coating to the electrode surface altered the curren t-density distribution on the surface of the electrode. Changes in the elec trode geometry had little effect on neural excitation patterns, and modelin g the electric held generated by sharply tipped microelectrodes using a the oretical point source was valid for distances >similar to 50 mum from the e lectrode tip. The results of this study suggest that a nearly uniform curre nt-density distribution along the surface of the electrode can be achieved using a relatively large surface area electrode (500-1000 mum(2)), with a r elatively blunt tip (3-6 mum radius of curvature), in combination with a th in (1 mum) moderately resistive coating (-50 Ohm m). (C) 2001 Biomedical En gineering Society.