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The goal of this experiment was to determine the electrical properties of the tissue reaction to implanted microelectrode arrays. We describe a new method of analyzing electrical impedance spectroscopy data to determine the complex impedance of the tissue reaction as a function of postimplantation time. A model is used to extract electrical model parameters of the electrode-tissue interface, and is used to isolate the impedance of the tissue immediately surrounding the microelectrode. The microelectrode arrays consist of microfabricated polyimide probes, incorporating four 50-mum-diameter platinum microelectrodes. The devices were implanted in the primary motor cortex of adult rats, and measurements were performed for 12 weeks. Histology was performed on implants at three time points in one month. Results demonstrate that the tissue reaction causes a rapid increase in bioimpedance over the first 20 days, and then stabilizes. This result is supported by histological data.