ELECTRICAL-PROPERTIES OF IMPLANT ENCAPSULATION TISSUE

The purpose of this study was to determine the electrical properties o f the encapsulation tissue that surrounds electrodes chronically impla nted in the body. Two four-electrode arrays, fabricated from either ep oxy or silicone rubber, were implanted in each of six adult cats for 8 2 to 156 days. in vivo measurements of tissue resistivity using the fo ur-electrode technique indicated that formation of the encapsulation t issue resulted in a significant increase in the resistivity of the tis sue around the arrays. In vitro measurements of tissue impedance using a four-electrode cell indicated that the resistivity of the encapsula tion tissue was a function of the tissue morphology. The tight layers of fibroblasts and collagen that formed around the silicone rubber arr ays had a resistivity of 627 +/- 108 Omega-cm (mean +/- SD; n = 6), wh ich was independent of frequency from 10 Hz to 100 kHz, and was signif icantly larger than the resistivity of the epoxy encapsulation tissue at all frequencies between 20 Hz and 100 kHz. The combination of macro phages, foreign body giant cells, loose collagen, and fibroblasts that formed around the epoxy arrays had a frequency-dependent resistivity that decreased from 454 +/- 123 Omega-cm (n = 5) to 193 +/- 98 Omega-c m between 10 Hz and 1 kHz, and was independent of frequency between 1 kHz and 100 kHz, with a mean value of 195 +/- 88 Omega-cm. The results indicate that the resistivity of the encapsulation tissue is sufficie nt to alter the shape and magnitude of the electric field generated by chronically implanted electrodes.