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New Thinking on the Healing and Integration of Implanted Electrodes

Buddy Ratner , PhD

Millions of medical devices made of synthetic or modified natural materials all trigger a similar reaction, the foreign body reaction (FBR). For materials that pass routine cytotoxicity assessment, biocompatibility is largely associated with a mild foreign body reaction, i.e., a thin, avascular, non-adherent foreign body capsule. This is particular important for electrode implants where this dense collagen capsule can impede electrical communication. This talk will have two parts. First, we demonstrate the monitoring of the FBR throughout its development using electrical impedance spectroscopy (EIS) in conjunction with the implantation of a microelectrode array. Second, strategies developed at the University of Washington that can improve healing and integration will be discussed.

We have performed experiments in vitro, ex ova and in vivo (rat) with an implanted microelectrode array. In vitro, we used a reservoir of phosphate buffered saline into which selected proteins were introduced that adsorb onto the electrode surface. Three proteins were studied and each was found to affect the EIS results differently. We have investigated the foreign body response ex ova using the chick chorio-allantoic membrane (CAM) model. Following implantation of the electrode array the chick CAM exhibited a response similar to the mammalian foreign body response. Finally, implantation in rat jaw muscle permitted an in vivo assessment of electrode performance with time, and provided a model to test the effects of various electrode coatings on healing and integration.

Based on studies over the past 10 years at the University of Washington, a class of biomaterials will be described that readily integrates into tissue and stimulates spontaneous reconstruction of tissue. The material is made by sphere-templating of synthetic polymers. All pores are identical in size and interconnected. Studies from our group have shown optimal healing (as suggested by induced vascularity and minimal fibrosis) for spherical pores of approximately 30 micron size. The integrative healing effect noted is independent of biomaterial – similar results are observed with sphere-templated silicone rubber and pHEMA hydrogel. In addition, surface chemical modification of the hydrogel with carbonyl diimidazole (CDI), or immobilization on the hydrogel of collagen I or laminin did not change the healing response. Good healing results have been seen upon implantation in skin (subcutaneously, percutaneously), heart muscle, sclera, skeletal muscle, bone and vaginal wall. The results obtained to date suggest that these sphere-templated polymers will also find application for healing of implanted electrodes.

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