In vivo assessment of tissue compatibility and functionality of a polyimide cuff electrode for recording afferent peripheral nerve signals

Minimally-invasively implantable spiral cuff electrodes coated with polyimide (PI) for insulation in combination with suitably designed amplifiers were recently reported to allow signalling of electroneurograms (ENGs). These cuff electrodes can be used to record the respiratory drive after implantation on the phrenic nerve, but PI can cause epineurial fibrosis, fiber loss, and limited reproducibility of recordings. This study aimed to explore the tissue reaction in response to a flexible tripolar cuff electrode embedded in a thin (10 μm) PI insulating carrier after implantation around the sciatic nerves of rats (n=4) at the branching into the N. peroneus communis, the N. tibialis and N. cutaneus surae caudalis. 28 days after implantation the electrode functionality was proven and ENG signals recorded. In addition, transverse sections of the implantation site were analysed after hematoxylin-eosin (HE) and Weigert ́s fibrin staining for changes in the tissue morphology and the number of myelinated nerve fibres. 28 days after implantation the electrode contact to the nerve still was sufficient for signal recording and no changes in the nerve morphology and the number of myelinated nerve fibres could be noted. However, cuff functionality for nerve signal transfer was limited by a fibrous capsule, which covered the whole electrode and which was composed mainly of tightly packed fibroblasts and fibrin. The formation of such a fibrous capsule is known to be caused by the foreign body response and shows a limited tissue compatibility which might be mainly related to the PI insulating material. Further studies will address the investigation of alternative elastic matrix-materials to achieve a strong integration of the electrode in the nerve tissue and in this way a long term functionality of the cuff electrode.