Stability and Biocompatibility Tests of Alginate as Cochlear Implant Coating Material

Inner ear trauma caused by cochlear implantation is a severe clinical problem. It was shown that an electrode alginate coating can reduce the insertion forces in vitro. The grade of the alginate viscosity can be adjusted by using different metal ions for cross-linking the salts of the alginic acid. The aim was to investigate the stability and inner ear biocompatibility of alginate using different in vitro established cross-linkers. Alginate beads were cross-linked in either calcium chloride (CaCl2) or barium chloride (BaCl2) solution. The beads were cultivated in artificial perilymph and stability and swelling were observed for 13 months. Ototoxicity was tested on cochlear whole mount explants from neonatal rats. Neomycin served as positive control to induce hair cell damage and explants without any addition served as negative control. The beads and explants were co-cultured for 48 hours and the hair cell survival was analysed microscopically. Neomycin treatment induced an extensive inner and outer hair cell loss. Neither CaCl2 nor BaCl2 cross-linked alginate beads caused any damage to the hair cells. Even though the same volume of alginate and cross-linkers were used, in CaCl2 cross-linked beads were initially almost double the size of in BaCl2 cross-linked beads. None of the cross-linked alginate beads had a significant volume change within 3 months being cultured in artificial perilymph. After 3 months the CaCl2 cross-linked beads swelled massively and dissolved within one week whereas BaCl2 cross-linked alginate beads remained unchanged until month 13 after culture start. Alginate beads gelled with both cross-linkers are biocompatible with the inner ear sensory epithelium. Both cross-linkers ensure a stable gelation of alginate but a swelling followed by degradation of the in CaCl2 cross-linked beads occurred after 3 months. For coatings, which need to be long term stable, BaCl2 should be chosen whereas CaCl2 may be more suitable for applications where limited stability is needed and the swelling is not affecting the surrounding tissue. Therefore, BaCl2 cross-linking of alginate may be the best choice for cochlear implant coating.