Swellable coatings for hearing aid applications

The problem of acoustic feedback in hearing aids could be solved potentially by applying a compliant hydrogel to the outer surface that would conform to the ear canal and block feedback. With this objective, several formulations of hydrogels were developed and their swelling and mechanical properties investigated. Hydrogel formulations were polymerized from hydroxyethyl methacrylate (HEMA) and N-vinyl-pyrrolidone (NVP), with various photo-initiators, crosslinkers, and swelling agents. The hydrogel that swelled most rapidly and yet remained undissolved in water had a monomer composition of 40 mol% HEMA, 60 mol% NVP, with 1 wt% polyethylene glycol dimethacrylate as a crosslinker, and 0.5 wt% 2,2-dimethoxy-2-phenyl-acetophenone as the photo-initiator. The tensile modulus, strength, hardness, and durability of the dry hydrogels were not a strong function of composition. In the swollen state, the mechanical properties were much reduced. The potential use of these materials on hearing aids has been discussed in this article.