Aspects of silicone rubber as encapsulant for neurological prostheses. Part 2: Adhesion to binary oxides

The paper presents some measurements of the hydrothermal stability of experimental adhesive joints in water at 100 degrees C. The joints are between one adhesive silicone rubber and ten metal or metal-oxide adherends, all combinations of interest to the neurological prosthesis maker. The probable adhesion mechanism is then considered, in the search for some parameter by which the experimental results could have been predicted. Evidence is produced that physical adsorption plays little or no part in the adhesion, but that hydrothermal stability seems to be a function of the adherend ionic charge. In pursuit of this idea, the valency, the Slater potential and the iso-electric point for the surface (IEPS) of the adherend are examined as possible prediction parameters. It is concluded that, at least in neutral and acidified water, the IEPS is a promising predictor, with the benefit that it is experimentally determinable for adherends of unknown composition. The most stable joints seem to occur when the charge density on the adherend has an optimum negative value.