Scanning electron microscopy of platinum scala tympani electrodes following chronic stimulation in patients

Platinum (Pt) electrodes from three auditory prostheses (Cochlear Pty Ltd) were examined for evidence of corrosion following implantation periods of up to 1000 days. These devices were used for periods ranging from 1600 to 10,400 h and developed maximum charge densities of 0.257 microC mm-2 geom. per phase. Scanning electron microscopy of the surface of the 66 stimulated electrodes examined showed no evidence of definitive Pt corrosion. Their surface features were essentially identical to control (unstimulated) electrodes. In addition, there was no evidence of any change in the surface morphology of the Silastic carrier adjacent to the stimulating electrodes. These results indicate that Pt is a suitable electrode material for neural prostheses that use relatively large surface area electrodes (0.1-1.0 mm2) and low to moderate charge densities (0.01-0.26 microC mm-2 geom. per phase).