Electrodes for neural stimulation in cochlear implants necessitate and extremely high charge injection/emission capacity and good drawability to mimic the function and size of a real neuron, respectively. Multi-layered activated iridium is known to possess one of the largest charge carrier densities. However, iridium is expensive and brittle. On the other hand, titanium is relatively cheap, ductile and biocompatible. Recently, an attempt was made to develop a titanium-based electrode with an iridium-rich iridium-titanium alloyed zone by laser surface alloying. The present study is aimed at a detailed characterization of the microstructure, surface-chemistry and phase-distribution in the alloyed zone of such an electrode prepared by laser surface alloying of titanium with iridium. Laser surface alloying with an earlier determined optimum processing condition appears to develop an alloyed zone that can be conveniently divided into three regions with distinctly characteristic microstructure and composition. The influence of the laser surface alloying parameters on the morphology, identity and distribution of the phases are discussed. Accordingly, a metastable free energy-composition diagram (schematic) is proposed to account for the observed microstructure. Special etching following laser surface alloying seems capable of significantly increasing the real surface area of the alloyed zone. Finally, an attempt has been made to correlate the microstructure and composition of the alloyed zone or electrode-tip with the laser surface alloying parameters, and assess its suitability for neural stimulation.
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