New means have been investigated for the production of electrode devices (stimulation electrodes) which could be implanted in the human body in order to control pain, activate paralysed limbs or provide electrode arrays for cochlear implants for the deaf or for the relief of tinitus. To achieve this ion implantation and laser materials processing techniques were employed.
Ir was ion implanted in Ti-6Al-4V alloy and the surface subsequently enriched in the noble metal by dissolution in sulphuric acid. For laser materials processing techniques, investigation has been carried out on the laser cladding and laser alloying of Ir in Ti wire. A particular aim has been the determination of conditions required for the formation of a two phase Ir Ir-rich and Ti-rich microstructure which would enable subsequent removal of the non-noble phase to leave a highly porous noble metal with large real surface area and hence improved charge carrying capacity compared with conventional non porous electrodes.
Evaluation of the materials produced has been carried out using repetitive cyclic voltammetry, amongst other techniques. It has been found that pure Ir can be activated by repetitive cycling in both PBS and 1N H2SO4 solutions. For Ir ion implanted in Ti-6Al-4V alloy (nominal dose 2 x 1016 atoms/cm2), the optimum time of immersion in 1N H2SO4 for the enrichment of the surface with Ir was found to be 60 hrs. The value of charge density supported by this electrode after activation by repetitive cycling in both PBS and 1N H2SO4 was close to 10 mC/cm2 which is sufficient for some types of stimulation electrode but still an order of magnitude less than that supported by pure Ir treated in the same way. This limitation appeared to be connected to the number of Ir atoms at the surface. For the case of ion implantation with Ir at a higher dose (l x 1017 atoms/ cm2) the optimum time of immersion in 1N H2SO4 for the enrichment of the surface with Ir was reduced to 30 hrs. However, little improvement in the current carrying ability of the oxidised surface compared with the material ion implanted at a dose of 2 x 1016 atoms/ cm2 was achieved.
For laser alloyed Ir on Ti wire, it has been found that differences in the melting point and density of the materials makes control of the cladding or alloying process difficult. Investigation of laser process parameters for the control of alloying and cladding in this system was carried out and a set of conditions for the successful production of two phase Ir-rich and Ti-rich components in a coating layer with strong metallurgical bonding to the Ti alloy substrate was derived. The laser processed material displays excellent potential for further development in providing stimulation electrodes with the current carrying capacity of Ir but in a form which is malleable and hence capable of formation into smaller electrodes with improved spatial resolution compared with presently employed electrodes.