2.1.1.1.5. Surface Characteristics and Biocompatibility of Tantalum

In the present device, Ta powder is packed onto the end of the Ta stem forming a slug, in a process called sintering. This increases the surface area for charge storage and allows the discharge of currents larger than would be possible using a solid tantalum pellet. One might question whether a sintered slug would produce a more intense reaction because of its rough surface. However, the surface roughness of various forms of Ta has been shown not to make a difference, probably because the pore sizes are very small and the grains are individually smooth rather than sharp at cellular dimensions. Meenaghan et al.¹ compared the tissue reactions to various forms of Ta implanted into the subcutaneous paravertebral region of the monkey. Ta slugs were prepared in various ways: mill finished, metallurgically polished, electrochemically oxidized, and sintered with porous surface. Within the groups of devices manufactured using different methods, one subgroup was glow-discharged and one was not. The study showed that regardless of surface roughness, the tissue response was remarkably similar. However, tissues around implants that were glow-discharged showed a significantly greater cellular response. Meenaghan et al.¹ postulated the surface energy of the implant is perhaps more important than the surface texture. Other studies have supported this study, showing that porous Ta has no greater effect than non-porous materials on tissue reaction and that in some cases its performance is better.

1. Meenaghan, M.A., Natiella, l.R., Moresi, l.L., Flynn, H.E., Wirth, l.E., & Baier, R.E. (1979). Tissue response to surface treated tantalum implants: preliminary observations in primates. Journal of Biomedical Materials Research, 13, 631-643.

2. Guarda, F., Galloni, M., Assone, F., Pasteris, V., & Luboz, M.P. (1982). Histological reactions of porous tip endocardial electrodes implanted in sheep. International Journal of Artificial Organs, 5(4), 267-273.

6.2.Iridium (Ir)

6.2.1.1.Physical/Chemical properties

Iridium and its alloys with platinum are materials with mechanical properties similar to stainless steel. Iridium has excellent electrical conduction properties in the bulk metal and its interface with saline body fluids. The electrical characteristics of iridium electrodes have been well-documented.^1,2 Excellent long-term biological performance is also characteristic of iridium and other noble metals because the release of metal ions to tissues is minimal. Iridium is described in the CRC Handbook of Chemistry and Physics as the most corrosion-resistant element known. Iridium has been used to coat various materials to decrease problems associated with corrosion of the underlying materiaI^3,4.

1. Beebe, X., & Rose, T.L. (1988). Charge injection limits of activated iridium oxide electrodes with 0.2 ms pulses in bicarbonate buffered saline. IEEE Transactions on Biomedical Engineering, 35, 494-495.

2. Robblee, L.S., Mangaudis, M.J., Jasinsky, E.D., Kimball, A.G., & Brummer, S.B. (1986). Charge injection properties of thermally prepared iridium oxide films. Material Research Society Symposium Proceedings, 55, 303-310.

3. Buchanan, R.A., Lee, I.S., and Williams, J.M. (1990). Surface modification of biomaterials through noble metal ion implantation. Journal of Biomedical Materials Research, 24,309-318.

4. Lee, I.S., Buchanan, R.A., and Williams, J.M. (1991). Charge-injection densities to iridium and iridium ion implanted TI-6AI-4V with relevancy to neural stimulation. Journal of Biomedical Materials Research, 25, 1039-1041.