Conventional Powder Metallurgy Process and Characterization of Porous Titanium for Biomedical Applications

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TISSUE degradation associated with traumas and diseases and its replacement is currently an important public health problem. For the bone tissue, degradation is evident in the bone density reduction of patients as young as 30 years old, which implies a strength reduction up to 40 pct that could be increased by both the cyclic load degradation and the surface wear of joints. Of all biomaterials used for bone replacement, it is recognized that both commercially pure titanium (c.p. Ti) and Ti6Al4V alloy, medical grade, are the materials that show the best in vivo behavior because of their excellent balance between mechanical, physical-chemical, and biofunctional properties. However, they have the following three disadvantages that, in many cases, compromise the implants and prosthesis liability: (1) The stiffness of titanium is higher than the bone, which produces the stress shielding phenomenon, promoting the bone resorption around the implant; (2) despite its high osteointegration capability, titanium is surrounded Y. TORRES and J.A. RODRI´GUEZ, Associate Professors, and I. NIETO, Fellowship, are with the Departmento de Ingenierı´ a Meca´nica y de los Materiales, E.T.S. de Ingenierı´ a, Universidad de Sevilla, 41092 Sevilla, Spain. Contact e-mail: [email protected] J.J. PAVO´N, Associate Professor, is with Grupo BIOMAT, Programa de Bioingenierı´ a, Universidad de Antioquia, Medellı´ n, Colombia. Manuscript submitted September 29, 2010. Article published online May 3, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B

by a fibrous tissue because of its bioinert behavior, which is related with many loosening events; and (3) more studies are needed about its liability from damage prevention criteria because this is the only admissible criteria for biomaterials design. Regarding the stressshielding problem, some developments of both biocomposites and porous titanium implants still do not reach suitable equilibrium between mechanical and biofunctional properties.[1–4] Several previous works showed that is possible to match the stiffness of cortical bone using different techniques to fabricate porous titanium samples.[5–15] However, there is a lack of studies about the real effect of this porosity on other important mechanical properties, i.e., mechanical strength and fatigue life, and about the relationships between both the porosity and microstructure with the mechanical properties. Porosity percentage must be controlled to reduce the implant stiffness without any undesirable influence in mechanical resistance. From a powder metallurgy (PM) point of view, this could be reached by controlling compacting pressure, sintering temperature, and even using some special additives as space holders. To improve both bone ingrowth and osteointegration, the pores size and morphology must be controlled, especially in the surface, which will be also critical for the fatigue resistance of the implant. In this work, porous samples were obtained by a PM conventional technique. The influence of the main sintering conditions, compacting pressure an