Porous Titanium for Biomedical Applications: Evaluation of the Conventional Powder Metallurgy Frontier and Space-Holder Technique

Abstract

Titanium and its alloys are reference materials in biomedical applications because of their desirable properties. However, one of the most important concerns in long-term prostheses is bone resorption as a result of the stress-shielding phenomena. Development of porous titanium for implants with a low Young’s modulus has accomplished increasing scientific and technological attention. The aim of this study is to evaluate the viability, industrial implementation and potential technology transfer of different powder-metallurgy techniques to obtain porous titanium with stiffness values similar to that exhibited by cortical bone. Porous samples of commercial pure titanium grade-4 were obtained by following both conventional powder metallurgy (PM) and space-holder technique. The conventional PM frontier (Loose-Sintering) was evaluated. Additionally, the technical feasibility of two different space holders (NH4HCO3 and NaCl) was investigated. The microstructural and mechanical properties were assessed. Furthermore, the mechanical properties of titanium porous structures with porosities of 40% were studied by Finite Element Method (FEM) and compared with the experimental results. Some important findings are: (i) the optimal parameters for processing routes used to obtain low Young’s modulus values, retaining suitable mechanical strength; (ii) better mechanical response was obtained by using NH4HCO3 as space holder; and (iii) Ti matrix hardening when the interconnected porosity was 36–45% of total porosity. Finally, the advantages and limitations of the PM techniques employed, towards an industrial implementation, were discussed.

Porous Titanium Cylinders Obtained by the Freeze-Casting Technique: Influence of Process Parameters on Porosity and Mechanical Behavior

“…These studies allowed us to evaluate the influence of the parameters that control the manufacturing process (ΔT, diameter and mold material) on the porosity: volumetric In this work, we use cylindrical molds of different materials (alumina and Teflon ® ) and diameters (8,12,15 and 20 mm for alumina and 12 mm for the Teflon ® mold, respectively). The molds filled with the slurry prepared above are subjected to temperature gradients (∆T), (27,32,37 • C), by varying the cold surface and keeping constant the hot one (22 • C) controlled in two different ways, to induce directed freezing. On the one hand, using a low-cost home-made device (cylindrical expanded polystyrene container) (Figure 1a), designed and manufactured to be capable of housing a cryogenic solution (ice and salt, in different proportions) that allows the desired temperature in the cold spot (−5, −10, −15 • C, the bases of the six molds).…”

“…The dynamic Young's modulus values, E d , was estimated. On the one hand, Young's modulus has been estimated according to Equation (1), which corresponds to the experimental relationship between E d and the total porosity; it has been already employed and described in [27]:…”

“…Ti (~110 GPa [28]), P T is the percentage of total porosity of the sample and F f the shape factor, calculated as it has been described in the Materials and Methods section. Furthermore, the yield strength, σ y , has been also estimated for the three main areas of the cylinder calculated following Equation (3), which has been adapted from the equation proposed in [27]:…”

“…Ti (~650 GPa [31]) and P i is the percentage of interconnected porosity of the sample, as it has been described above. Since geometrical models are not able to predict the mechanical properties, it has been reported this equation to calculate the required values [27].…”

Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications

“…These variables have a great influence in the microstructure of the PM samples, for example their porosity (content, size, and morphology), the grain size and sinterability (quality of the necks between the metal particles), as well as their behavior in service (stiffness, yield stress, fatigue life and wear and corrosion resistance). The processing conditions used were chosen in order to obtain mechanical properties (Young's modulus and yield strength) similar to those of the cortical bone (20-25 GPa and 150-180 MPa, respectively) [45][46][47][48].…”