Novel titanium foam for bone tissue engineering
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Titanium foams fabricated by a new powder metallurgical process have bimodal pore distribution architecture (i.e., macropores and micropores), mimicking natural bone. The mechanical properties of the titanium foam with low relative densities of approximately 0.20–0.30 are close to those of human cancellous bone. Also, mechanical properties of the titanium foams with high relative densities of approximately 0.50–0.65 are close to those of human cortical bone. Furthermore, titanium foams exhibit good ability to form a bonelike apatite layer throughout the foams after pretreatment with a simple thermochemical process and then immersion in a simulated body fluid. The present study illustrates the feasibility of using the titanium foams as implant materials in bone tissue engineering applications, highlighting their excellent biomechanical properties and bioactivity.
I. INTRODUCTION
Pure titanium and some of its alloys are widely used as implant materials under load-bearing conditions in dentistry and orthopedics.1 Titanium and some titanium alloys are better received by human tissue when compared to the receptivity of other metal materials (e.g., SUS316L stainless steel and Co–Cr–Mo-type alloys, due to their lower modulus, superior biocompatibility, and corrosion resistance).2 Previous investigations3,4 showed that TiO2 gel prepared by a sol-gel process induces bonelike apatite formation on its surface in simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma. Therefore, titanium and some of its alloys can form an apatite layer via TiO2 gel formation on their surfaces in the body’s environment and bond to living bone through the apatite layer. However, most metallic implant materials in use today, including titanium and its alloys, suffer interfacial instability with host tissues, biomechanical mismatch of the elastic moduli, production of wear debris, and no supply of blood. Recently, substantial efforts have been made in development of new titanium alloys specifically tailored for biomedical applications with a lower modulus, such as Ti–12Mo– 6Zr–2Fe (TMZF),5,6 Ti–15Mo–5Zr–3Al,7 Ti–15Zr– 4Nb–2Ta–0.2Pd,8 Ti–15Sn–4Nb–2Ta–0.2Pd,8 “completely biocompatible” Ti–13Nb–13Zr,9,10 new -type biomedical Ti–29Nb–13Ta–4.6Zr,11 and the minimum elastic modulus Ti–35Nb–5Ta–7Zr alloys.12 However, the elastic moduli of all the titanium alloys are still higher than those of human bone, especially cancellous bone. Recently, there has been an increasing interest in fabricating porous scaffolds that mimic the architecture of human bone; osteoblasts obtained from the patient’s hard tissues can be expanded in culture and seeded onto the J. Mater. Res., Vol. 17, No. 10, Oct 2002
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scaffolds, gradually integrating with the new bone tissues in vitro and/or in vivo.13–17 The scaffolds or threedimensional (3D) foams provide necessary supports for cells to proliferate and maintain their differentiated function, and their architecture defines the ultim
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