NiTi foams are unique among biocompatible porous metals because of their high recovery strain (due to the shape-memory or superelastic effects) and their low stiffness facilitating integration with bone structures. To optimize NiTi foams for bone implant applications, two key areas are under active study: synthesis of foams with optimal architectures, microstructure and mechanical properties; and tailoring of biological interactions through modifications of pore surfaces. This article reviews recent research on NiTi foams for bone replacement, focusing on three specific topics: (i) surface modifications designed to create bio-inert porous NiTi surfaces with low Ni release and corrosion, as well as bioactive surfaces to enhance and accelerate biological activity; (ii) in vitro and in vivo biocompatibility studies to confirm the long-term safety of porous NiTi implants; and (iii) biological evaluations for specific applications, such as in intervertebral fusion devices and bone tissue scaffolds. Possible future directions for bio-performance and processing studies are discussed that could lead to optimized porous NiTi implants.

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A new powder metallurgy technique for creating porous NiTi is demonstrated, combining liquid phase sintering of prealloyed NiTi powders by Nb additions and pore creation by NaCl space-holders. The resulting foams exhibit well-densified NiTi-Nb walls surrounding interconnected pores created by the space-holder, with controlled fraction, size, and shape. Only small amounts of Nb (3 at.%) are needed to produce a eutectic liquid that considerably improves the otherwise poor densification of NiTi powders. NiTi-Nb foams with 34-44% porosity exhibit high compressive failure stress (>1,500 MPa), ductile behavior (>50% compressive strain), low stiffness (10-20 GPa), and large shape-memory recovery strains. These thermomechanical properties, together with the known biocompatibility of the alloy, make these open-cell foams attractive for bone implant applications.

Porous NiTi was produced by sintering pre-alloyed NiTi powders (with small Ni addition to form Ni-rich composition) with NaCl powders which are removed to create 40-60 vol.% macropores which are open to the surface, blocky in shape, and 100-400 lm in size. The microporosity present between the NiTi powders is infiltrated by an in situ created NiTi-Nb eutectic liquid which, after solidification, densifies the NiTi powders into dense struts. This processing technique allows for separate control of the macroporous structure, and the densification and composition of the NiTi struts.