Powder Metallurgical Near‐Net‐Shape Fabrication of Porous NiTi Shape Memory Alloys for Use as Long‐Term Implants by the Combination of the Metal Injection Molding Process with the Space‐Holder Technique

Abstract

A new method was developed for producing highly porous NiTi for use as an implant material. The combination of the space‐holder technique with the metal injection molding process allows a net‐shape fabrication of geometrically complex samples and the possibility of mass production for porous NiTi. Further, the porosity can be easily adjusted with respect to pore size, pore shape, and total porosity. The influence of the surface properties of powder metallurgical NiTi on the biocompatibility was first examined using human mesenchymal stem cells (hMSCs). It was found that pre‐alloyed NiTi powders with an average particle size smaller than 45 μm led to the surface properties most suitable for the adhesion and proliferation of hMSCs. For the production of highly porous NiTi, different space‐holder materials were investigated regarding low C‐ and O‐impurity contents and the reproducibility of the process. NaCl was the most promising space‐holder material compared to PMMA and saccharose and was used in subsequent studies. In these studies, the influence of the total porosity on the mechanical properties of NiTi is investigated in detail. As a result, bone‐like mechanical properties were achieved by the choice of Ni‐rich NiTi powder and a space‐holder content of 50 vol% with a particle size fraction of 355–500 μm. Pseudoelasticity of up to 6% was achieved in compression tests at 37 °C as well as a bone‐like loading stiffness of 6.5 GPa, a sufficient plateau stress σ25 of 261 MPa and a value for σ50 of 415 MPa. The first biological tests of the porous NiTi samples produced by this method showed promising results regarding proliferation and ingrowth of mesenchymal stem cells, also in the pores of the implant material.

Production of Nitinol Wire from Elemental Nickel and Titanium Powders Through Spark Plasma Sintering and Extrusion

“…Most production of PM-NiTi uses pre-alloyed powder that are then consolidated either by hot isostatic pressing (HIP) or metal injection molding (MIM) to form the desired near-net shape ( Ref 1,[3][4][5]. Research work using elemental powders as feedstock are inconclusive as these samples did not receive any thermo-mechanical working to fully develop their microstructures and mechanical properties ( Ref 2,[9][10][11]14,16,17).…”

Template-based fabrication of nanoporous metals

“…27 In the case of aluminum, uniaxial pressing during sintering accelerated the densification process and adjusted the final structure porosity (5-35%). 27 In some cases, space-holder particles are used during sintering which can later be selectively removed through melting (e.g., PMMA polymer 29 ), dissolution (e.g., saccharose 29 ), or evaporation processes (e.g., NaCl salt crystals, 28 Mg powder in Al…”

Open Celled Porous Titanium

“…Space holder Water dissolution with magnetic stirrer at 20-80 °C for 2-6 hrs Very good solubility in water & low cost.Very hard to use in MIM [42] [ 43] Poly(methyl methacrylate)…”