Fatigue behaviour of NiTi shape memory alloy scaffolds produced by SLM, a unit cell design comparison.

Selective laser melting (SLM) is an additive manufacturing technique able to produce complex functional parts via successively melting layers of metal powder. This process grants the freedom to design highly complex scaffold components to allow bone ingrowth and aid mechanical anchorage. This paper investigates the compression fatigue behaviour of three different unit cells (octahedron, cellular gyroid and sheet gyroid) of SLM nitinol scaffolds. It was found that triply periodic minimal surfaces display superior static mechanical properties in comparison to conventional octahedron beam lattice structures at identical volume fractions. Fatigue resistance was also found to be highly geometry dependent due to the effects of AM processing techniques on the surface topography and notch sensitivity. Geometries minimising nodal points and the staircase effect displayed the greatest fatigue resistance when normalized to yield strength. Furthermore oxygen analysis showed a large oxygen uptake during SLM processing which must be altered to meet ASTM medical grade standards and may significantly reduce fatigue life. These achieved fatigue properties indicate that NiTi scaffolds produced via SLM can provide sufficient mechanical support over an implants lifetime within stress range values experienced in real life.

Potential of amorphous microporous silica for ibuprofen controlled release.

Amorphous microporous silica (AMS) xerogel materials were synthesized in an acid-catalyzed sol-gel process. The porosity of AMS was adapted by varying sol-gel synthesis parameters including the molar hydrolysis ratio (r-value), HCl:Si molar ratio, the type of silicon alkoxide source and the solvent. AMS particles of millimeter size were loaded with ibuprofen, by heat treatment and melt impregnation. In vitro release experiments were performed in simulated gastric and intestinal fluid. The release kinetics were critically depending on the AMS particle size distribution and the micropore diameter. The release was interpreted as configurational diffusion in the AMS micropores. The stability of unloaded and ibuprofen loaded AMS material upon storage was investigated using nitrogen physisorption, DSC analysis and in vitro release experiments. Ibuprofen loaded AMS formulations show remarkable stability, which can be attributed to the presence of ibuprofen molecules in the channels, functioning as scaffolds to support the pore structure.

Microstructure of surface and subsurface layers of a Ni-Ti shape memory microwire.

The microstructure of a 55 microm diameter, cold-worked Ni-Ti microwire is investigated by different transmission electron microscopy techniques. The surface consists of a few hundred nanometer thick oxide layer composed of TiO and TiO2 with a small fraction of inhomogeneously distributed Ni. The interior of the wire has a core-shell structure with primarily B2 grains in the 1 microm thick shell, and heavily twinned B19' martensite in the core. This core-shell structure can be explained by a concentration gradient of the alloying elements resulting in a structure separation due to the strong temperature dependence of the martensitic start temperature. Moreover, in between the B2 part of the metallic core-shell and the oxide layer, a Ni3Ti interfacial layer is detected.

Critical overview of Nitinol surfaces and their modifications for medical applications.

Nitinol, a group of nearly equiatomic shape memory and superelastic NiTi alloys, is being extensively explored for medical applications. Release of Ni in the human body, a potential problem with Nitinol implant devices, has stimulated a great deal of research on its surface modifications and coatings. In order to use any of the developed surfaces in implant designs, it is important to understand whether they really have advantages over bare Nitinol. This paper overviews the current situation, discusses the advantages and disadvantages of new surfaces as well as the limitations of the studies performed. It presents a comprehensive analysis of surface topography, chemistry, corrosion behavior, nickel release and biological responses to Nitinol surfaces modified mechanically or using such methods as etching in acids and alkaline solutions, electropolishing, heat and ion beam treatments, boiling in water and autoclaving, conventional and ion plasma implantations, laser melting and bioactive coating deposition. The analysis demonstrates that the presently developed surfaces vary in thickness from a few nanometers to micrometers, and that they can effectively prevent Ni release if the surface integrity is maintained under strain and if no Ni-enriched sub-layers are present. Whether it is appropriate to use various low temperature pre-treatment protocols (< or = 160 degrees C) developed originally for pure titanium for Nitinol surface modifications and coatings is also discussed. The importance of selection of original Nitinol surfaces with regard to the performance of coatings and comparative performance of controls in the studies is emphasized. Considering the obvious advantages of bare Nitinol surfaces for superelastic implants, details of their preparation are also outlined.

Surface oxidation of NiTi shape memory alloy.

Mechanically polished NiTi alloy (50 at% Ni) was subjected to heat treatment in air in the temperature range 300-800 degrees C and characterised by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Thermogravimetry measurements were carried out to investigate the kinetics of oxidation. The results of thermodynamic calculations were compared to the experimental observations. It was found that NiTi alloy exhibits different oxidation behaviour at temperatures below and above 500 degrees C. A Ni-free zone was found in the oxide layer for oxidation temperatures of 500 degrees C and 600 degrees C. The oxidation at 500 degrees C produces a smooth protective nickel-free oxide layer with a relatively small amount of Ni species at the air/oxide interface, which is in favour of good biocompatibility of NiTi implants. The oxidation mechanism for the NiTi shape memory alloy is discussed.

Metallic surface modification.

The potential beneficial effect of metal surface treatment using electrochemical polishing on stent thrombogenicity and neointimal hyperplasia was evaluated in a rat A-V model and a porcine coronary model. Thrombogenicity of polished stents (n=6) was compared to non-polished stents (n=5) in a rat A-V shunt model using 125I-fibrinogen and 51Cr-labelled platelets. Total clot weight after 30 min was significantly lower in the polished stents (32.1+/-2.8 vs 18.1+/-4.4: p<0.001). Also, 125I-fibrinogen deposition was significantly lower in the polished stents (1.30+/-0.07 vs 0.66+/-0.04: p<0.001). Platelet deposition was, however, not significantly reduced (12.7+/-3.4 vs 9.87+/-1.9, NS). Subsequently, the effect of electrochemical polishing on neointimal hyperplasia was evaluated in a porcine coronary model. Polished (n=10) and non-polished stents (n=10) were randomly implanted in the right coronary artery of healthy pigs. Neointimal hyperplasia was significantly decreased in the polished stents (0.56+/-0.28 vs 0.94+/-0.34 mm2: p<0.01).

Concept of the homemade coronary stent: experimental results and initial clinical experience.

To defray the escalating cost of coronary stenting, we handmade a balloon expandable coil stent with stainless steel wire. Preliminary comparison with the Palmaz-Schatz stent showed that, when implanted in porcine iliac arteries, there was no difference in immediate angiographic results or in the degree of foreign body reaction at 6 wk. Subsequently, a total of 73 stents were implanted in 52 patients, either as a bailout device (54%) or for suboptimal angiographic results (46%). All but two implantations were successful. The postprocedural regimen consisted of heparin 1,000 IU/hr, aspirin 250 mg daily, and ticlopidine 500 mg daily. In-hospital complications were limited to two groin hematomas, one necessitating blood transfusion. Importantly, stent thrombosis was not observed. While 6-mo follow-up is pending, we already conclude that a balloon expandable coil stent can be handmade easily at low cost and implanted safely in patients.