Additive Manufacturing (AM) gets increasing attention for Nitinol processing for medical devices because it provides the opportunity to circumvent many of the challenges associated with conventional machining of Nitinol as well as the tailoring of patient-specific implants. Frequently used AM techniques for Nitinol are powder-bed technologies such as Selective Laser Melting (SLM) which is a suitable method for creating complex parts. However, functional Nitinol parts are strongly influenced by the powder properties. In particular, the oxygen content is a critical factor when components with medical grade Nitinol are required. Trace amounts of oxygen form an oxide covering the powder surface with a nanometric thickness as well as complex structure and composition due to the high specific surface energy of the powder. The oxide layer is depending on ambient conditions during powder manufacturing as well as powder handling. The present work provides characterization of the oxide layer and the microstructure for a pseudoelastic Ni 50.8 Ti 49.2 Nitinol alloy powder for AM by TEM investigations. Furthermore, for an estimation of the powder oxide layer thickness, the calculated oxide layer thickness resulting from the oxygen content and the particle size is compared with the measured oxide layer thickness by an EDX line-scan.

Nitinol is frequently used as raw material for cardiovascular endoprosthesis, due to their high biocompatibility and properties such as psedoelasticity. Typically, the highest stress/strain that Nitinol based devices undergo occurs during crimping. These mechanical stresses can affect the functional fatigue behaviors. Nitinol devices are crimped to small diameters to deliver to the anatomical location of interest. Thereby, the microstructural and mechanical changes in Nitinol tubes generated by mechanical load conditions are studied, to monitor the pseudoelastic properties as a function of load time. Consequently, a permanent load test is carried out by means of a fixture with parallel clamps, in order to generate deformation of the cross-sectional area of the tube. The procedure consists of deforming 5 mm long samples with 7 mm outer diameter tubes and a wall thickness of 0.5 mm in straight annealed condition at room temperature. For this test, o-shaped samples of single ingot were analyzed. The test was carried out with strains of 2 %, 4 %, 6 % and 8 % to simulate different device crimp strains. Parts were evaluated after time intervals up to six months under load. Afterwards the geometrical changes of the samples were measured. Additionally, the samples were characterized by DSC and XRD measurements.

Transcatheter heart valve replacement is a key advancement in the cardiovascular device industry and provides an alternative to open surgical procedures for patients that suffer from severe symptomatic stenosis and/or regurgitation. The functions and boundary conditions of the four heart valves are unique and must be considered separately. It is essential that the structural durability of these high-risk valve replacement implants is thoroughly assessed through testing and analysis. As such, ISO 5840 outlines a comprehensive device durability approach that incorporates worst-case boundary conditions, computational stress/strain analyses, and benchtop fatigue testing. The present study is focussed on 100,000,000-cycle fatigue testing of custom-designed “diamond-shaped” coupons of process-optimized high purity VAR/EBR Nitinol. Benchtop testing was coupled with finite element analysis (FEA) and microstructural characterization to provide an in-depth understanding of durability.

Nitinol staples are in forefront of internal fixation devices. The application of staples in comparison with other fixation methods such as plates, screws or pins has benefits of being less invasive and simpler. There is a huge potential of using Nitinol in the orthopedic field, but so far only limited research done on material properties of Nitinol in combination with fatigue life of Nitinol staples. Earlier work has shown significant impact of grain size on fatigue life of Nitinol sheet. The aim of this study was a comparative investigation on the fatigue behavior of semi-finished Nitinol flat continuous rolled sheets with staple shaped samples and optimization of Nitinol sheet for the use in the biomedical implant under product-related conditions. Samples based on a generic staple design used in the internal fixation of the musculoskeletal system and four-point bend test methods were used to conduct this study. Monotonic force-displacement tests at different maximum force conditions were conducted, to understand the relationship between microstructure in Nitinol sheets and fatigue life. To get a better understanding of these correlations is primary objective of this investigation.

The continuous rolling of Nitinol alloys is a metalworking process with the ability to produce large quantities of sheet with uniform properties for the use in actuation applications in motion systems with cyclic loads. Great advantages of continuous rolling in comparison with other manufacturing methods are the cold work and heat treatment steps and their ability to influence the properties of the product and keep them in a very tight window over the width and the length of the process. Those tightly controlled properties are key-requirements to use the continuous rolled Nitinol material for subsequent automated processes like stamping in progressive dies or deep- drawing. It is also required for efficient reel-to-reel laser or EDM cutting. The primary objective of this work is to evaluate and obtain the properties of Nitinol continuously flat-rolled sheets and optimization of the process parameters by fatigue evaluation.