Nitinol self-expanding stents are used to treat disease in all areas of the human gastrointestinal (GI) tract. Chemistry along the GI tract changes both spatially and temporally with pH ranging from 1.2 in the stomach to 8.5 in the common bile duct. A variety of secretions add ions, digestive enzymes, and proteins to the environment along the GI tract. Establishing absolute acceptance criteria for corrosion resistance of metallic implants presents a unique challenge in this constantly changing environment. In this study, cyclic potentiodynamic polarization corrosion tests were conducted over several years at multiple labs in accordance with the requirements of ASTM F2129. Braided Nitinol wire stents with thermal oxide surface representing a range of GI tract stents with good clinical history were tested in solutions selected to simulate the target anatomy in the human gastrointestinal tract. The results suggest that acceptance criteria for devices tested in simulated vascular environments do not reflect requirements for successful devices used in the varied chemical environments of the GI tract. Breakdown voltage acceptance criterion need to consider the clinical application and in-body use environment. Inherent test variability must also be considered when demonstrating statistical equivalence to clinically successful comparator devices or fixed value specifications.

Although improvements in fatigue performance with increasing Nitinol microstructural purity have been previously characterized, there is limited information on whether corrosion resistance is impacted by reductions in inclusion size and distribution. The objective of this study is to characterize the surface oxide for different Nitinol microstructural purities and determine its influence on corrosion susceptibility. To assess the surface oxide, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) were performed on Nitinol heart valve frames with a variety of purities and surface finishes.

Compressive loading of shape memory alloys (SMA) is gaining considerable attention in recent years due to the improved fatigue life compared to tensile loading. This can be beneficial in applications such as dampers, actuators, and particularly elastocaloric cooling. SMA elements, however, tend to buckle under compressive loading and their stability can be enhanced by utilizing properly designed holders, i.e., structures that support SMA elements and prevent them from buckling. On the other hand, these supporting structures are in contact with SMA elements, which can cause wear and their premature failure, intensified by the lateral expansion of material under compression. In current literature, a majority of experiments are focused on reciprocating sliding wear of tungsten carbide or variations of bearing steel balls/discs/pins/rings on NiTi plates as well as on comparison of wear performance of NiTi with other materials. The aim of this present work is to theoretically and experimentally study tribological conditions between the tube and supporting element (bushing) and to find the most compatible material to NiTi in order to minimize wear, provide adequate structural support, and finally to enhance the overall fatigue behavior.

The surface structure of medical implants and their chemical state are extremely important for their lifetime and reliability. There are problems with the degradation of NiTi implants due to structural fatigue, localized tribo-corrosion, and inconsistent hemocompatibility. These issues potentially can be solved by surface texturing by controlled short laser pulse treatments with a multibeam approach explored in this study. One of the unique surface textures in nanoscale is represented by introducing laser induced periodic surface structures (LIPSS) into the implant surface. The LIPSS formation involves the excitation of surface plasmon polaritons and material surface reorganization. Ripples with periodicity less than 1 ?m along with the catalytic activity of oxide surface with "rutile nanohairs" can significantly reduce bacterial film adhesion while promoting surface endothelialization and hemocompatibility. The morphological texturing of the surface allows for tuning the wetting properties from extreme hydrophobicity to hydrophilicity. Reduction of friction and wear of material surfaces can be achieved by introducing textures that reduce the contact friction area. The geometry of the LIPSS and dimples maintains an adhesive film of liquid among moving parts. Short laser "beam-shaped" pulses were applied in this work to NiTi surfaces. The results indicate that LIPSS processing of NiTi surface with controlled height profiles and periodicity gives rise to chemisorbed hydrocarbon molecules on rutile oxide layer, which leads to super-hydrophobicity and a beneficial antibacterial effect. Ultrashort laser pulse micromachining does not affect the microstructure and martensitic phase transformation. The corrosion resistance of LIPSS textured NiTi surface is not degraded, and the process reduces friction area and maintains an adhesive film of liquid between the moving parts.