Effects of tissue digestion solutions on surface properties of nitinol stents

Analysis of explanted medical implants can provide a wealth of knowledge about device safety and performance. However, the quality of information may be compromised if the methods used to clean tissue from the device disturb the retrieved condition. Common solutions used to digest tissue may adversely affect the surface of the device and its severity can be material and processing dependent. In this study, two groups of stents made from the same material (Nitinol) were shape set in a salt pot (SP) or further processed by mechanical polishing (MP) and then immersed in one of three tissue digestion solutions (TDS): nitric acid (HNO₃ ), sodium hydroxide (NaOH), or papain enzyme (papain). Nickel (Ni) ion concentrations were measured for each stent-TDS combination and post-immersion stent surface constituents, morphology and oxide depths were compared to baseline samples. Exposure to the HNO₃ TDS resulted in relatively high Ni ion release and surface damage for both stent types. Papain TDS induced a greater Ni ion release than NaOH TDS, however, both were significantly lower than HNO₃ . The NaOH TDS increased the oxide layer thickness on MP stents. In contrast, all other stent immersions resulted in thinner oxide layers. For the Nitinol finishes used in this study, HNO₃ is not recommended while papain and NaOH solutions may be appropriate depending on the post-retrieval analysis performed. This study elucidates the importance of preliminary testing for TDS selection and how the surface finish can affect the sensitivity of a material to a TDS. 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 331-339, 2018.