A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 5 3 8 FEATURE temperature. Computational modeling of medical devices containing Nitinol—especially of fatigue loading, such as that shown in Fig. 1—can be complex and requires the use of a Nitinol-specific constitutive law. Furthermore, given the sensitivity of Nitinol to thermal processing and surface treatments, the guidance recommends that experiments to determine constitutive law parameters or constant life curves (i.e., for fatigue safety factor calculations) be conducted on test specimens representative of the final manufactured device. Given the unique sensitivity of Nitinol to pre-strain (e.g., as occurs when crimping a stent onto a delivery catheter), which can increase or decrease a Nitinol device’s fatigue performance[4,5], the guidance also highlights the need to take pre-strain into account when estimating fatigue safety factors. Finally, although also relevant for conventional materials, the guidance emphasizes the importance of the computational methodology used to calculate mean and alternating strain[6] as well as validation of the computational model. As the use of computational modeling in medical devices increases, it will become increasingly important to document adequate credibility of computational models for their context of use[7,8]. CORROSION Passive device alloys, such as Nitinol, are potentially susceptible to corrosion through a number of different mechanisms: pitting, crevice, uniform, galvanic, and fretting corrosion. The corrosion properties of a Nitinol device are directly influenced by its manufacturing processes. For example, thermal treatments used to create desired thermomechanical properties tend to grow the native TiO2 surface layer. The result can include the formation of nickel-rich phases and microstructural defects within the surface oxide that may significantly degrade the resistance of the material to one or more corrosion mechanisms. Removal of this thermally grown oxide through surface processing steps such as chemical etching, mechanical polishing, or electro- polishing, followed by passivation, tends to result in a thin (< 10 nm) and uniform oxide layer that provides imparts corrosion resistance to the alloy. Based on these observations, the guidance recommends different corrosion evaluations depending on the processing history of the Nitinol device components and device design. These evaluations are captured by the flow chart shown in Fig. 2, which illustrates that all prolonged and permanent contacting devices that contain Nitinol should be assessed using potentiodynamic polarization testing per ASTM F2129. If the results pass a predefined acceptance criterion and an established surface processing method is used to remove the thermal oxide that may develop during heat treatments, no further assessments are necessary. However, if either of these criteria are not met, the guidance recommends conducting an extended in vitro assessment to evaluate nickel leaching kinetics using ASTM F3306. Because device design considerations may also enhance corrosion and metal ion release, galvanic corrosion testing per ASTM F3044 is recommended in scenarios where the Nitinol component(s) is in contact with dissimilar metals. Finally, if fretting corrosion is a concern (e.g., overlapping devices), microscopic evaluations can be conducted following accelerated durability testing or as described in device-specific standards and/or guidance documents. BIOCOMPATIBILITY AND LABELING The potential for Nitinol to release nickel ions means there may be biocompatibility considerations in addition to those found in FDA guidance “Use of International Standard ISO 10993-1 Biological Evaluation of Medical Devices—Part 1: Evaluation and Testing within a Risk Management Process.” Specifically, there is a need for a risk assessment to be performed comparing in vitro nickel release to tolerable intake (TI) valueswhennickel ion release testing is performed.While the guidance recommends a systemic non-oral TI of nickel of Fig. 1 — A triangular-shaped inclusion is present at the fatigue crack initiation site for a Nitinol specimen subjected to cyclic fatigue loading. Fig. 2 — Corrosion testing paradigm flow chart. 5
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