PoliTo(BIO)Med Lab, Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Turin, Italy.
National Research Council, Institute of Condensed Matter Chemistry and Technologies for Energy (CNR-ICMATE), Lecco, Italy.
J Mech Behav Biomed Mater. 2024 Oct;158:106653. doi: 10.1016/j.jmbbm.2024.106653. Epub 2024 Jul 6.
The remarkable mechanical properties of nickel-titanium (NiTi) shape memory alloy, particularly its super-elasticity, establish it as the material of choice for fabricating self-expanding vascular stents, including the metallic backbone of peripheral stents and the metallic frame of stent-grafts. The super-elastic nature of NiTi substantially influences the mechanical performance of vascular stents, thereby affecting their clinical effectiveness and safety. This property shows marked sensitivity to the primary parameters of the heat treatment process used in device fabrication, specifically temperature and processing time. In this context, this study integrates experimental and computational analyses to explore the potential of designing the mechanical characteristics of NiTi vascular stents by adjusting heat treatment parameters. To reach this aim, differently heat-treated NiTi wire samples were experimentally characterized using calorimetric and uniaxial tensile testing. Subsequently, the mechanical response of a stent-graft model featuring a metallic frame made of NiTi wire was assessed in terms of radial forces generated at various implantation diameters through finite element analysis. The stent-graft served as an illustrative case of NiTi vascular stent to investigate the impact of the heat treatment parameters on its mechanical response. From the study a strong linear relationship emerged between NiTi super-elastic parameters (i.e., austenite finish temperature, martensite elastic modulus, upper plateau stress, lower plateau stress and transformation strain) and heat treatment parameters (R > 0.79, p-value < 0.001) for the adopted ranges of temperature and processing time. Additionally, a strong linear relationship was observed between: (i) the radial force generated by the stent-graft during expansion and the heat treatment parameters (R > 0.82, p-value < 0.001); (ii) the radial force generated by the stent-graft during expansion and the lower plateau stress of NiTi (R > 0.93, p-value < 0.001). In conclusion, the findings of this study suggest that designing and optimizing the mechanical properties of NiTi vascular stents by finely tuning temperature and processing time of the heat treatment process is feasible.
镍钛（NiTi）形状记忆合金的卓越机械性能，尤其是其超弹性，使其成为制造自扩张血管支架的首选材料，包括外周支架的金属骨架和支架移植物的金属框架。NiTi 的超弹性特性对血管支架的机械性能有显著影响，从而影响其临床效果和安全性。这种特性对用于制造器件的热处理过程的主要参数（尤其是温度和处理时间）表现出显著的敏感性。在这种情况下，本研究通过调整热处理参数来设计 NiTi 血管支架的机械特性，将实验和计算分析结合在一起。为了达到这个目的，使用量热法和单轴拉伸试验对不同热处理的 NiTi 丝样品进行了实验表征。随后，通过有限元分析评估了由 NiTi 丝制成的金属框架的支架移植物模型在不同植入直径下产生的径向力的机械响应。支架移植物作为 NiTi 血管支架的实例来研究热处理参数对其机械响应的影响。研究结果表明，NiTi 超弹性参数（即奥氏体结束温度、马氏体弹性模量、上平台应力、下平台应力和相变应变）与热处理参数之间存在很强的线性关系（R > 0.79，p 值 < 0.001），适用于采用的温度和处理时间范围。此外，还观察到支架移植物在扩张过程中产生的径向力与热处理参数之间（R > 0.82，p 值 < 0.001）以及支架移植物在扩张过程中产生的径向力与 NiTi 的下平台应力之间（R > 0.93，p 值 < 0.001）存在很强的线性关系。总之，本研究的结果表明，通过精细调整热处理过程的温度和处理时间来设计和优化 NiTi 血管支架的机械性能是可行的。
