Abstract：
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 (R2 > 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 (R2 > 0.82, p-value < 0.001); (ii) the radial force generated by the stent-graft during expansion and the lower plateau stress of NiTi (R2 > 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.
摘要：
镍钛(NiTi)形状记忆合金的卓越力学性能,特别是它的超弹性，将其确立为制造自扩张血管支架的首选材料，包括外周支架的金属主干和支架移植物的金属框架。NiTi的超弹性本质上影响血管支架的机械性能，从而影响其临床疗效和安全性。此属性显示了对器件制造中使用的热处理工艺的主要参数的显着敏感性，特别是温度和处理时间。在这种情况下，本研究整合了实验和计算分析，以探索通过调整热处理参数设计NiTi血管支架的机械特性的潜力。为了达到这个目的,使用量热和单轴拉伸测试对不同热处理的NiTi线材样品进行了实验表征。随后，通过有限元分析,根据在各种植入直径下产生的径向力评估了采用NiTi丝制成的金属框架的支架移植物模型的力学响应.支架移植物用作NiTi血管支架的说明性案例，以研究热处理参数对其机械响应的影响。从研究中，NiTi超弹性参数之间出现了很强的线性关系(即，奥氏体完成温度,马氏体弹性模量,上高原应力，较低的高原应力和转变应变）和热处理参数（R2>0.79，p值<0.001），适用于温度和处理时间的范围。此外,观察到以下之间的强线性关系:(i)支架移植物在扩张期间产生的径向力和热处理参数(R2>0.82,p值<0.001);(ii)支架移植物在扩张期间产生的径向力和NiTi的较低平台应力(R2>0.93,p值<0.001)。总之,这项研究的结果表明，通过微调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。
