血管支架疲勞壽命提升之設計概念與實驗驗證

Abstract

血管支架植入技術自從1990年發明以來，一直受到醫學界的廣泛關注。然而，血管支架在植入血管以後，容易因為血壓引起的脈動以及日常身體的活動而產生徑向週期性的變化，尤其是在身體周邊的血管中更為明顯，這些週期性、反覆不斷的運動，容易造成血管支架疲勞破壞，甚至造成支架結構的斷裂。因此，近年來這些因疲勞產生破壞的支架儼然成為一項在醫學界中重要的課題。在本篇研究中提出了一項有趣的支架設計概念，運用調整支架之幾何形狀，漸進地減縮支架支撐結構的寬度，來分散原本在支架彎曲結構上的應力集中，使其延伸到原本毫無承受應力的支撐結構上，進而增加支架的疲勞壽命。為分析此支架設計對於疲勞安全係數的影響以及其他重要的機械性質，運用有限元素法搭配Goodman疲勞壽命分析對支架進行量化的分析與比較，模擬結果顯示以此創新設計的幾何形狀進行優化之血管支架，在疲勞安全係數方面大幅提升至原本的4倍之多。此概念性支架雛型品，乃是藉由一連串製程而完成的，運用脈衝式光纖雷射對鎳鈦圓管進行切削加工，其次使用多次熱處理擴張使其定型並消除內部應力，最後進行電化學拋光以增加其表面精度。本研究也藉由現有疲勞測試機的概念，製造專為測試血管支架之疲勞性質所設計的旋轉疲勞測試機，並對支架雛型品進行實驗以驗證模擬的結果。實驗結果指出經過此創新設計的支架能有效地提升其疲勞壽命，完美符合模擬結果所顯示的趨勢，不管是在常溫下抑或是體溫下進行實驗，經過此設計的支架其疲勞斷裂圈數皆相較於標準支架提升了6~7倍之多，此外，在徑向力測試的結果也指出，其徑向力之數值相對於標準的支架僅僅下降不到20%，比起以往為了增加疲勞壽命，大幅犧牲徑向強度的設計，有著顯著性的提升。綜合以上模擬以及實驗的結果，本研究在血管支架設計方面上提供了極具前瞻性的方向，成為未來提升血管支架疲勞強度研究的基石。

Stenting procedure has received great attention from the medical society since its introduction in 1990s. However, these intravascular stents could suffer from various repetitive motions due to pulsatile blood pressure and daily body activities after implantation, especially in the applications of peripheral arteries. Such motions oscillate the stents repeatedly, leading to the potential risk of stent fracture and fatigue failure. Such fatigue-related stent fracture has drawn much attention within the medical society in recent years and as a result, stent fatigue performance has become a major issue for stent design. In this paper, an intriguing stent design concept aimed at enhancing fatigue life is proposed. The concept of this design is to shift the highly-concentrated stresses away from the crown region and re-distribute them along the stress-free bar arms by tapering the strut width. Finite element models (FEA) and Goodman fatigue life analysis were developed to evaluate the mechanical integrity and fatigue safety factor of the stent to various loading conditions. Simulation results show that the fatigue safety factor of our novel stent design jumped to 4 times that of a conventional stent. Conceptual stent prototypes were first cut by a pulsed-fiber optic laser module, followed by a series of expansions and heat treatments to gradually shape the stent to its final size and lastly, processed by electrochemical polishing to refine their surface roughness. A rotating bending fatigue tester was built for this study and stent fatigue tests were conducted for proof of concept. Experimental results show that our novel stent concept successfully enhanced the fatigue life as designed. The fatigue cycle number of our novel stent increased by 6-7 times that of a conventional stent, which agreed well with the trend predicted by the simulations, no matter whether the stents are tested in room temperature or in body temperature. Furthermore, the result of radial force test also indicates that compared to a conventional stent, which sacrifices even more radial strength for the sake of increasing the fatigue life, our pioneering stent only decreases slightly under 20% in radial strength. In conclusion, the findings of this paper provide an excellent guide to the optimization of future stent design to greatly improve stent fatigue life.