## 4 Discussion  
A balloon-expandable coronary artery stent undergoes large deformation during crimping to an outside diameter that is small enough to allow its delivery to the target blood vessel and then during expansion by a balloon to an inside diameter large enough to restore blood flow. In providing the scaffolding function in the lesion area, a sufficient radial strength is required to maintain the lumen opening and to withstand the dynamic loading due to the natural pulsation of blood pressure. Therefore, a good stent should have high flexibility to allow easy delivery, high radial strength for scaffolding, and minimum recoil to ensure adequate blood flow. Evaluation of the stent mechanical performance includes deformation behaviors during crimping and expanding, as well as radial strength and flexibility determined by performing bending tests. Once the outside diameter of a stent to be delivered to the lesion site is determined, the flexibility and strength of the stent are dependent on the following three main factors: material, strut thickness, and stent pattern. The material selection is generally based on biological and biomechanical considerations. Within the scope of biocompatible materials, especially blood compatible metallic materials, the material choices are fairly limited. In recent years, for improved safety and efficacy, there has been a general trend of making the stent strut thinner. When a thinner strut is applied in stent design, a better deliverability, lower disturbances to blood flow, and a lower risk of restenosis can indeed be achieved. On the other hand, the strength and longitudinal integrity of the stent are considerably reduced. Considering the biodegradability of magnesium and its alloys and the relatively low Young's modulus of these materials in comparison with the [PERSON]'s moduli of stainless-steel and Co-Cr-based alloys, the strut thickness must be kept at a rational level. Thus, for biodegradable magnesium alloy stents, stent pattern design becomes a particularly important means of adjusting the stents mechanical performance. Towards stent design optimization, both bench test and FEA are necessary for developing an in-depth understanding of deformation behavior specifically for cardiovascular stents in relation to the stent pattern design. In this study, planar meshes with two existing stent patterns, as well as the planar mesh with a newly proposed pattern, considering the specific mechanical characteristics of magnesium alloys, were subjected to stretching tests and FEA to reveal the meshes responses to stretching. It was based on the idea that the circular expansion of a tubular stent could be translated into the planar stretching of a planar stent mesh to simplify the procedure of specimen preparation and to reduce the associated costs. To represent the mechanical behavior of tubular stents, the stretching direction of the planar specimens would, of course, have to be the same as the circular direction in the expanding tests of \begin{table}
\begin{tabular}{c c c c c} \hline  & **Stent A** & **Stent B** & **Stent C** & **PLLA Stent**[19] \\ \hline Bending stiffness (Nmm\({}^{2}\)) & 60.1 & 42.1 & 41.5 & 2.9 \\ \hline \end{tabular}
\end{table} 
Table 1: Bending stiffness values of the three magnesium alloy stents and a bioresorbable poly(L-lactide) (PLLA) stent at a deflection of 0.6 mm. the tubular stents. Then, the stretching force applied during the stretching tests to cause deformation would be related to the strength of the tubular stents and the stretching displacement would be related to the amount of deformation of the tubular stents during expansion until the target deployment diameter was reached. The results obtained from the present study confirmed that the stretching curves of the planar specimens could indicate the deformation behavior of the stents of particular designs. First, the length-normalized force was related to the scaffolding strength of the real stent. The larger the length-normalized force, the higher the scaffolding strength.