## 3 Results
### Flexibility of the Tubular Stents  
The flexibility of a stent is an important performance index, as it relates to the ease of navigation through the tortuous blood vessel during delivery and the readiness to adapt itself to the blood vessel wall to achieve full conformity. In this research, the flexibility of the stents with the three patterns was predicted using FEA and the results were compared with those obtained from the three-point bending tests. Figure 19 shows the equivalent plastic strain distributions of the stents upon unloading after three-point bending, as predicted by the FEA. During bending, deformation was concentrated in the middle portion of the stent, where the curvature changed markedly. Strains in Stents A and C were concentrated mostly at the connections of the peak or valley portion with the axial link. However,  
Figure 18: Radial loading curves of Stents B and C after expansion to inside diameters of (**a**) 2.5 and (**b**) 3.0 mm. Stent B had strain concentrations in the connecting section and the U-shaped segments of stent rings. The FEA revealed that when the deflection applied was less than 0.6 mm, plastic deformation was accumulated in the connection section. With increasing bending deflection, the rings of Stent B in the circular direction became squashed. Figure 20 shows the profiles of the three stents after three-point bending and unloading. The bending deformation of Stents A and C occurred primarily at the axial links and the profiles of the deformed stents were in general quite uniform. However, the profile of Stent B exhibited poor uniformity, especially at large deflections. Near the section that had contact with the upper load applicator, the U-shaped segments in the upper half sank into the lumen, whilst the U-shaped segments in the lower half started to protrude. The non-uniform deformation was persistent even after unloading, which would mean that the deformed Stent B might lead to the bulging of the blood vessel or the injury of the inner wall of the blood vessel, once deployed. Obviously, such a risk should be avoided at the stent pattern design stage. Figure 19: Equivalent plastic strain distributions of (**a**) Stent A, (**b**) Stent B, and (**c**) Stent C upon unloading after three-point bending to a deflection of 1.4 mm. Figure 21 shows the bending force-deflection curves of the magnesium alloy AE21 stents with the three design patterns during loading and then during unloading. When the deflection was lower than 0.6 mm, the loading curve of Stent A had a significantly larger slope than the loading curves of Stents B and C. It suggested that Stent A had an inferior flexibility. The loading curves of Stents B and C were nearly identical, when the deflection was less than 0.6 mm, indicating similar flexibility, when they were forced to fit to small curvatures. However, when the deflection was larger than 0.6 mm, the differences in bending force appeared. The bending force applied to Stent B became higher than that applied to Stent C. Given that Stent B had a much lower radial strength than Stent C, with increasing bending force, the circular rings of Stent B could not sustain the local stresses and the U-shaped segments deformed to accommodate part of the deflection. The strain distribution, as shown in Figure 19b, also showed the deformation in the U-shaped segments in Stent B. In other words, it was the initial deformation of the stent rings that had contact with the upper load applicator that had resulted in the rising bending force of Stent B when the deflection was larger than 0.6 mm. Figure 21: Three-point bending force-deflection curves of the three stents. Figure 20: Profiles of the three stents after three-point-bending to a deflection of 1.4 mm and unloading. Bending flexibility can be represented by bending stiffness: the larger bending stiffness, the smaller flexibility.