## 3 Results  
### Stretching Behavior of the Planar Specimens  
Strain distributions in unit A and unit B at a displacement of 0.1 mm revealed by the FEA are shown in Figure 7. The accumulative strain in unit B appeared to be much smaller than that in unit A. The variation of the stiffness coefficient with displacement is shown in Figure 8. As clearly visible in Figure 8, the stiffness coefficient tends to reach a stable level after a certain displacement. The stable stiffness coefficient, when Pattern A was used, was about four times that when Pattern B was used. Figure 8: Displacement-dependent stiffness coefficients, \(k_{\mathrm{a}}\), \(k_{\mathrm{b}}\), and \(k_{\mathrm{c}}\), of the single unit of Patterns A, B, and C, respectively. Figure 7: Strain distributions in (**a**) unit A and (**b**) unit B subjected to the same deformation to a displacement of 0.1 mm. max abs: maximum principal. Figure 9 presents the speckle images of the deformed planar stent patterns, captured by a charge-coupled device (CCD). Considering the computational efficiency of DIC and mesh movement within the field of view, part of the stent mesh with uniformly distributed speckles, as highlighted by the rectangular boxes in Figure 9, was chosen to calculate the strain distribution. When the stretching displacement reached 1.0 mm for all the three stent design patterns, the major strain distribution of the stent mesh with Pattern A exhibited distinct strain concentrations at the peak and valley bows (highlighted by a triangle in Figure 9a), especially at the inside surface of the bow. However, the overall strains in Patterns B and C were relatively low. Large strains were located at the corners of the U-shaped segments of Pattern B, and at the corners connecting the axial links and the bottoms of the small U-shaped segments of Pattern C, as highlighted by the triangles in Figure 9b,c. The evolution of fracture, from the initiation and propagation of tiny cracks to final fracture along with increasing displacement, was tracked during the stretching tests. A crack, highlighted by a circle in Figure 9a, started its initiation at the inside surface of the bow, and grew across the bridge and caused the fracture of the strut. The single unit of Pattern C (Figure 1) was selected to build an FE model as well. The maximum strain at a displacement of 0.1 mm in unit C was 0.127, being larger than that in unit B (0.015) and smaller than that in unit A (0.165). The stable stiffness coefficient \(k_{\text{C}}\) of unit C was about twice as high as the value of unit B and only half that of unit A. The stiffness coefficient of the whole mesh of Pattern C was \(\frac{17}{12}k_{\text{C}}\). These values led to an expectation that the stent with Pattern C would have balanced mechanical performance in structural stiffness and deformability between that of Pattern A and that of Pattern B. During the stretching tests, the force increased as the deformation of the stent mesh became larger. The force was normalized by the initial stent axial length \(l_{\text{a}}\) (Figure 3), referred to as the length-normalized force. As shown in Figure 10, there were two distinguishable stages in the  
Figure 9: Speckle images and major strain distributions of the stent meshes during the stretching tests of the stents with: (**a**) Pattern A, (**b**) Pattern B, and (**c**) Pattern C. Triangle: strain concentration; Circle: Fracture. length-normalized force-displacement curves: the elastic deformation stage and the plastic deformation stage. At each of the stages, the length-normalized force was linearly related to the displacement. Toward the end of the plastic deformation stage, final fracture occurred. The length-normalized force-displacement curves of the three stent patterns differed significantly from each other. The slopes at the two deformation stages of the stretching curve of Pattern A were obviously larger than those of Patterns B and C. Among the three patterns, Pattern C exhibited the lowest slopes in the stretching curve.