**6.3.1 Design objective for the shape modification**

In the previous section, it was shown that although a stent suitable for the assumed symptoms could be designed using the proposed design method, further modifications were still required. Two kinds of design objectives are set up for shape modification.

*Objective for the mechanical stimulus*: as shown in Fig. 18, the contact force on the normal part of the artery is concentrated at both ends of the stent. This force concentration provokes neointimal thickening from the hyperplasia of smooth muscle cells. Therefore, the contact force must be reduced at both ends of the stent. However, this force should be larger than a certain limit so that stent functions on the lesion.

*Objective for the blood flow*: as stated above, the normal part of the artery is expanded to become much larger than the target diameter. This expansion state of the artery causes stagnation and vortices of blood flow. As a result, stent thrombosis may be induced. It is very important to reduce stagnation and vortex creation to decrease the risk of stent thrombosis. Therefore, the vascular wall should be expanded flatly by the insertion of the stent. The flat expansion of the artery can prevent the generation of stagnation and vortices in the stented artery.
