**1. Introduction**

180 Mechanical Engineering

Upcraft, S. & Fletcher, R. (2003). The rapid prototyping technologies. *Assembly Automation*,

Atherosclerosis is one of the most prominent diseases that induce dysfunction of circulation, and it is a disease of large and medium size arteries. If cholesterol presenting at high concentration in a blood injures an intima, a white corpuscle, i.e. a monocyte, goes into the intima and mutates into a foam cell. Then, smooth muscle cells migrate from the media to the intima, and they grow proliferously there. Based on these phenomena, cholesterol and other lipid materials accumulate in the intima. Atherosclerosis has become a serious problem in the developed countries that are aging. Therefore, countermeasures to the atherosclerosis have become important. Although there are various medical treatments for the atherosclerosis, a stent placement has received much attention as a minimally invasive procedure for vascular stenotic lesion based on the coronary atherosclerosis, the arteriosclerosis obliterans, etc. A stent is a cylindrical tube-shaped medical device that can expand the stenotic lesion in a blood vessel continuously. When considering the expansion method of a stent, two types are available. One is a self-expanding type that can expand by itself when released from the sheath of a catheter. Another is a balloon-expandable type that must be expanded forcibly using a balloon catheter. Because the self-expanding stent continues to expand to the memorized diameter at the stenotic lesion, it has the long-term patency of a vascular wall. In the present study, the main target is the self-expanding type.

Recently, the severe problem of in-stent restenosis has arisen in a blood vessel with a stent placed and left in it. In-stent restenosis results from the neointimal thickening in the blood vessel based on the hyperplasia of smooth muscle cells. The hyperplasia of smooth muscle cells is caused by a mechanical stimulus from the stent to the vascular wall. The drugeluting stent (DES) containing immunosuppressive agents is already in clinical use to resolve this problem (Morice et al., 2002). It can be said that the DES is more effective in preventing the development of restenosis than a bare metal stent (BMS). However, it can be said that the DES does not help to improve the life prognosis or to prevent myocardial infarction (Babapulle et al., 2004; Kastrati et al., 2007; Lagerqvist et al., 2007). It is also reported that the DES might cause deterioration in the life prognosis, although the BMS does not (Nordmann et al., 2006). Depletion of immunosuppressive agents has been pointed

Design and Evaluation of Self-Expanding Stents

Table 1. Dimensions of target stents

Fig. 1. Schematic view of the measuring method

length of each target stent are summarized in Table 1.

SENDAI SD10

SENDAI SD8

**2.2 Radial compression test and stent stiffness in radial direction** 

polyethylene terephthalate (PET) film 12 m thick. Test temperature is 34 C ± 1 .

Suitable for Diverse Clinical Manifestation Based on Mechanical Engineering 183

expanding stents made of NiTi shape memory alloy, namely Nitinol. Name, diameter, and

Protege® GPS™ 10 GPS 80 Zilver® ZIL 80

JOSTENT® SelfX JSX 60 Bard® Luminexx™ BLU 100

The stent stiffness in radial direction was measured by using the radial compression test machine designed by reference to the method proposed by (Duda et al., 2000). A stent is mounted on the polytetrafluoroethylene stage with slit and wrapped in a sheet. As illustrated in Fig. 1, one end of the sheet is fixed, and the other end is pulled by the linear actuator (ESMC-A2; ORIENTAL MOTOR Co., LTD., Tokyo, Japan). By applying tensile force to the sheet, the stent is compressed in its radial direction. This tensile force can be measured by using the load cell (LUR-A-200NSA1, load rated capacity: 200 N; KYOWA ELECTRONIC INSTRUMENTS CO., LTD., Tokyo, Japan). In addition, the reduction of the stent diameter is measured by using the LED displacement sensor (Z4WV; OMRON Corporation, Tokyo, Japan). The sheet to wrap the stent consists of a polyethylene film 50 m thick and a

(Duda et al., 2000) defined two kinds of forces for evaluating the scaffolding property of a self-expanding stent. One of the defined forces is chronic outward force, which is necessary to subtract 1 mm from a stent diameter. The other, namely radial resistive force, is needed to subtract 1 mm from a stent diameter. (Yoshino et al., 2008) defined the radial stiffness based on the radial pressure exerted on a stent for evaluating the scaffolding property. Based on these evaluation indicators, the stent stiffness in radial direction is defined as follows.

,

*p f*

2

*<sup>F</sup> <sup>K</sup> r l* 

*s s*

(1)

Stent Code Diameter (mm) Length (mm)

10

8

80

80

out for longer use. When using a DES, such serious problem as side effects occurring by drugs must be considered as well. As described above, there have been many reports about the use of a DES to prevent in-stent restenosis. However, there have been few studies to prevent in-stent restenosis by designing and modifying a BMS itself. Most of studies have been undertaken to try improvement or optimization of the BMS. Shape, location, and mechanical properties of a stenotic lesion depend on each patient. Optimization, which derives one specified stent shape, is not always the best for the patient. It is thus necessary to design a stent shape suitable for each patient. Using a suitable stent can reduce the risk of in-stent restenosis. However, there has been no study that has tried to design a stent shape in response to each patient's symptom.

For providing a bare metal stent with lower risk of in-stent restenosis, two objectives were set up. The first objective of our research is establishment of a method to design a stent for each patient's symptom. The second objective is establishment of a method to select a suitable stent from commercially available stents based on their mechanical properties. In this chapter, we describe the design method and selection method of a stent suitable for patient's symptom based on mechanical engineering.
