# Shape Memory Polyurethane and its Composites for Various Applications
## 2 Shape Memory Polyurethane in Different Applications
### Cardiovascular Implants  
Cardiovascular implants such as stents, grafts, etc., are applicable to repair or replace diseased arteries in living body. As the occurrence of chronic and cardiovascular disease is increasing in society, the commercial market for these implants is escalating. Millions of patients have benefited by the  
Figure 14: Illustration for the use of zwitterionic moieties in the self-healing of polyurethane [62]. use of cardiovascular implants recently [78]. Their basic function is to work as artificial conduit or replacement for non-functional veins. As an artificial conduit, cardiovascular implants should have the ability to mimic the mechanical and biological behavior of an artery which helps to sustain a prolonged healing process [79; 80]. A suitable cardiovascular implant can be derived from basic understanding of structural and functional properties of artery as they are involved in both blood and soft tissue interaction [81]. In the development of implants, design optimization is perceived as one of the most critical step. Therefore, it is always a booming area for the scientific community. Recent technological advancement opened the door for the development of new and innovative implants which can improve the diagnosis process of the patients [82]. Use of a shape memory metal alloy such as an NiTi alloy for the development of vascular devices like stents, occlusion coils has attracted the scientific community's attention [83; 84]. Devices made of metal and its alloys have several advantages. However, they are too stiff and their design prevents them from the navigation in arteries. In the context of implants development, shape memory polymers can easily be processed/molded into the required size and shape using different techniques [85; 86] along with externally stimulated actuation. Several researchers have developed SMP for implants using different materials such as palm oil [87], PCL-PLA with Fe\({}_{3}\)O\({}_{4}\)[88], perfluoropolyether [89], etc. Therefore, there is a requirement for devices which have access and delivery into the tortuous arteries made of polymers having flexibility and a high shape recovery ratio. In this perspective, [PERSON] and coworkers [90] have proposed a stent prototype developed using biocompatible SMP [91]. The stent was prepared using the dip coating technique on a stainless steel pin followed by drying in vacuum. In order to create the mesh pattern on the prepared stent, the laser etching method was applied to 50% of the solid stent (Figure 15). They found that the developed laser etched stent showed full shape recovery with an expansion ratio of 2.7 and 1% axial shortening after crimping. In addition, the device did not begin to self-expand during navigation into the artery and it can show the controlled actuation by an external heat source. In another work, [PERSON] and coworkers [92] developed a material by blending polyurethane and PCL by melt mixing in Brabender internal mixer in different ratios (70/30). They found that the material's shape memory can be simply actuated with the help of body temperature which is a desired property for the implants such as stent. The screw stent made of blended material can be deformed at higher temperature (40 \({}^{\circ}\)C) and be fixed at lower temperature (10 \({}^{\circ}\)C) for 2 hours followed by the recovery of the initial shape upon exposure to water at 37 \({}^{\circ}\)C. The applicability of the same screw stent was tested in a segment of human femoral vein at human body temperature (Figure 16). The stent (6 mm diameter) was placed into a 4 mm diameter vein after compressive shape fixing using delivery instrument. After submerging vein with stent into water at 37 \({}^{\circ}\)C, the stent was found to recover to its original shape and expand the wall of vein. They also concluded that the prepared material was biocompatible in nature as determined from the adhesion and proliferation of bone marrow mesenchymal stem cells.