# Shape Memory Polyurethane and its Composites for Various Applications
## 2 Shape Memory Polyurethane in Different Applications
### Bone Tissue Engineering  
More than 4.5 million reconstructive surgical procedures are reported to be executed yearly worldwide in response to the bone injuries [36]. A constant increase in diseases such as osteoporosis, tumors, and osteonecrosis has also been witnessed due to disordered lifestyle responsible for annihilation of bone tissues. In addition, traumatic injuries or ineffective healing after traumatic injuries leads to the destruction of bone tissues. Generally, our body heals the bone fractures naturally using different bone cells. The process of bone healing begins with bleeding and as the blood flows down to the bone fracture site, it forms a clot resulting in swelling in the broken area. Further, the dead cells and germs are cleaned up by a force of white blood cells and the blood clot starts to reduce. This triggers  
Figure 4: (**a**) Shape memory polyurethane strip attached on a fabric and direction of the force generated due to the activation of strip’s shape memory behavior, (**b**) direction of the force generated by strip attached on a circular shape, (**c**) model of leg with varying circumference showing pressure required for treatment of leg ulcer [30]. the osteoclasts cells to remove the broken dead bone rubbles followed by the formation of fibro cartilage tissues with the help of chondroblasts cells which hold the broken bone ends together. Simultaneously, osteoblasts and fibroblasts start building the bone tissues and producing collagen fibers, respectively, in combination and produce tissue called as soft callus. The soft callus begins to harden with time which is called as bony callus and the excessive bone tissue reabsorbed by osteoclasts leads to a healed and healthy bone. The time taken for the healing process may differ on the basis of different conditions such as general health condition, presence of infection, age of the person, blood supply condition, and the type of breakage. The small defects in the bones are healed naturally by the growth, renewal, and repair mechanism as explained previously. However, in case of bone defects bigger than the critical size, the healing process turns out to be crucial and it becomes necessary to develop new bone regeneration techniques using tissue engineering methods. Tissue engineering is mainly based on three fundamental things which constitute a template for the tissue formation which is a porous three-dimensional scaffold made of osteoinductive biomaterials, cells for tissue generation, and growth factors. The porous scaffolds should have the capability to support cell adhesion, proliferation, and differentiation promoted by growth factors. The structural and mechanical integrity of the scaffold are the vital parameters to facilitate and regulate the daily cellular processes and bone healing. In addition, the degradation rate of biomaterials should be consistent with the growth rate of new tissues. Therefore, designing the scaffold for the purpose of regenerating target tissue may be regarded as the most important part in tissue engineering. Bone tissues can be repaired using scaffolds made of different ceramics, metallic and polymeric materials [37] such as polylactic acid (PLA) [38], polycaprolactone (PCL), polyglycolic acid (PGA), polypropylene fumarate (PPF), polyurethane, polyhydroxyalkanoates (PHA), silk fibroin, chitosan, alginate, cellulose, etc. [39]. Different aspect of biopolymers as a bone substitute is discussed elsewhere [40]. In the process of designing scaffolds and their implantation in human body, it is always beneficial to have precisely matched dimensions with the contour of bone defect. Therefore, polymers or their composites with shape memory properties can contribute significantly and scientists and researchers have developed different materials for the same application. Due to advantage of shape memory programming, polyurethane and its composites can maintain a temporary structure of interest and recover as per the need after incorporation in bone via minimal invasive bone repair [41].