# Shape Memory Polyurethane and its Composites for Various Applications
## 1 Introduction  
The unique capability to deform in predefined conditions and recover back to its original shape upon being triggered by different stimuli such as heat, radiation, pH, hydration, and mechanical, magnetic, and electrical influences, etc., make polyurethane a potential candidate for several applications. Polyurethanes which are versatile materials with extraordinary characteristics, inherently have two macro-phase separated domains due to the presence of a urethane bond between soft and hard segments, alternatively. However, the urethane bond is formed due to the chemical reaction between hard segment (i.e., diisocyanate) and soft segment (i.e., polymeric diol). The capability of shape memory comes from the concept of these soft and hard segments in polyurethane. The presence of hard segment provides the mechanical strength which is responsible for it to remember the initial shape after deformation whereas the energy for dissipation is stored by soft segments providing the desired force to polyurethane in order to revive it to its initial form by the action of external stimuli [1]. The soft and hard segments in polyurethane govern the transitions behavior triggered by radiation, sound, heat, etc., resulting in the shape memory effect. The shape memory effect in polyurethane and its composites can be one-way, two-way, triple or quadruple based on the number of transition temperatures and soft and hard contents [2; 3; 4].  
The shape memory ability makes polyurethane a strong candidate for several applications such as self-tightening sutures, sensors, self-healing, implants, actuators [5; 6], wearable electronics, etc. It was first developed by [PERSON] and his group [7; 8]. However, it has been experienced by the scientific community that pristine polyurethane does not suffice the properties required for the desired applications. Therefore, they have come up with an idea to incorporate foreign materials such as polymer, fillers, etc., in the polymer matrix which can induce the desirable property for targeted application. Incorporation of the fillers in polyurethane matrix results in enhancedproperties such as shape memory ability, self-healing, electrical property, sensing ability, induction of bone formation, antibacterial, biocompatibility, etc. It is also observed by the scientific community that incorporation of fillers in optimized volume does not adversely affect inherent properties of polyurethanes. Incorporation of fillers such as chitosan, carbon nanotubes, lignin, hydroxyapatite, etc., can be done using blending with other polymers, bulk mixing, or in situ reactions.  
Such modifications also enhance the shape memory properties along with other properties such as mechanical, electrical, and biological activities in polyurethanes. These enhanced properties of polyurethane and its composites render this material suitable for several applications. The current review collates the knowledge on various applications of shape memory polyurethane (SMP) and its composites with a special focus on human health applications such as the shielding of electromagnetic interference, medical bandage development, bone tissue engineering, self-healing, implants development, etc. For additional understanding related to history, mechanism and characterization of shape memory polymers, readers are advised to go through the other scholarly reviews [1; 9; 10; 11].