# Shape Memory Polyurethane and its Composites for Various Applications
## 2 Shape Memory Polyurethane in Different Applications
### Self-Healing
Focusing on this advantage, [PERSON] et al. [67] developed a novel zwitterionic multi-shape-memory polyurethane (ZSMPUs) using N-methyldiethanolamine as soft segment, hexamethylene diisocyanate (HDI) as hard segment and 1,3-propanesultone (PS). They used PS to graft zwitterionic side chains onto the backbone of polyurethane via a ring opening mechanism which forms zwitterionic units. Normally, zwitterions are the dipolar ions with zero net electric charge having two or more functional groups containing at least one positive and one negative charges [75; 76; 77]. Introduction of the zwitterions may help to enhance biocompatibility, hydrophilicity, and multi shape memory ability of cross-linked polyurethanes. Exposure to moisture rich conditions (30 \({}^{\circ}\)C and 80%  
Figure 12: Conceptual representation of built-in synergetic reversible self-closing and intrinsic self-healing capability of polymer through dynamic reversible covalent bonding along with shape memory property [65]. RH) for few minutes makes the wounded polyurethane sample soft and sticky and slight pressure leads to healing. Moreover, the developed interface of healing zone was also diminished upon heating the sample at 50 \({}^{\circ}\)C for 2 hours. It was also concluded that the healing process does not affect the shape memory ability of prepared polyurethane (Figure 13). They ascribed this healing process to the supramolecular zwitterionic interaction which takes part in holding the sample together before rebuilding of the regular entangled network across the wounded surfaces. Figure 13: (**A**) Self-healing behavior of ZSMPU8 along with (**B**) its shape memory ability and (**C**) the stress-strain curve with healing cycle [67]. Similarly, [PERSON] and coworkers [62] reported the preparation of zwitterion polyurethane using N,N-bis (2-hydroxyethyl)) isonicotinamide as soft segment and HDI as hard segment along with the incorporation of zwitterions into the backbone. Integration of zwitterions was done by way of a ring opening of PS. The developed material contained pyridine type sulfobetains responsible for the strong electrostatic interactions which also acted as physical crosslinkers. It is known that the shape memory ability was based on the phase transition and, in this case, the aggregation of zwitterions influenced the phase transition behavior of polyurethane. They also concluded that the increase in zwitterions leads to enhanced self-healing efficiency in most humid environments without any additive and external energy. It was also concluded that the hydration of the material increases ionic mobility whereas electrostatic forces improve upon dehydration. During hydration, the zwitterions may migrate to a wounded surface and hold the surfaces together via increased crosslinking upon dehydration (Figure 14). Development of polyurethanes with self-healing ability can also be done by incorporation of liquid crystalline material. In this scenario, [PERSON] and coworkers [66] integrated 4-octocyloxy benzoic acid (OOBA) into PEG based polyurethane using solution phase mixture. They found an enhanced self-healing ability of polyurethane along with a shape memory property and proposed a new mechanism of heat induced bleeding. Bleeding of liquid crystal (OOBA) at the wounded surface, due to the enhanced fluidity of molecules, upon heating at 130 \({}^{\circ}\)C and forming crystals via crystallization process results in self-healing or heat induced healing of damage. They stated that the developed composite can be used as flexible printed circuits, self-healing concrete, and self-protecting textiles.