# Shape Memory Polyurethane and its Composites for Various Applications
## 2 Shape Memory Polyurethane in Different Applications
### Pressure Bandage Application  
In biomedical application, the wound and venous leg ulcer treatment is indeed a significant problem [23]. Ulcers can be developed to persons who are affected by the reduced blood flow in leg due to diabetes, rheumatoid disease, due to aging, low physical activity, etc. [24]. The increased sugar level in blood and fatty substance level on the internal wall of veins can cause progressive occlusion of the vessels [25; 26]. The healing of an ulcer can be induced by several methods whereas the application of gradual external pressure can be extremely helpful [27]. The skin and vascular changes can be minimized or reversed by driving fluids using external compression from the interstitial spaces into the vascular and lymphatic compartments [28]. It is found from the research that the external compression should be applied in gradual fashion in an upward direction with highest pressure at the ankle [29]. The compression can be employed by the application of pressure to govern fluid retention and suppress the swelling of the lower limb. Adequate pressure can be produced using pressure bandages due to the differences in their structure and rubber/elastic yarn content. However, several factors such as shape and circumference of the limb can affect the ability of normal pressure bandages to generate sustained compression. SMPs and their composites may serve as alternatives to produce a new class of pressure bandages. SMPs have several advantages including large strain, low cost, biocompatibility, easy processability and may thus be employed in the pressure bandage application. An inherent property of the SMP is to generate strength for recovery through the release of deformation energy upon external stimuli and the same strength can be used to generate the pressure on any circular objects. [PERSON] et al. [30] attempted to use the property of shape memory polymer for the pressure bandage application. They proposed two designs of bandages (Figure 3) which consisted of pre-stretched strips of temperature responsive SMPs with low transition temperature (around 50 \({}^{\circ}\)C) attached with fabric either with fixed strain of different length or fixed length of different strains. The developed bandage can be wrapped around the leg and the strips will shrink to their original shape upon application of external heat using a hair drier or a hot towel. The shrinkage of the SMP provides a coaxial force acting inwards (Figure 4a,b) with pre-defined pressure and its distribution for the venous leg treatment. The typical required pressure for the leg ulcer treatment at ankle is around 40 mmHg which reduces to 15 mmHg at the upper part of leg (Figure 4c). Figure 3: Proposed design of shape memory polyurethane/fabric bandages: (**a**) bandage consisting of strips with fixed strain of different length and (**b**) fixed length of different strains [30]. The aforementioned study used polycaprolactone diol as a soft segment, diisocyanate as a hard segment, and butanediol as a chain extender in the polyurethane preparation. The developed polyurethane had a maximum strain over 800% with 6.2 MPa stress and 82% recovery ratio with a transition temperature around 45-50 \({}^{\circ}\)C. They found from the experiments that the bandage with highly strain stored an increased amount of mechanical energy leading to the higher recovery force generating higher pressure upon heat treatment. Their results demonstrated that SMP bandages are proficient to maintain the required pressure for relatively longer duration. The immersion of bandage in water for two hours at room temperature had no measurable effect, therefore, sweating and body fluid may not affect its performance significantly. The inventors of this pressure bandage have notified that the generated pressure may drop during use and can be readjusted by any non-skilled person at home using hot towel or hair drier which considerably simplifies the treatment.