Shape Memory Applications

Potential applications for shape memory PU exist in almost every area of daily life from self-repairing auto bodies to kitchen utensils, from switches to sensors, from intelligent packing to tools [98]. Other potential applications are drug delivery [99], biosensors, biomedical devices [100,101], microsystem components [102], and smart textiles [103]. Because PU can be made biodegradable, they can be used as shortterm implants so removal by surgery can be avoided. Some important applications are discussed next. 

Shape memory PU and polymers in general have tremendous applications in biology and medicine [104, 105] especially for biomedical devices which may permit new medical procedures. Because of the ability to memorise a permanent shape that can be substantially different from an initial temporary phase, a bulky device could be introduced into the body in a temporary shape (e.g., string) that could go through a small laparoscopic hole and then be expanded on demand into a permanent shape at body temperature. 

Shape memory PU has been proposed as a candidate for aneurysm coils [106]. An intracranial aneurysm can go undetected until the aneurysm ruptures, causing 

Recently, the concept of cold hibernated elastic memory utilising SMP in open cellular structures was proposed for space-bound structural applications [108]. The concept of cold-hibernated elastic memory can be extended to various new applications such as microfoldable vehicles, shape determination and microtags [109]. Recent studies on shape memory PU-based conductive composites using conducting polymers and carbon nanotubes show considerable promise for application as electroactive and remote sensing actuators [110]. 

The previous discussion indicates the tremendous application of shape memory PU. Extensive work has been carried out for the development of shape memory PU in the last few years, and is reviewed next. 

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Liu, C., and Mather, R T. 2003. Thermomechanical characterization of blends of poly(vinyl acetate) with semicrystalline polymer for shape memory applications. Proceedings of Annual Technical Conference SPE, 61st, 2 1962-1966. 

Iyengar PK et al (2013) Polymethyl methacrylate nanofiber-reinforced epoxy composite for shape-memory applications. High Perform PolymCT 25(8) 1000-1006... 

Figure 16 A tandom copolymer for shape memory applications. Reproduced with permission from Ahn, S. Deshmukh, P. Kasi, R. M. Macromolecules 2B 0, 43,7330. ...

Transformations from disordered to ordered solid solutions do also occur in some further binary alloy systems, namely, Au-Cd, Au-Cr, Au-Mn, Au-Nb, Au-Pd, Au-V, and Au-Zn [1-3]. The martensitic transformations associated with the ordering in the Au-Cd and Au-Zn systems are relevant for shape memory applications and are also accompanied with considerable strengthening effects. The transformations in the other alloy systems listed above are, in part, relevant for particular functional applications, but little is known about the impact of the transformations on (mechanical) properties. 

For shape memory applications, crystals formed upon cooling of the semicrystalline polymer act as physical crosslinks by which a permanent or equilibrium shape can be set. In a complementary fashion, the miscible amorphous phase governs temporary shape fixing. In turn, crystallization kinetics, crystallite size, and degree of crystallinity collectively determine kinetics of permanent shape setting and shape memory performance characteristics. Thus, it is possible to tailor the shape memory properties of a system by varying the composition and the thermal history of such blends[9], leading us to the present study focused on crystallization kinetics from melt-miscible crystalline-amorphous blends. 

Shaped activated carbons, 4 747 Shaped refractories, 6 491 Shaped-tube electrolytic machining (STEM), 9 599-600 Shape-memory alloys biomaterials, 3 741-750 Shape-memory alloys (SMAs), 22 339-354, 708t, 711-713, 721t applications of, 22 345-353 crystallography of, 22 341-345 ferrous, 22 342t future outlook for, 22 353 magnetically controlled, 22 712 nonferrous, 22 342t one-way, 22 712... 

See also Smart polymers applications of, 22 355 biodegradable networks of, 22 364 cyclic and thermomechanical characterization of, 22 358—362 defined, 22 355-356 examples of, 22 362-364 molecular mechanism underlying, 22 356-358, 359t Shape-memory rings, 22 351 Shape-memory springs, in virtual two-way SMA devices, 22 346-347 Shape-memory stents, 22 352 Shape, of fiber polymers, 77 174-175. 

The PPDX-fr-PCL diblock copolymers were recently synthesized [111] and apart from the references already mentioned, only the contribution of Lendlein and Langer [112] deals with chemically similar materials, although structurally quite different since they employed multiblock copolymers of PPDX and PCL with very low molecular weights to prepare shape memory polymers for biomedical applications. 

One of the most commonly used medical devices is the stent, (Fig. 21.1), small metallic structures that are expanded in blood vessels, functioning to maintain the patency (freedom from obstruction) of the vessel in which it is placed. Although the first use of stents was in vasculature (blood vessel systems), more recent applications include, for example, implantation between two vertebrae to increase the rigidity of the spine. A typical vascular stent is placed in its anatomical location and then either plastically deformed/expanded (stainless steel) or allowed to expand to a predetermined size, as a consequence of shape memory (nitinol). 

Other materials have been found that have shape memories. Shape-memory polymers are plastics that have properties similar to SMAs. Pliable materials have recently been found that can be stretched up to twice their normal length, yet regain their shape when heated. Having a smart plastic material opens up applications of a softer, more flexible nature, such as clothing. 

MusollT, Andre. Shape Memory Alloys. Available online. URL http // www.smaterial.com/SMA/sma.html. Accessed May 28, 2009. This richly illustrated and highly informative Web site describes shape-memory alloy from the perspective of models, crystallography, simulation, applications, and research. 

Finally, metallic fibers find some limited applications as reinforcement in composites. They are generally not desirable due to their inherently high densities and because they present difficulties in coupling to the matrix. Nonetheless, tungsten fibers are used in metal-matrix composites, as are steel fibers in cement composites. There is increasing interest in shape memory alloy filaments, such as Ti-Ni (Nitanol) for use in piezoelectric composites. We will discuss shape-memory alloys and nonstructural composites in later chapters of the text. 

As previously mentioned, the nickel—titanium alloys have been the most widely used shape memory alloys. This family of nickel—titanium alloys is known as Nitinol (Nickel Titanium Naval Ordnance Laboratory in honor of the place where this material behavior was first observed). Nitinol have been used for military, medical, safety, and robotics applications. Specific usages include hydraulic lines capable of F-14 fighter planes, medical tweezers, anchors for attaching tendons to bones, eyeglass frames, underwire brassieres, and antiscalding valves used in water faucets and shower heads (38,39). Nitinol can be used in robotics actuators and micromanipulators that simulate human muscle motion. The ability of Nitinol to exert a smooth, controlled force when activated is a mass advantage of this material family (5). 

Shape-memory alloys (e.g. Cu-Zn-Al, Fe-Ni-Al, Ti-Ni alloys) are already in use in biomedical applications such as cardiovascular stents, guidewires and orthodontic wires. The shape-memory effect of these materials is based on a martensitic phase transformation. Shape memory alloys, such as nickel-titanium, are used to provide increased protection against sources of (extreme) heat. A shape-memory alloy possesses different properties below and above the temperature at which it is activated. Below this temperature, the shape of the alloy is easily deformed due to its flexible structure. At the activation temperature, the alloy can be changed by applying a force, but the structure resists this deformation and returns back to its initial shape. The activation temperature is a function of the ratio of nickel to titanium in the alloy. In contrast with Ni-Ti, copper-zinc alloys are capable of a two-way activation, and therefore a reversible variation of the shape is possible, which is a necessary condition for protection purposes in textiles used to resist changeable weather conditions. 

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