Shape stents

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. 

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). 

Figure 21.1. Stents are used to open arteries of the heart blocked by atherosclerotic plaques (A) a balloon and stent are placed across the plaque (B) the balloon is expanded, leaving the stent to prop open the artery (C) restenosis is the process wherein scar tissue builds up around the stent, again causing a flow restriction. A balloon is required for stainless steel, whereas a nitinol stent will expand on its own, due to the shape memory property of nitinol. (From Ref. 11, with permission.)...

Stent therapy in STEMI remained intriguing because it allows for establishment of vessel patency and the ability to protect the culprit lesion. Several trials evaluated PCI versus PTCA, all showing a marked benefit in the combined endpoints of mortality and the requirement of revascularization with stenting [47-49]. The STENT PAMI trial helped shape the future of STEMI treatment. Overall mortality was not significantly different between the two groups however, the combined end point of mortality, reinfarction, stroke, or revascularization was positive at 6 months owing to a marked increase in the requirement for revascularization in the PTCA... 

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. 

The variety of medical applications for shape memory alloys is impressively hroad. These alloys are already used as stents inserted into blocked arteries, as vena-cava filters, as orthodontic devices, and in eyeglasses. 

Molecular biology began to take shape in the 1940s. In those days the aim of the field became transparent molecular biology was to discover and characterize the molecular constituents of cells (Stent 1968 Hess 1970). And that is certainly what it has been doing very successfully over the last 60 years or so. 

Before insertion, the stents are crushed down so they can be fed into the artery through a thin tube. As the crushed stent heats up in the blood it returns to its original shape. 

One interesting alloy of titanium and nickel, called Nitinol, exhibits shape-memory properties. Below a particular temperature (the transformation temperature), the crystal structure of the alloy is such that it can be plastically deformed (martensitic). As the alloy is heated, the crystal structure alters to one that is more ordered and rigid (austenitic), and the deformed metal reverts to its original shape. This effect has been exploited in a number of devices, including a stent (a device used to hold open passageways such as arteries). The stent is placed inside a small-diameter catheter for insertion into the body, where it expands on being warmed to bod y temperature. 

FIGURE 19.4 Molecular mechanism and macroscopic effect of a shape-memory polymer, (a) Schematic representation of the thermally induced shape-memory effect of a polymer network with (b) Shape recovery of a stent with T = 52° in water at 37°C. The stent gradually changed from its... 

L. Xue, S. Dai, Z. Li, Biodegradable shape-memory block copolymers for fast self-expandable stents, Biomaterials 31 (2010) 8132-8140. 

G.M. Baer, W. Small IV, T.S. Wilson, W.J. Benett, D.L. Matthews, J. Hartman, D.J. Maitland, Fabrication and in vitro deployment of a laser-activated shape memory polymer vascular stent, Biomed. Eng. Online 6 (2007) 43. 

Small, W., Buckley, P. R., Wilson, T., Benett, W. J., Hartman, J., Saloner, D. Maitland, D. (2007) Shape memory polymer stent with expandable foam a new concept for endovascular embolization of fusiform aneurysms. IEEE Transactions on Biomedical Engineering, 54, 1157—1160. 

Wache, H. M., Tartakowska, D. J., Hentrich, A., and Wagner, M. H. 2003. Development of a polymer stent with shape memory effect as a drug delivery system. Journal of Materials Science Materials in Medicine 14 109-112. 

Guidewires, mechanical heart-valve housings and struts, biologic heart-valve stents, vascular stents, vena cava umbrellas, artificial heart housings, pacemaker leads, leads for implantable electrical stimulators, surgical staples, supereleastic properties of some nickel-titanium formulations, shape memory properties of some Ni titanium formulation, radiopaque markers... 

Biodegradable polymers play a prominent role in the family of SMPs. This is due to their biodegradabdity because in many of the medical applications the related devices with SM function (e.g., sutures, catheters, stents) should be present only temporarily in the human body. The other, not less important, aspect is that the switch temperature for shape programming (Tf.j. g) can well be matched with that of the body. 

Their work aimed at enhancing the recovery stress and accelerating the shape recovery process. Stents with fast recovery at T = 37 °C were also produced form poly(s-caprolactone-co-D,L-lactide) which not only has the appropriate but also degrades better than the reference PL A [84]. 

Wang, J., and Zhou, S. (2012) A shape memory stent of poly(e-caprolactone-co-DL-lactide) copolymer for potential treatment of esophageal stenosis. J. 

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