Degradation of long-term implants is of course a major problem, however, probably the most difficult problem relating to the use of these materials, and in fact all polymers, in vivo is that they are prone to interaction with the biological entity. Thus encapsulation by various biological components is a common problem, this may present as tissue growth, or it may present as calcification; for example urethral stents commonly become encrusted with materials such as calcium phosphate (Singh et al., 2001). Calcification is also a problem for heart valves and other blood contacting uses (Vyavahare et al., 1997), though here the development of blood clots is likely to be particularly problematic.

Crucial to the use of polyurethane in blood contacting applications is the prevention of blood clot formation, which of course could have fatal consequences. Even though, this material is considered to have good blood compat-ability, the development of even small blood clots can have serious medical consequences. Thus many attempts have been made to improve the haemo-compatability of polyurethanes, and in particular a commercially available poly(urethane-urea) based on MDI and ethylene diamine or 4,4'-diaminodi-phenylmethane, with a poly(tetramethylene oxide) flexible segment, known as Biomer (see below), has been used for the fabrication of devices such as artificial hearts and vascular grafts. It would appear that the effectiveness is somewhat influenced by the way the material is processed (Jagur-Grodzinski 1999; Orang et al., 1996), though this generally involves repetitive solvent casting, depositing a series of layers; and various surface treatments have also been effective in this regard. However, the anti- thrombogenic properties have been particularly enhanced by a range of chemical modifications to the surface. This can be done in a number of ways, but of particular interest is the addition of heparin




Carboxylate units on Polymer Surface




Amino units on Polymer Surface

Heparin units on Polymer Surface

Fig. 3.12 Covalent attachment of Heparin onto the surface of a polyurethane elastomer



to the system either using non-covalent interactions or by covalent attachment using a chemically modified polymer (Goddard and Hotchkiss, 2007) as shown in Fig. 3.12. Here a functionalized polyurethane is aminated and subsequently reacted with heparin (Alferiev et al., 2006). There is considerable interest in this area, and it seems likely that a combination of high-quality processing and a range of surface treatments, offer a real solution to the problems associated with long term implantation of polyurethane materials in blood contacting environments. This will of course subsequently offer a range of additional options (particular interest is shown in polymer replacements for heart components for example) for medical practitioners.

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