Further developments in this area were focused on the substitution of the polyether macrodiol by novel hydrocarbon, polycarbonate or siloxane macrodiols ( Gunatillake et al., 2003) or a combination of these which in general are responsible for the flexibility of the SPUs ( Król, 2007). Historically, biostable polyurethanes were first developed by using polyether type of polyol and different aromatic diisocyanates.
The properties of polyurethanes as those shown in Figure 1 depend on the various types of monomers that are used during their manufacturing (see Table 1). Standard two-step reaction to prepare segmented poly (urethane)s and poly(urethane-urea)s During this stage, additional urethane functional groups are formed when using a diol chain extender whereas ureas are produced when a diamine is used. In the second step, the low molecular weight chain extender is used to link the prepolymer segments yielding a high molecular weight polymer. Here, the characteristic urethane linkages are formed through the reaction between the isocyanate groups and the hydroxyl-terminated end groups of the polyol.
In the first step, an excess of the diisocyanate reacts with the soft segment polyol to form the prepolymer. In the two steps method for SPU synthesis, a prepolymer is first obtained and then chain extended as illustrated in Figure 1. These materials are thermoplastic block copolymers of the (AB)n type consisting of alternating sections of hard segments, composed of a diisocyanate and a low molecular weight diol chain extender, and soft segments, generally composed of various types of polyols, also called macrodiols. In general, PU’s can be prepared in one shot process or more commonly by a two step method, especially for the case of segmented polyurethanes (SPU’s). Polyurethanes (PU’s) properties depend both on the method of preparation and the monomers used. Emphasis is also made on the mechanism of degradation under various conditions and the techniques used for following the changes in their properties. In this chapter, general aspects of polyurethanes chemistry are presented first and then, the various types of degradations that can affect these polymers both in vivo and simulated in vitro conditions. In this way, polyurethanes for biomedical applications can be classified in two main types, according their relative stability in the human body as either biostables or biodegradables.
Therefore many research work have been focused on varying the chemical composition to enhance biostability or more recently to control the biodegradability of polyurethanes depending on specific applications in the cardiovascular field ( Bernacca et al., 2002 Stachelek et al., 2006 Thomas et al., 2009 Wang et al., 2009 Hong et al., 2010 Arjun et al., 2012 Styan et al., 2012).
Traditionally, segmented polyurethanes (SPUs), have been used in cardiovascular applications ( Kuan et al., 2011) as permanent devices such as pacemaker leads and ventricular assisting devices however, due to their great chemistry versatility, SPUs can also be tailored to render biodegradable systems for the tissue engineering of vascular grafts and heart valves. Polyurethanes are a family of polymers used in a variety of biomedical applications but mainly in the cardiovascular field due to their good physicochemical and mechanical properties in addition to a good biocompatibility.
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