48 R. Tran et al.
biomedical materials since the 1960’s.
In addition to good biocompatibility, their
controllable and diverse mechanical properties make them ideal biomaterials.
The typical applications of polyurethanes in medicine over the past years have
included pacemaker leads, catheters, artiﬁcial heart prostheses, and coatings for
silicone breast implants.
These applications require that the material remain
stable inside the body for long periods of time. Subsequently, all traditional
polyurethanes have been designed to be biostable and not degrade easily in vivo.
By using polyether soft segments, a more hydrolytically stable material was
produced, which increased the stability of the polymer in an in vivo setting.
However, the polyether soft segments proved to be more susceptible to oxidation.
The oxidative effects lead to unwanted degradation of the material. Due to the
toxic pre-cursors used in the polyurethane synthesis, the degradation of these
biostable polyurethanes would cause the release of carcinogenic compounds inside
the body. For example, toluene diisocyanate is one of the most commonly used
diisocyanates in the synthesis of biostable polyurethanes. Upon degradation of
the urethane bonds, it results in the formation of toluene diamine, which has been
shown to be carcinogenic. The effect of oxidation and subsequent degradation
of the polyether–urethanes led to the development of oxidation and hydrolysis
resistant polycarbonate based polyurethanes.
Due to these complications, the interest in the hydrolytically unstable
polyester based urethanes has increased over the last decade. Currently, the
primary degradable polyurethanes used as a biomaterial in tissue engineering
include polyester–urethanes, polyether–urethanes, and polyester–ether urethanes.
Alternatively, hydrolytically labile bonds may be introduced in the hard seg-
ment to control the degradation rate of the polyurethane to suit a particular
Faster degradation rates can also be obtained by making the
polyurethane degradable, both hydrolytically and enzymatically.
types of biodegradable polyurethanes are discussed in the following sections.
184.108.40.206 P olyester–urethanes
Polyester–urethane is a term used to describe a polyurethane comprising of a
polyester based soft segment. Different polyesters such as poly(L–Lactide) (PLA),
poly(ε–caprolactone) (PCL), poly(vinyl alcohol) (PVA), and poly(glycolic acid)
(PGA) have been used by various researchers for the synthesis of polyester–
urethanes with different properties.
220.127.116.11 PCL–based polyester urethanes
Poly(ε–caprolactone) (PCL) diol has been used by various researchers to synthe-
size polyester–urethanes with a wide range of properties. Different polyester–
urethanes can be obtained by varying the molecular weight of the PCL–diol, the
ratio of hard segment and soft segment, and the properties of monomers used in
The low glass transition temperature of PCL (T
the polymer to be in an amorphous or semi-crystalline state at use temperature,
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