Biodegradable Elastomeric Polymers and MEMS in Tissue Engineering 47
Figure 3.6. SEM images of scaffold surface indicate the presence of well defined pores (A)
and even cell distribution of cells on the scaffold (B and C). For color reference, see page 258.
was performed to evaluate the foreign body response to CUPEs. Tissue samples
were explanted at 1 week and 4 week time points, and examined histologically
using H&E staining. At the 1 week time point, the fibrous capsules surrounding
the CUPE implants were thinner than those surrounding the PLLA implants
(Fig. 3.5). At 4 weeks, the fibrous capsule thickness was reduced for both the
CUPE and PLLA implants, and was found to be comparable for both polymers.
These results indicated that CUPE had a weaker acute inflammatory response and
a similar chronic inflammatory response compared to PLLA.
We have also fabricated thin 3-D porous soft and elastic scaffold sheets (150
μm thick) by a simple freeze–drying method (Fig. 3.6(A)). Based on a scaffold–
sheet tissue engineering strategy, we proposed the use of CUPE scaffold sheets for
tissue regeneration. The thin scaffold sheets allowed even cell seeding, growth,
and distribution (Figs. 3.6(B) and 3.6(C)), as the cells did not have to penetrate too
deep within the scaffold. Soft scaffolds would also facilitate scaffold assembly into
various shapes through folding, rolling, trimming, and bending. The mechanical
strength of CUPE scaffolds would allow surgical handling and bioreactor training
for the seeded scaffolds.
3.3.2 Polyurethanes
Polyurethanes are segmented block co-polymers, which consist of a soft and hard
segment. The soft segment is composed of a macrodiol, and the hard segment is
a combination of a diisocyanate and a chain extender. Typically, the macrodiol
is usually a difunctional polyester or polyether segment, and a low molecular
weight diol or diamine is used as a chain extender. The segmented architecture is
responsible for the unique mechanical properties of polyurethanes. The partially
crystallized hard segments act as virtual crosslinks to give polyurethanes their
high tensile strength and elasticity.
Polyurethanes are a class of polymers which have been extensively used as
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48 R. Tran et al.
biomedical materials since the 1960’s.
61
In addition to good biocompatibility, their
controllable and diverse mechanical properties make them ideal biomaterials.
62
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.
61,63
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
application.
6466
Faster degradation rates can also be obtained by making the
polyurethane degradable, both hydrolytically and enzymatically.
67
The different
types of biodegradable polyurethanes are discussed in the following sections.
3.3.2.1 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.
3.3.2.2 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 synthesis.
64
The low glass transition temperature of PCL (T
g
60
C) allows
the polymer to be in an amorphous or semi-crystalline state at use temperature,
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