Biomedical Engineering Reference
In-Depth Information
As this happens, mechanical properties are rapidly lost and the local
pH at the site of implantation reduces rapidly because the degradation
products are acids, even if they are naturally occurring acids.
The rapid release of acidic products during degradation can cause
inflammatory responses, but may be mitigated through careful design
of composites (Section 9.3), where the autocatalytic process can be
compensated by bioactive glass components that increase pH on disso-
lution. In any case, assessment of the potential toxicity or inflammatory
reactions as a consequence of polymer degradation is always required.
Polyhydroxyalkanoates (PHAs) belong to a type of microbial
polyesters being increasingly considered for tissue engineering [6].
PHA can exist as homopolymers of hydroxyalkanoic acids, as well as
copolymers of two or more hydroxyalkanoic acids. The chemically
different structure of these polymers allows the development of polymer
systems with different properties that affect their degradation rates in
biological media as well as their mechanical properties. A major obstacle
in expanding the use of polymers of the PHA group is their availability,
with only two types of PHA, namely poly(3-hydroxybutyrate)
(P3HB) and poly(3-hydroxybutyrate- co -hydroxyvalerate), being readily
available.
Relevant properties of some of the synthetic polymers most widely
used in tissue engineering are listed in Table 9.2. For applications in
hard tissue repair and load-bearing sites, these polymers on their own
are too weak, this being one of the reasons for their combination with
bioactive glasses in composites, as discussed next.
9.3 COMPOSITE SCAFFOLDS CONTAINING
BIOACTIVE GLASS
This section will discuss specifically the materials science and technology
of composites based on the combination of biodegradable polymers and
bioactive glass particles, added as filler or coating to the polymer matrix,
for development of bone tissue scaffolds (Figure 9.4) [7].
The combination of degradable polymers and inorganic bioactive
particles in the form of a scaffold is promising for bone repair, as it
not only allows the combination of the 'right' biomaterials but also
using a polymer matrix makes it easy to process the composite into
a scaffold with a highly interconnected 3D pore network. Polymers
can be easily fabricated to form complex shapes and structures, yet, in
general, they lack a bioactive function (e.g. strong bonding to bone), and
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