Biomedical Engineering Reference
In-Depth Information
7.3 Biodegradable and bioactive ceramics:
second generation bioceramics
as it was indicated in Section 7.1, between almost bioinert and resorbable
ceramics, there is another important group of bioceramics whose surface
reacts with living tissues. these ceramics are usually referred to as bioactive
and, after a series of chemical processes, form a mechanically strong bond
with the living tissues. thus, these bioceramics are suitable for many clinical
uses when bonding of the biomaterial and the living tissues is required. in
this section, the most important resorbable and bioactive ceramics will be
described comparatively. in addition, it must be considered that decreasing
the particle size or increasing the hydrophilic character can make a bioactive
material resorbable.
2, 28
the most widely used biodegradable ceramics are based on calcium
phosphates and calcium sulphates.
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indeed, both have in common a certain
degree of reactivity that promotes positive interactions with living tissues.
Biodegradable ceramics are designed to fulfil specific body functionality
for a given period of time, aiding the self-repair processes of the living
organism. after that they must be resorbed. this could be an ideal behaviour
because it avoids the problems associated with a long residence of a synthetic
biomaterial in the body. the critical aspect in the design of biodegradable
ceramics is to adjust (slow down) the kinetics of the ceramic degradation,
which is usually quicker than that of living tissue formation. Moreover, it
must be considered whether the decrease in the mechanical properties of
ceramics during the resorption process could impede the required functionality.
table 7.4 includes important second generation bioceramics and their main
clinical applications.
the search for bioactive ceramics yielded promising results in the 1980s.
Larry Hench pioneered the field when he put an imaginative idea into practice,
that is, that certain glasses in contact with living tissue will be able to bond
to bone.
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These bioactive ceramics react with physiological fluids, but only
at the level of the surface of material, forming an apatite-like biologically
active layer. in the presence of living cells, this apatite can form new bone that
tightly bonds bioactive material and osseous tissues. the more characteristic
examples of bioactive ceramics are hydroxyapatite and some compositions
of glasses and glass-ceramics.
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For medical applications, bioactive materials are provided in different
formats: powder, porous monoliths, dense monoliths, injectable mixtures
and coatings. they have excellent features in terms of biocompatibility
and bioactivity, but they are brittle, rendering it impossible to use them for
repairing large osseous defects. However, these ceramics are excellent for
filling small defects, where the rate of bone regeneration is the main concern
and where mechanical properties are just a secondary aspect.
