Biomedical Engineering Reference
In-Depth Information
10.6 POLYMER CHOICE AND PROPERTY CONTROL
IN HYBRIDS
For biomedical applications, when a bioactive and biodegradable hybrid
is being designed to combine the bioactivity of silicate glass with the
toughness of biodegradable polymers, the hybridization can yield a
group of synthetic materials appropriate for a wide range of biomedical
applications like repair of soft or hard tissue, protecting membranes,
and scaffolds, to name but a few.
When a high proportion of bioactive silica is required, hybrids are com-
monly prepared via a sol-gel route. The starting reagents are dissolved
in a solvent together with some catalyst for hydrolysis and condensation
to yield precursor colloidal solution. The system then goes through the
sol state (colloidal solution) before gelation. Porous networks can also
be introduced into the hybrid and the porous structure will be tuned
so as to achieve optimum cell attachment and growth (Chapter 12).
The constituents of the hybrids primarily control their properties. Wide
ranges of organic polymers, regardless of being naturally or syntheti-
cally originated, are applicable as well as the inorganic metal-oxygen
fragments (e.g. Si-O or Ti-O) and cationic species. Hence, it is possible
to fabricate an enormous variety of hybrids. Yet other factors will tune
the hybrid properties, a few examples of which are listed in Table 10.2.
Only small modifications of the starting agents and their mixing ratios
will drastically change the mechanical and chemical properties.
Polymer choice is governed by a number of factors. The hybrid should
degrade at a controllable and preferably linear rate. Then, in the course
of degradation, silicate species like SiO
4
4
−
will be released from the
hybrid into the body fluid and hence stimulate representation of some
genes or cell growth in the nearby soft or hard tissue. Another advantage
Table 10.2
Some factors to control the property of hybrids.
Factors
Notes
Constituents
Starting materials, their reactivity
Ingredients
Catalysts, chelating agents
Composition
Mixing ratios of the constituents
Polymerization
Homo- or hetero-polymerization, sometimes controlled
by catalysts or coupling/chelating agents
Microstructures
Porosity, pore size, cluster size
