Biomedical Engineering Reference
In-Depth Information
4.1.2
Boom-Up Approach
4.1.2.1 Wet chemical processes
These include colloidal chemistry, hydrothermal methods, sol-gels,
and other precipitation processes. Generally, solutions of different
ions are mixed in well-deined quantities and under controlled
conditions of heat, temperature, and pressure to promote the
formation of insoluble compounds, which precipitate out of solution.
Among the advantages of the established wet chemistry methods,
are good control over microstructure and particle morphology.
The sol-gel process involves the evolution of inorganic networks
through the formation of a colloidal suspension (sol) and gelation of
the sol to form a network in a continuous liquid phase (gel) [28]. The
precursors for synthesizing these colloids consist usually of a metal
or metalloid element surrounded by various reactive ligands. The
starting material is processed to form a dispersible oxide and forms
a sol in contact with water or dilute acid. Removal of the liquid from
the sol yields the gel, and the sol/gel transition controls the particle
size and shape. Calcination of the gel produces the oxide.
Sol-gel processing refers to the hydrolysis and condensation of
alkoxide-based precursors such as Si(OEt)
4
(tetraethyl orthosilicate,
or TEOS). The reactions involved in the sol-gel chemistry based on
the hydrolysis and condensation of metal alkoxides M(OR)
z
can be
described as follows:
MOR + H
2
O
→
MOH + ROH (hydrolysis) (4.1)
MOH + ROM
→
M-O-M + ROH (condensation) (4.2)
The production of glasses by the sol-gel method permits
preparation of glasses at far lower temperatures than is possible
by using conventional melting. It also makes possible synthesis of
compositions that are dificult to obtain by conventional means
because of problems associated with volatilization, high melting
temperatures, or crystallization. In addition, the sol-gel approach
is a high-purity process that leads to excellent homogeneity. The
most successful applications utilize the composition control, micro-
structure control, purity, and uniformity of the method combined
with the ability to form various shapes at low temperatures. Finally,
the sol-gel approach is adaptable to producing ilms and ibers as
well as bulk pieces.
