Biomedical Engineering Reference
In-Depth Information
(a)
(b)
100 nm
200 nm
Figure 3.1
The versatility of the sol-gel process: (a) scanning electron microscope
of a bioactive sol-gel glass disc made under acid catalysis; and (b) transmission
electron microscope image of silica nanoparticles.
very exciting. For example, silicates and bioactive glasses can be made
that are either mesoporous or nanoparticles (Figure 3.1). The difference
in the sol-gel processing to produce the two materials is simply the
catalyst used in making the sol.
This chapter will describe the sol-gel process, discuss its benefits and
disadvantages over the melt-quench route, and describe some appli-
cations for the glasses. The most common sol-gel derived glasses for
biomedical applications are silica-based (SiO
2
) glasses, and therefore
the emphasis in this chapter is on silica sol-gel processing. However,
phosphate glasses have also been produced using the sol-gel process
(Chapter 4). More details can be found in [1-3].
3.2 WHY USE THE SOL-GEL PROCESS?
The motivation for sol-gel processing is primarily the potentially higher
purity and homogeneity and the lower processing temperatures associ-
ated with sol-gels compared with traditional glass melting or ceramic
powder methods. The goal of sol-gel processing is to control the glass
formation during the earliest stages of processing, producing uniquely
homogeneous structures.
The purity and homogeneity of dense gel silica made by the hydrolysis
and condensation of alkoxide precursors are superior to other silica glass
processing methods. The ability to produce optics with nearly theoretical
limits of optical transmission, lower thermal expansion coefficients and
