Bioactive Glass-Based Scaffolds for Bone Tissue Engineering - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 201

the extremely high equilibrium liquidus temperature of SiO 2 , (1713 C), and the
extremely high viscosity of silicate melts with high SiO 2 content. The processing
costs are considerable, due to the energy costs and use of platinum crucibles.
Melt-derived bioactive glasses have been used successfully as bone-filling
materials in orthopedic and dental surgery but their poor mechanical strength and
low toughness limit their application in load-bearing positions. However, one
method suggested to improve the mechanical strength of these bioactive glasses is
their transformation into glass-ceramics [ 72 ]. Glass-ceramics are partially crys-
tallized glasses produced by heating the parent bioactive melt-derived glass powder
above its crystallization temperature, usually at about 610-630 C[ 73 , 74 ].
Sintering of glass powder is one way to fabricate glass-ceramic scaffolds. During
the occurrence of crystallization and densification, the microstructure of the parent
glass shrinks, porosity is reduced and the solid structure gains mechanical strength
with increasing crystallization [ 62 ].
Low-temperature sol-gel processing offers an alternative to the conventional
glass and melting process [ 75 ]. This process involves the synthesis of an inorganic
network by mixing the metal alkoxides in solution, followed by hydrolysis,
gelation, and low-temperature firing to produce a glass. The sol-gel processing of
a silicate glass involves hydrolysis of alkoxide precursors, such as tetraethylor-
thosilicate (TEOS), to form a colloidal solution (sol). Polycondensation of silanol
(Si-OH) groups continues after hydrolysis is complete, beginning the formation of
the silicate (-Si-O-Si-) network [ 76 ]. As the network connectivity increases,
viscosity increases and a gel is formed. The gel is then subjected to controlled
thermal processes of aging to strengthen the gel, drying to remove the liquid
byproduct of the polycondensation reaction and thermal stabilization/sintering to
remove organic species from the surface of the material [ 77 ]. After these thermal
treatments, the sol-gel powder is derived via crushing and milling. Inherent in this
process is the ability to modify the network structure through controlled hydrolysis
and polycondensation reactions. Structural variation can thus be obtained without
compositional changes. Because the glasses can be prepared from gels by heat
treatment at relatively low temperatures (600-700 C), most of the disadvantages
of high-temperature processing can be eliminated with much higher control over
purity. Also, sol-gel processing offers the potential advantages of ease of powder
production, a broader range of bioactivity, and a better control of bioactivity by
changing microstructure through processing parameters. Sol-gel-derived bioactive
glasses provide excellent matrices for entrapping a variety of organic and inor-
ganic compounds and biologically important molecules [ 78 ].
In addition, sol-gel bioactive glasses with compositions varying over a wide
range have demonstrated bioactivity in vitro and in vivo, because they usually have
high specific surface area and Si-OH groups which could accelerate the surface
crystallization of HCA. Moreover, bioactive glasses prepared via sol-gel always
have an interconnected mesoporous structure, with pores of about 5-10 nm in
diameter. A macroporous sol-gel bioactive glass with two simultaneous pore
classes: i.e., larger than 100 lm and about 5-10 nm, has been proposed as a suitable
bone scaffold material [ 33 ]. However, it is very difficult to produce macroporous