Biomedical Engineering Reference
In-Depth Information
sol-gel glasses with a pore size larger than 100 lm because of the large shrinkage
during sol-gel processing. Jie et al. [
79
] reported a foaming method to prepare
macroporous sol-gel bioactive glasses with pores larger than 100 lm. In the last
few years, the successful application of high relative humidity during gel drying has
made it possible to fabricate macroporous structures using a pore former such as
polyvinyl alcohol (PVA) [
80
]. Jones [
81
] has carried out extensive work developing
sol-gel derived bioactive scaffolds exhibiting nano-structured topography.
Recent key papers [
82
-
88
] and an informative review [
78
] highlighting the
potential of the sol-gel technology in the field of bone-tissue scaffold development
can be consulted for completeness.
4 Bioactive Glass-Ceramic Scaffolds
4.1 Fabrication and Microstructures
The limited strength, brittleness and low fracture toughness (i.e., ability to resist
fracture when a crack is present) of bioactive glasses obtained either via the
melting route or sol-gel processes have so far prevented their use for load-
bearing implants [
13
,
62
,
73
,
109
], and thus the repair and regeneration of large
bone defects in load-bearing anatomical sites (e.g., limbs) remain a clinical/
orthopedic challenge [
110
]. Recent developments related to bone TE try to
overcome this problem by fabricating architectures and components carefully
designed on different length scales, i.e., from the macroscale, mesoscale, and
microscale down to the nanometer scale [
60
,
111
], including both multifunctional
bioactive glass composite structures and advanced bioactive glass-ceramic
scaffolds exhibiting oriented microstructures, controlled porosity and directional
mechanical properties [
60
,
91
,
93
,
94
,
98
,
102
], as discussed in the following
paragraphs. Most studies (summarized in Table
1
) have mainly investigated the
mechanical properties, in-vitro and cell biological behavior of glass-ceramic
scaffolds. Scaffolds exhibiting compressive strength [
91
,
94
] and elastic modulus
values [
93
,
94
] above those of cancellous bone and close to the lower limit of
cortical bone have been developed.
The ''replication'' or ''polymer-sponge'' fabrication process is one of the suc-
cessful methods introduced for producing bioactive glass-ceramic scaffolds [
72
].
The foams are manufactured by coating a polyurethane or polyester foam with a
glass particle slurry. The polymer foam determines the final scaffold macro-
structure, and thus serves as a sacrificial substrate for the glass coating. The slurry
infiltrates the polymer structure and adheres to the surface of the polymer. Excess
slurry is squeezed out leaving a glass coating on the struts of the foam. After
drying, the polymer is burned out and the glass is sintered to the desired density.
The process replicates the macrostructure of the sacrificial polymer, and results in
a distinctive pore microstructure within the macrostructure (Fig.
4
). In addition to
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