Biomedical Engineering Reference
In-Depth Information
(e) be produced by a process that allows the scaffold to be shaped
to fit a range of defect geometries and be up-scalable for mass
production; and
(f) be sterilisable and meet the regulatory requirements for clini-
cal use.
There are numerous synthetic bone graft materials as commercial
products, but none that fulfil all the criteria listed above. Metals are not
bioactive and generally do not degrade, although magnesium is being
investigated as a potentially biodegradable metal for stent applications.
Bioceramics (calcium sulfates and calcium phosphates) are usually cho-
sen because of their long track record in clinical use. Bioactive glasses
have the potential to fulfil most of these criteria. But currently their
commercial products are limited to particles. Why is this?
12.4 BIOGLASS AND THE COMPLICATION OF
CRYSTALLISATION DURING SINTERING
All processing techniques for making porous scaffolds from particles
involve sintering. Sintering is the fusion of particles at high temperature
(Figure 12.3). Raising the temperature above the glass transition
temperature (
T
g
) causes local flow of the glass and allows particles
to fuse. However, to maintain the amorphous glass structure and
properties, the temperature must not be raised above the crystallisation
onset temperature (
T
c,onset
). Unfortunately for the original Bioglass
composition (46.1mol% SiO
2
, 24.4mol% Na
2
O, 26.9mol% CaO
and 2.6mol% P
2
O
5
), the low silica content causes
T
g
and
T
c,onset
to
be too close together, so it is not possible to sinter the glass without
crystallising, which leads to the formation of a glass-ceramic and
reduction in bioactivity. Therefore, only Bioglass particles are available
commercially (Figure 12.1). Only now have researchers been able to
understand the relationship between glass structure, composition, glass
Glass particles
Fused particles
HEAT
Pore left due to incomplete sintering
Figure 12.3
Schematic of the principles of sintering glass particles.
