Biomedical Engineering Reference
In-Depth Information
Melt glasses
early bioactive glasses, prepared by quenching a melt, contained SiO
2
and P
2
O
5
as network formers and CaO and na
2
O as network modifiers.33
33
Many advances in the development of bioactive glasses and other bioactive
materials were reached by the
in vitro
assays of bioactivity performed in
fluids mimicking human blood plasma. The most common
in vitro
solution
to evaluate the bioactive response is so-called simulated body fluid (SBF),
proposed by Kokubo
et al
.
89
SBF is an aqueous acellular solution with a
composition of inorganic ions almost equal to human blood plasma. in these
tests the formation on the glass surface of a layer of HCa nanocrystals is
indicative of a positive bioactive response because the glass will bond with
the living tissues when implanted.
2
In vitro
tests allowed comparison of the
reactivity of glasses with different kinetics of bioactive response as well as
to in depth study of the mechanism of bioactive bond formation. thus, the
essential role of the initial silica-rich layer formed in the glass surface was
shown to be due to the ionic interchange of calcium and sodium ions in the
glass and protons in the solution. this layer attracted the calcium, phosphate
and carbonate ions in solution to form an amorphous calcium phosphate layer,
which crystallizes into biologically active HCa nanocrystals. this HCa layer
attracts the inorganic moieties that promote new bone formation.
2
Sol-gel glasses
the sol-gel synthesis of bioactive glasses allows their bioactive response to
be tailored and accelerated.
90
In a sol-gel glass (SSG), textural properties
can be modified to control their bioactive response. The sol-gel method is
more versatile because it allows bioactive coatings, fibres and highly porous
monoliths to be used as scaffolds for tissue engineering including growth
factors, polypeptides, and so on.
the sol-gel route is more time consuming than the traditional quenching
of a melt but requires noticeably lower temperatures. CaO-P
2
O
5
-SiO
2
has
been the most widely system of SSgs studied,
36, 40, 91-97
with some additions,
MgO, ZnO, or removals, P
2
O
5
, to modify mechanical properties, sorption
rates or the interaction with osteoblasts.
98-104
CaO plays an essential role in
the texture and the bioactive response of a SSg.
105, 106
Ca
2+
ions leaching
from the glass to the medium increase both the silanol (Si-OH) group
concentration at the surface, which favours the formation of the HCa layer,
and the supersaturation of the solution with respect to apatite. in addition,
differences were found in the
in vitro
formation of HCa depending on the
presence or not of P
2
O
5
in the glass.
107
CaO-SiO
2
glasses exhibit higher
initial reactivity. However, HCa nanocrystal formation required 7 days.
In contrast, in CaO-P
2
O
5
-SiO
2
glasses, amorphous calcium phosphate
