Biomedical Engineering Reference
In-Depth Information
TABLE 9.4
Bioactive Glass-Ceramic Compositions [12, 13]
BioactiveGlass-CeramicSystem
Comment
Na
2
O-K
2
O-CaO-SiO
2
-P
2
O
4
Traded under the name Ceravital
®
, has
enhanced mechanical properties due to
apatite precipitation
MgO-CaO-SiO
2
-P
2
O
5
Known as A-W glass-ceramic, has enhanced
mechanical properties due to both apatite
and wollastonite precipitation
At the time of writing, this composition is the
most commonly used glass-ceramic for
clinical applications
Na
2
O-K
2
O-CaO-CaF
2
-SiO
2
-P
2
O
4
Has enhanced mechanical properties due to
canasite precipitation
Na
2
O-K
2
O-MgO-CaO-SiO
2
-P
2
O
4
-CaF
2
Traded as Ilmaplant
®
, has enhanced
mechanical properties due to both apatite
and wollastonite precipitation
In an attempt to improve the mechanical properties of bioactive glass compositions,
Brohmer and coworkers developed the first bioactive glass-ceramic: Ceravital
®
[12]. With a
composition similar to Hench's 45S5 composition glass, it had half the bioactivity of 45S5.
Its mechanical strength was lower than dense HA but higher than 45S5 and as a result
used for minimal load-bearing applications such as middle ear ossicular chain replace-
ment. Improvements in mechanical properties were made by Kokubo and coworkers who
developed A-W glass-ceramic (commercially traded under the name Cerabone A/W
®
).
Based on the MgO-CaO-SiO
2
-P
2
O
5
system, thermal treatment lead to the precipitation of
apatite (oxluroapatite-Ca
10
(PO
4
)
6
(OF
2
)) and wollastonite phases (β-CaSiO
3
) resulting in a
fine-grained glass-ceramic with mechanical properties similar to that of human cortical
bone. However, its bioactivity was half that of 45S5 composition. Interestingly, unlike 45S5
glass, after 10 days of immersion in simulated body fluid, Ca-P phases were precipitated
on the surface of the glass without the prescience of a silica layer, prompting Kokubo to
conclude that precipitation of Ca-P and not the formation of a silica layer was the prerequi-
site for direct implant bonding with bone tissue. Further improvements in bioactivity have
lead to compositional modifications resulting in glasses based on Na
2
O-K
2
O-CaO-CaF
2
-
SiO
2
-P
2
O
4
systems and Na
2
O-K
2
O-MgO-CaO-SiO
2
-P
2
O
4
-CaF
2
systems (see Table 9.4).
However, their inherent precipitation of specific phases has limited their applications as
coating materials because it is difficult to precisely control the formation of phases dur-
ing the coating process. As monoliths, thermal processing steps (i.e., lower temperatures
to form nuclei followed by ramping up to higher processing temperatures to grow crys-
tal phases without coarsening) often results in temperature gradients and cracking due
to internal stresses. Subsequently, coatings are difficult to produce since they fail due to
cracking and delamination before or during in vitro testing.
Sol-Gel Bioactive Glasses
Sol-gel bioactive glasses couple controlled bioactivity with controlled resorbability (facili-
tating controlled release of ionic dissolution products) to elicit a specific cellular and genetic
response capable of improved tissue growth, replacement, and eventual regeneration.
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