Biomedical Engineering Reference
In-Depth Information
Table 2.2
Bioglass (45S5) composition in molecular and weight percentages.
Composition
SiO
2
Na
2
O
CaO
P
2
O
5
Mol%
46.13
24.35
26.91
2.60
Wt%
45.00
24.50
24.50
6.00
solution rises. Increases in pH can cause the calcium and phosphate ions
that are naturally present in body fluid to come out of solution. Phos-
phorus and calcium are also released into the physiological environment
by the glass. Accompanied with the pH rise, this drives the formation of
HCA at the glass surface, which is chemically similar to the inorganic
part of bone.
The mechanism for dissolution and bone bonding of a bioactive glass
proposed by Larry Hench is a multi-stage process involving the following
steps [7, 8]:
1. rapid ion exchange of alkali-metal cations (e.g. Na
+
,Ca
2
+
)with
H
+
from body fluid;
2. loss of soluble silica (effectively silicic acid), leaving behind -Si-OH
bonds;
3. condensation and repolymerisation of the Si-OH bonds to create
a silica-rich (cation-depleted) layer;
4. migration of Ca
2
+
and [PO
4
]
3
−
groups from inside the glass and
from the body fluid, forming an amorphous calcium phosphate
layer that grows on the silica-rich layer surface; and
5. crystallisation of the amorphous layer by incorporation of OH
−
and [CO
3
]
2
−
from solution to form HCA.
After the formation of crystalline HCA, the following steps were pro-
posed by Hench [8] to occur:
6. biological moieties absorbed in HCA layer;
7. macrophage action;
8. attachment of stem cells;
9. differentiation of stem cells;
10. generation of matrix; and
11. crystallisation of matrix.
A compositional diagram of the SiO
2
-Na
2
O-CaO system is shown
in Figure 2.1 [8]. All compositions have a fixed 6wt% of phosphate
