Biomedical Engineering Reference
In-Depth Information
Table 6.1
Compositions of some common bioactive glass (wt%).
Glass
B
2
O
3
Na
2
OCaO K
2
O
MgO
SiO
2
P
2
O
5
CuO SrO ZnO Fe
2
O
3
45S5
0
24.50 24.50
0
0
45.00
6.00
0
0
0
0
13-93
0
6.00 20.00 12.00
5.00
53.00
4.00
0
0
0
0
13-93B3 53.00
6.00 20.00 12.00
5.00
0
4.00
0
0
0
0
1B
17.00 24.00 23.90
0
0
29.30
0
0
0
0
0
2B
33.10 23.40 23.40
0
0
14.40
0
0
0
0
0
3B
48.60 22.90 22.90
0
0
0
0
0
0
0
0
Cu-3
52.79
5.98 19.92 11.95
4.98
0
3.98
0.40
0
0
0
CS
51.73
5.86 19.52 11.71
4.88
0
3.90
0.40 2.00
0
0
CSZ
51.20
5.80 19.32 11.59
4.83
0
3.86
0.40 2.00 1.00
0
CSZF
50.88
5.76 19.20 11.52
4.80
0
3.84
0.40 2.00 1.00
0.40
hydrated, the glass begins to dissolve, and a silica-rich layer forms on
the surface of the glass. The low solubility of silica in body fluids is
the reason for the silica layer formation. Calcium ions from the glass
diffuse through the silica-rich layer and chemically bond with phosphate
present in the body fluids, forming an amorphous calcium phosphate
layer [1]. Over time, the calcium phosphate layer crystallizes into HCA.
The formation of the HCA is what allows tissues like bone to chemically
bond to the bioactive glasses [2].
Bioactive borate glasses, unlike silicate glasses, form HCA directly on
the surface of the underlying unreacted glass [3], without forming a
borate-rich layer. This is because the borate is readily soluble in body
fluids, similar to the phosphate glasses (Chapter 4). The degradation
products of the glass can be passed through the body naturally, predom-
inantly through urine [4]. The lack of a diffusion layer allows borate
glasses to react to completion without a significant reduction in the dis-
solution kinetics [5]. To establish how borate and silicate glasses differ
in behavior, a set of glasses were designed based on the original 45S5
Bioglass
®
composition, where different proportions of the silica were
replaced by borate. Figure 6.2(a) illustrates the weight loss (after reac-
tion in dilute phosphate solution at 37
◦
C) from: 45S5 silicate (0B); two
borosilicate glasses (1B and 2B), in which one-third and two-thirds of
the silica in the 45S5 composition was replaced by borate, respectively;
and a borate glass (3B), in which all the silica was replaced by borate.
Only the borate glass (3B) dissolved fully to the theoretical weight loss
(
58%), as the silica-containing glasses essentially stopped well short
(35% and 42%, respectively).
The kinetics of each glass are modeled in Figure 6.2(b) and show that
the 3B glass reaction is the most rapid to completion, and has reaction
∼
