Biomedical Engineering Reference
In-Depth Information
hurdle caused by the confl ict between the properties of biodegradability and mechanical reliability,
which is discussed in Sections 1.3.2.3 and 1.3.3.4.
1.3.2.2 Biodegradability
The basic constituents of most bioactive glasses are SiO
2
, Na
2
O, CaO, and P
2
O
5
. The well-known
45S5 Bioglass contains 45% SiO
2
, 24.5% Na
2
O, 24.4% CaO, and 6% P
2
O
5
, in weight percent. The
bioreactivity of the material is composition-dependent. Hench and coworkers [22] have systemati-
cally studied a series of glasses in the four-component systems with a constant 6 wt.% P
2
O
5
content.
This work is summarized in the ternary SiO
2
-Na
2
O-CaO diagram shown in Figure 1.1. In region A,
the glasses are bioactive and bond to bone. In region B, glasses are nearly inert when implanted.
Compositions in region C are resorbed within 10-30 days in tissue. In region D, the compositions
are not technically practical.
The key advantage of bioactive glasses that makes them promising scaffold materials is the
possibility of controlling a range of chemical properties and thereby the rate of bioresorption. The
structure and chemistry of glasses, in particular sol-gel derived glasses [47,48], can be tailored at
a molecular level by varying either composition, or thermal or environmental processing history. It
is possible to design glasses with degradation properties specifi c to a particular application of bone
tissue engineering.
1.3.2.3 Mechanical Properties
A primary disadvantage of bioactive glasses is their low fracture toughness (Table 1.4) because of
their amorphous structure. Hence, many researchers sintered bioactive glasses at their crystalliza-
tion temperatures in order to improve the mechanical performance of these materials. However,
it was reported that crystallization of bioactive glasses could decrease the level of bioactivity [70]
SiO
2
B
I
C
A-WGC
(high P
2
O
5
)
1
8
5
2
0
A
D
Na
2
O
CaO
FIGURE 1.1
Compositional dependence (in wt.%) of bone bonding and soft tissue bonding of bioactive
glasses and glass-ceramics. Bioactivity index
I
B
is defi ned as
I
B
=
100/
t
0.5
, where
t
0.5
is the time taken for 50%
of the interface to bond to bone. All compositions have a constant 6 wt.% of P
2
O
5
. In region A, the glasses are
bioactive and bond to bone. In region B, glasses are nearly inert when implanted. Compositions in region C are
resorbed within 10-30 days in tissue. In region D, the compositions are not technically practical. In the region
where
I
B
>
8 (called region E), soft tissue bonding occurs. Apatite-wollastonite glass-ceramic (A-WGC) has
higher P
2
O
5
content [22].
