Biomedical Engineering Reference
In-Depth Information
concentration, and is typically two to four times higher than that of
blood. Additional property data for these and other REAS glasses can
be found elsewhere [4, 6, 8].
An important property of REAS glass for
in vivo
use is their out-
standing chemical durability. Since the radioisotope is a chemical and
physical part of the glass structure, as opposed to a surface coating, the
radioisotope can only escape from a target organ if the glass dissolves in
the body fluids during the time it is radioactive.
Because of their excellent chemical durability in the biological envi-
ronment, the release or leakage of the radioisotope from a REAS glass is
so small as to be physiologically unimportant, in most applications. Sev-
eral thousand patients with primary liver cancer have been treated with
radioactive yttrium aluminosilicate (YAS) glass microspheres (40wt%
yttria) for over 20 years with no reported incident of excessive leakage
of the
90
Y from the liver due to the dissolution of the YAS glass.
13.4 BIODEGRADABLE GLASSES: RARE EARTH
BORATE/BOROSILICATE GLASSES
There are certain biological applications, such as radiation synovectomy
of arthritic joints, where it is desired for the glass delivery vehicle to
degrade gradually in the body once it is no longer radioactive. While
not in commercial use at this time, alkali borate (B
2
O
3
) and borosilicate
glasses containing RE oxides, along with other cations, are known to
be biodegradable [9, 10]. Some typical compositions of such glasses are
given in Table 13.2 and more compositions can be found in Refs [11, 12].
Table 13.2
Composition (wt%) of selected biodegradable glasses and their
reaction rate in phosphate-buffered saline (PBS) solution.
Oxide
DyLB3-10 HoLB3-10 DyLB3-30 DyLAB5 DyLAB10
LS-Y
Li
2
O
11.3
11.3
8.8
6.1
5.6
13.7
B
2
O
3
78.7
78.7
61.2
58.4
53.4
-
Dy
2
O
3
10.0
-
30.0
23.7
23.1
-
Al
2
O
3
-
-
-
6.5
12.6
-
MgO
-
-
-
1.5
1.5
-
SiO
2
-
-
-
3.8
3.7
59.2
Y
2
O
3
-
-
-
-
-
11.4
Ho
2
O
3
-
10.0
-
-
-
-
6 (37
◦
C)
0.3 (21
◦
C) 0.07 (22
◦
C)
11 (37
◦
C)
Reaction
-
-
rate
a
(
0.4 (42
◦
C)
μ
m/h)
a
The rate at which a surface layer develops on the surface of a microsphere when it is immersed
in PBS solution, pH
=
7.4, at the temperature indicated [9, 10].