Biomedical Engineering Reference
In-Depth Information
Vaddiraju et al.
(
2009
) have very recently analyzed the role of H
2
O
2
outer diffusion on
implantable glucose sensor performance. These authors point out that the outer membrane
in implantable glucose sensors significantly influences the performance of the sensors by
governing the diffusion of various participating species. These authors correlated the role
of the outward H
2
O
2
diffusion through the outer membrane of glucose sensors to their sensi-
tivity. In electrochemical biosensors, particularly first-generation Clark-type glucose sensors,
the flavoenzyme glucose oxidase (GO
x
) is immobilized on a working electrode. The FAD
redox cofactor of GO
x
catalyzes the oxidation of glucose to glucolactone (
Vaddiraju et al.,
2009
):
Glucose
þ
GO
x
ð
FAD
Þ!
gluoclactone
þ
GO
x
ð
FADH
2
Þ
ð
7
:
7a
Þ
GO
x
ð
FADH
2
Þ!
O
2
þ
GO
x
ð
FAD
Þþ
H
2
O
2
ð
7
:
7b
Þ
The H
2
O
2
generated is detected amperometrically on a working electrode, which correlates
the current generated to the glucose concentration.
In
in vivo
applications the concentration of oxygen is rather low which leads to signal satu-
ration for higher glucose concentration determination.
Vaddiraju et al. (2009)
report that the use of outer membranes leads to:
(a)
a decrease in biofouling (
Wilson and Gifford, 2005
) and
(b)
a minimization of temperature-induced variations in sensor responses as a result of
enzyme reaction kinetics (
Jablecki and Gough, 2000
).
The use of outer membranes, however, does lead to an increase in the response time and a
decrease in sensitivity (
Wilson and Gifford, 2005
). Furthermore,
Mercado and Moussey
(1998)
draw attention to calcification-induced permeability changes and degradation of the
outer membrane.
Figure 7.11
shows the binding and dissociation of 6 mM glucose to the implantable glucose
sensor (
Vaddiraju et al., 2009
). A dual-fractal analysis is required to adequately describe the
binding and the dissociation kinetics. The values of (a) the binding rate coefficient,
k
, and the
fractal dimension,
D
f
, for a single-fractal analysis, (b) the binding rate coefficients,
k
1
and
k
2
,
and the fractal dimensions,
D
f1
and
D
f2
, for a dual-fractal analysis, (c) the dissociation rate
coefficient,
k
d
, and the fractal dimension,
D
fd
, for a single-fractal analysis, and (d) the disso-
ciation rate coefficients,
k
d1
and
k
d2
, and the fractal dimensions,
D
fd
, and
D
fd2
for a dual-
fractal analysis are given in
Tables 7.6
and
7.7
. It is seen that the affinity,
K
1
(
¼
k
/
k
d
) and
K
2
(
¼
k
2
/
k
d
) values are 31.2 and 15.4, respectively.
It is of interest to note that for the binding phase, as the fractal dimension increases by a
factor of 2.07 from a value of
D
f1
equal to 1.45 to
D
f2
equal to 3.0, the binding rate
