Page 420 - Handbook of Biomechatronics

The Artificial Pancreas 415

1980s. Sensor performance was further improved with the use of modified
electrodes and tailored membranes. The first electrochemical blood glucose
monitor for self-monitoring of diabetic patients was pen-sized and was
launched in 1987 as ExacTech by Medisense Inc. Its success led to a revo-
lution in the health care of diabetic patients (Yoo and Lee, 2010).
In the process, the mediator is oxidized at a solid electrode with an
applied positive potential. The rate of the electron transfer reaction is given
by the Butler-Volmer equation. However, when the potential is sufficiently
large, the mediator reaching the electrode reacts rapidly and the reaction
becomes diffusion controlled. The current flow then follows the Cottrell
equation (Bronzino, 2006).

nFAD 1=2 C
i ¼ p ffiffiffiffi (3)
πt

where i (A) is the current, n is the number of electrons to oxidize one mol-
2
ecule of the mediator, F is Faraday’s constant (96,485C/mol), A (cm ) is the
3
2
electrode area, D (cm /s) is the diffusion coefficient, C (mol/cm ) is the ini-
tial concentration, and t (s) is the time. The current will therefore decay with
the square root of the time, which means that the strongest chemical signal
occurs at the start, in contrast to the color reactions in which the color
becomes more intense as the time increases. However, when a voltage is first
applied to the electrodes, the dipole moments of the solvent molecules align
with the electric field on the electrode and this provides an interfering cur-
rent for a short period. The measurement process needs to wait until this has
decayed before readings are taken.
The first commercial test strip from MediSense was based on the oxida-
tion of ferrocene at 0.6V. Unfortunately at this voltage it also oxidizes other
molecules present in the blood such as ascorbic acid. This is corrected for by
incorporating an additional electrode on the strip that does not contain any
GOx. The newer generation strips function slightly differently with the glu-
cose dehydrogenase (GDH) enzyme not reacting with oxygen, allowing the
phenanthroline quinine (PQ) mediator to be oxidized at 0.2V that is lower
than the oxidation potential of most interfering substances. This reaction is
described by the following equations (Bronzino, 2006).

+
Glucose + GDH=NAD ! GDH=NADH + gluconolactone
+
GDH=NADH + PQ ! GDH=NAD + PQH
2
PQH ! PQ + electrons reaction atsolid electrode surfaceÞ
ð
2