Biomedical Engineering Reference
In-Depth Information
The main reason for not using humoral factors is the diculty (or impos-
sibility) of performing real-time measurement of their concentrations in the
blood or in tissues.
One method that would permit real-time measurement of the humoral
factors is measurements based on the electrochemical measurement technique
[170,171]; alternatively, a sensor such as the ion-sensitive field effect transistor
(ISFET [172]) or the enzyme field effect transistor (EnFET [173]) could be
used. Both of these are basically electrochemical measurements.
In this section, we would like to introduce our attempt to develop a system
that allows real-time sensing of catecholamine concentration, which is one of
the most important humoral factors involved in the control of the circulatory
system of the living body, as well as providing feedback on the factors involved
in the control of an artificial heart system.
As mentioned above, we adopted an electrochemical technique in this
study to measure catecholamine concentration. The main reason is that this
technique permits real-time measurement of the electrolyte concentration in
solution; in addition, the technique has already been used for the measure-
ment of catecholamines in the brain.
Figure 4.44 shows the principle underlying the method of electrochemical
measurement. If only electrode A (or B) is dissolved in the solution, the
current increases by
H
A
(
H
B
) due to the oxidation of electrolyte A (B)
when the applied voltage is greater than
ε
A
(
ε
B
) (the oxidation potential for
electrolyte A (B)). Similarly, if both A and B are dissolved in the solution,
the electric current increases by
H
A
and (
H
A
+
H
B
) when the applied voltage
is greater than
ε
A
and
ε
B
, respectively. Thus, the increase in electric current
due to the oxidation of electrolyte B is equal to the difference between the
electric currents when the electric potentials are
ε
A
and
ε
B
.
Figure 4.45 shows the experimental setup of our experiment for measu-
ring catecholamine concentrations and providing subsequent feedback to the
artificial heart driving system. The sensing system is composed of a working
electrode consisting of a carbon fiber, an Ag-AgCl reference electrode, and a
potentiostat. The electrodes were immersed in phosphate-buffered solution or
goat plasma, and the catecholamine concentration of the solution (or plasma)
was then varied. Changes in the electric current resulting from the ampero-
metric measurement of the catecholamines were determined by the system,
and the information was transferred to a personal computer.
The operating parameters of the pneumatically driven artificial heart sy-
stem were altered in accordance with the algorithm for changes in the cate-
cholamine concentration (the catecholamine concentration and the alteration
of the driving parameters were measured every 2 s).
The results of this study indicated that the system worked well for the
measurement of catecholamine concentration in phosphate-buffered solution.
Figure 4.46 shows the correlation between the electric current and the
adrenaline concentration in the phosphate-buffered solution. The minimum
