Biomedical Engineering Reference
In-Depth Information
9.4 Transducing Elements
A biosensing element is closely integrated within a physicochemical transducer or
transducing microsystem. The nature of the interaction of the biological element
with the analyte of interest impacts the choice of transduction technology. For ex-
ample, signals from the ionic movement can be transduced using bioelectrodes.
However, if there is no ionic movement in analyte-biosensing interactions, then
alternative strategies have to be developed. A successful biosensing system will
require the development of effective methods of sampling, alongside a biological
model of the process. Different transducer technologies are described next.
9.4.1 Electrochemical Biosensors
An approach to exploiting the bioelectrode technology is to incorporate a sensing
element that could also play a role in electron transfer. Electrochemical transducers
are the most established transduction methods. Although some electrodes approach
ideal nonpolarizable behavior at low currents, there is no electrode that behaves
in an ideal nonpolarizable condition. Consequently, the interfacial potential of the
counterelectrode in the two-electrode system varies as the current is passed through
the cell. This problem is overcome by using a three-electrode system (Figure 9.9),
in which the functions of the counterelectrode are divided between the reference
and working electrode. The reference electrode ensures that the potential between
the working and reference electrodes is controlled and the current passes between
the working and auxiliary electrodes. The current passing through the reference
electrode is further diminished by using a high input-impedance operational ampli-
fier for the reference electrode input. One previously used reference electrode was
the saturated calomel electrode (with a large surface area mercury pool). However,
since the current passing through the reference electrode in the three-electrode sys-
tem is many orders of magnitude lower than the current that passes through the two
electrode system, the requirements for the reference electrode are less demanding;
Figure 9.9
Electrochemical sensors: (a) three-electrode system and (b) Clark electrodes.
