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280 MICROSENSORS
Figure 8.47 (a) Typical I-V characteristics of a DC SQUID magnetometer and (b) schematic
arrangement of an integrated SQUID. After Koch (1989)
their main use is when they can be integrated into another microcomponent, such as a
MEMS device. For example, a magnetic microsensor could be used in a micromotor to
either monitor or control the movement of the rotor. Ideally, the magnetic microsensor
should be placed within the feedback arm of the closed-loop control of a microactuator
or MEMS device.
8.6 BIO(CHEMICAL) SENSORS
The general topic of chemical sensors is well covered in the standard literature on sensors
(Madou and Morrison 1989; Gopel et al. 1989–1998; Taylor et al. 1996).
The basic components of a bio(chemical) 19 sensor are illustrated in Figure 8.48 and
comprise a chemically sensitive layer interfaced to a sensing transducer. The analyte
molecules interact with the chemically sensitive layer and produce a physical change that
is detected by the transducer and are converted into an electrical output signal.
The nature of this interaction is determined by the type of material used and can be
either a reversible process or an irreversible reaction (see Figure 8.49). In a reversible
binding reaction, the analyte is typically bound to specific sites within the sensitive layer,
and when the external concentration is removed, the analyte molecules dissociate and
there is no net change. An example of this would be the adsorption and desorption
of an organic vapour in a polymeric material. In an irreversible reaction, the analyte
undergoes a chemical reaction catalysed by the sensitive layer and therefore is consumed
in the process. In this case, removal of the external analyte concentration still reverses
the process but the associated time-constant may be considerably longer. The irreversible
19
A bio(chemical) sensor can be either a biosensor or a chemical sensor. In general, a biosensor either detects
biological material or uses it in the active layer.
