Biomedical Engineering Reference
In-Depth Information
solids, tissues, and cells, by directly illuminating the object or by using waveguide
optics including optical fibers. Firefly luciferase is a very expensive enzyme, only
obtainable from the tails of wild fireflies. Thus, the use of immobilized luciferase
greatly reduces the cost of these analyses.
Optical biosensors have been adopted throughout clinical diagnostics and life
science research due to their short response time and sensitivity compared with
other techniques. Furthermore, using optic fiber technology, some internal parts of
the body can be accessed that are inaccessible by other techniques. Antibodies are
immobilized at the terminus of a fiber optics probe for use in vitro and in vivo as-
sessment. A single fiber is used to transmit the excitation radiation into the sample
and collect the fluorescence emission from the antigen. However, optical biosensors
have drawbacks in sample mixtures that are turbid. Some chromophores are also
unstable; thus, assay procedures involving nonchromogenic species may be useful.
Some optical biosensors require the use of target amplification such that the signal
is enhanced to a measurable level. Of these amplification methods, the polymerase
chain reaction (PCR) has emerged as a technique for the detection of nucleic acids.
However, PCR has several limitations including a time requirement for sample
processing and amplification, false positives from sample or reagent contamina-
tion, and high instrumentation and labor costs.
9.4.3 Acoustic Transducers
Piezoelectric crystals (discussed in Chapter 8) utilized in ultrasound imaging are
also used as transducers. The utility of the piezoelectric crystal as a mass sensor
arises from the linear relationship between the change in the mass (or viscosity)
at the crystal surface and the change in its oscillating frequency. The vibration of
piezoelectric crystals produces an oscillating electric field in which the resonant
frequency of the crystal depends on its chemical nature, size, shape, and mass. By
placing the crystal in an oscillating circuit, the frequency is measured as a function
of the mass. When the change in mass (
m
) is very small compared to the total mass
of the crystal, the change in vibrational frequency (
f
) of the crystal in the circuit
relates to
m
as follows:
Δ
bf
2
A
Δ
m
(9.10)
Δ=
f
where
A
is the area of the crystal and
b
is a constant determined by the crystal
material and thickness. Since
b
and
A
are constant for a crystal, the oscillating
frequency of the crystal changes linearly with the change in mass on the crystal at
a particular frequency. Acoustic transducers utilize this unique property in a wide
variety of configurations. When antibodies or antigens are immobilizing on to the
surface of the crystal, the binding of the antigen or the antibody in the sample
changes the crystal mass. The analyte amount is determined by correlating a fre-
quency change to the mass. Immobilized proteins (enzymes and antibodies) for the
detection of atmospheric pollutants have proven useful in a number of applications.
Formaldehyde in the concentration range of 1-100 parts per billion is assayed in
