Biomedical Engineering Reference
In-Depth Information
the membrane of certain cells in the nose that generate different signal pat-
terns when different smells are perceived. The brain has learned to inter-
pret these patterns, to recognize them and connect them to circumstances in
our direct environment, for example, making the observation that someone
recently baked bread. These receptors are based on the principle that a small
volatile molecule present in the atmosphere near the receptor can dock to the
receptor because it fits with respect to shape and charge distributions. The
docking results in a shift of charges in the receptor, which in turn gives rise
to a conformational change in the receptor molecule. These changes trigger
a cascade of effects in the cell that eventually causes a neuron to fire and the
brain to become aware of the docking event.
By linking these receptors to a semiconductor surface, it is also possible to
detect the charge shifts in the receptor when docking takes place through
a small change in the impedance of the semiconductor. By combining dif-
ferent receptors on a large number of patches on the semiconductor, it is in
principle, possible to create a device that is sensitive to a number of selected
volatiles. As always, it is much less easy to develop this principle in prac-
tice. The problems encountered include the conformational changes of the
proteins when they are taken out of their usual working environment, that
is, embedded in the cell membrane, and covalently bond them to a hard
surface.
In practical applications, the sensors of course need not detect all sub-
stances the human nose can detect. They probably will be targeted at mole-
cules that are relevant in the process for which they are intended. In fact these
molecules could be different from the ones the human nose is sensitive for.
At first instance, receptors from other species, such as dogs or insects, could
probably be used; however, later on, when nanoscience leads to new receptor
architectures that are suitable for ligands that no species has receptors for, or
which are more suitable for the detection of specific ligands in combination
with the unfavorable conditions of the silicon-based detection systems, com-
pletely new, artificial receptor molecules could also be implemented.
The electronic nose is just one example of new sensor principles that come
within reach through the application of micro- and nanotechnologies. Many
of them are based on the same principle of mimicking biological processes.
Although they are usually not as good as their biological counterparts,
they do outperform more traditional sensors in sensitivity, specificity, and
accuracy.
Electronic noses are still under development. It will require some major
breakthroughs regarding aspects such as the conformational changes these
molecules exhibit when taken out of their normal operating environment,
the orientation of the receptors away from the surface that they are bound
to, fine tuning the affinity between ligand and receptor to prevent the ligand
from becoming permanently bound to the receptor (which would make the
sensor a one-time-use device) or to prevent the occurrence of too little bind-
ing, etc.
