Biomedical Engineering Reference
In-Depth Information
system is the goal of all sensor designs; phenomena that need to be sensed include various
types of radiation (from simple light to more damaging high-energy ionizing radiation),
mechanical stress (or impending failure), chemical and biological toxins, pressure, tempera-
ture, position, and sound. The human body has sensors that are capable of detecting nearly
all of these factors—eyes detect position, visible light, and motion; ears detect sound; the
skin contains vast numbers of pressure and temperature sensors; the nose and tongue are
complicated chemical sensors; and the immune system of the body has the ability to detect
and react to large numbers of pathogenic microbes. Conventional technology can detect
some of these factors better than others. However, the sophisticated mechanisms provided
by biology often cannot be duplicated using those same, conventional means. Detection of
complex pathogens often requires equally complex—intelligent—sensor technologies, espe-
cially those capable of highly specific chemical interactions: alternative, innovative, and
intelligent solutions are necessary.
The current need for sensors that detect a wide variety of chemical and biological tox-
ins cannot be met through conventional sensor technology for a variety of reasons. The
basic approach for most chemical sensors is to turn to highly specific molecular interac-
tions, where the target molecule for detection is sensed through chemical interaction with
a very specific reactant. The limitation of such detection schemes is that any given sensor
can detect only a narrow range of chemicals, and similar molecules of lesser or no toxicity
might result in false positives. Chemical noses offer some improvement, although they
are, in essence, a large number of sensors operating in parallel, each monitoring the pres-
ence of a specific class of compounds. And such sensors are incapable of detecting the vast
majority of toxic compounds, industrial pollutants, and harmful microorganisms that are
deleterious to human health.
Biological molecules—specifically enzymes, proteins, and DNA—are unique, in that
they have benefited from eon upon eon of natural selection and evolution, which has
resulted in highly optimized properties custom-tailored for specific biological functions.
These molecules have evolved to function in a wide range of environmental conditions,
often with efficiencies unmatched by nonbiological or synthetic methods. Intelligent selec-
tion by the researcher can bring their novel functionalities to device applications. Often
these properties offer comparative advantages over more traditional approaches to device
design and construction. In fact, proteins and other biomolecules pose a unique solution to
the problem of size versus function, in that they represent a class of extremely sophisticated
molecules that perform specific functions under what can be a fairly broad range of condi-
tions. From the standpoint to designing improved sensors, it follows that a better approach
to detecting both biological molecules, as well as molecules that interact with biological
molecules (or organisms), is to incorporate molecules of biological origin into the sensors
themselves. There are many examples of naturally occurring biological interactions that can
be harnessed in detection schemes; complementary DNA interactions, enzyme-substrate
complexes, and antibody-antigen complexes offer just a few examples. The advantage
offered by these sorts of systems is one of specificity—the tighter the interaction between
the target molecule (i.e., the molecule that is the target of the sensor) and the detector, the
better the sensor's response will be. DNA is perhaps the best example of such interactions,
which are both strong and highly specific. As such, a wide variety of sensors for biological
organisms has been developed, which are capable of detecting the presence of specific
DNA or RNA sequences characteristic of disease-causing bacteria or virus. Many of these
detection schemes produce fluorescence once the target sequence binds to its complement
from the target organism (i.e., fluorescence resonance energy transfer, or FRET). In general,
proteins are widely sensitive to the presence of many types of molecules, through both spe-
cific and nonspecific binding. This is especially true for enzymes, which participate in a
large variety of specific interactions with small molecules, predominantly defined through
