Biomedical Engineering Reference
In-Depth Information
(so-called oligonucleotides), which are immobilized on the substrate surface in
discrete spots. Each spot contains several millions of identical oligonucleotides
to increase sensitivity. The sample to be tested contains single-stranded ge-
netic chains (DNA fragments, mRNA, or cDNA), and they are usually labeled
with a fluorescent dye. The genetic chains that match the immobilized oligonu-
cleotides bind to the spots on the substrate. The biochip is then illuminated
with a suitable wavelength, so that the fluorescence light pattern of the dif-
ferent spots allows the determination of the type and concentration of target
genetic chains in the sample.
12.2.2 Proteins (Antigens)
Since the genetic information contains the instructions that proteins are pro-
duced in the body, the identification of proteins - so-called proteomics - is
even more important than genomics. For this reason, the utilization of biochips
for protein analysis is growing at a rate of about 29%, while the growth of
DNA-chips is only about 18% [12].
A protein biochip is similar to a DNA-chip, but instead of oligonucleotides,
protein probes (antibodies) for the target proteins (antigens) are immobilized
on the individual spots on the biochip's surface. However, protein biochips
are more di
cult to handle, less specific, and less sensitive than DNA-chips
for several reasons [4]: Proteins are unstable, and they can easily be dena-
tured at solid-liquid and liquid-air interfaces. Today's production techniques
for protein probes produce only low-a
nity capture antibodies, making the
coupling of the antigens to the antibodies not very specific. Since antibodies
have usually rather large surface areas for interaction, they show important
cross-reactivity between target proteins. DNA samples can readily be am-
plified using PCR (polymerase chain reaction), while no such amplification
process is known for proteins.
The optical principles employed in protein biochips include fluorescence as
well as evanescent wave sensing (see Sect. 12.4).
12.2.3 Specific Organic Molecules
The same principle used for DNA and protein biochips can be employed, in
principle, to determine the concentration of various organic molecules that are
of biochemical importance. Suitable probe molecules are fixed on the surface of
a biochip. Corresponding target molecules then attach to the probe molecules,
and their presence can be measured using the same optical techniques as used
for the other types of biochips.
12.2.4 Cell Gene Products (cDNA, Proteins)
A cell microarray consists of a number of spots with different types of genetic
information, such as DNA strands, plasmids, or adeno viruses. Mammalian
