Biomedical Engineering Reference
In-Depth Information
As already mentioned the choice of base station is dependent on two
parameters. Basically one parameter is the chosen sensor type; the other is the nature
of the chosen analyte which can be DNA, proteins, or small molecules.
Depending on the type of sensor the hardware has either to provide optical or
electronic parts. For the optical read-out mainly optical parts are used within the
base unit [
12
]. For an excitation of the microarray a high-power LED with a
wavelength peak of 627 nm (bandwidth 26 nm) is installed. Before reaching the
polymeric wave guide the laser beam is shaped with a collimator and cylindrical
lenses to fit in size and shape with the prism. Rectangular to the polymeric
waveguide an uncooled CCD-camera is located which detects with varying inte-
gration times a picture of the illuminated microarray. In the current set-up a read-
out area of 6 9 10 mm
2
was chosen which is equal to 240 spots with a pitch of
500 lm. To avoid interference with the light used for excitation an interference
filter with a bandwidth of 663-737 nm was chosen. By varying the integration
time the sensitivity of the assay can be tuned. This is especially important for
multiparameter microarrays hence the assay performance of two parameters can
differ. Despite the fact that it is possible to vary the integration times up to one and
a half minute, for normal immunoassays integration times between 0.3 and 5 s are
sufficient.
For data analysis, the differently integrated pictures are used. Through guiding
spots on the array a grid is automatically fitted to the pictures and each spot can be
analyzed. After normalization of the determined values by means of integration
time and their comparison with a calibration curve values can be given in terms of
medical units. Of special importance in this case are the calibration curves which
have to be lot-specific and will be transmitted with every new batch of reagents
and cartridges.
Comparably to the optical base unit the electrochemical base unit has to include
the hardware necessary for the detection and measurement of the signal generated
by redox cycling [
13
]. In this regard, a potentiostat is inserted which is responsible
for measuring the redox-active species. To do so the potentials of the electrodes
are switched in a certain frequency and the integrated potentiostat carries out
chronoamperometric measurements. For data analysis, the measured current which
rises over time is recorded and the slope used for the quantification.
Both systems can be used for the detection of a binding event which is the case
for antigen measurements or serological analysis. Nevertheless, the detection of
DNA from a sample includes an additional step since the amount of DNA in a
sample is too low for its direct analysis. Hence, an amplification step is necessary
to gain a sufficient concentration for its detection within a microarray. In this
regard, a polymerase chain reaction (PCR) has to be carried out. A normal PCR
includes various heating and cooling steps. The main challenge for the technology
is to provide the heating and cooling rates necessary for a PCR-reaction. Hence, a
heating and cooling on chip is not sufficient. Therefore, the read-out device was
expanded to include in addition to the detection function also the possibility for
delivering the temperature rates necessary. This is obtained within the base unit by
a peltier element which is lowered down onto the PCR-chamber (located on the
