Biomedical Engineering Reference
In-Depth Information
cartridge is inserted into the read-out unit and processes such as washing, labeling,
incubation, and read-out are automatically performed within 15 min.
The platform has been designed to be modular; it therefore allows a choice
between an optical and an electrochemical sensor. Both sensors can measure
different analytes in parallel depending on the used capture molecules which are
immobilized on the surface of the sensor. While the electrochemical sensor is able
to measure 16 different analytes at once, the optical sensor offers the possibility to
measure as many biomarkers as necessary for the particular application (up to
500). To obtain such high numbers of parameters, a microarray is used [
3
]. In a
microarray different capture molecules are deposited on a surface in small spots
having diameters of around 50 lm and volumes in the nL-range.
For the detection of small molecules, proteins or antibodies either antibodies as
capture molecules or antigens can be spotted. After addition of the sample from
which the analytes can bind to the capture molecule different washing and labeling
steps are performed to measure the amount of bound analyte by means of a
fluorescence signal. This signal is measured and quantified within the read-out and
processing unit in this case with a CCD-camera and software which quantifies the
intensity of each spot.
For the detection of DNA the steps to be performed include sample preparation,
sample amplification, labeling, and detection steps which have to be integrated
within the Lab-on-Chip system. Within such systems purification of DNA may be
carried out in two ways: After disruption of the cells either nanobeads bearing
DNA-strands for purification or silica matrices may be used. After both purifica-
tion procedures the purified DNA has to be amplified. The most common method
for amplification is the polymerase-chain reaction (PCR), in which cooling and
heating cycles are used for annealing, elongation, and denaturation of new DNA-
strands with a polymerase. In the case of the ivD-platform an external peltier
element is used for cooling and heating since the high rates cannot be accom-
plished within the cartridge itself. For detection, also a microarray now bearing
DNA-probes is used.
Although in the case of the Fraunhofer ivD-platform the degree of integration
can be further improved the system was chosen to be as simple as possible. This
guarantees an easy transfer of already established assays.
2.3 Integration on the Chip: ''Active Arrays''
An even more elegant way by means of biochemical integration especially for
DNA-detection is given by the concept of ''active arrays'' (von Nickisch-Rosenegk
et al. [
29
]. Here, the reverse primers are immobilized on a substrate as ''capture
molecules.'' During the annealing step the templates from the sample bind to the
primer and are elongated. After denaturation of the elongated DNA-strand a new
primer anneals and the strand will be elongated again. Since the first primer is
immobilized on the sensor the final PCR product can directly be measured
