Biomedical Engineering Reference
In-Depth Information
Fig. 3 Pumping sequence of the different reservoirs. a Cartridge prior to pumping. b Arrows
indicating flow of liquids from reservoir to sensor field. c Pumping of sample reservoir into sensor
field
on the application not every reservoir is used and hence a decrease in size is possible
reducing both costs and waste.
To sum up, the self-contained active microfluidic cartridge offers various
advantages compared to other systems. They are as follows:
• Small sample and reagent volumes;
• Adaption of standard biomedical assays such as immunoassays;
• No error-prone external interface for pumping and reagents required;
• Just an electrical current is needed for the activation of pumping.
2.3 Possible Sensors: Optical or Electrochemical
Within the concept modularity was one of the main design aspects. The design was
chosen to be open for various types of sensors. Hence, the cartridge offers two
different ''free spaces'' for the integration of, so far, an optical and an electro-
chemical sensor (Fig.
4
).
Both sensors require an immobilization of capture molecules for the particular
application. In most cases these will be antibodies specific to an analyte but also
antigens, DNA or artificial receptor molecules can be immobilized on the various
surfaces. These sensors have a heterogeneous sandwich assay type, meaning an
immobilized capture molecule binds the analyte. After a washing step a secondary
antibody is flushed over the sensor which can bind from the opposite side of the
bound antigen. Depending on the assay a detection antibody labeled with a dye or
an enzyme is often used which binds to the secondary antibody and hence
''shows'' the amount of bound analyte. Besides the detection of antigens as
described by an immobilized antibody also other assay types such as serological
assays or DNA hybridization experiments can be performed on the surface of the
sensors.
The optical sensor is based on measuring a fluorescence signal of a microarray
which is deposited on a substrate. To maintain the cheap producibility of the whole
chip it was decided to make the substrate out of a polymer. After various material
