diseases like Alzheimer’s and cancer. They have proven to be a breakthrough
technology that has rendered the analysis of proteins after the work of Yalow and
Berson, who employed radioimmunoassays to determine peptide hormones in the
1950s [ 82 ]. Immunoassays have since then evolved considerably. The sensitivity
limits have been decreased to picomolar concentrations using monoclonal anti-
bodies, labeling techniques, and signal transduction and data acquisition devices.
The immunoassays are increasingly ported onto MF formats. MF bioassays have
been used in routine analysis of clinically observed symptoms, frequent monitoring
of disease progression and continuous assessment of therapeutic efficacy. Two
major technologies that are being currently used to fabricate biarrays are
(1) microspot array and (2) mosaic array. The most commonly developed immu-
noassay method is the spot array. It is a simple method whereby many probe spots
are printed onto a chip substrate. This bioarray is then tested against a single
sample, which flows on top of the array. With this method, a large amount of
data can be obtained from using a single sample [ 83 ]. The mosaic array is an
immunoassay method that allows for multiple probes to react with various samples
[ 84 ]. Rather than having a single sample react with multiple probes, this method has
several samples tested against multiple probes simultaneously. Such robust analysis
has the potential of early; possibly even presymptomatic, diagnosis of a disease. MF
immunoassays can be classified into two homogeneous or heterogeneous assays. In
homogeneous assays, the probes and the samples will be mixed in liquid phase; this
method requires a separation step to detect the presence of the reaction products. In
heterogeneous assays, the samples in the liquid phase interact with the probes
immobilized on a solid phase. While both of these assays are in practical use,
heterogeneous assays are much more widespread due to the ease of washing away
unbound particles from the solid substrate. Heterogeneous immunoassays usually
take the form of the ELISA [ 83 ]. Two major areas where immunoassays are now
playing a vital role for analysis are cancer detection and cardiovascular biomarker
detection. In these fields MF immunoassays have now enabled early diagnosis of
disease, resulting in the continuous monitoring of the progress of disease and
response to therapy. Legendre et al. reported work on the design and development
of an MF device for diagnosis of T-cell lymphoma [ 85 ]. The system accepts a
whole blood sample as the input, extracts the DNA, amplifies target sequences of
the T-cell receptor gene, and electrophoretically resolves the products for detection
of a signature consistent with monoclonality. Diercks et al. fabricated a MF device
that measures multiple proteins (tumor necrosis factor, CXC chemokine ligand
2, interleukin 6 and interleukin at pgmL^1 concentrations in nanoliter volumes
[ 86 ]. Antibody-coupled polystyrene microspheres labeled with embedded
fluorophores were used to detect the analyte (proteins). Optical detection of cap-
tured analyte was performed off-chip using a confocal microscope, which proved to
be a disadvantage in terms of device portability.
8 Biological Applications of Microfluidics System 213