which the flow of the sample is joined perpendicularly by a confinement makeup
flow of buffer. Since the flow is laminar, the makeup flow effective confines the
sample into a thin layer on the capture zone (Fig.9.1).
2.3 Heterogeneous Immunoassays on Microbeads
2.3.1 Magnetic Microbeads
To significantly increase the surface area to volume ratio, microbeads are often used
in microfluidic immunoassays. Microbeads can also serve as a simple vehicle for
reproducibly manipulating antibodies and their delivery to specific locations
[ 40 ]. Microbeads can be magnetic or non-magnetic, which often determines how
microbeads are implemented in the microfluidic system.
The use of magnetic microbeads in microfluidic immunoassays is becoming
more popular as their manipulation is greatly facilitated by the application of a
magnetic field, allowing easy capture of analyte from the sample mixture. Typical
magnetic bead immunoassays employ antibody-coated magnetic beads that are
immobilized on the surface of the device, [ 41 – 43 ] providing an increased capture
or sensing surface. The most efficient use of magnetic beads however is by
resuspending and dispersing them in the sample solution to reduce the diffusion
distances between the analyte and antibody. For microfluidic systems, collecting
and resuspending magnetic beads at specific stages of the assay requires more
specialized and complex functionalities such as micropumps, active mixers and
microcoils for generating localized magnetic field [ 44 , 45 ].
confinement flowsample flowCy5-labeled anti-rabbit IgG immobilized rabbit IgGFig. 9.1 Schematic illustrating the concept of flow confinement. A sample flow is joined by a
confinement makeup flow in a perpendicular orientation, leading to a double flux of sample and
confinement flow in a planar sheet geometry, effectively confining the sample flow into a thin layer
of higher velocity near one surface of the microchannel. Taken from [ 39 ]. Copyright© 2002
American Chemical Society
228 A. Ng