chambers that perform many analytical functions, including sample/reagent trans-
port, dilution, mixing and separation, all within a disc format.
The main characteristic of microfluidic systems on a CD is the use of centrifugal
force controlled by the rotation speed of the CD. The centrifugal force is to
overcome the capillary or surface force which prevents the liquid contained in
one chamber from exiting. As the centrifugal force increases from the center
towards the outer edge of the CD, sequential pumping of liquid can be achieved.
Similar to all pressure-driven flow methods, centrifugal force-driven flow is not
affected by the physiochemical properties of the fluid. In addition, because of the
CD format, these devices can be easily adapted with optical detectors similar to the
mechanism of a CD player.
Centrifugal force-driven microfluidics have attracted attentions and have been
implemented for carrying out homogeneous immunoassays for pathogen antigen
detection [ 74 ]. A number of commercial assays have also been developed. Gyrolab
used a packed bed of microbeads and performed multiple reagent delivery steps
controlled by centrifugal forces to perform immunoassays [ 75 ]. The bed of
microbeads formed an assay surface on which reagents flow through for analyte
capture and washing, followed by optical detection using a laser directly on the
device.
The ability of centrifugal-based microfluidic to implement on-chip blood sepa-
ration is particularly attractive. Lee et al. [ 76 ] developed a system for performing
bead-based immunoassay for the detection of host-generated antibody against
hepatitis B virus and in particular, the system was capable of plasma separation.
The control of flow between chambers was done by an interesting microvalve
design involving the use of ferrowax that can form valves that are normally open
or closed. Ferrowax is paraffin wax with embedded iron oxide nanoparticles, which
upon heating by a low intensity IR laser melts at an accelerated rate, and therefore
can be individually actuated. The immunoassay is fully automated started from
whole blood, and can be completed in less than 30 min.
A recent centrifugal microfluidic platform reported by Koh et al. performed an
innovative sedimentation-based immunoassay for detection of botulinum toxin
[ 77 ]. This platform first mixed a whole blood sample with capture antibody-
conjugated microbeads and fluorescently-labeled detection antibody. Binding of
the analyte between the capture and detection antibody formed a sandwich and
fluorescent immunocomplex. Subsequently, the centrifugal device separated the
beads with the immunocomplex from the blood sample through a density media,
which effectively washes the beads and remove all interfering agents. As the bead
pellet at the end of the channel located at the out edge of the disk, fluorescent signal
can be measured and the analyte quantified (Fig.9.3).
234 A. Ng