network to adsorb the analyte onto a PDMS-coated glass slide. Another PDMS
bearing H-shaped microchannels for reagent delivery was then bonded to the slide.
The sequential delivery of reagent solutions was performed automatically by a
programmable voltage sequencer. This method was able to detect bacterial lysate
antigen and achieved a detection limit of 3μg/mL. Simultaneous detection of
human antibodies against two bacterial species in serum was also possible with
this automatic system [ 61 ].
Electric force-driven immunoassay could have better binding kinetics than
pressure-driven assays. Hu et al. [ 62 ] built a two-dimensional computation model
to simulate the mass transport and binding between the antigen in the bulk fluid flow
and the immobilized antibody on the surface of the microchannel. The model
showed that, due to the uniform, plug-like fluid flow profile as a result of electro-
osmosis, assays using electric force-driven flow indeed exhibited superior reaction
kinetics than pressure-driven ones. However, there are strict design constraints for
the construction of electric force-based system. The substrate material should be
non-conductive, other electrical breakdown or short circuit might occur. Further-
more, the choice of buffer and reagents used in such systems must be conductive.
Although most buffers in biochemical assays contain salts and therefore conduc-
tive, an ionic strength that is too high might cause Joule heating, which significantly
affects the conformation of biomolecules and hence antibody-antigen recognition,
as well as causing denaturing and aggregation of other biomolecules present in the
sample matrix, particularly in complex biological samples such as blood and urine.
Another electric force-based fluid transport strategy is electrowetting, whereby
liquid, in contrast to continuous flow in channels, are handled as discrete droplets,
in the presence of voltage sequences applied to an electrode array insulated by a
hydrophobic dielectric layer [ 63 ]. Electrowetting is often combined with magnetic
microbead-based immunoassays for liquid droplet manipulations, separation of
bound and unbound analyte and washing steps. Sista et al. [ 43 ] developed a
microchip containing a sample droplet and a reagent droplet containing capture
antibody-conjugated magnetic microbeads and detection antibody. Electrowetting-
based manipulation moved and merged the two droplets, and after an incubation of
2 min the droplet containing the sandwich immunocomplex on microbead was
delivered over a magnet, followed by capture by the magnetic field. Unbound
analyte and other components were then removed by splitting the excess liquid
from the beads within the droplet, and a new droplet containing fresh buffer was
introduced over the beads for washing, and the process repeated several times to
ensure complete washing (Fig.9.2). Detection was achieved by introducing a
droplet containing detection reagents. The entire analysis using electrowetting
fluid handling was completed in 7 min.
9 On-Chip Immunoassay for Molecular Analysis 231