with the capture antibody. Using mouse IgG as a model analyte, this method
reached a detection limit of 250 pg/mL when operating in continuous-flow mode.
The majority of microbead-based strategies only employed the outer surface of
the beads for antibody immobilization. Unique implementation of the bead-based
immunoassay such as the work by Yang et al., [ 55 ] in which superporous agarose
beads with diameters ranging from 10 to 80μm were used as a solid support. The
superporous agarose beads were covalently conjugated to Protein A, which immo-
bilize the capture antibody in a favourable orientation. Using these superporous
beads as immobilization materials, the immunoassay detected goat IgG, as a model
analyte, with a detection limit of 100 pg/mL, a 10 times improvement over
conventional bead-based microfluidic assays. The large surface area attributed to
the inner matrix of the agarose beads, as well as the lowered fluidic resistance due to
the porosity of the beads have led to the enhanced sensitivity of the assay.
3 Fluid Driving and Handling Technologies in Microfluidic
Systems
Fluid transport plays an integral role in microfluidic immunoassays, and the per-
formance of the fluid driving and handling system directly affect the result of the
assays. To miniaturize the fluidic operations analogous to the steps in conventional
immunoassays, [ 56 ] many strategies for on-chip liquid transport have been
reported. The force for fluidic transport modalities can be categorized into electric,
pressure and power-free passive forces.
3.1 Electric Forces
Electric forces for fluid flow are generated by electroosmotic pumping based on
electroosmotic flow (EOF). EOF is the flow of the bulk fluid resulting from the
movement of the solvated ions. A layer of fluid containing a build-up of solvated
ions is attracted to the oppositely charged walls. When an electric field is applied,
the solvated ions and the waters of hydration are driven toward the oppositely
charged electrode. The movement of the solvated ions drags the bulk fluid via
viscous forces, forming a uniform plug-like flow [ 57 ]. Using this mechanism, fluid
handling such as flow initiation, stopping and direction can be easily controlled by
the applied electric field, and do not require valves or pumps. Furthermore, con-
trolling fluid flow by electric field alone facilitates automation.
The first application of electric force-driven fluid flow in immunoassays was by
Rooij et al. [ 58 , 59 ] and further developed by Gao et al. [ 60 , 61 ]. For example, Gao
et al. developed a microfluidic chip for detection of multiple microbial antigens
based on electroosmotic pumping [ 60 ]. The microchip first used a microfluidic
230 A. Ng