antibody complexes are detected, or an increase in signal. In non-competitive mode,
there is an excess of antibodies. The antigen are first “captured”, and subsequently
detected by a second set of labeled antibodies,viarecognition of a distinct recogni-
tion site (epitope) of the antigen. The formation of this antibody-antigen-antibody
complex is described as a “sandwich” immunoassay, where the signal is proportional
to the antigen concentration.
Conventional immunoassays, in particular heterogeneous immunoassays, are
performed in microtiter plates with 96 or more sample wells, and typically involve
a series of sample/reagent introduction, washing, mixing and incubation steps. The
process often requires at least several hours and could last for up to days to perform
a single experiment. The lengthy analysis time is mainly due to the long incubation
step, attributed to the limitation in mass transport for all the reagents to migrate
from within the solution to the antibody-coated surface, as the association rate of
the antigen to the antibody is relatively rapid [ 1 ]. Robotic systems can be used to
carry out repetitive fluid handling, thereby reducing hands-on time and improving
the throughput, but require significant investment in infrastructure, as well as high
maintenance efforts.
Miniaturization of the immunoassay within microfluidic systems represents
an attractive solution to shortcomings and limitations in conventional immunoas-
says. Common microfluidic systems are built by networks of channels with dimen-
sions in the μm range. Fluids flow in a laminar manner at these scales,
exhibiting neighbouring parallel fluidic streams where mixing occurs only through
diffusion [ 2 ]. Due to the much smaller scale, mass transport is more efficient, and the
increased surface to volume ratio further accelerates the antibody-antigen complex
formation. The smaller dimensions of the channel significantly reduce the amount of
reagent and sample required. Fluid handling are mostly automated through the
design of channel networks, the use of valves and other features, thereby simplifying
experimental procedures, improving reproducibility, reducing analysis time,
increasing throughput and lowering the operating cost.
This chapter focuses on a number of main developments in microfluidic immu-
noassays in since 2000. Representative works are highlighted, presented and cate-
gorized in three areas: microfluidic immunoassay formats, fluid driving and
handling technologies, and multiplexing approaches.
2 Immunoassay Formats in Microfluidic Systems
Both homogeneous and heterogeneous immunoassay formats (discussed above)
have been developed into microfluidic-based assays. In homogeneous formats,
analyte bound and free antibodies are both in solution, and can be distinguished
by their mobility in the microfluidic channels; whereas in heterogeneous formats,
all antibodies are immobilized on the surface of the microfluidic device, or on beads
introduced in the device. This section will discuss a number of microfluidic
implementations in immunoassays based on electrophoresis-based homogeneous
format, and surface- and bead-based heterogeneous formats.
224 A. Ng