3.2 Pressure-Driven Fluid Handling Modalities
The most popular fluid handling modality for microfluidic immunoassay is by
pressure-driven flow. In contrast to electric-force driven flow, pressure-driven
modalities do not have strict requirement on the conductivity of the solution and
substrates, and therefore are compatible with a range of samples, reagents and
device materials. On the other hand, pressure-drive flow exhibits a less uniform,
parabolic flow profile, which results in the dispersion of the liquid plug, and
therefore is considered less useful for separations [ 64 ].
The simplest setup for pressure-driven flow can be created by leaving the inlet
open to atmospheric pressure and applying vacuum at the outlet using a vacuum
pump, similar to a sucking action. The fluid can also be pushed by applying a
positive pressure at the inlet and leaving the outlet to atmospheric pressure. In
low-cost microfluidic chip designs, actuation by applying pressure using the hand
or thumb have been demonstrated [ 65 , 66 ].
Fig. 9.2Process flow of the heterogeneous immunoassay using electrowetting for fluid transport.
(a) Delivery of sample to the immunoassay reagents, (b) immunocomplex formation. (c) immo-
bilization of magnetic beads, (d) removal of unbound analyte and reagents (e) addition of fresh
wash buffer. Taken from [ 77 ]. Copyright©The Royal Society of Chemistry 2008
232 A. Ng