specificity (It is the ability of a paper based assay test to correctly exclude the
healthy individuals) of the dengue detection, a methodology has been developed
which combines reverse transcription loop-mediated isothermal amplification
(RT-LAMP), paper based device and fluorescence based colorimetric
detection [ 74 ].
5.1.1 Basic Principles of Fluid Flow in Paper Micro-fluidics
The flow through the paper occurs due to capillary action. The capillary action is
governed by three main variables i.e., cohesive force, surface tension and adhesive
force (Fig.2.23). Capillary action happens only when the adhesive force is greater
than cohesive force. When liquid is brought into contact with a dry paper, it will
start absorbing liquid because there is an unbalanced pressure difference within the
bulk phases [ 75 ]. The fluid can rise against gravity and it stops when the hydrostatic
pressure balances the interfacial pressure difference.
Let us consider capillary rise in a cylindrical tube of inner radius r.
The equilibrium height h of the liquid within a capillary is obtained by
Δp¼2 γLVcosθ
rð 2 : 1 ÞThe hydrostatic pressure is given as:Fig. 2.22Device for the stacking lateral flow immunoassay; (a) The exploded view of the device.
The test assembly consists of a sample pad, a reagent pad, a flow regulator, a test strip and an
absorbent pad. The test assembly is housed in the cassette; (b) The assembled stacking flow
device; (c) Photograph of the stacking flow device prototyped with a 3D printer (Reproduced from
Yi et al. [ 74 ] with permission from the Royal Society of Chemistry)
60 G. Bhatt et al.