Miniaturized Detection Systems - Waterborne Pathogens: Detection Methods and Applications - page 314

where ∆ P is the pressure difference and R is the channel resistance. 8 The
pressure drop can be provided by opening the outlet to atmospheric pres-
sure and applying positive pressure at the inlet, via e.g. a diaphragm pump or
gravity, by suspension of solution reservoirs above the microfluidic system. If
gravitational flow is utilized the pressure drop is given by

Δ P = ρg Δ h

(10.8)

where ρ is the density, g is the gravity acceleration, and ∆ h is the height dif-
ference. For a circular channel the resistance is given by

R = 8 η l
π r 4

(10.9)

whereas for a rectangular channel with an aspect ratio close to 1 the resis-
tance is given by

(10.10)

(for high or low aspect ratio channels the term within the square brack-
ets vanishes) where η is the fluid viscosity, l is the length, r is the radius, h
is the height, and w is the width of the channel. 1 Other pressure-driven
means of pumping include various different types of peristaltic pumps,
fabricated on-chip. 24,25 Micropumping has even been performed using
cardiomyocytes. 26
Electro-osmotic pumping is an alternative method of fluid transport. 21
The main advantages are that fluid flow can be controlled by switching
voltages on and off, thus negating the need for valves, and that the flow
profile is plug-like. However, this method is only compatible with certain
solutions, i.e. with a conductive solvent, and requires hydrophilic surfaces.
Additionally, demixing of solutions due to different electrophoretic mobili-
ties can occur.

10.1.3.2. Valves
Valves are essential to be able to control flows within microfluidic systems. 1
The two main strategies for creating valves in elastomeric microfluidic
devices with pressure-driven flows are the use of responsive materials and
channel deformation. 21 Responsive materials include conductive poly-
mers, 27 microspheres and hydrogels. 28 Samel et al. have demonstrated a
single use valve using thermally responsive microspheres which increase
their volume upon heating. 29 Hydrogels are polymer gels which undergo