Page 191 - An Introduction to Microelectromechanical Systems Engineering

170 MEMS Applications in Life Sciences

Pumping in Microfluidic Systems

Examples of flow channels used in microfluidics are rectangular trenches in a
substrate with cap covers on top, capillaries, and slabs of gel, having cross-sectional
dimensions on the order of 10 to 100 µm and lengths of tens of micrometers to
several centimeters. For microfluidic biological analysis, fluid drive or pumping
methods include applied pressure drop, capillary pressure, electrophoresis, electro-
osmosis, electrohydrodynamic force, and magnetohydrodynamic force; the first
four are common. Pressure drive, the most familiar from the macroscopic world, is
simply the application of a positive pressure to one end of a flow channel. Alterna-
tively, a negative pressure (vacuum) can be applied to the other end. Due to drag at
the walls, the flow is slowest at the edges, increasing in a parabolic profile to a maxi-
mum at the center [see Figure 6.1(a)].
Another familiar pumping force is the wicking action of small-diameter capillar-
ies. This force is due to surface tension (i.e., the surface energy of the system can be
lowered if the solid-gas interface is replaced by a solid-liquid interface). Capillary
action is commonly used to load liquid into a channel. After insertion of the end of a

Velocity is near Velocity is maximum
zero at walls at center of channel

Inlet
pressure Oulet
pressure
Flow
(a) Pressure-driven flow
V
Electric field

Ions move in opposite directions in the liquid

(b) Electrophoretic flow

V Velocity is constant
Electric field across channel

Charge Mobile ionic Mobile surface ions
on wall surface charge Flow drag bulk fluid along
surface
(c) Electroosmotic flow
Figure 6.1 Three types of pumping used in microfluidics: (a) pressure drive, in which a pressure
forces the volume fluid to flow; (b) electrophoretic flow, in which ions of opposite polarity in solu-
tion flow in opposite directions under the effect of externally applied electric field; and (c) electro-
osmotic flow, in which an electric field moves the mobile ion sheath of the surface double layer,
dragging the volume in the channel along with it.