Biomaterials for MEMS

116 V. Bazargan and B. Stoeber

human intervention and reduces the risk of sample loss. Another advantage of

the compactness of mocro TAS is faster completion of a sequence of laboratory

processes. In addition, the time required for many analytical steps and chemical

reactions scale favorably at small length scales, as will be described in more detail

further below. The small size and reduced power consumption of microTAS makes

them attractive for portable analytical instruments for the biomedical field or for

environmental monitoring and control.

The transport and precise control of small volumes of fluid on a microfluidic

chip requires adequate flow control devices such as pumps, valves and mixers

that provide the required performance and are easily integrated. In addition,

these flow control devices should not compromise the biological samples through

electrical or mechanical interventions; furthermore, the biocompatibility of the

flow environment such as the channel walls is a concern, and correspondingly,

biocompatibility of materials is an important field of research for the microTAS

community. The following discussion will concentrate on flow control mecha-

nisms for microfluidic devices with an emphasis on thermally responsive fluids

for microflow manipulation.

6.2 TRANSPORT IN MICROFLUIDIC CHANNELS

Typical dimensions L of the height and width of microfluidic channels range

from several microns to several 100s of microns. Aqueous liquids that are most

commonly transported in microfluidic systems have a density ρ = 1000 kg/m

3

and a shear viscosity μ =10

−3

Pa · s. With a typical flow velocity U = 1 mm/s,

flow in microchannels is characterized by a very low Reynolds number

Re = ρ LU/ μ ≈ 10

−2

<< 1. The Reynolds number relates inertial forces and

viscous forces of a fluid. A low Reynolds number corresponds to high viscous

forces relative to the inertia forces in a flow. Consequently, microfluidic flow

is dominated by viscous forces, in most cases inertia can be neglected.

1,2

The

flow regime in microfluidic channels can therefore be considered laminar in most

cases

3

; while turbulence is not impossible in microfluidic devices, it is rather

uncommon.

2,4,5

The P´eclet number, Pe = LU/ D , is a dimensionless number that describes the

ratio of convection to diffusion, and it can be defined either for molecular diffusion

Pe

m

or for thermal diffusion Pe

T

, using the molecular diffusion constant D or the

thermal diffusivity α, respectively. The Peclet number can also be interpreted as

the ratio of a characteristic diffusion time scale to a characteristic convection time

scale. The molecular diffusion coefficient is low especially for liquids, such as

D =6· 10

−10

m

2

/s for molecular diffusion of sodium phosphate in water.

6

The

associated P´eclet number Pe ≈ 100 indicates high convective effects compared

to molecular diffusion. However, in general there is no characteristic order of

magnitude for the P´eclet number in microfluidic devices.

7

In absence of turbulence

in microfluidic systems as mentioned above, molecular diffusion is important

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