Biomaterials for MEMS

Flow Control in Biomedical Microdevices using Thermally Responsive Fluids 117

for mixing, which can become rather difficult for long diffusion time scales.

8

Depending on the particular application purely diffusive mixing might be either

desired, especially where controlled mixing or separation of the species is needed,

or not sufficient, when rapid mixing is required. Several rapid mixing strategies

have been proposed for microfluidic devices that are effective despite the diffusion

process being relatively slow.

9−11

For the thermal P´eclet number heat transfer is analogous to mass transfer. With

a much larger thermal diffusivity than molecular diffusivity, heat diffusion is often

more significant than convection so that heat diffusion can be considered very

effective over the typical short length scales of microfluidic devices. With a thermal

diffusivity of water α =1.41· 10

−7

m

2

/ s, a typical time scale t ∼ L

2

/ α =0.7ms

for heat diffusion can be achieved across a 10 μm deep channel.

6.3 FLOW CONTROL MECHANISMS

6.3.1 Microvalve Principles

As described in Section 6.2, fluids are expected to behave quite differently in

microfluidic systems as compared to large scale systems because the different

forces governing flow scale differently. Therefore it is difficult to transfer flow

control concepts from macroscopic systems to microscale flow devices. This

led to the development of new flow control strategies for small-scale systems

as conventional flow control concepts cannot be simply miniaturized. At the

same time, microfluidics allows taking advantage of forces and effects that are

negligible in large scale systems, thus opening new opportunities for innovation.

The category of microvalves that are closest to their large scale counterparts simply

obstruct a fluid conduit by moving a solid into the flow path; some of the actuation

methods including electrostatic, piezoelectric, pneumatic, thermopneumatic, and

electromagnetic actuation are more effective at the microscale than in large-scale

systems.

12−17

Other examples include, passive micro check valves

16

that use drag

forces rather than inertial forces as well as microflow control strategies based on

surface tension.

25

A common disadvantage of many of these designs is the require-

ment for complicated fabrication schemes, unacceptable leakage rates, complex

external actuation, and the risk of irreversible blockage of the valve.

A different approach to valve actuation has been achieved using environmen-

tally responsive polymers that undergo a volumetric or phase change in response

to changes in temperature or concentrations. These concepts lead to fast actuator

response taking advantage of the relatively fast propagation of temperature or

concentration changes on the microscale that was discussed in Section 6.2. In

one of the first examples of this approach, Fr´echet and co-workers

20,21

grafted a

temperature-sensitive polymer (poly N-isopropylacrylamide) to the pore surfaces

of a porous polymer monolith in a microchannel. As the device temperature

transitioned through the lower critical solution temperature, the grafted polymer

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