118 V. Bazargan and B. Stoeber
chain would undergo reversible swelling and collapse, leading to closing or
opening of the pores of the polymer monolith, thus opening or closing the channel
to flow. Mastrangelo and co-workers
23,24
included paraffin in a sealed reservoir;
upon heating the high volumetric expansion of the paraffin was used to actuate a
membrane that then obstructed a flow channel, acting as a valve. This method
separated the actuator from the fluid to be controlled, allowing a wide range
of applications. A different category of environmentally responsive flow control
methods uses hydrogels, which are either anchored inside microfluidic channels,
or are transported with the flow.
6.3.2 Hydrogel-based Microvalve Principles
As mentioned in the previous section, a wide variety of environmentally respon-
sive fluids has been employed for flow control purposes in microfluidic devices.
The actuation of the fluids occurs via a change in temperature,
2637,101,115
exposure
to light,
38
a change in pH or ionic strength of the fluid
3951
and leads directly
to a change in viscosity, volume, or a phase transition. Most of these hydrogel
microvalve principles take advantage of the favorable scaling of their underlying
physical principles of operation at small length scales.
So-called environmentally responsive fluids or smart hydrogels were first
reported to exhibit a significant expansion as a function of the pH value of the
surrounding fluid by Kuhn et al. in 1948.
48
This work was followed by work
demonstrating the reversible volume change of these materials as a function of salt
concentration in large scale systems in 1950.
49
The hydrogels investigated in these
studies were able to change their volume in a reversible and reproducible manner
by more than one order of magnitude, which corresponds to the largest volume
change known for solid state materials. These studies, as well as the following
work by T. Tanaka
47
at the beginning of the 1980s, inspired the development of
other hydrogels with sensitivity to particular environmental parameters and the
application of these materials in medicine, for pharmaceuticals, in sensors and
for tissue engineering. Meanwhile, the potential of these hydrogels to control or
regulate the transport of fluids was investigated and demonstrated through the
first macrovalve for flow control at the milliliter and liter range.
50
However, the
major disadvantage of hydrogel-based valve principles was their slow response.
47
The first technologically relevant application of smart hydrogels to flow con-
trol was achieved with the pH-sensitive microvalve presented by Beebe et al. in
2000,
46
which relied on the swelling of a hydrogel in response to a change in
pH, and used in situ photo patterning of the hydrogel in a microfluidic channel.
The authors achieved a relatively fast valve response (less than 10 seconds), and
demonstrated the use of hydrogel valves for autonomous microflow control. Since
the swelling of hydrogel is diffusion controlled, the response time of these valves is
mainly determined by the characteristic size of the actuator
42
and scales favorably
at small length scales as discussed in Section 6.2. Specifically functionalized
hydrogels have been developed for applications as microvalves in response to
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