Page 38 - Optofluidics Fundamentals, Devices, and Applications

Basic Micr ofluidic and Soft Lithographic Techniques 19

allows for syringes (and syringe pumps) to be coupled easily to
microfluidic channels.

2-6-2 Valves and Pumps
Several groups have used the elasticity of PDMS in the actuation of
valves and pumps [19]. The valves operate by applying a force that
pinches a fluidic channel closed at a precise location. Compression of
the channels can be introduced in various ways, including: fluid pres-
sure [38,39], torque actuation from embedded machine screws [40] or
solenoids [41], expansion of a hydrogel [42], magnetic actuation [43],
or the thermal response of shape-memory alloys [44]. Takayama et al.
have also used the pins of a piezoelectric Braille display as valves in
microfluidic systems [45].
Quake valves are perhaps the most commonly used microfluidic
valves in elastomeric devices. The Quake valve is a three-layer micro-
fluidic structure, consisting of a flow channel in one layer separated
by a thin elastomeric membrane from a (usually perpendicular) con-
trol channel in the layer above. The application of pressurized air to
the control channel closes the flow channel. These valves are compat-
ible with soft lithography, and can be used in parallel at high densities
because of their small footprint. Their fabrication and operation are
complicated, however, and require costly and bulky off-chip infra-
structure (computer-controlled pneumatic actuators, gas distribution
system, etc.). These valves are sometimes overkill for simple microflu-
idic applications that require only one, or a small number, of valves.
TWIST and solenoid valves developed by our group are simpler
to construct and operate, and are suitable for situations that require
only small number of valves [40,41]. To construct a TWIST valve, a
small machine screw is introduced directly above a microfluidic
channel in a PDMS device. Rotation of the screw results in downward
motion of the screw and compression of the underlying channel, and
therefore the closing of the channel. To construct a solenoid valve, a
cylindrical, push-type solenoid is placed directly on top of a channel.
To focus the force from the solenoid onto a small area, a small bead is
inserted between the armature of the solenoid and the top of the
channel. Applying a voltage to the solenoid actuates the valve.
Recently Hulme et al. showed that it is possible to fabricate these
valves [pneumatic (Quake-like), screw (TWIST-like), and solenoid
valves] en masse, ahead of time, and then positioned and embedded
in microfluidic devices as needed [41] (Fig. 2-5). These valves are suit-
able for systems in which they are needed only in small numbers, and
in which fabrication of an integrated system is not required. Since the
valves are prefabricated using a standardized procedure, uniform
operation of the valves is possible. The disadvantage of this type of
valves is the need for component-level assembly and a relatively
large footprint for each valve.