Microfluidics - Introduction to BioMEMS - page 151

(a) (b)

Closed

Open

0 kPa -60 kPa

0 kPa -60 kPa

Open Open

Closed Closed

500 µm

-60 kPa

0 kPa

FIGURE 3.39 PDMS.“doormat”.microvalves..(From.Kazuo.Hosokawa.and.Ryutaro.Maeda,.“A.pneu-
matically-actuated.three-way.microvalve.fabricated.with.polydimethylsiloxane.using.the.membrane.
transfer.technique,”. J. Micromech. Microeng. .10,.415,.2000..Reproduced.with.permission.from.the.
Institute.of.Physics.)

wall separating two microchannels (hence the name “doormat”). As negative pressure is applied
to the control channel, the membrane pad delects downward and the two sides of the micro-
channel communicate under the wall ( Figure 3.40 ). his design can be fabricated entirely with
SU-8 photolithography, bypassing the need for photoresist relow and allowing for microchan-
nels with rectangular cross-sections.
An example of its operation is shown in Figure 3.40 . When the membrane is not delected
(oval pads in Figure 3.40 ), the valve is closed and no mass transport of red dye occurs (i.e., the
valve is closed at rest). Application of vacuum to the control line delects the membrane down-
wards, allowing mass transport (be it convective transport, i.e., low, or difusive transport, or
both).
he doormat design has none of the aforementioned limitations of the Quake microvalve.
Because the valve is actuated from the channel loor, it can actuate both deep and shallow chan-
nels. he valve design beneits from all the advantages of SU-8 microluidics, namely: (a) the
channel height is speciied independently of its width; (b) the channels have rectangular cross-
sections (greatly facilitating luid dynamics modeling—both analytical and numerical); and
(c) the roofs are lat (allowing for phase-contrast microscopy of cells within the channels).
Microchambers enclosed by doormat microvalves have volumes that are deined only by pho-
tolithography (not by a deformed PDMS wall), so metering is limited simply by the resolution
of the photolithographic process and, because the SU-8 sidewalls are straight, calculation of
the volumes is trivial ( Figure 3.40 ). Finally, because the valves are closed at rest, the devices

(a) Closed (b) Open (a) Closed

PDMS

Control channel

Control channel

100 µm

Microvalve

FIGURE 3.40 Metering.of.nanoliter.volumes.with.PDMS.“doormat”.microvalves..(From.Nianzhen.
Li,. Chia-Hsien. Hsu,. and. Albert. Folch,. “Parallel. mixing. of. photolithographically. deined. nanoliter.
volumes.using.elastomeric.microvalve.arrays,”. Electrophoresis .26,.3758-3764,.2005..Figure.con-
tributed.by.Greg.Boggy.)