Biomedical Engineering Reference
In-Depth Information
a
120
100
80
60
40
20
0
Colloid composite
Nanoshell composite
Nanoshell
Colloid
Nanoshell
Colloid
Laser on
Laser off
0
10
20
30
40
50
60
70
Time (min)
b
120
100
80
60
40
20
0
0
Colloid composite
Nanoshell composite
Nanoshell
Nanoshell
Colloid
Colloid
Laser on
Laser off
100 µm
10
20
30
40
50
60
70
Time (min)
FIGURE 3.48
Optomechanical. microvalves.. (From. Scott. R.. Sershen,. Glennys. A.. Mensing,. Marie.
Ng,.Naomi.J..Halas,.David.J..Beebe,.and.Jennifer.L..West,.“Independent.optical.control.of.micro-
luidic.valves.formed.from.optomechanically.responsive.nanocomposite.hydrogels,”.
Adv. Mater.
.17,.
1366-1368,.2005..Adapted.with.permission.from.John.Wiley.and.Sons.)
3.8.1.13 Single-Use Microvalves
For some applications, to ensure that the valve never closes again, it may be beneicial to design
single-use valves that are closed at irst, open once, and stay open forever aterwards. In 1999,
Robert Langer and colleagues at MIT designed a silicon-based “drug-release” microchip con-
taining etched cavities that were capped with “sacriicial” gold membranes (
Figure 3.49
). he
cavities could be illed with drug-containing solutions before being sealed with the membranes.
he membranes could be electrochemically dissolved on-demand with the application of a small
voltage (~1 V) pulse, so that each voltage pulse controlled the release of one dose (~25 nL) of
“drug” for each cavity present in the microchip.
Sacriicial membranes composed of SU-8 and silicon nitride have also been reported. As
with Langer's sacriicial gold membrane, these microvalves are opened when heat or electrical
current is applied to the sacriicial membrane, resulting in a change in membrane integrity and
rupture of the membrane. As the microvalve is opened, debris from the disintegration of the
sacriicial membrane may be released into the luid, potentially contaminating the system. In
addition, actuation of the microvalve may induce localized heating near the electrodes if power
and timing are not properly calibrated.
A sacriicial physical barrier may not be needed if all that is needed is a single-use micro-
valve—surface tension and wetting may do the job. “
Capillary burst microvalves
” passively
control luid low by increasing capillary resistance inside the microchannel. his is typically
accomplished by an abrupt change in the geometry or surface chemistry of the microchannel.
he high surface energy as a result of the abrupt channel change traps the liquid meniscus at the
interface of the valve. For example, as seen in “Lab-CD” platforms (see
Figure 3.36
in Section
3.8.1.2), a straight microluidic channel may empty into a valve area with diverging sidewalls.
he luidic resistance then greatly increases at the interface between the straight channel and the
angled valve sidewall, pinning the liquid meniscus at the straight capillary channel. he liquid
meniscus forms a bulge into the valve area characterized by the equilibrium contact angle, θ
e
,
