Biomedical Engineering Reference
In-Depth Information
attractive optical and mechanical properties and can be easily molded and bonded at temperatures
less than 100
C.
A valve utilizing a surface micromachined PPy bilayer actuator
15
has been reported, where PPy-Au
hinge moved a rigid plate of benzocyclobutene (BCB) polymer to open or block a microfl uidic channel
fabricated with SU-8 photosensitive epoxy and PDMS. However, there are a few problems associated
with the bilayer mode of actuation. The mechanical force that can be applied by the tip of the bilayer,
which is the point that undergoes the most physical displacement, is very small—some orders of mag-
nitude smaller than the pressure generated in the PPy fi lm. Also, delamination of the polymer from the
substrate is very common, since the bonding between the polymer fi lm and the electrode is mostly due
to noncovalent interactions.
10
As discussed in Section 13.2, PPy electropolymerized in the presence
of NaDBS can undergo reversible volume change of 30% or more in the direction normal to the sub-
strate.
16
This volume change, if realized in a thick micropatterned PPy if lm, can be directly applied in
a variety of micromechanical systems. Fabrication and operation of a microvalve utilizing anisotropic
volume change of PPy(DBS) and PDMS microfl uidics is described in Section 13.3.2.
°
13.3.2 D
IRECT
-M
ODE
P
OLYPYRROLE
-PDMS M
ICROVALVE
A property of PDMS that is particularly useful in the design of a microvalve is that it is possible
to fabricate thin PDMS membranes by spin-coating technique. These membranes are very fl ex-
ible and form a nonpermanent watertight seal when pressed against another smooth surface. A
valve design
3
implemented with a PDMS microfl uidic system consists of two chambers: working
channel for biological or chemical analyte separated by a thin PDMS membrane from the elec-
trolyte bath containing the PPy microactuator and a gold CE (see Figure 13.13). This valve oper-
ates as follows: an application of voltage between WE, consisting of PPy(DBS) deposited on a
gold conductor, and CE, which is a patterned gold fi lm, produces a redox reaction involving Na
+
ions in the electrolyte. As ions move into the polymer, it swells and pushes the membrane sepa-
rating two chambers into the working channel. This action seals the working channel completely
and results in a closed state for the valve. Under application of reverse voltage, polymer shrinks,
allowing the PDMS to come back into original position, thus opening the working channel.
As mentioned above, polymerizing PPy(DBS) on a fl at gold electrode results in a very signifi -
cant problem. After two or three cycles, the polymer completely delaminates from the electrode
due to swelling parallel to the substrate, which happens simultaneously with volume change that is
normal to the substrate. As discussed earlier, this lateral volume change is far smaller than the one
in the vertical direction, but it is enough to detach the polymer from the substrate. For this reason,
PPy was deposited on electroplated gold posts of 8-9 µm height (see Figure 13.14). These posts
were electroplated through photoresist wells, resulting in a T-shaped cross-section.
(A)
(B)
40
µ
m
7
µ
m
PDMS (polydimethylsiloxane)
−
transparent silicone elastomer
Microchannel molded in PDMS
Polypyrrole
Gold electrodes
Electrolyte (NaDBS)
Glass substrate
FIGURE 13.13
Microvalve design. Working channel is open (A) and is closed by the expanding polypyr-
role (B).
