Biomedical Engineering Reference
In-Depth Information
An analogous method was used to make the microchannel that would act as an electrolyte
bath. SU-8(50) photoresist was spin-coated onto a silicon wafer with 41 µm thickness. The pho-
toresist was then patterned according to the parameters that the manufacturer supplied, making
100 µm wide and 20 mm long strips with a wider area (0.5 mm at the widest point) in the middle
to accommodate the actuator with CE and electrolyte bath. PDMS was then mixed as described
above, spin-coated onto this mold master at 1000 rpm, and then heat-cured as above. This treatment
produces a PDMS membrane about 85 µm thick in the resistance-free areas and 44 µm thick on top
of the SU-8(50) pattern.
The PDMS chip containing the Y-shaped working channel was then bonded to the PDMS
membrane still on the silicon wafer by both exposing to oxygen plasma and pressing them together
after visual alignment on a mechanical stage. The PDMS assembly was then demolded from the
second silicon master.
The fi nal step of the process was to drill fl uid access openings into the PDMS assembly and
align it with the active PPy actuator on the glass substrate (see Figure 13.16).
Plasma bonding was not used in this step, since pressing PDMS against a glass surface pro-
duces a reversible watertight seal and enables reuse of either passive or active part of the device.
The electrolyte channel was then fi lled up with 0.1 M aqueous solution of NaDBS, and voltage was
applied between WE and CE to produce a change in volume of the PPy(DBS) actuator.
13.3.2.3 Microvalve Operation
Actuation was carried out between
V
WE
-
V
CE
= −
2.6 V and
V
WE
-
V
CE
=
0.0 V, which roughly cor-
respond to
1 and 0 V versus an Ag/AgCl reference. Voltage was limited to these values because
hydrogen bubbles start to form below
−
2.6 V, and it is possible to permanently damage the polymer
above 0 V. It became apparent that it would be highly desirable to have a REF-integrated on-chip
next to the microactuator to enable application of accurate voltages, as the voltage between WE and
CE drifted occasionally. Nevertheless, despite the lack of integrated REF, it was possible to close
and open the working channel (see Figures 13.16B and 13.16C). As seen from the micrographs,
the hemispherical cross-section of the channel enabled a complete seal at the maximum volume
increase in the PPy(DBS) actuator. The operation was then quantitatively verifi ed under a profi lometer
using an open PDMS electrolyte bath with no microchannel, allowing access for the profi ling stylus.
The actuation proceeded as follows: fi rst, the polymer expanded to the thickness of 45 µm from the
original thickness of 34 µm with 32% vertical volume change, after application of
V
WE-CE
= −
−
2.6 V.
0.0 V, the PPy returned to the thickness of 41 µm. It
was then possible to reversibly alternate the height of the microactuator between 41 and 45 µm by
alternating the voltage between
Under subsequent application of
V
WE-CE
=
2.6 and 0 V (although not always returning to the same height due
to voltage drifts in the absence of REF). Thus, the reversible volume change in direction normal to
the substrate is 12% of the original volume. This volume change took 10-15 s. This volume change
was signifi cantly lower than that reported for a continuous thick PPy fi lm or a micropatterned thin-
ner fi lm (1-1.5 µm)—80% nonreversible original volume change and 30% subsequent reversible
volume change. The difference in measured volume change is likely due to the fabrication method
used—instead of polymerizing the PPy on a fl at substrate, PPy was deposited onto a protruding
gold post to prevent delamination. It is possible to hypothesize that anisotropic morphology of PPy
is oriented normal to the substrate and is thus actually oriented parallel to the wafer surface in PPy
surrounding the post while oriented normal to the wafer surface in PPy on top of the post. The large
volume change then occurs not only in the normal direction but also in the lateral direction in the
Au post design. This characteristic was confi rmed by video microscopy, which showed reversible
lateral volume expansion increasing the diameter of PPy actuator by 4 µm, or about 6% half of
the volume expansion in the normal direction but still much larger than the lateral expansion in
PPy fi lms that was grown conventionally on a fl at substrate.
−
