Soft Lithography Replication of a Bioinspired Unidirectional Wicking Channel
Transcript of Soft Lithography Replication of a Bioinspired Unidirectional Wicking Channel
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Soft Lithography Replication of a Bioinspired
Unidirectional Wicking ChannelRyan Blumenstein
Ethan [email protected]
Sara [email protected]
Washington University in St. LouisMechanical Engineering and Materials Science
Micro-Electro-Mechanical Systems IMEMS 5801
Abstract Capillary filling methods, and directional
wicking techniques in particular, are powerful and
widely-used tools in passive microfluidic devices, as the
surface adhesion experienced by a microflow becomes
large relative to viscous and inertial body forces. Here, we
replicate a previously described unidirectional wicking
channel topography inspired by the rain-harvesting
behavior of the Texas horned lizard. Previously
fabricated by laser-engraving PMMA, wicking channels
are here fabricated by a PDMS soft lithography processwell-suited to batch processing. In addition to replicating
the previously studied channel geometry, we take
advantage of the resolution capabilities of photo- and soft
generally grouped as actuated and passive microfluidic
devices according to whether they utilize an external energy
source or spontaneous filling phenomena resulting from
device design. The former group includes all manner of
pumps, typically displacement- or centrifugal-style pumps, as
well as electric- or magnetic-field actuation, while the latter
includes devices that drive flow with chemical gradients,
osmotic pressures, permeation, and capillary forces. Actuated
microfluidic devices are capable of producing higher and
more consistent flow rates with the greatest control, butpassive devices offer greater portability, scalability, and low
power consumption [1]. For this reason, passive microfluidic
devices are of particular interest in many applications where
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d i b bl h h b
molecular analysis, biodefense, molecular biology, and
microelectronics [4]. One of the most common uses of
capillary action in microfluidics devices is in capillary
pumping. In a capillary pump, the surface attraction between
the working fluid and device surface spontaneously draw thefluid through a working channel without the need for external
pumping. An example of a device that uses capillary pumping
is shown below inFigure 2.Much work, both numerical and
experimental, has been done to model the dynamics of
capillary pumps and to develop tunable designs to control
filling rates with various capillary geometries [1,5].
FIGURE 1
MAGNITUDE OF INERTIAL,VISCOUS,AND GRAVITATIONAL FORCESRELATIVE TO INTERFACIAL FORCES AS FUNCTIONS OF CHANNEL SIZE AND
VELOCITY [6]
By controlling feature size, shape, spacing, and alignment,
some wicking devices have been designed to produce
anisotropic filling, allowing for directional control of fluid
motion [7,8,9,10]. Most such devices use micropatterned
posts or nanohairs [11,12] on a surface or channel to produceasymmetrical conditions that favor movement of a fluid
surface in one direction. Some wicking topographies achieve
sufficient directional selectivity to effectively halt fluid flow
in the reverse direction. These devices are referred to as
liquid diodes, as their selectivity resembles an electrical
diodes ability to allow current in only one direction. An
example of the wicking behavior of such a device is
illustrated inFigure 4.Anisotropic wicking topographies and
unidirectional liquid diodes are powerful tools for the
design of microfluidic devices, providing greater flow control
and even allowing for the construction of simple logic circuits
[13].
FIGURE 4
THIS EXAMPLE OF A LIQUID DIODE CHANNEL TOPOGRAPHY ALLOWS FLOW
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from damp sand [16]. The skin of these lizards produces
capillary wicking with a strong directional selectivity
towards the lizards mouth (seeFigure 3), making the lizards
skin an excellent natural model for unidirectional wicking
topographies. As a result, P. cornutum has inspired highlysuccessful attempts to replicate the anisotropic wicking
behavior in microfluidic devices [17,18,19].Figure 5shows
a schematic and photograph of a polymethyl methacrylate
(PMMA) biomimetic wicking channel for unidirectional
transport based on observations of the Texas horned lizards
rain-harvesting behavior and morphology. This channel was
found to allow fluid flow in the forward direction while
halting fluid spread in the reverse direction [18].
FIGURE 5
ABIOMIMETIC CHANNEL FOR UNIDIRECTIONAL WICKING,INSPIRED BY THE
RAIN-HARVESTING ADAPTATIONS OF THE TEXAS HORNED LIZARD [18]
In this investigation, we attempt to replicate the
unidirectional wicking topography shown inFigure 5 through
a soft lithography fabrication process. In addition, we
investigate the effects of channel width and feature size by
creating similar channels at varying scales. Compared to
laser-engraving, soft lithography has the advantage of being
suitable to batch processing. By replacing laser-engraving
with soft lithography and experimenting with varying
channel scales, we aim to design an optimized unidirectionalwicking device potentially suited to production at scale.
METHODS
I
reasons. A 5:1 ratio would dictate a channel depth of 1500 m
for the geometry described above, which was not feasible for
the photolithographic methods used to produce the channel
molds. With the methods and materials available, 450 m
was considered a reasonable maximum channel depth, givingan aspect ratio of only 1.5:1.
FIGURE 6
CHANNEL GEOMETRY AT 6:1SCALE,INCLUDING CHANNEL,WICKING SCALEFEATURES,LOADING PAD,AND 1AND 5MM MEASUREMENT FEATURES
In addition to reproducing previously described results, wewished to investigate the effect upon wicking behavior of
different feature sizes. This was of particular interest as prior
biomimetic devices inspired by the wicking behavior of the
Texas horned lizard have used features approximately six
times the size of the lizards scales[18]. The
photolithographic and soft lithographic are not bound by the
same size limitations as the laser-engraving methods
employed previously, allowing for the production of similar
geometry at much smaller scales. We therefore designed
similar wicking channels at one-half and one-sixth of the size
shown inFigure 6 to produce wicking features at approximate
ratios of 1:1 and 3:1 compared to the biological model. The
characteristic widths for these channels were 50 and 150 m,
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the channels labeled a-d are hereafter referred to as 1-4,
respectively).
FIGURE 7
SECTION OF A NOVEL ARRAY OF WICKING SCALE FEATURES,WITH
ALTERNATING ROWS INVERTED
FIGURE 8
THE PHOTOMASK DESIGN CONTAINS ALL CHANNELS,INCLUDING A)THE 3:1
CHANNEL, B) THE 6:1, C) THE 1:1, AND D) THE 3:1 ARRAY, ARRANGED TO
same spin coating procedure a second time to achieve a total
photoresist thickness of 450 m. The thick wafer was
subsequently soft baked again at 65Cfor 8 minutes and 95C
for 105 minutes.
Pattern transfer from mask to wafer was performed byexposure to 365nm light in a Karl Suss MJB3 Mask Aligner
(Figure 9). According to the data sheet, the thin wafer
required an exposure dosage of 350 mJ/cm2 and the thick
wafer required 500 mJ/cm2. However, both wafers were
intentionally overexposed (11.46 mW/cm for 45 s/80 s,
respectively) to ensure complete pattern transfer. Both wafers
underwent post exposure bake (PEB) at 65C for 5 minutes
and then at 95C for 15 minutes for the thin wafer and 30
minutes for the thick wafer. Following PEB, the wafers were
developed in MicroChem SU-8 developer for approximately
30 minutes (thin) and 45 minutes (thick).
FIGURE 9
KARL SUSS MJB3 MASK ALIGNER USED TO EXPOSURE PHOTORESIST
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again degassed in the vacuum chamber until all bubbles were
sufficiently removed: this was a longer process for the thicker
photoresist master than the thinner one. The devices were
cured for 1 hour at approximately 70C in a Quincy Lab
Model 20 Lab Oven. Finally, each device was removed witha scalpel and bonded to a glass slide.
FIGURE 10
VACUUM CHAMBER USED TO DEGAS PDMS(VACUUM PUMP NOT
PICTURED)
IV.
Surface functionalization
In order to achieve the recommended contact angle range
between 60-80 [18], the devices were plasma treated in aplasma cleaner (Harrick Plasma PDC-001-HP) with a
maximum power of 30 W. According to Figure 11, it was
determined that a plasma treatment of 3 minutes at maximum
FIGURE 11PDMS/DICONTACT ANGLE AS A FUNCTION OF PLASMA TREATMENT DOSE
(TIME AT 70 W)AND AIR EXPOSURE TIME [22]
RESULTS AND DISCUSSION
I. Photolithography
The SU-8 2075 photoresist was highly viscous, which
presented challenges in producing an even coating. Spin
coating produced a sizable edge bead on both wafers. After
pre-exposure baking, wrinkling defects became apparent in
the edge bead of the thick wafer, as seen inFigure 12.This
did not appear to impact the exposure, since the middle of the
wafer was free of defects.
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appeared to be fine, but after the pre-exposure bake there
were several elliptical depressions on the wafer. These can be
seen on the left side of Figure 13. These patterns would
interfere with the exposure, because they cover a significant
portion of the wafer.
FIGURE 13
THIN WAFER AFTER PRE-EXPOSURE BAKE SHOWING DEPRESSIONS IN
PHOTORESIST LAYER AFTER SOFT BAKING
The pre exposure bake for the two wafers took longer than
expected. Based on the manufacturers guidelines, the SU-8
for the thin wafer should have been 45 minutes at 95C, and
the thick wafer should have been 45 minutes after each spin.
However, after the prescribed time, both wafers were still
very tacky. The thin wafer still had the defects described
above. In order to be sure the wafer would not stick to the
photomask in the photolithography stage, the surface should
be only slightly tacky. The thicker wafer was soft baked for
a total of 150 minutes and the thinner wafer was soft bakedfor a total of 145 minutes. After this extended bake the wafers
were less tacky, and after cooling overnight both wafers were
significantly less tacky.
thin-coated wafer (design thickness 225 um) ranged from
300-400 um (seeappendix A: Photoresist Thickness Data).
Channel 1 (6:1) had significant issues due to it overlapping
with the defects on the photoresist. The height of this channel
was consistently below 100 um, so the device created withthis master would not be effective. The thicker wafer had a
design thickness of 450 um, but our measurements ranged
from 500-650 um. The higher-than-expected thickness of the
channels increased the capillary aspect ratios, which may
have improved the devices wicking capabilities.
Overall, the developed features appeared well defined with
few defects. There are a few notable exceptions, most notably
in thin Channels 1 (3:1) and 3 (1:1). Channel 1 was
significantly thinner than expected due to the mask
overlapping with the defect on the wafer, but the pattern
seemed to transfer well. As seen inFigure 14,the features are
crisp but there is a rough texture in the channel.
FIGURE 14
PRMASTER CHANNEL 1THIN (3:1),SHOWING ROUGHNESS IN THE CHANNEL
Channel 3 thin (1:1) showed precise pattern transfer for most
of the length (Figure 15), but had significant defects on
portions of the device (Figure 16). The severity of defects on
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FIGURE 16PRMASTER CHANNEL 3THIN (1:1),SHOWING SIGNIFICANT DEFECTS ON THECHANNEL
For the thick wafers, all of the masters seemed to be defect-
free enough to produce usable devices. There were issues
with Channel 2 thick (6:1) where the corners of the
trapezoidal features were darker than the center, as seen in
Figure 17.This is most likely due to incomplete etching of
the photoresist for the entire height of the feature. The middle
of the feature is completely etched, but the corners were lessexposed and therefore rounded off instead of etching to a
point.
Channel 3 thick (1:1) showed wavy surface distortion across
the width of the channel, as seen inFigure 18.The features
appear to be mostly intact, so was unclear how this defect
would affect the performance of the resulting PDMS device.
For additional images of the photoresist-patterned masters,
seeAppendix B: Supplementary Images.
properties (i.e. Youngs modulus), but should not have a
significant effect on the surface chemistry, contact angle, or
other fluidic behavior.
FIGURE 18
PRMASTER CHANNEL 3THICK (1:1),SHOWING DISTORTION IN AREAS OF
THE CHANNEL
After degassing the PDMS in the beaker, the PDMS was still
filled with air bubbles after being poured onto the wafers.
After another round of degassing, the PDMS appeared to be
free of air bubbles. After removing the PDMS from thewafers, the devices were examined for defects with a
microscope. Most of the defects observed resulted from flaws
in the PR master; however, several additional defects
occurred during the casting process. We observed warping of
the PDMS on the edge of Array 4 thin (3:1) (Figure 19). Most
of the warping occurred near the scale bars and loading pads,
so this defect was not believed to significantly influence fluid
flow in the device.
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leaving a dark hole behind. This was the only defect on the
device, so it should not have a significant impact on fluid
flow.
FIGURE 20
PDMSCHANNEL 3THICK (1:1),SHOWING DEFECT ON FEATURE AND
SURFACE WAVINESS
FIGURE 21
PDMSARRAY 4THICK (3:1),SHOWING MISSING FEATURE DEFECT
we used a diluted ethanol solution. A 35% ethanol solution
produced a contact angle of 635. However, the channel was
still too hydrophobic, as no channel filling was observed. A
50% ethanol solution resulted in a contact angle of 515,
which induced fluid filling on most devices.After preliminary testing, we discovered the fluid did not
flow well in the thin channels. There was either no
movement, or the fluid flowed over the features rather than
in the channels. The thicker channels exhibited better wicking
behavior, which is unsurprising. The thick channels have a
greater aspect ratio (depth:width), which means that filling
will be influenced more by the fluid interactions with the
channel walls than with the floor. Since our interest is in
measuring how the shape of the features determines the fluid
flow, this is preferable. Channel 2 (6:1) exhibited slower
filling speed than the other channels, while Channel 3 (1:1)
tended to be the quickest. This result is expected due to the
nature of capillary action: at small channel sizes and flow
velocities, surface forces are dominant over body forces.
Smaller features have a higher surface area to volume ratio,
so the surface force pulls the liquid more effectively.
Taking all of this into account, we decided to focus on three
devices: Channel 1 thick (3:1), Channel 3 thick (1:1), and
Array 4 thick (3:1). Two trials were performed in bothdirections on each device. The speed was found by measuring
the time the fluid took to travel 7.1mm, the distance between
two lines of college ruled lined paper underneath the device.
We planned to use the distance markers on the edge of the
channel, but they were difficult to see on the videos of the
trials. Measured filling speeds are recorded inTable 1; full
data are presented inAppendix C: Microfluidic Testing Data.
TABLE 1MEASURED DEVICE FILLING SPEEDS
Channel 1 Thick (3:1) Array 4 Thick (3:1)
Speed (mm/s) Speed (mm/s)
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the 7.1mm mark in the forward direction. To increase the
speed so it was easier to measure, a higher concentration of
ethanol was used to decrease the contact angle.Most of our data indicates the fluid flows faster in the reverse
wicking direction. This may be due to many factors,including contact angle of the fluid, definition of the features,
or material property differences between PDMS and PMMA.
We had considerable difficulty controlling the contact angle
of the fluid, and we found the flow through the devices was
sensitive to the contact angle. When the device was too
hydrophobic, the fluid would bead up on the loading pads
instead of entering the channels. When the device was too
hydrophilic, the fluid would flow quickly through the entire
channel without being pinned as designed. With the 50%
ethanol solution, we observed the fluid being pinned in the
same manner as the researchers who created the scale design,
as seen inFigure 22.In part a) the filling front approaches the
capillary at the top, while it is pinned on the bottom. Then in
b) and c), the filling front has advanced through the capillary
while still being pinned at the top and bottom of the channel.
Finally in d) the fluid from the capillary joins with the fluid
pinned at the bottom, joining the two streams and advancing
the filling front. In the reverse wicking direction, the fluid
should not have had the same pinning and capillary fillingaction. The filling front did appear to be pinned, as seen in
Figure 23.
FIGURE 23IMAGE TAKEN OF PINNING IN REVERSE WICKING DIRECTION AT 1:1SCALE
CONCLUSIONS
This investigation attempted to produce a biomimetic
unidirectional wicking channel from previously published
descriptions of similar such devices. This topography isinspired by observations of the rain-harvesting morphology
and behavior of the Texas horned lizard, whose skin is able
to transport water to its mouth for ingestion. Such
unidirectional wicking behavior is of great interest in
microfluidic applications where close control of filling
direction and rate may be required.
We recreated previously successful wicking topographies in
a PDMS casting instead of engraved PMMA. This approach
offers greater control over feature size and definition,particularly at smaller scales, and could potentially allow for
production at scale; however, channel depth is more limited,
which is problematic for larger geometries. In addition, the
natural hydrophobicity of PDMS must be overcome by
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in greater surface adhesions greater relative to viscous and
inertial body forces.
Unexpected, however, was the result that filling proceeded
preferentially in the reverse direction, which was expected to
halt the advance of a filling front. No conclusive explanationis offered for this result, which is inconsistent with previous
results [17,18,19]. An excessively low contact angle may
help to explain why the device failed to halt filling in the
reverse direction, but does not account for the filling rate
being greater in reverse than in the forward direction. Further
investigation is necessary to explain the reverse filling
observed and devise improved methods for more robust
control of wicking directionality.
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12. Chu, K.-H., Xiao, R. & Wang, E. N., Uni-directional
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surfaces.Nature Materials9, 413:417 (2010).
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Microfluidic logic gates and timers.Lab on a Chip7,1449-1453 (2007).
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micro-channels controlled by channel topography.J.
Colloid and Interface Science404, 169-178 (2013).
15. Sherbrooke, W. C., Integumental water movement and
rate of water ingestion during rain harvesting in the
Texas horned lizard, Phrynosoma cornutum.Amphibia-
Reptilia25, 29-39 (2004).
16. Sherbrooke, W. C., Scardino, A. J., de Nys, R. &
Schwarzkopf, L., Functional morphology of scale
hinges used to transport water: convergent drinking
adaptions in desert lizards (Moloch horridus and
Phrynosoma cornutum).Zoomorphology126, 89-102
(2007).
17. Comanns, P. et al., Moisture harvesting and water
transport through specialized micro-structures on the
integument of lizards.Beilstein Journal of
Nanotechnology2, 204-214 (2011).
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transport: the Texas horned lizard as a model for a
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Photoresist: Processing Guidelines for SU-8 2025, SU-
8 2035, SU-8 2050 and SU-8 2075, Available at
h // i h
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APPENDIX A:PHOTORESIST THICKNESS DATA
TABLE 2
WAFER 1MEASURED THICKNESSES (TARGET:225 M)
Channel Top Middle Bottom Average
1 - - - -
2 265 318 326 303
3 - 233 - -
4
412
359
389
386.7
Note: - denotes no measurement taken of visibly flawed regions
TABLE 3
WAFER 2MEASURED THICKNESSES (TARGET:450 M)
Channel Top Middle Bottom Average
1 614 660 673 649
2 556 616 620 597.3
3 540 549 599 562.7
4 482 510 524 505.3
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APPENDIX B:SUPPLEMENTARY IMAGES
Photoresist Master
PRMASTER CHANNEL 2THIN (6:1),SHOWINGSUCCESSFUL PATTERN TRANSFER
PRMASTER ARRAY 4THICK (3:1),CLOSE UP OFSMALLEST FEATURE
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PDMS Devices
PDMSCHANNEL 1THIN (6:1),SHOWING RESULTSOF SUCCESSFUL CASTING
PDMSCHANNEL 2THIN (3:1),SHOWING RESULTSOF SUCCESSFUL CASTING
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14 December 7, 2015
APPENDIX C:MICROFLUIDIC TESTING DATA
Video # Channel
#
Ethanol
Conc.
(%)
Direction
(away/towards
ID)
Time (1
segmt,
s)
Time (2
segmt,
s)
Speed, seg.1
(mm/s)
Speed, seg.2
(mm/s)
Video Notes Testing notes
2306 4 70 t 5.12 n/a 1.38671875 fill @ moderate rate ~ length
2305 3 50 t 1.55 6.4 4.580645161 2.21875 filled ~50%, then stagnation
2303 4 70 a 3.7 21.16 1.775 0.671077505 filling, decent rate (~ channel)
2301 3 50 a 2.66 7.48 2.669172932 1.898395722 goes to 3 lines filled whole channel
2299 1 50 t 12.8 n/a 0.5546875 preferential filling in reverse (slight advantage in distance, noticeablyfaster)
2298 1 50 a 20.97 n/a 0.338578922
2296 3 50 a 2.22 5.25 3.198198198 2.704761905 rapid filling whole length
2294 1 50 a 19.09 n/a 0.371922472 n/a fill approx. length
2292 4 50 t n/a n/a n/a n/a runs for more than 60s add 30 both sides; slight preference for rev direction
2289 1 50 t 7.72 38 0.919689119 0.373684211 two channels run atdifferent speeds
some filling
2287 3 50 t 1.2 3.26 5.916666667 4.355828221 rapid filling whole length
2280 4 50 a n/a n/a n/a n/a takes 30s to travel halfsegment, no progressfor another 30s
photo shows end of filling front @ stasis
2279 4 50 t 27 n/a 0.262962963 n/a only one portion ofarray reaches 1 seg
2278 2 35 both n/a n/a n/a n/a some flow in towarddirection, drop in
middle
filling both dir; noticeably faster in rev dir
2277 1 50 both drop in middle filled both directions, preerentially in reverse direction
2276 thin 4 both
2275 4 both better capillary filling - no clear directional preference
2274 3 both spread in reverse direction, strongly preferential
2273 2 both beading - no flow
2272 1 both little flow; contact angle: ? (pic @ 4:33)
2270 1 both