soft lithography replication of a bioinspired unidirectional wicking channel

7/25/2019 Soft Lithography Replication of a Bioinspired Unidirectional Wicking Channel

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Soft Lithography Replication of a Bioinspired

Unidirectional Wicking ChannelRyan Blumenstein

[email protected]

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.

REFERENCES

1.Zimmermann, M., Schmid, H., Hunziker, P. &Delamarche, E., Capillary pumps for autonomous

capillary systems.R. Soc. of Chemistry7, 119-125

(2007).

2. Wikisource contributors, 1911 Encylopaedia

Britannica/Capillary Action, Available at

https://en.wikisource.org(2015).3. Washburn, E. W., The dynamics of capillary flow.

Phys. Rev17, 273-283 (1921).

4. Whitesides, G. M., The origins and the future of

microfluidics.Nature368-373, 05058 (2006).

5. Saha, A. A., Mitra, S. K., Tweedie, M. & Roy, S.,

Experimental and numerical investigation of capillary

flow in SU8 and PDMS microchannels with integrated

pillars.Microfluidics and Nanofluidics7(4), 451-465

(2009).

6. Gunther, A. & Kreutzer, M. T., in Micro Process

Engineering, Vol. 1: Fundamentals, Operations and

Catalysts edited by Hessel V Renken A Schouten

12. Chu, K.-H., Xiao, R. & Wang, E. N., Uni-directional

liquid spreading on asymmetric nanostructured

surfaces.Nature Materials9, 413:417 (2010).

13. Toepke, M. W., Abhyankar, V. V. & Beebe, D. J.,

Microfluidic logic gates and timers.Lab on a Chip7,1449-1453 (2007).

14. Feng, J. & Rothstein, J. P., One-way wikcing in open

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).

18. Comanns, P. et al., Directional, passive liquid

transport: the Texas horned lizard as a model for a

biomimetic 'liquid diode'.J. R. Soc. Interface12,

20150415 (2015).

19. Buchberger, G. et al., Bio-inspired microfluidic

devices for passive, directional liquid transport: Model-

based adaption for different materials.Procedia

Engineering120, 106-111 (2015).20. MicroChem, SU-8 2000 Permanent Epoxy Negative

Photoresist: Processing Guidelines for SU-8 2025, SU-

8 2035, SU-8 2050 and SU-8 2075, Available at

h // i h
https://en.wikisource.org/https://en.wikisource.org/http://www.microchem.com/http://www.microchem.com/http://www.microchem.com/https://en.wikisource.org/


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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

soft lithography replication of a bioinspired unidirectional wicking channel

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