SURFACE-MICROMACHINED IN-CHANNEL PARYLENE DUAL VALVES FORUNPOWERED MICROFLOW REGULATION
Po-Jui Chen1, Damien C. Rodger1,2, Ellis Meng3, Mark S. Humayun2, and Yu-Chong Tai1
1
California Institute of Technology, Pasadena, CA 91125 USA
2
Keck School of Medicine, University of Southern California, Los Angeles, CA 90033 USA
3University of Southern California, Los Angeles, CA 90089 USA
ABSTRACTThis paper presents the world’s first in-channel parylene dualvalved microfluidic system for unpowered microflow regulation.
Incorporating a normally-closed and a normally-open passivecheck valve in a back-to-back configuration inside a microchannel,
the dual-valved system has successfully regulated thepressure/flow rate of air or liquid without power consumption orelectronic/magnetic/thermal transduction. By exclusively using
parylene C as the structural material, the fabricated valves havehigher flexibility to shunt flows in comparison with otherconventional thin-film valves. In addition, the parylene-based
devices are completely biocompatible/implantable, and provide an
economical paradigm for fluidic control in integrated lab-on-a-chipsystems. Testing results have successfully demonstrated that themicroflow regulation of the dual-valved system can achieve
mmHg pressure – mL/min flow rates for air and mmHg pressure –μL/min flow rates for water. This regulation range is suitable forcontrolling biological conditions in human healthcare, such as
regulating elevated intraocular pressure (IOP) in glaucoma patients.

INTRODUCTION
Micro-electro-mechanical systems (MEMS) is an enabling
technology to build various microfluidic devices (e.g., channels,valves, pumps, mixers, etc.) for micro-total-analysis systems(μTAS) and lab-on-a-chip applications [1][2]. Among suchdevices microvalves are the key component to systematicallycontrol microflow for reliable operation.
Microvalves aredesigned to be active or passive, with passive valves (check valves)
having advantages of simple operation, zero power consumption,and ease of integration. Based on their actuation mechanism,micromachined check valves are categorized as normally-open(Fig. 1(a), fluidic channel is always open unless there is sufficient
backward pressure) [3] or normally-closed (Fig. 1(b), fluidic
channel is blocked until pressure is above cracking pressure) [4][5]valves. However, both types of check valves are not ideal. For
normally-open valves the cracking pressure to conduct forward
flow is minimal, while there is always a leakage problem whenbackflow is applied. For normally-closed valves the backflow
leakage can be reduced, but the cracking pressure turns out to be
so large that reasonable flow rate is not achievable with lowpressure loading. These problems are inevitable using current
MEMS fabrication technologies.
Even though micromachined check valves are not ideal,
different design schemes can be implemented to realize different
fluidic behaviors from those in a single valve. In this work, anovel dual-valve configuration has been developed to achieve
passive microflow regulation by incorporating a normally-closed
check valve and a normally-open check valve together inside amicrochannel. This configuration inherently regulates pressureand flow rate of fluids without any power consumption. In
addition to implementing similar design concepts from previous

work [3][4], both the design and fabrication of normally-closed
and normally-open check valves have been modified and improved
so that the in-channel dual-valved system is capable of controlling
flows in the mmHg pressure – μL/min flow rate range, further
controlling biological conditions in human healthcare. For
example, a dual-valved device can be implanted in a glaucoma
patient’s eye to facilitate regulation of his or her elevated IOP.
Such a configuration can also be integrated in lab-on-a-chipsystems for refined flow regulation in life science research.

DESIGN
In order to realize flow regulation, the back-to-back dualvalved configuration illustrated in Fig. 1(c) was developed. By
placing a normally-closed valve in forward flow operation at theinlet and placing a normally-open valve in backward flow
operation at the outlet of a microchannel, the overall fluidic
behavior combines the dashed regions of Fig. 1(a) and Fig. 1(b),resulting in the curve shown in Fig. 1(c). In this configuration, thenormally-closed check valve dominates the fluidic control in termsof conducting flows through the channel at pressures higher than
the threshold point. The normally-open check valve works inreverse leakage mode so that it serves as a fluid stopper, in whichflow at pressures higher than the cut-off point is blocked. If theindividual check valves are well-tuned, this passive in-channel
dual-valved microsystem is able to spontaneously regulatepressure/flow rate in the designated range of forward flows
without active actuation. Additionally, this configuration is leakproof to backward flows due to the normally-closed valve atchannel inlet. Because the dual valves are required to be
integrated in-channel for this back-to-back operation, they were
designed to be surface-micromachined for fabrication convenience.

(a)

(b)

(c)
Figure 1. Concept of dual-valved microflow regulation.
Combining the single valve behavior shown in the dashed regionsof (a) and (b) in a “back-to-back” configuration results in thepassive pressure/flow rate regulation shown in (c).

Fig. 2 shows the designs of micro check valves. A freestanding membrane suspended by tethers acts as the normallyopen check valve, in which forward flow can be easily achieved
while reserve leakage happens in backflow [3]. As for thenormally-closed valve, it needs a pre-stressing force to block theinlet orifice so forward flow can not pass through without
sufficient pressure loading. Accordingly, a vacuum-collapsed
sealing method [4] is implemented. As a chamber is collapsed bya pressure difference between the chamber pressure and theambient pressure, the diaphragm connected to the chamber issnapped downward and pressed against the valve seat, resulting in
the dynamic behavior of a normally-closed check valve. Toensure that the cracking pressure of the normally-closed valve iswell controlled, its mechanical design was verified by iterative
FEMLAB™ (now called COMSOL Multiphysics™) simulations(Fig. 3) to estimate the valve performance. Depending on the useof straight-armed or twisted-armed tethers to suspend themembrane, single check valves can have different structural
rigidities and their fluidic behaviors (flow resistance, cracking
pressure, etc.) are correspondingly altered. Parylene C (poly-paraxylylene C) was selected as the structural material because of its
mechanical flexibility (Young’s modulus ~ 4 GPa), chemicalinertness, biocompatibility (USP Class VI grade), andCMOS/MEMS process compatibility. Furthermore, conformal
parylene deposition takes place at room temperature in a vacuum(~ 22 mTorr), which facilitates the vacuum-collapsed sealing of
the normally-closed check valve.
Vacuum-collapsed sealing

FABRICATION
The fabrication process illustrated in Fig. 4 started withgrowing a 2 μm wet oxide on a standard silicon wafer. The
backside of wafer was then bulk-etched by KOH, leaving 25 μm
membranes for fluidic coupling ports. Afterwards, a multi-layer
surface-micromachining technology was utilized to build frontside
structures including a series of photoresist and parylene C layers.
The sacrificial photoresist layers were hard-baked at 120 °C foredge-smoothening and degassing. All parylene C layers wereroughened by oxygen plasma treatment to reduce stiction in thefabricated devices. Using this process, valve seats, check valves,
and a integrated microchannel were fabricated. After etchingaway the backside silicon membrane to open through holes andstripping photoresist with acetone, an additional frontside paryleneC coating was finally applied to accomplish the vacuum-collapsed
sealing of the normally-closed valve.

Thermal oxidation
Backside oxide patterning
KOH silicon etching

Frontside oxide removal
1st parylene deposition and patterning (5 µm)

Free-standing membrane

1st sacrificial photoresist coating and patterning
2nd parylene deposition and patterning (2 µm)

Figure 2. Cross-section of normally-closed (left) and normallyopen (right) micro check valves. Arrows in normally-closed valve
denote the dragging direction when the chamber is collapsed byvacuum sealing.

2nd sacrificial photoresist coating and patterning
3rd parylene deposition and patterning (2 µm)

3rd sacrificial photoresist coating and patterning
4th parylene deposition and patterning (5 µm)

Silicon membrane etching
Photoresist stripping
5th parylene deposition for vacuum sealing (1.5 µm)

SiliconSiO2Parylene

Photoresist

Figure 4. Fabrication process flow.
Figure 3. FEMLAB™ simulations to estimate pre-stressing forceof the vacuum-collapsed chamber (top) and resultant thresholdpressure applied to valve membrane (bottom) to shunt fluids. By
coupling these simulations the dynamic behavior of a complete
normally-closed check valve was determined.

During fabrication the normally-closed valve was the mostcomplex component. A three-step partial exposure lithography
was applied to a single sacrificial photoresist layer in order toconstruct a three-level base featuring a vacuum-collapsed chamber,a valve-seat contact, and a etching/sealing channel of a normally-closed valve (Fig. 5). A 7-μm-high chamber was connected to a 2μm-high channel in order to realize final photoresist stripping and1.5-μm-thick parylene vacuum sealing through that channel. A 5μm photoresist spacer between the valve and the valve seat wasmade to control the pre-stressing force and threshold pressure of
the fabricated valve by its height difference to the chamber.
Planarization by spin-coating thick photoresist over the three-levelstructure was then needed to create a flat valve membrane foruniform device performance. Fig. 6 shows the fabricated inchannel parylene dual valves.

Etching/sealing
channel (2 μm)

Valve-seat
contact (5 μm)

Vacuum-collapsedchamber (7 μm)

TESTING AND DISCUSSION
Microfabricated in-channel single valves and dual valves
were tested using the fluidic setup illustrated in Fig. 7. A
customized
testingjig
wasusedtoprovideamacroscale/microscale fluidic interface with diced chips so thatmicroflows could be introduced to the devices through theirbackside fluidic ports. Compressed air or water served as the fluid
source. On the front end of the jig, a pressure regulator and apressure gauge were connected to control the injection pressure of
the fluids. The tuning resolution was up to 0.01 psi (~ 0.5 mmHg)
even without using hydrostatic pressure generated by height
difference of liquid column. Flow rate measurement and fluid
collection was done on the back end of the jig. Based on thedifferent viscosities of the fluids, flow rate measurement wasperformed via different methods. Air flow rate was measured witha flow meter due to its large value (mL/min range), while waterflow rate was measured by recording the marching velocity of thewater column front in the capillary due to its relatively small flow
rates (on the order of μL/min). Using these techniques, thepressure/flow rate behavior of the devices for the different fluids
could be accurately monitored.

20 µm
100 µmFigure 7. Schematic of fluidic interface setup for device testing.

(a)

(b)
Figure 5. Three-level sacrificial photoresist layer coating using
three-step partial exposure lithography: (a) Cross-section
schematic and top-view micrographs of the fabricated structure; (b)3-D profile scan of (a).

50 µm

50 µm

Figure 6. Micrographs of the fabricated in-channel parylene dual
valves (top view). (left) Normally-closed micro check valve. (right)
Normally-open micro check valve. The diaphragm tethers can be
designed as straight arms or twisted arms depending on
performance requirements.

Pressure/flow rate testing results are shown in Fig. 8 and Fig.
9 using air and water as the working fluids, respectively. The
tested valves were of the twisted-arm tether type in order to have amore compliant mechanical response. In single valve operation,
the fabricated normally-closed check valve demonstrated aremarkably low cracking pressure around 0.2 psi (~ 10 mmHg),
which was in good agreement with FEMLAB™ simulations. This
was a result of the fluidic pressure being applied on central area ofthe flexible membrane, thus effectively overcoming the prestressing force of the vacuum-collapsed chamber and shunting thefluids to the channel. Furthermore, there was no backflow leakage
to the normally-closed valve. For the normally-open check valve,the cracking pressure was not obviously observed and the forward
flow resistance was significantly lower compared to the normallyclosed valve, providing a higher flow rate with the same degree ofpressure loading. In terms of reverse leakage, the backward flowrate peaked at more than 2 mL/min for air and 2 μL/min for water.
This leak rate started to decrease when the pressure loading was
higher than 0.6 psi and was greatly suppressed with pressureloading higher than 1 psi, which implies that the valve membrane
was in firm contact with valve seat. The breakdown pressure wasconfirmed at more than 25 psi with no obvious leak rate within thisrange of pressure operation. Microfluidic pressure/flow rate
regulation was successfully performed in dual valve operation.
Combining the single check valve behaviors, the dual-valved
microsystem was able to regulate pressure in a range of 0-1 psi (~0-50 mmHg) with measured flow rates of up to 0.7 mL/min for air
and 1.3 μL/min for water. These small flow rates were due to alarge fluidic resistance obtained by the dual check valves with amicrochannel in the test configuration. As expected, no reverse
leakage was observed with pressure loading greater than 25 psi.

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Figure 8. Device testing with air as the working fluid: (a) Single
normally-closed check valve; (b) Single normally-open check valve;(c) Dual valve operation for microflow regulation.

Figure 9. Device testing with water as the working fluid: (a)Single normally-closed check valve; (b) Single normally-opencheck valve; (c) Dual valve operation for microflow regulation.

CONCLUSIONREFERENCES

A parylene-based unpowered dual-valved microflow
regulation system has been successfully developed.
By
incorporating a normally-closed and a normally-open check valve
in back-to-back configuration inside a microchannel, fluidpressure/flow rate can be systematically controlled. Because ofpassive operation in check valves, only mechanical elements wereinvolved and no power consumption/transduction was required. A
multi-layer surface-micromachining technology was used toconstruct the integrated dual-valved channels. Fabrication andcharacterization of single check valves and dual valves have beendemonstrated. In single valve testing, both normally-closed andnormally-open check valves achieved fluidic behaviors in good
agreement with design and simulation. Excellent pressure/flowrate regulation was obtained in dual valve testing. Flow rates of
0.7 mL/min for air and 1.3 μL/min for water were obtained within
a 0-50 mmHg pressure range. With this unique microflow
regulation performance, our biocompatible/implantable dualvalved system can be exploited to conduct life science researchand control biological conditions such as regulating elevated IOPof glaucoma patients.

[1] A. van den Berg and T. S. J. Lammerink, "Micro TotalAnalysis Systems: Microfluidic Aspects, Integration Conceptand Applications," Topics in Current Chemistry, vol. 194, pp.
21-50, 1997.

ACKNOWLEDGEMENTS
This work was supported in part by the Engineering ResearchCenters Program of the National Science Foundation under NSF
Award Number EEC-0310723. The authors especially thank Mr.
Trevor Roper for his fabrication assistance.

[2] S. Shoji, "Fluids for Sensor Systems," Topics in Current
Chemistry, vol. 194, pp. 163-188, 1998.
[3] X. Q. Wang, Q. Lin, and Y.-C. Tai, "A Parylene Micro CheckValve," Technical Digest, The 12th IEEE International
Conference on MicroElectroMechanical Systems (MEMS1999), Orlando, FL, USA, Jan. 17-21, 1999, pp. 177-182.
[4] X. Q. Wang and Y.-C. Tai, "A Normally-Closed In-Channel
Micro Check Valve," Technical Digest, The 13th IEEE
International Conference on MicroElectroMechanicalSystems (MEMS 2000), Miyazakli, JAPAN, Jan. 23-27, 2000,pp. 68-71.
[5] J. Xie, X. Yang, X. Q. Wang, and Y.-C. Tai, "Surface
Micromachined Leakage Proof Parylene Check Valve,"
Technical Digest, The 14th IEEE International Conference on
MicroElectroMechanical Systems (MEMS 2001), Interlaken,
Switzerland, Jan. 21-25, 2001, pp. 539-542.