Sensors & Actuators: B. Chemical 297 (2019) 126776

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Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb

Novel monolithic “Slightly-Open doormat” (SOD) valve enables eﬃcientfabrication of highly-scalable microﬂuidic gas-on-gas multiplexer

T⁎

Priyan Weerappulia,b,c, , Taisuke Kojimab,c,d, Shuichi Takayamab,c,d, Amar Basua,eaDepartment of Biomedical Engineering, Wayne State University, Detroit, MI 48202, USA
Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109, USAc
Biointerfaces Institute, University of Michigan, Ann Arbor, MI 48109, USAd
Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USAe
Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI 48202, USAb

A R T I C LE I N FO

A B S T R A C T

Keywords:
Microﬂuidics
Elastomeric valve
Monolithic PDMS valve
Large-Scale integration
Multiplexer
Microvalve
Normally-Closed valve
Normally-Open valve

The use of monolithic normally-closed (NC) valves within microﬂuidic systems is currently limited by the poor
scalability of mitigation strategies developed to prevent the formation of a permanent bond between the valve‘seat’ and elastomeric membrane during fabrication. Herein we report the highly-scalable design and characterization of a slightly-open doormat (SOD) valve that exhibits properties traditionally associated with NC
valves including operability at low actuation pressures, and the capacity to produce a complete seal. In addition,these valves are comparable in their scalability and in responsiveness, and exhibit a capacity to eliminate resting
channel resistance that is absent in their NC counterparts. We demonstrate the utility of this novel valve design
through the design and operation of a gas-on-gas multiplexer containing 32 device valves controlled by amultiplexer containing 160 SOD valves using only 10 external solenoid valves and a single common pressuresource.

1. Introduction
In academic parlance, the term microﬂuidics refers to a discipline
concerned with the study and development of engineered systems
capable of eﬀectively handling sub-milliliter volumes of ﬂuid. The
progeny of technologies pioneered with pulled glass capillary tubes(e.g. the Coulter counter); most modern microﬂuidic systems are fabricated using the silicone polymer polydimethylsiloxane (PDMS) and,despite tending toward planar geometries, boast greater degrees ofoperational complexity than their antecedents owing to the ability ofthis material to support elaborate channel architectures and integrated
control elements [1]. Of the numerous control elements developed todate, perhaps none has proven as transformational as the elastomericmicrovalve – a monolithic control element that enables the movementof ﬂuid through a channel to be interrupted, diverted, or metered by thedistension of a ﬂexible membrane.
The taxonomy of microvalve designs can be classiﬁed into two
distinct sub-groups, based upon their resting state: normally-open (NO)and normally-closed (NC) (Fig. 1). NO microvalves, of which the Quake
bilayer microvalve represents the most common embodiment, consist ofpaired hemi-circular channels cast in separate layers of PDMS [2]. By⁎

stacking these layers such that the channels are oriented orthogonal toone another, an increase in pneumatic pressure within one channel (thecontrol channel) may be made to produce distension of the diaphragmlike region separating it from the other channel (the ﬂow channel)[2,3].
NC microvalves (Fig. 1), of which the Hosokawa and Maeda‘doormat’ valve represents the most common embodiment [4], diﬀer
from NO microvalves in that while they are also comprised of orthogonally oriented paired channels, these channels are assembled invertical juxtaposition to one another (both channeled sides facing inward), and are bonded to opposite sides of a third elastomeric diaphragm layer. These valves diﬀer, also, in that ﬂow channels are discontinuous at points where valves occur. The presence of these
discontinuities forces the movement of ﬂuid within a channel to either
stagnate upstream of these points, or to continue downstream by distension of the elastomeric diaphragm layer. Though a subtle modiﬁcation in design, the presence of this discontinuity eﬀectively allows
NC valves to be operated at lower pressures and higher frequencies thantheir NO counterparts, and to better isolate upstream and downstream
ﬂuid compartments.
Despite their advantages, NC valves have seen relatively infrequent

Corresponding author at: Department of Biomedical Engineering, Wayne State University, Detroit, MI 48202, USA.
E-mail address: pweerapp@umich.edu (P. Weerappuli).

https://doi.org/10.1016/j.snb.2019.126776
Received 9 December 2018; Received in revised form 2 July 2019; Accepted 3 July 2019
Available online 03 July 20190925-4005/ Published by Elsevier B.V.

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Fig. 1. A schematic of the SOD valve (left) withthe control channel height (CCH), elastomeric
membrane thickness (MT), actuation/control
chamber width (CCW), gap-height (GH), valve
seat (VS), and ﬂow channel height (FCH) indicated. Closure of the valve is achieved bymodulating the pressure within the actuationchamber (P2 ) such that P2 > PATM and the
downward force exerted on the elastomeric
membrane by P2 exceeds the upward force
applied to the membrane by the upstream
source pressure (PS ). Cross-sectional diagramsof the SOD valve in relation to conventional
NC- and NO-valves (right) is included to illustrate the fundamental diﬀerence between these
designs.

perpendicular displacement of the diaphragm.
The diﬀerence between the SOD valve and its NC counterpart lies in
the gap height, representing the oﬀset, between the height of the valve
seat and the height of the surrounding channel architecture (Fig. 1).
The presence of this oﬀset allows the SOD valve to retain the mostdesired properties of NC valves (i.e. operability at low actuation pressures, compatibility with rectangular channel geometries, insensitivity
to channel height), while beneﬁting from a fabrication technique thatallows it to be integrated within microﬂuidic devices at quantities anddensities typically associated with NO valves.

use, due to challenges with membrane stiction during fabrication. The
bonding of PDMS layers is commonly achieved through the treatment ofbonding surfaces using oxygen plasma [5,6]. Exposure to oxygen
plasma results in a depletion of surface CH3 and CH2 functional groups,
and a concomitant enrichment of surface silanol (Si−OH) groups [7].
When two bonding surfaces are brought into contact with one another,silanol groups on opposing surfaces undergo condensation reactions –forming a permanent bond between these surfaces. The challenge thispresents for the fabrication of NC valves stems from the contact thatmust be made between the plasma-exposed channel discontinuity (thevalve ‘seat’) and the plasma-exposed elastomeric membrane without
these structures forming a permanent bond.
Historically, several approaches have been taken to address thischallenge including the use of microcontact stamps to deposit residual
PDMS oligomers on valve seat regions following plasma treatment [8];the use of a ‘shadow mask’ [9] or solid ‘shelter’ [10] to exclude surface
regions from being exposed to plasma during treatment; the depositionof metallic/polymeric sacriﬁcial materials in regions from where they
may be removed following bonding of the two layers [11]; the use ofpartially-cured PDMS membranes that will be prone to stick to the valve
ceiling during ﬁnal device assembly [12]; and the application of negative pressure during bonding to prevent contact between these regions until complete hydrophobic recovery has occurred [13,14].
Though each of these techniques has proven satisfactory for fabricating
devices containing small numbers of valves (< 50), they become impractical in systems where larger numbers of valves (> 100 per chip)are required.
To address this challenge, we have developed a novel ‘slightly-open’
doormat valve (SOD valve) capable of achieving a complete seal, and of
regulating the ﬂow of a pressurized gas across a broad range of supply
pressures. To demonstrate the utility of this valve design, we havefabricated gas-on-gas multiplexers capable of selectively distending
elastomeric membranes by the venting of individual gas-ﬁlled channels.
To our knowledge, this work represents the ﬁrst description and demonstration of a SOD valve and gas-on-gas multiplexer.

2.2. SU-8 mold fabrication
Fabrication of the multiplexer devices described herein, and of the
SOD-valve in general, occured in two stages: multilayered mold fabrication (Fig. 2A) and device fabrication (Fig. 2B). During the ﬁrst ofthese, a silicon wafer (University Wafer, South Boston, MA, USA) was
coated with one of several epoxy-based photoresists: SU-8 2 (for molds
having target gap heights ranging from 1.5 to 5 microns), SU-8 50 (formolds having target gap heights ranging from 10 to 15 microns) or SU-8
2025 (for molds having target gap heights ranging from 20 to 40 microns) (MicroChem Corp, Newton, MA, USA). These resists were spun at
500 rpm (10 s) and 500–4000 rpm (30 s) to produce features with target
heights ranging from 5 to 40 μm using a spin processor (WS-400-6NPPLITE, Laurell Technologies Corporation, North Wales, PA, USA). Spin
protocols used to fabricate molds with speciﬁc gap heights were derived
from recommended spin velocities provided by the photoresist manufacturer, and the resulting gap heights were conﬁrmed using a stylus
proﬁlometer (Dektak XT, Bruker Corporation, Billerica, MA, USA). The
SU-8 coated wafer was then placed on a hot plate that was ramped fromroom temperature (RT) to a surface temperature of 95 °C. The hot plateremained at this temperature for 1–5 minutes, at which point it was
shut oﬀ to allow its surface (and the wafer) to gradually return to RT(Fig. 2A, left).
A ﬁlm photomask (CAD/Art Services, Inc, Bandon, OR, USA) was
then used in conjunction with a mask aligner system (HTG HybridTechnologies Corp, San Jose, CA) to selectively expose the coated wafer
surface to conventional UV radiation (lamp calibrated 20 mW/cm2 at
365 nm) for 10–45 seconds. Following UV exposure, the wafer washeated to 95 °C, cooled, and then immersed in SU-8 developer solution(MicroChem Corp, Newton, MA, USA) for 1–10 min s (Fig. 2A, left).
Residual developer solution was then removed using compressed
nitrogen gas, and the wafer was again spin-coated with a layer of SU-8
photoresist (i.e. SU-8 2025/2075). This step, and its use of more viscous
SU-8 formulations, was intended for the fabrication of features at target

2. Methods
2.1. SOD valve design and working principle
The control of ﬂuid movement through a SOD valve is aﬀorded by
the actuated contact between an elastomeric diaphragm and a patterned discontinuity (the valve seat) within the ﬂuid ﬂow channel. The
presence of this discontinuity and diaphragm forces the ﬂuid to either
stagnate upstream of this barrier, or to spill over it by causing the2

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Fig. 2. The fabrication workﬂow illustrating the fabrication of both the SU-8 mold (A) and multi-layer PDMS device (B).

heights of 50–100 μm. For a target thickness of 100 μm, SU-8 2075 wasserially spin-coated at 500 rpm (10 s) and 2100 rpm (30 s) while, forchannels having a target thickness of 50 μm, SU-8 2025 was spin-coatedat 500 rpm (10 s) and 1750 rpm (30 s). The coated wafer was thenplaced on a hot plate, ramped to 95 °C, and left for 9–20 min s before
allowing it to return to RT (Fig. 2A, right).
Once cooled, the non-crosslinked SU-8 was removed from alignment
marks positioned near the periphery of the wafer using a circular headswab (ITW Texwipe, Kernersville, NC, USA) dipped in SU-8 developer
solution. The alignment marks and workﬂow utilized herein were developed by Dr. David Lai (unpublished). The wafer was then placed on a
hot plate, ramped from RT to 95 °C, and left for 5–10 min s. This secondary heating/cooling step ensured that any residual developer solution was removed from the regions surrounding the alignment marks,
and consequently minimized the stiction and transference that could
otherwise occur between the coated photoresist and the ﬁlm mask
during the UV exposure step. Following UV exposure and development(Fig. 2A, right) performed as was done for the ﬁrst SU-8 layer, theﬁnished mold was placed in a leveled gravity convection oven (DX-400,
Yamato Scientiﬁc America, Santa Barbara, CA, USA) for 8–10 hrs at
120 °C and, upon returning to room temperature, was placed in a desiccator for 1 h in the presence of vaporized tridecaﬂuoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane (United Chemical Tech., Bristol, PA,USA) to ensure passivation of the mold surface.
Fig. 2. Fabrication of Two-Layer Mold and Assembly of PDMS Device.

2.3. PDMS device fabrication
Fabrication of the PDMS device (Fig. 2B) occurred in two stages:
fabrication of the patterned channel layers, and fabrication of theelastomeric membrane. During the fabrication of the patterned channel
layers, a homogenized 1:10 preparation of PDMS curing agent to prepolymer (Sylgard 184, Dow Corning, Midland, MI, USA) was poured
onto the silicon/SU-8 mold to a depth of 2–4 mm. The uncured elastomer/mold was then placed in a leveled gravity convection oven at
60 °C overnight.
During fabrication of the elastomeric membrane, freshly prepared
PDMS was spin-coated onto pre-silanized glass slides (silanization ofthese slides was performed in the same manner as the silanization of thesilicon/SU-8 mold described previously) to a target thickness of 20 μm
using a CEE-200 spin processor (Brewer Science, Rolla, MO, USA).
These coated slides were then cured in a leveled gravity convection
oven at 120 °C overnight to ensure complete elastomer curing. This
curing process mitigated progressive changes in mechanical properties
that could otherwise result from PDMS aging [15,16], and the risk offabrication defects owed to the use of partially cured elastomeric
membranes [15].
The cured PDMS membrane was then bonded via plasma oxygensurface treatment using a vacuum plasma system (Covance MP,
FemtoScience, Hwaseong-si, Gyeonggi-do, South Korea) to the patterned layer containing the ﬂow channels. The resulting assembly wasthen placed in a leveled gravity convection oven at 60 °C for 1–2 hrs,
and then carefully removed from the glass slide using a razor blade. By

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functioned as a vent – remaining at atmospheric pressure (PATM). Twoperpendicular channels positioned on either side of the valve wereconnected to piezoelectric pressure sensors (Model ASDXACX030PAAA5, Honeywell, NJ, USA) to enable monitoring of pressure changes
immediately upstream (PU) and downstream (PD) of the valve during its
operation (Fig. 3A). All connections between the device and external
instrumentation (i.e. the liquid dispensing unit and pressure sensors)
were made by clear connective tubing (Tygon™ E-3603, Saint-Gobain
Performance Plastics, Akron, OH, USA). Analog pressure data was acquired via a multifunction I/O device (USB-6009, National Instruments,
Austin, TX, USA), and analyzed using a custom MATLAB script(v2017a, Mathworks, Natick, MA, USA).
All experiments, unless speciﬁed otherwise, were performed using
compressed nitrogen gas (Item UN1066, Airgas/Air Liquide, Radnor,
PA, USA). Additional experiments were performed using compressedcarbon dioxide gas (Item UN1013, Airgas/Air Liquide, Radnor, PA,
USA), compressed oxygen gas (Item UN1072, Airgas/Air Liquide,
Radnor, PA, USA), and compressed atmospheric air (source: ﬁlteredintake).
For characterizing the frequency with which the SOD and NC valves
could be actuated, the sensitivity of the pressure-based method wasfound to be insuﬃcient at higher (> 2 Hz) frequencies. This, based
upon our initial results, is likely owed to the capacitance of the tubing
used to connect the pressure sensors to the test device as the sensitivity
seemed to improve when shorter lengths of tubing were used, and tofurther degrade when longer lengths of tubing were used. Owing to thecuring this assembly at 60 °C we were able to avoid the temperatureinduced shrinkage of patterned PDMS features – relative to their casting
mold dimensions – that has been observed when PDMS is cured at
higher temperatures [17,18].
This assembly was then bonded to the patterned layer containing
the control channels by ﬁrst treating both bonding surfaces with localized plasma using a Corona SB wand (Elveﬂow, Paris, France). The
importance of using this method for treating the bonding surfaces inlieu of vacuum plasma (as was done when bonding the ﬂow channel
layer to the membrane) lay in the ability for corona-treated surfaces tobe bonded and de-bonded multiple times in the moments immediately
following treatment [19]. This enabled slight misalignments that occured when manually aligning the control channel layer and ﬂow
channel/membrane layer assembly to be corrected prior to formation ofa permanent bond. The completed device assembly was then placed in a
gravity convection oven at 120 °C overnight to ensure the formation ofa permanent bond between all layers.

2.4. Test device designs and assembly of experimental systems
To characterize the functionality and properties of the SOD valve, aseries of test devices comprised of single SOD valves positioned at the
midpoint of a straight gas-ﬂow channel were fabricated (Fig. 3A). The
inlet port was connected to a gas pressure source (PS) supplying compressed air at 10, 15, or 20 psi via a pressure regulator unit (Model
1500, Teli Dispensing Asia Ltd., Hong Kong), while the outlet port

Fig. 3. Utilizing a test device (A), the ability of the SOD valve to achieve and maintain a complete seal was conﬁrmed by applying positive pressure at the inlet while
the SOD valve was in a closed state, and monitoring changes in the downstream pressure sensor (PD ). An increase in downstream pressure relative to atmosphericpressure was not observed until the SOD valve was toggled to an open state. (B) SOD valves with gap heights (GH) ranging from 5 to 40 μm were tested tocharacterize the eﬀect of GH upon the threshold actuation pressure required to achieve closure of the SOD valve. Complete valve closure was observed for GHs <20 μm, with the threshold pressure required for actuating the valve being < PS . (C). A comparison of the threshold actuation pressure required for closure of the SOD
and NC valves indicated no signiﬁcant diﬀerence between these valve designs. (D). The ability to fabricate SOD valves in a manner that doesn’t result in an increase in
ﬂuidic resistance was veriﬁed by fabricating devices with valve widths (VW) greater than the surrounding channel width (CHW). (E) The toggle frequency range
across which the SOD valve and NC valve could be operated (F) and hysteresis (both plots are normalized relative to PS ) observed during these changes in valve state(G) were characterized to provide a dynamic comparison of the relative responsiveness of each valve architecture.
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actuation pressure at which either of these ratios became equivalent to
1 (as would be expected when the valve reached a closed state)(Fig. 3D). For all actuation (P2 ) and source (PS ) pressures tested,
PTh ≤ PS when the SOD valve gap height is below 20 μm – with nosigniﬁcant diﬀerence in PTh when comparing NC and SOD valves with
gap heights as large as 5 μm (Fig. 3D).

superior sensitivity and temporal responsiveness of conductivity measures relative to gas pressure measures [20], test devices containing
SOD or NC valves were perfused with a conductive 40 mM KCl solution
while 300 μL pipette tips were used to position Ag/AgCl electrodes(Item No. EP05, World Precision Instruments, Sarasota, FL, USA) in the
PU and PD ports on either side of the valve. A continuous measure of thecurrent between these electrodes allowed the state of the valve to beassessed. The current recorded ﬂuctuated between a high peak value
when the valve was held in an open state, and a low baseline value
when the valve was held in a closed state. Current recordings wereacquired using an electrometer (Model 6514, Keithley Instruments Ltd.,
Cleveland, OH, USA). To enable the stable actuation of the valve at highfrequencies, a function generator (Model AFG320, SONY/Tektronix,
Beaverton, OR, USA) was used to provide a 10 V square wave signal toactuate an analog pneumatic valve (Model ITV0010-2BL, SMC,
Chiyoda, Tokyo, Japan) connected in-line between an upstream pressure regulator (Model 1500, Teli Dispensing Asia Ltd., Hong Kong) andthe test device. All connections were made using clear connective
tubing (Tygon™ E-3603, Saint-Gobain Performance Plastics, Akron, OH,
USA) and blunt dispensing needle tips (Models JG18-0.5/JG20-0.5,
Jensen Global, Santa Barbara, CA, USA).

3.3. SOD valves may be designed to eliminate valve-associated baseline
channel resistance
Conventional NC valves typically increase channel resistance due tothe resting contact between the elastomeric membrane and the valve‘seat’. This contact force must be overcome for ﬂow to proceed throughthe valve. SOD valves, by eliminating this resting contact and supporting a range of widths and gap-heights, may be designed such thatthe resistance of the valve does not exceed the baseline hydraulic resistance of the channel. To demonstrate this phenomenon, the ratio
between the upstream and downstream pressures was recorded for test
devices containing valves with 5 μm gap heights and channel widths
ranging from 200 μm to 3.2 mm. By calculating the pressure drop acrossthese valves (in their resting state), we demonstrate that for a ﬁxed gap
height, increasing the width of the valve can eﬀectively increase thehydraulic radius of the channel such that the pressure drop across thevalve approaches zero (Fig. 3E).

3. Results and discussion
3.1. The SOD valve can achieve a complete seal

3.4. SOD valves may be actuated at frequencies comparable to NC valves
To determine if the SOD valve could achieve a complete seal, theoutlet port of a test device (Fig. 3A) was plugged using a PDMS-ﬁlled
pipette tip. Baseline pressure measurements recorded following theplacement of the pipette tip showed PS = PU = PD = PATM . Sourcepressures ranging from 6 to 20 psi (in 2 psi increments) were then applied while the SOD valve was held in a closed state by 30 psi pressure
supplied via the control channel (P2 ). Pressure measurements conﬁrmedthat PU = PS and PD = PATM across the range of supply pressures tested.
Importantly, these relationships were maintained even when the
duration of the applied source pressure exceeded 5 min (Fig. 3B). This
indicates that the valve has, indeed, achieved a complete seal and is not
simply leaking at a low rate – as such a leak would be expected to
produce an increase in PD relative to PATM with time. This observation issigniﬁcant as the ability to generate a complete seal in channels having
a rectangular cross section is generally regarded as a fundamental
functional advantage of classic NC valves relative to NO valves.
To determine whether the functionality of the SOD valve was dependent upon the gaseous species used to generate the source or control
pressures, this experiment was repeated using compressed nitrogen,carbon dioxide, oxygen, and atmospheric air (data not shown). For all
experiments, both valve chambers were perfused by a single gaseous
species. No species-dependent diﬀerences were observed in the functionality of the SOD valve.

The ability to reliably toggle the state of NC valves between fully
open and closed states at low pressures, and frequencies ≥1 Hz, presents
a distinct functional advantage relative to NO valves where actuation athigh frequencies requires the use of either shallow ﬂow channels orhigh actuation pressures. To compare the frequency range across which
SOD and NC valves may be operated, test devices containing SOD
valves (GH = 5 μm) or NC valves were toggled between open andclosed states at frequencies ranging from 0.5 to 300 Hz. Complete closure was observed at frequencies ranging from 0.5 to 2 Hz, while partial
closure without loss of synchrony between the valve state and appliedpressure waveform was achieved at frequencies as high as 300 Hz.
Importantly, there was no ostensible diﬀerence in the frequency range
across which the SOD valve could be toggled relative to the NC valve,and no ostensible diﬀerence in the frequency range across which full or
partial valve closure was observed. (Fig. 3F).
Hysteresis was observed in the operation of both SOD and NC valves
when using a pressure-based method for assessing valve closure.
Though the magnitude of hysteresis observed in both valves was
minimal, greater hysteresis was observed in the operation of NC valves
relative to SOD valves (Fig. 3G). This hysteresis may be owed to thestiction that occurs between the membrane and valve seat when thevalve is in its resting state as the maximum frequency at which the NC
valve could be toggled in a liquid-driven/wet state (where the liquidpresent may act to lubricate the contact between the membrane andvalve seat) does not ostensibly diﬀer from that of the NC valve. The
maximum actuation frequency of both valves at frequencies exceeding
1–2 Hz could not be accurately measured in a gas-driven/dry state test
device.

3.2. Threshold actuation pressure is dependent on gap height
To determine if, and at what approximate threshold actuationpressure (P2 = PTh ), the SOD valve achieved a complete seal, PS was held
at 10, 15, or 20 psi while P2 was stepped from 6 to 25 psi in 2 psi increments. Following each step increase, the relative pressures at PS , PU ,PD , and PATM were allowed to equilibrate and were then recorded for
30 s. Linear regression was performed to conﬁrm the system hadreached a dynamic equilibrium (the predicted slope of a line of best ﬁt
being 0 ± 0.001), whereupon the resulting time series data was used to
calculate and plot the ratio PS / PU as a function of the actuation pressure(P2 ) (Fig. 3C). SOD valves with gap heights ≥ 40 μm were fabricated,but a complete seal could not be achieved with these devices across therange of actuation pressures tested (Fig. 3C). By modeling the ratios
PU / PS and PD/ PATM as functions of P2 , we then attempted to estimate thetrue threshold pressure for the SOD and NC valves by calculating the

3.5. SOD valves enable the simple fabrication of gas-on-Gas multiplexers
To demonstrate the fabrication eﬃciency and functional utility of
the SOD valve, we designed an elastomeric balloon array wherein asource pressure suﬃcient to constitutively distend (inﬂate) all balloon
elements within the array could be selectively vented to allow for therelaxation (deﬂation) of individual balloon elements using a gas-on-gas
multiplexer (Fig. 4A). The functional unit of this system is a straightchannel that originates at a common inlet and terminates at a uniqueoutlet held at atmospheric pressure. Immediately upstream of each5Sensors & Actuators: B. Chemical 297 (2019) 126776

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Fig. 4. (A) A simpliﬁed schematic of the gas-on-gas multiplexer where pressure supplied via a common inlet port (PS ) is distributed to four unique ﬂow channels thatterminates at a unique outlet port (PATM ). Each ﬂow channel contains one balloon element (yellow circle) upstream of the SOD valve region. The state of each SOD
valve can be toggled by a series of external solenoid valves (P2A − D ). (B) The functional element of the gas-on-gas multiplexer is a ﬂow channel connecting a commonpressure source (PS ) to a unique outlet port (PATM ). The overhead schematic of this functional unit (top) illustrates the relative positions of the inlet (PS ), balloon
element (yellow circle), SOD valve region, external solenoid input (P2 ), and outlet (PATM ). The side-view schematics illustrate how closure of the SOD valve via theexternal solenoid valve (P2 ) can raise the pressure in the upstream region of the ﬂow channel to the source pressure (middle), while opening of the SOD valve canallow for relaxation of the balloon element by allowing the pressure in the channel to vent via the outlet port (PATM ) (bottom). (C) The scalable multiplexer control
scheme allows for the selective venting of n ﬂow channels (black) using 2log2n external solenoid valves (orange circles). The schematics shown illustrate thescalability of this system (where 2 ﬂow channels can be regulated using 2 external valves, 4 ﬂow channels can be regulated using 4 external valves, and 8 ﬂow
channels can be regulated using 6 external valves) and how the closure of a speciﬁc sequence of solenoid valves can allow an operator to selectively vent single ﬂow
channels. Red solenoid inputs and SOD valves are in an ON (positive pressure) state while gray inputs and SOD valves are in an OFF (atmospheric pressure) state. The
green ﬂow channels represent the venting channels as deﬁned by the sequence of external solenoid valves shown. Blue arrows represent the direction of the pressure
gradient – arrows point from the pressure source (P2 ) to the channel outlets (PATM ). (D) Photographs illustrating the design and operation of the elastomeric balloon
array and gas-on-gas multiplexer device containing 160 SOD valves capable of toggling the state of 32 ﬂow channels using 10 external solenoid valves. In the upperdevice layer, red regions represent channels connected to external solenoid valves that are in an ON (positive pressure) state, while gray regions represent channels
connected to external solenoid valves that are in an OFF (atmospheric pressure) state (For interpretation of the references to colour in this ﬁgure legend, the reader isreferred to the web version of this article).

greater than the pressure drop between the balloon element and thecommon pressure source. That, in other words, the pressure driven gas
ﬂow supplied to each balloon element from the source is vented fasterthan it is supplied – thereby ensuring that the pressure at a balloon
element never reaches the threshold pressure required to distend thevalve membrane when its corresponding ﬂow channel is actively
venting. To achieve this, the device reported herein was designed suchthat the ratio between the calculated hydrodynamic resistance upstream and downstream of each balloon element was approximately 15.
To assess the fabrication eﬃciency of multiplexers containing SOD
valves, over 20 devices (each device containing 160 SOD valves) were
fabricated (Fig. 4D). Of these, all devices (containing over 3200 valves
in total) were found to be fully functional.

outlet is a sequence of SOD valves that can be actuated via externalsolenoid valves. When one or more of these SOD valves are in a closed
state, the pressure within the upstream channel region approaches thesource pressure supplied at the inlet port. When these SOD valves are in
an open state, the upstream channel becomes continuous with theoutlet, allowing the channel pressure to approach the outlet pressure.
By positioning a single balloon element between the inlet and the SOD
valve region of each functional unit, we enabled the pressure withineach element to be toggled by changing the state of the downstream
SOD valves (Fig. 4B). Each functional unit was designed to accommodate 5 SOD valves, arranged according to a binary control scheme
capable of toggling the state of n balloon elements using 2log2n externalsolenoid valves (Fig. 4C) [21].
The ability to toggle the state of each balloon element requires thatthe pressure drop between the balloon element and outlet/vent is6

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4. Conclusions

References

The work reported herein represents, to our knowledge, the ﬁrst
description of a SOD valve, and of a functional (and highly scalable)
gas-on-gas multiplexer. The scalability of this design is owed to the useof a novel strategy for mitigating the risk of bonding between the valve‘seat’ and elastomeric membrane that currently limits the fabricationeﬃciency and scalability of monolithic NC valves, and to the minimal
design constraint presented by the need to balance the ratios between
the upstream and downstream channel resistance of the valves being
operated using the gas-on-gas multiplexer. Within the scope of the work
reported herein, this approach has allowed us to design and operate adevice containing 32 node valves controlled by a multiplexer containing 160 SOD valves using only 10 external solenoid valves and asingle common pressure source.
Our observation that the functionality of the SOD valve is solely
dependent upon the pressure diﬀerential between the ﬂow/regulated(PS ) and control/regulating (P2 ) valve chambers – and not upon thechemical species present within either chamber – suggests these valves
could be utilized to enable high throughput applications where thecontrolled delivery of speciﬁc gaseous species may be desired. For suchapplications, however, the permeability of PDMS to multiple gaseous
species [22,23] may necessitate the use of appropriate mitigation
strategies to prevent mixing across PDMS barriers (e.g. the elastomeric
membrane) where, for example, one gaseous species is used to regulate
another. In addition, as the complexity of microﬂuidic systems increases – particularly in areas of high-throughput analyses and soft
robotics, the need for eﬃcient valving (both in terms of fabrication andoperability) could be addressed by the SOD valve.

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Author contributions
P.W. developed, designed, and fabricated devices. P.W. and T.K.
performed experiments and data analysis. P.W. wrote the manuscript,
and all authors contributed equally to editing the manuscript.
Acknowledgements
The authors extend their gratitude to Dr. David Lai (GSK), whodeveloped the multi-layer mold fabrication technique described herein,
and to Mr. Arpith Vedhanayagam, who assisted with conducting andtroubleshooting multiple experiments. The authors are also grateful to
Mr. Stephen Robinson, Dr. Sung-Jin Kim and Dr. Sasha Cai Lesher-Perez
for thoughtful conversations and shared insight throughout the conceptual development and practical execution of the work reported
herein.
This work was supported by the National Science Foundation
Division of Chemical, Bioengineering, Environmental, and Transport
Systems (CBET) under award numbers 1236764 and 1512544.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.snb.2019.126776.

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