characterizations of gas purge valves for liquid alignment and
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Birck and NCN Publications Birck Nanotechnology Center
8-2012
Characterizations of gas purge valves for liquidalignment and gas removal in a microfluidic chipHan-Shengh ChuangNatl Cheng Kung University
Raviraj ThakurPurdue University, Birck Nanotechnology Center, [email protected]
Steven T. WereleyPurdue University, Birck Nanotechnology Center, [email protected]
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Chuang, Han-Shengh; Thakur, Raviraj; and Wereley, Steven T., "Characterizations of gas purge valves for liquid alignment and gasremoval in a microfluidic chip" (2012). Birck and NCN Publications. Paper 881.http://dx.doi.org/10.1088/0960-1317/22/8/085023
Characterizations of gas purge valves for liquid alignment and gas removal in a microfluidic
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J. Micromech. Microeng. 22 (2012) 085023 (8pp) doi:10.1088/0960-1317/22/8/085023
Characterizations of gas purge valves forliquid alignment and gas removal in amicrofluidic chipHan-Sheng Chuang1,2, Raviraj Thakur3 and Steven T Wereley3
1 Department of Biomedical Engineering, National Cheng Kung University, Tainan, Taiwan2 Medical Device Innovation Center, National Cheng Kung University, Tainan, Taiwan3 Birck Nanotechnology Center and Mechanical Engineering, Purdue University, West Lafayette,IN, USA
E-mail: [email protected]
Received 3 April 2012, in final form 29 May 2012Published 9 July 2012Online at stacks.iop.org/JMM/22/085023
AbstractTwo polydimethylsiloxane (PDMS) gas purge valves for excessive gas removal in generallab-on-a-chip applications are presented in this paper. Both valves are devised based on athree-layer configuration comprising a top layer for liquid channels, a membrane and a bottomlayer for gas channels. The pneumatic valves work as a normal gateway for fluids when themembrane is bulged down (open state) by vacuum or pushed up (closed state) by pressure. Inthe closed state, the air in front of a liquid can be removed through a small notch or apermeable PDMS membrane by compressing the liquid. The purge valve with a small notchacross its valve seat, termed surface-tension (ST) valve, can be operated with pressure under11.5 kPa. The liquid is mainly retained by the surface tension resulting from the hydrophobicchannel walls. In contrast, the purge valve with vacuum-filled grooves adjacent to a liquidchannel, termed gas-permeation (GP) valve, can be operated at pressure above 5.5 kPa. Basedon the principle of gas permeation, the excessive air can be slowly removed through the ventgrooves. Detailed evaluations of both valves in a pneumatically driven microfluidic chip wereconducted. Specifically, the purge valves enable users to remove gas and passively alignliquids at desired locations without using sensing devices or feedback circuits. Finally, a rapidmixing reaction was successfully performed with the GP valves, showing their practicability asincorporated in a microfluidic chip.
S Online supplementary data available from stacks.iop.org/JMM/22/085023/mmedia
(Some figures may appear in colour only in the online journal)
Introduction
Excessive gas removal in microchannels is of great interestto lab-on-a-chip (LoC) applications, such as low mixingefficiency [17], poor temperature uniformity in PCR [13], orinhibitive reactions [8]. Because bubble formation is a typicalissue commonly taking place in microchannels, unprocessedair bubbles or plugs may cause mass transport problems. As aresult, excessive gas removal is usually regarded as a prioritywhen dealing with microfluidic operations. With hydrophobicmicrocapillary vents [7], Lee et al [11] showed nanolitermetering, transportation, merging and biochemical reactions
on a microfluidic chip. Tiny liquid droplets were translated bylow air pressure (<3.5 kPa) generated from vents. Similarly,residual air was removed through the vents. However, thisdevice is only eligible for some limited circumstances due tothe lack of physical valves. Another approach removing airbubbles through gas permeable PDMS channels was reportedby Kang et al [9]. The concept relies on applying pressureover air/liquid phase flow to squeeze the air out of the liquidchannel. The group successfully predicted the purge behaviorwith a gas permeable model and a correction factor. Urbanskiet al [19] devised a latch valve to get rid of excessive mineraloil in oil/water flow for metering. Similar work can also be
0960-1317/12/085023+08$33.00 1 © 2012 IOP Publishing Ltd Printed in the UK & the USA
J. Micromech. Microeng. 22 (2012) 085023 H-S Chuang et al
(A)
(B)
Figure 1. Schematic of the microfluidic gas purge valves. Bothvalves are operated under a closed state. (A) Surface-tension (ST)valve; (B) gas-permeation (GP) valve.
applied to air/liquid flow. This partially closed latch caughtand retained emulsified aqueous samples, yet allowed theimmiscible mineral oil to continuously flow along the channel.However, a pressure range different from the pressure actingon the normal valves has to be carefully adjusted to achievethe partially closed state. In addition, leakage is very likely tobe a problem in the device. It is worth noting that removingair in microchannels also enables passive liquid alignment ata desired location without a sophisticated sensing system. Incontrast, the past detections of fluid locations still relied onsensors based on the change of electrical signals [3, 14]. Theadditional capability therefore allows accurate dispensing aswell as metering in many biochemical assays.
In this paper, we devised and characterized two types ofgas purge valves based on the valve proposed by Mathies’group [5]. The three-layer pneumatic valve is used in a widevariety of LoC applications due to simplicity. In such a valve,a flexible membrane is sandwiched between two functionallayers, isolating liquid channels and gas channels. A valve isdefined ‘closed’ when the membrane seals a liquid channelbut turns ‘open’ when the sealing state breaks. With multiplevalves deployed in a microfluidic chip, one is able to handlesophisticated fluid behavior using programmable controls[6, 18]. In addition to the basic functions, a gas purge valveis designed to remove excessive gas from a liquid channel,thus preventing air bubbles and aligning liquids withoutsensors or feedback circuits. Two types of purge valvessubjected to different operating mechanisms were fabricatedand studied (see figure 1). The ST valve features a smallnotch on the valve seat and resists the fluid flow dueto high surface tension. When surface tension dominates,the fluid interface will be retained at the notch valve.When the driving pressure is higher than the threshold, the
fluid will leak through the notch. For the current study,a notch with 150 μm cumulative width and 4.66 μmdepth was designed to bear pressure under 11.5 kPa.The GP valve has several finger-like grooves adjacent to aliquid channel and is vacuum vented. As a result, it can workat pressure higher than 5.5 kPa. Since both the devices are onthe basis of Mathies’ valve, they can also be used as normalvalves as long as being operated within the designed pressureranges.
Methods and materials
Both the purge valves were comprised of three layers ofPDMS (Sylgard 184, Ellsworth Adhesives) structures: liquidchannel, membrane and gas channel. The liquid and gaschannels were fabricated using replica molding of PDMSwith SU8 micromolds. For the GP valve, a standard single-step lithography procedure was followed using SU8 2025(MicroChem). For the ST valve, fabricating a notch (∼4.5 μm)in single-step lithography presented substantial challenges interms of high aspect ratio features and necessitated high-resolution masks. Therefore, an alternative, called multilayerSU8 lithography protocol [12], was followed to create thedesired notch. The notch was fabricated by spin-coatingSU8 2002 at 500 rpm and the liquid channel layer wasbuilt on top of it using SU8 2025. Finally, both layerswere developed simultaneously. The details of the processcan be found in the supplementary material (available atstacks.iop.org/JMM/22/085023/mmedia).
The PDMS prepolymer was prepared by mixing Sylgard184 and cure agent in a ratio of 10:1. Conventionalphotolithography was used to fabricate SU8 2025 mastermicromolds for both the liquid and the gas channels. Both thePDMS layers were cast from the molds and cured in an oven at65 ◦C for 1 h. A membrane was fabricated by spin-coating thesame PDMS prepolymer on a 4′′ silicon wafer at 1250 rpmand curing at 65 ◦C for half an hour. The final thicknessof the membrane was 100 μm. The gas channel layer waspermanently bonded on the membrane using oxygen plasmaand then peeled off from the wafer. A complete microfluidicchip was built after bonding the liquid channel layer with thegas layer/membrane composite. Alignment between the liquidchannel and the relevant gas channel was conducted under amicroscope. The dimensions of each valve were 600 μm indiameter, 48 μm in depth and 150 μm across the valve seat.The liquid channel and the gas channel were 300 and 100 μm inwidth, respectively. A three-layer notch valve with two notchescan be seen in figure 2. In the case of double-notch ST valve,each notch measured 75 μm in width and 4.66 μm in depth.
For the ST valve, the major force that decelerates orretains the air/liquid interface is surface tension. According toYoung’s equation, the pressure to retain an air/liquid interfacein a rectangular channel is expressed as
Pd = 2(h + w)σ cos(π − θc)
hw, (1)
where θ c is the contact angle of the liquid on a PDMS surface,σ is the surface tension of the liquid, h and w are the heightand width of the notch, respectively. The retaining pressure
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(A) (B ) (C )
Figure 2. (a) Image of a double-notch ST valve with trapped airphase. (b) The interface is retained at the notch when the drivingpressure is less than the threshold. (c) The liquid leaks through thenotch when the pressure exceeds the threshold.
is related to the contact angle of liquid and the size of notch.Usually a hydrophobic material, such as PDMS, accompaniedwith a tiny channel can create a higher surface-tension force.For instance, the theoretical threshold pressure in a PDMSchannel (θ c = 100◦) with two notches is 11.5 kPa. In theory, anotch can be designed to withstand different threshold pressurevalues by varying the cross-sectional area, yet a practical limitmay exist due to the potential PDMS membrane deformationin the case of extremely shallow notches.
For the GP valve, air is squeezed to the grooves adjacentto the liquid channel through the PDMS membrane. Due tothe porous nature of PDMS matrix, the elastomer allows gasmolecules to slowly migrate in and out of the material, yetretains liquid inside the channel. In general, the permeationcan withstand large pressure but needs longer time to removegas compared with the ST valve. A model derived fromthe gas-permeation equation [10] can be used to predict theremaining permeation area as a function of time:
A(t) = WL0 exp
[αP(p2 − p1)
db
T
273
76
Patmt
], (2)
where α is an empirical correction factor, P is the permeationof the material, p1 and p2 are the pressure values before andafter the PDMS matrix, b is the thickness of the membrane,d is the depth of the channel, W is the channel width, L0 isthe initial length of the gas in the channel and t is the elapsedtime. Basically, the purging rate reduced exponentially andvaried with the pressure difference (p2 − p1), the thicknessof the membrane and the inherent material properties. Theprocess can be accelerated by increasing pressure differenceor reducing membrane thickness in the current setup.The equation facilitates users to design a valve that can removegas in a reasonable timeframe.
In addition to the two dominant equations above, someother forces might play minor roles in the purging actions.
For instance, the hydraulic resistance caused by the frictionbetween channel walls and liquid molecules is proportional tothe channel length and flow rate according to the Hagen–Poiseuille equation. The driving pressure is progressivelydepleted when a fluid travels a long distance or flows witha high velocity. In the current setup, however, the pressureloss is negligible (<100 Pa) compared with the drivingpressure. Moreover, the decrease in fluid velocity due to highlycompressed air in the last stage of the purging process in theGP valve can further mitigate the pressure loss.
Gas removal by surface tension
Both the purge valves, unless mentioned otherwise, wereinvestigated in a closed state. A microchannel with input andoutput reservoirs and the ST valve were fabricated as per thepreviously described protocols. A water-based solution stainedwith a food color (ESCO Foods Inc.) was employed here asa working fluid and was placed in the input reservoir. Theoutput reservoir was connected to a vacuum pump (ThomasDiaphragm Pump, 5002VD/1,0/N/DC) to create a drivingforce for fluid movement. The pump was connected to a dcpower supply (Extech Instruments, 382260) and the pressuredifference was controlled by adjusting the dc voltage. The setpressure values were observed to fluctuate between ± 0.2 kPadue to the motorized diaphragm pump.
Figure 2 demonstrates one of the experimental results forthe double-notch ST valve. The leakage point was measuredby varying the fluid driving pressure and was observed toreach a value around 12 ± 0.2 kPa, which is close tothe predicted threshold (11/5 kPa). It should be noted thatsome other factors, such as hydraulic resistance and contact-line pinning [4, 21], though minor in the current setup, mayalter the threshold if the boundary condition is changed.Accordingly, the actual threshold may be higher than thetheoretical value. The influences of both effects on the currentthresholds are estimated to be less than 1%, respectively.To deal with the contact-line pinning, a measured contactangle should be used instead of a theoretical value. Thealignment was found to be fast compared to the GP valve.The variation of the air/liquid interface velocity acrossthe valve seat over location (x) from the notch valve isshown in figure 3 (see supplementary movie (available atstacks.iop.org/JMM/22/085023/mmedia)). The start of thenotch is located at the origin (x = 0). Measurements upstreamof the notch (x < 0) as well as downstream of the notch (x >
0) were recorded. The interface velocity drops to zero rapidlyas the driving pressure is less than the threshold, whereas theinterface slows down in the notch but resumes velocity afterleaving the notch.
Gas removal by permeation
Quantification of gas removal in the GP valve was alsoinvestigated. A series of experimental images showing theoperation are depicted as insets in figure 4. Unlike the STvalve, excessive air is removed through the porous PDMSmatrix with higher pressure acting on the liquid. Although the
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J. Micromech. Microeng. 22 (2012) 085023 H-S Chuang et al
Figure 3. Plot of the air/liquid interface velocities under three driving pressure conditions. The interface stops at the notch at low pressure(6 ± 0.2 kPa and 9 ± 0.2 kPa), but leaks through the notch at driving pressure (12 ± 0.2 kPa) higher than the threshold. The inset shows aclose-up of the interface velocity measured in the notch region (0 � x � 100).
Figure 4. Comparison of the analytical prediction and theexperimental measurement. The channel width and depth are fixed;therefore the volume of the residual air between the valve seat andthe air/liquid interface is simplified to the change of length. Theinset images from left to right represent the air/liquid interface atdifferent time steps: (a) 0 s (b) 20 s (c) 40 s (d) 65 s.
GP valve is designed for high pressure, the purging processis time-consuming. According to equation (2), the purgingtime is proportional to the contact area with air. The purgingefficiency drops significantly when less air remains in thechannel (i.e. decrease in the contact area). In addition tothe hydraulic resistance, the highly compressed air in the laststage may in part contribute to the decline of the efficiency.Air in front of a fluid is compressed initially due to low densitybut becomes more incompressible as the air density escalates.The process can be accelerated by increasing either the drivingpressure or reducing the membrane thickness. The GP valveis equipped with vent grooves adjacent to a liquid channel,so that the pressure difference is increased. The finger-like
geometry is adopted here to extend the contact area in thelast stage and prevent collapse of the structure. All of theaforementioned factors contribute to the acceleration duringthe purging process of the GP valve. The analytical modelshows good agreement with the experimental data when α is13 and a pressure difference of 38 kPa is applied (figure 4).
Evaluations of the GP valve in a microchannel
The GP valve was incorporated into a microchannel for someoperating tests. The driving force was generated by an on-chip peristaltic pump. Four parameters were evaluated basedon their influence on the displacement and fluid velocity. Theresults are discussed as follows.
(i) Pressure difference. This section shows the comparisonof purge valves with and without an upstream pressuresource. The upstream pressure was generated from anon-chip peristaltic pump. When the pump was idle, theliquid moved slowly due to the low-pressure differencefrom the vacuum filled grooves. However, the purgingaccelerated while the pump was activated. The pumpwas able to provide a time-averaged pressure value ofseveral tens of kilo-Pascals. A three-fold saving in timewas observed when a pump was used (figure 5(A)). Thecorresponding velocities are shown in figure 6(A). Flowdriven by the pump moved faster during the purgingprocess. On average, the pump-driven flow maintaineda mean velocity three-fold higher than the pumpless flow.Without a driving force, the flow lost its momentum andbecame stagnant at the last vent groove. In contrast, thepump-driven flow kept moving till it approached the valveseat.
(ii) Location of the liquid plug. The purging rate is alsosubjected to the location of air/liquid interface. The
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J. Micromech. Microeng. 22 (2012) 085023 H-S Chuang et al
(A) (B )
(C ) (D )
Figure 5. Displacement of liquid plug as a function of time. The blue stripes stand for the relative positions of vent grooves in themicrochannel. (A) Purging with and without a pump. (B) Purging upstream (far away from the purge valve) and downstream (next to thepurge valve). (C) Purging with different pumping frequencies. (D) Purging with different lengths of liquid plug.
purging efficiencies in the upstream (∼9.28 mm awayfrom the purge valve) and downstream (next to the purgevalve) regions were measured (figure 5(B)). Upstream, theliquid moved relatively fast (∼132.72 ± 39.69 μm s−1)owing to abundant air as a cushion as well as a largecontact area in the microchannel. However, the air cushionappeared to be unreliable, resulting in velocity fluctuationsin this region. In general, air is more compressibleinitially and becomes less compressible after being highlysqueezed when the pumping rate is higher than thepurging rate. The porous PDMS walls are necessaryfor promoting the purging process. However, this effectdecreases exponentially according to the permeationequation (equation (2)). In the last stage, hydraulicresistance depletes only some of the driving pressure(<100 kPa). Also, the air compressibility and contactarea of channel walls play minor roles in the purgingprocess. The vent grooves therefore become the only exitfor the excessive air, resulting in a decline in the flowvelocity (figure 6(B)). This unique characteristic makesthe processing time escalate when an air/liquid interfacemoves near the GP valve.
(iii) Pumping frequency. Changing the pumping frequencyis a major means for a peristaltic pump to vary flowvelocity. In principle, a higher pumping frequency resultsin faster fluid flow. To characterize the relationship,
the velocities subjected to three frequencies, 0.5, 1 and2 Hz, were measured. As shown in figures 5(C) and6(C), no meaningful difference is observed betweenthe frequencies considering the rate of change ofdisplacement as well as velocity. All of the linearcorrelation coefficients between any two cases are higherthan R2 = 99.4%, implying very similar trends. Thecause is attributed to the highly compressed air in thelast stage and the nature of the peristaltic pump. Asdiscussed in the previous sections, the flow velocitydeclined on approaching the GP valve due to slowpermeation. The compressed air caused backpressurewhen excessive pressure was accumulated in the channelbetween the air/liquid interface and the valve seat. Sincethe accumulated pressure was usually stronger than thefrequency change by the peristaltic pump, no significantimprovement in velocity was measured even though thefrequency was increased. The excessive pressure thenreleased back to the pump, decreasing the pumpingefficiency. This situation occurs in different phasesdepending on different pumping sequences. However,the phenomenon becomes negligible when the air/liquidinterface remains far away from the purge valve.
(iv) Length of the liquid plug. The last parameter shows theinfluence resulting from the length of the liquid plug. Aliquid plug is essential when metering or quantification
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Figure 6. Flow velocity as a function of location. (A) Purging with and without a pump. (B) Purging upstream (far away from the purgevalve) and downstream (next to the purge valve). (C) Purging with different pumping frequencies. (D) Purging with different lengths ofliquid plug.
is required. A non-plug fluid and two plugs measuring3.04 and 4.64 mm in length were compared underthe same conditions. As shown in figures 5(D) and 6(D),the purging appears to be independent of the length of theliquid plug. There are no distinguishable differences inthe liquid displacement as well as the velocity. Similar tothe previous pumping frequency, the minimal correlationcoefficient is R2 = 99.2%, implying the same behavioralpattern. This result implies that the hydraulic resistancedue to the friction between the liquid and the solid wallsis negligible as the air/liquid interface is very close tothe purge valve (slow velocity). In other words, only gaspermeation dominates the purge process in the last stage.
Liquid alignment for rapid mixing
Manipulation of liquid alignment before mixing in amicrofluidic chip using GP valves was performed. As shownin figure 7, two liquid plugs stained with different food colorswere used to facilitate the visual observation. To minimizehuman errors, the operations were automated with a self-developed LabVIEW R© (National Instruments) program. Inthe chip, the plugs were translated by peristaltic pumps.The pump was composed of three normal Mathies’ valves
Table 1. Mixing efficiency at different mixing cycles.
Zeroth First Second Third Fourthcycle cycle cycle cycle cycle
Efficiency 41.88 ± 94.28 ± 93.14 ± 94.38 ± 94.49 ±(%) 13.21 3.92 5.64 4.16 3.81
and operated in an orderly fashion. The details regardingthe pump and the chip were discussed in the relativepapers [1, 2]. For the mixing, the green plug was first translatedto a designated location (figures 7(A)–(C)) and so was thered plug (figures 7(D)–(E)). Without actively sensing, thealignment was achieved by pumping both the liquid plugs for acertain period of time till they approached the valve seats. Thetime required for each alignment was calibrated in advanceand was estimated to be shorter than 1.5 min. After bothplugs were well aligned (figure 7(F)), the valves were openedsimultaneously and both plugs were squeezed into the thirdchannel for chaotic mixing (figure 7(G)). Figures 7(H) and (I)show the statuses of the mixture at the first cycle and the secondcycle, respectively. The insets in figures 7(H) and (I) indicatethe completion of the mixing. Note that a fluorescent dyewas added in one of the liquid plugs during the quantificationof mixing efficiency (see supplementary material (availableat stacks.iop.org/JMM/22/085023/mmedia) for details). The
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J. Micromech. Microeng. 22 (2012) 085023 H-S Chuang et al
(A) (B ) (C )
(D ) (E ) (F )
(G ) (H ) (I )
Figure 7. A series of experimental images demonstrating the rapid liquid mixing in microchannels. (A)–(F) Two different foodcolors are aligned and then sent to a third channel for mixing. The alignment for each plug takes about 80 s. (G)–(I) Withoutthe obstruction of air bubbles, the mixing is completed rapidly within two cycles (see the supplemental file (available atstacks.iop.org/JMM/22/085023/mmedia)). The insets in images (G)–(I) illustrate the mixing profiles at different stages.
complete mixing was achieved rapidly after the first cyclewithout air bubbles or air gaps between plugs. A list of themixing efficiency from the beginning to the fourth cycle isshown in table 1. The definition of the efficiency can bereferred to the prior literature [22]. The mixing efficiency(mean ± standard deviation) is poor before the liquid plugstarts running loops. However, it swiftly exceeds 90% andreduces uncertainty after the first cycle. The results confirmthat rapid mixing could be realized with the deployment of thepurge valves. Compared with other mixing strategies reportedin the literature [15, 16, 20], the mixing rate of the currentdevice at Re = 0.32 is 0.2. This mixing rate is very competitiveamong the existing techniques. The rapid mixing capabilityis obtained from breaking continuous flow into liquid plugs,therefore sharing the same mechanism with the so-calleddroplet mixing in the digital microfluidics.
Conclusion
Two types of unique gas purge valves eligible for twodistinct pressure ranges are presented in this study. Theproposed devices not only work as a gateway for normal flowcontrols but also remove excessive air for liquid alignmentwithout active sensing elements. The ST valve can eitherretain liquids with small pressure (<11.5 kPa) or slow downfluid flow when the driving pressure exceeds the threshold.The functional mechanism of the ST valve is the surfacetension induced by hydrophobic channel walls. The analyticalthreshold is 11.5 kPa under the current setup. In contrast,the mechanism that drives the GP valve is gas permeationbased on the porosity of PDMS. Finger-like grooves deployedadjacent to a liquid channel are connected to vacuum toremove excessive air. The employment of PDMS membraneallows the GP valve to be operated with higher pressure
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(>5.5 kPa). According to the permeation equation, highpressure accelerates the purging process. Additionally, the GPvalve was evaluated with four parameters. The purging ratevaried with the location of air/liquid interface as well as thepressure difference (i.e. a pump in this case). The interfacevelocity was high when the liquid remained far away fromthe purge valve or the pressure difference across the PDMSmembrane was increased. Hydraulic resistance may alter theefficiency as well. However, the effect was estimated to benegligible in the current setup. Surprisingly, the pumpingfrequency and the length of liquid plug showed no significantinfluence on the purging rate. The results suggested that gaspermeation primarily dominates the purging process as theliquid approaches the purge valve. Eventually, a demonstrationof rapid mixing using the GP valves showed a practicalapplication. Since air bubbles were not trapped between liquidplugs, the mixing was rapidly completed after only one cycle.The deployment of purge valves in microfluidic chips isbelieved to benefit lab-on-a-chip applications.
Acknowledgments
The authors thank the National Science Council grant100-2218-E-006-036 for supporting this work. The authorsalso acknowledge the technical support from BirckNanotechnology Center at Purdue Discovery Park.
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