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Micro throttle pump employing displacement amplification in an elastomeric substrate

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 15 (2005) 18311839 doi:10.1088/0960-1317/15/10/007

Micro throttle pump employingdisplacement amplication in anelastomeric substrateI D Johnston, M C Tracey, J B Davis and C K L Tan

Science and Technology Research Institute, University of Hertfordshire, College Lane,Hateld, Hertfordshire, AL10 9AB, UK

E-mail: i.d.johnston@herts.ac.uk

Received 18 April 2005, in nal form 6 July 2005Published 9 August 2005Online at stacks.iop.org/JMM/15/1831

AbstractWe report a micro throttle pump (MTP) with enhanced throttling resultingfrom benecial deformation of its elastomer substrate. In the MTP reported,this has doubled the effective deection of the piezo electric (PZT) actuator with a consequent ve-fold enhancement of throttling ratio. This mode of throttling has been modelled by nite element method and computationaluid dynamic techniques whose predictions agreed well with experimentaldata from a throttle test structure; providing typical throttling ratios of 8:1 atlow pressures. The improved throttles have been incorporated in aprototype, single PZT, MTP, fabricated with double-depth microuidics,which pumped both water and a suspension of 5 m polystyrene beads at amaximum ow rate of 630 l min 1 and a maximum back-pressure of

30 kPa at a pumping frequency of 1.1 kHz. This represents an approximateve-fold enhancement of both performance metrics compared to our previous single PZT device.

Symbols

low frequency pumping efciency Rt throttling ratio

1. Introduction

We have previously reported [ 1] a new mechanism of microuid pumping exploiting throttling: the use of variablecross-section ow-constrictions to regulate uid ow. Suchconstrictions predicate the use, in part or wholly, of elasticsubstrate materials. Whilst throttles never actually close,straightforward calculations indicate that modest closed-to-open ow resistance ratios (throttling ratios) can yieldpumping efciencies close to those of classic, fully closingvalves. The fact that throttles do not actually close, allied totheir construction from elastic materials, makes them highlysuited to the pumping of solid phase suspensions, as we havedemonstrated [ 2].

Recently [ 3], throttling has been employed to form aprecision microuid dosing device fabricated from a rigid

silicon V-groove channel and an upper, sealing layer of poly(dimethylsiloxane) (PDMS) that was deformed into thechannel to achieve variable throttling. Pneumatically operatedthrottles [ 4], referred to as leaky valves, have also beenreported to demonstrate a number of microuid operations.These devices employed thin, deformable sidewalls dividinguid and pneumatic control channels: the sidewalls beingpneumatically deformed inwards so as to throttle the uidchannel. As currently reported, these throttles are slavevalves requiring an external master valve to modulate thepneumatic signal.

In the context ofMTPs, a numberof routes to performanceenhancement exist. In this paper we report enhancingperformance by increasing throttling ratio by means of a two-depth casting method that allows us to implement a shallowweir-like throttle between deeper, low ow resistanceinterconnects and a pump chamber. By adopting a shallowweir structure we directly employ piezoelectric action inthe depth direction rather than employing it indirectly viaPoissons ratio as we had previously done with slot throttles,which are deep, narrow channels. Two depths also allow us toemploy the deeper depth to reduce absolute throttle resistance

0960-1317/05/101831+09$30.00 2005 IOP Publishing Ltd Printed in the UK 1831

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Figure 1. FEM model of elastomer cube whose dark top faces are compressed downwards. To the right the resulting amplieddisplacement in the free central region can be seen. The arrows show the relative magnitudes of displacement.

(as distinct from throttling ratio) which is advantageous inpump applications.

Early in the work, we employed nite element modellingwith the objective of determining the exact behaviour of thecomposite structure given the previously established nonlinear dependence of ow resistance with the throttle gap (which is

fundamental to throttle performance). Modelling gave resultsin which the predicted throttle open and closed gaps were,unexpectedly, muchgreater andsmaller respectively than thosesolely due to the vertical upward and downward movement of the PZT causing the change. This effect is wholly explicablein terms of the conventional deformation of the PDMS, with aPoissons ratio close to 0.5, resulting in an opposing, upward,displacement of the weir surface. This effect was clearly of considerable interest as it approximately tripled the anticipatedthrottle deection andhence, in conjunction with the nonlinear relationship between throttle gapand throttling ratio, the effecthad the potential to signicantly increase throttling efciency.

Typically, actuation techniques employed in micro electro

mechanical systems (MEMS) are innately limited to providingsmall direct displacements. Various techniques have beenreported to increase displacement by means of mechanical or geometric advantage for MEMS applications [ 5 7]. Clearly,such transformations are achieved at the expense of theultimate force that can be developed. The effect discussedhere employs a distributed transformation mechanism in anelastic structure to provide a similar effect. The performanceof such mechanisms is frequently dependent not only on themechanism employed but also on correctly matching the inputand output systems they transform between.

1.1. Displacement amplication in elastomer substrates

The following simple example assists in envisaging theprincipleby which exaggerateddeformation canbe obtained inelastomers with linear, isotropic elastic moduli and Poissonsratios close to 0.5.

Consider an elastomer cube, as shown in gure 1 (left),in contact with six bounding surfaces that offer only normalreactive forces (that is to say that wall shear during thedeformation of the elastomer is absent). The upper facecontains a shallow recess that is free of contact with thesurfacelocal to it but with the recesses four vertical faces subjectedto the same boundary condition as the bounding surfaces.

Now consider deforming the cube in the followingmanner. Let ve bounding surfaces be xed and let thesixth, upper face be displaced downwards so as to compress

the elastomer. Lateral deection within the material inresponse to this axial movement manifests itself as an upwardsdeformation of the free surface at the base of the upper recess as shown in gure 1 (right). Under the conditionspertaining in this example, the local reduction in elastomer volume due to thedownwards movement of thebounded upper

face closely approximates the elastomer displaced upwardsfrom the unbounded base of the recess.By reapportioning the relative sizes of the constrained and

free surfaces we can usefully control the extent of the linear deections of the latter. This effect is analogous to hydraulicsand can be loosely termed solid hydraulics. However,this analogy can be misleading and, as we will discusssubsequently, more realistic bounding surface conditions canconsiderably modify actual deformation behaviour.

Creating an enhanced movement over a relatively smallarea of elastomer surface in the manner described representsa useful general addition to elastomer microstructure designtechniques. An interesting variant upon this theme hasrecently been applied to microvalves [ 7] where a low elasticmodulus, elastomer is cast into a dened void within a morerigid polymer substrate. The resulting composite substrate isdeformed by a PZT. By virtue of careful design, the elastomer lled-void is compressed, in an analogous manner to thatdiscussed here, and hence caused to preferentially deform ina predetermined direction thereby concentrating both pressureand travel so as to actuate a seal valve.

2. Throttle design and characterization

This paper concerns enhancement of MTP performance bymeans of optimizing throttling ratio. Accordingly, the designand modelling reported here is restricted to static, steady statethrottle modelling rather than addressing the more complexlong term challenge of dynamically modelling the compositeMTP.

Given some underlying design constraints, discussedsubsequently, we approached throttle design andcharacterization in three steps. Firstly we modelled thedeformation of the structure by means of nite elementmethod (FEM) techniques with the objective of quantifyingthe extra weir deection contributed by substrate deformation.This allowed us to establish the dimensions of a candidate testthrottle design. Secondly, we simulated a simplied second-order deformation of the throttle due to the effect of uidpressures developed across the weir when controlling ow.Thirdly, we extracted the prole of the candidate designs

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Figure 2. Vibrometer plot of the pumps upper surface at 800 Hz and whilst pumping water, displaying the complex bi-directional exureof the composite device.

throttle aperture and employed this to dene a computationaluid dynamic model of the throttle in both deformed states.From this model we established the volumetric ow at lowoperating pressure through the throttle under both conditionshence allowing us to calculate the low-pressure throttlingratio.

Throttles fall within the uid dynamic category of shortducts [ 8] within which, microuid ows are highly sensitiveto both their exact dimensions (which we exploit with micro-throttles) and other issues such as surface irregularities [ 9]. Inview of these sensitivities, we validated our throttle modellingpredictions against the measured throttling performance of aseparate throttle test structure whose piezoelectric deectionhad been characterized by laser interferometry. The teststructurealso allowed us to assess throttling at higherpressuresthan those we can currently accurately model.

2.1. General design principles

Whilst instructive, our earlier description of displacementamplication was deliberately simplied. The modelleddevice includes four major renements. Firstly, it incorporatesbounded surfaces that do display wall shear. Secondly it isthin with a width-and-length to thickness aspect ratio of circa 11:1. Thirdly, only the lower surface of the PDMS isa fully bounded surface because the upper surface is attachedto a deformable thin glass layer, and the outer four sides of thePDMS are unbound. Finally, and importantly, the modelleddevice includes a more complex bi-directional deectionof theupper surface (as we proceed to discuss), and not a uniformcompressive load.

The PZTglassPDMS structures we employ do notgenerate a net compressive force acting downwards so asto compress the PDMS in the manner previously discussed.This is due to the PZT not acting with respect to a commonreference plane shared with the base of the PDMS. Instead,opposing regions of the PZTglass concurrently ex upwardsand downwards with respect to their resting plane resulting incorresponding regions of local tensile and compressive stressas indicated in gure 2. By placing distinct uid elements inthese regions we can implement ow sequencing with a singlePZT. Previous modelling and development has indicated thata 12.5 mm diameter PZT is typically well suited to a circa18 mm square glassPDMS substrate: we have employed

these dimensions in order to facilitate overall operation of theMTP reported later in this paper.

Figure 2 displays a vibrometric measurement of the deection of an operational MTP obtained using aPolytec PSV400-series scanning laser vibrometer (LambdaPhotometrics Ltd, Harpenden, UK). The image indicates thepresence of a complex eld of compressive and tensile stressesbelowthe upper face (that is to sayin themicrostructureregion)of the PDMS. This behaviour, whilst exploited in our singlePZT pumps, makes it far from straightforward to subjectivelyassess whether the necessary local compression to result indisplacement amplication of the weir in our devices can beachieved. In particular, it was anticipated that the use of a thinsubstrate rather than the unit aspect ratio example previouslypresented, would be key to achieving this. FEMmodelling wastherefore employed to gain insight into the weirs behaviour

within the composite device.

2.2. FEM modelling: PZT actuated throttle deformation

This section limits itself to the structurally static case wherethere is no uid within the throttle structure. This limitationmeans that there are no internal pressures generated within theinternal uid pathway of the device due to either throttlingaction or the intrinsic ow resistance of the structure. Suchpressures would introduce second-order deformations of theintentionally compliant structure (as has been specicallyreported for pressure sensing in microuidic devices [ 10])as we discuss subsequently. Accordingly, the objective of this phase of the modelling was to examine the enhancementof throttle operation due to substrate deformation under ideal conditions. In practice this will reasonably dene theactual behaviour of the throttle under low internal pressureoperation.

Our previous work established the use of FEM modellingas a means of accurately predicting the upward and downwarddeection of a piezo-electric disc 12.7 mm in diameter and0.410 mm thickness (type T216-A4NO-273X, Piezo SystemsInc.) bonded to a borosilicate microscope slide cover slip18 mm square and 0.110 mm thick with this composite, inturn, bonded to a layer of PDMS (Dow Corning Sylgard184) 18 mm square and 1.5 mm thick. The elastic moduliused were 52 GPa, 73 GPa and 1 MPa respectively for thePZT disc, glass and PDMS and the corresponding values of

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Figure 3. Geometry of modelled quadrant of the micro-throttle structure used for FEM analysis, including the PDMS layer, glass layer andPZT. On the right is a magnication of the central weir region.

Figure 4. A FEM model contour map of deections in the region of the weir after a number of optimizations. To the left the throttle is inthe closed (compressed) position and to the right it is in the open (stretched) position.

Poissons ratio were 0.31, 0.30 and 0.4999. Thermal strainsof 0.000129 were used to simulate the piezoelectric strainsgenerated by the maximum working voltage of 180 V. Itshould be noted that the PZTs employed are intrinsic bimorphsdisplaying symmetric, bidirectional deection when subject to

equal magnitude positive and negative potentials.In particular, earlier modelling showed that the freedeection of the centre of a free disc of 19.1 m isreduced approximately vefold through being bonded to glass,however as a consequence of the relatively low modulus of PDMS, thesubsequent bonding of this composite to the PDMSlayer causes very little further reduction. More specically inthis application, whilst the PDMS layer has little effect of the deection of the PZT / glass, we are seeking to capitalizeon the effect of the latter causing much more signicantdeformational changes in the layer and in the surfaces of microstructures cast into the layer. Some optimization of the geometry shown in gure 3 aimed at achieving a largeworking gap when open and a small gap when closed gaveweir dimensions of 1000, 100 and 100 m for the cross-streamwidth, length in the ow direction and depth respectively.Clearancebetween thetop surface of theweir andthe undersideof the glass cover slip which lids the channel was xed as15 m. The local channel width of 1000 m tapered to a widthof 400 m in the channels leading to and from the throttlingzone in order to provide anchoring or bound-surface tothe PDMS in order to facilitate displacement amplicationand retain the necessary, predicable, bidirectional exureprinciples that we have previously reported.

ANSYS 7.1 (ANSYS Inc.) software was used workingwith tetrahedral elements and 25 000 nodes. Figure 3shows the geometry employed, two planes of symmetry wereexploited to optimize node density.

Figure 5. Predicted throttle gap at the centre of the weir resultingfrom the vertical displacement of the glass only, and fromdisplacement of the glass and the elastomer. Note, the static gap is15 m.

The model displayed that a downward movement of thePZT / glass compositeso as to close thethrottle causedthe topsurface of the weir to arch upwards and the base of thechannel on both sides to move upwards also, shown in gure 4.The process is identical, with reversed directions, on thePZT / glass upstroke.

As detailed in gure 5, the maximum downwardsmovement of the PZT / glass is 3.6 m at the intersection of the axes of symmetry; the maximum upwards movement of the weir top surface is 5.1 m so that locally the interveninggap reduces from 15 m to 6.3 m. This corresponds to anincrease in gap closure due to substrate deformation of circa240%. Conversely, at the end of the weir the overall gapis virtually unchanged at 14.7 m. Upward movement of the PZT / glass composite to open the throttle reverses the

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Figure 6. FEM model examining secondary deformations of the throttle structure caused by the effect of a pressure differential across theweir. To the left the pressure differential is 5 kPa and to the right it is 50 kPa.

sense, but retains the magnitude, of all the closed throttledeections.

2.3. FEM modelling: second-order asymmetric weirdeformation

The objective of the FEM modelling was to investigate andoptimize a throttle design intended to exploit displacementamplication. However, it is an inescapable consequence of exploiting the deformability of elastomer substrates that wemayalso experience second-order deformation of thesubstrateunder actual operating conditions. In this case our primaryconcern relates to deformation of the weir due to uid pressuredifferentials existing across it: these could result in the weir bending over and hence, prima facie , increase the effectivethrottle gap.

Once again, this would have been less than trivial to modelfully. Accordingly, we developed a simplied model basedupon the earlier work but with the additional condition that theweir wall on the input side is subjected to a uniform pressure.This pressure in this model does not vary according to thedeformation of the weir. The objective of this modelling wasto establish subjectively whether subsequent computationaluid dynamic (CFD) modelling of a simplied (in the senseof a uid ow path) inexible structure based upon the resultsof the preceding idealized FEM modelling could be extendedto higher operating pressures.

Figure 6 shows ANSYS generated views of the weir atinput pressures of 5 kPa and 50 kPa. As we have discussed,given the simplications implicit in this model, it is onlyappropriate to make qualitative observations. Subjectively,we observe that the weir does indeed deform sideways under pressure, interestingly however, careful examination of thedata shows that the left hand lip of the weir, whilst beingdisplaced to the right, also actually rises, by virtue of theweirs left face being under tensile stress due to the appliedpressure. This insight reinforced our view that a full CFDmodel of weir deformations effect on throttling performancewould be far from trivial and conrmed to us, that a teststructure provided the more appropriate route for examininghigh pressure behaviour at this point.

2.4. CFD modelling

As previously discussed, we have restricted CFD modellingto the prediction of ow resistance (and hence throttling

Figure 7. Geometry employed for the CFD modelling to predictvolumetric ow rates through the throttle under low pressureconditions.

when modelled in both states) within a structure that hasbeen statically deformed without any further second-order deformation due to internal uid pressure. In other words,the modelled structure is a solid snap-shot of a throttleddevice in one of its two states. As a consequence of this and the subjective conclusions of the second phaseof our FEM modelling, our objective here was to obtaina prediction of likely throttling ratio under low pressureconditions approximating to those modelled in the idealcase. Accordingly, our modelling assumes a ow structure of xed internal dimensions resulting from our original designand the FEM-computed deformation of it solely due topiezoelectrically induced stresses and the PDMS substratesresulting deformation.

Given our restricting the CFD model to the rst-order case we sought the limited objective of approximating lowpressure operation throttling ratios. It was also our intentionto gain some insight into ow behaviour over the weir whilstaccepting that the weir was rather idealized due to our rst-order conditions.

Having calculated the weirglass gaps for the open andclosed states from our FEM modelling, we employed theFlotran (ANSYS Inc.) CFD package to predict steady stateow conditions at low pressures. Examination of the problemindicatedthat one, axial,planeof symmetry could be exploited.We did not exploit the second plane through the centre of theweir as we had for FEM as the ow patterns each side of the weir were unlikely to be symmetric. Figure 7 shows theresulting 26 000 node model.

Having veried the model we proceeded to characterizethrottling ratios, in terms of volumetric ow rate ratios of a Newtonian uid with a dynamic viscosity of 0.001 Pa s

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(i.e. water) resulting in throttling ratios of 9.1 at 1 kPa and 7.9at 5 kPa.

Modelling indicated that ow rate / pressure functionthrough the composite structure deviated from linearity athigher pressures due to the formation of both up and downstreamvorticesadjacent to thethrottle. Modellingalso showedthat local Reynolds numbers of up to 60 would exist above theweir in the closed state.

2.5. Test throttle fabrication

Photomask-pairs were designed with Tanner L-Edit (Tanner Research, CA, USA) to produce, after le conversion,postscript les for 4040 dpi laser plotting. We employed thelm masks directly. The moulds were fabricated on 3 inch,test-grade, silicon wafers from two layers of SU-8 (SU-8 2025and 2050 Microchemcorp, MA, USA) with general processparameters as per the manufacturers recommendations.

The double-depth process sequence ensures a planar surface for spinning the second layer by means of exposing,

but not developing, a preceding, and now buried rst layer prior to spinning the thicker second layer over it. By thismeans, developed features in the rst layer do not interferewith the second layers integrity. The thin 15 m thickSU-8 layer was spun with a Suss Gyrset photoresist spinner.The mask included test points to allow layer thickness tobe subsequently conrmed with a high performance, 1 mreading accuracy, digital dial-gauge. It was then pre-exposurebaked and subsequently patterned with a Suss MJB3 contactaligner. The wafer was then post-exposure baked. The thick100 m layer was then spun over, so as to bury, thethin layer, and the composite was pre-exposure baked. Thewafer was then returned to the mask aligner and the second

photomask was aligned to the patterned rst layer followed byappropriate exposureof the second layer. Layer alignmentwasachieved by aligning thesecondmask to thesubtlydiscolouredfeature regions due to the previously employed rst mask:whilst requiring operator skill, this is possible and avoids thenecessity for an extra alignment mark fabrication step on thewafer prior to the main processing. For the second exposurewe employed a modication of our MJB3 to allow multi-dose exposure to avoid any heating effects. The compositewafer was then post-exposure baked and developed accordingto the normal manufacturers instructions for single layer wafers.

The PDMS microstructures were then cast from Dow

Corning SylgardR

184 PDMS processed in the manner described in our previous paper [ 2].

2.6. Throttle assembly

The test throttle was assembled as follows. Firstly a 1.2 mmthick soda-lime glass microscope slide was cut to 18 mm 20 mm and diamond drilled to match the designs connectionvias with barbed 1.6 mm diameter stainless steel tubingconnectors. It was then washed with dilute Decon90 (DeconLabs), rinsed and ultrasonicated with DI water, and blow-driedwith ltered nitrogen.

An 18 mm square, 110 m thick, borosilicate coverglass(Menzel-Gl aser, Braunschweig, Germany) was then cleanedwith dilute Decon90, thoroughly rinsed with DI water, and

Figure 8. Sectional schematic of a PZT operated test throttlestructure.

Figure 9. SEM of PDMS test throttle structure.

blow-dried with nitrogen. The upper face of the coverglasswas coated with circa 250 nm of chrome by evaporation toprovide electrical connection to the lower PZT electrode. Thetop face of the microscope slide and the bottom face of thePDMS were then UV-Ozone treated with a PSD-UVT system(Novoscan Technologies Inc., USA) using ambient air for 3 min. The components were then aligned and brought intocontact. This was then repeated for the microstructured topface PDMS and coverglass. Figure 8 shows a schematicof the assembled throttle. The throttle was then baked at90 C for 2 h to complete the PDMSglass bonding. The12.7 mm diameter, PZT bimorph disc (Piezo Systems, T216-A4NO-273X) was then bonded centrally to the top of thecoverglass with conductive epoxyadhesive(Chemtronics, GA,USA) such that the PDMS throttle element (see gure 9) wasdirectly under the centre of the PZT. The tubing connectorswere then xed with standard-grade epoxy adhesive. Finally,ne connecting wires were bonded with conductive adhesiveto both the upper face of the PZT and the perimeter of thechromed coverglass. The structure was then rested for oneday to ensure full adhesive curing.

2.7. Test throttle evaluationThe throttle was characterized by mass transfer to a Sartorious210s precision microbalance at a series of hydrostaticallydetermined pressures between 1 kPa and 50 kPa and voltagesbetween 30 V and 180 V.

To form a contiguous uid circuit to the output reservoir on the balance, the test throttle was primed via a Teon tubewhich interconnected the pump and an input reservoir-bottle,pressurized to 20 kPa with nitrogen, which was lled withltered, degassed, DI water. Once primed, the input reservoir wasvented to atmosphere andthe pressure acrossthe pump dueto the height difference of the uid in the reservoirs was zeroedby adjusting the height of the input reservoir with respect tothe output reservoir to achieve zero mass transfer.

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Figure 10. Experimentally measured throttling ratio as a function of vertical (peakpeak) deection of the PZT centre.

Figure 11. Experimentally measured throttling ratio as a function of applied uid pressure across the testing system.

As with composite micropumps, throttling performanceis also intimately related to the external ow resistances of tubing in the associated measuring system. In order to assessthe test throttle objectively the resistances of the tubing weremeasured and allowed for.

From laser interferometric measurements we havepreviously reported [ 2] we have determined that our PZT glassPDMSglass structures with this general form,materials properties and dimensions display a linear displacement / voltage relationship, normal to the centralaxis of the PZT, of approximately 0.018 m V 1. Thisinformation allows us to represent the throttles ow rate voltage characteristic, and hence, throttling ratio in terms of PZT deection. Figure 10 displays this information.

We also took advantage of the test throttle to measurethrottling ratio as a function of operating pressure, which willbe one of the constituents of the ensuing MTPs overall ow-pressure characteristic, see gure 11.

The modelled throttling ratios within the low pressureregion agree quite well with the actual performance of the testthrottle as presented in table 1.

The test throttle displays a declining, though still veryuseful, throttling ratio with increasing pressure, which weattribute to weir deformation as indicated by our FEMmodelling. The CFD modelling highlighted the presenceof eddy ows about the weir; however their objectiveinterpretation is limited by the predicted deformation of the

Table 1. Comparison of predicted and experimentally measuredthrottling ratios of the test throttle structure at low pressures.

Pressure Modelled Experimental(kPa) throttling ratio throttling ratio

1 9.1 8.25 7.9 7.2

weir at higher pressures that will in turn alter the ow patternaround the weir. It is likely that both mechanisms contributeto throttling degradation. However they will also both interactthus making their modelling non-trivial. These observationslead us to conclude that rapidly prototyped test structureswere a more appropriate tool to assess higher pressure throttleperformance in the context of this paper.

The advantages of the new throttle designs measured8.2:1 throttle ratio are particularly apparent when contrastedwith the maximum, 1.8:1 (at 4 kPa), throttling ratio of our previously reported, slot throttle [ 1]. Equation (1),derived from an expression we have previously reported [ 1],denes low frequency pumping efciency ( ) in terms of throttling ratio ( Rt), for an MTP with zero resistance externalconnections.

=Rt 1Rt + 1

. (1)

This expression predicts a pumping efciency of 78% under low pressure low frequency conditions (as discussed in our previous paper [ 1]) if two such throttles were integrated withinan MTP.

3. MTP design

Having experimentally validated the behaviour of the newthrottle design, the throttles were incorporated into an MTPdesign. The design employs the si ngle-piezo operatingprinciple we have previously reported [ 2]. It is important toappreciate that this MTP is a single PZT design incorporatingboth a pump chamber and a pair of microthrottles operatingin antiphase, as is required for pumping. This is achieved byexploiting an area of reversed exure at the interface betweenthe PDMS and the thin, upper, glass layer to which the PZTis bonded. This reversed exure occurs beyond the perimeter of the PZT and by appropriate placement of two throttles andthe pump chamber it is possible to implement pumping. For clarity, gure 12 demonstrates this.

Figure 13 shows a cast PDMS micropump substrate, thethrottles are identical to those employed in the throttle teststructure and shown in gure 8. The pump chamber diameter of 3 mm is unchanged from the preceding design.

The microstructured PDMS pump substrate wasfabricated using the same double depth process sequenceemployed in the fabrication of the test throttle. Deviceassembly varied from that used for the test throttle only inthe positioning of the PZT above the microstructure. In order to achieve an optimum bi-directional exure of the compositestructure the PZT is positioned 1 mm off centre in the directionof the pump input [ 2] (towards the feed via at the top right ingure 13). The fully assembled MTP is shown in gure 14.

As we have reported [ 2], unexpected bubble formationwas prevented by mildly restricting the drive waveform rise

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Figure 12. Simplied cross section displaying the bi-directional deformation resulting from the PZT exure on the elastomerglasssubstrate and the relative positions of microstructures in the opposing regions of exure.

Figure 13. SEM of the double depth PDMS micro throttle pumpstructure.

Figure 14. An assembled MTP.

and fall times by using a simple series resistor to form a200 s time-constant with the PZT capacitance.

4. Results: pumping

Measurements commenced by carefully characterizing theMTP with DI water. The drive-frequency / forward-pumpingrate relationship was determined by running the pump under microcontroller control. Microbalance data were processedin real time by software written in Labview R (NationalInstruments) to obtain mass ow rate values for each drivefrequency.

Pumping rate at zero back-pressure as a function of frequency is shown in gure 15. The pumps bead pumpingability was then assessed in a similar manner employing5 m diameter polystyrene beads (4025A, Duke ScienticCorp, CA, USA) as supplied at a concentration of 4.5 107 beads ml 1, also shown in gure 15. The data shown ingure 16 display the pumping-rate / back-pressure relationshipfor both water and beads, characterized at the broad peakpumping frequency of 1.1 kHz.

Figure 15. Experimentally obtained pumping rate for both water and an aqueous bead suspension as a function of drive frequency.

Figure 16. Pumping rate as a function of back-pressure for theseparate cases of an aqueous bead suspension and pure water at adrive frequency of 1100 Hz. The curve t is for water.

Figures 17 and 18 display the drive voltage dependency of pumping rate and back pressure respectively, both measuredat a xed frequency of 1.1 kHz.

These results show that the new MTP displays anapproximate ve-fold increase in both ow rate and back-pressure when compared to its predecessor. Other than therevised weir-throttle, the device is of identical construction tothe previous MTP. Whilst a full model of single-PZT, MTPshas yet to be developed, it seems reasonable, given that onlythe throttles have changed, to attribute the reported MTPsperformance enhancement to the revised throttles.

We note that the overall form of the MTPs owrate / frequency function is similar to that of its predecessor and again displays a series of mild ripples, with pump-rateminima at multiples of approximately 350 Hz. The cause

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Micro throttle pump employing displacement amplication in an elastomeric substrate

Figure 17. Pumping rate as a function of drive voltage.

Figure 18. Peak back-pressure as a function of applied voltage.

of these effects is as yet uncertain, but it is suspected thatthe underlying cause may actually be due to enhancementof pumping at drive frequencies between the ripples due toharmonics of the drive waveform feeding energy into resonantmodes of thecomposite structureand thus enhancingpumping.Laser vibrometry indicates a rst major resonance at circa850 Hz.

5. Conclusions

We have demonstrated the use of displacement amplication,due to constructive substrate deformation, to enhancemicrostructural deformation in an elastomeric substrate of uniform material properties. This has allowed us to obtainsignicantly more displacement than would, prima facie , beexpected from a given actuator, in this case a PZT. Whenemployed to enhance a single-PZT micro-throttle this hasallowed us to achieve throttling ratios of over 8:1, a major improvement over our previous microthrottles.

Employing these higher performance microthrottles wehave enhanced the performance of single PZT MTPs by a

factor of approximately ve in terms of both ow rate and backpressure whilst retaining our previous devices bidirectionalexuresingle PZT operation, solid-phase compatibility,straightforward assembly sequence and compactness.

We anticipate that this design of MTP could be optimizedsignicantly without introducing any fundamentally newfeatures. To this end we are currently extending modelling toinclude the effects of displacement amplication upon pumpchambers.

In general, we now consider that microthrottles displaysufcientthrottling ratiosto be considered in other uid controlapplications: in particular those intended forbead, particle andcell suspension handling.

Acknowledgments

We would like to thank our UH colleagues Gordon Herbertfor his assistance in SU-8 processing and Dr Xian Wei Liufor providing the SEM images of PDMS microdevices. Wealso thank Roger Traynor of Lambda Photometrics for bothproviding access to the Polytec PSV400-series scanning laser vibrometer andhisassistance in conducting themeasurements.

References

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