Micropumps for extreme pressures - DiVA portal278862/...Design of micropumps for extreme pressures...

Micropumps for extreme pressures

By

S. Svensson

Supervisor:L. Klintberg

Examiner:K. Hjort

Revised: October 7, 2009

Abstract

The objective of this thesis was to improve a paraffin actuated micropump design, tobe able to pump against extreme pressures (above 100 bar). This was accomplished byinitially studying the membrane activation, using video capturing. The micropump hasbeen improved to withstand pressures high enough, to enable use in an high-performanceliquid chromatography (HPLC) system. The micropump has been shown to pump againstback pressures up to 150 bar, with a positive net-flow. This should be compared with thepreviously recorded maximum back pressure of 50 bar. The pumping against high backpressures was possible due to an increased understanding of the sealing of the membranes.This resulted in a new design that was manufactured and characterised. Without clampingthe pump was measured to manage back pressures of 10 bar, and then starting to leakin a bond at the flow channel. With supporting clamping, the managed back pressuresincreased ten folded.

When measured on the different valves, pressure above 200 bar has been possibleto withhold. Although the valves were below their maximum limit, the pressure was notpossible to be further increased due to a limitation in the equipment, i.e. risk of damagingthe connections. When examined after pressurised at extreme pressures (above 100 bar)several times, no signs of fatigue or damage of the membrane was seen.

A new behaviour of the valves was discovered. Above certain pressures some designsself sealed, i.e. withholding the pressure after the voltage was turned off. For these valvesthe pressure had to be released by some other means.

Keywords – Micropump, Micromechanics, Paraffin, Actuator,

High-pressure pumping, Steel membrane

Sammanfattning

Baserat pa en tidigare design av Boden m.fl. har en peristaltisk mikropump utvarderasoch omdesignats for att klara extrema tryck (over 100 bar). Mikropumpen anvander sig avtre pumpkammare, vilka aktiveras i en viss ordning for att astadkomma en pumpcykel.Paraffin anvands som aktuatormaterial och aktiveras genom resistiv uppvarmning i enkopparledare, innesluten i en kavitet. Nar paraffin smalter okar volymen kraftigt vilketfar membranen i pumpen att rora sig. Dimensionerna for mikropumpen ar 30x15x1 mm.I examensarbetet har fem nya design-utforanden av den kanalstencil som ar i kontakt medmembranet tagits fram och utvarderats.

Tidigare har pumpen klarat av att pumpa mot ett tryck pa 50 bar. Ett polyimid-membran anvandes da men visade sig plastiskt deformeras i kontakt med motytan. Dettaresulterade i god tatning, men en opalitligt slagvolym vid aktivering. For att komma bortfran plasticeringen byttes membranet ut mot ett stalmembran.

Stalmembranet visade sig vara mer begransat i sina rorelser och storsta delen avutbojningen skedde i mitten av membranet. Tryckmatningar visade pa att ett inlopps/utloppshal placerat mot membranets mitt klarade tryck svara att uppna med experimentutrust-ningen (> 200 bar), medan ett hal placerat mot membranets rand medforde lackage redanstrax over 20 bar. Den har upptackten var orsaken till de nya designerna dar inlopps- ochutloppshalen placerades dels pa lika avstand fran membranets mitt, men aven dar haletriktat mot pumpens utlopp placerades centralt. Tanken bakom den senare designen varatt det palagda mottrycket ligger pa utloppet, medan inloppet kommer att befinna sigvid atmosfarstryck.

Nagot overraskande uppenbarades ett nytt fenomen da de nya designerna utvarder-ades. Tidigare har halvmaneformade hal anvants vid membranets rand for att ledavatskan vidare fran en pumpkammare till nasta. I de nya designerna anvandes utes-lutande cirkulara hal vilka visade sig ha en tendens att lasa sig nar de placeras langt utmot kanten pa membranet. Resultatet av detta var att ventilerna over ett visst tryck slottatt och holl tatt aven nar spanningen stangdes av, tills trycket lattades pa annat hall.

Ventilerna klarade att tata mot tryck over 200 bar (och har vid ett tillfalle belas-tats upp till 320 bar innan en sakerhetssparr i den externa pump som anvandes for attbygga upp trycket loste ut). Da kopplingarna hade svart att klara tryck over 250 barbegransades tryckmatningarna till 200 bar, men ventilerna kan med storsta sannolikhetbelastas betydligt mer. Under tryckmatningar och pumpexperiment har pumpen plac-erats i en fixtur med mojlighet att klamma at over pumpkammarna. Detta gor att enfog som visat sig vara pumpens svaga lank halls ihop och pumpen klarar hogre tryck.

Design of micropumps for extreme pressures

De tryckmatningar som avbrutits vid 200 bar har alla genomforts utan detta stod. Narmikropumpen lats pumpa mot ett mottryck klarade den dock inte tryck over 10 bar.

Med atklamning betedde sig mikropumpen battre och har visat sig kunna pumpa mottryck over 100 bar. Ett positivt flode har registrerats med ett mottryck pa 150 bar. Enligtvad vi kanner till ar detta det hogsta tryck en membrandriven mikropump nansin klaratav. Faktum ar att det endast existerar en typ av mikropumpar, med elektroosmos somdrivningsmekanism, som har rapporterats klarat hogre mottryck. Pumpar som anvanderelektroosmos begransas dock av kravet pa att vatskan som pumpas maste innehalla joner.Mikropumpen i detta arbete har inte nagon sadan begransning, aven om endast vattenannu har utvarderats for pumpen.

Med de uppmatta mottrycken uppnaddes malet med examensarbetet, som var attpumpa mot extrema tryck (definierade till tryck over 100 bar). Det finns en stor forbattringspo-tential hos pumpen och att na mottryck over 200 bar borde inte vara omojligt.


Contents

1 Introduction 11.1 In a world of micropumps . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 HPLC Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Previous work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.4 Main objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Theory 62.1 Actuation using paraffin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 The pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 The membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 Initial membrane deflection . . . . . . . . . . . . . . . . . . . . . . 72.3.2 Plasticising of the membrane . . . . . . . . . . . . . . . . . . . . . . 8

2.4 Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.5 Pump design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.6 Activation cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.7 Supply of energy through resistive heating . . . . . . . . . . . . . . . . . . 112.8 Time constant for membrane activation . . . . . . . . . . . . . . . . . . . . 122.9 Material selection of the membrane . . . . . . . . . . . . . . . . . . . . . . 122.10 Theory of deflecting membrane . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Design 173.1 Design O (original design) . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2 Design I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.3 Design II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.4 Design III (IV & V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4 Experimental 204.1 Video capture of membrane activation . . . . . . . . . . . . . . . . . . . . 204.2 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.2.1 The fixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2.2 Pressure sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2.3 Flow sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2.4 External pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214.2.5 Labview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21


4.3 Pumping against back-pressure . . . . . . . . . . . . . . . . . . . . . . . . 234.4 Valve measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.5 Optical surface profilometry . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5 Results 255.1 Directional dependency of steel membranes . . . . . . . . . . . . . . . . . . 255.2 Membrane activation - Images and video . . . . . . . . . . . . . . . . . . . 255.3 Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5.3.1 Voltage dependency . . . . . . . . . . . . . . . . . . . . . . . . . . . 315.4 Pumping against back-pressure . . . . . . . . . . . . . . . . . . . . . . . . 31

5.4.1 Unclamped micropump . . . . . . . . . . . . . . . . . . . . . . . . 315.4.2 Clamped pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.5 Flow and activation of a pump-cycle . . . . . . . . . . . . . . . . . . . . . 355.5.1 Activation time of a steel valve . . . . . . . . . . . . . . . . . . . . 355.5.2 Deflection of outlet membrane . . . . . . . . . . . . . . . . . . . . . 35

6 Discussion 396.1 A small pump with great potential . . . . . . . . . . . . . . . . . . . . . . 396.2 Placement of channels towards the membrane . . . . . . . . . . . . . . . . 396.3 Material selection for the membrane . . . . . . . . . . . . . . . . . . . . . . 406.4 Steel compared to polyimide as a membrane . . . . . . . . . . . . . . . . . 406.5 Motivation of the new designs . . . . . . . . . . . . . . . . . . . . . . . . . 406.6 Trapped air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.7 Bonding between channel- and inlet-stencils . . . . . . . . . . . . . . . . . 426.8 Limitations in the equipment . . . . . . . . . . . . . . . . . . . . . . . . . 426.9 Expansion within the system . . . . . . . . . . . . . . . . . . . . . . . . . . 436.10 Self-sealing valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436.11 Blocked channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.12 Plateau in pressure measurements . . . . . . . . . . . . . . . . . . . . . . . 446.13 The activation time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446.14 Frequency dependency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456.15 Future improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

7 Conclusions 46

Acknowledgements 47

References 48

Chapter 1

Introduction

The aim of this thesis is to design a micropump for extreme pressures based on a patenteddesign now owned by GE Healthcare. The micropump has previously been reported tomanage a back-pressure of 50 bar (0.5 MPa), but to be able to use it in a high-performanceliquid chromatography (HPLC) system, pressures above 100 bar is of interest. The pumpdesign concerned in this thesis is a thermally actuated displacement-micropump basedon the principle and design described by Boden et al. [1], although it differs in severalaspects, as later described. The main focus is to pump against high back-pressures, notto achieve high flow-rates.

1.1 In a world of micropumps

The creativity of engineering micropumps (active parts of sub-cm size) knows no bound-aries. The utilised mechanisms for micropumps ranges from mechanical pumps to pumpsusing ionic transportation as an actuator mechanism, although very few pumps are ofactual use in the world outside the laboratory [2]. Since there is a wide variety of mi-cropump designs, they need to be categorised. The most common way would be to differbetween the non-mechanical and the mechanical pumps [3].

The non-mechanical pumps directly provides the driving energy to the working mediumusing a physical phenomenon (e.g. magnetism, gravity, Coulomb forces). The mechanicalpumps on the other hand usually adds the energy by applying pressure on the medium.The function of an actuator in mechanical pumps is to apply a force on a membrane thatwill do the actual work on the medium. The mechanical pumps often include valves whichwill have the advantage/disadvantage that the flow will periodically stop. The majority ofmicropumps today are still mechanical due to the robust designs of these structures, eventhough the best flow-rates at highest back-pressures have been achieved using valve-lesspumps. A comparison by Boden, figure 1.1, gives a good overview of the different pumpsbased on their actuation mechanism.

Looking at the valve-less designs, one of the advantages of these micropumps is thatmost of the designs will result in a continuos flow. There are also no (or at least a minimalamount of) moving parts, which reduces the risk of fatigue failures and leakages. Among

1

Design of micropumps for extreme pressures 2

Figure 1.1: A comparison of the different micropumps based on the actuation mechanismby Boden. The star marks the performance of the first design of the steel-micropump [1].

these pumps, at least three are worth mentioning. The first one is a mechanical valve-lessmicropump designed by Olsson et al. [4]. This pump is milled out of two 0.5 mm thickblocks of brass. The pump contains two parallel connected pumping chambers and isactivated using four piezoelectric elements. This is a famous design because the conceptentered new lands by removing the valves and using shaped inlet and outlet channels(denoted nozzle and diffuser) to direct the flow, figure 1.2(a). This pump achieved flowrates of 16 ml min−1 against a back-pressure of 0.17 bar. The downside with this pump(also being one of its main features) is the lack of valves towards the ”outside”. This makesthe pump useless as a dispenser and it is not possible to pump against high pressuresbecause there is nothing to prevent the back-flow.

The second valve-less pump worth mentioning is one using electroosmotic flow as thedriving mechanism, figure 1.2(b). The electroosmotic design is based on a spontaneouselectric double layer, created at the interface between a surface and a liquid. At the surfaceof a glass capillary, spontaneously created positive charges will adhere to the anions inthe liquid. As a result the cations will be distributed more towards the middle of thecapillary. By applying a voltage parallel to the walls, the cations will move towards thenegative charge dragging the neutral particles in the liquid with them [5]. This pumpingmechanism relies on the pumped liquid being an ionic solution. Electroosmotic pumpsare reported to achieve flow-rates of several µl min−1 at back-pressures reaching above200 bar [6]. There has even been reports of pressures around 500 bar using a more complexvariant of the electroosmotic principle denoted packed-bed electroosmotic pump [7].

The pump design does not have to be complex. A much simpler valve-less designrelies on gravity to ”pump” the liquid in a steady flow using the principle of potential


!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]_abcdefghijklmnopqrstuvwxyz|~

Nozzle Diffuser

PZT-activated membrane

(a)


+ + + + ++ + + +

+ + +

+

+

+

+

+

+

+

- - - ------- -

- - - ------ --

N

N N

N

N

N

NN

N

N

N

NN

N

N

N

(b)

(c)

Figure 1.2: (a) The principle of the valve-less micropump designed by Olsson et al. [4].The shapes of the inlet and outlet directs the flow generated by the movement of a mem-brane activated by a piezoelectric element.(b) Electroosmotic principle. The negatively charged ions will be attracted towards thewalls of the glas capillary leaving the positively charged ions in the middle. By applying avoltage perpendicular to this gradient, the positively charged ions will be moved draggingalong the molecules in the fluid.(c) A principle sketch of the gravity pump. The liquid is pushed through the microfluidicsystem using the potential energy (U) of the liquid.


energy [8], figure 1.2(c). This pump, as well as many other valve-less pumps, have thedisadvantage that they can only pump against low back-pressures.

Because the main objective for many micropumps is in biomedical applications (e.g.drug delivery), a well defined pumped volume is of great importance. In those applications,the pressure is not in a region where it would be the main design consideration. For amicropump to be used as an implantable device, the highest pressure to overcome in vivois 1 bar [2]. However, there are applications where the ability to manage a high pressureis the primary interest.

1.2 HPLC Systems

High-performance liquid chromatography, or high pressure liquid chromatography (HPLC)is a technique to separate different components in a fluid, figure 1.3. A mobile phase isflowed through a stationary phase, often consisting of well defined particles [9] containedin a stainless steel tube. Apart from amplifiers, computer, ports etc. the system mainlyconsists of three parts: a pump, a separation column and a detector. The pressure inthe system is primarily generated in the column (considered by many to be the heartof the system), that consists of some type of porous structure whose task is to make ithard for molecules to pass [10]. This column is usually operated at high pressures (wellabove 100 bar) which makes it sensitive to uneven flow rates, due to the risk of damagingthe column [11]. The column will separate the molecules depending on for example theirsize, and will result in different times for different molecules to pass through the systemto the detector. The result is a chromatogram with the intensity as a function of time.The time for a molecule to pass through the system is specific for that molecule, makingseparation possible. It is thereby of interest to have as constant flow rate as possible. Thefunction of the pump in an HPLC-system is to provide enough pressure and to accomplisha constant flow through the column. Using displacement pumps, the constant flow ratebecomes an obstacle that has to be managed. This can be accomplished by using severalpumps connected in parallel, and thereby even out the flow.

It is often of interest to minimise components to increase the speed of a specific task.By reducing the size of the particles in the column, it is possible to reduce the time forthe analysis due to the reduced diffusion time for the molecules. Smaller particles willalso result in a more uniform flow and will therefore give a better analysis. Directly fromthe definition of pressure (force divided by area) it can be seen that a smaller particlesize in the column will render a higher back pressure. The pressures in HPLC-systemsare usually in the region of 70 - 400 bar (40 MPa), but may be up to 1000 bar [13]. Toachieve these pressures a large external pump is used. This solution works fine if used ina laboratory, but to make the analysis system mobile a small pump that can achieve highpressures would be of interest.


Figure 1.3: A commercial HPLC-system [12].

1.3 Previous work

The first micropump, designed by Boden et al. [14], was mainly manufactured in poly-meric materials, with the exception of a glass lid and copper coated polyimide heaters.This pump was able to build up a pressure of 2 bar without clamping (a support froma fixture). When clamped it was able to pump up a pressure of 9.2 bar. Although thiswas considered a high pressure at the time, the pump was not long term stable and theevolution of the pump went towards a steel micropump. The first steel design used thesame kind of polyimide membrane as was being used in the polymeric pump. This versionof the metal pump could build up a pressure of 50 bar before a leakage was observed inthe joints between the top stencil and the flow channel [1].

The polyimide membranes used were not able to cope the high pressures, but plas-ticised into the channels. This gave the pump an unpredictable flow-rate which is veryundesirable for most applications. Afterwards a number of different membrane materialshave been tested, although not fully evaluated. Among them there are two different steels,showing the most promising results.

1.4 Main objectives

In order to achieve pumping pressures above 100 bar, the main objectives that are to beaccomplished:

• Understand the sealing of the polyimide and steel valves.

• To be able to pump against high pressures, the valves have to be designed to with-stand and seal at these pressures.

• Manage 100 bar using stainless steel membranes, without plastic deformation.

Chapter 2

Theory

2.1 Actuation using paraffin

The common candle wax (paraffin) has some amazing properties. When heated up fromroom temperature it will undergo a phase transition around 45 C (possible to varyingbetween -100 C and 100 C) dependent on the molecular length of the polymer chains,resulting in a volume expansion in the region of 10 - 20 % of the original volume [15].Paraffin has a high bulk modulus (i.e. the volume change for a certain applied pressure)about 1.3 GPa [16]. This can be compared with the bulk modulus of air in the regionof 0.1 MPa [17]. The high bulk modulus of melted paraffin implies a low compressibility,hence the material can withstand high pressures without significant volume change.

The high bulk modulus and large volume expansion of paraffin, together with the lowworking temperature gives paraffin the potential of being a powerful actuator. Paraffinhas a high heat capacity and low thermal conductivity. This implies that paraffin is mostuseful as an actuator material in miniaturised systems.

2.2 The pump

The micropump in this thesis is a peristaltic pump, and consists of three cavities with adiameter of 2 mm. The cavities are enclosed by a backplate and a membrane, connectedin series, figure 2.1. The cavities are filled with paraffin and inside each cavity a heateris located. When the paraffin is melted it expands, exerting a pressure against the sur-roundings. If the steel-walls and backplate are considered stiff, the volume expansion willdeflect the membrane. If enough paraffin is melted, the deflection of the membrane willbe large enough to seal the channel and using this principle for all three membranes apumping cycle can be achieved. The amount of paraffin that needs to be melted to sealthe channel will set the lower limit of the power needed for the pump.

The three cavities will each have a specific role in the pumping cycle. The two outermembranes will work as valves, while the membrane in the middle will dispense the liquid.The membranes are connected by channels above the cavities.

6


(a)

(b)

Figure 2.1: (a) A cross-section of the old pump design. Top down: A top-stencil anda channel stencil in stainless steel, a 50 µm thick PI membrane (yellow) with an initialdeflection of 40 µm, a cavity-stencil, a heater (red), a cavity-stencil and at the bottom aback-stencil. (b) Left to right: Back stencil, cavity stencil, heater stencil, cavity stencil,membrane, channel stencil, top stencil.

2.3 The membrane

2.3.1 Initial membrane deflection

When melted, paraffin is introduced into the cavity containing the heaters, the paraffinwill contract due to the phase-shift that occurs during solidification. This will result in anunder-pressure if there is no air to even out the pressure. If the walls and bottom of thecavity can be regarded as stiff, the membrane will be the compensating component whichresults in the initial deflection of the membrane. This deflection will find its equilibriumwhen either the membrane is deflected enough to let the paraffin shrink its 10-20 %, or ifthe force needed to pull down the membrane further is higher then the yield strength ofparaffin, the paraffin will be plasticised. This initial deflection is typical measured to bein the region of 15 µm for stainless steel and 50 µm for polyimide, the two membranespreviously used in this pump. When the paraffin is melted again, the membrane will goback to its original flat shape.


2.3.2 Plasticising of the membrane

The previously evaluated polymers showed a prone behaviour of plasticising by the pres-sure during the expansion of paraffin. The channel stencil will act as a stop for themembrane during the expansion, but this support does not exist in the holes for the inletand outlet. A too ductile membrane will as a result be pressed into the channel and beplasticised. This behaviour has been observed for the polymeric materials but also forsteel grade 304. The deformation will in the case of steel result in a changed deflectionby a few µm, while the polymeric materials will have a changed deflection of more than50 µm.

2.4 Manufacturing

Utilising the CAD (Computer Aided Design) technique the blueprints for the differentstencils are sent to an external company, HP Etch, Sweden, to be etched out of either100 or 200 µm thick steel sheets. The etching technique used is an industrial processcalled photochemical machining (PCM). The use of this technique allows large scale batchproduction of the steel stencils. Except for the heaters (etched out of copper coatedpolyimide sheets) and the membrane, this is the material used in the pump. To bondthe steel sheets together a 7 µm parylene coating is used. The coating is performed byan external company PlasmaParylene, Germany. The bonding is done in two steps, thefirst at 200 C is done in a vacuum oven for 30 minutes. The second heat treatment isdone at 240 C for 30 minutes. The heaters used in the pump is a structure etched outof a copper coated polyimide sheet, and the heaters are also coated with parylene forthe bonding process. The heaters are manufactured at the Angstrom Laboratory usingstandard lithography, wet and dry etching [1].

To bond the separate stencils together, a fixture is used where the stencils are stackedin order. The whole pump is bonded in one step excluding the backplate, which is laterglued.

One of the previous problems with the pump has been paraffin leakage. Due to theisolation gap at the copper heaters, a channel exists for the paraffin to ”escape”. Thishas to be sealed and this is done after the pump is bonded together using a low viscosityepoxy glue (EPO-Tek 301-2). The paraffin is introduced to the cavities from the bottomby filling the cavities with melted paraffin. Afterwards, the excess paraffin is scraped offand a back-stencil is glued onto the pump using Loctite 407. The last step is to solderthe wires onto the copper pads.

To increase the pumped volume, shaped membranes were in some experiments used.To achieve this, the membrane was pushed against a fixture with three circular elevationsof 200 µm each.


Figure 2.2: The micropump beside a swedish 1 SEK-coin.

2.5 Pump design

The micropump is 15 x 30 mm and 1 mm thick, figure 2.2. Compared to the previousdesign [1], the amount of paraffin has decreased to 1/3 of the original amount due to theprevious amount of paraffin being over-dimensioned. The first metal pump used three0.2 mm thick cavity-stencils. Today only two 0.1 mm thick cavity-stencils are used. Thechannel and top-stencil are more rigid than the cavity-stencil being 0.2 mm thick. Thestandard diameter of the holes for the channels used is 0.3 mm while the paraffin cavitieshave a diameter of 2 mm.

2.6 Activation cycle

The pump uses a common pumping cycle [18] consisting of six phases which can bedivided into two stages (dispensing and filling), figure 2.3(a). Following the strokes seenin figure 2.3(b), the first phase is the main dispensing of the liquid by the activation ofthe chamber-membrane (T1). This is followed by the activation of the outlet-membranegiving a second stroke (T2). In this phase all three membranes are sealed and the secondstage is initiated when the pump is filled. At first the inlet membrane is opened creating anunder-pressure filling the cavity with liquid (T3). This is followed by the deactivation ofthe chamber-membrane (T4), filling the pump even more. To prevent the higher pressureat the outlet from creating a flow straight through the pump, the inlet membrane is closed(T5). The final phase is the preparation of the dispensing phase and the outlet membraneis deactivated (T6). This release of the outlet membrane will allow liquid at the outside


to move in the opposite direction, giving a momentary negative flow.

Ideally the opening and closing of the membranes would be instantaneous, but theheating and cooling of the paraffin are time consuming processes, being the bottle necksin the driving cycle. The cycle time used in previous pump experiments has usually beenin the region of 5 s, but this is far from an optimised speed.

The membrane activation is illustrated in figure 2.3(c).


0

1

Pumpcycle

Period

Activated

Inlet

Outlet

Chamber

T1 T2 T3 T4 T5 T6

(a)


Flow

Time

T1 T2

T3-T5 T60

(b)

(c)

Figure 2.3: (a) A schematic of the pumpcycle. (b) The flow of a whole cycle. (c) Aschematic over the membrane activation by expansion of paraffin for the different periodsof the pump cycle.

The frequency is defined as [19]:

f =1

T(2.1)

where T is the period of a whole pump cycle. A more detailed definition would be tosum the period time of all the cycle-steps and denote the individual parts with an indexi. This yields the following equation:

f =∑ 1

Ti

(2.2)


where i = 1, 2, 3, 4, 5 and 6, denoting the previously described periods of a whole cycle.Looking at the flow of the cycle (figure 2.3(b)) there are three parts moving liquid at theoutlet-side of the pump. The volume moved at T2 and T6 (when the outlet membraneis opened and closed) will be the same and thereby cancels each others’ contribution tothe pumped volume. The total pumped volume can thereby be approximated to thedispensed amount of liquid in the middle chamber. Assuming the shape of the membraneto be equal to a spherical cap, the volume can be described by the following equation [20]:

V =1

6πδ(3a2 + δ2) (2.3)

δ is the height of the cap (or in the case of the membrane the deflection) and a is theradius of the bottom circle in the spherical cap.


R

δ

a

Figure 2.4: A spherical cap.

If the volume V will be dispensed every cycle the flow rate q will be described by:

q = fV (2.4)

For the pump there is an optimal frequency that will give the highest flow rate, figure2.5. At higher frequencies the time is not enough to heat the needed amount of paraffinor cool the paraffin to open the membranes, decreasing the performance of the pump. Atlower frequencies the potential of the paraffin cavities is not fully used, and the paraffinis heated too much, resulting in a waste of energy and in worst case damage to the pump.

2.7 Supply of energy through resistive heating

The driving mechanism of the pump relies on resistive heating from a current passingthrough a copper circuit. The resistivity of the heaters will result in an emission of heat.Joules law for thermal power gives an expression for the power P (not to confuse with p


Figure 2.5: A principal graph of the typical frequency dependency of the peristaltic mi-cropump for atmospheric pressure.

which in this thesis is used denoting pressure):

P = UI =U2

R(2.5)

2.8 Time constant for membrane activation

During activation and deactivation there is a system specific delay before the responseactually goes to the desired output. An example is the combustion engine in your car(the rpm does not react instantaneous with the throttle). The declination of the responseduring deactivation is often possible to fit to an exponential expression:

x(t) = x(0)e−t/τ (2.6)

x(t) is the momentarily response, x(0) the initial response, t is the time and τ is atime constant. For the case when t = τ , the exponential becomes e−1 and x(t)/x(0) isthereby 0,367. This gives that the time it takes for the signal to become 37% of its initialvalue will be equal to the time constant τ , and this value can be used to compare differentsystems.

2.9 Material selection of the membrane

Commonly used micropump membrane materials ranges from soft materials like poly-dimethylsiloxane (PDMS) to hard and brittle materials like silicon. Depending on thedesign of the pump, the most suitable material will differ. If a large pumped volume isthe main interest, a stiff metal (or silicon) membrane would be a bad choice compared toa flexible polymer. On the other hand, a stiff metal membrane can support a higher load.Table 2.1 shows the basic material data for a couple of interesting materials.


Table 2.1: Properties of different materials. 1Dupont.com 2Matweb.com

Material Youngs modulus [GPa] Tensile Strength [MPa]Polyimide1 2.5 61Stainless steel 3042 193-200 215Stainless steel 3012 212 205Titanium Grade 52 114 880

The initial design of the pump [14] used polyimide (PI) as a membrane material. Thegeneral properties of this material includes a yield strength of 61 MPa and a Young´smodulus of 2.5 GPa which is maintained up to 230 C. Translating these numbers intowords, PI can be described as a rather flexible material that can withstand high loadsconsidering that it is a polymer. Both 50 µm and 25 µm thick membranes have previouslybeen used in the paraffin pump, but is now replaced since they plasticise. Compared tocommonly used construction materials like stainless steel, PI has a low yield strength andwill thereby plasticise at relatively low stresses. The consequence will be that PI hasan unreliable stroke. To get around the problem of deformation of polymeric materials,elastomeres (e.g. PDMS) could be used. However, when pushed against the oppositesurface containing the channels, the elastomere would be pushed into the channels by themelted paraffin, due to the inability of the material to support applied loads. Although thematerial would not permanently deform, it would eventually break due to the elongation.

Steel is a very versatile material. It has both a high ductility and a high yield strength.This makes steel a good choice in many applications. The downside of this versatility isthat there is almost always another material more suited for a specific task. The knowl-edge of steel is very broad and the properties can be varied using different hardeningmechanisms (Hall-Petch strengthening, solid solution strengthening, precipitation hard-ening and cold working). Many engineers find comfort in using steel as a constructionmaterial due to the insensitivity to local composition variations due to the ductile natureof the material. Due to its versatility, there are many different categories of steel. Themost common is the regular stainless steel usually denoted 18-8 or 304. It is an alloy of18 % chromium and 8 % nickel. When in contact with an oxygen rich atmosphere, thechromium will form a homogeneous layer of chromium oxide at the surface, preventingthe oxygen to come in contact with the rest of the material, and thereby protecting thebulk from corrosion [21].

A steel exposed to external stresses will be affected by phenomena like creep and fa-tigue. This is a consequence of dislocation movements and diffusion inside the material.One of the main advantages of using stainless steel as a membrane material is that theprotective layer of chromium is non-reactive, making the material highly corrosion resis-tant. Due to the inertness of steel, it has been one of the main materials in implants,and steel is considered to be fairly bio-compatible (depending on the definition) [22]. Thismakes a surface coating of the membrane towards the pumped fluid unnecessary, although


the membranes used today is parylene coated prior to the bonding process. Disregardingthe heaters, 18-8 is the construction material in the micropump. Looking at figure 2.7in the theory-chapter, showing the different stresses of a simple 10 µm thick membrane,it can be seen that for a deflection of 12 µm the stresses at the edge of the membraneis dangerously close to the yield strength of the material with the present design. Thisdeflection is usually the initial deflection of an unactuated 304-membrane.

One of the biggest problems with the metal membranes may be considered to befatigue. After long runs cracks appear at the membrane even though the stresses is belowthe yield strength. It is estimated that 90 percent of all failures of metallic applicationsis the result of a fatigue failure [23]. The stress at failure in a steel material is a functionof the number of cycles. This is not a linear relation and the curve will even out at aspecific value of the applied stress. The value is usually in the region above 35 - 60 %of the yield strength for steel. As a construction parameter this value is often a betterparameter than the yield strength of a metallic material. 301, another variation withinthe stainless steel family, has similar properties as 304, but is modified to have a higherfatigue strength.

Titanium is used in applications where high strength is desired in combination witha low weight. Although pure titanium is a relatively soft material, it is possible to reachtensile strengths up to 800 MPa with titanium alloys (Grade 5, Ti6Al4V). The limit wherethe fatigue strength goes to infinity is about 160 MPa, which is in the region of the tensilestrength of steel. A favourable property with titanium as a membrane material would beits lower Youngs modulus (E), which is in the region of 110 GPa for almost all alloys.The consequence of the lower Youngs modulus and high fracture strength is a materialthat is more durable than steel, but also more prone to deflection, i.e. a membrane thatwill give a larger stroke and that can withstand higher stresses. However, titanium isan expensive material with a limited number of suppliers, especially for thin alloy foils.Because of this, the membrane material used in this thesis was stainless steel grade 301.

2.10 Theory of deflecting membrane

By simplifying the membrane to a circular disc (see figure 2.6) the membrane stress isdescribed by the following three equations at some key points [24].

aδ

h

p

Figure 2.6: Cross-section of a deflecting membrane fixed at the edges.


The radial stress at the edge of the membrane is described by

σbending,edge =3pa2

4h2(2.7)

The tangential stress at the edge will be

σt,bending,edge =3µpa2

4h2(2.8)

At the center the tangential stress is described by

σt,center =3(1 + µ)pa2

8h2(2.9)

where a and h is the radius and thickness of the membrane respectively, µ is thePoissons ratio for the material used and p is the pressure evenly distributed along themembrane.

The characteristic equation for a flat membrane for small displacements (the deflectionbeing less than a third of the membrane thickness) is given by

p =16Eh3δ

3(1 − µ2)a4(2.10)

δ is the variable denoting the deflection of the membrane and E is the Youngs modulusof the membrane material. This equation only takes the bending stresses into account.The characteristic equation for the tensile stresses of a deflecting membrane is describedby

p =7 − µ

3(1 − µ)

Eh4

a4

δ3

h3(2.11)

Worth noticing is that the tensile stresses increases with the cubic power of the deflectioncompared to the linear relation of the bending stresses towards the deflection. By themethod of super-positioning, equation 2.10 and 2.11 can be combined to an equationwhich takes into account both bending stresses and the tensile stresses:

p =Eh4

a4(

16δ

3(1 − µ2)h+

(7 − µ)δ3

3(1 − µ)h3) (2.12)

The material constants related to this equations is mainly the Youngs modulus (E)and Poissons ratio µ. These two values will represent the stiffness of the material andthe ratio of which a material will expand in one direction when compressed in anotherdirection. One critical variable not mentioned in these equations is the tensile strength ofthe material σy. This is the maximum stress that a specific material can withstand beforeit starts to plasticise.

Figure 2.7 shows the stresses of equation 2.7 - 2.9 calcuated for a 10 µm thick stainlesssteel membrane with a radius of 1 mm. As seen in the figure, the radial stress at the edgesis the largest and is dangerously close to the tensile stress of stainless steel (215 MPa) fora deflection in the region of the measured deflection.


2 4 6 8 10 12 14

50

100

150

200

250

Stress (Deflection) for SS 304

Deflection [µm]

Str

ess

[MP

a]

Stress at centerRadial stress at edgeTangential stress at edge

Figure 2.7: A graph of equation 2.7 - 2.9, calculated for different values of the deflectionfor a 10 µm thick stainless steel membrane with a radius of 1 mm.

Chapter 3

Design

There are currently six different designs of the channel stencils that is in contact withthe membrane: the design used by Boden [1] and design I - V that was developed andevaluated as a part of this thesis, shown in figure 3.1. The used design of the individualpumps occurring in this thesis are summarised in table 3.1

Inlet Outlet

(a) (b)

(c) (d)

Figure 3.1: The placement of the holes in relation to the paraffin cavities for differentdesigns of the channel stencils. The dashed circles are the cavities and represent, fromleft to right, the inlet, chamber and outlet. (a) The original design with crescent shapedholes towards the channels. (b) Design I (c) Design II (d) Design III, IV and V

3.1 Design O (original design)

This is the original design of the channel stencil that was used with the polyimide mem-brane. The inlet and outlet are circular holes and they are located directly above thecentre of the membranes. The holes towards the channels has the shape of a crescent, andis located near the rim of the membrane, figure 3.1(a). The pump design is symmetrical,i.e. the pump can be used to pump in both directions.

17


Table 3.1: The used designs of the different micropumps occurring in this thesis.

Micropump DesignL56 OL80 IIIL81 IL82 IIL83 VL84 IVL87 II

3.2 Design I

The crescent shaped holes are replaced by circular holes with a diameter of 0.3 mm, andthe holes at the inlet and outlet are distributed at an equal distance of 0.3 mm from thecentre of the cavities, figure 3.1(b). Symmetrical.

3.3 Design II

The holes at the inlet and outlet are at equal locations as in design I with the diameterof 0.3 mm. The holes in the middle chamber are placed at a distance of 0.3 mm from thecentre, figure 3.1(c). Symmetrical.

3.4 Design III (IV & V)

The idea behind design III is based on the results (5.1) indicating that a hole at the rimwill manage a lower pressure than a hole in the middle, figure 3.1(d). The outlet holeis placed at the membrane centre, whereas the inlet hole is placed close to the rim, at adistance of 0.65 mm from the membrane centre, figure 3.1(d). This removes the symmetryof the design, and the pump is thereby limited to pump in one direction.

Design IV uses the same placement of the holes as design III, with the only differencethat a diameter of 0.5 mm is used instead of 0.3 mm as in design III.

Design V has the same dimensions as design III. The holes located at the centre ofthe membranes has a cavity with a diameter of 0.6 mm etched out at the upper side ofthe channel stencil, figure 3.2, allowing the channel stencil to yield at some extent.



Top stencil

Channel stencil

Figure 3.2: The cavity above the center holes within the channel-stencil of design V.

Chapter 4

Experimental

The capabilities of the micropump was evaluated by letting the micropump work againstan applied pressure built up by an external pump. The same set-up was used to charac-terise the individual valves with the difference that the flow sensor was removed in thevalve measurements.

4.1 Video capture of membrane activation

A separate steel fixture was fabricated with a milled opening in the middle to allow lightpassing through. One of the long sides was chamfered at an approximate angle of 45 to al-low illumination from the side. The fixture was placed underneath a microscope equippedwith a video-recorder connected to a computer. Using the built-in Windows Movie Makerat the maximum possible resolution (720x540) the live footages were obtained. The top-stencil and channel-stencil of the micropump was replaced by a channel-stencil milled outin polymethylmethacrylate (PMMA), which is optically transparent and makes it possi-ble to watch the membrane during activation. Due to the optical properties of steel, atransparent polyimide membrane was used at first to evaluate the melting behaviour ofparaffin. Later a stainless steel membrane was used to evaluate the behaviour of the steelmembrane compared to the polyimide membrane.

4.2 Setup

Standard HPLC connectors from Upchurch Scientific was used and assembled accordingto figure 4.1(a). The pump was mounted inside a fixture to get support by an externalclamping over the channel structure. The fixture also had holes to fit connectors towardsthe tubing. The applied pressure was monitored by connecting the inlet to a pressuresensor. For the flow measurements a flow sensor was connected between the pressure-sensor and micropump to monitor the strokes and thereby also the flow. The signalsgenerated from the pressure sensor and flow sensor were monitored using the Labview 8.5software.

20


The resistance of the heaters was measured using a Fluke 21 multimeter.

4.2.1 The fixture

The fixture consisted of two aluminium blocks that was screwed together, figure 4.2. Thepump was placed in between the two blocks with the inlet and outlet facing the two drilledholes for fluidic connections. At the bottom plate, a spring-loaded piston supplied theclamping force. The level of the clamping is regulated by a screw underneath the plate.If desired, the cylinder will exert maximum force up to 600 N at maximum stroke.

To adjust the temperature surrounding the pump, the fixture was placed on a tem-perature regulated plate. Due to the good thermal conductivity of aluminium, the entirefixture will have approximately the same temperature. To minimise the temperature dif-ference as much as possible, heat-conducting paste is applied in the interface between thefixture and the plate.

4.2.2 Pressure sensor

The pressure sensor used is a Keller PA-11 mounted onto a 3-way junction. It uses aconstant current of 1 mA and the voltage varies linear with applied pressure (at a rate of0.039 V/bar). The limit of the sensor is 400 bar and the output signal is monitored. Theconstant current was acquired from a TTi QL355TP power supply.

4.2.3 Flow sensor

The flow sensor used is a Sensiron SLG1430. The sensor was connected in the fluidicsystem using capillaries with a diameter of 75 µm. The range of the sensor is ±40 µl min−1

with a resolution of ±7 nl min−1, and the sampling rate used was 6.25 Hz.

4.2.4 External pump

When measuring which pressures the membrane can withstand, an external pump wasused for pressure build-up. In the first experiments a syringe pump (Harvard InstrumentsPHD 2000) was used. However, since the pressure limit was about 65 bar it was laterreplaced by a Pharmacia P-3500 pump. Even though the micropump can build up pres-sures of its own, the pumped amount is so small that it would take several hours to reacha pressure of a few bar due to the elasticity of the fluidic system.

4.2.5 Labview

A Labview-program was designed and used to monitor the system, but also to drive themicropump, figure 4.3. Two separate cards were used: a NI SCB-68 for monitoring in-signals and a NI TBX-68 connected to an external circuit for amplification of the outputcurrents. The voltage of ±5 V for the amplification circuit was acquired by a FarnellPDD3502A Power Supply. Due to the design of the amplification circuit, a voltage drop


(a)

1. External

Pump

2. Pressure

Sensor3. Flow

Sensor

4. Micropump

mounted

inside fixture

Computer with

Labview 8.5

7. NI SCB-68

8. NI TBX-68

Amplifier

Input signals

Output signals

Inlet

Outlet

Fluidic Tubing

Input signals

Output signals

(b)

Figure 4.1: (a) Photo of the set-up. (b) Principal diagram. The set-up: 1. HPLC pump2. Pressure sensor 3. Flow sensor 4. Micropump mounted inside fixture 5. Temperatureregulating plate 6. Power supply 7. Signals to Labview (DAQ) 8. Signals from Labview


Figure 4.2: The fixture in which the pump is mounted for experiments.

of 50 mV was present between the set value in Labview and the heater. The drivingfrequency and voltage can be set separately for the different heaters.

In the latest version of the Labview-program, the flow, applied voltages over the heatersand pressure could be registered simultaneous making it possible to determine the responseto a membrane activation at a certain pressure.

4.3 Pumping against back-pressure

By letting the P-3500 pump build up a pressure against the pump outlet, the ability of themicropump to pump against a back-pressure was evaluated. This eliminated the need ofthe micropump to build up the pressure by itself (which is a very time consuming process).The P-3500 pump can be stopped without losing the built up pressure (although a smallpressure drop is noticed at higher pressures). When the desired pressure was reached,the P-3500 pump was paused and the pressure and flow logged to a text-file. The flowwas measured in two ways: using a flow sensor, and by measuring the time to pump aspecified amount through a capillary at the inlet.

4.4 Valve measurements

The outlet membrane was activated with a DC current at the heater. By letting theexternal pump build up a pressure, the limit of the membrane was tested. There aretwo different ways to pressurise the membrane: from the outside and inside, figure 4.4.The main difference between these two cases is the applied stresses at the bonds that apressure inside the pump implies.


Figure 4.3: The user interface of the program used for control and monitor of the pump.The pressure and flow rate was monitored in graphs as well as the applied voltages.

4.5 Optical surface profilometry

To determine the membrane deflection, a white-light interferometer (WYKO NT1100)was used. The objective used was at 10x magnification.

Outlet

Pressurisation

from inside

Pressurisation

from outside

Figure 4.4: The two different ways of pressurising the membrane.

Chapter 5

Results

5.1 Directional dependency of steel membranes

When pressurising the outlet membrane of the pump L56 with crescent shaped chan-nels, showed in figure 3.1(a), the maximum pressure obtained differed depending on howthe membrane was pressurised, figure 5.1. Five measurements were conducted, threepressurising from above (upper diagram) and two from inside (lower diagram). Whenpressurised from above, it was possible to achieve pressures over 100 bar while pressuresfrom inside had a maximum around 25 bar. After each experiment the valve was checkedso that it sealed by pressurising it with a syringe. The resistance of the activated heaterwas measured to be 1.8 Ω and the applied voltage was in the two first measurements 0.9 Vbut was afterward increased to 1.0 V.

5.2 Membrane activation - Images and video

The design of the channel-stencil shown is a remake of the crescent shaped holes. The lidwas milled out of a PMMA (plexiglass) sheet. This implied that larger holes had to beused due to the limiting mechanical properties of PMMA.

During deactivation it can be seen that the membrane is going down instantaneous,long before the major part of the paraffin is solid, figure 5.2. Paraffin is melting frominside to rim.

The behaviour of polyimide membranes and steel membranes have a major difference.The polyimide membranes are flexible and thereby manage to seal along all of the cavity,whereas the sealing capability of the steel membranes are limited to the largest movementat the centre of the cavity.

5.3 Valves

The outlet membrane was pressurised for the five new designs by building up a pressureuntil either the membrane or the equipment failed. The pressure was in this case applied

25


200 250 300 350 400 450 500 550 600 650 7000

50

100

150

L56 − Pressurized from outside

Time [s]

Pre

ssur

e [b

ar]

Measurement 1, 50 µl min−1Measurement 2, 50 µl min−1Measurement 3, 20 µl min−1

500 600 700 800 900 1000 11000

5

10

15

20

25

L56 − Pressurized from inside

Time [s]

Pre

ssur

e [b

ar]

Measurement 4, 20 µl min−1Measurement 5, 20 µl min−1

Figure 5.1: Pressure measurement of the pump L56 (Old design). The membrane waspressurised from the two channel openings. The disturbances in the graphs were laterfound out to be due to the electrical field emitted from a nearby placed lamp.

from the outside, figure 4.4, and hence the joints of the pumps were loaded in a minimalway. During all of the valve measurements, the pressure was built up with the PharmaciaP-3500 pump at the rate of 10 µl min−1. All the valve measurements have been conductedwith the pump unclamped.

The outlet membrane of design I (L81) sealed at 0.75 V with a heater resistance of2.5 Ω, figure 5.3. No leakage was observed through the whole experiment, although asmall pressure drop was observed at 180 bar. At 200 bar the experiment was aborted dueto the risk of breaking the ferrules on the tubings. When the voltage was turned off themembrane was still sealing, preventing the pressure from going down. This self-sealingbehaviour was later pinpointed to occur between 35-45 bar. To release the pressure theflow sensor had to be opened.

Design II (L82) had a resistance of 2.2 Ω and sealed at 0.95 V. This valve only managedto withstand a pressure of 120 bar after the voltage was increased to 1.05 V. This was dueto a leakage between the ferrules and the pump. The ferrules were tightened, figure 5.4but the leakage in the ferrules was still present and no further investigations were madeto try to increase the pressure.

For design III (L80), the membrane was tested to seal at an applied voltage of 1.05 Vwith a resistance of 2.5 Ω, figure 5.5. The membrane was sealed during the whole measure-ment with no visible leakages observed. At 230 bar the ferrules were pushed off the tubeby the high pressure. This gave an upper limit of the rest of the pressure measurementsand experiments were aborted above 200 bar.


(a) (b)

(c)

Figure 5.2: Solidification of paraffin. (a) Activated heater. (b) The paraffin solidified atthe rim of the cavity and at the membrane. (c) Completely solidified.


0 5 10 15 200

20

40

60

80

100

120

140

160

180

200

L81

Time [min]

Pre

ssur

e [b

ar]

Flowrate 10 µl min−1 at 0.75 V

Figure 5.3: Valve measurement of the pump design I (L81). At 200 bar the measurementwas stopped due to the risk of damaging the ferrules at higher pressures. No leakages werenoticed although a strange bump occured in the measurement around 180 bar. When thevalve was deactivated the pressure remained constant.

0 2 4 6 8 10 12 14 160

20

40

60

80

100

120

L82

Time [min]

Pre

ssur

e [b

ar]


Figure 5.4: Valve measurement of design II (L82). The membrane started leaking justabove 120 bar and gave in at 125 bar.


0 2 4 6 8 10 12 14 16 18 200

50

100

150

200

L80

Time [min]

Pre

ssur

e [b

ar]


Figure 5.5: Valve measurement of design III (L80). The pressure was increased until theferrules burst at 230 bar. No leakage was observed during the build up.

Design IV (L84) sealed at 1.55 V with the resistance of 2.3 Ω and reached 200 barwithout visible leakage, figure 5.6. When the membrane was deactivated the pressuredropped instantaneous.

Design V (L83) sealed at 0.96 V and reached the limit of 200 bar, where the experimentwas aborted to save the ferrules, figure 5.7. The resistance was measured to be 2.1 Ω.When the valve was deactivated it was still sealing keeping the pressure constant andhad to be lowered to 15 bar to open the valve. The pressure was released by opening thepressure sensor.

The results of the pressure measurements on the valves are summarised in table 5.1.

Table 5.1: Summary of the different designs. *No data due to failures in the ferrules.

Design Maximum pressure [bar] Self-seal [bar]I >200 35-45II 120 (inside 103) -III >200 *IV >200 -V >200 15


0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

200

L84

Time [min]

Pre

ssur

e [b

ar]


Figure 5.6: Valve measurement of the pump design IV (L84). The membrane withstood200 bar and the experiment was aborted. When the valve was deactivated the pressuredropped instantaneously.

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

160

180

200

L83

Time [min]

Pre

ssur

e [b

ar]


Figure 5.7: Valve measurement of design V (L83). The membrane reached 200 bar and theexperiment was aborted. When the valve was deactivated the pressure remained constantand the pressure had to be lowered by opening the pressure sensor.


5.3.1 Voltage dependency

The outlet of design II (L82) was also pressurised from the inside, inducing higher stressesat the bonds between the top stencil and channel stencil, figure 5.8. Four measurementswere conducted varying the voltage. The valves sealed at the initial voltage of 1.0 V, witha measured resistance of 2.2 Ω. The voltage was increased to 1.3 V in steps of 0.1 V. Themeasured maximum pressure before leakage was increased from 40 bar to 103 bar. Thefourth measurement (not included in figure 5.8) resulted in a failure of the bond betweenthe channel stencil and the top stencil.

500 550 600 650 700 750 800 8500

20

40

60

80

100

120L82 Inlet pressurized

Time [s]

Pre

ssur

e [b

ar]

1.0 V1.1 V1.2 V

Figure 5.8: Pressure measurements for different voltages.

5.4 Pumping against back-pressure

The pumping experiments against an applied back pressure were conducted accordingto the method described in section 4.3. Both unclamped and clamped pumping experi-ments were conducted and evaluated. Although clamped pumping is preferred, problemsemerged when the clamping pressure was applied at the active parts of the pump. Thisresulted in that the channel stencil was pressed against the membrane, and thus blockingthe flow. To avoid this, the membrane was shaped using a stamp with circular bumps,pressed against the membrane foil before bonding.

5.4.1 Unclamped micropump

Initially, residual air in the channels of the pumps caused some problems during pumpingexperiments, resulting in problems with pumping against as low back-pressure as 1 bar.


This was solved by flushing ethanol through the pump.As in the pressure measurements the Pharmacia P-3500 was used to build up pressure

in the oppsite direction in relation to the pumping direction of the micropump. Twoexperiments were conducted using a frequency of 0.2 Hz.

At first the L82 was used driven with the voltages 1.3 V, 1.1 V and 1.1 V at theinlet, outlet and pump chamber respectively. The pump had a positive flow up to a back-pressure of 6 bar before the joints between the top-stencil and the channel-stencil broke,figure 5.9. The resistance of the heaters were measured to be 1.8 Ω, 2.2 Ω and 2.0 Ω forthe inlet, outlet and chamber respectively.

The second experiment used L83 and managed a back-pressure of 10 bar before thepump started to leak between the top-stencil and the channel-stencil, figure 5.10. Theapplied voltages was 1.1 V, 1.1 V and 1.3 V at the inlet, outlet and chamber respectively.The resistances were measured to be 1.9 Ω, 2.0 Ω and 2.0 Ω at the inlet, outlet andchamber respectively.

5.4.2 Clamped pumping

To examine the performance of the micropump two variables were evaluated, the appliedvoltage and the driving frequency.

Varying Voltage

Using the pump of design II (L87), with a shaped membrane to allow clamping withoutblocking the channels, a positive flow rate was observed for pressures up to 140 bar, figure5.11. Three measurement series were conducted with different voltage on the heaters,table 5.2. The pump operated at a frequency of 0.4 Hz at 19.5 C. The resistance of theheaters was for the outlet, inlet and chamber: 2.4 Ω, 2.2 Ω and 2.1 Ω.

The voltage on the heaters were initially set to 0.95 V and achieved flow rates upto 0.31 µl min−1. The flow rate decreased linearly with the applied pressure and above100 bar the measured flow rate was unreliable. Looking at the inlet tube it was confirmedthat the pump did not move any liquid in the positive direction above 110 bar.

When the voltage was increased to 1.05 V a maximum flow rate of 0.33 µl min−1 wasrecorded. The micropump managed a back-pressure up to 130 bar.

At a voltage of 1.15 V the maximum flow rate was further increased to 0.35 µl min−1.The pump managed a maximum back-pressure of 140 bar. When looking at the inlettube at 150 bar, the pump was seen having a high positive flow rate although the flowsensor registered a high negative flow rate, indicating leakage in the connections betweenthe pump and the flow sensor.

Varying frequency

Another group of measurements were conducted with the same pump (L87), figure 5.12.Using the highest voltage from the previous measurements (1.15 V), the frequency wasvaried between 0.4 Hz and 0.7 Hz at an operation temperature of 19.5 C. An increase


0 100 200 300 4000

0.5

1

1.5

2

L82− Pumped volume

Time [s]

Vol

ume

[µl]

(a)

0 100 200 300 4000

2

4

6

8

L82− Pressure

Time [s]

Pre

ssur

e [b

ar]

(b)

0 50 100 150 200 250 300 350 400−5

−4

−3

−2

−1

0

1

2

3

4

L82 − Flow Measurement

Time [s]

Flo

w [µ

l min

−1]

(c)

Figure 5.9: The pump result for L82. (a) Pumped volume (b) Pressure (c) Stroke


0 100 200 300 400 5000

0.5

1

1.5

L83− Pumped volume

Time [s]

Vol

ume

[µl]

(a)

0 100 200 300 400 5000

5

10

15

L83− Pressure

Time [s]

Pre

ssur

e [b

ar]

(b)

0 50 100 150 200 250 300 350 400 450 500−10

−8

−6

−4

−2

0

2

4

6

8

10

L83 − Flow Measurement

Time [s]

Flo

w [µ

l min

−1]

(c)

Figure 5.10: The pump result for L83. (a) Pumped volume (b) Pressure (c) Stroke


Table 5.2: The maximum pressure, power and flow rate with different voltages for thepump L87.

Voltage [V] Power [W] Maximum back-pressure [bar]

Maximum flowrate [µl min−1]

0.95 V 0.74 110 0.311.05 V 0.90 130 0.331.15 V 1.07 140 0.35

in frequency resulted in a linearly increased flow rate, with a maximum of approximately0.65 µl min−1 at 0.6 Hz, measured at the inlet capillary.

Driving the pump at 0.4 Hz the flow rate was with a 95% confidence interval estimatedto be 0.38±0.03 µl min−1 within 0-130 bar. At 0.5 Hz the flow rate was estimated tobe 0.49±0.03 µl min−1 within 0-80 bar. Between 0-100 bar for 0.6 Hz the flow ratewas estimated to be 0.63±0.05 µl min−1. At 0.7 Hz the pump managed to pump atatmospheric pressure at the rate of 0.28 µl min−1, deviating from the linear behaviourand was back-flowing already at 10 bar.

5.5 Flow and activation of a pump-cycle

The voltage applied on the different membranes were logged in a pumping experiment,using L83, and plotted in a diagram against the flow measured at the same moment, figure5.13. The voltages and flow were normalised in relation to their maximum values of themeasurement. It was seen that the activation of the membrane was not instantaneous,but delayed due to the required heating of the encapsulated paraffin.

5.5.1 Activation time of a steel valve

By applying a constant flow through the pump, the response of the flow was measuredwhen a membrane was activated, figure 5.14. Using equation 2.6, the time-constant τ

(response-time in a rough approximation) was calculated to be 1.1 s.

5.5.2 Deflection of outlet membrane

A deflection of 14 µm was measured at the outlet membrane of L82, figure 5.15. Eventhough the membrane was pressurised above 100 bar at several occasions, there werealmost no signs of permanent deformation. However, a small mark can be noticed wherethe membrane is in contact with the channel, being a sub-µm mark.


0 50 100 150−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4Pumping against applied pressure of L87

Pressure [bar]

Flo

w [µ

l min

−1]

0.95 V1.05 V1.15 V

Figure 5.11: The flow measurements for L87 at different applied back-pressures for threedifferent voltages at the heaters at 0.4 Hz.

0 20 40 60 80 100 120 140 160−0.2

−0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Pressure [bar]

Flo

w [µ

l min

−1]

Varying frequency − L87

0.4 Hz0.4 Hz − Flow sensor0.4 Hz − Measurement 20.5 Hz0.5 Hz − Flow sensor0.6 Hz0.6 Hz − Flow sensor

Figure 5.12: Flow measurements of L87 when varying the frequency at constant voltageof 1.3 V. The flow was measured both with a flow sensor and by measuring the time topump a certain amount of liquid through a capillary.


258 259 260 261 262 263 264 265 266 267 268−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

Time [s]

Nor

mal

ised

vol

tage

/flow

FlowInletOutletChamber

Figure 5.13: The normalised flow in relation to the activation of the three membranesfrom a measurement of the pump L83.

415 420 425 430 4350

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time [s]

Nor

mal

ised

vol

tage

/flow

Activation time

FlowVoltageτ

Figure 5.14: A magnified response of the activation of a membrane.


Figure 5.15: The surface topography of the outlet membrane of L82 using Wyko.

Chapter 6

Discussion

6.1 A small pump with great potential

The micropump in this thesis is the best of its kind concerning pumping against high backpressures. In comparison with other micropumps today, like piezoelectrical or thermop-neumatical pumps, utilising similar ideas, a pressure around 1 bar is considered a highpressure (figure 1.1). This micropump has pumped against a pressure of 150 bar.

However, there is one type of pump, the electroosmotic, that is capable of pumpingagainst back pressures higher than the micropump in this thesis. Electroosmotic pumpsrequire ionic working fluids in order to function. The pump developed in this thesis isto our knowledge the only micropump that can pump over 100 bar for potentially anysolution, which opens up the possibility to use the micropump in HPLC systems.

6.2 Placement of channels towards the membrane

The pumping experiments that reached back pressures above 100 bar were conductedusing design II. This design has both its holes at an equal distance from the centre of themembrane. This pump still has the symmetric features of the old design, and is able topump in any direction. However, if bidirectionality can be omitted, this is not the optimalhole placement considering high pressures. The best sealing is achieved at the centre ofthe membrane, and by locating the hole facing the outlet in the middle, the ability tohold against high pressure will increase even further. Design III, IV and V are all basedon this idea, but have not been fully evaluated due to the self sealing behaviour of thevalves.

Valve measurements have also indicated that a higher voltage has to be used whenpressurising the membrane from inside. Instead of compressive stresses on the bonds, thebonds will be pulled apart requiring the membrane to have a larger deflection to seal.When the membrane is pressurised from above, an additional support will be achieved bythe applied pressure, holding the structure together. The upper limit of the valves wasnot possible to test due to the limitation of the equipment. However, if clamped properlythe inlet valve and outlet valve should both manage similar pressures, well above 200 bar.

39


6.3 Material selection for the membrane

As a membrane, the ideal material would be a stiff material with a high yield strength.In general, polymers do not have this property, thereby lacking the ability to hold up thestrong force exerted by paraffin during its expansion when melted.

Two main candidates were considered: steel, and an alloy of titanium, grade 5 (Ti6Al4V).Grade 5 is a high strength material with half of the Youngs modulus compared to steel.This is a favourable combination, and attempts has been made to acquire this materialin a foil as thick as the steel foil used. However, one has to consider the price and avail-ability of the materials. It was soon obvious that grade 5 was too expensive to acquirein a suitable thickness, leaving steel as the most suitable high strength material at themoment. It should be added that after the membrane steel was changed from grade 304to grade 301, the membrane has never failed even once due to fatigue or tensile stresses.The earlier problems with plastic deformation has become negligible, due to the usage ofstainless steel (grade 301) membranes.

6.4 Steel compared to polyimide as a membrane

One of the old problems with the pump before the steel membrane was used was perma-nent deformation of the polyimide membrane. As previously mentioned, this resulted inan irregular stroke, but polyimide had one main advantage. Being flexible compared tosteel, the polyimide membrane allows the expanding paraffin to seal along the whole mem-brane making the placement of the holes towards the membrane less important. Whenusing the same hole placement with steel membranes, the membrane appeared to haveproblems of sealing towards the rim of the membrane, having most of its displacement inthe middle.

6.5 Motivation of the new designs

Liquids, including water are in general considered to be incompressible. This implies thatif a force is applied in one point of a container filled with water, all other points will feelthe same increase of force (Pascal’s law). Using this reasoning for the pump, one aspectof the initially bad performance in pumping against a back-pressure may be explained.

If the pump is considered to be a closed compartment with three outlets, each closedby a freely moving piston, the force applied by the back pressure will be transmitted tothe inlet chamber when the outlet membrane and channel membrane is not activated.To be able to pump the inlet membrane must withstand a higher pressure than the backpressure, otherwise the membrane will be pushed open, allowing the fluid to flow throughthe micropump in the negative direction.

Looking at the results from the first valve measurements, the half-moon shaped holesdid not manage pressures above 25 bar when pressurised from inside and when comparingthe the upper and lower graphs of figure 5.1, it was indicated that the location of the


holes towards the membrane plays an important role. This resulted in five new designsto evaluate.

However, the 20 bar limit was the theoretical upper limit of the old design, but forthe new designs, other problems made the actual capacity of the pump even worse.

6.6 Trapped air

The working principle of the pump is based on the assumption of a non-compressiblestructure, using a non-compressible working fluid. During normal pumping, using designII and IV, some troublesome discoveries were made. Although the valves of the inletand outlet could be pressurised well above 100 bar, the pump initially did not manageto pump against more than a back pressure of 3 bar at best. The negative flow ratewas pin-pointed to the position where the inlet valve opens after all three membranes areactivated. Just as the inlet valve opens, a water column shoots back. The water columnvolume seemed to be twice as large as the largest stroke measured. Consequently, thepump would, even under optimal conditions, only manage a forward stroke amountingto half the back-shooting volume, resulting in a negative flow rate. This was somewhatsurprising, as the valves could manage impressive pressures. Based on the fact that allthree membranes are activated, it seems unlikely that the water will originate from outsidethe outlet. Thus, the water must come from the interior of the pump.

Trapped air within the pump could explain this behaviour. Without pin-pointing theexact location of this air, it seems possible that the smaller space between the channels, atthe membrane and the channel stencil (due to the low initial deflection of the membrane)will have the highest flow resistance. The change from crescent-shaped holes to circularholes could also be one reason. However, this small gap will reduce the flow throughthe channel in the channel stencil, making it very hard to flush out the trapped air.When exposed to the external pressure outside the outlet, the air would be compressedby the pressure. After the outlet valve seal, the air will expand when the inlet opens, andthe pressure inside of the pump drops to atmospheric pressure. The compression of thetrapped air is proportional to the pressure, which would give a pressure-level where theexpanded air would push out liquid in the same rate as the stroke of a membrane, andthereby give the pump no pumping capabilities, or even negative flows.

Different approaches to solve this problem has been attempted (filling of water invacuum, flushing with the high viscous liquid PluronicTM known to wet steel surfaces)but what showed to be the best solution was flushing ethanol through the micropumpwhen the pump was placed in an ultrasonic bath. For the micropumps newer than L80this method was used and this procedure increased the pumping capabilities by a factorten, but also revealing other limitations.


6.7 Bonding between channel- and inlet-stencils

The major problem at the moment, seems to be insufficient bonding between the topstencil and the channel stencil. When examining L82 after the pump was disassembled,the parylene layer did not adhere to either of the steel surfaces, and was possible toremove in one unharmed piece, figure 6.1. The reason for this behaviour is at the momentunknown. The parylene-coating has previously proven to give a good bonding betweensteel surfaces, although some problems have occasionally appeared. For the most recentproblems there are some possible explanations: either a residual contamination from theetching of the stencils during manufacturing, or a process mishap during the coating ofthe parylene layer. There were some problem with the coating process, and the stencilsused in this project had been recoated to get the right parylene thickness.

If the problems in creating the parylene coating is neglected for a moment, the bondbetween the top stencil and the channel stencil is the most harshly treated joint in allof the structure. The fact that this bond always is the first to break (even though thewhole pump is bonded using parylene) points toward the relatively low stresses acting onthe other joints. If pumping against extreme pressures without clamping will be possible,this joint has to be strengthened. The pumping results of L87 (5.4.2) strengthens thisstatement. L87 uses the same design and stencils from the same batch as L82 with theexception of having a shaped membrane providing the possibility of using clamping whichrelieves the stresses at the joints. This improvement along with the removal of the trappedair made it possible to pump against back-pressures up to 150 bar.

Figure 6.1: When the L80 micropump was disassembled, it was found that the parylenelayer had a bad adhesion to the stainless steel. In a good bonding, the parylene is expectedto stick to both surfaces and be ripped apart when the stencils are separated.

6.8 Limitations in the equipment

As well as the challenge of building a strong structure, the task of testing the structureto its limit has shown to be as challenging. Previously, ferrules of polyetheretherketone(PEEK) has been used showing a prone behaviour of deformation by the threads of the


steel fixture. This resulted in constant leakage problems, and when tightened to hard, theflow would be blocked by the channel stencil pushing against the membrane. This wassolved by letting the workshop create new ferrules in steel. These proved to be a significantimprovement, by allowing much higher pressures to be applied, and the leakages in theconnections became much less frequent.

The improvement of the ferrules brought another problem to attention, previouslyhidden in leakages. The syringe pump used was unable to generate pressures above 60bar. This pump was replaced by a HPLC pump with a maximum back-pressure limit of400 bar.

With the ability of pressurising at extreme pressures, (above 100 bar) another limita-tion appeared when the membranes were pressurised in an attempt to break them. Beforeeven a slightest sign of breakdown in the membranes was seen, the fittings at the end ofthe tube were pushed off the PEEK tubes by the high pressure. This limited the pressuremeasurements to 200 bar to prevent damaging the fittings.

6.9 Expansion within the system

Looking at the pumping experiment of L83 (section 5.10), the stroke is seen to increasewith the increased pressure. This has usually been an expected behaviour with an airbubble within the channels. Before this experiment, the air within the micropump wasremoved, which was seen to be a success due to all air bubbles pouring out of the outlet.When taking the constant flow rate into account, problems with air seems unlikely. Usu-ally, an air bubble will cause the flow rate to decrease with increased pressure, which is notthe case in the experiment. Instead the increased strokes could be explained by a moreresponsive system at higher pressures. At lower pressures expansion within tubes andair may even out some of the flow reaching the sensor. However, this explanation coherebadly with the observations. When the pressure-limit was reached the pump started toleak in the bond between the channel stencil and the top stencil (see section 6.7). Thisimplies that the increased stroke may be caused by an expansion in the parylene bond,yielding a larger stroke due to the larger cavity above the membrane and possibility of themembrane to deflect over its relaxed position. Evidently the stresses in the bond becameto large resulting in a failure just below 10 bar.

6.10 Self-sealing valves

During the measurements of the new designs, a new and unexpected behaviour was in-troduced at fairly low pressures. Some of the valves became self-sealed, and managed towithhold the pressure when deactivated. Only a small declination of the pressure wasnoticed, probably due to a small back-flow in the external pump (P-3500) used to buildup the pressure, or a small leakage in some of the connections. There are currently twoseparate ideas explaining this behaviour:

The first, is that the high pressure is enough to partly redistribute the liquid paraffin


which will be pushed out towards the edges sealing the adjacent opening. When theheaters is turned off the paraffin closest to the rim and furthest away from the heaterswill solidify first, and the valve may withhold the shape even when the membrane isdeactivated.

The second explanation is that the high pressure pushes the channel and top stencilagainst the membrane, which will block the flow. This seems to be a less probableexplanation, because of the thick stencils used in comparison to the membrane thickness,and size of the features. There has been no detailed investigation of this problem, althoughit is a nice feature of the valves that they can be used as both active valves, but also havingthe possibility to seal when turned off. When used in this way, the usually highly energyconsuming valves may be an energy efficient solution.

6.11 Blocked channels

One major obstacle in achieving the high pressure pumping has been (beside bad bonding)the problems in using clamping. The clamping holds the pump together when pressurised,and prevents the micropump from bending with applied pressures. Looking at the crosssection of the pump (figure 2.1), the gap at the channel towards the rim of the membraneis small compared to the maximum deflection in the middle. One should consider thatthe structure shown in the picture is a polyimide membrane, which has a deflection fivetimes larger than the deflection of a steel membrane (typically the steel membrane isdeflected approximately 10 µm in the middle). At the rim, the deflection is much smaller(approximately just one or two µm at best), and it does not require much to seal thisgap. The problem with blocked channels was solved by shaping the membranes, allowingclamping to be used.

6.12 Plateau in pressure measurements

During the pressurisation of valves a strange drop of 1-5 bar appears in some of themeasurements. No leakages was noticed during that moment and afterwards the pressurebuild-up continues with the same inclination as before. What this movement is caused byis unclear although the best explanation is that the PEEK tube is moved in the connection.

6.13 The activation time

The activation time constant was estimated using the exponential function used for ex-plaining the discharge of a capacitor connected in series with a resistor. The time constantwas estimated to be 1 s. This is a relatively fast activation time, although there are valveswith an activation time of just a few ms. Again, speed is not the best feature of the mi-cropump, but rather the high pressures possible to manage. The design have on the otherhand a favourable scaling with size, and paraffin valves have been reported to be drivenat frequencies as high as 30 Hz [25]. The valves concerned in this thesis could for instance


be used in small hydraulic systems. The limit of the valves is still unknown, but paraffinis expected to manage pressures of at least 500 bar. However, if the flow rates would haveto be increased, several micropumps could be connected in parallel, giving the possibilityof high flow rates at high back pressures.

6.14 Frequency dependency

During the measurements of L87 (section 5.4.2), the pump stalled at 0.7 Hz being unableto pump at more than atmospherical pressure. When the frequency was lowered, thepump was still unable to pump with a constant stroke. When the pump was allowed tocool for a couple of minutes, the pump was able to operate again at 0.4 Hz, and anothermeasurement was conducted at that frequency. The flow rate was increased compared tothe first measurement. Probably due to harsh treatment of the pump, being pressurisedand forced to pump at pressures up to 150 bar several times. This could have slightlydeformed the membranes, which would have increased the pumped volume.

6.15 Future improvements

Most of the pumps have been pressurised several times at high pressures and survivedunharmed, sealing at the same voltage as before. There are occasions where the microp-ump have been running for days, and when examined afterwards showed to be relativelyunaffected. This indicates that except for some minor flaws, the micropump has a durabledesign, and when a micropump has failed, it has either been the bad bonding at the topstencil or a paraffin leakage at the heaters. The potential of the pump is great, and withsome adjustments the performance will be increased even more.

To even out the flow of the pump, several pumps could be connected in parallel. Themicropump has a well defined stroke, and would allow for a precise control of the dispensedamount of liquid with an even flow.

The driving sequence of the pump is far from optimised. Due to the many changesof design, and the requirement for a characterisation of each new pump, this would bepointless before the final pump-design is set. Due to this, the flow rates and back-pressurespossible to manage today is below the actual potential of the micropump.

To address the problem of being unable to use clamping, the location inlet holes can bechanged at the top stencil. By moving the holes from above the cavities, towards the edgesof the top stencil, hopefully the channels will be prevented from getting blocked when theferrules are tightened and the clamping is applied. Another interesting possibility willbe to glue fittings directly onto the top stencil, making it possible to evaluate the pumpwithout the usage of a fixture.

The unquestionably largest improvement of the pump would be to get rid of theparylene bondings and make the pump more rigid. This would probably allow the pumpto reach extreme pressures unclamped.

Chapter 7

Conclusions

The micropump was redesigned and pumping against back-pressures above 100 bar wasperformed, which was the aim of this thesis. Pressure above 100 bar is a prerequisitefor pumps that are going to be used in HPLC, and the potential of reaching even higherback-pressures has been showed during valve measurements.

The previous problem with plasticity of the membranes has been reduced. Even thoughpressurised above 100 bar several times, no severe damage of the membrane was noticed.

Five different valve-designs of the micropump have been measured to withstand pres-sures above 200 bar. Some of the new valve designs were shown to be self-sealing above acertain pressure when pressurised unclamped, managing to withhold the applied pressurewhen the voltage was turned off.

A new design of the micropump, with circular holes equally distributed from themembrane centres, was able to pump against applied back-pressures up to 150 bar withapplied clamping (applied support). Unclamped the pump was recorded to pump againsta pressure up to 10 bar.

The weakest link up to date in the micropump has been pin-pointed to be the bondbetween the channel stencil and the top stencil.

A major problem in the processing technique became apparent. Residual air-pocketsafter manufacturing initially limited the pumping capabilities to just a few bar. This wassolved adding an extra process step: priming the channel with ethanol.

To prevent leakages in the connection to the micropump when reaching high pressures,the PEEK fittings towards the pump had to be replaced by metal fittings. This enabledpressures up to 200 bar without leakages.

46

Acknowledgements

I would like to thank my supervisor Lena Klintberg for her unlimited source of patienceand wisdom in guiding and sharing the knowledge of science, and prof. Klas Hjort forgiving me this great opportunity to be a part of this project.

I would also like to thank Sam Ogden for the painstaking work of detoxicating mythesis aside from the support and guidance in the lab. I would like to thank GunjanaSharma for the help, sharing of ideas, and keeping the mood up in the lab. I would alsolike to send my gratitude to Ake in the workshop, and Janne for his help with all theelectrical problems.

I would like to thank GE Healthcare, Uppsala for financing the project, support, andmost of all for saving the day by lending us the P-3500 pump.

Finally I would like to thank the Division of Material Science, The Department ofEngineering Sciences at the Angstrom Lab for the welcoming and friendly environment,being a good foundation of creativity.

47

References

[1] Roger Boden, Klas Hjort, Jan-Ake Schweitz, and Urban Simu. A metallic micropumpfor high-pressure microfluidics. Journal of Micromechanics and Microengineering,18(11), 2008.

[2] D J Laser and J G Santiago. A review of micropumps. Journal of Micromechanicsand Microengineering, 2004.

[3] A. Nisar, Nitin Afzulpurkar, Banchong Mahaisavariya, and Adisorn Tuantranont.Mems-based micropumps in drug delivery and biomedical applications. Sensors andActuators B, 2007.

[4] Anders Olsson, Goran Stemme, and Erik Stemme. A valve-less planar fluid pumpwith two pump chambers. Sensors and Actuators, 1995.

[5] Carl H Hamann, Andrew Hamnett, and Wolf Vielstich. Electrochemistry, pages 128–130. Wiley-VCH, 2nd edition, 2007.

[6] Lingxin Chen, Yafeng Guan, Jiping Ma, Guoan Luo, and Kehui Liu. Applicationof a high-pressure electro-osmotic pump using nanometer silica in capillary liquidchromatography. Journal of Chromatography, pages 19–24, 2005.

[7] Lingxin Chen, Yafeng Guan, Jiping Ma, Xin Shu, and Feng Tan. Theory, controlsparameter and application of the packed-bed eletroosmotic pump. Chinese ScienceBulletin, 48:2572–77, 2003.

[8] Xiaoyue Zhu, Leonard Yi Chu, Bor han Chueh, Mingwu Shen, Bhaskar Hazarika,Nandita Phadke, and Shuichi Takayama. Arrays of horizontally-oriented mini-reservoirs generate steady microfluidic flows for continuous perfusion cell culture andgradient generation. Analyst, 129:1026–31, 2004.

[9] Daniel C. Harris. Quantitative Chemical Analysis, pages 713 – 718. W. H. Freemanand Company, New York, fifth edition, 2000.

[10] Marvin C. McMaster. HPLC A practical user’s guide, pages 3–9. John Wiley andSons, Inc., 2nd edition, 2007.

[11] Marek Noga, Filip Sucharski, Piotr Suder, and Jerzy Silberring. A practical guide tonano-lc troubleshooting. Journal of Separation Science, 30(14):2179–2189, 2007.

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[12] http://upload.wikimedia.org/wikipedia/en/4/4b/agilent1200hplc.jpg. 2009-09-02.

[13] Bernard A. Olsen, Bryan C. Castle, and David P. Myers. Advances in hplc technologyfor the determination of drug impurities. TrAC - Trends in Analytical Chemistry,25(8):796 – 805, 2006.

[14] Roger Boden, Marcus Letho, Urban Simu, Greger Thornell, Klas Hjort, and Jan-Ake Schweitz. A polymeric paraffin actuated high-pressure micropump. Sensors andActuators A, 2006.

[15] Roger Boden. Microactuators for Powerful Pumps. PhD thesis, Uppsala University,2008.

[16] Stefan Johansson. Actuator materials and microactuation. Department of Engineer-ing Sciences, Uppsala.

[17] Carl Nordling and Jonny Osterman. Physics Handbook for Science and Engineering,page 183. Studentlitteratur, Lund, seventh edition, 2004.

[18] F Goldschmidtboing, A Doll, M Heinrichs, P Woias, H-J Schrag, and U T Hopt. Ageneric analytical model for microdiaphragm pumps with active valves. Journal ofMicromechanics and Microengineering, pages 673–683, 2005.

[19] Carl Nordling and Jonny Osterman. Physics Handbook for Science and Engineering,page 224. Studentlitteratur, Lund, seventh edition, 2004.

[20] Lennart Rade and Bertil Westergren. Mathematics Handbook for Science and Engi-neering, page 74. Studentlitteratur, Lund, fifth edition, 2004.

[21] Carl H Hamann, Andrew Hamnett, and Wolf Vielstich. Electrochemistry, pages 227–230. Wiley-VCH, 2nd edition, 2007.

[22] Clemens van Blitterswijk. Tissue Engineering, page 256. Academic Press, 2008.

[23] Jr William D. Callister. Materials Science and Engineering An Introduction, pages211–214. John Wiley and Sons, Inc., 2003.

[24] Mario Di Giovanni. Flat and corrugated diaphragm design handbook. Marcel Dekker,inc, 1982.

[25] E.T. Carlen and C.H. Mastrangelo. Surface micromachined paraffin-actuated mi-crovalve. J. Microelectromech. Syst. (USA), 11(5):408 – 20, 2002.

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