Microfluidics: Electrokinetics and Fabrication

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Microfluidics: Electrokinetics and Fabrication

Dr. Thara SrinivasanLecture 18

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Lecture Outline: Part I

• Reading• From S. Senturia, Microsystem Design, Chapter

13, “Fluids,” p.339-349.

• Today’s lecture, Part I• Electrolytes and Electrokinetic Effects• Electroosmosis• Electrophoresis

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5Electrolytes

• Electrolytes, solutions of ionic species, have unique flow possibilities because electric fields can be used to direct the flow.

• Total charge density of solution and charge density far from bounding surfaces

• Charge density near a wall

( ) 02 =∇∑= φρ andCqzi

ieie

ερφ e−=∇2

∑−=∇−

−

i

TBkoeqiz

iie eCzq )(

02

φφ

εφ )( yφφ =

y

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Ionic Double Layers• Electrolyte in contact with insulating

solid surface• Inner and outer Helmholtz layers are

compact layers of adsorbed ions• Diffuse layer of ions has compensating

net charge

+

- - - - - - - - - -+

++ ++++

++

- --

+ +

++

- -+

Bulk solution (+ = -)

Diffuse layer (+ > -)

Compact layer (+ < -)

potentialzetae

w

DLy

w

,

ˆ

ςφφφ

==

−

3

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5Potential

• For small potential variations, expand exponential • Reference location is in neutral region• Variation in potential of ionic solution has exponential

dependence

=

∑==∇

≈∇

−=∑−=∇

−

−

φ

εφ

φ

φφφε

φφ

ˆ

ˆ

ˆ

ˆ,ˆ

02

212

2

0

ˆ

02

iii

B

ed

i

TBkeqiz

ieie

CzTkqL

eCqzq

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Debye Length

• Debye length (LD) is decay length ~ back to neutral 3 LD from wall• Ionic strength ↑ LD ↓• LD in pure water _____, in 1 mM KCl _____, in 1M KCl

_____.• MEMS channels typically much wider than LD: ______.

∑=i

iiB

e

d

CzTkq

L 02

21ε

4

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5Electroosmosis

• Diffuse double layer has net charge• Electrostatic body force

applied tangentially to diffuse double layer drags fluid along wall

• Glass channels have negative charge (Si-OH-), diffuse layer has net positive charge and shows net flow towards cathode

• Surface-mediated flow

+

- - - - - - - - - -

+++ +

+++++

- --

+ +

++

- -+

Bulk solution

Diffuse layer

Compact layer

Cat

hode

(-)

Ano

de (+

)

Ele

ctric

Fie

ld

glass

Diffuse layer (-)(+)

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xexx EU

dxdP

DtDU ρηρ +∇+−= 2

Electroosmotic Flow• E field (up to 1 kV/cm)

• Ex is applied voltage divided by fluidic path length• Does not disrupt double layer

3LD

U0

DLy

D

xwx

DLy

D

we

eLE

dyUd

eL

−

−

=

−=

ησ

σρ

2

2

5

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5Electroosmotic Flow

• No-slip at channel walls• Plug flow profile for 99.9% of cross section

• Typical experiment, double layer thickness δ = _____.• No shear throughout most of volume

3LD

U0

xEOFDxw

DLy

Dxwx

EUandLEU

eLEU

µη

ση

σ

==

−=−

maxmax

1

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Electrophoresis• Ionic species such as DNA,

protein segments and amino acids migrate under E-field

• For particle with electrophoretic mobility µEP steady-state speed UEP reached when accelerating force is balanced by frictional force from medium

• Generally µEOF > µEP (for SiO21.25 -3×)

• Since different chemical species have different µEP it is possible to separate them while they are being carried by electroosmoticflow

rq

rfandUfqEEU

EP

EPx

xEPEP

πηµ

πηµ

6

6

=

===

q = r =η =

6

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5Chip Electrophoresis Set-Up

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Chip Electrophoresis

• Sample injection

Sample well V1 (+)

Sample Waste V2 = 0

Buffer Well V3 > VJ

Buffer wasteV4 > VJ

Junction

Separation channel

1

11

121

2

AL

R

VRR

RV

e

toJ

J

σ=

+=

7

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5Chip Electrophoresis

• Sample flow

Sample well V1 < VJ

Sample Waste V2 < VJ

Buffer Well V3 (+)

Buffer wasteV4 = 0

U0

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Chip Electrophoresis

• Sample flow and separation

Sample well V1 < VJ

Sample Waste V2 < VJ

Buffer Well V3 (+)

Buffer wasteV4 = 0

U0

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Fabrication Techniques for Microfluidics

Picture credit: Fluidigm

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Lecture Outline: Part II

• From reader:• Becker, H and Locascio, L., “Polymer Microfluidic Devices,”

Talanta, Vol. 56, 2002, pp. 267-87.• Boone, T. et al., “Plastic Advances in Microfluidic Devices,”

Analytical Chemistry A, February 2002, pp 79-86.

• Today’s lecture, Part II• Replication vs. Direct Techniques• Polymer Replication Techniques• Master Fabrication• Electroplating• Back-End Processes

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Materials for Microfluidics• Important material properties for microfluidic chips

• Micromachinability• Surface charge • Molecular adsorption• Optical properties

• Why polymers?• Polymer granules: _____ cheaper than Si, glass or quartz• Replication processes suitable for automated production

• Injection-molded CD costs: ___________.• Relatively low capital cost compared to silicon IC equipment

• Benefits• Biocompatible, plastics molding is well-developed technology, may not

need coating to suppress EOF, chips may be cheap enough to be disposable (better, recyclable)

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Replication vs. Direct Techniques• Direct fabrication: each part must be made separately

+ Can create certain mechanical structures difficult to create with molding

– Expensive in materials and labor– Routine access to cleanroom for fab and bonding± For economical use, re-use chip

• Replicated microsystems:

+ Cost-effective in materials, continuous processing possible with higher yields and reproducibility

± Back-end processes required for each product; may be able to do wafer level assembly

± Chips may be single-use– Moving parts difficult to create

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5Choosing a Chip Material

• Silicon ~ for special research applications

• Glass ~ for research, or if you can use existing chip, or for organic solvents or high temperatures

• Thermoplastics ~ general industry trend for high-volume, disposable chips

• Elastomers ~ for research applications, quick turnaround with rapid prototyping

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Direct Fabrication• Typical glass microfabrication

process• Etch mask deposition (Cr or Au thin

film) and patterning• Etch mask etching (KI/I2 for Au or

K3Fe(CN)6/NaOH for Cr)• Substrate etching with dilute HF/NH4F• Ultrasonic drilling of access holes into

the cover plate before bonding• Thermal bonding of glass cover plate to

seal microchannels • Attachment of “sipper” capillaries into

the chip reservoirs• Coating of glass channel walls to

suppress EOF Caliper Technologies

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5Direct Fabrication

• Jensen group, MIT • DRIE on silicon substrates

• Mastrangelo group, U. Michigan• Sacrificial etching process to fabricate parylene-C polymer

channels• Rossier et al., École Polytechnique, Lausanne

• Plasma etching of channels into polyimide substrate

• Prof. Luke Lee, UCB• Low energy ion beam etching (Ar+) to make high aspect ratio

Teflon structures• Micronics, Redmond, WA

• Laser cutting of channels on thin film laminate

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• Replication vs. Direct Techniques• Polymer Replication Techniques• Master Fabrication• Electroplating• Back-End Processes

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5Polymer Replication

• Molding process• Fabricate a mold insert • Fill the mold with polymer• Harden the polymer • Demold the piece

• Polymer microfluidic chips• Credit card size to 10×20 cm2,

0.5-2 mm thick• Microchannels:

• Reservoirs: 1-3 mm in diameter, drilled through card (0.2-15 µL)

• Types of polymers• Thermoplastics ~ polystyrene,

polycarbonate• Elastomers ~ silicone• Thermosets ~ epoxy, acrylic

Aclara Biosciences

Gyros

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• Casting• Unpolymerized compound is poured into mold

and allowed to polymerize

• Hot embossing• Molding compound is introduced into an open

mold and formed under heat and pressure

• Injection molding• Heat-softened plastic resin is forced into a

mold cavity by high pressure

Polymer Molding Techniques

Åmic

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5Important parameters in polymer molding.

Glass transition temperature

Temperature range at which polymer changes from hard glassy material to soft (not melted) material

Melt temperature

Thermal expansion coefficient

Change in length or volume accompanying change in temperature

Hardness Measured using indentation at a specific applied force

Elasticity Ability of polymer to return to original shape after deformation

Temperature at which polymer melt flows, generally much higher than Tg. Some polymers do not melt but remain soft until they decompose.

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Casting

• “Soft lithography”• Dow polydimethylsiloxane (PDMS) is elastomer of choice• PDMS is poured into silicon stamp and allowed to cure at 40-70°C• Once cured, polymer is peeled off stamp• Bonding may be done using simple conformal contact to plastics,

glass, or reaction between two PDMS layers• Multilayer structures possible

Fluidigm

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5Multilayer Soft Lithography

• Rapid turnaround time possible – good for academic research • Demonstration at Fluidigm website

Whitesides group, Harvard

http://www.fluidigm.com/tech.htm#

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Hot Embossing

• Imprinting tool: silicon stamp or metal electroform

• Hard plastic sheet is stamped in a hydraulic press

• P maintained for minutes, T just above Tg to soften plastic

Mild

endo

, Ger

man

y

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5Injection Molding

• Process• Metal electroform mold made

from silicon or glass master• Electroform mounted in mold

insert• Molten thermoplastic polymer

injected into warm mold at high T and P

• Once part cools and solidifies, mold is opened and part is ejected

• Small features and low aspect ratios easy; high aspect ratios harder

Aclara

Steag Lilliput chip

• Injection molding vs. Casting• Injection molding gives

rapid cycle time• Casting gives best

replication fidelity

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• CD fabrication+ Standardized

handling/automation+ Superior flatness, low

thickness variations, high precision in multilevel depths

+ Multiple designs on single tool– Final part usually obtained by

dicing– Limitations due to circular

format–

• CD features• Injection-molding of

PC:

• After molding, disc is metallized and lacquered

• Pits of 9 lengths, smallest is:

• 2 billion pits per disc

Åmic

Gyros

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5Important parameters in polymer chip operation.

Electroosmotic pumping

Controllable surface charge, good insulator for HV operation (dielectric strength, electrical resistance)

Joule heating Thermal conductivity

Chemical properties Chemical resistance, permeability,analyte adsorption and biocompatibility

Optical detection High transmission and low background fluorescence

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Plastics Pro and Con

• Varying surface charge and charge density• Depends on polymer type and fabrication process

• For example, laser-ablation gives higher EOF than hot embossing• Location of charge in PMMA different for casting vs. hot embossed

channels• Can control using surface treatments or may not be necessary

• Background fluorescence• Biggest detection concern working with plastics and short-

wavelength excitation (488 nm)• Confocal epifluoresence rejects background• Can use red or near IR-absorbing fluorophores

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5Master Fabrication

• Fabricate master to serve as mold insert or precursor to electroplated mold insert• Batch techniques: photolithography plus etching• Serial techniques such as mechanical micromachining

• Serial techniques• Laser cutting• Laser-assisted chemical etching• Micro electro discharge machining (µEDM)• Stereolithography

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Batch Techniques• Photolithography plus etching

• Wet etching ~ isotropic or anisotropic channel profiles

• Dry etching ~ plasma or DRIE processing

• Patterned photoresist ~ SU-8 epoxy

Åmic

Si masters

SU-8 features

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5Fabrication Techniques for

Microfluidics


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Electroplating• Set-up and operation

• Anode, cathode, aqueous-metal solution, and power supply.

• When power is on, cations in solution are attracted to cathode. At the cathode, Ni+2

gain e-‘s and are deposited onto the cathode surface.

• Simultaneously, Ni is electrochemically etched from the anode to give ions for the aqueous solution and electrons for the power supply

• H+ also gain e-‘s from the cathode form bubbles of H2 (g)

• Fabrication details• Conductive seed layer

needed

Judy

→ →

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5Electroplating

• Film thickness not limited by mask thickness (PR thickness)

• Plating mask must achieve good step coverage • Adhesion layer is needed (Ni adheres poorly to Si)

CJ Kim group, UCLA

Nickel Plating Solution

Material Quantity

Nickel Sulfate 200g/L

Nickel Chloride 5 g/L

Boric Acid 25 g/L

Saccharin 3 g/L

Judy

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LIGA• LIGA stands for lithography, electroforming and molding (in German)• Deep X-ray lithography used; need synchrotron source • Metal electroform may be used as mold tool for plastics molding

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5Fabrication Techniques for

Microfluidics


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Back-end Processes

• Tasks• Drilling sample wells• Surface modification• Electrical interconnect• Bonding of a sealing layer• Machining ~ drilling and dicing

• Drilling sample wells • High cost step - done by inserting pins into mold tool• Definition using etching instead is desirable

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5Surface Modification and Interconnect

• Surface modification to tune hydrophobicity/hydrophilicity and control sample adsorption• Wet chemical techniques• Plasma treatments

• Electrical interconnect: metallization• Insert metal pins into reservoirs• Evaporation, sputtering• Plating• Screen printing of conductive ink

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Sealing

• Sealing microchannels• Silicon and glass ~ fusion or anodic bonding• Thermoplastics

• Low temperature thermal annealing using same polymer or one with lower Tg with or without P

• Pressure-sensitive adhesive

• Elastomers • Reversible seal upon contact• Permanent seal using a chemical reaction, Fluidigm, or

by plasma oxidation, Whitesides group

Microfluidics: Electrokinetics and Fabrication

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