Fabrication of PDMS Microfluidic

Devices

James M. Spotts

2008 Microfluidics

CourseInstitute for Systems Biology

November 17, 2008

Have fun, ask questions and play!

Some background regarding PDMS Microfluidics

•

“Early”

microfluidic

devices made from hard materials (e.g. glass, silicon) using MEMS (microelectromechanical

structure) techniques

•

Used extensively in microelectronics industry –

mature field

•

Etching or additive deposition of ceramic material (UHV Techniques)

•

Expensive equipment

•

Large practical/financial barriers to rapid prototyping

•

Difficult to make

mechanical

seals/valves

•

High Pressures

or Electrophoresis

was used to move fluid within channels

Channel Crossing Creates a Valve

Historic Limitations of “Early”

Microfluidics

(Pre Mid-1990s)

•

Fluid moving within micron-scale hard wall channels experience very large

fluidic resistance to motion

•

Redwoods among best engineered fluidic systems on earth

•

Able to move water 300 feet vertically against gravity

1st

Breakthrough for PDMS Microfluidics

Occurred in Mid-1990s

George Whitesides’

Group (Harvard)

•

Adapted one of the simpler steps of MEMS process (photolithography) to dramatically simplify the fabrication of microfluidic

devices

•

Used photolithographic techniques to pattern microfluidic

networks onto silicon wafer to create a (+) mold

•

Cast (-) replica of fluidic network using silicone rubber (PDMS)

PDMS = Poly(dimethyl)siloxane

PDMS

Photoresist

Silicon

•

Replica molding of micron-sized microfluidic

channel features

•

Many useful applications can be implemented quickly and inexpensively

•

Course example: Dynamic Chemical Gradient chip

•

Flexible, but mechanically strong (rubber)

•

Polymer has excellent working properties (Sets at RT in 4-6 hours)

•

Transparent (240 nm to 1100 nm) –

Useful for microscopy

•

Highly gas permeable -

Air Is easily displaced from fluidic network through the device by low pressure

Can blind-fill dead end channels

Advantages of PDMS for Microfluidics

Outstanding for aqueous media applications

Blind-Fill Dead-End PDMS Microfluidic

Channel (100 μm Wide x 25 μm High)

Air being forced out through the PDMS material

Otherwise impossible with purely “hard wall”

microfluidics

Real-Time

• Highly

permeable to water vapor (Evaporative losses esp. at high temp)

• Very Hydrophobic –

Non-Ideal substrate for cell culture

• Surface chemically inert (C-H bond very strong)

• Not compatible with organic solvents (PDMS swells) –

Limits assays

Unfavorable Properties of PDMS

(Unger et al, 2000)

Unger MA, Chou HP, Thorsen

T, Scherer A, Quake SR.Science. 2000 Apr

7;288(5463):113-6.

Breakthrough #2PDMS-Based Microfluidics

Using Multilayer Soft Lithography

Channel Crossing Creates a Valve

Key Development:

•

Figured out how to bond different layers of “cured”

PDMS together to form overlapping stacks of fluidic channels

•

Region of overlap formed a valve

Steve Quake’s Group (Caltech, now Stanford)

How To Bond Together PDMS Layers?

Mark UngerSteve Quake

•

Normally, use Catalyst : Base at ratio of 1:10

•

Used “Off Ratio”

protocol to bond different PDMS layers

Partially Cure“Thick”

Layer Using1:5 Catalyst : Base

Partially Cure“Thin”

Layer Using1:20 Catalyst : Base

•

Then layer the 2 partially cured 1:5 and 1:20 components

•

Belief: Interface will be 1:10 on average

•

Likely that excess catalyst from 1:5 layer rapidly diffuses thru thin (25-45 mm) 1:20 layer(s)

Curved

ProfileChannel Network

Rectangular ProfileChannel Network

~35-μm high

Sealing Substrate

What do you gain by layering channel networks?

The ability to create valves

Valve actuation

by pressurizing

fluid(H2

O, Buffer, Glycerol, Fluorinert

FC-40)

ΔP↑

Thick layer (~0.3 -

0.5 cm)

Thin Membrane (10 μm)

Cross-Sectional Representation

•

Easily fabricated valves and peristaltic pumps

•

Density of hundreds of valves / cm2

•

Fast actuating, pneumatic or hydraulic valves requiring low actuating pressures (<25 PSI)

•

Cheap & easy

fabrication

techniques

(Microfluidics

not just for hard-core engineering labs anymore)

Key advantages of PDMS multilayer soft lithography

Revolutionized Microfluidics

Optimal PumpPattern

New 4-Stroke Pump: POWERFUL

“Pinch & Push”Pattern

Truly Peristaltic

•

35-μm

High AZ-50XT Channels –

Greater Volume Displacement/Pulse

•

Air Bubble in Channel Does Not Stall Pumped Fluid Motion

“Nuts and Bolts”

of PDMS device operation and fabrication

RTV-615 (GE)

& Sylgard

184 (Dow)

Pt-CatalyzedHydride Transfer Reaction

Exact polymer composition for both is unknown (proprietary)

Therefore, difficult to troubleshoot material-related issues

2 Types of PDMS used for MSL

Advantages / Disadvantages of RTV-615 versus Sylgard

184

1)

RTV-615 (GE)

•

Favored for MSL (Quake)

More robust layer-to-layer bonding

Easier to establish layer-to-layer bonding protocol – less finicky wrt bake times

•

Poor quality control (reputation as being “dirty”)

Batch-to-batch variability in bonding characteristics

Therefore, new fabrication parameters need to be developed for each new batch purchased (advice: buy in bulk)

Anecdote: Fluidigm throws out 90% of RTV-615 that it receives

2)

Sylgard

184 (Dow Corning): The “cleaner”

of the 2

•

Less widely used for multi-layer devices –

often single-layer channels

Layer-to-layer bonding protocol more difficult to establish

More fabrication failures

•

Used in most papers reporting mammalian cell culture in PDMS devices

2 Types of valve actuation schemes used in MSL devices

1)

“Push-up”

2)

“Push-down”

•

3 course designs are all ‘Push-down”

•

What does this mean?

“Flow”

Layer (1:5)

“Control”

Layer (1:20)

“Bonding”

Layer (1:20)

Glass Substrate

Valve Actuation Scheme: Push-up Design

Valve Actuation by Pressurizing Fluid(H2

O, Buffer, Glycerol, Fluorinert

FC-40)

ΔP↑

Pinch-Point

Top “Thick”

layer is the Flow

Layer with Rounded

Channels

Cross-sectional Representation

Valve Actuation Scheme: Push-down Design

Valve Actuation by Pressurizing Fluid(H2

O, Buffer, Glycerol, Fluorinert

FC-40)

Features: May or may not use 3rd

bonding layer.

Flow layer-to-glass bonding poor unless use O2

plasma bonding

Valve actuation less robust relative to “Push-up”

style

ΔP↑

“Flow”

Layer

“Control”

Layer

Glass Substrate

Top (thick) layer now the “Control”

layer

Cross-sectional Representation

What will you be doing in the practical today and tomorrow?

You will be issued Two 4”

Diameter Molds to fabricate 3 “Push-down”

devices in duplicate

Control Mold –

Thick PDMS Layer Flow Mold –

Thin PDMS Layer

25-μm high rectangular profile 13-μm high curved profile

1

1

2

2

3 3

1

1

2

2

3 3

Cautions Regarding Molds

•

Use wafer tweezers when handling wafers whenever possible, or hold wafer by outside edges

•

Do not touch photoresist

features. They can be easily damaged.

•

Silicon wafer, while strong, is brittle•

Will crack if apply too much pressure (i.e. when cutting with scalpel, when peeling away thick PDMS layer)

•

Do Not Drop•

Wafer may crack, or the photoresist

can become scratched/damaged

•

DO NOT clean molds with solvents, even water•

FLOW Mold photoresist

(SPR-220) is very soluble in virtually any organic solvent

•

Control Mold (SU-8) more amenable to cleaning, but avoid doing so

•

Wafers are cleaned by baking on thin layer of 1:10 PDMS then peeling away

How to fabricate a PDMS microfluidic

device

Step 1: Cast the 2 Molds

“Control”

Layer1:5 Base : Catalyst

“Flow”

Layer1:20 Base : Catalyst

•

1:5 layer will need to be “de-gassed”

under vacuum for about 1 hour

•

Partially cure both at 80°C for 1 hour 15 minutes

Pour PDMS over moldto thickness of 3–5 mm

Spin-Coat PDMS ontomold to thickness of

~25-30 μm

“Control”

Mold

“Flow”

Mold

Spin-coating –

just what it sounds like

Example of Spin Curve

•

Pour 1:20 PDMS onto “Flow”

mold, then spin both in spin-coater at defined rpm to create PDMS layer of desired thickness

•

Excess PDMS gets spun-off the mold wafer

Can determine spin-curve by:

1)

Cutting apart completed devices and measuring features on confocal

2)

Weighing the amount of PDMS on wafer after spinning at given speed •

(know density of PDMS and area, therefore can solve for height)

Step 2: Cut out and peel away thick “Control”

layer & align to thin “Flow”

layer

You will have lots of opportunities

to practice the alignment step

Alignment marks have been placed within each design for assistance

Note: This is NOT an all or nothing step

Layers will be peeled apart and set back down multiple times to “walk”

the layers into proper alignment (takes practice…)

Line up crosses in “Flow”

layer within crosses in “Control”

layer

Goal: Have “Control”

valves properly placed wrt

“Flow”

channels

Alignment Marks

Step 3: Bake aligned devices overnight at 80°C

Everyone should aim to get to here by the end of today

Finishing the device: Creating the lab-to-chip interface

“Flow”

and “Control”

LayerAligned and Bonded Together

Flip Upside-Down

Punch MarkImpressionsFrom Molds

Punched Holes(Thru-Holes)

Step 4: Punch 64 μm (0.025”) diameter holes for fluidic connections

Peel away from “Flow”

wafer and

“Control”punch marks

“Flow”

punchmarks

Note: I usually punch holes with the whole 4“

diameter PDMS slab intact. You can also cut out each individual device before punching

Aim to punch just downstream of the punch marks

Step 5: Dice all 6 devices

Step 6: Seal punch holes by O2

plasma bonding devices to glass microscope slides

This is a one-shot deal –

You cannot reposition device on glass once they touch

Bottom surface facing up

Expose both glass surfaces and bottom surface of device to O2

Plasma

Touch the treated surfaces together to create a strong, permanent bond

Insert 0.025”

OD pins connected to Tygon

tubing containing water

After baking plasma bonded device for minimum of 4 hours, you are ready to hook up your chips

Brief overview to the course designs you will be fabricating

Overview of how molds are designed and fabricated

AutoCad

Design

Fabricate PDMS DeviceCreate Masks/Fabricate Molds

(Requires Cleanroom)

Flow Layer Mask

Control Layer Mask

Design to Device

20,000 dpi Transparency

Complete 4”

wafer design

Common to designchip using AutoCAD

Draw in “real-space”

units (microns)

Each photoresisttype/height drawnon its own layer(think transparency)and is assigned a unique color

5”

square border for mounting onto glass plate

Device Design

Each “transparency”

will become its own mask

(-) Resist (SU-8)Square Profile1.4 μm HighFlow Layer

(-) Resist (SU-8)Square Profile28 μm HighFlow Layer

(-) Resist (SU-8)Square Profile25 μm HighControl Layer

(+) Resist (AZ50-XT)Round Profile27 μm HighFlow Layer

AutoCAD design for each layer is converted to a binary line drawing

Shading Alternates Light –

Dark from the Outside of the 5”

Border InPhase Changes Upon Crossing Closed Contour

2 types of photoresists: Positive and Negative

•

Positive Resists

•

Whatever surface is exposed to UV light will be removed

during subsequent development step

•

Used for “Flow”

layer molds to produce rounded channel profiles

•

Negative Resists

•

Whatever surface is exposed to UV light will be retained

during subsequent

development step

•

Used to produce rectangular channel profiles

How to create a mask? Print out as 20,000 or 40,000 dpi transparency•

E-mail binary line drawing for each layer to printer•

File is converted to mask•

Shading alternates light –

dark from the outside of the 5”

border to the center-most nested feature

•

Black/white phase changes upon crossing closed contour

Positive Mask

How to create a negative mask?

Double Border Changes Phase

Negative Mask

Fabrication of Microfluidic

Molds by Multi-Step Photolithography

4”

Diameter Elemental Silicon Wafer Spin-Coat with Shipley SPR-220Photoresist, and Soft-bake

Align Mask and UV-Expose Resist

Develop SPR-220 Photoresist Re-Flow SPR-220 Photoresist, and Burn-in Features

Post-bake SPR-220 Photoresist

Spin-Coat with SU-8 100 Photoresist,Align Mask and UV-Expose Resist

Develop SU-8 100: Mold Completed

13 μm

100 μm

Can Fabricate Channels of Multiple Profiles and Heights on a Single Mold

Something you must, must, must

do when creating a mask for the thick PDMS layer

•

Thick layer features will

shrink by 1.5%

when thick layer PDMS is peeled away from the mold

•

Therefore, the mask design(s) for the thick layer is/are SCALED UP by 1.5%

(scale features by multiplying by 1.015)

•

“Push-Down”

Design: Scale Control Layer

features (1 Mask)

•

“Push-Up”

Design: SCALE ALL FLOW LAYER MASK FEATURES

Implication: Molds for “Push-down”

devices CANNOT be used to make “Push-up”

devices and vice-versa

Have fun, ask questions and play!