Introduction to Microfluidics By LNF Microfluidics ... · Introduction to Microfluidics By ... 0.5mg/√Hz noise floor ... Peel off and cut 4. Bond to glass 1.17. PDMS micromolding

LNF Microfluidics workshop 2014

Introduction to Microfluidics

By

Pilar Herrera-Fierro

3/10/2014 LNF Microfluidic Workshop 2014

• Intro to BioMEMS

• Surface properties and modifications

• Soft Lithography

• Making the mold: SU-8 or Si

• metal deposition

• Basic microfluidics design considerations


EECS 509 BioMEMS

What Are MEMS Microsystems?

• Micro Electro Mechanical Systems Are Miniature, Multifunctional Microsystems

Consisting of Sensors, Actuators, and Electronics.

They Are Built Using Micromachining Technologies.

• Micromachining Is an Enabling Technology That

Allows Formation Of Physical, As Well As

Electronic, Devices.

• Micromachining Uses Many of the Standard

Silicon IC Fabrication Techniques.

EECS 509 BioMEMS

Surface-Micromachined Acceleration Sensor (Accelerometer) For

Air Bag Deployment, Manufactured by Analog Devices, Inc.

Photos Courtesy Analog Devices, Inc.

~1

-2m

m

ADXL05:

±5g operating range

1000g survivability

0.5mg/√Hz noise floor

Only a few $!

• Undercut polysilicon shuttle mass

• Differential capacitance sensing

• Force-balanced operation

EECS 509 BioMEMS

BioMEMS

• Implementation of MEMS (Micro Electro Mechanical

Systems) to Bio-related areas

• Fluid delivery system at micro/nano-liter scale

• Multi-physics, multidisciplinary and cross-fields

www.calipertech.com BioMEMS, Lab-on-a-chip

EECS 509 BioMEMS

BioMEMS

• Biomedical MEMS

• Biosensors

• Biotelemetry

• Drug delivery

• Precision surgery

• Minimally-invasive

therapy

• Physical sensors

Deals in vivo with the host

anatomy

• Biotechnological MEMS

• Gene sequencing

• Functional genomics

• Drug discovery

• Pharmacogenomics

• Diagnostics

• Pathogen detection

Deals in vitro with the

biological samples of the

host

Future BioMEMS: Combination of MEMS for in vitro Diagnostics with in vivo Therapy

- Biology Perspective -

EECS 509 BioMEMS

Applications of BioMEMS

Advancement in molecular biology Have brought medical research into molecular level

Advancement in nanotechnology Manipulation of scale in molecular size possible

The applications

- Biological analysis

- Medical diagnosis

- Antigen/Antibody screening

- Chemical analysis and synthesis

- Drug discovery

- Drug screening

- • • • •

EECS 509 BioMEMS

Biomedical Applications of MEMS

Implantable Systems

Functional muscular stimulation (restore limb movement)

Auditory, and Visual Prostheses

Overcome disabilities such as Parkinson and Epilepsy

Pain control, Bladder control, Drug Delivery Systems,

Biological Fluid Analysis Systems

DNA Analysis

Blood Testing/Typing

Chemical/Biological Analysis

Cell-Based Assay Chips

Patient Health Monitoring Measure Patient Health Signs (Activity, breathing, chemistry,…)

Patient Health Service (drug delivery, …)

Environmental Sensing

Air quality

Water quality, and drug dosing

- MEMS & Microsystem Perspective

-

EECS 509 BioMEMS

Courtesy of Prof. Carlos Mastrangelo

THERMAL REACTION

DROP METERING

SAMPLE LOADING

GEL LOADING

SEPARATE DETECT

Integrated DNA Analysis

• Multiple components • Multiple reactions/separations • Decrease size/volume

EECS 509 BioMEMS

Single-Cell Assay Microsystem

Flow direction

Capture site

Captured cell

Actuation membrane

Concentration Generator

Microchamber Selection Logic

Peristaltic Pump

High-throughput Parallel Cell Assay at Single Cell Resolution

- Optimal stem cell culture & transplantation

- Cancer drug screening

Microfluidic Logic Network Microchamber Array for Single-Cells

Prof. E. Yoon, University of Michigan

EECS 509 BioMEMS

Neuro Implants Si based Bio-MEMS applications

Neuro-circuit interaction

Chemical delivery

Issues with long term implant – bio compatibility

Stanford

Robo-hobo: A rat instructed

via a wireless receiver and

brain implant to walk along

a railroad track.

IEEE Spectrum, Aug., 2002

EECS 509 BioMEMS

Drug Delivery Bio MEMS example

Science 2001, MIT

Nature 1999, MIT

http://web.mit.edu/cheme/langerlab/

http://www.sciencemag.org/cgi/content/full/293/5527/58/F2

EECS 509 BioMEMS

Advantages of Microsystems

Small samples

- Nanoliter quantities without evaporative loss

Multiplexing

- Discovery biotech puts a premium on high throughput, enables

genomics, proteomics

Integration, Performance, Speed

- Highly integrated systems possible

- Many analysis method work better as they are scaled down

- Scaling down dramatically improves speed of analysis

Portability (small size), Low reagent and power consumption (low cost)

New types of analysis, new effects to exploit

- Serial chromatographies, dielectrophoresis, surface tension

EECS 509 BioMEMS

Limitations of Microsystems

Techniques dependent on inertia are problematic

- Centrifugation, mixing

Physical state of analytes and carrier solvents can’t change

- Liquid only or gas only systems - solids clog, bubbles unstable and irreproducible

- No precipitation allowed

Interface with macro world

- Reagent reservoirs, sample introduction, detection

Mass transfer rates are tiny

- Mixing generally only occurs by diffusion

Non specific binding - high surface area to volume ratio

Microscale phenomena not fully understood

EECS 509 BioMEMS

Alternative Materials

Materials requirement for BioMEMS is different from those for typical MEMS.

Desired properties of BioMEMS materials

- Biocompatible

- Chemically modifiable

- Surface modifiable

- Easy to fabricate

- Economically viable (cheap… for throw away devices)

- ….

Si, glass and now more toward polymers…

EECS 509 BioMEMS

Types of BioMEMS Devices

Biomaterials

DNA chip cDNA, oligomer

Protein chip Enzyme, antibody, antigen

Cell chip Microorganism, animal cell, neuron

Applications

Bio-electronic device Biocomputing, bio-memory

Implantable chip Prosthetic device, bioinstrumentation

Lab-on-a-chip -TAS, screening

Biosensor Diagnostics, analysis

Microfluidics

EECS 509 BioMEMS

Microfluidics Characteristics

Low Reynolds’s Number, Re

- Laminar flow

- Difficult to mixing

Large surface to volume ratio - Surface effect dominant – bio-surface modification

- Microchannel – pressure drop

Small fluid volume: pL to mL - Nano or micro dispenser

- Diffusion

Fluidic driving - Electroosmotic force

- External pressure force

Surface Properties and Modifications


Hydrophilic vs Hydrophobic Surface

• Hydrophilic: from the Greek (hydros), meaning water, and φιλια

(philia), meaning love.

– Hydrophilic substances can seem to attract water out of the air,

the way salts (which are hydrophilic) do. Sugar, too, is hydrophilic,

and like salt is sometimes used to draw water out of foods.

– A hydrophilic molecule or portion of a molecule is one that is

typically charge-polarized and capable of hydrogen bonding,

enabling it to dissolve more readily in water than in oil or other

hydrophobic solvents.

• Hydrophobic: from the Attic Greek (hydro), meaning water, and

phobos, meaning fear.

– Hydrophobicity is the physical property of a molecule (known as a

hydrophobe) that is repelled from a mass of water.

– Hydrophobic molecules tend to be non-polar and, thus, prefer

other neutral molecules and non-polar solvents.


Contact Angle - Wetting • Contact angle is the angle, conventionally measured through the

liquid, where a liquid/vapor interface meets a solid surface.

• It quantifies the wettability of a solid surface by a liquid via the Young equation.

• A given system of solid, liquid, and vapor at a given temperature and pressure has a unique equilibrium contact angle.

• If the solid–vapor interfacial energy is denoted by gSG, the solid–liquid interfacial energy by gSL, and the liquid–vapor interfacial energy (i.e. the surface tension) by gLG, then the equilibrium contact angle qC is determined from these quantities by Young's Equation:

0 = gSG – gSL + gLGcos qC


Rame-Hart 200 Contact Angle Goniometer

Typical Contact Angles • Contact angles are extremely sensitive to contamination; values

reproducible to better than a few degrees are generally only obtained under laboratory conditions with purified liquids and very clean solid surfaces.

• Bare metallic or ceramic surfaces: ~0o

The liquid molecules are strongly attracted to the solid molecules then the liquid drop will completely spread out on the solid surface.

• Hydrophilic: < 90o

Silicon dioxide, silicon nitride, etc.

• Hydrophobic: > 90o

Silicon, most polymers, etc.

Highly hydrophobic surfaces made of low surface energy (e.g. fluorinated) materials may have water contact angles as high as ~120°

• Some materials with highly rough surfaces may have a water contact angle even greater than 150°, due to the presence of air pockets under the liquid drop. These are called super hydrophobic surfaces


How to Change the Surface Property?

• By coating and patterning a layer of film, you can change the surface property, especially inside a give microfluidic channel.

• Hydrophobic surface formation – By depositing Teflon or SAM (self-assembled monolayer) coating such as FDTS. Also by patterning the surface to customize hydrophobicity.

• Hydrophilic surface formation – By plasma treatment of the surface, you can change the surface from hydrophobic to hydrophilic (e.g. plasma treatment of PDMS). However, the surface property change is typically temporal and its property degrades over time.


Channel Patterning - Superhydrophobicity

• Wetting Wenzel regime are ‘‘sticky’’ in that drops of water tend to adhere to them more than a flat surface of the same type.

• Those following the regime of Cassie and Baxter are ‘‘slippy’’ and allow drops of water to roll off more easily than an equivalent flat surface.

Hierarchical structure is necessary to have high contact angle but also

essential for the stability of the composite interface (water-solid and water-

air).


Superhydrophobic Surfaces in Biology

• a) Lotus leaf (Nelumbonucifera), b) Hillock bush leaf, (Melaleuca hypericifolia), c) Middle of upper side of a common pond skater (Gerris lacustris), and d) Lichen Lecanora conizaeoides showing high roughness with inset showing water drop WCA 155 +/- 4o. (Source: Soft Matter 2008, 4, 224-240)


Lithographic Surface Modification

• (a) Photolithographic towers, (b) Indented square posts, (c) Diced silicon wafer, (d) Photolithographic towers, (e) Silicon nano-towers, (f) Laser-modified SU8 surface, (g) SU8 towers, (h) Silicon islands and (i) Silicon nanowires grown on those silicon islands. (Source: Soft Matter 2008, 4, 224-240)


PDMS Surface Contact Angle

10l droplets on PDMS hydro-phobic surfaces with droplet contact angles.

100 μm 150 μm

Day 1 Day 6

Growth of a SUM159 sphere shown

over the course of 6 days culture.

White circles outline the growing

SUM159 cells.

1) Silicon wafer 2) Photoresist coating 3) Photolithography

4) Silicon DRIE with

photoresist masking

5) PDMS casting over

the etched silicon

6) PDMS peeling-

off from the silicon

mold

Prof. Euisik Yoon, University of Michigan

(MicroTAS, 2011)


Channel Coating

Hydrophilic microchannel of PDMS and silicon dioxide

Hydrophobic microchannel of PDMS

and FDTS SAM coated silicon surface

Prof. Euisik Yoon, University of Michigan (Biomed Microdevices, 2005) 3/10/2014 LNF Microfluidic Workshop 2014

Capillary Force in Microchannel

• Surface 1: PDMS, Surface 2: SiO2

• Channel can be either hydrophilic or hydrophobic, depending on channel width.

• W > 5m, channel becomes hydrophilic.(Channel depth is 20m.)


Include hydrophobic region for degassing

Drug can be introduced into the channel without generating bubbles because the air leaks out through a hydrophobic ventilation channel while drug is injected to the hydrophilic channel.


Soft Lithography


Photolithography vs. Soft Lithography

Soft lithography

Use of photons for

patterning

(Optical process)

Use of a “soft” (flexible) mold

for patterning

(Physical process)


1. Photolithography

2. Pour polymer precursor(s)

and cure

3. Peel off and cut 4. Bond to glass

1.17. PDMS micromolding PDMS Micromolding Process


SU-8 mold

PDMS

Si wafer

PDMS

(+release agent)

Photoresist (SU8) master

30 µm

PDMS

Replica

• Multiple replicas

• Inexpensive

1.17. PDMS micromolding SU-8 Master & PDMS Replica


Structural integrity of PDMS walls

• Typically, structures with a high aspect ratio (>5:1 height/width)

do not replicate well.

• Some features smaller than 100nm can be replicated.


Structure Collapse of PDMS

• Lateral collapse: Commonly known as “pairing” when H/L > 5 • Sagging: Recessed structure when H/L <0.5

Delamarche, et al., Adv. Mater. (1997)


Remember this when choosing SU-8 type

Polymer with a backbone of Si-O-Si or “siloxane”

Inexpensive

Very elastic and soft

Optically transparent down to 300 nm

Surface is hydrophobic

Self-seals by conformal contact

Inert, but can be oxidized, etched, and derivatized

Biocompatible

Swells when exposed to solvents

High permeability to gases and fluids

Expands a lot with temperature (100 times more than silicon)

Si

O OO

Si

CH3

CH3

CH3

CH3

1.17. The magic of PDMS Properties of PDMS

(Polydimethyl Siloxane)

Two Methyl

Groups on

Silicon


Patterning Techniques Using Soft Lithography

• Replica Molding (REM)

• Micro Contact Printing (CP)

or Microstamping

• Micromolding in Capilaries (MIMIC)

or Microfluidic Patterning

• Microtransfer Molding (TM)

or Stencil Patterning


Replica Molding (REM)

• Additional duplication of pattern transfer from a soft mold

Prof. Whitesides, Harvard (Science 1996) 3/10/2014 LNF Microfluidic Workshop 2014

Microcontact printing (mCP)

1. Ink

2. Transfer

PDMS as a transparent rubber

Material is “added” where stamp contacts

surface


Microfluidic Patterning or Micromolding in capillaries (MIMIC)

1. Fill

2. Remove microchannels

microchannels

Material is added where stamp does not contact the surface

• Immobilization of material

• Procedure for removal of microchannels

• Deposit or etch


Polyurethane Structures on Si/SiO2

• UV-curable polyurathane prepolymer introduced in capillary channels.

• Connected patterns (b), multiple thicknesses mold (c), free standing film after

being dissolved in HF

Prof. Whitesides, Harvard (JACS 1996) 3/10/2014 LNF Microfluidic Workshop 2014

Microfluidically-Patterned Polyurethane 3D

Structures

Or 3D printer?

• Channels filled with UV-curable polyurethane precursor, ant then

exposed to UV to cure the precursor into polyurethane

• Manually stacking 3D structures Prof. Albert Folch, Biomed. Microdevices, 2000


Fabrication of PDMS Stencils MicroTransfer Molding (μTM)

Prof. A. Folch, U. of Washington (Langmuir 2002) 3/10/2014 LNF Microfluidic Workshop 2014

PDMS/PDMS Bonding – sealing channels

• Using Partially Cured PDMS

• Using Uncured PDMS as Adhesive

• Oxygen Plasma

PDMS

Partially-Cured

PDMS

PDMS

PDMS or

Glass

Uncured

PDMS


Oxygen plasma treatment of PDMS

• Reactive oxygen radicals (O+) attack methyl groups (Si-CH3) on the surface.

• Condensation reaction between silanol groups (Si-O-H) will form siloxane (Si-O-Si) to bond two PDMS layers.


Will

bind to

glass

Multi-Level Microfluidic Channels

M. Zhang, Lab Chip 2010 3/10/2014 LNF Microfluidic Workshop 2014

Fabricated Tactile Sensor Module

Electrode

Spacer

22mm 22mm

Bump

1mm

Air channel

Prof. E. Yoon, JMEMS 2006 3/10/2014 LNF Microfluidic Workshop 2014

Fabricated Tactile Sensor Module

Electrode

Spacer

22mm 22mm

Bump

1mm

Air channel


SU-8 Mold fabrication


Silicon - Si • Semiconductor, group IV

• Diamond Lattice unit cell

• Each atom shares its 4 valence electrons with 4

neighboring atoms

• There are 5x1022 atoms/cm3 in the Si lattice

• The crystalline nature of Si influences many of

its properties

From Fundamentals of Microfabrication by M. Madou

Tetrahedral Bonding of Si

Face-Centered

Cubic (FCC) unit cell Diamond Lattice


Miller Indices for a Simple Cubic Structure

• (xyz) values are the inverse of the coordinate of the intercepts of a given plane with the three axes;

• For example (100) represents the plane that intersects the x axis and runs parallel to the yz plane.

• [xyz] is a given crystal direction and represents the direction of the vector perpendicular to the plane (100).

• As we will see later, properties of Si change along these different planes.

x

y

z

1

1

1 x

(xyz)

(100)

Plane y

z

1

1

1 x

(110) Plane y

z

1

1

1

(111) Plane

<100> Direction

<110> Direction

<111> Direction


Si Wafer Marking and Designation • Standard designations have been developed by creating “flats” on Si

wafers to represent their doping type (n- or p-), and wafer orientation.

There are two types of flats: major flat (typically at the bottom of the

wafer, and the minor flat, which may be present on the side of the wafer, as illustrated below.


Why Do We Care About Orientation?

Create a pattern in a mask on a (100) wafer. The

mask edge (assume rectangular shape) is

aligned to the <110> direction. (100)

If the etch proceeds for a long time,

the 4 {111} planes meet in an

inverted pyramid shape.

(111)

Planes

54.7

° Slight undercutting

under the mask

(100)

Surface

Because (111) planes etch much slower, the

etch front practically stops on these planes,

while other planes continue to etch.

<110>


Photolithography how to make the SU-8 mold

• The process of printing a given 2D pattern onto a thin film layer

• This is a photographic process that requires a photosensitive material

“photoresist”, and a “mask” that permits exposure of only defined

regions to the incident radiation

• The mask:

– Typically made of a glass plate (soda lime or quartz glass) that is

transparent to UV light

– The pattern of interest is created on the glass using a thin (<1µm)

metal film such as chromium (Cr) or gold (Au)

– The mask plate has the pattern of interest repeatedly printed on it.

– Mask polarity can be designed to either allow incident radiation

pass through the patterned regions (i.e., dark field), or pass through

the field regions outside of the patterned areas (I.e., clear field)

Pattern of Interest

Pattern Generation

(PG) Pattern repeated on a glass plate

Step &

Repeat


• The photoresist (PR) : – A polymer whose chemical properties change when it is exposed to incident

radiation, typically UV light. Note that PR cannot be exposed to temperatures above about 200°C because it burns (note that this is a polymer like plastic).

– The PR can be then developed in a “developer” like the standard photographic process;

– Two different results can be obtained depending on the type of PR used: • Positive PR: This type of PR is removed (etched away) in the developer solution only in areas

that have been exposed to UV radiation • Negative PR: This type of PR is hardened (and therefore cannot be removed) in the developer

solution in areas that have been exposed to UV radiation.

– PR is typically in liquid form that can be spun onto a silicon wafer at speeds of a few thousand RPM’s. This spinning process creates a uniform film thickness in the range of 1-100’s of microns.

– After application, the PR is baked at 90-100°C to remove the solvents – The PR is now ready to be exposed and developed.

Photolithography


Photolithography (SU-8)

Si Wafer

Deposited film

to be

patterned

Spin and Soft

Bake PR

Align mask

on wafer

Expose PR to

UV thru mask

Develop PR,

hard bake (100-

120°C)


Photolithography: Positive vs. Negative PR (SU-8)

Clear-Field Mask Dark-Field Mask

Positive PR’s are typically used because they are easier to work with and use

less corrosive developers and chemicals

+ve PR +ve PR -ve PR


Tilted Exposure

Prof. S. Lee, KAIST (JMM 2007) 3/10/2014 LNF Microfluidic Workshop 2014

Maskless Photolithography

Direct Laser Writing • Raster Scanning of SU8

Courtesy of Prof. Alber Folch, University of Washington


• Also known as greyscale

• Heidelberg Micro PG 501

• 2micron min feature

• Fast writing

• The material to be deposited is placed in a vacuum chamber, it is somehow heated so it melts and evaporates. The vapor phase molecules land on the target wafer and form a thin film.

• Two basic approaches to evaporation are:

– Thermal Evaporation

– Electron-beam (E-beam) evaporation

• Thermal evaporation is the easiest of all and requires the simplest system. The film to be deposited is placed inside a crucible inside a vacuum chamber. The crucible is heated until the material evaporates.

• Heating of the crucible can be done in several ways, including resistive and inductive heating.

• The temperature required to evaporate the material depends on the vapor pressure of the material and on the pressure.

• Typically the material should have a vapor pressure of more than 10mTorr.

• The pressure in the chamber typically ranges from 0.1-1 µTorr

• Vapor pressure of different materials is shown on the next page.

Thin Film deposition (contacts)

Physical Vapor Deposition: Evaporation


• In e-beam evaporation, the material is heated and melted using a high-energy electron beam.

• With e-beam, higher temperatures can be achieved. Therefore, a wider range of materials can be deposited.

• Because of the higher temperatures possible with e-beam, in addition to refractory metals with low vapor pressure, it is possible to deposit some insulators such as oxides and glass.

• The evaporation rate is higher at lower pressures.

• Since the crucible is not heated as much, there is less contamination possibility using e-beam evaporation than thermal evaporation.

• Most materials used in the IC industry these days, use e-beam evaporation.

• Evaporation in general does not have a very good step coverage and the process is “line of sight”, as illustrated below.

Physical Vapor Deposition: Evaporation

Si Thinner (or discontinuous)

Film On Edge

Deposited Film Arriving Atoms


• A plasma is generated by applying a RF signal 5-15kV in a pressure range of 10-100 mTorr.

The gas used is typically Ar.

• The plasma creates Ar ions and electrons.

• Ar ions bombard the source knocking off source atoms.

• The source atoms get deposited on the substrate (target).

• Features:

– Wide variety of materials, including metals, insulators, and semiconductors

– When deposited in a reactive environment with oxygen, oxides can be deposited

– One can deposit multiple materials in a single pump down (meaning we do not need to

break vacuum for next material) since multiple sources can be placed in the same

chamber. It is also possible to deposit alloys and compounds like silicides (MoSi, TaSi)

– The films are deposited with better step coverage than e-beam so there is film continuity

going over steps.

Physical Vapor Deposition: Sputtering

+

-

RF Source

13.56MHz

Target

Wafer

Source

Plasma

Vac

uu

m


Thin Film Issues

• Adhesion

• Diffusion Barrier/Interface

• Ohmic Contacts

• Step Coverage

• Electromigraion

• Stresses


Isotropic Wet Silicon Etching (ISE)

• Si can be etched isotropically (equal etch rate in all directions) in a mixture of HF+HNO3, and some acetic (vinegar) acid (this mixture is sometimes referred to as HNA) This etch was developed in the 1960’s as the solid-state circuits industry was working on the development of beam-lead technology for building high-density circuits.

• The etch removes n- or p-type silicon in all directions at about the same rate.

• The etch does depend on agitation. The more agitation there is during the etch the larger the undercut under the masked regions.

• The etch attacks most materials (HF is nasty acid that attacks many materials). For shallow etches, silicon nitride can be used as a mask, for deep etches sometimes it is required to use Cr/Au. HF does not significantly attack silicon directly but the HF+HNO3 mixture is a strong etchant of Si.

Si substrate

With No Agitation

Mask

(usually nitride)

Si substrate

With Agitation


Bulk-Micromachining of Silicon

Anisotropic Si etching

<111>

Bottom of etch pit

If etch stops early, the wafer will

have tapered sidewalls.

Etch reaches bottom of wafer when

the mask opening is wide

<111>

54.7° thic

kness


Dry (Plasma) Etching • Dry etching is also a chemical etching technique, but employs gases instead of liquids. The gases

are ionized in an RF glow discharge (plasma) and the specific chemical species are used to etch the thin film.

• The gases used to create the plasma determine the etch rate, etch profile, and selectivity. Many gases are used to etch different thin films. A few of them, and the material they can etch are listed below.

• One of the main disadvantages of dry (plasma) etching is the worse selectivity it has with respect to both the mask (typically PR), and to different layers.

• Dry etching is now very

commonplace.

Plasma

Wafers

Chamber

RF

Power

Material Etch Gas

Si/Poly-Si CF4, SF6, ..

SiO2 CHF3, CF4/H2, CF4/O2

Si3N4 CF4/O2

Organics O2, O2/CF4, O2/SF6

Al BCl3


EECS 509 BioMEMS 67

Bulk Micromachined Silicon Microchannels

Chen and Wise,

Transducers 1995

Tjerkstra, et al., MEMS 1997

EECS 509 BioMEMS 68

Micro-Fluidic Channel – Design considerations

Microfluidics: Dimension < 1 mm

Nanofluidics: Dimension < 1 mm

Newtonian fluid (applying Navier-Stokes Equation) - Coefficient of viscosity is constant over all shear stress

- Incompressible, Steady (time-invariant) flow

- Fully developed flow

Laminar flow due to low Reynolds number - No turbulence Mixing becomes difficult

Priming bubbles very difficult - Surface tension > buoyant force

Capillary and viscous forces become dominant.

Electrophoretic, Electroosmotic flows more easily handled than pressure driven flow.

EECS 509 BioMEMS 69

Reynolds Number

Ratio of the inertial force to the viscous force

Turbulent flow : unsteady flow

Laminar flow : steady flow

Transitional value (Re): 2,000~3,000

cm)(g/sec viscosity:

ter)(4A/perimediameter hydraulic :channel of area sectional-cross :A

rate flow c volumatri:

density :

Re

hD

Q

hDA

Qh

vD

EECS 509 BioMEMS 70

Mechanism of Mixing

Mixing was produced by mechanical and molecular

physical process

Macro-scale: Dominated by mechanical process

- Turbulence, stirring

Micro-scale: Dominated by molecular process

- Laminar, diffusion

EECS 509 BioMEMS 71

Fabrication of Microfluidic Channels - Summary

Silicon and Glass

- Conventional micromachining, bonding, etching

- Expensive

Plastics

- Injection molding using thermoplastic materials such as

polystyrene, polypropylene, etc.

- Inexpensive

PDMS

- Good research material but not amenable to inexpensive mass

production for commercial devices.

Hydrogels

- Highly porous polymeric matrices: collagen, Matriegel, agarose,

PED, etc.; Easily molded from a PDMS mold for 3D cell culture.

Paper

- Extremely inexpensive, natural embedded capillary pump

EECS 509 BioMEMS 72

See you in the lab

Documents

Introduction to Microfluidics By LNF Microfluidics ... · Introduction to Microfluidics By ... 0.5mg/√Hz noise floor ... Peel off and cut 4. Bond to glass 1.17. PDMS micromolding