SCALING EFFECTS IN A MICROFABRICATED ELECTRICAL FIELD-FLOW FRACTIONATION SYSTEM WITH AN INTEGRATED DETECTOR

by

Bruce Kent Gale

A dissertation submitted to the faculty of

The University of Utah

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Department of Bioengineering

University of Utah

May 2000

Copyright © Bruce Kent Gale 2000

All Rights Reserved

SUPERVISORY COMMITTEE APPROVAL FORM

FINAL READING APPROVAL FORM

ABSTRACT

In this work the complete motivation, design, fabrication, and characterization of a microscale electrical field- flow fractionation (ElFFF) system is presented. The general theory for field flow- fractionation (FFF) systems is reviewed and demonstrates that miniaturization, while providing some advantages, does not purport to improve the capabilities of FFF systems. A closer look, specifically at the ElFFF system, though, indicates the numerous potential advantages for miniaturizing ElFFF systems. The advantages from miniaturization are explored thoroughly and a determination is made that a microscale ElFFF system could provide numerous benefits such as improved resolution and reduced analysis times, as well other advantages. The components involved in ElFFF systems are then reviewed and a design for a microscale ElFFF system is developed. The fabrication for a m-ElFFF system is outlined and the micromachining processes used in fabrication described. A completed m-ElFFF system is demonstrated and characterized with regard to most of the important parameters regarding ElFFF systems. These parameters include plate heights, resolution, equilibration times, electrode time constants, and effective electric fields, as well as several other parameters. The capabilities of the fabricated system are demonstrated by retaining and separating polymer particles by both size and surface charge, polymer particles with attached proteins, and by retaining and separating whole blood. Thus, the microscale system is shown to function in accordance with theory, and to meet the expectations developed before the system was fabricated.

In addition to the m-ElFFF system, the design and fabrication of a microscale electrical impedance detector is presented. The detector is characterized with regard to its detection abilities using three different modes of operation. Each of these modes, either DC, AC, or impedance spectroscopy, has their own unique advantages and disadvantages, but each is clearly able to detect the type of particles currently of interest for analysis in ElFFF systems. The detector is shown to improve the operation of the m-ElFFF by reducing plate heights in the complete system. The detector in electrical impedance spectroscopy mode also demonstrates the potential for making particle size measurements, further increasing the analytical capabilities of the complete m-ElFFF system.

TABLE OF CONTENTS

ABSTRACT......................................................................................................................... iv

LIST OF SYMBOLS........................................................................................................... viii

ACKNOWLEDGMENTS...................................................................................................... x

Chapter

1 INTRODUCTION..................................................................................................... 1

Scope of Work...................................................................................................... 1

Bioinstrumentation................................................................................................. 2

Electrical Field- Flow Fractionation......................................................................... 5

Micromachining..................................................................................................... 8

Biochemical Analysis System Detectors................................................................ 11

Chapter Outlines.................................................................................................. 16

2 THEORY AND SCALING EFFECTS...................................................................... 17

Introduction......................................................................................................... 17

General FFF Theory............................................................................................. 20

ElFFF Theory...................................................................................................... 37

Scaling Effects..................................................................................................... 52

Conclusion........................................................................................................... 70

3 m-ElFFF SYSTEM DESIGN AND FABRICATION.................................................. 72

Introduction to Microfabrication............................................................................ 72

ElFFF System Components................................................................................... 81

m-ElFFF System Parameters and Design............................................................... 84

m-ElFFF Channel Fabrication............................................................................... 104

Packaging.......................................................................................................... 110

Fabrication Results............................................................................................. 115

Preliminary Biocompatibility Testing.................................................................... 122

Conclusion......................................................................................................... 126

4 m-ElFFF SYSTEM CHARACTERIZATION.......................................................... 127

Introduction........................................................................................................ 127

Methods............................................................................................................ 128

Results and Discussion....................................................................................... 132

Conclusion......................................................................................................... 173

5 IMPEDANCE DETECTOR CHARACTERIZATION............................................ 176

Introduction........................................................................................................ 176

Theory............................................................................................................... 179

Fabrication......................................................................................................... 185

DC Mode Operation........................................................................................... 185

AC Mode and Impedance Spectroscopy.............................................................. 198

Conclusions........................................................................................................ 213

6 BIOLOGICAL ANALYSIS................................................................................... 219

Introduction........................................................................................................ 220

Methods............................................................................................................ 222

Results.............................................................................................................. 223

Conclusion......................................................................................................... 228

7 CONCLUSION..................................................................................................... 230

Macro and Micro Systems Compared.................................................................. 231

Future Work and Limitations of this Study............................................................ 233

Appendices

A NUMERICAL SOLUTIONS FOR STOKES FLOW............................................... 244

B WATER DYNAMICS IN VERY LOW ASPECT RATIO CHANNELS................. 258

REFERENCES.................................................................................................................. 274

LIST OF SYMBOLS

a.................................................................................................................................... radius

b...................................................................................................................... channel breadth

c.......................................................................................................................... concentration

c₀................................................................................................................ initial concentration

d................................................................................................................................ diameter

d_i......................................................................................................... steric inversion diameter

e....................................................................................................................... electron charge

f.......................................................................................... friction coefficient or friction factor

g_x.............................................................................................................. gravity in x-direction

h................................................................................................... gridline spacing in x-direction

i.................................................................................................................................... current

i.................................................................................................... gridline number in x-direction

j................................................................................................... gridline number in y-direction

k.............................................................................................................. Boltzmann’s constant

k................................................................................................... gridline spacing in y-direction

l................................................................................................. induced sample layer thickness

m............................................................................................. number of gridlines in x-direction

n............................................................................................. number of gridlines in y-direction

n_i⁰........................................................ initial number of active ions per volume (number density)

p................................................................................................................................. pressure

s............................................................................................................................ scale factor

t........................................................................................................................................ time

t_m.............................................................................................................. membrane thickness

t_r......................................................................................................................... retention time

t₀................................................................................................................................ void time

u, u_x................................................................................................. flow velocity in x-direction

v.................................................................................................................................. velocity

v_y...................................................................................................... flow velocity in y-direction

ávñ........................................................................................................... average flow velocity

ávñ_zone...................................................................... average flow velocity for a zone of particles

w......................................................................................................... plate separation distance

w_z..................................................................................................... flow velocity in z-direction

w_1/2..................................................................................................... peak width at half height

x................................................................................................................. coordinate direction

x_DL.......................................................................................................... double layer thickness

y................................................................................................................. coordinate direction

z.................................................................................................................................... charge

z................................................................................................................. coordinate direction

A.......................................................................................... distance from origin to peak elution

C........................................................................................................................... capacitance

C_B................................................................................................................... bulk capacitance

C_DL.................................................................................................... double layer capacitance

C_p............................................................................................................ specific heat capacity

D............................................................................................................... diffusion coefficient

D_h................................................................................................................ hydraulic diameter

E.............................................................................................................. electric field strength

E_eff.............................................................................................. effective electric field strength

E_y.................................................................................................................. Young’s modulus

H............................................................................................................................ plate height

H_D....................................................................................... diffusion contribution to plate height

H_i.................................................................................. instrumental contribution to plate height

H_n............................................................................... nonequilibrium contribution to plate height

H_p..................................................................... sample polydispersity contribution to plate height

J_D............................................................................................................... flux due to diffusion

J_F.................................................................................................... flux due to the applied field

K....................................................................................................................................... gain

L........................................................................................................................ channel length

L_e...................................................................................................................... entrance length

N.................................................................................................................... number of plates

P.................................................................................................................................... power

Q...................................................................................................................... thermal energy

Q.................................................................................................................... volume flow rate

R......................................................................................................................... retention ratio

.............................................................................................. average value of retention ratio

R_B....................................................................................................... bulk electrical resistance

R_e............................................................................................................... electrical resistance

Re................................................................................................................ Reynolds’ number

R_h................................................................................................... hydraulic Reynolds’ number

R_s.............................................................................................................................. resolution

S’.............................................................................................................. applied field strength

S_d.............................................................................................................. size selectivity index

T........................................................................................................................... temperature

U.......................................................................................................................... drift velocity

V_e....................................................................................................................... elution volume

V₀.......................................................................................................................... void volume

V.................................................................................................................................. voltage

V_eff.................................................................................................................. effective voltage

Z............................................................................................................................. impedance

Z_T..................................................................................................................... total impedance

a............................................................................................................................ aspect ratio

b................................................................................................. membrane deflection constant

g.............................................................................................................. steric transition factor

e............................................................................................................................. permittivity

e_{DL.............................................................................................................................} permittivity of the double layer

e_{r......................................................................................................................................................} relative permittivity

e₀....................................................................................................... permittivity of free space

f................................................................................................................... field susceptibility

z.......................................................................................................................... zeta-potential

h................................................................................................................................ viscosity

h₀....................................................................................................................... initial viscosity

k.......................................................................................................................... Debye length

l......................................................................................... nondimensional retention parameter

m........................................................................................................... electrophoretic mobility

r_B........................................................................................................ bulk electrical resistivity

r_e................................................................................................................ electrical resistivity

r_m................................................................................................................................. density

s.................................................................................................................. standard deviation

....................................................................... average standard deviation along the time axis

t.......................................................................................................................... time constant

............................................................................................................................. frequency

ACKNOWLEDGMENTS

There are a number of organizations and individuals that I would like to thank for their help on this project. I could not have completed this work without their support. Specifically, I would like to acknowledge financial support from a National Science Foundation graduate research fellowship, a Whitaker Foundation internship, and a University of Utah Technology Innovation Grant. Amoco Chemical Corporation and Microchem Corporation both graciously donated items to further this work. Many thanks are in order for the staff at HEDCO Microelectronics Laboratory. Their long hours and dedicated service kept the research on track. I would also like to thank the secretaries and staff in the Bioengineering Department office for their constant willingness to help and their ability to get things done quickly.

I would especially like to thank all the members of the Micro Instrumentation Research Lab for their constant support, good ideas, and restful entertainment. I would specifically like to thank Thayne Edwards for his help with the finite element modeling, Ian Papautsky for his help with the microfluid flow studies, and Jeff England for his help with the data acquisition system.

I would like to express appreciation to my supervisory committee for their insight, suggestions, and constant dose of reality. Thanks to Tim Ameel for a listening ear and a profound interest in the people around him. Thanks to Karin Caldwell for being the example of a world-class scholar, an enthusiastic researcher, an able administrator, and a caring person. I would especially like to thank my advisor Bruno Frazier for his bright vision of the future, his willingness to take on a confused student, and his desire to make any situation beneficial to all involved. It wasn’t an accident that we arrived and departed together.

Most importantly, I would like to thank my wife Chela for her constant support, patience, and forgiveness, without which none of this work would have been possible. I would also like to thank my parents for instilling and encouraging a desire to learn and for teaching the value of work. Many thanks to my brother Adam whose competitive nature (and mine) brought out the best in both of us. Lastly, I would like to thank God for blessing me with a multitude of abilities and opportunities.

CHAPTER 1

INTRODUCTION

Collecting biological samples for diagnosis or other analysis, sending them off to a lab, and then waiting for hours, days, or even weeks for results is a typical scenario for doctors, patients, and other scientists in health care, chemical analysis, environmental monitoring and other fields. Typically, medical and chemical analyses must be performed in dedicated labs because the analysis systems are bulky, slow, expensive, delicate, and often complex. There is a great demand for speeding up these analysis systems and moving them to the location where the results are needed. A h