Dielectrophoresis

Theory, Methodology and Biological Applications

Ronald R. Pethig

• Publisher: Wiley
• Year: 2017
• ISBN:
  9781118671412
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Comprehensive coverage of the basic theoretical concepts and applications of dielectrophoresis from a world-renowned expert.

• Features hot application topics including: Diagnostics, Cell-based Drug Discovery, Sensors for Biomedical Applications, Characterisation and Sorting of Stem Cells, Separation of Cancer Cells from Blood and Environmental Monitoring
• Focuses on those aspects of the theory and practice of dielectrophoresis concerned with characterizing and manipulating cells and other bioparticles such as bacteria, viruses, proteins and nucleic acids.
• Features the relevant chemical and biological concepts for those working in physics and engineering

1
Placing Dielectrophoresis into Context as a Particle Manipulator

1.1 Introduction

For those interested in etymology, deciphering the origin and hence probable meaning of compound words ending in phoresis is relatively straightforward. Based on Greek translation, such compound words imply something to do with ‘carrying things around’. For example, stating that electrophoresis describes an object being carried (i.e., moved) by an electrical effect is therefore an acceptable definition. For a particle to be set into motion requires the imposition on it of an external force. An example is the buoyancy force acting on a particle suspended in a fluid – the particle will either sink or rise under the action of gravity, depending on whether its specific density is greater or less than that of the surrounding fluid. If the particle finds itself in a flowing fluid, it will also experience a viscous drag force and be accelerated to the speed of the local fluid flow. The particle can be solid or take the form of a fluid droplet or gas bubble. This book's focus is the use of dielectrophoresis as a means to spatially manipulate bioparticles such as cells, bacteria, viruses, proteins and nucleic acids. In May 2013, in the United States, two sessions were devoted to this subject at an international conference on Advances in Microfluidics and Nanofluidics. In the flyer that promoted the conference, it was stated:

As dielectrophoresis (DEP) is arguably one of the fundamental pillars of microfluidic manipulation and given the continued advances in this mature field, we will be organising special sessions on dielectrophoresis with the aim of promoting interaction between researchers that work on fundamentals and applications of DEP across different communities and disciplines.

Various methods can be used to manipulate particles in a microfluidic device, so what justification is there to state that dielectrophoresis can be singled out as ‘one of the fundamental pillars’? Why is DEP considered an important topic for a conference on microfluidics and nanofluidics? Why devote a whole book to the subject? Some answers are provided in this chapter by reviewing those forces that can be used to manipulate bioparticles in microfluidic devices. It is not intended as a comprehensive review, but covers sufficient ground to set dielectrophoresis into context and highlight some of its special features and advantages. Bearing in mind that an increasing number of scientists trained in the biomedical fields are entering the subject area, the text is written in a style intended to be suitable for an interdisciplinary readership. To help maintain the word flow, boxes and worked examples are used in this chapter (and throughout the book) to divert the more formal and quantitative details away from the main text.

Dielectrophoresis is the induced motion of a particle when it is placed in an electric field gradient. In Chapter 2, we find that one advantage of this method is that it scales favourably with a reduction in dimensions of the electrodes used to generate the electric field. It is therefore ideally suited for applications in microfluidic devices designed to perform, for example, as an electronically controllable ‘laboratory on a chip’ or ‘micro-total analysis’ system. Although the terms are often used interchangeably, lab on chip is used to describe devices that integrate several laboratory processes, whereas micro-total analysis systems are considered to integrate all laboratory processes required for an analysis. For both cases, fluid flow in one or more channel networks, fabricated into or from a single solid substrate, is an essential element of the analytical or preparative function of the device [1–6]. It is also generally accepted that to qualify as a microfluidic device, at least one of its fluidic dimensions should be in the range 1 μm ∼1 mm.

The fundamental features and potential advantages of using microfluidic devices for biomedical assays and processes will now be outlined.

1.2 Characteristics of Micro-Scale Physics

A simple form of a microfluidic device would be, for example, a channel etched into a glass substrate of length 1 cm and internal cross section 10 μm × 10 μm, equipped with an inlet and outlet fluid port. One envisaged application of such a simple structure would be to study how thrombocytes (platelets) in a flowing fluid interact with immobilized proteins. The proteins can be immobilized by coating them onto the internal surfaces of the channel. With our specified dimensions the channel has an internal volume of 10⁻⁹ dm³ (1 nL). One small droplet of water that leaks from a tap has a volume about 20 000 times larger than this! Physical effects or forces, such as surface tension that controls the size of a water droplet, may play relatively minor roles in our macro-scale world of activity, but can dominate in microfluidic devices. The ability to accommodate such forces, either by minimizing their disruptive effects or using them to advantage, is an important aspect of the design and operation of a microfluidic device.

The following are practical examples of dominant physical phenomena at the micro scale:

• Microfluidic devices tend to have a large ratio of their surface area to volume. Consider a spherical chamber of radius R. This has a surface area of 4πR² and a volume of (4πR³)/3. The ratio of these two parameters is 3/R. Therefore, as the radius R decreases the ratio of surface area to volume increases. For example, a 10 dm × 10 dm × 10 dm cube has a surface-to-volume ratio of 40 m⁻¹, whereas for the 1 cm × 10 μm × 10 μm channel considered above, this ratio increases to 4 × 10⁵ m⁻¹. Scaling down the dimensions of a fluidic device thus provides the opportunity for suspended particles to interact with a large surface area. This can represent a desired outcome, as in the study of platelet-protein interactions, or lead to an undesirable result such as the adventitious adherence of cells to the internal walls of narrow-bore tubing.
• In micro devices, capillary action and other surface energy effects can be greater than gravitational forces. This can result in an upward or transverse fluid movement, or even block downward fluid flow in a capillary.
• A small drop of fluid placed in the inlet of a microfluidic device can evaporate very rapidly.
• Fluids that are brought together in a microfluidic circuit do not mix easily. Any mixing tha...

Table of contents

• Cover
• Title page
• Copyright
• Dedication
• Index of Worked Examples
• Preface
• Nomenclature
• 1 Placing Dielectrophoresis into Context as a Particle Manipulator
• 2 How does Dielectrophoresis Differ from Electrophoresis?
• 3 Electric Charges, Fields, Fluxes and Induced Polarization
• 4 Electrical Potential Energy and Electric Potential
• 5 Potential Gradient, Field and Field Gradient; Image Charges and Boundaries
• 6 The Clausius–Mossotti Factor
• 7 Dielectric Polarization
• 8 Dielectric Properties of Water, Electrolytes, Sugars, Amino Acids, Proteins and Nucleic Acids
• 9 Dielectric Properties of Cells
• 10 Dielectrophoresis: Theoretical and Practical Considerations
• 11 Dielectrophoretic Studies of Bioparticles
• 12 Microfluidic Concepts of Relevance to Dielectrophoresis
• Appendices
• Author Index
• Subject Index
• EULA