Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
1999-01-26
2001-07-24
Warden, Jill (Department: 1743)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S643000
Reexamination Certificate
active
06264815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus and method for testing or investigating particles present in a fluid using dielectrophoresis, for example to determine the dielectrophoretic characteristics, or to identify the presence and/or relative concentration of a particular type or types of particle in the fluid.
2. Description of the Related Art
Dielectrophoresis (DEP) is the translational motion of a particle caused by polarisation effects in a non-uniform electric field. Unlike electrophoresis, no overall electrical charge on the particle is necessary for DEP to occur. Instead, the phenomenon depends on the magnitude and temporal response of an electric dipole moment induced in the particle, and on the force produced as a consequence of the electric field gradient acting across the particle. The magnitude of the dielectrophoretic force F
dep
on a spherical particle of radius a is given by:
F
deo
=
2
π
a
3
ϵ
m
Re
[
(
ϵ
p
*
-
ϵ
m
*
)
(
ϵ
p
*
+
2
ϵ
m
*
)
]
∇
E
_
2
(
1
)
where ∈
m
is the absolute permittivity of the suspending medium, ∇{overscore (E)} signifies the gradient in the electric field, and ∈
p
* and ∈
m
* are complex permittivities of the particle and its surrounding medium, respectively. The complex permittivity is given by ∈*=∈−j&sgr;/&ohgr;, where ∈ is the absolute permittivity, &sgr; is the electrical conductivity, &ohgr; is the angular frequency of the electric field and j={square root over (−1+L )}. The term Re indicates that the real part of the expression within the square brackets of equation (1) is to be taken.
For particles suspended in a uniform aqueous electrolyte, the permittivity and conductivity of the suspending medium usually remains approximately constant over the frequency range 100 Hz to 100 MHz, whereas for the particles themselves these parameters can vary significantly. The term (∈
p
*−∈
m
*) can therefore be positive or negative, and thus over an extended frequency range a particle can exhibit both positive DEP (movement towards areas of high field strength) and negative DEP (movement towards areas of low field strength).
Differences in the dielectrophoretic frequency response of particles can be used to selectively separate them by dielectrophoresis. An example of a particle separator which operates on this principle is described in International Patent Application WO-A1-9422583 in which a fluid containing two types of particles flows over electrodes producing a non-uniform electric field which is controlled so that the two types of particle experience different resultant forces and the fluid flow can remove one particle type preferentially. This separator can thus separate dielectrophoretically different particles or cells, but to be used effectively the dielectrophoretic behaviour of the two different particle types should be already known.
Pin-plate electrodes have been used for this purpose to determine the dielectrophoretic characteristics of particular particle types, but the procedures are laborious and time consuming.
Gascoyne et al Meas. Sci. Technol. 3 (1992) at pages 439 to 445, determines the DEP behaviour of 200-300 particles, specifically mammalian cells, by automatic image analysis. However, as with the use of pin-plate electrodes the DEP response of the cells can only be measured at a single frequency at a time. Because the frequency range of interest in DEP is relatively large (typically 100 Hz to above 10 MHz), and several data points per decade may be required, many single-frequency experiments need to be obtained for a sufficiently wide spectrum. Using this method it is therefore cumbersome and time-consuming to obtain a dielectrophoretic spectrum over a large frequency range, so as to facilitate the observation or manipulation of cells or particles.
There is therefore a need for a better way of determining the characteristics of particles in a fluid and/or of identifying one or more particle types present.
SUMMARY OF THE INVENTION
According to the present invention there is provided apparatus for testing particles present in a fluid comprising a chamber, a series of spaced electrodes in the chamber, means for applying electrical inputs of different frequencies to the respective electrodes to generate different dielectrophoretic fields in respective regions adjacent the electrodes, and means for detecting the presence of particles in the respective regions.
Preferably means are also provided for varying other parameters in the chamber which can affect the dielectrophoretic response of the particles. Such parameters can include the electrical conductivity and/or permittivity of the material in the chamber and/or its pH value. Preferably also means are provided for adjusting the voltage of the electrical inputs to the electrodes.
Additionally or alternatively, other forces may be used to enhance the movement of the particles. These may include hydrodynamic, ultrasonic, electrophoretic or optical forces.
The apparatus may be operated, for example, to determine the parameters which are appropriate for the separation and/or identification of a particular particle type in the fluid, or to differentiate between two particular types of particle present or to analyse a mixture of several particle types.
The regions in which the particles are to be detected will depend upon the geometrical configuration of the apparatus and the conditions at which it is operated. In one form of the invention, the electrodes are directed towards a further electrode or electrodes at a common or ground potential and the main areas of interest will lie in the spaces between the tips of the series of electrodes and the common electrode. Additionally or alternatively, the regions between adjacent electrodes of the series are to be investigated.
The means to detect the presence of particles in each of said regions may comprise a source of electro-magnetic radiation which is transmitted through the chamber to impinge upon particles present in the electrode gaps, and sensing means to detect the transmitted radiation not absorbed by said particles. There may be respective radiation sources for each region of interest adjacent the series of electrodes, or beam deflecting means may direct the radiation for a single source through the regions successively.
The electromagnetic radiation source may be a laser and the detector may be a charge coupled device (CCD). Alternatively, a video camera can be provided to monitor the radiation transmitted and a light source other than a laser can be employed. Automated image analysis means can then be used to interpret images thus obtained.
Other means to detect the presence of particles may include current and/or voltage sensing circuits, connected in series with each of said electrodes, and arranged so as to detect variations in field characteristics and/or impedance fluctuations within the electrode gaps. The information may then be used to indicate the presence of particles adjacent to the electrodes. Automatic electronic switch means may be provided for switching such sensing circuits between the electrodes. This may be effected sequentially.
Any of these detection techniques may additionally employ means for obtaining information about the temporal dielectrophoretic response, that is to say, the speed at which particles move to or from different dielectrophoretic field regions. Because the speed of movement of the particles is directly related to the forces acting on them, and because those forces are also related to the field characteristics, such temporal information (eg. rate of arrival of particles) may help to corroborate other measurements or may be used independently to identify and/or characterise particles.
The series of electrodes may be configured as a series of elongate fingers in a comb-like array with their tips directed towards a common electrode in the form of a lin
Markx Gerardus Hendricus
Pethig Ronald
BTG International Limited
Pillsbury & Winthrop LLP
Starsiak Jr. John S.
Warden Jill
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