The present exemplary embodiment relates to instruments
or devices for collecting and sorting particles or samples, particularly from liquid
or gaseous media. The exemplary embodiment finds particular application in conjunction
with the separation and detection of biological agents, and will be described with
particular reference thereto. However, it is to be appreciated that the present
exemplary embodiment is also amenable to other like applications.
Bio-agents dispersed either in aerosol form or in water
are typically in such low concentrations that they are below the limit of detection
(LOD) of even the most sensitive detection schemes. Yet, the ingestion of even a
single bacterium may lead to fatal consequences. Accordingly, regardless of whether
the sample is derived from aerosol or water collection, there exists a need to further
concentrate the sample prior to detection.
Aerosol and hydrosol collection schemes typically sample
large volumes of air at very high rates (150 kL/min and up), and use a cyclone-impactor
design to collect particles having a size in the threat range and capture them in
a wet sample of 5-10 mL volume. This supernatant is then used as the test sample
for agent detection. In order to use currently available detection strategies, it
would be desirable to further concentrate the hydrosol by another two orders of
magnitude. For example, this could be achieved by collecting all the bio-particles
in the sample volume within a smaller volume of 50-100 µL.
Contaminants in water are typically treated by several
filtration steps to recover the sample for agent testing. After initial pre-filtration
to remove larger vegetative matter, the sample is further concentrated by two to
three orders of magnitude using ultra-filtration. This method of tangential flow
filtration (TFF) is laborious as it may require multiple sequential steps of TFF;
each step utilizing a filter of molecular weight (MW) cut-off that is 3-6X lower
than the MW of the target molecules, and recycling of the retentate. The limiting
factor for TFF is system loss, where there is a cut-off below which it may not provide
any further improvement in concentration. The retentate at the end is approximately
a 50 mL volume to be presented to the detector. It would be particularly desirable
to further concentrate the retentate by up to another three orders of magnitude.
Field Flow Fractionation (FFF) is a technique that allows
the separation of particles of different charge to size ratios (q/d) in a flow channel.
This technique is useful in many fields ranging from printing to biomedical and
biochemical applications. Separation is achieved because particles with different
q/d ratios require different times to move across the flow channel, and therefore
travel different distances along the flow channel before arriving at a collection
wall. To obtain well-defined and separated bands of species with different q/d values,
the particles are typically injected through a narrow inlet from the top of the
channel. Total throughput depends on the inlet geometry and flow rate, which in
turn affects the q/d resolution of the system.
FFF relies upon the presence of a field perpendicular to
the direction of separation to control the migration of particles injected into
a flow field. The separated components are eluted one at a time out of the system
based on retention times, and are collected in a sequential manner. The separations
are performed in a low viscosity liquid, typically an aqueous buffer solution, which
is pumped through the separation channel and develops a parabolic velocity profile
typical of Poissieulle flow. The process depends on controlling the relative velocity
of injected particles by adjusting their spacing from the side walls. Particles
with higher electrophoretic mobility or zeta potential will pack closer to the walls
and therefore move slower than those that are nearer the center of the channel.
In effect, particles move at different rates through the system based on zeta potential
and size. Use of different separation mechanisms such as thermal, magnetic, dielectrophoretic,
centrifugation, sedimentation, steric, and orthogonal flow has given rise to a family
of FFF methods. Although satisfactory in many respects, there remains a need for
an improved FFF separation technique.
The present exemplary embodiment contemplates a new and
improved system, device, cells, and related methods which overcome the above-referenced
problems and others.
In a first aspect, the exemplary embodiment provides a
traveling wave grid system comprising a first traveling wave grid, a second traveling
wave grid downstream of the first wave grid, and a transition region extending between
the first and second traveling wave grids. The transition region includes a collection
of arcuate traces. The transition region is adapted to transport and cause convergence
of a particle stream from the first grid to the second grid.
In a further embodiment the transition region includes chevron traveling wave grids.
In another aspect, the exemplary embodiment provides a
method for differentiating and optionally collecting particles according to size
from a sample of particles. The method comprises providing a traveling wave grid
system including a first traveling wave grid and a second traveling wave grid. The
first and second traveling wave grids are oriented at an angle with respect to each
other. The angle ranges from about 10° to about 170°. The method comprises
introducing a sample containing particles of different sizes onto the first traveling
wave grid. The method further comprises operating the traveling wave grid system
to thereby transport the particles along the first and second traveling wave grids.
Upon undergoing a change in direction corresponding to the angled orientation of
the first and second traveling wave grids, the particles separate into at least
two groups according to size of the particles.
In a further embodiment the first traveling wave grid and the second traveling wave
grid are oriented at an angle of from about 45° to about 135° with respect
to each other.
In a further embodiment the first and second grids are oriented at an angle of about
90° with respect to each other.
In a further embodiment particles undergo the change in direction along a longer
distance than other particles, are smaller in size than the other particles.
In a further embodiment larger particles have shorter turning radii than smaller
In yet a further aspect, the exemplary embodiment provides
a method for differentiating and optionally collecting particles according to size
from a sample of particles. The method comprises providing a traveling wave grid
including a provision for selectively adjusting a sweep frequency of an electrical
voltage signal applied to the grid. The method also comprises introducing a sample
containing particles of different sizes on the traveling wave grid. The method further
comprises operating the grid at a first sweep frequency whereby particles of a first
size are displaced from one region of the grid to another. And, the method comprises,
operating the grid at a second sweep frequency different than the first sweep frequency
whereby particles of a second size, different than the first size, are displaced
from one region of the grid to another.
In a further embodiment the first sweep frequency is higher than the second sweep
In a further embodiment the particles displaced from use of the first sweep frequency
are smaller than the particles displaced from use of the second sweep frequency.
In a further aspect, the exemplary embodiment provides
a purification cell adapted to remove and classify particles from a sample. The
cell comprises a concentration chamber including a first traveling wave grid, a
separation chamber including a second traveling wave grid, and a focusing channel
extending between the first and second traveling wave grids. The focusing channel
includes a third traveling wave grid. The second and third traveling wave grids
are oriented at an angle of from about 10° to about 170° with respect
to each other. The separation chamber further includes a collection of compartments
adapted to receive particles of different sizes. The collection of compartments
are aligned across the second traveling wave grid.
In a further embodiment the third traveling wave grid is a chevron traveling wave
In a further embodiment the separation chamber includes one or more chevron traveling
In a further embodiment the number of chevron traveling wave grids corresponds to
the number of compartments.
In a further embodiment the compartment nearest the focusing channel receives particles
of the largest size within the sample upon operation of the cell. In a further embodiment
the purification cell further comprises:
BRIEF DESCRIPTION OF THE DRAWINGS
- a recirculation loop extending between the concentration chamber and the separation
chamber, the recirculation loop including a fourth traveling wave grid.
FIGURE 1 is a schematic of a traveling wave array that
concentrates and directs a stream of particulates to a desired location.
FIGURE 2 illustrates a traveling wave array that may stagnate
with moderate mass loading.
FIGURE 3 is a schematic of an exemplary embodiment traveling
wave array where the blank regions denote curvilinear grids for particle focusing.
FIGURE 4 is a schematic of another exemplary embodiment
of a stagnation-resistant traveling wave array.
FIGURE 5 is a schematic of yet another exemplary embodiment
traveling wave array.
FIGURE 6 is a collection of three micrographs spanning
the width of a curvilinear traveling wave grid showing the degree of curvature and
resultant focusing in a stream of differently sized particulates undergoing a change
in direction in accordance with the exemplary embodiment.
FIGURE 7 is a schematic of another exemplary embodiment
traveling wave array showing separation of the focused particle stream.
FIGURE 8 illustrates a separation strategy in accordance
with the exemplary embodiment.
FIGURE 9 is a schematic of a purification cell in accordance
with the exemplary embodiment.
FIGURE 10 is a schematic of another recirculating purification
cell in accordance with the exemplary embodiment.
FIGURE 11 is a schematic of a purification device integrated
with a modified field flow fractionation cell for continuous separation in accordance
with the exemplary embodiment.
Currently there are no other effective methods to concentrate
very dilume amounts of bio agents (or bio molecules) in a liquid sample beyond the
typical concentrations achieved by centrifugation and ultrafiltration. Centrifugation
at high speed (10,000 rpm) may be used to pellet out large numbers of particles
such as bacteria; however, it is not readily portable. The exemplary embodiment
device is able to process the retentate after ultrafiltration and provide further
concentration by a factor of a hundred or greater. In addition, few devices are
available that can handle the volume typically associated with purification or bio-enrichment
operations. Lab-on-chip (LOC) devices may handle only minute volumes. The exemplary
embodiment device can readily handle such large volumes
More specifically, the exemplary embodiment provides various
unique traveling wave array configurations that can be utilized to optimize device
operation and specifically, to maximize mass transport and to minimize congestion.
The exemplary embodiment also provides various methods for sample separation in
a liquid medium. And, the exemplary embodiment provides purification cells utilizing
cascaded traveling wave grids that provide functions of concentration, focusing,
The term "traveling wave grid" or "traveling wave array"
as used herein, collectively refers to a substrate, a plurality of electrodes to
which a voltage waveform is applied to generate the traveling wave(s), and one or
more busses, vias, and electrical contact pads to distribute the electrical signals
(or voltage potentials) throughout the grid. The term also collectively refers to
one or more sources of electrical power, which provides the multi-phase electrical
signal for operating the grid. The traveling wave grids may be in nearly any form,
such as for example a flat planar form, or a non-planar form. The non-planar form
can be, for example, in the form of an arcuate region extending along the outer
wall of a cylinder. The non-planar grid could be in the form of an annular grid
defined within an interior region of a tube. Traveling wave grids, their use, and
manufacture are generally described in the previously noted U.S. patents.
As referred to herein, the various exemplary embodiment
traveling wave grid systems comprise one or more chevron grids. The term "chevron"
as used herein refers to a pattern of electrodes or traces constituting the traveling
wave grid or portion thereof, in which a significant portion of the traces, and
typically all traces, are arcuate and also arranged in a concentric fashion. Typically,
the arcuate traces are also arranged such that they are defined about one or more
center points that are located upstream from the intended direction of particle
flow during operation of the collection of traces. This configuration, relative
to the direction of flow, serves to maintain direction of the stream and reduce
dispersion of particulates in the flowing stream.
Another aspect of the traveling wave grid or array system
described herein is that the grids are in certain applications, oriented at some
angle with respect to each other. This orientation aspect is actually with regard
to the intended (or actual) direction of travel of particulates on one grid relative
to the direction of travel of particulates on another grid. Generally, the angle
between adjacent grids or regions of grids can be from about 10° to about 170°,
more particularly from about 45° to about 135°, and often about 90°.
In certain applications, the exemplary embodiment utilizes the directional change
of particle flow streams to differentiate, separate, and/or classify the particles.
A traveling wave array can comprise adjacent rectilinear
and chevron grids 10 and 50, respectively as shown in FIGURE 1. The rectilinear
grid 10 transports particulates laterally from a first edge 12 to a second edge
14 where the chevron grid 50 induces a directional turn to move the particulates
into a sample well 70 where field extraction can be used to collect the particulates
thus increasing their concentration. The chevron grid 50 also serves to focus the
resulting particle stream as the stream, when disposed on the chevron grid 50, tends
to move at right angles to the direction of the stream on grid 10. The width of
the present embodiment chevron grid 50 is about 3 mm and is easily congested when
sample concentration exceeds 40 mg/L.
FIGURE 2 depicts a stagnation situation with a concentrated
sample of 3 µm and 6 µm diameter polystyrene beads. In the traveling wave
array 100 comprising a rectilinear grid 110 and a chevron grid 150, the chevron
grid 150 is relatively narrow. The beads are collected on the left edge of the grid
110 along region 114 and cannot continue to travel along the chevron grid 150 to
the sample well (not shown) due to the high density of particles. The reason for
the congestion is evident from FIGURE 2. The transition from the rectilinear grid
to the chevron grid is analogous to that of a multi-lane highway converging into
a much narrower lane. The width of the rectilinear grid 110 is about 5 cm so the
compression factor to 3 mm is in excess of a factor of sixteen (16). Since transport
is from the bottom of the chevron grid 150 to the top (as shown in FIGURE 2), the
probability for congestion increases as the particulates approach the sample well.
Congestion is a stagnation condition in which the abundance of particulates leads
to multi-layered transport which becomes inefficient due to drop-off of the transport
To mitigate against this condition and to increase the
mass flow rate (which would be useful for biomedical applications where higher concentrations
would be involved), the exemplary embodiment provides several versions of improved
systems of traveling wave grids. Generally, in accordance with the exemplary embodiment,
a system of traveling wave grids or arrays is provided that comprise a first traveling
wave grid which is typically in the form of a rectilinear grid, a second traveling
wave grid, which can be in the form of either a rectilinear grid or a chevron grid,
or some other type of grid, and a transition region extending between the first
and second grids. As noted, the first and second grids are oriented at an angle
with respect to each other. The transition region is a traveling wave grid, or portion
thereof, which serves to efficiently assist in transporting particulates from one
grid to another, and preferably also promotes the change in direction of the particulates.
Specifically, FIGURE 3 illustrates a traveling wave array
200 in accordance with the exemplary embodiment comprising a rectilinear grid 210
in communication with a chevron grid 250 having an angled interface region 260.
The distal end 264 of the interface region 260 has a greater area or width than
the proximal end 262 of the region 260. That is, with respect to the direction of
flow of particulates on the chevron grid 250, the width of the interface region
260 decreases with the direction of flow. FIGURE 3 illustrates the use of a converging
radial traveling wave array for the transition region 260. A characteristic of the
array of FIGURE 3 is an overlapping path as particles in one region of the grid
210 overlap with particles in certain regions of the chevron grid 250.
FIGURE 4 depicts a traveling wave array 300 comprising
a rectilinear grid 310 in communication with a chevron grid 350 having an angled
interface region 360. The distal end 364 of the region 360 has a smaller area or
width than the proximal end 362 of the region 360. In contrast to the configuration
of FIGURE 3, the array of FIGURE 4 features an interface region 360 having a width
that increases with the direction of flow of particulates on the chevron grid 350.
In the array of FIGURE 4, a converging radial traveling wave array is also depicted,
however, with minimal overlapping paths. The array of FIGURE 4 is particularly beneficial
in that congestion is minimized and overlapping paths of traveling particles are
FIGURE 5 illustrates another traveling wave array 400 comprising
a first grid 410 that utilizes a plurality of arcuate electrodes 405, and a second
grid 450 which can be in the form of a chevron grid or a rectilinear grid. In this
version of the exemplary embodiment, the first grid 410 is in essence, a transition
region in itself. FIGURE 5 illustrates another strategy for a single converging
radial traveling wave array. This array features a relatively shortly travel distance
for faster concentration.
In FIGURES 3-5, the shaded area indicates the noted transition
regions and can be in the form of expanded chevron grid regions emanating from the
sample well inlet. All three configurations open up many lanes into the sample well.
Expanding the chevron grid regions allows more gradual convergence of the particle
streams over a larger approach angle span.
The exemplary embodiment also provides strategies for particle
separation. Most particulates have a native charge dependent on pH which leads to
a Coulomb force, but may also polarize in a non-uniform field. The induced dipole
moment (Clausius-Mossotti) is:
where a is the particle radius, &egr;particle is the particle dielectric
constant, and &egr;fluid is the fluid dielectric constant. For low
frequencies, &egr; is real. The dipole force is given by:
Experiments on both Bacillus thuringiensis spores
and polystyrene beads in the 200 nm to 10 µm size range show that electro-kinetic
transport is a balance of electro-osmotic flow (EOF), electrophoresis, and dielectrophoresis
In one aspect, the exemplary embodiment separates particles
by varying the traveling wave sweep frequency. The characteristic transport of traveling
waves is synchronous below a threshold sweep frequency and an asynchronous mode
above that. The distinction is the balance of Coulomb and dielectrophoretic forces
against drag whereby some particles are able to keep up and others are not. This
trait is retained for a fluidic environment, especially for larger and more dipolar
particles. A sample mixture of 1 µm, 3 µm, and 6 µm polystyrene beads
demonstrates that at 3Hz, all beads in the size range are transported. At 4 Hz,
some larger beads are stagnated by being trapped at traces. The reason is that their
displacement is shorter than the pitch of the traveling wave array so that they
are trapped in a situation where they move back and forth between the traces. At
6 Hz, all beads are trapped. This frequency sensitivity may be exploited in a separation
method. The strategy is to scan down in frequency to selectively move the more mobile
particles out of the mixture in sequential fashion.
In another aspect, the exemplary embodiment separates particles
by bending or turning a particle stream around a corner. Specifically, this mode
of separation involves moving the particle stream around a corner where the traveling
wave grids transition such that the fields also reflect a change in direction. This
strategy is motivated by the observation that when particles of various sizes concentrate
into a sample well, they appear to have different turning radii depending on their
relative size. FIGURE 6 shows three micrographs A, B, and C spanning the width of
a chevron grid region. The results are for a sample mixture of 1 µm, 3 µm
and 6 µm polystyrene beads. The 6 µm beads take a tighter turn around
the corner as is evident from the micrograph C. The smaller 1 µm and 3 µm
beads take a wider turn as depicted in micrographs A and B. The reason is that the
dielectrophoretic force scales with volume (r3) so larger beads experience
immediate effects of the turning field and are able to turn faster.
Referring to FIGURE 7, a traveling wave grid 500 in accordance
with the exemplary embodiment was utilized to further investigate and implement
this phenomenon. The array 500 comprises a chevron grid 550 and a rectilinear grid
510. A way to test the separation capability, albeit only an approximation, using
the exemplary embodiment separation strategy is to operate the array 500 in reverse.
A 100 µL volume of concentrated mixture of 1, 3 and 6 µm particles is
introduced into the sample well at a first end 554 of the chevron grid 550 and the
traveling wave grids 510 and 550 are operated in reverse to move the sample out
into the main rectilinear grid 510. Specifically, the particulates are transported
from the first end 554 to a second end 552 of the chevron grid, and then from or
near a first end or region 512 of the rectilinear grid 510 to a second end or region
514 of that grid 510. The path of the larger 6 µm particles is denoted by arrow
530. The path of the smaller 3 µm particles is denoted by arrow 540. The particles
that change direction are generally larger in size than particles that undergo the
same change in direction but along a longer distance. The particle mixture in the
relatively narrow channel of the chevron grid 550 is transported and focused by
the radial traveling wave array, i.e. the chevron grid 550, and injected into a
separation cavity with a linear traveling wave array, i.e. the rectilinear grid
510 moving particles upward. The relatively smaller beads or particles such as the
3 µm size beads move faster and arrive first. The larger 6 µm beads or
particles move slower and can react to directional change in a shorter distance
in sweeping around the corner such as denoted by D.
FIGURE 8 shows the results of this trial where the 1, 3,
and 6 µm beads are distributed over a 1 cm wide swath. Specifically, the path
of the 6 µm particles is noted by arrow 530. The path of the 3 µm particles
is noted by the arrow 540. And, the path of the 1 µm particles is noted by
the arrow 545. It is significant to note that both the paths of 6 µm and 3
µm particles underwent a 90° change in direction around corner D, within
a 0.5 cm span. This result is impressive considering that the chevrons are facing
a direction such that they tend to be dispersive rather than focusing. The low sample
density in the rectilinear chamber also requires microscopy to visualize the sample
The exemplary embodiment also provides a purification cell.
The combination of the noted traveling wave grid layouts and sample separation strategies
may be incorporated together with the concentration and focusing aspects of the
device to provide a purification cell 600 as shown in FIGURE 9. The purification
cell 600 includes a concentration chamber 610, a focusing channel 650, and a separation
chamber 670, 680. The top 680 of the separation chamber may be divided into a lateral
row of compartments 682, 684, 686, 688, and 690 to collect an increasing range of
particle sizes proceeding from left to right. For example, relatively large sized
particles constitute the stream denoted by arrow 672, which are subsequently collected
in compartment 690. Intermediate sized particles constitute the stream denoted by
arrow 674, which are subsequently collected in compartment 688. And relatively small
sized particles in stream 676 are collected in compartment 686. Streams of finer
sized particles can be collected in one or both of the compartments 682 and 684.
The traveling wave arrays in the separation chamber may be a continguous layout
of chevrons to focus particulates in the different size ranges into the designated
collection compartments at the top. The focusing section 650 forms a narrow stream
which will result in improved separation performance. Representative dimensions
for each portion or component of the cell 600 are provided on FIGURE 9.
FIGURE 10 shows another exemplary embodiment traveling
wave array 700 where a connecting bridge is utilized and disposed between the top
to close the loop on the cell. This strategy allows the contents of one of the collected
compartments to be re-circulated to result in increased purification. The purification
cell 700 includes a concentration chamber 710, a focusing channel 750, a separation
chamber 770, 780, and a connecting bridge 740. The top of the separation chamber
may be divided into a collection of compartments 782, 784, 786, 788, and 790 to
collect an increasing range of particle sizes proceeding from left to right. For
example, relatively large size particles constitute the stream denoted by arrow
772, which are subsequently collected in compartment 790. Intermediate sized particles
constitute the stream denoted by arrow 774, which are subsequently collected in
compartment 788. And relatively smaller sized particles in stream 776 are collected
in compartment 786. Streams of finer sized particles can be collected in one or
both of compartments 784 and 782. The connecting bridge 740 can be utilized to selectively
return particles of a particular size or size range, to the concentration chamber
710 if further processing is desired.
For large sample volumes, the exemplary embodiment purification
cell may be incorporated into the mFFF cell geometry as shown in FIGURE 11. Specifically,
the cell 800 comprises a concentration chamber 810, upper and lower regions 880
and 870 of a separation chamber, and a focusing channel 850 extending between the
concentration chamber 810 and the lower region 870 of the separation chamber. The
upper region 880 of the chamber, includes a collection of compartments for retaining
particles of different sizes, as described in conjunction with FIGURES 9 and 10.
Specifically, the cell 800 includes two spaced apart substrates or plates 820 and
830, one of which defines an inlet 822 for an inlet stream E, and an outlet 824
for an outlet stream F. As previously described with the configurations of FIGURES
9 and 10, the upper region 880 of the separation chamber includes a plurality of
compartments 882, 884, 886, 888, and 890 for collecting particles of different sizes
or size ranges. The operation of the purification cell is as follows. A sample stream
E enters the cell 800 via inlet 822. The entering sample flows into the concentration
chamber 810. A compression field moves particulates downward to the near vicinity
of the lower surface where the traveling wave grid disposed therein transports the
stream and components therein, toward the focusing channel 850. Once the sample
is in the channel 850, the chevron traveling wave grid extending therein, transports
and directs the sample to the noted separation chamber. The orientation of the separation
chamber is generally transverse to the direction of flow of the sample in the focusing
channel 850. As the stream enters the lower region 870 of the separation chamber,
as previously described, the particulates separate into discrete streams 872, 874,
and 876. The largest particles collect in compartment 890. Smaller sized particles
collect in the other compartments. The remaining portion of the stream exits the
cell 800 at outlet 824 as stream F.
The various purification cells of the exemplary embodiment
can employ cascaded functions of concentration, focusing, and separation. The cells
can feature a constant volume design, a flow-through configuration with increasing
volume, or utilize a constant volume with a recirculating transport to achieve higher
The advantages of the exemplary embodiment include but
are not limited to new traveling wave grid configurations to increase mass flow
and to minimize congestion and stagnation; the provision of new strategies for separation;
and the provision of a purification cell which can handle tens of milliliters as
compared to existing methods which are complicated and only handle up to several
Potential applications of the exemplary embodiment include
but are not limited to pre-concentrators for front-end detection in bio-defense
applications; water supply monitoring for utilities; food toxicology; blood plasma
separation; cell enrichment; and protein purification.