# Patent

## Title
Apparatus for separating particles utilizing engineered acoustic contrast capture particles

## Abstract

An apparatus for separating particles from a medium includes a capillary defining a flow path therein that is in fluid communication with a medium source. The medium source includes engineered acoustic contrast capture particle having a predetermined acoustic contrast. The apparatus includes a vibration generator that is operable to produce at least one acoustic field within the flow path. The acoustic field produces a force potential minima for positive acoustic contrast particles and a force potential minima for negative acoustic contrast particles in the flow path and drives the engineered acoustic contrast capture particles to either the force potential minima for positive acoustic contrast particles or the force potential minima for negative acoustic contrast particles.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 11/784,928, filed Apr. 9, 2007, the contents of which is incorporated by reference herein in its entirety.


STATEMENT OF GOVERNMENT INTEREST
This invention was made with Government support under Contract Number DE-AC51-06NA25396 awarded by the United States Department of Energy. The Government has certain rights in the invention.


TECHNICAL FIELD
The present invention relates, in general, to field-based separation of particles in a medium utilizing acoustic pressure.
BACKGROUND
It was realized early in ultrasonic transport phenomena that particles with different mechanical properties (density and compressibility) can be separated in a solution. Particles in an ultrasonic standing wave field that are more dense and less compressible than the background medium are transported to a different spatial location than particles that are less dense and more compressible than the background medium, producing a true binary separator based upon mechanical properties. Ultrasonic radiation pressure has been demonstrated as a viable means to manipulate and locally trap particles in microfluidic environments. In application, the acoustic radiation pressure force depends upon frequency of excitation, pressure amplitude within the medium, and the density/compressibility contrast between the particle of interest and the host medium. Within an applied ultrasonic standing wave, particles experience a drift force resulting from acoustic radiation pressure that transports the particles to a position within the applied standing wave that corresponds to minima in the acoustic radiation pressure force potential. In general, the location of this minima is located in a different spatial location for particles that are more dense and less compressible than the background medium in comparison to particles that are less dense and more compressible. The technique has been successfully demonstrated in particle conditioning experiments involving trajectory steering, agglomeration, retainment, mixing, selective retainment and deposition of cells on a surface.
Separation utilizing acoustic radiation pressure has not been readily implemented in biological problems since most biological particles of interest (e.g. red blood cells, white blood cells, bacteria, spores, etc.) all are more dense and less compressible than water. Recent biological assays have implemented means to separate analytes from sample solutions by taking advantage of their physical properties. For instance, positively charged silica surfaces are used to capture highly negative charged DNA molecules from complex samples. More recently, capture particles have been employed to capture biological particles of interest. This has been especially true for magnetic field-base purification methods where magnetic particles are used to capture particles of interest from within a sample and then separated from the sample using magnetic fields.
It is desirable, therefore, to provide an apparatus and method for separating particles within a medium utilizing engineered capture particles and acoustic radiation pressure.
SUMMARY
An apparatus and a method for separating particles from a medium includes a capillary defining a flow path therein that is in fluid communication with a medium source. The medium source includes engineered acoustic contrast capture particle having a predetermined acoustic contrast. The apparatus includes a vibration generator that is operable to produce at least one acoustic field within the flow path. The acoustic field produces a force potential minima for positive acoustic contrast particles and a force potential minima for negative acoustic contrast particles in the flow path and drives the engineered acoustic contrast capture particles to either the force potential minima for positive acoustic contrast particles or the force potential minima for negative acoustic contrast particles.
The engineered acoustic contrast capture particles may have a negative acoustic contrast and may have a density/compressibility ratio less than that of the medium source. Alternatively, the engineered acoustic contrast capture particles may have a positive acoustic contrast and may have a density/compressibility ratio greater than that of the medium source.
Alternatively, the medium source includes particles having positive acoustic contrast and particles having negative acoustic contrast and the acoustic field drives the positive acoustic contrast particles to the potential minima for positive acoustic contrast particles and the negative acoustic contrast particles to the potential minima for negative acoustic contrast particles. The positive acoustic contrast particles may be bioparticles. The force potential minima for positive contrast particles may be a pressure node and the force potential minima for negative contrast particles may be a pressure antinode.
Alternatively, the engineered acoustic contrast capture particles are functionalized to bind to corresponding targets in the medium. The engineered acoustic contrast capture particles may bind to the targets with antibodies. The targets may be biological molecules or particles. The vibration generator may be a transducer or a line-drive element.
Alternatively, the acoustic field is a dipole acoustic field and the force potential minima for positive acoustic contrast particles may be located along a center of the flow path and the force potential minima for negative acoustic contrast particles may be located adjacent a wall of the capillary. Alternatively, the acoustic field is an axisymmetric acoustic field and the force potential minima for negative acoustic contrast particles may be located along a center of the flow path and the force potential minima for positive acoustic contrast particles may be located adjacent a wall of the capillary.
Alternatively, the capillary is an inner capillary disposed within an outer capillary and the vibration generator may be disposed adjacent the outer capillary. The capillary may be quartz, glass, or plastic. The apparatus may further comprise a laser beam for analysis of the particles in the medium. The vibration generator may be capable of alternately producing a dipole acoustic field and an axisymmetric acoustic field.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
FIG. 1 is a schematic of contrast factors of various materials;
FIGS. 2a, 2b and 2c are schematic views of an apparatus in accordance with the present invention shown in various stages of acoustic field excitation;
FIGS. 3a and 3b are microscopic photographs showing bioparticles and capture particles in a capillary of the present invention; and
FIG. 4 is a schematic view of an apparatus in accordance with the present invention shown with an analyzing laser.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The acoustic radiation pressure force on a compressible, spherical particle of volume V in an arbitrary acoustic field can be written in terms of an acoustic radiation pressure force potential U:




  U
  =
  
    
      4
      3
    
    ⁢
    π
    ⁢
    
      
    
    ⁢
    
      
        a
        3
      
      [
      
        
          
            (
            
              
                β
                o
              
              ⁢
              
                
                  〈
                  
                    p
                    2
                  
                  〉
                
                2
              
            
            )
          
          ⁢
          
            f
            1
          
        
        -
        
          
            3
            2
          
          ⁢
          
            (
            
              
                
                  ρ
                  o
                
                ⁢
                
                  〈
                  
                    v
                    2
                  
                  〉
                
              
              2
            
            )
          
          ⁢
          
            f
            2
          
        
      
      ]
    
  




Here, a is the particle radius, βo is the compressibility of the surrounding fluid, and ρo is the density of the surrounding fluid. The pressure and velocity of the acoustic field in the absence of the particle are described by p and v, respectively, and the brackets correspond to a time-averaged quantity. The terms f1 and f2 are the contrast terms that determine how the mechanical properties of the particle differ from the background medium. They are given by:



  
    
      f
      1
    
    =
    
      1
      -
      
        
          β
          p
        
        
          β
          o
        
      
    
  




  
    
      f
      2
    
    =
    
      
        2
        ⁢
        
          (
          
            
              ρ
              p
            
            -
            
              ρ
              o
            
          
          )
        
      
      
        (
        
          
            2
            ⁢
            
              ρ
              p
            
          
          -
          
            ρ
            o
          
        
        )
      
    
  



The subscript p corresponds to intrinsic properties of the particle. The force F acting on a particle is related to the gradient of the force potential by:

F=−ΔU 

Particles will be localized at positions where the potential U displays a minima.
For acoustic wavefields comprised of plane waves, the potential minima of U occur at either pressure nodes or antinodes depending upon the signs of f1 and f2. Most particles and cells of interest in a water or aqueous background medium have properties that yield positive values of f1 and f2. These particles are termed positive acoustic contrast particles. Under the plane wave approximation, these particles will migrate to the pressure nodes (velocity antinodes) in the system. Examples of these particles in water-based buffers include erythrocytes, leukocytes, bacteria, yeast, or any other particles where density and compressibility contrast terms f1 and f2 are positive. Materials such as fat globules and gas bubbles yield negative values of both f1 and f2 and are termed negative contrast particles. Under the plane wave approximation, these particles will move to the location of a pressure antinode. This system can be used as a binary particle separator that spatially localizes particles into discrete locations in the wavefield (pressure nodes or antinodes) based upon positive or negative acoustic contrast of the particles relative to the background medium.
When the wavefield takes on greater complexity than described by plane wave approximations (e.g. cylindrical or spherical waves), the location of the localization of the particles within the wavefield is determined by the minima of the force potential U. In these cases, the pressure nodes and antinodes do not generally occur at the same spatial locations as the potential minima. But, similar to the plane wave case, the particles can generally be separated in a binary manner based upon their intrinsic properties relative to the background medium. The binary spatial positions where positive and negative acoustic contrast particles localize is now determined by minima in the force potential U.
The contrast factors f1 and f2 demonstrate the significance of the relative values of the intrinsic properties of the particles and the medium. As shown in FIG. 1, for most particles in bio-research and biomedical applications including, but not limited to, erythrocytes, phagocytes, spores, yeast, fungi, and bacteria, as well as plastics, sand, common sediments, and metals, the contrast term is positive for particles in an aqueous background and all particles in a given sample are forced to the location of the same potential minima in the standing wave field. Therefore, acoustic binary separation of most biological particles in an aqueous solution is not feasible without adjusting the properties of the background medium. Separation of lipids from blood by using their negative acoustic contrast relative to the other blood components has been demonstrated by others.
The engineered acoustic contrast capture particles, when attached to bioparticles of interest, shifts the effective density and compressibility of the bioparticle/capture particle construct from positive to negative or negative to positive depending upon the preferred separation criteria relative to other particles in the system. As an example, an engineered capture particle is designed with low density and high compressibility relative to an aqueous medium. The particle is functionalized with anit-CD45 ligands and introduced into a blood sample to bind to CD45 markers on leukocytes. Under conditions where the blood sample is suspended in an aqueous buffer, both the erythrocytes and leukocytes have positive acoustic contrast and under the action of an acoustic standing wave, are forced to the same potential minima. By attaching an engineered particle with negative acoustic contrast to the leukocyte, the capture particle/leukocyte construct takes on negative acoustic contrast and be advantageously separated from the positive contrast population of erythrocytes.
Referring now to FIGS. 2a-2c, apparatus in accordance with the present invention is indicated generally at 10. The apparatus 10 includes capillary 12 defining therein flow path or stream 14 in fluid communication with a medium source (not shown) within walls 16 of capillary 12. Capillary 12 is preferably constructed from quartz, glass, plastic, or any suitable material. Preferably, capillary 12 is circular in cross-section but those skilled in the art will appreciate that the cross-section of capillary 12 may be of any suitable geometry including, but not limited to, elliptical, square, rectangular or the like. The medium source may supply water, or any suitable liquid to flow path or stream 14, as will be appreciated by those skilled in the art. Capillary 12 is fully disposed within outer capillary 18. Vibration generator 20, such as a transducer the like, is disposed adjacent or mounted to an exterior surface of capillary 18 and is capable of producing at least one acoustic or displacement field within flow stream 14 of capillary 12. Alternatively, vibration generator 20 is a line-drive element, a displacement generator, or any other type of vibration generator adjacent or mounted to the exterior surface of capillary 12 capable of producing at least one acoustic or displacement field within flow path or stream 14 including, but not limited to, a contact aperture, a line contact, and a full encompassment of capillary 12, as will be appreciated by those skilled in the art. Under different symmetry excitations of the displacement field, the position of a force potential minima for positive contrast particles can be located along the central axis or flow stream 14 of capillary 12 or the force potential minima for positive contrast particles can be located at a position along wall 16. In these situations, the force potential minima for negative acoustic contrast particles will be located at the position not occupied by positive acoustic contrast particles (e.g. positive acoustic contrast particles along the central axis of flow stream 14 corresponds to negative acoustic contrast particles at a position along capillary wall 16 or negative acoustic contrast particles along the central axis of flow stream 14 corresponds to positive acoustic contrast particles at a position along capillary wall 16).
Disposed within flow stream 14 are bioparticles 22 and engineered acoustic contrast capture particles 24. As shown in FIG. 2a, with no acoustic field being generated by vibration generator 20 (i.e. the acoustic field is inactive), bioparticles 22 and negative contrast capture particles 24 are homogeneously distributed within flow stream 14 of capillary 12. As shown in FIG. 2b, when a dipole acoustic excitation and resulting dipole acoustic field is implemented by vibration generator 20, bioparticles 22 are focused to the center of flow stream 14 of capillary 12 (force potential minima for positive acoustic contrast particles) and engineered acoustic contrast capture particles with negative contrast 24 are transported to capillary wall 16 (force potential minima for negative acoustic contrast particles). Conversely, as shown in FIG. 2c, when an axisymmetric excitation and resulting axisymmetric acoustic field is implemented by vibration generator 20, all bioparticles 22 are sent to capillary wall 16 (force potential minima for positive acoustic contrast particles) and engineered acoustic contrast capture particles with negative contrast 24 are focused to the central axis of flow stream 14 of capillary 12 (force potential minima for negative acoustic contrast particles).
In a non-limiting example, background bioparticles 22 comprising 6.mu.m latex spheres and engineered acoustic contrast capture particles with negative contrast 24 comprising hollow, glass microspheres having low density, high compressibility characteristics were utilized in apparatus 10. Upon activation of vibration generator 20 generating a dipole acoustic field, such as shown in FIG. 2b, latex spheres 22 are focused to the center of flow stream 14 of capillary 12 while hollow glass spheres 24 are transported to wall 16 of capillary 12. This is shown in FIGS. 3a and 3b. A microscope with a narrow depth of field (not shown) is utilized to demonstrate the location of positive and negative contrast particles in apparatus 10. In FIG. 3a, the focal plane of the microscope intersects the central axis of flow stream 14 of capillary 12 where positive contrast particles 22 (latex spheres) have been focused along the axis and engineered negative contrast particles (glass microspheres) can be seen along capillary wall 16. In FIG. 3b, the focal plane of the microscope intersects an uppermost boundary of inner capillary wall 16. Here, negative contrast particles 24 (glass microspheres) are located adjacent capillary wall 16. Before activation of the acoustic field, particles 22 and 24 were homogenously distributed throughout capillary 12, such as shown in FIG. 2a. 
Referring now to FIG. 4, in this embodiment, engineered acoustic contrast capture particles with negative contrast 24′ are functionalized with a specific coating to bind a target particle of interest, such as by functionalizing and coating engineered negative contrast capture particles 24′ with antibody 26. Negative contrast capture particles 24′ and antibodies 26 are mixed with a sample, incubated, and are bound to target cells 28. Target cells 28 are preferably biological molecules or particles. Upon flowing the sample through flow stream 14 of capillary 12 excited in an axisymmetric vibration mode by vibration generator 20, engineered negative contrast capture particles 24′ with captured target cells 28 are focused to the center of flow stream 14 of capillary 12 for analysis by laser beam 30 (flow cytometry) or separation. The sample background particles (as they are bioparticles similar to bioparticles 22 shown in FIGS. 2a-2c) are forced to wall 16 of capillary 12. In a flow cytometer (not shown), laser beam 30 is focused to probe only those particles that transport along the axis of flow stream 14 of capillary 12.
Apparatus 10 preferably includes vibration generator 20 capable of driving capillary 12 in both axisymmetric and dipole modes, advantageously allowing for separate analysis of target particles 28 and remaining sample background particles 22. Engineered negative acoustic contrast capture particles 24 are transported to the position of a minima of the force potential for negative contrast particles in an acoustic field. Conversely, all other bioparticles 22 are transported to the position of a minima of the force potential for positive contrast particles in an acoustic field. In a flow cytometer, functionalized, engineered negative contrast capture particles 24′ can be mixed with a sample. The untrapped sample or specific binding to capture particles 24′ can be analyzed by placing or designing either the force potential minima for positive acoustic contrast particles or negative acoustic contrast particles at the center of the sample analysis flow stream 14.
Apparatus 10 advantageously takes advantage of the fact that most bioparticles, such as bioparticles 22 shown in FIG. 2a through FIG. 4, possess positive acoustic contrast and can be spatially isolated at a specific predetermined location defined by the force potential minima for positive acoustic contrast particles in an acoustic field, as the only particles that have negative contrast are acoustically similar to lipids/fats. By engineering capture particles 24 with a negative acoustic contrast, it is possible to separate engineered negative contrast capture particles 24 and 24′ from biological particles 22 (and most other background particles 22) in a sample, as shown in FIG. 4. Under the action of an acoustic field, biological particles 22 will transport to the minima in the force potential for positive contrast particle, while engineered negative contrast capture particles 24 will transport to the minima in the force potential for negative contrast particles. In general, the spatial location defined by the force potential minima of positive and negative acoustic contrast particles are isolated from one another or at distinct predetermined locations within flow stream 14 of capillary 12.
Apparatus 10 advantageously employs engineered acoustic contrast capture particles with negative contrast 24 and 24′ having a density/compressibility ratio less than that of the medium source (i.e. particles 24 and 24′ are less dense and more compressible than the background fluid medium in the medium source, such as water). If the medium in flow stream 14 of capillary 12 is not water, the contrast properties of engineered acoustic contrast capture particles 24 and 24′ may be advantageously adjusted according to the medium to take on either negative or positive contrast values. Engineered acoustic contrast capture particles 24 and 24′ are used to capture bioparticles of interest 28 in a sample. When used in conjunction with an acoustically driven cylindrical channel, engineered acoustic contrast capture particles 24 and 24′ can be forced to the center of flow stream 14 or wall 16 of capillary 12 and separated from the remaining sample constituents. This is very effective in new assays for flow cytometry that require target cells or constituents to be separated from a background. Alternatively, engineered acoustic contrast particles 24 and 24′ have a positive acoustic contrast and have a density/compressibility ratio greater than that of the medium source (i.e. particles 24 and 24′ are more dense and less compressible than the background fluid medium in the medium source, such as water).
Apparatus 10 may be advantageously utilized in commercial applications including, but not limited to, applications involving the separation of specific targets in biological samples.
Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above and/or in the attachments, and of the corresponding application(s), are hereby incorporated by reference.

## Claims

1. A system comprising:
a capillary defining a flow path therein,
a fluid medium source coupled to the capillary, the fluid medium source configured to supply a fluid medium to the capillary;
a vibration generator coupled to the capillary; and
a plurality of engineered negative acoustic contrast capture particles disposed in the fluid medium, each of the engineered negative acoustic contrast capture particles having a lower density/compressibility ratio than the density/compressibility ratio of the fluid medium.

2. The system of claim 1 wherein the vibration generator is configured to produce an acoustic field within the flow path of the capillary.

3. The system of claim 1 wherein the vibration generator is configured to produce a force potential minima in the fluid medium, the force potential minima driving the engineered negative acoustic contrast capture particles to the minima.

4. The system of claim 1 wherein the engineered negative acoustic contrast particles comprise a coating to bind a target particle in the fluid medium to the engineered negative acoustic contract particles.

5. The system of claim 1 further comprising a plurality of positive acoustic contrast particles, the positive acoustic contrast particles having a higher density/compressibility ratio than the density/compressibility ratio of the fluid medium.

6. The system of claim 5 wherein the vibration generator is configured to produce a first force potential minima in the flow path and a second force potential minima in the flow path, the first force potential minima configured to drive the engineered negative acoustic contrast particles to the first force potential minima, and the second force potential minima configured to drive the positive acoustic contrast particles to the second potential minima.

7. The system of claim 1 wherein the capillary is a cylindrical capillary.

8. The system of claim 5 wherein the positive acoustic contrast particles comprise bioparticles.

9. The system of claim 4 wherein the target particle has a negative acoustic contrast.

10. The system of claim 1 wherein the engineered negative acoustic contrast particles are glass microspheres.

11. A method of separating particles in a fluid medium, the method comprising:
flowing the fluid medium in a capillary, the fluid medium comprising a plurality of engineered negative acoustic contrast particles and one or more targets; and
imparting to the fluid medium, via a vibration generator coupled to the capillary, acoustic pressure,
wherein the acoustic pressure is configured to generate a first force potential minima in the fluid media, the first force potential minima configured to drive the engineered negative acoustic contrast particles to the first force potential minima.

12. The method of claim 11 wherein the engineered negative acoustic contrast particles have a density/compressibility ratio that is less than the density/compressibility ratio of the fluid medium.

13. The method of claim 11 wherein the fluid medium comprises a plurality of positive acoustic contrast particles.

14. The method of claim 13 wherein the vibration generator is configured to generate a second force potential minima in the fluid media, the second force potential minima configured to drive the positive acoustic contrast particles to the second force potential minima.

15. The method of claim 13 wherein the positive acoustic contrast particles are bioparticles.

16. The method of claim 13 wherein the positive acoustic contrast particles have a density/compressibility ratio that is greater than the density/compressibility ratio of the fluid medium.

17. The method of claim 11 wherein the engineered negative acoustic contrast particles are glass microspheres.

18. The method of claim 11 wherein the engineered negative acoustic contrast particles comprise a coating to bind the one or more targets to the engineered negative acoustic contrast particle.

19. The method of claim 18 wherein the one or more targets have an acoustic contrast that has a lower density/compressibility ratio than the fluid medium.

20. The method of claim 18, wherein the one or more targets have a negative acoustic contrast.