Patent Publication Number: US-2010117007-A1

Title: System and method for separating micro-particles

Description:
RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 11/087,174, filed Mar. 22, 2005; which is a Continuation of application Ser. No. 09/843,902, filed Apr. 27, 2001; which is related to and claims priority from Provisional Application Ser. No. 60/248,451, filed Nov. 13, 2000, the contents of which are incorporated by reference herein in its entirety as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to systems and methods for separating micro-particles and/or nano-particles. More particularly, this invention relates to systems and methods for separating micro-particles and/or nano-particles by using a light source to create a separation force on the particles based on their physical properties. 
     BACKGROUND OF THE INVENTION 
     At the present, there are sorting methods to separate particles, such as cells and other biological entities, based on their size, density, and charge, but none that sort based on optical dielectric properties. For example, laser tweezers have been described that use the interaction of light with a particle to move the particle around. However, in this case, a priori knowledge of which particle to move is required for the tweezers to be used as a sorting mechanism. In other words, tweezers are more of a ‘manipulation and/or transportation’ tool, rather than a ‘sorting’ tool. Thus, current methods and systems for separating particles require prior identification of the particles to be separated. 
     There is a need for a system and method for separating particles which does not require prior identification of the particles to be separated There is also a need for a system and method for separating particles which does not damage the particles. 
     SUMMARY OF THE INVENTION 
     These needs and others are satisfied by a system and method for separating particles according to the present invention which comprises means for creating a light intensity pattern in the vicinity of the particles and means for moving the light intensity pattern with respect to the particles. The means for creating a light intensity pattern can comprise a light source for producing two light beams aimed to interfere with each other in the vicinity of the two particles. 
     In one embodiment, the system comprises a beam splitter and a reflector. In this embodiment, the light source is configured to produce a light beam aimed at the beam splitter. The beam splitter is configured to split the light beam into a first light beam directed toward the particles and a second light beam directed toward the reflector. The reflector is configured to redirect the second light beam toward the particles such that the first and second light beams interfere creating a light intensity pattern in the vicinity of the particles. 
     An actuator can be connected to the reflector for moving the reflector to move the light intensity pattern. Alternatively, the actuator can be connected to the light source and beam splitter for moving the light source and beam splitter. 
     It is also possible to move the particles relative to the light intensity pattern to create the moving light intensity pattern. In order to do this, the particles can be carried on a slide connected to an actuator configured to move the slide relative to the light intensity pattern. 
     The light intensity pattern can also be moved by using a phase modulator to modulate the phase of one of the two light beams with respect to the other. This causes the light intensity pattern created by the interference of the light beams to move spatially. The phase modulator can be place in the path of either the first light beam or second light beam. Alternatively, an amplitude modulator can be used, in which case the interference pattern will move temporally. 
     Any material that responds to optical sources may be utilized with these inventions. In the biological realm, examples would include cells, organelles, proteins and DNA, and in the non-biological realm could include metals, semiconductors, insulators, polymers and other inorganic materials. 
     Preferably, the light source comprises a laser producing a light beam having a wavelength of between 0.3 μm and 1.8 μm. Using a light beam in this wavelength range minimizes the chance that damage will be caused to the particles if they are living cells or biological entities. Even more preferably, the light beam wavelength range could be 0.8 μm and 1.8 μm. Good, commercially available lasers are available which produce a light beam having a wavelength of 1.55 μm. 
     In an alternative embodiment, the system comprises a light source and an optical mask. The light source is configured for producing a light beam directed through the optical mask toward the particles. The optical mask creates a light intensity pattern in the vicinity of the particles. An actuator can be connected to the light source and optical mask for moving the light source and optical mask to create a moving light intensity pattern. Alternatively, the optical mask can be specially configured for producing a moving light intensity pattern in the vicinity of the at least two particles. Another alternative is to include a phase modulator positioned in the light beam path for modulating the phase of the light beam to create a moving light intensity pattern. 
     In yet another embodiment the system can comprise a plurality of light sources positioned adjacent to each other for producing a plurality of light beams directed toward the particles The light beams can be aimed to slightly overlap each other to create a light intensity pattern. An actuator can be included for moving the plurality of light sources, thus causing the light intensity pattern to move spatially. Alternatively, the light beams can be dimmed and brightened in a pattern for creating a temporally moving light intensity pattern. 
     A method for separating particles according to the present invention comprises the steps of. applying a light source to create a light intensity pattern, exposing particles to the light intensity pattern producing force on each particle and moving the light intensity pattern with respect to the particles causing the particles to move with the light intensity pattern at velocities related to their respective physical properties. If the particles have different physical properties they will move at a different velocity causing the particles to separate. 
     Preferably, the step of applying a light source comprises interfering at least two optical light beams as discussed herein with respect to one embodiment of a system according to the present invention. 
     Alternatively, the step of applying a light source can comprise using an optical mask to create the light intensity pattern. The optical mask can comprise an amplitude mask, a phase mask, a holographic mask, or any other suitable mask for creating a light intensity pattern. 
     In another embodiment of a method according to the present invention the step of applying a light source can comprise periodically dimming and brightening a plurality of light sources to create the light intensity pattern. 
     Preferably, the light intensity pattern comprises at least two peaks and at least two valleys. The light intensity pattern can be periodic, sinusoidal, nonsinusoidal, constant in time, or varying in time. If the light intensity pattern is periodic, the period can be optimized to create separation between particles. 
     In one embodiment, the method comprises moving the light intensity pattern at a constant velocity. The velocity of the light intensity pattern can be optimized to cause separation based on the physical properties of the particles. 
     In an alternative embodiment, the method comprises allowing the at least two particles to separate, and then suddenly “jerking” the light intensity pattern to cause particles with different physical properties to fall into different valleys of a potential pattern created by the light intensity pattern. 
     The method light intensity pattern can be tuned to a resonant frequency corresponding to the physical properties of one type of particles to optimize separation of that type of particle. The light intensity pattern can be applied in multiple dimensions and the period of the light intensity pattern can be varied in each dimension. 
     The particles can be carried in a medium, such as a fluidic medium, which can be either guided or non-guided. If the medium is guided it can include fluidic channels. 
     The method can also include superimposing a gradient onto the light intensity pattern. The gradient can be spatially constant or varying and can comprise temperature, pH, viscosity, etc. Additional external forces can also be applied, such as magnetism, electrical forces, gravitational forces, fluidic forces, frictional forces, electromagnetic forces, etc., in a constant or varying fashion. 
     A monitoring and/or feedback system can also be included for monitoring the separation between particles and providing feedback information as to separation and location of particles. 
     Further object, features and advantages of the present invention will become apparent from the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B,  2 A,  2 B and  3  are block diagrams of various embodiments of a system according to the present invention; 
         FIG. 4A  is a graphical depiction of an optical grating produced light intensity pattern generated by a system according to the present invention. 
         FIG. 4B  is a graphical depiction of an energy pattern corresponding to the light intensity pattern of  FIG. 4A . 
         FIG. 4C  is a graphical depiction of a potential energy pattern corresponding to the light intensity pattern of  FIG. 4A . 
         FIGS. 5A ,  5 B and  5 C are a graphical depiction of a moving potential energy pattern generated by a system and method according to the present invention. 
         FIG. 6  is an enlarged sectional view of a fluidic micro-channel with a graphical depiction of the moving light intensity pattern of  FIG. 4A  superimposed in the fluidic micro-channel. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, a system and method for separating particles is described that provides distinct advantages when compared to those of the prior art. The invention can best be understood with reference to the accompanying drawing figures. 
     Referring now to the drawings, a system according the present invention is generally designated by reference numeral  10 . The system  10  is configured to generate a moving light intensity pattern that produces a force on the particles to be separated. The force causes the particles to move at velocities related to certain physical properties of each particle, such as the particle&#39;s optical dielectric constant. Particles with different physical properties will move at different velocities causing the particles to separate based on their physical properties. 
     One embodiment of a system  10  according to the present invention is shown in  FIG. 1 . In this embodiment, the system  10  comprises a light source  12 , a beam splitter  14 , and a reflector  16 . A motor  18  can be connected to the reflector  16  for moving or rotating the reflector  16 . A control system  19  is connected to the motor  18  for controlling operation of the motor  18  and thus movement of the reflector  16 . 
     The particles to be separated can be placed in a medium on a slide  22 . In one embodiment of the invention, the slide  22  includes a non-guided fluidic medium, such as water. In another embodiment, shown in  FIG. 6 , the slide  22  includes fluidic channels  500 ,  502  and  504  through which the particles  410 ,  412  travel. 
     The medium can be non-guided or guided. One example of a guided medium is a medium comprising fluidic channels as is well known in the art. 
     The light source  12  is positioned to produce a light beam  24  that is aimed at the beam splitter  14 . The beam splitter  14  splits the light beam  24  into two light beams  26 ,  28  and directs one of the light beams  26  toward the reflector  16  and the other light beam  28  toward the slide  22 . The reflector  16  redirects light beam  26  toward the slide  22 . The light beams  26 ,  28  are focused near the particles and aimed to interfere with each other to create a light intensity pattern near the particles. 
     The motor  18  can be used to move or rotate the reflector  16 , which causes the light intensity pattern to move in space. A control system  19  is connected to the motor  18  to control operation of the motor  18 . By moving the light intensity pattern in space and keeping the slide  22  fixed, forces created on the particles by the light intensity pattern cause the particles to move at velocities related to each particle&#39;s physical properties as described herein. The particles can also be caused to move by fixing the light intensity pattern in space and mechanically moving the slide  22  carrying the particles. This causes the light intensity pattern to move in space relative to the particles. 
     Alternatively, motor  18  can be connected to the light source  12  and beam splitter  14 . In this embodiment the light source  12  and beam slitter  14  can be moved or rotated by the motor  18 . This causes light beam  28  to move relative to light beam  26 , which, in turn, causes the light intensity pattern to move. 
     In another embodiment, shown in  FIG. 1B , the light intensity pattern is moved by modulating the relative phase of the light beams  26 ,  28 . In this embodiment, a phase modulator  20  is positioned in the path of light beam  26 . The phase modulator  20  is configured to modulate the phase of light beam  26  relative to the phase of light beam  28 . A control system  19  is connected to the phase modulator  20  for controlling operation of the phase modulator  20 . Alternatively, the phase modulator  20  can be positioned in the path of light beam  28  for modulating the phase of light beam  28  relative to the phase of light beam  26 . 
     Modulating the phases of light beams  26  and  28  relative to each other causes the light intensity pattern created by the interference of light beams  26  and  28  to move. Moving the light intensity pattern relative to the particles creates forces on the particles related to the physical properties of each particle. As described above, these forces will cause particles with different physical properties to move at different relative velocities. 
     Alternatively, an amplitude modulator can be used instead of the phase modulator  20 . The amplitude modulator can be used for modulating the amplitude of the light beams  24 ,  26 ,  28  thus creating a moving light intensity pattern. 
     Preferably, the light source  12  comprises a laser for producing light beams  26  and  28  coherent with respect to each other. Alternatively, two light sources could be used to produce light beams  26  and  28 . 
     In applications where the particles are biological material or living cells, it is preferable that the laser produce light beams  26 ,  28  having a wavelength of between 0.3 μm and 1.8 μm so as not to generate excessive heat that could damage the particles. More preferably, the laser would produce light beams  26 ,  28  having a wavelength of greater than 0.8 μm. Very good lasers are commercially available which produce light beams  26 ,  28  having a wavelength of 1.55 μm and would be appropriate for use in a system  10  according to the present invention. Alternatively, the light source  12  can produce incoherent light beams  26 ,  28 . 
     In another embodiment of the invention, shown in  FIG. 2A , the system  110  comprises a light source  112  and an optical mask  114 . A motor  116  can be connected to the light source  112  and optical mask  114  for moving or rotating the light source  112  and optical mask  114 . A control system  119  is connected to the motor  116  for controlling operation of the motor  116  and thus movement of the light source  112  and optical mask  114 . In this embodiment, the light source  112  produces a light beam  118  that is aimed through the optical mask  114  toward a slide  120  holding the particles to be separated. 
     The optical mask  114  is configured to create a light intensity pattern near the particles. The motor  116  can be used to move or rotate the light source  112  and optical mask  114  thus causing the light intensity pattern to move. Alternatively, the light intensity pattern can be fixed in space and the slide  120  can be moved producing relative motion between the light intensity pattern and the particles. 
     The optical mask  114  can comprise an optical phase mask, an optical amplitude mask, a holographic mask or any similar mask or device for creating a light intensity pattern. In another alternative embodiment, the optical mask  114  can be specially configured to produce a moving light intensity pattern. This type of optical mask  114  can be produced by writing on the mask with at least two light beams. In essence, one light beam writes on the mask to create the light intensity pattern and the other mask erases the mask. In this embodiment, a new light intensity pattern is created each time the mask is written upon. 
     In the embodiment shown in  FIG. 2B , a phase modulator  122  is used to create the moving light intensity pattern. The phase modulator  122  is positioned between the light source  112  and the optical mask  114  such that light beam  118  is directed through the phase modulator  112 . A control system  119  is connected to the phase modulator  122  for controlling operation of the phase modulator  112 . 
     In yet another embodiment, shown in  FIG. 3 , the system  10  comprises a plurality of light sources  212  positioned adjacent to each other such that they produce light beams  214  directed toward a slide  216  holding the particles to be separated. In one embodiment, the light sources  212  are aimed to create light beams  214  that overlap each other to produce a light intensity pattern. 
     An actuator  218  can be attached to the light sources  212  for moving or rotating the light sources  212  to move the light intensity pattern with respect to the slide  216 . A control system  219  is connected to the actuator  218  for controlling operation of the actuator  218 . For example, motors (not shown) can be attached to each of the light sources  212 . The light intensity pattern can also be moved relative to the slide  216  by modulating phase, moving the slide  216  relative to the light sources  212  or in any other described herein. 
     Alternatively, the light sources  212  can be aimed such that the light beams  214  slightly overlap each other near the slide  216 . A light intensity pattern can be created by switching the light sources to be dimmed and brightened in certain patterns to give the appearance of a moving light intensity pattern. For example, in one embodiment the light sources  212  are dimmed and brightened such that at any given moment in time, whenever one light source is bright, all adjacent light sources are dim and when the first light source is dim the adjacent light sources are bright. 
     In operation, focusing a light beam in the vicinity of a particle causes the light beam to interact with optical dipoles inside the particle. Maximum intensity of a light beam is achieved at the focal point of the beam. The particle tends to move toward the point of maximum intensity of the light beam because the minimum energy for the overall system is achieved when the dipoles of the particle reside where the maximum intensity of the light beam occurs. 
     A system according to the present invention, such as those described infra, are configured to create a variable light intensity pattern.  FIGS. 4A ,  4 B, and  4 C show a periodic light intensity pattern  400 , the force  402  exerted on a particle by the light intensity pattern  400 , and the potential  404  exerted on a particle by the light intensity pattern  400 , respectively. The light intensity pattern  400  shown in  FIG. 4A  is sometimes referred to as an optical grating. 
     Particles subjected to the light intensity pattern  400  of  FIG. 4A  tend to move toward the peak intensity points  406 . The wells  408  of the potential pattern  404  shown in  FIG. 4C  represent points where the overall system energy is at a minimum. Thus, a particle will tend to move toward the wells  408  of the potential pattern  404 . 
     Light intensity patterns  400  created according to the present invention can comprise at least two peaks  406  and at least two valleys  407 . Suitable light intensity patterns  400  can be periodic, sinusoidal, nonsinusoidal, constant in time or varying in time. If the light intensity pattern  400  is periodic, the period can be optimized to create separation between particles exposed to the light intensity pattern  400 . For example, for large particles the period length can be increased to increase the size of wells  408  in the corresponding potential pattern  404  to accommodate the large particles. 
       FIGS. 5A ,  5 B and  5 C show two particles  410 ,  412  exposed to a potential pattern  406 . In this figure, particles  410  and  412  are of similar size and shape but have different dielectric constants. 
     As described infra, moving the light intensity pattern  400  and consequently the potential  406  created by the light intensity pattern  400 , relative to particles  410 ,  412  exposed to the light intensity pattern  400  causes the particles  410 ,  412  to move at velocities related to the physical properties of the particles  410 ,  412 . For example, the force acting on a particle is proportional to the dielectric constant of the particle. More specifically, the force is proportional to (E p −E m )/(E p +2 E m ). Thus, two particles  410 ,  412  of similar size and shape having different dielectric properties will travel at different velocities when exposed to a moving light intensity pattern  400 . 
     The potential  406  created by the light intensity pattern  400  causes the particles  410 ,  412  to move toward wells  408  in the potential pattern  406 . Because the light intensity pattern  400 , and consequently the potential pattern  406 , are moving, the particles  410 ,  412  “surf’ on waves created in the potential pattern  406 . The waves include peaks  414  of high potential and wells  408  of low potential. 
     The particles  410 ,  412  move with the potential pattern  406  at velocities related to the particles  410 ,  412  physical properties. One such physical property is the dielectric constant of the particles  410 ,  412 . Because the dielectric constants of particles  410  and  412  are different, they will move at different velocities when exposed to the potential pattern  406  created by the light intensity pattern  400 . 
     In one embodiment, the light intensity pattern  400 , and consequently the potential pattern  406 , is moved at a constant velocity. The velocity can be optimized to cause separation of the particles  410 ,  412  based on the particles&#39;  410 ,  412  physical properties. For example, a maximum velocity exists for each particle  410 ,  412  such that if the maximum velocity is exceeded, the peak  414  on which the particle  410  or  412  is “surfing” will pass the particle  410  or  412  causing the particle  410  or  412  to fall into the preceding well  408 . 
     In this embodiment, a velocity is chosen between the maximum velocities of particles  410  and  412 . Assuming the maximum velocity of particle  412  is higher than the maximum velocity of particle  410 , when exposed to the potential pattern  406  shown in  FIG. 5A , particle  412  will “surf” on peak  414  and particle  410  will fall behind into well  408  thus separating particles  410  and  412  based on their physical properties. 
     In another embodiment is shown in  FIGS. 5B and 5C . In this embodiment, particles  410  and  412  are exposed to potential pattern  406  for a predetermined amount of time to allow the particles  410 ,  412  to separate slightly as shown in  FIG. 5B . Once the particles  410 ,  412  have separated slightly, the potential pattern  406  is “jerked” forward a predetermined difference such that the particles  410 ,  412  are positioned on opposites sides of peak  414 . Once the particles are positioned on opposite sides of peak  414 , the forces exerted on the particles  410 ,  412  cause them to fall into wells  408  on opposite sides of peak  414  thus separating the particles  410  and  412  based on their physical properties. 
     In one application of the invention, shown in  FIG. 6 , a moving light intensity pattern  400  can be superimposed onto a fluidic channel guided medium  506  having fluidic channels  500 ,  502  and  504 . The channels  500 ,  502  and  504  are arranged in a T-shape with the light intensity pattern  400  being superimposed on the branch of the “T” (i.e. the junction between channels  500 ,  502 , and  504 ). 
     The particles  410 ,  412  travel from channel  500  into the light intensity pattern  400 . The light intensity pattern  400  is configured to move particles  410  and  412  in different directions, as described infra, based on the particles&#39;  410 ,  412  physical properties. In this case, the light intensity pattern  400  is configured to move particle  412  into channel  502  and particle  410  into channel  504 . In this manner, the particles  410 ,  412  can be separated and collected from their corresponding channels  504 ,  502 , respectively. 
     Using an application such as this, the light intensity pattern can be configured to move particles  410  having a physical property below a certain threshold into one channel  504  and particles  412  having a physical property above the threshold into the other channel  502 . Thus, various particles can be run through channel  500  and separated based on a certain threshold physical property. Multiple fluidic channel guided mediums  500  can be connected to channels  502  and/or  504  to further sort the separated particles  410 ,  412  based on other threshold physical properties. 
     Additional optimization can be done to facilitate particle sorting. For example, each particle  410 ,  412  has a specific resonant frequency. Tuning the wavelength of the light intensity pattern  400  to the resonant frequency of one of the particles  410  or  412  increases the force exerted on that particle  410  or  412 . If, for example, the frequency of the light intensity pattern is tuned to the resonant frequency of particle  412 , the velocity at which particle  412  travels is increases, thus increasing the separation between particles  410  and  412 . 
     Other forces can also be superimposed onto the particles  410 ,  412  to take advantage of additional differences in the physical properties of the particles  410 ,  412 . For example, a gradient, such as temperature, pH, viscosity, etc., can be superimposed onto the particles  410 ,  412  in either a linear or non-linear fashion. External forces, such as magnetism, electrical forces, gravitational forces, fluidic forces, frictional forces, electromagnetic forces, etc., can also be superimposed onto the particles  410 ,  412  in either a linear or non-linear fashion. 
     The light intensity pattern  400  and/or additional forces can be applied in multiple dimensions (2D, 3D, etc.) to further separate particles  410 ,  412 . The period of the light intensity pattern  400  can be varied in any or all dimensions and the additional forces can be applied linearly or non-linearly in different dimensions. 
     A monitoring system, not shown, can also be included for tracking the separation of the particles  410 ,  412 . The monitoring system can provide feedback to the system and the feedback can be used to optimize separation or for manipulation of the particles  410 ,  412 . 
     It will be apparent to those skilled in the art that modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except as may be necessary in view of the appended claims.