Abstract:
A method of inoculating a culture medium including providing a droplet including a single cell type on a droplet actuator and inoculating a culture medium with the droplet. A method of providing a metabolically useful substance to a cell culture, including providing a droplet actuator including a cell culture droplet loaded thereon, the sample droplet including cells and a cell culture medium, and a second droplet comprising a metabolically useful substance. The method also includes conducting one or more droplet operations to combine the cell culture droplet with the second droplet on the droplet actuator. Related methods, droplet actuators, and systems are also provided.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Patent Application Nos. 61/013,535, entitled “Manipulating cells in a droplet actuator,” filed on Dec. 13, 2007; and 61/091,637, entitled “Manipulating cells in a droplet actuator,” filed on Aug. 25, 2008; the entire disclosures of which are incorporated herein by reference. 
     
    
     GRANT INFORMATION  
       [0002]    This invention was made with government support under DK066956-02 awarded by the National Institutes of Health of the United States. The United States Government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION  
       [0003]    The inventions relates to methods, devices and systems for sorting cells, inoculating culture media, replenishing culture media, growing cells, and testing cell cultures. 
       BACKGROUND OF THE INVENTION  
       [0004]    Droplet actuators are used to conduct a wide variety of droplet operations, such as droplet transport and droplet dispensing. A droplet actuator typically includes two surfaces separated by a gap. One or both surfaces include electrodes for conducting droplet operations. The gap typically includes one or more filler fluids that are relatively immiscible with the droplets. Droplets may, for example, be reagents and/or droplet fluids for conducting assays. In wide variety of applications, such as the production of antibodies and assaying stem cells, samples within droplet actuators may include cells to be manipulated and, therefore, there is a need for new approaches to manipulating cells within a droplet actuator. 
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]    The invention provides a method of inoculating a culture medium. The method may include providing a droplet including a single cell type on a droplet actuator and inoculating a culture medium with the droplet. The inoculating step may involve conducting one or more droplet operations to bring the droplet into contact with the culture medium. The droplet including a single cell type may be provided by (a) providing a droplet actuator including a sample droplet loaded thereon, the sample droplet including cells of multiple cell types; (b) dispensing a sub-droplet from the sample droplet; (c) analyzing the sub-droplet to determine whether the sub-droplet includes a single cell type; (d) repeating steps (b) and (c) until one or more droplets each including a single cell or a single cell type is identified. 
         [0006]    The invention also provides a method of providing a droplet including a single cell type, the method including: providing a droplet actuator including: a sample droplet loaded thereon, the sample droplet including cells of multiple cell types; a bead droplet including one or more beads having affinity for a specific one of the cell types; conducting one or more droplet operations to combine the bead droplet with the sample droplet, thereby permitting cells of the specific one of the cell types to bind to the beads; conducting a droplet based washing protocol to separate the beads bound to cells of the specific one of the cell types from cells of other cell types. 
         [0007]    Further, the invention provides a method of providing a droplet having a modified distribution of cell types, the method including: providing a droplet actuator including a sample droplet loaded thereon, the sample droplet including a first distribution of cells of multiple cell types; activating a series of electrodes to elongate the droplet, thereby providing an elongated droplet; applying a dielectrophoretic gradient along the elongated droplet; deactivating an intermediate one of the series of electrodes to divide the droplet into two or more sub-droplets, each such sub-droplet having a distribution of cells that differs from the first distribution of cells of multiple cell types. At least one of the sub-droplets provided may include a cell type that is enriched relative to the sample droplet. The cell culture droplet and the second droplet may be situated between droplet actuator substrates in proximity to a droplet operations surface. The cell culture droplet may be substantially surrounded by a filler fluid. 
         [0008]    In another embodiment, the invention provides a method of providing a metabolically useful substance to a cell culture. A droplet actuator may be provided, including: and a cell culture droplet loaded thereon, the sample droplet including cells and a cell culture medium; a second droplet including a metabolically useful substance. The method may include conducting one or more droplet operations to combine the cell culture droplet with the second droplet on the droplet actuator. The cell culture droplet may be situated between droplet actuator substrates in proximity to a droplet operations surface. The cell culture droplet and the second droplet may be situated between droplet actuator substrates in proximity to a droplet operations surface. The cell culture droplet may be substantially surrounded by a filler fluid. The cell culture droplet and the second droplet may be substantially surrounded by a filler fluid. 
         [0009]    The invention provides a method of growing cells on a droplet actuator. The method includes providing a droplet actuator including a cell culture droplet including a cell culture medium and one or more cells; and maintaining the droplet at a temperature suitable for causing the cells to grow in the cell culture medium on the droplet actuator. The cell culture droplet may be situated between droplet actuator substrates in proximity to a droplet operations surface. The cell culture droplet may be surrounded by a filler fluid. The cells may include cells bound to beads. 
         [0010]    The invention also provides a method of providing a hybridoma. A droplet actuator may be provided including: a B-cell droplet including a B-cell; and a myeloma cell droplet including a myeloma cell. The method may involve conducting droplet operations to combine the B-cell droplet with the myeloma cell droplet under conditions suitable to cause the fusion of the B-cell with the myeloma cell to produce a hybridoma. The B-cell droplet is situated between droplet actuator substrates in proximity to a droplet operations surface. The myeloma cell droplet is situated between droplet actuator substrates in proximity to a droplet operations surface. The hybridoma may be grown and tested on the droplet actuator. The B-cell droplet may be surrounded by a filler fluid. The myeloma cell droplet may be surrounded by a filler fluid. 
         [0011]    In a further embodiment, the invention provides a method of monitoring a cell culture. The method may include providing droplet actuator including a cell culture droplet including a cell culture medium and one or more cells; conducting one or more droplet operations to dispense a sample droplet from the cell culture medium; and testing the sample droplet for one or more target substances. The cell culture droplet may be situated between droplet actuator substrates in proximity to a droplet operations surface. The sample droplet may be dispensed using electrode-mediated droplet operations between droplet actuator substrates in proximity to a droplet operations surface. Testing may be effected using steps including electrode-mediated droplet operations between droplet actuator substrates. The cell culture droplet may be substantially surrounded by a filler fluid. The sample droplet may be substantially surrounded by a filler fluid. 
         [0012]    Testing may be effected while the sample droplet is substantially surrounded by a filler fluid. Testing may involve conducting one or more electrode-mediated, droplet-based assays on the droplet actuator. The target substances may include metabolically useful substances. 
         [0013]    One or more droplet operations may be used to replace the sample droplet from the cell culture droplet with a replacement droplet added to the cell culture droplet, the replacement droplet including one or more metabolically useful substances. The replacement droplet is dispensed and transported from a droplet actuator reservoir by electrode mediated droplet operations into contact with the cell culture droplet. The replacement droplet may be selected to replace one or more specific substances identified as deficient in the testing step. The testing and replacement of one or more target substances may be automated. The testing may include quantifying one or more metabolic substances. 
         [0014]    The invention also provides a method of monitoring a cell culture including: providing cell culture including a cell culture medium and one or more cells and a fluid path to a droplet actuator; providing a cell culture droplet from the cell culture to the droplet actuator via the fluid path; testing the sample droplet for one or more metabolically useful substances. The method may also include replacing one or more metabolically useful substances identified as deficient by the testing step. 
       Definitions 
       [0015]    As used herein, the following terms have the meanings indicated. 
         [0016]    “Activate” with reference to one or more electrodes means effecting a change in the electrical state of the one or more electrodes which, in the presence of a droplet, results in a droplet operation. 
         [0017]    “Bead,” with respect to beads on a droplet actuator, means any bead or particle that is capable of interacting with a droplet on or in proximity with a droplet actuator. Beads may be any of a wide variety of shapes, such as spherical, generally spherical, egg shaped, disc shaped, cubical and other three dimensional shapes. The bead may, for example, be capable of being transported in a droplet on a droplet actuator or otherwise configured with respect to a droplet actuator in a manner which permits a droplet on the droplet actuator to be brought into contact with the bead, on the droplet actuator and/or off the droplet actuator. Beads may be manufactured using a wide variety of materials, including for example, resins, and polymers. The beads may be any suitable size, including for example, microbeads, microparticles, nanobeads and nanoparticles. In some cases, beads are magnetically responsive; in other cases beads are not significantly magnetically responsive. For magnetically responsive beads, the magnetically responsive material may constitute substantially all of a bead or one component only of a bead. The remainder of the bead may include, among other things, polymeric material, coatings, and moieties which permit attachment of an assay reagent. Examples of suitable magnetically responsive beads are described in U.S. Patent Publication No. 2005-0260686, entitled, “Multiplex flow assays preferably with magnetic particles as solid phase,” published on Nov. 24, 2005, the entire disclosure of which is incorporated herein by reference for its teaching concerning magnetically responsive materials and beads. The fluids may include one or more magnetically responsive and/or non-magnetically responsive beads. Examples of droplet actuator techniques for immobilizing magnetically responsive beads and/or non-magnetically responsive beads and/or conducting droplet operations protocols using beads are described in U.S. patent application Ser. No. 11/639,566, entitled “Droplet-Based Particle Sorting,” filed on Dec. 15, 2006; U.S. Patent Application No. 61/039,183, entitled “Multiplexing Bead Detection in a Single Droplet,” filed on Mar. 25, 2008; U.S. Patent Application No. 61/047,789, entitled “Droplet Actuator Devices and Droplet Operations Using Beads,” filed on Apr. 25, 2008; U.S. Patent Application No. 61/086,183, entitled “Droplet Actuator Devices and Methods for Manipulating Beads,” filed on Aug. 5, 2008; International Patent Application No. PCT/US2008/053545, entitled “Droplet Actuator Devices and Methods Employing Magnetic Beads,” filed on Feb. 11, 2008; International Patent Application No. PCT/US2008/058018, entitled “Bead-based Multiplexed Analytical Methods and Instrumentation,” filed on Mar. 24, 2008; International Patent Application No. PCT/US2008/058047, “Bead Sorting on a Droplet Actuator,” filed on Mar. 23, 2008; and International Patent Application No. PCT/US2006/047486, entitled “Droplet-based Biochemistry,” filed on Dec. 11, 2006; the entire disclosures of which are incorporated herein by reference. 
         [0018]    “Droplet” means a volume of liquid on a droplet actuator that is at least partially bounded by filler fluid. For example, a droplet may be completely surrounded by filler fluid or may be bounded by filler fluid and one or more surfaces of the droplet actuator. Droplets may, for example, be aqueous or non-aqueous or may be mixtures or emulsions including aqueous and non-aqueous components. Droplets may take a wide variety of shapes; nonlimiting examples include generally disc shaped, slug shaped, truncated sphere, ellipsoid, spherical, partially compressed sphere, hemispherical, ovoid, cylindrical, and various shapes formed during droplet operations, such as merging or splitting or formed as a result of contact of such shapes with one or more surfaces of a droplet actuator. For examples of droplet fluids that may be subjected to droplet operations using the approach of the invention, see International Patent Application No. PCT/US 06/47486, entitled, “Droplet-Based Biochemistry,” filed on Dec. 11, 2006. In various embodiments, a droplet may include a biological sample, such as whole blood, lymphatic fluid, serum, plasma, sweat, tear, saliva, sputum, cerebrospinal fluid, amniotic fluid, seminal fluid, vaginal excretion, serous fluid, synovial fluid, pericardial fluid, peritoneal fluid, pleural fluid, transudates, exudates, cystic fluid, bile, urine, gastric fluid, intestinal fluid, fecal samples, liquids containing single or multiple cells, liquids containing organelles, fluidized tissues, fluidized organisms, liquids containing multi-celled organisms, biological swabs and biological washes. Moreover, a droplet may include a reagent, such as water, deionized water, saline solutions, acidic solutions, basic solutions, detergent solutions and/or buffers. Other examples of droplet contents include reagents, such as a reagent for a biochemical protocol, such as a nucleic acid amplification protocol, an affinity-based assay protocol, an enzymatic assay protocol, a sequencing protocol, and/or a protocol for analyses of biological fluids. 
         [0019]    “Droplet Actuator” means a device for manipulating droplets. For examples of droplet actuators, see U.S. Pat. No. 6,911,132, entitled “Apparatus for Manipulating Droplets by Electrowetting-Based Techniques,” issued on Jun. 28, 2005 to Pamula et al.; U.S. patent application Ser. No. 11/343,284, entitled “Apparatuses and Methods for Manipulating Droplets on a Printed Circuit Board,” filed on filed on Jan. 30, 2006; U.S. Pat. No. 6,773,566, entitled “Electrostatic Actuators for Microfluidics and Methods for Using Same,” issued on Aug. 10, 2004 and U.S. Pat. No. 6,565,727, entitled “Actuators for Microfluidics Without Moving Parts,” issued on Jan. 24, 2000, both to Shenderov et al.; Pollack et al., International Patent Application No. PCT/US2006/047486, entitled “Droplet-Based Biochemistry,” filed on Dec. 11, 2006, the disclosures of which are incorporated herein by reference. Methods of the invention may be executed using droplet actuator systems, e.g., as described in International Patent Application No. PCT/US2007/009379, entitled “Droplet manipulation systems,” filed on May 9, 2007. In various embodiments, the manipulation of droplets by a droplet actuator may be electrode mediated, e.g., electrowetting mediated or dielectrophoresis mediated. Examples of other methods of controlling fluid flow that may be used in the droplet actuators of the invention include devices that induce hydrodynamic fluidic pressure, such as those that operate on the basis of mechanical principles (e.g. external syringe pumps, pneumatic membrane pumps, vibrating membrane pumps, vacuum devices, centrifugal forces, and capillary action); electrical or magnetic principles (e.g. electroosmotic flow, electrokinetic pumps piezoelectric/ultrasonic pumps, ferrofluidic plugs, electrohydrodynamic pumps, and magnetohydrodynamic pumps); thermodynamic principles (e.g. gas bubble generation/phase-change-induced volume expansion); other kinds of surface-wetting principles (e.g. electrowetting, and optoelectrowetting, as well as chemically, thermally, and radioactively induced surface-tension gradient); gravity; surface tension (e.g., capillary action); electrostatic forces (e.g., electroosmotic flow); centrifugal flow (substrate disposed on a compact disc and rotated); magnetic forces (e.g., oscillating ions causes flow); magnetohydrodynamic forces; and vacuum or pressure differential. In certain embodiments, combinations of two or more of the foregoing techniques may be employed in droplet actuators of the invention. 
         [0020]    “Droplet operation” means any manipulation of a droplet on a droplet actuator. A droplet operation may, for example, include: loading a droplet into the droplet actuator; dispensing one or more droplets from a source droplet; splitting, separating or dividing a droplet into two or more droplets; transporting a droplet from one location to another in any direction; merging or combining two or more droplets into a single droplet; diluting a droplet; mixing a droplet; agitating a droplet; deforming a droplet; retaining a droplet in position; incubating a droplet; heating a droplet; vaporizing a droplet; cooling a droplet; disposing of a droplet; transporting a droplet out of a droplet actuator; other droplet operations described herein; and/or any combination of the foregoing. The terms “merge,” “merging,” “combine,” “combining” and the like are used to describe the creation of one droplet from two or more droplets. It should be understood that when such a term is used in reference to two or more droplets, any combination of droplet operations that are sufficient to result in the combination of the two or more droplets into one droplet may be used. For example, “merging droplet A with droplet B,” can be achieved by transporting droplet A into contact with a stationary droplet B, transporting droplet B into contact with a stationary droplet A, or transporting droplets A and B into contact with each other. The terms “splitting,” “separating” and “dividing” are not intended to imply any particular outcome with respect to volume of the resulting droplets (i.e., the volume of the resulting droplets can be the same or different) or number of resulting droplets (the number of resulting droplets may be 2, 3, 4, 5 or more). The term “mixing” refers to droplet operations which result in more homogenous distribution of one or more components within a droplet. Examples of “loading” droplet operations include microdialysis loading, pressure assisted loading, robotic loading, passive loading, and pipette loading. Droplet operations may be electrode-mediated. In some cases, droplet operations are further facilitated by the use of hydrophilic and/or hydrophobic regions on surfaces and/or by physical obstacles. Droplet operations may be discrete flow operations, in which each overall operation involves discrete steps, and each discrete step is mediated by the one or more electrodes upon which the droplets reside and/or adjacent electrodes. In certain cases, discrete flow droplet operations may involve movement of droplets through a surrounding filler fluid, as compared to movement of filler fluid to cause droplet movements. 
         [0021]    “Filler fluid” means a fluid associated with a droplet operations substrate of a droplet actuator, which fluid is sufficiently immiscible with a droplet phase to render the droplet phase subject to electrode-mediated droplet operations. The filler fluid may, for example, be a low-viscosity oil, such as silicone oil. Other examples of filler fluids are provided in International Patent Application No. PCT/US2006/047486, entitled, “Droplet-Based Biochemistry,” filed on Dec. 11, 2006; and in International Patent Application No. PCT/US2008/072604, entitled “Use of additives for enhancing droplet actuation,” filed on Aug. 8, 2008. 
         [0022]    “Immobilize” with respect to magnetically responsive beads, means that the beads are substantially restrained in position in a droplet or in filler fluid on a droplet actuator. For example, in one embodiment, immobilized beads are sufficiently restrained in position to permit execution of a splitting operation on a droplet, yielding one droplet with substantially all of the beads and one droplet substantially lacking in the beads. 
         [0023]    “Magnetically responsive” means responsive to a magnetic field. “Magnetically responsive beads” include or are composed of magnetically responsive materials. Examples of magnetically responsive materials include paramagnetic materials, ferromagnetic materials, ferrimagnetic materials, and metamagnetic materials. Examples of suitable paramagnetic materials include iron, nickel, and cobalt, as well as metal oxides, such as Fe 3 O 4 , BaFe 12 O 19 , CoO, NiO, Mn 2 O 3 , Cr 2 O 3 , and CoMnP. 
         [0024]    “Washing” with respect to washing a magnetically responsive bead means reducing the amount and/or concentration of one or more substances in contact with the magnetically responsive bead or exposed to the magnetically responsive bead from a droplet in contact with the magnetically responsive bead. The reduction in the amount and/or concentration of the substance may be partial, substantially complete, or even complete. The substance may be any of a wide variety of substances; examples include target substances for further analysis, and unwanted substances, such as components of a sample, contaminants, and/or excess reagent. In some embodiments, a washing operation begins with a starting droplet in contact with a magnetically responsive bead, where the droplet includes an initial amount and initial concentration of a substance. The washing operation may proceed using a variety of droplet operations. The washing operation may yield a droplet including the magnetically responsive bead, where the droplet has a total amount and/or concentration of the substance which is less than the initial amount and/or concentration of the substance. Other embodiments are described elsewhere herein, and still others will be immediately apparent in view of the present disclosure. 
         [0025]    The terms “top” and “bottom” are used throughout the description with reference to the top and bottom substrates of the droplet actuator for convenience only, since the droplet actuator is functional regardless of its position in space. 
         [0026]    When a liquid in any form (e.g., a droplet or a continuous body, whether moving or stationary) is described as being “on”, “at”, or “over” an electrode, array, matrix or surface, such liquid could be either in direct contact with the electrode/array/matrix/surface, or could be in contact with one or more layers or films that are interposed between the liquid and the electrode/array/matrix/surface. 
         [0027]    When a droplet is described as being “on” or “loaded on” a droplet actuator, it should be understood that the droplet is arranged on the droplet actuator in a manner which facilitates using the droplet actuator to conduct one or more droplet operations on the droplet, the droplet is arranged on the droplet actuator in a manner which facilitates sensing of a property of or a signal from the droplet, and/or the droplet has been subjected to a droplet operation on the droplet actuator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0028]      FIG. 1  illustrates a cell sorting process conducted in a droplet actuator; 
           [0029]      FIG. 2  illustrates a process of sorting droplets in a droplet actuator by the types of cells contained therein; 
           [0030]      FIGS. 3A and 3B  illustrate side views of a first and second step, respectively, of a method of using a droplet actuator for separating different types of cells; 
           [0031]      FIG. 4  illustrates a process for merging a droplet containing one or more cells with a droplet of, for example, a reagent; 
           [0032]      FIG. 5  illustrates a cell incubation process for growing cells in a droplet actuator, e.g., growing cells from a single cell; 
           [0033]      FIG. 6  illustrates a cell fusing process of merging droplets that contain different types of cells; 
           [0034]      FIG. 7  illustrates a process of separating different cell types by use of beads in a droplet actuator; 
           [0035]      FIG. 8  illustrates a cell incubation process of growing cells on beads in a droplet actuator; and 
           [0036]      FIG. 9  illustrates a liquid exchange process in a cell culture reservoir. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    The invention provides methods of manipulating cells within a droplet actuator. For example, by use of operations, such as, dispensing droplets from a cell suspension, analyzing the number of droplets in the dispensed droplet, merging the droplet with other droplets containing either specific reagents or other cells, detecting a property of the droplet, and incubating the droplet at a particular temperature. Embodiments of the invention provide a wide variety of techniques, of which the following are examples: (1) sorting droplets by the number of cells in a droplet, (2) sorting droplets by the types of cells in a droplet, (3) merging cell-containing droplets with reagent droplets, (4), incubating cell-containing droplets in order to grow more cells, (5) fusing droplets with different types of cells in a single droplet, (6) separating a single droplet with different types of cells into multiple droplets, each with a reduced number of cell types, (7) growing cells on beads via incubation, (8) culturing cells in a culture reservoir, and (9) performing liquid exchange in a cell-containing culture reservoir. 
       8.1 Sorting Cell-Containing Droplets 
       [0038]      FIG. 1  illustrates a cell sorting process  100  conducted in a droplet actuator. Droplets are dispensed from a parent droplet or reservoir containing a suspension of cells and dispensed droplets are sorted by the number of cells contained therein.  FIG. 1  shows an arrangement of electrodes  110 , e.g., electrowetting electrodes, in the droplet actuator. A sensor  114  is provided for detecting the number of cells in a droplet. Sensor  114  may be any suitable detection mechanism for detecting the number of cells in a droplet. Examples include optical detection mechanisms, electrical detection mechanisms, and florescence-based detection mechanisms. Cells may be labeled to facilitate detection. A sample reservoir contains a volume of sample liquid  118  that contains a quantity of cells  122 . Droplet operations are used to dispense and transport droplets from the sample, such as a droplet  126 . Each dispensed droplet may include a random number of cells. Dispensed droplets are transported along electrodes  110  and into sensing proximity with sensor  114 . 
         [0039]    In one example scenario, the droplets of interest are those droplets that contain a single cell  122  only and any droplets that contain no cells  122  or two or more cells  122  are discarded or returned to the sample. In this example, when a droplet arrives at sensor  114 , the number of cells  122  that are contained therein is determined. In this example, when single-cell droplets, such as single-cell droplets  130 , are detected, single-cell droplets  130  are transported along a certain electrode path for further processing. In contrast, when droplets that contain no cells  122  or two or more cells  122 , such as droplets  134 , are detected, droplets  134  are transported along a certain different electrode path that returns droplets  134  back to the source volume of sample liquid  118  or, alternatively, to a waste reservoir (not shown). The parent droplet may have a concentration of cells selected to statistically (e.g., using Poisson distribution statistics) maximize the number of dispensed droplets including single cells. Droplet operations may be used to dilute excessively concentrated parent droplets in order to improve or maximize the occurrence of dispensed droplets with single cells. 
         [0040]    Cell sorting process  100  of sorting droplets by the number of cells is not limited to targeting and processing single-cell droplets only. The target droplets of interest may contain any desired number of cells depending on the intended purpose of the droplet/cell operations within the droplet actuator. For example, two-celled droplets may be targeted and all others are discarded, one- or two-celled droplets may be targeted and all others are discarded, and so on. 
         [0041]      FIG. 2  illustrates a process  200  of sorting droplets in a droplet actuator by the types of cells contained therein.  FIG. 2  shows an arrangement of electrodes  210 , e.g., electrowetting electrodes, wherein the location of a sensor  214  is arranged along a transport path for detecting the cell type in a droplet. Sensor  214  may be any suitable detection mechanism for detecting the cell type in a droplet, such as, but not limited to, optical detection mechanisms, electrical detection mechanisms, and florescent-based detection mechanisms. A sample reservoir contains a volume of sample liquid  218  that contains a quantity of various types of cells. In one example, sample liquid  218  contains a quantity of a first cell type  222  and a quantity of a second cell type  224 . Droplet operations are used to dispense droplets that contain a random number and cell type and transport the dispensed droplets into proximity with sensor  214 . 
         [0042]    In one example scenario, the droplets of interest are those droplets that contain the first cell type  222  only and any droplets that contain no cells at all or at least one of the second cell type  224  are discarded. Therefore, when a droplet arrives at sensor  214 , the type(s) of cells contained therein is determined. In this example, when droplets that contain one or more of the first cell type  222  only, such as droplets  226 , are detected, droplets  226  are transported along a certain electrode path for forming a sample volume  230  that contains the first cell type  222  only. By contrast, when droplets that contain no cells at all or at least one of the second cell type  224 , such as droplets  234 , are detected, droplets  234  are transported along a different electrode path for forming a waste volume  238  that may contain both the first cell type  222  and the second cell type  224 . Alternatively, an electrode path (not shown) may be provided for forming a sample volume that contains the second cell type  224  only. 
         [0043]    In another example, the sorting process is used to enrich the concentration of one cell type relative to another cell type. For example, any droplet containing the target cell type may be sorted to one location while any droplet not containing the target cell type may be sorted to a second location. Thus, the first location is enriched with the target cell type while the second location is depleted of the target cell type. This process can be repeated any number of times to achieve a desired level of purification. When the target cell type is labeled, for example, with a fluorescent tag, the sensor may simply need to detect whether or not any signal is present in the droplet to perform this process. For more concentrated cell suspensions the sensor may be used to detect whether the total signal of the droplet exceeds a certain threshold indicating whether the droplet is enriched or depleted of the target cell type. The process can be repeated many times over so that even a relatively small enrichment at each step can produce a substantial amount of purification. 
         [0044]    Cell sorting process  200  is not limited to processing two types of cells only. Any number of types of cells may be detected and sorted accordingly into any number of cell type-specific sample volumes. By use of a cell sorting process, such as cell sorting process  200 , the invention provides a method of providing droplets with enriched or pure concentrations of pre-selected cell types. 
         [0045]      FIGS. 3A and 3B  illustrate side views of a first and second step, respectively, of a method of using a droplet actuator  300  for separating different types of cells. Droplet actuator  300  includes a top plate  310  and a bottom plate  314  that are arranged with a gap therebetween. A set of electrodes  318 , e.g., electrowetting electrodes, are associated with bottom plate  314 . A quantity of sample fluid  322  is provided in the gap of droplet actuator  300 . Additionally, sample fluid  322  contains a quantity of cells  326  of interest that are intermixed with a quantity of other types of cells  330 . Furthermore, when the dielectric properties of the different types of cells within sample fluid  322  are different, certain electrodes  318  may be used to manipulate certain cells by use of dielectrophoresis (DEP). DEP is the lateral motion imparted on uncharged particles (e.g., cells) as a result of polarization that is induced by non-uniform electric fields (e.g., induced via electrodes  318 ). For example,  FIG. 3A  shows a certain electrode  318  that is near one end of the slug of sample fluid  322  is energized in a manner that corresponds to the dielectric properties of the cells  326  of interest. In doing so, the cells  326  of interest are attracted and immobilized (due to DEP) near one end of the slug of sample fluid  322 , as shown in  FIG. 3A , while the other types of cells  330  that have different dielectric properties are not attracted. 
         [0046]      FIG. 3B  shows that once the cells  326  of interest are attracted and immobilized (due to DEP) near one end of the slug of sample fluid  322 , a droplet splitting operation may occur in order to create a droplet  334  of sample fluid that contains substantially the cells  326  of interest only. By use of the method shown in  FIGS. 3A and 3B , cells of interest are separated from unwanted cells via splitting. In another embodiment, DEP may be used to enrich a droplet with cells of interest, and a cell sorting method such as the method described with respect to  FIG. 2  may be employed to further isolate a specific cell type. 
         [0047]    In an alternative embodiment, different types of beads that have different affinities for different types of cells may be provided within sample fluid  322 . In one example, certain beads within sample fluid  322  may have an affinity for the cells  326  of interest and substantially no affinity for the other types of cells  330  and, thus, the cells  326  of interest only bind to these certain beads. Additionally, the beads may have different magnetic properties, for example, by having magnetically responsive beads of different sizes, by providing a mix of magnetically responsive beads and non-magnetically responsive beads, and any combination thereof As a result, a magnetic field strength that corresponds to the beads that have an affinity for the cells  326  of interest may be applied in order to attract and immobilize the target beads near one end of the slug of sample fluid  322 . Again, a subsequent droplet splitting operation may occur in order to create a droplet  334  of sample fluid that is enriched for the cells  326  of interest or contains substantially the cells  326  of interest only. 
       8.2 Merging Droplets Containing Cells 
       [0048]      FIG. 4  illustrates a process  400  for merging a droplet containing one or more cells with a droplet of, for example, a reagent.  FIG. 4  shows an arrangement of electrodes  410 , e.g., electrowetting electrodes, along which a cell-containing droplet, such as a cell-containing droplet  414 , and a droplet of reagent, such as reagent droplet  418 , may be manipulated. In particular, a first step of cell merging process  400  shows cell-containing droplet  414  and reagent droplet  418  being transported toward one another along electrodes  410  via electrowetting. A second step of cell merging process  400  shows a merged droplet  422 , which is cell-containing droplet  414  and reagent droplet  418  that have been combined into a single droplet. The reagent may, for example, include a nutrient or other reagent for which the cell has a metabolic requirement, a drug or other molecule used to perform a treatment on the cell, such a lysis reagent, or any chemical useful for performing an analysis on the cell. 
       8.3 Incubating Cells in Droplets 
       [0049]      FIG. 5  illustrates a cell incubation process  500  for growing cells in a droplet actuator, e.g., growing cells from a single cell.  FIG. 5  shows an arrangement of electrodes  510 , e.g., electrowetting electrodes, along which a cell-containing droplet, such as a cell-containing droplet  514  may be manipulated. In particular, a first step of cell incubation process  500  shows cell-containing droplet  514  that contains, for example, a single cell only. A second step of cell incubation process  500  is a temperature control step that maintains cell-containing droplet  514  at a temperature that promotes cell growth. The second step shows an incubated droplet  518 , which is a droplet that contains multiple cells that have grown over time from the single cell. By use of a cell incubation process, such as cell incubation process  500 , cells can proliferate within a droplet actuator. By use of a cell sorting process as described above with respect to  FIGS. 1 and 2 , and an incubation process, droplets may be obtained having a substantially pure population of cell types. 
       8.4 Fusing Cells in Droplets 
       [0050]      FIG. 6  illustrates a cell fusing process  600  of merging droplets that contain different types of cells.  FIG. 6  shows an arrangement of electrodes  610 , e.g., electroweffing electrodes, along which a droplet that contains a first cell type, such as a droplet  614 , and a droplet that contains a second cell type, such as droplet  618 , may be manipulated. In particular, a first step of cell fusing process  600  shows droplet  614  and droplet  618  being transported toward one another along electrodes  610  via electrowetting. A second step of cell fusing process  600  shows a fused droplet  622 , which is droplet  614  and droplet  618  that have been combined into a single droplet that contains both the first and second types of cells, e.g., fusion of a B-cell are with a myeloma cell to produce an antibody-producing hybridoma. In anther example, a fusing process, such as cell fusing process  600 , may be used in the in vitro fertilization (IVF) process, i.e., fusing a sperm cell with an egg cell. 
       8.5 Using Beads for the Manipulation of Cells 
       [0051]      FIG. 7  illustrates a process  700  of separating different cell types by use of beads in a droplet actuator.  FIG. 7  shows an arrangement of electrodes  710 , e.g., electroweffing electrodes, along which a droplet that contains, for example, a first and second cell type, such as a cell-containing droplet  714 , and a droplet that contains beads, such as bead-containing droplet  718 , may be manipulated. In particular, the beads of bead-containing droplet  718  may be, for example, magnetically responsive beads. Examples of suitable magnetically responsive beads are described in U.S. Patent Publication No. 2005-0260686, entitled, “Multiplex flow assays preferably with magnetic particles as solid phase,” published on Nov. 24, 2005. Additionally, the beads of bead-containing droplet  718  may have an affinity for a certain cell type. In one example, the beads of bead-containing droplet  718  may have an affinity for the first cell type only and substantially no affinity for the second cell type. 
         [0052]    A first step of cell separation process  700  shows cell-containing droplet  714  and bead-containing droplet  718  being merged along electrodes  710  using electrode-mediated droplet operations. A second step of cell separation process  700  shows a merged droplet  722 , which is cell-containing droplet  714  and bead-containing droplet  718  that have been combined into a single droplet that contains both the first and second cell type along with the beads. The second step of cell separation process  700  also shows that the first cell type within merged droplet  722  bind to the beads because the beads have an affinity for the first cell type only. By contrast, cells of the second cell type do not bind to the beads and, thus, remain substantially suspended within merged droplet  722 . A third step of cell separation process  700  illustrates a droplet-based wash procedure using wash buffer droplet  724  that is used to remove the unbound second cell type while the beads are restrained in place. The result is a cell-containing droplet  726  that has a substantially pure cell type. The droplet  730  of unbound cells may be subjected to further droplet operations and/or other processing or analysis. 
       8.6 Growing Cells 
       [0053]      FIG. 8  illustrates a cell incubation process  800  of growing cells on beads in a droplet actuator.  FIG. 8  shows an arrangement of electrodes  810 , e.g., electrowetting electrodes, along which a droplet that contains a certain cell type, such as a cell-containing droplet  814 , and a droplet that contains beads, such as bead-containing droplet  818 , may be manipulated. In particular, the beads of bead-containing droplet  818  may be, for example, magnetically responsive beads. Additionally, the beads of bead-containing droplet  818  may have an affinity for the particular cell type within cell-containing droplet  814 . 
         [0054]    A first step of cell incubation process  800  shows cell-containing droplet  814  and bead-containing droplet  818  being transported toward one another along electrodes  810  via electrowetting. A second step of cell incubation process  800  shows a merged droplet  822 , which is cell-containing droplet  814  and bead-containing droplet  818  that have been combined into a single droplet that contains both the cells and the beads. The second step of cell incubation process  800  also shows that the cells within merged droplet  822  bind to the beads because the beads have an affinity for the particular cell type. A third step of cell incubation process  800  is a temperature control step that maintains merged droplet  822  at a temperature that promotes cell growth. The third step of cell incubation process  800  shows an incubated droplet  826 , which is a droplet that contains multiple cells that have grown over time upon the surface of the beads. By use of a cell incubation process, such as cell incubation process  800 , cells can proliferate within a droplet actuator. In particular, the beads provide a means for growing cells on surfaces other than the droplet actuator surface so that the cells can be subsequently manipulated in the droplet actuator. 
         [0055]    Embodiments of the invention may be provided for culturing cells on a droplet actuator. A cell-containing droplet, such as a droplet that contains one or more cells and/or cell-types, may be transported using droplet operations into contact with a cell culture medium. The cell culture medium may be included in a cell culture reservoir or well. When necessary, the cell culture medium may be in contact with the atmosphere or with a sub-atmosphere on the droplet actuator. The droplet actuator may include or be associated with a heating element configured to heat the cell culture medium to an appropriate temperature for incubation. 
         [0056]      FIG. 9  illustrates a liquid exchange process  900  in a cell culture reservoir.  FIG. 9  shows an arrangement of electrodes  910 , e.g., electrowetting electrodes, which fluidically connect a fluid reservoir  914  and a cell culture droplet  918 . The arrangement is useful, for example, for performing a liquid exchange process supplying reagents, such as reagents metabolically useful substances, to cell culture droplet  918 . Fluid reservoir  914  may contain, for example, a volume of reagent fluid  922 . Cell culture droplet  918  may contain, for example, a volume of cell culture medium  926  that contains a quantity of cells  930 . Cells  930  may be immobilized within cell culture droplet  918 . In one example, cells  930  may be bound to magnetically responsive beads that are within cell culture droplet  918 , whereby the magnetically responsive beads may be magnetically immobilized. Similarly, non-magnetically responsive beads may be physically immobilized, e.g., using one or more physical barriers as described in International Patent Application No. PCT/US08/74151, filed on Aug. 25, 2008, entitled “Bead Manipulations on a Droplet Actuator,” the entire disclosure of which is incorporated herein by reference. Any mechanism for immobilizing or retaining cells  930  within cell culture droplet  918  is suitable. Liquid may be exchanged using droplet operations for merging nutrient-containing droplets into contact with cell culture droplet  918 . In some cases, droplet splitting operations may also be used to remove droplets including reduced quantities of such nutrients from the cell culture droplet  918 . 
         [0057]    In one example, by use of droplet operations, droplets of reagent fluid  922  may be dispensed from fluid reservoir  914  and transported along electrodes  910  and into cell culture droplet  918 . By introducing reagent fluid  922  into cell culture medium  926  of cell culture droplet  918 , reagent fluid  922  is exchanged with cell culture medium  926 . Subsequently, one or more droplets  934 , which are formed of a mixture of reagent fluid  922  and cell culture medium  926 , are transported away from cell culture droplet  918 ; all the while, cells  930  are held immobilized within cell culture droplet  918 . In alternative embodiments, cells  930  are not immobilized. 
         [0058]    Example purposes of a liquid exchange process, such as liquid exchange process  900 , may include, but are not limited to, delivering in a metered fashion various substances, such as metabolically useful substances, drugs or chemicals, to cell culture medium  926  of cell culture droplet  918 , changing the PH concentration of cell culture medium  926  of cell culture droplet  918 , changing the concentration of cells  930  within cell culture medium  926  of cell culture droplet  918 , and any combinations thereof. 
       8.7 Inoculation of a Cell Culture Medium 
       [0059]    The droplet actuator of the invention may include a cell culture medium arranged in sufficient proximity to one or more droplet operations electrodes to permit a droplet comprising a cell to be introduced to the culture medium. The culture medium itself may be composed on the droplet actuator by combining various droplets including medium components. The culture medium may or may not be subject to droplet operations. In accordance with the invention, a culture medium may be provided on the droplet actuator. A droplet including one or more cells may be transported via droplet operations into contact with the culture medium. The inoculated culture medium may be incubated on the droplet actuator. A droplet may be contacted with a viscous culture medium and removed from the culture medium in order to capture one or more cultured cells, e.g., using the techniques described in International Patent Application No. PCT/US08/74151, filed on Aug. 25, 2008, entitled “Bead Manipulations on a Droplet Actuator,” the entire disclosure of which is incorporated herein by reference. 
       8.8 Testing Cells 
       [0060]    Cells on a droplet actuator may be tested using a wide variety of techniques. A cell may be produced on the droplet actuator and tested on the droplet actuator. A cell may be supplied from an external source to the droplet actuator for testing. A reporter assay may be conducted using droplet operations on the droplet actuator to determine whether a gene of interest is being expressed. A RT-PCR assay may be conducted using droplet operations on the droplet actuator using material extracted from the cells using a droplet-based extraction protocol to determine the presence and quantity of mRNA for the gene of interest. An immunoassay may be conducted using droplet operations on the droplet actuator to determine the presence and the amount of protein produced. An enzymatic assay may be conducted using droplet operations on the droplet actuator to determine the activity of the protein. Two or more of these assays or assay types may be conducted on a single droplet actuator. 
         [0061]    The results of a combination of the foregoing assays would show the relationship between the expression of the gene, the amount of protein product and the activity of the protein. Cells may be treated with pathogens, therapeutic agents or other test substances or conditions, and the foregoing assays may be conducted to elucidate the effect of the test substance on the cell. 
       CONCLUDING REMARKS 
       [0062]    The foregoing detailed description of embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention. This specification is divided into sections for the convenience of the reader only. Headings should not be construed as limiting of the scope of the invention. The definitions are intended as a part of the description of the invention. It will be understood that various details of the present invention may be changed without departing from the scope of the present invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation, as the present invention is defined by the claims as set forth hereinafter.