Abstract:
A microfluidic device provides high throughput generation and analysis of defined three-dimensional cell spheroids with controlled geometry, size, and cell composition. The cell spheroids of the invention mimic tumor microenvironments, including pathophysiological gradients, cell composition, and heterogeneity of the tumor mass mimicking the resistance to drug penetration providing more realistic drug response. The device is used to test the effects of antitumor agents.

Description:
BACKGROUND 
       [0001]    Numerous cancer model systems are now available to investigate disease mechanisms and to screen therapies. However, current preclinical tools to investigate cancer biology and potential tumor treatments still have significant problems. To investigate tumor cell biology and drug efficiency, it would be advantageous to utilize an ex vivo cell culture system that mimics the microenvironment of a tumor. Available two-dimensional (2D) tissue culture models and monolayer cultures lack the realistic complexity of tumor microenvironments and cannot represent tumor heterogeneity or the resistance to drug penetration, which are critical parameters. The tumor microenvironment plays a critical role in cell differentiation, growth and metastatic processes and greatly impacts therapeutic efficacy. Secretory growth factors from cancer-associated fibroblasts have been shown to stimulate proliferation of tumor cells and appear to play a crucial role in disease progression and establishment and maintenance of the tumor microenvironment. Additional features of tumor microenvironments include evasion by components of the immune system. It has been suggested that the presence of immune cells allows tumor cells to be more aggressive and that, without the presence of immune cells, cancer stem cells may not efficiently progress. 
         [0002]    Continuous progress in tissue engineering has made possible the development of various 3D scaffolds and bioreactor systems, which have improved the diversity, fidelity, and capacity of culture models in cancer research. Among the 3D in vitro culture systems that have been developed, tumor spheroid or sphere cultures are becoming a popular approach to obtain and maintain the functional phenotype of human tumor cells. Interestingly, many recent reports state that CSC populations are particularly and/or exclusively maintained in sphere culture. However, today only a small number of cell sphere systems are sufficiently well characterized to resemble the tumor-like 3D cytoarchitecture as well as simulate the pathophysiological micro-milieu and tumor cell responses of the in vivo tumor state. Unfortunately, some so-called spheres and even spheroids described in the literature are no more than loose aggregates of cancer cells that easily detach, cannot be manipulated or transferred, and may lack both cell—cell and cell—matrix interactions, but also lack a true spherical geometry. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention provides a microfluidic platform for high throughput generation and analysis of clearly defined 3D cell spheroids with uniform geometry and versatility in terms of the spheroid sizes and cell composition. The cell spheroids of the invention resemble the tumor microenvironment, including pathophysiological gradients, cell composition and heterogeneity of the tumor mass mimicking the resistance to drug penetration providing more realistic drug response. 
         [0004]    One aspect of the invention is a microfluidic device for the formation and analysis of cell spheroids. The device includes: a first inlet for an oil, a second inlet for a first aqueous suspension of cells, and a third inlet for a polymerization mediator; a nozzle formed by a T-shaped intersection of two or more of the first, second, and third microchannels; and an incubation chamber containing a plurality of microchambers configured in a two-dimensional array. The first inlet is fluidically connected to a first microchannel; the second inlet is fluidically connected to a second microchannel; and the third inlet is fluidically connected to a third microchannel. The nozzle is capable of producing aqueous droplets suspended in the oil; the aqueous droplets contain the cells and the polymerization mediator. The incubation chamber is fluidically connected to the nozzle, and is capable of accepting and delivering the aqueous droplets individually into the microchambers. 
         [0005]    Another aspect of the invention is a cell spheroid comprising two or more cell types adhered to an essentially spherical polymer scaffold. 
         [0006]    Still another aspect of the invention is a kit including the microfluidic device described above, together with one or more reagents, cells, or polymers, and instructions for use. 
         [0007]    Yet another aspect of the invention is a method of making a plurality of cell spheroids. The method includes the steps of: (a) providing the microfluidic device described above, together with an oil, a first cell suspension containing a polymer precursor, and a polymerization mediator; (b) flowing the oil, first cell suspension, and polymerization mediator into the first, second, and third inlets, respectively, whereby aqueous droplets suspended in the oil are formed by the nozzle of the microfluidic device, the droplets containing cells of the first cell suspension, the polymer precursor, and the polymerization mediator; (c) allowing the polymer precursor to polymerize to form polymer scaffolds in the aqueous droplets, whereby a cell spheroid is formed in each droplet; and (d) distributing the cell spheroids into the microchambers of the microfluidic device. 
         [0008]    Still another aspect of the invention is a method of monitoring a cell spheroid for the effect of a test substance. The method includes the steps of: (a) providing a microfluidic device as described above, wherein the device contains an array of cell spheroids in the incubation chamber of the device; (b) perfusing the incubation chamber with an aqueous solution containing the test substance; and (c) monitoring the cell spheroids. 
         [0009]    Further embodiments of the invention are summarized in the following list of items.
       1. A microfluidic device for the formation and analysis of cell spheroids, the device comprising:
           a first inlet for an oil, a second inlet for a first aqueous suspension of cells, and a third inlet for a polymerization mediator; the first inlet fluidically connected to a first microchannel, the second inlet fluidically connected to a second microchannel, and the third inlet fluidically connected to a third microchannel;   a nozzle formed by a T-shaped intersection of two or more of the first, second, and third microchannels, the nozzle capable of producing aqueous droplets suspended in the oil, the aqueous droplets comprising the cells and the polymerization mediator; and   an incubation chamber comprising a plurality of microchambers configured in a two-dimensional array, the incubation chamber fluidically connected to the nozzle and capable of accepting and delivering said aqueous droplets individually into said microchambers.   
           2. The microfluidic device of item 1, further comprising a fourth inlet for a second aqueous suspension of cells, the fourth inlet fluidically connected to a fourth microchannel, the fourth microchannel fluidically connected to the second microchannel at a junction capable of forming a cell mixture comprising cells from the first and second cell suspensions.   3. The microfluidic device of item 2, further comprising a fifth inlet for a third aqueous suspension of cells, the fifth inlet fluidically connected to a fifth microchannel, the fifth microchannel fluidically connected to the second and/or fourth microchannels such that a cell mixture is formed, the cell mixture comprising cells from the first, second, and third cell suspensions.   4. The microfluidic device of any of the preceding items, further comprising an optically transparent window covering the microchambers, wherein the device is configured to permit light microscopic observation of cells in the microchambers.   5. The microfluidic device of any of the preceding items, further comprising one or more liquid perfusion pathways for perfusion of one or more of said microchambers.   6. The microfluidic device of item 5, wherein each microchamber can be separately perfused.   7. The microfluidic device of item 5, wherein one or more groups of microchambers can be separately perfused.   8. The microfluidic device of any of the preceding items, further comprising one or more additional inlets, one or more additional microchannels, one or more outlets, and/or one or more valves, pumps, mixing zones, incubation chambers, vacuum channels, ports, heaters, vents, reservoirs, reagents, or waste chambers.   9. The microfluidic device of any of the preceding items, wherein the microchambers each have a diameter in the range from about 70 microns to about 900 microns.   10. The microfluidic device of any of the preceding items, wherein the microchambers are configured as substantially spherical spaces connected via microchannels.   11. The microfluidic device of item 10, wherein the microchannels connected to the microchambers have a diameter in the range from about 50 microns to about 400 microns.   12. The microfluidic device of item 10, wherein the diameter of the microchannels is less than the diameter of the microchambers.   13. The microfluidic device of item 9, wherein the microchambers are configured as docking stations within the incubation chamber.   14. The microfluidic device of any of the preceding items, further comprising one or more cell spheroids disposed in one or more of said microchambers.   15. The microfluidic device of item 14, wherein the cell spheroids each comprise two or more different cell types.   16. The microfluidic device of any of items 14-15, wherein the cell spheroids comprise a polymer scaffold.   17. The microfluidic device of any of the preceding items which is fabricated from polydimethylsiloxane and glass.   18. A cell spheroid comprising two or more cell types adhered to an essentially spherical polymer scaffold.   19. The cell spheroid of item 18, comprising three or more cell types.   20. The cell spheroid of item 19, wherein the three cell types are a tumor cell or model therefor, an immune cell, and a stromal cell.   21. The cell spheroid of item 18 or item 19, wherein the polymer is selected from the group consisting of alginate, agarose, collagen, chitosan, and polyethylene glycol.   22. The cell spheroid of any of items 18-21, further comprising a cell adhesion peptide.   23. The cell spheroid of any of items 18-22 having a diameter in the range from about 50 microns to about 900 microns.   24. The cell spheroid of any of items 18-23 comprising a necrotic core.   25. A plurality of the cell spheroids of any of items 18-24 configured as an array.   26. The plurality of cell spheroids of item 25, wherein the array is encased in a microfluidic device capable of incubating and perfusing the cell spheroids.   27. A kit comprising a microfluidic device of any of items 1-17; one or more reagents, cells, or polymers; and instructions for use.   28. A method of making a plurality of cell spheroids, the method comprising the steps of:
           (a) providing the microfluidic device of item 1, an oil, a first cell suspension comprising a polymer precursor, and a polymerization mediator;   (b) flowing the oil, first cell suspension, and polymerization mediator into the first, second, and third inlets, respectively, whereby aqueous droplets suspended in the oil are formed by the nozzle of the microfluidic device, the droplets comprising cells of the first cell suspension, the polymer precursor, and the polymerization mediator;   (c) allowing the polymer precursor to polymerize to form polymer scaffolds in the aqueous droplets, whereby a cell spheroid is formed in each droplet; and   (d) distributing the cell spheroids into the microchambers of the microfluidic device.   
           29. The method of item 28, further comprising:
           (e) interrupting the flow of oil in the cell incubation chamber of the microfluidic device and flowing an aqueous solution into the incubation chamber, whereby the cell spheroids are washed.   
           30. The method of item 29, further comprising:
           (f) initiating a flow of cell culture medium through the incubation chamber; and   (g) placing the device into an environment suitable for survival and/or growth of the cells in the cell spheroids.   
           31. The method of item 30, further comprising:
           (h) allowing the cells in the cell spheroids to proliferate.   
           32. The method of any of items 28-31, further comprising providing in step (a) a second cell suspension and a microfluidic device that further comprises a fourth inlet fluidically connected to a fourth microfluidic channel which is fluidically connected to the second microchannel at a junction capable of forming a cell mixture comprising cells from the first and second cell suspensions; whereby the second cell suspension mixes with the first cell suspension in step (b), and whereby the formed aqueous droplets comprise a mixture of cells from the first and second cell suspensions.   33. The method of any of items 28-31, further comprising providing in step (a) second and third cell suspensions and a microfluidic device that further comprises a fourth and fifth inlets fluidically connected to fourth and fifth microfluidic channels, respectively, which are fluidically connected to the second microchannel at a junction capable of forming a cell mixture comprising cells from the first, second, and third cell suspensions; whereby the second and third cell suspensions mix with the first cell suspension in step (b), and whereby the formed aqueous droplets comprise a mixture of cells from the first, second, and third cell suspensions.   34. The method of any of items 28-33, wherein the polymer scaffolds comprise one or more polymers selected from the group consisting of alginate, agarose, collagen, chitosan, and polyethylene glycol.   35. The method of any of items 28-34, wherein the first cell suspension in (a) further comprises a cell adhesion peptide.   36. The method of any of items 28-35, wherein the polymerization mediator in (a) is a solution containing a divalent or trivalent cation.   37. The method of any of items 28-36, wherein the flowing of oil in (b) is at a rate in the range from about 150 μL/hr to about 500 μL/hr, wherein the flowing of first cell suspension in (b) is at a rate in the range from about 75 μL/hr to about 150 μL/hr, wherein an alginate scaffold is formed in (c), and wherein the polymerization mediator is a 0.1 to 1 M calcium salt solution and its flowing in (b) is at a rate in the range from about 1 μL/hr to about 20 μL/hr.   38. The method of any of items 28-37, wherein the first cell suspension in (a) comprises alginate at a concentration from about 0.25% w/v to about 2% w/v.   39. The method of any of items 28-38, wherein the oil comprises a vegetable oil, a silicone oil, or a mineral oil.   40. A method of monitoring a cell spheroid for the effect of a test substance, the method comprising the steps of:
           (a) providing the microfluidic device of item  14 , wherein the device comprises an array of cell spheroids in the incubation chamber of the device;   (b) perfusing the incubation chamber with an aqueous solution comprising the test substance; and   (c) monitoring the cell spheroids.   
           41. The method of item 40, wherein the step of monitoring comprises determining a change in survival, growth, and/or proliferation of cells within the cell spheroids.   42. The method of any of items 40-41, wherein the cell spheroids comprise tumor cells.   43. The method of any of items 40-42, wherein the test substance is an antitumor agent.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0067]      FIGS. 1A-1C  show different embodiments of the microfluidic device of the invention. 
           [0068]      FIG. 2A  is light micrograph of the nozzle area of a microfluidic device of the invention during the formation of aqueous spheres in oil.  FIG. 2B  is a light micrograph of the collecting zone proximal to the nozzle, showing the collection of freshly formed microspheres and their distribution into the microchannels and microchambers of the incubation chamber of the device. 
           [0069]      FIG. 3  shows a light micrograph of two rows of interconnected microchambers of a microfluidic device of the invention. The microchambers each contain a single microsphere of polymerized alginate and MCF7 cells. 
           [0070]      FIG. 4A  shows a scanning electron micrograph (SEM) of an alginate microsphere polymerized in the absence of cells using a microfluidic device of the invention.  FIG. 4B  shows an SEM of an alginate microsphere copolymerized with MCF7 cells using a microfluidic device of the invention. 
           [0071]      FIG. 5  shows the viability of MCF7 adriamycin sensitive 3D spheroids in a microfluidic chip over 14 days. 
           [0072]      FIGS. 6A-6F  show the analysis of doxorubicin effects on MCF7 cell spheroids in a microfluidic device incubation chamber.  FIGS. 6A and 6B  show spheroids of adriamycin resistant MCF7 cells incubated without ( FIG. 6A ) and with ( FIG. 6B ) 12.8 μM doxorubicin.  FIG. 6A  is after 96 hr incubation;  FIG. 6B  is after 48 hr incubation with doxorubicin.  FIGS. 6C and 6D  show spheroids of adriamycin sensitive MCF7 cells incubated without ( FIG. 6C ) and with ( FIG. 6D ) 12.8 μM doxorubicin.  FIG. 6C  is after 96 hr incubation;  FIG. 6D  is after 48 hr incubation with doxorubicin.  FIGS. 6E and 6F  show spheroids of adriamycin sensitive MDF7 cells co-cultured with HS5 bone marrow fibroblasts. The spheroids were incubated without ( FIG. 6E ) and with ( FIG. 6F ) 12.8 μM doxorubicin.  FIG. 6E  is after 96 hr incubation;  FIG. 6F  is after 48 hr incubation with doxorubicin. 
           [0073]      FIGS. 7A-7D  compare the effect of doxorubicin on 2D cell monolayers and 3D cell spheroids containing MCF7 cells.  FIG. 7A  shows changes in viability between 2D monolayers and 3D spheroids of MCF7 adriamycin resistant cells upon treatment with varying concentrations of doxorubicin (800 nM to 12800 nM).  FIG. 7B  shows changes in viability between 2D monolayers and 3D spheroids of MCF7 adriamycin sensitive cells upon treatment with varying concentrations of doxorubicin (800 nM to 12800 nM).  FIG. 7C  compares the viability of MCF7 adriamycin resistant and sensitive cells as 3D spheroids upon treatment with varying concentrations of doxorubicin (800 nM to 12800 nM).  FIG. 7D  compares cell viability in 3D MCF7 adriamycin sensitive and MCF7-HS5 co-cultured spheroids upon treatment with doxorubicin at 3200 nM and 12800 nM. 
           [0074]      FIG. 8  shows the results of treating with different chemotherapeutic regimens in co-cultured MCF7 and HS5 cell spheroids. The co-cultured cell spheroids were incubated either with doxorubicin (DOX) along or concurrently with DOX and paclitaxel (PCT), and cell viability was determined. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0075]    The present invention provides devices and methods for preparing and analyzing three-dimensional (3D) cell spheroids for investigation of cell microenvironments, including cell-cell and cell-extracellular matrix interactions and the effects of pharmaceutical and biological agents on cell viability, growth, and development. The invention is particularly useful for screening antitumor agents and their combinations using a combination of co-cultured cell types in a controlled 3D configuration that realistically mimics the ability of chemotherapeutic agents to attack small early stage metastatic growths in a cancer patient. 
         [0076]    As used herein, “cell spheroid” refers to any generally round collection of cells bound to a substantially spherical polymer scaffold. The size of a cell spheroid can vary in the range from about 50 microns (μm) to about 900 microns in diameter, and is substantially determined and delimited by the size of the polymer scaffold to which the cells are bound. The methods of the invention, described below, permit cell spheroid size to be varied by the design of the microfluidic device used to produce them. Larger cell spheroids having a diameter of about 500 microns or more can develop a necrotic center due to the lack of availability of nutrients or buildup of waste products in the core. In general, larger cell spheroids have three layers: a core which may be necrotic, a middle layer of viable and substantially stationary cells, and an outer layer of migrating cells. 
         [0077]    The strategy of forming cell spheroids according to the present invention is to first form a series of aqueous droplets in an oil (such as mineral oil, silicone oil, or a vegetable oil, the oil optionally including a low concentration of a surfactant to improve flow characteristics) using a nozzle containing a T-shaped junction in a microfluidic device (i.e., a device for handling fluids that has at least one channel of diameter in the range from 1 to 999 microns). The droplets are substantially spherical, and their aqueous contents include a suspension of one or more types of individual cells and an initially non-polymerized form of a polymer suitable for mimicking fibrous elements of the extracellular matrix of a mammalian tissue. The droplets may also include a polymerization mediator or catalyst, which is a chemical agent that reacts with a polymer precursor in the droplet to form a 3D polymer scaffold within the droplet, such as a microbead composed of an essentially spherical network of fibers. The droplets as formed also include one or more cells or mixtures of different types of cells. The cells can be any type of cell including, for example, tumor cells (including tumor stem cells and model tumor cells), cells of a cell line or culture, cells from a patient, immune cells such as lymphocytes or macrophages, stromal cells, or fibroblasts. The cells preferably adhere to the polymer scaffold and grow, differentiate, and/or proliferate within the droplet to form a cell spheroid. 
         [0078]    An exemplary polymer is alginate, which can be supplied as a soluble solution of sodium alginate, into which is mixed, and the nozzle of the microfluidic device during droplet formation, a CaCl 2  solution which serves as polymerization mediator. The Ca 2+  ions (or any other suitable divalent or trivalent cation that promotes alginate polymerization) cause the formation of a network of polymerized alginate fibers within the droplets within minutes after mixing at the nozzle, resulting in formation of a polymer scaffold for cell attachment. Many other suitable polymers and corresponding polymerization mediators can be used. For example, the polymer can be formed from collagen (polymerized by a pH elevation), agarose (polymerized by a temperature reduction), polyethylene glycol (PEG, polymerized using UV light directed at an appropriate zone of the microfluidic device), or chitosan. 
         [0079]    Sodium alginate (a salt of alginic acid, a naturally occurring polysaccharide) can be utilized for cell encapsulation and has advantages including biocompatibility, mechanical resistance, formation of a hydrogel at physiological pH, and optimal pore size for nutrient and gas exchange. Alginate can be modified chemically or physically, e.g., modified with cell adhesion peptides such as GRGDY to facilitate cellular interaction and adhesion. The concentration range of alginate for cell encapsulation is preferably 0.25%-2% w/v in complete cell growth media. 
         [0080]    Polyethylene glycol (PEG) is a crosslinked polyether that has good biocompatibility and low immunogenicity. Many PEG derivatives capable of polymerization by free radical methods are available. For example, PEG can be functionalized with acrylate and methacrylate groups at the chain ends. 2-hydroxy-2-methylpropiophenone can be used as photoinitiator for polymerization by UV light provided and focused through a microscope. The concentration range of PEG for cell encapsulation is preferably 0.25%-10% w/v in complete cell growth media. 
         [0081]    Agarose is a linear polysaccharide consisting of alternating residues of β-1,3-linked-D-galactose and α-1,4-linked 3,6-anhydro-L-galactose. Aqueous agarose solutions form gels upon cooling, due to the aggregation of double helices formed by the physical entanglement of anhydro bridges on the individual molecules. Solutions of low-gelling temperature agarose are particularly useful for cell encapsulation. At moderately high concentrations, they are liquid at a temperature of 37° C., and below 20° C. they gel, and upon heating to 37° C. they remain gel-like. The concentration of agarose used for cell encapsulation is preferably 0.5% -10% w/v in complete cell growth media. 
         [0082]    Collagen enhances cellular activities including attachment and proliferation through interactions between the Arg-Gly-Asp (RGD) domains of collagen and the integrin receptors in the cell membrane. In addition, collagen is a major component of the extracellular matrix, and has low antigenicity and high hydrophilicity. Collagen solution can be prepared in 0.1-0.02 M acetic acid at a concentration of 2-20% w/v. The concentration of collagen used for cell encapsulation is preferably 2%-20% w/v in complete cell growth media. 
         [0083]    Chitosan is a naturally occurring non-mammalian cationic polymer is used for 3D cell culture. It is biocompatible and lacks immunogenicity. Chitosan has a hydrophilic surface promoting cell adhesion, proliferation, and differentiation. Chitosan can be mixed with acetic acid at a ratio of 2:1 by weight to form a homogeneous chitosan solution. The concentration of chitosan used for cell encapsulation is preferably 0.5%-10% w/v in complete cell growth media. 
         [0084]    Once cell spheroids are formed within the microfluidic device, then can be deposited into an array of wells, microchambers, or docking stations where the cells can be monitored for viability, growth, proliferation, development, motility, intercellular interaction, and interactions with the polymer scaffold or with extracellular matrix components. A device containing the spheroids can be placed in to a typical cell culture incubator for a period of hours, days or weeks and removed periodically for monitoring. The device can be configured to fit onto a standard stage of an inverted light microscope, such as a fluorescence microscope, and the device also can optionally include a transparent window covering the incubation chamber containing the spheroids, so as to permit non-disruptive microscopic observation of the spheroids. The device also can optionally include a separate perfusion pathway to allow perfusion of the spheroids individually, collectively, or in groups with desired media containing a variety of agents. Such agents can include, for example, known or candidate antitumor agents, peptides, cytokines, antibodies, aptamers, nucleic acids, nucleotides, siRNA, antisense RNA, cell adhesion molecules or inhibitors of cell adhesion such as RGD peptides, receptor agonists or antagonists, labeled compounds such as fluorescent compounds or antibodies. 
         [0085]    A microfluidic device according to the invention can have a number of possible configurations. Generally, however, it will include three or more inlets for the introduction of fluid into a fluid pathway or channel of the device, three or more interconnected microchannels, a nozzle for the formation of individual aqueous droplets in an oil, and an incubation chamber for the cell spheroids produced at the nozzle. The device may also include one or more outlets for removal of a fluid from the device, one or more mixing zones, one or more filters, one or more microchannels or docking stations for incubation and/or treatment of individual spheroids, and one or more perfusion channels for introduction and removal of cell culture media and/or other solutions containing agents for treatment or analysis of the spheroids. The microchambers or docking stations can be arranged in an array of 1000 or more ordered positions for monitoring and analysis. Microchambers can be essentially spherical, or cylindrical, or have a different shape. The device may also include one or more valves, pumps, vacuum channels, ports, heaters, vents, reservoirs, reagents, or waste chambers, or any combination thereof. 
         [0086]    The flow rates required for each of the fluid inputs into the microfluidic device can vary depending on the design of the device and the concentrations of components such as the cell concentration, the polymer precursor concentration, and the polymerization mediator (e.g., calcium chloride) concentration. Exemplary flow rates are as follows. For the flow of oil into the oil inlet: 150-500 μl/hr, 150-400 μl/hr, or 200-500 μl/hr. For the flow of cell suspension into the cell inlet: 75-150 μl/hr, 75-100 μl/hr, or 100-150 μl/hr. For the flow of calcium chloride solution, 1-20 μl/hr or 1-10 μl/hr. Suitable flow rates can be readily ascertained and optimized by routine experimentation with a given device. 
         [0087]    An important parameter used to determine the size of the cell spheroids is the size of the microchambers and connecting microchannels of the incubation chamber. The size (i.e., diameter) of the microchambers limits the size of the spheroids, and polymer scaffolds, to slightly less than that of the microchamber. Microchamber size can be, for example, any value from about 70 to about 900 microns. Docking sites are typically somewhat larger, in the range from about 600 to about 900 microns. The diameter of the microchannels is less than that of the microchambers, such as from about 50 to about 300 microns, and somewhat larger for docking site embodiments, such as about 200 to about 400 microns. 
         [0088]    Fabrication of the microfluidic device can be by any method known in the art. A common and suitable method is to use a “soft” lithography method to pattern a template for the upper portion of the device, which is then cast from polydimethylsilane (PDMS) and peeled from the template. The PDMS portion contains the channels and other structural and fluid handling features of the device. The PDMS portion is then subjected to plasma treatment and then adhered to glass, such as a glass microscope slide. Holes may be drilled into the PDMS portion of the device as appropriate to provide inlets and outlets. 
         [0089]      FIGS. 1A-1C  describe three exemplary embodiments of the microfluidic device of the invention. In  FIG. 1A , first inlet  10  is intended for the introduction of the oil phase. The first inlet is connected via a short microchannel to optional filter  12 , which outputs to optional mixing zone  14 , which in turn connects to nozzle  15 . Second inlet  20  can be used for either a cell suspension or a calcium solution, as can third inlet  30 . Both second and third inlets are connected via microchannels to optional mixing zones  24  and  34 , which then are connected via microchannels to additional inputs that, together with the output of the oil microchannel, form the T-junction of the nozzle (rectangle on  FIG. 1A ). The intersection of three substantially perpendicular inlet microchannels forms the nozzle. The flow rates are controlled so that, in this design, the oil stream breaks the cell stream and forms aqueous droplets containing the cells and alginate, and optionally the calcium. Alternatively, the droplets can first be formed and ordered into the array, and subsequently the calcium solution is allowed to flow over the droplets in the array, causing gelation from the outside inward, forming either an encapsulating shell or a fully polymerized microbead. The output of the nozzle feeds into a collecting space  41  at the top of incubation chamber  40 . The lower end of the collecting space provides a series of funnels aligned with the columns of microchambers  42 , each of which will receive the droplets that cell spheroids and eventually retain a single cell spheroid. Outlet  45  at the lower end of the incubation chamber is used to collect fluid that has passed through the incubation chamber. Perfusion pathway  50  is a microchannel surrounding the incubation channel. The perfusion pathway contains perfusion inlet  55 , which together with outlet  45  can be used to perfuse the microchambers and the incubation chamber. 
         [0090]      FIG. 1B  shows an embodiment that differs from that in  FIG. 1A  in providing second cell suspension input  25 , which is used to mix two different types of cells for co-incubation in the cell spheroids. The embodiment shown in  FIG. 1C  has similar inputs to that of  FIG. 1B , i.e., first and second cell suspension inputs, but using a different physical layout, and demonstrates that a variety of different physical layouts may be used. More importantly, the incubation chamber of the device shown in  FIG. 1C  provides an array of docking stations, rather than microchannels. The docking sites are intended to collect and entrap droplets that form cell spheroids, particularly larger spheroids of 600-900 microns in diameter. In this design, the oil stream breaks the two cell streams at the nozzle, and the droplets are collected at the docking sites, following which the calcium solution can flow over the droplets to gel the alginate. Cell spheroids can be on the order of about 50 to about 900 microns in diameter, such as from about 50 to about 600 microns or about 70 to about 800 microns for microchamber embodiments, and about 600 to about 900 microns for docking site embodiments. 
         [0091]      FIG. 2A  shows a light microscopic image of the nozzle area during the formation of aqueous droplets. In this example, oil is flowing in from the microchannels shown at the left and top sides of the image, and aqueous cell suspension is flowing up from below. A series of newly formed aqueous droplets immersed in the oil stream is flowing off to the right. In  FIG. 2B  a group of freshly formed droplets is gathered in the collection zone of the incubation chamber and is being sorted by the funnel structures on the right into the columns of microchambers connected by short microchannels.  FIG. 3  shows a portion of two columns of microchambers, each of which contains a single cell spheroid (here, MCF7 cells in an alginate scaffold).  FIG. 4A  shows a close-up SEM image of a pure alginate scaffold formed in the microfluidic device in the absence of cells.  FIG. 4B  shows an SEM image of a portion of a cell spheroid containing both alginate scaffold and attached MCF7 cells. 
         [0092]    The invention also contemplates methods of making a plurality of cell spheroids. The method includes providing an embodiment of the microfluidic device described above, together with an oil, a cell suspension comprising a polymer precursor, and optionally a polymerization mediator, if such is required to form a scaffold from the polymer precursor. The oil, cell suspension, and optionally the polymerization mediator are flowed into first, second, and third inlets of the device. The nozzle of the device forms aqueous droplets suspended in the oil. The droplets contain cells of the cell suspension, the polymer precursor, and optionally the polymerization mediator. In a variation of the method, the polymerization mediator is added subsequent to the formation of the aqueous droplets. The polymer precursor is allowed to polymerize to form polymer scaffolds in the aqueous droplets, whereby a cell spheroid is formed in each droplet. The droplets are then distributed into the microchambers of the microfluidic device. In a variation of the method, the droplets are distributed into the microchambers prior to polymerization of the polymer scaffold. In another variation of the method, the flow of oil is stopped and an aqueous solution such as a cell culture medium is flowed into the incubation chamber, whereby the cell spheroids are washed. The gelation of the spheroids can be performed either before or after this step. 
         [0093]    After cell spheroids with scaffolds are in place in the array of microchambers or docking sites in the incubation chamber, a flow of cell culture medium can be initiated through the incubation chamber, such as by using a perfusion channel of the device. The device is then placed into an environment suitable for survival and/or growth of the cells in the cell spheroids, such as a convention incubator used for cell culture. Preferably, continuous flow of cell culture medium is maintained while the cells in the device are in the incubator. Over a period of hours to days, cells in the spheroids grow and proliferate. 
         [0094]    The matured cell spheroids are useful for studies of a variety of agents or test substances, such as antitumor agents. The microfluidic device containing the microspheroids in the incubation chamber are perfused with an aqueous solution, such as a culture medium, containing the test substance. The cell spheroids are then monitored using a suitable technique, such as fluorescence microscopy, a cell viability assay, or other method to determine a state of interest of the cells. The microfluidic device of the invention can be used to screen different antitumor agents against the tumor cells of a particular patient, such as a human or other mammalian subject, to determine an effective agent or combination of agents for chemotherapeutic intervention for the patient. The device also can be used for basic studies of cell interactions, cell-matrix interactions, or for the development of new antitumor agents. 
       EXAMPLES 
     Example 1 
     Formation of Cell Spheroids in a Microfluidic Device 
       [0095]    The microfluidic device depicted in  FIG. 1A  was used to prepare spheroids of MCF7 breast cancer cells. The three inlets of the device were simultaneously fed with mineral oil containing 3% v/v of Span 80 (a surfactant), a suspension of MCF7 cells at 7-10 million cells/mL and containing 2% w/v sodium alginate in Dulbecco&#39;s Modified Eagle Medium (DMEM) containing 10% v/v fetal bovine serum and 1% v/v antibiotic antimycotic solution, and a 4% w/v CaCl 2  solution. The solutions were introduced simultaneously into the device using syringe pumps. The flow rates were 300 μL/hr for the oil, 75 μL/hr for the cell suspension, and 10 μL/hr for the calcium solution. The size of the microchambers was 200 microns, and the average size of the cell spheroids produced was 170+/−25 microns. After the spheroids were produced, the flow of oil, cell suspension, and CaCl 2  solution was stopped, the incubation chamber of the device was continuously perfused with cell culture medium, and the device was placed in a cell culture incubator maintained at 37° C. and 95% air, 5% CO 2 . 
       Example 2 
     Cell Viability 
       [0096]    The viability of cells in cell spheroids incubated in a microfluidics device of the invention was determined using the LIVE/DEAD viability/cytotoxicity assay for mammalian cells by Life Technologies (Cat No: L-3224). The kit included two dye components: calcein-AM as an indicator of live cells and ethidium homodimer-1 as an indicator of dead cells. For live cells, calcein-AM was cleaved by esterase enzymes to form the green fluorescent dye calcein in the cytoplasm of the cells. For dead cells, the compromised cell membranes allowed ethidium homodimer-1 to permeate and bind to the nucleic acids in the nuclei of the cells, which then emitted a red fluorescence. 
         [0097]    3D spheroids of MCF7 adriamycin sensitive cells, a breast cancer cell line, were formed in a microfluidic device of the invention as described in Example 1. The cell spheroids were housed in the microfluidic device for 14 days and continuously perfused with fresh cell culture medium, and the cell viability was checked after 1, 4, and 14 days using the LIVE/DEAD assay. As can be seen in  FIG. 5 , there was no statistically significant change in the cell viability over a period of 14 days, after which about 99% of the cells were alive. 
       Example 3 
     Sensitivity of Cell Spheroids to Antitumor Agents 
       [0098]    The cell viability assay described in Example 2 was used to ascertain the sensitivity to doxorubicin of MCF7 breast cancer cells in cell spheroids present in a microfluidic device of the invention. 
         [0099]      FIGS. 6A and 6B  show adriamycin resistant MCF7 cell spheroids at 96 hr incubation without doxorubicin ( FIG. 6A ) and at 48 hr incubation with 12.8 μM doxorubicin ( FIG. 6B ). The red fluorescence in  FIG. 6B  indicates cell toxicity of the doxorubicin.  FIGS. 6C and 6D  show adriamycin sensitive MCF7 cell spheroids at 96 hr incubation without doxorubicin ( FIG. 6C ) and at 48 hr incubation with 12.8 μM doxorubicin ( FIG. 6D ). Comparing the red fluorescent nuclei staining in  FIGS. 6B and 6D , it can be seen that adriamycin sensitive MCF7 cells were more susceptible to doxorubicin than the adriamycin resistant cells.  FIGS. 6E and 6F  show cell spheroids containing a mixture of adriamycin sensitive MCF7 cells and HS5 fibroblasts.  FIG. 6E  shows the viability result after 96 hr in the absence of doxorubicin, and  FIG. 6F  shows the result after 48 hr incubation with 12.8 μM doxorubicin. 
         [0100]    The results obtained from the viability study described above, and similar studies employing lower doxorubicin concentrations, were compared to doxorubicin sensitivity in 2D cell cultures; the comparison is shown in  FIGS. 7A-7D . In  FIG. 7A , it can be seen that for the 12.8 μM concentration of doxorubicin there was a statistically significant difference in cell viability (p&lt;0.05) between 2D and 3D cultures, indicating that the tumor cells were more susceptible to drug treatment in a 2D environment compared to a 3D environment. Similarly, in  FIG. 7B  it can be seen that, for most concentrations of doxorubicin tested (from 1600 nM to 12800 nM), there was a statistically significant difference in cell viability (p&lt;0.05) between the 2D and 3D cultures. As seen in  FIG. 7C , the adriamycin sensitive MCF7 cells and the adriamycin resistant MCF7 cells had high and comparable survival rates in spheroids. It appears that the microenvironment and the 3D architecture of cell spheroids play an important role in protecting tumor cells from the effects of chemotherapeutic agents. In  FIG. 7D , it is evident that the addition of fibroblasts, which provide a supporting cell component to the spheroid cultures, caused a significant increase in the overall cell survival rate in response to doxorubicin. 
         [0101]    The sensitivity of co-cultured MCF7 and HS5 cells to a combination of antitumor agents (i.e., 12.8 μM doxorubicin with and without 12.8 μM paclitaxel, 48 hr incubation) was also investigated. As shown in  FIG. 8 , there was a statistically significant drop in viability upon combination treatment as compared to the single drug treatment regimen. 
         [0102]    As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. Any recitation herein of the term “comprising”, particularly in a description of components of a composition or in a description of elements of a device, can be exchanged with “consisting essentially of” or “consisting of”. 
         [0103]    While the present invention has been described in conjunction with certain preferred embodiments, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein. 
         [0104]    This application claims the priority of U.S. Provisional Application No. 62/017,318 filed 26 Jun. 2014 and entitled “Novel 3D In Vivo Mimicking Microenvironment to Deliver Cell Based Therapy or/end Evaluate New Therapeutics and In Vivo Cell Delivery Methods”, the whole of which is hereby incorporated by reference.