Abstract:
A device for assembling aggregations of adherent cells includes a gripper moveable within an assembly vessel that fixes aggregations of adherent cells at a membrane of the gripper and, by movement of the gripper, assembles aggregations of cells on a separate membrane within the vessel, thereby creating a three-dimensional assembly of aggregations of cells that fuse and can be employed in surgical procedures as a unitary tissue of adherent cells. The aggregations of cells, as assembled, can assume three-dimensional configurations distinct from any one of the component aggregations of cells assembled.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/846,703, filed on Jul. 16, 2013. 
         [0002]    The entire teachings of the above application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    First reported in 2005, bio-printers (adapted inkjet printers) were developed to try and meet the challenge of printing 3D organs, but they have had very limited success (3-17). They fabricate structures via a dropwise printing of cells with an extracellular matrix (ECM) material, which serves as the “bio-glue”. The bio-glue gels within minutes, but the cells require tens of hours to attach to the ECM. Recently, bio-printers have become commercially available (EnvisioTec, Organovo, Inc). However, success is limited to simple structures such as a single tube or an array of spheroids (17). The structures survive by passive diffusion and none even begin to approach the complexity, nor cell density of an organ. Bio-printers are also limited by slow throughput inherent in the small size/simplicity of their building materials as well as the vast number of building units that must be deposited. Bio-printers deposit (one at a time) a drop of either a spheroid (˜1,000 cells) or liquid ECM. Our single honeycomb building part has 6×10 6  cells, equivalent to 6, 000 spheroids. Bio-printers are not creating thick structures with sufficient density of cells to require perfusion. They are creating structures of modest thickness, high ECM content and low cell density that do not require perfusion. 
         [0004]    Current pick-and-place instruments from the electronics industry are not suitable, nor could they be easily modified since our building must always occur within an aqueous environment of cell culture medium. We also investigated microbiology instruments for picking bacterial colonies and these were deemed not suitable because they locate a colony and punch out a small plug of agarose and dispense this plug (with colony) to a 96 well plate. These instruments (e.g., Hudson Robotics) are designed for very high throughput, do not have the precision we need, would certainly damage our tissues and cannot grip, let alone perfuse a growing organ. Hence, there is no off the shelf pick and place device available which we can modify for our intended research projects. 
         [0005]    Therefore, a new device and method are that overcome or minimize the above-referenced problems. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention generally is directed to a device and method for assembling aggregations of adherent cells. 
         [0007]    In one embodiment, the invention is a device for assembling aggregations of adherent cells that includes an assembly vessel. A gripper is movable within the assembly vessel. The gripper includes a gripper housing defining a gripper chamber and at least two openings, a gripper membrane over one of the openings, a conduit extending from another of the openings of the gripper, and a support at the gripper housing that controls the position of the gripper within the housing. A perfusate source is in fluid communication with the conduit extending from the gripper housing. A build support is fixed within the assembly vessel that includes a build housing defining a build chamber and at least two openings, a support membrane over one of the openings, and a conduit extending from another of the openings of the build support to the perfusate source. 
         [0008]    In another embodiment of the invention, a method of assembling aggregations of adherent cells includes the step of securing a first aggregation of cells to a gripping membrane by directing a perfusate through the gripping membrane. The gripping membrane is moved to a build membrane opposing the gripping membrane, and the first aggregation of cells is transferred from the gripping membrane to the build membrane by directing perfusate across the first aggregation of cells in a direction toward the build membrane and then through to build membrane. A second aggregation of cells is secured to the gripping membrane by directing perfusate through the gripping membrane. The gripping membrane is moved to the first aggregation of cells at the build membrane, and the second aggregation of cells is transferred from the gripping membrane to the first aggregation of cells by directing perfusate across the second aggregation of cells and the first aggregation of cells and then through the build membrane, whereby the first and second aggregations of cells are stacked on the build membrane, thereby assembling the aggregation of cells. 
         [0009]    The device and method described herein are in the field of tissue engineering, namely the in vitro engineering of thick tissues of high cell density. “Thick tissue,” as that term is defined herein, means tissues that are greater in thickness than 200 microns. 
         [0010]    “High cell density,” as that term is defined herein, means at least about 10 8  cells/ml. An example of tissue having “high cell density” is the human liver. The number of cells in the human liver is estimated to be ˜240 billion (Bianconi et al. An Estimation of the Number of Cells in the Human Body.  Annals of Human Biology,  40, 463-471, 2013). The volume of the liver, which needs to be estimated for purposes of partial hepatectomy, is ˜2 liters (Heinemann et al., Standard Liver Volume in the Caucasian Population.  Liver Transplantation and Surgery  5: 366-368, 1999). Thus, cell density in a real liver is 10 8  cells/ml. 
         [0011]    The device of the invention does not rely on bio-inks that might be toxic and need to be washed out of a construct. Also the device of the invention can employ large living parts that have very high cell density. The living parts are formed by cells aggregating with each other (cell-driven self-assembly). 
         [0012]    The invention, however, is not limited to thick tissue and high cell density; it can be employed to pick, place and perfuse materials that are not “thick” or of “high cell density.” 
         [0013]    It assembles relatively large 3D tissues/organs layer-by-layer using a controllable low level suction head to pick up living microtissue building parts and place them onto other microtissue building parts in precise locations, while maintaining perfusion as these parts fuse and the living structure is built. This is a versatile building platform that can grip multi-cellular building parts of any size, shape and cell type. Large living building parts in the shape of a honeycomb and, when stacked, the aligned lumens of these honeycomb parts will form channels that enable perfusion of the organ under construction. Success at breaking this “sound barrier” and the ability to build organs in vitro has a far-reaching impact in the field of tissue engineering as well as many other areas of research that use animals. Many of these programs have an unmet need to create new more complex 3D in vitro models (test beds) that more accurately mimic the complexity of in vivo. In addition to reducing the use of animals in research, these models are less expensive and more amenable to investigation. The device and method of the invention can be employed to construct complex 3D test beds of tissues of specified shape and size to study these cellular and molecular events; and to understand the transport of drugs and small molecules. 
         [0014]    Investigating and modeling the 3D transport of drugs into tissues, the effects of drugs known to inhibit efflux pumps such as Pgp (P-glycoprotein is an efflux drug transporter), that move small molecules and drugs out of cells, quantitative 3D model and algorithm will facilitate discovering new, more effective inhibitors of drug efflux transporters, 2D cell culture does not adequately mimic drug transport in vivo, which is, more often than not, through multiple layers of different cells. The device also constructs complex 3D test beds of different cell types, layering of cells and composite microtissues of different cell types (normal &amp; pathologic) into desired shapes. 
         [0015]    The device assembles/engineers large 3D tissues/organs layer-by-layer using a controllable, low-level fluid suction head to pick up living building parts and place them onto other building parts in precise locations while maintaining perfusion as this living structure is built. This is a versatile building platform that can grip multi-cellular building parts (of any size and shape), image the part it has gripped and then precisely place this part onto a stack of living building parts to effect the layer-by-layer engineering of a solid organ. Each living part has carefully designed lumen structures and is composed of tens of millions of cells formed in specific geometries designed to be stacked and used to build a large 3D tissue/organ complete with a branched tubular (vascular) network for perfusion. Each living part can be designed to have lumens of different sizes and when these building parts are stacked, their lumens can align to form a branching tubular network that can be perfused. An example of a suitable prior art building part is a large honeycomb structure ( FIG. 1 ), as described in Tejavibulya, N., et al., Directed Self-Assembly of Large Scaffold-Free Multi-Cellular Honeycomb Structures,  Biofabrication  3, 1-9, 2011. The honeycomb-shaped building part can be made by seeding mono-dispersed cells into specially designed non-adhesive agarose micro-molds. Within twenty-four hours, the cells in this scaffold-free environment aggregated and self-assembled a multi-cellular structure in the shape of the honeycomb. The contiguous multi-cellular honeycomb formed around agarose posts which directed the formation of lumens in the honeycomb. This tissue sheet is 2 cm end-to-end in the x, y dimensions and less than 200 μm in the z dimension. Thus, each cell in the honeycomb receives adequate oxygen and nutrients because it is no more than 100 μm away from the top surface, bottom surface or the surface of a nearby lumen. Using CAD and rapid prototyping, micro-molded hydrogels of virtually any size and shape can be designed and these micro-molds direct self-assembly of living cells into the final shape of the building part. This process works for over fifty different cell types including primary human cells from a variety of tissues and organs. And further, these living building parts will readily fuse with one another to form even larger prior art living structures, as described, for example, in Livotti and Morgan Tissue engineering Livotti, C. M., and Morgan, J. R. Self-Assembly and Tissue Fusion of Toroid-Shaped Minimal Building Units.  Tissue Eng.  16: 2051-2061, 2010 (PMCID: PMC2949232). 
         [0016]    Shown in  FIG. 2 , are two prior art toroid-shaped parts of liver cells that have fused within two days published in Livotti and Morgan. Fusion does not occur between building parts made using conventional tissue engineering approaches where cells are seeded onto a scaffold. Because the parts are made entirely of cells with no added scaffold, they fuse with each other via the same cell-to-cell contacts that drove the self-assembly of the original part from millions of individual cells. Thus, we have a process that can potentially make an indefinite number of building parts of any design in two days, and these parts can be fused within four days to form a larger structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
           [0018]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0019]      FIG. 1  is a photograph of one embodiment of a prior art aggregation of cells configured as a honeycomb, and suitable as a viable building unit by the device and method of the invention. The honeycomb included about 6×10 6  cells formed in a micro-mold and stained for viability after 24 hrs. Viable cells are green. Dead cells are red. Shown is the merged red and green fluorescent stitched image. Bar 1800 μm. (1). 
           [0020]      FIG. 2  are photographs of Toroids (about 20,000 cells each) suitable for use in the present invention and undergoing fusion in all dimensions. Two prior art self-assembled toroids placed adjacent on flat nonadhesive agarose fused in the x-y (horizontal) plane. Images at days 0, 2, 4 ( FIGS. 2A-2C ). Bars 200 μm. 
           [0021]      FIG. 3  is a schematic representation of one embodiment of a device of the invention, wherein an aggregation of cells is supported by a staging support. 
           [0022]      FIG. 4  is the embodiment of the device of  FIG. 3 , wherein a gripper of the invention has been lowered to the aggregation of cells supported by the staging support. 
           [0023]      FIG. 5  is the embodiment of the device of  FIG. 4 , wherein the gripper has been raised from the staging support while gripping the agglomeration of cells. 
           [0024]      FIG. 6  is the embodiment of the device of  FIG. 5 , wherein the gripper has been rotated about a vertical axis to thereby bring the aggregation of cells into proximity of a build support of the invention. 
           [0025]      FIG. 7  is the embodiment of the devices of  FIG. 6 , wherein the gripper has been brought into essentially vertical alignment with the build support by movement of the assembly vessel in a horizontal (x,y) plane. 
           [0026]      FIG. 8  is the embodiment of the device of  FIG. 7 , wherein the gripper has been lowered to the build support to thereby place the aggregation of cells on the build membrane. 
           [0027]      FIG. 9  is the embodiment of the device of  FIG. 8 , wherein the gripper has been raised after release of the aggregation of cells on the build support membrane. 
           [0028]      FIG. 10  is the embodiment of the device of  FIG. 9 , wherein the vessel has been moved in a horizontal plane to prepare the gripper for rotation about the vertical axis prior to gripping a second agglomeration of cells. 
           [0029]      FIG. 11  is the embodiment of the device of  FIG. 10 , wherein the gripper has been rotated about the vertical axis to thereby align the gripper with the staging support for gripping the second agglomeration of cells. 
           [0030]      FIG. 12  is the embodiment of the device of  FIG. 11 , wherein a second agglomeration of cells has been placed on the staging support. 
           [0031]      FIG. 13  is the embodiment of the device of  FIG. 12 , wherein the gripper has been lowered to the second agglomeration of cells at the staging support. 
           [0032]      FIG. 14  is the embodiment of the device of  FIG. 13 , wherein the gripper has been raised while gripping and lifting the second agglomeration of cells off of the staging support. 
           [0033]      FIG. 15  is the embodiment of device of  FIG. 14 , wherein the gripper has been rotated about a vertical axis to bring the second aggregation of cells within the proximity of the build support and the first aggregation of cells. 
           [0034]      FIG. 16  is the embodiment of the device of  FIG. 15 , wherein the assembly vessel has been moved in a horizontal plane to align the gripper and second aggregation of cells within the build support and first aggregation of cells. 
           [0035]      FIG. 17  is the embodiment of the device of  FIG. 16 , wherein the gripper and the second aggregation of cells have been lowered vertically to bring the second aggregation of cells into contact with the first aggregation of cells. 
           [0036]      FIG. 18  is the embodiment of the device of  FIG. 17 , wherein the gripper has been vertically raised to leave the second aggregation of cells on the first aggregation of cells, thereby forming an assembly of aggregations of cells of the invention. 
           [0037]      FIG. 19  is a perspective view of an assembly of aggregations of adherent cells of one embodiment of the invention. Stacking and alignment of honeycomb-shaped parts creates channels for perfusion. 
           [0038]      FIG. 20  is a perspective view of one embodiment of an assembly aggregation of cells of the invention that includes rod-shaped building parts that define channels for perfusion. 
           [0039]      FIGS. 21A-21D  are photographs of different views of an embodiment of a gripper suitable for use in the device of the invention. The gripper includes a round membrane (dia 12 mm, 3 μm pores). The different views show membrane gripping surface ( FIG. 21A ) and side view of capped polystyrene cylinder with connector for pump tubing ( FIG. 21D ). 
           [0040]      FIG. 22  is a photograph of another embodiment of the device of the invention. Shown is the bio-gripper head held in place by a manual x, y, z micromanipulator positioned over the objectives of an inverted microscope. The bio-gripper head (or gripper) is immersed in the cell culture medium of the build area (lid removed) and is attached to a peristaltic pump that creates controllable fluid suction to grip and dispense microtissues (aggregations of adherent cells). A microtissue to be gripped is positioned under the bio-gripper by moving the build area using the x, y (horizontal) microscope stage. The bio-gripper is then lowered along its z (vertical) axis to contact the microtissue and fluid suction grips the microtissue to the membrane. The bio-gripper with attached microtissue is retracted and the microscope stage moved to the target. The bio-gripper is lowered and fluid flow reversed to dispense the microtissue on its target. 
           [0041]      FIG. 23  is a series of photographs of live/dead staining of control ( FIGS. 23A-23C ) and gripped ( FIGS. 23D-23F ) H35 spheroids. Scale bar 100 microns. 
           [0042]      FIGS. 24A-24D  are photographs of live/dead staining of ungripped control ( FIGS. 24A ,  24 B) and gripped ( FIGS. 24C ,  24 D) KGN toroids, 25,000 cells/toroid. Scale bar 200 microns. 
           [0043]      FIGS. 25A-25D  are photographs of live/dead staining of ungripped control ( FIGS. 25A ,  25 B) and gripped ( FIGS. 25C ,  25 D) KGN toroids, 30,000 cells/toroid. Scale bar 200 microns. 
           [0044]      FIGS. 26A-26D  are photographs of live/dead staining of gripped KGN toroids after fusion;  FIGS. 26A ,  26 B  30 , 000  cells/toroid,  FIGS. 26C ,  26 D  40 , 000  cells/toroid. Scale bar 500 microns. 
           [0045]      FIG. 27  is a photographic (Brightfleld) image of four KGN toroids stacked on a 330-micron outer diameter capillary tube. Scale bar 300 microns. 
           [0046]      FIGS. 28A-28E  are photographic grayscale brightfield side view images of a toroid stack around a 170-micron outer diameter capillary tube, demonstrating fusion of individual toroids into a single tissue over time (FIG.  28 A=0 hrs, FIG.  28 B=12 hrs, FIG.  28 C=24hrs, FIG.  28 D=48 hrs, FIG.  28 E=72 hrs). Scale bar 500 microns. This is new data not in original filing. Attached is the figure from our manuscript we are submitting, below is the figure legend. 
           [0047]      FIGS. 29A-D  are a series of photographs illustrating formation of an aggregation of cells employed as a building part in another example of the invention. 
           [0048]      FIGS. 30A-E  are a series of photographs representing assembly of building-part honeycomb microtissues (comprised of 250,000 MCF-7 cells) that were picked, placed, and stacked onto a build head by the method of the invention. Close-up overhead photos of a building sequence show a stack of two, three, and four honeycombs ( FIGS. 30A , B and C, respectively). A close-up side view photo of a stack of three honeycombs on the build head is shown at the bottom left ( FIG. 30D ). An angled top view photo of stack of three honeycombs on a build head is shown on bottom right ( FIG. 30E ). The approximate time to stack four honeycombs was 15 minutes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    The invention generally is directed to a device and method for assembly aggregation of adherent cells. The invention is also directed to three-dimensional assemblies adherent cells. 
         [0050]      FIG. 1  (Prior art) is one embodiment of aggregations of adherent cells according to the method of the invention, employing a device of the invention. Specifically, shown in  FIG. 1  is an orbital honeycomb of about six times ten to the sixth cells formed in a micro-mold and stained for viability after 24 hours. The viable cells are shown in green. Dead cells are shown in red. The image shown is the emerged red and green fluorescent stitched image. 
         [0051]    In another embodiment, shown in  FIG. 2  (Prior art), the aggregation of inherent cells is in the form of a toroid, as shown in  FIG. 2 , of about 20,000 cells each, which undergo fusion in all directions.  FIG. 2  is an image of two self-assembled toroids placed adjacent on flat non-adhesive agarose fused in a plane. The images are shown at days 0,2,4 ( FIGS. 2A through 2C ). 
         [0052]    Aggregations of cells, such as are shown in  FIGS. 1 and 2 , that are suitable for use in by the device and in the method of the invention, can be formed by a method known in the art, such as is described, for example, in U.S. Pat. No. 8,361,781 B2, issued Jan. 29, 2013, by Morgan et al., the entire teachings of which are incorporated herein by reference. 
         [0053]    Examples of suitable cells for use by the device and the method of the invention include many different cell types including but not limited to primary cells including hepatocytes, cardiomyocytes, kidney cells, pancreatic cells, fibroblasts, myocytes, epithelial cells, corneal epithelial cells, stromal cells, stem cells, induced pluripotent stem cells, smooth muscle cells, muscle cells, chondrocytes, neural cells, ligament cells, tendon cells, ovarian cells, thyroid cells, parathyroid cells, and also many different kinds of cell lines including but not limited to MCF-7 cells, KGN cells, HEK cells, 3T3 fibroblasts, HepG2 cells, HepG2C3A cells, H35 cells. 
         [0054]    One embodiment of a device of the invention for assembling aggregations of adherent cells is shown in schematic form in  FIG. 3 . As shown therein,  FIG. 3  includes device  10  of the invention for assembling aggregations of adherent cells. Device  10  includes assembly vessel  12  defining inlet  14  and outlet  16 . A suitable material of construction of assembly vessel can include, for example, plexiglass. In one embodiment, assembly vessel  12  is formed of plexiglass. In a specific embodiment, assembly vessel  12  is transparent. Assembly vessel  12  is in fluid communication with perfusate source  18  at inlet  14  and outlet  16  through conduits  20 ,  22 , respectively. Perfusate pump  24  at conduit  20  is employed to control the rate of recirculation of perfusate  26  through assembly vessel  12  and perfusate source  18 . Perfusate source  18  is a suitable source of perfusate to sustain the aggregations of cells within assembly vessel  12 , such as is known in the art. Examples of suitable perfusates recirculated through perfusate source include, for example, cell culture medium such as DMEM Dulbecco&#39;s Modified Eagles&#39; media, all of which, are well known in the art. In another embodiment, not shown, pumps are employed to recirculate perfusate through components within assembly vessel without use of a separate perfusate source vessel. 
         [0055]    Gripper  28  within assembly vessel  12  includes gripper housing  30  defining gripper chamber  32  and at least two openings  34 ,  36 . Suitable materials of construction of gripper housing include, for example, polystyrene and glass. Gripper membrane  38  is affixed over an opening  36 , as shown in  FIG. 1 . Examples of suitable membranes for use as gripper membrane at gripper housing  30  include, Millicell cell culture inserts (EMD Millipore, Billerica, Mass.) (12 mm diameter, 10 mm height), which have a polycarbonate membrane with track-etched 3-micron (gripper) or 8-micron (build) pores. Also those versed in the art will know there are other porous membranes as well as porous structures besides membranes that can be used to grip tissues. In one embodiment, gripper membrane  38  has a pore size diameter of about 3 μm and a pore density of about 2×10 6  pores/cm 2 . In another embodiment, the pore size diameter is about 8 μm and a pore density of about 1×10 5  pores/cm 2 . Gripper housing  30  is supported by support  40  at gripper housing  30 . In the embodiment shown in  FIG. 3 , support  40  also operates as a conduit extending from opening  34  of gripper housing  30  and provides fluid communication between chamber  32  defined by gripper housing  30  and perfusate source  18  through conduit  42 , and pump  44 . Optionally, three-way valve  46 , which is controlled by controller  48  at the intersection of conduits  40 , 42  and  50 , is included and provides an option for reversing the flow of perfusate through conduit  40  by terminating flow through conduit  42  and pump  44 , and opening fluid communication between perfusate source  18  and gripper  28  through conduit  50  and pump  52 . Alternatively, when the pump is a peristaltic pump, or positive displacement pump, manifold and three-way valve are not needed, and flow is reversed at conduit by simply reversing the operation of the pump. 
         [0056]    Conduit  40  is fixed to micromanipulator  54  which, in turn, is supported by post  56  mounted to rigid external support  58 . Micromanipulator  54  is controlled by controller  48  and, upon actuation by controller  48 , rotates about post  56 , thereby causing rotation of gripper  28  about major longitudinal axis  60  extending through post  56 . Micromanipulator  54 , also upon actuation of controller  48 , is movable along major longitudinal axis  60  of post  56 , thereby raising and lowering gripper  28  within assembly vessel  12 . Visualization device  62 , such as a microscope at conduit  40  is directed toward gripper  28  and, by virtue of transparency of the material of gripper housing  30  and gripper membrane  38 , images aggregations of cells within assembly vessel  28  and below gripper membrane  38 . Visualization device  62  is operated by controller  48 . Visualization device  62  will move with movement of conduit  40  supporting gripper  28 . Alternatively, or optionally, in another embodiment, not shown, at least one visualization device is located at at least one of a transparent bottom or side of assembly vessel  12 . 
         [0057]    Staging support  64  is fixed within assembly vessel  12  at bottom portion  66  of assembly vessel  28 . Staging support  64  includes staging housing  68  defining staging chamber  70  and at least two openings  72 ,  74 . Staging membrane  76  extends over and seals opening  72  of staging housing  68 . Staging membrane  76  can be of the same or a different type of material or porosity than the gripper membrane. Conduit  78  extends from other opening  74  of staging housing  68  and through pump  80  to perfusate source  18 . The material of construction of staging housing  68  and staging membrane  76  can be the same as that of gripper  28 , although they need not be transparent. Optionally, rigid support or mold, not shown, is fixed to staging membrane  76  in order to assist in retaining an aggregation of cells at staging membrane  76 . 
         [0058]    Build support  82  is fixed within assembly vessel  12 , as is staging support  64 . Build support  82  includes build housing  84  defining build chamber  86  and at least two openings  88 ,  90 . Build membrane  92  extends over and seals opening  88  of build housing  84 . Suitable membranes include those employed as the gripper membrane, such as a membrane having a pore size diameter of about 8 μm and a pore density of about 1×10 5  pores/cm 2 . The build membrane can be of the same or different type of material or porosty than the gripper membrane and the staging membrane. Conduit  94  extends from another opening  90  of build housing  84  and extends through pump  96  to perfusate source  18 . Optionally, rigid support or mold (not shown) is fixed at build membrane  92  to assist in support of an aggregation of cells at build membrane  92 . Preferably, staging membrane  76  and build membrane  92  are in a common plane. Preferably the common plane in which support membrane  76  and build membrane  92  lie is transverse to and, most preferably, normal to the major longitudinal axis  60  of post  56  extending from support  58 . 
         [0059]    Assembly vessel support  98  is fixed to assembly vessel  12 . Assembly vessel support  98  controls movement of assembly vessel  12  in a plane essentially normal to major longitudinal axis  60  of post  56  extending from support  58 . The position of assembly vessel  12  by virtue of assembly vessel support  98  is controlled by controller  48 . 
         [0060]    In one embodiment of the invention, device  10  of the invention represented in  FIG. 3  is employed to conduct an embodiment of the method of the invention by moving aggregations of cells resting on one or more staging supports  64  within assembly vessel  12  to build support  82  to thereby obtain a stack of aggregations of adherent cells on build support  82 . In one embodiment, the method of the invention includes the following steps. 
         [0061]    Aggregation of cells  100  is deposited on staging membrane  76 . In one embodiment, assembly vessel  12  contains a plurality of staging supports  64 , each of which support at a respective staging membrane  76  at least one aggregation of cells. Aggregations of cells are either deposited at staging membranes or grown at staging membranes by a suitable method, such as is described in U.S. Pat. No. 8,361,781 B2, issued Jan. 29, 2013, by Morgan et al., the entire teachings of which are incorporated herein by reference in their entirety. The rate of perfusate flow across aggregation of cells  100  at staging membrane  76  by direction of perfusate  26  from assembly vessel  12  through staging membrane  76  and into staging support  64  is sufficient to sustain aggregation of cells  100  with nutrients and oxygen. Alternatively, aggregation of cells  100  is not supported by staging support  64 , but, rather, is grown in a separate vessel (not shown) and transferred to assembly vessel by a suitable means, wherein they lie essentially randomly at the bottom of assembly vessel  12 . In this latter embodiment, visualization device  62  and gripper  28  can be employed to locate and then selectively secure individual aggregations of cells to gripper membrane  38  for transfer to build support  82 . 
         [0062]    Returning to the method employed by the apparatus shown in  FIG. 3 , visualization device  62  is employed to identify aggregation of cells  100  lying on staging membrane  76  within assembly vessel  12  by use of controller  48 . Once identified, support  40 , including visualization device  62  and gripper  28 , are lowered by actuation of micromanipulator  54  to cause movement of micromanipulator  54  along major longitudinal axis  60  of post  56  by controller  48  until gripper membrane  38  is adjacent to aggregation of cells  100  at staging membrane  76 , as shown in  FIG. 4 . 
         [0063]    Perfusate  26  is directed from assembly vessel  12  through gripper membrane  38  and conduit  40 ,  42  by actuation of pump  44 , whereby perfusate  26  is directed from gripper housing  30  through conduits  40 ,  42  and pump  44  to perfusate source  18  causes a direction of flow of perfusate  26  through gripper membrane  38  at a rate and velocity that is greater than that of perfusate  26  directed through staging membrane  76  into staging housing  68  and pump  80  back to perfusate source  18 . As a consequence, aggregation of cells  100  will preferentially be directed toward gripper membrane  38 . The rate of flow of perfusate  26  is sufficient to sustain the aggregation of cells  100  at gripper membrane  38 . 
         [0064]    As shown in  FIG. 5 , micromanipulator  54  is actuated by controller  48  to thereby cause micromanipulator  56  to move upward along major longitudinal axis  60  of post  56 , causing gripper  28 , in turn, to move away from staging membrane  76  while aggregation of cells  100  remains at gripper membrane  38 , thereby raising aggregation of cells  100  away from staging membrane  76 . 
         [0065]    As shown in  FIG. 6  micromanipulator  54  is then rotated about major longitudinal axis  60  of post  56  by operation of controller  48 , thereby also causing gripper  28  and visualization device  62  to rotate about major longitudinal axis  60  of post  56 , whereby gripper  28  and aggregation of cells  100  at gripper membrane  38  are brought into relatively close proximity to build support  82 . 
         [0066]    As shown in  FIG. 7 , assembly vessel support  98  is actuated by use of visualization device  62  and controller  48  to bring aggregation of cells  100  into alignment with build membrane  92 . Assembly vessel support  98 , as stated above, operates to move build membrane  92  in a plane that is transverse to and, preferably, normal to major longitudinal axis  60  of post  56  about which gripper  28  rotates by actuation of micromanipulator  56 . 
         [0067]    As shown in  FIG. 8 , micromanipulator  54  is then actuated to move along the major longitudinal axis  60  of post  56 , thereby lowering visualization device  62  and gripper  28  until aggregation of cells  100  is adjacent build membrane  92 . Perfusate  26  is directed from assembly vessel  12  through build membrane  92  and through build support housing  84  and conduit  94  by pump  96  back to perfusate source  18  at a rate sufficient to cause aggregation of cells  100  to be retained at build membrane  92 . The rate of flow of perfusate from assembly vessel  12  across aggregation of cells  100  and build membrane  92  into build housing  84  is also sufficient to sustain the cells with sufficient nutrients and oxygen. 
         [0068]    Three-way valve  46  is then actuated to terminate flow of perfusate  26  from assembly vessel  12  through gripper membrane  38  and gripper housing  30 , thereby terminating the force of flow through gripper membrane  38  that causes adherence of aggregation of cells  100  to gripper membrane  38 . Optionally, three-way valve  46  is actuated to provide fluid communication between conduit  50  and pump  52 , thereby reversing the flow of perfusate  26  so that, rather than perfusate  26  being directed from gripper  28  through perfusate source  18 , perfusate  26  is directed from perfusate source  18  through pump  52 , three-way valve  46  and gripper  28  into assembly vessel  12 , providing a direction of flow that directs aggregation of cells  100  away from gripper membrane  38 . Alternatively, when a peristaltic, or positive displacement valve is employed, a three-way valve is not necessary, and flow through conduit can be stopped or reversed simply by stopping or reversing operation of the peristaltic or positive displacement pump. 
         [0069]    As can be seen in  FIG. 9 , micromanipulator  54  is then actuated to move along longitudinal axis  60  of port  56 , thereby raising visualization device  62  and gripper away  28  from build support  82 , leaving aggregation of cells  100  at build membrane  92 . At this time, if not already done, three-way valve  46 , or other suitable means, depending on the type of pump employed, can be actuated to terminate flow of perfusate  26  from perfusate source  18  to gripper  28 , if the three-way valve  46  is set to cause perfusate to be so directed. 
         [0070]    As shown in  FIG. 10 , assembly vessel support  98  is then actuated to move assembly vessel  12  and, consequently, staging support  64  and build support  82 , in a plane essentially transverse to the longitudinal axis  60  of post  56 . As shown in  FIG. 11 , micromanipulator  54  is actuated to cause rotation of micromanipulator  54  about the major longitudinal axis  60  of post  56 , thereby aligning gripper membrane  38  with staging support  64 . 
         [0071]    As shown in  FIG. 12 , the next, or second, aggregation of cells  102 , either by prior placement of second aggregation of cells  102  on staging support  69  previously employed, or by actuation of assembly vessel support  98 , is vertically aligned with assistance of visualization device  62  with gripper membrane  38 . 
         [0072]    As shown in  FIG. 13 , gripper  28  and, thereby, gripper membrane  38  are lowered by actuation of micromanipulator  54  to cause gripper membrane  38  to contact second aggregation of cells  102  at staging membrane  76 . Three-way valve  46 , or other suitable valve, depending on the type of pump used, is then actuated to cause perfusate  26  to be directed from assembly vessel  12  through gripper membrane  38  and gripper housing  30  through conduits  40 ,  42  and pump  46  to perfusate source  18  at a rate greater than the rate at which perfusate  26  is directed through staging membrane  76 , pump  80  and conduit  78  to perfusate source  18 , thereby causing second aggregation of cells  102  to preferentially adhere to gripper membrane  38 . Also, the rate at which perfusate  26  is directed across second aggregation of cells  102  and gripper membrane  38  is sufficient to sustain second aggregation of cells  102 . 
         [0073]    As shown in  FIG. 14 , micromanipulator  54  is then actuated to raise gripper  28 , with second aggregation of cells  102  adhering to gripper membrane  38 , away from staging membrane  76 . Micromanipulator  54  is then actuated to rotate about the major longitudinal axis  60  of post  56 , to thereby bring gripper  28  and second aggregation of cells  102  within the proximity of first aggregation of cells  100  previously deposited at build membrane  92 , as shown in  FIG. 15 . Assembly vessel support  98  is then actuated to cause alignment, with the assistance of visualization device  62  and controller  40 , of first aggregation of cells  100  at build membrane  92  with second aggregation of cells  102  at gripper membrane, as shown in  FIG. 16 . 
         [0074]    Gripper  28 , with second aggregation of cells  102 , is then lowered until second aggregation of cells  102  is contacting first aggregation of cells  100 , as shown in  FIG. 17 . 
         [0075]    Three-way valve  46 , or another type of valve, as appropriate, is then actuated again, as described previously, with respect to the first aggregation of cells  100  placed at build membrane  92  and, as shown in  FIG. 18 . Alternatively, pump  44  is shut off. In either case, second aggregation of cells  102  is released from gripper membrane  38 , or drawn away from gripper membrane  38  by flow of perfusate  26  directed across second aggregation of cells  102 , first aggregation of cells  100 , and build membrane  92  into build support chamber  86  and back to perfusate source  18 . Gripper  28  is then raised, leaving second aggregation of cells  102  upon first aggregation of cells  100 , thereby assembling aggregations of cells  104  in a stack on build support  82 . 
         [0076]    The above process is then repeated to build a stack of aggregations of cells  104  at build support  82  until a suitable number of aggregations of cells have been assembled on build support  82 . The number of aggregations of cells assembled on build support  82  is indefinite. For example, one strategy is to define the minimum number that defines a stack which is two, such as for an artificial cornea. Another embodiment, for example, would be building a liver for humans. The human liver has about 240 billion cells. A large honeycomb part may have close to 10 million cells, so to build a liver would require picking and placing about 24,000 parts of this size. Larger parts would mean fewer stacks. A third embodiment would be to build a “mega” organ not for transplantation but rather for the in vitro synthesis and secretion of valuable products, such as recombinant proteins. These man-made mega organs would be like bio-manufacturing facilities and so could have even more stacks. 
         [0077]    Following assembly of a suitable number of layers of aggregations of cells, the assembly can be removed from assembly vessel  12  for suitable use. Alternatively, the assembly of aggregations of cells  104  can remain within vessel and perfused by perfusate that is conducted through assembly of aggregations of cells  104 , such as through openings defined by the aggregations of cells  104  assembled on build support  82  through build support housing  84  and back to perfusate source  18 , thereby allowing the assembly of aggregations of cells  104  to remain in place by virtue of the flow of perfusate  26  from assembly vessel  12  through build support  82  and providing sufficient nutrients to maintain the assembled aggregations of cells  104  for a period of time sufficient to cause the assembly of aggregations of cells  104  to fuse. The fused assembly of aggregations of cells  104  can then be removed from assembly vessel  12  for subsequent processing and use, such as surgical use as tissue. 
         [0078]    In one embodiment, the assembly of aggregations of cells  104  is conducted in a manner to cause openings defined by the aggregations of cells to substantially align. In another embodiment, the aggregations of cells are stacked in a manner that does not cause the openings defined by each aggregation of cells to be aligned. In a still further embodiment, different shapes of aggregations of cells are assembled to thereby cause formation of a stack of aggregations of cells that assumes a three-dimensional character, such as that of a frustum, pyramid, or other three-dimensional shape (not shown) that, upon fusion of the layers of aggregations of cells assembled, will form a unitary three-dimensional tissue suitable for use as, for example, replacement tissue of a body part. 
         [0079]      FIG. 19  shows a schematic of how vascular channels are formed when honeycomb parts are stacked. Alternatively, as shown in  FIG. 20 , vascular channels can be created by cross stacking building parts in the shape of rods as described. Flow rates of Q=1 cm 3  /min are sufficient for griping the structures without any damage. Where permeability of membrane is k=nπd 4 /128=6×10 −10  cm 2 , for example, the pressure drop across the membrane is estimated to be ΔP=QμL/kA=102 Pa. Hence, negative pressures of about 100 Pa typically are enough to grip H35 spheroids. These pressure drops not enough to induce rupture of the microtissues. 
         [0080]    The device and method of the invention will now be demonstrated by the following experimental demonstration, which is not intended to be limiting in any way. 
       EXEMPLIFICATION 
     Example 1 
     Materials and Methods 
     Micro-Molded Hydrogels and Microtissue Formation 
       [0081]    Agarose gels were cast from 3D Petri Dish® micro-molds (Microtissues, Inc., Providence, R.I.). Powder UltraPure™ Agarose (Invitrogen, Carlsbad, CA) was sterilized by autoclaving and dissolved via heating in sterile water to 2% (weight/volume). Molten agarose was pipetted into each micro-mold and air bubbles were removed by agitation with a sterile spatula. After setting, gels were separated from the micro-mold using a spatula, transferred to twelve-well tissue culture plates, and equilibrated for at least 4 hours with several changes of culture medium. Micro-molds with two different recess geometries were used to produce agarose gels to create spheroid or toroid microtissues. Round recesses for spheroids were 800 μm in diameter and contained 81 recesses per gel. Toroidal recesses were 1400 μm in diameter with a central agarose peg of 600 μm and contained 36 features per gel. 
         [0082]    Rat hepatoma (H35) and human ovarian granulosa (KGN) cells were expanded in Dulbecco&#39;s Modified Eagle Medium (DMEM) (Invitrogen) supplemented with 10% fetal bovine serum (FBS)(Thermo Fisher Scientific, Waltham, Mass.) and 1% penicillin/streptomycin (Sigma-Aldrich, St. Louis, Mo.). Cultures were maintained in a 37° C., 10% CO 2  atmosphere. Cells were trypsinized, counted, and re-suspended to the desired cell density for each experiment. 190 μL of cell suspension was pipetted into the rectangular seeding chamber above the recesses of each micro-molded agarose gel. Spheroid gels were each seeded at a concentration of 1,250 cells per spheroid feature. Toroid gels were each seeded at concentrations varying from 25,000 to 35,000 cells per toroid feature. Samples were then incubated for approximately twenty minutes to allow cells to settle into recesses before 2 mL of medium was slowly added to each well. Medium was exchanged every other day. 
       Live-Dead and Cell Tracker Staining 
       [0083]    For Live-Dead staining, microtissues were incubated with a mixture of 2 mL phosphate-buffered saline (PBS) with 4 μM of ethidium homodimer-1 and 1 μM calcein AM (Invitrogen) for 75 minutes at 37° C. Microtissue viability was assessed via fluorescent imaging using a Zeiss Axio Observer Z1 equipped with an AxioCam MRm camera with AxioVision Software (Carl Zeiss Micro-Imaging, Thornwood, N.Y.) and an X-Cite 120 fluorescence illumination system (EXFO Photonic Solutions, Ontario, Canada). 
         [0084]    For Cell Tracker staining, cells were washed with serum-free medium and incubated for 45 minutes in either DMEM with 5 μM CellTracker™ Green (Invitrogen) or DMEM with 5 μM CellTracker™. Plates were washed with PBS and the labeled cells were trypsinized, counted and seeded into micro-molds to form labeled microtissues. 
       Fabrication of Device Instrument 
       [0085]    The bio-gripper head of the device instrument, shown in  FIGS. 21A-21D  was fabricated using a Millicell cell culture insert (EMD Millipore, Billerica, Mass.) (12 mm diameter, 10 mm in height) with a polycarbonate membrane with track-etched 3-micron pores. The three small feet on the bottom of the cylindrical insert were removed. A 4-mm hole was drilled into the side of the insert and a polypropylene elbow joint fitting (2.4 mm ID)(Cole-Parmer Instrument Co., Chicago, Ill.) was connected using plastic cement (The Testor Corp., Rockford, Ill.). The top or open end of the insert was sealed with a standard 18 mm circular micro cover glass (VWR Scientific, Inc., West Chester, Pa.) using epoxy (ITW Devcon, Danvers, Mass.). The elbow joint was connected to a segment (14 cm) of stiff polystyrene tubing that was attached to a manual x, y, z micromanipulator (Narishige, Nikon, Japan) mounted on a Nikon Diaphot inverted microscope with manual x, y stage control and its condenser removed. The bio-gripper&#39;s micromanipulator was mounted directly above and in line with the objectives. The other end of the polystyrene tube was fitted with a male-male tubing joint that was connected to tubing run through a Multistaltic® peristaltic pump (Haake Buchler Instruments, Inc., Saddle Brook, N.J.) capable of reversible flow rates up to 2 mL/minute. The other end of the tubing was placed into the build chamber, a 150-mm Petri dish containing cell culture medium, thus creating a closed system for recirculation of culture medium during operation of the device. 
         [0086]    The device was operated as follows. Microtissues (spheroids or toroids) released from their micro-molds were deposited into the build chamber and brought into view using the microscope&#39;s x, y stage. The bio-gripper head, submerged in cell culture medium, was lowered down onto the microtissue. Proximity of microtissue and the membrane of the gripper head were evident when both were in focus. The peristaltic pump was run at 1 mL/min to grip tissues. After gripping, the bio-gripper head was raised, with both the microtissue of interest and the membrane moving out of focus. 
         [0087]    To place the microtissues, the microscope&#39;s x, y stage was adjusted so that the intended target was brought into position under the bio-gripper head with gripped microtissue. The head was lowered, and when it had reached the appropriate distance on the z-axis, flow across the membrane was reversed to facilitate release of the microtissue. Bio-gripper heads were exchanged with each experiment to mitigate the effect of membrane clogging from debris. Side-views of the microtissue being lowered onto its target were obtained using a dissecting microscope mounted perpendicular to the z-axis and was useful for more precise control of the distance between gripped microtissue and target. 
       Instrument Fabrication 
       [0088]    To build a manually operated device instrument, the condenser of an inverted microscope was removed and mounted directly above the objectives an x, y, z micromanipulator holding a bio-gripper head ( FIG. 22 ). The bio-gripper head was fabricated from a cylindrical polystyrene cell culture insert with a membrane (3 μm diameter pores). The insert top was capped off with a glass coverslip and a side port added to attach tubing ( FIGS. 21A-21D ). The controllable fluid suction force of the bio-gripper head was created by the action of a peristaltic pump pulling culture medium through the membrane. The porous membrane (of area A=πD 2 /4=0.6 cm 2 , pore size d=3 μm, thickness L=22 μm and pore density n=3×10 6  pores/cm 2 ) was transparent when wet enabling visualization of the microtissue when gripped. Permeability of the membrane was approximately k=nπd 4 /128=6×10 −10  cm 2 , and the pressure drop across the membrane at low flow rates (Q=1 cm 3 /min) were estimated to be Δp=QμL/kA=102 Pa. The bio-gripper head was a modular piece that was easily exchanged and could be custom designed for microtissues of varying sizes and shapes 
       Gripped Microtissues Were Viable 
       [0089]    To determine if gripping altered the viability of microtissues, H35 spheroids (1250 cells/spheroid) were gripped, moved and dispensed into dishes coated with agarose to prevent spheroid adhesion. Control ungripped spheroids were kept in parallel dishes and subjected to all the same treatments except gripping. Spheroids were then stained with Live/Dead ( FIG. 23A-23F ). From these images, there was no significant difference in the viability between gripped and non-gripped spheroids. 
         [0090]    To determine if larger more complex structures could be gripped, toroids of KGN cells (25,000, 30,000, and 35,000 cells/toroid) were made. The toroids were loaded into the build area, gripped and deposited into an agarose coated culture dish submerged in the build area. Control (non-gripped toroids) and gripped toroids were stained Live/Dead ( FIGS. 23A-23F ,  24 A- 24 D). There was no breakage of the toroid structure and there were no differences in viability between gripped and control toroids at any of the seeding densities tested. 
         [0091]    Large Sheets of Toroids Were Formed and Gripped: 
         [0092]    To determine if the device could safely manipulate even larger more complex microtissues, sheets of fused toroids were formed. After 15 hours of self-assembly, toroids (˜30,000 to 40,000 cells/toroid) were released from their micro-molds into 60-mm dishes that had been coated with agarose. After the toroids settled, the dishes were tilted causing the toroids to collect on one side of the dish and contact each other. Twenty four later, the toroids had fused into a contiguous sheet of toroids. Gripping, moving and releasing these sheets did not fracture the sheet or alter its viability ( FIGS. 25A-25D ). The same flow rate (1 mL/min) was used to grip individual microtissues and the sheet of toroids. During the fusion process, the lumens of the toroids narrowed, less so for the toroids with 30,000 cells versus 40,000 cells. 
         [0093]    Toroids Were Stacked: 
         [0094]    To test the ability of the device to stack toroids, KGN toroids (35,000 cells/toroid) were gripped and then released over small diameter capillary tubes (330 or 170 μm outer diameter) embedded in and protruding upward from agarose. One at a time, toroids were gripped and transported so that their lumens were aligned in the x, y plane with the outer diameter of the capillary tubes. The z distance between the bio-gripper&#39;s membrane and the end of the capillary tube was approximated by observing the capillary tube catching the toroid as the tube was moved in the x, y plane relative to the toroid. Upon alignment, the toroid was released by reversal of flow through the membrane. Careful approximation of the membrane and the capillary tube in the z direction minimized occurrences of the toroid not successfully being released onto the capillary tube. By repeating this procedure, an initial stack of toroids was placed around the large diameter capillary tube ( FIG. 26 ). The procedure was repeated with the smaller diameter capillary tube (170 μm) and taller stacks were made containing up to 16 toroids. Manual stacking of each toroid required less than 5 minutes. Stacked toroids were incubated at 37° C. and side view images taken to determine if the stacked toroids fused ( FIG. 27 ). Over time, the toroids fused as shown by the closing of small gaps and the melding and flattening of the round edges of the toroids. The stack of toroids also contracted their lumens and appeared to attach to the capillary tube. The height of the stack of 16 toroids is greater than 3.5 mm.  FIGS. 28A-28E  show different toroid stacks formed by the method of the invention. 
       Example 2 
       [0095]    Honeycombs were gripped and stacked to determine if a device of the invention (“Bio-P3”) could handle even larger and more complex building parts, we prepared multi-cellular honeycombs were prepared ( FIGS. 29A-D ). Shown in  FIGS. 29A-D  are photographs of a micro-mold ( FIG. 29A ) and a micro-molded agarose gel prepared from mold ( FIG. 29B ). After equilibration in cell culture medium, micro-molded agarose gel was seeded with 250,000 MCF-y cells that self-assembled a multi-cellular honeycomb structure in the gel within 24 hours ( FIG. 29C ). The cells were harvested 48 hours after seeding. Brightfield image of multi-cellular honeycomb with seven lumens after release from the gel is shown in  FIG. 29D . Scale bar 500 microns. The method of preparation of multicellular honeycombs is more completely described in U.S. Pat. No. 8,361,781, the teachings of which are incorporated by reference in their entity. Honeycomb microtissues were introduced into the holding pen of the Bio-P3 instrument. They were subsequently gripped one at a time, moved into position and deposited onto the build head ( FIG. 30 ). A stack of four honeycombs was assembled, with fair alignment of lumens through the four tissues.  FIGS. 30A-E  are a series of photographs representing assembly of building-part honeycomb microtissues (comprised of 250,000 MCF-7 cells) that were picked, placed, and stacked onto a build head by the method of the invention. Close-up overhead photos of a building sequence show a stack of two, three, and four honeycombs ( FIGS. 30A , B and C, respectively). A close-up side view photo of a stack of three honeycombs on the build head is shown at the bottom left ( FIG. 30D ). An angled top view photo of stack of three honeycombs on a build head is shown on bottom right ( FIG. 30E ). The approximate time to stack four honeycombs was 15 minutes. This most directly demonstrates the potential of the Bio-P3 device, to assemble a large (&gt;2 mm in smallest dimension), multi-lumen, high-density (˜1 million cells total) tissue construct. 
         [0096]    The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
         [0097]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.