Abstract:
The present disclosure provides a method of fabricating cell, such as stem cell, arrays on a carrier where the surface energy of the carrier has been modified and patterned so that only areas of low contact angle are wet by a water based cell solution. The patterned cell solution when applied to the carrier surface then self assembles into a 3 dimensional micro pattern on the carrier that mimics the surface topography of mammalian organs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of U.S. application Ser. No. 62/186,670, filed on Jun. 30, 2015, the disclosure of which is incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    Contact angle is one of the common ways to measure the wettability of a surface or material. Wetting refers to the study of how a liquid deposited on a solid (or liquid) substrate spreads out or the ability of liquids to form boundary surfaces with solid states. The wetting is determined by measuring the contact angle, which the liquid forms in contact with the solids or liquids. The wetting tendency is larger, the smaller the contact angle or the surface tension is. A wetting liquid is a liquid that forms a contact angle with the solid, which is smaller than about 90°. A non-wetting liquid creates a contact angle from between 90° to 180° with the solid. 
         [0003]    The contact angle is an angle that a liquid creates with a solid surface or capillary walls of a porous material when both materials come in contact together. This angle is determined by both properties of the solid and the liquid and the interaction and repulsion forces between liquid and solid and by the three phase interface properties (gas, liquid and solid). Cohesion and adhesion forces that are intermolecular forces describe those interactions. The balance between the cohesive forces of similar molecules such as between the liquid molecules (i.e., hydrogen bonds and Van der Waals forces) and the adhesive forces between dissimilar molecules such as between the liquid and solid molecules (i.e., mechanical and electrostatic forces) will determine the contact angle created in the solid and liquid interface. The traditional definition of a contact angle is the angle a liquid creates with the solid or liquid when it is deposited on it. Contact angle is defined by Young&#39;s equation that is shown in  FIG. 1 . 
         [0004]    However in practice it is better to refer to  FIG. 2  which shows how a water based liquid will behave when applied to a surface where there are both hydrophobic and hydrophilic patterns pre-deposited onto a carrier such as glass, metal or plastic. 
       SUMMARY 
       [0005]    The present disclosure provides for high and low solid surface free energy patterns to contain and direct a cell solution to form micro 3 dimensional structures that mimic mammalian tissue topography such as heart, musculoskeletal, and liver organs. 
         [0006]    In one embodiment, the present disclosure provides a method of fabricating cell arrays on a carrier where the surface energy of the carrier has been modified and patterned so that only areas of low contact angle are wet by a water based cell solution. The patterned cell solution, e.g., a stem cell solution, when applied to the carrier surface then self assembles into a 3 dimensional micro pattern on the carrier that mimics the surface topography of mammalian organs. In one embodiment, induced pluripotent stem cells that differentiate into cardiomyocytes are applied to a carrier with a pre patterned high contact angle ink and only the low contact angle areas of the carrier are wet and form 3 dimensional structures which induce the cells to beat with regularity on the carrier surface. 
         [0007]    The present disclosure thus provides a method of forming micro 3 dimensional cell, e.g., stem cell, arrays on a carrier surface by utilizing pre-defined (predetermined) printed ink patterned into 2 dimensional coating geometries deposited onto a carrier such as glass, plastic or metal. 
         [0008]    Also provided is a method of fabricating cell, e.g., stem cell, arrays on a carrier surface where the pre-defined (predetermined) surface energy patterns direct the cell solution to self-assemble into a micro 3 dimensional geometry. 
         [0009]    Further provided is a method of fabricating cell, e.g., stem cell, arrays on a carrier surface where the pre-defined surface energy patterns have a contact angle with water of greater than 90 degrees and in one embodiment greater than 120 degrees. 
         [0010]    More particularly, the present disclosure shows a method of utilizing pluripotent stem cell derived cardiomyocytes that can be patterned into linear arrays utilizing pre-defined surface energy geometric patterns, wherein the cells subsequently beat in rhythm after deposition onto a carrier surface. 
         [0011]    These and other advantages of the disclosure will become apparent to those of reasonable skill in the art from the following detailed description, as considered in conjunction with the following examples and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0012]      FIG. 1  shows the how to mathematically describe contact angle. 
           [0013]      FIG. 2  shows a comparison between hydrophobic and hydrophilic contact angles. 
           [0014]      FIG. 3  shows a method in accordance with one embodiment. 
           [0015]      FIG. 4  shows a substrate with various 2-D hydrophobic structures deposited thereon; 
           [0016]      FIG. 5  illustrates stem cell material deposited in the valleys between the 2-D ink of  FIG. 4  in accordance with various embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The following discussion is directed to various embodiments of the disclosure. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
         [0018]    In the embodiments disclosed herein, the surface energy of a substrate is modified before depositing a cell liquid (e.g., an ink), e.g., a stem cell liquid, thereon. The term “surface energy” refers to a property of a material that draws surface molecules inward. In some embodiments, the surface energy of the substrate carrier surface on which the cell liquid is to be deposited is modified so as to approximately match the surface energy (surface tension) of the cell liquid itself. By approximately matching the surface&#39;s surface energy to that of the cell liquid, the cell liquid adheres to the desired regions and does not adhere to the remaining regions which may have a much lower surface energy or higher contact angle. In other embodiments, the surface energy of the regions on which the cell liquid is not to adhere is modified to reduce its surface energy. Then, when the cell liquid coats the substrate surface, the liquid adheres only to the regions whose surface energy was not reduced. These embodiments are described in greater detail below. 
         [0019]    The embodiments described herein permit the formation of 3-D cell, e.g., stem cell differentiated cells, into geometries (e.g., as thin as 250 microns or smaller) to be formed on a substrate and formed at ambient conditions. Further, the substrate material used may include silicon, glass, acrylate, polyimide, elastomers, polycarbonate, polyethylene terephthalate (PET), and the like. The substrate may be rigid or flexible if desired. 
         [0020]    As used herein, the term “pattern” is generally used to refer to the desired pattern of the living cells formed by the cell liquid. The pattern may include straight lines (e.g., a set of spaced, parallel lines) or any arbitrary pattern or 3-D formation of cellular material. 
         [0021]      FIG. 3  illustrates an embodiment of the method  100  in which the surface energy of areas of the substrate is modified to approximate the surface energy of the cell liquid. The substrate areas so modified are the areas where the cell material formed from the cell liquid is to remain thereby forming cell pathways across the substrate. To the extent possible, some of the actions depicted in  FIG. 3  may be performed in a different order from that shown and some actions may be performed in parallel, not sequentially. 
         [0022]    At  102 , the method comprises altering the surface energy of the desired areas of the substrate surface (e.g., the areas in which cell material is desired to be formed). Depositing, on the substrate surface, a substance such as a super hydrophobic ink so as to increase the water contact angle from below 90 degrees to over 115 degrees and for example greater than 120 degrees, can perform this action. In some embodiments, the deposited material has a water contact angle of about 120 to about 160 degrees. A suitable material to deposit on the substrate surface includes fluorinated polymers available from Cytonix Inc. Altering the surface energy of the desired areas may entail decreasing the surface energy of those areas of the substrate surface by at least 20%, 25%, 30%, 35%, 40%, 45%, 50% or more.  FIG. 2  depicts a side view of a substrate  130 . 
         [0023]    At  104 , the method comprises depositing a cell doped liquid that spontaneously forms into micro three-dimensional (3-D) structures on the surface in the hydrophilic untreated regions of the surface of the substrate. Such structures may be of any shape or size. In some embodiments, such structures can take on any shape as determined by the high and low energy pattern deposited onto the surface such as lines, squares, triangles, or any polygon type structure.  FIG. 4  shows a side view of the substrate  130  on which 2-D super hydrophobic ink patterns  132  are deposited. The 2-D structures  132  form valleys  134  there between. The surface energy of the 2-D structures may approximate the surface energy of the altered regions of the substrate and may in one embodiment be formed of fluoroacrylate as well. In some embodiments, the surface energy of the 2-D structures  132  is within 2%, 5%, 10%, 12% or 15% of the surface energy of the substrate surface. 
         [0024]    The 2-D structures  132  comprise raised or protruding structures that delimit the width and shape of the desired cell pattern. In some embodiments, the structures  132  may have a height (H 1 ) of 250 microns (or about 100 to 200, 200 to 300 or 300 to 400 microns) a width of 50 microns (or about 25 to 50, 50 to 75, 75 to 100, 100 to 250 microns or more) and a distance (D 1 ) between ridges of 50 microns (or about 25 to 50, 50 to 75, 75 to 100, 100 to 200, or 200 to 500 microns or more). The structures may also have a height of a few nanometers to several hundred microns. The distance D 1  defines the pitch of the cell pattern. 
         [0025]    At  106 , the method comprises depositing a cell doped liquid (e.g., an ink) on to the desired areas. The cell liquid chosen in this step should have a surface energy (surface tension) approximately equal to the surface energy of the unaltered regions of the substrate  134 . In some embodiments, the cell liquid has a surface contact angle of less than 90 degrees and in one embodiment less than 50 degrees. In some embodiments, the liquid&#39;s surface contact angle may be less than 35 degrees. The cell liquid in one embodiment is a mixture of growth factors, nutrients, and adhesion promoters that are easily dispensed by techniques well known in the art. For example, the cell liquid may be a cardiomyocytes mixed with the appropriate aforementioned additives. In some embodiments, the depositing (printing) of the cell liquid is performed using an inkjet printer, or automated pipetting system. The print gap, ink volume, print speed, etc. are adjustable based on the application at hand and thus may be varied as desired. 
         [0026]      FIG. 5  shows that the cell doped liquid  140  readily settles into the valleys  134  between the predefined super hydrophobic printed ink patterns. The close match between the surface energy of the substrate and that of the cell liquid causes the liquid  140  to settle in the valleys in a generally constant depth fashion. Because the surface energy of the substrate  130  and 2-D structures  132  is not excessively low, the cell liquid does not form beads. Because the surface energy of the substrate is not too high, the liquid does not spread too quickly. If the surface energy of the substrate was too high, the liquid would likely cover and adhere to the tops of the 2-D structures  132  themselves which would be undesirable in some cases. 
         [0027]    At  108  in  FIG. 3 , the method further comprises forming a seed stem cell layer using the deposited stem cell liquid. This action can be performed by allowing the deposited stem cell liquid to settle (e.g., for a few hours) on the substrate and allow the stem cells to adhere to the surface. At  110  in  FIG. 3  the now formed stem cell plate is then placed into an incubator to facilitate cell growth and viability. 
       Examples 
       [0028]    The following examples are provided to illustrate the exemplary methods for practicing the present invention. 
       Example 1 
       [0029]    iPS-derived cardiomyocytes were deposited onto two surface modified glass slides as in  FIG. 3 . The predefined hydrophilic wells had the same area of the ones on a standard 96-well plate (about 0.32 cm 2 ). The wells were seeded with 600,000 live cells/cm 2 , meaning, 1.92E+05live cells/well. Three adhesion promoter materials, matrigel vs. gelatin/fibronectin vs. laminin were used as substrate cell binding options. 
       Example 2 
       [0030]    The stem cell doped liquid was seeded with iPS-derived cardiomyocytes at the same density and conditions on an 8-well plastic (Permanox) standard chamber slide to see how they would compare as a reference standard. 
         [0031]    The slides together with a 6 cm 2  empty plate containing tissue culture-grade water were all placed inside a large 15 cm 2  tissue culture dish to create an additional saturated atmosphere to avoid too much evaporation. 
         [0032]    One day after seeding the cells were attached in all the wells. It appeared that there were more cells rounded up in the patterned slide vs. the unpatterned chamber slide. From the aforementioned examples it appears that using glass is not optimal for binding iPS based cardiomyocytes and so polymer based substrates may be employed to facilitate good cell adhesion and growth to hydrophobic and hydrophilic patterned substrates. 
         [0033]    The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.