Abstract:
The invention provides method of fabricating a scaffold comprising a fluidic network, including the steps of: (a) generating an initial vascular layer for enclosing the chamber and providing fluid to the cells, the initial vascular layer having a network of channels for fluid; (b) translating the initial vascular layer into a model for fluid dynamics analysis; (c) analyzing the initial vascular layer based on desired parameters selected from the group consisting of a characteristic of a specific fluid, an input pressure, an output pressure, an overall flow rate and combinations thereof to determine sheer stress and velocity within the network of channels; (d) measuring the sheer stress and the velocity and comparing the obtained values to predetermined values; (e) determining if either of the shear stress or the velocity are greater than or less than the predetermined values, and (f) optionally modifying the initial vascular layer and repeating steps (b)-(e). The invention also provides compositions comprising a vascular layer for use in tissue lamina as well as a medical devices having a vascular layer and kits.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of PCT international application Ser. No. PCT/US2008/004872, filed Apr. 14, 2008, designating the United States and published in English on Oct. 23, 2008 as publication WO 2008/127732 A3, which claims priority to U.S. Provisional Application Ser. No. 60/923,312, filed Apr. 12, 2007 and U.S. Provisional Application Ser. No. 60/923,474, filed Apr. 12, 2007. The entire contents of the aforementioned patent applications are incorporated herein by this reference. 
     
    
     STATEMENT OF GOVERNMENT SUPPORT 
       [0002]    Some of the work described herein was sponsored by the National Institutes of Health, Grant Nos. 1 F32 DK076349-01 and T32DK07754-09. The U.S. Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    Although organ transplantation has emerged as viable treatment for patients with end stage organ disease, there is a uniform organ shortage in the United States and worldwide. Patients awaiting liver, lung and heart transplants often die before they receive an organ due to the long transplant waiting times. Artificial organs could be used to assist or even replace organs as a solution to the organ shortage. 
         [0004]    Development of a tissue engineered solid organ such as a liver or kidney is typically dependent on two main components—the parenchymal cells and a vascular network to supply oxygen and nutrients to the parenchymal cells. The diffusion distance of oxygen and nutrients from a blood vessel through tissue is very short (e.g., a few hundred microns). If cells, such as hepatocytes are grown in a three-dimensional container and placed in the body near a blood vessel, only the cells in close proximity to the blood vessel will survive. Over time, new blood vessels may grow into the implanted cells, however, many of the cells that are far from the existing blood vessels will die without immediate blood supply. 
         [0005]    Present designs provide a vascular network as a central part of the scaffold for a tissue engineered solid organ. The vascular network serves as the blood supply to deliver oxygen and nutrients to the other cells which are also placed in the scaffold to give the organ its function (e.g., hepatocytes for a tissue engineered liver). This approach allows a vascular network to be designed for the particular organ from the inlet vessels which are anastomosed to the native circulation to the smallest vessels which perfuse the parenchymal cells. This tissue engineered organ is implanted with blood vessels already adequately located in proximity to the parenchymal cells. This allows a thick, solid organ such as the liver, lung, heart, kidney or other organs or tissues to be created and implanted. 
         [0006]    In the body, blood vessels which supply organs typically enter the organs as one single vessel (typically an artery) and then branch in a pattern, reducing their diameter and greatly increasing their surface area until they form the smallest vessels known as capillaries. The capillaries supply the cells of the organ with oxygen and nutrients and remove waste products. From the capillaries, the vessels coalesce in a similar branching pattern to exit the organ often as a single vessel (typically a vein). There is a need in the art for tissue engineered organs having such a physiological vasculature network to provide sustained function following implantation. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the subject technology to provide a tissue engineered organ which has a structure similar to natural organs and is capable of similar performance for sufficient periods of time without malfunction. Preferably, the tissue engineered organ will have low thrombogenicity and a high packing efficiency. 
         [0008]    It is envisioned that the subject technology may be used to replace an organ, in vivo or ex vivo, assist an organ, temporarily replace an organ and ascertain the efficacy and safety of a drug on human cells. 
         [0009]    One aspect of the subject technology provides a method of fabricating a scaffold comprising a fluidic network. The method includes the steps of: (a) generating an initial vascular layer for enclosing the chamber and providing fluid to the cells, the initial vascular layer having a network of channels for fluid; (b) translating the initial vascular layer into a model for fluid dynamics analysis; (c) analyzing the initial vascular layer based on desired parameters selected from the group consisting of a characteristic of a specific fluid, an input pressure, an output pressure, an overall flow rate and combinations thereof to determine sheer stress and velocity within the network of channels; (d) measuring the sheer stress and the velocity and comparing the obtained values to predetermined values; (e) determining if either of the shear stress or the velocity are greater than or less than the predetermined values, and (f) optionally modifying the initial vascular layer and repeating steps (b)-(e). The initial vascular layer may be fabricated from collagen. 
         [0010]    Another aspect of the subject technology is directed to a composition comprising a vascular layer for use in a tissue lamina. The vascular layer includes a substrate defining a network of channels having at least one input channel and at least one output channel and at least two intermediate channels at least partially connecting the at least one input channel and the at least one output channel, each channel having a height and a width, wherein the intermediate channels are formed in accordance with Murray&#39;s law by varying said height and width with respect to adjacent portions of the input and output channels. 
         [0011]    The subject technology also provides an artificial vascular network including a substrate defining a network of channels having at least one input channel and at least one output channel and at least two intermediate channels at least partially connecting the at least one input channel and the at least one output channel. Each channel has a height and a width, wherein the intermediate channels are formed in accordance with Murray&#39;s law by varying said height and width with respect to adjacent portions of the input and output channels. The artificial vascular network is prepared by a process including the steps of: (a) fabricating a substrate defining a network of channels, wherein the channels provide fluid to cells; (b) translating the network of channels into a model for fluid dynamics analysis; (c) analyzing the network of channels based on desired parameters selected from the group consisting of a characteristic of a specific fluid, an input pressure, an output pressure, an overall flow rate and combinations thereof to determine sheer stress and velocity within the network of channels; (d) measuring the sheer stress and the velocity and comparing the obtained values to predetermined values; and (e) determining if either of the shear stress or the velocity are greater than or less than the predetermined values. 
         [0012]    Another aspect is a medical device for assisting or replacing an organ including a header layer having a nozzle to connect to a vessel and defining a distribution network in fluid communication with the nozzle, and a first vascular layer having a substrate defining a vascular network of channels in fluid communication with the distribution network, the vascular network including at least one input channel that bifurcates repeatedly into portions, which rejoin to form an output channel, the input and output channels having a height and a width, wherein the bifurcated portions are formed in accordance with Murray&#39;s law by varying said height and width with respect to adjacent portions of the input and output channels. Another layer defines a chamber for holding parenchymal cells that is configured to receive oxygen and nutrients from a fluid in the vascular layer. 
         [0013]    A membrane separates the vascular layer from the parenchymal chamber. Preferably, the membrane is semi-permeable and the pore size of the membrane is smaller than the cell diameters, thus, cells will not be able to pass through (i.e., a low permeability for animal cells), while low molecular weight nutrients, gases and fluids can pass through (i.e., a high permeability for small compounds), thereby providing adequate cell-to-cell signaling. Cell sizes vary but in general, the cell sizes are in the range of microns. For example, a red blood cell has a diameter of about 8 μm. Preferably, the average membrane pore size is on a submicron-scale to ensure effective screening of the cells. For lung application and the like, the membrane should also allow passage of carbon dioxide, oxygen and like gases therethrough. 
         [0014]    A vascular network for creating tissue engineered organs in accordance with the subject technology follows the same approach as the blood vessels within a natural organ. The replication of physiological design principles in vascular networks is referred to herein by the phrase “biomimetic vascular networks”. 
         [0015]    The subject technology described herein includes the theory, concepts, design, manufacturing, testing and applications of biomimetic vascular networks. These vascular networks have primary application as a central part of a scaffold to create a tissue engineered structure such as an organ or other mammalian tissue. There are additional applications of this technology, for example, as a tool, e.g., a platform for drug discovery, development and/or evaluation (e.g., toxicity, safety and/or efficacy) and as a platform for in vitro or in vivo research and testing. 
         [0016]    It should be appreciated that the present invention can be implemented and utilized in numerous ways, including without limitation as a process, an apparatus, a system, a device, a kit (e.g., a kit comprising one of the platforms described herein and instructions for use), a method for applications now known and later developed or a computer readable medium. These and other unique features of the system disclosed herein will become more readily apparent from the following description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Other aspects and many of the attendant advantages of this invention will readily be appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0018]      FIG. 1  illustrates a perspective view of a bifurcating portion of a vascular network in accordance with the subject technology; 
           [0019]      FIG. 2  illustrates a perspective view of a trifurcating portion of a vascular network in accordance with the subject technology; 
           [0020]      FIG. 3  is a top view of a vascular network design in accordance with the subject technology; 
           [0021]      FIG. 4  is a more detailed top view of a portion of the vascular network design of  FIG. 3 ; 
           [0022]      FIG. 5  is an exploded view of implant components using the vascular network design of  FIG. 3  in accordance with the subject technology; 
           [0023]      FIG. 6  is an assembled perspective view of the implant of  FIG. 5 ; 
           [0024]      FIG. 7  is an assembled cross-sectional view of an implant using a simple vascular network design in accordance with the subject technology; 
           [0025]      FIG. 8  is cross-sectional view of another implant with multiple vascular layers in accordance with the subject technology; 
           [0026]      FIG. 9  is a graph of the results of the in vitro blood tests and the analysis of blood flow through the implant of  FIG. 6 ; 
           [0027]      FIG. 10  a perspective view of another vascular network design in accordance with the subject technology; 
           [0028]      FIG. 11  is a detailed view of the vascular layer of  FIG. 10 ; 
           [0029]      FIG. 12  is a graph of the results of the in vitro blood tests and the analysis of blood flow through an implant using the vascular layer of  FIG. 10 ; 
           [0030]      FIG. 13  is a vascular network design which utilizes a repeating polygonal pattern in accordance with the subject technology; 
           [0031]      FIG. 14  is a portion of a vascular network created with a collagen film in accordance with the subject technology; 
           [0032]      FIG. 15  is an implant in accordance with the subject technology in place as a liver assist device; and 
           [0033]      FIG. 16  is an implant in accordance with the subject technology in place as a lung assist device. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0034]    The present invention overcomes many of the prior art challenges associated with tissue engineered vascular networks and artificial solid organs. The advantages, and other features of the technology disclosed herein, will become more readily apparent to those having ordinary skill in the art from the following detailed description of certain embodiments taken in conjunction with the drawings which set forth representative embodiments of the present invention and wherein like reference numerals identify similar structural elements. 
         [0035]    It is to be understood that the subject technology is not intended to be limited to the particular constructs and methods described in the described embodiments, as one skilled in the art can extend the concepts involved using variations which are obvious after reading the present disclosure. Although any methods and materials similar or equivalent to those described herein may be useful in the practice of the subject technology, certain compositions, films, methods and materials are described below. All relative descriptions herein such as top, bottom, left, right, up, and down are with reference to the Figures, and not meant in a limiting sense. 
         [0036]    The process of creating an optimal vascular network design that performs sufficiently is aided by identifying and learning from the fundamental structure of blood vessels in the body. The vascular network of arteries, capillaries and veins is complex. However, the basic structural principles can be utilized within the limits of currently available manufacturing processes. There are several fundamental principles of blood vessels which have been utilized in the subject technology and incorporated into the design of the biomimetic vascular networks described herein. The concepts and the resulting designs are also advantageously refined using computational fluid dynamics (CFD) analysis. 
         [0037]    Blood vessels in the body have a particular relationship between the diameter of a parent vessel and the diameters of the resulting daughter vessels known as Murray&#39;s Law. Generally, Murray&#39;s Law states that the cube of the diameter of the parent vessel is equal to the sum of the cubed diameters of all daughter vessels. For a bifurcating channel, Murray&#39;s Law is expressed as d 0   3 =d 1   3 +d 2   3 , where d 0  is the diameter of the parent vessel and d 1  and d 2  are the diameters of the daughter vessels. The reference of diameter in regard to a rectangular channel refers to the hydraulic diameter of the channel as defined as hydraulic diameter=4*(cross-sectional area of channel)/(channel perimeter). 
         [0038]    The principle of Murray&#39;s law was described in the 1930&#39;s and has since been confirmed in many studies. The principle of Murray&#39;s law is to optimize the efficiency of mass transport of the blood and control the shear stress within the vessels or channels. Shear stress in blood vessels within the body is tightly controlled within a physiologic range. Shear stress for arteries is typically on the range of 15 to 70 dynes/cm 2  and for veins the shear stress is typically on the range of 1-5 dynes/cm 2 . Vessel bifurcations have shear stress which is fairly uniform over the entire network. 
         [0039]    Controlling shear stress is very important in minimizing the formation of thrombus within the network and ensuring that oxygen and nutrient exchange occurs within normal physiologic parameters. The biomimetic principle of designing for appropriate shear stress was employed in the design of the vascular networks described herein to reduce or eliminate thrombosis with the networks. 
       A Vascular Network Design 
       [0040]    Referring to  FIG. 1 , a bifurcating portion  10  of a representative simple vascular network is shown in perspective view. The network portion  10  includes a substrate  11  having channels formed therein. A parent vessel or channel  12  bifurcates into two daughter vessels  14 . According to Murray&#39;s law, if the daughter vessels  14  are equal, then Murray&#39;s law simplifies to d 0   3 =2(d 1   3 ). For example, if the parent vessel  12  is 1000 um in diameter than each of the daughter vessels  14  would be 794 um in diameter. The daughter vessels  14  further bifurcate into smaller vessels  16 . According to Murray&#39;s law, the resulting smaller vessels  18  and  20  would be 630 um in diameter for this example. 
         [0041]    The smaller vessels  16  rejoin to form a larger vessels  18 , which then join to form vessel  20 . In one embodiment, the smaller vessels  16  are most representative of the capillary vessels found in the body. However, the parent channels  12 ,  14  may participate in nutrient, gas, and waste exchange in a similar manner as the smallest channels  16 . Likewise the channels  18 ,  20 , which are formed from the smallest channels  16 , are somewhat analogous to capillaries or venules or veins in the body. Similarly, the channels  18 ,  20  may also participate in nutrient, gas and waste exchange like the smallest channels  16 . 
         [0042]    The vessels  12 ,  14 ,  16 ,  18 ,  20  are formed from linear structures and substantially rectangular. In another embodiment, the vessels  12 ,  14 ,  16 ,  18 ,  20  may be circular or elliptical nature. In the event that the vessels  12 ,  14 ,  16 ,  18 ,  20  are rectangular, the vessels  12 ,  14 ,  16 ,  18 ,  20  may have an aspect ratio of 1:1, e.g., the width and depth are equal. In another embodiment, the vessels  12 ,  14 ,  16 ,  18 ,  20  may have an aspect ratio of: 1:2; 2:1; or the like. Furthermore any aspect ratio such as the range of 100:1 or 1:100 may be considered depending on the particular application of the vascular network. 
         [0043]    Referring to  FIG. 2 , a perspective view of a trifurcating portion  30  of a vascular network is shown. The trifurcating portion  30  also includes a substrate  31  that forms a parent vessel  32 , which divides into two equal daughter vessels  34  and a larger daughter vessel  36  as commonly occurs in the body. In this event, Murray&#39;s law is d 0   3 =d 1   3 +d 2   3 +d 3   3 . Preferably, Murray&#39;s law is applied so as to achieve an adequate capillary channel density so the maximum distance between channels  32 ,  34 ,  36  does not exceed the maximum diffusion distance of oxygen and nutrients. 
         [0044]    In one embodiment, the maximum distance between channels would not exceed 40 um. In another embodiment this maximum distance would not exceed 500 um. In another embodiment, this maximum distance would be in the range of 200 um to 300 um. In another embodiment, a majority of the channels would fall within a maximum distance and a minority of the channels would have a distance therebetween higher than the preferred maximum. A network needs to both follow or approximate Murray&#39;s law and the other design principles described herein and achieve a branching strategy which results in achievement of the desired maximum distance between channels. 
         [0045]    Generally, the bifurcation angle between the parent vessel and the daughter vessels in the body is related to the relative diameters of the daughter vessels. These relations are described in the literature. In principle, if there are two daughter vessels  14  which are equal in diameter as shown in  FIG. 1 , the preferred angle  22  between an axis  24  of the parent vessel  12  and the daughter vessel is approximately 45 degrees. 
         [0046]    Still referring to  FIG. 2 , when one or two daughter vessels  34  are smaller than another daughter vessel  36 , the preferred angle  38  between the axis  40  of the parent vessel  32  and the daughter vessels  34  becomes larger than 45 degrees and approaches 90 degrees as the daughter vessels  34  become much smaller than the parent vessel  32  and other daughter vessel  36 . Following these biomimetic principles of bifurcation angles helps to achieve uniform blood flow with minimal shear disruption and hence platelet activation. 
         [0047]    In the embodiment of  FIG. 2 , the smaller daughter channels  34  are orientated such that a bottom  42  of the channel  34  is not on the same plane with respect to the bottom  44  of the parent channel  32  but the respective tops of channels  32 ,  34  are on the same plane. As a result, an edge  46  is created in the transition between channels  32 ,  34 . Additionally, vertical edges  48  are created in the same transition. The edges  46 ,  48  are potential area for shear stress concentrations, flow separation, stasis or turbulence within the device. 
         [0048]    Still referring to  FIG. 2 , the bottom  44  of the parent channel  32  includes a radius of curvature or fillet  50  on each side. In the absence of a radius of curvature  50 , there is lower blood velocity in the corner area than in other portions of the channel  32 . The addition of the fillets  50  creates a more uniform velocity in the channel  32 . 
         [0049]    The size, shape and position of the fillets  50  are selected using computational fluid dynamics (CFD) analysis, which is a tool used to analyze fluid flow to predict the behavior of a fluid within a defined model. One version of a CFD tool is the FLOWORKS® module within the SOLIDWORKS® 3D CAD software available from SolidWorks Corporation of Concord, Mass. The vascular network designs are modeled in three dimensions and then analyzed with CFD analysis to iterate and evaluate performance of the features with respect to selected parameters. 
         [0050]    Modification of the design elements of the networks from known biomimetic principles may occur, however there are goals or criteria of the flow of fluid through the network which are established and used as a guideline or boundary condition. For example, target shear stress, inlet pressure, outlet pressure and resulting flow rate can be the primary goals of the design. Other goals of the design may include minimizing flow separation, minimizing areas of low flow velocity and/or stagnation. 
         [0051]    In one embodiment, the boundary conditions defined for the CFD analysis of the network are a combination of inlet pressure, outlet pressure and flow rate. The analysis focuses on blood in a non-Newtonian model as the fluid flowing through the vascular network. After the CFD module completes the analysis, the results are reviewed, specifically the pressure drop across the network, the flow within the network, the shear stress on all of the walls of the network, and the velocity within the channels including the uniformity of velocity. If any of the parameters fall outside a target range, then the design is modified and the analysis is repeated. 
         [0052]    For example, if the shear stress within a certain area is too high, that area of the design is modified and the analysis run again. The iterative sequence of reviewing an analysis, determining an area of the design which is not optimal, changing the design and running the analysis again is repeated to optimize many of the features of the design. For example, the fillet or defined curvature of each edge for optimal flow can be varied in response to CFD analysis. Referring again to  FIG. 2 , the dimensions of the fillets  50  relative to that of the channel  32  were optimized using CFD analysis. 
         [0053]    Furthermore, the fillets  46 ,  48  and  49  in the transition between channel  32  and channel  34  are each different parameters. Serial iterations of design changes, CFD analysis and review of results can optimize not only each fillet but the bifurcation as a unit so the flow at the convergence of channels  32 ,  34  and  36  is uniform with minimal shear stress changes and, thus, minimal risk for blood clot formation. As a result of the iterative process, branching channel design can be formed with minimal flow disturbances. 
         [0054]    Referring now to  FIGS. 3 and 4 , top views of a vascular network design  60  are illustrated. The vascular network design  60  replicates the basic structure of the vascular system of the liver, which is arranged in a radial hexagonal pattern. The vascular network design  60  includes a substrate  61  forming channels  62  with hexagonal areas or liver lobules  64  (best seen in  FIG. 4 ) intermediate the channels  62 . Each hexagonal area of a liver has several (usually six) blood vessels at the periphery of the lobule which undergo branching toward a central vein. The basic design for the vascular network design  60  was organized similar to a liver lobule. The vascular network design  60  has six radially spaced inlet channels  66  which undergo multiple bifurcations towards a central draining vein  68 . 
         [0055]    Unlike the liver lobule, which is a tightly packed structure of blood vessels and cells, one can clearly observe that in certain points of the vascular network design  60 , there is a significant distance between adjacent channels. A goal of the vascular network design  60  was to take a step towards the fundamental structure of a liver lobule but keep the vascular network branching pattern simple to understand the utility of the biomimetic design principles. Other designs may pursue a much denser pattern to achieve effective delivery of oxygen and nutrients. 
         [0056]    Accordingly, in the planar vascular network design  60  of  FIG. 3 , the inlet channels  66  and outlet central vein  68  are oriented orthogonal to the branching channels  62 . The channels  62  are rectangular with an aspect ratio of 1:1 and hydraulic diameters calculated according to Murray&#39;s law. The channels  62  start with initial channels  70  in fluid communication with the inlet channels  66 . The initial channels  70  successively branch, in a bifurcating manner, into successively smaller channels  72 ,  74 ,  76 ,  78 . The smallest diameter channels  78  are preferably 200 um in diameter while the initial channels  70  from the inlet are 608 um in diameter. Since the channels  62  undergo simple bifurcations, the bifurcation angles are 45 degrees for each. Similarly, the channels  62  rejoin to form successively larger channels  80 ,  82 ,  84 ,  86 , which also have bifurcation angles of 45 degrees with the largest rejoining channels  86  being in fluid communication with the outlet central vein  68 . 
         [0057]    Generally, in the body, the smaller the blood vessel diameter, the shorter the blood vessel, with capillaries being the shortest blood vessels. More specifically, the literature enumerated lengths of several vessels according vessel diameter from 4 mm diameter arteries to 8 um diameter capillaries. Using this data in a spreadsheet program, a 3 rd  order polynomial equation was determined through best fit analysis to derive a biomimetic length for the different diameter channels in the vascular network design  60 . For a blood vessel of diameter x, the length y was determined by the equation, y=−1E-09x3+8E-06x2+0.0259x+0.1226. 
         [0058]    For the vascular network design  60 , the smallest channels  78  were 200 um in diameter and the biomimetic length derived from the previously stated equation was used to determine a biomimetic length of 6.79 mm. The branches  70 ,  72 ,  74  and  76  which preceded the smallest channel  78  and the branches  80 ,  82 ,  84 , and  86  which followed, could not be constructed with their biomimetic lengths due to the size constraints of the mold. Therefore, the biomimetic length of the smallest channel  78  was used and the lengths of the other channels ( 70 ,  72 ,  74 ,  76 ,  80 ,  82  and  84 ) were scaled to 39.2% of their biomimetic length to enable the entire vascular network to fit within the defined mold size of six inches in diameter. Thus, the vascular network design  60  uses proportionally smaller lengths while preserving length relationships between channels of different diameters. In another embodiment, the biomimetic lengths of all of the channels could be scaled in an equal fashion. 
         [0059]    The shear stress in arteries is between 15 and 70 dynes/cm 2 . The channels  62  are designed in response to blood flow under CFD analysis to form curves which minimize concentrations of shear stress. Through an iterative design process using results from repeated CFD analysis, the curvature of the vascular network design  60  was improved to minimize concentrations of shear stress. The vascular network design  60  also keep at least some of the input channels  70 ,  72 ,  74 ,  76 ,  78  in the physiological shear stress range for arteries. Additionally, as described above, the channels  62  may have radii of curvature or fillets, particularly at the points of bifurcation, to improve the flow characteristics. In short, all of the channels  62  can be evaluated and refined using results from repeated CFD analysis to minimize the variation of shear stress. 
         [0060]    The venous system in the body has comparatively larger diameters and lower shear stress than the arterial system. Accordingly, the shear stress within the venous system is typically 1 to 5 dynes/cm 2 , which is lower than the shear stress in the arterial system. The vascular network design  60  has lower shear stress in the output channels  80 ,  82 ,  84 ,  86  to minimize resistance and mimic the venous shear stress values. There is a balance between achieving a low shear stress value to replicate the human veins and having too slow of blood flow such that thrombus may be initiated. Thus, the output channels  80 ,  82 ,  84 ,  86  are scaled up in diameter compared to the inlet channels  70 ,  72 ,  74 ,  76 ,  78  to achieve a shear stress value of generally between 6 and 10 dynes/cm 2 . 
         [0061]    In other embodiments, the shear stress values for the inflow portion of a vascular network may correspond to normal arterial shear stress values and the outflow portions of the network may correspond to normal venous shear stress values. The degree of scaling up of the diameters of the venous system may be in the range of 1% to 50% of the corresponding inflow diameters. In another embodiment, the degree of venous scaling may be in the range of 5% to 15% of the corresponding venous diameters. Although the vascular network design  60  has a branching pattern in the outflow portion which closely replicates the branching pattern in the inflow portion, in other embodiments, the branching pattern of the inflow and outflow portions may not be similar. Furthermore, the branching pattern between portions of the inflow areas of the network may be different than other areas of the inflow network. Likewise in other embodiments, the branching pattern between portions of the outflow areas of the network may be different than other areas of the outflow network. 
       Process for Creating the Vascular Network Design 
       [0062]    Previous vascular networks for tissue engineered organ development were manufactured utilizing molds created using the photolithography process and replica casting using silicone as described in U.S. patent application Ser. No. 10/187,247, filed Jun. 28, 2002 and U.S. patent application Ser. No. 10/983,213, filed Nov. 5, 2004. Photolithography is only able to create a single depth of channel or multiple depths with vertical step transitions. In contrast, the vascular network design  60  has many different diameters of channels and these channels may achieve the most uniform blood flow if the channels have an aspect ratio not equal to 1:1. Furthermore, a uniform depth transition between the channels can minimize shear stress changes. Minimizing shear stress changes and associated flow disturbances will minimize clotting within the resulting device. In this regard, the device can be characterized as have low thrombogenicity. 
         [0063]    Although using photolithography to create a mold to manufacture the vascular network design  60  is possible, a different manufacturing process was used, namely electrical discharge machining (EDM) or micro-machine tool equipment available from Microlution, Inc. of Chicago, Ill. In the EDM process, an electrode in the shape of the reverse or mirror image of the vascular network design  60  is created out of graphite using traditional milling machining processes. The electrode is then used to vaporize metal in the desired pattern to create a metal or vascular mold (not shown). The mold is preferably a positive mold, thus the channel features are ridges projecting from a base. To create the vascular network design  60 , a material is placed over the mold in a manner in which the material takes the shape of the projected network pattern. The mold is then removed from the material or substrate  61 , leaving the imprint of the vascular network design  60  in the material. 
       The Assembled Implant Using the Vascular Network Design 
       [0064]    Referring now to  FIG. 5 , an exploded view of an implant  100  using the vascular network design  60  is shown. The implant  100  includes a perforated layer  110  and a header layer  120 , which mount on top of the vascular network design  60  to substantially enclose the channels  62  as shown in  FIG. 6 . The vascular network design  60  has inlets  66  that accept inflow in a direction orthogonal to the branched pattern of channels  62  in the vascular network design  60 . In order to direct blood flow from a blood vessel and into the inlets  66  of the vascular network design  60 , the header layer  120  distributes blood from a single central inlet  122  via radial channels  124 . 
         [0065]    On the radially outward end, the radial channels  124  form an opening  126 , which aligns with a respective passthrough hole  112  in the perforated layer  110 . The passthrough holes  112 , in turn, align with the inlets  66  of the vascular network design  60  so that blood flows from the single central inlet  122  to the inlets  66 . Blood passes through the header layer central inlet  122 , the header layer radial channels  124 , the passthrough holes  112  in the perforated layer  110 , into the inlets  66  of the vascular network design  60 , through the branched pattern of channels  62 , and out the central opening  68  of the vascular network design  60 . 
         [0066]    Referring to  FIG. 7 , an assembled cross-sectional view of the implant or scaffold  100  using the simplified vascular network design  60  is shown. The blood enters the scaffold  100  via an inlet nozzle  130 . The flow path of the blood is designated by arrows “a”. The blood passes through a header layer  120 , which separates and directs the blood radially outward through channels  124 . The blood then flows through the passthrough hole  112  in the perforated layer  110  and into the inlets  66  of the vascular layer  60 . The blood is then directed through the channels  62  of the vascular network design  66 , where the blood collects in a central outlet  68 . The blood then proceeds through the outlet nozzle  132 . 
         [0067]    Cells particular to the type of tissue which is being generated (e.g., hepatocytes for liver) are positioned in at least one parenchymal chamber  172  defined by a cellular layer  170 . The parenchymal chamber  172  is separated from the vascular network channels  62  by a semi-permeable membrane layer  110   a  (see  FIG. 7 ). The membrane can be formed from a physiological source (e.g., derived from a living tissue), or from a biologically compatible, nondegradable material such as cellulose, PolyDiMethylSiloxane (PDMS), PolyMethylMethacrylate (PMMA), PolyEtherSulfone (PES), PolySulfone (PS), PolyCarbonate (PC), or from a degradable material such as PLGA, PolyCaproLactone (PCL) or Biorubber, but the invention is not so limited. “Parenchymal cells” include the functional elements of an organ, as distinguished from the framework or stroma. Parenchymal cells can include but are not limited to smooth or skeletal muscle cells, myocytes, fibroblasts, chondrocytes, adipocytes, fibromyoblasts, ectodermal cells, including ductile and skin cells, hepatocytes, kidney cells, liver cells, cardiac cells, pancreatic islet cells, cells present in the intestine, and other parenchymal cells, stem cells including adipose or bone derived mesenchymal stem cells, embryonic stem cells and induced pluripotent stem cells, osteoblasts and other cells forming bone or cartilage, and hematopoietic cells. The cells are not limited to parenchymal cells but can be many other kinds of cells (stem or progenitor cells). 
         [0068]    As blood flows though the vascular network layer  60 , oxygen and nutrients diffuse across the membrane layer  110   a  to nourish the cells within the chamber  172 . Waste generated by the cells in the chamber  172  can diffuse back across the membrane layer  110   a  and into the channels  62  of the vascular network layer  60 . The chamber  172  may have one or more inlets  174  where the cells can be infused, injected or otherwise inserted into the chamber  172  of the implant  100 . The flow path of the cells is designated by arrows “b”. 
         [0069]    In the embodiment of  FIGS. 5-7 , there are six radially spaced channels  124  on the header layer  120  that align with six inlets  66  of the vascular network design  66 . Single central inlet  122  and outlet  68  align with the nozzles  132 ,  134  but other configurations may be utilized such as two inlets and two outlets. In another embodiment, there may be four inlets and one outlet. In another embodiment, the number of inlets may be in the range of 1 to 120. In another embodiment, the number of outlets may be in the range of 1 to 120. 
         [0070]    The implant  100  has the nozzles  130 ,  132  for interconnecting with the blood vessels of the body. The inlet nozzle  130  preferably permanently mounts to the central inlet  122  of the header layer  120  and the outlet nozzle  132  permanently mounts to the outlet  68  of the vascular network design  60 . The nozzles  130 ,  132  may utilize a standard artificial vascular graft, such as a PTFE graft. The vascular graft may be secured to the connectors or nozzles  130 ,  132  by intrinsic compression of the graft around the respective nozzle  130 ,  132 , such as by one or more sutures, clamps, adhesive, locking devices or any combination there of. In another embodiment, a graft may be incorporated directly into the scaffold material without attaching to a nozzle. A graft may be directly secured to a component of the scaffold such as the header layer  120  by one or more sutures, clamps, adhesive, locking devices or any combination there of. 
       Another Assembled Implant 
       [0071]    Referring to  FIG. 8 , another assembled implant  200  is shown in cross-sectional view. As will be appreciated by those of ordinary skill in the pertinent art, the implant  200  utilizes similar principles to the implant  100  described above. Accordingly, like reference numerals preceded by the numeral “2” instead of the numeral “1” are used to indicate like elements whenever possible. The primary difference of the implant  200  in comparison to the implant  100  is having multiple vascular layers or vascular network designs  260   a ,  260   b  joined to a single header layer  220 . Each vascular network design  260   a ,  260   b  couples to respective semi-permeable membrane layers  210   a ,  210   b  and cellular layers  270   a ,  270   b.    
         [0072]    The number of vascular layers may vary such as within the range from 2 to 1000 or more. The number and size of the layers for a particular implant depends on the particular tissue, and the amount of tissue required, and the size of the patient. The number of scaffolds may also be increased to accommodate the needs of the patient. 
         [0073]    For multiple vascular layers  260   a ,  260   b , the inlets  266   a ,  266   b  are redesigned to balance flow between the layers  260   a ,  260   b . For example, the inlets  266   a ,  266   b  may taper. Alternatively, each vascular layer  260   a ,  260   b  may be fed by only half of the radial channels  224  of the header layer  220 . As shown in  FIG. 8 , the vascular layer  260   a  has inlet holes  266   a  which extend there through in a tapered manner. Thus, blood enters both the channels  262   a ,  262   b  of both vascular layers  260   a ,  260   b  by partially passing through the vascular layer  260   a  into the vascular layer  262   b  below. 
         [0074]    In use, blood from the patient enters the implant  200  via the inlet nozzle  230  and proceeds into the header layer  220 , where the blood is directed radially via the distribution channels  224 . The blood proceeds into through the passthrough holes  212  in the perforated layer  210  and into the inlets  266   a ,  266   b  of the vascular layers  260   a ,  260   b.    
         [0075]    With more than one vascular layer  260   a ,  260   b , a portion of the blood flow continues vertically downward past the upper vascular layer  260   a , through respective openings  212   a  in the adjacent semi-permeable membrane layer  210   a  and parenchymal layer  270   a  and into the inlets  266   b  of the lower vascular layer  260   b . As noted above, to facilitate even distribution of blood between the vascular layers  260   a ,  260   b , the respective inlets  260   a ,  260   b  and openings  212   a  may taper. Alternatively, select radial channels  224  may feed different vascular layers  260   a ,  260   b.    
         [0076]    Within the vascular layers  260   a ,  260   b , the blood is directed through the bifurcating channels  262   a ,  262   b  into central collecting outlets  268   a ,  268   b . The central collecting outlets  268   a ,  268   b  extend through the underlying semi-permeable membrane layers  210   a ,  210   b  and cellular layers  270   a ,  270   b  to discharge the blood through the outlet nozzle  232 . The outlet nozzle  232  may direct the blood back into the blood vessel of the patient or to another device for further processing prior to reentry. The semi-permeable membranes  210   a ,  210   b  are adjacent to the vascular network layers  260   a ,  260   b  to separate the vascular network layers  260   a ,  260   b  from the adjacent parenchymal layers  270   a ,  270   b , but still allow oxygen and nutrients to pass. The parenchymal layers  270   a ,  270   b  form chambers  272   a ,  272   b , which contain cells corresponding to the tissue which the implant  200  is supplementing or replacing. 
         [0077]    In operation, there may be more than one implant or scaffold with a plurality of vascular layer and other layers. The number of implants or scaffolds may be in the range of 1 to 50. In another embodiment the number implants or scaffolds may be in the range of 2 to 8. The implants may have multiple inlets and outlets. For example, a series of inlets may be utilized to supply blood to five vascular layers. The implants may also be connected in a parallel and/or serial configuration between the supply blood vessel and the return blood vessel. Bifurcated vascular grafts may be used as necessary. The number of scaffolds and thus cellular components may be different for each patient and thus a different size or configuration of device may be used for each patient. The chambers  272   a ,  272   b  have two inlets  274   a ,  274   b  where the cells can be infused along the flow path indicated by arrow b, injected or otherwise inserted into the chamber  272   a ,  272   b  of the implant  200 . 
       Implant Performance 
       [0078]    Implants  100  according to  FIG. 6  have been built and tested. The mold for the vascular layer  60  was built with the EDM process as described previously. The mold for the header layer  120  was built using a traditional milling process. The perforated layer  110  was formed using a standard Petri dish as a mold with a punch process to create the holes  112 . The nozzles  132  were created using a traditional machining process. The implant  100  was fabricated from a polydimethylsiloxane (PDMS), a silicone like material, as the material for the header layer  120 , the vascular layers  60  and the perforated layer  110 . The nozzles  132  may be made with polycarbonate or nylon. For assembly, the header layer  120 , the vascular layers  60  and the perforated layer  110  were adhered to one another with oxygen plasma bonding. 
         [0079]    The implants  100  were tested with anti-coagulated sheep blood flowing at various flow rates and the inlet pressures of the device were measured. The results were compared to the expected results of the inlet pressure according to CFD analysis.  FIG. 9  is a graph of the results of the in vitro blood tests and the analysis of blood flow through the implant  100  using computational fluid dynamics. The in vitro flow results and the CFD results had good correlation, especially at the design point of a 6 mmHg pressure drop across the implant  100 . There was a less than 5% variation between the in vitro performance and the expected CFD data. 
       Another Vascular Layer 
       [0080]    Referring to  FIGS. 10 and 11 , a mold  360  for fabricating another vascular layer is shown in perspective view. The mold  360  consists of raised features where the vascular layer consists of channels. As will be appreciated by those of ordinary skill in the pertinent art, a vascular layer fabricated from mold  360  utilizes similar principles to the vascular layers  60 ,  160 ,  260  described above. The primary difference of the vascular layer fabricated from mold  360  in comparison to the vascular layers  60 ,  160 ,  260  is an increased density of channels  362 . 
         [0081]    The branching network of channels  362  in the vascular layer fabricated from mold  360  has two inlets  364  and two outlets  366 . A main channel  368  extends from each inlet  364  into daughter channels  370  through a series of trifurcations and ultimately ends in a bifurcation. The daughter channels  370  then branch off into repeating capillary-like subunits  372 . 
         [0082]    Each subunit  372  is similar although significant variation may be appropriate for certain applications. Each subunit  372  has an initial inlet channel  386  followed by a series of three bifurcations to form a series of successively smaller channels  384 ,  382 ,  380  (as best seen in  FIG. 11 ). In one embodiment, the smallest channels  380  are 100 um across and 100 um deep and all of the channels  386 ,  384 ,  382 ,  380  have an aspect ratio of 1:1. 
         [0083]    After the smallest channels  380 , the subunits  372  coalesce into successively larger channels  378 ,  376 ,  374 ,  388  and the flow collects in two outlets  366 . The vascular layer  360  may also have vertically orientated channels at the area of the inlet and outlet so multiple layers can be assembled. It is also envisioned that a resulting implant (not shown) would have a header layer, a perforated layer, nozzles, and/or vascular grafts or tubing leading to vascular grafts and the like. The vascular grafts may further be anastomosed to a blood vessel in the body. 
       More Implant Performance 
       [0084]    The vascular design layer fabricated from mold  360  of  FIGS. 10 and 11  was prototyped. The mold  360  created was a positive feature mold. The mold  360  was created in delrin using micromilling technology. Using the mold  360 , vascular layers were created using PDMS, which was poured over the mold  360 , cured and removed. A second layer (not shown) was bonded to the top of the open vascular network to create a closed vascular network. The two layers were bonded together using oxygen plasma bonding. Silicone tubing was adjoined to the inlets  364  and outlets  366 . An external barbed connector and T connector were used to uniformly direct inflow into the two inlet areas and collect outflow from the two outlet areas. 
         [0085]    In vitro testing was performed on the vascular network using anticoagulated sheep blood. The blood was pumped through the vascular network using a syringe pump and the inlet pressure to the vascular layer fabricated from mold  360  was recorded over a range of flows.  FIG. 12  is a graph of the in vitro testing with blood compared to a data point from CFD analysis. The CFD analysis results for the design point of a differential pressure of 16 mmHG yielded a flow rate of 6.35 mmHg are shown on the graph. 
         [0086]    The in-vitro testing required approximately a 20% higher inlet pressure to achieve a similar flow rate. CFD analysis is useful to predict with reasonable accuracy the inlet pressure for a given flow rate through the vascular network fabricated from mold  360 . At the flow rate of 6.35 mmHg, the shear stress within the vascular network is controlled within a physiologic range. The flow disturbances are minimized as a result of refining the bifurcation angles and fillets for the design. 
         [0087]    The vascular networks disclosed herein demonstrate fundamental design principles which may be useful for generating a wide array of scaffolds for tissue engineering. The designs related to the general design for a liver lobule but could be applied to other organ scaffolds also. The vascular networks may be tailored such that the pressure across the scaffold is matched with the design of the scaffold so there is adequate blood flow in the scaffold to supply oxygen and nutrients to the cells. 
       A Liver Application 
       [0088]    Referring now to  FIG. 13 , a vascular network design  400  which utilizes a repeating polygonal pattern similar to that of a liver is shown schematically. In another embodiment, the vascular network design may be arranged in a repeating radial pattern similar to that of  FIGS. 3-8 . 
         [0089]    In the liver, polygonal (e.g., hexagonal) liver lobules are arranged in a repeating pattern to achieve a high density configuration. The vascular network design  400  includes multiple polygonal structures  402 , each polygonal structure  402  having multiple vascular inputs  404  arranged in a radial pattern around a central vascular output  406 . The vascular network design  400  also would include a branching network  408  of channels between the vascular inputs  404  and the central vascular output  406 . The branching vascular network design  400  may be similar to the design illustrated in  FIGS. 3-8  in other respects. 
         [0090]    In another embodiment, the branching vascular network design may follow the design principles as outlined above yet accomplish a high density of vascular channels. For example, the polygonal structures  402  may be positioned adjacent to one another such that the vascular inputs  404  branch into multiple channels which are part of the network of channels for multiple polygonal structures. The diameters of these vascular inputs may be different from vascular inputs positioned at the edge of the design which may only branch into channels which contribute to a single polygonal structure. The polygonal structures may replicate in a way that nine polygonal structures combine to create a single vascular layer. In another embodiment, the number of adjacent polygonal structures which create a vascular layer may be in the range from 1 to 100. 
         [0091]    As described above, the flow from the patient may enter the scaffold through a single feature such as an inlet nozzle. The vascular network design  400  may have a header system (not shown), which evenly distributes the inlet flow from a single feature into all of the vascular inputs  404 . The header system may incorporate one or more header layers to achieve even flow distribution and incorporate features to minimize thrombus formation. In a similar fashion, there may be a header system which collects the flow from the vascular outputs  406  and diverts it into a single outlet feature such as a nozzle. 
         [0092]    Referring to  FIG. 15 , an implant  600  in accordance with the subject technology is shown in place as a liver assist device. The tissue engineered implant  600  is connected between the portal vein  602  and the inferior vena cava  604  of the liver  606 . In a patient with liver failure, their portal pressure is usually 10 mmHg or higher and the inferior vena cava pressure is often 0-2 mmHg. Thus, the differential pressure (e.g., the difference of the inlet and outlet pressures) may be 8 mmHg. Thus the flow through the scaffold for a tissue engineered liver must be adequate with approximately low pressures. 
         [0093]    In another embodiment, a vascularized scaffold for a tissue engineered liver may incorporate portal venous blood inflow and arterial blood inflow into the scaffold and may have a common venous outflow. The scaffold may be analogous to the human liver which has both portal venous and hepatic artery inputs. The hepatic artery primarily supports the biliary cells within the liver and hepatocytes in the outer portion of the hepatic lobule. There may be arterial inputs within a vascular layer in the same region as the portal venous inputs. The resulting branching network from the arterial inputs may not extend to and connect directly with a central output. The arterial network may connect directly with the branching network from the portal vein. 
         [0094]    In still another embodiment, the arterial vascular network would go through sufficient branching and have sufficient pressure drop as to not significantly change the pressure within the portal venous channels when connected thereto. The branching arterial network may be a different layer, or a different plane or otherwise remote from the branching portal vein network. The branching arterial network may also connect directly with a central draining output which may also include output from the portal venous network. 
         [0095]    In another embodiment, if a vascularized scaffold is created for a tissue engineered lung, the vascularized scaffold may be positioned between the pulmonary artery and the left atrium. Depending on the degree of pulmonary hypertension of a patient with end stage lung disease, the mean differential pressure between the pulmonary artery and left atrium may be 12 to greater than 20 mmHg. A vascularized lung scaffold may need to have sufficient flow for a differential pressure in this range. In another embodiment, the vascularized scaffold may be tailored to create at least a portion of a tissue engineered kidney, pancreas, skeletal muscle, heart, intestine, bladder, tongue or soft tissue. 
       A Lung Application 
       [0096]    Referring to  FIG. 16 , an implant  700  in accordance with the subject technology is shown in place as a lung assist device. The tissue engineered implant  700  is connected between the pulmonary artery  702  and the left atrium of the heart  704 . In this position, the tissue engineered lung implant  700  augments the function of the lung  706 . An external pack  708  provides oxygen, air or another gas or gas mixture to the implant  700 . In use, blood flows through the implant  700  including the vascular layer(s) as described in  FIG. 6 . The oxygen or other gas flows through the parenchymal layer, separated from the vascular layer by a gas permeable membrane. Oxygen diffuses from the parenchymal layer across the membrane and into the blood. Likewise carbon dioxide diffuses from the blood across the membrane and into the parenchymal layer. The flow of gas through the parenchymal layer washes the carbon dioxide out of the parenchymal layer. This exchange of oxygen and carbon dioxide between the blood and parenchymal layer performs the fundamental functions of a lung, which is the oxygenation of blood and removal of carbon dioxide from the blood. For the tissue engineered lung application, the semi-permeable membrane  110   a  of  FIG. 7  is permeable to oxygen and carbon dioxide. The membrane can be porous, non-porous or a combination of porous and non-porous portions. For example, the membrane may be a porous material such as polycarbonate covered with a very thin non-porous but gas permeable material such as silicone. In another embodiment, the membrane may be a resorbable material such as collagen. 
         [0097]    Furthermore, the membrane may be covered with cells. In one embodiment the portion of the membrane adjacent to the vascular layer may be covered with endothelial cells. In another embodiment, these endothelial cells may be lung endothelial cells or non-lung endothelial cells which express carbonic anhydrase on their membranes. Carbonic anhydrase is an enzyme which converts bicarbonate into carbon dioxide. In the blood most of the carbon dioxide occurs in the form of bicarbonate. In the lung, the bicarbonate is quickly converted to carbon dioxide by carbonic anhydrase and then can diffuse into the air spaces of the lung, the alveoli. In another embodiment, the membrane can be covered with endothelial cells as described above on the side adjacent the vascular layer and covered with lung epithelial cells (Type I, Type II or both) on the parenchymal side of the membrane. For the tissue engineered lung application, the total surface area of the vascular layer and parenchymal layer interface is sufficient to have exchange of oxygen, carbon dioxide or both to augment the lung function of a patient. 
         [0098]    In one embodiment, the scaffold may be placed within the body as previously described. In another embodiment, the scaffold may be placed outside the body with vascular or other connections (e.g., biliary, respiratory and the like) interfacing with the scaffold by protruding through the skin or another opening in the body. An externally placed scaffold may be useful for temporary support of an organ such as the liver or lung. In another embodiment, a pump may be may be positioned in the circuit between the patient and an external scaffold to augment the flow of blood through the scaffold. Types of pumps might include a roller pump, centrifugal pump or piston pump. For example, a temporary lung assist device may interface with a patient in a such a way that two separate venous catheters supply the blood to the scaffold and return blood from the scaffold. Due to inadequate pressure drop between two veins, an external pump may be added to effect blood flow through the scaffold. 
         [0099]    The material for the vascular network may be composed of a material which allows the attachment of cells. In another embodiment, the material may allow the attachment of vascular cells such as endothelial cells, smooth muscle cells, and fibroblasts. In another embodiment, the material may be a non-resorbable material, a resorbable material or a combination of non-resorbable and resorbable materials. In another embodiment the material may be a combination of resorbable materials or a combination of non-resorbable materials. A representative, but not exhaustive, list of resorbable materials or biodegradable polymers for construction of vascular scaffolds is shown in Table 1. 
         [0000]    
       
         
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Aliphatic polyesters 
               
               
                   
                 Bioglass 
               
               
                   
                 Carboxymethylcellulose 
               
               
                   
                 Cellulose 
               
               
                   
                 Chitin 
               
               
                   
                 Citrate 
               
               
                   
                 Collagen 
               
               
                   
                 Copolymers of glycolide 
               
               
                   
                 Copolymers of lactide 
               
               
                   
                 Elastin 
               
               
                   
                 Fibrin 
               
               
                   
                 Hydrogel 
               
               
                   
                 Modified proteins 
               
               
                   
                 Nylon-2 
               
               
                   
                 PLA/polyethylene oxide copolymers 
               
               
                   
                 PLA-polyethylene oxide (PELA) 
               
               
                   
                 Poly (amino acids) 
               
               
                   
                 Poly (trimethylene carbonates) 
               
               
                   
                 Poly(alklyene oxalates) 
               
               
                   
                 Poly(butylene diglycolate) 
               
               
                   
                 Poly(hydroxy butyrate) (PHB) 
               
               
                   
                 Poly(n-vinyl pyrrolidone) 
               
               
                   
                 Poly(ortho esters) 
               
               
                   
                 Polyalkyl-2-cyanoacrylates 
               
               
                   
                 Polyanhydrides 
               
               
                   
                 Polycyanoacrylates 
               
               
                   
                 Polydepsipeptides 
               
               
                   
                 Polydihydropyrans 
               
               
                   
                 Poly-dl-lactide (PDLLA) 
               
               
                   
                 Polyesteramides 
               
               
                   
                 Polyesters of oxalic acid 
               
               
                   
                 Polyethylene Glycol 
               
               
                   
                 Polyethylene Oxide 
               
               
                   
                 Polyglycan Esters 
               
               
                   
                 Poly(Glycerol Sebacate) 
               
               
                   
                 Polyglycolide (PGA) 
               
               
                   
                 Polyiminocarbonates 
               
               
                   
                 Polylactides (PLA) 
               
               
                   
                 Poly-l-lactide (PLLA) 
               
               
                   
                 Polyorthoesters 
               
               
                   
                 Poly-p-dioxanone (PDO) 
               
               
                   
                 Polypeptides 
               
               
                   
                 Polyphosphazenes 
               
               
                   
                 Polysaccharides 
               
               
                   
                 Polyurethanes (PU) 
               
               
                   
                 Polyvinyl alcohol (PVA) 
               
               
                   
                 Poly-β-alkanoic acids 
               
               
                   
                 Poly-β-malic acid (PMLA) 
               
               
                   
                 Poly-ε-caprolactone (PCL) 
               
               
                   
                 Pseudo-Poly(Amino Acids) 
               
               
                   
                 Starch 
               
               
                   
                 Trimethylene carbonate (TMC) 
               
               
                   
                 Tyrosine based polymers 
               
               
                   
                 Glycolide/l-lactide copolymers (PGA/PLLA) 
               
               
                   
                 Glycolide/trimethylene carbonate copolymers (PGA/TMC) 
               
               
                   
                 Lactide/tetramethylglycolide copolymers 
               
               
                   
                 Lactide/trimethylene carbonate copolymers 
               
               
                   
                 Lactide/ε-caprolactone copolymers 
               
               
                   
                 Lactide/σ-valerolactone copolymers 
               
               
                   
                 L-lactide/dl-lactide copolymers 
               
               
                   
                 Methyl methacrylate-N-vinyl pyrrolidone copolymers 
               
               
                   
                 PHBA/γ-hydroxyvalerate copolymers (PHBA/HVA) 
               
               
                   
                 Poly hydroxyalkanoate polymers (PHA) 
               
               
                   
                 Poly-β- hydroxypropionate (PHPA) 
               
               
                   
                 Poly-β-hydroxybutyrate (PBA) 
               
               
                   
                 Poly-σ-valerolactone 
               
               
                   
                   
               
             
          
         
       
     
         [0100]    In another embodiment, the thickness of the material of the scaffold may be very thin. The thickness of the scaffold material used to create a patterned vascular layer may be in the range of 0.1 um to 1000 um thick. In another embodiment, the thickness of the scaffold material may be in the range of 1 um to 10 um thick. The scaffold or supporting tissue of organs often comprises collagen as an abundant component. 
       Collagen Layers 
       [0101]    An embodiment of this construct may include collagen films as the primary scaffold material. Collagen is the primary component of the extracellular matrix in solid organs and other tissues including blood vessels. The strength of collagen is highlighted by the durability of human tissues. For example, human veins typically have a burst pressure of several thousand mmHg yet are so thin they are partially transparent. The strength of thin collagen films may allow the creation of a vascularized scaffold with minimal scaffold component mass to maximize the amount of functional tissue present in a particular volume of the scaffold. Cells readily adhere to collagen and may adsorb and remodel the collagen to optimize the scaffold prior to or following implantation. 
         [0102]    Collagen may also be constructed in a thin film, sheet, or other porous or non-porous construct. In a embodiment, a collagen thin film, sheet or other construct will have a degree of porosity. This porosity may be uniform in size or spacing. The pores sizes may only allow small molecule in the range 1 to 500 kilodaltons. In another embodiment, the pores may be in the range of 1 micron to 100 microns. The porosity of the collagen network may be sufficient to allow the diffusion of oxygen, carbon dioxide, proteins, carbohydrates, fats, drugs, or any other biologically active across the collagen network. The porosity of the collagen may be adjusted to meet the required diffusion for a particular target tissue such as the heart, liver or lung. 
         [0103]    Given the porosity of the collagen, an additional membrane between the vascular channel and the cells of the tissue or organ may not be required. The cells of the target tissue, for example, myocardial cells, could be placed directly on the vascular network. A second layer of vascular network could be added on top of the cells so each area with cells of the target tissue may be in contact with more than one vascular network layer. Successive vascular networks could be staggered in such a way that the vascular network patterns are not precisely overlying one another. The out of alignment arrangement may allow the cells of the tissue between the vascular network layers to be closer to a respective channel than if the layers were precisely aligned. The collagen films may be very thin and may be patterned to form the desired branching patterns and chambers. 
         [0104]    Manufacturing a vascular network pattern out of collagen may be achieved in a number of methods. A collagen film, sheet or other thin structure may be formed into a pattern consistent with a vascular network pattern, such as the networks shown above. Once a patterned network is created in collagen, a second collagen film, sheet or other thin structure which is not patterned may be bonded to the patterned collagen network, thus creating a sealed or closed collagen vascular network which has a network of channels consistent with a desired network design. The patterned network created in collagen may have channels, which are the desired depth of the final channel. 
         [0105]    In another embodiment, the patterned network created in collagen may have half of the desired depth of the final channel. The patterned network may be bonded to a similarly patterned network, where the result is a closed collagen vascular network with the desired depth of the final channel. Although the closed collagen vascular network may be created in a such a manner that the cross section of the channel is rectangular at one point in the fabrication, upon filling the vascular network with a fluid, the resulting cross section of the channel may be circular, oval, elliptical or may retain a rectangular shape. 
         [0106]    Referring to  FIG. 14 , a portion  502  of a vascular network  500  created with a collagen film, sheet or other thin structure is shown. The portion  502  shows a vascular channel  504  with a single bifurcation design. The collagen material  502  has a total thickness, which may be less than the inner diameter  506  of the channel  504 . For example, the thickness of the collagen material or portion  502  may be 10 um and the inner diameter  506  of the vascular channel  504  may be 100 um. The vascular network  500  could be manufactured by creating a patterned collagen film or sheet and bonding an unpatterned collagen film or sheet thereto. The vascular network  500  could also be manufactured by bonding two patterned collagen films together as described above. 
         [0107]    The manufacturing of a patterned collagen film may accomplished by starting with a positive mold of the desired vascular network. Such a mold has ridges corresponding to the desired channels of the network. It is a negative of the desired structure. A solution, suspension, colloid or other mixture containing collagen may be poured or otherwise placed over the mold. The collagen mixture may then be air dried or vacuum dried. The drying may occur at or above ambient temperature. 
         [0108]    After drying, the collagen film then may be separated from the mold resulting in a patterned collagen film. In another embodiment, the collagen mixture may then be gelled by incubation. The resulting gel may be air dried or vacuum dried. The resulting collagen film may be removed from the mold and retain the pattern and features of the mold. A patterned collagen film may be bonded to a flat or other patterned collagen film using adhesive, dehydrothermal cross-linking adjacent collagen sheets, chemically cross-linking adjacent collagen sheets or photochemically cross-linking collagen with a photosensitizing dye. In the embodiment where a patterned film is placed adjacent to an unpatterned or flat collagen film, the patterned film or the unpatterned may be dry, partially dry or hydrated. A degree of hydration of one or more of the films may result in a desirable degree of coaptation of the non-channeled portion of the patterned film to the corresponding flat film. 
         [0109]    Thin films (e.g., having thicknesses of less than 10 um) that are patterned or unpatterned may have peripheral support elements to aid in handling. These elements may assist in the handling of thin films during patterning, coaptation, bonding, stacking and attachment of other components in the manufacturing process. The peripheral support elements may include a mesh material, a filament, a plurality of filaments, a substantially solid material or a solid material. The support elements may be temporary and removed prior to implantation of the scaffold within the body of a living being. The support elements may be resorbable or non-resorbable and may be implanted into the body of a living being within the scaffold. The inlet tubes and header components may also be composed of collagen materials including collagen tubes. 
         [0110]    The collagen components may be formed of collagen from bovine, porcine, equine, ovine, human or other mammalian sources. Any collagen type or any combinations of collagen types may be used. In another embodiment, the patterned or unpatterned films may be composed of a mixture of collagen and non-collagen components. In another embodiment, the films may be a mixture of collagen types (e.g., types I, III, IV and VII). In another embodiment, the non-collagen components may be materials derived from natural membranes or one or more extracellular matrix proteins or any other naturally occurring portions or components of the extracellular tissues in a living being. These extracellular matrix proteins or components may be one or more of the following; fibrin, elastin, fibronectin, laminin, hyaluronic acid, heparin sulfate or chondrotian sulfate. In one embodiment, a patterned sheet which is bonded to an unpatterned sheet may be of different material composition than the unpatterned sheet. In another embodiment, the material for the patterned and flat films is a combination of collagen type I, collagen type IV, fibrin and fibronectin. 
         [0111]    In another embodiment, scaffold in accordance with the descriptions above may be used to ascertain the efficacy of drugs on human cells. For example, a scaffold may be fabricated to determine metabolism of a test agent in a certain kind of tissue. The scaffold could incubate the test agent and an enzyme, and form an enzyme-substrate complex between the enzyme and the test agent. As a result, one could detect one or more metabolites of the test agent. For additional example, see U.S. patent application Ser. No. 10/215,600 filed on Aug. 9, 2002 and Ser. No. 11/183,115 filed on Jul. 15, 2005. 
       INCORPORATION BY REFERENCE 
       [0112]    All patents, published patent applications and other references disclosed herein are hereby expressly incorporated in their entireties by reference. It should be understood that the foregoing disclosure and description of the present invention are illustrative and explanatory thereof and various changes in the size, shape, and material composition, as well as in the description of the embodiments, may be made without departing from the spirit of the invention as defined by the appended claims.