Abstract:
The disclosure describes methods of making high-strength NDGA collagen and associated methods of preparing collagen preparatory material and medical bioprostheses.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority to U.S. Provisional Application Ser. No. 60/882,065, filed Dec. 27, 2006, U.S. Provisional Application Ser. No. 60/883,408, Filed Jan. 4, 2007, and U.S. Provisional Application No. 60/890,660, filed Feb. 20, 2007, the contents of which are hereby incorporated by reference as if recited in full herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to biomedical materials. 
       BACKGROUND OF THE INVENTION 
       [0003]    Koob et al. have described methods of producing nordihydroguairetic acid (NDGA) polymerized collagen fibers of tensile strengths similar to that of natural tendon (e.g., about 91 MPa) to make medical constructs and implants. See, Koob and Hernandez,  Material properties of polymerized NDGA - collagen composite fibers: development of biologically based tendon constructs , Biomaterials 2002 January; 23 (1): 203-12, and U.S. Pat. No. 6,565,960, the contents of which are hereby incorporated by reference as if recited in full herein. 
       SUMMARY OF EMBODIMENTS OF THE INVENTION 
       [0004]    Embodiments of the present invention are directed to improved methods of producing biocompatible NDGA polymerized fibers. Some embodiments are directed at producing high-strength NDGA polymerized fibers used to make implantable biocompatible constructs, implants and/or other prostheses. 
         [0005]    Some embodiments are directed to methods of manufacturing nordihydroguairetic acid (NDGA) polymerized collagen fibers. The methods include: (a) treating collagen with a solution comprising NDGA; (b) drying the NDGA treated collagen while holding the collagen in tension for a period of time; (c) washing the dried NDGA treated collagen in a solution to remove unreacted soluble NDGA cross-linking intermediates; (d) drying the NDGA treated collagen while holding the collagen in tension for a period of time; and (e) repeating steps (a)-(d) at least once to produce high-strength NDGA polymerized collagen fibers. 
         [0006]    In some embodiments, the methods can also include, after steps (a)-(d) are repeated at least once, forming a bioprosthesis using the high strength NDGA polymerized fibers. In some embodiments, the bioprosthesis can be a ligament bioprosthesis that has a tensile strength of between about 180-280 MPa, and a stiffness and dynamic flexibility that meets or exceeds that of a natural ligament. In other embodiments, the bioprosthesis can be a tensile strength between about 180-280 MPa, and a stiffness and dynamic flexibility that meets or exceeds that of a natural tendon. 
         [0007]    Still other embodiments are directed to biomedical implants. The implants include at least one high-strength synthetic NDGA polymerized collagen fiber. 
         [0008]    In some embodiments, the at least one fiber is a plurality of fibers, and the bioprosthesis is a ligament bioprosthesis that has a tensile strength of between about 180-300 MPa, and a stiffness and dynamic flexibility that meets or exceeds that of a natural ligament. In other embodiments, the at least one fiber is a plurality of fibers, and the bioprosthesis has a tensile strength between about 180-300 MPa, and a stiffness and dynamic flexibility that meets or exceeds that of a natural tendon. 
         [0009]    Yet other embodiments are directed to medical kits for a tendon or ligament repair, augmentation or replacement. The kits include a high-strength NDGA collagen fiber construct and a sterile package sealably enclosing the NDGA collagen fiber construct therein. 
         [0010]    Among other things, the NDGA collagen fiber construct can be a ligament bioprosthesis that has a tensile strength of between about 180-300 MPa a tendon bioprosthesis that has a tensile strength of between about 180-300 MPa. 
         [0011]    Still other embodiments are directed to medical kits that include an implantable medical device comprising NDGA collagen fiber derived from echinoderm collagen; and a sterile package sealably enclosing the device therein. The NDGA collagen fibers may have an average tensile strength of about 100 MPa. 
         [0012]    Additional embodiments are directed to medical kits that include a device comprising NDGA collagen fiber derived from porcine collagen and a sterile package sealably enclosing the device therein. The NDGA collagen fibers may be high-strength fibers. 
         [0013]    Other embodiments are directed to medical kits that include a device comprising NDGA collagen fiber derived from caprine collagen and a sterile package sealably enclosing the device therein. The NDGA collagen fibers may be high-strength fibers. 
         [0014]    Other embodiments are directed to methods of organizing collagen before cross-linking. The methods include: (a) purifying donor collagen preparatory material; (b) dialyzing the purified collagen preparatory material a plurality of times; and (c) forming a substantially clear gel using the dialyzed collagen material thereby indicating improved organization of collagen fibrils. 
         [0015]    The dialyzing can be carried out three times against dionized water (DI) in a volume ration of between about 30:1 to about 100:1, for between about 30-90 minutes. Typically, each dialyzing is carried out against dionized water (DI) in a volume ration of about 60 to 1 for about 40 minutes. 
         [0016]    Further features, advantages and details of the present invention will be appreciated by those of ordinary skill in the art from a reading of the figures and the detailed description of the embodiments that follow, such description being merely illustrative of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a flow chart of operations that can be used to carry out embodiments of the invention. 
           [0018]      FIG. 2  is a flow chart of operations that can be used to carry out embodiments of the invention. 
           [0019]      FIG. 3  is a flow chart of operations that can be carried out before cross-linking for improved organization of collagen fibrils in collagen preparatory material according to embodiments of the invention. 
           [0020]      FIG. 4  is a schematic illustration of an NDGA-treated fiber held in tension during a drying operation according to embodiments of the present invention. 
           [0021]      FIG. 5  is a schematic illustration of a medical kit comprising a high-strength NDGA-treated collagen construct according to embodiments of the invention. 
           [0022]      FIG. 6  is a graph of tensile strength (MPa) of high strength NDGA fibers relative to other fibers, including prior NDGA fibers, according to embodiments of the invention. 
           [0023]      FIG. 7  is a graph of tensile strength (MPa) of fibers using collagen from different sources according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The present invention now is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0025]    Like numbers refer to like elements throughout. In the figures, the thickness of certain lines, layers, components, elements or features may be exaggerated for clarity. Broken lines illustrate optional features or operations unless specified otherwise. 
         [0026]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. As used herein, phrases such as “between X and Y” and “between about X and Y” should be interpreted to include X and Y. As used herein, phrases such as “between about X and Y” mean “between about X and about Y.” As used herein, phrases such as “from about X to Y” mean “from about X to about Y.” 
         [0027]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity. 
         [0028]    It will be understood that when an element is referred to as being “on”, “attached” to, “connected” to, “coupled” with, “contacting”, etc., another element, it can be directly on, attached to, connected to, coupled with or contacting the other element or intervening elements may also be present. In contrast, when an element is referred to as being, for example, “directly on”, “directly attached” to, “directly connected” to, “directly coupled” with or “directly contacting” another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” another feature may have portions that overlap or underlie the adjacent feature. 
         [0029]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. The sequence of operations (or steps) is not limited to the order presented in the claims or figures unless specifically indicated otherwise. 
         [0030]    The terms “implant” and “prosthesis” are used interchangeably herein to designate a product configured to repair or replace (at least a portion of) a natural tendon, ligament or other tissue of a mammalian subject (for veterinary or medical (human) applications). The term “implantable” means the device can be inserted, embedded, grafted or otherwise chronically attached or placed on or in a patient. The term “agitate” and derivatives thereof refer to mixing the components in a vessel by moving, shaking, vibrating, oscillating, rotating, centrifuging or other movement types, including combinations of the movement types. 
         [0031]    The term “dynamic flexibility” means that the bioprosthesis is able to perform at least as well as the target tissue undergoing repair, such as a natural ligament or tendon, so as to be able to dynamically stretch and compress, and typically allow some torsion, to behave at least as well as the repaired or replaced target tissue. 
         [0032]    The collagen can be of any form and from any origin. The collagen can be any of the identified collagen genotypes, for example, the interstitial fiber forming collagen types I, II and III, as well as any other substantially fiber forming types of collagen, for example collagen VI. The collagen can be acid soluble collagen or pepsin solubilized collagen. The collagen can be from mammalian cells synthesized in vitro. The collagen can be from molecularly engineered constructs and synthesized by bacterial, yeast or any other molecularly manipulated cell type. For example, the collagen can be sea cucumber dermis collagen, bovine, caprine, porcine, ovine or other suitable donor mammal, marine animal collagen such as chinoderms, molecularly engineered collagen, or gelatin (e.g., in any suitable form including solid, gel, hydrogels, liquids, or foams). In addition, the collagen can be digested with a protease before the oxidizing and polymerizing steps. The collagen can be in the form of microfibrils, fibrils, natural fibers, or synthetic fibers. The polymeric material, e.g., collagen, can be solubilized, dissolved or otherwise transferred into an acid solution, for example, acetic acid (e.g., about 0.01M to about 1.0M, typically about 0.5M), hydrochloric acid (between about pH 1 to about pH 3, typically about pH 2.0), or any other suitable acid at appropriate concentration (e.g., about pH 1.0 to about pH 3.0, typically about pH 2.0). The collagen can also be dissolved in a neutral buffered solution either with or without salts, e.g., phosphate buffer at about pH 7.0, phosphate buffered saline at about pH 7.0. The phosphate buffer can be at any concentration of sodium phosphate between about 0.01 and 0.5, but more typically between about 0.02 and about 0.1M. The buffer can also be any buffer, including, but not limited to, sodium acetate, HEPES, or MOPS. The collagen can be present in a quantity that is at least about 0.1% to about 10%, typically between 0,1% to about 5% (e.g., about 0.1, 0.2, 0.3, 0.4, 1.0, 2.0, 4.0%) by weight per volume before dialyzing, or by weight per volume in the neutral buffer solution before fibrillogenesis and fiber formation. In the dried fiber, collagen can be between about 50-100% (e.g., at least about 75%, 90%, 95% or 100%) before crosslinking. 
         [0033]    Collagen “microfibrils,” “fibrils,” “fibers,” and “natural fibers” refer to naturally-occurring structures found in a tendon. Microfibrils are about 3.5 to 50 nm in diameter. Fibrils are about 50 nm to 50 μm in diameter. Natural fibers are above 50 μm in diameter. A “synthetic fiber” refers to any fiber-like material that has been formed and/or chemically or physically created or altered from its naturally-occurring state. For example, an extruded fiber of fibrils formed from a digested tendon is a synthetic fiber but a tendon fiber newly harvested from a mammal is a natural fiber. Of course, synthetic collagen fibers can include non-collagenous components, such as particulates, hydroxyapatite and other mineral phases, or drugs that facilitate tissue growth or other desired effects. See, U.S. Pat. No. 6,821,530, incorporated herein by reference above. For example, the fibers and/or constructs formed from same, can include compositions that can contain carbon nano-tubes, zinc nano-wires, nano-crystalline diamond, or other nano-scale particulates; and larger crystalline and non-crystalline particulates such as calcium phosphate, calcium sulfate, appetite minerals. For example, the compositions can contain therapeutic agents such as bisphosphonates, anti-inflammatory steroids, growth factors such as basic fibroblast growth factor, tumor growth factor beta, bone morphogenic proteins, platelet-derived growth factor, and insulin-like growth factors; chemotactic factors such fibronectin and hyaluronan; and extracellular matrix molecules such as aggrecan, biglycan, decorin, fibromodulin, COMP, elastin, and fibrillin. In some embodiments, the fibers and/or fiber-derived constructs can contain cells, engineered cells, stem cells, and the like. Combinations of the above or other materials can be embedded, coated and/or otherwise attached to the fibers and/or construct formed from same. 
         [0034]    Properly processed NDGA polymerized fibers are biocompatible as discussed in U.S. Pat. No. 6,565,960, incorporated by reference hereinabove.  FIG. 1  illustrates operations that can be used to form high-strength collagen fibers. The term “high-strength” refers to fibers having an average tensile strength of at least about 150 MPa, such as between about 180 MPa and 350 MPa, and typically, for bovine, porcine or caprine based “donor” collagen, between about 180 MPa and 280 MPa, such as about 279 MPa (measured on average). The fibers may also have suitable stiffness and strain yield. In general, the fibers formed from the compositions and processes of the invention can have a stiffness of at least about 200 MPa (e.g., at least about 300, 400, 500, or 600 MPa), and a strain at failure of less than about 20% (e.g., less than about 15 or 10%). The fibers may be formed with a relatively thin diameter, such as, for example about a 0.08 mm dry diameter (on average) and about a 0.13 mm wet diameter (on average). 
         [0035]    To measure these physical properties, any suitable apparatus having (1) two clamps for attaching to the fiber(s), (2) a force transducer attached to one of the clamps for measuring the force applied to the fiber, (3) a means for applying the force, and (4) a means for measuring the distance between the clamps, is suitable. For example, tensiometers can be purchased from manufacturers MTS, Instron, and Cole Parmer. To calculate the tensile strength, the force at failure is divided by the cross-sectional area of the fiber through which the force is applied, resulting in a value that can be expressed in force (e.g., Newtons) per area. The stiffness is the slope of the linear portion of the stress/strain curve. Strain is the real-time change in length during the test divided by the initial length of the specimen before the test begins. The strain at failure is the final length of the specimen when it fails minus the initial specimen length, divided by the initial length. 
         [0036]    An additional physical property that is associated with the extent of cross-linking in a composition is the shrinkage temperature. In general, the higher the temperature at which a collagenous composition begins to shrink, the higher the level of cross-linking. The shrinkage temperature of a fiber can be determined by immersing the fiber in a water or buffer bath, raising the temperature of the water or buffer bath, and observing the temperature of the water or buffer bath at which the fiber shrinks. Tension on the fiber may be required for observing the shrinkage. The shrinking temperature for the compositions of the invention can be at least about 60 degrees C. (e.g., at least 65 or 70 degrees C.). 
         [0037]    For compositions that are not elongated in shape, such as in a disk, the fracture pressure in compression loading can be an indication of physical strength. The fracture pressure is the minimum force per area at which a material cracks. 
         [0038]    It is believed that high-strength fibers allow for improved or alternative bioprosthesis constructs and/or medical devices. For example, high-strength fibers may be particularly suitable for bioprostheses suitable for tendon and/or ligament repair, augmentation, and/or replacement. A biomaterial with increased strength over that of natural tissue (muscle and the like) can allow for a bioprosthesis that has a smaller cross-sectional area than that of the natural tissue being replaced or repaired. The smaller area can improve the function of the bioprosthesis as a scaffold for neo-tendon or ligament in-growth, which may augment strength and/or long term survival rate of the repair. The use of high-strength fibers on medical devices and constructs may also offset or reduce the effects of stress concentration factors that reside at regions of integration in adjacent tissue such as bone. 
         [0039]    Referring to  FIG. 1 , some methods include obtaining or harvesting pepsin-solubilized collagen from a donor source. The harvested collagen can be treated using a solution comprising NDGA to polymerize the collagen (block  10 ). The NDGA treated collagen can then be dried while the collagen is held in tension for a desired period of time (block  20 ). The typical tension force during at least part of the drying operation is between about 2-4 grams weight per fiber. The “dried” collagen can then be placed in a liquid bath or solution (typically an ethanol solution) and washed to remove any unreacted soluble NDGA cross-linking intermediates (block  30 ). That is, after the NDGA polymerization process, the NDGA treated collagen fibers can be washed in an ethanol solution (typically including phosphate buffered saline) to remove potential cytotoxins due to leachable reaction products. After washing, the collagen can then be dried again while held in tension (block  40 ). This sequence can be repeated at least once (block  50 ); typically only two repetitions are needed to achieve the desired tensile strength. 
         [0040]    The drying may be at room temperature, typically at between about 50° F. (10° C.) to about 80° F. (27° C.) or may be carried out at suitable, low heating temperatures, below about 105° F. (40.6° C.), with or with out the aid of forced gas flow (e.g., fans to blow air). Different drying times and temperatures may be used during a single drying event or between drying events. The drying can be carried out in a sterile and/or suitable clean-room environment and/or sterilized after the process is completed before or after packaging. The collagen may be partially or substantially totally dried. In some embodiments, the collagen is not required to be completely dry before the next step. The desired period of drying time can be between about 1-5 hours, typically about 2 hours for a typical amount of collagen (block  22 ). The washing can include agitating the NDGA-treated collagen in a solution of between about 50-95% ethanol, typically about 70% ethanol, in an amount of at least about 50 ml of 70% ethanol per gram of dry fiber. 
         [0041]    The tensile force can be provided as shown in  FIG. 4 , by clamping, pinching, rolling or otherwise attaching one end portion  200   e   1  of a fiber  200  to a rod or other holding member  205  and attaching at least one weight  210  to an opposing end portion  200   e   2 . A single weight may be used for more than one fiber or each fiber may use more than one weight. Other tensioning mechanisms or configurations may also be used. Substantially horizontal, angled or other non-vertically oriented tensioning systems may be used. In some embodiments, weights can be applied to both opposing end portions of the fiber(s) to generate the desired tension. A typical weight of about 2-10 grams per fiber, depending on the extruded fiber size, can be appropriate. 
         [0042]      FIG. 2  illustrates operations that can be used to form NDGA-treated collagen fibers according to other embodiments of the invention. As shown, donor collagen material is obtained and purified as appropriate (block  100 ). The donor material can be from any suitable source.  FIG. 6  illustrates different fiber tensile strengths (average) obtained using different donor collagen sources. The purified collagen preparatory material is dialyzed, incubated, then placed in a fiber-forming buffer that is then extruded (block  105 ). 
         [0043]      FIG. 3  illustrates operations that can be used to form improved organization of collagen fibrils using a dialyzing process. As noted above, preparatory donor collagen material can be purified (block  60 ). The purified collagen preparatory material is dialyzed a plurality of times in a selected liquid for a desired period of time (block  65 ). The dialyzing is typically repeated three times (block  72 ). The dialyzing can be carried out against dionized (DI) water in a volume ratio of between about 30:1 to about 100:1, typically about 60 to 1, for between about 30-90 minutes, typically about 40 minutes (block  66 ). The dialyzing can form a substantially clear gel of collagen fibrils indicating good organization (block  70 ), where opacity indicates less organization. The organization can help improve tensile strength of subsequently cross-linked fibers. 
         [0044]    The dialyzed collagen material can be incubated for a desired time before placing in a fiber-forming buffer (block  75 ). The dialyzed gel can be cross-linked to provide collagen fibers for medical constructs (block  76 ). The polymerization (e.g., cross-linking) can be carried out using NDGA and the resultant NDGA treated collagen fibers can be relatively thin, such as, for example, about 0.08 mm dry diameter (on average) (block  77 ). 
         [0045]    The incubation may be for at least about 24 hours, typically 24-48 hours, and may be at room temperature of between about 15-30° C., typically about 25° C. The dialysis process can be used before cross-linking for subsequent use with any suitable cross-linking materials, to promote collagen organization, such as, for example, and the process is not limited to NDGA, but may be useful with other materials, including, for example, glutaraldehyde. The dried collagen fiber can also be treated with other methods to improve the tensile properties of the fiber. The dried collagen fibers produced by the method(s) described herein can be cross-linked with agents such as glutaraldehyde, formaldehyde, epoxy resins, tannic acid, or any other chemical agent that produces covalent cross-links between collagen molecules within fibrils or between fibrils. Alternatively, the dried fiber can be treated to induce cross-linking between collagen molecules such as, but not limited to, one or more of a carbodiimide treatment, ultraviolet irradiation either with or without carbohydrates to initiate glycation adducts, and dehydrothermal treatment coupled with any of the aforementioned methods. 
         [0046]    The fiber-forming buffer can include about 30 mM NaH 2 PO 4 , 140 mM NaCl, in a volume ratio of about 60 to 1, for between about 12-24 hours, typically about 16 hours at a slightly elevated temperature of about 37° C. The extrusion can be directed to enter directly or indirectly into an aqueous bath, such as a water or saline bath, and hung from one end portion. To remove from the bath, the extruded material can be lifted out of the bath at a slow rate of less than about 5 mm/min, typically about 1 mm/min. The extruded fibers can then be dried (block  110 ). To dry, the fibers may be hung or otherwise held for at least about 5 hours, typically for at least about 6 hours, such as between about 6-10 hours. 
         [0047]    Referring again to  FIG. 2 , as shown, the extruded dried fibers can be hydrated in a liquid buffer solution (block  120 ). The hydration can be for between about 30 minutes to about 3 hours, typically about 1 hour, in a solution of at least 50 ml of buffer (such as 0.1 M NaH 2 PO 4 , pH 7.0) per gram of dry fiber. In some embodiments, the pH of the phosphate buffered solution can be increased to above pH 7 to a pH of about 11 or between 7-11, e.g., a pH of about 8.0, 9.0, 10.0 or 11.0. The hydrated fibers in the buffer solution can then be combined with a liquid solution comprising (dissolved) NDGA (block  130 ). About 30 mg/ml of the NDGA can be dissolved in about 0.4 NaOH prior to combining with the buffer/fiber solution. The dissolved NDGA solution can be added in an amount of between about 3-4 mg NDGA per ml of buffer, such as about 0.1 M NaH 2 PO 4 . The NDGA and fiber solution can be agitated, shaken, centrifuged, rotated, oscillated or otherwise moved and/or mixed for a length of time (block  140 ), typically between about 12-48 hours, such as at least about or about 16 hours. As discussed above with respect to  FIG. 1 , the fibers can then be removed and held in tension (e.g., hung or stretched), during a drying operation (block  150 ), typically lasting at least about 2 hours. The (partially or wholly) dried fibers can then be washed to remove unwanted reaction products (block  160 ). Typically, the fibers are washed (agitated) in about 70% ethanol as also discussed above, then held in tension during a drying operation (block  170 ). The steps  120 - 170  can be repeated (block  175 ). 
         [0048]      FIG. 5  illustrates a medical kit  250  that includes a medical device or implant  225  with at least one NDGA-treated collagen fiber  200 . The kit  250  may include other components, such as, for example, a container of surgical adhesive, sutures, suture anchors, and the like. The device or implant  225  may be held hydrated in a flexible sealed package of sterile liquid  230 . The kit  250  may include a temperature warning so that the construct  225  is not exposed to unduly hot temperatures that may degrade the implant. A temperature sensor may optionally be included on the package of the kit (not shown) to alert the clinician as to any excessive or undue temperature exposure prior to implantation. For example, it may be desirable to hold or store the kit  250  (and implant or device  225 ) at a temperature that is less than about 37° C. and/or 100° F. prior to implantation. The kit  250  may be packaged in a housing with a temperature controlled or insulated chamber  250   c  to facilitate an appropriate temperature range. 
         [0049]    Embodiments of the invention can form implants and/or medical devices using NDGA collagen fibers with different tensile strengths from a single source type, e.g., NDGA-treated bovine collagen, with both low strength, such as less than about 90 MPa tensile strength, typically between about 10 MPa and 90 MPa, and high strength fibers and/or using NDGA-treated collagen from more than one source type (e.g., bovine and echinoderm). 
         [0050]    The present invention is explained in greater detail in the following non-limiting Examples. 
       EXAMPLES 
       [0051]      FIG. 6  illustrates average tensile strength of NDGA-treated collagen fibers that can be produced according to embodiments of the invention. In some embodiments, the fibers can be produced by the below described 12 step process. The reference to the lower strength “NDGA fibers” in  FIG. 6  refers to prior art fibers as described in U.S. Pat. No. 6,565,960 and/or Koob and Hernandez, Material properties of polymerized NDGA-collagen composite fibers: development of biologically based tendon constructs, Biomaterials, 2002 January; 23 (1): 203-12.
   1. Purify Type I collagen from 8-9 month old fetal bovine tendons as previously described (Koob and Hernandez, Biomaterials 2002, supra).   2. Dilute the purified collagen prep with 3% acetic acid to a final concentration of 0.2% (weight/volume).   3. Place the purified collagen prep in 3.2 mm diameter dialysis tubes and dialyze 3 times against DI (di-ionized) water in a volume ration of 60 to 1 for 40 minutes each time.   4. Incubate in DI water for 36 hours at room temperature (25° C.)   5. Place in fiber-forming buffer of 30 mM NaH 2 PO 4 , 140 mM NaCl in a volume ratio of 60 to 1 for 16 hours at 37° C. This causes the collagen to form a gel within the dialysis tubes.   6. Extrude the collagen fiber gel into a water bath, hang from one end and lift out of the water bath at a rate of 1 mm/min and allow drying for at least 6 hours.   7. Hydrate the dried fibers for 1 hr in at least 50 ml of buffer (0.1 MI NaH 2 PO 4  pH 7.0) per gram of dry fiber.   8. Dissolve 30 mg/ml NDGA in 0.4 N NaOH   9. Add the dissolved NDGA solution to the buffer and fibers (use 3.33 mg NDGA per ml of 0.1 M NaH 2 PO 4 ). Agitate for 16 hours   10. Hang for 2 hrs with a 6.7 gram weight clamped to the bottom to provide tension while drying.   11. Place in 50 ml of 70% ethanol per gram of dry fiber and agitate for 2 hrs of washing to remove any unreacted, soluble, NDGA cross-linking intermediates.   12. Dry again as per step 10 and repeat the NDGA treatment as per steps 7 through 9 above.     
         [0064]      FIG. 7  illustrates different donor or starting collagen materials, including bovine, caprine, porcine and echinoderm produced according to the methods described herein and treated with NDGA having associated average tensile strength using manufacturing methods described herein. In the case of the echinoderm collagen fibers, the collagen fibrils were produced by water extraction of the sea cucumber dermis producing intact native fibrils according to published methods. See, Trotter, J. A., Thurmond, F. A. and Koob, T. J. (1994)  Molecular structure and functional morphology of echinoderm collagen fibrils . Cell Tiss. Res. 275; 451-458. 
         [0065]    NDGA treated collagen constructs have biocompatibility, suitable biomechanical properties and the potential for biologic in-growth of native tissue for long-term stability. 
         [0066]    The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.