This application is a Divisional of U.S. patent application Ser. No. 10/729,146, filed on Dec. 4, 2003, entitled COMPRESSED HIGH DENSITY FIBROUS POLYMERS SUITABLE FOR IMPLANT, which issued on Mar. 13, 2012 as U.S. Pat. No. 8,133,500, which is assigned to the same assignee as this invention, and whose disclosure is incorporated by reference herein.
BACKGROUND OF THE INVENTION
Despite the growing sophistication of medical technology, repairing and replacing damaged tissues remains a costly, and serious problem in health care. Currently, implantable prostheses for repairing tissues are made from a wide number of synthetic and natural materials. Ideally, these prosthetic material should be chemically inert, biocompatible, noncarcinogenic, capable of being secured at the desired site, suitably strong to resist mechanical stress, capable of being fabricated in large quantities in the form required, sterilizable, and free of viruses or other contaminating agents. Examples of tissue that can be treated with implantable prostheses include dura mater, tendon (e.g., rotator cuff, anterior cruciate, etc.) and rectic abdominus muscle due to herniation.
A wide variety of prosthetic materials have been used, including tantalum, stainless steel, Dacron, nylon, polypropylene (e.g., Marlex), microporous expanded-polytetrafluoroethylene (e.g., Gore-Tex), dacron reinforced silicone rubber (e.g., Silastic), polyglactin 910 (e.g., Vicryl), polyester (e.g., Mersilene), polyglycolic add (e.g., Dexon), and cross-linked bovine pericardium (e.g., Peri-Guard). To date, no single prosthetic material has gained universal acceptance.
Metallic meshes, for example, are generally inert and resistant to infection, but they are permanent, do not generally adapt in shape as a skeletal structure grows, and they shield the healing tissues from the stresses that may be necessary to generate fully functioning tissue. Non-resorbable synthetic meshes have the advantage of being easily molded and, except for nylon, retain their tensile strength in the body. Their major disadvantages are their lack of inertness to infection, the occasional interference with wound healing, and that they are often long-term implants. Absorbable meshes have the advantage of facilitating tissue in-growth and remodeling at the site of implantation, but often do not have the short-term or long-term mechanical strength necessary for the application.
Both U.S. Pat. No. 4,948,540, granted to Nigam and U.S. Pat. No. 5,206,028 granted to Li, disclose a collagen membrane suitable for medical uses. In the case of Li, the membrane is constructed in a fashion to make it easier for implantation, by ensuring the membrane is not transparent, and not slippery. Both patents begin by providing a solution of collagen, which is freeze-dried, cross-linked, and then compressed. Li then utilizes a second cross-linking, freeze-drying and compression step. The initial cross-linking step locks the fibers into a specific orientation. The compression step merely reduces the porosity within the sheet without inducing fiber migration that would substantially improve the strength of the composition. A second cross-linking step is necessary to hold the sheet in its compressed conformation. What is needed is a sheet with improved strength, capable of maintaining its structural competence without the need of multiple freeze-drying and cross-linking steps.
In U.S. Pat. No. 6,599,524 granted to Li, there is disclosed a membrane sheet having oriented biopolymeric fibers. The membrane is manufactured with oriented parallel fibers formed around a rotating mandrel. The rotations of the mandrel as the fibers are added results in the orientation of the fibers. The membrane is then compressed to drive out excess liquid, and cross-linked, resulting in a membrane with directionally oriented fibers. This material is only aligned in a single direction and must be laminated with binding agents in order to create a functional device. Additionally, such a device does not provide gradients such as those seen in natural tissues. What is needed is a method that allows for layering that occurs at the microscopic as well as the macroscopic level as part of a one step process and more closely represents the layered structure of natural connective tissues.
Prosthetic devices are used in the repair, augmentation, or replacement of articulating organs. For example, the rotator cuff (i.e., shoulder joint) is made up by a combination of the distal tendinous portion of four muscles: the supraspinatus, subspinatus, subscapularis and the teres minor. Proper functioning of this tendonous cuff, depends on the fundamental centering and stabilizing role of the humeral head with respect to sliding action during lifting and rotation movements of the arm. A tear in the rotator cuff tendons is a common injury that can be caused by constant friction from repetitive overhead motion, trauma, or age-related degeneration that can narrow the space between the clavicle and the top of the scapula.
To repair large tears of the rotator cuff, it is desirable to use a scaffold or graft material to help support the damaged tissue and guide its repair. Several types of materials have been used for such procedures. Wright Medical (Memphis, Tenn.) markets a product known as GraftJacket, which is manufactured by Lifecell Corporation (Branchburg, N.J.) from human cadaver skin. Human cadaverous tissue products can be difficult to obtain and have the potential for disease transmission. Tissue Sciences (Covington, Ga.) markets a product known as Permacol, which is comprised of cross-linked porcine dermis. DePuy (Warsaw, Ind.) markets the Restore Patch which is fabricated from porcine small intestine submucosa. Biomet (Warsaw, Ind.) markets a product known as CuffPatch another porcine small intestine product. The CuffPatch and the Restore Patch products provide biocompatible scaffolds for wound repair but they are complicated to manufacture, as they require the lamination of multiple layers of submucosal tissues to gain the strength needed for these applications. Fabrication of such patches from porcine small intestine submucosa are described in U.S. Pat. Nos. 4,902,508 Badylak et al. and 5,573,784 Badylak et al.
Additional applications for prosthetic devices exist in the form of membrane patches. The spinal cord and brain are covered with a protective membrane that is known as the dura mater. The integrity of the dura mater is critical to the normal operation of the central nervous system. When this integrity is intentionally or accidentally compromised (e.g., ruptured, severed, damaged, etc.), serious consequences may ensue, unless the membrane can be repaired. Typically, dura tissue is slow to heal. To enhance the healing process, graft materials can be utilized to guide the regeneration of the tissue. Repairing damaged membranes has largely focused on implantable materials known as dural substitutes, which are grafted over the damaged dura mater and are designed to replace and/or regenerate the damaged tissue.
Thus, there is a need for an effective dura substitute that would be biocompatible, sufficiently noninfectious (e.g., purified, etc.) to prevent the transmission of disease, conformable, available in a variety of sizes, high in tensile strength, inert, suturable, and optionally capable of forming a water-tight seal.
Researchers have experimented with a wide variety of substances to act as dura substitutes. Autologous grafts of tissue, such as pericardium, can be effective as a dura substitutes; however, autologous tissue is not always available and it posses additional costs and risks for the patient. Cadaverous dura mater has also been used but like autologous tissues, cadaverous tissues can be difficult to obtain. Tutogen Medical Inc. (West Paterson, N.J.) markets a product known as Tutoplast dura mater, which is obtained from human cadavers. Processed human cadaveric dura mater has been implicated in the transmission of cases of the fatal Creutzfeldt-Jakob disease. Other products overcome this shortcoming by using alternate materials. The Preclude Dura substitute, manufactured by W. L. Gore (Newark, Del.), is an inert elastomeric fluoropolymer material. The material is biocompatible but is a permanent implant and does not resorb over time. Dural substitutes comprising collagen have been also been explored as described in U.S. Pat. No. 5,997,895 (Narotam et al.). Integra Lifesciences Corporations (Plainsboro, N.J.) distributes a product known as DuraGen. The product is manufactured from bovine achilles tendon and is a pliable porous sheet. Although the material is resorbable and biocompatible, the integrity of the material is not sufficient enough to withstand suturing to the wound site.
The present invention overcomes these suturing and other difficulties of the materials currently available and provides a structure capable of being adapted to a wide variety of surgical applications.
Other applications for the implantable prosthesis of this invention, in the form of a surgical mesh, include pelvic floor disorders such uterine and vaginal vault prolapse. These disorders typically result from weakness or damage to normal pelvic support systems. The most common etiologies include childbearing, removal of the uterus, connective tissue defects, prolonged heavy physical labor and postmenopausal atrophy. Many patients suffering from vaginal vault prolapse also require a surgical procedure to correct stress urinary incontinence that is either symptomatic or latent.
Another embodiment of the present invention is directed to devices useful as prosthetic menisci, and in vivo or ex vivo scaffolds for regeneration of meniscal tissue.
The medial and lateral menisci are a pair of cartilaginous structures in the knee joint which together act as a stabilizer, a force distributor, and a lubricant in the area of contact between the tibia and femur. Damaged or degraded menisci can cause stress concentrations in the knee thereby creating abnormal joint mechanics and leading to premature development of arthritic changes.
In the prior art, treatment of injured or diseased menisci has generally been both by surgical repair and by tissue removal (i.e., excision). With excision, regeneration of meniscal tissue may not always occur. Allografting or meniscal transplantation is another method of replacement, which has been previously tried.
This approach has been only partially successful over the long term due to the host's immunologic response to the graft and to failures in cryopreservation and other processes. Alternately, menisci have been replaced with permanent artificial prostheses such as Teflon and polyurethane. Such prostheses have been selected to be inert, biocompatible, and structurally sound to withstand the high loads which are encountered in the knee joint. Typically, these permanent implants do little to encourage the regeneration of the damaged host tissue. Therefore, what is needed is an improved prosthetic meniscus composed of biocompatible materials, which are biocompatible, compliant, durable, and suitable to acts as a temporary scaffold for meniscal fibrocartilage infiltration and regeneration of the host tissue.
In U.S. Pat. No. 5,184,574 granted to Stone and U.S. Pat. No. 6,042,610 granted to Li, there is disclosed a meniscus replacement material, manufactured by shape molding collagen fibers within a mold via application of low pressure by a piston prior to or after drying. Stone requires the step of applying freezing cycles to the material. The fibrous materials achieve densities of 0.07-0.5 g/cc. Hydrated fibers at these density range from a free flowing liquid slurry to a loose dough-like material unable to maintain a shape. Freezing and possibly lyophilizing of the material is necessary to remove it from the mold and cross-linking solutions are applied to it while still in the frozen or lyophilized state so that it does not warp. Fiber orientation may be obtained by applying a rotating force to the piston in order to form a circumferential orientation. However, this orientation occurs only in areas directly in contact with the rotating piston. What is necessary is a fibrous construct with sufficient integrity to be handled without the necessity of freezing and/or lyophilizing and that can be implanted without the requirement of cross-linking, if desired. Additionally, this construct lacks any consistency throughout the thickness of its structure, being able to create oriented fibers only at the periphery.
Another embodiment of the present invention is directed to devices useful as prosthetic ligament, and in vivo or ex vivo scaffold for regeneration of ligament tissue and to methods for their fabrication.
The anterior cruciate ligament (ACL) of the knee functions to resist anterior displacement of the tibia from the femur during flexure. The ACL also resists hyperextension and serves to stabilize the fully extended knee during internal and external tibial rotation. Partial or complete tears of the ACL are common. The preferred treatment of the torn ACL is ligament reconstruction, using a bone-ligament-bone autograft (e.g., from the patient's patellar tendon or hamstring tendon). Cruciate ligament reconstruction generally provides immediate stability and a potential for immediate vigorous rehabilitation. However, ACL reconstruction is not ideal; the placement of intraarticular hardware is required for ligament fixation; anterior knee pain frequently occurs, and there is an increased risk of degenerative arthritis with intraarticular ACL reconstruction. Another method of treating ACL injuries involves suturing the torn structure back into place. This repair method has the potential advantages of a limited arthroscopic approach and minimal disruption of normal anatomy. A disadvantage of this type of repair is that there is generally not a high success rate for regeneration of the damaged tissues due to the lack of a scaffold or other cellular inductive implant.
Another embodiment of the present invention relates to devices useful as a prosthetic intervertebral disc. The intervertebral disc plays an important role in stabilizing the spine and distributing the forces between the vertebral bodies. In the case of a damaged, degenerated, or removed disc, the intervertebral space collapses over time and leads to abnormal joint mechanics and premature development of arthritis.
In the prior art, discs have been replaced with prostheses composed of artificial materials. The use of purely artificial materials in the spine minimizes the possibility of an immunological response. Such materials must withstand high and repeated loads seen by the spinal vertebral joints, early attempts focused upon metallic disc implants. These efforts met with failure due to continued collapse of the disc space and or erosion of the metal prosthesis into the adjacent bone.
SUMMARY OF THE INVENTION
The current invention is directed to a general prosthesis, which, when implanted into a mammalian host, undergoes controlled biodegradation accompanied by adequate living cell replacement, such that the original implanted prosthesis is remodeled by the host's cells before it is degraded by the host's enzymes and/or by hydrolosis. The device of the subject invention is structurally stable, pliable, semi-permeable, and suturable.
Embodiments of this invention can be utilized to repair, augment, or replace diseased or damaged organs, such as rotator cuff injuries, dura defects, abdominal wall defects, pericardium, hernias, and various other organs and structures including, but not limited to, bone, periosteum, perichondrium, intervertebral disc, articular cartilage, dermis, epidermis, bowel, ligaments, tendon, vascular or intra-cardiac patch, or as a replacement heart valve.
The device if this invention could be used for sling procedures (e.g., surgical methods that place a sling to stabilize or support the bladder neck or urethra). Slings are typically used to treat incontinence. Additionally, in the form of a surgical mesh, the device can be used for such applications as hernia and dura repair.
In another embodiment, this invention provides a ligament repair or replacement prosthesis that is biocompatible, is able to withstand ACL forces, and promotes healing of the injured tissues by acting as a scaffold for cellular infiltration. Another embodiment of this invention is to provide an improved disc replacement or prosthesis that is biocompatible, does not interfere with normal vertebral segment motion, is able to withstand normal spinal column forces, does not wear into the surrounding bone, promotes regrowth of intervertebral disc material and acts as a scaffold for fibrocartilage infiltration.
The tissue repair implant of this invention, functioning as a substitute body part, may be flat, tubular, hollow, solid, or of complex geometry depending upon the intended use. Thus, when forming the structure of the prosthesis of this invention, a mold or plate can be fashioned to accommodate the desired shape.
Flat sheets may be used, for example, to support prolapsed or hypermobile organs by using the sheet as a sling for those organs or tissues (e.g., bladder or uterus). Tubular grafts may be used, for example, to replace cross sections of tubular organs such as esophagus, trachea, intestine, and fallopian tubes. These organs have a basic tubular shape with an outer surface and a luminal surface. In addition, flat sheets and tubular structures can be formed together to form a complex structure to replace or augment cardiac or venous valves and other biological tissue structures.
The tissue repair implant of the present invention may be rendered porous to permit the in-growth of host cells for remodeling or for deposition of the collagenous layer. The device can be rendered “non-porous” to prevent the passage of fluids if necessary or the porosity can be adjusted to create a membrane capable of selective permeability. The degree of porosity will affect mechanical properties of the implant, and these properties are also affected by processing (as will be discussed).
The mechanical properties include mechanical integrity such that the tissue repair implant resists creep for the necessary period of time, and additionally is pliable (e.g., has good handling properties) and suturable. The term “suturable” means that the mechanical properties of the layer include suture retention, which permits needles and suture materials to pass through the prosthesis material at the time of suturing of the prosthesis to sections of native tissue. During suturing, such prostheses must not tear as a result of the tensile forces applied to them by the suture, nor should they tear when the suture is knotted. Suturability of tissue repair implant, i.e., the ability of prostheses to resist tearing while being sutured, is related to the intrinsic mechanical strength of the prosthesis material, the thickness of the prosthesis, and the tension applied to the suture. The mechanical integrity of the prosthesis of this invention is also in its ability to be draped or folded, as well as the ability to cut or trim or otherwise shape the prosthesis.
In another embodiment of the invention, reinforcing elements (e.g., threads, fibers, whiskers, textiles, etc.) are incorporated into the tissue repair implant for reinforcement or for different rates of remodeling. Thus, the properties of the tissue repair device can be varied by the geometry of the thread used for the reinforcement. Additionally thread constructs such as a felt, a flat knitted or woven fabric, or a three-dimensional knitted, woven or braided fabric may be incorporated between layers or on the surface of the construct. Porous, non-fibrous sheets of polymer foam may also be incorporated between layers or on the surface of the construct. Such polymer foams can be made by methods known in the art such as particulate leaching or solvent freeze-drying methods.
An embodiment of the present invention may be made by the following steps: providing a mixture comprising a plurality of fibers, a lubricant, and a suspension fluid, with the suspension fluid filling a void space between said fibers and subjecting said mixture to at least one compressive force. The compressive force causes the migration and alignment of said fibers; and may remove substantially all of the suspension fluid from said mixture. The mixture may further comprise a biologically active agent, or a reinforcing agent.
Additionally, the compressive forces may reduce the void space between the fibers, and the lubricant may assist fiber movement during compression, and be in the form of a liquid or a solid, and may be provided in a carrier fluid. The suspension fluid flow may also cause plates of oriented fibers to be formed.
The compressive force may be applied by a molding surface, thereby creating a shaped fibrous member in said mold. Additionally, or alternatively, the material may be machined, allowing the fabrication of complicated shapes.
In a preferred embodiment, at least a portion of said compressed mixture may be cross-linked by exposure to a cross-linking agent. This process will affect the strength and resorption rate of the implant. Additionally, the strength may be tailored by a reinforcing element, such as particulates, threads, fibers, whiskers, textiles, rods, meshes, or combinations thereof. The function or properties of the implant may also be affected by additives, such as ceramics, polymers, cells, biologically active agents, liquids, surfactants, plasticizers, and combinations thereof.
DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts fibrous dough prior to and after compression.
FIG. 2 depicts a change in fiber orientation and inter-fiber void space as the fibrous dough is compressed.
FIG. 3 depicts fibrous dough prior to and after compression.
FIG. 4 depicts compression of fibrous dough as it passes through rollers.
FIG. 5 depicts three-dimensional compression of fibrous dough.
FIG. 6 depicts compression of a cylindrical mass of fibrous dough.
FIG. 7 depicts incorporation if reinforcing materials within compressed fibers.
FIG. 8 depicts incorporation of particulates, biologics within the compressed fibrous matrix.
FIG. 9 depicts incorporation of microstructures within the compressed fibrous matrix.
FIG. 10 depicts a hemostatic tract plug of compressed fibrous matrix.
FIG. 11 depicts hemispherical cups of compressed fibrous matrix.
FIG. 12 depicts a selectively compressed ring of fibrous matrix surrounding a non-compressed fibrous matrix.
FIG. 13 depicts selective compression of a fibrous matrix.
FIG. 14 depicts compressed fibrous constructs useful surgical applications.
FIG. 15 depicts the surgical application of a compressed fibrous construct.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiments of this invention include compressed, biodegradable, fibrous compositions for application to a tissue site in order to support, promote or facilitate new tissue growth. One aspect of this invention is a fibrous component (e.g., collagen, elastin, chitosan, alginate, hyaluronic acid, polyglycolic acid, polyurethane, silk, etc.; see table 1) that provides unique mechanical and physical properties, as will be discussed. Such fibrous components in slurry form may be pre-processed into a fibrous dough or paste by removal of a portion of suspension fluid, as known in the art, prior to formation into a compressed conformation, as will be disclosed. An example is in the form of an interlaced matrix described in U.S. patent application Ser. No. 10/601,216 filed on Jun. 20, 2003 and assigned to the same assignee of the present invention, which is incorporated by reference herein. The material is a natively cross-linked collagen such as Semed F produced by Kensey Nash Corporation (Exton, Pa.).
The fibrous dough is dehydrated/desolvated by applying a compressive force in such a manner as to reduce the inter fiber space by removing at least a portion of the suspension fluid. In a preferred embodiment substantially all of the suspension fluid is removed. Unlike unaltered or natural matrices (e.g., dermis, small intestine submucosa, etc.), the thickness, porosity, fiber-density, fiber-orientation, fiber-length, fiber composition and component-ratio (e.g., Collagen to Elastin ratio), as a non-limiting example, can be controlled with the current invention.
To improve the migration of fibers and prevent clumping during the compressive process it is preferred to incorporate a percentage (e.g., 0%-50% by mass of fibers) of one or more lubricants (e.g., biocompatible oils, hydrogels, liquid polymers, low-molecular weight polymers, glycosaminoglycans, surfactants, waxes, fatty acids, fatty acid amines and metallic stearates such as zinc, calcium, magnesium, lead and lithium stearate, etc.) into the fibrous dough suspension. A lubricant is defined as a substance, which is capable of making surfaces smooth or slippery. These characteristics are due to a reduction in friction between the polymers to improve flow characteristics and enhance the knitting and wetting properties of compounds. Said lubricant may be liquid or solid and may be suspended or dissolved in a carrier solvent (e.g., water, alcohol, acetone, etc.). Additionally the lubricant may only become lubricious under compressive force or change in temperature. The lubricant may remain in its entirety in the final invention; may be partially removed in the dehydration/desolvation process; or, may be washed out or removed by methods known in the art during further processing. Lubricants that remain in the final invention may be biologically active agents or may form microstructures. Preferred lubricants include Tween-80, hyaluronic acid, alginate, glycerin or soluble collagen with the most preferred being acid soluble collagen such as Semed S produced by Kensey Nash Corporation (Exton, Pa.).
Additional ways in which to add lubricity include physically or chemically altering the surface of the fibers making up the composition. Such alterations can be achieved through chemical or physical attachment of a lubricious substance to the fibers, temperature induced phase changes to the surfaces of the fibers or partial solubilization of the fibers through alteration of the pH and/or conductivity of the free fluid or use of a percentage of solvent for the fibers within the free fluid. Other methods of creating lubricity are known to those skilled in the art, and are embraced by this disclosure.
During the compression step of the fibrous dough, the fibers align themselves into layered or plate-like structures. As the inter fiber void space is collapsed, the displaced fluid is forced outward and begins to flow out of the device. The flow may play a role in aligning the fibers in that direction. The rate of flow is directly affected by rate and duration at which compression occurs. This phenomenon occurs throughout the structure and results in aligned fibrous layers or plates separated by fluid planes. These planes facilitate migration within the structure, allowing the fibers within a single layer to move without interference from fibers in a different layer.
The compression induced fluid migration may occur three-dimensionally, thereby dissecting planes in the structure as it runs into resistance. Additionally the fluid may be forced through narrow passageways in the fibrous mats and begin creation of a new plane at a different level within the construct. Thus it is possible to create a structure wherein the planes do not traverse the entire length of the device, but instead exist as multiple fissures located randomly within the construct and each fissure can be defined by fibrous plates having an aligned fiber orientation unique from that of neighboring fissures. The plates themselves may be organized in a random, oriented, or aligned fashion. As compression continues, the lubricant reduces the friction, allowing the aligned fibers within the plates or planes to slide across each other and nest in the most compact orientation. Additional compression brings the plates of fibers in closer contact, allowing them to become locked into a compact anisotropic structure, although the material may be isotropic in two dimensions.
Unlike existing state of the art sheets, this layering occurs at the microscopic as well as the macroscopic level as part of a one step process and more closely represents the layered structure of natural connective tissues. Additionally, the amount of fiber compaction within a plate or layer and the spacing between the plates or layers can be controlled by the force applied and the amount of time allowed for equilibration at a specific force. The preferred force applied is from 0.01 tons/square inch to 100 tons/square inch with the most preferred force being in the range of 0.2 tons/square inch to 2.0 tons/square inch. This amount of force is in excess of state of the art methods used merely to extract fluid and concentrate the fibers into workable dough-like material. Existing methods do not induce fiber migration or layer formation. Devices created under such conditions as described above do not require additional steps such as freezing and/or cross-linking within molds to be handled. The preferred amount of equilibration time is in the range of about less than one minute to more than 500 minutes with a more preferred range of about 1 minute to 60 minutes.
The use of wicking materials such as paper towels/sponges or fluid removal systems such as screens or vacuum systems prevent excessive pooling of fluid in any single area of the structure during compression. If fluid is allowed to accumulate, it can create craters or voids within the structure. If the fibers surround these pools of fluid succumb to the compressive forces, a rip or discontinuity in the structure will form as the fluid is forcibly expelled. Strategic location of fluid exit pores within a mold can be used to create unique directional flows that in turn align the fibers within a layer or plate. In this way the fibers forming plates at each level can be oriented in the same direction or turned at any conceivable angle to each other. Although orientation of fibers from plate to plate may be organized or random, fiber orientation within a plate is organized with the fibers running predominantly parallel to each other. Molds with fluid vacuum assist further improve control of fiber orientation. Additionally, materials such as threads and screens provide avenues for fluid escape. As the fluid flows along the length of the threads and screens, the fibers adjacent to them are aligned parallel to them. Use of porous rods or porous hollow tubes that can be extracted or left in place as reinforcement can also be used to facilitate uniform fluid removal. If the fluid extraction tubes are removed, long channels will be left that can be utilized for purposes such as suture line conduits.
As the inter fiber space is reduced and the free fluid within the dough is expelled, the overall porosity of the compressed composition is reduced towards a theoretical zero point. The amount of porosity as well as the size of the pores dictates whether the device functions as a tissue matrix or barrier. Additionally, the physical/mechanical properties are highly influenced by the amount of inter fiber space. Another factor affecting the mechanical and physical properties of the composition is the use of additives (e.g., surfactants, plasticizers, particulates, porosifiers, meshes, etc.).
In a preferred embodiment, the method of preparing the high-density fibrous matrix involves: providing a fibrous material; contacting said fibrous material with a suspension fluid and a lubricant; applying a compressive force within one or more dimensions that partially dehydrates/desolvates the fibrous material. Subsequently, the fibrous material may be cross-linked. It may be further desirable to provide a directed means of egress for the suspension fluid during compression, as previously discussed. Additionally use of a fibrous suspension having interlaced, interlocked fibers may be desirable.
In another embodiment, the partially dehydrated fibrous matrix is fully dried (e.g. vacuum dried, freeze-dried, air-dried.) after which it may be cross-linked. It may be further desirable to rehydrate/resolvate the fibrous matrix to facilitate incorporation of cross-linking agents, plasitisizers, surfactants, biologically active agents, microstructures, cells or other materials. If desired the sheet may again be dried.
Any method of compression known by those skilled in the art is conceivable for this invention, including, but not limited to, using hydraulically or pneumatically powered platens or pistons to compress the fibrous matrix material. Other methods include but are not limited to using a screw or an arbor press to compress the material, using centrifugation to extract fluid and compress the fibers, or forcing the material between rollers.
The structure of the fibrous matrix material is also influenced by the amount of compressive force applied to the material. The amount of compression may change the porosity of the fibrous matrix material. The pore size distribution will also be affected by the amount of compression as the fibrous matrix material may be compressed so that only certain areas have collapsed, or so that all areas collapse. The direction of compression in relationship to the original structure of the fibrous matrix material will also affect the structure of the compressed fibrous matrix material. For example, if the initial fibrous matrix material has long parallel fibers, a force applied could be used to force the fibers together in a parallel fashion or bunch up the fibers as the force attempts to shorten the length of the fibrous composition.
Compression of the fibrous matrix material can be controlled to create various structural patterns within the material; likewise, the mechanical properties of the material may be altered to meet specific requirements. The amount of compression is directly related to the tear strength of the material. If a medical device fabricated from the compressed material is not in the form of a sheet, the compressed material can be compressed three-dimensionally to form the desired shape. If the medical device is axially loaded, the compressed material may be compressed in one direction to optimize the mechanical properties of the material in that direction.
If not compressed initially into the final shape, after being compressed and removed from the compression device, the fibrous matrix material may be machined into a new shape or design with various features. Machining processes are well known to those skilled in the art. (e.g., punching, coring, milling, sawing, lathing, etc.) Additionally, the compressed fibrous matrix may function as a component of a larger device and if not attached during the compression step, may be attached to components by methods known to those in the art (e.g., gluing, stapling, sewing, etc.).
The inventors have discovered that after the compressive dehydration/desolvation process the resultant material has mechanical properties, including tear strength, superior to those of non-compressed materials that have been cross-linked. Not being confined to a single theory, it is believed that the high compressive forces will create weak chemical linkages aside from the physical interaction of the fibers. This permits the current invention to be utilized in applications that initially require specific tear strength but where it is desirable for the device to be quickly degraded away after fulfilling its initial function such as dura repair. The current invention can be cross-linked, either chemically (e.g., EDC) or by non-chemical methods (e.g., dehydrothermal (DHT)) know to those skilled in the art, for applications requiring strength for an extended period of time, such as hernia repair.
The inventors have further discovered that a non-compressed or mildly compressed sheet can be cross-linked, completely or only at the surface, by a first method after which it is fully compressed and cross-linked by a second method. The first cross-linking restricts motion of the fibers during the compression step, retarding an increase in the footprint of the sheet. Even thought the sheet is cross-linked in the non-compressed state, the addition of a lubricant facilitates migration and shifting of the partitions making up the sheet. This allows thick sheets to achieve the same fiber density per unit volume as thin sheets.
Highly compressed sheets of collagen fibers placed into cross-linking solutions have formed a tough cross-linked skin around a minimally non-cross-linked center. The center of such sheets are easily separated forming a shell, pocket or bladder. The permeability of the bladders varied depending upon the initial compression. Low compression produced bladders that slowly allowed dyed fluid to exude. Moderate compression allowed water to pass through but filtered out the larger dye molecules. High compression created a barrier to fluid water but slowly allowed the escape of water vapor. Such a phenomenon was not evident in DHT cross-linked sheets.
Such devices would be useful for tissue engineering applications associated with bladder, intestine, tendons, ligaments and vessels, as well as the creation of rotator cuff patches, hernia repair sheets, orbital implant coverings, graft wraps and the formation of anti-adhesion devices. The shell of material could be filled with ceramics or polymers useful in bone repair or used as containment devices for injection of settable polymers or ceramics. Additionally, the center could be filled with fluids prior to or after implantation for applications such as controlled drug delivery or the creation of shock absorbing vessels useful for breast implants, fat pad replacement or meniscus and disc repair or replacement.
Restricted contact of cross-linking solutions with the surfaces of collagen devices control the degree of cross-linking in fibrous, non-fibrous, compressed and non-compressed materials. For example, restricted contact can be achieved by placing shaped, fully hydrated, collagen dough into a cross-linking solution. The cross-linking solution slowly displaces hydration fluid at the periphery but does not immediately come into contact with the hydrated material in the center. As the material continues to sit in the cross-linking solution a gradient begins to form with a greater amount of cross-linking occurring at the surface and lesser amounts of cross-linking occurring toward the center.
Additionally, a second type of cross-linking could be introduced after drying to create a bi-phasic cross-linking (e.g., DHT, chemical vapors, radiation). Devices having such unique cross-linkings would be useful in tissue-engineering applications involving multi-phasic tissues such as cartilage and skin or could function as in-vivo cell culture vessels capable of protecting foreign cells, such as islet cells from a different person or animal, from attack by the recipients\' immune system.
The central portion of units cut from compressed collagen sheets having only the surface cross-linked swell when in contact with excess aqueous fluids. A small amount of fluid hydrates the sheet and creates thin flexible units. Only after being placed in contact with excess fluid does the sheet begin to swell. The swelling can be delayed by minutes to hours depending upon the initial thickness, magnitude of compression, and the amount of cross-linking at the surface. This creates a large central porosity suitable for cell migration and/or delayed drug or biologics delivery, centered between two low-porosity protective sheets. Such a device would also be suitable in applications requiring implantation through a small opening that will swell to full size after becoming fully hydrated by body fluids.
The fibrous matrix material may be compression molded into an initial or final design of a medical device. If the device has complicated geometry, various features may be machined after compression molding. The material and mechanical properties of the final device can be altered by the temperature of the molds, the amount of overall compression, the design of the mold, etc. The fibrous matrix material may be compressed before molding, or all the compression may occur during the molding process. The direction of compression before or during compression molding will also affect the mechanical properties of the device. For example, a cylinder of fibrous dough material may be three-dimensionally compressed to improve the mechanical properties and then compression molded into a threaded bone screw. Additionally, the cylinder of fibrous material could be compressed into a cone shape providing a gradient of compression. Such gradients would be useful for multi-phasic tissue or multi-phasic drug delivery.
The implantable prosthesis of the present invention may be sterilized by any method known in the art. (e.g., exposure to ethylene oxide, hydrogen peroxide gas plasma, e-beam irradiation, gamma irradiation, etc.) The sterilization minimizes the opportunity of infection to occur as a result of the implant.
In the preferred embodiment of the invention, the fibrous prosthesis is manufactured from a resorbable material, although this is not meant to exclude the use of non-resorbable polymers, minerals and metals within the final structure.
Different polymers, molecular weights, additives, processing methods, cross-linking methods and sterilization methods can be used to control the resorption rates of resorbable polymers and is well know by those skilled in the art. For example, reconstituted collagen fibers degrade faster than natively cross-linked collagen fibers and collagen that has not been cross-linked degrades faster than cross-linked collagen. Additives such as ceramics capable of increasing the localized pH also increase the rate of degradation, as do chemotactic ground substances that attract cells to the localized area. Resorption rates can be adjusted to be shorter for applications that require mechanical strength for only a short period of time or longer for applications that require mechanical strength to be present for a longer duration. Examples of resorbable polymers that can be formed into fibers and used to form the prosthesis are shown in Table 1. These materials are only representative of the materials and combinations of materials that can be used as prosthetic material and this table is not meant to be limiting in any way.
For the purposes of promoting an understanding of the principles of this invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the embodiments and elements of the embodiments. It must be understood that no limitation of the scope or applications of the invention is thereby intended. For ease of understanding, fibers are represented in the drawings by simple crossed lines, by no way does this indicate that they may not be interconnected, interwoven, interlaced or entangled, or that the final structure is porous or non-porous, organized or random, and/or reticulated, except as otherwise noted. In theory, the compressed fibrous structure could in fact be produced through the compression of a single continuous fiber.
Referring now to the drawings, FIG. 1 shows the fibrous matrix material before and after compression. Before compression, shown in FIG. 1A, the fibrous matrix material 100 comprises a large percentage of void space surrounding the fibers 110. The fibers 110 form a structure composed mostly of inter fiber void space 120. After being compressed, shown in FIG. 1B the compressed porous matrix material 130 contains the same amount of fibrous material 140; however, the sacrificed, inter fiber void space 150 has resulted in a reduced porosity in the material. It should be noted that the inter fiber void space in this figure and all other figures may contain a lubricant as has been discussed.
In another embodiment depicted in FIG. 2A, the fibrous matrix material 200 is placed between two compressive devices 210 (e.g., platens, pistons, etc.), which may or may not be heated or cooled. Heating can be used for such purposes as to modify the fibers (e.g.,—denature, soften, melt), increase the rate of fluid evaporation, fuse the fibers once compressed, or improve the activity of any lubricant. Cooling can be used for such purposes as protecting the fibers from excessive heat during compression or to induce phase change or thickening of the suspension fluid and/or lubricant. The fibers 220 and the inter fiber void space 230 define the structure of the fibrous matrix material. In FIG. 2B, the top compressive device 215 is lowered to compress the fibrous matrix material 240 while the compressive device 210 remains stationary. A gradient is formed starting at the top of the material where fibers 250 are forced together, reducing the interfiber void space 230, while the fibers 220 in the lower part of the material retain their conformation. This can be employed to create an implant for biphasic tissues such as bone or cartilage. Two gradients can be formed by compressing the fibrous matrix material 260 with both compressive devices 215 at the same time, as shown in FIG. 2C. The top and bottom surfaces have a majority of compressed fibers 250. The next top and bottom layer of fibers 280 will be mildly compressed and have a reduced inter fiber void space 270. The middle of the material 260 will have fibers 220 that maintain their original inter fiber void space 230. This can be employed to create an implant for a triphasic tissue such as the skull that transitions from cortical bone to cancellous bone and back to cortical bone. As shown in FIG. 2D, if compressive devices 215 continue to exert force the material 290 could be evenly compressed with no gradients. The compressed fibers 250 and inter fiber void space 295 will be evenly distributed, or nearly so, through the material 290. Continued compression by compressive devices 215, as shown in FIG. 2E initiates migration of fully compressed fibers 296 in the material 297. This further reduces the inter fiber void space 298. This is useful in the creation of sheet implants having superior strength and finely controlled porosity to replace those currently manufactured for such applications as dura, tendon and hernia repair.
It is envisioned that desired percentages of porosity or desired pore distribution can be controlled based on the amount and method of compression. Specific pore volumes or densities may promote different types of tissue ingrowth (e.g., bone or vascular tissue ingrowth). Based on desired porosity or density, the fibrous matrix material may act as a cellular scaffold for various uses in tissue engineering.
In another embodiment as illustrated in FIG. 3A, an amorphous mass of fibrous dough 300 containing fibers 310 and inter fiber void space 320 is compressed to form an anisotropic sheet material 330 shown in FIG. 3B. The fibers 340 begin to align in the radial direction as force 350 induces migration of the fibers 340 collapsing the inter fiber void space 360.
Another form of compression is illustrated in FIG. 4. An amorphous mass 400 containing fibers 410 and inter fiber void space 420 is drawn through rollers 430. This drawing motion compresses and aligns fibers 440 while simultaneously reducing the inter fiber void space 450. The rollers could also be aligned circumferentially around the mass and used to draw the material into an elongated cylinder (not shown).
Another form of compression utilizes centrifugal force to compress fibers in an outward direction onto a porous structure. For example, the fibers could be forced out against a spinning porous drum creating a cylinder of compressed fibrous material (not shown). The drum could contain any number of contours or structures that would form corresponding negatives and positives in the fibrous material. Such a method could be used to create detailed anatomical structures such as the cheek, nose or ear. Additionally, this process could be used to create multi-layered constructs or embed materials such as sutures, particulates or meshes into the fibrous constructs. In another preferred embodiment, the above formed multi-layer construct is placed over a mandrel and further compressed creating a structure useful for tissue engineering applications such as vascular grafts, where each layer corresponds to the individual layers within an artery. In another embodiment, the above mandrel is replaced by a series of fibers or threads, which may or may not be woven or spun together, wherein the compressed fibrous material interpenetrates/interdigitates the series of fibers or threads, locking them into a conformation suitable for tendon, ligament or muscle repair.
In another embodiment as illustrated in FIG. 5, a sphere 500 of fibrous matrix material is three-dimensionally compressed by force 510. Fibers 520 separated by inter fiber void space 530 create the sphere\'s 500 structure. After being compressed, the porosity, inter fiber void space and size of the sphere 540 are decreased. Unlike two-dimensional compression, the fibers 550 have not collapsed into thin layers. The three-dimensional compression caused each fiber 550 to fold or coil as the inter void space 560 was reduced. This embodiment could be used as a device to promote staged delivery of biologically active agents or it could be split in half to create a chin or cheek implant, for example. A polymeric material could be placed in the center to release a biologically active agent (not shown). This embodiment may also be used to create a cell based implant wherein the cells supported in the non-compressed center of the device are protected from the body\'s immune system by the collapsed porous exterior. The center could also be hollowed out by using a central core material (e.g., ice, polymer, salt, etc) that could function similar to a porosifying agent and be removed after compression and replaced with cells (not shown). This would be particularly useful in supporting and protecting transplanted tissue (autograft or xenograft) such as islet cells capable of producing insulin. While the compressed fibers 550 would prevent immune cells from entering the sphere 540 and destroying the islet cells, oxygen and nutrients would readily pass through the compressed inter fiber space 560. In turn, waste product and insulin would pass out of the sphere.
A modified three-dimensional compression is illustrated on a cylinder 600 of fibrous matrix material in FIG. 6. Like the sphere 500, the cylinder 600 is composed of fibers 610 separated by inter fiber void space 620. Compression can be applied to the cylinder 600 by applying force around the circumference of the cylinder 600 while restricting elongation (or increasing) of its height. This type of three-dimensional compression would cause the compressed cylinder\'s 630 fibers 640 to pack together as the inter fiber space 650 is reduced. If elongation is encouraged, the fibers would draw out as the inter fiber space is reduced (not shown). Depending on the amount of compression, and direction of fiber migraton, the fibers 640 could define thin channels running parallel to each other throughout the height or width of the cylinder 630. Devices like this would be useful as orthopedic rods or nerve guides. Placement of one or more removable solid rods in the center of the mass would allow for the formation of one or more lumen within the cylinder. Uses would include tissue engineering of vessel and nerves as well as any other tubular tissue.