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"Brittle Biocompatible Composites and Methods" in Patent Application Approval Process

February 13, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- A patent application by the inventors Fortini, Arthur J. (Pasadena, CA); Kaplan, Richard (Pacoima, CA); Borwick, Gregory (Reseda, CA), filed on March 14, 2013, was made available online on January 30, 2014, according to news reporting originating from Washington, D.C., by VerticalNews correspondents.

This patent application is assigned to Ultramet.

The following quote was obtained by the news editors from the background information supplied by the inventors: "Various synthetic biocompatible composite materials had been previously proposed for use as orthopedic biomaterials. It had been proposed that these materials should serve as structures into which new bone may grow.

"Reticulated composites formed by the chemical vapor deposition (CVD) of tantalum metal on reticulated carbon foam have long been used for in vivo implantation. See Kaplan U.S. Pat. No. 5,282,861, which is hereby incorporated herein by reference as though fully set forth hereat. Such reticulated composite materials have been sold for orthopedic uses under the designation, 'Trabecular Metal.' Trabecular Metal is a trademark of Zimmer, Inc. The carbon substrates generally comprised an open network of ligaments joined together at nodes. See, for example, FIG. 1. The tantalum generally conformed to and fully encapsulated the carbon substrate in previous reticulated composites. Tantalum typically occupied between approximately 15 and 20 volume percent of the bulk volume of the implant, with the balance being approximately 3 volume percent carbon substrate, and approximately 77 to 82 volume percent interconnected void space. See, for example FIGS. 2, 3, and 14. In such reticulated composites the strength and ductility of the composites were approximately the same as those of the tantalum coating. The carbon substrate contributed very little to the strength and ductility properties in those composites. The carbon substrates were generally vitreous carbon. Conventionally, reticulated carbon foam had been coated with ductile tantalum in such an amount that the strength and ductility properties of the ductile tantalum dominated the properties of the resulting composite. See, for example, FIG. 2 and FIG. 3. The resulting reticulated tantalum foam implant was strong and deformed rather than fracturing when subjected to a load.

"Conventionally, tantalum deposits were generally formed under conditions where elements that might cause embrittlement of the tantalum were kept to a low concentration in the deposit to maintain the highest degree of ductility. A small amount of other elements may have been included in such coatings for the purposes of strengthening or otherwise improving the properties of the pure tantalum. The resulting tantalum deposits were both ductile and strong, as well as being very fracture resistant and biocompatible. These properties of ductility, fracture resistance, and strength had previously been believed to be required of a reticulated tantalum-carbon composite that was intended for an orthopedic use. These properties allowed the reticulated composite to be used as a structural replacement for natural bone where a load bearing structure was desired.

"The tantalum coating process conventionally involved, for example, chemical vapor infiltration (CVI) procedures carried out to form a completely enveloping deposit of tantalum on reticulated carbon skeletons. CVI is a form of chemical vapor deposition (CVD) wherein the deposited metal is infiltrated into a porous body, and deposited on the interior surfaces of that body. CVI procedures were used to deposit the tantalum because they resulted in the production of a closely controlled tantalum deposit that reliably exhibited highly desirable physical and biocompatible surface properties. See, for example, the enlarged portion of FIG. 3 for a visual description of a nano-textured, ductile, tantalum surface that exhibited exceptionally good biocompatibility. The term 'nano' as used herein means less than 1 micron for purposes of this disclosure and the attached claims. These surface properties encouraged rapid bone ingrowth into a reticulated implant. These surface properties also promoted good adhesion between the new in-grown bone and the reticulated implant. The risk of the patient's body rejecting the resulting implant was minimized. Reticulated tantalum composite structures were osseointegrated rapidly and effectively in the human body.

"The incorporation of small amounts of impurities, such as, for example, oxygen, into the tantalum deposit on the brittle reticulated substrate results in a slightly less ductile but generally stronger tantalum deposit. Amounts of impurities in excess of these small amounts may degrade the physical and surface properties of the tantalum to such a degree that it was previously believed to be unsuitable for implantation purposes. Brittle tantalum deposits could result from the presence of impurities during formation of the tantalum deposit or during subsequent elevated temperature processing in the presence of embrittling agents such as hydrogen. Such degradation of physical properties was generally regarded as undesirable, because it impaired the strength and load bearing properties of the resulting implant.

"Tantalum deposits that were conventionally laid down by chemical vapor infiltration (CVI) processes during the formation of ductile tantalum composites exhibited micro- and nano-textured surfaces. Under moderate magnification, the prior micro-textured surfaces generally exhibited features including a chaotic jumble of angular terraces, blocks, facets, and walls projecting from the surface of the tantalum coating. The individual elements of the features, on average, usually had peak to valley heights of approximately 1 to 5 microns. See, for example, FIG. 6 or FIG. 3. Under higher magnification, the surfaces of the individual elements exhibited approximately regular nano-patterns of terraces, walls, and newly nucleated crystals. See, for example, FIG. 7. These nano-patterns generally projected from the surface of the individual elements by approximately 300 to 500 nanometers. See, for example, the enlarged area in FIG. 3. There are certain applications where the characteristic strength and ductility of conventional reticulated tantalum precludes, or at least substantially restricts its use. For example, sometimes the particular shape and size requirements of an implant are not known until surgical procedures have exposed the site where the implant is to be inserted. The surgeon must be able to shape and size the implant during the surgery to fit the needs of the patient. Because of the necessity to maintain certain operating room conditions, and finish the surgical procedure within certain time constraints, the surgeon must be able to shape and size an implant manually with just the tools that are normally available in an operating room.

"The characteristic strength and ductility of conventional tantalum foam are such that it is very difficult, if not impossible, for a surgeon to manually shape and size it during an operation. Conventional reticulated tantalum undergoes ductile deformation, that is, it smears instead of fracturing. Generally, more force than can usually be applied manually is required to cut a conventional reticulated tantalum implant. The machines and procedures that are normally used to shape reticulated tantalum objects without deforming or smearing them are not suitable for use during surgery in an operating room environment.

"It had been previously proposed to manufacture small tantalum flakes from a thin ductile sheet of tantalum for use in electrical capacitors. According to a previously proposed manufacturing procedure, thin tantalum sheets were hydrided to form a brittle tantalum or tantalum hydride foil, the brittle foil was reduced to small flakes by milling, and the resulting brittle flakes were rendered ductile by dehydriding them. See McCracken et al. US 2008/0233420, published Sep. 25, 2008, which is hereby incorporated herein by reference as though fully set forth hereat.

"It had also been previously proposed to use open-cell metal structures as implants, and to shape them immediately before or during an operation. See Nies US 2010/0185299, published Jul. 22, 2010, which is hereby incorporated herein by reference as though fully set forth hereat. No method of shaping the metal structures is proposed, suggested, or taught by Nies.

"It had further been previously proposed that separate structural elements in the shape of caltrops could be interlocked with one another in an array so the array could resist shear stress in essentially all directions. See Black et al., U.S. Pat. No. 5,676,700, patented Oct. 14, 1997, which is hereby incorporated herein by reference as though fully set forth hereat. No method for forming the caltrop shape is proposed, suggested, or taught by Black.

"Reticulated brittle substrates, of the general type shown in FIG. 1 had been previously proposed for use in ductile reticulated tantalum composite implants. See also, for example, Stankiewicz U.S. Pat. No. 6,103,149, which is hereby incorporated herein as though fully set forth hereat. This patent described embodiments of reticulated vitreous carbon foam that were proposed for use as a substrate in reticulated tantalum composite implants. Reticulated carbon foam made according to this patent, or otherwise, is very brittle. It deforms very little before fracturing when loaded in any of tension, compression, or shear, and it exhibits very little fracture toughness. Vitreous carbon, the material of which carbon foam is conventionally comprised, exhibits good biocompatibility properties. Stankiewicz teaches the production of reticulated carbon skeletons containing generally spheroidal pores with aspect ratios between approximately 0.8 and 1.2 from polymeric foam. The spheroidal pores may be made more prolate or oblate, and the porosity of the skeletons decreased by compressing the polymeric foam.

"In the event of any inconsistencies, contradictions, or contra indications between the teachings herein and the teachings of any reference that is incorporated herein by reference, the teachings herein shall control."

In addition to the background information obtained for this patent application, VerticalNews journalists also obtained the inventors' summary information for this patent application: "Tantalum has been used as an orthopedic biomaterial in reticulated implants at least in part because of its strength and ductility. It is a very good load bearing metal. Also, it is very biocompatible. When properly prepared, CVD tantalum exhibits a micro- and nano-textured surface that promotes bone ingrowth and adhesion. Other ductile metals, such as, for example, niobium and titanium have also been used as orthopedic biomaterials. Because of its strength and ductility, tantalum has not been used where morselization or manual shaping of a monolith is required.

"According to embodiments of the invention, reticulated monolithic tantalum and other biocompatible metal composites exhibit physical properties that permit these monolith composites to be morselized, or sized and shaped manually. Certain reticulated tantalum and other biocompatible metal composite embodiments exhibit surfaces with characteristic ductile metal biocompatibility properties, but with strength and fracture physical properties that are more characteristic of the brittle substrate than of the ductile metal. Such embodiments fracture easily, and may be morselized or worked by manual sizing and shaping. In further embodiments certain morselized composites exhibit caltrop shaped configurations. Morselized or manually shaped monolithic composites exhibit excellent biocompatibility characteristics with micro- and nano-textured surfaces that promote rapid bone ingrowth and adhesion. Additional embodiments may involve applying additional tantalum or other materials to the morselized composites to alter their physical or biocompatibility properties. Such additional deposits may, for example, fully encapsulate the brittle substrate, alter the proportion of ductile tantalum in the composite sufficiently to influence the strength and ductility of the morselized composite, or add other materials that improve the biocompatibility of the composites or promote bone ingrowth.

"Monolith composites may be shaped by a surgeon during surgery. Embodiments are brittle enough and/or have a low enough strength that a surgeon can shape them by hand during surgery using only the tools that are normally available in an operating room environment. These include hand carving, chiseling, and sawing tools as well as small hand-held grinders, saws, drills, Dremel tools ('Dremel' is a registered trademark of Robert Bosch Tool Corp.), and other power tools. Certain embodiments are brittle enough that they may be manually reduced in size and shaped by fracturing. Such embodiments primarily fracture rather than undergoing plastic deformation. The fractured material may generally be removed, and the remaining custom-shaped monolith may be implanted in a patient.

"Reticulated tantalum and other biocompatible composites intended for use as surgeon shapeable implants include a brittle, biocompatible, reticulated substrate, with a small controlled amount of ductile tantalum or other ductile metal deposited on the surface of the substrate. There is enough tantalum or other metal in the surface deposit on the brittle substrate to provide a biocompatible micro- and nano-textured surface that promotes the ingrowth and adhesion of new bone. The tantalum or other metal deposit does not include enough tantalum or other metal to prevent the reticulated composite from fracturing. When worked, the reticulated tantalum or other metal composite primarily undergoes fracturing rather than plastic deformation, thus allowing it to be manually shaped by a surgeon with the tools that are normally available in an operating room. In certain embodiments the reticulated composite undergoes some degree of plastic deformation in addition to fracturing, but not enough to render it manually unworkable.

"The amount of tantalum or other metal contained in conventional reticulated composites is such that the metal generally determines the machinability, and at least the mechanical properties of strength, fracture toughness, and ductility. The brittle properties of the substrate are able to largely dominate the physical fracture, strength and ductility properties of the reticulated tantalum or other metal composite only when the metal content is substantially lower than in conventional reticulated tantalum composites. Even when the tantalum or other volume percent is so low that the substrate dominates the physical properties of the reticulated composite, the biocompatible, ingrowth, and adhesion properties of such a composite will be dominated by the surface properties of the thin tantalum or other metal coating. Thus, a surgeon will be able to utilize a substantially monolithic implant that is physically shapeable in the environment in which the surgeon must work, and yet will exhibit substantially all of the superior biocompatibility properties that are normally associated with ductile tantalum or other metal implants. Very thin ductile tantalum coatings contribute some strength and ductility to such reticulated tantalum composites, but not so much strength or ductility or fracture toughness that it will prevent the material from being manually shaped in the operating room.

"Certain embodiments of a method of manufacturing comprise selecting a skeleton that is reticulated, biocompatible, and substantially monolithic. The skeleton comprises a reticulated three-dimensional network of ligaments interconnected at nodes to define pores and windows between said pores. The skeleton has from approximately 3, or 10, or 20, or 40 to 300, or 200, or 150, or 100 pores per inch, a fracture strength in compression of less than approximately 150 pounds per square inch, and a void volume of at least approximately 66 percent. A deposit of ductile biocompatible metal is formed on the skeleton in an operation that comprises practicing a chemical vapor infiltration procedure, for example as described by Kaplan U.S. Pat. No. 5,282,861, to form a reticulated composite. The operation is carried out under conditions wherein the deposit exhibits a surface adapted to promote bone ingrowth and adhesion. The deposit has a stand-alone deformation tensile strength of at least approximately 20,000 pounds per square inch. The deposit is limited to an amount at which the reticulated composite has a fracture strength in compression of less than approximately 1000, or 1200, or 2000 pounds per square inch. The deposit may comprise tantalum, or niobium, titanium, or other biocompatible metals, or their alloys.

"Certain embodiments of a method of using a reticulated composite monolith comprise commencing an in vivo surgical procedure on a living being that will require that a portion of the living being's bone be replaced by a customized scaffold. A reticulated composite monolith that comprises a brittle biocompatible substrate and a ductile metal coating that is biocompatible and has a 3-dimensional network of ligaments interconnected at nodes forming pores and windows between said pores with pore diameters ranging from approximately 400 to 7,500 microns, or from approximately 400 to 900 microns is selected. The windows range from approximately 30 to 4500 microns in diameter, or from approximately 30 to 400 microns in diameter. The reticulated composite monolith has substantially no deformation strength, a fracture strength in compression of less than approximately 1000, or 1200, or 2000 pounds per square inch, and the biocompatible surface characteristics of a ductile metal such as, for example, tantalum. The customized scaffold is formed by manually shaping the reticulated composite monolith during the in vivo surgical procedure. The manually shaping includes fracturing the reticulated composite monolith to change its shape. Fracturing the reticulated composite monolith expose the biocompatible substrate where the reticulated composite monolith is fractured. No significant plastic deformation of the monolith takes place during this shaping operation. Fracturing of the ligaments and nodes accounts for substantially all of the changes in shape that this manual shaping step accomplishes. The substrate is exposed at the location where the ligaments or nodes are broken. The composite debris that is broken away from the monolith is generally removed before the customized scaffold is implanted.

"Certain embodiments of an article of manufacture comprise a reticulated composite monolith comprised of a 3-dimensional network of ligaments interconnected at nodes to define pores and windows between the pores. The reticulated composite monolith is comprised of biocompatible materials including a substrate and a ductile metallic coating on the substrate. The reticulated composite monolith has a fracture strength in compression of less than approximately 1000, or 1200, or 2000 pounds per square inch, and said ductile metal coating having surface characteristics that promotes bone growth and adhesion.

"Certain embodiments of an article of manufacture include a reticulated composite monolith that comprises a skeleton of vitreous carbon coated with from approximately 1 to 10 volume percent of ductile tantalum, or from approximately 2 to 5 volume percent ductile tantalum.

"Certain embodiments of a reticulated composite monolith that is biocompatible comprise a skeleton. The skeleton comprises vitreous carbon, and is substantially monolithic. The skeleton comprises a reticulated three-dimensional network of ligaments interconnected at nodes to define generally spheroidal pores and windows between the spheroidal pores. The spheroidal pores range in diameter from approximately 400 to 7500 microns, or between approximately 400 and 900 microns. The windows range from approximately 30 to 4500 microns in diameter, or between approximately 30 and 400 microns. The skeleton has a stand-alone fracture strength in compression of less than approximately 150 pounds per square inch. A metal coating on the skeleton comprises ductile biocompatible metal. The metal coating has a stand-alone deformation tensile strength of at least approximately 20,000 pounds per square inch, and a submicron-textured surface that promotes ingrowth and adhesion of new bone. The metal coating is present in an amount sufficient to provide the textured surface, and proportioned relative to the skeleton such that the reticulated composite monolith has a fracture strength in compression of less than approximately 1000, or 1200, or 2000 pounds per square inch.

"According to further embodiments, a composite caltrop that is biocompatible comprises a brittle substrate comprised of vitreous carbon having a stand-alone fracture strength in compression of less than approximately 150 pounds per square inch. A metal coating on the brittle substrate is comprised of ductile biocompatible metal. The metal coating has a stand-alone deformation tensile strength of at least approximately 20,000 pounds per square inch, and a submicron-textured surface that promotes ingrowth and adhesion of new bone. The metal coating is present in an amount sufficient to provide such a submicron-textured surface, and proportioned relative to the brittle substrate such that the caltrop has a fracture strength in compression of less than approximately 1000, or 1200, or 2000 pounds per square inch. The caltrop has peripheral surfaces, at least some of which are broken. The brittle biocompatible substrate is exposed where the peripheral surfaces are broken.

BRIEF DESCRIPTION OF THE DRAWINGS

"Further advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description of the preferred embodiments and upon reference to the accompanying drawings in which:

"FIG. 1 is a millimeter scale image of a prior reticulated vitreous carbon skeleton that is suitable for use as a brittle substrate according to the present invention;

"FIG. 2 is a millimeter scale image of a prior CVD deposited tantalum coating on a reticulated vitreous carbon substrate such as that shown in FIG. 1;

"FIG. 3 is a millimeter scale image of a prior CVD deposited tantalum coating on a reticulated vitreous carbon substrate with an enlarged nano-scale image of a small area of the surface of this tantalum coating; and

"FIG. 4 is a diagrammatic flow chart of certain of the steps carried out according to an embodiment in accordance with the present invention.

"FIG. 5 is a millimeter scale image of various typical 3-dimensional, spiny, multi-axial particles (caltrops) formed by crushing an uncoated reticulated vitreous carbon substrate.

"FIG. 6 is a micron scale image of a prior CVI deposited ductile tantalum coating (approximately 14 volume percent tantalum) on a reticulated vitreous carbon substrate showing the micro-scale roughness;

"FIG. 7 is a higher magnification image of a part of the coating shown in FIG. 6 showing the nano-scale roughness;

"FIG. 8 is a micron scale image of a cross-section of a ductile tantalum coating (approximately 3 volume percent tantalum) on a vitreous carbon substrate according to the present invention showing the thickness and micron scale roughness of the ductile tantalum coating;

"FIG. 9 is a micron scale image of a thin ductile tantalum coating (approximately 4 volume percent tantalum) according to the present invention showing the micron scale and sub-micron scale roughness of the tantalum surface;

"FIG. 10 is a micron scale image of another embodiment of a thin ductile tantalum coating (approximately 2 volume percent tantalum) according to the present invention showing the micron scale and sub-micron scale roughness of the tantalum surface;

"FIG. 11 is an image of morselized particles of various sizes that were obtained by crushing a ductile tantalum coated (approximately 1 to 5 volume percent tantalum) reticulated composite according to the present invention;

"FIG. 12 is an image of a prior proposed reticulated structure wherein the pores are regular in size and shape, and are angular in outline;

"FIG. 13 is an enlarged image of section 13-13 in FIG. 12;

"FIG. 14 is an image of a section of a billet comprising a prior CVI deposited coating of ductile tantalum (at least approximately 14 volume percent) on a reticulated vitreous carbon substrate, illustrating the part of the billet that is described as a pore or cell 10, and the part of the billet that is described as the window 12 between the pores or cells;

"FIG. 15 is a stress-strain curve obtained by compressing a billet of prior reticulated ductile tantalum coated vitreous carbon with approximately 14 volume percent tantalum;

"FIG. 16 depicts two stress-strain curves obtained by compressing reticulated thin ductile tantalum coated billets according to the present invention wherein the values along the x-axis is shown in inches of movement of the compressing element in the testing apparatus, which is proportional to strain;

"FIG. 17 is stress-strain curve similar to FIG. 16 of three further embodiments according to the present invention; and

"FIG. 18 is an image of a partially crushed cylindrical billet measuring approximately one-half by one-quarter inches, wherein this billet comprises an embodiment of a thinly tantalum coated (approximately 3 volume percent tantalum) reticulated vitreous carbon substrate according to the present invention.

"FIG. 19 is an image of typical caltrop shaped particles obtained from machining a billet of reticulated ductile tantalum coated (approximately 1 percent tantalum) vitreous carbon on an end mill."

URL and more information on this patent application, see: Fortini, Arthur J.; Kaplan, Richard; Borwick, Gregory. Brittle Biocompatible Composites and Methods. Filed March 14, 2013 and posted January 30, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=1100&p=22&f=G&l=50&d=PG01&S1=20140123.PD.&OS=PD/20140123&RS=PD/20140123

Keywords for this news article include: Surgery, Ultramet, Bone Research, Nanotechnology, Emerging Technologies, Chemical Vapor Deposition.

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