RESILIENT THIN FILM TREATMENT OF SUPERELASTIC AND SHAPE MEMORY METAL COMPONENTS

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to wear resistant thin films and their manufacture, and more particularly to a method for applying a wear resistant thin film to superelastic and shape memory metals.

[0002] Various metal alloys are known which exhibit "superelastic" and/or shape memory qualities (geometry restoration). These materials can be elastically deflected to a far greater degree than ordinary alloys. These properties make superelastic and shape memory alloys especially useful for the construction of medical tools, devices, and implants.

[0003] One category of medical implants are stents, which are small, tubular devices that are implanted into arteries to hold them open so that blood can flow freely through it. A stent is manufactured in an extended condition. It is then collapsed and inserted into the selected artery and moved to the location of a blockage. The stent is ihen expanded to the desired diameter. While stents are effective in opening arteries, they are subject to chemical attack within the body, and their base alloys may be incompatible with biological structures.

BRIEF SUMMARY OF THE INVENTION

[0004] Accordingly, it is an object of the invention to provide a durable anti-wear thin film for superelastic and shape memory alloys.

[0005] It is another object of the invention to provide a wear-coated stent for medical uses.

[0006] It is another object of the invention to apply thin film on structures fabricated from memory metals that become the shaped scaffolding and the structural support for lissue growth in bioengineered tissue

manufacturing and natural tissue regeneration (either externally cultured or internally grown).

[0007J It is also desired to create a flexural coating or covering that improves the biocompatibility and corrosion resistance of devices constructed from memory metal

[0008] These and other objects are met by the present invention, which according to one embodiment provides a coated component including: a metallic member comprising superelastic or shape memory properties, or a combination thereof; and a thin film consisting essentially of carbon in a noncrystalline microstructure disposed on an outer surface of the metallic member.

[0009] According to another embodiment of the invention, the thin film has a flexuraf capability of at least about 8%.

[0010] According to another embodiment of the invention, the metallic member comprises an alloy of nickel and titanium.

[0011] According to another embodiment of the invention, a plurality of members are arranged in a lattice-like structure.

[0012] According to another embodiment of the invention, a stent includes: a lattice structure of metallic members comprising superelastic or shape memory properties, or a combination thereof; and a thin film consisting essentially of carbon in a non-crystalline microstructure disposed on an outer surface of the lattice structure.

[0013] According to another embodiment of the invention, a medical implant includes: a non-organic functional portion; a lattice structure attached to the functional portion, the lattice structure comprising metallic members having superelastic or shape memory properties, or a combination thereof; and a thin film consisting essentially of carbon in a non-crystalline

microstructure disposed on an outer surface of the lattice structure; the lattice structure serving as a scaffold for the growth and integration of body tissues into the implant.

[0014] According to another embodiment of the invention, the functional portion is a caged ball heart valve structure.

[0015] According to another embodiment of the invention, the functional portion is a synthetic blood vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

[0017] Figure 1 is a schematic perspective view of a stent treated in accordance with the present invention;

[0018] Figure 2 is a cross-sectional view of a portion of the stent shown in Figure 1 ;

[0019] Figure 3 is a schematic side view of a thin film treated apparatus for use with the present invention;

[0020] Figure 4 is a schematic side view of a synthetic blood vessel treated in accordance with the present invention; and

[0021] Figure 5 is a schematic side view of a replacement heart valve treated in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, Figures 1 and 2 depict an exemplary stent 10 constructed in accordance with the present

invention. The stent 10 has a lattice-fike construction of slender, elongated members 12. In the expanded condition, the stent 10 is generally cylindrical. It should be noted that the present invention is equally applicable to other types of implants and components. The stent 10 is made from an alloy exhibiting superelastic and/or shape memory properties. One example of a suitable material is an alloy of nickel and titanium generally referred to as NITINOL. NITINOL exhibits shape memory characteristics, and when suitably heat treated, also has superelastic properties.

[0023] The entire surface of the stent 10 has a thin film 14 of a carbon- based material deposited thereon. This thin film material is essentially pure carbon, has a non-crystalline microstructure, and exhibits a flexural capability of approximately 8% or better. The carbon structure and bond layer enable the thin film 14 to endure significant vibration and deformation without cracking or detaching from the substrate or delaminating. Such thin films may be obtained from BioMedFlex LLC, Huntersville, NC, 28078. Known memory and superelastic materials typically exhibit an operational deformation of up to 8%. To the extent that future materials are developed with elastic and restorative properties in excess of 8%, the thin film 14 will accommodate these materials due to the customizable nature of the thin film 14 and the current exhibited flexural nature and strain rate demonstrated in other applications.

[0024J It is believed the thin film 14 is able to reinforce and strengthen memory and superelastic materials. The thin film 14 possesses a sufficiently strong bond layer and thin film layer as to add to the structural integrity and restorative nature of the substrate material. The thin 14 film possesses an ultimate tensile strength in excess of 200,000 PSI. The flexural capability and strong local bond attachment will cause the thin film 14 to form a composite like structural reinforcement layer on the substrate body.

[0025] Similarly, the flexural nature of the thin film 14 and bond layer will produce a skin reinforcement effect that will be able to bridge small

substrate imperfections and weak areas that may constitute weak points or stress risers. The thin film 14 will reinforce these focally weak areas and allow the body to better withstand the rigors of low and high cycle fatigue applications by minimizing or reducing the effects of local substrate weak points. The outcome will be to hamper the initiation and propagation of surface cracks and defects. The result will be an improved or elevated endurance limit and therefore better component integrity

[0026] Figure 3 illustrates a thin film apparatus 16 for applying the thin film 14 to the stent 10. The thin film apparatus 16 is a chemical vapor deposition (CVD) apparatus of a known type. It includes a processing chamber 18 which receives the workpiece, a hydrocarbon gas source 20, an RF field generator 22 of a known type, and a vacuum pump 24.

[0027] The thin film process proceeds as follows. First, the untreated stent 10 is plasma cleaned in a known manner to eliminate any foreign material or contaminants from the surface thereof. The thin film 14 is then deposited all over the exterior of the stent 10 and the members 12 using a plasma assisted chemical vapor deposition (CVD) process. The RF field which generates the plasma is specially manipulated so that the thin film material is deposited "around the corner" of the members 12. That is, the thin film process does not require a direct line-of-sight to the interior-facing portions of the members 12 to achieve a satisfactory thin film thereon.

[0028] Once the thin film cycle is complete, the stent 10 is removed from the thin film chamber 18. Because of the high flex capability of the thin film 14, the stent 10 may be inserted into an artery and expanded without cracking or loss of the thin film 14. The finished thin film 14 acts as a biocompatible and inert layer over the base material of the stent 10. The thin film 14 is resistant to scratches and wear and acts as a barrier against biofluids, chemicals, moisture, etc. The thin film 14 exhibits very low surface friction, which reduces wear and damage to surfaces (e.g. artery walls) in contact with stent 10, causes less buildup and adhesion of other materials,

and facilitates extraction because it does not tend to adhere to other materials. The this film treated stent 10, due to the applied benign surface treatment, will not activate blood clotting or adhesion agents and related blood constituents (platelets or inflammation response agents).

[0029] It is known to apply anti-inflammatory or antibiotic coatings to the stent 10 to create so-called "drug-eluting" stents. While these coatings are medically effective, they also have a tendency to dissolve, thus exposing the base material of the stent 10. In contrast to the prior art, the stent 10 with the hard carbon thin film 14 will remain protected even when the drug coatings (when and if the two are combined) wear away. The resilient hard carbon thin film also can stand alone as the sole anti-inflammatory surface treatment on a stent.

[0030] In addition to stents, It is also possible to apply the thin film to structures fabricated from memory metals for use as shaped scaffolding and structural support for tissue growth in bioengineered tissue manufacturing and natural tissue regeneration (either externally cultured or fnternally grown). In the case of structural scaffolding or framework, the thin film would be added to the superelastic material to enhance biocompatibility and improve corrosion resistance.

[0031] For example, Figure 4 depicts an exemplary implantable synthetic blood vessel 30. The vessel 30 has a tube portion 32 with a generally cylindrical wall of a biocompatible material, and a scaffold portion 34 attached or incorporated in one end of the tube portion 32. The scaffold portion 34 is configured as a lattice-like construction of slender, elongated members 36. The scaffold portion 34 is made from an alloy exhibiting superelastic and/or shape memory properties. One example of a suitable material is an alloy of nickel and titanium generally referred to as NITINOL. The entire surface of the scaffold portion 34 has a thin film of a carbon-based material (described above) deposited thereon.

[0032] Figure 5 illustrates a an exemplary implantable heart valve 38, which includes a ball 40 captured in a wire-frame cage 42 that is attached to a sealing ring 44. The sealing ring 44 is configured as a lattice-like construction of slender, elongated members 46. The sealing ring 44 is made from an alloy exhibiting superelastic and/or shape memory properties. One example of a suitable material fs an alloy of nickel and titanium generally referred to as NITINOL. The entire surface of the scaffold portion 44 has a thin film of a carbon-based material (described above) deposited thereon.

In use, the scaffold portion 34 of the vessel 30 or the valve 38 would be implanted into a patient. The construction of slender, elongated members forms a support for growth and integration of the body's tissues into the implant. This allows the non-organic functional portion of the implant (i.e. the tube portion 32 or the caged valve ball 40) to be secured fixed in the patient. Antibiotic or antiflammatory coatings describe above may also be used.

[0033] The foregoing has described a thin-film coated component, apparatus for applying a thin film to such a component, and a method for applying such a thin film. It is believed that the thin film described herein will provide enhanced biocompatibility, improved corrosion protection, reduction of localized release of metal constituents through chemical and mechanical retention, tolerance of normal healing cell growth, and reduced inflammation. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.