Patent Application
20080249597
The present invention relates to a thin film tissue repair matrix made from a super elastic material such as Nitinol for use within a patient's body or on as part of a skin graft onto a patient's body.
A thin film tissue repair matrix is an implantable device used to provide structural support within a patient to help repair a patient's organs. These support device applications include: lung repair, pleurodesis, hernia repair, skin grafts, etc. The tissue repair matrixes must be flexible and have surface areas that are large enough to provide the necessary support for the internal organ. In order to provide a large flexible surface area, tissue repair matrix is frequently a mesh or woven structure.
A problem with the prior art tissue repair matrix is that they cannot be compressed for insertion through a small minimally invasive hole and then expanded within the patient prior to that can cause trauma within the patient. What is needed is a tissue repair matrix that can be compressed for insertion into a patient through a minimally invasive hole in the patient and then expanded for implantation within the patient.
The present invention is an improved tissue repair matrix for implantation into a patient. The tissue repair matrix is made of a thin film of super elastic alloy such as Nitinol that is formed by vapor deposition. The matrix is cut into a pattern that provides porosity and may have features that promote adhesion of the tissue. The repair matrix has a first smaller surface area in a compressed state which allows insertion into the patient through a minimally invasive hole. After the tissue repair matrix is inserted into the patient it is expanded to a larger surface area in an expanded state. The tissue repair matrix is implanted into the patient in the expanded form as a “bandage” support surface for body tissue. The porous matrix provides a surface on which new tissue grows in a damaged area to help the patient heal. Applications for the inventive repair matrix include: lung repair, pleurodesis, hernia repair and skin grafting.
The repair matrix may have a planar surface or a three dimensional surface. If the inventive tissue repair matrix is used as a planar member, the super elastic alloy is vapor deposited onto a planar substrate and machined with the desired repair matrix pattern. In a three dimensional embodiment, the repair matrix may deposited in the planar form and then converted into a three dimensional shape through a deformation and heat treating process. Alternatively, the super elastic alloy may also be vapor deposited onto a three dimensional substrate so that it does not require post deposition heat treatment to obtain the required shape. A three dimensional repair matrix may be desirable for specific medical applications. For example, in a lung repair application, the shape of the repair matrix may correspond to the surface of the lung that is being repaired.
The repair matrix may have a plurality of hooks or barbs that provide anchors for the repair matrix to adhere to the organ being repaired. The hooks are sharp pointed features that are cut in the thin film of the repair matrix. In the first smaller compressed state, the hook may be flush with the thin film so the point of the hook is protected. This allows the repair matrix to be inserted into the patient without having any sharp points exposed. When the repair matrix expands to the second larger area, the hooks may bend away from the repair matrix. The bent hooks engage the organs of the patient and also provide a hole in the repair matrix for ingrowth.
The repair matrix may also include amorphic circles that are attached to the outer edges that make the inventive tissue repair matrix a-traumatic to the implanted patient. The amorphic circles may also have hooks (described above) and holes which allow for in-growth after the tissue repair matrix has been implanted within the patient. The holes in the amorphic circles can also be used as suture points that are used to secure the tissue repair matrix to the desired location within the patient.
The tissue repair matrix may be cut into an intricate pattern of interconnected struts, loops and bridges. Bending of the struts, loops and bridges allows the tissue repair matrix to transform in area between a compressed state and an expanded state. After the tissue repair matrix is fabricated, it can be coated with polymers, therapeutic agents, bioactive materials or radio-opaque materials depending upon the application.
The tissue repair matrix is made from a “super elastic” metal alloy such as Nitinol. Super elastic metal alloys have the physical characteristics of being extremely elastic when cooled to the martensitic molecular phase. In this phase, the inventive tissue repair matrix can be compressed into a small volume without springing back to its expanded shape. In the compressed form, the gaps separating the struts, loops and bridges are very small so that the adjacent struts are in very close proximity to each other. In addition to being compressed in a planar manner, the tissue repair matrix may also be bent or rolled out of plane in an accordion manner to further compresses the tissue repair matrix.
Before the repair matrix is implanted in the desired area within the patient, it is heated to change the super elastic alloy to the austenitic phase causing the tissue repair matrix to expand. The final austenite transition temperature may be about 24° C. to about 37° C. so that exposure to the patient's body heat causes the desired transformation. In the expanded state, the struts, loops and bridges bend so the repair matrix expand to a larger area with larger gaps between the components. After the repair matrix is fully expanded, it has a sufficient area to provide support to the internal organs and can be attached to the patient through in-growth or sutures through the holes in the amorphic circles.
The foregoing and other aspects of the present invention will best be appreciated with reference to the detailed description of the invention in conjunction with the accompanying drawings, wherein:
The present invention is directed towards a tissue repair matrix which is made from a thin film of super elastic alloy. Although the super elastic alloy is described as Nitinol (Ni—Ti alloy), other alloys with similar super elastic properties may be used. Very thin Nitinol film stock is commercially available from a number of suppliers including Nitinol Devices & Components, Fremont Calif.
Alternatively, the Nitinol thin film stock may be formed through a vapor deposition process. Vapor deposition, as used herein, refers to any process of depositing metals and metal compounds from a source to a substrate or target by dissipating metal ions from the source in a vaporous medium. Examples of vapor deposition processes that may be used to make the present invention include evaporation vapor deposition, sputtering deposition, chemical vapor deposition, etc.
In the evaporation vapor deposition process, vapor is generated by heating a source material to a temperature to cause the vaporization thereof. The evaporating metal atom leaves the surface of the Nitinol source material in a straight line. Therefore, the highest quality deposition layers are deposited when the source-to-substrate distance is less than the mean path distance between collisions of the vaporized metal and the surrounding vacuum chamber. The substrate may be rotated or translated during the evaporation process so that a uniform Nitinol layer is deposited on the substrate.
In the sputtering process, a source material is placed in a vacuum chamber with a substrate material. A radio-frequency power source gives the substrate a positive charge relative to the Nitinol source material. The source material is bombarded with inert gas ions from an ion beam or a plasma discharge to cause the source material to dislodge. These dislodged atoms are then deposited onto the substrate to form the thin film layer.
In the chemical vapor deposition process, reactant gases that may be diluted in a carrier gas are injected into a reaction chamber. The gas mixture is heated and the atoms are deposited on a substrate. The deposition continues until the desired thickness is formed. The thickness of the deposited Nitinol used to make the super elastic alloy tissue repair matrix may range from about 0.0001 to about 0.1 inch.
The thin film vapor deposited Nitinol can be planar or have a three dimensional shape. Thus, the substrate that the Nitinol is deposited on can be a planar or three-dimensional surface. In order to simplify the removal of the deposited Nitinol from the planar or three-dimensional substrate, a release layer may be applied to the substrate prior to the Nitinol vapor deposition.
The Nitinol sheet stock may be cut into the desired fully expanded tissue repair matrix pattern while in the austenitic phase or in the martensitic phase. The phase of the Nitinol material is temperature dependent. In general, the austenitic transition temperature Af is about 24° C. to about 37° C. At temperatures above the austenitic transition temperature, the Nitinol will be in the Austenitic phase. At lower temperatures, the Nitinol may be fully or partially in the martensitic phase. If the tissue repair matrix is cut in the martensitic phase, it can then be maintained in the expanded shape while it is heat treated to convert the Nitinol to the austenitic phase. The tissue repair matrix can then be cooled to transform the tissue repair matrix to the martensitic phase before the tissue repair matrix is compressed into a compact state for implantation in a patient. The austenitic phase shape is the shape that the tissue repair matrix will attempt to assume whenever it is heated above the austenitic transition temperature.
With reference to
In an embodiment, a photochemical etch process may be used to cut the desired pattern into the Nitinol sheet. This process can include various process steps that are generally known as photolithography. A photoresist layer is deposited onto the Nitinol sheet and exposing the photosensitive layer to a pattern of light that matches the desired pattern that the sheet is to be cut into. The light chemically alters the exposed areas of the photoresist layer and a chemical reaction is used to remove the portions of the photosensitive layer that were not exposed to light. An etch process then cuts through the areas of the Nitinol that are not covered by the photoresist to form the patterned tissue repair matrix. The remaining photoresist is removed to produce the finished patterned tissue repair matrix.
In an embodiment, the tissue repair matrix may be made from a plurality of adjacent elongated planar strips 52(a)-52(d) that are secured adjacent to each other across the length of the tissue repair matrix 50 by a plurality of bridges 70. Although four adjacent elongated strips are shown, tissue repair matrixes with any number of elongated strips can be made. The elongated strips 52 each include a plurality of longitudinal struts 60 and a plurality of loops 62 that connect the adjacent struts 60. The adjacent struts 60 are connected with loops 62 in an alternating pattern at opposite ends of the struts 62, forming a serpentine or “S” shaped pattern. In the compressed state, the loops 62 are substantially semi-circular and appear to be about a 180° bend. The space between the struts 60 is very small because the 180° bends of the loops 62 cause the struts 60 to be compressed close to each other.
The tissue repair matrix may also includes a plurality of amorphic circles 91 that are attached along the outer edges of the tissue repair matrix 50. Along the short sides of the tissue repair matrix 50, the amorphic circles 91 are attached to the loops 62. Along the long sides of the tissue repair matrix 50, the amorphic circles 91 are attached at a terminal point along the longitudinal length of the outermost strut in the planar strip 52(a)-52(d). The rounded surfaces of the amorphic circles 91 and loops 62 eliminate any sharp external features and make the tissue repair matrix 50 a-traumatic when implanted into a patient. The amorphic circles 91 are round features that may also have a smooth rounded edge, such as a “bull-nose” edge to further remove any sharp surfaces. The amorphic circles 91 will preferably have a rounded ring shape rather than a cylindrical shape with sharp edges. Because the repair matrix is very thin, this edge rounding may not be noticeable or necessary.
Holes 93 in the amorphic circles 91 provide areas for ingrowth to stabilize the tissue repair matrix 50 implanted into a patient. Alternatively, the holes 93 in the amorphic circles 91 may also be used to suture the tissue repair matrix 50 to tissue within the patient. The tissue repair matrix may be used as a physical graft structure or to provide physical support to organs within a patient. The amorphic circles 91 can range in diameters from about 0.001 to about 0.250 inch. The holes 93 are concentric with the amorphic circles 91 and may be proportional in diameter. The diameters of the holes 93 may range from about 10% to about 90% of the diameter of the amorphic circle 91. Although the amorphic circles 91 and holes 93 are illustrated only around the perimeter, the amorphic circles 91 can also be attached to any interior loop 62 of the tissue repair matrix 51 as shown in
In alternative embodiments, some of the amorphic circles 91 may be replaced with rounded structures that provide the same a-traumatic edges to the tissue repair matrix 50 as the amorphic circles 91. These rounded structures may be spheres, ovals, rounded rectangles, rounded triangles, a rounded “T” end, etc. Like the amorphic circles 91, these rounded structures can be attached to the loops 62 anywhere in the tissue repair matrix 50, within any of the bridges 70 or at the struts 60 at the ends of the elongated strips 52(a)-52(d). The rounded structures may also have holes formed through their centers for ingrowth or sutures.
In another embodiment, the repair matrix may include various mechanisms such as barbs and holes that are used to improve the adhesion of the repair matrix to the patient. The barbs and holes may be placed on any portion of the inventive repair matrix including the struts, loops, bridges and amorphic circles. With reference to
In the preferred embodiment, the hook 95 is cut during the fabrication of the repair matrix and heat treated as described above so that it assumes the bent shape in the expanded austenitic phase. As discussed above, the repair matrix is cooled to a martensitic phase and compressed prior to implantation in the patient. In the compressed state, the hook 95 is deformed to be flush the hole 97. After the repair matrix is inserted into the patient and the phase changes to the austentic phase, the repair matrix assumes the expanded state and the hooks 95 bend away from the holes 97 to improve the adhesion to internal organs.
Although the bridges 70 appear to be straight structures connected to loops 62 on adjacent planar strips at an angle as shown in
The geometry of the struts is also designed to better distribute strain throughout the tissue repair matrix and minimize the opening size between the struts, loops and bridges. The number of struts, loops and bridges as well as the design of these components are important factors when determining the working properties and fatigue life properties of the tissue repair matrix. A tissue repair matrix that has a larger quantity of smaller sized struts per elongated strip improves the mechanical properties of the tissue repair matrix by providing greater rigidity than sheets made with fewer and larger struts. For example, a tissue repair matrix where the ratio of the number of struts per elongated strip to the strut length L (in inches) that is greater than 400 has increased rigidity.
After the tissue repair matrix is cut to the desired pattern, surface processing can be performed. The tissue repair matrix may be passivated by exposing the Nitinol to oxygen to form a layer of metal oxide which helps to prevent corrosion. The tissue repair matrix may also be polished to remove any rough surfaces through processes such as: mechanical polishing, electro polishing or chemical mechanical polishing. This polishing removes any sharp surfaces that may have been formed during the tissue repair matrix cutting processes.
Alternatively, the tissue repair matrix may be textured to improve the ingrowth after implantation or improve the adhesion of coatings applied to the tissue repair matrix. The texturing can be through photochemical etching, sand blasting, tumbling, etc. These textured surfaces can then be coated with different materials that will improve the implanted performance. These chemical coatings are generally intended to improve the biocompatibility of the tissue repair matrix within the patient's body by enhancing ingrowth, preventing rejection and resisting infection. These surface coatings include polymers, therapeutic agents and bioactive materials.
In an embodiment, some of the tissue repair matrix may also be coated with a radio-opaque material that is detectable with x-rays. The radio-opaque materials may alternatively be attached to the Nitinol tissue repair matrix by laser welding, adhesives, mechanical fasteners, etc. After the tissue repair matrix has been implanted within the patient, the implant area can be x-rayed to determine the exact position of the tissue repair matrix. If the tissue repair matrix is improperly positioned, the error can be detected and corrected.
After the tissue repair matrix 50 is cut and all surface coatings are applied, it is ready for use. The tissue repair matrix 50 is cooled below the martensitic transformation temperature to change the Nitinol to a super elastic material. The martensitic transformation temperature Mf may be between about 0° to about 15° C. In the martensitic phase, the interconnected struts 60, loops 62 and bridges 70 of the tissue repair matrix 50 can be compressed into a small area as shown in
Although the tissue repair matrix 50 is shown in
To implant the tissue repair matrix into a patient, the compressed repair matrix is held by a delivery apparatus and is inserted through a small incision cut through the skin of the patient. The repair matrix is then fully expanded before being permanently or temporarily implanted in the patient. The expansion of the tissue repair matrix inside the patient results from a molecular transformation of the metal alloy from the martensitic phase to the austenitic phase which results from the increased temperature inside the patient's body. The patient's body heat converts the phase of the Nitinol material into the austenitic phase. As the molecular structure of the metal alloy changes to the austenitic phase, the tissue repair matrix decompresses into its expanded shape.
With reference to
After being fully expanded inside the patient, the tissue repair matrix 51 is positioned and secured in the patient using other medical instruments. The tissue repair matrix 51 may be attached within the patient's body by ingrowth through the holes 93 in the amorphic circles 91 and the gaps G between the struts 60, loops 62 and bridges 70. Alternatively, sutures may be sewn through the holes 93 to secure the tissue repair matrix 51 in place. After the tissue repair matrix 51 is implanted, all surgical tools are removed so the patient can heal.
As seen from
In contrast, when stress is applied to a specimen of a metal such as Nitinol exhibiting super elastic characteristics at a temperature above which the austenite is stable (i.e. the temperature at which the transformation of martensite phase to the austenite phase is complete), the specimen deforms elastically until it reaches a particular stress level where the alloy then undergoes a stress-induced phase transformation from the austenite phase to the martensite phase. As the phase transformation proceeds, the alloy undergoes significant increases in strain but with little or no corresponding increases in stress. The strain increases while the stress remains essentially constant until the transformation of the austenite phase to the martensite phase is complete. Thereafter, further increases in stress are necessary to cause further deformation. The martensitic metal first deforms elastically upon the application of additional stress and then plastically with permanent residual deformation.
If the load on the specimen is removed before any permanent deformation has occurred, the martensitic phase specimen will elastically recover and transform back to the austenite phase. The reduction in stress first causes a decrease in strain. As stress reduction reaches the level at which the martensite phase transforms back into the austenite phase, the stress level in the specimen will remain essentially constant (but substantially less than the constant stress level at which the austenite transforms to the martensite) until the transformation back to the austenite phase is complete, i.e. there is significant recovery in strain with only negligible corresponding stress reduction. The alloys are structurally stronger and more rigid in the austenitic phase than the martensitic phase. After the transformation back to austenite is complete, further stress reduction results in elastic strain reduction. This ability to incur significant strain at relatively constant stress upon the application of a load and to recover from the deformation upon the removal of the load is commonly referred to as super elasticity or “shape memory.” See for example, U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamoto et al.).
The transition between martensite and austenite phases can be controlled by the material temperature. The shape material is fully martensitic when it is colder than the final martensitic transition temperature Mf and fully austenitic when the material is heated above the final austenitic transition temperature Af. The alloy may be partially martensitic and partially austenitic at temperatures between the final martensitic transition temperature Mf and the final austenitic transition temperature Af. These shape memory alloys are stronger in the full austenitic phase than in the martensitic state, but no longer have the super elastic property. When a shape memory alloy structure is heated, it reverts, or attempts to revert, to its original heat-stable shape.
The super elastic metal alloys may comprise nickel, titanium and additional elements such as: niobium, hafnium, tantalum, tungsten and gold. The ratio of the nickel and titanium in the super elastic alloy will alter the martensite/austenite transition temperatures. An alloy having more than 50.5 atomic % nickel has a complete transition temperature from the martensite phase to the austenite phase (Af) below human body temperature, so that austenite is the only stable phase at body temperature. The alloy preferably has an Af in the range from about 24° C. to about 37° C. The Mf is about 25 to 50 degrees C. lower than the Af.
Because these super elastic alloys are capable of extreme deformation, it is desirable to design products that will not exceed the maximum allowable strain during use. In trying to minimize the maximum strain experienced by the struts, loops and bridges, the present invention utilizes a structural geometry that distributes strain to areas of the tissue repair matrix which are less susceptible to failure. For example with reference to
It is also desirable to minimize local strain concentrations on the bridge 70 and bridge connection points 72, 74. This can be accomplished at the outset through efficient utilization of materials in the struts 60, loops 62 and bridges 70 increases the strength and the ability of the inventive tissue repair matrix 50 to provide structural support. These strain concentrations can also be minimized by utilizing the largest possible curvature radii in the bridges 70 while maintaining feature widths that are proportional to the applied forces. Another way to minimize the strain concentrations is to minimize the maximum open area between the struts 60, loops 62 and bridges 70 in the tissue repair matrix in the expanded state.
These design characteristics are illustrated in
Nitinol can withstand extremely large amounts of elastic strain deformation, so the above features are well suited to a tissue repair matrix made from this alloy. This feature allows for maximum utilization of Nitinol or other material capabilities to enhance radial strength, improve tissue repair matrix strength uniformity, improves fatigue life by minimizing local strain levels and improves tissue repair matrix apposition in irregular organ wall shapes and curves.
Another design feature that improves the uniform expansion of the tissue repair matrix is the angle of the bridges that connect the adjacent elongated sections of the inventive tissue repair matrix. As the tissue repair matrix is transformed from its compressed state to its expanded state, strains are applied to the struts and loops. The forces of the expanding struts and loops are delivered to the bridge ends and alter the angle of the bridges with respect to the loops to which they are connected. As shown in
In an alternative embodiment illustrated in
In addition to changing the direction of expansion, the alignment of the elongated strips will also influence the mechanical properties of the repair matrix in the expanded state. With reference to
According to the description herein, the inventive tissue repair matrix can be altered for many different implantation applications by changing the lengths and number of elongated sections. The inventive tissue repair matrix can be built for very specific applications including: lung repair, pleurodesis, hernia repair, skin grafting and other organ repair applications.
Although particular embodiments of the present invention have been shown and described, modification may be made to the device and/or method without departing from the spirit and scope of the present invention. The terms used in describing the invention are used in their descriptive sense and not as terms of limitations.
