Patent Application
20090018644
This invention relates to endoprostheses, and more particularly to stents.
The body includes various passageways such as arteries, other blood vessels, and other body lumens. These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is typically a tubular member that is placed in a lumen in the body. Examples of endoprostheses include stents, covered stents, and stent-grafts.
Endoprostheses can be delivered inside the body by a catheter that supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen.
The expansion mechanism can include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon-expandable endoprosthesis. The balloon can be inflated to deform and to fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn.
In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded, e.g., elastically or through a material phase transition. During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.
In one aspect, an endoprosthesis is disclosed that can include a member capable of supporting a body passageway. The member can have a surface region overlying a bulk region, and can include a shape memory alloy and boron. The concentration of boron in the surface region can be greater than the concentration of boron in the bulk region.
In some embodiments, the shape memory alloy can include titanium, niobium, or a combination thereof. In some embodiments, the shape memory alloy can include a Ni—Ti alloy.
In some embodiments, the surface region can include boride intermetallic phases. For example, the surface region can include titanium boride phases, niobium boride phases, or a combination thereof. In some embodiments, the surface region can be at least 1 nanometer thick and/or no more than 5 microns thick. In some embodiments, the surface region can have a concentration at least 1 weight percent boron and a thickness of between 1 nanometer and 3 microns. The surface region can also have a surface region concentration of up to 30 weight percent boron and a thickness of between 1 nanometer and 3 microns. The concentration of boron in the member can decrease in the thickness direction from the surface region to the bulk region.
In some embodiments, the endoprosthesis can also include a carbon layer overlying layer the surface region, carbides embedded within the surface region, or a combination thereof. For example, titanium carbide can be embedded within the surface region.
In some embodiments, the member capable of supporting a body passageway can be a stent. For example, the stent can be a self-expanding stent and/or a superficial femoral artery stent. In some embodiments, the member can further include portions comprising the shape memory alloy but lacking boron.
In another aspect, a method of producing an endoprosthesis is disclosed that includes heat setting a shape member alloy endoprosthesis into a predetermined expanded state and implanting boron ions into the surface of the endoprosthesis. In some embodiments, the boron ions can be implanted by plasma ion immersion implantation.
In some embodiments, the boron ions can be selectively implanted into the surface of the heat set endoprosthesis to produce portions that include a surface comprising the shape memory alloy and boron, and portions that include a surface comprising the shape memory alloy but lacking boron.
In some embodiments, the method can further include incorporating carbon into or onto the surface of the heat set endoprosthesis.
Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
Referring to
Stent 10 can include a shape memory alloy. The shape memory alloy can include titanium, niobium, or a combination thereof. For example, the shape memory alloy can be a Ni—Ti alloy, such as nitinol. Other examples of suitable shape memory alloys can include Ti—Pd alloys, Ti—Pd—Ni alloys, Ni—Ti—Cu alloys, Ti—Nb—Al alloys, Hf—Ti—Ni alloys, Ti—Nb, and Ni—Zr—Ti alloys.
Stent 10 can include a surface region and an underlying bulk region. Both regions can include the shape memory alloy. In addition, the concentration of boron can be greater in the surface region than in the bulk region, thereby creating a boron enriched surface. Boron or boron ions can be implanted using a number of techniques, some of which are discussed below. The Boron can be implanted in the surface to achieve a surface concentration of boron of up to 30% atm. The concentration of boron in the overall stent mass can depend on the thickness of the stent. The implantation dose of Boron can be about 4.8×10(17) boron/cm to achieve a 30% atm boron in the surface. The stent can have an overall boron concentration of between 0.005 and 0.5 weight percent boron (e.g., between 0.01 and 0.1 weight percent boron). The boron enriched surface region can be at least 1 nanometer thick and/or no more than 5 microns thick (e.g., between 1 nanometer and 3 microns). In some embodiments, a surface region having a thickness between 1 nanometer and 3 microns can have a boron concentration of between 1 weight percent boron and 30 weight percent boron. The boron concentration can decrease in the thickness direction of the member from the boron enriched surface region to the bulk region. The depth of the boron penetration in the thickness direction of the member can be selected to ensure that the stent retains sufficient shape memory properties to enable it to perform the role of supporting a body passageway. In some embodiments, the addition of the boron will not alter the modulus of elasticity and/or the ultimate tensile strength of the member by more than five percent.
Implanted boron or boron ions can combine with the shape memory alloy of the stent to form boride intermetallic phases (e.g., TiB or NbB). The boride intermetallic phases can impart increased surface hardness and fatigue resistance to the stent. For example, implanting boron or boron ions into the surface of a Ni—Ti alloy stent can result in the formation of titanium boride intermetallic phases along the surface. By implanting boron or boron ions into the surface of other shape memory alloys, other boride intermetallic phases can be formed.
By selectively enriching the surface of stent 30 or stent 40 with boron, the cracking of the stent can occur in predetermined areas and in a predetermined pattern. For example, the non-boron enriched stent portions can be arranged to transition the stent from a closed stent structure to a set of separate rings or to a spiral shape. The cracking of predetermined stent portions can relieve strain in other areas of stent 30 or stent 40.
Boron can be implanted into the surface of the shape memory alloy of the stent by a variety of methods, including plasma ion immersion implantation (PIII) or beamline ion implantation of boron ions into the surface of the stent.
In some embodiments, the stent can be heat set into a desired expanded state and boron ions can be implanted into the surface of the stent. Shape setting can be achieved over a wide temperature range from 300 C to 900 C. For example, heat treating temperatures for a binary NiTi alloys are sometimes chosen in the narrower range of 325 to 525 C to achieve a combination of physical and mechanical properties (e.g., 510 C). For example, a binary NiTi alloy can be heat treated at 510 C for a time of between 5 minutes to 30 minutes.
In some embodiments, the boron ion implantation can be preformed at a temperature up to about 250 C (e.g., between room temperature and 200 C). For example, the implantation process can be a room temperature process that does not alter the heat set of the stent. In some embodiments, a heated boron ion implantation process can be limited to no more than about 2 hours.
Any of the stents illustrated in
Stents 10, 20, 30, or 40 can be of any desired shape and size (e.g., superficial femoral artery stents, coronary stents, aortic stents, peripheral vascular stents, gastrointestinal stents, urology stents, and neurology stents). Depending on the application, the stent can have a diameter of between, for example, 1 mm to 46 mm. In certain embodiments, a coronary stent can have an expanded diameter of from 2 mm to 6 mm. In some embodiments, a peripheral stent can have an expanded diameter of from 5 mm to 24 mm. In certain embodiments, a gastrointestinal and/or urology stent can have an expanded diameter of from 6 mm to about 30 mm. In some embodiments, a neurology stent can have an expanded diameter of from about 1 mm to about 12 mm. An abdominal aortic aneurysm (AAA) stent and a thoracic aortic aneurysm (TAA) stent can have a diameter from about 20 mm to about 46 mm.
In some embodiments, the stent 10, 20, 30, or 40 can be a superficial femoral artery stent. Superficial femoral artery stents can be subject to repeated large strains during use, for example due to the bending of a knee. The strain resistance of a superficial femoral artery stent can be improved by implanting boron into the surface of a shape memory alloy making up the superficial femoral artery stent.
In use, a stent can be used, e.g., delivered and expanded, using a catheter delivery system. Catheter systems are described in, for example, Wang U.S. Pat. No. 5,195,969, Hamlin U.S. Pat. No. 5,270,086, and Raeder-Devens, U.S. Pat. No. 6,726,712. Stents and stent delivery are also exemplified by the Sentinol® system, available from Boston Scientific Scimed, Maple Grove, Minn.
In some embodiments, stents can also be a part of a covered stent or a stent-graft. In other embodiments, a stent can include and/or be attached to a biocompatible, non-porous or semi-porous polymer matrix made of polytetrafluoroethylene (PTFE), expanded PTFE, polyethylene, urethane, or polypropylene.
In some embodiments, stents can also include a releasable therapeutic agent, drug, or a pharmaceutically active compound, such as described in U.S. Pat. No. 5,674,242, U.S. Ser. No. 09/895,415, filed Jul. 2, 2001, and U.S. Ser. No. 10/232,265, filed Aug. 30, 2002. The therapeutic agents, drugs, or pharmaceutically active compounds can include, for example, anti-thrombogenic agents, antioxidants, anti-inflammatory agents, anesthetic agents, anti-coagulants, and antibiotics.
In some embodiments, stents can be formed by fabricating a wire including a boride enhanced surface, and knitting and/or weaving the wire into a tubular member.
All publications, references, applications, and patents referred to herein are incorporated by reference in their entirety.
Other embodiments are within the claims.
