GRADUALLY EXPANDING STENT

FIELD OF THE INVENTION

[0001] The present invention relates to stents. More particularly, the present invention relates to a gradually expanding stent.

BACKGROUND OF THE INVENTION

[0002] Stents are tubular shaped devices that are often placed, deployed, or implanted within passages, conduits, or lumens in the body as a treatment for various conditions in the lumens. For example, a stent may be implanted in a blood vessel for treatment of a stenosis, occlusion, stricture, or aneurysm in the vessel. Other lumens within which stents may be implanted include, for example, various sections of the urinary tract and bile ducts. The implanted stent may serve to reopen the lumen, or to reinforce or maintain the open state of the lumen. [0003] Typically, a stent is designed with two states, with a different stent diameter associated with each state. For example, a stent may be inserted into a lumen while the stent is in an unexpanded, or contracted, state. In the contracted state, the diameter of the stent may be small enough to enable insertion via a catheter or other means into the lumen. Once the stent has been inserted into the lumen, the diameter of the stent may be enlarged so that the stent is in an expanded, opened state. In the expanded state, the diameter of the stent may be sufficiently large so that the stent may provide a desired function, such as increasing the size of an opening in the lumen and maintaining the opening.

[0004] Several methods for expanding the diameter of a stent are known in the art. Some stents require use of a separate expanding device in order to mechanically apply a radially outward force on the walls of the stent so as to expand the stent to its expanded state. For example, the walls of the stent may include a collapsible and expandable wire mesh or coil. An expanding device, such as an inflatable balloon, may be initially inserted into the cavity of the stent in its contracted state. Pumping air or another fluid into the balloon may cause the balloon to expand. Expanding the balloon may then apply a radially outward force on the stent, thus enlarging the diameter of the stent. Such a stent is described by, for example, Palmaz in US 4,733,665.

[0005] Other stents are designed to be self-expanding, without the need for a mechanical expanding device. For example, a stent may be constructed with elastic or resilient elements that tend to expand the stent. In order to collapse the stent and maintain the stent in a contracted state, a compressing force is applied. Removing the compressing force, for example, by removing a sheath that surrounds the stent, may enable a resilient element to expand the stent. Another possibility is to employ shape memory materials in constructing the stent. A shape memory material may be deformed. However, once a triggering stimulus is applied to the material, for example, heat or a magnetic field, the material reverts to its original shape. For example, Dotter in US 4,503,569 describes a stent that is shaped in the form of a helically wound coil and is constructed out of a shape memory alloy. The stent is initially fashioned in an expanded state. Prior to implantation, the coil is then wound more tightly in order to reduce its diameter. After implantation, the temperature of the stent is raised to a transition temperature of the shape memory alloy. For example, a heated fluid may be injected into a blood vessel into which the stent was implanted. Upon heating to the transition temperature, the shape memory alloy coil reverts to its original shape, with a larger diameter.

[0006] The stents described above may expand from their contracted states to their expanded states in a relatively short period of time, for example within a few seconds. Rapid expansion of a stent within a lumen such as a blood vessel may cause trauma to the lumen or to tissue surrounding the lumen. For example, smooth muscle cells lining a blood vessel in which a stent is rapidly expanding may tend to resist an abrupt enlargement of the blood vessel. Inability of the walls of the blood vessel to rapidly acclimatize to the rapidly expanding diameter of the vessel may lead to, or aggravate, damage to the wall of the vessel. Such damage may lead to, or accelerate, re-occlusion or restenosis of the blood vessel. Restenosis of the vessel may require performance of an additional invasive intervention in the treated area within a relatively short period of time.

[0007] It is an object of the present invention to provide a stent that is capable of expanding at such a rate so as not to contribute to damage to the wall of a lumen in which the stent is implanted.

[0008] Other aims and advantages of the present invention will become apparent after reading the present invention and reviewing the accompanj'ing drawings.

SUMMARY OF THE INVENTION

[0009] There is thus provided, in accordance with some embodiments of the present invention, an expandable stent that includes a generally tubular structure made of a plurality of separately expandable elements, the elements arranged such that expansion of an element of the separately expandable elements incrementally expands a transverse dimension of the stent.

[0010] Furthermore, in accordance with some embodiments of the present invention, an element of the separately expandable elements includes a resilient member and a biodegradable restrainer for restraining the resilient member in a bent state.

[0011] Furthermore, in accordance with some embodiments of the present invention, the biodegradable restrainer includes polymers of lactones.

[0012] Furthermore, in accordance with some embodiments of the present invention, an element of the separately expandable elements comprises shape memory alloy. [0013] Furthermore, in accordance with some embodiments of the present invention, the shape memory alloy is temperature activated.

[0014] Furthermore, in accordance with some embodiments of the present invention, the shape memory alloy is magnetically activated.

[0015] Furthermore, in accordance with some embodiments of the present invention, an element of the separately expandable elements includes piezoelectric actuators.

[0016] Furthermore, in accordance with some embodiments of the present invention, an element of the separately expandable elements includes magnetic actuators.

[0017] Furthermore, in accordance with some embodiments of the present invention, the stent is drug eluting. [0018] There is further provided, in accordance with some embodiments of the present invention, a method for enlarging a lumen. The method includes providing an expandable stent with a generally tubular structure made of a plurality of separately expandable elements, the elements arranged such that expansion of an element of the separately expandable elements incrementally expands a transverse dimension of the stent. The method further includes inserting the expandable stent in a contracted state into the lumen and delivering the expandable stent to a target location. The method

further includes separately causing the elements to incrementally expand a transverse dimension of the stent.

[0019] Furthermore, in accordance with some embodiments of the present invention, an element of the separately expandable elements includes a resilient member and a biodegradable restrainer for restraining the resilient member in a bent state.

[0020] Furthermore, in accordance with some embodiments of the present invention, the biodegradable restrainer comprises polymers of lactones.

[0021] Furthermore, in accordance with some embodiments of the present invention, an element of the separately expandable elements comprises shape memory alloy. [0022] Furthermore, in accordance with some embodiments of the present invention, the method further includes activating the shape memory alloy by heating the element.

[0023] Furthermore, in accordance with some embodiments of the present invention, the method further includes magnetically causing the element to expand.

[0024] Furthermore, in accordance with some embodiments of the present invention, the method further includes piezoelectrically causing the element to expand.

[0025] Furthermore, in accordance with some embodiments of the present invention, the stent is drug eluting.

BRIEF DESCRIPTION OF THE DRAWINGS [0026] In order to better understand the present invention, and appreciate its practical applications, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention. Like components are denoted by like reference numerals.

[0027] Fig. 1 is a side view of a stent in a contracted state, in accordance with embodiments of the present invention.

[0028] Fig. 2 shows the stent shown in Fig. 1 after an initial expansion.

[0029] Fig. 3 shows the stent of Fig. 2 after degradation of biodegradable restraining elements.

[0030] Fig. 4 shows the stent of Fig. 3 after folded beams of shape memory alloy have reverted to their unfolded state.

[0031] Fig. 5 illustrates a method for enlarging a lumen in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS [0032] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, modules, and/or units have not been described in detail so as not to obscure the invention.

[0033] A stent in accordance with embodiments of the present invention is designed to expand from a collapsed or contracted state to an expanded state in a gradual manner. The stent includes a generally tubular structure. During expansion of the stent, the diameter or other transverse dimension of the stent is increased. The stent, while in a contracted state, may be inserted into a lumen of a patient's body, for example, the lumen of a blood vessel. For example, a contracted stent may be inserted into the lumen using a catheter.

[0034] After insertion, the stent may be enlarged or expanded in stages to an expanded state. For example, expanding the stent within a lumen may restore an open pathway in a lumen of a body conduit, such as a blood vessel, in which stenosis or occlusion has occurred. When remaining in the lumen, the stent may assist in maintaining the opening of the lumen.

[0035] An initial expansion of the stent may be performed to enlarge the stent to a partially expanded state. For example, the initial expansion may be performed soon after insertion of the stent into a lumen in order to anchor the stent in place. For the purpose of the initial expansion, the stent may be provided with standard plastic or resilient elements. For example, for a stent provided with plastic elements, the stent may be partially expanded by plastically deforming the elements using a conventional angioplasty balloon system. If, for example, the stent is provided with flexed resilient elements, the initial expansion may be performed by removing a restraining sheath from around the stent.

[0036] Once the stent is inserted at a desired location, for example, in a lumen, and after partial expansion of the stent, the stent may be gradually enlarged to an expanded state of the stent. In addition, the stent may gradually release into the lumen a drug or other substance that had been adsorbed or otherwise incorporated into the stent, in a manner similar to drug eluting stents known in the art.

[0037] A stent in accordance with embodiments of the present invention is provided with structural elements that enable gradual expansion of the stent. Each of the elements may be expanded individually, separately and independently of the other elements. Separate expansion of an individual element may cause incremental expansion of the stent.

[0038] For example, the stent may be provided with elements that interact with an expected environment within a lumen so as to enable expansion of the stent. For example, the stent may be provided with one or more resilient structural elements. Each resilient element may be held in a compressed state by a restraining device constructed using a biodegradable material. Exposure to an environment inside a lumen may cause the biodegradable restraining device to fully or partially dissolve or degrade. When the biodegradable restraining device degrades, the previously restrained resilient element may expand, thus further incrementally enlarging the size of the stent. Several such sets of resilient elements and biodegradable restraining devices may be provided. Each restraining element may be designed to degrade at a rate that may vary from element to element. In this manner, each restrained resilient element may expand separately, thus enlarging the stent gradually.

[0039] Alternatively or in addition, the expansion may take place in response to exposure of a periodically applied trigger or stimulus. An applied stimulus may include, for example, heat, electromagnetic fields or radiation, or ultrasonic waves. The applied stimulus may interact with one or more components of the stent to cause controlled expansion of that component. Expansion of the component may enlarge the stent. The interacting components may include, for example, components made of shape memory alloy, magnetic actuators, or piezoelectric actuators. The application of the stimulus may be controlled so as to enlarge the stent at a sufficiently gradual rate so as to avoid or minimize damage to the wall of the lumen. For example, a period of time over with the expansion may take place may range from several hours to several months.

[0040] Gradual expansion of the stent may reduce or prevent trauma to the walls of a lumen inside which the stent was implanted. Gradual expansion of the stent, causing gradual enlargement of the lumen, may allow the walls of the lumen to gradually acclimatize to its slowly expanding diameter. Such gradual acclimatization may result in a reduced rate of re-occlusion or restenosis of the lumen.

[0041] Reference is now made to the accompanying Figures.

[0042] Fig. 1 is a side view of a stent in a contracted state, in accordance with embodiments of the present invention. Stent 10 may represent an entire stent or a longitudinal section of a longer stent. Stent 10 includes one or more elastic assemblies 12. In addition to elastic assemblies 12, stent 10 may optionally include one or more structural members 14 that may retain an approximately fixed shape during expansion of stent 10. Fixed structural members 14 and elastic assemblies 12 are arranged about a hollow central cavity 16 surrounding, and possibly coaxial with, stent longitudinal axis 15. Central cavity 16 may be approximately cylindrical in shape, with a circular, elliptical, or other suitably shaped cross section. Central cavity 16 may be bent with a longitudinal curvature. As stent 10 expands from a contracted state to an expanded state, the diameter, or other suitable transverse dimension specifying the width of central cavity 16, is increased.

[0043] Each elastic assembly 12 may include one or more plastic or resilient deformable elements 18. Plastic and resilient elements of stents are known in the art. A plastic element may be deformed from one shape to another when a suitable force is applied. For example, a standard angioplasty balloon may be inflated inside central cavity 16 of stent 10. Inflating the balloon inside central cavity 16 may apply a radial outward force on elastic assembly 12. The outward force applied to elastic assembly 12 may cause a plastic deformable element of elastic assembly 12 to alter its shape from a collapsed or folded configuration to an opened or unfolded configuration. Altering the shape of the plastic deformable element may cause stent 10 to partially expand.

[0044] A resilient deformable element in elastic assembly 12 may be in a compressed, bent, or folded stressed state when stent 10 is in its contracted state. Thus, removing a compressing restraint may enable the bent resilient deformable element to revert to an unstressed state. When reverting to its unstressed state, the bent or folded elements may expand, partially expanding stent 10. For example, stent 10 may be initially inserted into

a lumen while sheathed in a sleeve or sheath. The sheath may apply an inward force on stent 10 that resists the tendency of a resilient element to expand stent 10. After insertion of stent 10 into a lumen, the sheath may be withdrawn from around stent 19. Withdrawing the sheath may enable the resilient deformable element to revert to an unstressed state. Thus, withdrawing the sheath may enable stent 10 to partially expand.

[0045] Fig. 2 shows the stent shown in Fig. 1 after an initial expansion. Deformable elements 18, which were in a folded state when stent 10 was in a contracted state (Fig. 1), are partially unfolded. For example, one or more of deformable elements 18 may include a plastic element that is unfolded during initial expansion by means of application of an outward force, such as by inflating a balloon in central cavity 16. Alternatively or in addition, one or more of deformable elements 18 may include a resilient element that reverts to an unfolded state during initial expansion when a restraining element, such as a sheath, was removed from around stent 10.

[0046] After deformable elements 18 are partially unfolded, gradual expansion elements 20 remain folded. One or more of gradual expansion elements 20 may be designed to expand sequentially at a time after the initial expansion.

[0047] For example, in an embodiment of the present invention, one or more of gradual expansion elements 20 may include a resilient element that is restrained from expanding by means of a restraining or encapsulating element, or restrainer, such as a thread, film, tape, sheath, or adhesive layer. The restrainer is at partially constructed of a biodegradable material. The material may preferably be biocompatible. The biodegradable material is designed to dissolve, become absorbed, or otherwise degrade under long-term exposure to suitable conditions. Such conditions may include the conditions commonly found within a lumen within which stent 10 is implanted. Such materials are described by, for example, Pitt et al. in US 4,379,138 and Buscemi et al. in US 5,551,954, and include, for example, polymers of lactones. Physical and chemical properties of the restrainer, such as, for example, mechanical dimensions and degree of polymerization, may be varied. Varying physical and chemical properties of the restrainer may influence the rate of degradation in a predictable manner. Thus, by varying the rate of degradation among gradual expansion elements 20, each gradual expansion element 20 may be released by its restrainer at a different time. Sequential release and expansion of gradual expansion elements 20 may result in gradual expansion of stent 10.

[0048] Fig. 3 shows the stent of Fig. 2 after degradation of biodegradable restraining elements. Gradual expansion elements 20 have been expanded.

[0049] In addition to resilient elements restrained by biodegradable restrainers, or as an alternative to restrained resilient elements, one or more of gradual expansion elements 20, for example those labeled 20a, may be at least partially constructed of shape memory alloy. The shape memory alloy may preferably be biocompatible. An example of a biocompatible shape memory alloy is nickel-titanium alloy. For example, each shape memory element 20a may have been folded when its temperature was below a transition temperature of the shape memory alloy. Activating a shape memory element 20a by heating it to its transition temperature may restore shape memory element 20a to its unfolded, expanded state. Selective activation of individual shape memory elements 20a may cause selective expansion of the heated shape memory elements 20a. Selective separate expansion of individual shape memory elements 20a may cause stent 10 to expand gradually at a controlled rate. For example, an individual shape memory element 20a may be heated by directing energy, such as focused electromagnetic radiation or ultrasound waves, on a single shape memory element 20a. Fig. 4 shows the stent of Fig. 3 after folded beams of shape memory alloy have reverted to their unfolded state. As shown in Fig. 4, shape memory element 20a has reverted to a straightened shape that does not include any bends or folds. Such a straightened shape may provide additional structural strength to stent 10. For example, additional structural strength provided by straightened shape memory element 20a may resist inward pressure exerted by walls of a lumen in which stent 10 is implanted.

[0050] Alternatively or in addition, shape memory elements 20a may include a magnetically activated material such as a shape memory alloy that reverts to its original shape when exposed to a magnetic field. Selective application of a suitable magnetic field to a magnetically activated shape memory element 20a may expand the element, causing controlled expansion of stent 10. Alternatively or in addition, expansion elements may include piezoelectric actuators that move in response to an introduced or induced electric current. Selective induction or introduction of an appropriate electric current in the actuator may cause controlled expansion of the stent.

[0051] Complete expansion of all gradual expansion elements in the stent may expand the stent to its fully expanded state. For example, a stent intended for implantation in a

blood vessel may expand from a diameter of about 1.6 mm in its contracted state, to a diameter of about 5 mm in its fully expanded state.

[0052] Fig. 5 illustrates a method for enlarging a lumen in accordance with embodiments of the present invention. A lumen enlargement method in accordance with embodiments of the present invention includes implanting a gradually expandable stent into the lumen and gradually expanding the stent. A gradually expandable stent may be inserted into a lumen, such as a blood vessel (step 30). For example, an end of a catheter may be inserted into the lumen, and the stent may be inserted into the lumen via the catheter. The stent may then be guided to a target location within the lumen (step 32). Initial expansion of the stent may then be performed, in a manner depending on the structure of the stent (step 34). For example, a balloon may be inflated within the stent in order to perform the initial expansion, or a restraining element such as a sheath may be removed from the stent, enabling an elastic restoring force to initially expand the stent. The stent may then remain within the lumen for an extended period of time. During this time, the stent is expanded gradually. For example, biodegradable restrainers may degrade and enable elastic restoring forces of elastic elements to gradually expand the stent. Alternatively or in addition, a stimulus or other activation process may be applied selectively to individual sections of the stent, separately of other sections, at predetermined times. For example, electromagnetic radiation or ultrasound waves may be focused on a section of the stent to cause the section to enlarge the stent. Selective sequential activation of individual sections of the stent at predetermined times may cause the stent to expand gradually. Gradual expansion of the stent may enlarge the lumen (step 36).

[0053] It should be clear that the description of the embodiments and attached Figures set forth in this specification serves only for a better understanding of the invention, without limiting its scope.

[0054] It should also be clear that a person skilled in the art, after reading the present specification could make adjustments or amendments to the attached Figures and above described embodiments that would still be covered by the present invention.