Bioabsorbable metal medical device and method of manufacture

[0001] The present invention relates generally to cutting, welding, brazing and ablation techniques, and more specifically to techniques that employ a laser/fluid jet to cut, weld, braze or ablate bioabsorbable metal medical devices such as stents.

[0002] Stents and stent delivery devices are employed in a number of medical procedures and as such their structure and function are well known. Stents are used in a wide array of bodily vessels including coronary arteries, renal arteries, peripheral arteries including iliac arteries, arteries of the neck and cerebral arteries as well as in other body structures, including but not limited to arteries, veins, biliary ducts, urethras, fallopian tubes, bronchial tubes, the trachea, the esophagus and the prostate.

[0003] Stents are typically cylindrical, radially expandable prostheses introduced via a catheter assembly into a lumen of a body vessel in a configuration having a generally reduced diameter, i.e. in a crimped or unexpanded state, and are then expanded to the diameter of the vessel. In their expanded state, stents support or reinforce sections of vessel walls, for example a blood vessel, which have collapsed, are partially occluded, blocked, weakened, or dilated, and maintain them in an open unobstructed state. To be effective, the stent should be relatively flexible along its length so as to facilitate delivery through torturous body lumens, and yet stiff and stable enough when radially expanded to maintain the blood vessel or artery open. Such stents may include a plurality of axial bends or crowns adjoined together by a plurality of struts so as to form a plurality of U-shaped members coupled together to form a serpentine pattern.

[0004] There are two types of stents that are presently utilized: permanent stents and bioabsorbable stents. A permanent stent is designed to be maintained in a body lumen for an indeterminate amount of time. Permanent stents are typically designed to provide long-term support for damaged or traumatized wall tissues of the lumen. There are numerous conventional applications for permanent stents including cardiovascular, peripheral, urological, gastrointestinal, and gynecological applications.

[0005] Bioabsorbable stents may advantageously be eliminated from body lumens after a predetermined, clinically appropriate period of time, for example, after the traumatized tissues of the lumen have healed and a stent is no longer needed to maintain the integrity of the lumen. The conventional bioabsorbable materials from which such stents are made are selected to resorb or degrade over time, thereby eliminating the need for subsequent surgical procedures to remove the stent from the body lumen if problems arise.

[0006] One technique that is employed to manufacture stents is laser cutting. Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter and U.S. Pat. No. 5,906,759 to Richter. Laser cutting usually involves the use of a pulsed laser beam and a stent preform such as a tubular preform that is positioned under the laser beam and moved in a precise manner to cut a desired pattern into the preform using a servo motion controlled machine tool. Laser cutting is sometimes performed in an oxygen atmosphere to assist in the process. An example of a conventional laser for cutting a stent is a highly focused pulsed Nd:YAG laser which has a pulse duration in the range of approximately 0.1 to 20 milliseconds. The laser produces a relatively large melt zone and heat affected zone (HAZ) on the metal.

[0007] To reduce the size of the heat affected zone, cutting and processing systems have been developed that incorporate a water column and laser. For example, SYNOVA Inc., of Lausanne, Switzerland, has developed a laser-microjet that uses a laser beam that is contained within a water jet arranged as a parallel beam, similar in principle to an optical fiber. The SYNOVA laser-microjet relies on a low pressure water column to contain the laser, to reduce force applied to the work piece, to act as a cooling mechanism and to remove cutting debris. In U.S. Pat. No. 6,696,666, a laser microjet is employed to manufacture a stent. A second water jet is directed through the tubular preform to deflect the laser beam, thereby preventing damage to the interior wall of the tubular preform.

[0008] In accordance with the present invention, a method and apparatus is provided for processing a medical device formed from a bioabsorbable metallic material. The method begins by generating a beam of radiation onto the bioabsorbable metallic material. The radiation beam is transmitted through a fluid medium and onto a heat affected zone (HAZ) of the bioabsorbable metallic material to thereby cool the HAZ and reduce a concentration of oxygen surrounding the HAZ.

[0009] In accordance with one aspect of the invention, the bioabsorbable material is a magnesium alloy.

[0010] In accordance with another aspect of the invention, the magnesium alloy is selected from the group consisting of lithium-magnesium and sodium-magnesium.

[0011] In accordance with another aspect of the invention, the magnesium alloy includes a rare-earth element.

[0012] In accordance with another aspect of the invention, the magnesium alloy includes yttrium.

[0013] In accordance with another aspect of the invention, a proportion of magnesium in the alloy is greater than about 90%.

[0014] In accordance with another aspect of the invention, the fluid medium comprises a fluid column that is parallel to the radiation beam.

[0015] In accordance with another aspect of the invention, the radiation beam and the fluid column comprise a laser/fluid jet.

[0016] In accordance with another aspect of the invention, the fluid comprises water.

[0017] In accordance with another aspect of the invention, the fluid includes a dissolved gas that displaces dissolved oxygen.

[0018] In accordance with another aspect of the invention, the radiation is applied to cut the material.

[0019] In accordance with another aspect of the invention, the radiation is applied to weld or braze together first and second components of the material.

[0020] In accordance with another aspect of the invention, the radiation provides a surface treatment to the material

[0021] In accordance with another aspect of the invention, the material is a tubular preform.

[0022] In accordance with another aspect of the invention, the medical device is a stent.

[0023] In accordance with another aspect of the invention, the medical device is a filter device.

[0024] In accordance with another aspect of the invention, the radiation beam is generated by a laser source.

[0027] The present invention applies laser processing techniques to fabricate from a bioabsorbable metallic material a wide variety of medical devices including, without limitation, stents, filter devices (e.g. vena cava filters, carotid and cardiac embolic protection filters), neurovascular coils, electrical stimulation leads, and the like. As discussed in detail below, a fluid is applied to the heat affected zone (HAZ) of a magnesium alloy-based material to prevent the magnesium from igniting by cooling the material and displacing oxygen surrounding the HAZ. For purposes of illustration only and not as a limitation on the invention, the present invention will be described in terms of stents formed from a cylindrical metal mesh that can expand when pressure is internally applied. One example of such a stent, described below, is shown in FIG. 1. Of course, the present invention is equally applicable to a wide variety of other types of stents including, without limitation, various balloon-expandable and self-expanding stents, as well as those formed from a sheet or tube into spiral, coil or woven geometries, either open or closed cell.

[0028] The present invention may be employed in a variety of different laser or other electromagnetic radiation processing techniques that includes but is not limited to laser cutting techniques. For example, the invention is applicable to laser welding, and laser brazing techniques in which a laser or other electromagnetic beam is applied to a joint for the purpose of securing one element of a medical device, such as the strut of a stent, for example, to another element of the bioabsorbable medical device such as another strut. The invention is also applicable to laser ablation techniques to provide a surface treatment such as texturing roughening, polishing, and the like or to form a feature on or within any portion of the bioabsorbable medical device.

[0029] Having reference to FIG. 1, there is shown an exemplary bioabsorbable stent 10. The stent generally comprises a plurality of radially expandable cylindrical elements 12 disposed generally coaxially and interconnected by elements 13 disposed between adjacent cylindrical elements 12. The cylindrical elements 12 have an undulating pattern. The particular pattern and number of undulations per unit of length around the circumference of the cylindrical element 12, or the amplitude of the undulations, are chosen to fill particular mechanical requirements for the stent 10 such as radial stiffness.

[0030] Each pair of the interconnecting elements 13 on one side of a cylindrical element 12 can be placed to achieve maximum flexibility for a stent. In this example the stent 10 has three interconnecting elements 13 between adjacent radially expandable cylindrical elements 12, which are 120 degrees apart. Each pair of interconnecting elements 13 on one side of a cylindrical element 12 are offset radially 60 degrees from the pair on the other side of the cylindrical element. The alternation of the interconnecting elements 13 results in a stent that is longitudinally flexible in essentially all directions. Various other configurations for the placement of interconnecting elements 13 are possible. However, the interconnecting elements 13 of an individual stent typically should be secured to either the peaks or valleys of the undulating structural elements 12 in order to prevent shortening of the stent during the expansion thereof. Additional details concerning the particular stent depicted in FIG. 1 as well as variations thereof are shown, for example, in U.S. Pat. No. 5,514,154.

[0031] The bioabsorbable stent 10 is formed of a bioabsorbable metal alloy. Bioabsorbable metal alloys useful for stents include zinc-titanium alloys, and magnesium alloys, such as lithium-magnesium, sodium-magnesium, and magnesium alloys containing rare earth metals. Some examples of bioabsorbable metal alloys are described in U.S. Pat. No. 6,287,332 and U.S. Appl. Serial No. 2004/0220660.

[0032] As discussed in U.S. Appl. Serial No. 2004/0098108, one particular magnesium alloy that may be employed has a magnesium proportion greater than 90%. In addition the magnesium alloy contains yttrium in a proportion of between 4% and 5% and neodymium as a rare earth element in a proportion of between 1.5% and 4%. The remaining constituents of the alloy are less than 1% and are formed for the major part by lithium or zirconium.

[0033] If lithium-magnesium alloys are employed, lithium hydroxide and magnesium hydroxide are to be expected as decomposition products, which can both be considered non-toxic and biocompatible. However, these decomposition products are poorly soluble and, with the absorption of carbon dioxide convert to carbonates, which are also poorly soluble. To overcome this problem, other combinations of magnesium alloys may be employed, such as a sodium-magnesium alloy, for example. Sodium hydroxide possesses a high solubility and the sodium dissolves. The other decomposition product, magnesium hydroxide, forms a fine precipitate that may deposit without risk in the developing vascular skin.

[0034] One problem that arises when laser cutting is used to form a magnesium alloy stent is that the alloy easily oxidizes and combusts readily. This problem is exacerbated since laser cutting is often performed in an oxygen atmosphere. To overcome this problem, the present invention employs a fluid during the cutting process to both cool the cutting area and reduce the oxygen concentration in the cutting zone. The combination of cooling and oxygen isolation can prevent the magnesium from igniting. In some cases the dissolved oxygen content should be less than about 0.5 mg/L.

[0035] FIG. 2 shows a system for processing and/or cutting a stent preform 20 using a laser beam. The stent preform 20 may be in the form of a tube (as shown), a sheet or any other shape of material into which a stent design is cut. The preform 20 is processed or cut by the application of laser energy, indicated by arrow 22. Laser energy 22 is transmitted through a column or stream of water or other solution or fluid referred to hereinafter as a fluid column 23. Fluid column 23 behaves similarly to a fiber optic in that the fluid column 23 allows the laser energy 22 to be focused and transmitted therethrough. The combined stream of laser energy 22 and water or fluid column 23 is collectively referred to herein as a jet 25. As previously mentioned, the use of a hybrid laser/water jet for the purpose of cutting is known and laser/water jet mechanisms are commercially available from SYNOVA Inc., of Lausanne, Switzerland.

[0036] As is shown in FIG. 2, jet 25 is directed to the exterior of the processing side 14 of the tubular preform 20. Jet 25 is supplied by a laser-jet mechanism 26 such as is available from SYNOVA Inc. Mechanism 26 includes a laser, which may be any type of laser suitable for use in processing tubular members. For example, laser energy 22 may be energy that is supplied by a YAG laser, IR laser, UV laser, CO₂ laser, diode laser, etc. or any combination thereof.

[0037] In addition to providing a natural cooling mechanism as well as aiding in drawing debris away from the cutting site, the fluid column 23 creates an envelope around the cutting site to expel oxygen and prevents an oxygen concentration buildup great enough to allow the magnesium from igniting.

[0038] The fluid column 23 may be composed of a variety of substances. For example, fluid column 23 may be primarily water, mineral oil, and the like. While a wide range of fluids may be employed, those that are highly flammable such as gasoline and toluene should be avoided. If water is employed, it should have a relatively low level of dissolved oxygen in it. In this regard it may be helpful to add dissolved CO₂ (or another gas such as an inert gas) into the water so that oxygen is displaced, thereby further preventing ignition.

[0039] In some cases, fluid column 23 may be a stream of liquid having CO₂ or other gaseous bubbles and/or micro-bubbles therein. In yet another example, fluid column may be a solution of liquid having material dissolved therein, such that the resulting solution has some degree of energy reflecting and/or absorbing characteristics.

[0040] Laser energy 22 may be provided, for example, by a Nd:YAG or CO₂ laser operating at a wavelength of, e.g., 1,064 nm and 10,600 nm, respectively. The particular wavelength that is selected should be matched to the fluid so that its transmittance through the fluid is maximized, or is at least sufficiently great to perform the cutting or other processing task. For the fluids mentioned above these wavelengths will generally range from visible to infrared wavelengths (i.e., about 400 nm to 3 microns). The laser source may be an ultra-fast laser operating on a femtosecond or picosecond timescale. Alternatively, a laser operating at a wavelength of about 193 nm or 248 nm or laser diodes such as those operating at wavelengths between about 800 to 1000 nm may be employed. In some embodiments, diode pumped fiber laser may be employed in which the diode provides energy to pump or stimulate a gain element such as a rare-earth element doped in the fiber. The present invention, however, is not limited to laser sources. More generally, any other appropriate source of electromagnetic energy that is capable of cutting or otherwise processing a preform may be employed in the present invention.

[0041] Other operational parameters of the laser jet mechanism 26 may be adjusted as known in the art to yield optimal cutting or other processing results. Generally speaking, with respect to the intensity of the energy, an intensity of greater than about 10⁶ Watts/cm² will be required for cutting while an intensity of greater than about 10³ Watts/cms² will be required for welding.

[0042] In some embodiments of the invention, instead, or in addition to the use of a fluid jet, the preform may be submerged in a fluid bath to achieve the necessary cooling and oxygen deprivation. While in these cases, the use of the fluid jet may not be necessary, it may nevertheless be advantageous to facilitate the removal from the cutting zone of particles or molten material that has been ablated from the preform. If employed, the particular fluid used in the fluid jet may or may not be the same as the fluid used in the fluid bath. For instance, in some cases it may be desirable to use a fluid for the jet that has a higher refractive index than the fluid in the bath so that it acts as a waveguide for the laser beam.