Patent Application
20070209947
This application is related to the application entitled “Method of Descaling Metallic Devices” by inventor Sanjay Shrivastava, filed on the same date herewith.
The present invention relates generally to providing an apparatus and method for electropolishing products made from metals, and in particular, electropolishing metallic medical devices such as stents, made of stainless steel, titanium, tungsten, nickel-titanium, tantalum, cobalt-chromium-tungsten, etc. While the apparatus and method are described herein as being applicable mainly to medical stents, in particular intravascular stents, the invention is not limited to such medical products. For example, the methods may be applied to electropolish metallic automotive or aerospace components.
Stents are generally tube-shaped intravascular devices placed within a blood vessel to maintain the patency of the vessel and, in some cases, to reduce the development of restenosis. The stents may be formed in a variety of configurations which are typically expandable since they are delivered in a compressed form to the desired site. Such a configuration may be a helically wound wire, wire mesh, weaved wire, serpentine stent, or a chain of rings. The walls of stents are typically perforated in a framework design of wire-like connected elements or struts or in a weave design of cross-threaded wire. Some stents are made of more than one material. The stent may be, for example, a sandwich of metals having outer layers of a biocompatible material, such as stainless steel, with an inner layer providing the radioopacity to the stent needed for tracking by imaging devices during placement. A stent made of such material may be, for example, a thin layer of titanium between layers of stainless steel. In forming such stents from metal, a roughened outer surface of the stent may result from the manufacturing process. It is desirable for the surface of the stent to be smooth so that it can be easily inserted and traversed with low friction through the blood vessels toward the site of implantation. A rough outer surface may not only cause increased frictional obstruction, but may also damage the lining of the vessel wall during insertion. Furthermore, smooth surfaces decrease the probability of thrombogenesis and corrosion.
Since the processing to form metallic stents often results in a product initially having undesirable burrs, sharp ends or debris and slag material from melting the metal during processing, as a first order treatment of the product, descaling of the surface is required in preparation of further surface treatment such as electropolishing.
The present invention is directed to an apparatus and method for electropolishing such stents after they have been descaled by an appropriate method, such as that disclosed in concurrently filed application Ser. No. ______. Descaling may include, for example, dipping the stent into a strongly acidic solution and thereafter ultrasonically cleaning the stent.
Electropolishing is an electrochemical process by which some of the surface metal is electrolytically dissolved. In general, the metal stent serves as an anode and is connected to a power supply while immersed in an electrolytic solution having a metal cathode connected to the negative terminal of the power supply. Current therefore flows from the stent, as the anode, causing it to become polarized. The rate at which the metal ions on the stent are dissolved is controlled by the applied and/or voltage. The positioning of the cathode relative to the stents is important so that there is an even distribution of current to the stent. According to the theory of electropolishing, the current density is highest at high points protruding from a surface and is lowest at the surface low points. Thus, the higher current density at the raised points causes the metal to dissolve faster at these points which thus levels the surface. Electropolishing therefore serves to smooth the surface, typically to the point where it is shiny and reflective.
The present invention provides an apparatus and process for electropolishing a plurality of metallic devices such as stents simultaneously to consistently produce smooth surfaces.
An apparatus is provided for simultaneously electropolishing a plurality of metallic stents comprising:
In one embodiment, the plate is stationary and the elongated members are accommodated by a moveable second plate capable of rotating about an axis substantially parallel to the axes of the elongated members and the motorized driver is engaged with the second plate.
In another embodiment, the plate is moveable and is engaged with the motorized driver and the elongated members are accommodated by a stationary second plate.
In yet another embodiment, the apparatus further comprises a second continuous cathode configured to be located in close proximity to each of the elongated members when the elongated members and cathode are immersed in the electrolytic solution and a second cathode current conducting member attached to the second cathode.
In a further embodiment, the two cathodes are tubular in shape and disposed substantially concentrically in the solution.
A method is also provided for simultaneously electropolishing a plurality of metallic stents comprising the steps of:
In one embodiment, the method further comprises the steps of
In one embodiment, the electrolytic solution comprises about 4 vol. % ethylene glycol, about 10 vol. % sulfuric acid and about 86 vol. % methanol.
In another embodiment, in step c) the voltage is supplied for a period in the range of about 25 to 50 seconds while the stents are immersed in the electrolytic solution.
In a preferred embodiment, steps b), c) and d) are repeated three times.
In an embodiment, the passivation solution comprises nitric acid.
In yet another embodiment, the ultrasound energy is applied for a period of about 2 minutes to the liquid at a temperature in the range of about 50° to 60° C.
The present invention is directed to an apparatus and method for electropolishing a plurality of metallic devices, in particular, metallic stents. The present invention is advantageous not only in that a plurality of devices can be simultaneously electropolished, but also that by providing rotation of each of the stents located in equivalent positions in close proximity to a continuous cathode, the stents, serving as anodes, are uniformly electropolished.
In one embodiment of the invention, the individual stents, in addition to being rotated within the electropolishing solution adjacent to the cathode, are also displaced along the cathode by rotation on a movable plate to which they are attached. This provides not only agitation of the electropolishing solution, but also ensures uniform exposure of each of the stents, as anodes, to the same cathode surface, thus averaging out any current collecting differences which may exist between different portions of the electrode surface.
In yet another embodiment, the stents, as anodes are rotated only on their individual axes and are attached to a stationary plate.
In yet another embodiment, the apparatus is provided with two concentric cathodes with the stents, as anodes, placed therebetween, thereby providing additional cathode surface area.
Referring to
At one end of the elongated members 10 there are respective toothed members 12 each engaged with toothed centrally placed stationary plate 13. Each of the members 10 is mounted to a movable toothed plate 14, which is driven by toothed member 15 connected to a motor 16. Each of the members 10 is freely rotatable along its own longitudinal axis. Thus, when toothed member 15 drives movable plate 14, the plurality of members 10 are moved in a circular motion within the electrolytic solution (not shown) at the same time toothed members 12 engaged with stationary plate 13 are also individually rotated. By rotation of the individual elongated members 10, a stent (not shown) attached to adapter 11, has all of its surfaces uniformly exposed to the cathode, which is a tubular structure, shown below, either larger than the circumference defined by the revolving members 10 or smaller than the circumference defined by the revolving members 10, or both, in the case of the two cathode embodiment described below. The apparatus in
Referring to
Referring to
Referring to
Referring to
Typical coronary stents may vary in a range from about 7 to 40 millimeters in length with a diameter in a range of about 1 to 7 millimeters. However, stents of larger or smaller size may be suitably accommodated.
In order to accomplish the electropolishing process, the stents, preferably all identical in length and diameter and design, are placed on one or more of the adaptors 11. The mounted stents are then immersed into the electropolishing solution and while immersed, the motor, such as shown in
In another configuration of the apparatus, the plate 14 may be stationary and the plate 13 may be mounted to be movable and driven by the motor 16. In such a configuration, the elongated members 10 will only rotate about their longitudinal axes, but will not revolve in the solution since plate 14 will be stationary.
It is desirable for the electropolishing process to be performed in stages. After one immersion in the electropolishing solution, typically lasting from about 20 to 60 seconds, the stents may be removed from the solution and washed, typically with alcohol. Then, the electropolishing may be repeated several times with each step followed by a rinse of the stents. Typically a suitable polishing process will comprise about four iterations of the electropolishing step. But more or fewer iterations may be suitable, depending upon the stents, electrolyte, voltage/amperage, speed of rotation and other process variations. Once the desired electropolishing is completed, the stents are removed from the electropolishing solution and from the electropolishing apparatus, rinsed and contacted with a passivation solution to ensure that no residual electropolishing solution remains on the stents. The stents are typically again rinsed and placed in a bath to which ultrasound energy is applied to complete the rinsing. A useful final rinse step will involve exposure for about two minutes in an ultrasound bath in the range of about 50 to 60° C.
A preferred electropolishing solution will comprise about 1 to 20 volume percent ethylene glycol, about 5 to 30 volume percent sulfuric acid and about 50 to 94 volume percent methanol. A useful electropolishing solution comprises about 4 volume percent ethylene glycol, about 10 volume percent sulfuric acid and about 86 volume percent methanol.
The following example is presented for the purpose of illustration and is not intended to limit the invention in any way.
Five dry identical stents are inserted onto five of the adapter on the receiver by sliding over the corkscrew or undulating contact points. While agitating the electropolishing solution (4 volume percent ethylene glycol, 10 volume percent sulfuric acid, 86 volume percent methanol) the stents are lowered on the apparatus into the electropolishing solution. The positive lead from the electrical source is attached to the apparatus and the motor is turned on to revolve and rotate the stents in the solution. When the cycle time has elapsed (depending on the size and type of stent) the stents are removed from the receiver and submerged in a container of methanol. Each stent is rotated while submerged. The stents are then re-immersed in the electropolishing solution for another polishing cycle. The polishing cycle is repeated between two to four polishing cycles. The stents removed from the adapters and placed into a purified water rinse for about 30 seconds. The stents are then removed and placed in Anapol PA/nitric acid passivation rinse bath for 2 minutes. The stents are removed from the bath and placed in a purified water bath for about 30 seconds. The stents are then placed in an ethanol rinse beaker and the beaker is placed in an ultrasonic bath to sonicate for about 2 minutes at around 55±5° C. The stents are then removed and strung onto a wire through the center of each stent and are dried with compressed air.
