Metal working – Method of mechanical manufacture – Assembling or joining
Reexamination Certificate
1999-03-08
2001-06-26
Bryant, David P. (Department: 3726)
Metal working
Method of mechanical manufacture
Assembling or joining
C029S527200, C029S527400, C623S001420, C623S001120, C623S001220
Reexamination Certificate
active
06249952
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to the manufacture of vascular stents, and more particularly to a method of making an otherwise self-expanding stent balloon expandable.
II. Discussion of the Prior Art
Various types of stents are described in the prior art and they generally fall into one of two classes, namely, self-expanding stents and balloon expandable stents. A common type of self-expanding stent is referred to as a “Wallstent® Endoprosthesis” and is described in U.S. Pat. No. 4,655,771 to Wallsten, which is incorporated herein by reference in its entirety. It comprises a braided tubular structure of metal wires or monofilament plastic strands. The tubular structure is characterized by a longitudinal shortening upon radial expansion.
In use, such a stent may be longitudinally extended to achieve a reduced radial diameter and placed within the lumen of a delivery catheter. The delivery catheter may then be advanced through the vascular system until its distal end is located proximate a stenotic lesion to be stented. The stent is then deployed out the distal end of the catheter and when unconstrained by the catheter, it self-expands into contact with the vessel with sufficient radial force so that it is maintained in the blood vessel for an extended period of time. A suitable delivery device for a Wallstent® Endoprosthesis is shown in U.S. Pat. No. 5,026,377 to Burton et al.
An example of a balloon expandable stent is the “Palmaz™” balloon expandable stent described in U.S. Pat. No. 4,733,665. A balloon expandable stent may comprise a fenestrated tube of material having a low modulus of elasticity and with substantially no memory property. The fenestrations through the wall of the tube are such that when the tube is placed over the deflated balloon on a balloon delivery catheter and then is routed to the location in the vascular system where it is to be used, the inflation of the balloon deforms the stent from a reduced diameter to a larger operating diameter. The balloon is then deflated and withdrawn, leaving the stent in place.
A need exists for a stent that has the self-expanding characteristics of a braided stent, but which can be delivered over a balloon for accurate positioning and deployment. The present invention fulfills that need.
SUMMARY OF THE INVENTION
The method of the present invention provides a balloon expandable stent that comprises a tube of braided wire having a relatively small outside diameter when in a longitudinally extended state and which normally self-expands to a larger outside diameter when in a longitudinally contracted state. As used herein, the “free state” of a self-expanding stent is the state that is reached when no external forces are applied thereto. This free states corresponds to a radially fully expanded state. The braided tube is coated with either a brittle material or a material that will readily soften when exposed to body fluids for initially restraining the braided wire tube from self-expanding to the larger outside diameter when external forces are removed. When the coated braided wire tube is concentrically disposed over the balloon of a stent delivery catheter and the balloon is inflated, the restraining force imposed by the brittle coating is effectively broken, allowing the device to self-expand.
It has also been found expedient to incorporate into the coating of the polymeric material an effective amount of water-soluble solid particles that, when dry act to reinforce the coating, and when placed in an aqueous medium, such as blood, will elute with time and thereby lessen the restraint imparted by the coating on the intersections of the wires comprising the braided wire tube, permitting expansion thereof. The particles may comprise a drug.
The method of manufacturing the balloon expandable stent comprises the steps of first braiding a plurality of resilient metal or plastic strands having a memory property into a tubular structure of a predetermined outer diameter. Then the resulting tubular structure is heat treated on a mandrel at a suitable temperature to cause stress relief (generally about 500-600° C. (For Elgiloy®) thereby causing the tubular structure to conform to the shape of the mandrel. Next, the tubular structure is stretched longitudinally to reduce its outer diameter and, while it is clamped and held in the stretched condition, the tubular structure is coated or otherwise treated with a brittle plastic or other material capable of freezing the intersections against movement. If a cross-linkable material containing a water soluble powder is to be used, the material is cross-linked either by application of heat and/or radiation, such that the coating holds the stent in its stretched state and precludes self-expansion thereof to its predetermined outer diameter, even when the force used to longitudinally stretch the tubular structure is removed. When used, the tubular structure may be concentrically positioned over an uninflated balloon on a balloon delivery catheter and, following the passage of the balloon and stent through the vascular system to the site of a lesion, the balloon is inflated to radially expand the tubular structure sufficient to rupture or disintegrate the coating and allow self-expansion of the tubular structure. The destruction of the integrity of the coating is achieved by the use of a hydrogel as the coating or the inclusion of soluble particles in the polymeric matrix and the subsequent exposure of the coated device to an aqueous medium.
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Bryant David P.
Sci-Med Life Systems, Inc.
Westman Champlin & Kelly P.A.
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