Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Heart valve
Reexamination Certificate
1999-03-10
2001-09-11
McDermott, Corrine (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Heart valve
C128S898000
Reexamination Certificate
active
06287338
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to pre-stressing, prior to use, devices incorporating materials subject to stress softening. In particular, the present invention relates to pre-stressing elastomer objects subject to stress softening. Even more particularly, the present invention relates to pre-stressing prosthetic heart valves made, at least partially, of an elastomer.
BACKGROUND OF THE INVENTION
Some materials (for example nylon, certain wool fibers, and elastomers) experience a phenomenon called “stress softening.” As a result of this phenomenon, the stress-strain curve of a material prior to being loaded is different from that of the material after being loaded and unloaded. For some materials, the properties of the first loading and any subsequent loading can be dramatically different. Elastomers are subject to stress softening in a behavior known as the “Mullins' Effect.”
Many devices include materials subject to stress softening. For example, prosthetic heart valves sometimes include materials subject to stress softening, including elastomers.
Ever since 1950, when blood oxygenators made open heart surgery feasible, it has been possible to treat some forms of heart disease by replacing one of the patient's heart valves with a prosthetic valve. Early heart valve prostheses included ball-and-cage valves and disc-and-cage valves in which a ball or a disc was housed in a cage. One side of the cage provided an orifice through which blood flowed either into or out of the heart, depending on the valve being replaced. When blood flowed in a forward direction, the energy of the blood flow forced the ball or disc to the back of the cage allowing blood to flow through the valve. When blood attempted to flow in a reverse direction, or “regurgitate”, the energy of the blood flow forced the ball or disc into the orifice in the valve and blocked the flow of blood.
A bi-leaflet valve comprised an annular valve body in which two opposed leaflet occluders were pivotally mounted. The occluders were substantially rigid and moved between a closed position, in which the two leaflets were mated and blocked blood flow in the reverse direction, and an open position, in which the occluders were pivoted away from each other and did not block blood flow in the forward direction. The energy of blood flow caused the occluders to move between their open and closed positions.
A tri-leaflet valve comprised an annular valve body in which three leaflets were mounted to a portion of the valve body, called a “stent,” located at the circumference of the annulus. When blood flowed in the forward direction, the energy of the blood flow deflected the three leaflets away from the center of the annulus and allowed blood to flow through. When blood flowed in the reverse direction, the three leaflets engaged each other in a coaptive region, occluded the valve body annulus and prevented the flow of blood. The valve leaflets were made from tissue, such as specially treated porcine or bovine pericardial tissue, or from man-made materials such as ceramic materials, elastomers or other biocompatible polymers.
SUMMARY OF THE INVENTION
The present invention is directed toward pre-stressing elastomer devices that can be stress softened. The design of the device utilizing the present invention uses properties of the materials after stress softening rather than before. Prior to implantation, the device is pre-stressed to subject it to the stress that it is expected to experience in use. Pre-stressing causes the device to experience stress softening and operate as designed.
In general, in one aspect, the invention features a method for manufacturing a prosthetic heart valve. The method comprises fabricating a heart valve. The valve has an inflow side and an outflow side. At least a portion of the heart valve comprises a material subject to stress softening. The material experiences stress when a pressure differential occurs between the inflow side and the outflow side. The method includes creating a stress in the material prior to use.
Implementations of the invention may include one or more of the following: Creating a pressure differential may comprise raising the stress to the point where the material subject to stress softening experiences the stress that it is expected to experience in use.
Creating a pressure differential may comprise raising the stress to the point where the material subject to stress softening experiences more stress than it is expected to experience in use. The method may comprise removing the stress. The method may comprise cyclically creating and removing the stress a predefined number of times. Creating the stress may comprise applying pressure to the outflow side of the valve with a fluid. The fluid may be a gas or a liquid. Creating the stress may comprise applying a weight to the valve. Applying a weight may comprise applying a weight to the outflow side of the valve or applying a weight to the inflow side of the valve. Creating the stress may comprise applying a vacuum to the valve, accelerating the valve, vibrating the valve, applying a static head of fluid to the outflow side of the valve, pumping fluid into a chamber on the outflow side of the valve, pumping fluid out of a chamber on the inflow side of the valve, or creating a pressure differential between the inflow side and the outflow side of the valve. The valve may have an annular portion supporting three leaflets and the method may further comprise supporting the annular portion of the valve when creating the stress. Supporting the annular portion of the valve may comprise placing an annular collar inside the annular portion of the valve.
In general, in another aspect, the invention features a method for manufacturing an object. At least a portion of the object comprises materials subject to stress softening. The method comprises pre-stressing prior to use the portion of the object comprising materials subject to stress softening.
In general, in another aspect the invention features an object comprising a flexible member. The flexible member comprises a material subject to stress softening. The flexible member is pre-stressed to the stress it is expected to experience in use.
Implementations of the invention may include one or more of the following. The material may comprise an elastomer. The object may comprise a valve. The valve may comprise a valve body and three flexible leaflets coupled to the valve body. The flexible member may be part of the valve. The flexible member may have a different performance after being pre-stressed than before being pre-stressed. The performance of the flexible member after pre-stressing may be closer to a desired performance of the flexible member than the performance of the flexible member before pre-stressing.
In general, in another aspect, the invention features a tri-leaflet heart valve comprising an elastomer portion. The elastomer portion is pre-stressed, prior to implantation, to the stress it is expected to experience in use.
Implementations of the invention may include one or more of the following. The elastomer portion may be pre-stressed, prior to implantation, beyond the stress it is expected to experience in use. The tri-leaflet heart valve may comprise an annular valve body and three leaflets coupled to the annular valve body. The annular valve body may comprise the pre-stressed elastomer portion. At least one of the three leaflets may comprise the pre-stressed elastomer portion.
In general, in another aspect, the invention features a method for pre-stressing a tri-leaflet heart valve to meet a performance requirement. The heart valve has an inflow side and an outflow side and comprises an elastomer portion subject to stress softening. The elastomer portion of the heart valve experiences stress when a pressure differential occurs between the inflow side and the outflow side. The method includes creating a stress in the elastomer portion. The method includes repeating a predetermined number of cycles: raising the stress to a predetermined amount and loweri
James Donald W.
Moe Riyad E.
Sarnowski Edward J.
Loo Blossom E.
McDermott Corrine
Stewart Alvin
Sulzer Carbomedics Inc.
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