Metal deforming – With temperature modification of tool or of specified...
Reexamination Certificate
2000-06-07
2001-08-28
Tolan, Ed (Department: 3725)
Metal deforming
With temperature modification of tool or of specified...
C072S342940, C072S370080, C072S466200
Reexamination Certificate
active
06279368
ABSTRACT:
BACKGROUND OF THE INVENTION
This application relates to heating and setting mandrels for use in manufacturing and more particularly, a mandrel for heating and setting a stent having limb members which provide for improved expansion characteristics.
The term stent generally refers to a prosthesis, which can be introduced into a corporeal lumen and expanded to support that lumen or attach a conduit to the inner surface of that lumen. Self-expanding stents are generally known in the art. During use, the self-expanding stent is compressed into a reduced size having an outer diameter substantially smaller than the stent in its expanded shape. The stent is held in its compressed state during its passage through the patient's vascular system until reaching the target treatment site, whereupon the compressed self-expanding stent may be deployed. While in its compressed state, stress is stored in the bends of the stent limbs. During deployment, the stresses in the stent limbs cause the stent to expand radially from its initially compressed state. Once in place, the radial extremities of the stent bear against the inside walls of the passageway, thereby allowing normal blood flow.
The processes of manufacturing self-expanding stents are also known in the art insofar as heating or annealing a stent upon a mandrel for purposes of setting a particular stent shape. Additionally, shape memorization processes utilizing mandrels are stent specific as each stent-type embody different design requirements. Previous attempts at heating or annealing simply involve mounting a stent upon a mandrel and exposing it to heat with little attention being paid to the shape that is set during the heating process. Because these previous attempts fail to control the shape created during the heating process, a less effective final stent is produced.
Most stents known in the art change diameter through the deformation of a small percentage of a length of the limbs defining the stent. Usually, this deformation occurs only at, or near, curved apices formed in stent limbs. The length of the limb that deforms and the magnitude of the deformation has a bearing on three important and interrelated characteristics of the stent: 1) the minimum diameter to which the stent can be compressed; 2) the radial stiffness or energy required to compress the stent; and 3) the maximum value of stress/strain experienced by the stent. Many other factors are also determinative of these characteristics including stent material, resting diameter, stent length, etc.; however, these other factors are assumed to be generally constant for a given stent design.
A stent having curved limb members can improve the above mentioned characteristics of the stent by spreading the deformation energy over a majority of the length of the stent limbs. This is in contrast to other stent designs that concentrate the deformation at or near the apices in the stent limb.
For example, to maximize radial stiffness and minimize a compressed diameter of a stent, limb elements defining the stent each can embody two curves of constant radius and opposite direction which meet at an inflection point. When such a stent is compressed, the two curved sections assume a nearly straight profile, the advantage of which is that the entire length of the curved portions store deformation energy and function to urge the stent radially outward.
In the event a stent having curved limb members is to be manufactured, in order to set a desired expanded configuration the stent is expanded over a cylindrical mandrel and heated. However, merely expanding the stent over a mandrel without additional controls or constraints, rarely results in limb elements having the desired profile. To wit, the end of the limbs may be provided with a smaller than desired radius of curvature whereas the portion of the limbs near an inflection point may have a much larger than desired radius of curvature. This results in producing a stent that embodies limbs which do not store stress in an optimal manner.
Therefore, what is needed and heretofore unavailable in the art is a mandrel for heating and setting a stent which facilitates the production of a desired stent profile as well as aids in evenly dispersing stresses along limb elements defining the stent during manufacturing. The present invention satisfies these and other needs.
SUMMARY OF THE INVENTION
Briefly, and in general terms, the present invention provides an improved heating/setting mandrel which substantially reduces the amount of stress inflicted upon the stent during the stent manufacturing process by having restraint mechanisms which properly shape the stent and effectively alleviate unwanted stresses imposed upon the stent limbs during the heating portion of the manufacturing process. Moreover, the mandrel construction of the present invention is relatively inexpensive to manufacture, is trouble-free and reliable in use, and attains improved and constant results in the manufacture of the an improved stent having curved limb elements.
Furthermore, the present invention provides increased radius of curvature values for the curved limb elements at or near the areas of a stent cell where limbs meet and decreased radius of curvature values near an inflection point of the stent limb. Therefore, the benefits of having stent limbs with one radius of curvature value along a first curve and the same but opposite radius of curvature value in a second curve is achieved.
In one aspect, the invention comprises a hollow cylinder made from a heat conducting material having restraint mechanisms to shape and hold in place a stent having curved limb elements. In this configuration, restraint mechanisms are strategically placed on the outer surface of the mandrel. These restraint mechanisms serve to force the stent into the desired shape and hold the stent limbs in place during the heating process.
In a preferred embodiment, the mandrel of the present invention embodies a hollow cylinder with a pattern of radially drilled holes on the outer surface of the mandrel that accepts a plurality of pins which constrain stent limbs in a desired configuration during the heating process. In another embodiment of the invention, the plurality of pins may be individually placed or made part of a mechanism that allows them to protrude and retract from the outer surface of the mandrel. In yet another embodiment, the mandrel of the present invention embodies a hollow cylinder with shallow grooves representative of a desired stent design pattern, the same being machined into the outer surface of the mandrel.
REFERENCES:
patent: 5707388 (1998-01-01), Lauterjung
patent: 5718724 (1998-02-01), Goicoechea et al.
patent: 5746765 (1998-05-01), Kleshinski et al.
patent: 5800519 (1998-09-01), Sandock
patent: 6071308 (2000-06-01), Ballou et al.
Escano Arnold M.
Pollock David T.
Endovascular Technologies, Inc.
Fulwider Patton Lee & Utecht LLP
Tolan Ed
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