U.S. patent application number 12/950799 was filed with the patent office on 2011-06-23 for method of peening metal heart valve stents.
This patent application is currently assigned to EDWARDS LIFESCIENCES CORPORATION. Invention is credited to Louis A. Campbell, James A. Davidson.
Application Number | 20110146361 12/950799 |
Document ID | / |
Family ID | 44149193 |
Filed Date | 2011-06-23 |
United States Patent
Application |
20110146361 |
Kind Code |
A1 |
Davidson; James A. ; et
al. |
June 23, 2011 |
Method of Peening Metal Heart Valve Stents
Abstract
Medical devices, such as percutaneous prosthetic heart valves
can include a stent or frame structure component. The stent can be
shot peened, laser peened, and/or ultrasonically peened, thereby
reducing surface abnormalities, improving surface appearance,
and/or increasing fatigue life of the device.
Inventors: |
Davidson; James A.; (San
Juan Capistrano, CA) ; Campbell; Louis A.; (Santa
Ana, CA) |
Assignee: |
EDWARDS LIFESCIENCES
CORPORATION
Irvine
CA
|
Family ID: |
44149193 |
Appl. No.: |
12/950799 |
Filed: |
November 19, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61289248 |
Dec 22, 2009 |
|
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Current U.S.
Class: |
72/53 |
Current CPC
Class: |
C21D 2221/00 20130101;
A61F 2220/0075 20130101; C21D 10/005 20130101; C21D 7/06 20130101;
A61F 2230/0054 20130101; A61F 2/2418 20130101 |
Class at
Publication: |
72/53 |
International
Class: |
C21D 7/06 20060101
C21D007/06 |
Claims
1. A method of making a prosthetic heart valve comprising:
providing a valve frame, wherein the frame comprises an inner and
an outer surface; peening at least a portion of the outer surface
of the frame, thereby improving fatigue resistance of the frame;
and securing valve leaflets to the frame.
2. The method according to claim 1, wherein peening comprises shot
peening with biocompatible beads.
3. The method according to claim 1, wherein peening comprises
changing the surface roughness of at least a portion of the outer
surface of the frame with uniformity sufficient to mask one or more
surface scratches, notches, and/or burrs.
4. The method according to claim 1, wherein peening comprises
selectively peening only areas of the outer surface of the frame
that are subject to tensile stress while in use.
5. The method according to claim 1, wherein peening comprises at
least one of laser peening and ultrasonic peening.
6. The method according to claim 1, wherein peening comprises
peening substantially the entire outer surface of the frame.
7. The method according to claim 1, wherein peening comprises
peening at least a portion of both the inner and outer surfaces of
the frame.
8. The method according to claim 1, wherein the frame is made of
Nitinol.
9. The method according to claim 1, wherein the frame is a radially
expandable and collapsible annular frame.
10. A method of making a prosthetic heart valve, comprising:
providing a radially collapsible and expandable frame for the heart
valve, wherein the frame comprises an inner and an outer surface;
peening at least a portion of the outer surface of the frame,
thereby altering one or more surface characteristics of the frame;
and mounting valve leaflets to the frame.
11. The method according to claim 10, wherein peening comprises
selectively peening only areas of the outer surface of the frame
that are subject to tensile stress while in use.
12. The method according to claim 10, wherein peening comprises one
or more of shot peening with biocompatible beads, laser peening
with a biocompatible overlay layer, and ultrasonic peening.
13. The method according to claim 10, wherein peening comprises
changing the surface roughness of at least a portion of the outer
surface of the frame with uniformity sufficient to mask one or more
surface scratches, notches, and/or burrs.
14. The method according to claim 10, wherein the frame is made of
a self-expanding, shape memory material.
15. A method for peening a medical device, comprising: providing a
radially collapsible and expandable annular frame, wherein the
frame comprises an inner and an outer surface; placing the frame on
a stabilizing device; peening at least a portion of the outer
surface of the frame, thereby altering one or more surface
characteristics of the frame; and coupling the frame to prosthetic
valve leaflets for implantation in a human subject.
16. The method according to claim 15, wherein peening comprises
changing the surface roughness of at least a portion of the outer
surface of the frame with uniformity sufficient to mask one or more
surface scratches, notches, and/or burrs.
17. The method according to claim 15 wherein substantially the
entire outer surface of the frame is peened.
18. The method according to claim 15, wherein peening comprises
shot peening with biocompatible beads.
19. The method according to claim 15, wherein substantially only
the portions of the frame that are subject to the greatest tensile
stresses are peened.
20. The method according to claim 15, further comprising peening at
least a portion of the inner surface of the frame.
Description
FIELD
[0001] The present disclosure relates to medical devices and
methods for manufacturing medical devices. Specifically, the
disclosure relates to improved heart valve stents and methods of
manufacturing the same.
BACKGROUND
[0002] In minimally invasive surgery techniques, medical devices,
such as prosthetic heart valves, can be crimped or otherwise
radially compressed onto a delivery catheter, positioned within a
patient's heart, and then expanded. Medical devices that are
implanted in this manner are typically guided through small and
circuitous vessels in order to reach their destinations. During
delivery, the devices typically must be flexible enough to undergo
some bending while maneuvering through the vessel, and yet resist
the formation of cracks. Smaller profiles while in the compressed
state allow for easier maneuvering through the patient's vessels.
Thus, it is desirable for devices such as heart valve stents to be
as small as possible, but without sacrificing mechanical
characteristics and strength.
[0003] Stents and other medical devices made of biocompatible metal
alloys, especially Nitinol, are commonly subject to fractures.
Fractures, such as fatigue-related fractures, are problematic for
the device's performance within the body. Further, scratches and
other surface abnormalities and irregularities can lead to fatigue
crack growth.
[0004] Some attempts have been made at combating these fracture
problems in medical devices. For example, increasing the stiffness
of the device can reduce its susceptibility to fractures, but, in
the case of heart valve stents, can also undesirably increase shock
loading to the valve leaflets. Other techniques such as annealing
or work hardening the devices can reduce residual tensile stresses,
but do not impact surface defects that can create crack initiation
points.
[0005] Thus, an improved heart valve stent and method of forming
heart valve stents are needed to increase the factor of safety
(resistance to fatigue) without simultaneously increasing the
stiffness and/or the thickness of the stent.
SUMMARY
[0006] Disclosed methods can improve the fatigue safety factor,
resistance to fatigue cracks, and/or the surface appearance of
medical devices such as heart valve stents, without increasing the
stiffness or thickness of the stent. According to some disclosed
embodiments, thinner stents having substantially the same fatigue
life as thicker stents can be formed, thus allowing for lower
profiles when the heart valve stent is radially compressed or
crimped onto a delivery system, and requiring less material for
manufacturing the stent.
[0007] One method of improving fatigue resistance in a medical
device comprises providing a frame (e.g., an annular frame such as
an annular heart valve frame), wherein the frame comprises an inner
and an outer surface, peening at least a portion of the outer
surface of the frame, thereby improving fatigue resistance of the
frame, and securing the frame to one or more leaflets, such as to
form a prosthetic heart valve. Such methods can be particularly
useful for, for example, percutaneous heart valve stents comprising
shape memory materials, such as Nitinol.
[0008] Peening can comprise shot peening, laser peening, and/or
ultrasonic peening. Shot peening can be done with biocompatible
shots in some embodiments. Similarly, laser peening can comprise
using a sacrificial overlay layer over at least a portion of the
frame, wherein the overlay layer is arranged between the frame and
a laser peening source, and the overlay layer does not compromise
biocompatibility of the device being peened.
[0009] In some methods, substantially the entire outer surface of
the frame can be subject to one or more types of peening. In other
embodiments, at least a portion of the outer surface of the frame
can be subject to one or more types of peening. Additionally or
alternatively, peening can be performed on at least a portion of
the inner surface of the frame. Further, peening can be directed to
one or more specific regions of the outer and/or inner surfaces of
the frame. For example, peening can be directed to one or more
areas of the inner and/or outer surfaces of the frame that are
subject to the greatest stresses or risk of fatigue crack
growth.
[0010] Disclosed methods can also alter surface characteristics of
medical devices, such as by improving the device's aesthetic
appearance and/or substantially eliminating or reducing score lines
or burrs. One method of altering surface characteristics of a
medical device comprises providing a radially collapsible and
expandable frame, wherein the frame comprises an inner and an outer
surface, and peening at least a portion of the outer surface of the
frame, thereby altering one or more surface characteristics of the
frame, and securing the frame to one or more valve leaflets. For
example, peening can uniformly increase surface roughness, thereby
masking or removing surface irregularities such as score lines,
scratches, notches, and/or burrs. In one method, peening comprises
roughening at least a portion of the outer surface of the frame
with uniformity sufficient to mask one or more surface scratches,
notches, and/or burrs. The surface characteristics of at least a
portion of the inner surface of the frame can be altered by
peening, in addition to or alternatively to at least a portion of
the outer surface of the frame.
[0011] Peening can comprise one or more of shot peening with
biocompatible beads, laser peening with a biocompatible overlay
layer, and ultrasonic peening.
[0012] Methods of peening medical devices are also disclosed. One
method for peening a medical device comprises providing a radially
collapsible and expandable frame, wherein the frame comprises an
inner and an outer surface, placing the frame on a stabilizing
device, peening at least a portion of the outer surface of the
frame, thereby altering one or more surface characteristics of the
frame, and coupling the frame to a prosthetic heart valve for
implantation in a human subject. Placing the frame on a stabilizing
device, such as securing or mounting the frame on a mandrel or a
delivery system component (e.g., a guidewire or delivery catheter)
can aid in positioning the frame appropriately for peening. For
example, a frame can be held substantially stationary on a mandrel,
while a shot peening nozzle moves around and over the surface of
the frame. Similarly, a frame can be secured in place on a mandrel
while a laser beam is directed over the surface of the frame for
laser peening.
[0013] The frame can be coupled to the rest of the prosthetic heart
valve (e.g., a leaflet structure) after one or more peening
processes have been performed. In one embodiment, the frame can be
coupled to a leaflet structure and a skirt to form a complete
prosthetic heart valve, the valve can be crimped onto a delivery
system component such as a guidewire (e.g., the frame is coupled to
the prosthetic valve prior to being placed on the delivery system
component), and the frame can be peened with one or more peening
techniques before delivering the prosthetic valve to the patient.
The frame can be subjected to one or more peening processes before
the prosthetic valve is crimped onto the delivery system component
and/or before being coupled to other prosthetic heart valve
components. For example, a self-expandable Nitinol frame can be
subjected to one or more peening processes, placed onto a delivery
system component, and then radially compressed, such as with a
restraining sheath, for implantation into a patient.
[0014] The foregoing and other objects, features, and advantages of
the invention will become more apparent from the following detailed
description, which proceeds with reference to the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 illustrates an exemplary prosthetic heart valve
including a stent.
[0016] FIG. 2 illustrates a heart valve stent.
[0017] FIG. 3 illustrates another embodiment of a prosthetic heart
valve including a stent.
[0018] FIG. 4 is a flow chart of one method of peening a metal
heart valve stent.
DETAILED DESCRIPTION
[0019] FIG. 1 illustrates an exemplary prosthetic heart valve 10
having a stent, or frame 12. Such exemplary valve is the subject of
co-pending application Ser. No. 12/480,603, which is hereby
incorporated herein in its entirety. Valve 10 and frame 12 are
configured to be radially collapsible to a collapsed or crimped
state for introduction into the body on a delivery catheter and
radially expandable to an expanded state for implanting the valve
at a desired location in the body (e.g., the native aortic valve).
Frame 12 can be made of a plastically-expandable material that
permits crimping of the valve to a smaller profile for delivery and
expansion of the valve using an expansion device such as the
balloon of a balloon catheter. Alternatively, valve 10 can be a
so-called self-expanding valve wherein the frame 12 is made of a
self-expanding shape memory material such as Nitinol. A
self-expanding valve can be crimped to a smaller profile and held
in the crimped state with a restraining device such as a sheath
covering the valve. When the valve is positioned at or near the
target site, the restraining device is removed to allow the valve
to self-expand to its expanded, functional size.
[0020] Implantable prosthetic valve 10 comprises a radially
collapsible and expandable frame or stent 12 and a leaflet
structure 14 comprising a plurality of leaflets 60. The leaflet
structure has a scalloped lower edge portion that is positioned
inside of and secured to the frame 12. The valve can further
include an annular skirt member 16, which can be disposed between
the frame 12 and the leaflet structure 14 such that the scalloped
lower edge portion can be attached to an inner surface of the skirt
member 16. Skirt 16 can be secured to the inside of frame 12 via
sutures 56. Suture 58 can secure the leaflet structure 14 to skirt
16. Each leaflet can have an upper edge, a curved lower edge and
two side flaps extending between respective ends of the upper edge
and the lower edge, wherein each side flap is secured to an
adjacent side flap of another leaflet to form commissures of the
leaflet structure. Each commis sure can be attached to one of the
commissure attachment posts, and a reinforcing bar can be
positioned against each side flap for reinforcing the attachments
between the commissures and the commissure attachment posts.
[0021] An exemplary frame 12, which is shown alone in FIG. 2 for
illustrative purposes, can comprise a plurality of angularly
spaced, axial struts or posts 18 that are interconnected by a
plurality of rows of circumferential struts. Posts 18 can be
interconnected via a lower row of circumferentially extending
struts 20 and first and second rows upper rows of circumferentially
extending struts 22 and 24, respectively. The struts in each row
desirably are arranged in a zigzag or generally saw-tooth like
pattern extending in the direction of the circumference of the
frame as shown. Adjacent struts in the same row can be
interconnected to one another as shown in FIGS. 1 and 2 to form an
angle A. Each pair of struts 22 connected at a common crown
structure 26 forms a cell with an adjacent pair of struts 24 in the
row above. Each cell can be connected to an adjacent cell at a node
32. Each node 32 can be interconnected with the lower row of struts
by a respective vertical (axial) strut 30 that is connected to and
extends between a respective node 32 and a location on the lower
row of struts 20 where two struts are connected at their ends
opposite crown structures 26. Openings 78 in posts 18 can be used,
among other things, to secure frame 12 to the leaflet
structure.
[0022] The prosthetic heart valve shown in FIGS. 1 and 2 is meant
to be one example, and not limiting in any way. As used throughout
this disclosure, the term "frame" can be any type of frame suitable
for use with a medical device such as a heart valve. Frames can be
substantially annular in some embodiments. In some embodiments, a
frame can be shaped from one or more arcs that are joined
together.
[0023] Another embodiment of an exemplary medical device is the
self-expandable prosthetic heart valve shown in FIG. 3. Such
exemplary valve is the subject of co-pending application Ser. No.
12/429,040, which is hereby incorporated herein in its entirety.
The valve 310 includes an expandable frame member, or stent, 312
that supports a flexible leaflet section 314. The valve 310 is
radially compressible to a compressed state for delivery through
the body to a deployment site and expandable to its functional size
shown in FIG. 3 at the deployment site. In certain embodiments, the
valve 310 is self-expanding; that is, the valve can radially expand
to its functional size when advanced from the distal end of a
delivery sheath.
[0024] As shown, the stent 312 can be formed from a plurality of
longitudinally extending, generally sinusoidal shaped frame
members, or struts, 316. The struts 316 are formed with alternating
bends and are welded or otherwise secured to each other at nodes
formed from the vertices of adjacent bends so as to form a mesh
structure. The struts 316 can be made of a suitable shape memory
material, such as the nickel titanium alloy known as Nitinol, that
allows the valve to be compressed to a reduced diameter for
delivery in a delivery apparatus and then causes the valve to
expand to its functional size inside the patient's body when
deployed from the delivery apparatus.
[0025] The stent 312 of FIG. 3 has an inflow end 326 and an outflow
end 327. The stent 312 can have a plurality of angularly spaced
retaining arms, or projections, in the form of posts 330 (three in
the illustrated embodiment) that extend from the stent upper
portion, near the outflow end 327. Each retaining arm 330 has a
respective aperture 332 that is sized to receive prongs of a
valve-retaining mechanism that can be used to form a releasable
connection between the valve and a delivery apparatus. In
alternative embodiments, the retaining arms 330 need not be
provided if a valve-retaining mechanism is not used.
[0026] Medical devices, such as the percutaneous heart valve stents
shown in FIGS. 1-3, can comprise any suitable material, and
preferably comprise a biocompatible metal alloy. Suitable materials
can include Nitinol, other nickel-titanium alloys, copper,
aluminum, titanium, nickel, platinum, tantalum, cobalt, chromium,
cobalt-chromium alloys, steel-based alloys such as stainless steel,
nickel-based alloys (e.g., a nickel-cobalt-chromium alloy such as
MP35N.TM.) polymers, or combinations or alloys thereof.
[0027] Disclosed improved heart valve stents and methods for making
the same can allow the manufacture of thinner heart valve stents,
using less material to achieve the same or better performance in
radial and crush force resistance, fatigue resistance (e.g.,
fatigue safety factor), and/or corrosion resistance. Moreover, the
crimped profile of the frame can be reduced, thereby providing a
lower profile valve assembly for percutaneous delivery to the
treatment location in the body. While the methods are disclosed
with reference to medical devices that are collapsible and
expandable, the methods are equally applicable to non-collapsible
medical devices, such as the stents of non-collapsible surgical
valves. For example, the methods disclosed herein can be used to
treat the metal stent or wireform of the valve disclosed in U.S.
Pat. No. 6,585,766, which is incorporated herein by reference.
[0028] One such method comprises peening to modify the mechanical
properties of heart valve stents. Peening can produce a compressive
residual stress layer within the stent and/or on the surface of the
stent that can increase fatigue life. Generally, the surface of a
heart valve stent can be shot peened by being mechanically impacted
or bombarded with a plurality of shots, such as small metal, glass,
aluminum oxide, or ceramic beads, spheres, or pellets, walnut
shells, or similar materials. In some embodiments, the surface of a
heart valve stent can be subject to sand and/or grit blasting. In
embodiments of peening a heart valve stent, the material (e.g., the
shots) can be forced, such as forced by compressed air through a
nozzle, onto the surface of a metal stent so as to impact the
surface and create plastic deformation in at least some regions of
the stent. Such impact on the surface can alter the structure of
the stent near the surface, such as, for example, by creating
compressive strains in the peened surface. Shot peening can result
in favorable compressive residual stress or reduce pre-existing
residual tensile stress, thus improving fatigue strength or fatigue
life.
[0029] For example, in some embodiments, the determined fatigue
safety factor of a heart valve stent can be increased from about
1.0 to a fatigue safety factor greater than about 1.0. In some
embodiments, the fatigue safety factor can be increased from about
1.0 prior to peening, to about 1.05 or greater, to about 1.2 or
greater, or to about 1.4 or greater.
[0030] At least some portion of the stent can be peened in the
disclosed embodiments, where the terms "peening" and "peened"
include shot peening, laser peening, and ultrasonic peening. In
some embodiments, substantially the entire surface of the heart
valve stent can be subject to peening. In other embodiments, only
certain regions of the stent are subject to peening. For example,
in some embodiments, peening can be focused on the crown structures
26 and/or the nodes 32 of FIGS. 1-2. In some embodiments, peening
can be focused on the axial posts 18 of FIGS. 1-2. In some
embodiments, regions of the heart valve stent that are most
susceptible to fatigue cracks can be peened to a greater extent
than other regions of the heart valve stent. In some embodiments, a
peening mask can be used to prevent specific regions of the heart
valve stent from being impacted by the shots and/or laser.
[0031] Disclosed methods of shot peening heart valve stents can
include varying the size, shape, hardness, and/or material of the
shots, the speed of the shots, the nozzle pressure firing the
pellets, the nozzle distance from the sample being peened, and/or
the duration of the process. Varying these parameters can allow at
least some control over the results of the peening. For example,
increasing the mass and/or incident velocity of the pellets can
increase the magnitude of the compressive stress imparted to the
stent material. Higher pellet mass and/or velocity can also allow
the process to obscure more significant surface defects that
otherwise can reduce the fatigue resistance of the stent. In some
embodiments, increasing the pellet mass and/or velocity can be
limited so as to substantially prevent dimensional distortion of
the stent such as warping the stent or removing too much material.
The appropriate particle hardness, mass, and velocity can be
determined through an empirical process for any given stent
material and design configuration.
[0032] The working distance (e.g., the nozzle distance from the
sample being peened) can also be varied. Reducing the working
distance can increase the effects of the peening. Changing one or
more parameters at a time can have various impacts on the effects
of the peening. For example using a shot or pellet comprising a
material of relatively low hardness can reduce the effects of the
peening as compared to using a shot or pellet comprising a material
of relatively high hardness. However, the effectiveness of peening
with a lower hardness pellet can be increased by, for example,
varying the velocity (e.g., increasing the velocity) of the pellet
and/or varying the working distance (e.g., decreasing the working
distance). Likewise, in some embodiments, the velocity of the
pellets can be decreased and/or the working distance can be
increased in order to lessen the effects of the peening.
[0033] In some embodiments, process parameters can be varied to
account for system requirements or limitations. For example, if a
peening system can only accommodate a relatively short working
distance, varying other parameters can change the effects of the
peening to overcome this limitation. For example, if the working
distance is not as great as desired, parameters such as the pellet
velocity can be increased to increase the effects of the peening.
Each of the process parameters can be increased or decreased in
order to create the desired effects on the piece being peened.
[0034] In some embodiments, the parameters of the process can be
varied for different portions of the stent. For example, some
regions of the stent can be subject to peening with smaller beads
at lower pressure and/or for less time than other regions of the
stent. The shot size, velocity, and/or peening duration can be
varied to result in desirable combinations of residual surface
stress and surface appearance. In some embodiments, areas of the
heart valve stent subject to tensile stress while the valve is in
use can be selectively peened, while areas of the heart valve stent
not subject to surface tensile stress while in use can have either
reduced or no peening. Such selective peening can be useful in
obtaining sufficient surface modification to improve fatigue
resistance while minimizing dimensional distortion of the
stent.
[0035] For example, shot diameter can be varied from less than
about 10 microns to greater than about 50 microns. Nozzle pressure
can be varied from less than about 10 psi to greater than about 100
psi, with specific embodiments using nozzle pressure between about
30 psi and about 80 psi. One or more nozzles can be used in some
methods. For example, in one method, a three nozzle array can be
used to shot peen a heart valve stent. The one or more nozzles can
be held at a constant distance from the stent, or the distance can
be varied throughout the duration of the peening. In some
embodiments, the nozzle or nozzles can be from about 0.5 cm or less
to about 10 cm or greater away from the stent. In some embodiments,
peening can be performed for less than about 1 second to greater
than about 5 seconds. Peening can be performed for a period of time
long enough to achieve desired result. For example, it can be
performed for a period of time long enough to achieve a desired
surface roughness, such as a roughness of from about 50 nm to about
500 nm. In some embodiments, the heart valve stent can be displaced
and/or rotated from its original position when peening was started.
For example, the heart valve stent can be rotated a certain number
of degrees at a time, for example, 30, 60, 90, or 120 degrees at a
time, and peening repeated after each rotation until the stent has
been rotated substantially 360 degrees and peened on all desired
surfaces.
[0036] The heart valve stent can be mounted onto a stabilizing
structure such as a mandrel to protect the stent's inner surface
during peening. The stent can be rotated while the shots are
projected towards the outer surface of the stent from a pressurized
shot source. Similarly, the stent can be rotated while one or more
laser beams are projected towards the outer surface of the stent
from a laser peening apparatus. In some embodiments, one or more
nozzles and/or laser beams can be moved across and/or around the
stent. The one or more nozzles and/or laser beams can be positioned
at an angle to the vertical axis of the stent, such as at an angle
of approximately 30 degrees to the vertical axis of stent. After
being peened, the stent can be removed from the mandrel and
cleaned, before being secured to one or more valve leaflets. In
alternative embodiments, the stent can be secured in place such
that at least a portion of both the inner and outer surfaces of the
stent are subject to peening. For example, a heart valve stent can
be secured by one or more of the posts 18 and at least some portion
of the inner and/or outer surfaces subjected to peening.
[0037] In some disclosed methods, a heart valve stent can be peened
before attachment to other components of a prosthetic valve (e.g.,
the leaflet structure 14 and the skirt 16). In alternative
embodiments, the stent can be peened after attachment to one or
more of the other components of a prosthetic valve. Some methods
provide for peening the heart valve stent before crimping it onto a
delivery system.
[0038] In some embodiments, a heart valve stent can be annealed
and/or subject to some other hardening technique before peening.
Such annealing can leave residual stresses on the stent surface
after the stent is formed. Peening can create compressive surface
stress which can be beneficial to the fatigue life of the heart
valve stent, and help to overcome the disadvantageous stresses
resulting from processes such as annealing. Thus, some methods of
peening heart valve stents can reduce adverse residual stress on
the surface of metallic stents as well as add favorable residual
stresses to surface irregularities, rendering them less detrimental
from a fatigue life perspective.
[0039] Cold drawing a wire used to form a stent structure can
result in surface damage or irregularities such as score lines,
scratches, notches, and/or intermittent burrs. Disclosed methods of
peening a heart valve stent can also aesthetically improve the
stent's surface appearance, such as by substantially eliminating or
reducing such surface irregularities. While peening can result in
what optically appears to be a rougher surface, peening can
actually result in a smoother surface based on quantitative
measurement methods such as profilometry.
[0040] Disclosed methods of peening heart valve stent surfaces can
create a more uniform surface that can improve fatigue life. In
some embodiments, peening the surface of a heart valve stent can
alter the surface of the stent, such as by removing stress risers.
In some embodiments, uniformly peening the surface can mask the
surface irregularities and result in improved fatigue resistance.
In some embodiments, application of the shots and/or laser beams is
substantially uniform over the entire surface of the heart valve
stent.
[0041] Furthermore, in some methods, peening can reduce deleterious
effects of processing techniques such as plating, decarburization,
corrosion, and grinding.
[0042] In these ways, the present disclosure can allow for the use
of a thinner stent, using less material (e.g., a heart valve stent
with a smaller profile in its crimped state) without sacrificing
fatigue resistance. A lower profile stent can save money on
materials required, and can also provide more flexibility and
easier tracking through the patient's vasculature during delivery.
As a result, complications associated with the incision site and
implantation can be reduced. Thinner stents can also reduce the
external diameter of a valve and/or allow increased flow area
through the valve for any given external diameter. Increased flow
area can reduce the work required by the patient's heart to pump
blood through the valve, thereby contributing to heart valve
optimization.
[0043] In some embodiments, peening does not affect the sterility
and/or biocompatibility of the stent surface. In these embodiments,
peening can be performed such that surface contamination is
substantially avoided. In some embodiments, peening can be used to
remove at least some adherent surface contaminants. Further, in
some methods, the shots can comprise biocompatible beads, such as
biocompatible metal or ceramic beads. Different bead materials can
be used depending on the material of the heart valve stent, if
desired. For example, a particular heart valve stent can be shot
peened using shots selected to be substantially the same or a
similar material as the heart valve stent itself. Similarly, a
particular heart valve stent can be shot peened using shots
selected to possess different mechanical characteristics from the
heart valve stent material. For example, selected shots can have a
greater or lesser hardness than the heart valve stent material, to
create desired results.
[0044] Some disclosed methods comprise using peening techniques
such as laser peening and/or ultrasonic peening. Laser peening can
create a high-pressure plasma that generates a shock wave that can
force the resulting compressive stress deeper into the surface of
the stent. In some embodiments, a tamping layer, such as a laminar
flow of water, can be provided over the surface of the stent during
peening. The water layer can substantially prevent the plasma
formed by laser peening from expanding, and thus can help drive the
energy into the stent surface.
[0045] Embodiments of laser peening a medical device can include
directing a laser beam onto a sacrificial overlay layer to induce a
pressure shock wave within the device that can be used to produce
one or more compressive residual stress regions on the surface of
and/or within the heart valve stent. Some methods of laser peening
a medical device such as a heart valve stent can comprise using a
sacrificial overlay or ablative layer, such as paint, HDPE, tape,
or any suitable biocompatible material. A laser apparatus that
includes a laser source having a beam intensity sufficient to
induce a pressure shock wave within the stent can be used to direct
one or more laser beams onto the stent surface or onto the
sacrificial overlay layer to produce one or more residual stress
regions within the stent. The overlay layer can absorb radiation
and act as a thermal barrier to protect the stent from thermal
effects generated during the laser peening process. In preferred
embodiments, the overlay layer comprises one or more biocompatible
materials that substantially do not compromise the biocompatibility
and/or sterility of the heart valve stent. In some embodiments, the
overlay layer can be patterned, to provide for laser peening of
selected regions of the heart valve stent, while preventing other
surfaces from being peened, such as by preventing transmission of
the shock wave into those portions of the stent.
[0046] Embodiments of ultrasonic peening a medical device can
include using a mist of microbeads inside a vibrating chamber.
Methods can combine high frequency impacts of these microbeads or
strikers with ultrasonic oscillation of the medical device. Methods
of ultrasonic peening can include bringing an acoustically tuned
body to resonance by energizing an ultrasonic transducer. The
energy generated from these high frequency impulses can be imparted
to the surface of a medical device through the contact of specially
designed steel pins. Transfer pins can be free to move axially
between the resonant body and the surface of the medical device
(e.g., the heart valve stent). Bringing the ultrasonic transducer,
pins, and other components into contact with a medical device can
acoustically couple them together, creating harmonic resonance that
can result in compressive residual stress, stress relief, and
surface improvements.
[0047] As with shot peening, parameters of laser peening and
ultrasonic peening can be varied to achieve different desired
results. For example, different wavelengths of lasers can be used
for different peening processes, or the frequency and/or wavelength
can be varied during a single peening process. In some embodiments,
laser beams can be provided having a frequency of from about 3 Hz
or less to about 6 Hz or greater. Frequencies for ultrasonic
peening can range from about less than 25 kHz to greater than about
55 kHz. The displacement amplitude of the resonant body can also be
varied, such as between about 20 microns and about 50 microns. The
above parameters as well as other parameters known in the art can
be varied and optimized for specific applications, such as will be
appreciated by those skilled in the art.
[0048] In some methods, laser peening or ultrasonic peening alone
can be used to improve fatigue resistance or surface properties of
heart valve stents. In alternative methods, laser peening and
ultrasonic peening can be used in combination with each other,
and/or in combination with shot peening to produce desired results.
Some methods can comprise cold straightening before, after, or
during a peening technique. Robotics and/or CNC machines can aid in
the peening process, whether shot peening, laser peening, and/or
ultrasonic peening is used. For example, the nozzle used to force
shots onto the surface of a heart valve stent can be robotically
positioned and moved over the surface of the stent.
[0049] While, as described above, many different methods of peening
a metal valve stent are disclosed, FIG. 4 illustrates a flow chart
of one exemplary embodiment. A metal stent, such as a Nitinol heart
valve stent can be provided (step 400). The valve stent can be
secured to a mandrel, or otherwise held in place such that at least
a portion of the inner and/or outer surface of the valve stent is
accessible by a peening tool (step 402). At least a portion of the
inner and/or outer surface of the valve stent can then be peened,
such as by shot peening, laser peening, and/or ultrasonic peening
(step 404). The valve stent can be rotated and/or repositioned to
access different portions of the surface, if necessary. Once
peening is complete, the valve stent can be removed from the
mandrel (step 406). The valve stent can then be secured to a
leaflet structure comprising one or more valve leaflets, without
requiring any further processing (step 408). In this way, peening
can be essentially the last processing step performed on the valve
stent before securing the valve stent to one or more valve leaflets
to form a prosthetic valve.
[0050] In view of the many possible embodiments to which the
principles of the disclosed invention may be applied, it should be
recognized that the illustrated embodiments are only preferred
examples of the invention and should not be taken as limiting the
scope of the invention. Rather, the scope of the invention is
defined by the following claims. We therefore claim as our
invention all that comes within the scope and spirit of these
claims.
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