U.S. patent application number 14/496085 was filed with the patent office on 2015-01-29 for laser cutting process for forming stents.
The applicant listed for this patent is ABBOTT CARDIOVASCULAR SYSTEMS INC.. Invention is credited to Li Chen, Yu-Chun Ku, Randolf Von Oepen, Travis Yribarren.
Application Number | 20150028008 14/496085 |
Document ID | / |
Family ID | 42123157 |
Filed Date | 2015-01-29 |
United States Patent
Application |
20150028008 |
Kind Code |
A1 |
Chen; Li ; et al. |
January 29, 2015 |
LASER CUTTING PROCESS FOR FORMING STENTS
Abstract
Systems and methods for improving the cutting efficiency and cut
profile of stent strut is provided. A means for altering the energy
distribution of a laser beam is provided, along with various ways
of controlling a laser to provide for improved strut configurations
are provided. A method for improved cutting speeds using a
combination of laser sources is also provided.
Inventors: |
Chen; Li; (San Jose, CA)
; Yribarren; Travis; (Campbell, CA) ; Von Oepen;
Randolf; (Los Altos, CA) ; Ku; Yu-Chun; (San
Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ABBOTT CARDIOVASCULAR SYSTEMS INC. |
Santa Clara |
CA |
US |
|
|
Family ID: |
42123157 |
Appl. No.: |
14/496085 |
Filed: |
September 25, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12699336 |
Feb 3, 2010 |
8872062 |
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14496085 |
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61149630 |
Feb 3, 2009 |
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61149664 |
Feb 3, 2009 |
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61149667 |
Feb 3, 2009 |
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Current U.S.
Class: |
219/121.72 ;
219/121.73 |
Current CPC
Class: |
B23K 26/0823 20130101;
A61F 2240/001 20130101; A61F 2/91 20130101; B23K 26/0626 20130101;
B23K 26/36 20130101; B23K 26/361 20151001; A61F 2002/91566
20130101; A61F 2/915 20130101; B23K 26/38 20130101 |
Class at
Publication: |
219/121.72 ;
219/121.73 |
International
Class: |
B23K 26/06 20060101
B23K026/06; B23K 26/36 20060101 B23K026/36; A61F 2/915 20060101
A61F002/915; B23K 26/38 20060101 B23K026/38 |
Claims
1. (canceled)
2. A method for shaping a laser beam for cutting a stent pattern
into a stent, comprising: providing a laser beam having a first
intensity distribution; and re-mapping the first intensity
distribution of the laser beam to a second intensity
distribution.
3. The method of claim 2, wherein the first intensity distribution
is a Gaussian intensity distribution.
4. The method of claim 2, wherein the second intensity distribution
is a top hat intensity distribution.
5. The method of claim 2, wherein the second intensity distribution
is a non-Gaussian intensity distribution.
6. A method for cutting a stent pattern into a tube, comprising:
remapping the intensity distribution of a laser beam to a
non-Gaussian intensity distribution; and applying the non-Gaussian
intensity beam to a tube to remove at least a portion of a wall
thickness of the tube.
7. The method of claim 6, wherein applying the beam to the tube
includes exposing the tube to multiple passes of the laser
beam.
8. The method of claim 6, wherein remapping includes providing a
non-Gaussian intensity distribution to the laser beam, the
non-Gaussian intensity distribution having at least one
characteristic resulting in removing material from the wall of the
tube in a selected configuration.
9. The method of claim 8, wherein the selected configuration
includes forming indentations on a surface of the tube.
10-20. (canceled)
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Provisional
Application No. 61/149,630, filed Feb. 3, 2009 and U.S. Provisional
Application No. 61/149,664, filed Feb. 3, 2009 and U.S. Provisional
Application No. 61/149,667, filed Feb. 3, 2009 incorporated by
reference in its entirety.
[0002] This application is also related to U.S. application Ser.
No. ______ entitled IMPROVED LASER CUTTING SYSTEM, filed Feb. 3,
2010, and U.S. application Ser. No. ______ entitled MULTIPLE BEAM
LASER SYSTEM FOR FORMING STENTS, filed Feb. 3, 2010.
BACKGROUND
[0003] 1. Field of the Invention
[0004] The present invention relates generally to implantable
medical devices and to a method for manufacturing implantable
medical devices. These implantable medical devices may also be
capable of retaining therapeutic materials and dispensing the
therapeutic materials to a desired location of a patient's body.
More particularly, the present invention relates to a method for
forming the structure of a stent or intravascular or intraductal
medical device.
[0005] 2. General Background and State of the Art
[0006] In a typical percutaneous transluminal coronary angioplasty
(PTCA) for compressing lesion plaque against the artery wall to
dilate the artery lumen, a guiding catheter is percutaneously
introduced into the cardiovascular system of a patient through the
brachial or femoral arteries and advanced through the vasculature
until the distal end is in the ostium. A dilatation catheter having
a balloon on the distal end is introduced through the catheter. The
catheter is first advanced into the patient's coronary vasculature
until the dilatation balloon is properly positioned across the
lesion.
[0007] Once in position across the lesion, a flexible, expandable,
preformed balloon is inflated to a predetermined size at relatively
high pressures to radially compress the atherosclerotic plaque of
the lesion against the inside of the artery wall and thereby dilate
the lumen of the artery. The balloon is then deflated to a small
profile, so that the dilatation catheter can be withdrawn from the
patient's vasculature and blood flow resumed through the dilated
artery. While this procedure is typical, it is not the only method
used in angioplasty.
[0008] In angioplasty procedures of the kind referenced above,
restenosis of the artery often develops which may require another
angioplasty procedure, a surgical bypass operation, or some method
of repairing or strengthening the area. To reduce the likelihood of
the development of restenosis and strengthen the area, a physician
can implant an intravascular prosthesis, typically called a stent,
for maintaining vascular patency. In general, stents are small,
cylindrical devices whose structure serves to create or maintain an
unobstructed opening within a lumen. The stents are typically made
of, for example, stainless steel, nitinol, or other materials and
are delivered to the target site via a balloon catheter. Although
the stents are effective in opening the stenotic lumen, the foreign
material and structure of the stents themselves may exacerbate the
occurrence of restenosis or thrombosis.
[0009] A variety of devices are known in the art for use as stents,
including expandable tubular members, in a variety of patterns,
that are able to be crimped onto a balloon catheter, and expanded
after being positioned intraluminally on the balloon catheter, and
that retain their expanded form. Typically, the stent is loaded and
crimped onto the balloon portion of the catheter, and advanced to a
location inside the artery at the lesion. The stent is then
expanded to a larger diameter, by the balloon portion of the
catheter, to implant the stent in the artery at the lesion. Typical
stents and stent delivery systems are more fully disclosed in U.S.
Pat. No. 5,514,154 (Lau et al.), U.S. Pat. No. 5,507,768 (Lau et
al.), and U.S. Pat. No. 5,569,295 (Lam et al.).
[0010] Stents are commonly designed for long-term implantation
within the body lumen. Some stents are designed for non-permanent
implantation within the body lumen. By way of example, several
stent devices and methods can be found in commonly assigned and
common owned U.S. Pat. No. 5,002,560 (Machold et al.), U.S. Pat.
No. 5,180,368 (Garrison), and U.S. Pat. No. 5,263,963 (Garrison et
al.).
[0011] Intravascular or intraductal implantation of a stent
generally involves advancing the stent on a balloon catheter or a
similar device to the designated vessel/duct site, properly
positioning the stent at the vessel/duct site, and deploying the
stent by inflating the balloon which then expands the stent
radially against the wall of the vessel/duct. Proper positioning of
the stent requires precise placement of the stent at the
vessel/duct site to be treated. Visualizing the position and
expansion of the stent within a vessel/duct area is usually done
using a fluoroscopic or x-ray imaging system.
[0012] Although PTCA and related procedures aid in alleviating
intraluminal constrictions, such constrictions or blockages reoccur
in many cases. The cause of these recurring obstructions, termed
restenosis, is due to the body's immune system responding to the
trauma of the surgical procedure. As a result, the PTCA procedure
may need to be repeated to repair the damaged lumen.
[0013] In addition to providing physical support to passageways,
stents are also used to carry therapeutic substances for local
delivery of the substances to the damaged vasculature. For example,
anticoagulants, antiplatelets, and cytostatic agents are substances
commonly delivered from stents and are used to prevent thrombosis
of the coronary lumen, to inhibit development of restenosis, and to
reduce post-angioplasty proliferation of the vascular tissue,
respectively. The therapeutic substances are typically either
impregnated into the stent or carried in a polymer that coats the
stent. The therapeutic substances are released from the stent or
polymer once it has been implanted in the vessel.
[0014] In the past, stents have been manufactured in a variety of
manners, including cutting a pattern into a tube that is then
finished to form the stent. The pattern can be cut into the tube
using various methods known in the art, including using a
laser.
[0015] Laser cutting of the stent pattern initially utilized lasers
such as the conventional Nd:YAG laser, configured either at its
fundamental mode and frequency, or where the frequency of the laser
light was doubled, tripled, or even quadrupled to give a light beam
having a desired characteristic to ensure faster and cleaner
cuts.
[0016] Recently, lasers other than Nd:YAG lasers have been used,
such as solid-state lasers that operate in the short pulse
pico-second and femto-second domains. These lasers provide improved
cutting accuracy, but cut more slowly than conventional lasers such
as the long pulse Nd:YAG laser.
[0017] The intensity of the light beam created by either
conventional long pulse or short pulse lasers such as pico-second
and femto-second lasers has a Gaussian distribution. A laser beam
having a Gaussian intensity distribution results in a beam having
higher energy intensity at the center of the beam spot, with
reduced energy as a function of distance from the center of the
beam spot. This results in a tapered cut when the laser beam cuts
through a material. In other words, the cut on the topside of the
material is wider than the exit of the laser beam through the
bottom side of the material.
[0018] When a laser having a Gaussian intensity distribution is
used to cut a stent strut the resulting tapered edge causes
difficulty in achieving overall dimensional stability after
electrochemical polishing. The tapered edge shape may also not be
ideal in carrying out its function when the stent is implanted in a
vessel, as the tapered strut may not be ideal in opposing the
vessel wall.
[0019] An additional problem with prior art systems that typically
have used lasers that generate long laser pulses with durations in
the microsecond range is that this type of laser removes material
using a mostly thermal process, with some degree of evaporation of
tubing material. In contrast, new lasers operate in the range of 10
pico-seconds (10.times.10.sup.-12 seconds) or shorter for stent
cutting, and remove material by way of ablation rather than a
thermal process.
[0020] The thermal process using long laser pulses can result in
molten material and slag, which may be redeposited upon the stent
surfaces, as well as surrounding surfaces of the cutting equipment.
The thermal process of the long pulse laser may also result in
production of a heat-affected zone in the stent tubing material.
This heat-affected zone, which occurs frequently when the stent
tube is cut by the long pulse laser in the presence of certain
reactive gases such as oxygen can result in embrittlement of the
stent material and thus decreases mechanical performance of the
stent material. In contrast, the short pulses of a pico-second or
femto-second laser removes material primarily through ablation
which results in minimal thermal damage and a reduction in the
amount of slag produced during the ablation process.
[0021] What has been needed, and heretofore unavailable, is an
efficient and cost-effective laser cutting system that provides for
improved cutting speeds and cut profiles. The present invention
satisfies these, and other needs.
SUMMARY OF THE INVENTION
[0022] In one aspect, the present invention is embodied in a system
that utilizes a laser beam that has been shaped using a shaping
module to modify the intensity profile of the laser beam. In other
aspects, the shaped laser beam has a more even intensity profile
across the relevant beam diameter than is typically delivered by a
non-shaped laser beam having a Gaussian profile. Control over such
a shaped laser beam produces cleaner surfaces and faster
fabrication times. Use of a shaped beam having an alternative
energy profile may also result in improved stent characteristics
that are advantageous to stent performance and function. For
example, shaped laser beam profiles may result in a steeper stent
sidewall than is obtainable using a laser beam with a Gaussian
intensity distribution, which may improve manufacturability and
performance of the stent. Moreover, use of shaped laser beams
formed in accordance with the various aspects of the present
invention may also result in improved stent cutting speeds, optical
characteristics, and drug retention characteristics. It will be
understood that the laser beam shaping technology described herein
can be used for forming other medical device components,
particularly where improved edge surfaces or component fits and the
like are required. For example, such a system could be used to
produce parts of a guidewire or catheter device. It may also be
used to provide for precise machining of pacemaker components.
[0023] In yet another aspect, the system of the present invention
includes a laser cutting system for cutting a stent pattern into a
stent, comprising a laser for producing a laser beam; a
laser-shaping module capable of altering the intensity profile of
the laser beam; and, a collimating lens.
[0024] In still another aspect, the present invention includes a
method from shaping a laser beam for cutting a stent pattern into a
stent, comprising providing a laser beam having a first intensity
distribution; and re-mapping the first intensity distribution of
the laser beam to a second intensity distribution.
[0025] In still another aspect, the present invention provides a
laser cutting process that enables greater control over the shape
of the stent strut. The improved dimensional control is achieved in
one aspect by offsetting the path of a laser beam from the central
axis of a stent tube. By offsetting the laser from the central axis
of the tube, the typical taper resulting from use of a Gaussian
laser beam may be virtually eliminated, producing a stent strut
with a much more rectangular shape.
[0026] In another aspect, more than one laser beam can be used,
with the more than one laser beams offset from the central axis of
the tube. In such an arrangement, the first laser beam may cut a
perpendicular stent wall on one stent strut while a second beam,
offset from both the central axis of the tube and the cutting axis
of the first laser beam, is used to cut a second stent wall.
[0027] In yet another aspect, a single laser beam can be used to
cut both sides of a stent strut to result in a rectangular strut.
The laser beam is moved from a first position having a first offset
angle to a second position having a second offset angle so that the
walls of each stent strut may be cut by the laser. In an
alternative aspect, the stent tubing may be moved relative to the
laser to accomplish the same result. In still another aspect, both
the laser and the tube may be moved simultaneously to one another
to achieve the same effect.
[0028] In a further aspect, the present invention includes a method
for cutting a stent pattern into a tube, comprising: remapping the
intensity distribution of a laser beam to a non-Gaussian intensity
distribution; and applying the non-Gaussian intensity beam to a
tube to remove at least a portion of a wall thickness of the
tube.
[0029] In yet a further aspect, applying the beam to the tube
includes exposing the tube to multiple passes of the laser beam. In
a still further aspect, remapping includes providing a non-Gaussian
intensity distribution to the laser beam, the non-Gaussian
intensity distribution having at least one characteristic resulting
in removing material from the wall of the tube in a selected
configuration. In still another aspect, the selected configuration
includes forming indentations on a surface of the tube.
[0030] In yet another aspect, the present invention includes a
system for cutting a stent pattern into a tube, comprising: a tube
mounted in a fixture, the tube having a central axis; a first laser
beam for cutting a portion of a pattern into the tube, the first
laser and tube arranged relative to one another such that the laser
beam is directed to a surface of the tube on an axis that is offset
from the central axis of the tube; a second laser beam for cutting
a second portion of the pattern in the tube, the second laser beam
and tube arranged relative to one another such that the laser beam
is directed to the surface of the tube on an axis that is offset
from the central axis of the tube and the axis of the first laser
beam.
[0031] In yet another aspect, the present invention includes a
system for cutting stent patterns into a tube, comprising: a tube
mounted in a fixture, the tube having a central axis; a laser for
providing a laser beam for cutting a portion of a pattern into the
tube, the laser beam and tube moveable with respect to one another
such that the laser beam is oriented in a first position relative
to the tube and the beam directed to a first surface of the tube on
an axis that is offset from the central axis.
[0032] In another aspect, the laser beam or tube are moved to a
second position such that the laser beam is directed to a second
surface of the tube, the axis of the laser beam being offset to the
central axis of the tube by an offset different from the offset
used to cut the first surface of the tube. In yet another aspect, a
rectangular strut is produced.
[0033] In still another aspect, the present invention includes a
system and method for cutting patterns into a tube using a series
of laser passes to cut the pattern. In one aspect, the system makes
a first cut of the pattern to a depth of less than the wall
thickness of the tube using a long pulse laser. One or more
additional passes are then performed using a short pulse laser,
such as a pico-second laser, to complete the cut. In yet another
aspect, a long wavelength pico-second laser is used to make the
first cut, then a short wavelength pico-second laser is used to
complete the cut.
[0034] In another aspect, the present invention includes systems
and methods where a long pulse laser and a short pulse laser are
mounted to a common base and the tube is passed first below the
long pulse laser and then below the short pulse laser in a
continuous cutting process.
[0035] In yet another aspect, the first cutting operation is
carried out on one laser cutting station and then the tube is moved
to a second cutting station where the cutting of the stent patter
is completed by a short pulse laser.
[0036] In still another aspect, the present invention includes use
of a laser capable of being configured to operate in a long pulse
mode to make a first cut and then being capable of being
reconfigured to operate in a short pulse mode to make a second
cut.
[0037] In yet another aspect, the present invention includes a
method of cutting a stent pattern into a tube, comprising: mounting
a tube is a fixture; cutting a pattern into the tube to a depth
less than a wall thickness of the tube using a long pulse laser,
the laser and fixture controlled by a computer to provide for
relative motion between the long pulse laser and the tube; and
cutting the pattern into the tube to a depth greater than the wall
thickness of the tube using a short pulse laser, the short pulse
laser and fixture controlled by the computer to provide for
relative motion between the short pulse laser and the tube.
[0038] In another aspect, the tube may be moved to a different
location before the pattern is cut by the short pulse laser.
[0039] In still another aspect, the long pulse laser and the short
pulse laser and fixture are mounted on a common base.
[0040] In a further aspect, the present invention also includes a
method for cutting a stent pattern into a tube, comprising:
mounting a tube is a fixture; configuring a laser to operate in a
long pulse mode; cutting a pattern into the tube to a depth less
than a wall thickness of the tube using the laser operating in the
long pulse mode, the laser and fixture controlled by a computer to
provide for relative motion between the laser and the tube;
configuring the laser to operate in a short pulse mode; and cutting
the pattern into the tube to a depth greater than the wall
thickness of the tube using the laser operating in the short pulse
mode, the laser and fixture controlled by the computer to provide
for relative motion between the laser and the tube.
[0041] In a still further aspect, the present invention includes a
method for a stent pattern having multiple cells into a tube,
comprising mounting a tube in a fixture, cutting each cell of the
pattern into the tube using a laser, completing the cutting of each
cell before beginning cutting of the next cell until all of the
cells are cut.
[0042] Other features and advantages of the invention will become
apparent from the following detailed description, taken in
conjunction with the accompanying drawings, which illustrate, by
way of example, the features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 is a partial view of a stent showing various elements
of a stent pattern.
[0044] FIG. 1a is a cross-sectional view of a portion of one of the
elements of the stent pattern.
[0045] FIG. 2 is a side view of a typical arrangement of a computer
controlled cutting station for cutting stent patterns into tubing
using a laser beam.
[0046] FIG. 3 is a graphical illustration of an embodiment of
layout of a laser system incorporating aspects of the present
invention.
[0047] FIG. 4 is a graphical illustration of a laser beam intensity
profile of a standard laser showing a typical Gaussian profile.
[0048] FIG. 5 is a prospective view illustrating a typical ablation
of a surface using a laser having the beam profile of FIG. 4.
[0049] FIG. 6 is a cross-sectional view of the surface of FIG. 5
showing a conical profile of the cut similar in shape to the
Gaussian profile of FIG. 4.
[0050] FIG. 7 is a graphical illustration of a laser beam intensity
profile of a laser beam shaped in accordance with aspects of the
present invention.
[0051] FIG. 8 is a cross-sectional view of an ablation profile in a
surface that was created using a laser beam having the beam
intensity profile of FIG. 7.
[0052] FIG. 9 is a cross-sectional view of stent struts of the
stent pattern of FIG. 1 cut by a laser beam having the Gaussian
intensity profile of FIG. 4.
[0053] FIG. 10 is a cross-sectional view of a stent strut cut by a
shaped laser beam having an intensity profile similar to that shown
in FIG. 8.
[0054] FIG. 11 is a graphical illustration of a laser beam
intensity profile of a laser beam shaped in accordance with an
alternative embodiment of the present invention.
[0055] FIG. 12 is a cross sectional view of an ablation profile in
a surface that was created using a laser beam having the beam
intensity profile of FIG. 11.
[0056] FIG. 13 is a cross-sectional view depicting the central axis
of a tubing used into which a stent pattern is to be cut using a
laser beam and also showing the beam path of a laser that is
directed at the stent tubing along a line that is offset from the
tubing central axis.
[0057] FIG. 14 is a cross-sectional view illustrating a stent strut
cut using the offset laser beam of FIG. 13.
[0058] FIG. 15 is a cross-sectional view of stent struts cut using
a multiple laser beam system, with one laser beam offset from the
central axis of the tubing, and a second laser being offset at a
different angle to the central axis of the stent tubing.
[0059] FIG. 16 is a cross-sectional view of an exemplary
rectangular stent strut cross-section produced using the multiple
offset laser beams shown in FIG. 15.
[0060] FIG. 17 is a cross-sectional view of another embodiment of
the present invention wherein a single laser is used and cuts at a
first angle which is offset from the central axis of the tube, and
is then moved to a second position at a second angle offset from
the central axis of a tube to make a second cut.
[0061] FIG. 18 is a cross-sectional side view of a tubing wall
illustrating the depth of cut of a first pass of a laser, and the
depth of the cut using a second pass of a laser.
[0062] FIG. 19 is a top perspective view depicting use of multiple
laser beams to cut a groove and through cut in the surface of a
tube.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0063] FIG. 1 is an enlarged perspective view of a stent 10
illustrating an exemplary stent pattern and showing the placement
of interconnecting elements 15 between adjacent radially expandable
cylindrical elements. Each pair of the interconnecting elements 15
on one side of a cylindrical element are preferably placed to
achieve maximum flexibility for a stent. In the embodiment shown in
FIG. 1, the stent 10 has three interconnecting elements 15 between
adjacent radially expandable cylindrical elements which are 120
degrees apart. Each pair of interconnecting elements 15 on one side
of a cylindrical element are offset radially 60 degrees from the
pair on the other side of the cylindrical element. The alternation
of the interconnecting elements results in a stent which is
longitudinally flexible in essentially all directions. Various
configurations for the placement of interconnecting elements are
possible. However, as previously mentioned, all of the
interconnecting elements of an individual stent should be secured
to either the peaks or valleys of the undulating structural
elements in order to prevent shortening of the stent during the
expansion thereof.
[0064] The number of undulations may also be varied to accommodate
placement of interconnecting elements 15, for example, at the peaks
of the undulations or along the sides of the undulations as shown
in FIG. 1.
[0065] As best observed in FIG. 1, cylindrical elements in this
exemplary embodiment are shown in the form of a serpentine pattern.
As previously mentioned, each cylindrical element is connected by
interconnecting elements 15. The serpentine pattern is made up of a
plurality of U-shaped members 20, W-shaped members 25, and Y-shaped
members 30, each having a different radius so that expansion forces
are more evenly distributed over the various members.
[0066] The afore-described illustrative stent 10 and similar stent
structures can be made in many ways. However, the preferred method
of making the stent is to cut a thin-walled tubular member, such
as, for example, stainless steel tubing to remove portions of the
tubing in the desired pattern for the stent, leaving relatively
untouched the portions of the metallic tubing which are to form the
stent. In accordance with the invention, it is preferred to cut the
tubing in the desired pattern by means of a machine-controlled
laser, as exemplified schematically in FIG. 2.
[0067] The tubing may be made of suitable biocompatible material
such as, for example, stainless steel. The stainless steel tube may
be Alloy type: 316L SS, Special Chemistry per ASTM F138-92 or ASTM
F139-92 grade 2. Special Chemistry of type 316L per ASTM F138-92 or
ASTM F139-92 Stainless Steel for Surgical Implants. Other
biomaterials may also be used, such as various biocompatible
polymers, co-polymers or suitable metals, alloys or composites that
are capable of being cut by a laser.
[0068] Another example of materials that can be used for forming
stents is disclosed within U.S. application Ser. No. 12/070,646,
the subject matter of which is intended to be incorporated herein
in its entirety, which application discloses a high strength, low
modulus metal alloy comprising the following elements: (a) between
about 0.1 and 70 weight percent Niobium, (b) between about 0.1 and
30 weight percent in total of at least one element selected from
the group consisting of Tungsten, Zirconium and Molybdenum, (c) up
to 5 weight percent in total of at least one element selected from
the group consisting of Hafnium, Rhenium and Lanthanides, in
particular Cerium, (d) and a balance of Tantalum
[0069] The alloy provides for a uniform beta structure, which is
uniform and corrosion resistant, and has the ability for conversion
oxidation or nitridization surface hardening of a medical implant
or device formed from the alloy. The tungsten content of such an
alloy is preferably between 0.1 and 15 weight percent, the
zirconium content is preferably between 0.1 and 10 weight percent,
The molybdenum content is preferably between 0.1 and 20 weight
percent and the niobium content is preferably between 5 and 25
weight percent.
[0070] The stent diameter is very small, so the tubing from which
it is made must necessarily also have a small diameter. Typically
the stent has an outer diameter on the order of about 0.06 inch in
the unexpanded condition, the same outer diameter of the tubing
from which it is made, and can be expanded to an outer diameter of
0.1 inch or more. The wall thickness of the tubing is about 0.003
inch or less.
[0071] Referring now to FIG. 2, the tubing 50 is put in a rotatable
collet fixture 55 of a machine-controlled apparatus 60 for
positioning the tubing 50 relative to a laser 65. According to
machine-encoded instructions, the tubing 50 is rotated and moved
longitudinally relative to the laser 65 which is also
machine-controlled. The laser selectively removes the material from
the tubing and a pattern is cut into the tube. The tube is
therefore cut into the discrete pattern of the finished stent.
[0072] The process of cutting a pattern for the stent into the
tubing is automated except for loading and unloading the length of
tubing. Referring again to FIG. 2, it may be done, for example,
using a CNC-opposing collet fixture 55 for axial rotation of the
length of tubing, in conjunction with a CNC X/Y table 70 to move
the length of tubing axially relatively to a machine-controlled
laser as described. Alternatively, the collet fixture may hold the
tube at only one end, leaving the opposite end unsupported. The
entire space between collets can be patterned using the laser. The
program for control of the apparatus is dependent on the particular
configuration used and the pattern to be cut by the laser.
[0073] In one embodiment, the invention includes an apparatus and
method for laser cutting a stent with a shaped laser beam to
produce beneficial stent characteristics. The embodiment utilizes a
laser beam-shaping module to modify the intensity profile of a
laser beam. The modified intensity profile results in beneficial
stent characteristics, such as better-formed sidewalls with a
cleaner cut surface. Using a modified intensity profile, the stent
surface may also be modified by using various beam shapes to
produce additional benefits, such as optical or coating retention
improvements.
[0074] Referring now to FIG. 3, an embodiment of an exemplary
pico-second laser system optical layout 100 is shown. In this
embodiment, a pico-second laser 105 generates a laser beam 110
having a Gaussian intensity distribution that is then is directed
through a shutter 115 onto a minor 120. Although shown in this
exemplary embodiment, one skilled in the art will appreciate that
mirror 120 is not necessary if the design requirements of the
system allow the laser and other optical components to be arranged
in a linear fashion.
[0075] The reflected beam then passes through a half wave plate
125, polarizer 130 and quarter wave plate 135, before entering a
beam expander 140. Beam expander 140 is used to control the spot
size of the beam. The laser beam 110 exits the beam expander and
enters a laser beam shaper, such as a LaseRemap, manufactured by
Lambda Research Inc. Within the beam shaper, the Gaussian intensity
distribution of the laser beam is transformed to an Airy pattern.
Passing through a collimating lens 150, the collimated Airy pattern
laser beam may then be reflected by a mirror 155, although minor
155 is not necessary to the performance of the invention, the
inclusion of such a mirror may be beneficial in compacting the
optical arrangement to reduce the size of the optical train. The
collimated Airy pattern laser beam undergoes a Fourier transform
through a focusing lens 160, resulting in a focused beam intensity
profile that appears similar to a top hat. This beam profile is
then directed onto the work piece 165.
[0076] The modified laser beam is then directed toward a work
piece, for example, a piece of tubing to be cut into a stent.
Alternatively, minor 155 may also be partially transmissive,
allowing the cutting of the work piece 165 to be observed by a
camera or other view device 180 that views the work piece through a
filter and lens assembly 170, 175.
[0077] In an alternative embodiment, collimating lens 150 may be
located after the beam 110 is reflected by mirror 155. This
arrangement may be advantageous depending on the overall layout of
the optical system and sizing requirements of the system.
[0078] The laser beam shaper may typically be formed of two fused
silica plano-convex aspheric lenses, arranged with the convex
surfaces facing each other. In this arrangement, the device takes
the general form of a Keplerian telescope, with a radially varying
magnification. Because of the optics of the beam shaper, the
Gaussian intensity distribution of the laser beam is modified to a
non-Gaussian distribution, as will be discussed below in more
detail. The distance between the two lenses of the beam shaper may
be changed to generate a variety of beam intensity profiles.
However, for reasons that will be discussed in more detail below,
an arrangement where the beam intensity profile takes the general
form of a "top hat" is generally preferred.
[0079] The focal length of the collimator has an effect on the
final spot size of the shaped laser beam. However, the final focal
spot size of the shaped beam is limited mainly by the beam
shaper.
[0080] Further details of a beam-shaping module in accordance with
aspects of the present invention are included in "Lambda Research
Optics, Inc."; "Aspheric Laser Beam Reshaper Applications Guide;"
by C. Michael Jefferson and John A. Hoffnagel; "Transformation of a
Gaussian Laser Beam to an Airy Pattern for use in Focal Plane
Intensity Shaping Using Diffractive Optics," by Kurt Kanzler; and
U.S. Pat. No. 6,975,458 issued to Kanzler entitled "Method and
Apparatus for Transformation of a Gaussian Laser Beam to Far Field
Diffraction Pattern." The subject of each of these references is
intended to be incorporated herein in their entirety.
[0081] FIG. 4 depicts a typical Gaussian laser beam intensity
profile for a laser that has not undergone shaping. As can be seen
in FIG. 4, the intensity towards the center of the beam is much
greater than the intensity at the edges of the beam spot. This drop
off in intensity occurs quite rapidly as a function of distance
from the beam center. As a result, the cut profile produced by
directing this laser beam toward a piece of material has a
similarly varied curvature, as the rate of material removal due to
ablation is dependent on the applied laser beam intensity.
[0082] An example of the cut profile produced by directing a laser
beam with Gaussian intensity profile is shown in FIG. 5. A Gaussian
intensity profile as depicted in FIG. 4 typically produces a
generally conical profile with the central portion of the cut area
being deeper than the edges of the cut area. As one skilled in the
art would expect, the profile of the cut area is similar to the
intensity profile of the laser beam. This is further illustrated by
FIG. 6, which shows a cross-section of a cut profile produced by a
laser beam having a Gaussian intensity profile. One important
aspect to be noted from FIG. 6 is that the cut profile shows
sidewalls 205 having a positive taper, that is, the area of the cut
on the top of the work piece where the laser beam is directed is
larger than at the bottom of the of the cut.
[0083] The effect of such a Gaussian intensity profile on a cut
stent can be seen by referring to FIG. 9. FIG. 9 shows a
cross-section of struts 305, 310 of a stent cut with a laser beam
having a Gaussian intensity profile. The sidewalls 315, 320 of the
stent struts 305, 310 are sloped, as would be expected from the cut
profile shown in FIG. 6. This results in a stent strut having a
non-uniform shape, such as, for example, and as illustrated in FIG.
6, having a top side that is larger that the side directly
opposite.
[0084] FIG. 7 depicts a laser beam intensity profile for laser beam
that has undergone reshaping in accordance with the principles of
the present invention. This profile illustrates one example of a
beam generated by the system of FIG. 3 wherein the beam shaper is
adjusted to produce a beam intensity having what is called in the
art a "top hat" or "flat top" profile. In such an intensity
profile, the intensity of the beam is relatively constant across
the diameter of the laser beam, unlike the intensity profile of the
Gaussian beam depicted in FIG. 4. Accordingly, the edges of the
shaped beam have approximately the same intensity as the center of
the beam, providing reduced intensity drop off at the beam's
edge.
[0085] FIG. 8 is a cross-sectional view of a tube wall that has
been cut using a shaped beam having an intensity profile similar to
the profile shown in FIG. 7. The flat top profile of the shaped
beam creates a cut with a relatively uniform depth across the cut
diameter. Further, due to the uniformity of the beam intensity
across the beam diameter, the edges 255 of the cut are much
steeper, that is, having less taper, as compared to the cut
produced by a Gaussian beam depicted in FIG. 9.
[0086] FIG. 10 shows a stent strut 400 cut with a laser beam having
a flat top intensity profile as is shown in FIG. 7. It is quickly
apparent that the stent strut 400 has a much steeper sidewall 405.
This steep sidewall 405 is preferred over tapered sidewalls such as
are shown in FIG. 9 because the resulting overall strut geometry is
much more uniform, as illustrated by the similar widths of strut
top 410 and strut bottom 415. Sloped or tapered sidewalls are
disadvantageous in that they produce non-uniform geometries that
can affect stent performance. Also, manufacturing processes such as
electro-polishing and sandblasting are better suited to non-sloped
stent struts because they will produce more desirable strut
geometries.
[0087] The shape of the beam intensity profile affects the
distribution of energy across the surface of the material being
cut, which has manufacturing and as-cut geometry implications. A
Gaussian beam profile inherently applies much more energy to the
center of the beam spot when such a laser beam is cutting material.
As a result, the center portion of the cut material will be heated
much more quickly than the edges of the cut material. This is an
inefficient energy distribution since the edges of the cut area
will be melted/ablated using a relatively low intensity of the
laser beam, increasing the total time necessary to cut through the
tubing wall. In contrast, a beam having a top hat or flat intensity
profile shaped in accordance with principles of the present
invention applies energy uniformly across the surface to be cut,
providing more efficient delivery of energy to the entire cut area
and resulting in faster fabrication times.
[0088] The intensity distribution within the beam cutting area may
also affect the quantity and flow characteristics of slag/debris
that is formed during the laser cutting process. It is believed by
the inventors that more slag/debris more is formed with a laser
beam having a Gaussian intensity profile then with a shaped laser
beam having a flat top intensity profile. Also, the slag/debris
formed with the Gaussian laser beam may flow in such a way that a
significant amount of slag remains on a stent strut after cutting.
This is disadvantageous in that such slag/debris contamination
requires additional time and resources for post-processing of the
as-cut stent.
[0089] Shaping the laser beam in accordance with the present
invention may result in other useful beam shapes. An example of
such an alternative beam shape is shown in FIG. 11. In this
example, the beam shaper is adjusted to provide a laser beam having
an intensity profile where the beam is more intense toward the
outer beam edges. Such an intensity profile appears to have "ears."
When such a shaped beam is applied to a material, the cut profile
resembles that shown in FIG. 12.
[0090] As shown in FIG. 12, a beam having an energy profile as
shown in FIG. 11 results in a greater depth of cut at the edges of
the cut profile than towards the center of cut. Such an intensity
profile may be advantageous in that fewer laser pulses may be
required to cut entirely through the stent tubing. Cutting the
tubing in this matter effectively results in ablating only a ring
of material through the full thickness of the work piece, leaving a
"plug" as a remnant that is then easily removed during post
process. One skilled in the art will understand that focusing the
intensity in such a manner will result in faster cutting of the
tube than if the beam intensity is shaped in such a manner that the
intensity is evenly distributed across the entire area of the
beam.
[0091] Additionally, a shaped laser beam having an intensity
profile as shown in FIG. 11 may be used to form a cut profile such
as that shown in FIG. 12. In this manner, craters, divots or
indentations 430 may be created in the surface of the tubing. Such
divots or indentation can be used to alter the absorption and
reflectance characteristics of the surface of the final stent or
work piece.
[0092] A shaped beam may also be used to mark the stent surface for
marketing or functional purposes, such as by producing a barcode, a
serial number, or a logo directly on the stent. Such marking could
also be used for data tracking. Since the shaped laser beam
intensity is well distributed, minimal material would need to be
removed to effectively mark the stent, resulting in little, if any,
change in the mechanical characteristics of the stent.
[0093] Divots or other features formed in the surface of a stent or
other device using a shaped laser beam may also be used as
reservoirs for the retention of drugs on stent surface. As shown in
FIG. 12, a divot or indentation 430 may be formed in the surface of
the stent by use of a shaped laser beam having an intensity profile
as illustrated in FIG. 11. Such a divot or indentation will have a
circumferential dip along the outer edge of the divot or
indentation, forming a well into which a drug or polymer may be
introduced. Such a profile results in improved retention of a drug
or polymer disposed within the well. Further, using a shaped laser
beam, the wells can be formed below the surface of the stent,
minimizing contact with the surrounding vessel wall during delivery
of the stent within a patient's vasculature.
[0094] Referring again to FIG. 9, a typical stent cross-section
formed using a Gaussian laser beam directed at the central axis of
the stent tubing is shown. As can be seen from FIG. 9, the edges of
the stent struts are not ideal, rather, they are tapered which is
due to the Gaussian energy distribution of the energy of the laser
beam. As explained previously, when such a laser beam is used to
cut a material, the profile of the cut will be tapered from top to
bottom reflecting the Gaussian distribution of intensity of the
laser beam. In other words, the diameter of the cut area is greater
at the top surface of the material being cut than the diameter of
the exit cut located at the bottom of the material being cut.
[0095] The tapered profile of a strut edge cut using a Gaussian
laser beam is not ideal, since it may result in difficulty in
achieving over dimensional stability after the stent strut is
electrochemically polished. Moreover, such a profile may not be
ideal in terms of the function of the stent strut, which is to
oppose a vessel wall when the stent has been implanted inside the
lumen of a vessel.
[0096] FIG. 13 shows a cross-section of a length of tubing 500 to
be formed into a stent that has been mounted into a collet fixture
for laser cutting. Typically, the tubing is positioned under the
laser beam such that the laser beam impinges upon the tubing along
a radius drawn through the tubing central axis, shown as reference
numeral 505. In one embodiment of the present invention, the tubing
and laser are instead mounted relative to each other such that the
beam path from the laser is no longer directed along a radius of
the central axis 505 of the tubing 500, but rather is directed at a
path 510 which is offset from the central axis 505 of the tube.
[0097] FIG. 14 depicts the result of cutting a stent pattern using
an off axis beam path 510. As shown, the taper along one edge 515
of the stent strut is steep and approximately perpendicular to the
inner strut surface, while the taper along edge 520 of a
neighboring strut is not approximately perpendicular, but rather
subtends some angle. Because it is desirable to produce a stent
strut having two perpendicular sides, a further modification to
this system may be made to achieve a completely perpendicular strut
cross-section.
[0098] One embodiment for achieving a completely perpendicular
strut cross-section is depicted in FIG. 15. In this embodiment, a
second laser is used to direct a laser beam along a separate path
525 which is offset from the central axis of the stent tube and
offset from the first path 510. Use of the two laser beams results
in cutting two steep sidewalls on adjacent stent struts 515, 522.
In this embodiment, the beam spot may be adjusted so that one beam
is used to cut one strut wall side and the second beam is used to
produce another side of the strut. In this manner, opposing strut
walls 515, 522 may be cut such that both strut walls are
perpendicular to the bottom and top edges of the respective struts.
As depicted in FIG. 16, such a strut 550 will have an approximately
rectangular cross-section, as depicted in FIG. 16, with each of the
four sides of strut forming an approximate right angle 555 with an
adjacent side of the strut. While the strut 550 is illustrated as
being square, one skilled in the art will understand that such a
strut may also be formed to have a rectangular shape.
[0099] FIG. 17 illustrates an alternative embodiment of the present
invention wherein a single laser beam may be used to form the strut
illustrated in FIG. 16. In this embodiment, a single laser 600 is
used to perform a first cut along a path 605 that is offset from
the radial axis of tube 500. Laser 600 is then moved relative to
the tube such that the beam then impinges the tube 600 along a
second path 610. Alternatively, the tube may be shifted relative to
the laser beam to achieve the same effect. In still other
alternative embodiments, the orientation of the tube 500 and the
laser 600 may be moved simultaneously to accomplish the same
result.
[0100] The process parameters used for pico-second ablation of a
stent pattern differ from those used in removing material using
conventional thermal process with long laser pulse duration. First,
the parameters used to set up the laser are different. Long pulse
lasers use a relatively high average power, low peak power and high
process gas pressure for stent cutting. Using such settings
typically produce stents with undesirable heat-affected zones,
rough sidewalls, molten material, and slag.
[0101] In contrast, a pico-second or femto-second laser capable of
ablating material uses low average power, high peak power and low
process gas pressure. Use of such a short pulse laser typically
results in a stent having cleaner edges with reduced heat-affected
zones, smoother sidewalls and negligible molten material and
slag.
[0102] Another difference between the use of a thermal process
lasers and the ablation process laser is that the motion control
program must be set up differently. Unlike lasers which utilize a
thermal process to cut the entire stent pattern in a single pass,
short pulse lasers that use ablation to cut the stent cut a
closed-loop path, commonly referred to as a cell, in the stent
pattern using multiple passes before moving onto the next cell.
This multiple pass cutting method is necessary because the short
pulse laser removes far less material with each pass than a thermal
process laser. Accordingly, the pattern cutting times are much
longer using a short pulse laser than have typically been
experienced using conventional long-pulse thermal processes.
[0103] There are many different combinations of movements that can
be used to translate a stent beneath a laser beam, or to translate
the laser beam over a stent tube. For example, a single-pass method
is typically used when the tubing is being cut by a long-pulse
thermal affect laser. However, such a single-pass method results in
significant heating of the tubing, producing heat affected zones in
the stent material as well as production of slag and debris.
[0104] The damage caused in the single-pass method may be reduced
by use of a multiple-pass method, wherein a significant portion of
the stent pattern, or even the entire stent pattern, is cut across
the stent using multiple passes of a laser or lasers. Using such a
method, the heat input can be reduced for any given pass and
consequently the heat-affected zone on the stent is reduced.
However, as the laser traverses the tubing in each pass, the
ablated material reduces the overall rigidity of the tubing, which
may result in a less precise cut of the pattern if the tube flexes
or bends during the cutting process.
[0105] One embodiment of the present invention utilizes a process
referred to hereafter as a cell-multiple-pass method to cut the
stent pattern into the stent. In this embodiment, only a single
cell of the stent pattern is cut into the tubing at a time. When
one cell of the stent is completed, the tubing is traversed to
allow the laser to cut the next cell. Using this method and a short
pulse laser, each cell may require multiple passes of the laser to
cut the pattern. This embodiment is advantageous when compared to
the single-pass method because there is less heat input to the
tubing for any given pass, thus resulting in a reduction of the
heat-affected zone created in the stent. Furthermore, the
cell-multiple-pass method is also advantageous over the other
multi-pass methods where the entire stent pattern of the stent is
cut into the stent using multiple passes of a laser in that the
cell-multiple pass method cuts only a single cell at the time,
leaving surrounding cells uncut, and thus contributing to the
overall rigidity of the tubing, which resulting in a more precise
cut during repetitive cutting.
[0106] While use of lasers with extremely short pulses, such as a
pico-second lasers, have been shown to be effective in generating
minimum-slag and sharp and smooth edges and sidewalls, the cutting
process does tend to require longer manufacturing times, because
less material is removed with each pass of the laser. For example,
it may take several times longer for a pico-second laser to cut
through stent tubing as it would take a nanosecond or microsecond
pulse laser.
[0107] Another embodiment of the present invention includes a
method for laser cutting of a stent using multiple passes of a
laser-cutting beam over the desired stent pattern. In this
invention, the first laser pass does not cut through the entire
material thickness, but instead forms a groove. For example, the
groove may be formed using a nanosecond pulse laser or a
microsecond pulse laser such as a fiber laser, and it may have a
depth of at least half the material thickness.
[0108] After the first pass is completed, one or more additional
laser passes are performed using a different laser, such as a short
pulse or pico-second laser that completes the cut and forms a
better surface finish on the sidewalls. The short pulse laser is
passed over the same stent pattern cut by the long pulse laser to
cut the material remaining in the base of the groove base to
complete cutting pattern through the entire thickness of material
being cut. Alternatively, a longer wavelength laser may be used for
the first pass or passes followed by use of a shorter wavelength
laser to make the final pass. The shorter wavelength laser may be
same laser as the longer wavelength laser, with the shorter
wavelength light obtained through a frequency conversion of the
longer wavelength light.
[0109] FIG. 18 is a cross-sectional side view of a tubing wall 700
illustrating the results of using such a multi-pass laser cutting
system. A first pass region 705 is cut into tube wall 700 using,
for example, a nanosecond pulse laser or a microsecond pulse laser.
As illustrated in FIG. 18, the first pass region may be at least
half of the thickness of the tube wall 700. A second pass region
710, which may include completion of the cut through the entire
thickness of tube wall 700, may be cut using a second laser, for
example, a short pulse laser, such as a pico-second laser. Use of
the short pulse laser results, in a much better wall configuration
and finish with a significant reduction in slag formation.
[0110] The consecutive laser passes may occur in series within the
same machine, such as is illustrated in FIG. 19. In the embodiment
shown FIG. 19, a laser cutting head may include two lasers rather
than one. The first pass laser 720 may be, for example, a long
pulse, thermal affect laser, such as a conventional Nd:YAG laser,
and the second pass laser 730 may be, for example a short pulse,
ablation type laser, such as a pico-second laser. A stent tubing
700 mounted in a movement assembly is traversed underneath both the
first pass and second pass lasers 720, 730. As the tubing is
traversed below the first pass laser 720, a groove 725 is formed.
As the tubing continues to be traversed, the groove 725 will
eventually fall beneath the second pass laser 720, which will
complete the cut 735 of the stent pattern through the thickness of
the tube 700. Those skilled in the art will understand that
alternatively, the lasers 720 and 730 may be traversed over the
tubing and still obtain the same result.
[0111] Alternatively, the consecutive laser passes may occur by
transferring the stent tube from a first laser cutting station
where the first laser pass is made to a second laser cutting
station where the second laser pass is made. Such a method requires
optics, photodetectors, and computer control necessary to ensure
that the second laser pass is performed directly over the groove
resulting from the first pass.
[0112] In yet another embodiment, a laser that is being capable of
being reconfigured from a long pulse laser to a short pulse laser
may be used. In this embodiment, the laser may be configured as a
long pulse laser and makes the first pass over the stent tubing,
cutting a significant portion of the finished depth of the pattern
cut. After the first stage of the laser pattern is cut using the
laser set in long pulse, thermal, mode, the laser is reconfigured
to operate the laser in a short pulse mode to complete an
additional pass or passes as necessary to cut the pattern all the
way through the stent tubing. Changing the configuration or mode of
such a laser requires altering various laser parameters, such as,
for example, laser pulse width and power level, which can easily be
done under control of a computer processor that is operating under
appropriate software command.
[0113] One advantage of the various embodiments incorporating or
combining a first pass long pulse laser and a second pass short
pulse laser is that it balances surface finish and speed of laser
cutting. Such a process is capable of creating a stent with an edge
quality similar to the edge of the stent shown in FIG. 16 with
little if any slag formation. However, since the first laser pass
can remove material at a faster rate than the second pass which is
performed using the short pulse pico-second laser, the two laser
passes can be completed faster than would be possible using a
single pass of a short pulse pico-second laser alone.
[0114] The various embodiments of the present invention provide for
improved control over stent strut geometry, which contributes to
greater dimensional stability and may enhance performance
characteristics such as wall opposition and stent retention when
the stent is implanted in the lumen of a vessel. The various
embodiments may also be used to produce other stent geometries,
including strut walls that are tapered either toward the inner or
outer surface of the stent.
[0115] The various embodiments of the present invention are also
advantageous in that they may utilize a laser beam that has been
shaped using a shaping module to modify the intensity profile of
the laser beam. In some embodiments, the shaped laser beam has a
more even intensity profile across the relevant beam diameter than
is typically delivered by a non-shaped laser beam having a Gaussian
profile. Control over such a shaped laser beam produces cleaner
surfaces and faster fabrication times. Use of a shaped beam having
an alternative energy profile may also result in improved stent
characteristics that are advantageous to stent performance and
function. For example, shaped laser beam profiles may result in a
steeper stent sidewall, which may improve manufacturability and
performance of the stent. Moreover, shaped laser beams may result
in improved stent cutting speeds, optical characteristics, and drug
retention characteristics. It will be understood that the laser
beam shaping technology described herein can be used for forming
other medical device components, particularly where improved edge
surfaces or component fits and the like are required. For example,
such a system could be used to produce parts of a guidewire or
catheter device. It may also be used to provide for precise
machining of pacemaker components.
[0116] It will be apparent from the foregoing that the present
invention provides a new and improved method and apparatus for
direct laser cutting of metal stents enabling greater precision,
reliability, structural integrity and overall quality, without
burrs, slag or other imperfections which might otherwise hamper
stent integrity and performance. Other modifications and
improvements may be made without departing from the scope of the
invention. Accordingly, it is not intended that the invention be
limited, except as by the appended claims.
* * * * *