U.S. patent application number 11/704076 was filed with the patent office on 2007-06-14 for embolic filters with controlled pore size.
This patent application is currently assigned to ev3 Inc.. Invention is credited to Kent D. Anderson, Marwane S. Berrada, Thomas L. Clubb, Richard S. Kusleika.
Application Number | 20070135834 11/704076 |
Document ID | / |
Family ID | 32770403 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070135834 |
Kind Code |
A1 |
Clubb; Thomas L. ; et
al. |
June 14, 2007 |
Embolic filters with controlled pore size
Abstract
The invention provides a device for filtering emboli from blood
flowing through a lumen defined by the walls of a vessel in a
patient's body comprising a filter element. The filter is
expandable from a collapsed configuration when the filter element
is restrained to an expanded configuration when the filter element
is unrestrained, and the filter element comprises a self-expanding
material having pores. When the filter element is in the expanded
configuration, the average pore size is from 30 to 300 microns and
the standard deviation of the pore size is less than 20 percent of
the average pore size.
Inventors: |
Clubb; Thomas L.; (Hudson,
MI) ; Kusleika; Richard S.; (Eden Prairie, MN)
; Anderson; Kent D.; (Champlin, MN) ; Berrada;
Marwane S.; (Minneapolis, MN) |
Correspondence
Address: |
POPOVICH, WILES & O'CONNELL, PA;650 THIRD AVENUE SOUTH
SUITE 600
MINNEAPOLIS
MN
55402
US
|
Assignee: |
ev3 Inc.
|
Family ID: |
32770403 |
Appl. No.: |
11/704076 |
Filed: |
February 8, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10354679 |
Jan 30, 2003 |
|
|
|
11704076 |
Feb 8, 2007 |
|
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Current U.S.
Class: |
606/200 |
Current CPC
Class: |
A61F 2230/0067 20130101;
A61F 2230/0006 20130101; D10B 2509/06 20130101; A61F 2230/008
20130101; D04C 1/02 20130101; A61F 2230/0008 20130101; D04B 1/16
20130101; D04C 3/48 20130101; A61F 2002/018 20130101; A61F 2/013
20130101; A61F 2/01 20130101; A61F 2230/0069 20130101 |
Class at
Publication: |
606/200 |
International
Class: |
A61M 29/00 20060101
A61M029/00 |
Claims
1. A device for filtering emboli from blood flowing through a lumen
defined by the walls of a vessel in a patient's body, comprising: a
filter element being expandable from a collapsed configuration when
the filter element is restrained to an expanded configuration when
the filter element is unrestrained, wherein the filter element
comprises a material having pores, and when the filter element is
in the expanded configuration, the average pore size is from 30 to
300 microns and the standard deviation of the pore size is less
than 20 percent of the average pore size.
2. A device of claim 1, wherein when the filter element is in the
expanded configuration, the average pore size is from 30 to 150
microns.
3. A device of claim 1, wherein when the filter element is in the
expanded configuration, the average pore size is from 100 to 140
microns.
4. A device of claim 1, wherein when the filter element is in the
expanded configuration, the average pore size is from 40 to 70
microns.
5. A device of claim 1, wherein when the filter element is in the
expanded configuration, the standard deviation of the pore size is
less than 15 percent of the average pore size.
6. A device of claim 1, wherein when the filter element is in the
expanded configuration, the standard deviation of the pore size is
less than 10 percent of the average pore size.
7. A device of claim 1, wherein when the filter element is in the
expanded configuration, the standard deviation of the pore size is
less than 5 percent of the average pore size.
8. A device of claim 1, wherein when the filter element is in the
expanded configuration, the standard deviation of the pore size is
less than 2 percent of the average pore size.
9. A device of claim 1, wherein when the filter element is in the
expanded configuration, the filter element has a percent open area
greater than 50 percent.
10. A device of claim 1, wherein when the filter element is in the
expanded configuration, the filter element has a percent open area
greater than 60 percent.
11. A device of claim 1, wherein when the filter element is in the
expanded configuration, the filter element has a percent open area
greater than 70 percent.
12. A device of claim 1, wherein when the filter element is in the
expanded configuration, the filter element has a percent open area
greater than 80 percent.
13. A device of claim 1, wherein the material having pores has a
tensile strength greater than 70,000 psi.
14. A device of claim 1, wherein the material having pores has a
tensile strength greater than 100,000 psi.
15. A device of claim 1, wherein the material having pores has a
tensile strength greater than 200,000 psi.
16. A device of claim 1, wherein the material having pores is made
of metal.
17. A device of claim 1, wherein the material having pores is made
of nitinol.
18. A device of claim 1, further comprising an elongate support
member and wherein the filter element is carried on a portion of
the elongate support member.
19. A device of claim 1, wherein the material having pores
comprises at least one minibraid.
20. A device of claim 1, wherein the material having pores
comprises two or more minibraids and the minibraids form a
weave.
21. A device of claim 1, wherein the material having pores
comprises at least one minibraid that is helically coiled.
22. A device of claim 1, wherein the material having pores
comprises at least one stranded wire.
23. A device of claim 1, wherein the material having pores
comprises at least one fenestrated wire.
24. A device of claim 1, wherein the material having pores
comprises cylindrical wires having different diameters and has a
graduated strand pattern.
25. A device of claim 1, wherein the filter element has proximal
and distal portions and a central portion, the filter element
having a shape in the expanded configuration which defines a cavity
having a proximal facing opening.
26. A device of claim 25, wherein the material having pores
comprises wires braided to form diamond-shaped pores.
27. A device of claim 26, wherein the material having pores
comprises only axially elongated diamond-shaped pores.
28. A device of claim 26, wherein the material having pores
comprises axially shortened diamonds in the proximal portion of the
filter element, square diamonds in the central portion of the
filter element, and axially elongated diamonds in the distal
portion of the filter element.
29. A device of claim 1, wherein the material having pores
comprises a knitted mesh.
30. A device of claim 1, wherein the material having pores
comprises braided wires and the material having pores was made by
martensitic braiding.
31. A device of claim 1, wherein the material having pores
comprises braided wires and the material having pores was made by
martensitic knitting.
32. A device of claim 1, wherein the material having pores
comprises braided wires and the material having pores was made by
wire removal.
33. A device of claim 1, wherein the material having pores
comprises braided wires and the material having pores was made by
adding wires during braiding.
34. A device of claim 1, wherein the material having pores
comprises braided wires and a portion of the wires is coated with a
polymer coating that reduces the average pore size in the coated
portion of the filter element.
35. A device of claim 1, wherein the filter element is an
electroformed filter.
36. A device of claim 1, wherein the filter element was made by
forming the filter into a mandrel.
Description
[0001] This application is a continuation of application Ser. No.
10/354,679, filed Jan. 30, 2003, the contents of which are hereby
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to devices used in a blood vessel or
other lumen in a patient's body. In particular, the present
invention relates to devices for capturing emboli and particulate
in a lumen.
BACKGROUND OF THE INVENTION
[0003] During vascular surgery or endovascular treatment of vessels
including thrombectomy, atherectomy, balloon angioplasty, and/or
stent deployment, debris such as plaque and blood clots can move
from the treatment site through a vein or artery and compromise the
flow of blood at a location removed from the treatment site. In
particular, various protection systems have been developed to
prevent such debris from embolizing in the vessel. Distal
protection devices include filters and occlusive devices (e.g.,
balloons) placed distally of the treatment site. Proximal
protection devices include filters and occlusive devices placed
proximally of the treatment site. In the case of filters, emboli
collect within or on the filter. The filter with captured emboli is
typically collapsed into a recovery catheter and the catheter
withdrawn from the patient's body.
[0004] The size or number of emboli that must be retained by the
filter in order to prevent clinically undesirable sequaelae is
unknown. This uncertainty adds to the complexity of designing a
filter with the appropriate characteristics. Small particles might
pass through the filter pores and lodge downstream in tissues where
they may cause tissue ischemia or tissue necrosis. In the heart,
blood can be drawn and measurements can be made to track enzyme
levels and determine myocardial damage. However, in the brain there
is no easy and inexpensive method to evaluate the effect of a
shower of emboli. Within the downstream tissue bed, there is a
statistical component to the consequences of an embolus. For
example, a 100 micron particle may lodge in a part of the brain
where few adverse consequences are detected clinically, or it can
lodge in a retinal artery, resulting in blindness in one eye.
Therefore, it may be necessary to adjust the filter characteristics
to suit the region of emboli filtration. A smaller pore size filter
may be needed if protecting the brain than protecting the heart or
kidney.
[0005] Embolic protection filters permit the passage of blood while
retaining emboli that are larger than the pore size of the filter.
Filter meshes are commonly made by incorporating holes in a polymer
film, by interweaving filaments, or by producing interconnected
porosity in a sheet of material (e.g., foam). It is difficult to
make an embolic protection filter with the appropriate combination
of pore size, pore area, embolic capacity, patency, mechanical
strength, low collapsed or retracted profile, and recovery
characteristics. Embolic filters made from polymer films commonly
have a narrow range of pore sizes but suffer from a low percent
open area because there is a limit to how closely the holes can be
placed. Too little spacing between holes can result in a weak film
that tears upon filter recovery. Foams tend to be bulky, thereby
compromising the collapsed profile, and they have low strength.
[0006] Interwoven meshes such as braids have the advantage of a
pore area which is a high percentage of the total mesh area,
excellent strength, and good flexibility, but tend to be made and
used in ways that result in a wide range of pore sizes. A wide
range of pore sizes is undesirable for a number of reasons. Patency
is influenced by pore size. Theoretically, blood can be sheared as
it flows through the pore, particularly at the edges of the pore
opening. Shearing of blood can activate platelets and initiate a
cascade of events that cause blood clotting. When filters are used
in the bloodstream, it is common for thrombus to form in the
vicinity of the smallest pores and no thrombus to form in the
vicinity of the largest pores. Flow through the filter is thereby
reduced because part of the filter becomes occluded. In addition,
while some filters have a reasonable average pore size, a wide
range of pore sizes in these filters may allow large particles to
pass through the large pores during either the capture or recovery
phase.
[0007] A need in the art remains for an embolic protection filter
having pores which are both small in size and which do not vary in
size beyond an acceptable range.
SUMMARY OF THE INVENTION
[0008] The invention provides an embolic filter that is designed to
provide the desired characteristics of controlled pore size, high
percentage of pore area, high embolic capacity, patency, mechanical
strength, low collapsed or retracted profile, and strength during
recovery.
[0009] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are intended to provide further explanation of
the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1A and 1B are schematic views of a distal protection
system, illustrating expanded and contracted configurations,
respectively, of a cup-shaped filter, and FIG. 1C is a schematic
view of an alternative distal protection system having a
windsock-shaped filter.
[0011] FIG. 2 is an enlarged view of minibraid material used in a
filter.
[0012] FIGS. 3A and 3B are side views of a minibraid suitable to
form the braid material shown in FIG. 2.
[0013] FIGS. 4A and 4B are side views of a minibraid suitable to
form the braid material shown in FIG. 2.
[0014] FIG. 5A is a side view of a conical filter formed from
minibraid.
[0015] FIG. 5B is an illustrative perspective view of the conical
filter of FIG. 5A.
[0016] FIG. 6A is a schematic view of a braid comprising coiled
wire, and FIG. 6B is an enlarged view of a section of the braid of
FIG. 6A. FIG. 6C is a schematic view of a braid having expanded
wires, and FIG. 6D shows an enlarged view of the braid of FIG. 6C.
FIG. 6E is a schematic view of a single filament coiled wire and
FIG. 6F is a schematic view of a multiple filament coiled wire.
[0017] FIG. 7A is a top view of a section of a fenestrated wire,
FIG. 7B is a detail view of a portion of the wire of FIG. 7A, and
FIG. 7C is a perspective view of the wire of FIG. 7A. FIG. 7D is a
top view of a tapered fenestrated wire. FIG. 7E is an enlarged view
of fenestrated wires used in a filter.
[0018] FIGS. 8A and 8B are partial planar views of a woven wire
fabric.
[0019] FIGS. 9A and 9B are side views of alternative embodiments of
the distal protection filter showing different pore sizes in the
filter mesh.
[0020] FIGS. 10A and 10C are side views of alternative embodiments
of the distal protection filter showing different pore shapes in
the filter mesh. FIG. 10B is a side view of a braid before it is
formed into a filter.
[0021] FIG. 11A is a perspective view of an alternative embodiment
of a distal protection filter, and FIG. 11B is a detailed view of a
portion of the filter in FIG. 11A.
[0022] FIGS. 12A and 12B are cross-sectional views showing the
arrangement of wires in a braid, and FIG. 12C is a perspective view
showing a section line for the views in FIGS. 12A and 12B.
[0023] FIGS. 13A and 13B are side views of alternative embodiments
of a distal protection filter of the invention.
[0024] FIG. 14 is a side view of an alternative embodiment of a
distal protection filter wherein the pore size is controlled by
adding wires during braiding.
[0025] FIG. 15A is a schematic illustration of wires winding around
a mandrel, FIGS. 15B and 15C are detailed side views showing the
arrangement of the wires on the mandrel, and FIGS. 15D, 15E, and
15F are alternative embodiments of a mandrel suitable for making an
embolic protection filter of the invention.
[0026] FIG. 16 is a perspective view of an electroformed
filter.
[0027] FIGS. 17A to 17C are cross-sectional views of a mandrel and
a filter material formed into the mandrel in a process for forming
a filter into a mandrel.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The terms "distal" and "proximal" as used herein refer to
the relative position of the guidewire, catheters, and filter in a
lumen. Thus, "proximal" refers to a location upstream from the
"distal" position. That is, the flow of a body fluid, such as
blood, moves from the proximal to the distal portions of the
device.
[0029] The invention encompasses the use of any filtration device
to be deployed in a lumen or vessel of a patient. Although the
examples relate generally to filter protection devices deployed
distal to a treatment site, the device can also be deployed
proximal to a treatment site in connection with interrupting or
reversing flow through the vessel. In the case of a proximally
deployed device, it will be advantageous to construct the device on
a hollow elongate member so as to preserve access to the treatment
site through the hollow member.
[0030] In a preferred embodiment, the distal protection system
comprises a catheter which is loaded with an elongate support
member or guidewire about which is disposed a distal protection
filter. The elongate support member is structurally similar to a
traditional guidewire in some respects. However, it is not used as
a means of navigating the patient's vascular system and, therefore,
does not need to be provided with all of the features of
flexibility and steerability as does a traditional guidewire. With
these differences in mind, the terms elongate support member and
guidewire may be used interchangeably herein. A floppy tip
(described further below) may be at the distal end of the elongate
support member or guidewire. Typically, the filter is introduced
into a blood vessel through an introducing catheter. Methods of
introducing guidewires and catheters and the methods for the
removal of such devices from vessels are well known in the art of
endovascular procedures. In a typical procedure using the device of
this invention, the elongate support member and filter are loaded
into an introducing sheath or catheter and moved into the vessel
and through the catheter to the treatment site. This is done
typically by advancing a first, or introduction guidewire, through
the vessel to the region of interest. A catheter is advanced over
the guidewire to the region of interest, and the guidewire removed.
Then the filter or other functional device carried by the elongate
support member is advanced down a catheter sheath to the region of
interest but within the catheter. The catheter sheath is withdrawn
to deploy (expand) the filter at the region of interest.
Alternatively, the filter is preloaded into a catheter and held in
place by an outer sheath of the catheter and they are together
advanced through the vessel to the region of interest without using
an initial guidewire. In this embodiment the catheter/filter
combination will be used to navigate through the vessel to the
region of interest. Then the catheter is withdrawn to deploy the
filter. In a second alternative, an introduction guidewire is
advanced to the region of interest, and the filter (contained in a
catheter) is advanced over the guidewire to the region of interest,
at which point the catheter is removed leaving the deployed filter
near the region of interest on the guidewire. In this embodiment
the filter is not comprised of an elongate support member as
previously defined, and the guidewire and/or filter may be
configured to preserve a spatial relationship between the guidewire
and the filter. For example, the guidewire may be configured to
prevent the filter from advancing beyond the distal end of the
guidewire.
[0031] In other embodiments of the invention, no catheter is
required for filter delivery. For example, the filter may be
stretched axially so as to reduce its diameter to a size suitable
for navigation through a vessel and across a treatment site.
[0032] In some embodiments of the invention, the device can include
an actuator instead of being self-expanding. Actuators include
struts, coaxial elongate elements, expandable elements such as
balloons, support frames, etc.
[0033] Typical dimensions of a filter used in the devices of this
invention range from 2 m to 90 m in length, and from about 0.5 m to
2 m in diameter before deployment, and from about 2 m to 30 m in
diameter after deployment. A typical guidewire is about 0.2 to 1.0
mm in diameter and ranges from 50 cm to 320 cm in length.
[0034] The components of the distal protection system are made from
biocompatible materials. Materials also may be surface treated to
produce biocompatibility. The elongate support member may be formed
of any material of suitable dimension, and generally comprises
metal wire. Suitable materials include stainless steel, titanium
and its alloys, cobalt-chromium-nickel-molybdenum-iron alloy
(commercially available under the trade designation Elgiloy.TM.),
carbon fiber and its composites, and engineered polymers such as
liquid crystal polymers, polyetheretherketone (PEEK), polyimide,
polyester, and the like. A shape memory or superelastic metal such
as nitinol is also suitable. The elongate support member may be
solid or may be hollow over some or all of its length.
[0035] The material used to make the filter or filter support
structure is preferably self-expanding. Suitable materials include
metals such as stainless steel, titanium and its alloys,
cobalt-chromium-nickel-molybdenum-iron alloy (commercially
available under the trade designation Elgiloy.TM.), carbon fiber
and its composites, and engineered polymers such as liquid crystal
polymers, polyetheretherketone (PEEK), polyimide, polyester, silk,
and the like. A shape memory or superelastic metal is particularly
suitable for those applications when it is desired for an element,
such as a filter, to assume a pre-determined three-dimensional
shape or for a guidewire to maintain a pre-determined curvature. A
shape memory or superelastic metal comprising nickel and titanium
known as "nitinol" is commercially available in various dimensions
and is suitable for use as both a guidewire and a filter. For
example, nitinol tubular braid can be heat set into a desired
shape, compressed for delivery to a site, and then released to
resume the heat-set shape.
[0036] The filter element has a body defining an interior cavity.
The filter body has a plurality of openings or pores such that,
when the filter element is in its deployed configuration within the
vessel lumen, fluid flows through the filter element and particles
of the desired size are captured inside the interior cavity of the
filter element.
[0037] The filter may comprise any material that is suitably
flexible and resilient, such as a mesh, i.e., a material having
openings or pores. The filter may comprise braided, knitted, woven,
or non-woven fabrics that are capable of filtering particles,
preferably having pore sizes from 30 to 500 microns. Woven or
non-woven fabrics may additionally be treated to fuse some or all
of the fiber intersections. The fabric may be spun or electrospun.
Suitable materials include those formed from sheets, films, or
sponges, polymeric or metallic, with holes formed by mechanical
means such as laser drilling and punching, or by chemical means
such as selective dissolution of one or more components. For
example, a suitable filter material is braided tubular fabric
comprising superelastic nitinol metal. Mesh fabric of nitinol
material can be heat-set to a desired shape in its expanded
configuration.
[0038] The material comprising the filter is preferably at least
partially radiopaque. This material can be made radiopaque by
plating, or by using core wires, tracer wires, or fillers that have
good X-ray absorption characteristics compared to the human body.
Radiopaque filters are described in U.S. patent application Ser.
No. 10/165,803, filed Jun. 7, 2002, entitled "Radiopaque Distal
Embolic Protection Device," the contents of which are hereby
incorporated by reference herein.
[0039] The embodiments of this invention, described in detail below
in connection with the figures, are suitable for use with various
distal protection systems that are known in the art. The filter may
have a windsock type shape. The construction, deployment and
retrieval of a filter having this shape is described, for example,
in U.S. Pat. No. 6,325,815 B1 (Kusleika et al.), the contents of
which are hereby incorporated by reference herein.
[0040] The filter may also be a cup-shaped or basket-shaped device
which forms a proximally facing opening when expanded. The
construction, deployment, and retrieval of such a filter is
described in WO 96/01591 (Mazzocchi et al.). This cup-shaped device
may generally resemble an umbrella or a parachute, having a
dome-like structure curving radially outwardly from the guidewire
or elongate support member. Other shapes may be equally suitable in
performing a filtering function, such as a conical shape, or a
relatively flat disc shape. The filter may include a filter basket
having a self-expanding radial loop designed to position the filter
basket within the vasculature and to hold the filter basket open
during deployment. Such a filter is described in EP 181 900 A2
(Oslund et al.). It will be appreciated that the shape of these
filtration devices shown in various embodiments are merely
illustrative and are not meant to limit the scope of the
invention.
[0041] Regardless of the shape of the filter, the filter preferably
is deployed using an elongate support member. This can be done in
various ways, and one or both of the proximal and distal ends of
the filter may be affixed to the elongate support member (by a
fixed element) or may be slidably disposed about the elongate
support member (by one or more sliding elements).
[0042] One type of sliding element comprises inner and outer
annular rings. The first ring fits within the second ring. The
inner diameter of the first ring is larger than the diameter of the
elongate support member so that the sliding element can slide over
the elongate support member. The sliding element can be affixed to
the filter fabric by placing the fabric between the first and
second rings. However, this is not meant to be limiting, and the
filter fabric can also be affixed to the sliding element by
adhesive, solder, crimping, or other means known in the art. The
sliding element may comprise any stiff material such as metal or
polymer and preferably the slider is radiopaque. Suitable materials
include stainless steel, titanium, platinum, platinum/iridium
alloy, gold alloy, polyimide, polyester, polyetheretherketone
(PEEK), and the like. Movement of a sliding element with respect to
the elongate support member can be facilitated by coating one or
both of the inside of the sliding element and the outside of the
elongate support member with a friction-reducing coating, such as
polytetrafluoroethylene or a lubricious hydrophilic coating.
[0043] Fixed elements include annular rings. Also included within
this meaning is an element that is crimped, adhered, soldered, or
otherwise fastened directly to the elongate support member. Also,
the filter fabric may be attached directly to the elongate support
member. In any event, the sliding and fixed elements (or any
attachment point) typically comprise radiopaque material to assist
in the placement of the filter. In addition, one or more radiopaque
markers may be positioned at various locations on the protection
device. These radiopaque markers or marker bands comprise a
material that will be visible to X-rays and they assist in
positioning the device.
[0044] Some distal protection filters include a floppy tip at a
distal portion of the guidewire or elongate support element. The
floppy tip provides an atraumatic and radiopaque terminus for the
device. An atraumatic tip prevents vessel injury during initial
placement or subsequent advancement of the device. A radiopaque tip
helps the physician verify suitable tip placement during
fluoroscopy. The floppy tip preferably comprises a springy or
resilient material, such as a metal (e.g., stainless steel, iron
alloys such as Elgiloy.TM., platinum, gold, tungsten, and shape
memory or superelastic metal such as nitinol) or polymer (e.g.,
polyetheretherketone (PEEK), polyimide, polyester,
polytetrafluoroethylene (PTFE), and the like). Springy materials
are desirable because they tend to retain their shape. The
physician will initially shape the tip, typically with a slight
curve, and then as the device is advanced through the body the tip
will be deflected as it encounters obstacles. It is desirable,
after the inevitable deflections during insertion, that the tip
restore itself to the pre-set shape. Polymeric materials
additionally may be reinforced with metals or other fillers. The
tip may be a monofilament or multifilament (such as a cable). The
floppy tip may be tapered or have a uniform diameter over its
length. The floppy tip may comprise a tube, or could have circular,
flat, or other cross-sections. It may be coiled. The tip may
comprise one or more elements (for example, parallel independent
structures). The tip may be polymer-coated or otherwise treated to
make the surface slippery. The floppy tip can be any desired
length.
[0045] The filter comprises biocompatible materials such as metals
and polymeric materials. Materials such as metals and polymeric
materials can be treated to impart biocompatibility by various
surface treatments, as known in the art. When wire is used, the
wire is selected on the basis of the characteristic desired, i.e.,
stiffness or flexibility, and the properties can depend upon both
the diameter of the wire and its cross-sectional shape. The size,
thickness, and composition of elastic materials are selected for
their ability to perform as desired as well as their
biocompatibility. It is to be understood that these design elements
are known to one of skill in the art.
[0046] Filters are typically constructed as described in U.S. Pat.
No. 6,325,815 B1. See column 3, line 63, to column 4, line 16; and
column 4, line 48, to column 5, line 36. The filter body typically
comprises a length of a braided tubular fabric, preferably made of
nitinol. The filter body is typically made by placing a braided
tubular fabric in contact with a molding surface of a molding
element which defines the shape of the desired filter body. By heat
treating the braided tubular fabric in contact with the molding
surface of the molding element, one can create a filter body having
virtually any desired shape.
[0047] Braiding is a process for producing a tubular interwoven
structure from individual strands. Braids are typically produced in
continuous lengths on commercially available braiding machines.
Some commercial products produced on braiding machines include
rope, shoelaces, and reinforcing jackets for electrical cable.
Medical products produced by braiding include stents, vascular
grafts, and catheter reinforcing layers.
[0048] In a typical braiding process for making a 72 stranded
braid, lengths of strands, such as wire, are wound onto bobbins. In
this example 72 bobbins are wound with wire. Each bobbin is loaded
into the carrier of a 72 carrier braiding machine. Typically
braiding machines for medical use have from 16 to 144 carriers or
more. Each wire is led through a tensioning mechanism in the
carrier and all wire strands are gathered at a common central
elevated position along the (typically vertical) axis of the
braiding machine, where they are fastened to a take-up mechanism.
The take-up mechanism may be a long mandrel arranged along the axis
of the braiding machine and onto which the braid is formed during
the braiding process. Once so configured, the carriers are rotated
relative to the axis of the braiding machine. The carriers are
rotated in a serpentine path, half of them moving clockwise and the
other half moving counterclockwise, so as to interweave the strands
in a programmed pattern. While the carriers are rotating, the
take-up mechanism advances the woven braid in a direction away from
the carriers. The combination of these motions produces a helix of
strands twisting in a clockwise direction along the mandrel,
interwoven with a helix of strands twisting in a counterclockwise
direction along the mandrel. In this manner continuous lengths of
braid are produced with an inside diameter of the braid equal to
the outside diameter of the braiding mandrel. The individual braid
strands, while still on the mandrel, can be twisted together after
the length of the mandrel has been braided. If desired, after
removing the mandrel from the braiding machine, the strands can be
heat-treated. In the case of nitinol strands, heat treatment on the
mandrel at about 525.degree. C. for 10 minutes or so can cause the
nitinol-braided fabric to remember the shape and size of the
mandrel when the nitinol is at rest.
[0049] The average pore sizes of filters of the invention
preferably range from 30 to 300 microns. In another preferred
embodiment, the average pore sizes range from 30 to 150 microns. A
pore size of about 120 microns is preferred for devices intended to
be used in connection with coronary procedures and a pore size of
about 50 microns is preferred for devices intended to be used in
connection with carotid or intracranial procedures. The variation
in pore size within the filter should be minimized. In preferred
embodiments of the invention, the standard deviation of the pore
size is less than 20 percent of the average pore size. In other
preferred embodiments, the standard deviation of the pore size is
less than 15, 10, 5, or 2 percent of the average pore size.
[0050] The percent open area of the filters of the invention is
preferably greater than 50 percent. In other preferred embodiments,
the percent open area is greater than 60, 70, or 80 percent. A
standard formula is used to calculate the percent open area of a
given design. The percent open area is calculated by dividing the
total pore area by the total filter area (including the pore
area).
[0051] The filters of the invention preferably are made of a
material having a tensile strength of greater than 70,000 psi (7031
kg/cm.sup.2), more preferably greater than 150,000 psi (14,062
kg/cm.sup.2), and more preferably greater than 200,000 psi (17,578
kg/cm.sup.2). Cast polymer films have a maximum tensile strength of
about 10,000 psi (703 kg/cm.sup.2); oriented polymer films have a
tensile strength as high as 50,000 psi (3516 kg/cm.sup.2), and
metal filters typically contain wires having a tensile strength of
from 70,000 to 300,000 psi (7031 kg/cm.sup.2 to 21,093
kg/cm.sup.2).
[0052] The various embodiments of the invention will now be
described in connection with the drawing figures. It should be
understood that for purposes of better describing the invention,
the drawings have not been made to scale. Further, some of the
figures include enlarged or distorted portions for the purpose of
showing features that would not otherwise be apparent. The material
comprising the filter (e.g., mesh or fabric with pores, as
described above) is indicated by cross-hatching in some of the
figures but is omitted from others for simplicity.
[0053] FIGS 1A, 1B, and 1C illustrate embodiments of various
filters in which the concepts of the present invention may be
applied. The application of these concepts is not, however, limited
to these embodiments and are equally applicable for use in any
filter where control of pore size is desirable. FIGS. 1A and 1B
illustrate schematic views of a distal protection system in which
elongate support member 5a carries filter 10a. The proximal end of
the filter is connected to a proximal sliding element 6 and the
distal end of the filter is connected to a distal fixed element 8.
The distal fixed element is connected at a fixed location on the
elongate support member while the proximal slider is configured to
slide freely over the elongate support member. Struts or tethers 7
attach to the body of the filter and to sliding element 6. The
elongate support member terminates distally at optional atraumatic
floppy tip 13. The filter is shown in its expanded deployed
configuration in FIG. 1A and in its contracted delivery
configuration in FIG. 1B. The figures show that the proximal
sliding element 6 travels over the elongate support member a
distance S when the filter is contracted to, for example, its
delivery configuration.
[0054] FIG. 1C illustrates another type of distal protection system
in which windsock-shaped filter 10c is attached to elongate support
member 5c which terminates at floppy tip 13c. The filter is
attached to support member 5c via proximal element 6c and distal
element 8c. Either one or both of these elements may be sliding or
fixed elements, as described above.
[0055] It is to be understood that the following embodiments are
useful for any shape or type of filter. For example, these
embodiments are useful for any filter deliverable by any manner to
a desired position in a body lumen where control of the desired
characteristics of the filter as set forth above is desired. In
particular, the invention includes both proximal and distal
filters.
Minibraid Filters
[0056] One way to control the pore size and pore distribution in a
filter is to use minibraids. FIG. 2 illustrates schematically a
self-expanding material 20 used to form a filter of this invention.
The self-expanding material 20 comprises braided strands of braided
nitinol. Minibraids 22 may be formed from eight strands of nitinol
wire (0.001 inch (25 micron) diameter) as described with respect to
FIGS. 3A and 3B. Alternatively, minibraids 22 may be formed as
described with respect to FIGS. 4A and 4B. It is noted that these
minibraids are easier to handle than unbraided wire of this
diameter. Twenty-four of these minibraids are braided together to
form a second braided, self-expanding material 20, in a 2/2 braid
pattern, having 192 filaments. The pore size of this second braid
is approximately 100 microns in a 4 mm braid diameter. The braided
material shown in FIG. 2 can be used to form a filter similar to
the one shown in FIG. 1. In preferred embodiments, the weave of the
filter is much tighter (see FIG. 2) than the weave shown in FIG. 1.
In the preferred embodiments in which the weave is tight, the pore
size and pore distribution are determined almost entirely by the
characteristics of the minibraid.
[0057] One of skill in the art recognizes that other diameters of
wire and other materials can be used to form the minibraids. For
example, 0.0007 inch (17.8 micron) diameter nitinol wire is a good
choice for forming the braided material shown in FIG. 2. For
purposes of this application, a minibraid has a diameter from 0.002
inch (51 micron) to 0.050 inch (1270 micron). In a preferred
embodiment, a minibraid has a diameter from 0.005 inch (127 micron)
to 0.020 inch (508 micron).
[0058] FIG. 3A is a side view of material 30 useful as a minibraid.
Material 30 is formed from eight small diameter wires 32 wound
around core wire 34. FIG. 3A shows 0.001 inch (25 micron) diameter
nitinol wires braided over a 0.005 inch (127 micron) aluminum core
wire. This material can be used to form a second braid, described
above for FIG. 2, after which the aluminum core wire 34 can be
dissolved away. Acid dissolution is preferred because it was found
that alkali (NaOH) dissolution caused the nitinol to become
embrittled. Core wire 34 alternatively may be a polymeric material,
such as polyester, which can be removed by heating. FIG. 3B is a
side view showing the shape of material 30 after the core wire has
been removed. Alternatively, the core wire can be retained in the
minibraid.
[0059] FIG. 4A is a side view of material 40, suitable for use as a
minibraid. In this embodiment, a tube ranging in diameter from
0.005 inch (127 micron) to 0.020 inch (508 micron) is etched to
form material 40, comprising segments 42. In forming the minibraid
from material 40, regions of a stainless steel hypotube are masked
and then the hypotube is etched using ferric chloride. Laser
etching can also be used to form a material of the desired
structure.
[0060] FIG. 4B shows a side view of material 40 in which etching
has occurred with core wire 44 added. Material 40 is then braided
to form a second braid structure suitable for use in this
invention, as shown above in FIG. 2 and as described further below.
Core wire 44 prevents material 40 from collapsing under tension
during the braiding operation. Core wire 44 can be removed as
described above in connection with FIG. 3 or can be retained in the
minibraid.
[0061] FIG. 5A shows a side view of a filter formed from minibraid
52 comprised of 0.001 inch (25 micron) diameter braided nitinol
wires 56. The nitinol wires 56 are braided on a core rod 54 of
diameter 0.005 inch (127 micron). The minibraid and core rod 54 are
then wrapped around a mandrel approximately 4 mm (millimeters) in
diameter and the nitinol is heat set on the mandrel by heating to
525.degree. C. for five minutes. If the core is polyester it can be
burned off during heat treatment. If the core is aluminum, it can
be dissolved away by acid treatment subsequent to the heat
treatment. FIG. 5A does not show the core rod 54. The mandrel has a
conical shape that forms a suitable filter for the practice of the
invention. In any event, the core rod preferably is removed and the
resultant conically shaped filter 50, illustrated in schematic
perspective view in FIG. 5B, is ready for use.
[0062] FIG. 5B shows a perspective view of the conically shaped
filter 50 formed from minibraids. Optionally, the minibraids can be
interwoven with an opposingly wound helix of solid or minibraid
filaments to provide additional structural integrity. The pore size
and pore distribution are determined almost entirely by the
characteristics of the minibraid because the spaces between the
minibraids are minimized. The structures shown in FIGS. 3, 4, and 5
can achieve pore sizes comparable to those described in connection
with FIG. 2.
Coiled Wire Filters
[0063] Another way to control the pore size and pore distribution
in a filter is to use coiled wire. FIGS. 6A to 6D illustrate a
coiled wire, in which one or more wires 63 are wound around a
central axis to produce coiled wire 62. The coiled wire can be
formed by twisting wires around a central core or, preferably, by
twisting wires without a central core as is commonly done in the
wire and cable industry. Although the embodiments shown have the
appearance of coils it is understood that suitable structures
include helical wound filaments, commonly referred to as stranded
wire or cable in the wire and cable industry. This coiled wire is
more flexible than a single wire of the same overall diameter.
Coiled wire 62 can itself be braided, woven, or wound in a helical
pattern to form a suitable filter material, depending upon the
desired characteristics. Preferably the coiled wire comprises
nitinol, which can be heat set to a desired configuration. FIG. 6A
shows braid 64 comprising coiled wires 62, and FIG. 6B is an
enlarged view of a section of braid 64. FIG. 6C illustrates coiled
wires 62 having coils 63 in an expanded configuration, and FIG. 6D
shows an enlarged view of the braid of FIG. 6C. Filters can be made
from a single filament coiled wire as shown in FIG. 6E or from
multiple filament coiled wire as shown in FIG. 6F. One could
achieve pore sizes in the range of 100 microns by braiding 24
coiled wires of 8 strands each followed by expanding the individual
coiled wires.
[0064] Coiled wire can be formed by coiling or twisting individual
strands around a core wire, as described above for minibraids. If
the individual strands are nitinol then the strands can be heat set
to memorize the shape on the core wire, and the core wire can be
removed by means such as acid dissolution. Coiled wire can next be
braided with the coiled wire in an unexpanded configuration by
tensioning the coils. After braiding, the tension on the coils can
be reduced or eliminated by axially compressing the braid, causing
the individual wires or strands within the coils to assume their
heat set memorized configuration. After the wires expand to their
expanded configuration, the average pore size and the pore size
distribution are smaller than that of the initially braided
structure.
Fenestrated Wire Filters
[0065] Yet another way to control the pore size and pore
distribution in a filter is to use fenestrated wire. FIGS. 7A to 7C
illustrate a fenestrated wire, that is, one in which windows have
been formed. FIG. 7A shows a top view of wire 72 in which have been
formed triangular-shaped windows 71. The windows are spaced
uniformly throughout the wire, though one of skill in the art
recognizes that the size, shape, and spacing of the windows could
be varied to achieve the desired porosity. FIG. 7B is a partial
detail view showing the dimensions of the wire (a, b, and c=0.0217
mm) and the windows (d=diameter of 0.0125 mm, e=60.degree.). FIG.
7C is a perspective view of wire 72.
[0066] FIG. 7D shows fenestrated wires 75 that taper in width. Such
tapered fenestrated wires can be braided to form a conical filter.
FIG. 7E is an enlarged view of fenestrated wires 72 used in a
filter 73. As shown in FIG. 7E, in one preferred embodiment, the
space between the individual wires 72 is minimized. In another
preferred embodiment, the space between the individual wires 72 is
approximately the same as the space within the windows 71 in the
fenestrated wires 72. In another preferred embodiment, the space
between the individual wires 72 varies from the minimum space
possible to approximately the same space within the windows 71 in
the fenestrated wires 72.
[0067] Fenestrated wire can be formed by masking and etching, laser
cutting, rolling using textured rolls, and other methods.
Fenestrated wire 72 can be braided, woven, or wound to form a
suitable filter material, depending upon the desired
characteristics. Ideally the density of the filter structure will
be such that the spacing between filter wires is on the order of
the size of the fenestrations. In the preferred embodiments in
which the weave is tight, the pore size and pore distribution are
determined in large part by the characteristics of the fenestrated
wire, which has a uniform pore distribution.
Graduated Strand Diameters Filters
[0068] Another way to control the pore size and pore distribution
in a filter is to use graduated strand diameters. FIGS. 8A and 8B
illustrate woven fabric suitable for use as a filter of this
invention, wherein the pore size can be controlled. It has been
found that interspersing a number of large diameter strands among
smaller diameter strands results in a filter fabric that has a high
self-expansion restoring force. That is, the filter assumes the
desirable size and configuration during deployment from its
compressed and contracted configuration.
[0069] To achieve fine pore size filters one can increase the
number of individual strands in a mesh, such as a braid, such that
they become closely spaced. To prevent excessively large compressed
diameters the individual strands must be kept small in diameter.
Due to the small strand diameters such filters have poor expansion
characteristics despite the large number of individual strands. To
overcome this limitation it has been found that good expansion
characteristics without excessive bulk can be achieved by
interspersing a limited number of large diameter filaments into a
mesh of fine filaments. FIG. 8A is a planar view of a section of
fabric in which 0.001 inch (25 micron) diameter wire 81, preferably
made of nitinol, is woven with a larger diameter wire 82 (0.003
inch (75 micron) diameter), resulting in uneven pore size 84a and
84b in the fabric. Though this embodiment may be useful, a
preferred embodiment is one in which wires of varying diameter are
woven together to produce uniformly-sized pores 84b. In FIG. 8B,
wires 81 of a first diameter (25 micron) are woven together with
wires 82 and 83 of two larger diameters (e.g., 50 and 75 micron,
respectively). This weave produces pores 84c of uniform size,
preferably ranging from 30 to 300 microns. A uniform size is
preferred for filtering out particulate and emboli of specific
dimensions. The presence of larger diameter wires contributes to an
improved restoring force in a filter. That is, the filter expands
to the desired structure and size when it is deployed. The
disadvantage of making a filter with many large wires can be that,
when the fabric is gathered together to form a filter, the diameter
of the gathered fabric becomes undesirably large. With this
construction, the diameter of the gathered fabric is close to that
of fabric comprising only small diameter wire.
Polymer Coated Filters
[0070] Yet another way to control the pore size and pore size
distribution in a filter is to use a polymer coating. FIGS. 9A and
9B illustrate a wire distal protection filter. FIG. 9A has a
silicone coating on a portion of it to reduce the pore size of the
filter and FIG. 9B is shown with no coating to illustrate the
effect of the coating on pore size. Filter 90 comprises wires 92.
The filter is disposed about an elongate support member 95,
attached by proximal element 96 and distal element 98, and has
floppy tip 93. A silicone (such as two part NuSil# thermoset
silicone composition, #MED 10-6640, available from NuSil
Technology, Carpinteria, Calif., having an uncured viscosity of
about 7000 centipoise and when cured 700% elongation & ultra
high tear strength) coating 91 covers a proximal region 92a of
filter 90, coating wires 92 of the filter and partially closing the
proximal pores 94a. In this manner, the pore size over the length
of the filter becomes more uniform, since the large pores of an
uncoated filter are made smaller and more similar in size to the
pores at the distal end. Preferably, the coating at the more
proximal portions of the proximal region 94a reduces a greater
percentage of the area of the pores 94a than the more centrally
located portions of the proximal region 94a. Such a coating results
in a more uniform pore distribution because the more proximal pores
are larger than the more centrally located pores in an uncoated
filter. It is to be understood that many materials could produce
the desired effect, and these include, but are not limited to,
other polymers such as polyurethane, latex rubber, thermoplastic
elastomers such as those sold under the tradename SANTOPRENE.RTM.,
available from Advanced Elastomer Systems, Akron, Ohio, butyl or
other rubbers, and hydrogels or other expanding polymers. For a
typical 72 wire braid of 0.0015 inch (38 micron) diameter wires, a
pore size maximum of less than 100 microns with a pore size range
of 20 to 100 or less microns should be achievable using this
construction. Alternatively, the coating could be applied to the
entire length of the wire filter and partially occlude
substantially all of the pores, or the coating could be selectively
applied to portions of the filter to effect selective pore size
reduction.
[0071] To produce a coating such as that shown in FIG. 9A, the
filter can be dipped in a polymer solution and withdrawn at a
programmed rate so as to deposit a film of polymer onto the filter
mesh. The filter may be rotated to prevent undesired pooling of the
coating. Variables such as % solids content, solvent used, relative
humidity, temperature, and others may be controlled to achieve the
desired coating properties. A primer may be applied to the mesh, or
the mesh may be surface modified, to aid with coating adhesion. To
prevent coating migration into the mesh of the filter where a
coating is not desired, the filter can be masked with low melting
temperature wax or any other masking material as known in the art.
The mask is later removed, for example, by heating in the case of a
low temperature wax. Alternatively, excess coating can be applied
and the excess later removed by means such as laser ablation.
[0072] When coatings such as that described in connection with FIG.
9A are applied to woven filamentous structures such as braids, the
filament crossings tend to become stiffened since the coating
adheres to the filaments in the region of the crossings. In some
embodiments filament crossing immobilization is desirable so that
pore size uniformity is achieved by preventing lateral or
through-thickness migration of filter mesh filaments. In other
cases it can be desirable to fracture the coating, particularly at
the filament crossings, in order to restore the flexibility of the
woven filter structure. Coating fracture can be accomplished by
repeated cycling of the coated structure between two filament
crossing angles, or alternatively, by embrittling the coating such
as by using solvents or by cooling the coating to below its glass
transition temperature followed by cycling at least once to
fracture the coating.
[0073] Coatings applied by dipping can produce a coating of
unacceptably large thickness. An applied coating can be ironed to a
reduced thickness by pressing the coating between molds prior to
solidification of cure of the coating or, in the case of
thermoplastic coatings, after the coating has been applied. For
example, a conical metal mesh with a thermoplastic coating can be
placed inside a heated conical metal mold cavity, and a conical
mandrel pressed into the coated conical mesh so as to heat form the
coating into a reduced thickness configuration.
[0074] Coatings applied by dipping can also produce coatings of an
unacceptably large mass. Alternatively, coatings can be applied
using vapor deposition techniques. For example, a thermoplastic
elastomer can be heated above melting and passed through a spray or
atomizing nozzle to produce a mist of molten polymer. The mist of
polymer, upon contacting a metal mesh at a temperature below the
freezing temperature of the polymer, will condense on and adhere to
the metal mesh. By adjusting process parameters, a thin coating can
be applied to all or part of the mesh. Alternatively, a polymer can
be dissolved into solution and atomized into a mist which is
brought into contact with a heated metal mesh. The mist, on contact
with the heated metal, will deposit on and adhere to the metal mesh
as the heat drives solvent from the mist. Coatings applied using
these techniques can be fractured or thinned out using techniques
described above.
Elongated Braid Filters
[0075] Another way to control the pore size and pore distribution
in a filter is to fashion a woven fabric so that pores of the
desired sizes or shapes are located in the desired places. FIG. 10A
illustrates filter 100a having three regions wherein there are
different diamond-shaped pores in regions 102a, 102b and 102c.
Region 102a has axially shortened diamonds, region 102b has square
diamonds, and region 102c has axially elongated diamonds. These
shapes are achieved by stretching a tubular braided structure over
a conical mandrel. In this example, a 3.5 mm diameter wire braid of
about 100 picks/inch (39 picks/cm) and 72 wires of 0.0015 inch (38
micron) diameter has been stretched over a 4 mm conical mandrel. In
use, wall apposition region 102a will contact the walls of the
lumen, substantial filtering will begin in region 102b, and emboli
will accumulate in capture region 102c. The placement of the area
of the square diamonds in the center of the filter results in a
lower variation in pore size than placing the square diamonds at
the proximal opening of the filter.
[0076] A design shown in FIG. 10C illustrates filter 100b having no
square diamonds. In a typical embodiment, a 6 mm diameter braid
100c made from 72 wires of 0.001 inch (25 micron) diameter and
about 100 picks/inch (39 picks/cm) (as shown in FIG. 10B) is formed
into a filter having a windsock shape with a 4 mm opening (as shown
in FIG. 10C). That is, the braid is elongated, thus producing
axially elongated diamonds and diameter reduction with consequent
reduction in size of the pores. In this example, the maximum pore
size, compared to a filter made with similar braid of 4 mm
diameter, has been reduced by 8% to 162 microns.
Knitted Mesh Filters
[0077] Another way to control the pore size and pore distribution
in a filter is to use a knitted structure, preferably
self-expanding nitinol. Knitting is generally defined as a fabric
produced by intermeshing loops of yam, wire, etc. Circular knits
and flat knits are preferred. Welt tops may be preferred in some
designs. Knitted structures can be made with more pore size
uniformity by knitting loops of a predetermined size and using a
given number of loops to form a given filter perimeter and a larger
number of loops to form a larger perimeter. It is desirable to knit
structures close to final geometry so as to minimize manufacturing
steps. For example, one could use a circular knitting machine to
knit a windsock shape. FIG. 11A illustrates the overall shape of a
filter 105 having a knitted structure made of wires 106. FIG. 11B
illustrates the mesh in detail and shows how the knitted loops can
be interwoven. Preferably, the filter would be knit with no loose
ends at the open end by looping the strands back at the edge. The
closed end could have loose ends which would be subsequently
clamped within a marker band or other structure, or that would be
bonded into a unitary structure such as a wire tip.
[0078] Ideally this knitted structure would be heat set to assume a
self-expanding characteristic. To assist with mesh expansion, it
would be desirable to use larger wires for a portion of the knit
strands. The large wires would assist in expansion of the knit
because the stiffness of the wires increases dramatically with wire
diameter. It may be desirable to use progressively changed wire
diameters in the knit so as to minimize pore size disruption
(similar to the graduated strand diameters filters above). The mesh
can be made radiopaque using the same approaches described
above.
Filters Made Using Martensitic Braiding or Martensitic Knitting
[0079] Another way to control the pore size and pore distribution
in a filter is to use martensitic braiding. In forming embolic
protection filters, it has been found that when braiding high pick
count braids (e.g., 200 picks per inch or about 78 picks per cm)
the stiffness of the wire limits the ability to achieve high pick
counts. This is because the wires weave over and under each other,
and the frequency of the curves in the wire increases as the pick
count increases. Wire stiffness will limit the achievable pick
count to a maximum even if the braiding machine is set to produce a
higher number of picks per inch. This is illustrated in FIGS. 12A
and 12B, in which a braid formed of wires 112a and 112b has been
placed over mandrel M. FIG. 12A shows a low pick count braid, FIG.
12B shows a high pick count braid, and FIG. 12C shows a braid on a
mandrel M and a section line S for the views in FIGS. 12A and
12B.
[0080] Nitinol wire is very desirable for use in these filters, and
it typically is woven at room temperature when in its austenitic
phase. For a typical nitinol wire with a ratio of nickel to
titanium of 1:1, the martensitic phase can exist when the nitinol
is cooled to below about -30.degree. C., although this temperature
can be varied substantially depending on the thermo-mechanical
history of the material. The martensitic phase is less stiff,
having a modulus of about 5.times.10.sup.6 psi, compared to the
austenitic phase's modulus of 8.times.10.sup.6 psi.
(34.47.times.10.sup.6 MPa vs. 55.12.times.10.sup.6 MPa). By cooling
the nitinol, preferably to below the martensite start temperature,
M.sub.s, and even more preferably to below the martensite finish
temperature, M.sub.f, as determined by differential scanning
calorimetry at zero stress, higher pick counts can be achieved
during braiding because the wires can bend more easily. Such
braids, when formed into filters can permit finer filter pore sizes
to be achieved.
[0081] Similar to martensitic braiding, martensitic knitting can be
used to control the pore size and pore distribution in a filter. As
discussed above, lowering the temperature of nitinol wire to the
martensitic region allows the wires to bend more easily. Wires that
bend more easily can be knitted into filters with finer pore
sizes.
Filters Made by Wire Removal
[0082] The pore sizes in embolic protection filters also can be
varied by removing portions of the wires comprising the filter. A
typical braided filter has smaller pore sizes at its distal end due
to its shape. It would be desirable to remove material to make
those pores the same size as those at the proximal end. For
example, a 72 wire braid could be made with half nitinol and half
stainless steel wires. Preferably adjacent strands would alternate
between these two materials. After forming the braid into the
desired filter shape, a region of the filter at the distal end is
placed in a solution that will dissolve the stainless steel wires.
For example, a 100 to 300 g/L solution of ferric chloride will
dissolve stainless steel and not damage the nitinol. FIG. 13A
illustrates a filter 120a in which the stainless steel wires 122b
at the distal end have been removed. Nitinol wires 122a remain.
[0083] Etching can be combined with various wire compositions to
produce a filter such as shown in FIG. 13B, with 72 wires, 48
wires, and 24 wires present in different portions of the filter.
For clarity, the individual wires are not shown in FIG. 13B. A 72
wire braid can be made comprising equal parts of nitinol, stainless
steel, and gold wire. The braid is made so that the wires alternate
within each helix (e.g., nitinol, stainless steel, gold, nitinol,
stainless steel, gold, etc.). The braid is removed from the
braiding machine and then placed on a mandrel and heat set into a
desired filter shape, such as a windsock shape. The proximal
element or band is crimped and swaged around the gathered braid,
but the distal element or marker is not permanently attached at
this time. The filter is then passivated in nitric acid solution to
remove nickel from the nitinol surface and to leave a corrosion
resistant oxidized titanium surface. Optionally, the proximal
one-third of the filter can be masked with wax or other suitable
impervious material that can later be removed easily.
[0084] The distal two-thirds of the filter is then immersed in a
solution of potassium iodide in iodine to dissolve the gold wire. A
20% sodium cyanide solution containing a small amount of hydrogen
peroxide is also suitable for this purpose.
[0085] The proximal two thirds of the filter then is masked, and
the distal one-third of the filter is immersed in ferric chloride
solution to dissolve the stainless steel. The mask material is then
removed, for example, by heating, and the entire structure is again
passivated in nitric acid solution. The distal marker or element is
then affixed to the 24 nitinol strands remaining at the distal end
of the filter.
[0086] It is preferred to use larger diameter nitinol wires than
are typically used in such filters in order to produce a filter
having sufficient self-expansion properties. The nitinol has to
overcome resistance to expansion by the non-shape-set stainless
steel and gold wires. For example, for a windsock-shaped filter
having a 4 mm opening, 0.002 inch (50 micron) nitinol wires, and
0.001 inch (25 micron) diameter stainless steel and gold wires are
suitable for use in this filter.
[0087] One of skill in the art recognizes that other wire
combinations can be used to form filters similar to those described
in FIGS. 13A and 13B. Also, wires can be removed by methods other
than by chemical etching. For example, laser cutting, plasma
etching, mechanical cutting, water jet cutting, and selective
embrittlement followed by fracture are all techniques that would
permit removal of a desired portion of wire or wires.
[0088] It also may be desirable to remove only a portion of a wire
in a region to create a more uniform pore size or an altered pore
size distribution. That is, material could be partially removed or
etched by the methods described above, thus increasing the pore
size in a region. The converse is also true, where material could
be added to fine diameter wires such as by sputtering or other
physical vapor deposition methods, by plating, or by coating with
various polymers, as described above.
[0089] A method to remove metal wire by etching is to immerse the
filter lengthwise (i.e., the braid formed into its filter shape)
and to withdraw it slowly from an etchant bath so that the distal
end of the filter remains in the bath the longest. Material removal
can be accelerated by flowing etchant over the filter, or to
otherwise agitate the etchant. The filter material can be flexed
and moved during etching to assure that all wire, even at wire
cross-overs, will be exposed to etchant.
Filters Made by Adding Wires During Braiding
[0090] Yet another way to control the pore size and pore
distribution in a filter is to add wires during braiding. FIG. 14
illustrates a windsock-shaped filter 170 in which the mesh fabric
of the filter is braided so that there are fewer wires, as measured
around the perimeter, at the distal end 174 than at the proximal
end 172 of the filter. This design results in a more uniform pore
size. To do this, a length of braid is made, and then the braiding
machine is stopped and more wires are added to the braiding
machine. The wire ends are looped though the existing braid. The
machine is then restarted and the braiding continues to make a
suitable length of braid. The filter is formed so that the portion
of the braid having the smaller number of wires forms the distal
end of the filter. There is a transition region, denoted by 173,
between the proximal and distal portions of the filter.
[0091] To form a suitable filter, the following procedure can be
used: (a) on a 48 carrier braiding machine, load 24 bobbins into
alternate carriers, 12 for the clockwise rotation carriers and 12
for the counter-clockwise rotation carriers; (b) braid a desired
length of 24 wire braid on the 48 carrier braider; (c) separately,
cut 12 lengths of wire and wind one end of each length onto a
bobbin such that the wire is wound onto two bobbins; (d) mount one
of these bobbins onto a clockwise carrier, loop the other bobbin
through the braid tent, through a cell formed by wire crossings,
mount the second bobbin onto a counter-clockwise carrier, and
remove any slack from the wire by tensioning the carrier; (e)
repeat for the other 11 lengths of wire; and (f) continue braiding
on the 48 carrier braiding machine.
[0092] Clearly, this procedure could be repeated so as to produce a
braid with several zones, each having more wires than the previous
one. It is also possible to intersperse the additional wires in
step (d) above at different heights in the braid tent. This would
produce a more gradual transition between zones of wires.
[0093] Alternatively, the braiding mandrel can be modified, as
shown in FIG. 15F. Mandrel 300f in FIG. 15F is shown with
pin-receiving holes 301. Pins 303 can be placed in the desired
positions so that a braid of the desired wire density and pattern
is formed. Braid is braided on the braiding machine until the braid
reaches the location of the holes 301 on the mandrel 300f. Pins 303
are then inserted into the holes and braid wires are added as
described above except that the wires are looped over the pins
rather than being looped through the braid tent. Braiding then
continues to form the desired length.
[0094] It would also be desirable to reduce the number of loose
wire ends in a length of braid. These ends can irritate, cut, or
puncture vessels and result in damage. FIG. 15A is a schematic
illustration showing wires 185 leading from carriers 315. The wires
wrap around mandrel 300a. To eliminate ends, wires can be mounted
on the braiding mandrel by looping the wires over pins 303, mounted
in pin-receiving holes 301, as shown in detail in FIGS. 15B and 15C
in two alternative patterns. Once all wires are looped and the
tension of the carriers adjusted braid can be produced in a
conventional way.
[0095] FIG. 15E shows a mandrel 300e modified to comprise
pin-receiving holes 301 that accept pins in two axial positions. To
use this mandrel braid is looped over the first row of pins as
described at 15F. Braiding is carried out until the braid length
formed on the mandrel reaches the second row of holes. More wires
are looped over the second row of pins and braiding recommences
until a structure of the desired length has been fabricated. It
will be recognized that multiple groupings of pins are possible,
for example, see FIG. 15D.
Electroformed Filters
[0096] Another way to control the pore size and pore distribution
in a filter is to form the filters by electroforming.
Electroforming can be used to make mesh filters for use in the
present invention. Electroforming permits the selection of the pore
sizes and distribution of pores. Electroformed mesh films are
commercially available from Buckbee-Mears in various pore sizes.
Such films may not be as flexible as materials comprising braids,
knits, weaves, and the like, but the films can be made very thin or
with preferential fold lines, thereby overcoming their flexibility
limitations. Such films also can be pleated to assist in the
collapse of a filter made from electroformed film. These films may
require the support of a secondary frame, either a separate
structure or an integral structure, to provide the film with
sufficient mechanical integrity to act as an embolic protection
filter. Such films can be made into the desired three-dimensional
shape by forming onto a substrate, such as a cone-shaped or a
dome-shaped mandrel. The films can be formed on a flat substrate
and the desired three-dimensional shape can be added by
post-processing. Alternatively, the films can be formed directly on
the three-dimensional substrate. FIG. 16 shows a perspective view
of an electroformed filter 350 having a smooth cylindrical proximal
portion 352 and a pleated distal portion 354 having holes 356 to
allow the flow of fluid. The filter 350 is disposed about an
elongate support member 355, attached by proximal element 357 and
distal element 353.
[0097] A process for making an electroformed filter could be as
follows. First, choose a substrate material, typically nickel or
stainless steel, preferably passivated, or metallized ceramic such
as indium tin oxide on glass. Second, coat the substrate with
photoresist. Third, deposit a pattern for the filter, typically
chrome on glass. Fourth, through contact or non-contact methods,
print the pattern onto the substrate and expose the photoresist to
light through the pattern. Photoresists can be positive action
(these are removed in the exposed areas) or negative action (these
remain in the exposed area). Fifth, develop the photoresist in a
suitable developer; excess photoresist is removed and one is left
with a conductive substrate with a photoresist pattern thereon.
Sixth, electroplate the substrate with the metal of interest.
Typically, an aqueous solution of metal salt is the source of the
electroplated metal. Useful metal salts include those of gold,
nickel, or platinum. Seventh, remove the patterned photoresist to
free the electroformed part from the substrate.
Filter Formed into Mandrel
[0098] The following process could be used to form filters for use
as embolic protection devices that have a narrow distribution of
pore sizes. A form, typically a mandrel, is etched (another
suitable process such as laser removal of the mandrel surface can
be used) with the filter pattern desired. The mandrel material is
removed to some depth under the intended filter surface. Material
removal depth can be varied for various portions of the filter.
[0099] Next, material is embedded into the
grooves/fissures/depressions/etc. in the form surface. In the case
of metals, the form can be filled by electroforming metal onto the
form surface, by filling the form texture with powdered metal
followed by sintering or other agglomeration methods, by metal
injection molding, by swaging or otherwise mechanically compressing
material into the texture, by using shock waves such as those
created by explosives, or by using electromagnetic energy. For
polymer filters the form texture can be filled using dipping,
casting, thermoforming, or other methods.
[0100] It is expected that excess material will be present above
the surface of the mandrel after the material is embedded into the
form. Excess material is removed form the form, typically by
grinding, leaving a filled form in the pattern of the desired
filter. The form can then be removed, typically by dissolving it or
melting it out. FIGS. 17A to 17C are cross-sectional views of a
mandrel and a filter material formed into the mandrel in a process
for forming a filter into a mandrel. FIG. 17A is a cross-sectional
view of a mandrel 360. FIG. 17B shows the filter material 362
deposited onto the mandrel 360. FIG. 17C shows the filter material
362 on the mandrel 360 after the excess filter material is
removed.
[0101] In addition to the methods described above, the pore sizes
of the filters described above can also be controlled by the
methods described in the U.S. patent application filed on the same
date as the present application and entitled "Embolic Filters
Having Multiple Layers and Controlled Pore Size" (Atty Docket:
EV31002US), the contents of which are hereby incorporated by
reference herein.
[0102] The above description and the drawings are provided for the
purpose of describing embodiments of the invention and are not
intended to limit the scope of the invention in any way. It will be
apparent to those skilled in the art that various modifications and
variations can be made without departing from the spirit or scope
of the invention. Thus, it is intended that the present invention
cover the modifications and variations of this invention provided
they come within the scope of the appended claims and their
equivalents.
* * * * *