U.S. patent application number 10/535422 was filed with the patent office on 2006-06-22 for medical instrument for soft tissue and method for manufacture thereof.
This patent application is currently assigned to Independent Administrative Institution National Institute For Materials Science. Invention is credited to Takao Hanawa, Takashi Kawabata, Daisuke Kuroda, Morihide Makino.
Application Number | 20060130934 10/535422 |
Document ID | / |
Family ID | 32321847 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060130934 |
Kind Code |
A1 |
Kuroda; Daisuke ; et
al. |
June 22, 2006 |
Medical instrument for soft tissue and method for manufacture
thereof
Abstract
Ferritic stainless steel was produced by the melting method. The
ferritic stainless steel was worked to the shape of a medical
device for living soft tissue to obtain a medical device body. The
medical device body was brought into contact with gas including
nitrogen at a 800.degree. C. treatment temperature or more to make
the ferritic stainless steel forming the medical device body absorb
the nitrogen and transform at least part of the ferritic stainless
steel to austenite.
Inventors: |
Kuroda; Daisuke; (Ibaraki,
JP) ; Hanawa; Takao; (Ibaraki, JP) ; Makino;
Morihide; (Tokyo, JP) ; Kawabata; Takashi;
(Saitama, JP) |
Correspondence
Address: |
ARMSTRONG, KRATZ, QUINTOS, HANSON & BROOKS, LLP
1725 K STREET, NW
SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Independent Administrative
Institution National Institute For Materials Science
2-1, SENGEN, 1-CHOME TSUKUBA-SHI
IBARAKI
JP
305-0047
Japan Lifeline Co., Ltd.
38-1 IKEBUKURO 2-CHOME TOSHIMA-KU
TOKYO
JP
171-0014
|
Family ID: |
32321847 |
Appl. No.: |
10/535422 |
Filed: |
November 11, 2003 |
PCT Filed: |
November 11, 2003 |
PCT NO: |
PCT/JP03/14307 |
371 Date: |
December 19, 2005 |
Current U.S.
Class: |
148/221 |
Current CPC
Class: |
A61B 2017/00575
20130101; A61B 2017/00606 20130101; C23C 8/26 20130101; A61L 31/022
20130101; C22C 38/001 20130101; C21D 1/18 20130101; C22C 38/38
20130101; A61B 17/0057 20130101; C21D 6/002 20130101; C21D 2211/001
20130101; C22C 38/22 20130101; C22C 38/04 20130101; C21D 9/0068
20130101; C22C 38/28 20130101; A61F 2/91 20130101 |
Class at
Publication: |
148/221 |
International
Class: |
C23C 8/26 20060101
C23C008/26 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2002 |
JP |
2002-337598 |
Claims
1. A method for producing a medical device for living soft tissue
having: a melting step of producing ferritic stainless steel by
melting method, a working step of working said ferritic stainless
steel to the shape of a medical device for living soft tissue to
obtain a medical device body, and a nitrogen absorption step of
bringing said medical device body into contact with a gas
containing nitrogen at a predetermined treatment temperature or
more to make said ferritic stainless steel forming said medical
device body absorb nitrogen to transform at least part of said
ferritic stainless steel to austenite.
2. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein said ferritic stainless
steel has as main ingredients Fe in an amount of 50 to 90 wt %, Cr
and/or Mn in amounts of 10 to 30 wt %, and Mo and/or Ti in amounts
of 0 to 10 wt %.
3. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein said ferritic stainless
steel has as main ingredients Fe in an amount of 65 to 80 wt %, Cr
and/or Mn in amounts of 15 to 25 wt %, and Mo and/or Ti in amounts
of 0 to 5 wt %.
4. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein said treatment temperature
is in a temperature range of 800 to 1500.degree. C.
5. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein said treatment temperature
is in the temperature range of 1100 to 1300.degree. C.
6. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein said ferritic stainless
steel is made to contain nitrogen in an amount of at least 0.5 wt
%.
7. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein said ferritic stainless
steel is made to contain nitrogen in an amount of at least 0.8 wt
%.
8. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein at least part of said
ferritic stainless steel is transformed to austenite to form a
two-phase structure of ferrite and austenite.
9. The method of production of a medical device for living soft
tissue as set forth in claim 1, wherein all of said ferritic
stainless steel is transformed to austenite.
10. The medical device for living soft tissue produced by a method
of production of a medical device for living soft tissue as set
forth in claim 1.
11. The method of production of a medical device for living soft
tissue as set forth in claim 2, wherein said treatment temperature
is in a temperature range of 800 to 1500.degree. C.
12. The method of production of a medical device for living soft
tissue as set forth in claim 2, wherein said treatment temperature
is in the temperature range of 1100 to 1300.degree. C.
13. The method of production of a medical device for living soft
tissue as set forth in claim 2, wherein said ferritic stainless
steel is made to contain nitrogen in an amount of at least 0.5 wt
%.
14. The method of production of a medical device for living soft
tissue as set forth in claim 2, wherein said ferritic stainless
steel is made to contain nitrogen in an amount of at least 0.8 wt
%.
15. The method of production of a medical device for living soft
tissue as set forth in claim 2, wherein at least part of said
ferritic stainless steel is transformed to austenite to form a
two-phase structure of ferrite and austenite.
16. The method of production of a medical device for living soft
tissue as set forth in claim 2, wherein all of said ferritic
stainless steel is transformed to austenite.
17. The medical device for living soft tissue produced by a method
of production of a medical device for living soft tissue as set
forth in claim 2.
Description
TECHNICAL FIELD
[0001] The present invention relates to circulatory system medical
devices, digestive tract medical devices, and other medical devices
for living soft tissue and methods for production of the same.
BACKGROUND ART
[0002] Medical devices used for the cardiovascular system,
including the cerebral circulatory system, etc. are required to
have blood compatibility, but are easily improved by improvement of
not only the materials, but also the surface properties etc.
However, in long-term implanted medical devices, surface coatings
easily break down leading to the appearance of the properties
inherent to the base metal of the medical devices.
[0003] In the past, as the metal serving as the base of such
medical devices, from the viewpoint of corrosion resistance,
SUS304, SUS316, and other materials have been frequently used. For
example, one type of medical device, a stent, is usually made of
stainless steel (Japanese Patent Publication (A) No. 9-117512,
Japanese Patent Publication (A) No. 11-99207, Japanese National
Disclosure (A) No. 12-501328, Japanese Patent Publication (A) No.
7-531, Japanese Patent Publication (A) No. 12-5321, etc.) Further,
recently, there are known to have been attempts to make medical
devices out of NiTi shape memory alloys.
[0004] However, there are reports that these metallic materials are
problematic in the effect of the Ni contained on the body. At the
present, metallic materials not including Ni and good in corrosion
resistance in the body are being sought. However, in general,
metallic materials superior in corrosion resistance are hard and
poor in workability.
DISCLOSURE OF THE INVENTION
[0005] The present invention was made in view of this situation and
has as its object the economic provision of a medical device for
living soft tissue not containing Ni, believed to be low in effect
of the body, low in cytotoxicity, superior in workability, and
superior in corrosion resistance, mechanical strength, and
biocompatibility.
[0006] The inventors engaged in intensive study and as a result
discovered that by producing ferritic stainless steel by the
melting method, working it into a medical device for living soft
tissue, then bringing it into contact with a gas containing
nitrogen to make the nitrogen be absorbed into the worked product
so as to transform at least part to austenite, it is possible to
provide a delicate, complicated medical device for living soft
tissue having superior workability, that this has superior
corrosion resistance while maintaining sufficiently superior
physical properties and practical properties, and that this has
superior performance and safety as a medical device for living soft
tissue, in particular a medical device for living soft tissue used
in contact with the blood or in contact with blood vessels and
thereby completed the present invention.
[0007] That is, the method for producing a medical device for
living soft tissue of the invention has a melting step of producing
ferritic stainless steel by the melting method, a working step of
working said ferritic stainless steel to the shape of a medical
device for living soft tissue to obtain a medical device body, and
a nitrogen absorption step of bringing said medical device body
into contact with a gas containing nitrogen at a predetermined
treatment temperature or more to make said ferritic stainless steel
forming said medical device body absorb nitrogen to transform at
least part of said ferritic stainless steel to austenite.
[0008] The mechanical strength, modulus of elasticity, and other
physical properties and the corrosion resistance are sharply
improved by bringing the steel into contact with an inert gas
containing nitrogen to make nitrogen be absorbed into the worked
product so as to transform at least part to austenite. Further, the
nitrogen is added to products made by ferritic stainless steel, so
working is easier than with austenitic stainless steel and a
product having the desired shape is easily obtained.
[0009] Therefore, a step requiring workability is performed by the
material ferritic stainless steel, worked to a medical device for
living soft tissue, then transformed to austenite to enable working
to a complicated medical device for living soft tissue and provide
a product superior in corrosion resistance and safety at a low
cost.
[0010] Note that the above technique of bringing a medical device
body worked to a desired shape into contact with an inert gas
containing nitrogen gas at a predetermined temperature or more
falls under the category of nitrogen absorption treatment
classified as a so-called solid phase absorption method. By heating
a medical device body to a predetermined temperature or more in a
gas atmosphere containing nitrogen, nitrogen is added to all or
part of the product.
[0011] Preferably, said ferritic stainless steel has as main
ingredients Fe in an amount of 50 to 90 wt % (preferably 65 to 80
wt %), Cr and/or Mn in amounts of 10 to 30 wt % (preferably 15 to
25 wt %), and Mo and/or Ti in amounts of 0 to 10 wt % (preferably 0
to 5 wt %). The ferritic stainless steel preferably does not
contain nickel.
[0012] Preferably, said treatment temperature is a temperature
range of 800 to 1500.degree. C., more preferably 1100 to
1300.degree. C. If the treatment temperature is too low, the
absorption of nitrogen in the ferritic stainless steel tends to
become insufficient, while if the treatment temperature is too
high, the physical properties tend to easily change.
[0013] Preferably, said ferritic stainless steel is made to contain
nitrogen in an amount of at least 0.5 wt %, more preferably at
least 0.8 wt %. If the amount of absorption of nitrogen in the
ferritic stainless steel is too small, the effect of the present
invention tends to become smaller.
[0014] Preferably, at least part of said ferritic stainless steel
is made austenitic to form a two-phase structure of ferrite and
austenite. Alternatively, all of said ferritic stainless steel is
transformed to austenite. By forming a two-phase structure of
ferrite and austenite or by transforming the entire amount to
austenite, the corrosion resistance, mechanical strength, and
biocompatibility are improved.
[0015] The medical device for living soft tissue according to the
present invention is produced by the above method of production of
the medical device for living soft tissue. The medical device for
living soft tissue is not particularly limited, but the effect of
the present invention is particularly great in the case of
circulatory system medical devices or digestive tract medical
devices. Circulatory system medical devices are medical devices
used for example inside living organs of the circulatory system
including the heart, central major blood vessels, peripheral blood
vessels, and cerebral blood vessels or in contact with the outside
walls of the organs. Further, digestive tract medical devices are
medical devices used inside the stomach, intestines, and other
parts of the digestive tract or in contact with the outside walls
of the organs. This is because this type of medical device is often
embedded in the body over a long period of time and the metal
forming the medical devices preferably does not contain Ni. As
specific examples of circulatory system medical devices, stents,
stent grafts, vein filters, occlusion coils, materials for
repairing abnormalities of the blood organs such as patent ductus
arteriosus or atrial septal defects, artificial valves, blood
vessel clips, etc. which are placed in the body for long periods of
three months or more may be mentioned.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic perspective view of a stent according
to a first embodiment of the present invention. FIG. 2A and FIG. 2B
are cross-sectional views of principal parts showing the state of
use of a stent. FIG. 3 is a schematic perspective view of a
prosthesis according to another embodiment of the present
invention. FIG. 4A is an X-ray diffraction pattern of a test piece
after nitrogen absorption treatment, while FIG. 4B is an X-ray
diffraction pattern of a test piece equivalent not subjected to
nitrogen absorption treatment. FIG. 5 is a graph of the correlation
showing the balance between strength and ductility between a test
piece after nitrogen absorption treatment and a test piece
equivalent not subjected to nitrogen absorption treatment.
BEST MODE FOR WORKING THE INVENTION
[0017] Below, the present invention will be explained in detail
based on the embodiments shown in the figures.
First Embodiment
[0018] As shown in FIG. 1, the stent 2 serving as a circulatory
system medical device according to the present embodiment is a
substantially cylindrically shaped stent placed in its entirety in
an arterial cavity of the body and has first stent elements 4 and
second stent elements 6. The first stent elements 4 and the second
stent elements 6 form the medical device body.
[0019] The first stent elements 4 are present along the
circumferential direction and have shapes expandable in outside
diameter. They are made of materials resistant to collapse after
expansion of the outside diameter. The shape enabling expansion of
the outside diameter is not particularly limited, but specifically
may be, along the circumferential direction, a wave shape,
peak-valley shape, a sine/cosine curve shape, a zigzag shape, a
string-of-pearl shape, a sawtooth shape, a pulse shape, or a
combination of the same or other repeating shapes etc. The material
resistant to collapse after expansion of its outside diameter, in
the present embodiment, is made ferritic stainless steel brought
into contact with a gas containing nitrogen at a predetermined
treatment temperature or more to make it absorb nitrogen to
transform at least part to austenite.
[0020] The second stent elements 6 are stent elements for
connecting in the axial direction a plurality of said first stent
elements 4 arranged in the axial direction. In the present
embodiment, they are comprised of the same material as the first
stent elements.
[0021] The width and/or thickness of the first stent elements 4 is
preferably 30 to 400 .mu.m, more preferably 50 to 100 .mu.m, while
the width and/or thickness of the second stent elements 6 is
preferably 20 to 100 .mu.m, more preferably 30 to 60 .mu.m. The
axial direction length L1 of the repeating units forming the first
stent elements 4 is not particularly limited, but preferably is 0.5
to 5 mm, more preferably 0.8 to 2 mm. The axial direction length L2
of the second stent elements 6 is not particularly limited, but
preferably is 0.5 to 5 mm, more preferably 1.5 to 3 mm. Note that
the second stent elements 6 do not necessarily have to be straight
lines parallel to the center axis of the stent and may also be
slanted lines, curves, or combinations of the same.
[0022] In the present embodiment, the second stent elements 6 are
arranged more sparsely along the circumferential direction than the
first stent elements 4. For example, it is preferable to arranging
two to six second stent elements 6 in the circumferential direction
of the first stent elements 4. Further, the second stent elements 6
may connect the peak parts of the repeating units of the first
stent elements 4 or may connect the valley parts or the peak parts
and the valley parts. Further, they may also connect the parts in
the middles of the peaks and valleys.
[0023] The dimensions of the stent 2 as a whole are suitably
determined in accordance with the objective of use etc. and are not
particularly limited, but for example when used for treatment of a
coronary artery, the outside diameter of the stent 2 at the time of
expansion is preferably 2 to 5 mm and the axial direction length is
15 to 40 mm. Further, in the case of a stent for treatment of a
peripheral blood vessel, the outside diameter at the time of
expansion of the stent 2 is preferably 3 to 10 mm, and the axial
direction length is 15 to 40 mm. Further, in the case of a stent
for treatment of the aorta, the outside diameter at the time of
expansion of the stent 2 is preferably 5 to 30 mm, and the axial
direction length is 30 to 100 mm.
[0024] The surfaces of the first stent elements 4 and second stent
elements 6 forming the stent 2 may be covered by a plating film
and/or biocompatible coating film. This is for improving the
biocompatibility. Further, as a plating film, a platinum or gold
plating film is used. The biocompatible coating film is not
particularly limited, but for example a polyethylene or other
olefin, polyimide or polyamide or other nitrogen-containing
polymer, siloxane polymer, or other ordinary polymer used for
medical purposes etc. may be used. Further, the coating film is not
limited to a polymer and may also be an inorganic coating film of
silicon carbide, pyrolitic carbon, diamond-like carbon, or other
carbon etc. Further, the surface of the stent 2 may also be treated
to be made hydrophilic. The surface of the stent 2 may also have
immobilized at it an enzyme, bio ingredient, or chemical for
preventing reconstriction. The film thicknesses are not
particularly limited, but the thickness of the plating film is for
example 0.05 to 5 .mu.m or so, while the thickness of the
biocompatible coating film is 0.1 to 10 .mu.m or so, preferably 0.5
to 5 .mu.m.
[0025] Next, the method of production of the stent of the present
embodiment will be explained.
[0026] First, the ferritic stainless steel serving as the material
of the medical device body comprised of the first stent elements 4
and second stent elements 6 is produced. This ferritic stainless
steel is produced by the melting method. The "melting method" means
the process of placing the materials forming the ingot into a
furnace and melting them to make them homogeneous. The conditions
at the time of melting are not particularly limited, but preferably
the furnace is heated in the vacuum state up to near 1600.degree.
C. and then the materials are added.
[0027] Next, the ferritic stainless steel produced by melting
method is worked to form a metal tube (thickness of 50 to 400
.mu.m).
[0028] Next, the metal tube is coated on its surface with a
photosensitive cross-linkable resist and is exposed by a reduction
projection exposure apparatus using a previously prepared master as
a photomask to transfer the master pattern to the resist. At this
time, the exposure is performed while rotating the metal tube.
After this, an ordinary method is used to dissolve away the
uncross-linked parts of the resist to form patterns of first stent
elements 4 and second stent elements 6 on the resist, then the
unnecessary metal parts are removed by etching to obtain the
medical device body. This medical device body is comprised of
ferritic stainless steel produced by melting method.
[0029] Next, this medical device body is brought into contact with
an inert gas containing nitrogen at a predetermined treatment
temperature or more to make the ferritic stainless steel forming
the medical device body absorb nitrogen and transform at least part
of, preferably all of, the ferritic stainless steel to austenite,
whereby the stent 2 shown in FIG. 1 is obtained.
[0030] Next, an example of use of the stent of the present
embodiment will be explained. As shown in FIG. 2A, the stent 2 is
first fit over the circumference of a balloon part 10 of a balloon
catheter 12 in the state compressed in the radial direction. In
that state, the balloon catheter 12 is inserted into a blood vessel
20 or other body cavity. After this, the stent is passed together
with the balloon part 10 of the balloon catheter 12 through the
inside of the blood vessel 20 bending by up to 90 degrees or more
and finally reaches a constricted part 22 of the blood vessel 20.
In the stent 2 according to the present embodiment, mainly the
second stent elements 6 easily bend to match the bent shape of the
blood vessel 20 and return to their original shapes after
positioning at the target constricted part 22. Therefore, the
ability of the stent 2 to follow bends and the insertion properties
are improved.
[0031] After this, as shown in FIG. 2B, the constricted part 22
expands together with the expansion of the balloon part 10. The
stent 2 also simultaneously expands outward in the radial
direction. After this, only the balloon catheter 12 is pulled out
from the blood vessel 20 so as to leave only the expanded stent 2
inside the expanded constricted part 22 and prevent reconstriction.
In the present embodiment, the first stent elements 4 in the stent
2 are parts for countering the force trying to return the
constricted part after expansion to its original form. They are
made of materials not easily collapsing, so can effectively prevent
reconstriction.
[0032] Further, in the present embodiment, the material is worked
to a medical device body, then the worked product is made to absorb
nitrogen so as to transform at least part to austenite. Therefore,
by performing a step requiring workability by the ferritic
stainless steel material, working this to a medical device, then
transforming this to austenite, it becomes possible to provide a
product able to be worked to a complicated medical device and
superior in corrosion resistance and safety.
[0033] Note that the specific form of the stent 2 is not limited to
the one shown in FIG. 1. A spiral shape, a hexagonal single-layer
stack structure, a hexagonal double-layer stack structure, and
various other shapes may also be considered.
Second Embodiment
[0034] As shown in FIG. 3, a prosthesis 30 serving as a circulatory
system medical device according to the present embodiment has wires
32 forming a mesh. By braiding the wires 32, as shown in FIG. 3, a
first disk 34 and second disk 36 and a recess 38 formed between the
two are formed whereby overall an hourglass shaped medical device
body is formed. FIG. 3 shows the state where the prothesis 30 is
expanded. It is designed to for example plug a hole naturally
formed in a body wall 40. The prosthesis 30 in the state before
passing through the hole of the body wall 40 is transported to the
hole of the body wall 40 by a catheter etc. in the state compressed
in outside diameter and then expanded there.
[0035] In the present embodiment, the wires 32 forming the medical
device body, in the same way as the first embodiment, are comprised
of ferritic stainless steel brought into contact with a gas
containing nitrogen at a predetermined treatment temperature or
more to make it absorb nitrogen and transform at least part to
austenite.
[0036] In the present embodiment, the material is worked to the
medical device body, then the worked product is made to absorb
nitrogen so as to transform at least part to austenite. Therefore,
by performing a step requiring workability by the ferritic
stainless steel material, working this to a medical device, then
transforming this to austenite, it becomes possible to provide a
product able to be worked to a complicated medical device and
superior in corrosion resistance and safety.
[0037] Note that the present invention is not limited to the
above-mentioned embodiments and can be modified in various ways
within the range of the present invention.
[0038] For example, the medical device is not limited to the
illustrated embodiments. Various medical devices for living soft
tissue may be considered.
[0039] Below, the present invention will be explained further based
on detailed examples, but the present invention is not limited to
these examples.
EXAMPLE 1 AND COMPARATIVE EXAMPLE 1
[0040] A vacuum arc melting furnace was used to first produce a 3.5
kg ingot of ferritic stainless steel (24 wt % of Cr, 2 wt % of Mo,
and the balance of substantially Fe). This was produced by heating
the furnace in a vacuum state to 1600.degree. C. and then adding
the materials.
[0041] This ingot was divided into four and cut to obtain 25
mm.times.25 mm.times.110 mm blocks, which were then hot forged at
1100.degree. C. and cold forged at room temperature to prepare rods
of a diameter of 9 mm.times.90 mm and sheets of a thickness of 1.5
mm.times.15 mm.times.15 mm. These rods were further machined to
prepare rod tensile test pieces. These two types of test pieces
were subjected nitrogen absorption treatment as explained below
using a nitridation system.
[0042] That is, each test piece was placed on a SUS304 mesh board,
degreased and washed by acetone, then inserted and arranged into a
nitridation part of a nitridation system which was then evacuated
to a vacuum to 2 Pa by a rotary pump. Next, the nitridation part
was charged at a flow rate of 2 L/min with an inert gas containing
nitrogen gas. The nitridation part was raised from room temperature
at a rate of 5.degree. C./min to 1200.degree. C. and the test piece
and nitrogen gas were brought into contact at 1200.degree. C. for
24 hours.
[0043] After the above nitrogen absorption treatment, the test
piece was quenched from 1200.degree. C. in ice water. The surface
scale was removed by polishing, then an X-ray diffraction system
was used to identify the microstructure. In this X-ray diffraction,
a CuKa tube was used to change from 2.theta./.theta.=40.degree. to
90.degree. by 1.degree./min increments. FIG. 4A shows the obtained
X-ray diffraction pattern. For comparison, FIG. 4B shows the X-ray
diffraction pattern of a test piece equivalent not subjected to
nitrogen absorption treatment. As will be clear from a comparison
with FIGS. 4A and 4B, it was confirmed that the test piece after
nitrogen absorption treatment became completely austenitic
stainless steel. The amount of addition of nitrogen was about 0.9
wt %.
[0044] Next, a 100 kN capacity Instron-type tensile tester was used
to conduct a tensile test on the test piece at a crosshead speed of
0.5 mm/min. The balance of the strength and ductility of a test
piece after nitrogen absorption treatment, a test piece equivalent
not subjected to nitrogen absorption treatment, and existing alloy
is shown in FIG. 5. As confirmed from FIG. 5, if performing
nitrogen absorption treatment, compared with existing alloy and a
test piece not subjected to nitrogen absorption treatment, the
balance of strength and ductility was superior. This tendency was
confirmed in the same way as the case where the test piece was a
cold rolled material not subjected to nitrogen absorption
treatment. The efficacy of nitrogen absorption treatment was
reconfirmed.
[0045] Further, each test piece was also evaluated for corrosion
resistance. Test pieces of the example of the invention, SUS316L
stainless steel test pieces, and test piece equivalents not
subjected to nitrogen absorption treatment piece were dipped in
different test solutions adjusted to 37.degree. C. and deaerated
using nitrogen, that is, a 0.9% NaCl solution, PBS(-) solution,
Hanks solution, and Eagle's MEM solution.
[0046] For each test solution, the test piece subjected to the
nitrogen absorption treatment could be confirmed to exhibit a
superior corrosion resistance compared with SUS316L stainless steel
and a test piece not subjected to nitrogen absorption treatment.
That is, the test pieces of the example of the invention were not
observed to have any pitting, while the SUS316L stainless steel
suffered from pitting.
EXAMPLE 2
[0047] In the same way as Example 1, steel sheet of a thickness of
1.5 mm.times.15 mm.times.15 mm was prepared, then the steel sheet
was rolled up and welded to form a tube shape. This was further
worked to form a tube of a thickness of 100 .mu.m and an outside
diameter of 1.5 mm.
[0048] This tube was cut by a YAG laser, fetomsecond laser, etc.
and chemically polished to obtain a stent 2 of the shape shown in
FIG. 1. This stent 2 was degreased and washed by acetone, then
brought into contact with nitrogen gas at 1200.degree. C. for 10
minutes. The nitrogen gas contact conditions at this time were the
same as Example 1.
[0049] This stent was mounted over a 3F polyamide balloon catheter
(1 mm outside diameter of balloon at the time of expansion) and
folded to give a stent outside diameter of 1.2 mm to reduce its
size.
[0050] Using the balloon, the folded stent was expanded by the
balloon catheter by a contrast agent solution and the expansion
pressure at that time measured. In this example, the expansion
pressure was 8 Pa. It could be confirmed that expansion was
possible with a relatively low expansion pressure.
[0051] Further, the stent of this example was expanded to an
outside diameter of 3 mm, then subjected to a water pressure of 10
atm using a cylindrical balloon to examine collapsing, but no
abnormalities were observed. The stent withstood even
pressurization to 20 atm and had durability sufficient for
practical use. Further, it was placed in a pressurizing tube and
measured for pressure and recoil. Even at 2 atm, there was only
slight recoil and a sufficient strength was exhibited. Further, it
was confirmed that after expansion of the stent, it was not easily
crushed.
[0052] Further, the stent was mounted at a stent delivery catheter
and inserted into a 3.phi. simulated circuit having three bends of
R=10, 10, 20. Smooth insertion was possible and there was no
problem. Further, it was confirmed that at the straight parts after
the bent parts, the stent was expanded, but no large looseness was
seen.
COMPARATIVE EXAMPLE 2
[0053] Other than not performing the nitrogen absorption treatment,
the same procedure was performed as in Example 2 to prepare a stent
which was then tested in the same way as Example 2. The expansion
pressure was 8 Pa or similar to the examples, but at a water
pressure of 10 atm, the stent ended up collapsing or crushing was
also observed and there were problems in terms of mechanical
strength.
[0054] As explained above, according to the present invention, it
is possible to economically provide a medical device for living
soft tissue not containing Ni, believed to be low in effect of the
body, low in cytotoxicity, superior in workability, and superior in
corrosion resistance, mechanical strength, and
biocompatibility.
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