Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Melting and Spinning Device and Melting and Spinning Method
Technical Field
This invention relates to a method and an apparatus for melt spinning a
medical
material implanted in a living body, for example, a strand of a biodegradable
polymer
material which forms a stent implanted in the vascular vessel of a living
body.
Background Art
When a stenosed lesion has occurred in the vascular vessel of a living body,
in
particular the blood vessel, such as arterial vessels, the percutaneous
transluminal
angioplasty (PTA) is performed by inserting a balloon mounted in the vicinity
of the
distal end of a catheter into the stenosed lesion and inflating this balloon
to expand the
stenosed lesion to keep the blood flowing.
Meanwhile, it is known that, even if PTA is applied, there is a high
probability
that re-stenosis is liable to occur at the once-stenosed portion.
In order to prevent this re-stenosis from occurring, the conventional practice
is
to implant a tubular stent on the site where the PTA has been performed. This
stent is
inserted in a contracted state into the blood vessel, subsequently dilated and
implanted
in this state in the blood vessel to support the blood vessel from its inside
to prevent
re-stenosis from occurring in the blood vessel. As this sort of the stent, the
metallic
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stent formed of such as stainless steel or a Ti-Ni based alloy, is now in use.
Meanwhile, the principal objective in implanting a stent in the blood vessel
in
PTA is to prevent acute coronary occlusion and to decrease the frequency of
re-stenosis. It has been reported that, since the acute coronary occlusion and
re-stenosis are the phenomenon which occurs during a predetermined time
period, so
that only transient therapy is needed. Consequently, the stent is required to
maintain
the function of supporting the blood vessel from inside for a predetermined
time
period, while it is more desirable that the stent is not left in the living
body as a foreign
substance.
If the metallic stent is implanted in the blood vessel, it is left
permanently, so
that, when re-stenosis occurs on the stent site, the stent frequently proves
an
obstruction to the operation of re-angioplasty. Moreover, the operation of
coronary-artery bypass graft is difficult to perform on the site of implanted
stent. Thus,
implanting the permanently persisting metallic stent offers various
inconveniences to
re-treatment.
In order to overcome the problem inherent in the metallic stent, such a stent
formed of a biodegradable polymer material has been proposed which is degraded
after
a lapse of a predetermined time, from the time it is implanted in e.g., the
blood vessel
of the living body to be then disappear by being absorbed in the living tissue
(JP Patent
No.2842943; JP Laying-Open Patent Publication H-11-57018).
The present inventors have proposed a stent comprised of a knitting obtained
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on knitting a strand of the biodegradable polymer material into a tubular form
(JP
Patent 2842943), and a stent obtained by bending a strand of a biodegradable
polymer
in a zigzag shape and wrapping it in a tubular form under a non-woven non-
knitted
condition.
With the use of the strand formed of a biodegradable polymer, it is possible
to
form a stent which exhibits mechanical characteristics sufficient to support
the vessel
in the inflated state for certain time duration and disappears after lapse of
the preset
time.
Since the stent formed of the strand of the biodegradable polymer can readily
be flexed and deformed, it can readily be delivered through the sinuous blood
vessel
so as to be implanted on the target site.
It should be noted that the biodegradable polymer material, as a high
molecular
material, differs in its degradation and absorption characteristics, and hence
in its
mechanical properties, depending on the molecular weight. For example, the
molecular
weight of the biodegradable polymer material, such as polylactic acid (PLA),
is
lowered by being melted and thermally decomposed. The degree that the
molecular
weight is lowered changes depending on the degree of thermal decomposition.
Thus,
if the melt spinning heating time of the same biodegradable polymer material
is
non-uniform, then the average molecular weight of the spun strand becomes
non-uniform. If the strand is non-uniform in its average molecular weight, its
degradation and absorption characteristics or mechanical properties undergo
localized
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variations.
If, with non-uniform average molecular weight, a stent is formed and implanted
in the vascular vessel, such as a blood vessel, the stent in its entirety
cannot be
degraded or absorbed evenly. Moreover, there is a fear that the stent formed
using this
sort of the strand cannot support the inner wall of the vascular vessel, such
as blood
vessels, with a uniform force, because the strand itself exhibits strength
variations.
Disclosure of the Invention
It is therefore an object of the present invention to provide a method and an
apparatus for spinning the strand, whereby it is possible to spin the strand
having
uniform mechanical properties and uniform degradation and absorption
characteristics
free from strength variations, that is, it is possible to spin the strand
having a uniform
average molecular weight and making it a suitable construction material for a
biodegradable stent.
The present invention provides a melt spinning apparatus for melt spinning a
strand of a biodegradable polymer material, forming a stent implanted in a
living body.
This comprises a vertically mounted cylinder, supplied with the biodegradable
polymer
material, a screw mounted in the cylinder coaxially, rotationally driven by a
rotational
driving unit and having at least one turn of a helical groove on its
peripheral surface,
and a nozzle mounted to the distal end of the cylinder and having a discharge
opening
coaxially with the cylinder. The biodegradable polymer material supplied into
the
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cylinder and melted by rotation of the screw is emitted vertically from a
discharge
opening in the nozzle for spinning the strand.
With the present melt spinning apparatus, the molten biodegradable polymer
material is fed by a screw in the vertical direction and emitted from a nozzle
for
spinning a strand, so that the strand has a uniform molecular weight
distribution is spun
as stagnation or non-uniform eddying currents of the biodegradable polymer
material
melted in the cylinder or the nozzle may be prevented from being produced.
Moreover, with the present melt spinning apparatus, there is provided a plural
number of heating units placed in juxtaposition along the axial direction of
the
cylinder, on the outer sides of the cylinder forming the melt mechanism for
melting the
biodegradable polymer material, for controlling the molten state of the
biodegradable
polymer material injected into the cylinder. The heating units are able to
perform
temperature control independently of one another.
The nozzle for discharging the molten biodegradable polymer material is kept
at a constant temperature by the heating units. By controlling the nozzle
temperature,
the temperature of the molten biodegradable polymer material discharged from
the
nozzle can be made constant.
According to the present invention, the biodegradable polymer material is
melted by a melt mechanism including a screw which is provided in a vertically
mounted cylinder coaxially and on the peripheral surface of which at least one
turn of
the helical groove is formed. The screw is rotated by a rotational driving
mechanism.
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The molten biodegradable polymer material is emitted in the vertical direction
through
a discharge opening in a nozzle provided coaxially with the cylinder for
spinning the
strand.
With the melt spinning method of the present invention, a spun filament having
a uniform molecular weight distribution may be produced.
Other objects, features and advantages of the present invention will become
more apparent from reading the embodiments of the present invention as shown
in the
drawings.
Brief Description of the Drawings
Fig.1 is a side view showing a melt spinning apparatus according to the
present
invention.
Fig.2 is a cross-sectional view showing the cylinder and the screw of a
melting
mechanism.
Fig.3 is a plan view showing a flow resistance plate mounted to the distal end
of the cylinder.
Fig.4 is a cross-sectional view showing a discharging unit at the distal end
of the
melt mechanism.
Fig.5 is a cross-sectional view showing a supply unit for supplying a polymer
material to the melting mechanism.
Fig.6 is a side view showing the screw that is placed in the cylinder forming
the
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melting mechanism.
Fig. 7 is a side view showing a supply control unit which is placed
between the melting mechanism and the discharging unit.
Fig. 8 is a cross-sectional view showing a set of gears forming the supply
control unit.
Best Mode for Carrying out the Invention
A melt spinning apparatus and method for melt spinning the biodegradable
polymer material using the melt spinning apparatus are now explained in
detail.
The melt spinning apparatus according to the present invention is of a
vertical
type in which a melt spinning unit is mounted vertically, as shown in Fig. 1.
The melt spinning apparatus, shown in Fig.1, includes a base plate 2, mounted
horizontally on a mounting surface, and the melt spinning unit 1 is supported
by
member 3a by a support pillar 3 mounted upright on the base plate 2. The melt
spinning unit 1 includes: a melt mechanism 4 which is supported by and
parallel to the
upstanding support pillar 3, a discharging unit 5 for discharging the polymer
material
melted by the melt mechanism 4, a supplying unit 6 for supplying the polymer
material
to the melt mechanism 4, and a rotational driving mechanism 7 for rotationally
driving
a screw 16 forming the melt mechanism 4.
The melt mechanism 4 which is in the melt spinning unit 1, includes a cylinder
8, as shown in Fig.2. The screw 16 is provided within and coaxially of the
cylinder 8.
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The screw 16 pressurizes the polymer material, injected into the cylinder 8
and
extrudes the pressurized material towards the distal end of the cylinder 8
while melting
it.
On the outer periphery of the cylinder 8, there is a plural number of heating
units
9 in juxtaposition along the axial direction of the cylinder 8. These heating
units 9 are
controlled independently of one another to enable the multi-stage control of
the
temperature axially of the cylinder 8.
On the distal end of the cylinder 8, there is mounted a connecting member 21
for connecting the discharging unit 5 to the cylinder. The discharging unit 5
is adapted
for discharging the melted polymer material. The connecting member 21 is ring-
shaped
and has at its center portion a flow resistance plate 22 including a plural
number of
through-holes 22a matching the axial direction of the screw 16, as shown in
Fig.3. The
molten polymer material, supplied from the distal end of the cylinder 8, by
rotating the
screw 16, is pressurized by the flow resistance when traversing the flow
resistance
plate 22. The pressurized polymer material is discharged towards the
discharging unit
from the distal end of the cylinder 8.
The diameter or the number of the through-holes 22a provided on the flow
resistance plate 22 is changed depending on the amount or supply rate of the
molten
polymer material supplied from the distal end of cylinder 8, on rotation of
screw 16,
or on the viscous resistance of the polymer material.
The flow resistance plate 22 may be of any shape provided that it affords the
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flow resistance to the polymer material supplied melted from the distal end of
the
cylinder 8 on rotation of the screw 16 to pressurize the polymer material.
The discharging unit 5, mounted by the connecting member 21 at the distal end
of the cylinder 8, includes a sprue bush 11, connected to the distal end of
cylinder 8,
and a nozzle 10 mounted to the distal end of the sprue bush 11, as shown in
Fig.4. The
nozzle 10 is secured to the distal end of the sprue bush 11 by a mounting
member 12.
Meanwhile, the nozzle 10 and the mounting member 12 can be the same.
The sprue bush 11, which is part of the discharging unit 5, supplies the
molten
polymer material from cylinder 8, to the nozzle 10 in a stable state at a
constant rate
of amount per unit time. A flow passage 1 la is in the center co-axially of
the cylinder
8, as shown in Fig.4. That is, the flow passage 1 la and the cylinder 8 are
placed
vertically with a common axis P 1. The flow passage 11 a is tapered moderately-
from
the vertically placed cylinder 8 towards the nozzle 10 so that the polymer
material
supplied melted from the cylinder 8 may be supplied in succession by
predetermined
amounts per unit time to the nozzle 10 without producing stagnation or eddying
currents.
The nozzle 10 includes a discharge opening l0a for discharging the polymer
material supplied melted from the sprue bush 11, as shown in Fig.4. The
discharge
opening IOa operates for controlling the diameter of the spun strand and is
formed by
an optimum diameter depending on the thickness of the spun strand. The
discharge
opening 10a is also formed to be coaxial with the flow passage 11 a. That is,
the
f r
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discharge opening 10a and the flow passage 1la are set upright coaxially as
the
cylinder 8.
Meanwhile, plural different nozzles with different diameters Rl at the
discharge
opening 10a may be provided and exchanged from time to time to spin the strand
with
different thicknesses.
On the outer periphery of the discharging unit 5, there is a heating unit 13
for
controlling the temperature of the discharging unit 5. This heating unit 13
controls the
temperature of the discharging unit 5 to control the temperature of the
polymer
material discharged from the nozzle 10.
The supplying unit 6 for supplying the molten polymer material to the melt
mechanism 4 includes a hopper 14 for loading the polymer material into the
cylinder
8 and a mounting unit 6a for mounting the unit 6 to the cylinder 8, as shown
in Fig.5.
On the outer periphery of the mounting unit 6a, there is provided a
temperature
controller 15 for controlling the temperature of the supplying unit 6. This
temperature
controller 15 keeps the polymer material, loaded into the hopper 14, at a
constant
temperature, and is comprised of a heating/cooling means.
The melt mechanism 4 is explained more specifically. Referring to Fig.6, the
melt mechanism 4 includes the screw 16, having a helically extending groove 17
on its
peripheral surface, is mounted coaxially within the cylinder 8. The screw 16
is
rotationally driven by the rotational driving mechanism 7 at the proximal end
where the
screw is connected. When the screw 16 is driven rotationally, the polymer
material,
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loaded into the cylinder 8 and melted by the heating units 9, is fed to the
distal end of
the cylinder 8.
Meanwhile, the helical groove of the screw used in the routine melt spinning
apparatus, is formed to a pitch subsequently equivalent to the screw diameter.
The
helical groove 17 of the screw 16 used in the melt spinning apparatus
according to the
present invention has a pitch Tp equal to one-half the diameter Sr of the
screw 16. By
forming the helical groove 17 in this manner, the dwelling time of the
injected polymer
material in the cylinder 8 can be protracted, such that melting can be
achieved reliably
by sufficient heating in the heating unit 9 even though the screw 16 is of a
reduced
length. By employing the screw 16, the screw length may be reduced, as a
result of
which the melt mechanism 4 including the cylinder 8 can be reduced in size.
It should be noted that the melt spinning apparatus of the present invention
may
be provided with a supply controlling mechanism 18, between the melt mechanism
4
and the discharging unit 5, for controlling the supply quantity of the polymer
material
in molten state, which is supplied to the discharging unit 5. This supply
controlling
mechanism 18 may be configured as shown for example in Fig.7. The supply
controlling mechanism 18, shown in Fig.7, includes a pressure detection means
19 for
measuring the pressure of the polymer material extruded from the melt
mechanism 4
and circulated in the molten state through a flow passage 18a, and a set of
gears 20 for
feeding the melted polymer material to the discharging unit 5. This supply
controlling
mechanism 18 detects the pressure of the polymer material flowing through the
flow
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passage 18a by the pressure detection means 19. The rotation of the set of
gears 20 is
controlled by this detection output to keep the pressure of the polymer
material flowing
through the flow passage 18a constant. By controlling the pressure of the
polymer
material flowing through the flow passage 18a at a constant magnitude, a
preset
constant quantity of the polymer material can be supplied to the discharging
unit 5.
A heating unit 23 is on the outer periphery of the portion of the supply
controlling mechanism 18, which controls the temperature of the polymer
material
flowing through the flow passage 18a at a preset temperature.
The melt spinning method employed by the melt spinning apparatus of the
present invention is now explained.
The present invention melt-spins the strand, formed of a biodegradable polymer
material used for forming a stent implanted in the living body. The melt spun
polymer
material, used herein, is the biodegradable polymer material. The
biodegradable
polymer material may be enumerated by polylactic acid (PLA), polyglycolic acid
(PGA), polyglactin (polyglycolic acid-polylactic acid copolymer),
polydioxanone,
polyglyconate (trimethylene carbonate-glycoid copolymer) and a polylactic
acid-e-caprolactone copolymer.
For spinning the polymer material, a pellet-like polymer material Pp is
charged
into a hopper 14 of the supplying unit 6. The polymer material, loaded into
the hopper
14, is supplied to the cylinder 8 of the melt mechanism 4.
In order for the polymer material, loaded into the hopper 14, to be quickly
K A
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supplied into the helical groove 17 formed in the screw 16 rotating in the
cylinder 8,
the polymer material needs to be in solid state. That is, the polymer
material, supplied
into the cylinder 8, needs to be controlled to a temperature not higher than
its melting
point (Tm) or softening point. For shortening the melt time in the melt
mechanism 4,
the polymer material, supplied to the cylinder 8, needs to be melted
immediately. Thus,
the temperature controller 15, provided in the supplying unit 6, sets the
temperature of
the polymer material, charged into the hopper 14, to a temperature at which
the
polymer material can be melted immediately as it maintains its solid state.
The polymer material, supplied into the cylinder 8 through the hopper 14, is
introduced into the helical groove 17 of the screw 16, rotated by the
rotational driving
mechanism 7, so as to be extruded towards the distal end of the cylinder 8, as
it is
heated by the heating units 9 provided on the outer periphery of the cylinder
8. As the
polymer material is extruded, the temperature of the polymer material is
controlled to
be lower than its thermal decomposition temperature so as not to cause
transmutation
of the polymer material. The polymer material, thus controlled to a
temperature not
higher than its thermal decomposition temperature, is positively extruded from
the
distal end of the cylinder 8 as it is kept in molten state without undergoing
transmutation.
The polymer material, extruded at the distal end of the cylinder 8 while in
its
molten state, is afforded with flow resistance by the flow resistance plate
22, in such
a manner that it is evenly pressurized by the through-holes 22a. The polymer
materia,
A ,
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thus pressurized, is supplied to the discharging unit 5.
Since the through-holes 22a formed in the flow resistance plate 22 are
oriented
vertically in order not to produce stagnation or eddying currents in the
polymer
material, the polymer material can be supplied to the discharging unit 5 as
the
molecular weight distribution is maintained to be constant.
If the melt spinning apparatus has the supply controlling mechanism 18 between
the melt mechanism 4 and the discharging unit 5, the molten polymer material,
extruded from the cylinder 8 of the melt mechanism 4, is maintained at a
constant
pressure by the supply controlling mechanism 18, so that it is controlled in
flow rate
at the discharging unit 5 and is reliably supplied to the discharging unit 5
at a constant
flow rate.
The polymer material supplied to the supply controlling mechanism 18 is heated
by the heating unit 23 provided on the outer periphery of the supply
controlling
mechanism 18 and hence is delivered to the discharging unit 5, reliably in its
molten
state. The heating unit 23 maintains the heating temperature at less than the
thermal
decomposition temperature so as not to cause transmutation of the polymer
material.
The molten polymer material, delivered from the melt mechanism 4 or the
supply controlling mechanism 18 to the discharging unit 5, is heated in the
sprue bush
11 by the heating unit 23 to a temperature less than the thermal decomposition
temperature. Since the flow path of the discharging unit 5 from the sprue bush
11 to
the nozzle 10 is oriented vertically, the polymer material flowing therein is
not
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subjected to stagnation or eddying currents. Since the polymer material,
maintained in
its molten state, may thus be supplied to the nozzle 10 through the vertical
flow path,
it can be discharged at the nozzle 10 as it maintained in the state of uniform
molecular
weight distribution, and hence the strand of the polymer material can be spun
with
uniform molecular weight distribution.
It should be noted that a monofilament strand may be spun because the sole
discharge opening 10a is formed through the nozzle 10 for extending in the
vertical
direction.
Industrial Applicability
With the melt spinning method and apparatus of the present invention, it is
possible to prevent stagnation or nonuniform eddying currents of the
biodegradable
polymer material in order to spin the strand into a uniform average molecular
weight.
That is, the strand of the biodegradable polymer material may be spun which is
uniform mechanical properties and degradation and absorption characteristics.
This
spun strand can be used to the utmost advantage for forming a stent inserted
into the
vascular vessel of the living body.
