An intra-luminal expander assembly for use with a catheter comprises an expandable tubular expander element having openings, for example slots, formed in it, and first and second fixation portions located at or towards its ends. The expander element is disposed around a core element which comprises a shape memory alloy which has been treated so that, when the temperature of the alloy is increased above its A.sub.s temperature, it displays a shape memory effect and the length of the core element decreases. The expander element is attached to the core element through the first fixation portion and oriented so that, as the length of the core element decreases when its temperature increases, the first fixation portion is caused to move relative to the second fixation portion in a direction towards the second fixation portion, and the second fixation portion being restrained against movement with the first fixation portion so that the length of the expander element decreases and its transverse dimension increases.

FIELD OF THE INVENTION 
This invention relates to an intra-luminal expander assembly, and to a 
method of implanting an expander assembly in a lumen. 
BACKGROUND TO THE INVENTION 
A lumen in a human or animal body, such as a blood vessel or a urinary 
tract, can require internal support to ensure proper flow of fluid in the 
lumen. For example, a lumen can become at least partially occluded, and 
support can be required to reestablish sufficient internal bore in the 
lumen for flow of fluid. 
Support for a lumen can be provided by implantation of a stent in the 
lumen. In many situations, this can allow a patient to resume normal 
activities without dependence on medical help, at least temporarily and in 
many situations indefinitely. 
It can also be desirable to provide support in a lumen temporarily, without 
necessarily implanting a stent in the lumen. This can be achieved by means 
of a catheter which can be inserted into a lumen, and which has a portion 
which can be inflated by means of fluid supplied to the inflatable portion 
through a hollow bore of the catheter. This technique is referred to as 
"balloon angioplasty" when applied to blood vessels. 
In processes in which inflatable catheters are used, the occlusion and the 
walls of the lumen are expanded and stretched by inflation of the 
catheter. The walls then remain in the stretched condition so as to remove 
or at least to reduce the occlusion, and to establish an increased flow of 
fluid in the lumen. 
Once inflated, the catheter completely blocks the lumen against flow of 
liquid. The expansion process must therefore be carried out quickly, and 
then the catheter must be deflated quickly to reestablish flow of fluid. 
This is particularly critical in blood vessels on or near the heart which, 
if deprived of blood flow for even short periods (sometimes less than 30 
seconds) can give rise to the condition known as "heart attack". 
The present invention provides an expander assembly which includes a shape 
memory alloy component, which exhibits a shape memory effect. Shape memory 
alloys are discussed in an article by L. McDonald Schetky in the 
Encyclopedia of Chemical Technology (edited by Kirk-Othmer), volume 20, 
pages 726 to 736. Subject matter disclosed in that document is 
incorporated in this specification by this reference to the document. Such 
alloys can exist in martensite and austenite phases. An article formed 
from the alloy while in the austenite phase can be deformed, after it has 
been cooled so that the alloy is in the martensite phase. If the 
temperature of the article is subsequently increased so that the alloy 
transforms back to the austenite phase, the article reverts to the 
configuration which it had before it was deformed. The transformation from 
austenite phase to martensite phase takes place over the temperature range 
M.sub.s to M.sub.r, and the transformation from martensite phase to 
austenite phase takes place over the temperature range A.sub.s to A.sub.f. 
SUMMARY OF THE INVENTION 
The present invention provides an expander assembly in which a shape memory 
alloy component is used to cause an expander element with openings formed 
in it, positioned around the component, to expand transversely, in use to 
expand and to support a lumen in which the assembly is positioned. 
In one aspect, the invention provides an intra-luminal expander assembly 
which comprises: 
(a) a tubular expander element having openings in it which allow it to be 
expanded transversely, and first and second fixation portions located at 
or towards opposite ends of the element, and 
(b) a core element comprising a shape memory alloy which has been treated 
so that, when the temperature of the alloy is increased above its A.sub.s 
temperature, it displays a shape memory effect and the length of the core 
element decreases, the core element extending through the expander 
element; 
the expander element being attached to the core element through the first 
fixation portion and oriented so that, as the length of the core element 
decreases when its temperature increases, the first fixation portion is 
caused to move relative to the second fixation portion in a direction 
towards the second fixation portion, and the second fixation portion being 
restrained against movement relative to the first fixation portion so that 
the length of the expander element decreases and its transverse dimension 
increases. 
The expander assembly of the invention has the advantage that materials of 
the expander element (which contacts the wall of the lumen) and the core 
element can be selected according to the requirements of the support 
required to be provided by the sheath and of the movement required to be 
imparted by the core. The material of the expander element can be selected 
to provide the appropriate physical support for the lumen. It can also be 
selected without restriction by any requirement for bio-compatibility 
during long term implantation, in contrast with stent assemblies in which 
the stent is formed from a shape memory alloy. 
Furthermore, the use of an expander element with openings formed in it 
allows fluid to flow along a lumen via the openings even when the expander 
element is located in the lumen and is expanding it. Problems arising from 
stopped flow of fluid, especially blood, when a balloon catheter is used 
are therefore avoided. The assembly of the present invention therefore 
allows the expansion to take place less hurriedly than in the case with an 
inflatable catheter. Slower expansion is considered likely also to give 
rise to the advantage of reduced damage to the tissue of the wall of the 
lumen. 
A further advantage of the assembly of this invention is that, as a result 
of the openings in the expansion element which allow it to expand, and 
which allow fluid flow through it, the amount of material in the expansion 
element is reduced compared with a device with an intact wall arranged for 
inflation. This facilitates packaging of the assembly, and delivery to the 
location in the lumen where it is to be deployed. 
The core element of the expander assembly may comprise a portion which is 
formed from a shape memory alloy which exhibits a shape memory effect and 
another portion. The two portions will generally be adjacent one another 
along the length of the core element. The portion which exhibits the shape 
memory effect will be configured to provide sufficient movement to cause 
the expander element of the assembly to change in configuration to support 
the lumen. 
Generally, the length of the portion of the core element which exhibits a 
shape memory effect will be longer than the expander element before it has 
been deformed. Generally, at least a portion, and preferably all, of the 
core element within the expander element will exhibit the shape memory 
effect. More preferably, the portion of the core element which exhibits 
the shape memory effect extends from one of the fixation portions of the 
expander element to beyond the other of the fixation portions. 
The portion of the core element which exhibits a shape memory effect 
preferably comprises an alloy based on a nickel-titanium alloy, optionally 
with other elements such as chromium, copper, iron and vanadium. An 
example of a particularly preferred alloy consists of 50.3% atomic per 
cent nickel and 49.7% atomic per cent titanium, which has been treated so 
that its transformation temperatures are approximately as follows (in 
.degree.C): 
M.sub.s 0 
M.sub.f -10 
A.sub.s 42 
A.sub.f 48 
Other shape memory alloys, such as those based on nickel-palladium or 
copper, may be used. When the core element comprises a portion which 
exhibits a shape memory effect and another portion, the other portion may 
comprise an alloy which exhibits pseudoelastic or superelastic properties. 
These properties of shape memory alloys are discussed in the Schetky 
article referred to above. The use of a pseudoelastic or superelastic 
alloy as a core in a catheter is disclosed in EP-A-141006. Subject matter 
disclosed in that document is disclosed in this specification by this 
reference to the document. 
The expander assembly can include a sheath which surrounds the core 
element, to which the second fixation portion of the expander element is 
attached so that the sheath and the expander element are oriented 
contiguously with respect to one another along the core element. This is 
particularly preferred when the shape memory effect portion of the core 
element includes a portion which protrudes from the expander element. The 
provision of a sheath allows movement of the core element relative to the 
expander element, even when the shape memory effect portion of the core 
element is longer than the expander element. 
The sheath is generally connected at or towards one end to the expander 
element. It can extend along substantially the entire length of the core 
element extending away from the expander element. Alternatively, it can 
extend along just a portion of the core element proximal to the expander 
element. In this case, the sheath can be connected to the core element, 
generally to a portion which does not exhibit a shape memory effect, for 
example by means of a mechanical connector (for example a crimped ferrule) 
or by means of a weld or solder joint. 
Preferably, the sheath comprises a helically wound wire. This has the 
advantage of allowing the expander assembly to flex in the portion in 
which the sheath is present, which can be advantageous when the expander 
assembly is to be manoeuvred along a tortuous path through lumina, for 
example through blood vessels, to a desired location. 
A connection between the expander element and the core element, and a 
sheath if present, is preferably made mechanically, for example by means 
of a crimped ferrule or by welding. For some applications, it can be 
preferred that the connection be breakable, so that the expander element 
can be left in situ in the lumen, while the core element is removed. This 
can be achieved, for example, using a dematable connection to the expander 
element, for example a screw-threaded connection or a bayonet connection, 
or a connection which can be released mechanically remotely. 
Preferably, the expander assembly includes a collar formed from the 
deformable material, which is located around the openings in the expander 
element. The provision of a collar has the advantage that the 
configuration of the expander element, after it has expanded transversely, 
can be controlled: the expander element will tend to expand in regions in 
which the material from which it is made is weakest. A collar can 
reinforce the expander element in selected regions as desired. The 
thickness of the material of the collar can vary along the length of the 
collar, so that the support given to the expander element changes along 
the length of the expander element. Preferably, the collar is located 
around the portion of the expander element in which the openings are 
provided so that that portion protrudes from at least one end of the 
collar. This has significant advantages in that the sleeve can provide 
selective support for the expander element to control its configuration 
when expanded, while also allowing fluids to flow along the lumen after 
the expander assembly has been expanded, the fluids flowing along the 
lumen through the expander element, via the openings therein. 
A further advantage of the use of a collar on the expander element is that 
it can encourage contraction of the expander element so that it can be 
released from the internal wall of the lumen for removal from the lumen. 
Preferably, the collar is formed from a polymeric material. Examples of 
suitable materials include medical grade silicone, and polyurethane. 
Preferably, the expander element is formed from a metal. The metal will be 
selected for properties which make it suitable for use in the desired 
application; for example, it will be bio-compatible. For some 
applications, it can be preferred for the metal to be selected with 
physical properties which allow it to be deformed by the core element so 
that it retains the deformed configuration and remains in it to support 
the lumen, without requiring any hold-out force by the core element. For 
example, the expander element may be deformed plastically by the core 
element as the length of the core element decreases. 
The expander element can for some applications require the core element to 
remain within it to provide a hold-out force to retain the expander 
element in its expanded configuration. This can be employed to allow the 
expander assembly to be removed from a lumen. For example, the expander 
element can be arranged to be capable of (a) being deformed by the core 
element so that its transverse dimension increases when the alloy of the 
core element transforms from martensite phase to austenite phase, and (b) 
deforming the core element so that its length increases when the alloy of 
the core element transforms from austenite phase to martensite phase, when 
the transverse dimension of the expander element decreases. This can be 
achieved particularly conveniently when the M.sub.s temperature of the 
alloy of the core element is less than body temperature. 
The expander element can be retained on the core element while it supports 
a lumen. The expander element can be used to deliver a stent, the stent 
being provided on the expander element to be expanded by the expander 
element as its transverse dimension increases. The expander element itself 
might be implanted in a lumen by the core element, to function as an 
implanted stent after the core element has been removed. 
The expander element can be formed from a stainless steel with appropriate 
resilient characteristics, such as found in 316 series alloys. The 
expander element can be formed from a shape memory alloy which exhibits 
pseudoelastic or superelastic properties, or a combination of the two. 
The expander element can be formed from a shape memory alloy, which 
displays the shape memory effect, or which exhibits pseudoelastic or 
superelastic properties. When the expander element displays the shape 
memory effect, it is preferred that the A.sub.s temperature of the alloy 
be higher than the temperature of a patient in whose body the element is 
to be applied. A stent assembly which includes a stent element formed from 
a shape memory alloy is disclosed in the application filed concurrently 
with this application, which bears the title "An intra-luminal stent 
assembly". Subject matter disclosed in that application is incorporated in 
this specification by this reference. 
It can be preferred for the expander assembly to include means for 
connecting the core element to a source of electrical power, so that 
current flows through the shape memory alloy of the core element which 
displays the shape memory effect. Generally, the core element can be 
connected directly to one terminal of a power supply at a convenient point 
along the length of the core element, generally at or towards a proximal 
end thereof. Another terminal of the power supply can be connected to the 
core element at or towards a remote end thereof by means of a conductor 
which extends along the core element and is insulated therefrom. The 
temperature of the core element can then be caused to increase so that it 
exceeds the A.sub.s temperature of the alloy by supplying power to the 
shape memory effect portion of the core element. 
The openings in the expander element can usefully be provided as slots 
which extend along at least a portion of its length, preferably 
approximately parallel to the longitudinal axis of the assembly. Other 
configurations of openings can be used, provided that they can accommodate 
sufficient transverse expansion of the expander element. For example, the 
openings might be provided as an array of slits, or of rhombuses so that 
the deformation of the expander element will then involve, to an 
approximation, bending of the arms which define the slits or the 
rhombuses. 
Preferably, the core element includes means for coupling with a source of 
inductively coupled power. This method of triggering the shape memory 
effect simplifies the assembly of the invention, by eliminating any 
requirement to connect components of the assembly to a source of 
electrical power. For example, it may include materials which couple with 
an inductive power source, such as a coating of a magnetic material. 
Suitable materials include iron. In this way, heating of the core element 
can be initiated externally by means of an inductive heat source. The 
expander assembly of the invention can also include means for heating the 
shape memory effect portion of the core element inductively, or heating 
the surface of the expander element that contact the walls of the lumen. 
Effects of the application of heat include increasing the temperature of 
the tissue to be expanded, which can reduce restenosis. 
The shape memory effect portion of the core element can be treated so that 
the A.sub.s temperature of the shape memory alloy is slightly below body 
temperature. In this way, a expander assembly can be inserted into a lumen 
while at a temperature below the A.sub.s temperature of the alloy, and the 
expander element can be caused to expand due to an increase in temperature 
of the core element when exposed to body temperature. 
In another .aspect, the invention provides a method of implanting an 
expander assembly in a lumen, which comprises: 
(a) locating in the lumen an expander assembly comprising 
(i) an expandable tubular expander element having openings in it which 
allow it to be expanded transversely, and first and second fixation 
portions located at or towards opposite ends of the element, and 
(ii) a core element comprising a shape memory alloy which has been treated 
so that, when the temperature of the alloy is increased above its A.sub.s 
temperature, it displays a shape memory effect and the length of the core 
element decreases, the core element extending through the expander 
element; and 
(b) causing the temperature of the core element to increase above the 
A.sub.s temperature of the alloy so that the lengths of the core element 
and of the expander element are made to decrease, and the transverse 
dimension of the expander element is made to increase.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows an expander assembly which includes a catheter by which an 
expander element can be delivered to a desired location in a lumen. The 
lumen may be, for example, a blood vessel such as a vein or an artery, or 
it may be a urinary tract. The expander element might be supplied to a 
blood vessel in the treatment of arteriosclerosis. The expander element 
might be supplied to the urethra in treatment of an enlarged prostate 
gland condition. 
The expander assembly comprises an elongate catheter 2. The catheter 
comprises a core element and a sheath. The core element extends throughout 
the length of the catheter, and can be controlled rotationally at the 
distal end by means of a control element 4. 
The core element comprises a shape memory alloy such as one based on a 
nickel-titanium alloy, optionally with one or more other elements. The 
distal end portion of the core element formed from the shape memory alloy 
is capable of exhibiting a shape memory effect. The length of the distal 
end portion of the core element can be reduced by heating the element so 
that its temperature exceeds the A.sub.s temperature of the alloy. 
The portion of the core element other than the distal end portion is formed 
from a shape memory alloy which exhibits the property of optimised 
elasticity, which is referred to in U.S. Pat. Nos. 4,772,112 and 
4,896,955. Subject matter disclosed in these documents is incorporated in 
this specification by this reference. 
Terminals 5, 6 are provided as part of the catheter for connection of the 
core element to a supply of electrical power, so that current can be made 
to flow through the core element, or at least the distal end portion 
thereof. 
FIG. 2 shows the distal end portion of the expander assembly. The core 
element 10 of the expander assembly comprises an end portion 12 formed 
from a shape memory alloy which exhibits a shape memory effect, and a 
proximal portion 14 to which the end portion is rigidly connected by means 
of a welded joint. 
An expander element in the form of a slotted sleeve 16 is located around 
the end portion 12 of the core element. The expander element is formed 
from a stainless steel. The expander element is capable of being expanded 
radially outwardly, and of contracting elastically to or towards its 
original configuration. 
The expander element has fixation portions 18, 20, at opposite ends of the 
slots 22. The expander element is connected to the end 24 of the core 
element by crimping. 
The expander assembly includes a sheath 26 formed as a helically wound 
wire. The sheath is connected to the core element at about the junction 
between the end portion 12 and the proximal portion 14 of the core element 
10. The connection is made by means of a ferrule 28. The core element is 
able to move within the sheath. 
The second fixation portion 20 of the expander element 16 is connected to 
the sheath by means of a ferrule. 
A collar 32 is located around the slotted portion of the expander element. 
The collar is located approximately centrally on the expander element, so 
that the slots 22 protrude from under the collar. The collar is formed 
from silicone polymer or a polyurethane. 
The expander assembly is inserted into a lumen with the shape memory alloy 
of the end portion 12 of the core element in its martensite phase. The 
expander assembly is manoeuvred until the end portion of it, with the 
expander element 16 located where it is to be disposed, for example to 
support the wall of the lumen at the site of an occlusion. The temperature 
of the shape memory alloy of the end portion 12 of the core element is 
then caused to increase to a temperature above the A.sub.s of the alloy. 
This causes the length of the end portion of the core element to decrease. 
This causes the ends of the expander element to move relative to one 
another, towards one another, so that the length of the expander element 
decreases. This causes the transverse dimension of the expander element to 
increase, as the slots formed in it open. 
The expander element 16 remains in its expanded configuration for as long 
as the shape memory alloy of the end portion 12 of the core element 
remains in the austenite phase. This will require power to be supplied 
continually to the core element for as long as the expander element is to 
remain expanded, unless the M.sub.s temperature of the alloy of the end 
portion is below body temperature. When the M.sub.s temperature of the 
alloy is below body temperature, disconnection of the core element from 
the source of electrical power can allow the expander element to contract 
radially, causing the length of the end portion of the core element to 
increase as it does so. This contraction can enable the expander assembly 
to be removed from the lumen. 
FIGS. 3A-3C show schematically how the expander assembly of the present 
invention can be used to widen a lumen 30 which is partially occluded by 
accumulated matter 32. 
FIG. 3A shows the expander assembly being passed along the lumen so that 
the end portion thereof is located in the vicinity of the occlusion 32. 
FIG. 3B shows the expander assembly while the expander element is expanded 
transversely, so that it is urged against the wall of the lumen, causing 
the occlusion 32 to be widened. 
FIG. 3C shows the expander assembly after the expander element has 
contracted, so that it can be removed from the lumen.