WO2006003451A2

WO2006003451A2 - Method for forming a tubular structure

Info

Publication number: WO2006003451A2
Application number: PCT/GB2005/002648
Authority: WO
Inventors: Anthony Walter Anson
Original assignee: Anson Medical Limited
Priority date: 2004-07-06
Filing date: 2005-07-06
Publication date: 2006-01-12
Also published as: GB0415152D0; WO2006003451A3

Abstract

A method for forming a tubular structure comprises providing a mould cavity having at least one removable former therein; adding a settable fluid to the mould cavity around said former; allowing the fluid to set; and removing said former so as to leave a tubular structure formed from the set fluid. The said tubular structure can be employed as an inner channel in an expandable medical device.

Description

Method for forming a tubular structure

The present invention relates to an expandable medical device and to a method of forming a tubular structure for use in such a device, hi particular it relates to a method whereby tubular structures may be produced using a material in a liquid form that can be caused to change to a solid by a chemical reaction. The liquid material is injected into a separable, closed cavity mould having internal channels conforming to a desired geometry. Within the internal pathways resides a removable core for forming a lumen. The injected liquid may be converted to a solid by means of a chemical reaction between two or more component chemicals.

Making diverse artefacts by moulding techniques has been established since the discovery that a metal or other substance when heated or otherwise manipulated, could be poured into a suitable mould in the form of a liquid. The liquid metal cools and solidifies, taking the shape of the cavity or aperture comprising the mould. Plastic injection moulding and reaction injection moulding, well known in the art, uses a similar principle. In the case of an injection moulding, a polymer is heated causing it to soften and become a viscous liquid. A portion of the liquid polymer is then injected into a separable, closed cavity mould. The polymer cools down and solidifies in the mould, which can then be separated to remove the solidified moulding. Reaction injection moulding, also well known in the art, uses a similar principle except that a catalysing and/or exothermic reaction causes solidification of the liquid. This has the advantage of not having to heat the liquid media before it is poured into a closed cavity mould.

Another well-known method to produce a solid structure from a liquid is that of metal casting. A suitable receptacle contains metal, which has been heated sufficiently to form a liquid. Said liquid is then poured into a mould and allowed to cool and solidify. The mould (commonly made of particular sand and binding resins) is configured to resist the effects of high temperatures, which may be more than 2000 degrees Celsius, depending upon the type of metal used.

Metal die-casting also well known in the art, and uses a closed cavity mould in a similar fashion to plastic injection moulding. However, the injectable media is in the form of a metal with a relatively low melting point such as zinc-based and aluminium alloys.

In a first aspect of the present invention there is provided a method for forming a tubular structure, the method comprising the steps of: (a) providing a mould cavity having at least one removable former therein,

(b) adding a settable fluid to the mould cavity around said former,

(c) allowing the fluid to set, and

(d) removing said former so as to leave a tubular structure formed from the set fluid.

Preferably, tubular structures are made of a settable media that in one form is liquid and in its other form is set, due to a chemical reaction upon mixing two or more components.

A liquid media is poured or injected into a mould, having two or more separable parts. The internal geometry of the mould is arranged to accommodate a removable component. This removable component is preferably a low melting point metal alloy. Said alloy can be arranged, depending upon its elemental composition, to have a melting point of 28 degrees Celsius to several hundred degrees Celsius. In this case, ideally the low melting point alloy has a melting point within the range 30 to 120 degrees Celsius. A removable component made of said low melting point alloy is arranged to have dimensions which are generally smaller than the internal dimension of the closed-cavity mould within which it resides.

If a settable media is now injected into the mould, it will fill the voids between the mould and removable component. A catalysing or polymerising reaction now takes place, causing the injected media to solidify. The mould assembly is now heated to a temperature at, or slightly above the melting point of the alloy insert. The alloy is then poured out of the mould in its liquid form. The separable mould is now disassembled, facilitating removal of the moulded part.

Such a moulding process may be applied advantageously to the manufacture of simple or complex tubular forms. For example, if a round, hollow tube with a series of round, hollow attachments arranged to be in-line and perpendicular to the axis of the structure, is required. If the main tube is required to be 25mm outside diameter and the attachment tubes 20mm diameter, both tube forms having a 2mm wall thickness. A two-part separable mould is then manufactured, having half of the form of the required structure in one half of the mould; a mirror form of the required geometry is generated in the other half-mould.

Another separable mould is then manufactured to make a low melting point alloy core. The diametral outer dimension of the core is 21mm for the larger "parent" tube and 16mm for the manifolded attachment tubes.

Low melting point alloy is then heated to change to a liquid form. A hypodermic syringe of suitable capacity can be used to scavenge the liquid metal. The syringe is then used to inject the alloy into the closed cavity. If a syringe is used for this, a Luer-type injection port is positioned appropriately to connect the internal volume of the mould. Equally, the metal alloy can be caused to flow using a hydrostatically derived pressure differential between the internal volume of the mould and the alloy. If a container, for the liquid metal is suspended above the mould and suitable pipes or tubes connect the liquid metal container to the mould, liquid metal will then flow into said mould, internal volume due to hydrostatic pressure. Other methods such as rotary liquid pumps, other piston\cylinder arrangements, Archimedean type screw-feeders or the effects of gravity can be realised to cause liquid metal flow into the mould cavity. After filling the mould with metal it will begin to cool and solidify; this can be accelerated by actively cooling the mould by conduction or convection means. The moulding of low melting point alloy can now be removed from the mould. It is then placed in the tubular structure mould: an arrangement to ensure sufficiently accurate registration of the metal core, relative to the larger cavities in the injectable mould. This can be facilitated by extending the length of each core member and reducing the diametral dimension at the terminal ends of each core. A receptacle is made in each half of the tube mould so that the low melting point alloy fits accurately in said receptacle. The core is now placed in one-part of the tube mould and other mould parts assembled around the core.

A liquid polymer is now injected into the mould assembly, surrounding the voids between low melting point alloy core and mould cavity. If the moulding is made from a two, or multi-part silicone elastomer, a reduction in the time taken to effect polymerisation may be gained by heating the mould. However, the elevated curing temperature must be below the softening or melting point of the low melting point alloy.

When the polymer forms a solid, the low melting point alloy is melted out of the mould, the mould is then disassembled and the cast artefact removed.

Prior to moulding, it may be advantageous to apply a barrier coating to the low melting point alloy casting to prevent any of the constituent parts of the alloy from transferring to the moulding. Alternatively, the low melting point alloy can comprise any other material capable of being a liquid at one temperature, a solid at another temperature. An example of one such material is wax. Other examples, such as water, mercury and alcohols that are liquids at room temperature, solids, when cooled to a freezing temperature. In a second aspect of the present invention there is provided an expandable device for insertion into a lumen, said expandable device comprising

(a) a body formed of at least one flexible material, said body being convertible from a collapsed condition in which it is of a size to be inserted into the lumen into an expanded condition in which the body is fixed relative to the lumen,

(b) at least one outer channel extending over at least a region of the body,

(c) at least one inner channel disposed inside said outer channel, and

(d) inlet means communicating with said at least one inner channel to enable a rigidifying material to be introduced into said inner channel so that, in use, at least said region of said body can be rigidified Whereby to maintain the body in its expanded condition.

The at least one inner channel is preferably formed by means of a method as described above.

In a preferred embodiment, the at least one inner channel is employed in an expandable device as disclosed in WO 99/00073 (in the name of the present applicant), the contents of which are incorporated herein by reference.

Another expandable device in which said at least one inner channel may be employed is disclosed in US 6395019 (Trivascular, Inc.).

The liquid inflating media is preferably selected so that it is capable of changing to a solid by polymerisation or catalysation. The inflated device now becomes a structure capable of resisting radial and longitudinal deformations.

Both WO 99/00073 and US 6395019 disclose inflatable devices for use in vascular repair or maintenance in the human or other mammalian species, particularly using endo vascular techniques. A separable inner tubular structure could be advantageously employed as a means to transport the injected media within the cavities resident inside the inflatable devices. This would give additional security against potential leakage of said injectable media.

A graft as described in the two cited patents, has a limitation regarding ultimate inflation pressures. These limitations are determined by strength of material, geometry and manufacturing techniques. If the inflatable cavities were lined with a tubular structure whose geometry was similar to the cavities' geometry, increased inflation pressures can be realised.

In the case of a tubular graft being constructed of a material capable of expanding or stretching within an elastic limit, an advantage can be gained by allowing a nominally sized graft (when fully inflated), to cover an increased diametral size range as the inflation pressure and consequent inflation forces can be increased. Grafts for the treatment of aneurysmal disease are invariably sized against the patient directly, from X- ray or ultrasound derived images, in the case of endovascular treatment.

Another example of an implanted medical device that would benefit from this present invention consists of a "Y" shaped tubular structure to facilitate vascular anastomosis. One of the side arms can be everted inside the main body of the device; this allows what appears to be a single tube, to be inserted into an artery through a small incision. The everted side-arm can now be reverted to produce a prominent extension from the artery. An artery or suitable graft can now be affixed about the extension, forming the side- anastomosis.

The "Y" shaped tube is formed within a split-mould having a removable core. A polymer in liquid form is injected into the mould assembly. The polymer now sets to form a solid, conforming to a "Y" shaped tubular structure. The removable core, preferably a low melting point alloy is heated to form a liquid that can be poured out of the mould. The split mould is then taken apart and the moulding removed.

The function of a low melting point alloy as described, is to act as a removable core in a moulding process to produce interconnected tubular structures. Alternatively, the low melting point alloy can be substituted by one of many commonly available metals or suitable polymers. Metals such as stainless steels, brass alloys aluminium and titanium in elemental or alloy forms and polymers such as PTFE, nylons, polycarbonate, acrylics, polysulphone, PEEK, PVC could all be used to form a core in tubular structures. To initiate such a process, a preferred metal or polymer having a nominally round or polygonal cross-section, is machined to length and to closely mate with an essentially gap-less contact at one or both ends of the material.

The example of a bifurcated anastomosis device would require two round sections of the core former material to connect at an acute angle and to have a means of maintaining the spatial relationship between core element and moulding. The two core forming elements consist of two straight members: one member has flat plain ends, the second member has one plain end face, the opposite end is machined with a profile that mimics the cross- section of member one. This profiled end when offered-up to member one, should be in intimate contact with half the circumference of member one. This intimate contact prevents or reduces the potential of a viscous pre-cured settable media perfusing into any gaps existing between the connected parts.

In a similar fashion to a low melting point alloy core, a split, two-part mould has a fabricated metal core placed within its mould cavity. The mould halves are furnished with registrations to ensure precise placement of the core assembly. The two mould halves are clamped together with the core assembly constrained within. A settable resin is now injected into the mould cavity expelling air and surrounding the core. The resin now sets to a solid and the mould halves are separated. Each part of the core assembly is withdrawn and the moulding removed. Certain settable resins will have an adhesive effect, making removal of the core element difficult. Under those circumstances, a release agent or other lubricious material can be employed on the mould cavity and core assembly. Examples of such release agents are silicon oils, PTFE, waxes and a range of lubricating-type oils and greases. Other lubricious agents are diamond-like carbon or PTFE.

The following illustrations show, by way of example, various tubular structure forms and the means to produce them.

Fig.l Example of interconnected tubular structure to be inserted within an inflatable tubular vascular graft.

1. Shows interconnected tube matrix 2. Shows one tubular termination and filler tube.

Fig 2. A tubular structure in the form of bifurcated graft.

1. Shows one tubular branch of a bifurcated structure 2. Shows second tubular branch

ig 3 A tubular, bifurcated graft split mould.

1. One-half of split, two-part mould. 2. One-half of split, two part mould

3. Registration dowel hole .

4. Moulding.

5. Riser to witness mould filling and air escape. 6 Inlet or injection port for settable media. 7. Removable core to establish lumen dimensions in moulding 8. Registration dowel hole.

Fig 4 A low melting point alloy removable core to make a bifurcated graft.

1. Shows a solid, round cross-section meltable or dissolvable core assembly.

2. Shows tow side arm cores to manufacture bifurcated tubular graft.

Fig.5 Tubular structure comprised of circular and straight parts.

1 Shows a semi-circular interconnected tube.

2. Shows a tube connected to a circular tube.

3. Shows a second tube connected to a circular tube.

Fig.6 A cast, low melting point alloy form used to generate the lumen of an interconnected tubular structure as shown in Fig 1.

1. Shows solid interconnected meltable or dissolvable structure of essentially round cross-section, being one part of a bifurcated arrangement. 2. Shows body of the meltable bifurcated structure.

3. Shows second bifurcation part.

Claims

1. An expandable device for insertion into a lumen, said expandable device comprising (a) a body formed of at least one flexible material, said body being convertible from a collapsed condition in which it is of a size to be inserted into the lumen into an expanded condition in which the body is fixed relative to the lumen,

(b) at least one outer channel extending over at least a region of the body,

(c) at least one inner channel disposed inside said outer channel, and (d) inlet means communicating with said at least one inner channel to enable a rigidifying material to be introduced into said inner channel so that, in use, at least said region of said body can be rigidified whereby to maintain the body in its expanded condition.

2. A device as claimed in claim 1 , having a series of outer channels forming an outer array and a series of inner channels forming an inner array.

3. A device as claimed in claim 2, wherein said array comprises a main channel with subsidiary channels extending at intervals transversely therefrom.

4. A device as claimed in any preceding claim in combination with a rigidifying material for introduction into said at least one inner channel through said inlet means.

5. A device as claimed in any preceding claim, wherein a series of outer channels is provided, said channels not being in communication, each channel in the series having its own inner channel with its own inlet means.

6. A method for forming a tubular structure for use as an inner channel in a device as claimed in any of the preceding claims, the method comprising the steps of: (a) providing a mould cavity having at least one removable former therein,

(b) adding a settable fluid to the mould cavity around said former,

(c) allowing the fluid to set, and

7. A method as claimed in claim 6, wherein the removable former is in the form of an array, or where a series of elongate formers is employed in the form of an array, to result in a series of connected tubes in the form of an array.

8. A method as claimed in claim 6 or 7, wherein the settable fluid comprises two fluid substances which, when mixed, eventually set to form a flexible solid substance.

9. A method as claimed in claim 8, additionally comprising the step of curing the settable fluid by heating it.

10. A method as claimed in any of claims 6 to 9, wherein the removable former is formed of a substance with a lower melting point than that of the settable fluid when in its set form, and wherein the step of removing said former comprises melting the former and removing it in liquid form.

11. A method as claimed in any of claims 6 to 10, wherein a release agent is applied to at least part of the former and/or the inner surface of the mould cavity in order to facilitate removal of the former after the settable fluid has set.

12. A tubular structure obtainable by a method as claimed in any of claims 6 to 11.

13. A medical device in combination with a tubular structure as claimed in claim 12.

14. A medical device formed from a tubular structure as claimed in claim 12.