WO2006136817A1

WO2006136817A1 - Electrospinning of fibres

Info

Publication number: WO2006136817A1
Application number: PCT/GB2006/002268
Authority: WO
Inventors: Anthony John Ryan; David Norton
Original assignee: The University Of Sheffield
Priority date: 2005-06-21
Filing date: 2006-06-20
Publication date: 2006-12-28
Also published as: GB2427382A; GB0512629D0

Abstract

A method for the production of an array of aligned fibres by electrospinning, which comprises forming a focused electrostatic field between a body having an orifice, maintained at a first electric potential, and a counter electrode, maintained at a second electric potential, the modulus of said second electric potential being greater than the modulus of said first electric potential, introducing a solution of a polymer in a volatile solvent into the electrostatic field through the orifice such that the polymer solution flows from the orifice towards the counter electrode in the form of a charged jet or stream, said jet or stream having a path length at least a portion of which is substantially straight, disposing a collector means in the path of the jet or stream at a position within the substantially straight portion thereof at which the solvent has evaporated sufficiently for the jet or stream to solidify, and moving the collector means relative to the jet or stream to deposit aligned fibres on the collector means.

Description

ELECTROSPINNING OF FIBRES

FIELD OF THE INVENTION

This invention relates to the electrospinning of fibres, and more particularly to a method for the electrospinning of axially aligned fibres, especially axially aligned nanofibres, and to axially aligned arrays thereof.

BACKGROUND OF THE INVENTION

The principles of electrospinning have been known for many years. Conventionally the method involves the application of a high voltage to a polymer solution contained within a syringe. A grounded counter electrode is disposed at an appropriate distance (gap). As the electric potential of the syringe is increased, the charged pendant drop of polymer solution formed at the tip of the capillary needle is deformed into a cone, known as a "Taylor cone". At a critical field strength, when the force due to the electric field overcomes the surface tension forces holding the droplet, the solution starts flowing in the form of a charged jet across the gap. A jet of polymer solution is propelled from the needle of the syringe towards the grounded counter electrode. Whilst in transit most of the solvent molecules evaporate away leaving a dried polymer fibre, which is collected. Along the "path length" of the jet a phenomenon known as "jet whipping" ensues as the jet becomes unstable in the electric field, and the different polymer strands in the jet separate out due to mutual repulsion, a phenomenon known as "splaying". The jet whipping and splaying lead to the collection of typically randomly oriented fibres (a non woven material). When the fibres reach the collecting grounded counter electrode their diameters are generally in the range of a few micrometres to nanometres.

Patents describing various methods for the production of non woven fibres by electrospinning include: US 2003/0017208, US 5522879, US 5376117, US 4904272, US 4842505, US 4657793, US 4323525, US 4230650, US 4127706, US 4069026, US 4044404, US 4043331 , US 3994258 and US 1975504.

A few patents describe the production of products comprising some degree of alignment of the fibres.

US 5024789 and US 4965110 describe an electrospinning method for producing tubular fibrous structures wherein the fibres are collected on an electrostatically charged mandrel, wherein the fibres take different paths from the source to the mandrel to produce a structure of smaller diameter fibres randomly oriented, larger diameter fibres and/or bundles of fibres circumferentially oriented and elongated voids circumferentially oriented.

US 4552707 describes a synthetic vascular graft made by electrospinning an organic polymeric material or a precursor thereof and collecting the spun fibres on a rotating mandrel, and controlling the speed of rotation of the mandrel such that a desired degree of anisotropy is present in the synthetic vascular graft.

US 4689186 describes the preparation of products having a tubular portion comprising electrospinning a fibre-forming liquid, the electrostatic field being distorted by the presence of an auxiliary electrode, preferably so as to encourage the deposition of circumferential fibres.

US 6592623 describes a muscle implant including an extracellular matrix made from electrospun polymer fibres. The fibres can be formed from collagen and can be oriented in a specific direction, layered, or programmed to be completely random and unoriented.

None of the above methods entirely overcome the effects of jet whipping and splaying. There is still, therefore, a need for a method for the production of highly aligned fibres, especially nanofibres, by electrospinning Li et al, Adv. Mater. 2004, 16, No. 4 describe the electrospinning of nanofibres as uniaxially aligned arrays and layer-by-layer stacked films, by the use of a grounded collector consisting of two conductive strips separated by an insulating gap of variable width. However jet whipping is still inherent in this method.

Natarajan et al, Applied Physics Letters describes the electrospinning of continuous aligned polymer fibres using a separation distance of about 1 to 3 cm between the tip of the capillary and the counter electrode.

The disclosures of all the above-mentioned patents and publications are incorporated herein by reference in their entirety for all purposes.

BRIEF SUMMARY OF THE DISCLOSURE

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example, "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. The present invention provides an improved method for the production of highly aligned fibres by electrospinning wherein the fibres are collected whilst traveling in a focused electrostatic field.

In a first aspect, the present invention provides a method for the production of an array of aligned fibres by electrospinning, which comprises forming a focused electrostatic field between a body having an orifice, maintained at a first electric potential, and a counter electrode, maintained at a second electric potential, the modulus of said second electric potential being greater than the modulus of said first electric potential, introducing a solution of a polymer in a volatile solvent into the electrostatic field through the orifice such that the polymer solution flows from the orifice towards the counter electrode in the form of a charged jet or stream, said jet or stream having a path length at least a portion of which is substantially straight, disposing a collector means in the path of the jet or stream at a position within the substantially straight portion thereof at which the solvent has evaporated sufficiently for the jet or stream to solidify, and moving the collector means relative to the jet or stream to deposit aligned fibres on the collector means.

In a second aspect, the present invention provides an electrospinning apparatus for the production of an array of aligned fibres, which comprises a body having an orifice, means for maintaining the body at a first electric potential, a counter electrode disposed at a distance from the orifice, means for maintaining the counter electrode at a second electric potential, the modulus of said second electric potential being greater than the modulus of said first electric potential, means for introducing a solution of polymer in a volatile solvent through the orifice into a focused electrostatic field formed in the gap between the orifice and the counter electrode, such that the polymer solution flows from the orifice towards the counter electrode in the form of a charged jet or stream, said jet or stream having a path length at least a portion of which is substantially straight, and a collector means disposed in the path of the jet or stream at a position within the substantially straight portion thereof at which, in use, the solvent has evaporated sufficiently for the jet or stream to solidify, and means for moving the collector means relative to the jet or stream to deposit aligned fibres on the collector means.

In a further aspect, the invention provides an array of aligned fibres produced by the method and/or apparatus of the invention.

By "an array of aligned fibres" in this specification is meant a structure in which two or more fibres are oriented in a substantially parallel direction over at least a substantial part of their length. In general the array of aligned fibres can have more than 10, more than 100, or more than 1000 fibres oriented in a substantially parallel direction over at least a substantial part of their length. Preferably the fibres are oriented in a substantially parallel direction over substantially their entire length. In a method according to the invention, the fibres can be produced to any desired length, for example, more than 1 mm, more than 10 mm, more than 100 mm, or more than 1000 mm. A wide range of fibre diameters can also be produced using a method according to the invention, although the method of the invention can be especially useful for producing very small diameter fibres in the nanometre range. In general, fibre diameters of from 50 nm to 100 μm, preferably from 50 nm to 50 μm, more preferably from 100 nm to 1000 nm, for example, around 400 to 500 nm may be produced using a method of the invention.

By "electrospinning" in this specification is meant a method in which fibres are formed from a solution or melt by streaming an electrically charged polymer solution or melt through an orifice.

The aligned fibres of the present invention comprise high molecular weight polymeric materials. These polymers, by virtue of their high molecular weight, form viscous solutions which can produce fibres, especially nanofibres, when subjected to an electrostatic potential. Polymers having molecular weights in the range of from 2000 g mol^"1 to 5 million g mol^"1, preferably fromi OOOO g mol^"1 to 2 million g mol^'1, more preferably from 50000 g mol^"1 to 1 million g mol^"1 can be used. Suitable general purpose high molecular weight polymers useful in the present invention include both synthetic and natural fibre forming polymers, for example, cellulose acetate, polystyrene, PVA, PEO, PVP, polyacrylamide, polyurethane, polycarbonate, PTFE, PE, PP, polyacrylate, Kevlar, PHB, polyaniline, poly (phenylene terephthalamide) and silk. Block copolymers and other thermoplastic elastomers are also useful in the present invention, for example, triblock copolymers comprising polystyrene, polybutadiene, hydrogenated polybutadiene, polyisoprene, hydrogenated polyisoprene, such as the Kraton family (SBS, SIS, SEBS), and multi-block copolymers such as polyester-ethers, polyether-urethanes and polyether-ureas.

Where the aligned fibres are intended for use in pharmaceutical applications, examples of suitable polymers include, poly(ethylene oxide), polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone, hyaluronic acid, alginates, carragenen, cellulose derivatives such as carboxymethyl cellulose sodium, methyl cellulose, ethylcellulose, hydroxyethyl cellulose, hydroxypropylcellulose, hydroxypropylmethyl cellulose, hydroxypropylmethyl cellulose phthalate, cellulose acetate phthalate, noncrystalline cellulose, starch and its derivatives such as hydroxyethyl starch, sodium starch glycolate, chitosan and its derivatives, albumen, gelatin, collagen, polyacrylates and their derivatives, poly(alpha-hydroxy acids) and their copolymers such poly(caprolactone), poly(lactide-co-glycolide), poly(alpha- aminoacids) and their copolymers, poly(orthoesters), polyphosphazenes, poly(phosphoesters), and polyanhydrides, or mixtures thereof..

For certain pharmaceutical applications the aligned fibres are preferably water-soluble. Water soluble polymers include but are not limited to, poly(ethylene oxide), polyvinyl alcohol, polyvinyl pyrrolidone, hyaluronic acid, alginates, carragenen, cellulose derivatives such as carboxymethyl cellulose sodium, hydroxyethyl cellulose, hydroxypropylcellulose, hydroxypropylmethyl cellulose, hydroxypropylmethyl cellulose phthalate, cellulose acetate phthalate, starch and its derivatives such as hydroxyethyl starch, sodium starch glycolate, dextrin, chitosan and its derivatives, albumen, zein, gelatin, and collagen. DNA, RNA and other naturally occurring polymeric materials comprising nucleic acids and polypeptides can also be used in the method of the invention.

Water insoluble polymers useful in pharmaceutical applications include but are not limited to, polyvinyl acetate, methyl cellulose, ethylcellulose, noncrystalline cellulose, polyacrylates and their derivatives, poly(alpha- hydroxy acids) and their copolymers such as poly(ε-caprolactone), poly(lactide-co-glycolid- e), poly(alpha-aminoacids) and their copolymers, poly(orthoesters), polyphosphazenes, poly(phosphoesters), and polyanhydrides.

Suitable volatile solvents for use herein include, but are not limited to, water, acetic acid, acetone, acetonitrile, methanol, ethanol, propanol, ethyl acetate,. propyl acetate, butyl acetate, butanol, N₁N dimethyl acetamide, N, N dimethyl formamide, 1-methyl-2-pyrrolidone, dimethyl sulfoxide, diethyl ether, disisopropyl ether, tetrahydrofuran, pentane, hexane, 2-methoxyethanol, formamide, formic acid, hexane, heptane, ethylene glycol, dioxane, 2- ethoxyethanol, trifluoroacetic acid, methyl isopropyl ketone, methyl ethyl ketone, dimethoxy propane, methylene chloride , or mixtures thereof.

Preferred solvents include dichloromethane, chloroform, tetrahydrofuran and N₁N dimethylformamide.

The solvent to polymer ratio is determined by the desired viscosity of the resulting solution. Preferably the viscosity of the solution is within the range of from 1.5 to 50000 cp, more preferably from 50 tolOOOO cp, especially from 100 to 5000 cp.

For electrospinning of a polymeric composition, important parameters, in addition to viscosity, are surface tension, and electrical conductivity of the solvent/polymeric composition. Preferably the surface tension of the polymer solution is within the range of from 70 to 500 mNm^"1, and its electrical conductivity is within the range of from 1 to 300 Son² mol^"1.

By a "focused electrostatic field" in this specification is meant an electrostatic field wherein the electric lines of force are relatively close together and divergence is kept to a minimum. Preferably the body having the orifice is maintained at ground potential and the counter electrode is a point source upon which the electrostatic field is focused. In this specification, the

"modulus of the electric potential" means the magnitude of the electric potential, whether the charge is positive or negative.

The counter electrode can, for example, be a pin or pointed conductive rod, which can be connected to a high-voltage generator. The potential difference between the body having the orifice and the counter electrode is preferably at least 1000V. Preferably the pin or pointed rod is maintained at an electric potential of from 1000 to 50000 V, more preferably from 5000 to 30000 V, most preferably from 15000 to 25000 V.

In conventional electrospinning it is customary for the high-voltage potential to be applied to the spinneret orifice and for the collector to be at ground potential. By reversing the polarity, and focusing the electrostatic field upon a point source counter electrode, it has been found that the jet or stream issuing from the orifice can be kept substantially straight over a path length of several centimetres, for example, from 1 to 70 cms, preferably from 10 to 70 cms, or more, without significant jet whipping.

In this specification, the term "orifice" means an opening or hole in a body through which the polymer solution can flow. Usually the body will be a hollow body, for example, a spinneret, or a hollow needle. Preferably the body is formed from a conductive material, for example, a metal, and is preferably a capillary orifice, for example, a capillary needle. Polymer sOlution can be supplied to the orifice from a reservoir by any suitable means, for example, by a pump or plunger mechanism that forces the polymer solution through the hollow body towards the orifice. When-the polymer solution exits the orifice it flows in the form of a jet or stream, under the influence of the electrostatic field, in the direction of the counter electrode. Because the electrostatic field is focused upon the counter electrode, the path length of the polymer solution is substantially straight with relatively little jet whipping or splaying, at least in the portion of the path length adjacent to the orifice. As the polymer solution progresses through the electrostatic field the solvent evaporates and the viscosity of the solution increases to a point at which the jet or stream can maintain its structural integrity. Where the jet or stream is capable of maintaining its structural integrity, and is preferably self-supporting, it is considered to be "solidified" in the context of the present specification.

According to the invention, the collector means is disposed in the path of the jet or stream at a position such that the jet or stream is substantially straight and has lost sufficient of the solvent by evaporation to be "solidified" as defined in this specification. It will be appreciated that, even though the jet or stream may form one or more fibres capable of maintaining their structural integrity, the fibres may still contain substantial amounts of solvent, which can continue to evaporate from the fibres after deposition on the collector means.

Preferably the collector means is disposed at a distance of from 1 to 60 cms, more preferably from 10 to 50 cms, from the orifice.

Any suitable substrate can be used as a collector means in the method and apparatus of the present invention, provided that it does not substantially distort the electrostatic field. The collector means can comprise, for example, a hollow spindle, having an axis of rotation at 90° to the direction of the electrostatic field. The spindle can have 2, 3, 4, 5, 6, 10 or more bars around its circumference for receiving the deposited fibres. Rotation of the spindle enables long fibres to be collected, extending around the complete circumference thereof. The collector means is preferably formed from an insulating material, for example, a plastics material such as PET, and is preferably rotated about insulated axles in order that it should not substantially distort the electrostatic field. Other forms of collector means are, of course, possible, and for example, the collector means could be a grid provided with translation means for moving the grid back and forth across the electrostatic field. In medical applications, the collector could also be shaped in the form of a desired object, for example, a nose, ear, or other anatomical structure.

Aligned fibres can be removed from the collector means by any suitable method, for example, by picking the fibres and rolling them onto a separate storage spindle or glass slide, or by lifting them off the collector means directly onto a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of a method and apparatus according to the invention will now be described, by way of example only, with reference to the accompanying Drawings in which:

Figure 1 shows a perspective view of an apparatus for reverse polarity electrospinning:

Figure 2 shows a perspective view of a first electrospinning apparatus in accordance with the invention:

Figure 3 shows a perspective view of a second electrospinning apparatus in accordance with the invention enclosed within an insulated enclosure:

Figure 4 shows a perspective view of the apparatus of Figure 3 showing aligned fibres collecting on the spindle:

Figure 5 shows a perspective view of a pair of spindles suitable for use in the present invention: Figure 6 shows scanning electron micrographs of polymer fibres collected (A) without the insulated box; and (B) with the insulated box:

Figure 7 shows light microscope pictures of block copolymer fibres formed using the apparatus of Figure 3 at a collector speed > 100 rpm: and

Figure 8 shows (A) a scanning electron micrograph of a polylactide polymer spun by a conventional electrospinning method; and (B) a light microscope image of a polylactide polymer spun using the apparatus of Figure 3.

DETAILED DESCRIPTION

Referring firstly to Figure 1 , there is shown an electrospinning apparatus, illustrated generally at 1 , comprising a syringe 2 having a reservoir 3 and a capillary needle 4. The syringe 2 is mounted upon a pump 5. The needle 4 is maintained at ground potential by a connecting ground wire 6. The needle 4 is demountable and can be replaced by other capillary needles having a range of diameters depending upon the viscosity of the polymer solution used and the desired fibre diameter. The counter electrode comprises a second needle 7, which is aligned with the capillary needle 4, and connected to a high-voltage DC source 8 by a wire 9.

Upon the application of the high-voltage to second needle 7, a jet 10 of polymer solution is ejected through the capillary needle 4. The path length of the jet 10 is straight for a distance D until it reaches the point 11 , at which it becomes charged and is repelled from the counter electrode needle 7. It has been found that the distance D is greater for less polar solvents and is also increased by increasing the gap between the capillary needle 4 and the counter electrode needle 7, and by increasing the applied voltage.

Turning now to Figure 2, there is shown a collector 12 interposed between the capillary needle 4 and the counter electrode needle 7. The collector 12 comprises a rotatable spindle 13, having insulated axles 14, and provided with a series of bars 15, arranged around its circumference. The spindle 13 is rotated by means of a high-speed electric motor 16. The collector 12 is positioned such that the jet 10 passes through it whilst still straight. Rotation of the spindle enables aligned, oriented fibres to be collected on the bars 15. The orientation of the fibres can be increased by increasing the rotation speed of the spindle 13 up to the point at which the circumferential velocity of the spindle exceeds the velocity of the jet 10.

Figure 3 illustrates a modified apparatus which is suitable for polymer solutions in less volatile or more polar solvents. For such solutions it is desirable that the path length of the jet should be as long as possible in order to allow for sufficient evaporation of the solvent and drying of the fibres. The modified apparatus is similar to that shown in Figure 2, except that the capillary needle 4, the collector 12 and the counter electrode 7 are enclosed in a semi-insulated enclosure 17. This increases the distance D by shielding the charged jet of polymer solution from earth points.

Figure 4 shows the apparatus of Figure 3 with a series of aligned fibres 18 collected on the bars 15 of the spindle 13.

Figure 5 shows two designs of spindle suitable for use in the apparatus of the present invention. The three-bar spindle 19 is suitable for a broad range of polymers and fibres, but polymers which form brittle fibres or fibres of very small diameter (<1 μm) may be broken as they are stretched over the spindle. In such cases, a spindle with more bars can be used in order that the gap over which the fibres are unsupported is decreased. The six-bar spindle 20 can overcome this problem.

EXAMPLE 1

Figure 6 (A) shows a scanning electron micrograph of polymer fibres collected using the apparatus of Figure 2, that is to say, without the semi-insulated enclosure, using a counter electrode voltage of 20 kV and a collector rotation speed of 300 rpm. Figure 6 (B) shows a scanning electron micrograph of fibres of the same polymer collected using the apparatus of Figure 3, using a counter electrode voltage of 20 kV and a collector rotation speed of 300 rpm. It can be seen that the fibres of Figure 6 (B) are of smaller diameter, and form a more dense aligned array.

Figure 7 shows light microscope pictures of a block copolymer electrospun using the apparatus of Figure 3. The counter electrode voltage was 17.5 kV and the collector speed of rotation was >100 rpm.

EXAMPLE 2

Figure 8 (A) shows a scanning electron micrograph of a polylactide polymer spun using a conventional electrospinning technique with a charged orifice and a grounded rotating mandrel collector. Figure 8 (B) shows a light microscope image of the same polymer spun using the apparatus of Figure 3, using an orifice at ground potential and a counter electrode voltage of 22.5 kV. It can be seen that the fibres of Figure 8 (B) are of smaller diameter, are more highly aligned and more densely deposited than the fibres of Figure 8 (A).

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method for the production of an array of aligned fibres by electrospinning, which comprises forming a focused electrostatic field between a body having an orifice, maintained at a first electric potential, and a counter electrode, maintained at a second electric potential, the modulus of said second electric potential being greater than the modulus of said first electric potential, introducing a solution of a polymer in a volatile solvent into the electrostatic field through the orifice such that the polymer solution flows from the orifice towards the counter electrode in the form of a charged jet or stream, said jet or stream having a path length at least a portion of which is substantially straight, disposing a collector means in the path of the jet or stream at a position within the substantially straight portion thereof at which the solvent has evaporated sufficiently for the jet or stream to solidify, and moving the collector means relative to the jet or stream to deposit aligned fibres on the collector means.

2. A method according to claim 1 , wherein the array of aligned fibres has more than 10 fibres oriented in a substantially parallel direction over at least a substantial part of their length.

3. A method according to claim 1 or 2, wherein the fibres are oriented in a substantially parallel direction over substantially their entire length.

4. A method according to any one of the preceding claims, wherein the fibres are more than 10 mm in length.

5. A method according to any one of the preceding claims, wherein the fibre diameter is from 100 nm to 1000 nm.

6. A method according to any one of the preceding claims, wherein the polymer has a molecular weight in the range of from 1000Og mol^"1 to 2g mol^"1 million.

7. A method according to any one of the preceding claims, wherein the polymer is selected from cellulose acetate, polystyrene, PVA, PEO, PVP, polyacrylamide, polyurethane, polycarbonate, PTFE, PE, PP, polyacrylate, Kevlar, PHB, polyaniline, DNA, poly (phenylene terephthalamide) silk, triblock copolymers comprising polystyrene, polybutadiene, hydrogenated butadiene, polyisoprene, hydrogenated polyisoprene, SBS, SIS, SEBS, polyester-ethers, polyether-urethanes and polyether-ureas.

8. A method according to any one of claims 1 to 6, wherein the polymer is selected from poly(ethylene oxide), polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone, hyaluronic acid, alginates, carragenen, cellulose derivatives including carboxymethyl cellulose sodium, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropylcellulose, hydroxypropylmethyl cellulose, hydroxypropylmethyl cellulose phthalate, cellulose acetate phthalate, noncrystalline cellulose, starch and its derivatives such as hydroxyethyl starch, sodium starch glycolate, chitosan and its derivatives, albumen, gelatin, collagen, polyacrylates and their derivatives, poly(alpha- hydroxy acids) and their copolymers including poly(caprolactone), poly(lactide-co-glycolide), poly(alpha-aminoacids) and their copolymers, poly(orthoesters), polyphosphazenes, poly(phosphoesters), and polyanhydrides, or mixtures thereof.

9. A method according to any one of claims 1 to 6, wherein the aligned fibres are formed from a water soluble polymer.

10. A method according to claim 9, wherein the water soluble polymer is selected from poly(ethylene oxide), polyvinyl alcohol, polyvinyl pyrrolidone, hyaluronic acid, alginates, carragenen, cellulose derivatives including carboxymethyl cellulose sodium, hydroxyethyl cellulose, hydroxypropylcellulose, hydroxypropylmethyl cellulose, hydroxypropylmethyl cellulose phthalate, cellulose acetate phthalate, starch and its derivatives including hydroxyethyl starch, sodium starch glycolate, dextrin, chitosan and its derivatives, albumen, zein, gelatin, and collagen.

11. A method according to any one of claims 1 to 6, wherein the polymer is DNA₁ RNA or a naturally occurring polypeptide or nucleic acid.

12. A method according to any one of claims 1 to 6, wherein the polymer is a water insoluble polymer.

13. A method according to claim 12, wherein the water insoluble polymer is selected from polyvinyl acetate, methyl cellulose, ethylcellulose, noncrystalline cellulose, polyacrylaf.es and their derivatives, poly(alpha-hydroxy acids) and their copolymers including poly (ε-caprolactone), poly(lactide-co-gly.colid- e), poly(alpha-aminoacids) and their copolymers, poly(orthoesters), polyphosphazenes, poly(phosphoesters), and polyanhydrides.

14. A method according to any one of the preceding claims, wherein the volatile solvent is selected from water, acetic acid, acetone, acetonitrile, methanol, ethanol, propanol, ethyl acetate, propyl acetate, butyl acetate, butanol, N₁N dimethyl acetamide, N₁N dimethyl formamide, 1-methyl-2- pyrrolidone, dimethyl sulfoxide, diethyl ether, disisopropyl ether, tetrahydrofuran, pentane, hexane, 2-methoxyethanol, formamide, formic acid, hexane, heptane, ethylene glycol, dioxane, 2-ethoxyethanol, trifluoroacetic acid, methyl isopropyl ketone, methyl ethyl ketone, dimethoxy propane, methylene chloride , dichloromethane, chloroform, tetrahydrofuran and mixtures thereof.

15. A method according to any one of the preceding claims, wherein the viscosity of the solution is within the range of from 50 tolOOOO cp.

16. A method according to any one of the preceding claims, wherein the surface tension of the polymer solution is within the range of from 70 to 500 rnNrn^"1, and its electrical conductivity is within the range of from 1 to 300 Son² mol-¹.

17. A method according to any one of the preceding claims, wherein the body having the orifice is maintained at ground potential and the counter electrode is a point source upon which the electrostatic field is focused.

18. A method according to claim 17, wherein the counter electrode is maintained at an electric potential of from 5000 to 30000 V.

19. A method according to any one of the preceding claims, wherein the jet or stream issuing from the orifice is kept substantially straight over a path length of from 1 to 70 cms, or more, without significant jet whipping.

20. A method according to any one of the preceding claims, wherein collector means is rotated relative to the jet or stream to deposit aligned fibres on the collector means.

21. A method according to claim 20, wherein the collector means has an axis of rotation aligned at substantially 90° to the direction of the electrostatic field.

22. A method according to any one of the preceding claims, wherein the collector means is disposed at a distance of from 1 to 60 cms from the orifice.

23. A method according to any one of the preceding claims, wherein the aligned fibres are removed from the collector means by picking the fibres and rolling them onto a separate storage spindle or glass slide, or by lifting them off the collector means directly onto a substrate.

24. A method for producing aligned fibres by electrospinning substantially as hereinbefore described.

25. An electrospinning apparatus for the production of an array of aligned fibres, which comprises a body having an orifice, means for maintaining the body at a first electric potential, a counter electrode disposed at a distance from the orifice, means for maintaining the counter electrode at a second electric potential, the modulus of said second electric potential being greater than the modulus of said first electric potential, means for introducing a solution of a polymer in a volatile solvent through the orifice into a focused electrostatic field formed in the gap between the orifice and the counter electrode, such that the polymer solution flows from the orifice towards the counter electrode in the form of a charged jet or stream, said jet or stream having a path length at least a portion of which is substantially straight, and a collector means disposed in the path of the jet or stream at a position within the substantially straight portion thereof at which, in use, the solvent has evaporated sufficiently for the jet or stream to solidify, and means for moving the collector means relative to the jet or stream to deposit aligned fibres on the collector means.

26. An apparatus according to claim 25, wherein means are provided to maintain the body having the orifice at ground potential, and wherein the counter electrode is connected to a high voltage generator.

27. An apparatus according to claim 25 or 26, wherein the counter electrode is a point source upon which the electrostatic field is focused.

28. An apparatus according to claim 27, wherein the counter electrode is a pin or pointed conductive rod, which can be connected to a high-voltage generator.

29. An apparatus according to any one of claims 25 to 28, wherein the body is a hollow body.

30. An apparatus according to claim 29, wherein the body is a spinneret, or a hollow needle.

31. An apparatus according to claim 29 or 30, wherein the orifice is a capillary orifice.

32. An apparatus according to any one of claims 25 to 31 , wherein the body is formed from a conductive material.

33. An apparatus according to any one of claims 29 to 32, wherein polymer solution is supplied to the orifice from a reservoir by a pump or plunger mechanism that urges the polymer solution through the hollow body towards the orifice.

34. An apparatus according to any one of claims 25 to 33, wherein the collector means comprises a rotatable hollow spindle.

35. An apparatus according to claim 34, wherein the collector means has an axis of rotation at 90° to the direction of the electrostatic field.

36. An apparatus according to claim 34 or 35, wherein the spindle has 2, 3, 4, 5, 6, 10 or more bars around its circumference for receiving the deposited fibres.

37. An apparatus according to any one of claims 34 to 36, wherein the collector means is formed from an insulating material.

38. An apparatus according to any one of claims 34 to 37, wherein the spindle is rotatable about insulated axles.

39. An apparatus according to any one of claims 25 to 33, wherein the collector means comprises a grid provided with translation means for moving the grid back and forth across the electrostatic field.

40. An apparatus according to any one of claims 25 to 39, wherein the collector means is disposed at a distance of from 1 to 60 cms from the orifice.

41. An apparatus for the production of aligned fibres substantially as hereinbefore described with reference to and as illustrated in the accompanying Drawings.

42. An array of aligned fibres produced using a method according to any one of claims 1 to 24 or using an apparatus according to any one of claims 25 to 41.