WO2009071909A1

WO2009071909A1 - Nanometre fibres

Info

Publication number: WO2009071909A1
Application number: PCT/GB2008/004027
Authority: WO
Inventors: Ata Yoosfinejad
Original assignee: Munro Technology Ltd
Priority date: 2007-12-06
Filing date: 2008-12-08
Publication date: 2009-06-11
Also published as: EP2231301A1; WO2009071909A8; GB0723895D0

Abstract

A spatially-ordered matrix array of nanometre fibres with nanometre-size voids therebetween, is provided, optionally deposited on a substrate support. The array may be produced by an electrospinning process in which the fibres are deposited on a substrate disposed on a collector including a plurality of electrically-conductive discrete nodes disposed thereon.

Description

NANOMETRE FIBRES

This invention relates to nanometre fibres and provides an array of such fibres suitable for the filtration of particles, in particular for particles in the nanometre size range, such as viruses. The fibres may also be useful for example in tissue and other medical engineering, catalysis, microelectronics, textiles and materials engineering.

For the filtration of particles in the sub-micron size range, fibres in the nanometre diameter range, such as 5nm, can be produced by an electrospinning technique and disposed in the form of a non-woven scaffold of randomly oriented fibres which may be used as a filtration medium. Currently most nanofibres are produced using the electrospinning technique. Electrospinning involves, in general terms, introducing a charged liquid polymer solution to an electric field, the solution being dispensed typically through a needle attached to a syringe at a voltage between 10-2OkV, and deposition of the solution on a conductive collector material at ground (OV) potential located between 10 and 30cm from the needle. The ejected polymer solution, as it emerges from the needle in the form of a pendant droplet maintained by surface tension, is initially deformed to a conical shape and, once the voltage exceeds a critical value, dependent on the chemical nature of the polymer solution, the electrostatic forces overcome the surface tension and the solution forms a fine charged jet which is deposited on the collector material as a continuous nanofibre. However, before deposition, the jet exhibits bending instabilities due to repulsive electrical forces within the jet, as a result of which the jet extends through spiralling loops which, as they increase in diameter, result in the jet growing longer and thinner until it eventually solidifies on the collector. Due to the bending instabilities, the fibres are deposited on the collector in a random manner and result in a fibrous mat having a random pore size between the individual fibres. The broad pore size distribution means that the mat is only efficient for filtration of micron and larger particles and, even then, there is no guarantee that individual particles will not be allowed to pass through the mat. The filtration media are therefore not totally reliable particularly for particles in the nanometer size range, such as viruses.

In the light of the actual and potential harm which can be created on a global scale from the spread of viruses such as avian flu and SARS, as well as the possibility of deliberate release of viruses or toxic compounds in the form of gases as a political weapon, it would be of great benefit to provide a filtration medium which was able reliably to filter particles in the nanometre size range. By way of example, the particle size of the H5N1 strain of bird flu virus is in the region of lOOnm.

In an attempt to create more uniform structures which avoid problems resulting from the random nature of fibres produced by normal electrospinning techniques, WO02/034986 describes an electrospinning technique to form non- woven materials made from hollow fibres (mesotubes and nanotubes), the fibres being oriented in a single direction, in which the fibres are deposited on a flexible substrate which is subsequently stretched in one direction to align the fibres in the stretch direction. However, although the fibres are oriented in one direction, the separation distance between the aligned fibres cannot be readily controlled, and the resultant array of fibres is still not appropriate for filtration and other purposes.

An object of the present invention is to provide nanometre fibres in a form suitable for use as a filtration means for nanometre-size particles and which avoids the problems of currently- available filters using mats formed from randomly deposited nanometre-size fibres.

In one aspect, the present invention provides a spatially-ordered matrix array of nanometre fibres with nanometre-size voids therebetween.

The nanometre fibres may be deposited on a substrate support or may be essentially self- supporting.

By "spatially-ordered matrix array of nanometre fibres" is meant that the fibres define between them regular geometric shapes, for example squares or other rectangles, diamonds or hexagons, preferably as a fibre matrix monolayer, with nanometre-scale spaces or voids between the individual fibres.

The matrix array of nanometre fibres is preferably produced by an electrospinning process in which the fibres are deposited on a substrate disposed on a collector including a plurality of electrically-conductive discrete nodes disposed thereon. The fibre matrix may subsequently be removed from the substrate. Depending, however, on the nature of the substrate and the intended end use, the matrix array may be left remaining on the substrate. In another aspect, the invention provides a process for producing an array of nanometre fibres, the process comprising electrospinning a fibre material to become deposited on a substrate disposed on a conductive collector means, in which the conductive collector means comprises a plurality of discrete electrically-conductive nodes, whereby the array of nanometre fibres constitutes a spatially-ordered matrix array with nanometre voids therebetween.

In yet a further aspect, the invention provides electrospinning apparatus for forming a spatially-ordered array of nanometre fibres, the apparatus including injection means for a liquid polymer material and collector means for the nanometre fibres, in which the collector means comprises an array of discrete electrically-conductive nodes arranged in a grid pattern.

In the present invention, the fibre material forming the nanometre fibres comprises either a molten polymeric material, any thermoplastic polymer being potentially suitable, or a solution of a soluble thermoplastic polymer in a suitable solvent.

In one embodiment, the electrically-conductive nodes formed on the collector comprise discrete target points for attraction of the electrospun fibre material, the individual points being disposed at desired locations on the collector in order to control the geometry and size of the pores. The individual nodes may, for example, be constituted by clusters of electrically conductive nanoparticles. The nanoparticles may be positioned on the collector by various means including an atomic force microscope probe. In another embodiment, the nodes may be formed on a silicon or other suitable conductive wafer to constitute an array of regularly aligned nodes by nonlinear laser lithography techniques.

In yet a further embodiment, mechanical means such as the textile technique of carding may be used as basis for providing a matrix of conductive nodes.

In using the collector including the electrically-conductive nodes in an electrospinning process, the fibre material is attracted either to a discrete node and settles laterally thereof or, where the nodes are arranged in aligned rows, the fibre material under deposition becomes attracted to the nearest node points and settles accordingly. The resulting matrix array of nanometre fibres avoids the need for multi-layer systems, for use in filtration, and can easily be incorporated in existing filtration apparatus, the mesh geometry of the array being readily altered to meet particular requirements.

Preferably, the substrate on which the matrix array is deposited comprises a skeletal material to ensure that the integrity of the array is maintained, while still allowing passage therethrough, and access to the matrix array of nanometre fibres, of a gaseous medium to be filtered or other medium to be processed, according to the end use required. The substrate may for example comprise a textile material which, although not necessarily being electrically conductive, will nevertheless permit transmission of attractive forces between the zero-potential nodes formed on the underlying collector and the charged polymer jet during the electrospinning process. Natural and/or synthetic materials may be used for the textile fabric, either woven or non-woven. Preferably, the material exhibits an essential smooth deposition surface with a substantial absence of upstanding ends, whereby the matrix nanofibres are in supported contact with the substrate surface rather than being held spaced slightly apart therefrom on upstanding ends.

In the process or apparatus according to the invention, the electrically-conductive nodes formed as an array on the collector means constitute a regular array pattern and preferably coincide with parallel and intersecting notional lines whereby the nodes define square, diamond or other regular geometric shapes to attract the nanofibres as they are formed from the liquid polymer under the influence of the applied electric field and as the polymer solidifies or the solvent evaporates. The resulting fibre array is controlled as to the orientation and spacing-apart of the individual fibres, as opposed to the fibres being randomly-oriented or randomly spaced apart, as in the known art. Control may be exercised by varying the speed of the substrate relative to the collector to alter the effective attractive forces applied to the nanometre fibres as they approach the substrate.

In producing filtration means according to the invention by an electrospinning process, the collector may comprise a cylindrical mandrel disposed for rotation about an essentially horizontal axis of rotation, the mandrel having an array of individual nodes disposed thereon and the substrate material being passed over the mandrel as the mandrel rotates while the liquid polymeric material is deposited thereon. The substrate material may thus be passed over the mandrel in a continuous process and wound on a receiving roll.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, of which:

Figure 1 is a diagram illustrating schematically an electrospinning process for manufacture of an array of nanofibres according to the invention;

Figure 2 shows a perspective view of the collector mandrel used in the process of Figure 1 to illustrate how the fibres are attracted to the collector;

Figure 3 shows a fragment of a continuous substrate illustrating a square pattern of deposited fibres; and

Figure 4 is similar to Figure 3 but illustrates a diamond pattern of deposited fibres.

Referring firstly to Figure 1, the liquid polymer, either as a melt or as a solution, is contained in syringe 11 the needle 12 of which is maintained at a positive electrical potential. A horizontally-disposed cylindrical rotatable target collector mandrel 13 having surface- mounted nodes 14 is mounted beneath the syringe and is maintained at zero or ground voltage. A fabric substrate material 15 is passed from a supply roll 16, over guide rollers 17, 18, the upper arcuate surface of the collector 13, and onwards over guide roller 19 to form a take-up roll 20. As the fabric substrate passes over the collector mandrel 13, the polymer liquid in the form of a nanofibre 21 is deposited on the surface of the substrate in a manner determined by the nodes 14 formed on the surface of the collector, as described in more detail with reference to Figure 2. In practice, the syringe needles may be provided on a cylinder which is rotatably mounted in a solvent/polymer reservoir to form thousands if not millions of individual fibres, the numbers of which may optionally be attenuated by the use of inert, for example, ptfe, baffle plates. A suitable apparatus for forming the nanometre fibres is the Elmarco Nanospider "NS Lab" electrospinner. Figure 2 shows an array of the nanofibre-scale nodes 14 formed for example by nonlinear laser lithography, a manufacturing process using a femtosecond laser (ultra fast or ultra short pulse lasers) enabling structure sizes of less than lOOnm to be achieved. The nodes are formed on a semiconductor silicon wafer, although other conductive materials could be used in place of silicon. The silicon wafer is applied to the surface of the collector mandrel 13 and the nanofibres 21 are attracted towards the nodes and settle on the substrate material in a grid pattern as illustrated in Figure 2. For simplicity, Figure 2 shows the grid pattern as though the fibres were deposited directly on the surface of the mandrel without the substrate being present; in practice, the fibres would be distributed on the surface of the substrate and would form a nanometre-scale matrix array disposed on and supported by the fabric substrate material, as shown in Figure 3, where the fibres define a square grid pattern, and in Figure 4, where the fibres form a diamond grid pattern. In Figures 3 and 4, the nodes 14 are shown at the positions of the nodes on the mandrel 13, below the substrate 15 as the substrate passes over the mandrel in the direction of the arrows at a linear speed matched with the rotational speed of the surface of the mandrel or, alternatively, at a different (faster or slower) speed to influence the fibres to assume a particular orientation as they approach the substrate according to the desired configuration of the matrix array of fibres. In this connection, the velocity of the nodes on the rotating mandrel will produce variable resultant attractive forces on the fibres.

As an alternative to the collector mandrel 13 shown in Figure 1, the rotary drum of a textile carding machine may be used as the collector, in which the fabric substrate is passed over the arcuate upper surface of the drum, from which the carding needles extend radially and act as attraction nodes for the polymer nanofibre in a manner analogous to that shown in Figure 2 for the laser-etched nodes.

Arrays of nanometres fibres according to the invention may be used for filtration purposes and in fibre-reinforced plastics materials, application of pesticides to plants, biomedical applications including tissue engineering scaffolds, bandages and drug release systems, protective clothing especially for chemical and biological protection, catalyst supports and for solar and light sails and mirrors in space, for example.

Claims

1. A spatially-ordered matrix array of nanometre fibres with nanometre-size voids therebetween.

2. A fibre matrix according to claim 1, in which the fibres are deposited on a substrate support.

3. A fibre matrix according to claim 2, in which the matrix array is produced by an electrospinning process in which the fibres are deposited on a substrate disposed on a collector including a plurality of electrically-conductive discrete nodes disposed thereon.

4. A process for producing an array of nanometre fibres, the process comprising electrospinning a liquid polymer fibre material to become deposited on a substrate disposed on a conductive collector means, in which the conductive collective means comprises a plurality of discrete electrically-conductive nodes, whereby the array of nanometre fibres constitutes a spatially-ordered matrix array with nanometre voids therebetween.

5. Electrospinning apparatus for forming a spatially-ordered array of nanometre fibres, the apparatus including injection means for a liquid polymer material and collector means for the nanometre fibres, in which the collector means comprises an array of discrete electrically- conductive nodes arranged in a grid pattern.

6. Apparatus according to claim 5, in which the electrically-conductive nodes formed on the collector comprise clusters of electrically conductive nanoparticles.

7. Apparatus according to claim 6, in which the nanoparticles are positioned on the collector by means of an atomic force microscope probe.

8. Apparatus according to claim 5, in which the nodes are formed on a silicon or other conductive wafer to constitute an array of regularly aligned nodes by nonlinear laser lithography.

9. Apparatus according to claim 5, in which mechanical means such as textile carding are used for providing the array of conductive nodes.

10. A fibre matrix according to claim 2 or claim 3 or a process according to claim 4, in which the substrate comprises a skeletal material.

11. A fibre matrix or a process according to claim 10, in which the substrate comprises a textile material.

12. A process according to claim 4 or apparatus according to any of claims 5 to 9, in which the collector comprises a cylindrical mandrel disposed for rotation about an essentially horizontal axis of rotation, the mandrel having an array of individual nodes disposed thereon.

13. A process according to claim 12, in which the substrate material is passed over the mandrel in a continuous process and wound on a receiving roll.

14. The use of an array of nanometre fibres according to any of claims 1 to 3 in filtration, biomedical engineering, microelectronics, materials engineering, bandages, drug release systems, protective clothing, catalyst supports or in solar or light sails and mirrors in space.