IMPROVEMENTS IN ELECTROSTATICALLY PRODUCED STRUCTURES AND METHODS OF MANUFACTURING THEREOF

Abstract: In apparatus comprising needles (l0), a mandrel (ll) and electrostatically charged plates (l2, l3, l4 and l5), variation of the electrostatic potential on the mandrel (ll) and the plates (l2, l3, l4 and l5) produces variation of the fibrous structure produced. In general, where the mandrel (ll) is significantly more attractive to the fibres, a microstructure of fibres of substantially uniform diameter is produced, these fibres being generally randomly orientated. Where the voltages on the mandrel (ll) and on the plates (l3 and l5) are varied to lessen the attraction of the mandrel (ll) relative to the plates (l3, l5) the fibrous structure is varied to produce fibres of a larger diameter than the original micro fibrous structure, this having a significant effect on characteristics of the fibrous structure.

Description: The invention relates to electrostatically produced structures, for example tubular vascular grafts, and to methods of manufacturing such structures. It has been proposed in the past to spin electrostatically fibrous structures of polymeric material such as polyurethane using an electrostatically charged, spinning mandrel as a fibre collector, a solution of the polymer being ejected towards the mandrel from capillary needles or other suitable means. The mandrel has in some circumstances been placed between charged electrodes to alter the electrostatic field created by the charged mandrel. The mandrel may be replaced by an alternative electrostatically charged collector if a tubular structure is not required. It has been found that synthetic fibrous structures built up in this way generally have fibres of a diameter not larger than lµm, and that the fibres are generally randomly orientated. Some directional bias can be induced by varying the speed of mandrel rotation, as indicated in our published British Patent Application Nos. 2l2l286A and 2l20946A. It has been found that the generally random nature of the fibrous structure and the small fibre size of lµm or less has produced a tubular structure which can be prone to kinking and can therefore be a problem when used as an arterial graft particularly where limb movement is involved. According to one aspect of the invention there is provided a tubular fibrous structure comprising small diameter fibres and substantially larger diameter fibres, said smaller diameter fibres being randomly oriented in the fibrous structure, said larger diameter fibres being embedded in a matrix of said small diameter fibres and said larger diameter fibres being generally oriented circumferentially with respect to the longitudinal axis of said tubular structure. The structure may comprise a multiplicity of elongate voids extending generally circumferentially with respect to the longitudinal axis of the structure. The small diameter fibres may have a diameter in the range 0.5µm to 2µm, for example lµm, and the larger diameter fibres may have a diameter in the range 2,µm to l5µm. The larger the fibre diameter, the more likely the fibre is to be orientated circumferentially. The structure may comprise an inner layer of fibres of the small diameter. The layer of fibres of larger diameter may be an intermediate layer, inside an outer layer of fibres of the small diameter. The inner layer may be of a polymeric material adapted to be compatible with contact with blood, and may be of a thickness in the range l0µm to 60µm, preferably 40µm. The intermediate layer may be of a thickness in the range 300µm to 2000µm. The outer layer may be of a thickness in the range of l0µm to 60µm, preferably 40µm. According to a further aspect of the invention, there is provided a tubular fibrous structure comprising small diameter fibres and substantially larger diameter fibres, said small diameter fibres being randomly oriented in the tubular structure, at least a portion of said larger diameter fibres being embedded in a matrix of small diameter fibres, and said larger diameter fibres being generally oriented circumferentially to the longitudinal axis of said tubular structure, said tubular structure having open areas or voids between the larger diameter fibres, said open areas or voids extending in the circumferential direction of the tubular structure whereby said structure has a low modulus in both compression and extension in the axial direction of said tubular structure. According to a further aspect of the invention, there is provided a method of electrostatically spinning a tubular fibrous structure using an electrostatically charged, spinning mandrel and an electrostatically charged grid means in the region of the mandrel to produce an electrostatic field, which method comprises the steps of introducing into said electrostatic field a fibreizable material, collecting on said mandrel a first portion of said liquid in the form of fibres attracted directly to the mandrel and a second portion of said liquid in the form of fibres having been deflected towards said grid means to follow a longer path to said mandrel than the fibres from said first portion of liquid, whereby a tubular fibrous structure is produced having fibres of different diameters and varying fibre orientations. At least some of said second portion of said liquid may contact the grid means. According to a still further aspect of the invention, there is provided a method of electrostatically spinning a tubular fibrous structure using an electrostatically charged, spinning mandrel and an electrostatically charged grid means in the region of the mandrel to produce an electrostatic field, which method comprises the step of selecting a desired potential for the mandrel, selecting a desired potential for the grid means, and introducing fibreizable material into the electrostatic field produced by the mandrel and the grid means whereby a tubular fibrous structure is produced having fibres of different diameters and varying fibre orientations. According to a still further aspect of the invention, there is provided a method of electrostatically spinning a tubular fibrous structure using an electrostatically charged, spinning mandrel and an electrostatically charged grid means in the region of the mandrel to produce an electrostatic field, and means for introducing fibreizable material into the electrostatic field, which method comprises the steps of arranging the grid means such that the electrostatically charged surface of the grid means nearest the fibreizable material introduction means lies no nearer thereto than the mandrel, selecting desired electrostatic potentials for the mandrel and the grid means, and introducing said fibreizable material into the electrostatic field, whereby a tubular fibrous structure is produced having fibres of different diameters and varying fibre orientations. The electrostatic spinning process may be started with the mandrel at a first mandrel voltage and the grid means at a first grid voltage and the mandrel and the grid voltages varied to cause a variation in the diameter and orientation of at least a portion of the fibres forming the tubular fibrous structure. The first mandrel voltage and the first grid voltage may be such as to produce fibres of a first diameter generally randomly orientated, and the mandrel and grid voltages may be varied by increasing the electrostatic charge on the grid means relative to the mandrel to produce fibres of a larger diameter than the first diameter as well as fibres of the first diameter, the fibres of larger diameter tending to be orientated generally circumferentially with respect to the longitudinal axis of the mandrel. The method may include a further step of returning the mandrel to the first mandrel voltage and the grid means to the first grid voltage for a period at the end of the process. The method may comprise the step of arranging and electrostatically charging the grid means such that a leading edge of the grid means lies forward of the mandrel with respect to the fibreizable material introduction means where fibres of said first diameter are required, and such that a leading edge of the grid means lies no nearer the fibreizable material introduction means than the mandrel when fibres of said second diameter are required. The grid means may comprise a first pair of coplanar grids or plates on one side of the mandrel and a second pair of coplanar grids or plates on the other side of the mandrel and in a plane parallel to the first pair of coplanar grids or plates, one grid or plate of each coplanar pair lying forward of the mandrel with respect to the fibreizable material introduction means, and the other grid or plate of each coplanar pair lying no nearer the fibreizable material introduction means than the mandrel, and the method may comprise the steps of electrostatically charging both grids or plates of each pair when fibres of said first diameter are required and electrostatically charging the second grid or plate only of each pair when fibres of a diameter larger than said first diameter are required. The mandrel may be charged to a voltage in the range 6 to 20kV, and an example of a preferred voltage for the mandrel, when of 4mm diameter, and the grid means is l2kV for the mandrel, 6kV for the grid means to produce fibres of the first diameter, 7kV for the mandrel, 9.2kV for the grid means when fibres of a larger diameter are to be produced. It will be appreciated, however, that these voltages are quoted merely by way of example as mandrel size, grid spacing and grid location have a fundamental effect on the voltage relationship between the mandrel and the grid means to produce different fibre production on the mandrel. The tubular fibrous structure may have different fibrous structures at different locations along its length, and this feature may be achieved by traversing the fibreizable material introduction means along the length of the mandrel and varying the electrostatic potentials of the mandrel and the grid means as the fibreizable material introduction means moves relative to the mandrel to produce different electrostatic fields for fibre collection and have different fibrous structures at different axial locations on the mandrel. This variation may be conveniently controlled by a microprocessor programmed to repeat a desired sequence of electrostatic charge variation. According to a further aspect of the invention, there is provided apparatus for electrostatically spinning a tubular fibrous structure, which apparatus comprises a mandrel to act as a collector for electrostatically spun fibres, means for electrostatically charging the mandrel and for varying the electrostatic charge thereon, means for rotating the mandrel, grid means in the region of the mandrel, means for electrostatically charging the grid means and for varying the electrostatic charge thereon, and means for introducing a fibreizable material into the electrostatic field, the surface of the grid means nearest to the fibreizable material introduction means lying no nearer thereto than the mandrel. The grid means may comprise a pair of parallel plates or grids arranged on each side of the mandrel. Leading edges of the plates or grids nearest the fibreizable material introduction means may be parallel, and the parallel leading edges may lie in a plane passing through or at least adjacent the mandrel. The apparatus may further comprise secondary grid means extending forwardly of the grid means with respect to the fibreizable material introduction means. The secondary grid means may comprise a pair of parallel plates. The apparatus may comprise means for electrostatically charging the grid means and the secondary grid means together, or the grid means alone. The apparatus may further comprise microprocessor means for controlling variation of the voltages on the mandrel and the grid means in accordance with a desired sequence to produce a tubular fibrous structure having desired characteristics. By way of example, one embodiment of a method of electrostatically spinning a tubular fibrous structure, of apparatus for carrying out the method and of a tubular fibrous structure will now be described with reference to the accompanying drawings, in which:- Figure l is a side view of apparatus according to the invention for spinning a tubular fibrous structure in a first mode of operation; Figure 2 is a side view illustrating the apparatus of Figure l in a second mode of operation; Figure 3 is a scanning electron microscope photograph of the microstructure of the surface of a tubular structure including fibres of different diameters; Figure 4 is a scanning electron microscope photograph of a section through the microstructure of a tubular fibrous structure having fibres of different diameters; Figure 5 is a graph of the change in diameter of a tubular structure having fibres of different diameters as a function of internal pressure; Figure 6 is a table of results of bending experiments on a fibrous structure having fibres of different diameter; and Figure 7 is a photograph of a fibrous structure having fibres of different diameters undergoing a bend test. Figures l and 2 illustrate apparatus for electrostatic spinning in two modes of operation. Figures l and 2 illustrate the apparatus diagrammatically. The general electrostatic spinning process has been described in several Patent specifications already published, for example earlier published British application Nos. 2l2l286A and 2l20946A. The apparatus consists of an array of capillary needles l0 mounted on a carrier (not shown) with means for moving the carrier in reciprocating fashion parallel to a mandrel ll. The means for moving the needle block is conveniently a motor. The mandrel ll in the embodiment illustrated is of 4mm diameter although it will be appreciated that other diameters may be used. There are means for rotating the mandrel at a variety of different speeds but typically the speed will be of several thousand revolutions per minute, a typical speed being 5000 r.p.m. The capillary needles l0 are supplied with polymeric material such as polyurethane or other suitable polymeric material in solution (although it is possible to use a suspension) and material emanating from the needles is attracted towards the mandrel ll by electrostatically charging the mandrel ll to a potential of several kilovolts with respect to the needles l0. The process results in the production of fibres which collect on the mandrel ll in a manner which has been described in specifications already published. It has already been proposed to have electrostatically charged grids on either side of the mandrel ll with respect to the needles l0 but the edges of the grids nearest the needles l0 have been significantly nearer the needles than has the mandrel. In the apparatus of Figures l and 2 pairs of plates l2, l3 and l4, l5 are provided. The plates l2, l3, l4 and l5 are conveniently of metallic sheeting although barred grids may be used if desired. In the mode of operation illustrated in Figure l, all four plates l2, l3, l4 and l5 are electrostatically charged to a voltage less than the voltage to which the mandrel ll is charged. Typical voltages for the spacing of the plates and the size of the mandrel are l2kV on the mandrel and 6kV on the plates l2, l3, l4 and l5. This results in an array of fibres leading from the needles l0 to the mandrel ll substantially as shown in Figure l and is much the same as known electrostatic spinning processes. However, by changing the mode of operation to that shown in Figure 2, the fibrous structure formed on the mandrel ll changes substantially. To produce the shape of fibre array emanating from the needles l0 shown in Figure 2, the forward plates l2 and l4 of the two pairs of plates are no longer electrostatically charged but are at the same potential as the needles l0. The rearward plates l3 and l5 having parallel leading edges which lie in a plane passing through or at least adjacent the mandrel ll are charged to a higher voltage than was the case in Figure l and the mandrel voltage is reduced. This causes fibres to be attracted more readily towards the rearward plates l3 and l5 and thence to the mandrel ll. Some fibres will be attracted directly to the mandrel ll. Fine adjustment of the voltages is required as if the plates l3 and l5 are at too high a potential, material from the needles will simply be attracted to the plates l3 and l5 and not necessarily reach the mandrel ll whereas if the mandrel voltage is too high relative to the plates l3 and l5, the fibrous structure will not differ significantly from that caused by the mode of operation described with reference to Figure l. Typical voltages for the Figure 2 mode of operation are 9.2kV on the plates l3 and l5 and 7kV on the mandrel ll. It will be appreciated, however, that the voltage relationship between the mandrel ll and the plates l3 and will differ depending on the plate size and spacing, the mandrel diameter and the spacing of the needles l0 from the mandrel ll and the plates l2, l3, l4 and l5. The fibrous structure produced in the mode of operation described with reference to and shown in Figure 2 differs significantly from the fibrous structure produced by the mode of operation described with reference to and shown in Figure l. In the Figure l mode of operation, the fibres are generally in the range 0.5µm to 2µm, and mostly of approximately lµm diameter, and are generally randomly orientated. In the Figure 2 mode of operation, however, while some fibres of diameter approximately lµm (in the range 0.5µm to 2µm) are still formed on the mandrel ll and their orientation random, fibres of a larger diameter are also produced, probably from the fibres whose path length to the mandrel ll is lengthened because of the different electrostatic field. Fibres up to a diameter of l5µm may be produced and, in general, the larger the fibre diameter, the more likely is that fibre to be arranged circumferentially with respect to the longitudinal axis of the mandrel ll. The larger diameter fibres are, in effect, more likely to be wound around the mandrel ll leading to circumferential orientation. Figure 3 illustrates the type of microstructure produced in the Figure 2 mode of operation, it being apparent that fibres of a diameter of approximately lµm extend randomly in direction over the structure whereas fibres of larger diameter extend generally circumferentially. It has also been found that the microstructure produced in the mode of operation of Figure 2 includes elongate voids in the structure which generally extend circumferentially with respect to the longitudinal axis of the mandrel ll and the tubular fibrous structure formed on it. Figure 4 illustrates the presence of the voids l6, Figure 4 being a scanning electron microscope photograph of a section through the microstructure. The presence of the voids l6 in the microstructure assists in resisting kinking of the structure upon bending. It will be appreciated that tubular fibrous structures having varying fibrous structures across their cross-section can be produced readily by altering voltages at the mandrel ll and plates l2, l3, l4 and l5 and by altering the mode of operation from the Figure l mode to the Figure 2 mode. It is also possible to feed solutions of different polymers to the needles l0 where different material characteristics are required. Thus a polymer having desirable characteristics for contact with blood and which may also be doped with, for example, a pharmaceutical can be introduced first of all in the Figure l mode to build up an inner layer for example of l0 to 60µm, preferably 40µm for contact with blood. Thereafter, simply by changing the polymer emanating from the needles l0 and the operational mode to that of the Figure 2 mode, the fibrous structure can be altered. Typically, the layer of the structure formed by the Figure 2 mode of operation would be between 300 and 2000µm thick. Finally, the Figure l mode could be used for an outer layer of perhaps l0µm to 60µm, preferably 40µm of the microstructure including fibres generally of lµm diameter randomly orientated. Simply changing voltages can thus be used to create changes in the fibrous structure across the cross-section of the tubes and, once a desirable characteristic for a tube has been achieved, repeats can be readily made by microprocessor control. Furthermore, it will be appreciated that tubular fibrous structures can be made with different characteristics along the length of the tubular structure by altering voltage or other characteristics as the needles l0 traverse along the mandrel ll. This too can readily be controlled by a microprocessor. A composite structure as described above including at least a layer of fibres including randomly orientated fibres of approximately lµm diameter together with larger fibres tending to be orientated circumferentially around the tube has the appearance of a lightly ridged, flexible hose. The structure offers little resistance to bending and will assume a very tight loop without kinking. It has greater axial compliance than a similarly dimensioned tubular fibrous structure made entirely in the Figure l mode of operation but, most importantly, when compressed along its axis, it shortens with a minimal tendency to buckle. Mechanical tests have been carried out to quantify these features of the graft. COMPARISON OF HOOP-AXIAL MODULI A simplified tensile test was made on specimens cut from a tubular structure made in the Figure 2 mode of operation and from a tubular structure made from a Figure l mode of operation. The Young's modulus of the "dogbone" shaped specimens was measured and the results are given below:- hoop modulus - Fig. l mode 5.4 × l0⁵ Pa hoop modulus - Fig. 2 mode 20.l × l0⁵ Pa axial modulus - Fig. l mode 9.7 × l0⁵ Pa axial modulus - Fig. 2 mode 2.8 × l0⁵ Pa Compared with a Fig.l mode structure, the circumferential modulus of a Fig. 2 mode structure is increased by a factor of four and the axial modulus is decreased by a similar factor. This shows that the preferential fibre alignment in the Fig. 2 mode structure significantly changes the hoop/axial ration of moduli which explains in part the good bending properties. These results are consistent with the fibre orientation observed by scanning electron microscope for a Fig. 2 mode structure. The static compliance of one of the Fig. 2 mode structures has been measured using a specimen 7cm in length, 3.7mm bore and 0.57mm wall thickness. The structure was pre-clotted with gelatin prior to being tested and was subjected to l5% axial strain. The internal pressure was ramped to 2000mm Hg and slowly brought back to zero, during which the change in diameter was repeatedly measured. The experiment was repeated at 20% and 25% axial strain. A graph of the change in diameter as a function of internal pressure at l5% axial strain is shown in Figure 5. The graph is essentially linear with only a small hysteresis. The compliance was calculated by measuring the external diameter at l20mm Hg and subtracting it from the diameter at 80mm Hg. This figure was divided by the diameter at l00mm Hg. The static compliance for each of the three experiments was:- at l5% extension 0.66% compliance at 20% extension 0.73% compliance at 25% extension 0.79% compliance AXIAL EXTENSION UNDER PRESSURE Owing to the decrease in the axial (longitudinal) Young's modulus of the Fig. 2 mode structure, the specimens will elongate when subjected to internal pressure. This extension was measured for a Fig. 2 mode structure and for a Fig. l mode structure of similar dimensions, the dimensions being increases in length by l5% compared with less than 4% for a wholly microfibrous, standard specimen made in the Fig. l mode. A l5% increase is thought to be within clinically acceptable limits. BENDING CHARACTERISTICS OF THE FIG. 2 MODE STRUCTURE. The bending characteristics of two typical specimens have been determined at zero and 80mm Hg internal pressure. The measurements are compared with results from Fig. l mode structures of similar dimensions. The term "bending diameter" describes the diameter of the smallest circle around which an unsupported structure will bend without kinking. Results of the bending experiments are set out in the table of Figure 6. The mean bending diameter of a Fig. l mode structure is 8cm. A structure of similar gross dimensions but modified by the inclusion of circumferentially aligned macrofibres by the Fig. 2 mode of operation is l.28cm. Figure 7 is a photograph of a similar specimen being tested and illustrates the advantageous bending characteristics. When the specimens were tested under an imposed internal pressure of 80mm of mercury, the mean bending diameters were 5.4cm and l.0cm respectively. It will be appreciated that the apparatus described and the types of structure made can be varied significantly depending on the desired characteristics. It will, of course, be understood that the present invention has been described above purely by way of example and modifications of detail can be made within the scope of the invention.

Claims: 1. A tubular fibrous structure comprising small diameter fibres and substantially larger diameter fibres, said smaller diameter fibres being randomly oriented in the fibrous structure, said larger diameter fibres being embedded in a matrix of said small diameter fibres and said larger diameter fibres being generally oriented circumferentially with respect to the longitudinal axis of said tubular structure. 2. A tubular fibrous structure as claimed in Claim l including a multiplicity of elongate voids extending generally circumferentially with respect to the longitudinal axis of the structure. 3. A tubular fibrous structure as claimed in Claim l or Claim 2 wherein the small diameter fibres have a diameter in the range 0.5µ to 2µm. 4. A tubular fibrous structure as claimed in any one of Claims l to 3 wherein the larger diameter fibres have a diameter in the range 2µm to l5µm. 5. A tubular fibrous structure as claimed in any one of Claims l to 4 wherein the ratio of the diameters of the larger diameter fibres to the diameters of the small diameter fibres is less than 30:l. 6. A tubular fibrous structure as claimed in any one of Claims l to 5 including at least one layer of fibres all of said small diameter. 7. A tubular fibrous structure as claimed in Claim 6 having a layer of fibres all of said small diameter on the inside of the structure. 8. A tubular fibrous, structure as claimed in Claim 7 wherein the layer of fibres of said small diameter has a thickness in the range l0µm to 60µm. 9. A tubular fibrous structure as claimed in Claim 8 wherein the layer of fibres of said small diameter has a thickness of 40µm. l0. A tubular fibrous structure as claimed in any one of Claims 6 to 9 having a layer of fibres of said small diameter on the outside of said structure. 11. A tubular fibrous structure as claimed in Claim l0 wherein the outside layer has a thickness in the range l0µm to 60µm. 12. A tubular fibrous structure as claimed in Claim ll wherein the outside layer has a thickness of 40µm. 13. A tubular fibrous structure as claimed in any one of Claims 6 to l2 wherein the larger diameter fibres in the matrix of said small diameter fibres lie in a layer of thickness in the range 300µm to 2000µm. 14. A tubular fibrous structure wherein at least an axial portion thereof is a tubular fibrous structure as claimed in any preceding Claim, the complete tubular fibrous structure having been made continuously by electrostatic spinning. 15. A tubular fibrous structure comprising small diameter fibres and substantially larger diameter fibres, said small diameter fibres being randomly oriented in the tubular structure, at least a portion of said larger diameter fibres being embedded in a matrix of small diameter fibres, and said larger diameter fibres being generally oriented circumferentially to the longitudinal axis of said tubular structure, said tubular structure having open areas or voids between the larger diameter fibres, said open areas or voids extending in the circumferential direction of the tubular structure whereby said structure has a low modulus in both compression and extension in the axial direction of said tubular structure. 16. A tubular structure according to Claim l5 wherein the inner surface of the structure is predominantly small diameter fibres to provide a smooth inner surface. 17. A tubular structure according to Claim l5 or Claim l6 wherein the outer surface of the structure is predominantly larger diameter fibres and voids. 18. A tubular structure according to Claim l5 or Claim l6 wherein the open spaces or voids are disposed on the outer surface of the structure. 19. A tubular structure according to any one of Claims l5 to l8 wherein the larger diameter fibres form circumferential reinforcing hoops, said hoops being disposed in spaced apart relationship along the length of the graft. 20. A tubular structure according to any one of Claims l5 to l9 wherein the inner surface of the structure is relatively smooth and the outer surface of the structure has circumferentially disposed spaced apart ridges. 2l. A tubular structure according to any one of Claims l5 to 20 wherein the open spaces or voids have a tapered depth with the wide portion of said spaces or voids being on the outer surface of the tubular structure. 22. A method of electrostatically spinning a tubular fibrous structure using an electrostatically charged, spinning mandrel and an electrostatically charged grid means in the region of the mandrel to produce an electrostatic field, which method comprises the steps of introducing into said electrostatic field a fibreizable material, collecting on said mandrel a first portion of said liquid in the form of fibres attracted directly to the mandrel and a second portion of said liquid in the form of fibres having been deflected towards said grid means to follow a longer path to said mandrel than the fibres from said first portion of liquid, whereby a tubular fibrous structure is produced having fibres of different diameters and varying fibre orientations. 23. A method of electrostatically spinning a tubular fibrous structure as claimed in Claim 22 wherein said fibres from said second portion of said liquid are coliected on said mandrel after at least some of said second portion has contacted said grid means. 24. A method as claimed in Claim 22 or Claim 23 wherein the fibres from said first and second portions are collected simultaneously. 25. A method of electrostatically spinning a tubular fibrous structure using an electrostatically charged, spinning mandrel and an electrostatically charged grid means in the region of the mandrel to produce an electrostatic field, which method comprises the step of selecting a desired potential for the mandrel, selecting a desired potential for the grid means, and introducing fibreizable material into the electrostatic field produced by the mandrel and the grid means whereby a tubular fibrous structure is produced having fibres of different diameters and varying fibre orientations. 26. A method as claimed in Claim 25 wherein the grid means is selected to have a higher potential than the mandrel. 27. A method of electrostatically spinning a tubular fibrous structure using an electrostatically charged, spinning mandrel and an electrostatically charged grid means in the region of the mandrel to produce an electrostatic field, and means for introducing fibreizable material into the electrostatic field, which method comprises the steps of arranging the grid means such that the electrostatically charged surface of the grid means nearest the fibreizable material introduction means lies no nearer thereto than the mandrel, selecting desired electrostatic potentials for the mandrel and the grid means, and introducing said fibreizable material into the electrostatic field, whereby a tubular fibrous structure is produced having fibres of different diameters and varying fibre orientations. 28. A method as claimed in any one of Claims 22 to 27 further comprising the steps of starting the electrostatic spinning process with the mandrel at a first mandrel voltage and the grid means at a first grid voltage, and varying the mandrel and grid voltages to cause a variation in the diameter and orientation of at least a portion of the fibres forming the tubular fibrous structure. 29. A method as claimed in Claim 28 wherein the first mandrel voltage and the first grid voltage are such as to produce fibres of a first diameter generally randomly orientated, and the mandrel and grid voltages are varied by increasing the potential of the grid means relative to the mandrel to produce fibres of a larger diameter than the first diameter as well as fibres of the first diameter, the fibres of the larger diameter tending to be orientated generally circumferentially with respect to the longitudinal axis of the mandrel. 30. A method as claimed in Claim 28 or Claim 29 comprising a further step of returning the mandrel to the first mandrel voltage and the grid means to the first grid voltage for a period at the end of the process. 3l. A method as claimed in Claim 29 or Claim 30 as dependent on Claim 29 comprising the step of arranging and electrostatically charging the grid means such that a leading edge of the grid means lies forward of the mandrel with respect to the fibreizable material introduction means where fibres of said first diameter are required, and such that a leading edge of the grid means lies no nearer the fibreizable material introduction means than the mandrel when fibres of said second diameter are required. 32. A method as claimed in Claim 3l where the grid means comprise a first pair of coplanar grids or plates on one side of the mandrel and a second pair of coplanar grids or plates on the other side of the mandrel and in a plane parallel to the first pair of coplanar grids or plates, one grid or plate of each coplanar pair lying forward of the mandrel with respect to the fibreizable material introduction means, and the other grid or plate of each coplanar pair lying no nearer the fibreizable material introduction means than the mandrel, which method comprises the steps of electrostatically charging both grids or plates of each pair when fibres of said first diameter are required and electrostatically charging the second grid or plate only of each pair when fibres of a diameter larger than said first diameter are required. 33. A method as claimed in any one of Claims 22 to 32 wherein the mandrel is charged to a potential in the range 6kV to 20kV. 34. A method as claimed in any one of Claims 22 to 33 wherein, for a mandrel of 4mm diameter, the mandrel is charged to 7kV and the grid means to 9.2kV to produce fibres of different diameters. 35. A method as claimed in any one of Claims 22 to 34 comprising the step of traversing the fibre introduction means along the length of the mandrel and varying the electrostatic potentials of the mandrel and the grid means as the fibreizable material introduction means moves relative to the mandrel to produce different electrostatic fields for fibre collection and hence different fibrous structures at different axial locations on the mandrel. 36. Apparatus for electrostatically spinning a tubular fibrous structure, which apparatus comprises a mandrel to act as a collector for electrostatically spun fibres, means for electrostatically charging the mandrel and for varying the electrostatic charge thereon, means for rotating the mandrel, grid means in the region of the mandrel, means for electrostatically charging the grid means and for varying the electrostatic charge thereon, and means for introducing a fibreizable material into the electrostatic field, the surface of the grid means nearest to the fibreizable material introduction means lying no nearer thereto than the mandrel. 37. Apparatus as claimed in Claim 36 wherein the grid means comprises a pair of parallel plates or grids arranged one each side of the mandrel. 38. Apparatus as claimed in Claim 37 wherein leading edges of the plates nearest the fibreizable material introduction means are parallel. 39. Apparatus as claimed in Claim 38 wherein the parallel leading edges lie in a plane passing through or at least adjacent the mandrel. 40. Apparatus as claimed in any one of Claims 36 to 39 further comprising secondary grid means extending forwardly of the grid means with respect to the fibreizable material introduction means. 4l. Apparatus as claimed in Claim 40 wherein the secondary grid means comprise a pair of parallel plates. 42. Apparatus as claimed in Claim 40 or Claim 4l comprising means for electrostatically charging the grid means and the secondary grid means together, or the grid means alone. 43. Apparatus as claimed in any one of Claims 36 to 42 comprising microprocessor means for controlling variation of the voltages in accordance with a desired sequence to produce a tubular fibrous structure having desired characteristics.

Inventors: BERRY, JOHN PHILLIP
Applicants: ETHICON INC., THE UNIVERSITY OF LIVERPOOL
Application year: 1986
Main classification: D04H1
IPC codes: D01F6, A61F2, D01D5, D04H1, D04H3