CA1103867A - Vascular prosthesis produced from electrostatically spun fibres - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A vascular prosthesis comprising a mat of fibres of 0.1 to 10 micron diameter prepared by electrostatically spinning an organic material and collecting the spun fibres on a suitable receiver, the prosthesis comprising a tubular portion having a bore of internal diameter of the order of 0.3 to 3 cm. and a porous component having a pore size of between 0.001µ and 500µ.

Description

~1~3~7 Thls lnvention rela~es ~o the ~roduction of fibrillar products by the electros~atic spinning of organic ma'erials~
The technique of e.lectrostatic splnn.ing of li~uids, i.ncludlng solutions containing a fihre-forming material, i~c~ known and has ~)een descrlbed in a number o patents as well as in the general literature.
The process oE electrostatic spinniny involves the introduction of a liquid into an electric f.ield, whereby the liquid is caused to produce ~ibres which tend ~o be drawn to an electrode. While being drawn from the liquid the fibres usually harden, which may lnvolve mere cooling (where the liquid is normally solld at room temperature, for ex~mple), chemical hardening tfor lS exam~le by treatment with a hardenlng vapour) or evapor~
ation oE solvent (for example by dehydration). The product fibres may be collected on a suitably loca~ed receiver and subsequently stripped from it~
The flbres obtained by the electrostatic spinning process are thin, of the order of 0.1 to 25 micron, preferably 0.5 to 10 micron and more preferably 1.0 to 5 micron ln diameter.
We have found that the fibres, if collccted to form a mat of appropriate thickness may~ because of the inherent poroslty o the mat so obtained, provide a non-woven material having a wide variety of applications, depending upon the composition of the f~bres, their density of deposition, their diameter, and their in-.

~33~7~7 hexent strength, and the thickness and shape of the mat.
` I-t is also possible ~o post-treat such mats with other materials to modify th~ir properties, for example to increase their strength or wa~er resistance.
Flbres havirlg different properties may be obtained by adjustLng their composition either by spinning a liquid containing a plurality of components, each of which may contribute a desired characteris~ic to the finished product, or by simultaneously spinning from diferent liquid sources fibres of different composition which are simultaneously deposited to form a mat having ~an intimately intermingled mass of fibres of diffe~ent materialO A further alternative is to produce a mat having a plurality of layers of different fibres (or fibres of the same material but with different charac-teristics e.g. diameter) depo~ited, say, by varying with time the fibres being deposited upon the receiving surface.
One way of effècting such a variation, for example, would be to have a moving receiver passing in succession sPts o~ spinnerets from which fibres are being electro-statically spun, sald fibres being deposited in succession as the receiver reaches an appropriate location relative to the spinnerets.
r~hus, the present invention provides a mat comprislng a plurality of flbres of organic material, said fibres being obtained by electrostatic ~pinning from a liquid comprising the material or a ~recursor thereof.

33~
Within the -term mat we include deposits of electro-statically spun f~bres in the ~orm oE three dimensional as well as two di~ensional articles.
According to one embodiment of the present invention we provlde a shaped mat of electrostatically spun fibres in a form ~ppropri.ate for use as a wound dressing.
A particular advantage o-E the use of materials made from the electrostatically spun fibres is that the fibres may be of very small diameter, to give a mat with small interstices and consequently a high surface area. Where the dressing is formed from a wettable polymer, blood or serum escaping from the wound tends to penetrate the dressing and the high surface area encourages clotting. Such dressings may be used as emergency dressings to halt bleading. As examples of suitable polymers we may mention polyurethanes. Where the dressing is formed from a non-wetting polymer a particular advantage is that if the interstices between the fibres are sufficiently small, averaging, say l to 100 l~, tissue fluids, including blood, tend not to permeate the dressing, so that the fluids are retained adjacent to the wound, where clotting will occur.
Subsequent r moval of such a dressing is facilitated by the absence of blood clot permeating the drsssing material. Furthermore such dressings have the advan~age that they are usually sufficiently porous to allow interchange of oxygen and water vapour between the atmosphere and the surface of the wound. As examples of .

'7 sul~able non-we-tting polymers we may mention saturated polyesters e.g. polyethylene terephthalate, fluorinated compounds~ particularly fluorinated hydrocarbons, e.g.
PTF~ and silicones.
Such dressin~s may, o;E course, be associàted with suitable supports or reinforcement, with mats o~, say, woven fib.res whic~l may ~lave other desirable properties, c)r with surface or othe.r treatment w.~th materials having antiseptic or wound-healing properties. Blood clotting, for example, may be encourayed by incorporating clotting accelerators or inducers in or on the mat and/or on materials associated therewith ln a wound dressing.
Other components with which the mat may he associated include water-proof layers intended to protect the mat from undesirable effects of moisture, dirt etc.
Preferabl.y the wound dresslng of the invention com-prises a mat of flexible non-absorbent, porous, hydrophobic material, together with a non absorbent backing layer.
Such a backlng layer is preferably made of hydrophobic material, but this is not essential. The dressing may also include means for applying pressure to the mat Such means may be, for example, a stretchable elastic bandage~
In a preferred embodiment the dressing.comprises a bac~ing layer, one s~lrface of which has an adhesive - facing, and on the same surface of the backing strip a porous mat of the material of the invention; optionally a pad of absorbent or non-absorbent material is located between the backing layer and the mat, , `

Non-adheren-t dr~sslngs according to the invention have been tested for efficacy by applying them to the surface of a woun~ on a rabbit involving loss of an area of full thickness skin, and observing the progress of healing in comparison with control wounds. A dressing made from the preferxed material allowed normal healing with little or no seepage of fluid through the dressing and minimal adherence o the dressin~ to the scab.
According to a further embodiment of the invention we provide a shaped mat comprising electrostatically spun fibres in the form of a lining or surfacing to a component-which may be in contact with body fluids such as blood and lymph~ Such mats may be tubular, or of irregular shape.
The difficulty of development of satisfactory blood and body tissue compatible surfaces on, say, the walls of artificial hearts and other circulatory assisting devices, as well as compatible linings to damaged nat~ral as well as artificial blood vessels, represents an obstacle to the development of safe artificial organs and tissues. We have found that the deposition upon the surfaces of such artificial organs and tissues of a lining of thin fibres of appropriate material may improve their blood and other ~issue fluid compatibility. It is desirable for this purpose, however, that the lining be very thin and the use of an electrostatically deposited fibrous coatlng has been shown to meet ~any of the critical requirements. The - primary desiderata includP
(a) very small fibre diameters (small in relation to cell dimensicns), so that fibre diameters of 0.1 micron to lO micron, and partic~,~larly 0.5 to 5 micron are particularly appropriate.
(b) rrhe lininy should be sufficiently porous to allow penetration of cells into it; ldeally therefore the average pore dimensLon should be of the order of 5 to 25 micron, preEerably 7 to 15 micron.
(c) The lining should preferably be of the order of , lO to 50 micron,in thickness.
- ~d) The lining should be capable of being bonded to the article to whlch it consti~utes a lining by some suitable means not involving a process destructive of ~he properties indicated above.
(e) The lining should contain no materials harmful to the body or to the body cells Qr ~luids which,may come into contact with it.
The technique of,electrostatlc spinning provides a ?O method of forming such linings to accord perfectly with the dimensiohs and contours of the articles to be coated by making the surface of the article or positive , or negative replica thereof the collector ln an electro-static spinning proces3.
~aterials suitable for the preparation of such linings include polymeric substances~and in particular inert, polymeric substances. As preferred substances we would mention fluorl~a~od hydrocarbons-, e.g. PTFE which -conveniently may be spun from a dispersion of the material in a suitable dispersing agent, and poly-urethanes which may be spun from solution.
In some applications the mat may be strong enough, or may be spun thicX enough, to be used without a supporting article i.e. it will no-t properly be described as a lining. Thus self-supporting tubular devices may be electrostatically spun; Eor exarnple vascular prosthetics may be prepared rom polytetrafluoroethylene or fxom polyurethanes.
The electrostaticaily spun products, for example tubes or other shaped items, may as described above, be of sufficient strength to be employed as such, without reinforcement. However, it is usually preferred that the material is reinforced, for example by applying to one surface of the product a reinforcing Iayer, which ` itself may be electrostatically spun, or by incorporating reinforcement within the wall of the product itself.
Thus, we have reinEorced elèctrostatically spun products by incorporating within the wall thereof a web wh1ch may be woven or non-woven, or an alternative arrangement of fibres. We particularly prefer to employ as rein-forcement a helix of suitable ibre, said helix being located within the walls of a tubular product comprising electros~atically spun fibrous mate~ial~ Although it is usual to enclose the reinforcement within the wall material we do not exclude the possibility of applying it to a surface of the product where its presence will ' .' ~ ' -$~ t7 not ~e disadvantageous, The thickness of reinforcement will be influenced inter alia by the ~hickness o the mat the location of the reinforcement and the reinforce-ment strength required. In general ~he thickness of the relnforcement will be less than ~hat of the mat, although ~here thl3 relnEorcement lie3 at a surface of the mat ancl may p~oject therefrom ths thickness of the reinforcement may be thicker,than that of the mat.
Generally the thickness of' the reinforcement (or of reinforcing flbres) will be of the order of o.l to lO times - ' the thickness of the mat preferably 0.2 to 0.8 times.
Sui~able reinforcing materials includP metallic, : polymeric or qlass fibre. Such electrostatically spun tubes and other prosthetics have the advantage over tubes used hitherto in this application in generating a thinner layer of encapsulating natural tissue so that smaller - dlameter tubes may be us0d without the tube becomi~g clogged by natural tissue.
The mats according to the present invention may be spun from a solutlon of or a dispersion of a polymer or its precursors. Polymers which may be conveniPntly spun from solution ihclude high molPcular'weight flbre forming thermoplas~ics; in particular we would mention poly-urethane~ polyamid~s and polyacrylonitrile. Polymers Z5 whlch may conveniently be spun from dispersion include polytetrafluoroethylene and polyesters as well as those listed above. ~s an example of a polymer preeursor which may be spun f rom solution we mention urea f ormaldehyde _ 9 _ which may be cross-linked subsequent to spinning by treatment with acid vapour Water soluble polymers, e g. polyvinyl alcohol, polyviny.l pyrrolidone, and polyethylene oxide, may be spun from aqueous solution While we do not exclude the possibility that mats prepared .Erom such materials may be used as prepared, preferably such mats are given at least a degree of insolubility in aqueous medium e.g. by cross-linking with a suitable reagent.
Where the mats are spun from a dispersion the spinning material comprises preEerably also an additional component which acts to enhance the viscosity of the suspension and to improve its fibre forming properties Most convenient for this purpose, we have found, is an additional organic polymeric material which subsequent to fibre formation, can, if desired, be destroyed during sinter-ing.
The spinning material, then, is a solution or suspension which comprises an organic polymer which is capable of forming a fibre and has cohesion properties such that the fibre form is retained during any post fibreization hardening until the fibre has hardened sufficiently not to lose its fibrous shape on detachment from a support where -this is appropriate.
Where mats are spun from solution they comprise point bonded fibres and are often strong enough for use without any further treatment.

.';, ~33~

Where mats are spun from dispersion they oten have a tendency to be friable,being mere agglomerations of discrete particles held together in the form of fibres by the additional organic polymeric component presen-t. P~eerably such mats are sintered so that the particles soten and flow into each other and the ~ibres may become point bonded. In the case of PTFE
sintering may con~eniently be carried out between 330C
and 450C, preferably be'tween 370C and 390C~
Sterilisation may proceed concurrently during the sintering process. The sin~ering temperature in the case of PTFE is usually suf~iciently high to destroy compl'etely,any undes,irable organ~c component in the final product e,g~ an additional organic polymeric ma-terial added solel~ to enhance the viscosity, or an emulsify-ing agent added to facilitate dispersion.
The additi.onal organic compcnent need be employed only in a relatively small proportion (usually within the range 0.001 to 12% and preferably 0,01 to 3%) by weiyht of the suspension, although the precise concen-tration for any particular applicakion can easi1y be determined by trial~ .

The degree of polymerisation of the additional organic component is preferably greater than about 2000 units linearly; a wi~e range of such polymers is available.
An important requirement is solubility of the polymex in the selected solvent,or suspending medium which is ~referably water. As examples o water-soluble polymeric compounds we may mention polyethylene oxide, poly-~ 3~7 acrylamide, polyvinyl pyrrolidone and polyvinyl alcohol; where an organic medium is employed to prepare the spinning material, either as the sole llquid solvent or as a component thereof, a further wlde range of organic polymeric compounds is available, for example polystyrene and polymathylmethacrylate.
~he degree of polymeri~ation of the polymer will be selected in the light of required solubility and the ability of the polymer to impart the desired properties of coheslon and vlscosity to the fibreizable liquid.
We have found that generally the viscosity of the fibreizable liquid whether due solely to the presence of the fibreizable polymer or partly contributed to by the additional oryanic polymer should be greater than O.l but not greater than 150 poise. Preferably it is between 0.5 to 50 poise and more preferably between l and lO poise, (vlscosities being measured at low shear rates). The viscosity required using a given additional organic polymèr will vary with the molecular welght o the polymer, i.e. the lower the molecular weight the higher the final viscosity needed. Again, as the molecular weight of the polymer is increas d a lower concentration of lt is required to give good fibreization. Thus, as examples we would mention that ln the preparation of polytetrafluoroethylene mats we have found that using a polyethylene oxide of MW lO0,000 as ~he additional organic polymer a concentration-of about 12% by weight relative to the PTFE content is 33~7 neeaea to glve satisfactory fisre zatiGn, whereas w.ith a MW of 300,C)OO a concentration OL l to ~ may be adequate. Again, at a MW of 600,000 a concentration of l to 4~ is sat-~sfactory, whiie at a MW of 4 x lO a concentration as low as 0.2~ may giV2 good fibreization.
The concentration of the fibrei~.able polymer will depend upon the amour1t required to provide adequate fibre propexties, and wl1l be influenced also by the~
need to produce a liquid of appropriate vi.scosity and speed of fibre hardening. Thus in the case of a dispersion we may use a concentration within the range 25% w/w to saturation, (in the case of a dispersion, 'saturation' means the maximum concentration which may be included without destroying the useful spinnability of the liqu1d) preferably 40 to 70~ and more preferably 50 to 60%, and in the case of a solution we` may use a concentration within the range lO to 60% w/w, preferably . 20 to 35% w/w.
It will be appreciated that the concentrat1on of the components m~st each be adjusted to ta~e account of the presence and concentration of any other and their relat1ve effects upon v'scosity~
The spinning material should have some electrical conduc-tivity, although this may vary between quite wide limits; for example we prefer to employ solutions having conductivity w1thin the range l x lO 6 to 5 x lO 2 mhos cm Any convenient method may be employed ~o bring the spinning ma~erial into the electrostatic field, for example we have supplied the spinning liquid to an appropriate position in the electrostatic field by feeding it to a nozzle Erom which i~ is drawn by the field, whereupon fibreization occurs. Any suitable apparatus can be employed for this purpose; thus we have fed the spinning material from a syrinqe reservoir ; to the tlp o an earthed syringe needle, the tip being located at an appropriate distance from an electrostatically charged surface. Upon leaving the needle the material forms fibre between the needle tip and the charged surface.
Droplets of the spinning liquid may be introduced into the field in other ways, which will be apparent to the skilled man, the only requirement being that they can be held within the field at a distance from the electrostatically charged surface such that fibreization occurs. For example they could be carried into the field on, say, a continuous carrier, e.g. a metal wire.
It will be appreciated that where the liquid is fed into the field through a nozzle, several nozzles may be used to increase the rate of flbre production.
Alternative means of bringing the fibreizable liquid into the charge field may be employed, for example a perforated plate (the perforations being fed with fibreizable liquid from a manifold) may be employed.

In one embodiment the surface to which the ~ibres are drawn is a continuous surface, as of a drum, over which passes a belt which may be withdrawn from the region of charge, carrying with it the fibres which have been formed and which have become attached thereto. Such an arranyement is shown in the attached drawings. In order that the invention may be better understood several embodiments will now be numbered by way of example only with reference to the accompanying drawings in which Figure 1 is a diagrammatic side view of apparatus for the continuous production of fibxes; Fi~ures 2 and 3 are perspective views of spun fibre collective surfaces; and Figure 4 is a pexspective view part in cu-t-away section of a wound dressing.
In Figure 1, 1 is an earthed metal syringe needle supplied from a reservoir with spinniny material at a rate related to the rate of fibres production, ~elt 2 is of gauze driven by a driving roller 3 and an idler roller ~ to which is fed an electrostatic charge from a generator 5 ~in the apparatus illustrated a Ven de Graaff machine).
Removal of the fibre mat 6 from belt 2 is by any convenient means, for example by suction or by air jet, or it may be removed by juxtaposîtion of a second belt, or a second roller~ Preferably it îs cut and liited of, In the Figure the mat is shown being picked u/p by a roller 7 rotating against the belt~
The optimum distance of the nozzle from the charged surface is determined quite simply by trial and error, We have found, for example, that using a potential of the order of 20 Kv a distance of 5-35 cm is suitable, but as the charge, nozzle dimensions, liquid flow rate,charged surface area etc, are varied ~

so the optimum dlstance may vary, and lt is most convenien-tly determined as describedO
Alternative methods of fibre col.lection ~hich may be employed lnclude the use of a lar~e rotating cylindrical collectins surface substantially as described, the fi]:~res being collected from another point on the surf~lce by a non-electrlcally conducti.ng pick-up means insl:ead of being carried away on the belt .[n a further embodiment the electrostatically charged surface may be the sides of a rotatiny tube, the tube being disposed coaxially with the nozzle and at an appropriate distance from it. Alternatively deposition of fibres and the ormation o a tube may occur on a cylindrical former. The former may be made from any of a variety of materials. A metallic former is preferred and alum.inium is particularly preferred. The tube may be removed from the former by a variety of methods. In particular it may be mentioned that a polyurethane tube is preferably peeled from an aluminium former while an aluminium former may be dissolved in sodium hydroxide solution to obtain a PTFE tube. To facilitate peeling the polyurethane tube fr?m the aluminium former, the latter may be conveniently covered with a layer of flexible polyurethane foam.
The electrostatic potential employed will usually be within the range 5 Kv to 1000 Kv, conveniently 10 - 100 Kv and preferably 10-50 Kv. Ary appropriate method of producing the desired potential may be . . , em~loyed. ~hus, we illustrate tne use of a conventional ~an de Graaff machine in Fiqure 1 bus other commexcially available and more convenient device:, are known and may ; be suitable.
It is, of course, important tha the electrostatic charge is not conducted from the charged surface and where the charged surface is contact~d with ancillary equipment, for example a fibre collecting belt, the belt should be made of a non-conducting material lo (although it must not, of course, insulate the charged plate from thematerial to be fibreized. We have found it convenient to use as the belt a thin Terylene (RTM) ¦ net of mesh size 3 mm wide). Obviously all supporting means, bearing etc. for the equipment will be insulated ~ 15 as appropriate. Such precautions will be obvious to ! the skilled man.
To allow hi~h production rates, hardening of the fibres should occur rapidly and this is facilitated by the use of concentrated fibreizing liquids (so that the r,inimum liquid has to be removed), easily volatile solvents ~or e~ample the liquid may be wholly or partly o, low boiling organic liquid) and relatively high temperatures in the vicinity of th~ fibre formation.
The use of a gaseous, usually air, blast, particularly ~i -25 using warm gas wil;l of~en accelerate hardening `~
t of the fibre. Carsful direction of the air blast may also be used to cause the flbres, after detachment, to lie ~n a desired position or direction. However, using '!, .', , ~L~.'t3~

conditlons as described in ~he Examples no particular precautions were needed to ensure ra~id hardenin~.
We found that durincJ its formation and travel from the nozzle to the bel1: sufficient hardenLng ~dehydration S in the case described) occurredat amDient temperature without the need Eor auxillary hardenlng treatment.
Mats preparecl according to the present invention may be between a few microns and a few centimetres thick, the choice of ~hîckness will depend on the particular application. Thus for a lininy the thickness may be between 5 ~ and 100 ~ r preferably between lO ~ and 50 and for a wound dressing the thickness may be between 25 ~ and l500 ~, preferably between 50 ~ and loOo ~.
The pore size of mats prepared according to the invention ~ay be between O.OOl ~ and 500 ~.
For linings the mat should be sufficiently porous ~o allow penetration of cells into it, preferably the average pore dimension should be of the order of 5 to 25 ~ , particularly preferably between 7 and 15 ~.
~0 For wound dressings the pore size will depend on ~he hydrophobicity of the polymer used and on the application i.e. whether adhe~ent or non adherent. Typical values of average pore dimension are, for an adherent poly urethane wound dressing 50 to lO0 ~ and for a non-adherent polytetrafluoroethylene wound dressing l to 50 ~.
The as-spun mats usually have porosities in the range 55% to 95%, which may be reduced to as low as 1%
by an appropriate compressive post-treatment. The yorosity will depend on the par~icular applicatiorl, typical porosity values are, for a lining 75~ and ior an adherent wound dressing 80% and a non-adherent wound dressing 60~i. By the term por~sity we mean the percentage of the total volume of th~ mat which is ~ree space.
Where disper~lons are employed as the spinning ' material, the particl2 size ma~ be between .01 ~ and 1 preferabl~ it is between .1 ~ and .3~, The high surface area of the mats according to the present in~ention affords a method of immobilising , a range of active moieties so that they are constrained to act at the site of application and do not percolate' throughout the body. Moieties which may be immobilised include enzymes,drugs and active carbon. These moieties may be added to the spinning solutions or,dispersions or the maks ma~ subsequently be treated with them., While in some applications a mat of nigh surface area i.e. fine fibres is needed, in others a mat of ~0 high porosity is needed. Our Canadian Patent ~o. 1065112 discloses methods fox obtaining a desired porosity/specific area combination, namely by addition of an electrolyte to the spinning material or by , post-spinning compression of the mat.
"

.

- 18a -~3~

Tne invention is particularly useful in providing novel synthetic blood vessels or components thereof.
Convenlently such synthetic vessels consist of tubes, preferably of circular cross~section, whlch may be of constant diameter along their length or May be of varying diarneter or configuration, for example they may taper or they may include constrictions or grooves to facili-tate their location~ Such tubes may be of dimensions and con-figuration appropriate to the intended function and location in which they are to be employed, for example as a replacement for a diseased blood vessel, and they may, of course, be prepared upon a mandrel of corresponding dimensions and configuration.
~uch synthetic vessels may be of the order of 0.3 to 3 cm, preferably 0.5 to 2 cm and more preferably 0.8 to 1.5 cm in internal diameter. The thickness of the wall of the vessel may vary between wide limits, and will dependinter alia upon the strength and elasticity required in the tube as well as the need for it to be convenient to locate and affix. Usually the thickness of the vessel wall will be within the range 0.5 to 5 mm9 preferably between l and 3 mm.
The synthetic vessel of the invention may be of any of a variety of configurations, for example it may be a straight tube, a loop, an anastomosis or it may bifurcate. Such-forms may be obtalned by spinning upon a mandrel of suitable shape.
~here appropriate the mandrel may be made removable from within the prepared vessel for example by collapse (where, say, an inflatable metal coated mandrel is used), by dissolution (where a soluble conductiny mandrel has been employed) or melting.

-- lg --.

The preferred vessels comprise fibres of an appropriate polyurethane selected from the wide range of materials available on -the basis of ease of fabrication, lack of toxicity, solubility, mecilanical properties, degree of biodegradabillty, etc. While it is preEerred that a completely polymerisecl polyurethane dissolved in a suitable solvent ~together with other additives as required) is used as the spinning solution we do not exclude the possibility of spinning incompletely polymerised polyurethane, completion of polymerisation being effected during or after spinning.
Thus for example it is possible to spin an incompletely polymerised polyurethane product onto an inflatable mandrel in one configuration, inflate the mandrel to stretch the formed product, for example to increase its porosity to a desired degree, and then to cure the product in its expanded condition.
The following examples illustrate the invention:-xample 1 .
The apparatus was as shown in Figure 1. The belt was of "Terylene" (RTM) net 15 cm wide, the nozzle diameter was ~25 mm, located 15 cm from the surface of the charged roll which had a diameter of 10 cm and width 16 cm.

, To 80 yms of an aqueous disperslon of PTF~ of number average me~lian particle si~e 0.22 microns (Standard Specific Gravlty of ~he polymer being 2.190) containing ~.6~ by weight, based on the weight of the S dispersion, of surfactant "Triton X100" (Rohm and Elaas) and having a PTFE solids conten~ o~ 60~ by weight was added 20 gms of a 10~ (by weight) ~queous sol~ltion of polyethylene oxide (PEO) oE average molecular weight (M~ of 2 x 105. The final composltion con~ained 48%
by weight PTFE and about 2% by weight. PEO (conductivity 1.8 x 10 4 mhos cm 1), The suspension was thoroughly mixed and fed to the nozzle by an earthed syringe injector. The electrode was charged to ~20 Kv and a fine jet of liquid was drawn from the nozzle and collected on the receiving surface. The fibres so collected were found to be dry and of even cross-section (1.0-2.0~). The fibres were very friable and were removed carefully from the collector, dried at 80C and then sintered on a bed of o titanium dioxide at 380C for 15 minutes. After this treatment the mat, 200 ~ thick, was found to have retained its fibrous structure having fibres between 1 and 2 ~ diameter and was quite strong.
The contac~ angle of ~he mat, measured by a modified method of Owens and Wendt, (Journal of Applied Polymer Science 1969 13 pp 1741-1747) was 137 and in the hydrostatic head ~est (BS 2823) a pressure of 50 cm of water failed to penetrate the mat.

~ 21 -*denotes Trade Mark A disc of the ma~ (1.3 cm diameter) was applied to -the surface of a wound on a rabbit involving loss of an area of full thickness skin. The rate of re-epithelialisation was slightly better than that of a comparable open wound. No seepage o body fluid into the mat was observed.
The preparat:Lon was xepeated to give an as-spun mat 800 ~ thick, having a porosity of 83% and a pore size distribution shown in Table 1. The mat was compxessed to a thickness of 300 ~ for 3 min at 100C
and 400 psi and then heated at 3B0C for 15 minutes.
The resul~ing mat was 400 ~ thick and had a porosity of 59~ and a pore size ~istribution as shown in Table ~.
Table 1 Pore Size Distributlon of As-5pun Mat Pore Dl~meter (~)% of Pores with Smaller Diameter ' . . 100 100

2 30 Table 2 Pore Size Distribution of Treated Mat Pore Diameter (~) ; 2S 15 85 ~ 2.5 75 1.2 50 ~ ~2 -~xampl _ Example 1 was repeated except that 1 gm of potassium chloride was added to the spinning composition to give a conductivity of 1.2 x 10 mhos cm . The resulting fibres, after sintering, had diameters of 0.5-1.4 Example 1 was repeated except tha~ the polyethylene oxide had an average molecular weight of 2 x 105. The resulting fibres, af~er sintering, had diameters of 0.9-1.6 ~ and the mat was 50 ~ thick.
The contact angle of the mat, measured as in Example 1, was 123 and the ma~ supported a 16.5 cm column of water.
Ex mple 4 Example 1 was repeated except that the collecting . surface was a metal gauze as shown in Figure 2, upon which the fibre mat was supported during the subsequent sintering proces~.
I Example S
¦ 2G The process of Example 1 was repeated using a spinning solution compr.ising a 25~ solution of a poly-~ u.rethane ("Daltof]ex" 330S),.in dimethyl for~lamid0~thyl i ethyl ketone (conductivity 1 x.10 6mhos cm i~ the c~l~e~ting surface being a metal tube ~10) having a sleeve (11~ of . 25 - flexible open~cell polyurethane foam (see Figure 3) r ~he tube being rotated at 100 rpm.
. The polyurethane fibres formed had an average diameter of 2-4 microns, and were collected in the form - ~3 -*denotes Trade Mark i.
,,, I

h ~7 of ~ tu~e which after completion of spinning to give a layer about 2 mm thick could be peeled from the foam.
Example 6 The process of Example 5 was repeated except that the product was collected as a flat mat 7S~ thick.
The contAct angle of the màt, measured as in ~xample 1, was 73 and the mat supported a 1.5 cm column of water.
A portion of tha mat was tested for efficacy as in Example 1. The healing wound appeared neat and tidy with the absence of any gross texture.
Example 7 The process of Example 1 was repeated using an aluminium tubular collector having walls 0.5 mm thick, the PTFE being collected directly onto the metal. A
first layer of PTFE was deposited on the collector, a close helix of ~ichromewlre (0.2 m~ diameter) was then applied over the PTFE, followed by another layer of PTFE ~lbres, and the entire composite tube, on the collector, sintered. The tube was then removed rom the aluminium collector by dissolving the latter in concentrated sodium hydroxide solution.
E~ample 8 The process of Example 7 was repeated using as the helical winding glass fibre of dlameter 0.02 mm.
Several layers of the glass fibre were employed. Tubes of diameters 1 to 10 cm have been prepared by the methods described in Examples 7 and 8.

*denotes Trade Mark - 24 -, .

~x am~ le g A lining to a former of irregular contour was ob~ained by employing a porous conducting ~ormer and applying suction to the surface away from that upon which fibres were deposited su~ficlent to cause the fibrous mat formed to conform ~o the contour o the foxmer. Such a llning could be attached to, say, an artlficial body component, by use of an appropriate adhesive, e.g. nylon in formic acid or polyurethane in DMF.

~ dust mask for pollen filtration was made by preparlng a pattern shaped accurately to fit over the nose and mauth, metalllsing the surface of the pattern to make it adequately conducting and depositing upon the pattern a mat about 3 mm th1ck of polyurethane fibres which after drying could be remove~ from the pattern.and provided an acc~rat~ly contour~d face mask which was both light in weight and fairly elastic.
~o ~ats obtained generally a~ described in Example 1, 10 cm in diameter and having average flbre diameter 1-2 microns r average pore si~e 5 microns and porosity about 80~ have been employed as air filters and. where, tha propertles o~ the polymer are appropriate,as fil ers for liquids e.g. water or beer. Where the ma~ is of hydrophobic material, e.g. PTFEJ pressure may be required to force an aqueou~ liquid through it. However a degree of hydrophobicity is desirabl~, for example where the r.`at is ernployed as a diaphr~gT?. for, s~ blood or oth~r liquid oxyg2nation.

Using a cylindrical s~ainless steel former (1~6 cm diameter) cha:rged to ~0 Kv, a iO% solution oE poly-urethane ("Daltolnold" 338E) in dime~hyl form~nide was electrostatically spun throucJh one neeclle a~ the xate of 0.7 g or PU/hour. The tuhular struc~ure produced had a wall thickn2ss of 0.4 mm, a total pore volume of 1800 mm3/g and a median pore radius of 9.4 microns and consisted of approxlmately 10 micron polymer nodules cross lin~ed together by 0.4 ~ diameter fibres.
A section o~ this product wa5 implanted by sut~ring into thP descendin~ aorta of a pig, lO days after lS which the pig was killed and the ~raft examined.
The gross indings showed the graft to be patent with no sign of intravascular thrombosis,i,e. the implant~d vessel was visibly still open. Histologically, there was evidence of connective tissue and capillary in~rowth between the fibres of the prosthesis.

Example 1 was repeated except tha~ a 10% by wei~ht solution of a polyamide (Maranyl A100)* in formic acid was used as the spinning material. The ~ibres collect~d were dry and had even cross-sections (0,06-0 5~).

The preparation was repeated using a 16% by ~eight solution of Maranyl A100* in formic. The fibres collected were dr~ and had even cross sections (0~70-2.8 ~), *denotes Trade Mark . . .
.: .

Exdmple 13 Example 1 was repeated except that a 12% by weight solutlon of polyacrylonitrll~ in ~1methyl formamide was used a~ the spinnlng m~teria~ he fibres collected were dry and had even cross-sectl!ons (0.8-1.4 ~1 Example 14 Ex~mple 1 wa~ repeated except that a 10~ by weight solution of a polyacrylonitr¦ile/vinylldene chloride co-polymer tViclan A85/02) ln tetrahydrofuran was used as the spinning material. The f1~xes collected were dry and of even cross-section (1.0-2.0~).
Example 15 A wound dresslng (Flgure 4) was prepared comprlslng a woven textlle backing 12 havlng an adhesive surface layer 13, a pad of absorbent materlal 14 covered by a mat lS of electrostatieally spun material prepared as descrlbed in Example 1. The adhesive 13 of the backlng is protected by cover strlps 16 which are removed prior to appllcatlon of the dressing.

*denotes Txade Mark.

I

27 ~
` ' . .. . .. ...... . . . ....

~,

Claims (11)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A vascular prosthesis comprising a mat of fibres of 0.1 to 10 micron diameter prepared by electrostatically spinning an organic material and collecting the spun fibres on a suitable receiver, the prosthesis comprising a tubular portion having a bore of internal diameter of the order of 0.3 to 3 cm and a porous component having a pore size of between 0.001µ and 500µ.

2. A vascular prosthesis according to Claim 1 in which the fibres comprise a polyurethane.

3. A prosthesis according to Claim 1 or 2 in which the fibres are of diameter 0.4µ to 10µ.

4. A prosthesis according to Claim 1 in the form of a loop.

5. A prosthesis according to Claim 1 in the form of an anastomosis.

6. A prosthesis according to Claim 1 in the form of a bifurcation.

7. A prosthesis according to Claim 1 comprising reinforcement in the form of helical fibres.

8. A method of preparing a vascular prosthesis comprising the step of electrostatically spinning a fibre forming composition, from a solution of or a dispersion of a polymer or its precursors and collecting the fibres of 0.1 to 10 micron diameter so formed onto a suitably shaped former or mandrel to produce a vascular prosthesis comprising a tubular portion having a bore of internal diameter of the order of 0.3 to 3 cm, the said solution or dispersion also containing a viscosity enhancing agent.

9. A method of preparing a vascular prosthesis according to Claim 8 wherein, subsequent to the collection of the said electrostatically spun fibres in tubular form, there is applied to at least part of the exterior of the said tubular form, a reinforcement.

10. A method of preparing a vascular prosthesis according to Claim 9, wherein the said reinforcement comprises fibres of organic polymer, glass, or metal,

11, A method of preparing a vascular prosthesis according to Claims 9 or 10 wherein further electrostatically spun fibre is deposited external of the said reinforcement.