WOUND DRESSINGS AND PROCESSES FOR THEIR PREPARATION

Abstract: Sterile net of elastomeric polyurethane, sterile wound dressings comprising an absorbent layer and a wound fac­ ing layer of the sterile net and methods of manufacture are described.

Description: WOUND DRESSINGS, COMPONENTS ThEREFORE AND PROCESSES FOR ThEIR PREPARATION The present invention relates to wound dressings, to polyurethane nets which may be used in wound dressings and to their manufacture. The use of integral polymer nets made of a polyolefin such as high density polyethylene and-polypropylene as wound facing materials for absorbent pads has been disclosed in British Patent Specification No. 1,142,323. The wound facing materials of these-dressings are relatively non-conformable. It would be desirable to have available more conformable wound facing substrates. It is also desirable that the components of a wound dressing, in particular the wound facing substrate, should be sterile. Certain polyurethane nets are disclosed in US Patent No, 3,913,510 as carpet backing materials which hold carpet tufts in position without the use of an adhesive. However, there is no suggestion that such net can be used for other purposes such as wound dressings. The skilled worker will appreciated that is not normal to obtain materials from the carpeting art to use as wound dressings. However it has now been discovered that nets#o# elastomeric polSurethane can -be employed to provide a highly conformable sterile wound facing. It has also been found that such nets can be sterilised so that wound dressings employing them can be provided in sterile as well as non-sterile form. The present invention provides a sterile net of elastomeric polyurethane. Normally and preferably the net will be an integral net (that is a net in.which the strands and judctures.are formed integrally, for example during manufacture). In another aspect the present invention provides a sterile wound dressing comprising an absorbent layer and a wound facing layer which wound dressing is characterised in that the wound facing layer comprises a sterile net of elastomeric polyurethane. As previously indicated the net used in the dressing is normally and preferably an integral net. Since the net is intended for use in contact with lesions, the elastomeric polyurethane employed will be non-toxic. It is desirable that the polyurethane employed in this invention has a sufficient moisture- vapour transmission rate to allow moisture from the wound area to transmit through the integral net material as well as through the interstices. Suitable polyurethanes will thus have a moisture transmission rate of at least 600gum and preferably at least 100Qg/m 2/24hrs. at 37.50C at 100% to 10% relative humidity difference in the form of a continuous film 25 microns thick. Particularly suitable elastomeric polyurethanes are linear polyurethanes for example those containing polyether or polyester groups. Certain apt elastomeric linear polyester polyurethanes are disclosed in U.S. Patent Specification No. 2,871,218. Certain apt elastomeric linear polyether polyurethanes are disclosed in U.S. Patent Specification No. 2,899,511. It is preferred to employ polyether polyurethanes and especially linear polyether polyurethanes. Favoured elastomeric linear polyurethanes include Estanes (registered trade mark) from B.F. Goodrich. Preferred polyurethanes include Estane 5714 F1, 5702 and 5703 .3¯ Suitable elastomeric polyurethanes have an elongation at break of 300% to 800 and preferably of 500% to 700% when tested according to ASTM D882 Estane 5714 F1 is an elastsmeric-linear polyether-polyurethane which has an elongation break of 560% when tested according to ASTM D882. A 25 micron thick-film of Estane 5714 F1 has a moisture vapour transmission rate of approximately 180Qg/m¯2124hrs/at 370C at 100% to 10% relative humidity differences Estanes-5702 and 5703 are polyester polyurethanes which have an elongation at break of 700% when tested according to ASTM D882. A 25 micron thick film of Estane 5702 has a moisture vapour transmission rate of approximately 1600g/mê /24hrs at 370C at 100% to 10% relative humidity difference. The elastomeric polyurethane can contain a filler as long as the filler employed does not adversely effect the elastomeric properties of the polyurethane. Suitable fillers can give the integral- polyurethane net and wound dressing of the invention a desirable.white textile appearance. The elastomeric polyurethane can contain up to-50 parts by weight.(but more suitably not more than 20 parts by weight) of titanium dioxide filler. The net of elastomeric polyurethane of# the wound dressing of the Invention can have any- convenient form depending on the.chosen arrangement of strand, puncture and hole areas and also their shapes -and relative size. In one-preferred form-the net consists essentially of longitudinal and transverse strands intersecting at right angles to give a square grid hole pattern. Suitable nets of this--type aptly have 4 to 40 strands per cm and- desirably 8 to 40 strands-per cm:and - preferably 8 to 24 strands per cm in both longitudinal and transverse directions. Fine integral elastomeric polyurethane nets of 8 to 40 strand counts have not been previously known but are now believed to be particularly suitable for use in contacting wound. Thus in a further aspect the invention provides sterile integral nets of elastomeric polyurethane which have a strand count of 8 to 40 per cm. Preferably such nets have a strand count of 8 to 24 per cm. Such nets are desirable since when used as a wound facing for example for an absorbent dressing the dimensions of the openings of the integral net are small enough to-prevent-the absorbent#-layer, especially when made of absorbent fibres, from protruding through the hole thus causing# irritation of the wound. Variations on the square grid pattern can give other desirable forms of -the integral net. Unequal density of strands -in either the longitudinal or transverse directions will give rectangular hole-areas. Continuous-parallel strands.in-one direction with a staggered arrangement of connecting strands in the other direction size give a "briekwork" -pat-tern. The strands in-one direction may be thicker than those.in the other direction. Such variation may be employed to produce nets which have different mechanical properties in the longitudinal or transverse direction. Other preferred forms of the nets can have strands at an angle to the longitudinal or transverse direction i.e. diagonal strands. Another preferred form of the integral polymer net can have a staggered arrangement of circular or approximately circular (e.g. hexagonal) arrangements of strands and hole areas. The net can be in the form of a mixed pattern of two or more of the arrangements. Suitable integral elastomeric polyurethane nets: for wound dressings of thesinvention have a plurality of openings with a maximum dimension of up to 2 mm, preferably from 0.05 mm to 1 mm, desirably from 0.1 to 0.5mm. Suitable nets have a thickness of 25 micron to 250 micron-, preferably 50 micron to 150 micron. Similarly suitable nets can have a weight per square metre of log to 80g, preferably 15g to 50g. Suitable integral nets of elastomeric polyurethane have an elongation at break of 100% to 800%, desirably -200% to 750% and preferably.300% to 7005 when measured as a 2.5cm wide strip at a 30cm/min strain rate at -200C. The integral net of elastomeric polyurethane should be capable of being sterilised independently or in combination with the other components of the wound dressing. Suitable methods of sterilising the net or a wound dressing containing the net include steam autoc#laving, ethylene oxide gas, electron beam irradiation and gamma irradiation. The absorbent layer of wound dressings of the invention can be selected# from absorbent materials used in conventional wound dressings. Suitable absorbent materials include knitted, woven and nonwoven fabrics containing cellulosic fibres, synthetic fibres or mixtures thereof. A favoured absorbent layer consists essentially of a thin layer of staple fibres containing 80% by weight of cotton fibres and 20% by weight--of-acrylic fibres lightly bonded-and laid on an open weave cotton gauze. The-absorbent layer may have a, thickness of approximately Gmm-and a weight per unit area of approximately 200gsm. To enable the wound dressing to function adequately the integral elastomeric polyurethane nets should be in contact with and preferably attached to or about the wound facing side of the absorbent layer. The net can be folded around and attached to the non-wound contacting side of the absorbent layer but it is preferred that the net is substantially coextensive with the absorbent layer and attached over the whole of the wound facing surface. Suitable methods of attaching the nets to the absorbent layer include heat sealing, adhesive bonding or forming the absorbent layer or net in situ whilst in contact with the other layer, Preferably a layer of-absorbent material is bonded to an integral net of elastomeric polyurethane to form a composite strip whi#ch is then cut into suitable size wound dressings.. The wound dressing of the invention-can be in various conventional forms. A-preferred wound dressing is in the form of a square or rectangular strip. Convenient sizes of the strips for treating wounds are Scm-x 5cm, 10cm x locum and 20cm x 20cm. Wound dressings of the invention can have a thickness of lmm to 20mm. Another preferred wound dressing is in the form of an adhesive dressing in which the absorbent layer in contact with the integral net of elastomeric polyurethane constitutes the absorbent pad of the dressing. The wound dressing and nets of the invention can be sealed in bacteria proof packages and the package sterilised by conventional methods such as ethylene oxide gas, steam autoclaving, electron irradiation and gamma irradiation. The invention also provides a method of forming an integral net of polyurethane in which an elastomeric polyurethane in a flowable state is cast onto a surface having a pattern of discrete raised areas and interconnected recessed areas, and treated to form a solid integral net. The flowable state of the elastomeric polyurethane can include solutions, dispersions, hot melts and powders. The term 'treated.' as used in the process of making the integral polymer-nets of the invention means the physical and/or chemical process which can convert the cast-polymer net from a flowable form to a solid form. If more than one process is involved then the process can be carried out simultaneously or consecutively as is convenient. In the process of the invention the elastomeric polyurethane can be cast in a molten state for example by hot melt extrusion. However it is preferred that the elastomeric polyurethane is cast as a solution. Thus the polyurethane employed in this invention is preferably a linear polyurethane. Suitable casting solutions may contain 15% to 35%, and preferably 20 to 305 by weight of elastomeric polyurethane. A favoured casting solution contains 20 to 25% by weight of Estane 5702 or Estane 5703 in acetone. Another favoured solution-contains 25%.to 30% of Estane 5714F in tetraydrofuran or mixtures of tetrahydrofuran and acetone. Conveniently the cast elastomeric-polyurethane is treated by passage through a hot oven in order to evaporate the solvent and form a solid integral net. However other treatments are also possible, These treatments include drying a liquid dispersion, cooling a molten polymer and crosslinking a polymer in a liquid or-solid form. Figure 1 illustrates a process of making the nets of the invention. A thermoplastic film (1) with an embossed pattern on its upper surface fed from roll (2) to coating head (3) where a solution (4) cast onto the recesses of the embossed sheet. The wet cast net (5) on the embossed sheet passed into an oven (6) where it is dried. The dried cast net (7) then separated from the embossed sheet (1) and wound up onto roller (8) where it is also interleaved with a release paper (9) fed in from roll (10). Figure 2 is a perspective view of a casting process used to form the nets of the invention. Figure 3 is a side view of the casting head and casting process- of Figure 2 with the side plates removed. In Figure 2 and Figure 3 the embossed film (1) having a pattern-on- its--surface of discrete raised areas (11) and interconnected recesses (12) coated with a polymer solution (4) by means of a coating head (3) to form a cast net (5). The coating head (3) has a flat bed (14), a spreading block (15) and side spreading -guides (16) which are next to the adjustable blade (17) fixed to the sides of the coating head (18). Most of the spreading head components can be made from-metal in conventional manner but in order to provide less friction against the film it is advantageous to make the spreading block and the guides or their film contacting surfaces- from a fluorocarbon polymer such as polytetraflaoroethylene.. It is preferred that the base of the spreading block should be large enough to span the discrete raised areas of the embossed film in order to prevent the block catching the recessed areas. The spreading block and adjustable blade can be replaced by a single broad based adjustable spreading device. Alternatively coating heads- using fixed or rotating roller doctor devices can be used. Figure 4 illestrates a plan view of an embossed pattern sheet suitable for forming a- net of the invention. Figure 5 is a cross section through line A1A of Figure 4. Figure 4 shows discrete raised areas (19#) arranged in a square pattern to give a square grid patt-ern of recesses (20).' In Figure 5 a section through line A-A -of Figure 4 shows the discrete raised areas (19) in the shape of truncated square pyramids and conical recesses (20). A particularly preferred embossed pattern sheet has 8 per cm raised areas in the form of square truncated pyramids lmm wide and 0.5mm high with sides sloping to a 600 conical angle and a corresponding longitudinal and transverse square grid recess O.25mm wide at the base and 0.75mm at the top. Other suitable embossed sheet patterns include those which have raised areas of a rectangular, diamond, circular or approximately circular (e.g.. hexagonal) shape so as to give the co#rresponding shape to the hole area in the integral net. Similarly the recessed areas in one direction can be of different depth-, different density or direction, or can be diagonal to the longitud inal direction corresponding shape or arrangement to the strand and/or juncture areas of the integral polymer net. Mixtures of two or more of these patterns can also be used. The casting surface may be in the form of a roller, an endless flexible belt or a length of flexible sheet material, It is preferred that the casting surface has release properties in order to enable the dried net to be removed from the casting surface. A preferred casting surface is a melt embossed sheet of a polyolefin polymer made by the method given in British Patent Specification No. 1055963. Example 1 A solution containing 30% by weight of Estane 5714F1 in tetrahydrofuran was cast into the recesses of a 15cm wide#melt'embos'sed- high density polyethylene sheet by means of the blade over flat bed spreading technique. The sheet had. a melt embossed pattern of 8 per cm.raised areas in the form. of square truncated pyramids lmm wide at their base and O..5mm high with sides sloping-to a solid conical angle of 600. The wet cast net on the embossed film was dried by passage through. a hot air circulating oven at a.temperature of 900C to 1000C for two minutes. The dried cast net was separated from the embossed film and wound onto a roller inter leaved--with a douba sided silicone release coated paper. The resultant cast integral elastomeric polyurethane net had the following properties: Weight (per mê) 40g; thickness 100 microns to 125 microns; hole size 0.3 to 0.4mm; tensile strength (g/2.5cm wide), machine direction 800 + 51, transverse direction 664 + 57; elongation at break(%), machine direction 389 + 24, transverse direction 374 + 24. The net was bonded to an absorbent layer weighing approximately 200g consisting of 80% cotton fibres and 20% acrylic fibres lightly bonded by an acrylic latex on a woven gauze base, by passage through the nip of pressure rollers heated to 1000C. The resultant strip cut into 5cm x Mcm dressings- and packed into steam autoclavable pouches formed by heat sealing a 59gsm laquered Kraft paper from Bemrose Flexible Packaging Ltd. The dressings were sterilised by steam autoclaving at 1150C for 30 minutes. Example 2 A solution containing 20% by weight of Estane 5702 in acetone was cast into the recesses of a 15cm wide high density polyethylene embossed sheet and dried in the same manner as Example 1. The resultant 'cast net had the-following properties: Weight (ner¯m2) 20 to 25g; thickness 50 microns; hole size 0.3 to 0.4mm. The. net was laminated to an absorbent layer and made into sterile dressings as Example 1. Example 3 A solution containing 205 by weight of Estane 5714 Fl in tetrahydrofuran was cast into the recesses of a 15cm wide melt embossed high density polyethylene sheet by means of the blade over flat bed technique. The sheet had a melt embossed pattern of 20 per cm raised areas in the form of square pyramids 0.4mm wide at their base, 0.3mm-high-and side -'sloping#to'.a solid angle-of OQC, arranged in staggered rows in the length of the sheet and parallel rows across the sheet i.e. in the form of a brick pattern. The wet cast net was dried in the same manner as Example 1. The resultant cast net had the following proper- ties: Weight (per m2) 14g; thickness 50 microns; hole size 0.1 to 0.2mm;. tensile strength (g/2.5cm wide), machine direction 488 + -73, transverse direction 493 + 74; elongation at break (%) machine direction 408 + 53, transverse direction 428 + 52. The net was laminated on an absorbent layer and made into sterile dressings of Example 1. Example 4 A solution containing 20% by weight of TiO2 filled Estane 5714 F1 (contains 15 p.h.r of TiO2) in tetrahydro furan was cast into a sheet and formed into a net as in Example 3. The resultant cast net had the following properties Weight (per mê) l9g thickness 50 microns; iole size 0.1 to 0.2mm; tensile strength (g/2.5cm wide), machine direction 313 + 33; transverse direction 312 + 26; elongation at break (%) Machine direction 307 + 44, transverse direction 364 + 44. The net was laminated to an absorbent layer and made into sterile dressings as in Example 1. Example 5 The net of.Example 4 was cut into 5cm x 5cm squares and packed and sterilised as the wound dressing of Example 1 to give a sterile net of elastomeric polyurethane. Example 6 A solution containing 20% by weight of Estane 5714 F1 in tetrahydrofuran was cast into the recesses of a 30cm wide melt embossed high density polyethylene sheet by means of a blade over flat bed coating technique The sheet had a melt embossed pattern of 6 per cm raised areas arranged in a diamond pattern of rows at an angle of-45 --to the#-iong.itudinal.#dir#ction#.of##the##heet in-the.. form of square truncated pyramids 1.3mm wide at the base and 0.5mm high with side sloping to a solid conical angle of 700. The wet cast net on the embossed film was dried by passage through a hot air oven at a temperature of 700C to 800 for 1 minute.. The dried cast 'net on the embossed film was wound onto:a roller. The resultant cast integral elastomeric polyurethane net had the following properties: Weight (per square metre) 37g; tensile strength (g/2.5cm wide) machine direction 536 + 33, transverse direction 526 + 39; elongation at break (%) machine direction 309 + 18, transverse direction 307 + 53. Individual 30cm x 40cm pieces of the cast net were placed between release paper sheets, packed into sealed bacteria proof autoclave sterilising bags (available from J. Dickinson a Co.) and sterilised by exposure to gamma irradiation, electron irradiation at 2.5Mrads or ethylene oxide gas. Samples of the sterilised net were tested for sterility by incubation in a broth for 7 days at a temperature of 300C. The lack of any bacterial growth indicated that the nets were sterile. The tensile strengths and elongations at break of the sterilised nets were not significantly different from that of the nets before sterilisation.

Claims: WHAT WE CLAIM IS: 1. A sterile net of elastomeric polyurethane. 2. A net as claimed in claim 1 which is an integral net of linear polyurethane. 3. A sterile integral net as claimed in claim 2 in which the linear polyurethane comprises a linear polyether pol-yurethane. 4. A sterile net as- cla,'imed in any of claims 1 to 3 in which the net has a strand count of 8 to 24. 5. A sterile net as claimed in any of claims 1 to 4 in which the net.has a plurality of openings of 0.05mm to lmm in size. 6. A sterile wound dressing which comprises an absorbent layer and a wound facing layer characterised in that the wound facing layer comprises an elastomeric polyurethane net of any of claims 1 to 5. 7. A sealed bateriazproof package containing a sterile net of any of claims 1 to 5. 8. A sealed bacteria-proof package containing a sterile wound dressing of claim 6. 9. A method of making a sterile net of any of claims 1 to 5 which comprises casting an elastomeric polyurethane in a flowable state onto a surface having a pattern of discrete raised areas and interconnected recessed areas and treating the cast net to form a solid. net. 10 A method of making a sterile bacteria-proof package of either of claims 7 or -8 which comprises sterilising the net or 'wound dressing within a sealed bacteria-proof - package..

Inventors: LLOYD, RONALD
Applicants: SMITH AND NEPHEW ASSOCIATED COMPANIES P.L.C.
Application year: 1981
Main classification: A61L15
IPC codes: A61F13, A61L15