WO1990014109A1

WO1990014109A1 - Dressings

Info

Publication number: WO1990014109A1
Application number: PCT/GB1990/000752
Authority: WO
Inventors: Nigel David Miller
Original assignee: Smith & Nephew Plc
Priority date: 1989-05-16
Filing date: 1990-05-16
Publication date: 1990-11-29
Also published as: AU638995B2; EP0472577A1; JPH04505268A; CA2051391A1; GB2248555A; GB9121450D0; AU5648790A; GB8911151D0; GB2248555B

Abstract

A hypoadherent wound dressing comprising a body-facing substrate, which may be a continuous or apertured film having a polymeric coating on the wound contacting surface, and having a peeling energy of not more than 20jm-2 and a moisture vapour transmission rate of greater than 500 grams per square meter per 24hrs at 37°C and a relative humidity difference of from 10 % to 100 %.

Description

DRESSINGS

This invention relates to dressings, particularly dressings for use with exuding wounds.

It is known that if an exuding wound is covered with a dressing there is a tendency for the dressing to adhere to the wound eschar. As the wound heals and dries the dressing adheres more tenaciously. Significant difficulties can occur when the dressing is removed. Not only can removal be an extremely painful experience but serious wound disruption can also occur. Significant trauma can be expected with apertured or porous dressings.

In International Patent Publication No. WO89/01346 there are described filmic dressings in which the body contacting surface of the polymer film has a low adherency defined by having a peeling energy of less than 30 joules per square meter (when determined by the test method described in J. Clinical Materials, Vol. 1 , 1986, pages 9 - 21).

We have now found that moisture permeable dressings having a wide variety of wound facing substrates may be coated with a polymer such that the wound contacting material has a very low level of adherency even when the wound has dried out and without the dressing losing its desirable moisture vapour transmitting properties.

The energy required to separate the dressing directly from wound eschar is much less than that required for conventional composite dressings.

Accordingly the present invention provides a hypoadherent dressing comprising a body-facing substrate having a polymeric coating on its body contacting surface and having a peeling energy of not greater than 20jm^-2 and a moisture vapour transmission rate of greater than 500 grams per square meter per 24 hours at 37°C and at a relative humidity difference of from 10% to 100%.

As used herein the peeling energy defines the adhesion between the substrate (film) and a gelatin substrate. The peeling energy is determined by the method described in J Clinical Materials, Vol 1 (1986) pp9-21 and is expressed in joules per square meter (jm-²) .

As used herein the moisture vapour transmission rate is determined by the Payne Cup Method. This method is described in European Patent Specification No. 0046071. The moisture vapour transmission rates (MVTR) are expressed in grams per square meter (g/m²) when measured over a 24 hour period at a temperature of 37°C and at a relative humidity difference of from 10% to 100%. When the test material is in contact with water vapour but not liquid water, the MVTR is referred to as the 'Upright MVTR' whereas when the test material is in contact with liquid water, the MVTR is referred to as the 'Inverted MVTR'.

The upright MVTR of the dressing is more suitably greater than 700g/m², preferably greater than 1000g/m², more preferably greater than 2000g/m². Under certain conditions it may be desired for the dressing to have upright MVTR's of greater than 5000g/m² eg. about 8000g/m² or greater. Alternatively, it may be desired for the upright MVTR to be less than 5000g/m² eg. about 2500g/m² or less to prevent too rapid egress of water vapour.

The substrates suitable for use in the dressings of the invention can be filmic materials made from, for example, a polymer or blend of polymers. The filmic substrate may be a continuous film or a discontinuous film, for example, an apertured film. Apertured films may be formed by simply perforating eg. by mechanical or flame perforation. Alternatively more sophisticated net-like structures can be produced by weakening the film in predetermined areas, eg. by embossing, followed by stretching in one or more directions to rupture the weakened areas.

The thickness of the film will generally be at least lO m. Films upto 250/m thick can be used with advantage. Suitably the film will be upto 175_/m thick preferably from 15, more preferably from 60, to 150 m thick.

The continuous films employed as the substrate for the coating are moisture vapour permeable and should have an upright MVTR of at least 500g/m². Aptly the upright MVTR of the film will be greater than 700g/m² and will preferably be greater than 1000g/m², more preferably greater than 2000g/m². Very high MVTR's, for example, greater than 5000g/m² and upto 14000g/m² or greater may be desirable where high moisture vapour permeabilities are required, for example, where the dressing of the invention is required for contact with a highly exuding wound such as an ulcer or a burn.

The films for use as the coating substrate in the dressings of the invention are suitably conformable materials, ie. they will readily conform to the shape of the body portion or wound to which the dressing is applied. Suitably the films will be made of linear polymer materials. Advantageously, the films will be extensible, preferably elastically extensible, so that they can accommodate any dimensional changes in other components of the dressing, without deforming or losing conformability of the dressing as a whole. Films having these properties may be produced from elastomeric polymers or polymer blends.

Suitable elastomers for use in the manufacture of continuous film include both hydrophilic and relatively non-hydrophilic elastomers such as polyurethanes, polyether-amide and polyester ether elastomer. Suitably continuous films of these elastomers include those disclosed in the aforementioned European Patent No. 107915.

Suitable relatively non-hydrophilic polyurethane elastomers for use in the film used in the invention include linear polyester polyurethane and linear polyether polyurethane elastomers for example those known as Estanes available from BF Goodrich (UK) Limited.

Apt grades are Estanes 5701, 5702, 5714 and 58201. An apt film for use in the invention is a 25 m thick polyether polyurethane (Estane 5714) continuous film.

Hydrophilic elastomers, for use in the invention, can aptly have a water content when hydrated of at least 5% by weight. Suitably such elastomer may absorb upto 70% by weight, desirably from 10% to 40% by weight and preferably from 20% to 30% by weight, for example 25% by weight, water when hydrated. Suitable hydrophilic linear polyurethane elastomers for use in the invention are described in United Kingdom Patent No. 2093190. An apt hydrophilic linear polyurethane elastomer for use in the invention which has water content when hydrated of 25% by weight is described in Example 2 of the United Kingdom Patent No. 2093190. An apt film of this hydrophilic linear polyurethane elastomer for use in the invention has a weight per unit area of approximately 20g/m².

Suitable polyether-amide elastomer films are disclosed in British Patent 1473972, French Patent Nos. 1444437 and 2178205 and United States Patent No. 3839245. An apt polyether-amide is known as Pebax 4011 RN00 available from ATO Chemical Products (UK) Limited. This polymer has a water content of approximately 55% when hydrated. An apt film of Pebax 4011 RN00 for use in the invention has a thickness of 70 m. Suitable polyether-ester elastomers for use in the invention are known as Hytrel available from Du Pont (UK) Limited. An apt grade is known as Hytrel 4056. An apt film of Hytrel 4056 for use in the invention has a thickness of 70yim.

Other apt materials for use in forming the film include di- and tri-block copolymers such as those of hard block materials such as styrene with softer block elastomeric materials such as isoprene or butadiene. Suitable materials in this group include block copolymers sold under the trade names KRATON and CARIFLEX.

Other apt elastomers for use in forming the apertured film include hydrocarbons such as polyethylene or polyisobutadiene, polyester ether, polyester amides, polyurethanes and other copolymers such as ethylene-vinyl acetate copolymers or mixtures of such elastomers. A suitable material which may be apertured is polyethylene pterephthalate sold under the trade name 'MELINEX' by ICI pic.

Other suitable elastomeric acrylic polymers such as a copolymer of alkoxy alkyl acrylate or methacrylates, as for example those described in United Kingdom Patent Specification No. 1280631 and ethylene-acrylic acid or acrylic ester polymers such as those sold under the trade name PRIMACOR and LOTADUR.

The material for forming the film may be a blend, suitably a blend of an elastomeric material with an incompatible more rigid polymer material. Such blends may comprise the above described elastomeric materials with a polyolefin such as polyethylene, polystyrene polycyclooctene or polypropylene.

Suitable blends include those of ethylene-vinyl acetate copolymers, polyurethane or polybutadiene with an incompatible polymers such as a polyolefin, for example polystyrene. Such blends are described for example in European Patent Specification Nos. 0141542 and 0046071 and in United Kingdom Patent Specification No. 2103537. Other preferred blends include those polyether-amides with high impact polystyrene, non-hydrophilic polyurethanes with hydrophilic polyurethanes, and polyurethane with linear low density polyethylene.

The ratio of elastomeric material to the other blend components is desirably chosen so that the elastomeric materials form the continuous phase of the blend and the other materials are in the discrete or discontinuous phase. Generally the elastomeric components will form at least 50% by weight of the blend. Aptly the blend will contain upto 90% by weight of the elastomeric components. Suitably the blend will contain between 55 and 85% of the elastomeric components. Certain ethylene polymers such as low density polyethylene and linear low density polyethylene exhibit some elastic properties therefore may be considered to be part of the elastomeric components of blends containing such materials.

Preferred blends of an ethylene-vinyl acetate copolymer comprise from 10% to 90% by weight of ethylene-vinyl acetate copolymer more preferably 20 to 80% by weight of ethylene-vinyl acetate copolymer. Typical examples of such a blend comprises a blend of 40 parts ethylene-vinyl acetate copolymer and 60 parts high impact polystyrene, or a blend of about 25 parts ethylene-vinyl acetate copolymer, about 13 parts high impact polystyrene and about 60 parts linear low density polyethylene or a blend of from 40 to 90 parts by weight of ethylene-vinyl acetate copolymer and 60 to 10 parts by weight of high impact polystyrene.

In addition to the use of continuous films the substrate for the polymer coating may be a discontinuous polymer film and may include films containing pores or micropores or apertured films such as those containing perforations or slits extending through the entire thickness of the film. Within the meaning of the term 'apertured film' are included the more sophisticated net-like structures produced by weakening the film in predetermined areas, eg. by embossing, followed by stretching in one or more directions to rupture the weakened areas. Alternatively films having a net-like structure may be formed by casting for example as described in European Patent Specification No. 50514.

In those cases where the coating substrate film is intended for use as an apertured film, the polymer may be applied to an already apertured film or applied to a continuous film and the coated continuous film thereafter apertured.

The body contacting surface of the dressing is rendered hypoadherent by the application of a polymer coating to reduce the peeling energy to not greater than 20jm^-2, aptly not more than 15jπr² and preferably not more than lOjπr².

Preferably the polymer employed for the coating material will be substantially linear in nature to confer a desired degree of flexibility to the coating. The coating polymer for use in the invention may contain hydrophilic residues, hydrophobic residues or both hydrophilic and hydrophobic residues. These residues may be incorporated either into a single polymer or by providing a blend of polymers. Aptly the coating polymer will be a hydrophilic polymer, such as a hydrophilic polyurethane and may contain upto 10% by weight, preferably upto about 5% by weight of a hydrophobic component such as a siloxane.

Apt coating polymers for use in the invention are urethane polymers, suitably polyether polyurethanes. The polymer will suitably be a random copolymer. In addition to the urethane residues the polymer will suitably contain siloxane residues. These may be reacted in with the polyurethane or are present as components of a polymer blend comprising the polyurethane.

The ether units for the preferred polyether polyurethane may be notionally derived from an alkylene diol, for example, containing upto six carbon atoms, such as ethylene diol, a propylene or a butylene diol. Preferably the polyurethane will contain CH₂CH₂0- units together with -CH₂CH₂CH₂0-, -CH₂CH(CH₃)0- or -CH₂CH₂CH₂CH₂0- units. More preferably the ether units in the polyurethane will contain -CH₂CH₂0- and -CH₂CH(CH₃)0- or -(CH₂)₄0- or mixtures thereof of which poly -CH₂CH(CH₃)0- blocks are preferred. In the preferred polyurethanes the mole ratio of poly(ethylene glycol) to poly[(prop or but)ylene glycol] derivable blocks present in the hydrophilic polyurethanes may vary from 1:1 to 1:30, more suitably from 1:2 to 1:10 and preferably from 1:2.5 to 1:4. The molecular weight of these blocks is aptly from 600 to 60000 and favourably from 900 to 4000, for example 1000 to 2000.

The polyurethane may contain di-isocyanate residues which may be residues of aromatic or aliphatic di-isocyanates such as 4,4'-diphenylmethane di-isocyanate, toluene di-isocyanate, 1,6-hexamethylene di-isocyanate, 4,4'dicyclohexylmethane di-isocyanate or the like. Favoured di-isocyanates for use in the hydrophilic polyurethane of this invention are 4,4'dicyclohexylmethane di-isocyanate (vhich is preferred) and 4,4'-diphenylmethyl di-isocyanate.

The urethane polymers for use in the coatings of the invention may also contain chain extruders such as diols, diamines or amino alcohols. Suitable chain extenders are aliphatic compounds such as ethane diol, butane 1,4-diol or butane 1,4-diamine. The siloxane residues which may be incorporated into the polyurethanes which are suitable for the coatings of the dressings of the invention can be polymerised together with the other polymer precursors.

Suitable siloxane containing materials included polyalkyl siloxanes or copolymers thereof. Preferred siloxane polymers are those based on polydimethyl siloxane.

The siloxane residues may be co-present with alkylene oxide residues. Thus at least a part of the ether component of the final polyurethane may be derived from oligomers containing both the siloxane and alkylene oxide units. Especially suitable oligomers are block copolymers of polyalkylene siloxane, such as polydimethyl siloxane and a polyalkylene polyol such as polyethylene or polypropylene glycol, a mixed polyalkylene polyol or a mixture of such polyalkylene polyols. Especially suitable block copolymer of this class are those marketed under the name Petrarch.

The polymer may be produced under conventional reaction conditions for producing polyurethanes and the final polymer taken up in appropriate solvent for it to be applied as a coating. The coating polymer may be applied to the substrate as a continuous coating or as a discontinuous coating. The coating weight of the coating polymer should be sufficient to maintain the integrity of the coating and yet not be so high as to reduce the moisture vapour permeability of the dressing to below 500g/m². Coating weights may be as high as 30gsm. Generally it has been found that satisfactory dressings are obtained with coating weights of less than 20gsm, suitably less than lOgsm and preferably between 0.1 to 5.0gεm.

The manner in which the coating is applied can depend largely in the choice of material to be coated. Coatings may be applied by for example spraying, screen printing or dip coating onto the as-formed substrate such as a continuous or perforated film. Alternatively the coating may be applied to a precursor for the substrate. For example a continuous coating may be applied to a continuous film and thereafter the coated substrate perforated.

The coatings may be applied as a spray onto the body-facing surface. Alternatively the coating may be applied as a discontinuous coating such as a net or patterned coating, for example in the form of dots or stripes. Coatings may be applied onto a portion of an adhesive coated substrate such as a film or tape to provide a non-adherent area surrounded by an adhesive coating. Alternatively dressings coated with the hypoadherent coating may be further provided with adhesive strips on one or more pairs of opposed edges. The adhesive may suitably be a pressure sensitive adhesive. Preferred forms include a highly moisture permeable backing layer coated with a discontinuous layer of adhesive and having a centrally disposed strip of hypoadherent polymeric coating.

Suitable adhesives include acrylic adhesives which are described in UK Patent Specification No. 2070631 and vinyl ether based adhesives such as those described in Composition A of UK Patent Specification No. 1280631.

The dressings of the present invention may be employed for covering wounds, particularly those wounds which produce large amounts of exudate initially. The dressings of the present invention may, with advantage, be employed for covering wounds such as burns, bed sores, surgical sites or skin grafts or wounds caused by disease or mechanical injury. Lesions on animals bodies may be treated by applying a dressing of the present invention to the body to cover the lesion.

The dressings of the invention may be in the form of simple films coated on one surface with a polymer coating to render them hypoadherent. In another embodiment the dressings of the invention may be a composite structure comprising a polymer film coated on the body-facing surface with a hypoadherent polymer coating and having at least one further layer over the non-coated side of the polymer film. This further layer may comprise an absorbent layer and may be bonded to the polymer film. Such a construction is favoured in those instances where the substrate polymer and the polymer coating are apertured to allow passage of wound exudate through the apertures into the absorbent. Such absorbent layers may be in the form of layers of gauze or a foam pad.

The dressings of the present invention may be packaged and sterilised in a conventional manner. Typically, the dressings of the invention may be sterilised by conventional ethylene oxide sterilisation procedures.

We have found that with the dressings of the invention constructions, such as apertured dressings or rapid drying, highly moisture vapour permeable continuous film dressings, which would otherwise be expected to be highly adherent to dried wound exudate, in fact, exhibit very low levels of adherency even when the wound is fully dried.

The invention will be illustrated by the following Examples:

Example 1

Preparation of Coating Polymer

An additive block terpolymer was prepared by bulk polymerisation of the following components:

Petrarch Siloxane Diol - MW1970 (a polydimethyl siloxane/polyethylene glycol block copolymer diol) - 0.015 mole

Butane 1,4- Diol - 0.035 mole

Desmodur W (Hexamethylene diisocyanate)- 0.050 mole

T₁₂ (Di-n-butyl tin dilaurate) catalyst 0.02% The siloxane diol, diisocyanate and catalyst were mixed together in a reactive vessel and left for 1 hour and temperature of 60°C, to form a viscous reaction product. The butane-l,4-diol was then added to the reaction product whilst stirring vigorously. Stirring was continuous and a colourless solid formed whereupon the solid was heated for a further 2 hours at 60^βC and thereafter allowed to cool overnight.

The cured solid product was taken up into a mixture of methylene dichloride and industrial methylated spirit (5:4 v/v) as 2% by weight solution.

Production of Dressing

The solution was sprayed, using an air-gun onto a commercially available perforated polyethylene terephthalate film dressing (sold by Smith & Nephew Medical Limited under the trade mark "Melolin") at a coating weight of about 6gsm.

The moisture vapour transmission rate of the dressing was greater than 500g/m² per 24 hours at 37°C and a relative humidity of from 10% to 100%.

An adhesion test was performed by first coating gelatin onto the coated "Melolin" product and onto an uncoated sample of "Melolin". Both samples were left to dry for 24 hours at 35°C. On testing the coated sample readily peeled and had a peeling energy of less than 20jπr², whereas particles of gelatin adhered to the uncoated sample and the gelatin surface was disrupted.

Example 2

Preparation of Coating Polymer

A hydrophilic polyurethane was produced by reacting 3 moles of polypropylene glycol (PPG 1025), 1 mole of polyethylene glycol (PEG 1500) and 10.46 moles of hexamethylene diisocyanate (Desmodur W) in a resin flask, maintained at 90°C. When all the reactants had melted 0.02% by weight of a catalyst (tri-n-butyl dilaurate) was added dropwise and the reaction continued for a further hour at 90°C after which the formed prepolymer was allowed to cool to 35^βC.

The prepolymer was taken up into solution into dichloromethane and 6 moles of piperazine added. The reaction mass gradually thickened over the next two hours.

A polymer blend was formed by mixing together the above described polyurethane with the siloxane modified polyurethane described in Example 1 in a weight ratio of 9:1 as 2% w/w solution in dichloromethane.

Preparation of the Dressing

The polymer blend solution was spray coated onto a polyurethane film to a coating weight of l.βgεm and the coating allowed to dry. The moisture vapour transmission rate of the coated film was greater than 500g/m² per 24 hours at 37^βC and a relative humidity difference of from 10% to 100%. A gelatin film was applied to the polymer coated film. A gelatin film was also applied to uncoated sample of the polyurethane film. Both film samples were then subjected to the adhesion test.

Upon testing the peeling energy for the coated polyurethane film was 20jm^-2 whereas that of the uncoated sample was 55 nr².

Claims

1. A hypoadherent wound dressing comprising a body-facing substrate having a polymeric coating on its body contacting surface and having a peeling energy of not greater than 20jm^-2 and a moisture vapour transmission rate of greater than 500g/m² per 24 hours at 37°C and a relative humidity difference of from 10% to 100%.

2. A dressing as claimed in claim 1 wherein the polymer coating is a continuous coating.

3. A dressing as claimed in claim 1 or claim 2 wherein the substrate is a continuous polymer film.

4. A dressing as claimed in claim 3 wherein the film has a moisture vapour permeability of greater than 500g/m² at 37^βC per 24 hours and relative humidity of from 10% to 100%.

5. A dressing as claimed in claim 1 or claim 2 wherein the substrate is a discontinuous film.

6. A dressing as claimed in any one of claims 1 to 5 wherein the substrate is an elastomer.

7. A dressing as claimed in claim 6 wherein the substrate is a polyurethane.

8. A dressing as claimed in any one of the preceding claims wherein the peeling energy of the coating is not more than 10jπr².

9. A dressing as claimed in any one of the preceding claims wherein the coating polymer is an elastomer.

10. A dressing as claimed in claim 9 wherein the coating polymer comprises a polyurethane.

11. A dressing as claimed in any one of the preceding claims wherein the polymer comprises residues of a siloxane.

12. A dressing as claimed in either claim 10 or claim

11 wherein the polymer is siloxane modified polyurethane.

13. A dressing as claimed in any one of claims 9 to

12 wherein the coating polymer is a blend of a polyurethane and a siloxane-containing polymer.

14. A dressing as claimed in any one of claims 11 to

13 wherein the polymer comprises upto 10% by weight of units derived from a siloxane.

15. A dressing as claimed in claim 13 comprising a layer of an absorbent material over the uncoated side of the polymer film.