WO2009019485A2

WO2009019485A2 - A wound dressing

Info

Publication number: WO2009019485A2
Application number: PCT/GB2008/002689
Authority: WO
Inventors: Hugh Semple Munro; Philip Andrews
Original assignee: First Water Limited
Priority date: 2007-08-06
Filing date: 2008-08-06
Publication date: 2009-02-12
Also published as: GB0715198D0; GB2464080A; GB201002858D0; GB2464080B; WO2009019485A3

Abstract

The present invention provides a wound dressing comprising (i) an absorbent layer portion capable of absorbing wound exudates and arranged to receive wound exudates in use, the absorbent layer portion comprising a hydrogel; and (ii) a backing layer which overlies the absorbent layer portion to a side, which, in use, is directed away from the wound, and wherein the backing layer has a moisture vapour transmission rate (MVTR) of from about 300 to about 1100 g/m2/24hours.

Description

A WOUND DRESSING

Field of the Invention

The present invention relates to a wound dressing suitable for use in the treatment of wounds and burns, particularly wounds and burns that produce a considerable amount of exudates.

Background and Prior Art

Wounds or lesions typically exude liquid material after formation and during the healing process. The sort of dressing that can be used effectively to cover a wound and allow it to heal depends on the type of wound and the amount of material it exudes. Wounds can be categorised according to their exudation. The general categorisation is into the three categories 'high exudation', 'medium exudation' and 'low exudation'. When selecting a dressing, a balance needs to be struck between the desire to remove exudate from the wound and maintaining an appropriate level of fluid in and around the wound to prevent it becoming too dry or too wet.

Recent developments in wound dressings include dressings having an absorbent, hydrophilic polymer-containing part and a backing. The function of the hydrophilic polymer-containing part is to absorb water and keep the wound in a suitably moist condition, which promotes healing. The hydrophilic polymer-containing part in some dressings of the prior art is a foamed hydrophilic polyurethane. The function of the backing is to prevent bacterial infection of the wound from external sources. The backing is therefore generally impermeable to liquids. However, to allow removal of moisture from the dressing, the backing is generally permeable to water vapour.

It has been found that some of the foamed or fibrous dressings of the prior art, particularly the polyurethane foam dressings, can cause problems when used on high or medium exudation wounds. For example, in hydrophilic polyurethane foams much of the water is stored within its pores (rather than in the polyurethane itself) and, once capacity is reached on absorption of a considerable amount of liquid exudate, the absorbed exudate has a tendency to leak back into the wound and onto the surrounding area from the dressing. This is detrimental to the healing process of the wound and may also cause infection. For this reason, the current views in the field are that, if one wishes to use absorbent dressings on medium or high exudation wounds, they should have a backing with a high MVTR to allow the water within the dressing to evaporate out of the dressing quickly, which should prevent absorbed exudate re-contacting the wound. This should also avoid maceration of the area surrounding the wound.

US 5445604 discloses a wound dressing for use with heavily exuding wounds. The dressing comprises a foamed, hydrophilic absorbent layer and a backing layer. An elastomeric apertured film for contact with the wound is disposed on the absorbent layer on the opposite side from the backing layer. The absorbent layer comprises a polyurethane foam, which adsorbs water, via the apertured film, by means of capillary action through the porous network. The water is largely retained within the pores of the polyurethane foam. The backing layer has a moisture vapour transmission rate of preferably at least 1000 g/m²/24hours, and up to 5000 g/m²/24hours (measured at 37

⁰C and at 100 % to 10 % relative humidity difference). This document does not disclose the use of a hydrogel in the dressing. In the present application, hydrogels include, but are not limited to, materials comprising hydrophilic polymers that can absorb at least 2.5 time their own weight in water into the material (as opposed to simply holding the water within the pores of a foamed material).

WO 00/41661 and EP-A-I 759 677 discloses a wound dressing having a fibrous absorbent layer, and a backing layer with a high vapour transmission rate. This document states that the MVTR of the backing layer should ideally be at least 3000 g/m²/24hours to avoid maceration of the skin surrounding the wound.

WO 98/31402 describes a wound dressing comprising a backing layer, an apertured wound facing layer and an intermediate absorbent layer. This document states that the backing layer suitably has a MVTR of at least 1000 g/m²/24hours, preferably at least 1600 g/m²/24hours.

GB 2 290 031 describes a dressing for moist wounds having, inter alia, a body layer of moisture-absorbent foam material and an outer backing or barrier layer. This document states that preferred values for the MVTR of the backing layer are greater than 1000 g/m²/24hours.

While the dressings of the prior art mentioned above are adequate in some circumstances, they still have their drawbacks and do not necessarily provide a desired balance between promoting the healing of a heavily exuding wound, while avoiding maceration of the wound or drying out of the dressing. It is an object of the present invention to overcome or mitigate at least some of the problems associated with the dressings of the prior art, or at least provide an alternative to the dressings of the prior art.

Brief Description of the Invention

The present invention provides a wound dressing comprising (i) an absorbent layer portion capable of absorbing wound exudates and arranged to receive wound exudates in use, the absorbent layer portion comprising a hydrogel; and (ii) a backing layer which overlies the absorbent layer portion to a side, which, in use, is directed away from the wound, and wherein the backing layer has a moisture vapour transmission rate (MVTR) of from about 300 to about 1100 g/m²/24hours.

The present inventors have found that the dressing of the present invention is particularly effective at treating medium and high exudation wounds. The dressing is able to maintain an appropriate amount of moisture within the wound to promote healing, while substantially avoiding both maceration of the surrounding skin and drying out of the dressing. The combination of a backing having the MVTR within the desired range and an absorbent layer comprising a hydrogel (particularly a hydrogel comprising one or more hydrophilic polymers carrying multiple pendant sulphonyl groups on each polymer molecule) has been found to be particularly effective at absorbing exudate, keeping the wound in a suitably moist condition to promote healing, while avoiding maceration of the skin around the wound.

In contrast to the expectations in the field, the present inventors have surprisingly found that hydrogel-containing dressings having backings with a very high transmission rate are not in fact suitable for wounds that exude a considerable amount of fluid. It has been found that, with wounds that exude a moderate or high amount of liquid, the hydrogel polymers in fact dry out relatively quickly. This can lead to the wound and/or surrounding skin adhering to the dressing, which is of course undesirable.

The present invention further provides a method of treating a wound, for example a chronic ulcerous skin lesion, in a human or non-human mammal, particularly a human, comprising contacting the wound for an effective period of time with the wound dressing of the present invention, the dressing being applied to the wound or burn such that the absorbent layer portion is disposed between the backing layer and the wound.

The effective period of time will vary from subject to subject, but generally speaking an effective period of time will be up to about six weeks, for example between about 3 days and 6 weeks, depending on the seriousness of the wound and whether it is acute or chronic. Regular changes of the dressing may be required, particularly with more serious and exuding wounds. The time between changes of dressing will generally be in the range of about 2 to about 7 days, preferably about 3 to about 7 days.

The absorbent layer portion comprises a hydrogel. The hydrogel preferably comprises a polymer derived from the polymerisation of substituted vinyl monomers, either alone or in combination with other monomers. Preferably, the substituted vinyl monomers are substituted with acid or ionic groups (which may, for example, be salts of acid groups or tertiary ammonium groups). The monomers may be acrylates or salts thereof, preferably sulfonated acrylates or salts thereof. The hydrogel may comprise one or more hydrophilic polymers selected from polymers of a polyalkylene glycol acrylate or a substituted derivative thereof; a polyalkylene glycol methacrylate or a substituted derivative thereof; acrylic acid and salts thereof; 2-acrylamido-2- methyl-propanesulphonic acid and salts thereof; acrylic acid (3-sulphopropyl) ester or a substituted derivative thereof or a salt thereof; diacetone acrylamide; a vinyl lactam; an optionally substituted N-alkylated acrylamide such as hydroxyethyl acrylamide; an optionally substituted N,N-dialkylated acrylamide; and N-acryloyl morpholine or a substituted derivative thereof. The polymers may be homo-polymers or co-polymers of the above monomers. Preferably, the absorbent layer portion comprises a hydrogel composition comprising a hydrophilic polymer carrying multiple pendant sulphonyl groups, and optionally multiple pendant carboxylic groups, on each polymer molecule. Such a hydrogel composition may be as disclosed in International Patent Publication No. WO2007/007115, which is incorporated herein by reference. In particular, the hydrogel may be as disclosed in International Patent Publication No. WO2007/007115 from line 21 on page 29 to line 2 on page 36.

Preferably, the hydrogel comprises a hydrophilic polymer of one or more of the following monomers: 2-acrylamido-2-methylpropane sulphonic acid (AMPS), acrylic acid (3-sulphopropyl) ester (SPA) and salts thereof. The salts may be any appropriate salts, for example sodium, potassium, lithium, caesium, calcium, magnesium, zinc or ammonium salts or mixtures thereof. In the case of copolymers, different counterions may be used in the different monomers for the copolymerisation.

The hydrogel composition has the capacity to absorb many times (e.g. at least about 2.5 times, for example at least about 5 times, for example at least about 10 times, for example between about 10 and about 50 times, and potentially up to about 250 times) its own weight of exudate or other fluid (e.g. water) in 24 hours. Therefore, the exudate management capacity of the composition can be selected according to the intended target patients and lesions for treatment. The hydrogel preferably has a water activity greater than 0.4, for example greater than 0.5, for example greater than 0.6, for example greater than 0.7, preferably greater than 0.8, preferably greater than 0.9, preferably greater than 0.95, preferably greater than 0.97 but less than 0.99 in the absence of maceration. In the presence of maceration the hydrogel preferably has a water activity less than 0.95, more preferably less than 0.9. In some instances the water activity of the hydrogel may be lower than 0.4. The hydrogel for use in the present invention may have a water activity in the range of 0.5 to 0.89. Detailed Description

The present invention further provides a method of treating a wound or lesion, for example a chronic ulcerous skin lesion, in a human or non-human mammal, particularly a human, comprising contacting the wound for an effective period of time with the wound dressing of the present invention, the dressing being applied to the wound or burn such that the absorbent layer portion is disposed between the backing layer and the wound.

Preferably, the MVTR of the backing layer is from about 400 to about 900 g/m²/24hours, more preferably from about 500 to about 800 g/m²/24hours.

The backing layer preferably has a water transmission rate (WTR) that is greater than the moisture vapour transmission rate (MVTR). This has been found to be advantageous when the dressing, particularly a dressing comprising having an internal cellular structure, has absorbed a considerable amount of fluid and is reaching its liquid-storing capacity; it allows increased water evaporation through the dressing to rapidly reduce its water content to a desirable level. Preferably, the WTR is at least about 700 g/sq.m/24hours and at most about 2000m/sq.m/24hours.

WTR and MVTR are known measurements in the field. The methods used to measure WTR and MVTR in the present application are similar to the methods disclosed on page 9 of EP-A-0091800, except that the temperature within the oven was 40 °C and the relative humidity within the oven was 55%. (In EP 0091800, "dry" MVP and "wet" MVP mean MVTR and WTR, respectively). Unless otherwise stated in this application, when measuring WTR and MVTR, 20 ml Calcium Saline (as specified in the Examples below) was used in Paddington Cups as supplied by The Surgical Materials Testing Laboratory (http://www.smtl.co.uk/). The oven used was a Mercia Scientific Humidity Cabinet, as supplied by Mercia, Stockton, UK. Further details of the test method used in the present invention are given in the Examples below.

The backing layer overlies the absorbent layer portion. It may be in contact with the absorbent layer, or one or more other materials (e.g. layers) may be disposed between the backing layer and the absorbent layer portion. Such other materials may, for example, comprise an adhesive layer, as is known to those skilled in the art. The backing layer may, for example, comprise polyurethane, e.g. in the form of a sheet. The polyurethane may be in foam form or in the form of a continuous layer (i.e. containing few, if any, voids). Suitable polyurethane backing layers are commercially available from Intelicoat Ltd, Wrexham, UK under the trade names Inspire 2317™, 2321 ™ and 2408 ™. The backing layer may extend beyond the margins of the absorbent layer portion, and may be provided with a skin adhesive portion to secure the dressing to the skin. The skin adhesive portion may be hydrogel in nature (for example a plasticised tacky hydrogel, which may be the same as or different from the hydrogel of the absorbent layer portion), or may be another type of skin adhesive selected from the many skin adhesives known in the wound dressing art.

The absorbent layer portion comprises a hydrogel. The absorbent layer portion may consist essentially of or consist of a hydrogel. A material that "consists essentially of a hydrogel includes, but is not limited to, a material comprising 10% or less by weight of non-hydrogel material, preferably 5% or less by weight of non-hydrogel material, most preferably less than 2% or less by weight of non-hydrogel material.

The absorbent layer portion may comprise a material having an internal cellular structure, i.e. a material containing internal cellular voids, which may, for example, be a foam. The material having the internal cellular structure may comprise a hydrogel, e.g. in the form of a hydrogel foam. The material having the internal cellular structure may consist essentially of or consist of a hydrogel. A hydrogel, which may be the same as or different from the hydrogel in the material having the internal cellular structure, may be present within at least some of the voids of the cellular structure. In a further alternative, the material having the internal cellular structure may comprise a non-hydrogel material, e.g. a non-hydrogel foam, with the hydrogel being present within the voids of the cellular structure/foam. Preferably the absorbent layer portion comprises, consists essentially of or consists of a foam comprising polyurethane, preferably a hydrophilic polyurethane foam. Suitable such foams are commercially available from Rynel (e.g. Medical grade hydrophilic polyurethane foam, 562) (www.rynel.com) and from Polymer Health Technology, Wales, UK (e.g. example 3019) (www.polyhealth.com). These hydrophilic foams and others used in the prior art such as Hypols are not hydrogels as the term is used in relation to the present invention.

The hydrogel present within the pores may or may not itself comprise internal cellular voids. The hydrogel within the pores may be a non-foamed hydrogel. The hydrogel present within at least some of the voids (e.g. essentially all) of the cellular structure may fill at least some (e.g. essentially all) and/or may coat the wall surfaces of at least some (e.g. essentially all) of the voids.

The absorbent layer portion may comprise a material having an internal cellular structure, as described above, with a partial or complete layer consisting essentially of or consisting of hydrogel covering the wound-facing side thereof (i.e. the side facing away from the backing layer). In other words, the hydrogel may be present as a layer on at least part of the wound-facing side of the material having an internal cellular structure. In this embodiment, the material having an internal cellular may or may not comprise a hydrogel. If this material does comprise a hydrogel, the partial or complete layer of hydrogel may comprise the same or a different hydrogel. The layer of hydrogel may be non-cellular (i.e. continuous and containing few, if any, voids).

The hydrogel may be formed in situ on or in the material having the internal cellular structure, e.g. by coating a hydrogel precursor material onto the material having the internal cellular structure and forming the hydrogel from the precursor material. Suitable hydrogel precursor materials are known to those skilled in the art. Formation of the hydrogel may involve polymerising and/or crosslinking the precursor material. The hydrogel may be coated on to the material having the internal cellular structure. The amount of hydrogel coated on to the material having the internal cellular structure is preferably more than about 50 g/m² and may be in the range of from about 100 g/m to about 1000 g/m², preferably about 150 to about 300 g/m², most preferably about 170 to about 230 g/m², most preferably about 200 g/m². The amount of hydrogel coated on to the material having the internal cellular structure is preferably less than about 500 g/m², more preferably less than about 300 g/m².

The absorbent layer portion may be non-cellular, (i.e. continuous and containing few, if any, voids). The absorbent layer portion may be a sheet comprising, consisting essentially of or consisting of hydrogel, that may, for example, have been coated on to the backing layer. The amount of hydrogel coated on to the backing layer is preferably more than about 50 g/m² and may be in the range of from about 100 g/m² to about 1000 g/m², preferably about 150 to about 300 g/m², most preferably about 170 to about 230 g/m², most preferably about 200 g/m². The amount of hydrogel coated on to the backing layer is preferably less than about 500 g/m², more preferably less than about 300 g/m².

The absorbent layer may be non- fibrous.

At least some of the hydrogel is preferably present on an exposed surface of the material having an internal cellular structure and therefore able to contact a wound.

The dressing may comprise a net member, which, in use, is disposed between the absorbent layer portion and the wound or lesion. The net member may be as described in EP 1 649 873 A2, which is incorporated herein by reference, for example as described in paragraphs [0052] to [0060] of EP 1 649 873 A2. Net members for use in wound dressings and for contacting the wound are known in the art. The net member will suitably be porous and may comprise a hydrophobic material.

The absorbent layer portion preferably can absorb at least about 2.5 times, for example at least about 5 times, for example at least about 10 times, for example between about 10 and about 50 times, and potentially up to about 250 times its own weight of exudate (e.g. water) or other fluid in 24 hours. The Hydrogel, Dressing and Treatment

The absorbent layer portion comprises a hydrogel, which may be termed a hydrogel composition. The expression "hydrogel" and like expressions, used herein, are not to be considered as limited to gels which contain water, but extend generally to all hydrophilic gels, including those containing organic non-polymeric components in the absence of water. The gel forming agent may, for example, be selected from natural hydrophilic polymers, synthetic hydrophilic polymers, gelling hydrophilic biopolymers and all combinations thereof. The term "hydrogel" is used herein regardless of the state of hydration, and therefore includes, for example, hydrogels that are in a dehydrated or anhydrous state or in a state of partial hydration.

Hydrogels are described in greater detail in Hydrogels, Kirk-Othmer Encyclopedia of Chemical Technology, 4^th Edition, vol. 7, pp. 783-807, John Wiley and Sons, New York, the contents of which are incorporated herein by reference.

The expression "polymer" and like expressions, used herein, includes homopolymers, copolymers and all mixtures and combinations thereof.

Hydrogels are, generally speaking, hydrophilic polymers characterized by their hydrophilicity (i.e capacity to absorb large amounts of fluid such as wound exudate) and insolubility in water: i.e. they are capable of swelling in water while generally preserving their shape.

The hydrophilicity is generally due to groups such as hydroxyl, carboxy, carboxamido, and esters, among others. On contact with water, the hydrogel assumes a swollen hydrated state that results from a balance between the dispersing forces acting on hydrated chains and cohesive forces that do not prevent the penetration of water into the polymer network. The cohesive forces are most often the result of crosslinking, but may result from electrostatic, hydrophobic or dipole-dipole interactions. The hydrogels in the present invention may include a hydrophilic polymer carrying multiple pendant sulphonyl groups on each polymer molecule.

Generally, the degree of sulphonylation of such a polymer is on average (number average) at least about one pendant sulphonyl group per linear 150 carbon atoms of the carbon atom backbone of the polymer, for example per linear 100 carbon atoms of the carbon atom backbone of the polymer, for example per linear 50 carbon atoms of the carbon atom backbone of the polymer, for example per linear 30 carbon atoms of the carbon atom backbone of the polymer, for example at least about one pendant sulphonyl group per linear 12 carbon atoms of the carbon atom backbone of the polymer, for example at least about one pendant sulphonyl group per linear six carbon atoms of the carbon atom backbone of the polymer. More preferably, the polymer will contain on average at least about two pendant sulphonyl groups per linear six carbon atoms of the carbon atom backbone of the polymer, for example up to about three pendant sulphonyl groups per linear six carbon atoms of the carbon atom backbone of the polymer. At the higher levels of sulphonylation it is preferred that pendant carboxylate groups will be substantially absent.

Most preferably, the polymer contains one pendant sulphonyl group per linear two carbon atoms of the carbon atom backbone of the polymer. Such a polymer is readily prepared by polymerising (meth)acrylic acid derivatives such as esters or amides using monomers containing one sulphonyl group per molecule. The sulphonyl groups may be present in acid, ester, salt or other suitable form, and may be covalently linked to the carbon atom backbone of the polymer. A suitable sulphonyl moiety is the -SO₃ ^" species, either in acid form (-SO₃H) or in salt form (-SO₃M), where M is a univalent metal counterion, or -SO₃MO₃S- where M is a divalent metal counterion), or the organic sulphate species (for example, -0-SO₃H in acid form, or in corresponding salt form). Suitable linking moieties include alkylene bridges, alkylene-ester bridges, -O- bridges and alkylene-amide bridges. The alkylene moieties may be straight or branched, saturated and preferably contain from 1 to about 8 carbon atoms.

Such hydrophilic polymers include, for example, polymers derived from (meth)acryloyloxyalkylsulphonates, polymers of sulpho-substituted acrylamides such as acrylamidoalkanesulphonic acids, polymers of salts of any of the foregoing (for example, alkali or alkaline earth metal salts or ammonium or quaternary organ- ammonium salts), or any combination thereof. Mixtures of such polymers with each other are also envisaged.

Such polymers may, if desired, be used together with sulpho-free polymers. Such other polymers, if present, may suitably be selected from homopolymers or copolymers of acrylic and methacrylic acid esters, including hydroxyalkyl (meth)acrylates, 2-(N,N-dimethylamino)ethyl methacrylate, polymers and copolymers of other substituted and unsubstiruted acrylamides, polymers and copolymers of N- vinylpyrrolidinone, and polyelectrolyte complexes.

The hydrogel may be present on at least part of the wound- or lesion-contacting surface of the absorbent layer portion. The hydrophilic polymer carrying multiple pendant sulphonyl groups, optionally with multiple pendant carboxylic groups, on each polymer molecule may be present at least at the lesion-contacting surface of the absorbent layer portion and/or the hydrogel composition, which may form at least part of the absorbent layer portion. If desired, the hydrophilic polymer carrying multiple pendant sulphonyl groups, optionally with multiple pendant carboxylic groups, on each polymer molecule may also be present in the internal bulk of the composition and/or the absorbent layer portion, and/or a sulphonyl-free polymer or combination of polymers may be present in the internal bulk of the composition and/or the absorbent layer portion.

Generally, the degree of carboxylation of such a polymer is on average (number average) at least about one pendant carboxylic group per linear 100 carbon atoms of the carbon atom backbone of the polymer, for example up to about one pendant carboxylic group per linear six carbon atoms of the carbon atom backbone of the polymer.

The hydrogel used in the present invention suitably comprises a substantially water- insoluble, slightly crosslinked, partially neutralized, gel-forming polymer material having the pendant sulphonyl groups, and optionally pendant carboxylic groups, in acid or salt form at least at its lesion-contacting surface. Such polymer materials can be prepared from polymerizable, unsaturated, acid- and ester-containing monomers. Any polymer to be present at the lesion-contacting surface of the absorbent layer portion and/or hydrogel composition will preferably contain pendant sulphonyl groups, for example -SO₃ ^" in acid or salt form, and optionally carboxylic groups in acid or salt form. Thus, such monomers include the olefinically unsaturated acids, esters and anhydrides which contain at least one carbon to carbon olefinic double bond. More specifically, these monomers can be selected from olefinically unsaturated carboxylic acids, carboxylic esters, carboxylic acid anhydrides; olefinically unsaturated sulphonic acids; and mixtures thereof.

Olefinically unsaturated carboxylic acid, carboxylic acid ester and carboxylic acid anhydride monomers include the acrylic acids typified by acrylic acid itself, methacrylic acid, ethacrylic acid, α-chloroacrylic acid, α-cyano-acrylic acid, β- methyl-acrylic acid (crotonic acid), α-phenyl acrylic acid, β-acryloxy-propionic acid, sorbic acid, α-chloro-sorbic acid, angelic acid, cinnamic acid, p-chloro-cinnamic acid, β-styryl-acrylic acid (l-carboxy-4-phenyl-l,3 -butadiene), itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fumaric acid, tricarboxy- ethylene and maleic acid anhydride and salts (e.g. alkali metal salts such as sodium, potassium and lithium salts) thereof. For forming any polymer to be present at the lesion-contacting surface of the hydrogel composition and/or the absorbent layer portion, the monomer or monomer mixture will preferably include a monomer containing pendant sulphonyl groups, e.g. -SO₃ ^" in acid or salt form.

Olefinically unsaturated sulphonic acid monomers include aliphatic or aromatic vinyl sulphonic acids such as vinylsulphonic acid, allylsulphonic acid, vinyltoluenesulphonic acid and styrene sulphonic acid; vinyl sulphobetaines such as SPDA (1-propanaminium N,N-dimethyl-N-[2-[(l-oxo-2-propenyl)oxy]-3-sulfo hydroxide, inner salt (available from Raschig); acrylic and methacrylic sulphonic acid such as sulphoethyl acrylate, sulphoethyl methacrylate, sulphopropyl acrylate, sulphopropyl methacrylate, 2-hydroxy-3-acryloxy propyl sulphonic acid, 2-hydroxy- 3- methacryloxy propyl sulphonic acid and 2-acrylamido-2-methyl-propanesulphonic acid and salts (e.g. ammonium or alkali metal salts, such as sodium, potassium and lithium salts, or alkaline earth metal salts, such as calcium or magnesium) thereof. The monomers may suitably be used in admixture with each other or with other monomers. In one particularly useful embodiment of the invention, a monomer which has a first counter-cation associated with it may be used in admixture with one or more monomer which has/have one or more second/further counter-cation(s) associated with it/them. The monomers in their anionic form (i.e. disregarding the counter-cation) may be the same or different. In this way, the proportions of different cations (e.g. alkali metal ions such as sodium or potassium, or ammonium ions) can be finely controlled in the resultant polymer (homopolymer or copolymer). The particular weight ratios of one monomer to the or each other monomer can be selected within wide limits by those skilled in the art, depending on the desired properties of the resultant hydrogel polymer. In a further embodiment of the present invention, two or more different types of anionic monomers (disregarding the countercation) may be used to form the hydrogel and each monomer may have the same countercation.

Further examples of suitable monomers for use in the present invention include: a polyalkylene glycol acrylate or a substituted derivative thereof; a polyalkylene glycol methacrylate or a substituted derivative thereof; acrylic acid and salts thereof (e.g. alkali metal salts such as sodium, potassium and lithium salts); 2-acrylamido-2- methyl-propanesulphonic acid and salts thereof (e.g. ammonium or alkali metal salts, such as sodium, potassium and lithium salts, or alkaline earth metal salts, such as calcium or magnesium); acrylic acid (3-sulphopropyl) ester or a substituted derivative thereof or a salt thereof (e.g. an alkali metal salt such as sodium, potassium or lithium salt); diacetone acrylamide (N-l,l-dimethyl-3-oxobutyl-acrylamide); a vinyl lactam (e.g. N-vinyl pyrrolidone or a substituted derivative thereof); an optionally substituted N-alkylated acrylamide such as hydroxyethyl acrylamide; and an optionally substituted N,N-dialkylated acrylamide; and/or N-acryloyl morpholine or a substituted derivative thereof. For forming any polymer to be present at the lesion-contacting surface of the absorbent layer portion and/or the hydrogel composition, the monomer or monomer mixture may include a monomer containing pendant sulphonyl groups, e.g. -SO₃ ^" in acid or salt form, and optionally carboxylic groups in acid or salt form.

Preferably, if the hydrogel comprises one or more hydrophilic polymer salts, it will comprise sodium ions, either alone or in combination with one or more other salt forms such as, for example, potassium, magnesium, zinc, calcium or ammonium salts. The ammonium salts may be organo-ammonium salts (i.e. containing primary ammonium, secondary ammonium, tertiary ammonium and/or quaternary ammonium cations). A combination of sodium and potassium counterions can be particularly suitable. Where a combination of counterions is present in the hydrogel, any multivalent counterion (e.g. one or more of magnesium, zinc, calcium) is suitably present in a total molar proportion of up to about 5 mol% relative to the monovalent counterions, such as the sodium ions. Two or more different countercations may be present in the hydrogel, the first of which is preferably the relatively more strongly hydrated according to the Hofmeister series of cations and the second is the relatively more weakly hydrated according to the Hofmeister series of cations. Preferably, the molar ratio of the first to the second countercations in the hydrophilic polymer is less than about 250:1, preferably less than about 200:1, for example less than about 100:1, for example less than about 80:1, for example less than about 50:1, and preferably more than about 2:1. For example, the ratio may be between about 2:1 and about 250:1, for example between about 5:1 and about 200:1, for example between about 5:1 and about 100:1, for example between about 7:1 and about 100:1, for example between about 10:1 and about 100:1.

The first cation may, for example, be sodium and the second may, for example, be selected from potassium, primary ammonium, secondary ammonium, tertiary ammonium and quaternary ammonium, or the first may be potassium and the second may be selected from primary ammonium, secondary ammonium, tertiary ammonium and quaternary ammonium.

The above monomers and monomer types may optionally include substituent groups. Optional substituents of the monomers used to prepare the hydrogels used in the present invention may preferably to selected from substituents which are known in the art or are reasonably expected to provide polymerisable monomers which form hydrogel polymers having the properties necessary for the present invention. Suitable substituents include, for example, lower alkyl, hydroxy, halo and amino groups.

In one particular form of the present invention, the hydrogel material may be free of uncrosslinked polymerised styrene sulphonates. In another particular form of the present invention, the hydrogel material may be free of any styrene sulphonate component, whether polymerised or unpolymerised and whether crosslinked or uncrosslinked. In another form of the invention, the hydrogel material and/or the absorbent layer portion may be free of hydrophilic polyurethane material. In another form of the invention, the hydrogel material and/or the absorbent layer portion may be free of hydrocolloids.

The hydrogel used in the present invention preferably comprises a flexible three- dimensional polymer matrix. The hydrogel may be present in a composite in association with one or more other component selected from other hydrogels, hydrocolloids and non-hydrogel polymers, for example a polyurethane hydrogel. The hydrogel or composite may be a plasticised three-dimensional matrix of cross-linked and/or entangled polymer molecules, and preferably has sufficient structural integrity to be self-supporting even at very high levels of internal water content, with sufficient flexibility to conform to the surface contours of mammalian, preferably human, skin or other surface with which it is in contact.

The hydrogel generally comprises, in addition to the cross-linked polymeric network, an aqueous or non-aqueous plasticising medium including an organic plasticiser. This plasticising medium is preferably present in the same precursor solution as the monomer(s).

The hydrogel or composite (e.g. a composite with a polyurethane hydrogel) or any portion thereof may be present as a foam, i.e. including a rigid cellular internal structure. Methods for obtaining such hydrogels are disclosed, for example, in WO-A- 03/077964, the disclosure of which is incorporated herein by reference.

The hydrogel composition may suitably be present as a thin sheet, preferably supported by a sheet support member to provide mechanical strength. The sheet support member for the hydrogel may, for example, be a thin scrim or net structure, for example formed of a synthetic and/or natural polymer such as polyethylene or polypropylene. The sheet support member for the hydrogel may overlie the hydrogel sheet on the major face of the sheet directed away from the lesion in use, or may be embedded within the hydrogel polymer. The sheet support member may, if desired, extend beyond the margins of the hydrogel composition and/or the absorbent layer portion, and may be provided with a skin adhesive portion to secure the dressing to the skin. The skin adhesive portion may be hydrogel in nature (for example a plasticised tacky hydrogel, which may be the same as or different from the hydrogel provided on the support member for the treatment according to the present invention), or may be another type of skin adhesive selected from the many skin adhesives known in the wound dressings art.

The hydrogel sheet may be part of a multi-layer composite, including further layers such as further hydrogels and/or other polymers and/or other sheet support members. For example, a breathable (air and/or moisture permeable) polymeric film (e.g. of polyurethane), which may if desired be present as a foam, may overlie the hydrogel sheet or composite on the major face of the sheet or composite directed away from the lesion in use. The breathable polymeric film may be or constitute part of the backing layer.

The hydrogel composition and other sheet components as desired may preferably be provided with a release layer (e.g. of non-stick paper or plastic, such as siliconised paper or plastic) to protect one or both major face of the sheet prior to use.

The dressing (e.g. hydrogel composition and other sheet components as desired) can constitute a dressing for a wound, e.g. a chronic ulcerous skin lesion which can, after removal of any release layer as appropriate, be applied to the lesion directly so that the major face, which may present at its surface the hydrogel of the absorbent layer portion, e.g. a hydrogel carrying pendant sulphonyl groups, is directed towards the lesion and contacts the lesion, preferably the wound bed and surrounding tissues.

If desired, conventional bandages, cloths or other protective fabrics or materials can subsequently be applied to encase the dressing and hold it in place on the lesion.

Particularly where the hydrogel is plasticised, there is very slight adhesion between the hydrogel dressing and the patient's skin or the lesion tissue. This has the beneficial effect that one nurse or other healthcare professional can apply the dressing and can then prepare any desired bandages, cloths or the like for subsequent application. The dressing of the present invention will remain in place because of the mild adhesion, even if the patient moves before the further bandages etc. are applied.

The precursor liquid can comprise a solution of the gel-forming polymer in a relatively volatile solvent, whereby the hydrogel is deposited as a residue on evaporation of the solvent, or - more preferably - the precursor liquid will comprise a solution of the monomer(s), cross-linking agent, plasticiser, and optionally water and other ingredients as desired, whereby the hydrogel is formed by a curing reaction performed on the precursor liquid after application to the substrate to which the hydrogel is to be applied.

Preparation of the Hvdrogel and Dressing

In the following discussion, the in situ polymerisation of the hydrogel will be discussed. The alternative solvent deposition method carried out on a pre- formed gel- forming polymer is well known and the details of that procedure do not need to be reproduced here. The hydrogel may be formed in situ on one or more components of the dressing, e.g. the backing layer, a material having an internal cellular structure (if present), or a release layer, if used. The further components of the dressing, to which the hydrogel has not already been adhered, can then be overlaid on the exposed face of the hydrogel, e.g. the release layer or backing layer, as appropriate.

The hydrogel may be prepared as described in International Patent Publication No. WO2007/007115 from line 4 on page 36 to line 27 on page 42. In particular, the hydrogel may be prepared using a photoinitiator, and may contain one or more cross- linking agents, organic plasticizers, surfactants, and other additives as described therein.

The polymerisation reaction is preferably a free-radical polymerisation with cross- linking, which may for example be induced by light, heat, radiation (e.g. ionising radiation), or redox catalysts, as is well known.

For example, the free radical polymerisation may be initiated in known manner by light (photoinitiation), particularly ultraviolet light (UV photoinitiation); heat (thermal initiation); electron beam (e-beam initiation); ionising radiation, particularly gamma radiation (gamma initiation); non-ionising radiation, particularly microwave radiation (microwave initiation); or any combination thereof. The precursor solution may include appropriate substances (initiators), at appropriate levels, e.g. up to about 5% by weight, more particularly between about 0.002% and about 2% by weight, which serve to assist the polymerisation and its initiation, in generally known manner. Preferred photoinitiators include any of the following either alone or in combination: Type I-α-hydroxy-ketones and benzilidimethyl-ketals e.g. Irgacure 651 (2,2- dimethoxy-2-phenylacetophenone). These are believed on irradiation to form benzoyl radicals that initiate polymerisation. Photoinitiators of this type that are preferred are those that do not carry substituents in the para position of the aromatic ring.

Preferred photoinitiators are 1 -hydroxycyclohexyl phenyl ketone, for example as marketed under the trade name Irgacure 184 by Ciba Speciality Chemicals; Irgacure 651 (2,2-dimethoxy- 2-phenylacetophenone); Darocur 1173 (2-hydroxy-2-propyl phenyl ketone); and mixtures of Irgacure 184 and Darocur 1173.

Photo-polymerisation is particularly suitable, and may be achieved using light, optionally together with other initiators, such as heat and/or ionising radiation. Photoinitiation will usually be applied by subjecting the pre-gel reaction mixture containing an appropriate photoinitiation agent to ultraviolet (UV) light. The incident UV intensity, at a wavelength in the range from 240 to 420nm, is typically greater than about lOmW/cm². The processing will generally be carried out in a controlled manner involving a precise predetermined sequence of mixing and thermal treatment or history.

The UV irradiation time scale should ideally be less than 60 seconds, and preferably less than 10 seconds to form a gel with better than 95% conversion of the monomers. Those skilled in the art will appreciate that the extent of irradiation will be dependent on a number of factors, including the UV intensity, the type of UV source used, the photoinitiator quantum yield, the amount of monomer(s) present, the nature of the monomer(s) present and the presence of polymerisation inhibitor. The precursor solution (pre-gel) containing the monomer(s) and preferably cross- linking agent, water, plasticiser, photoinitiator and optionally other components as described below, is initially laid down on a substrate. Where the hydrogel composition is to be prepared in sheet form, the substrate will be a sheet. It may suitably comprise the backing layer or a release layer and any desired sheet support member that may be interposed between the release layer and the hydrogel composition, or embedded within the hydrogel composition, in the finished dressing. In this way, the precursor solution can be polymerised is situ on the release layer, preferably with all or substantially all other components of the final dressing in place.

In one preferred embodiment, (on the one hand) the precursor solution in contact with the substrate to which it is to be applied and (on the other hand) the source of the polymerisation initiator (e.g. the radiation source) may move relative to one another for the polymerisation step. In this way, a relatively large amount of polymerisable material can be polymerised in one procedure, more than could be handled in a static system. This moving, or continuous, production system is preferred.

After completion of the polymerisation, the product is preferably sterilised in conventional manner. The sterile composite may be used immediately, e.g. to provide a skin-adhesive layer in an article, or a top release layer may be applied to the composite for storage and transportation of the composite.

If desired, certain ingredients of the hydrogel may be added after the polymerisation and optional cross-linking reaction. However, it is generally preferred that substantially all of the final ingredients of the hydrogel are present in the precursor solution, and that - apart from minor conventional conditioning or, in some cases, subsequent modifications caused by the sterilisation procedure - substantially no chemical modification of the hydrogel takes place after completion of the polymerisation reaction.

Monomers

The monomers are discussed in more detail above. Particularly preferred monomers include: the sodium salt of 2-acrylamido-2-methylpropane sulphonic acid, commonly known as NaAMPS, which is available commercially at present from Lubrizol as either a 50% aqueous solution (reference code LZ2405) or a 58% aqueous solution (reference code LZ2405A); the potassium salt of 2-acrylamido-2-methylpropane sulphonic acid (Potassium AMPS), which is available commercially at present from Lubrizol; the ammonium salt of 2-acrylamido-2- methylpropane sulphonic acid (Ammonium AMPS), which is available commercially at present from Lubrizol; acrylic acid (3-sulphopropyl) ester potassium salt, commonly known as SPA or SPAK (SPA or SPAK is available commercially in the form of a pure solid from Raschig); acrylic acid (3-sulphopropyl) ester sodium salt, commonly known as SPANa (SPANa is available in the form of a pure solid from Raschig); and SPDA. Acrylic acid (BASF) may be used as supplied or in partial or complete salt form where the salt counterion is an alkali metal (e.g. sodium or potassium), alkaline earth metal (e.g. calcium) or ammonium. Mixtures of any two or more of the above monomers may be used. When a mixture of the monomers is used, it may, for example, be a mixture of NaAMPS and SPAK, a mixture of NaAMPS and SPANa, a mixture of NaAMPS and Potassium AMPS, a mixture of NaAMPS and Ammonium AMPS, or a mixture of NaAMPS and acrylic acid. The relative amounts of the monomers in a mixture may be dictated by the desired ratio of counterions (e.g. potassium, sodium and ammonium) in the hydrogel, as well as the required properties of the copolymer, and may be selected easily by those skilled in the art, if necessary with routine testing of the copolymers prepared. See the discussion above, for information as to suitable molar ratios of sodium to potassium ions.

Cross-linking Agents

Conventional cross-linking agents are suitably used to provide the necessary mechanical stability and to control the adhesive properties of the hydrogel. The amount of cross-linking agent required will be readily apparent to those skilled in the art such as from about 0.01% to about 0.5%, particularly from about 0.05% to about 0.4%, most particularly from about 0.08% to about 0.3%, by weight of the total polymerisation reaction mixture. Typical cross-linkers include tripropylene glycol diacrylate, ethylene glycol dimethacrylate, triacrylate, polyethylene glycol diacrylate (polyethylene glycol (PEG) molecular weight between about 100 and about 4000, for example PEG400 or PEG600), and methylene bis acrylamide. Organic Plasticisers

The hydrogel may comprise one or more plasticiser, preferably one or more organic plasticiser. The one or more organic plasticiser, when present, may suitably comprise any of the following either alone or in combination: at least one polyhydric alcohol

(such as glycerol, polyethylene glycol, or sorbitol), at least one ester derived therefrom, at least one polymeric alcohol (such as polyethylene oxide) and/or at least one mono- or poly-alkylated derivative of a polyhydric or polymeric alcohol (such as alkylated polyethylene glycol). Glycerol is the preferred plasticiser.

An alternative preferred plasticiser is the ester derived from boric acid and glycerol. When present, the organic plasticiser may comprise up to about 45%, for example up to about 35%, for example up to about 25%, for example up to about 15%, by weight of the hydrogel composition.

Surfactants

Any compatible surfactant may optionally be used as an additional ingredient of the hydrogel composition. Surfactants can lower the surface tension of the mixture before polymerisation and thus aid processing. The surfactant or surfactants may be non- ionic, anionic, zwitterionic or cationic, alone or in any mixture or combination. The surfactant may itself be reactive, i.e. capable of participating in the hydrogel-forming reaction. The total amount of surfactant, if present, is suitably up to about 10% by weight of the hydrogel composition, preferably from about 0.05% to about 4% by weight.

The surfactant may, for example, comprise at least one propylene oxide/ethylene oxide block copolymer, for example such as that supplied by BASF PLC under the trade name Pluronic P65 or L64.

Other additives The hydrogel in the dressing of the present invention may include one or more additional ingredients, which may be added to the pre-polymerisation mixture or the polymerised product, at the choice of the skilled worker. Such additional ingredients are selected from additives known in the art, including, for example, water, organic plasticisers, surfactants, polymeric material (hydrophobic or hydrophilic in nature, including proteins, enzymes, naturally occurring polymers and gums), synthetic polymers with and without pendant carboxylic acids, electrolytes, osmolites, pH regulators, colorants, chloride sources, bioactive compounds and mixtures thereof. The polymers can be natural polymers (e.g. xanthan gum), synthetic polymers (e.g. polyoxypropylene-polyoxyethylene block copolymer or poly-(methyl vinyl ether alt maleic anhydride)), or any combination thereof. By "bioactive compounds" we mean any compound or mixture included within the hydrogel for some effect it has on living systems, whether the living system be bacteria or other microorganisms or higher animals such as the patient. Bioactive compounds that may be mentioned include, for example, pharmaceutically active compounds, antimicrobial agents, antiseptic agents, antibiotics and any combination thereof. Antimicrobial agents may, for example, include: sources of oxygen and/or iodine (e.g. hydrogen peroxide or a source thereof and/or an iodide salt such as potassium iodide) (see, for example Bioxzyme™ technology, for example in The Sunday Telegraph (UK) 26 January 2003 or the discussion of the Oxyzyme™ system at www.wounds- uk.com/posterabstracts2003.pdf); honey (e.g. active Manuka honey); antimicrobial metals, metal ions and salts, such as, for example, silver-containing antimicrobial agents (e.g. colloidal silver, silver oxide, silver nitrate, silver thiosulphate, silver sulphadiazine, or any combination thereof), hyperchlorous acid; or any combination thereof.

In the Bioxzyme system, a dressing comprises two hydrogels. One contains glucose based antibacterial compounds and the other contains enzymes that convert the glucose into hydrogen peroxide. When these are exposed to air and contacted together at a wound site, the enzyme- containing gel being adjacent the skin and the glucose- containing gel overlying the enzyme- containing gel, a low level steady flow of hydrogen peroxide is produced, which inhibits anaerobic bacteria. This antibacterial effect can be enhanced by the inclusion of a very low level of iodide (less than about 0.04%) in the hydrogel. The hydrogen peroxide and the iodide react to produce iodine, a potent antimicrobial agent.

Hydrogels incorporating antimicrobial agents may, for example, be active against such organisms as Staphylococcus aureus and Pseudomonas aeruginosa.

Agents for stimulating the healing of wounds and/or for restricting or preventing scarring may be incorporated into the hydrogel. Examples of such agents include growth factors such as TGF (transforming growth factor), PDGF (platelet derived growth factor), KGF (keratinocyte growth factor, e.g. KGF-I or KGF-2), VEGF (vascular endothelial growth factor), IGF (insulin growth factor, optionally in assiciation with one or more of IGF binding protein and vitronectin), e.g. from GroPep Ltd, Australia or Procyte, USA (see, e.g. WO-A-96/02270, the contents of which are incorporated herein by reference); cell nutrients (see, e.g., WO-A-93/04691, the contents of which are incorporated herein by reference); glucose (see, e.g., WO-A- 93/10795, the contents of which are incorporated herein by reference); an anabolic hormone or hormone mixture such as insulin, triiodothyronine, thyroxine or any combination thereof (see, e.g., WO-A-93/04691, the contents of which are incorporated herein by reference); or any combination thereof.

Additional polymer(s), typically rheology modifying polymer(s), may be incorporated into the polymerisation reaction mixture at levels typically up to about 10% by weight of total polymerisation reaction mixture, e.g. from about 0.2% to about 10% by weight. Such polymer(s) may include polyacrylamide, poly-NaAMPS, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP) or carboxymethyl cellulose.

Additional osmolite(s) may be included to modify the osmolality of the hydrogel. Osmolites may be ionic (e.g. electrolytes, for example salts which are readily soluble in the aqueous phase of the hydrogel to increase the ionic strength of selected cations or anions and hence the osmolality of the hydrogel). By selecting the ions present in an ionic osmolite, and particularly by selecting the cation so as to correspond or not with cationic counterions in the monomer(s) of the hydrogel, the ionic strength of certain anions (e.g. chloride) can be varied with fine control, without substantially changing the ionic strength of cations already present in very large amounts as counterions of the monomer(s).

Osmolites may be organic (non-ionic), for example organic molecules which dissolve in or intimately mix with the aqueous phase of the hydrogel to increase the osmolality of the hydrogel deriving from non-ionic species in the aqueous phase. Such organic osmolites include, for example, water-soluble sugars (e.g. glucose, fructose and other monosaccharides; sucrose, lactose, maltose and other disaccharides; or any combination of mono- and di-saccharides), polyhydric alcohols (e.g. glycerol and other polyhydroxylated alkanols).

Additive ingredients may serve more than one purpose. For example, glycerol may serve as an organic plasticiser and an osmolite.

The hydrogel used in the present invention preferably consists essentially of a cross- linked hydrophilic polymer of a hydrophilic monomer and optionally one or more comonomer, together with water and/or one or more organic plasticiser, and optionally together with one or more additives selected from surfactants, polymers, pH regulators, electrolytes, osmolites, chloride sources, bioactive compounds and mixtures thereof, with less than about 40%, for example less than about 10%, by weight of other additives.

For further details of suitable hydrogel material for use in the present invention, and its preparation, please refer to the following publications: PCT Patent Applications Nos. WO-97/24149, WO-97/34947, WO-00/06214, WO-00/06215, WO-00/07638, WO-00/46319, WO-00/65143 and WO-01/96422, the disclosures of which are incorporated herein by reference.

The water activity of the hydrogel or of the precursor solution (as measured, for example, by a chilled mirror dewpoint meter, Aqualab T3) is preferably between 0.05 and 0.99, more preferably between, 0.2 and 0.99, and even more preferably between 0.3 and 0.98, for example between 0.6 and 0.89. The osmolality of the precursor solution can therefore be used to optimise the hydrogel properties. Hydrogel can be a mixture of pre-formed polymers. "Hydrogel" is not considered to be limited to gels which contain water, but extend generally to all hydrophilic gels that are able to absorb water and includes gels containing non-polymeric components in the absence of (i.e. in place of) water.

The absorbent layer portion may comprise a hydrogel composition as defined in any one of the Examples disclosed in International Patent Publication No. WO2007/007115 from page 44 to page 84.

The dressing may be for the treatment of a chronic ulcerous wound. The dressing may be for the treatment of a wound classified as a medium or high exudation wound. A medium exudation wound includes, but is not limited to, a wound that exudes 8 to 15 g of exudate in a period of 24 hours. A high exudation wound includes, but is not limited to, a wound that exudes more than 15 g of exudate in a period of 24 hours. A low exudation wound includes, but is not limited to, a wound that exudes less than 8 g of exudate in a period of 24 hours. The weight of exudate is that taken up into a conventional absorptive dressing (i.e. a dressing that merely blots the exudate from the wound) when applied to the wound in a conventional manner.

The present invention further provides the use of a hydrogel in the manufacture of a wound dressing of the present invention for the treatment of a wound or a burn, wherein, in the treatment, the dressing is applied to the wound or burn such that the absorbent layer portion is disposed between the backing layer and the wound.

The present invention provides a layered composition for the treatment of a wound or burn, wherein the composition comprises (i) an absorbent layer portion capable of absorbing wound exudates and arranged to receive wound or burn exudates in use, the absorbent layer portion comprising a hydrogel; and (ii) a backing layer which overlies the absorbent layer portion to a side, which, in use, is directed away from the wound, and wherein the backing layer has a moisture vapour transmission rate (MVTR) of from about 300 to about 1100 g/m²/24hours. The layered composition may be a wound dressing as described herein. The present invention will now be described with reference to the following non- limiting Examples.

Examples

Example IA - Method for Measuring the Water Transmission Rate of a Material

Apparatus Needed:

Sample of material

MVTR circle template

Scissors

Balance

Paddington cups (original type, with black screw cap - pre-labelled alphabetically)

Purified water

Normal Saline solution (0.88 to 0.92 g NaCl and purified water to 10Og)

Calcium Saline solution *(0.81 to 0.85 g NaCl and 0.027 to 0.029 g CaCl₂ and purified water to 10Og) (NB: use analytical grade anhydrous salts to prepare the isotonic solutions)

Torque controlled screwdriver

Tray

Oven at 40°C, 55% RH

Test Method:

1. Lay the sample (e.g. the material for use as the backing layer or the absorbent layer portion) down on a flat surface, and draw a circle using the MVTR template. Cut the circle out. Cut two circles for each sample to be analysed. 2. Take a Paddington cup, place on the balance and record the mass. Ensure that the black screw top is secure. 3. Fill the Paddington cup with 2Og of test fluid (pure water, Normal Saline or Calcium Saline), and record the mass. Record which fluid is used for each test.

4. Remove the lid from the Paddington cup. 5. Remove the protective liners (if present) from the side of the sample that would be in contact with the skin, and carefully stick the sample down, ensuring that there are no wrinkles, that the central hole is covered with gel, but that the screw holes are not.

6. Remove the protective liners (if present) from the upper surface of the sample. Put the lid back on and tighten the screws using the screwdriver until a torque of 40 cN.m is reached. (Very soft fragile samples may not be as tightly fastened as stronger ones - ensure that the sample has not ripped). Make a note, if <40cN.m.

7. Record the mass of the Paddington cup. 8. Repeat for all samples.

9. When all the Paddington cups have been prepared, invert the cups, place them on a fray, and place the tray in the appropriate oven.

10. Record the time at which the samples are placed in the oven.

11. After 24 hours remove the samples, allow to equilibrate, then measure and record their masses.

12. Unscrew the black top and pour away any free fluid that is remaining in the Cup. (CARE MUST BE TAKEN - if the sample has broken up, strain the fluid through a sieve to retain any sample.)

13. Use tissue to dry any fluid remaining on the black top or the thread of the Paddington Cups.

14. Reweigh any sample and the drained Paddington Cup, and record.

* Note: Calcium Saline is a synthetic wound fluid also known as 'Solution A' in BP (1995) and BS EN 13726-1 :2002. Unless otherwise stated, all values for MVTR and WTR in the present application are measured using calcium saline. Example IB - Method for measuring the Moisture Vapour Transmission Rate of a Material

Apparatus Needed:

Sample of material

MVTR circle template

Scissors

Balance Paddington cups

Purified water

Normal Saline solution (0.88 to 0.92 g NaCl and purified water to 10Og)

Calcium Saline solution * (0.81 to 0.85 g NaCl and 0.027 to 0.029 g CaCl₂ and purified water to 10Og) (NB: use analytical grade anhydrous salts to prepare the isotonic solutions)

Torque controlled screwdriver

Tray

Oven at 40°C, 55% RH

Test Method:

1. Lay the sample down on a flat surface, and draw a circle using the MVTR template. Cut the circle out. Cut two circles for each sample to be analysed. 2. Take a Paddington cup, place on the balance and record the mass.

3. Fill the Paddington cup with 2Og of test fluid (pure water, Normal Saline or Calcium Saline), and record the mass. Record which fluid is used for each test.

4. Remove the lid from the Paddington cup. 5. Remove the protective liners (if present) from one side of the sample, and carefully stick the sample down, ensuring that there are no wrinkles, that the central hole is covered with gel, but that the screw holes are not. 6. Remove the protective liners (if present) from the upper surface of the sample. Put the lid back on and tighten the screws using the screwdriver until a torque of 40 cN.m is reached. (Very soft fragile gels may not be as tightly fastened as stronger ones - ensure that the gel has not ripped)

7. Record the mass of the Paddington cup.

8. Repeat for all samples.

9. When all the Paddington cups have been prepared, place the samples on a tray, with the sample facing upwards (i.e. non-inverted), and place the tray in the appropriate oven.

10. Record the time at which the samples are placed in the oven.

11. After 24 hours remove the samples, and measure and record their masses.

* Polyurethane sheets obtained from Intelicoat Ltd, Wrexham, UK that may be used in the present invention.

Example 2 - Manufacture of a Wound Dressing

A precursor solution comprising 70 parts by weight of a 58% aqueous solution of the sodium salt of acrylamidomethylpropanesulphonic acid (NaAMPS, LZ2405 Lubrizol), 30 parts glycerol and 0.14 parts of a 1 to 10 (by weight) mixture of

Daracure 1173 photoinitiator (Ciba Speciality Chemicals) and IRR280 cross-linker

(PEG400 diacrylate, UCB Chemicals) was dispensed from a slot die 120mm wide at a coat weight of 200g/m² onto a moving web of a 3029 medical grade hydrophilic polyurethane foam sample from Polymer Health Technology (Wales) laminated on one side to Inspire 2408 (Intelicoat) backing layer supported on a web of siliconised paper (Cotek) moving at 7m/s and cured with a NUVA Solo 30 medium pressure mercury arc lamp (GEW).

A 1 ml drop of saline solution was absorbed into the wound-facing (hydrogel coated side) of the wound dressing in less than 5s. This material gave acceptable performance with respect to the rate of dry out and total fluid handling.

Claims

1. A wound dressing comprising (i) an absorbent layer portion capable of absorbing wound exudates and arranged to receive wound exudates in use, the absorbent layer portion comprising a hydrogel; and (ii) a backing layer which overlies the absorbent layer portion to a side, which, in use, is directed away from the wound, and wherein the backing layer has a moisture vapour transmission rate (MVTR) of from about 300 to about 1100 g/m²/24hours.

2. A wound dressing according to claim 1, wherein the MVTR of the backing layer is from about 400 to about 900 g/m²/24hours.

3. A wound dressing according to claim 1 or 2, wherein the MVTR of the backing layer is from about 500 to about 800 g/m²/24hours.

4. A wound dressing according to any one of the preceding claims, wherein the backing layer comprises a polyurethane.

5. A wound dressing according to any one of the preceding claims, wherein the absorbent layer portion comprises a material having an internal cellular structure.

6. A wound dressing according to claim 5, wherein the material having an internal cellular structure consists essentially of a hydrogel.

7. A wound dressing according to claim 6, wherein a hydrogel, which may be the same as or different from the hydrogel in the material having the internal cellular structure, is disposed within at least some of the voids of the cellular structure and/or as a layer on at least part of the wound-facing side of the material having an internal cellular structure.

8. A wound dressing according to any claim 5, wherein the material having an internal cellular structure consists essentially of a non-hydrogel material and a hydrogel is present within at least some of the voids of the cellular structure and/or as a layer on at least part of the wound-facing side of the material having an internal cellular structure.

9. A wound dressing according to any one of claims 1 to 4, wherein the absorbent layer portion is essentially free of internal voids.

10. A wound dressing according to claim 9, wherein the absorbent layer portion consists essentially of a hydrogel.

11. A wound dressing according to claim 10, wherein the absorbent layer portion consists of a hydrogel.

12. A wound dressing according to any one of the preceding claims, wherein the absorbent layer portion can absorb at least about 2.5 times its own weight of water in 24 hours.

13. A wound dressing according to any one of the preceding claims, wherein the absorbent layer portion comprises a hydrogel composition comprising a hydrophilic polymer carrying multiple pendant sulphonyl groups on each polymer molecule.

14. A wound dressing according to anyone of the preceding claims, wherein the hydrogel comprises one or more hydrophilic polymers selected from polymer of a polyalkylene glycol acrylate or a substituted derivative thereof; a polyalkylene glycol methacrylate or a substituted derivative thereof; acrylic acid and salts thereof; 2- acrylamido-2-methyl-propanesulphonic acid and salts thereof; acrylic acid (3- sulphopropyl) ester or a substituted derivative thereof or a salt thereof; diacetone acrylamide; a vinyl lactam; an optionally substituted N-alkylated acrylamide such as hydroxyethyl acrylamide; an optionally substituted N,N-dialkylated acrylamide; and N-acryloyl morpholine or a substituted derivative thereof.

15. A wound dressing according to claim 14, wherein the hydrophilic polymer is a polymer of one or more monomers selected from 2-acrylamido-2-methylpropane sulphonic acid (AMPS), acrylic acid (3-sulphopropyl) ester (SPA) and salts thereof.

16. A wound dressing according to any one of the preceding claims, wherein the dressing is for the treatment of a chronic ulcerous wound.

17 A wound dressing according to any one of the preceding claims, wherein the dressing is for the treatment of a medium or high exudation wound.

18. A wound dressing as claimed in any one of the preceding claims, wherein the absorbent layer comprises a polyurethane foam having a hydrogel present within at least some of the pores of the foam and/or as a layer on at least part of the wound- facing side of the foam and the backing layer has an MVTR of from about 300 to about 1100 g/m /24hours, and wherein the hydrogel comprises a hydrophilic polymer carrying multiple pendant sulphonyl groups on each polymer molecule.

19. Use of a hydrogel in the manufacture of a wound dressing as defined in any one of claims 1 to 18 for the treatment of a wound or a burn, wherein, in the treatment, the dressing is applied to the wound or burn such that the absorbent layer portion is disposed between the backing layer and the wound.

20. Use of hydrogel comprising a hydrophilic polymer carrying multiple pendant sulphonyl groups on each polymer molecule, for the technical purpose of providing all or part of an absorbent layer portion of a wound dressing, which wound dressing also has a backing layer having an MVTR of from about 300 to about 1100 g/m²/24hours for maintaining in balance (a) a moist wound environment to promote the healing of the wound without maceration, (b) a moisture flow through the wound dressing to promote effective removal of exudate from the wound, and (c) a sufficient moisture level within the absorbent layer to prevent adherence of the dressing to the wound.

21. The use according to claim 20, wherein the dressing is as defined in any one of claims 2 to 15 and 18.

22. The use according to claim 20 or 21, wherein the wound is as defined in claim 16 or 17.