WO2019020829A1

WO2019020829A1 - Absorbent layer and method of manufacture

Info

Publication number: WO2019020829A1
Application number: PCT/EP2018/070528
Authority: WO
Inventors: Victoria Jody HAMMOND; John Kenneth Hicks; Ella Lynn MUMBY; Stephanie Jane NOBLE
Original assignee: Smith & Nephew Plc
Priority date: 2017-07-28
Filing date: 2018-07-30
Publication date: 2019-01-31
Also published as: GB201712170D0

Abstract

A method of manufacturing an absorbent layer for use in a wound product and an absorbent layer is provided. The method includes placing a layer of foam between two opposed pressing surfaces. The foam is compressed foam between the two pressing surfaces to reduce the thickness of the foam and the foam is dielectrically heated the foam during compression between the pressing surfaces.

Description

ABSORBENT LAYER AND METHOD OF MANUFACTURE

INTRODUCTION

The present invention relates to an absorbent layer for use in a wound product and a method of manufacturing such an absorbent layer. In particular, but not exclusively embodiments of the present invention relate to a method of manufacturing an absorbent layer for use in a wound product (e.g. a wound dressing or a wound filler), including compressing and dielectrically heating a layer of foam between two pressing surfaces. The resultant absorbent layer includes a graduated foam structure.

BACKGROUND

There are many variations of the standard type of wound dressing. Standard dressings often include an absorbent material and means to hold the material in place. It is often desirable for the wound dressing to conform to the patient to improve patient comfort. The term ^'conformable^' means that the dressing will conform to changes in dimensions and contours of the body portion to which the dressing is attached.

To improve conformability, it has previously been suggested to make the wound dressing as slim as possible. However, this can compromise the ability of the dressing to absorb wound exudate.

A need therefore exists for a more conformable wound dressing, which aptly at least matches the absorbency of existing wound dressings as well as being suitable for conforming around contoured body areas.

SUMMARY

A first aspect of the present invention relates to a method of manufacturing an absorbent layer for use in a wound product, the method comprising:

placing a layer of foam between two opposed pressing surfaces;

compressing the foam between the two pressing surfaces to reduce the thickness of the foam; and

dielectrically heating the foam during compression between the pressing surfaces. Each of the pressing surfaces may be the surface of a die or in some examples, one of the pressing surfaces may be a fixed surface (e.g. a table) and the other pressing surface may be the surface of a die or other suitable pressing element. Suitably, the step of dielectrically heating the foam includes passing high frequency electromagnetic waves between the pressing surfaces and thereby through the foam. As used herein "high frequency" refers to frequencies between around 1 MHz up to around 1 GHz.

The high frequency electromagnetic waves are suitably passed between the pressing surfaces at an angle substantially perpendicular to each of the pressing surfaces. In this way, the electromagnetic waves will pass through the foam substantially perpendicular to the lay direction of the foam.

The high frequency electromagnetic waves may suitably include high frequency radio waves. In other examples, the high frequency electromagnetic waves may alternatively or additionally include microwaves.

Suitably, the high frequency radio waves may have a frequency from about 3 MHz to about 300 MHz, or aptly from about 10 MHz to about 100 MHz.

By applying electromagnetic waves of suitable frequency to the foam, dipoles within the foam are excited and oscillate to align with the alternating field, thus generating heat, which can melt or soften the foam structure. It will be appreciated that the optimum frequency of the electromagnetic radiation may depend on the exact material of the foam. For a polyurethane (PU) foam, the frequency of applied electromagnetic radiation may aptly be from about 10 MHz to about 100 MHz.

In some examples, at least one or both of the pressing surfaces may be heated. This may help to heat and melt or soften the foam structure at the surfaces of the foam, allowing greater flexibility and variability in the foam structure across the absorbent layer. In some examples, at least one of the pressing surfaces may be heated to a temperature of from about 50 to about 150 degrees Celsius. After the foam is dielectrically heated, the foam is suitably maintained under compression between the pressing surfaces without dielectrically heating for a period sufficient to allow the foam to set (dwell time). For example, this may be a period of time suitable to allow the foam to cool to atmospheric temperature. In this way, once the electromagnetic waves are turned off (thereby ceasing dielectric heating) the foam is no longer subject to dielectric heating and remains under pressure in a compressed state between the pressing surfaces. In this fashion it is possible to heat, compress, and set the foam under compression.

The foam is suitably a dielectric material. For example, the foam may be a polyurethane, polyester, or polyoiefin foam. The foam is suitably an open-cell structured foam, for example a hydrocellular foam.

During compression, the thickness of the foam may be reduced by from about 20% to 80%. That is, the thickness of the foam after compression may be from about 20% to 80% of its initial thickness prior to compression. Aptly, the thickness of the foam may reduce by about 50% during compression.

Suitably, the foam is placed between (or 'sandwiched' between) sheets of siliconised paper prior to compression. The siliconised paper can help to create a barrier between the pressing surfaces and the foam, and focus the energy within the foam structure. This can help to prevent the foam from overheating and sticking to the pressing surfaces.

The layer of foam may be pre-shaped or pre-cast and in one example may be in the form of a sheet. The sheet may have an initial thickness of from approximately 1 to 8 mm and a compressed thickness of from approximately 0.2 to 3 mm, for example 0.5 mm.

According to a second aspect of the present invention there is provided a compressed foam, wherein the compressed foam comprises a cellular structure having relatively larger cells at first and second outer surface regions compared to relatively smaller cells at a central region between the first and second outer surface regions, when viewed in cross-section.

Suitably, an average cell size of the cellular structure gradually reduces from an outer surface of the foam towards the central region. Suitably, the average cell size is smallest at the central region.

Some examples of the present invention provide the advantage that the thickness of the wound product may be reduced compared to known wound products.

Some examples of the present invention provide the advantage that conformability of the wound product, in particular the absorbent layer, is improved compared to known wound products. In some examples, the conformability of the wound product may be improved without compromising the absorbent properties of the absorbent layer.

In some examples of the present invention, the absorbent layer may exhibit improved absorbent properties compared to known absorbent layers. BREIF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figures 1A to 1 C show manufacturing steps for manufacturing an absorbent layer according to one example;

Figure 2 illustrates a flow of an example of a method of manufacturing an absorbent layer;

Figure 3 illustrates a manufacturing process for manufacturing an absorbent layer according to another example;

Figure 4 illustrates a scanning electron microscope (SEM) cross-section of a 4mm

ALLEVYN (RTM) foam compressed to 1.3 mm;

Figure 5 illustrates a SEM cross-section of a 4mm ALLEVYN (RTM) foam compressed to 1 .6 mm using the method of Figure 2;

Figure 6 shows a diagrammatic cross section through an example of a wound product; and

Figure 7 shows a diagrammatic cross section through another example of a wound product. DETAILED DESCRIPTION

As used herein the expression "wound" may include an injury to living tissue that may be caused by a cut, blow, or other impact, typically one in which the skin is cut or broken. A wound may be a chronic or acute injury. Acute wounds occur as a result of surgery or trauma. They move through the stages of healing within a predicted timeframe. Chronic wounds typically begin as acute wounds. The acute wound becomes a chronic wound when it does not follow the healing stages resulting in a lengthened recovery. It is believed that the transition from acute to chronic wound can be due to a patient being immuno-compromised. Chronic wounds may include for example: Venous ulcers: Venous ulcers usually occur in the legs, account for the majority of chronic wounds, and mostly affect the elderly, Diabetic ulcers (typically foot or ankle ulcers, Peripheral Arterial Disease, Pressure ulcers, or Epidermolysis Bullosa (EB)). The wound may also include a deep tissue injury. The deep tissue injury is a term proposed by the National Pressure Ulcer Advisory Panel (NPUAP) to describe a unique form of pressure ulcers. These ulcers have been described by clinicians for many years with terms such as purple pressure ulcers, ulcers that are likely to deteriorate and bruises on bony prominences. In the following a wound product will be referred to in general terms. However, in certain examples of the present invention, the wound product is a wound dressing. Furthermore, in certain other examples of the present invention, the wound product is a wound filler. It will be appreciated that the wound product of the present invention may therefore be used as a wound dressing or a wound filler (where these uses should be considered as non-limiting), where certain examples for each use may provide further modifications specific to each particular use. Additionally, in other examples, the wound product may be a debrider or a wound cleaner.

Figs. 1 A to 1 C and Fig. 2 illustrate manufacturing steps included in manufacturing an absorbent layer for use in a wound product.

In the step shown in Fig. 1 A and at 201 , a layer of foam 102 is placed between a first pressing surface 104 and a second pressing surface 106. The pressing surfaces 104, 106 are opposed and are aptly spaced apart and parallel. In this example, the first pressing surface 104 is a table or bench top 105 (i.e. a static pressing surface) and the second pressing surface 106 is a surface of a die 107 (i.e. a movable pressing surface).

At step 202, and as shown in Fig. 1 B, a pressing head 108 presses the die 107 into the foam 102 to compress the foam 102 between the two pressing surfaces 104, 106. The pressing head 108 applies pressure to the die 107 in a direction perpendicular to the pressing surfaces 104, 106 and the surface of the foam 102, as illustrated by arrow A.

In this example, the initial thickness of the layer of foam (prior to compression) is around 2 mm. The foam is compressed to around 25 % (or reduced by 75 %) of its initial thickness to a compressed thickness of around 0.5 mm.

At step 203, following compression of the foam to the desired thickness, the foam layer 102 is dielectrically heated. During dielectric heating, compression of the foam between the two pressing surfaces 104, 106 is maintained.

Dielectric heating of the foam layer includes passing high frequency electromagnetic waves (illustrated by dotted lines 1 10 in Fig. 1 B) between the pressing surfaces 104, 106 and thereby through the foam 102. The electromagnetic waves may be generated by any method well known in the art. In this example, the electromagnetic waves are generated by the pressing head 108, which is a Radyne (RTM) welder head. The Radyne (RTM) welder head operates by passing an AC current through the cavity created between the two pressing surfaces to create the electromagnetic waves. The electromagnetic waves excite dipoles within the layer of foam material as they try to align with the alternating field. This creates molecular kinetic energy and thus thermal energy, generating heat from within the material. In this example the high frequency electromagnetic waves are high frequency radio waves and have a frequency of between about 10 MHz to 100 MHz.

The electromagnetic waves 1 10 are aptly passed between the pressing surfaces 104, 106 at an angle perpendicular to each of the pressing surfaces (and thus also perpendicular to the surfaces of the layer of foam). This helps to control the melting of the foam structure such that the resultant foam has a graduated structure having larger cells at the outer surface regions and relatively smaller cells in the central region when viewed in cross-section.

In this example, the foam layer 102 is dielectrically heated for a period of about 30 seconds. The time of dielectric heating of the foam may be selected so as to help ensure that the foam layer is heated enough to achieve the graduated structure but not so much that the absorbent cell structure of the foam is compromised. In most examples, it may be sufficient to dielectrically heat the foam for from around 3 seconds to 90 seconds. Aptly, the foam is dielectrically heated for from around 20 to 30 seconds.

After dielectric heating, the foam layer 102 remains compressed between the pressing surfaces 104, 106 for a period of time (setting time) to allow the foam layer to set under compression. That is, during this time there are no electromagnetic waves passing through the foam layer 102, allowing the foam to cool to a set compressed condition. Aptly, the setting time may be from around 30 seconds to 2 minutes.

Once the foam layer 102 is set, the pressure may be removed from the foam 102 by separating the pressing surfaces 104, 108. In this example, the die 107, is removed from the foam 102 leaving a compressed area 1 12 as shown in Fig. 1 C, which has taken the shape of the die 107.

The resultant compressed foam forms the absorbent layer and has a structure that is less compressed at the outer surface region and more compressed in the central region. The more compressed foam at the central region helps to improve the drape-ability and conformability of the foam, which can help when applying the foam to different body parts (for example, the ankle, the heel or the leg). However, the cells of the foam in the central region may become smaller and may therefore have a reduced ability to absorb fluid from the wound. The relatively uncompressed cells around the outer surface regions of the foam maintain their ability to absorb fluid and thus the resultant foam structure includes a relatively conformable core (central region) surrounded by more absorbent outer surface regions.

The foam layer may be formed from any of the absorbent foams conventionally used in wound dressings. Suitable foam materials include foamed hydrogeis formed from either natural or synthetic polymers or mixtures thereof. Examples of natural polymers are the cellulosics, alginates or agar. Examples of suitable synthetic polymers include polyurethane.

Aptly the foam includes a conformable foam. Suitable foams include, polyurethane or polyester foams. In one example, the foam may be a hydrophilic polyurethane foam. For example, the foam may be the hydrophilic polyurethane foam disclosed in European Patent No. EP 0099748. Another suitable foam is the absorbent foam dressing known as ALLEWN (Registered Trade Mark (RTM)) available from Smith & Nephew. A further suitable material for use as the absorbent layer is carboxylated butadiene styrene rubber.

With the methods described above, a single layer of foam is compressed from a first uncompressed condition to a second compressed condition.

In the second compressed condition, the single layer of foam is graduated in that the foam exhibits different regions of compression. The foam includes a cellular structure having relatively larger cells at first and second outer surface regions compared to relatively smaller cells at a central region between the first and second outer surface regions, when viewed in cross- section. The average ceil size of the foam may gradually reduce from the outer surfaces towards the central region. As such, the average cell size may be smallest at the central region of the foam.

The outer surface regions of the foam (when viewed in cross section) may include more rounded ceils, whilst the cells in the central region of the compressed foam, may include flattened cells (i.e. cells that are more irregular and flatter in shape).

The majority of the cells within the foam (for example, at least 60%, or at least 70%, or at least 80%, or at least 90%) remain open in the compressed condition. Depending on the specific energy applied during dielectric heating and the duration of dielectric heating, the foam material in the central region may become soft enough such that the cells in the central region begin to close to form a closed cell structure. Aptly, at least some of the cells in the central region remain open to more easily facilitate the passage of fluid through the absorbent layer. For example, at least 20 %, or at least 30 %, or at least 40 % of the cells in the central region are open in the compressed condition. It will be appreciated that the specific duration of dielectric heating and energy applied during dielectric heating can be adjusted according to the desired characteristics of the compressed foam.

The absorbent layer (compressed foam) described above has potential uses in a range of wound products (or exudate management dressings). For example, a wound dressing may include a similar dressing construction as outlined in Figs. 6 and 7.

Fig. 6 illustrates an example of a wound dressing 600. The wound dressing 600 includes an absorbent layer 602 in the form of a compressed foam as described above. A wound contact layer 604 is positioned beneath the absorbent layer and contacts the wound, in use. The wound contact layer 604 may be in the form of a perforated film having an adhesive silicone coating on the wound contacting side. A breathable film 606 is positioned over the absorbent layer 602 and is coupled to the wound contact layer 604 around the periphery to seal the absorbent layer 602 therebetween. The breathable film is suitably fluid impermeable and moisture vapour permeable.

Fig. 7 illustrates another example of a wound dressing 700. This example is similar to the example shown in Fig. 6 but includes an additional acquisition/distribution layer 708 between the absorbent layer 602 and the wound contact layer 604. The distribution/acquisition layer 708 functions to draw moisture from the wound and distribute the moisture to the absorbent layer 708, where the moisture may be stored. In this way, the distribution/acquisition layer 708 helps to control moisture levels at the wound site. The distribution/acquisition layer may be formed from lateral wicking fibres, for example cellulose fibres. Additional layers may also be present in either of the wound dressings described above. For example, the wound dressings may also include at least one of a superabsorber or a masking layer. Combinations of an absorbent layer (compressed foam), acquisition/distribution, superabsorber and masking layers are envisaged in different combinations and constructions, and may be in between a wound contact layer and a breathable film.

The dressings may include of a layer of soft, hydrophilic graduated polyurethane foam, compressed to about 1 to 3 millimetres thick, bonded to a semipermeable polyurethane film. Although this film is suitably permeable to moisture vapour, it may also provide an effective barrier to water or wound exudate and also may prevent the passage of microorganisms through the back of the dressing.

The wound contact surface of the dressing may be covered with a perforated film, designed to prevent the dressing from adhering to granulating tissue. The absorbent layer (compressed foam) of the present invention is highly absorbent, and strike-through is prevented by the semipermeable film backing.

When applied to an exuding wound, the dressing will absorb excess fluid but maintain the wound surface in a moist condition providing a micro-environment that is conducive to healing.

In other examples, the absorbent layer may be used as part of a negative pressure therapy dressing. Negative pressure dressings include a negative pressure source, e.g. a pump, which is coupled to the dressing to provide negative pressure to the wound bed. A seal layer may be provided over the wound to help maintain negative pressure at the wound. The absorbent layer may be used as a wound filler to be placed in a wound cavity below the seal layer. Alternatively or additionally the absorbent layer may be used at the surface of the wound to help distribute negative pressure to the wound and to absorb wound exudate. As well as in the wound management industry, there is the potential to use graduated compressed foams where a combination of product conformability and absorption are desirable; for example in hygiene or first aid products, or in padded safety equipment.

With the above described examples, an absorbent layer (compressed foam) may be manufactured that exhibits improved conformability compared to uncompressed foam. Using the absorbent layer in a wound product also allows for a more conformable wound product compared to previously known wound products. The fluid handling on a wound model of dressings including the compressed foam is more desirable to that of dressings containing high heat/pressure compressed foam. Dressings including compressed foams display a fluid absorbency pattern similar to that of uncompressed foam rather than the capillary action of absorption observed for dressings containing high heat/pressure compressed foam. Various modifications to the above described examples may be possible. For example, although the pressing surfaces described above include one static pressing surface and one movable pressing surface, in other examples both pressing surfaces may be movable pressing surfaces. For example, both pressing surfaces may be surfaces of respective dies, which may be moved towards each other to compress the foam therebetween.

The initial thickness of the foam may be different depending on the intended properties of the absorbent layer. For example, the initial thickness of the foam may between about 1 mm to 8 mm, or aptly between about 2 mm to 6 mm, for example around 2 mm, or around 4 mm, or around 6mm.

It will be appreciated that different initial thicknesses of foam may be compressed to different thicknesses. For example, a foam layer having an initial thickness of around 8 mm may be compressed to a thickness from about 1 mm to about 4 mm. in another example, the foam may have an initial thickness of around 6 mm and may be compressed to around 4 mm thickness. In another example, a foam layer having an initial thickness of about 4 mm may be compressed to a thickness from about 0.5 mm to about 2 mm. During compression, the thickness of the foam may be reduced by from about 20 % to about 80 % of the initial thickness. Aptly, the thickness of the foam may be reduced by from about 50 % to about 80 % of the initial thickness (in other words, the foam is compressed to a thickness that is from about 20 % to 50 % of the initial thickness).

Other method steps may also be included in manufacturing the absorbent layer. For example, in addition to the steps described in relation to Fig. 1 and Fig. 2, the foam layer 102 may be placed between sheets 302, 304 of siliconised paper (see Fig. 3). The foam layer may be sandwiched between the sheets 302, 304 prior to compression between the pressing surfaces. This can help to control the heat and energy being delivered to the foam. The sheets can help to create a barrier between the pressing surfaces and the foam. This can help to focus the energy within the foam structure and can help to prevent the foam from overheating and sticking to the pressing surfaces.

In some methods of manufacture, one or both of the pressing surfaces 104, 106 may be heated prior to, or during the compression step and/or the dielectric heating step. This can help to warm the foam prior to dielectricaily heating the foam, which can gently soften the cellular structure of the foam. Aptly, one or both of the pressing surface may be heated to between 50 and 150 degrees Celsius. This range may be particularly suitable for the ALLEVYN (RTM) foam discussed above. However, it will be appreciated that different thermoplastic materials may require heating outside this range in order to alter the structure of the foam.

EXAMPLES

Example 1 (Comparative)

A hydrocellular foam consisting of hydrophilic polyurethane and polyethylene glycol (ALLEVYN (RTM)) foam was compressed using two heated plates (heated to between 120 and 150 degrees Celcius). The aim of the compression was to create foam thinner than 2 mm to fit within current manufacturing processes.

Although it was found that the compressed foam was more conformable than the original foam, it was also found that the absorbency of the foam decreased in line with the level of compression. The absorbency testing and visual conformability of the foams implied that if standard 4 mm ALLEVYN (RTM) foam is compressed to 2 mm thick, the total absorbency will be similar to that of standard uncompressed 2 mm ALLEVYN (RTM) foam but the conformability will be better.

Wound model testing was carried out on dressings including the heat compressed foams and although some fluid was absorbed by capillary action, a significant amount pooled underneath the pad which was deemed to be unacceptable. The fluid handling of 4 mm foam compressed to 2 mm was poorer than that of standard 2 mm foam.

Scanning electron microscopy (SEM) of the foam compressed between heated plates showed that the structure of the foam had been evenly compressed through the cross section, and the top and bottom aspects of the foam showed a flattened structure see Fig. 4.

Compressing foam using heat and pressure only caused an even flattening of the gas cells across the foam structure. This was found to impair the fluid uptake into the foam when tested in a dressing construction on a wound model.

Example 2

Radiofrequency (RF) power (dielectric heating), pressure and a low heat were used in combination to compress the same hydrocellular foam of Example 1 to investigate whether the resulting foam structure and wound model fluid uptake were different to that of the high heat/pressure compressed foams.

Radiofrequency technology is used for welding polyurethanes using Radyne (RTM) welders. A trial was run at Premier Tooling Systems, Lancashire, United Kingdom. ALLEVYN (RTM) foam was compressed using heat, pressure and RF power to achieve the desired end foam thicknesses. Standard ALLEVYN (RTM) foam was sandwiched in between sheets of siliconised paper and placed on the table of a Radyne (RTM) foot welder. A flat conductive metal die heated to around 65 degrees Celsius was placed on top of the foam and paper. The die was then lowered until it was the desired distance from the table. RF power was transmitted through the foam whilst it was under compression to dielectrically heat the foam, then the RF switched off and the compression maintained for a dwell time to allow the foam to set.

After the trial, SEM images of the compressed foam were compared to those previously obtained for the foams compressed in Example 1. It was noted that the cross-sectional structure of the RF compressed foams had a more open cell structure at the outer edges and a higher degree of compression in the middle (see Fig. 5) as compared to the even cross- sectional compression for the heat/pressure compressed foams (see Fig. 4). The surface aspects of the RF compressed foams showed open round gas cells (Fig. 5), compared to the flattened cells in the images of high heat/pressure compressed foams (Fig. 4). The combination of open and compressed cells in the RF samples leads to improved conformability compared to uncompressed foam, and with a better ability to manage fluid on a wound model than high heat/pressure compressed foam. Example 3

Wound dressings were manufactured using the compressed foam of Example 2, using the dressing constructions in Figures 6 and 7. A volunteer sample assessment and wound model testing was conducted and confirmed that the combination of open and compressed cells in the RF samples leads to improved conformability compared to uncompressed foam, and with a better ability to manage fluid on a wound model than high heat/pressure compressed foam. In particular, the fluid distributed more evenly into the RF compressed foam and showed less tendency to pool underneath the dressing. In other words, the fluid was taken up more readily into the RF compressed foam. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1 . A method of manufacturing an absorbent layer for use in a wound product, the method comprising:

placing a layer of foam between two opposed pressing surfaces;

dielectrically heating the foam during compression between the pressing surfaces.

2. A method according to claim 1 , wherein the step of dielectrically heating the foam comprises passing high frequency electromagnetic waves between the pressing surfaces and thereby through the foam.

3. A method according to claim 2, wherein the high frequency electromagnetic waves are passed between the pressing surfaces at an angle substantially perpendicular to each of the pressing surfaces.

4. A method according to claim 2 or claim 3, wherein the high frequency electromagnetic waves comprise high frequency radio waves.

5. A method according to claim 4, wherein the high frequency radio waves have a frequency from about 3 MHz to about 300 MHz, or from about 10 MHz to about 100 MHz.

6. A method according to any preceding claim, further comprising heating at least one of the pressing surfaces.

7. A method according to claim 6, wherein at least one of the pressing surfaces is heated to a temperature of from about 50 to about 150 degrees Celsius.

8. A method according to any preceding claim, further comprising placing the foam between sheets of siliconised paper prior to compression.

9. A method according to any preceding claim, wherein following dielectrically heating the foam, the foam is maintained under compression without dielectrically heating for a period sufficient to allow the foam to set.

10. A method according to any preceding claim, wherein the foam is a polyurethane, polyester, or polyoiefin foam.

1 1 . A method according to any preceding claim, wherein during compression, the thickness of the foam is reduced by from about 20% to 80%.

12. An absorbent layer obtained by the method of any of claims 1 to 1 1 .

13. An absorbent layer comprising a compressed foam, wherein the compressed foam comprises a cellular structure having relatively larger cells at first and second outer surface regions compared to relatively smaller cells at a central region between the first and second outer surface regions, when viewed in cross-section.

14. An absorbent layer according to claim 13, wherein an average cell size of the cellular structure gradually reduces from an outer surface of the foam towards the central region.

15. An absorbent layer according to claim 14, wherein the average cell size is smallest at the central region.

16. A wound dressing comprising an absorbent layer according to any of claims 12 to 15.

17. A wound dressing according to claim 16 further comprising a breathable film and a wound contact layer.

18. A wound dressing according to claim 16 or 17 wherein the absorbent layer is disposed between the breathable film and the wound contact layer.

19. A wound dressing according to claim 18 further comprising an acquisition or distribution layer disposed between the absorbent layer and the wound contact layer.