GB2502057A

GB2502057A - Oxygen generating and carbon dioxide absorbing wound dressing

Info

Publication number: GB2502057A
Application number: GB1208359.8A
Authority: GB
Inventors: John Hurst; Nigel Parker
Original assignee: EMCO PACKAGING SYSTEMS Ltd
Current assignee: EMCO PACKAGING SYSTEMS Ltd
Priority date: 2012-05-14
Filing date: 2012-05-14
Publication date: 2013-11-20
Also published as: GB201208359D0; WO2013171467A1

Abstract

A wound dressing which, on hydration by water vapour derived from the wound and perspiration from the enclosed skin area, generates oxygen and absorbs carbon dioxide to provide supra-atmospheric oxygen within a headspace atmosphere of a wound site under treatment. An outer cover 1, with low water vapour and low oxygen transmission rates and carrying a self-adhesive coating, encloses an upper envelope, having a water vapour and gas impermeable backing sheet 2 and a water vapour and gas permeable but liquid water impermeable face sheet 3 and containing a fill material 6 formulation including sodium carbonate peroxyhydrate, and a lower envelope 4 containing a fill material 7 capable of absorbing high volumes of aqueous liquids.

Description

AN OXYGEN GENERATING AND CARBON DIOXIDE ABSORBING WOUND

DRESSING

Field of the Invention

The invention relates to a combined oxygen generating and carbon dioxide absorbing wound dressing, incorporating a water vapour and gas permeable envelope, containing a water vapour activated mixture.

This device has been designed so that when applied to the wound, water vapour, produced both by the wound itself and by perspiration from the skin area enclosed within the dressing, permeates the envelope and initiates and maintains oxygen generation.

Description of the Related Art

In the economically advanced western nations, the increase in longevity together with the rise in obesity and associated diabetes has led to some nearly two million or roughly one in two hundred Europeans suffering from chronic wounds such as pressure induced sores and venous leg ulcers, as well as diabetic lesions. With these populations projected to show increased longevity into the foreseeable future, the number of suffers is likely to rise substantially.

Chronic wounds can take from weeks to even years to heal, with over 30% taking more than two years and many never healing. This inevitably leads to considerable suffering and consequential incapacity for those afflicted.

Despite the availability of many treatment systems, clinicians remain obstructed and frustrated by both the persistent slow healing and the increasing treatment costs of these wounds.

Of the treatment systems available, however, one that has demonstrated efficacy, with the promise of further beneficial development is the continuous, topical deployment of oxygen, at supra-atmospheric levels, to the wound itself.

The present invention is a wound dressing, in a generally conventional format, which integrally incorporates a water vapour and gas permeable envelope containing a homogenous mixture of compounds, which, when activated by water vapour, generates oxygen.

When the dressing in the present invention is applied, the wound site and the surrounding area of skin is fully enclosed, creating a restricted headspace which effectively controls the movement of water vapour, both from the wound itself and that from the perspiration produced by the surrounding skin, so that it permeates into the above envelope and initiate oxygen generation.

The oxygen thus generated diffuses out of the envelope and into the wound site headspace atmosphere, continuously bathing the wound itself in pre-determined, supra-atmospheric levels of oxygen

Summary of the Invention

According to the invention there is provided a wound dressing, as shown in Fig. 1 and described in detail later, comprising: -an outer fabric cover with a moderately low water vapour transmission rate (WVTR) and oxygen transmission rate (OTR), which carries a self-adhesive coating in order to allow it to be affixed to the wound site, enclosing the same, a first envelope, in sachet format, integrally secured within the outer fabric sheet, containing a fill material, which is activated by water vapour to generate oxygen and absorb carbon dioxide and wherein the first, or oxygen generating (OG) envelope has an outer or backing sheet comprising a non-woven support layer laminated to a water vapour and gas impermeable polymer film and an inner, cover face or control' sheet comprising a non-woven polymer support layer laminated to a water vapour and gas permeable but liquid water impermeable, polymer film, a second envelope, in sachet format, integrally secured within the outer fabric layer, containing super-absorbent material, which is in direct contact with the wound and wherein the second or wound exudate absorbing (WEA) envelope comprises a fully liquid and gas permeable, non-woven polymer coverface and backing sheet, a peelable cover face for the complete dressing, of a liquid water, water vapour and gas impermeable polymer laminate, attached to the edge facing of the outer fabric layer. and designed both to maintain the sterility of the WEA envelope and prevent water vapour access to the OG envelope The inventors understand that the WVTR and OTR of the control' layer of the OG envelope must be great enough to provide sufficient gas and water vapour permeability without allowing the permeability of liquid water.

The inventors also understand that the selection of the material composition and gauges of the components of the control' layer of the first envelope can be used to control the WVTR and OTR of this layer and thus regulate both the level of oxygen within the wound site headspace atmosphere and the overall volume of the latter.

The control' layer may have a WVTR of lOg to 200g1m2 in 24 hours at 1 bar and 23 °C and 85% relative humidity and an OTR in the range 650cm3-20,000cm3Im2 in 24 hours at 1 bar at 23°C.

The inventors understand that the wound types that the device is designed to treat require dressings to be changed at relatively short intervals. In order to minimize redundancy in performance capacity, which will aid in both reducing the weight and size of the dressing and consequently contribute to patient comfort in application, the mass of the fill material formulation will be closely tailored to meet the wound site headspace atmosphere control required during these intervals.

In relation to the above, the inventors have achieved significant improvements in the performance, during the operational life of the product, by introducing control' layer selection to optimally regulate the WVTR and OTR of the OG envelope against a limited range of wound requirements. The inventors have, moreover, additionally discovered that further benefits may be obtained by selectively modifying the relative composition of the fill material formulation.

The fill material formulation may comprise an active ingredient that generates oxygen and absorbs carbon dioxide when activated by water vapour as well as a water vapour control system comprising ingredients that absorb water vapour in order to regulate the reaction of the active ingredient, wherein the active ingredient is present in an amount from 55% to 75%, and the water vapour control ingredients are present in an amount from 23% to 40%.

The introduction of the control layer, with the additional control over the WVTRs, effectively supplements the water vapour control function of the fill material formulation, thus allowing, in some applications, the percentage composition of those component compounds exercising this function to be reduced. The major consequence of this is the ability to reduce the weight and size of the dressing.

In a preferred embodiment, the permeable support layer is a non-woven polyethylene fabric. The density may be 45 gIm2-55g1m2.

A polyurethane adhesive may be used to bond the layers of the laminate together.

In another aspect of the invention there is provided a wound dressing, comprising: an OG envelope, impermeable to liquid water and permeable to water vapour and gases; a fill material contained in the OG envelope, which is activated by water vapour to generate oxygen and absorb carbon dioxide; and wherein the OG envelope is in two parts with an upper sheet of a laminate material comprising a permeable support layer and a high barrier to water vapour and gas transmission layer, the barrier layer having a WVTR rate of lOg to 15g1m2 in 24 hours at 1 bar and 23 °C and 85% relative humidity and an OTR in the range 70cm3- 1 OOcm3/m2 in 24 hours at 1 bar at 23°C, wherein the OG envelope is in two parts with a lower cover face or sheet of a laminate material comprising a permeable support layer and a liquid water impermeable but water vapour and gas permeable layer, this latter layer having a WVTR of lOg to 140g/m2 in 24 hours at 1 bar and 23 °C and 85% relative humidity and an OTR in the range 1 50cm3-20,000cm3/m2 in 24 hours at 1 bar at 23°C.

In a further aspect of the invention there is provided a wound dressing device, comprising: an OG envelope, impermeable to liquid water and permeable water vapour and gases; a fill material contained in the envelope, which is activated by water vapour to generate oxygen and absorb carbon dioxide; wherein the fill material formulation comprises: an active ingredient that generates oxygen and absorb carbon dioxide when activated by water vapour; and water vapour control ingredients that absorb water vapour to regulate the reaction of the active ingredient, wherein the active ingredient is present in an amount from 30%to 75% and the water vapour control is present in an amount from 25% to 70%.

in its deployment, the oxygen generating, wound dressing device in this invention is similar to the standard absorbent dressings conventionally applied to chronic wounds in that it is first removed from its sealed, protective, liquid water and gas impermeable storage pack and the cover face sheet removed to expose the self adhesive facing of outer fabric cover.

The oxygen generating wound dressing is then placed carefully over the wound site (Fig 2), ensuring that the adhesive facing (10) of the outer fabric has made good contact with the skin surrounding the wound (8) and that the latter is fully enclosed.

Water vapour from both the wound itself and perspiration from the surrounding area of enclosed skin (9) rapidly elevates the relative humidity (RH) within the wound site head space, diffuses through the WEA envelope and permeates the OG envelope, where it initiates a reaction within the fill material formulation to generate oxygen, the latter gas now diffusing out of the OG envelope, passing through the WEA envelope and into the head space.(Fig. 2.), increasing the percentage oxygen composition of the atmosphere contained towards the pre-determined target optimum.

Overpressure of the wound by continuous delivery of oxygen to the wound site headspace atmosphere, is avoided by the intentional, limited escape of oxygen through the composite fabric cover sheet in the restricted surface area available in the in the immediate vicinity of its adhesion to the skin surrounding the wound site.

Detailed Description

An embodiment of the invention is a wound dressing, as shown in the sectional side elevation in Fig. 1., having an outer fabric cover of a composite material (1), with both a low water vapour transmission rate (WVTR) and moderately low oxygen transmission rate (OTR), to allow limited loss of oxygen from the wound-site head space atmosphere in order to prevent over pressure on the wound itself. This intentional escape of oxygen is controlled both by the selection of the OTR of the composite material of the fabric cover and the restricted surface area available for gas transmission.

The outer fabric cover comprises a composite water impermeable but water vapour and gas permeable layer, which in combination have a WVTR of lOg to 200g1m2 in 24 hours at 23 °C and 85% relative humidity and an OTR in the range 400cm3- 800cm31m2 in 24 hours at 1 bar at 23°C.

The outer fabric cover additionally carries a self-adhesive coating in order to allow it to be affixed to the wound site, ensuring that the latter is enclosed.

Within the outer fabric cover, integrally contained and secured, are two envelopes of the same overall dimensions and both having a general sachet format. These are positioned in horizontal alignment, with one above the other (Fig. 1.).

The upper, OG envelope contains a fill material (6), including sodium carbonate peroxyhydrate, which is capable, on hydration, of both generating oxygen and subsequently absorbing carbon dioxide, being designed to maintain a predetermined percentage composition of oxygen and carbon dioxide within the enclosed headspace of the wound area This OG envelope is fabricated from an upper or backing sheet (2) comprising a non-woven polymer support layer laminated to a water vapour and gas impermeable polymer film and a lower, cover face or control' sheet (3) comprising a non-woven, polymer support layer laminated to a water vapour and gas permeable but liquid water impermeable polymer film.

A principal requirement of the upper, OG envelope, as a sub-device of the invention, is to operate effectively within the presence of free aqueous liquids.

The lower of the two envelopes contains a fill material (6) of super-absorbent acrylate beads laminated within layers of polypropylene fabric, which is designed to fully absorb and retain wound exudates in order to allow the wound itself to remain relatively dry.

The lower, WEA envelope is fabricated from a medical grade, non-woven PP fabric (4), or a material of equivalent specification and performance.

The upper OG envelope of the embodiment contains a fill material formulation of a mixture of four compounds. The formulation of the fill material of the device, in terms of the percentage composition of the constituent compounds, is designed to deliver the wound site headspace atmosphere control required and is thus specific to the nature of the wound application concerned, with a limited range of formulations available to meet these particular requirements. Specific formulations are described below.

The mass of the fill material formulation of the OG envelope of the device, while avoiding excessive redundancy, is designed to deliver the performance life required and is thus specific, within the limited range discussed above, to the size and particular nature of the wound requiring treatment.

In order to allow the fill material formulations to perform optimally, the control layer of the OG envelope of the device is required to be impermeable to liquid water but to have both an appropriately high WVTR and OTR.

To achieve this, the material, from which the control membrane of the OG envelope is fabricated, is a bi-laminate with an outer polymer film layer supported by an inner layer of a dense, non-woven polymer fabric.

In order to retain fully the fill material formulation selected, both dry and hydrated, in the presence of free liquid water, the OG envelope of the device has a upper or backing sheet of a laminate material comprising an inner permeable support layer and a liquid water and gas impermeable outer layer, The lower or control' sheet, however, comprises an inner permeable support layer and an outer water impermeable but water vapour and gas permeable layer, which in combination have a WVTR of lOg to 200g/m2 in 24 hours at 23 °C and 85% relative humidity and an OTR in the range 650cm3-20,000cm3/m2 in 24 hours at 1 bar at 23°C.

The WVTR may be measured by employing a commercially available water permeation analyser such as the MOCON PERMATRAN-W Model 3/33, which is designed to test this property of packaging films and has integral automatic relative humidity generation. The machine measures according to the ASTM standard F -1249.

The OTR may be measured by employing a commercially available oxygen transmission rate analyser, such as the MOCON OX-TRAN Model 1/50, which is designed to test this property of packaging films. The machine measures according to the ASTM standard F-1927.

In a particular embodiment, the backing sheet of the OG envelope is a bi-laminate material, comprising an outer, permeable layer of a non-woven, opaque 45 g/m2 - 55g/m2 polyethylene fabric, bonded with a polyurethane adhesive to an inner, liquid water, water vapour and gas impermeable polymer film The above polymer film layer is polyethylene terephthalate (PET), also known as polyester, with a thickness of 2Opm -25pm The cover face or control' sheet of the OG envelope is a bi-laminate material, comprising an outer, permeable layer of a non-woven, opaque 45 g/m2 -55g/m2 polyethylene fabric, bonded with a polyurethane adhesive to an inner, water impermeable but water vapour and gas permeable layer of polymer film The above polymer film layer, depending on the water vapour and gas transmission requirements demanded by the nature of the wound to be dressed, is either PA, with a thickness of 10pm -20pm or FE, with a thickness of 10pm -2Opm or polypropylene (PP) with a thickness of 10pm -2Opm or appropriately performing co-extrusions of any of the above polymers. For many applications, PA is preferred.

The supporting layer in both the backing and control' sheets is TYVEK; a permeable, non-woven, high density polyethylene (HDFE) fabric, which in this case will have a density of 45g/sm -55gIsm. This material is not an effective barrier to water vapour, oxygen or carbon dioxide. For preference the grade of TYVEK would be TYVEK 2FS, a medical specification with microbial barrier properties.

The inner and outer layers of the cover face control' sheet are bonded together with a polyurethane adhesive, or a material of equivalent properties and performance, the composition and thickness of which will have a negligible effect on either the WVTR or OTR of the inner layer or the laminate in general.

In its simplest construction format, the OG envelope material is fabricated into a sachet, which allows the inward passage of water vapour, the outward passage of oxygen and the inward passage of carbon dioxide; the latter two gases at both a relatively high rate and a pressure that may slightly exceed ambient atmospheric.

In this format, the OG envelope of the device can be manufactured using existing vertical, or horizontal, form, fill and seal technology, where the seals themselves can be heat bonded or dielectrically or ultrasonically welded.

In this format, the OG envelope of the device is intended to be placed with precision and secured within the outer fabric cover of the wound dressing.

The lower WEA envelope of the embodiment contains a fill material in the form of a die-cut, multi-laminate pad comprising layers of a medical, liquid water permeable, wound contact specification, non-woven PP fabric, between which are dispersed, at low density, fine beads of super-absorbent acrylate, also of a medical, wound contact

specification.

The WEA envelope itself is fabricated, in both its caver and backing sheets, from a medical, liquid water permeable, wound contact specification, non-woven PP fabric.

In this format, the WEA envelope in the device can be manufactured using existing vertical, or horizontal, form, fill and seal technology, where the seals themselves can be heat bonded or dielectrically or ultrasonically welded.

In this format, the WEA envelope in the device is intended to be placed with precision and secured within the outer fabric cover of the wound dressing.

The device is designed for deployment as a self-adhesive wound dressing, in conventional format for the protection and treatment of chronic wounds such as pressure induced sores and venous leg ulcers, as well as diabetic lesions. in which there is the possibility of the presence of free water and where the oxygen composition of the wound site headspace atmosphere is required to be maintained at predetermined supra-atmospheric levels and that of carbon dioxide at atmospheric or sub-atmospheric levels.

The criteria, which this device aims at, are:-ease of application to the wound site, with no additional training beyond that already required for the conventional dressing of chronic wounds to protect the wound site from physical contact to protect the wound site from chemical and microbial contamination to effectively remove wound exudates to allow the wound to remain dry.

to protect the wound site by retaining fully the fill materials and any reaction products produced, to be flexible in that it can be applied at any orientation that the location of the wound requires, to be comfortable when worn by the patient to produce and maintain pre-determined supra-atmospheric levels of oxygen within the wound site headspace atmosphere, to effect and maintain atmospheric or sub-atmospheric levels of carbon dioxide within the wound site headspace atmosphere, To address the latter two requirements, alternative formulations of the fill material may be used.

In the device, where high levels of both oxygen generation and carbon dioxide absorption are required, the fill material formulation comprises a mixture c of the following: -sodium carbonate peroxyhydrate, in the form of fine granules Bentonite BR, in the form of a fine powder.

sodium chloride, as a fine crystalline powder sodium carbonate, in the form of fine granules The particle sizes of the component compounds of the fill material formulation have been selected to deliver optimum performance in terms of reaction initiation and delivering a wound site head space atmosphere with supra-atmospheric percentage composition of oxygen..

The formulation of the fill material, in terms of the percentage composition of the constituent compounds, is designed to effect the wound site headspace atmosphere control required and is thus specific to the type of wound treated and, within a limited range, will vary between specific wound type applications In deployment, the following compounds, where included in the formulation, are activated by contact with water, in the form of water vapour, which is present in the wound site headspace atmosphere, namely: sodium carbonate peroxyhydrate, sodium carbonate, The oxygen generating reaction formula as follows: -H20 Na2CO3.3/2H20 4 Na2CO3 + 3/2H202 4 Na2CO3 + 3/2H20 + 3/402 sodium carbonate peroxyhydrate Thus, sodium carbonate peroxyhydrate constitutes an oxygen generating compound within the fill material formulation since on hydrolysis it generates oxygen.

The carbon dioxide absorbing reaction is as follows: -Na2CO3 + CO2 + H20 -* 2NaHCO3 sodium carbonate Thus, sodium carbonate constitutes a carbon dioxide absorbing component within the fill material formulation since since on hydrolysis, it absorbs carbon dioxide In order to control the rates of the reactions and allow them to continue over a pre-determined, extended period, the availability of water is regulated by both the selection of the gauge and material of the contrOl layer of the upper envelope within the dressing and the predetermined percentage composition of the Bentonite BP and sodium chloride components within the till material formulation. Both are strongly hydroscopic and absorb water vapour trom the wound site headspace atmosphere much more readily than does either the sodium carbonate peroxyhydrate or sodium carbonate, effectively restricting the amount of water available, in the form water vapour, to the latter compounds.

Thus, the combination of Bentonite BP and sodium chloride constitutes a control formulation.

The Bentonite BP component, as a finely particulate material, has a very large capacity to absorb water, as water vapour, from the atmosphere.

Bentonite clays are aluminosilicates of alkali metals. They have three dimensional crystalline frameworks of tetrahedral silica or alumina anions strongly bonded at all corners. These structures contain -Si-O-Al-linkages that torm surface pores of uniform diameter and enclose regular internal cavities and channels of discrete sizes and shapes, depending on the chemical composition and crystal structure structures of the specific zeolite involved. The pore sizes range from 2.0 to 4.3 Angstroms and the enclosed cavities both the mineral cations and water molecules, the latter being capable of desorption as well as adsorption.

A particular percentage composition of the fill material formulation, where carbon dioxide in the wound site headspace atmosphere is to be maintained at atmospheric or sub-atmospheric levels is: -sodium carbonate peroxyhydrate 55% to 75%, preferably 60% to 70% Bentonite BP 18% to 30%, preferably 20% to 25% sodium chloride, 5% to 10%, preferably 6% to 8% sodium carbonate 2% to 5%, preferably 3% to 4% In a particular embodiment, the following percentage composition of the fill material formulation may be used: sodium carbonate peroxyhydrate 67.0% Bentonite BP 22.0% sodium chloride, 8.0% sodium carbonate 3.0% Apart from the composition, the mass of a fill material formulation in the upper envelope is also determined by the particular demands placed on the wound site headspace atmosphere by the by the size, type and nature of the chronic wound concerned and the requirements the latter imposes on the former in order to maintain the predetermined, optimum gas composition desired.

The dimensions of both the upper and lower envelopes and thus the respective, effective surface areas will reflect the size of the chronic wound to be dressed and treated. It is anticipated by the inventors that the device in the present invention will be produced in a limited but appropriate range of sizes Experiments have shown that wound dressings containing the oxygen generating envelope as an integral component as described can both develop and maintain elevated, supra-atmospheric levels of oxygen and control carbon dioxide levels at atmospheric or sub-atmospheric levels within the headspace atmosphere of the wound site the pack atmosphere for periods exceeding eight days.

In particular, the selective use of polymers and the gauges available in the control' sheet of the OG envelope to regulate water vapour transmission allows a degree of precision in offering a range of performances that more closely meet the particular dressing requirements of the wounds under treatment.

Claims

CLAIMS1. An oxygen generating and carbon dioxide absorbing wound dressing, comprising an outer fabric cover, S a first envelope contained within the outer cover, the contents of which are activated by water vapour to generate oxygen and absorb or generate carbon dioxide; and a second envelope contained within the outer cover, the contents of which are capable of absorbing high volumes of aqueous liquids, wherein the outer fabric cover is comprised of a composite material with both a low water vapour transmission rate and moderately low oxygen transmission rate, the latter to allow a controlled escape of oxygen from the wound-site head space atmosphere in order to prevent over pressure on the wound itself, wherein the outer fabric cover carries a self-adhesive coating in order to allow it to be affixed to area of the wound site, ensuring that the latter is fully enclosed.
2. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 1, wherein the outer fabric cover having a water vapour transmission rate of lOg to 200g1m2 in 24 hours at 23 °C and 85% relative humidity and an oxygen transmission rate in the range 400cm3-800cm3/m2 in 24 hours at 1 bar at 23°C.
3. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 1, wherein the first envelope contained within the outer cover is in sachet format and fabricated with the cover face sheet and the backing sheet each of a different material.
4. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and has a cover face sheet that is impermeable to liquid water and permeable to water vapour and gases.
5. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and has a cover face sheet comprising an inner permeable support layer and an outer water impermeable but water vapour and gas permeable layer, which in combination have a WVTR of lOg to 200g/m2 in 24 hours at 23 °C and 85% relative humidity and an OTR in the range 650cm3-20,000cm3Im2 in 24 hours at 1 bar at 23°C.
6. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and has a cover face sheet of a bi-laminate material, comprising an outer, permeable layer of a non-woven, opaque 45 g/m2 -55g1m2 polyethylene fabric, bonded with a polyurethane adhesive to an inner, control' layer of water impermeable but water vapour and gas permeable polymer film.
7. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and has a cover face sheet of a bi-laminate material, where the above inner or control' layer of polymer film is either polyamide, with a thickness of 10pm -2Opm or polyethylene, with a thickness of 10pm -2Opm or polypropylene with a thickness of 10pm -2Opm or an appropriately performing co-extrusion of any of the above polymers.
8. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and has a cover face sheet of a bi-laminate material, in which a polyurethane adhesive, or an adhesive of equivalent properties and performance, bonds the two layers of the laminate together.
9. An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and contains a fill material formulation which comprises: lOan active ingredient, sodium carbonate peroxyhydrate, that generates oxygen and absorbs carbon dioxide when activated by water vapour; and water vapour control ingredients, Bentonite BR and sodium chloride, that competitively absorb water vapour to regulate the reaction of the active ingredient, wherein the active ingredient is present in an amount from 55% to 75%, and the water vapour control ingredients are present in an amount from 23% to 40%.11 An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and contains a fill material formulation which comprises: a carbon dioxide absorbing ingredient, sodium carbonate from 2% to 5%.12.An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and contains a fill material formulation, which comprises: 55% to 75% by weight sodium carbonate peroxyhydrate; 18% to 30 % by weight Bentonite BP, 5% to 10% by weight sodium chloride, and 2% to 5% by weight sodium carbonate.13.An oxygen generating and carbon dioxide absorbing wound dressing according to claim 3, wherein the first envelope contained within the outer cover is in sachet format and contains a fill material formulation, which may, in particular comprise: sodium carbonate peroxyhydrate 67.0% Bentonite BP 22.0% sodium chloride, 8.0% sodium carbonate 3.0% 14.An oxygen generating and carbon dioxide absorbing wound dressing according to any preceding claim, wherein the first envelope contained within the outer cover is in sachet format and contains a fill material formulation, in which the particle size ranges of the component compounds have been selected to allow optimum performance in terms of reaction initiation and timely delivery of a wound site headspace atmosphere with a supra-atmospheric percentage composition of oxygen..15.An oxygen generating and carbon dioxide absorbing wound dressing according to any preceding claim, wherein the first envelope contained within the outer cover is in sachet format and contains a fill material formulation, which does not require an integral or endogenous source of water, in the form of water vapour, to initiate the oxygen generating reaction, the latter being initiated soley by water vapour derived from both the wound itself and the perspiration from the enclosed area of skin surrounding the woundS