Moisture Vapour Permeability

Sample
(gm⁻²/24 hrs)

Description
Results
Mean

Non perforated silicone
373
405
367
382

wound contact layer

Aptly, the layer in the dressing which controls moisture vapour transmission rate (the silicone layer described in this specification is a suitable example of such a layer) has a moisture vapour permeability measured in gm-2/24 hrs of between 350 and 410. Aptly, the average moisture vapour permeability is around 380 gm-2/24 hrs. It will be appreciated that those skilled in the art may well consider a figure of less than 500 gm-2/24 hrs to amount to an impermeable layer. It will be appreciated that according to certain embodiments of the present invention a layer having a moisture vapour permeability in the range noted above can be provided at any suitable location in the wound dressing so as to control moisture vapour permeability and thus the environment of the tissue site. Aptly, this layer can be provided by a silicone adhesive wound contact layer.

An example of a suitable pressure sensitive adhesive is a Wacker silres PSA 45 adhesive. According to certain embodiments of the present invention a non-adhesive silicone layer may be used such as a Dow Corning 7-4107 elastomeric membrane. It will be understood that certain embodiments of the present invention are not restricted to the specific materials described for the silicone layer.

A layer 105 of porous material may be located above the wound contact layer. This porous layer behaves as a transmission layer which allows transmission of fluid including liquid and gas away from a wound site into upper layers of the wound dressing. In particular, the transmission layer 105 ensures that an open air channel can be maintained to communicate negative pressure over the wound area even when the absorbent layer has absorbed substantial amounts of exudates. The layer should remain open under the typical pressures that will be applied during negative pressure wound therapy as described above, so that the whole wound site sees an equalised negative pressure. The layer 105 is formed of a material having a three dimensional structure. For example, a knitted or woven spacer fabric (for example Baltex 7970 weft knitted polyester). Other materials could of course be utilised.

Aptly, the transmission layer comprises a 3D polyester spacer fabric layer including a top layer (that is to say, a layer distal from the wound-bed in use) which is a 84/144 textured polyester, and a bottom layer (that is to say, a layer which lies proximate to the wound bed in use) which is a 100 denier flat polyester and a third layer formed sandwiched between these two layers which is a region defined by a knitted polyester viscose, cellulose or the like monofilament fibre. Other materials and other linear mass densities of fibre could of course be used.

Whilst reference is made throughout this disclosure to a monofilament fibre it will be appreciated that a multistrand alternative could of course be utilised.

The top spacer fabric thus has more filaments in a yarn used to form it than the number of filaments making up the yarn used to form the bottom spacer fabric layer.

This differential between filament counts in the spaced apart layers helps control moisture flow across the transmission layer. Particularly, by having a filament count greater in the top layer, that is to say, the top layer is made from a yarn having more filaments than the yarn used in the bottom layer, liquid tends to be wicked along the top layer more than the bottom layer. In use, this differential tends to draw liquid away from the wound bed and into a central region of the dressing where the absorbent layer helps lock the liquid away or itself wicks the liquid onwards towards the cover layer where it can be transpired.

Aptly, to improve the liquid flow across the transmission layer (that is to say perpendicular to the channel region formed between the top and bottom spacer layers, the 3D fabric is treated with a dry cleaning agent (such as, but not limited to, perchloro ethylene) to help remove any manufacturing products such as mineral oils, fats and/or waxes used previously which might interfere with the hydrophilic capabilities of the transmission layer. Aptly, an additional manufacturing step can subsequently be carried in which the 3D spacer fabric is washed in a hydrophilic agent (such as, but not limited to, Feran Ice 30 g/l available from the Rudolph Group). This process step helps ensure that the surface tension on the materials is so low that liquid such as water can enter the fabric as soon as it contacts the 3D knit fabric. This also aids in controlling the flow of the liquid insult component of any exudates.

A layer 110 of absorbent material may be provided above the transmission layer 105. The absorbent material which may be a foam or non-woven natural or synthetic material and which may optionally include or be super-absorbent material forms a reservoir for fluid, particularly liquid, removed from the wound site and draws those fluids towards a cover layer 140. The material of the absorbent layer also prevents liquid collected in the wound dressing from flowing in a sloshing manner. The absorbent layer 110 also helps distribute fluid throughout the layer via a wicking action so that fluid is drawn from the wound site and stored throughout the absorbent layer. This helps prevent agglomeration in areas of the absorbent layer. The capacity of the absorbent material should preferably be sufficient to manage the exudates flow rate of a wound when negative pressure is applied. Since in use the absorbent layer experiences negative pressures the material of the absorbent layer is chosen to absorb liquid under such circumstances. A number of materials exist that are able to absorb liquid when under negative pressure, for example superabsorber material. The absorbent layer 110 may typically be manufactured from ALLEVYN™ foam, Freudenberg 114-224-4 and/or Chem-Posite™11C-450.

Aptly, the absorbent layer is a layer of non-woven cellulose fibres having super-absorbent material in the form of dry particles dispersed throughout. Use of the cellulose fibres introduces fast wicking elements which help quickly and evenly distribute liquid taken up by the dressing. The juxtaposition of multiple strand-like fibres leads to strong capillary action in the fibrous pad which helps distribute liquid. In this way, the super-absorbent material is efficiently supplied with liquid. Also, all regions of the absorbent layer are provided with liquid.

The wicking action also assists in bringing liquid into contact with the upper cover layer to aid increase transpiration rates of the dressing.

The wicking action also assists in delivering liquid downwards towards the wound bed when exudation slows or halts. This delivery process helps maintain the transmission layer and lower wound bed region in a moist state which helps prevent crusting within the dressing (which could lead to blockage) and helps maintain an environment optimised for wound healing.

Aptly, the absorbent layer may be an air-laid material. Heat fusible fibres may optionally be used to assist in holding the structure of the pad together. It will be appreciated that rather than using super-absorbing particles or in addition to such use, super-absorbing fibres may be utilised according to certain embodiments of the present invention. An example of a suitable material is the Product Chem-Posite™ 11 C available from Emerging Technologies Inc (ETi) in the USA.

Optionally, according to certain embodiments of the present invention, the absorbent layer may include synthetic stable fibres and/or bi-component stable fibres and/or natural stable fibres and/or super-absorbent fibres. Fibres in the absorbent layer may be secured together by latex bonding or thermal bonding or hydrogen bonding or a combination of any bonding technique or other securing mechanism. Aptly, the absorbent layer is formed by fibres which operate to lock super-absorbent particles within the absorbent layer. This helps ensure that super-absorbent particles do not move external to the absorbent layer and towards an underlying wound bed. This is particularly helpful because when negative pressure is applied there is a tendency for the absorbent pad to collapse downwards and this action would push super-absorbent particle matter into a direction towards the wound bed if they were not locked away by the fibrous structure of the absorbent layer.

The absorbent layer preferably comprises a layer of multiple fibres. Aptly, the fibres are strand-like and made from cellulose, polyester, viscose or the like. Aptly, dry absorbent particles are distributed throughout the absorbent layer ready for use. Aptly, the absorbent layer comprises a pad of cellulose fibres and a plurality of super absorbent particles. Aptly, the absorbent layer is a non-woven layer of randomly orientated cellulose fibres.

Super-absorber particles/fibres may be, for example, sodium polyacrylate or carbomethoxycellulose materials or the like or any material capable of absorbing many times its own weight in liquid. Aptly, the material can absorb more than five times its own weight of 0.9% W/W saline, etc. Aptly, the material can absorb more than 15 times its own weight of 0.9% W/W saline, etc. Aptly, the material is capable of absorbing more than 20 times its own weight of 0.9% W/W saline, etc. Aptly, the material is capable of absorbing more than 30 times its own weight of 0.9% W/W saline, etc.

Aptly, the particles of superabsorber are very hydrophilic and grab the fluid as it enters the dressing, swelling up on contact. An equilibrium is set up within the dressing core whereby moisture passes from the superabsorber into the dryer surrounding area and as it hits the top film the film switches and the fluid vapour starts to be transpired. A moisture gradient is established within the dressing to continually remove fluid from the wound bed and ensure the dressing does not become heavy with exudate.

Aptly the absorbent layer includes at least one through hole located so as to underly the suction port. As illustrated in FIG. 1 a single through hole can be used to produce an opening underlying the port 150. It will be appreciated that multiple openings could alternatively be utilised. Additionally should more than one port be utilised according to certain embodiments of the present invention one or multiple openings may be made in the super-absorbent layer in registration with each respective port. Although not essential to certain embodiments of the present invention the use of through holes in the super-absorbent layer provide a fluid flow pathway which is particularly unhindered and this is useful in certain circumstances.

Where an opening is provided in the absorbent layer the thickness of the layer itself will act as a stand-off separating any overlying layer from the upper surface (that is to say the surface facing away from a wound in use) of the transmission layer 105. An advantage of this is that the filter of the port is thus decoupled from the material of the transmission layer. This helps reduce the likelihood that the filter will be wetted out and thus will occlude and block further operation.

Use of one or more through holes in the absorption layer also has the advantage that during use if the absorbent layer contains a gel forming material, such as superabsorber, that material as it expands to absorb liquid, does not form a barrier through which further liquid movement and fluid movement in general cannot pass. In this way each opening in the absorbent layer provides a fluid pathway between the transmission layer directly to the wound facing surface of the filter and then onwards into the interior of the port.

A gas impermeable, but moisture vapour permeable, cover layer 140 preferably extends across the width of the wound dressing. The cover layer, which may for example be a polyurethane film (for example, Elastollan SP9109) having a pressure sensitive adhesive on one side, is impermeable to gas and this layer thus operates to cover the wound and to seal a wound cavity over which the wound dressing is placed. In this way an effective chamber is made between the cover layer and a wound site where a negative pressure can be established. The cover layer 140 is sealed to the wound contact layer 102 in a border region 200 around the circumference of the dressing, ensuring that no air is drawn in through the border area, for example via adhesive or welding techniques. The cover layer 140 protects the wound from external bacterial contamination (bacterial barrier) and allows liquid from wound exudates to be transferred through the layer and evaporated from the film outer surface. The cover layer 140 typically comprises two layers; a polyurethane film and an adhesive pattern spread onto the film. The polyurethane film is moisture vapour permeable and may be manufactured from a material that has an increased water transmission rate when wet.

The absorbent layer 110 may be of a greater area than the transmission layer 105, such that the absorbent layer overlaps the edges of the transmission layer 105, thereby ensuring that the transmission layer does not contact the cover layer 140. This provides an outer channel 115 of the absorbent layer 110 that is in direct contact with the wound contact layer 102, which aids more rapid absorption of exudates to the absorbent layer. Furthermore, this outer channel 115 ensures that no liquid is able to pool around the circumference of the wound cavity, which may otherwise seep through the seal around the perimeter of the dressing leading to the formation of leaks.

In order to ensure that the air channel remains open when a vacuum is applied to the wound cavity, the transmission layer 105 is preferably sufficiently strong and non-compliant to resist the force due to the pressure differential. However, if this layer comes into contact with the relatively delicate cover layer 140, it can cause the formation of pin-hole openings in the cover layer 140 which allow air to leak into the wound cavity. This may be a particular problem when a switchable type polyurethane film is used that becomes weaker when wet. The absorbent layer 110 is generally formed of a relatively soft, non-abrasive material compared to the material of the transmission layer 105 and therefore does not cause the formation of pin-hole openings in the cover layer. Thus by providing an absorbent layer 110 that is of greater area than the transmission layer 105 and that overlaps the edges of the transmission layer 105, contact between the transmission layer and the cover layer is prevented, avoiding the formation of pin-hole openings in the cover layer 140.

The absorbent layer 110 is positioned in fluid contact with the cover layer 140. As the absorbent layer absorbs wound exudate, the exudate is drawn towards the cover layer 140, bringing the water component of the exudate into contact with the moisture vapour permeable cover layer. This water component is drawn into the cover layer itself and then evaporates from the top surface of the dressing. In this way, the water content of the wound exudate can be transpired from the dressing, reducing the volume of the remaining wound exudate that is to be absorbed by the absorbent layer 110, and increasing the time before the dressing becomes full and should be changed. This process of transpiration occurs even when negative pressure has been applied to the wound cavity, and it has been found that the pressure difference across the cover layer when a negative pressure is applied to the wound cavity has negligible impact on the moisture vapour transmission rate across the cover layer.

An orifice 145 is provided in the cover film 140 to allow a negative pressure to be applied to the dressing 100. A suction port 150 is sealed to the top of the cover film 140 over the orifice 145, and communicates negative pressure through the orifice 145. A length of tubing 220 may be coupled at a first end to the suction port 150 and at a second end to a pump unit (not shown) to allow fluids to be pumped out of the dressing. The port may be adhered and sealed to the cover film 140 using an adhesive such as an acrylic, cyanoacrylate, epoxy, UV curable or hot melt adhesive. The port 150 is formed from a soft polymer, for example a polyethylene, a polyvinyl chloride, a silicone, or polyurethane having a hardness of 30 to 90 on the Shore A scale.

An aperture is provided in the absorbent layer 110 beneath the orifice 145 such that the orifice is connected directly to the transmission layer 105. This allows the negative pressure applied to the port 150 to be communicated to the transmission layer 105 without passing through the absorbent layer 110. This ensures that the negative pressure applied to the wound site is not inhibited by the absorbent layer as it absorbs wound exudates. In other embodiments, no aperture may be provided in the absorbent layer 110, or alternatively a plurality of apertures underlying the orifice 145 may be provided.

A filter element 130 that is impermeable to liquids, but permeable to gasses is provided to act as a liquid barrier, and to ensure that no liquids are able to escape from the wound dressing. The filter element may also function as a bacterial barrier. Typically the pore size is 0.2 μm. Suitable materials for the filter material of the filter element 130 include 0.2 micron Gore™ expanded PTFE from the MMT range, PALL Versapore™ 200R, and Donaldson™ TX6628. Larger pore sizes can also be used but these may require a secondary filter layer to ensure full bioburden containment. As wound fluid contains lipids it is preferable, though not essential, to use an oleophobic filter membrane for example 1.0 micron MMT-332 prior to 0.2 micron MMT-323. This prevents the lipids from blocking the hydrophobic filter. The filter element can be attached or sealed to the port and/or the cover film 140 over the orifice 145. For example, the filter element 130 may be moulded into the port 150, or may be adhered to both the top of the cover layer 140 and bottom of the port 150 using an adhesive such as, but not limited to, a UV cured adhesive.

It will be understood that other types of material could be used for the filter element 130. More generally a microporous membrane can be used which is a thin, flat sheet of polymeric material, this contains billions of microscopic pores. Depending upon the membrane chosen these pores can range in size from 0.01 to more than 10 micrometers. Microporous membranes are available in both hydrophilic (water filtering) and hydrophobic (water repellent) forms. In some embodiments of the invention, filter element 130 comprises a support layer and an acrylic co-polymer membrane formed on the support layer. Aptly the wound dressing 100 according to certain embodiments of the present invention uses microporous hydrophobic membranes (MHMs). Numerous polymers may be employed to form MHMs. For example, PTFE, polypropylene, PVDF and acrylic copolymer. All of these optional polymers can be treated in order to obtain specific surface characteristics that can be both hydrophobic and oleophobic. As such these will repel liquids with low surface tensions such as multi-vitamin infusions, lipids, surfactants, oils and organic solvents.

MHMs block liquids whilst allowing air to flow through the membranes. They are also highly efficient air filters eliminating potentially infectious aerosols and particles. A single piece of MHM is well known as an option to replace mechanical valves or vents. Incorporation of MHMs can thus reduce product assembly costs improving profits and costs/benefit ratio to a patient.

The filter element 130 may also include an odour absorbent material, for example activated charcoal, carbon fibre cloth or Vitec Carbotec® Q2003073 foam, or the like. For example, an odour absorbent material may form a layer of the filter element 130 or may be sandwiched between microporous hydrophobic membranes within the filter element.

The filter element 130 thus enables gas to be exhausted through the orifice 145. Liquid, particulates and pathogens however are contained in the dressing.

In operation the wound dressing 100 is sealed over a wound site forming a wound cavity. A pump unit (not shown) applies a negative pressure at a connection portion 154 of the port 150 which is communicated through the orifice 145 to the transmission layer 105. Fluid is drawn towards the orifice through the wound dressing from a wound site below the wound contact layer 102. The fluid moves towards the orifice through the transmission layer 105. As the fluid is drawn through the transmission layer 105 wound exudate is absorbed into the absorbent layer 110.

Turning to FIG. 2 which illustrates a wound dressing 100 in accordance with an embodiment of the present invention one can see the upper surface of the cover layer 140 which extends outwardly away from a centre of the dressing into a border region 200 surrounding a central raised region 201 overlying the transmission layer 105 and the absorbent layer 110. As indicated in FIG. 2 the general shape of the wound dressing is rectangular with rounded corner regions 202. It will be appreciated that wound dressings according to other embodiments of the present invention can be shaped differently such as square, circular or elliptical dressings, or the like.

When a negative pressure is applied to the port 150 from a pump 230 via conduit 220, the negative pressure is communicated to the wound cavity below the cover layer. This negative pressure is thus experienced at the target wound site. Fluid including air and wound exudate is drawn through the wound contact layer and transmission layer 105. The wound exudate drawn through the lower layers of the wound dressing is dissipated and absorbed into the absorbent layer 110 where it is collected and stored. Air and moisture vapour is drawn upwards through the wound dressing through the filter layer and out of the dressing through the suction port. A portion of the water content of the wound exudate is drawn through the absorbent layer and into the cover layer 140 and then evaporates from the surface of the dressing.

FIG. 3 illustrates a cross-section of a portion of the dressing shown in FIGS. 1 and 2. In particular, FIG. 3 illustrates a magnified view of the wound contact layer 102 which includes a lower surface 101 and multiple perforations 104 formed as through holes. An upper surface 104 of the wound contact layer abuts a first layer 300 of the transmission layer 105. A further, upper, layer 301 of the transmission layer 105 is spaced apart from the first layer. The first and further layers of the transmission layer are kept apart in a spaced apart relationship by multiple mono-filament fibre spacers 302 which act as resilient flexible pillars separating the two layers of the transmission layer. The upper layer 301 of the transmission layer is adjacent a lower surface of the absorbent 110 which, for example, is formed as a pad of fibrous cellulose material interspaced with super-absorbent particulate matter.

The absorbent layer 110 holds liquid collected during the application of negative pressure therapy. By having this layer in fluid communication with, and preferably in contact with, the layer of the transmission layer, the region of the transmission layer 105 is kept at a moist environment. This helps avoid build-up and crusting of the exudate during use.

FIG. 4 illustrates a view of an underside of the wound dressing. More particularly, a lower surface 101 of the wound dressing 100 is part of a wound contact layer 102. Aptly, the wound contact layer 102 is a polyurethane layer or polyethylene layer or other flexible layer which is perforated. Aptly, materials are selected from the Triboelectric series of materials which become negative in charge when brought into contact with other materials. Such materials can be, but are not limited to, polyester, polyurethane, saran wrap, polyethylene, polypropylene, silicone film and/or vinyl (pvc) or the like. As illustrated in FIG. 4, the perforations 104 are through holes in the wound contact layer. The perforations may be formed as slits or slots or substantially circular holes. Aptly, the circular holes have a size ranging from 0.025 mm to 1.5 mm. The opening of each perforation has a cross-sectional area which is referred to as an open area. The open area provided by the openings when related to the total area presented by the underside of the dressing is sufficiently small so as to help prevent tissue ingrowth into the wound dressing, allowing wound exudate to flow into the dressing, and to maintain a level of hydration below the dressing and provide access of oxygen to the surface of the underlying skin in use whereby scar formation is prevented or reduced. Aptly, the open area of openings is around 20% or less of a total area. Aptly, the open area is around 17.5% or less of a total area. The open area is large enough to enable full communication of negative pressure to the underlying tissue. That is to say there is little or no pressure difference on either side of the layer.

As illustrated in FIG. 4, each opening 104 in the wound contact layer 101 is a through hole and reveals a respective region of the transmission layer lying within the dressing on a side of the wound contact layer distal from the wound/tissue in use. Aptly, the transmission layer material is selected from a material in the Triboelectric series which becomes negative in charge and will thus tend to attract electrons when brought into contact with other materials.

In use, location of a wound dressing having a silicone adhesive wound contact layer may help restore a barrier function of the stratum corneum of an underlying wound. This helps reduce or minimise transepidermal water loss (TEWL) which would otherwise occur. By helping to reduce TEWL stimulation of the various stages in a normal healing process, which would lead to excessive collagen production and abnormal scarring, may be prevented wholly or partially. Provision of a limited open area in the wound contact layer may enable such TEWL to be controlled so as to assist in an anti-scarring and yet may also enable negative pressure therapy to be applied to the underlying tissue.

This contemporaneous application of negative pressure therapy may help apply pressure simultaneously to the tissue. This may assist in reducing scar formation. Negative pressure application may also help contract and/or compress the whole underlying tissue. Compression may also help prevent upward overgrowth of tissue. This may in turn help to reduce the raised appearance of a scar. Contraction may help with a position of the epidermis of an underlying wound region. This may thus reduce the need for excess tissue growth which might otherwise lead to scar tissue formation.

Aptly, the simultaneous application of NPT also increases build-up of electrostatic charge between the dressing and tissue. The interaction between negatively charged ions of the dressing and ionic charges of the tissue may help contribute to wound and scar healing.

Aptly, having a silicone adhesive as a wound contacting surface layer enables the topical application of silicone oils which may leak from the adhesive layer. Aptly, cotemporaneous application of compression forces adhesive forces and a dressing which helps buffer external shear forces means that a particularly optimum environment is provided to assist in scar free healing/tissue formation. This may lead to improved inhibition of scar formation.

FIG. 5 illustrates a first, upper surface 500 and a further, lower surface 502 of a transmission layer 105 according to an embodiment of the present invention. In the embodiment illustrated in FIG. 5 fibres 503 of a woven layer extend between the first surface 500 and the further surface 502. It will be appreciated that according to further embodiments of the present invention if a foam layer is used as a transmission layer 105 the connected strands forming the foam will act as spacer elements. As illustrated in FIG. 5 in a relaxed mode of operation, that is to say when in use, no negative pressure is applied to the wound dressing or negative pressure is applied to the wound dressing but no external force acts on the wound dressing then the fibres 503 extend substantially perpendicular to the upper and lower surfaces keeping the surfaces in a spaced apart substantially parallel configuration.

FIG. 6 illustrates the transmission layer 105 when an external force is exerted on the outside of the dressing. The external force can be a compressive force indicated by arrow A and/or a lateral force illustrated by arrow B in FIG. 6. As indicated either a compressive force or a lateral force acts to cause the fibres 503 to lean to one side. This causes the upper and lower surfaces to become laterally offset with respect to each other as well as causing the thickness of the layer to reduce from a separation distance r indicated in