Abstract:
Embodiments disclosed within herein relate to a wound dressing, suitable for use in negative pressure wound therapy, comprising a body of porous material, the body of porous material comprising a plurality of cuts which provide regions of flexibility within the body. Further embodiments relate to methods of manufacturing and method of use of such wound dressings. In addition, the wound dressing can comprise a retaining mechanism removably coupled to the material and configured to retain the material in the expanded conformation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of U.S. patent application Ser. No. 12/443,169, filed Nov. 17, 2009, entitled LATTICE DRESSING, which is a National Phase Application of PCT Patent Application No. PCT/US2007/079529 filed Sep. 26, 2007, entitled LATTICE DRESSING, which claims priority to U.S. Provisional Application No. 60/826,922, filed Sep. 26, 2006, entitled LATTICE DRESSING. This application is also a continuation-in-part of U.S. patent application Ser. No. 13/202,974, filed Oct. 5, 2011, entitled DRAPEABLE WOUND DRESSING, which is a National Phase Application of PCT Patent Application PCT/GB2010/000293, filed Feb. 19, 2010, entitled DRAPEABLE WOUND DRESSING, which claims priority to Great Britain Application No. 0903032.1, filed Feb. 24, 2009, entitled DRAPEABLE WOUND DRESSING. The contents of the aforementioned applications are hereby incorporated by reference in their entireties as if fully set forth herein. The benefit of priority to the foregoing applications is claimed under the appropriate legal basis, including, without limitation, under 35 U.S.C. §119(e). 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This application describes embodiments of apparatuses, methods, materials, methods of manufacture, and systems for the treatment of wounds, specifically to aid in packing and treating wounds. 
         [0004]    This application further relates to improved wound dressing materials. In particular this application relates to wound dressing materials with improved drapeability, which allows them to more easily conform to the shape of a surface over which they are placed. Especially, but not exclusively, the present application relates to foam materials, especially those suitable for use in negative pressure wound therapy (NPWT). 
         [0005]    2. Description of the Related Art 
         [0006]    Wound dressings are commonly manufactured from sheet materials. These materials include transparent films and adhesives and opaque foams and fibres. These materials generally retain their transparency or opacity upon fluid absorption, the exception being carboxymethylcellulose-based hydrofibre (which is initially opaque and becomes transparent when wet). Wound dressings which rely upon a foam element for exudate management are opaque and require removal and exchange for a new dressing during every wound inspection. This is a disadvantage of opaque dressings, for example, including a foam element. Foam-based dressings also have limited extensibility due to the mechanical properties inherent in a material of foam structure; this can cause problems when attempting to dress locations of high curvature. In spite of these performance disadvantages, the use of foam-based wound dressings or dressing elements dominates woundcare. This may be due to their excellent absorbency, low dry weight, cut-ability and mouldability. Limited extensibility is not restricted to foams; this is also a property of the films commonly applied in medical devices. 
         [0007]    The single-axis lattice cutting of monolithic materials, including the sheet materials utilised in wound dressings has been disclosed in (GB821,959). However, once cut, these materials are not extended in any manner until post-application to the wound, and only then as a result of the flexing of the limb or other part of the body on which the dressing is worn. Whilst this type of dressing does enable some ease of movement for the patient, the material only permits minimal visibility of the wound. Additionally, the size of apertures and thus the permeability of the material, particularly important for the release of exudates from the wound, cannot be controlled. 
         [0008]    There is a need for a wound dressing which can be established in an extended conformation both prior to application and during application to a wound. 
         [0009]    There is a need for a wound dressing which can be retained within a defined extended conformation both prior to application and during application to a wound. 
         [0010]    There is a need for a wound dressing which allows visualisation of the wound immediately upon application of the dressing. 
         [0011]    There is a need for a wound dressing which has a controlled aperture profile, which enables control of exudate egress. 
         [0012]    There is a need for a wound dressing which has a controlled contractile profile for promoting wound closure. 
         [0013]    NPWT is a relatively new treatment for open wounds. Typically in NPWT the wound cavity or surface is filled or covered with a material that allows the transmission of a partial vacuum (i.e. does not completely collapse) to the wound bed when a negative pressure is applied to the wound area, and also allows fluids to pass from the wound bed towards the source of negative pressure. There are two primary approaches to NPWT, gauze or foam types. The gauze type (also referred to as the Chariker-Jeter technique) involves the use of a drain wrapped in gauze topped by a sealed dressing. The foam type involves the use of foam placed over or in the wound. The present invention is directed primarily, but not exclusively, towards the foam type of NPWT. 
         [0014]    In foam based NPWT the wound is filled or covered with a porous, compressible foam packing material and covered over and sealed with flexible sheet (a drape) that is fairly impermeable to fluids. A tube is inserted under or through the drape into the wound site and its distal end is connected to a vacuum source (commonly a pump). The wound cavity, enclosed by the drape and tissue, contracts under the force of atmospheric pressure and compresses the packing material visibly. Gross tissue movement ceases after a few tens of seconds and fluid flow from the wound (withdrawn from the tissue) ensues. The fluid is transmitted through the packing material and up the vacuum tube to a collection receptacle positioned between the distal end of the tube and the vacuum source. The material mechanically supports the tissue to which it is applied, and also allows the free flow of fluids away from the site when a vacuum is applied, even when compressed. A good material for this application is hydrophobic, reticulated polyurethane foam of very high free internal volume. However, articles of high free internal volume tend to be poorly drapeable due to the requirement for their structure to mechanically support their high free internal volume, and this is the case in current foams applied in NPWT. 
         [0015]    The foams, which in other regards are near optimal for NPWT, are very poorly conformable to the site of application especially prior to the application of a covering drape and vacuum being applied. This is true for cavity wounds of concave geometry, and surface wounds, e.g. to the extremities, of convex geometry. 
         [0016]    Monolithic articles of high free internal volume are able to support their own external dimensions by virtue of their mechanical properties, i.e. they are relatively stiff. This factor has the side effect of making such solid objects poorly drapeable and this is not desirable for all applications. For some applications, the mechanical integrity of the structure is not required at the scale of the monolith itself 
         [0017]    For medical applications involving the application of articles to the body, both good drapeability and good resistance to compression under loading are desirable attributes. 
       SUMMARY OF THE INVENTION 
       [0018]    The visibility through an opaque material can be increased by the creation of apertures through it. However, the creation of apertures, for example by hole-punching, has the disadvantages of resulting in the generation of waste and also the mechanical weakening of the material. By forming through slits in a material, these slits being capable of expanding to form apertures on extension of the material, we have been able to achieve both increased visibility of the wound and increased extensibility of the material, without significant material waste. In this manner, it is also possible to achieve extension of the slit to form a circular hole without mechanically weakening the material. 
         [0019]    Therefore, according to an aspect of the invention there is provided a lattice comprising a material having at least one slit, the slit being capable of expansion into an aperture, the lattice having 
         [0020]    i) a first conformation, wherein the slit is substantially closed, and 
         [0021]    ii) a second conformation, wherein the lattice is stretched by the application of an extensive force expanding the slit to form an aperture having a first dimension. 
         [0022]    Material having a slit or plurality of slits is referred to as a lattice, and prior to extension of the material, the lattice is referred to as being in a “first conformation” or “initial conformation” or “closed lattice conformation” wherein the slits are substantially closed. A substantially closed slit is a slit which has a conformation that allows no or substantially no visual inspection of the wound through the lattice with the naked eye. 
         [0023]    Preferably, the material is a polyurethane foam or a silicone elastomer, for example. 
         [0024]    The cutting of a largely two-dimensional or flat material and a three-dimensional material to form slits can be achieved by any means known in the art, for example laser cutting or blade pressing. The cutting method can be a batch process or continuous process. The cutting arrangements of two-dimensional material are preferably two-dimensional and along the x and y plane. The cutting arrangements of three-dimensional material are preferably three-dimensional and along the x, y and z planes. 
         [0025]    The material may be cut so that the slits form a parallel, staggered, patterned or random arrangement. 
         [0026]    Cuts are not restricted to any particular geometry; they may be straight-edged or curved. Straight-edged cuts include uni-directional arrangements, where all cuts are parallel, and multi-directional arrangements, where all cuts are non-parallel. Curved-cuts include arrangements radiating in an outwardly direction from a central point of the material and describing a circumference of a circle. 
         [0027]    Generally speaking, the greater the number of cuts and thus excise of material, the more flexible and extensible will be the lattice formed from the material. However, the geometry of the cutting arrangement will also alter the mechanical properties of the material. Each different cutting geometry will provide a lattice having somewhat different mechanical properties and thus provide lattices of different flexibility and extensibility. 
         [0028]    When cutting slits into the material to form the lattice, it is preferable that a minimum amount of material is removed. This avoids the detrimental effect on the structural integrity, and more particularly the mechanical strength of the lattice formed, which would otherwise happen where a larger amount of material was removed as in the case of the prior art where material is removed to form holes. 
         [0029]    Preferably, the amount of material removed to form the lattice is therefore, for example, less than about 50% of the initial material volume, more preferably less than about 10% of the initial material volume, and ideally, less than about 1% of the initial material volume. Preferably, the excise of material to form the lattice gives rise to no or substantially no visual removal of material. 
         [0030]    When the slits have been formed by cutting the material, the surface area of the resulting lattice can be extended, to about 25%-75% greater than the surface area of the original material. Extension of the lattice opens the slits forming a plurality of apertures/structural voids of a first dimension. The lattice in this extended form is referred to as an open lattice or having an “open lattice conformation”, “second conformation” or “extended conformation”. 
         [0031]    The open lattice is established by an extensive force prior to or immediately prior to and during attachment to the site of application, for example, a wound site. 
         [0032]    The lattice has a top or non-wound contacting surface and a bottom or wound contacting surface. The open lattice can be made from any suitable extendible material, for example, polyurethane foam. 
         [0033]    The extensive force can be applied to the lattice in a unidirectional, bidirectional or isotropical manner and can be achieved by any means known to the skilled artisan, for example by drawing between rollers running at differential speeds. A process of extending the material in a controlled manner is considered particularly advantageous as this allows apertures/structural voids of predetermined dimensions and of particular interest to be formed. 
         [0034]    Preferably, the lattice is extended to 25-75% of its maximum geometrical extension limit. The maximum geometrical extension limit is defined as the point at which the rate of change of the open volume or area of the apertures is zero or substantially zero. More preferably, the lattice is extended to maximise the open volume or area of the apertures, which may not necessarily be related to the maximum geometric extension. 
         [0035]    Preferably, once the lattice has been extended into its second conformation, it can be retained in this conformation by the use of a retaining means. The retaining means may be adhered to a surface of the open lattice in any conventional manner. The retaining means can be a physical entity, such as a structural element, for example. 
         [0036]    In an embodiment of the invention, the structural element can be a mechanically stiff backing layer, for example, the backing layer being stiff enough to maintain the extended open lattice conformation prior to and during application of the lattice to the wound site. The backing layer may comprise any suitable material, but is preferably plastic or paper. 
         [0037]    Additionally, the backing layer is easily releasable from the material. The release of the backing layer may be facilitated by a release tab or gripping portion associated with the backing layer. 
         [0038]    The backing layer can be applied directly or indirectly to the open lattice, thus retaining and supporting it in the extended, or second or open lattice conformation prior to and during application. The backing layer has a sufficient mechanical stiffness to resist the inherent contractile force of the open lattice when the extensive force is removed, thereby retaining the lattice in the extended or open lattice conformation. Once the open lattice has been applied to the wound site the backing layer is removed. On removal of the backing layer, the contractile force is transferred to the wound site promoting closure of the wound. Removal of the backing layer also causes the surface area of the lattice and the volume of the voids established therein by the extensive force to shrink. 
         [0039]    The extensive force applied to the lattice to form the open lattice is translated into a contractile force when the extensive force is removed. In embodiments of the invention more than about 10% of the extensive force is translated to a contractile force upon removal of the retaining means. In preferred embodiments of the invention more than about 50% of the extensive force is translated to a contractile force upon removal of the retaining means. 
         [0040]    In this manner, a contractile force can be applied to the site of application. The use of such a lattice at a wound site, either directly or indirectly (for example, as part of a wound dressing) has the advantage of promoting wound closure, as a result of the contractile forces pulling the edges of the wound together. 
         [0041]    By controlling the lattice and open lattice geometry (both the cut and the orientation of extension), it is possibly to control the geometry of the contractile force. 
         [0042]    Upon removal of the retaining means, the open lattice preferably recovers more than 1% of the difference between its dimensions prior to extension and those same dimensions post-extension. More preferably, the open lattice recovers more than about 5% of the difference, and ideally recovers more than 10% of the difference. 
         [0043]    In this embodiment of the invention, the greater the extension of the lattice the greater will be the contractile force on removal of the backing layer. 
         [0044]    The invention is also concerned with the application of the open lattice both internally and externally of the body and on normal or injured tissue such that, following application of the open lattice, the extensive force can be released and transmitted to the attached tissue as a compressive force. 
         [0045]    For the contraction of linear wounds, for example post-operative surgical incisions, lattices extensible along a single axis are preferred, as these lattices will impart contraction along a single axis, which when the lattice is appropriately position on the wound will be directed perpendicular to the line of the incision. For the contraction of two-dimensional wounds, for example chronic wounds such as pressure ulcers, lattices extensible along multiple axes are preferred. 
         [0046]    Preferably, the lattice is extensible in a mechanically isotropic manner, enabling wound contraction to be directed towards the centre of the wound. 
         [0047]    Alternative embodiments provide for the backing layer to be fixed to the non-wound contact surface of the open lattice. In this case the backing layer is a transparent material so that visual inspection of the wound is possible through the apertures of the lattice. It will be appreciated that in this embodiment, no contractile force is applied to the wound from the open lattice. 
         [0048]    In another embodiment of the invention, the lattice can be of a material which allows it to also function as the retaining means. For example, the lattice can be made from so-called SMART materials (also referred to as shape-memory materials). The shape of SMART materials can be altered in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH, electric or magnetic fields. In embodiments of the invention, the material is a SMART material. The lattice formed from the SMART material is extended into and retained in the second or open lattice conformation by the application of one or more external stimuli. Post-application of the open lattice to a wound site, further controlled exposure of the open lattice to an external stimulus would result in the open lattice fully or partially contracting to its original, “remembered” dimensions or first conformation. In a wound site, the open lattice could also be initiated or encouraged to contract by exposure to the moisture content of a wound exudate. 
         [0049]    In another embodiment of the invention, the structural element can be a transparent film, for example a polyurethane film, fixedly or removably applied to the non-wound contact surface of the open lattice or to the non-wound contact surface and wound contact surface of the open lattice. The film is not as structurally rigid or stiff as the backing layer so that it can not hold the open lattice in a sufficiently extended form to provide a contractile force to promote closure of the wound if removed. Instead, the film keeps a more relaxed open conformation so that the slits remain apertures to allow visual inspection of the wound. The film of this embodiment also acts as a bacterial barrier which is particularly important when the lattice is administered to external wounds. 
         [0050]    In further embodiments, both the film and backing layer may be used. The backing layer forms the external non-wound contacting surface and may be removable or fixed. 
         [0051]    In yet a further embodiment, the lattice can be extended by hand. Gripping portions or tabs maybe provide at a periphery of the material to give the hand purchase, for example, aiding extension of the lattice by hand. The gripping tabs can be located to allow a more effective extension of the lattice and formation of apertures or structural voids from the slits. Once the lattice has been applied to the wound site the gripping portions are released. On release of the gripping portions, the contractile force of the open lattice is transferred to the wound site promoting closure of the wound. Release of the gripping portions also causes the surface area of the open lattice and the volume of the voids established therein, by the extensive force, to shrink. After the lattice has been secured to the wound site, the gripping portions can be removed from the lattice by cutting, for example. This will prevent the free ends of the gripping portions from snagging. Preferably, the lattice also has a polyeurethane film attached to both the wound and non-wound contact surfaces of the lattice. The polyurethane film will extend with the lattice to form an open lattice and will also act as a bacterial barrier. 
         [0052]    In embodiments of the invention the lattice is a wound dressing. In alternative embodiments of the invention the lattice can form a part (for example a layer) of a wound dressing. 
         [0053]    In an example of the lattice being incorporated into a wound dressing, a moisture permeable top-film can be applied to the non-wound contacting surface of the lattice (for example, by heat lamination), and the removable backing layer applied to the top-film. The top film can be a polyurethane film, for example. Optionally, a perforated layer of polyurethane film can be applied to the wound contact surface of the polyurethane foam lattice. The polyurethane film applied in this way will prevent the polyurethane foam sticking to the wound which may otherwise occur. 
         [0054]    Therefore, according to a further aspect of the invention there is provided a wound dressing consisting of or comprising the lattice of the invention. 
         [0055]    In an embodiment of this aspect of the invention the lattice material is a polyurethane foam and the retaining means is a backing sheet adhered temporarily to a non wound contacting surface of the lattice (for example, by a suitable adhesive or surface energy). The backing sheet is of a plastic material (for example, a polymeric film) or paper material (for example, reinforced paper or cardboard). The backing sheet is removed from the open lattice once the wound dressing has been applied to the wound by any suitable means known in the art, for example, by adhesive, sutures, staples or topical pressure. 
         [0056]    Preferably, the material is sufficiently elastic to allow a return, unaided or unhindered, of the open lattice to the first conformation, wherein the slits are substantially closed, after being stretched, deformed, compressed, or expanded. Materials conventionally utilised in wound dressings, such as foams (for example, polyurethane foam), silicone-based material (for example, a silicone elastomer), hydrofibre, films, non-woven and woven materials, demonstrate such elastic properties and are suitable materials. 
         [0057]    Such elasticity results in the material exerting a spring-like contractile force following the removal of the extensive force, for example, by the removal of the retaining means. This contractile force results in the open lattice forming a “third conformation” or “contracted conformation”, in which the apertures/structural voids are contracted to a smaller, second dimension. 
         [0058]    In embodiments of the invention the material is a monolith. 
         [0059]    According to a further aspect of the invention there is provided a wound dressing comprising a material having; 
         [0060]    i) an initial conformation, 
         [0061]    ii) an expanded conformation resulting from the application of an extensive force to the material, wherein the removal of the extensive force causes a contraction of the material, and 
         [0062]    iii) the wound dressing including means for retaining the material in the expanded conformation. 
         [0063]    The material of this aspect of the invention is sufficiently expandable and contractible (partially or fully) from and to an initial conformation, without the requirement for the provision of slits. For example, the material can have elastic properties which closely resemble, for example, LYCRA® (Invista, US). 
         [0064]    In an embodiment of this aspect of the invention, the material is provided with at least one slit and preferably with a plurality of slits. 
         [0065]    According to a further aspect of the invention there is provided a method of promoting the closure of a wound, the method comprising the steps of; 
         [0066]    a) providing a material having a at least one slit to form a lattice, the slit being capable of expansion into an aperture, the lattice having: 
         [0067]    i) a first conformation, wherein the slit is substantially closed, and, 
         [0068]    ii) a second conformation, wherein the slit is expanded into an aperture having a first dimension, 
         [0069]    iii) retaining the lattice in the second conformation prior to application to a wound site, 
         [0070]    iv) applying the lattice in its second conformation to the wound site, and, optionally, 
         [0071]    c) allowing the lattice to retract from the second conformation towards the first conformation after application to the wound site. 
         [0072]    In embodiments of the invention the open lattice can be applied directly to a wound. In alternative embodiments of the invention the open lattice can form a part (for example a layer) of a wound dressing which is applied to a wound. 
         [0073]    The invention is particularly suited for application to topical and internal wounds, for example traumatic injuries, surgical incision wounds and open chronic wounds. Surgical wounds include those that are the result of plastics or maxillofacial operations, mastectomy or caesarean section. 
         [0074]    The direction of the contractile force is influenced by the geometry of cut in the material to form the lattice, the geometry of extension of the lattice and the geometry of the attachment points between the lattice or wound dressing and the tissue. 
         [0075]    The open lattice or wound dressing can be applied to the wound site using any suitable technique and attachment means known in the art, for example, adhesive, sutures, staples or topical pressure. Topical pressure can be provided by compression bandaging or atmospheric pressure acting upon a cavity of reduced pressure relative to the external atmosphere. Attachment can be achieved at specific locations on the open lattice or wound dressing or may cover the entire surface thereof. For topical applications, attachment is preferably achieved by an area exceeding 50% of the total area of the open lattice or wound dressing in contact with the site of application. More preferably, for topical applications, attachment is achieved by a pressure sensitive adhesive, for example an acrylate-based adhesive. Typically, the adhesive forms a layer on the wound contact surface of the open lattice or wound dressing. 
         [0076]    According to a further aspect of the invention there is provided a method of manufacturing a wound dressing comprising the steps of, 
         [0077]    (a) providing a first material, 
         [0078]    (b) establishing the material in an extended conformation by applying an extensive force to at least part of the material, and 
         [0079]    (c) retaining the material in the extended conformation by use of a retaining means. 
         [0080]    Preferably, the method of manufacture includes the step of forming at least one slit and more preferably a plurality of slits in the material to form a lattice. According to yet a further aspect of the invention there is provided a method of manufacturing a wound dressing comprising the steps of, 
         [0081]    (a) providing a first material, and 
         [0082]    (b) forming at least one slit in said first material to form a lattice. 
         [0083]    Preferably, the method of manufacture includes the steps of, 
         [0084]    (a) applying an extensive force to at least part of the lattice to establish the lattice in an extended or open conformation, and 
         [0085]    (b) retaining the lattice in the extended or open conformation by use of a retaining means. 
         [0086]    Examples of materials for use as the first material, include foams, such as polyurethane foam and silicone-based elastomers. 
         [0087]    An example of a suitable material for use as the retaining means is a polymeric film, such as polyurethane film. Other suitable materials include polyester, polyethylene and polypropylene which can be perforated or extruded net. 
         [0088]    In some embodiments, there is provided a wound dressing comprising a body of porous material, the body of porous material comprising a plurality of cuts which provide regions of flexibility within the body. 
         [0089]    While the plurality of cuts provide regions of flexibility within the body, they are not capable of rendering portions of the body frangible, such that the portions are relatively easily severable from the body. 
         [0090]    Preferably the porous material is a wound packing foam suitable for use in negative pressure wound therapy (NPWT). Particularly suitable foams for NPWT include polyurethane foam, typically reticulated polyurethane foam of very high free internal volume, e.g. 80% or higher, preferably 90% or higher free internal volume. Typical foams used in NPWT have porosities in the range 30-60 ppi (pores per inch) and mean pore diameters in the range 300-800 μm. However, other suitable foams are known in the art and may be equally employed. In general suitable foams have an open porous structure, to allow transmission of the negative pressure to the wound bed, and sufficient mechanical strength to prevent the negative pressure (typically approximately 80-125 mm Hg below ambient atmospheric pressure) from collapsing the structure of the foam. 
         [0091]    Suitably the cuts are slits. Herein this section and elsewhere in this specification, the term slit is intended to mean a cut which is generally long and thin, and preferably straight and linear. In practice, slits in foam are typically effectively 2-dimensional as the resilience of the foam means that the slit is essentially closed unless the material is stretched. Suitably the slits are from 10 mm to 70 mm in length, preferably from 20 mm to 50 mm, especially from 25 mm to 40 mm. Slits of around 30 mm have been found to be particularly effective in a typical NPWT, though there is of course scope to vary this. 
         [0092]    Providing cuts, e.g. slits, confers macroscopic flexibility, while not substantially affecting the microscopic mechanical properties of the body, i.e. to resist compression under negative pressure. This flexibility allows the body of porous material to drape more easily, i.e. to conform to the shape of the wound to be dressed. 
         [0093]    In a preferred embodiment, the body comprises at least one linear series of slits, each slit being separated from an adjacent slit by a gap. By “linear series of slits” it is intended to mean a plurality of generally linear slits, each slit generally aligned along a straight line in a series, with a gap separating each slit from adjacent slits in the series. Generally it is preferred that the gaps are regularly sized. 
         [0094]    Preferably the body comprises two or more parallel linear series of slits, each linear series being spaced from the adjacent linear series. Preferably the spacing between the adjacent linear series is regular. 
         [0095]    It will be clear to the person skilled in the art that the size of the gap between the slits, and the spacing between adjacent series, must be sufficient such that the material retains structural integrity sufficient for it to be handled, used in therapy and removed thereafter without breaking up. If the gaps and spacing were too small, the body would be too weak to achieve this. On the other hand, there is a desire to maximise the size and density of the slits to maximise drapeability. It is preferred that the minimum size of the gaps and/or spacing should certainly be no less than the average pore diameter. It is more preferred that gaps and/or spacing are at least 5 times the average pore diameter; given that the average pore diameter for NPWT foam is in the range of 300-800 μm, this gives a gap or spacing of 1500 to 4000 μm, i.e. 1.5 to 4 mm. A gap of around 3 mm has been found to be particularly effective in a typical NPWT, though there is of course scope to vary this. 
         [0096]    It is preferable that the spacing between adjacent linear series of slits is kept reasonably small to provide the desired amount of flexibility and hence drapeability. Accordingly, it is preferred that the spacing is not more than 50 times the average pore diameter (typically from 15 mm to 40 mm depending on pore density), preferably not more than 30 times than average pore diameter (typically from 9 mm to 24 mm depending on pore density). A spacing of around 3 mm has been found to be particularly effective in a typical NPWT, though there is of course scope to vary this. 
         [0097]    It is preferred that, where two or more parallel linear series of slits are provided, adjacent series are linearly offset relative to each other, i.e. it is preferred that the slits and gaps in adjacent series do not line up, but are staggered. To put it another way, adjacent series may be out of phase with each other. In one embodiment adjacent series are offset such that the centre point of a slit in one series is aligned approximately with the gap in an adjacent series—similar to the way layers of bricks are offset in a wall. There is of course scope to vary the amount of offset. 
         [0098]    This offset of adjacent series of slits allows for particularly good drapeability. When the body is curved the slits open up to form a lattice type structure which is particularly suited to provide good flexibility, and hence drapeability. 
         [0099]    The slits may suitably pass completely through the thickness of the body. Alternatively the slits may pass only partially though the thickness of the body, provided they pass through far enough to provide the desired flexibility to the body. In general it is preferred that the slits pass at least half way through the thickness of the body, preferably at least three quarters of the way through the body, and especially substantially all the way through the body. 
         [0100]    The present application is particularly suited to a wound dressing material comprising a body of porous material which is relatively thin, although it could be used to impart flexibility to a body which is thick. In particular, the present invention is particularly suited to a sheets of porous material which have a thickness of from 5 mm to 75 mm, preferably from 10 mm to 50 mm, especially from 15 to 40 mm, most preferably from 20 to 35 mm. In a very thin sheet (less than 5 mm) drapeability of the sheet is not such an issue, and where the thickness is greater than about 75 mm, providing a plurality of cuts becomes less effective as a means of introducing flexibility. Thus the ranges set out above represent sheets of thicknesses which are particularly well suited to the present invention. The other dimensions of the sheet are not particularly significant, although it may be observed that NPWT foam is typically sold in generally cuboid sheets with the dimensions of the edges of largest face being between 100 and 200 mm, e.g. a sheet measuring 100×200×30 mm is fairly typical. 
         [0101]    It is generally preferred that the cuts are provided passing through the shortest dimension of the body. In particular it is preferred that the cuts (e.g. slits) are provided passing between the two largest faces of a cuboid body (e.g. sheet), and especially that the cuts are perpendicular to the largest faces. 
         [0102]    Where two or more parallel linear series of slits are provided, as set out above, this provides for good flexibility in a single direction of curve. When a body is curved around a surface, the inside of the body is subject to compression and the outside of the body is subjected to tension (inside and outside being defined relative to the curve). In wound dressings according to the present invention the cuts allow the body to stretch when under tension, the cuts allowing deformation of the shape of the body (e.g. into a lattice), and this allows the body to easily adapt to the desired curve. Where the cuts comprise slits in a single orientation, the body will be well adapted to curving in a direction which causes tension perpendicular to the slits, but will be less well adapted to curving in another direction, i.e. where the tension produced is parallel to the slits. Therefore, the body is well suited to drape around a body with a single curvature (e.g. generally cylindrical), but not so well suited to draping around a more complex curved object such as a generally spherical shape. 
         [0103]    In a further embodiment of the present invention, the body comprises a second set of slits passing at least partially through the body in a different orientation (preferably perpendicular) to the slits in the at least one linear series of slits described above (the “first set of slits”). It is preferred that the second set of slits also pass between, and are perpendicular to, the largest faces of the body. Additional slits provided in this manner allow the body to curve more easily in a second direction thus making it more suited to adapting to a complex curved surface, i.e. curved in two planes. 
         [0104]    In a preferred embodiment of the invention at least some of the slits in both orientations intersect. Suitably the intersection of the slits effectively provides a two-dimensional (2-D) slit. Preferably the 2-D slit has an H shape, with a two parallel slits in one orientation being intersected at their mid-points by a third slit in a perpendicular orientation. Such an H-shaped 2-D slit is well suited to the provision of a relatively dense array of 2-D slits in the body. For example, rows of H-shaped slits, each adjacent slit within the row alternating in orientation by 90 degrees, can be packed closely together, while obeying the minimum spacing requirements set out above. 
         [0105]    In a preferred embodiment of the invention, the body comprises a first set of parallel linear series of H-shaped slits in a first orientation and a second set of parallel linear series of H-shaped slits in a second orientation. Preferably the second orientation is perpendicular to the first orientation. Preferably adjacent series within a set are offset relative to one another. Such an arrangement allows very close packing of the H-shaped slits, whilst maintaining a suitable spacing between neighbouring slits to retain the overall structural integrity of the body. To allow close packing of the slits it is preferred that the length of the two parallel slits (the “sides”) of the H-shaped slit are shorter than the perpendicular slit (the “cross-piece”) of the H-shaped slit. For example, the “cross-piece” may be from 10 mm to 70 mm in length, preferably from 20 mm to 50 mm, especially from 25 mm to 40 mm, most preferably 30 mm and the “sides” are preferably around 6 mm shorter than the “cross-piece”, especially around half the length of the “cross-piece”. Where the “sides” of the H are shorter than the “cross-piece”, it allows the “sides” of H-shaped slits in one orientation to nest within the area bounded by “sides” and “cross-piece” of the H-shaped slit in the other orientation. This allows for very close packing of the H-shaped slits. 
         [0106]    It should be noted that while a body having an array of H-shaped 2-D slits is a preferred embodiment of the present invention, it is possible to achieve a suitable 2-D array using other shapes, such as cruciform slits, or the like. 
         [0107]    It should be noted that the cuts, e.g. slits, of the present invention can be provided by cutting a preformed body of foam, or the cuts could be provided during the process in which the body is manufactured (e.g. moulding). It is preferred that the cuts are provided by die cutting a preformed body. 
         [0108]    In a further aspect, the present invention provides a method of manufacturing a wound dressing, the method comprising the steps of; 
         [0109]    providing a body of a porous material; and 
         [0110]    providing a plurality of cuts which provide regions of flexibility within the body. 
         [0111]    It is preferred that the cuts are provided by die cutting using an array of suitably sized and shaped blades to provide the desired cuts. 
         [0112]    Details of preferred cuts, e.g. slits, are set out above, and it will be obvious to the person skilled in the art how to provide a suitable blade to achieve the desired cut. 
         [0113]    The cuts may be provided all the way through the body, or they may only pass partially through the body. 
         [0114]    The cuts may be formed in a batch, or they may be formed in a flow process. 
         [0115]    In a further aspect the present invention provides a method of dressing a wound comprising administering a wound dressing as set out above to the wound. 
         [0116]    Preferably the method provides the step of applying a negative pressure to the wound through the wound dressing material, i.e. the method is NPWT. In general this can be achieved by providing a substantially fluid impermeable sheet over the wound and wound dressing, thus defining a sealed volume, and applying a negative pressure inside said sealed volume. The seal need not be completely hermetic, but should be sufficient to allow a suitable negative pressure to be sustained. The source of negative pressure, e.g. a pipe form a vacuum pump, can be provided at a position such that it draws fluids from the wound bed through the wound dressing material. 
         [0117]    Suitably the negative pressure is in the range of from 80 to 125 mm Hg below ambient atmospheric pressure. 
         [0118]    In a further aspect the present invention provides the use of a wound dressing material as set out above in wound treatment, especially NPWT. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0119]    Other features and advantages of the present invention will be apparent from the following detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
           [0120]      FIG. 1  demonstrates the extension of a slit, in three stages, under an extensive force, indicated by the arrows F, and in a direction perpendicular to the longitudinal axis of the slit. The intermediate or second stage shows that the slit has been expanded to form a circle. 
           [0121]      FIG. 2  is a plan view of a cutter for use in the manufacture of a first embodiment of lattice according to the invention. 
           [0122]      FIG. 3  is a plan view of the lattice formed using the cutter of  FIG. 2 , the lattice shown here in the open lattice or second conformation on application of a uni-directional extensive force applied perpendicular to the longitudinal axis of the slits. 
           [0123]      FIG. 4  is a plan view of a further cutter having a different cutting profile to the cutter of  FIG. 2  for use in the manufacture of a second embodiment of lattice according to the invention. 
           [0124]      FIG. 5  is a plan view of the lattice formed using the cutter of  FIG. 4 , the lattice shown here in the open lattice or second conformation on application of a bi-directional extensive force applied perpendicular and parallel to the longitudinal axis of the slits. 
           [0125]      FIG. 6  is a schematic in plain view of yet a further a cutter, having an alternative cutting profile to the cutter of  FIGS. 2 and 4 , for use in the manufacture of a third embodiment of the lattice according to the invention. 
           [0126]      FIG. 7  is a plan view of the cutter manufactured according to the schematic of  FIG. 6 . 
           [0127]      FIG. 8  is a plan view of the lattice formed using the cutter of  FIG. 7 , the lattice shown here in the open lattice or second conformation on application of a bi-directional extensive force applied perpendicular and parallel to the longitudinal axis of the slits. 
           [0128]      FIG. 9  is a plane view of the lattice in the first conformation where the slits are substantially closed. The lattice is opaque and the slits allow for no or substantially no visual inspection across the lattice. 
           [0129]      FIG. 10  shows an array of blades adapted to form slits in a body of a wound dressing material according to the present invention; 
           [0130]      FIG. 11  shows a body according to the present invention curved in a first direction; 
           [0131]      FIG. 12  shows a body according to the present invention curved in a second direction; 
           [0132]      FIG. 13  shows a comparison of a body of foam according to the present invention with an un-cut body of foam; 
           [0133]      FIG. 14  shows a second array of blades adapted to form cuts in a body of a wound dressing material according to the present invention; 
           [0134]      FIG. 15  shows a body according to the present invention cut with the array of blades of  FIG. 14 ; and 
           [0135]      FIG. 16  shows the body of  FIG. 15  curved in two dimensions. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0136]    Like reference numbers refer to corresponding parts throughout the drawings, description and examples. 
       EXAMPLE 1 
       [0137]    To create the wound dressing of example 1, a perforated sheet of polyurethane film was applied to the wound contact surface of a sheet of 4 mm depth polyurethane foam (Allevyn, Smith &amp; Nephew Medical Limited). 
         [0138]    The wound contact surface is that surface which is placed adjacent to or in direct contact with the wound. The non-wound contact surface is that surface which is remote from or opposite the wound contact surface. 
         [0139]    A cutter of specification shown in  FIG. 2  (Cutter blade with 15 mm length blades, linear spacing 5 mm, vertical spacing 5 mm) was used to cut slits in the sheet of polyurethane foam to form a lattice. The cutting action also formed slits in the polyurethane film. 
         [0140]    Following cutting, an extensive force was applied to the lattice in a direction perpendicular to the longitudinal axis of the cuts or slits to produce an open lattice as shown in  FIG. 3 . A moisture permeable top-film was heat laminated to the non-wound contact layer or surface of the open lattice. To the top-film, a polymeric film release sheet of sufficient mechanical stiffness to resist the contractile force of the open lattice was applied. After applying the release sheet the extensive force applied to the lattice was removed and the extended or open lattice was retained in the open lattice conformation by the release sheet. 
       EXAMPLE 2 
       [0141]    To create the wound dressing of example 2, a perforated sheet of adhesive polyurethane film was applied to the wound contact surface of a sheet of 4 mm depth polyurethane foam (Allevyn, Smith &amp; Nephew Medical Limited). The adhesive surface of the film was covered by a siliconised release paper. A cutter of specification shown in  FIG. 2  (Cutter blade with 15 mm length blades, linear spacing 5 mm, vertical spacing 5 mm) was used to cut slits in the sheet of polyurethane foam to form a lattice. The cutting action also formed slits in the polyurethane film and siliconised release paper. 
         [0142]    The wound contact surface is that surface which is placed adjacent to or in direct contact with the wound. The non-wound contact surface is that surface which is remote from or opposite the wound contact surface. 
         [0143]    Following cutting, the siliconised release paper was removed and an extensive force was applied to the lattice in a direction perpendicular to the longitudinal axis of the cuts or slits to produce the open lattice pattern as shown in  FIG. 3 . A new sheet of siliconised release paper was then attached to the wound contact surface of the perforated adhesive film. A moisture permeable top-film was heat laminated to the non-wound contact layer or surface of the open lattice. To the top-film, a polymeric film release sheet of sufficient mechanical stiffness to resist the contractile force of the open lattice was applied. After applying the release sheet the extensive force applied to the lattice was removed and the extended or open lattice was retained in the open lattice conformation by the release sheet. 
       EXAMPLE 3 
       [0144]    To demonstrate the effectiveness of the wound dressing of example 2, the siliconised release paper was removed from the perforated adhesive film and placed, adhesive side down, upon intact skin. The polymeric film release sheet was then removed. A uni-directional contractile force was generated on the skin, upon removal of the polymeric film release sheet, and in a direction perpendicular to the axis of the cuts. 
       EXAMPLE 4 
       [0145]    To create the wound dressing of example 4, a perforated sheet of polyurethane film was applied to the wound contact surface of a sheet of 4 mm depth polyurethane foam (Allevyn, Smith &amp; Nephew Medical Limited). A cutter of specification shown in  FIG. 4  (Cutter blade with 15 mm length blades, linear spacing 5 mm, vertical spacing 2.5 mm) was used to cut slits in the sheet of polyurethane foam to form a lattice. The cutting action also formed slits in the polyurethane film and siliconised release paper. 
         [0146]    The wound contact surface is that surface which is placed adjacent to or in direct contact with the wound. The non-wound contact surface is that surface which is remote from or opposite the wound contact surface. 
         [0147]    Following cutting, the lattice was extended along two axes, x and y, as shown in  FIG. 4 . The extensive force was applied perpendicular and parallel to the longitudinal axis of the slits to produce an open lattice structure. A moisture permeable top-film was heat laminated to the non-wound contact surface of the open lattice. To the top-film, a polymeric film release sheet of sufficient mechanical stiffness to resist the contractile force of the open lattice was applied. After applying the release sheet the extensive force applied to the lattice was removed and the extended or open lattice was retained in the open lattice conformation by the release sheet. 
       EXAMPLE 5 
       [0148]    To create the wound dressing of example 5, a perforated sheet of adhesive polyurethane film was applied to the wound contact surface of a sheet of 4 mm depth polyurethane foam (Allevyn, Smith &amp; Nephew Medical Limited). The adhesive surface of the film was covered by a siliconised release paper. A cutter of specification shown in  FIG. 4  (Cutter blade with 15 mm length blades, linear spacing 5 mm, vertical spacing 2.5 mm) was used to cut slits in the sheet of polyurethane foam to form a lattice. The cutting action also formed slits in the polyurethane film and siliconised release paper. 
         [0149]    The wound contact surface is that surface which is placed adjacent to or in direct contact with the wound. The non-wound contact surface is that surface which is remote from or opposite the wound contact surface. 
         [0150]    Following cutting, the siliconised release paper was removed and the lattice was extended along two axes, x and y, as shown in  FIG. 4 . The extensive force was applied perpendicular and parallel to the longitudinal axis of the slits to produce an open lattice structure. A new sheet of siliconised release paper was then attached to the wound contact surface of the perforated adhesive film. A moisture permeable top-film was heat laminated to the non-wound contact layer or surface of the open lattice. To the top-film, a polymeric film release sheet of sufficient mechanical stiffness to resist the contractile force of the open lattice was applied. After applying the release sheet the extensive force applied to the lattice was removed and the extended or open lattice was retained in the open lattice conformation by the release sheet. 
         [0151]    The lattice of the wound dressing of example 5, having the polymeric film release sheet removed, it shown in  FIG. 5 . 
       EXAMPLE 6 
       [0152]    To demonstrate the effectiveness of the wound dressing of example 5, the siliconised release paper was removed from the perforated adhesive film and placed, adhesive side down, upon intact skin. The polymeric film release sheet was then removed. Upon removal of the polymeric film release sheet, a contractile force was generated on the skin acting towards the centre of the dressing. 
       EXAMPLE 7 
       [0153]    A similar process to that described for the wound dressing of example 5 is employed to create the wound dressing of example 7. However, in this case, a cutter of specification shown in  FIGS. 6 and 7  was used to cut the slits. As can be seen from  FIG. 6 , the blades have three cutting edges. A long cutting edge of 15 mm in length bridging two shorter cutting edges of 7 mm in length. The two shorter cutting edges being parallel to each other and perpendicular to the longer cutting edge. Each blade has a spacing with an adjacent blade which describes a square area having a side length of 3.75 mm. This spacing is demonstrated by the shaded square portion in  FIG. 6 . 
         [0154]    The lattice of the wound dressing of example 7, having the polymeric film release sheet removed, it shown in  FIG. 8 . 
         [0155]    An array of blades ( 10 ) mounted on a board is shown in  FIG. 10 . Each blade ( 12 ) is a straight thin blade 30 mm long, and having a depth of approximately 30 mm. The blades are arranged in 20 parallel linear series of blades ( 16 , 18 ), each series comprising a row of blades ( 12 ) arranged longitudinally, with a gap ( 14 ) of 3 mm between each blade ( 12 ) in the series. Each series is spaced from the adjacent series by a 3 mm spacing ( 15 ). Furthermore, adjacent series ( 16 , 18 ) are staggered relative to one another such that the gap between the blades on one series ( 16 ) aligns with the midpoint in the adjacent series ( 18 ). Accordingly, the blades within the array ( 10 ) are arranged like the bricks in a wall. Given this offset arrangement, it is convenient that at the end of a series where a full 30 mm blade would extend beyond the dimension to be cut, blades of 15 mm length are provided; this allows for a neater array—once more, this is akin to half bricks at the end of a row in a wall. Full length blades could be used at the ends, provided they would not be problematic in the cutting process. 
         [0156]    A body of NPWT foam ( 20 ) measuring 200×125×30 mm is cut using the array ( 10 ). It is cut by driving the array of blades ( 10 ) through the body ( 20 ) in a die cutting operation. This can be achieved using a press, typically a hydraulic press (not shown), also known as a clicker press. The blades are driven perpendicularly into and through the largest face of the body ( 20 ), and perpendicular thereto, to form a plurality of slits therein. The slits ( 21 ) formed are arranged in a plurality of parallel linear series ( 26 , 28 ) of slits, each comprising slits ( 21 ) 30 mm long separated by gaps ( 22 ), where material is left un-cut, which are 3 mm long. Each series is separated by a spacing ( 24 ) 3 mm in width. When the body ( 20 ) is curved, as shown in  FIG. 11 , the slits ( 21 ) open up to form a lattice structure. Tension in the outer region of the body ( 20 ) as a result of the curving process is relieved through deformation of the body ( 20 ) which is facilitated by the slits ( 21 ) provided therein. The arrangement of parallel offset linear series of linear slits is particularly suited to this as it form a regular lattice structure, as shown in  FIG. 11 . 
         [0157]    In the embodiment shown in  FIG. 11 , an additional partial cut ( 30 ) has been made running the length of the middle of the largest face of the body ( 20 ), perpendicular to the slits. This allows the body ( 20 ) to be easily split in two if this is desirable. 
         [0158]      FIG. 12  shows another body ( 40 ) cut using the array of blades of  FIG. 10 , this time without the additional cut ( 30 ). The body has been curved in a different manner to that in  FIG. 11 . In this case the body has been bent back on itself along its longest side, i.e. the 200×300 mm face has been curved back on itself. The body ( 40 ) has opened via the slits ( 42 ) into an open lattice structure. This type of curving of the body ( 40 ) is not generally useful for a wound dressing application, but does serve to demonstrate the flexibility and strength of the body ( 40 ). 
         [0159]      FIG. 13  further demonstrates the ability of a body according to the present invention ( 40 ) to drape over a surface, in this case a leg, when compared to an uncut body ( 46 ). 
         [0160]      FIG. 14  shows an array ( 50 ) of blades adapted to form cuts in a body of foam in two orientations, the orientations being perpendicular to each other. As with the array ( 10 ) in  FIG. 10 , the blades have a depth of 30 mm. However, in the array ( 50 ) comprises H-shaped blades ( 52 ) comprising a first blade element 30 mm long ( 54 ) (also termed “cross-piece”), with second ( 56 ) and third ( 58 ) blade elements (also termed “sides”) 15 mm long located at the end of the first blade element ( 54 ), each end of the first blade element intersecting with the midpoint of the second and third blade elements, thus defining a “wide H-shaped” blade. The array is made up of first set of eleven parallel linear series of H-shaped blades in a first orientation (called X for convenience) and a second set of eleven parallel linear series of H-shaped blades in a second, perpendicular orientation (called Y for convenience). Adjacent series within each set are offset in exactly the same manner as for linear blades. As can be seen from  FIG. 14 , the blades are spaced and arranged such that a close packing of the blades as achieved, but each blade is always approximately 5 mm or so from the nearest neighbouring blade. It can be seen that the “side” of a blade in the X-orientation nests within the region defined by the “cross-piece” and “sides” of a blade in the Y-orientation. Such an array is suited to forming slits in a body to allow draping in two planes. 
         [0161]      FIG. 15  shows a body ( 60 ) formed by cutting with the array of  FIG. 14 . The slits ( 62 ) are formed by pressing the array of blades ( 50 ) through the body ( 60 ) in the same manner as described above. H-shaped slits ( 62 ) are formed in the body ( 60 ) corresponding to the array of blades ( 50 ). As shown in  FIG. 16 , the body ( 60 ) is well adapted to curving in complex shapes. 
         [0162]    It should be noted that the present description has focused on bodies formed by a batch die cutting process. There are of course numerous ways of forming cuts in a body of porous material (e.g. laser cutting, high pressure liquid cutting), or the cuts could formed when the body itself is formed (e.g. during a moulding process). Furthermore, these methods could be applied in a flow process rather than a batch; this might be more efficient for large production runs. All such variations are within the scope of the present invention. 
         [0163]    Furthermore, it should be noted that, while the exemplified embodiments form particularly preferred embodiments with excellent drapeability, it is quite possible that other arrangements of cuts will provide satisfactory results.