Abstract:
Methods and devices for management of fluids during skin graft transplantation are disclosed. In one aspect of the invention, absorbent substrates for transplanting skin grafts are disclosed having a surface adapted to contact at least one excised skin graft and engage said graft for removal from a donor site; a sealing member at least partially surrounding an absorbent material disposed within the enclosure; wherein at least a portion of the skin-contacting surface is porous and in fluid communication with the absorbent layer to capture fluids.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/860,822, filed Jul. 31, 2013 entitled “Absorbent Substrates For Harvesting Skin Grafts”, and U.S. Provisional Application No. 61/782,385, filed Mar. 14, 2013, entitled “Absorbent Dressing With Hybrid Drape”, the disclosures of which are incorporated by reference in their entirety. 
     
    
     FIELD 
       [0002]    The present invention relates generally to medical treatment systems and, more particularly, but not by way of limitation, to absorbent dressings, systems, and methods for harvesting and transplanting skin grafts. 
       BACKGROUND 
       [0003]    Skin is the largest organ of the human body, representing approximately 16% of a person&#39;s total body weight. Because it interfaces with the environment, skin has an important function in body defense, acting as an anatomical barrier from pathogens and other environmental substances. Skin also provides a semi-permeable barrier that prevents excessive fluid loss while ensuring that essential nutrients are not washed out of the body. Other functions of skin include insulation, temperature regulation, and sensation. Skin tissue may be subject to many forms of damage, including burns, trauma, disease, and depigmentation. 
         [0004]    Skin grafts are often used to repair such skin damage. Skin grafting is a surgical procedure in which a section of skin is removed from one area of a person&#39;s body (autograft), removed from another human source (allograft), or removed from another animal (xenograft), and transplanted to a recipient site of a patient, such as a wound site. As with any surgical procedure, skin grafting involves certain risks. Complications may include graft failure, rejection of the skin graft, bleeding, fluid accumulation or infection at either the donor or recipient site. Additionally, when an autograft is taken from one area of a person&#39;s body to produce the graft, some degree of trauma occurs at the donor site. If the recipient site is a large wound or otherwise damaged skin region, the trauma at the donor site can be significant. 
         [0005]    Techniques have been developed for harvesting a large number of smaller grafts, e.g., so-called micrografts, to reduce the trauma at the donor site. By removing only a fraction of the skin at a donor site and leaving regions of healthy skin surrounding the excised regions, a large amount of skin for transplantation can be obtained with less discomfort. Micrograft harvesting can also reduce the healing time and risk of infection. 
         [0006]    Harvesting of skin grafts can be accomplished in many different ways. One common technique for harvesting a skin graft involves the application of suction to separate a surface portion of the skin, e.g., the epidermis and a basal cell layer, from the underlying dermis. Harvesting of suction blisters typically also involves a heat source to facilitate blister formation. 
         [0007]    Various devices are available for generating and harvesting micrografts. For example, the Cellutome™ skin harvester is available from Kinetic Concepts, Inc. of San Antonio, Tex. The Cellutome™ system includes a head that provides a source of reduced pressure (vacuum), and optionally a heater element, and a harvester configured for placement on a target region of a patient&#39;s skin. The harvester is further adapted to form a sealing engagement with the head such that the target region of skin is embraced within an evacuated chamber. The Cellutome™ harvester further includes at least one alignment plate having a plurality of holes through which skin blisters can be raised in the presence of negative pressure; and a cutting plate having at least one cutting surface for cleaving skin blisters after they are formed within the chamber. 
         [0008]    Typically, micrograft harvesters rely upon a support or substrate to lift the excised blisters from the device. The substrate is then applied to a recipient site so that the plurality of micrografts can be assimilated as transplanted tissue. Ideally, the grafts will expand and coalesce to complete the healing process. 
       SUMMARY 
       [0009]    Methods and devices for management of fluids during skin graft transplantation are disclosed. In one aspect of the invention, absorbent substrates for transplanting skin grafts are disclosed having a surface adapted to contact at least one excised skin graft and engage said graft for removal from a donor site; a sealing member at least partially surrounding an absorbent material disposed within the enclosure; wherein at least a portion of the skin-contacting surface is porous and in fluid communication with the absorbent layer to capture fluids. 
         [0010]    In one embodiment of the invention, absorbent substrates for transplanting skin grafts are disclosed including a base layer having a surface adapted to contact at least one excised skin graft and to engage said graft for removal from a donor site; a sealing member peripherally joined to the base layer and defining an enclosure therebetween; and an absorbent material disposed within the enclosure; wherein at least a portion of the base layer is porous and in fluid communication with the absorbent layer to capture fluids. 
         [0011]    In certain embodiments the substrate includes a base layer configured for positioning in a chamber of a skin graft harvesting device and, optionally, the base layer is further configured to capture a plurality of skin grafts at the same time. 
         [0012]    The base layer of the absorbent substrate preferably includes a biocompatible material, e.g., a material selected from the group of silicones, silicone gels, soft silicones, hydrocolloids, hydrogels, polyurethanes, polyurethane gels, polyolefins, polyolefin gels, hydrogenated styrenic copolymers, hydrogenated styrenic copolymer gels, foamed gels and combinations thereof. In some preferred embodiments, the base layer includes silicone. 
         [0013]    In certain embodiments, a skin graft contacting portion of the base layer has an average thickness between about 50 microns and about 10 millimeters, preferably in some cases, between about 500 microns (μm) and about 1000 microns (μm). The skin graft contacting portion of the base layer should also be flexible enough to conform to the shape of the harvester and/or the recipient site. For example, the skin graft contacting portion of the base layer can have a stiffness between about 5 Shore OO and about 80 Shore OO. 
         [0014]    In another aspect of the invention, the substrate further includes an adhesive associated with at least a part of the base layer to engage the graft. When used with multi-graft or “micrograft” harvesters, the substrate can further include a pattern of adhesive sites on a skin contacting surface of the base layer, each site arranged and configured to engage a corresponding skin graft raised by a skin graft harvesting device. For example, the substrate can carry a pattern of adhesive sites printed on at least a portion of a skin-contacting surface of the base layer. In certain embodiments, the adhesive can include an acrylic adhesive. 
         [0015]    In another aspect of the invention, the base layer can include a plurality of openings to provide passageways from fluid transport from the recipient site to the absorbent material. The openings (e.g., pores) can be spaced apart from each other. In certain embodiments, the openings are generally circular. The openings can have an average cross-sectional dimension ranging from about 0.1 nanometers to about 1 millimeter, or preferably an average cross-sectional dimension ranging from about 1 nanometer to about 100 micrometers. In other embodiments, the pores can be elongated or grid-like and their minor dimension can range from about 0.1 nanometers to about 1 millimeter, or preferably from about 1 nanometer to about 100 micrometers. 
         [0016]    In another aspect of the invention, the base layer is patterned to define a plurality of skin graft capture sites and the base layer further includes a network of pores disposed between at least some of the capture sites. Again, the pores (disposed between capture sites) can be circular or elongated and have an average cross-section dimension (or a minor dimension, in the case of elongated pores) ranging from about 0.1 nanometers to about 1 millimeter, or preferably ranging from about 1 nanometers to about 100 micrometers. 
         [0017]    In yet another aspect of the invention, the substrate can also include at least one wicking layer disposed in the enclosure and adapted to distribute fluid to the absorbent material. For example, the substrate can include at least a first wicking layer disposed in the enclosure between the base layer and the absorbent material. Alternatively, or in addition to the first wicking layer, the substrate can include one or more additional wicking layers (e.g., a second wicking layer) disposed in the enclosure between the absorbent material and the sealing member. In certain embodiments, the first and/or second wicking layer can have a grain structure adapted to wick fluid along a surface of the wicking layer. 
         [0018]    In certain embodiments, the absorbent material is a plurality of absorbent layers, and one or more of the additional absorbent layers are positioned in fluid communication between a first wicking layer and a second wicking layer. The substrate can also include at least one intermediate wicking layer disposed in fluid communication between the absorbent layers. In certain embodiments, a peripheral portion of a first wicking layer can be coupled to a peripheral portion of a second wicking layer to provide a wicking layer enclosure surrounding the absorbent layer between the first and the second wicking layers. 
         [0019]    In another aspect of the invention, the absorbent material can include a hydrophilic material that is adapted to absorb fluid and the sealing member is liquid impermeable. For example, the sealing member can include a water-impermeable polyurethane component. 
         [0020]    In an alternative embodiment, a unitary absorbent structure can be employed rather than a separate base layer and absorbent material. For example, an absorbent substrate according to this embodiment can include an absorbent material having a surface adapted to contact at least one excised skin graft and engage said graft for removal from a donor site. The absorbent material can further include a sealing member surrounding and defining an enclosure for the absorbent material and substantially sealing the absorbent material except for the skin contacting surface; and wherein at least a portion of skin contacting surface of the absorbent material is porous to capture fluids. 
         [0021]    In yet another aspect of the invention, the substrate can further include at least one port for coupling to the reduced pressure source to extract accumulated fluids from the substrate. The port can further include a valve, e.g., a check valve or one-way valve, to prevent backflow of extracted fluids. The port can further include a conduit providing fluid communication between the absorbent material or at least one wicking layer within the chamber and an external fluid receptacle. 
         [0022]    In another aspect, the substrate can include at least one removable backing for handling the substrate prior to positioning it in a skin graft harvester. The substrate can further include at least one removable backing for handling the substrate prior to positioning it at a recipient site. For example, the substrate can include at least a first removable backing associated with the base layer for handling the substrate prior to positioning it in a skin graft harvester and a second removable backing for handling the substrate and an associated skin graft prior to positioning it at a recipient site. 
         [0023]    In another embodiment, a skin graft transplantation substrate is provided including a base layer, an adhesive, a sealing member, a first wicking layer, a second wicking layer, an absorbent layer, and a conduit interface. The base layer has a periphery surrounding a central portion and a plurality of apertures disposed through the periphery and the central portion, wherein the base layer is adapted to cover the skin graft transplantation site and tissue surrounding the site. The sealing member has a periphery and a central portion, the periphery of the sealing member being positioned proximate the periphery of the base layer such that the central portion of the sealing member and the central portion of the base layer define an enclosure. The first wicking layer and the second wicking layer are each disposed in the enclosure. The absorbent layer is positioned in fluid communication between the first wicking layer and the second wicking layer. A peripheral portion of the first wicking layer is coupled to a peripheral portion of the second wicking layer providing a wicking layer enclosure surrounding the absorbent layer between the first and the second wicking layer. The conduit interface is positioned proximate to the sealing member and in fluid communication with the dressing. 
         [0024]    In another aspect, a system is provided for draining a skin transplantation site including a substrate or dressing and a reduced-pressure source. The substrate or dressing is adapted to provide reduced pressure and/or to store fluid extracted from the site. The substrate or dressing includes a base layer, an adhesive, a sealing member, a first wicking layer, a second wicking layer, an absorbent layer, and a conduit interface. The base layer has a periphery surrounding a central portion and a plurality of apertures disposed through the periphery and the central portion. The central portion of the base layer is adapted to be positioned proximate the transplantation site and the periphery of the base layer is adapted to be positioned proximate the tissue surrounding the transplantation site. Further, the periphery of the base layer is adapted to surround the transplantation site, and the apertures in the base layer are adapted to be in fluid communication with site and the tissue surrounding the transplantation site. In certain embodiments, adhesive can be stored or preloaded into apertures in the base layer such that upon placement of the base layer onto the transplantation site, the adhesive is released. (A two-part lower backing can also be employed such that a first (inner) portion of the lower backing is removed when the substrate is joined to a skin graft harvester and a second outer portion of the backing subsequently removed to facilitate peripheral adhesion at the transplantation site.) The sealing member has a periphery and a central portion, the periphery of the sealing member being positioned proximate the periphery of the base layer such that the central portion of the sealing member and the central portion of the base layer define an enclosure. The first wicking layer and the second wicking layer are each disposed in the enclosure. The absorbent layer is positioned in fluid communication between the first wicking layer and the second wicking layer. The conduit interface is positioned proximate to the sealing member and in fluid communication with the dressing. The reduced-pressure source is adapted to be coupled in fluid communication with the conduit interface to provide reduced pressure to the dressing. 
         [0025]    In another aspect of the invention, methods are disclosed for fluid management during skin transplantation. The methods can include the steps of contacting at least one skin graft with an absorbent substrate, the substrate comprising a base layer having a surface adapted to contact and engage at least one excised skin graft and a sealing member peripherally joined to the base layer and defining an enclosure therebetween; and an absorbent material disposed within the enclosure; deploying the substrate at a recipient site such that a skin graft that is engaged by the base layer contacts the recipient site; and maintaining the substrate in contact with the recipient site to facilitate transplantation of the graft and removal of fluids. 
         [0026]    In another aspect, the methods of the present invention can include maintaining the absorbent substrate at the recipient site, and further, removing excess fluids at the recipient site by extraction into the absorbent material of the substrate. The methods can be practiced by providing a plurality of pores in the base layer to provide a fluid communication path between a recipient site and the absorbent material within the substrate and, optionally, deploying at least one wicking layer within the substrate to distribute fluids captured from a recipient site to different regions of the absorbent material. 
         [0027]    In certain embodiments, the methods can further include a step of coupling the substrate to a reduced pressure source to facilitate fluid extraction and, optionally, draining accumulated fluids from the absorbent material into a fluid extraction receptacle or deploying a one-way valve between the absorbent material and the fluid extraction receptacle. 
         [0028]    Other aspects, features, and advantages of the illustrative embodiments will become apparent with reference to the drawings and detailed description that follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    A more complete understanding of this specification may be obtained by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
           [0030]      FIG. 1  is a schematic, perspective bottom view of an illustrative embodiment of a absorbent substrate for management of fluids during skin graft transplantation; 
           [0031]      FIG. 2  is a schematic, perspective top view of the absorbent substrate of  FIG. 1 ; 
           [0032]      FIG. 3  is a schematic, side view of the absorbent substrate of  FIG. 1 ; 
           [0033]      FIG. 4  is a partial cross-sectional view of an absorbent substrate according to the invention; 
           [0034]      FIG. 5A  is partial cross-sectional view of another absorbent substrate according to the invention in which the baser layer and absorbent material are a unitary structure. 
           [0035]      FIG. 5B  is schematic perspective bottom view of the substrate of  FIG. 5A . 
           [0036]      FIG. 6  is a schematic, side view of an alternative embodiment of an absorbent substrate according to the invention having a port for coupling to a reduced pressure source or external fluid drainage receptacle; 
           [0037]      FIG. 7  is a schematic, perspective top view of a skin graft harvester for use with the absorbent substrate; 
           [0038]      FIG. 8  is a schematic, perspective top view of the skin graft harvester of  FIG. 6  with the head component removed and the cutter mechanism exposed; 
           [0039]      FIG. 9  is a schematic, perspective top view of the skin graft harvester of  FIG. 6  with an absorbent substrate according to the invention deployed in the harvester to capture skin grafts. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    In the following detailed description of non-limiting, illustrative embodiments, reference is made to the accompanying drawings that form a part hereof. Other embodiments may be utilized and logical, structural, mechanical, electrical, and chemical changes may be made without departing from the scope of this specification. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. The following detailed description is not to be taken in a limiting sense, with the scope of the illustrative embodiments being defined by the appended claims. 
         [0041]    The term “micrograft” as used herein is intended to encompass skin grafts that have a width or length less than a millimeter, more preferably, less than 100 microns. A micrograft is an excised skin segment having at least one dimension parallel to the skin surface that is less than a millimeter, preferably less than 100 micrometers, more preferably in some applications less than 10 micrometers. The minimum width or length is preferably less than 500 micrometers, preferably less than 100 micrometers or less than 50 micrometers or less than 10 micrometers or less than 1 micrometer. For example, a micrograft can be generally circular, oval or oblong in a plane parallel to the skin surface and have a diameter or major axis that ranges from about 1 millimeter to 0.01 micrometers, or from about 100 micrometers to about 0.1 micrometers, or more preferably from about 50 to 1 micrometers. Micrografts also typically have a depth dimension that extends at least through the epidermis and preferably in some applications encompasses at least one layer of basal cells. The depth can range from about 500 micrometers to about 0.1 micrometers, preferably from about 100 micrometers to about 1 micrometer. 
         [0042]    The term “harvesting” as used herein is intended to encompass the removal of one or more skin grafts from an skin graft generating device, such as, for example, a suction blister micrograft generator, as well as the transplantation of such skin grafts and any intermediate steps, such as culturing, expanding, stretching, treating or otherwise preparing a skin graft for transfer to a recipient site. 
         [0043]    The terms “generally circular” and “circular” are used interchangeably herein to describe openings that are round, oval or otherwise form closed polygonal shapes having a major dimension (width or diameter) that is less than 5 times the minor dimension (width or diameter) of the shape. Preferably the major dimension is less than 3 times, or less than 2 times, the minor dimension. 
         [0044]    The term “about,” as used herein, refers to variations in a numerical quantity that can occur, for example, through measuring or handling procedures in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of compositions or reagents; and the like. Typically, the term “about” as used herein means greater or lesser than the value or range of values stated by 1/10 of the stated values, e.g., ±10%. For instance, a concentration value of about 30% can mean a concentration between 27% and 33%. The term “about” also refers to variations that would be recognized by one skilled in the art as being equivalent so long as such variations do not encompass known values practiced by the prior art. Each value or range of values preceded by the term “about” is also intended to encompass the embodiment of the stated absolute value or range of values. Whether or not modified by the term “about,” quantitative values recited in the claims include equivalents to the recited values, e.g., variations in the numerical quantity of such values that can occur, but would be recognized to be equivalents by a person skilled in the art. 
         [0045]    Referring to the drawings,  FIG. 1  depicts an embodiment of an absorbent substrate  10 , viewed from the bottom, showing a base layer  12  and a peripheral removable backing  14 . The base layer  12  further includes a plurality of sites  16  for capturing skin grafts. The entire base layer  12  or just the sites  16  can be treated, e.g., coated with an adhesive, to make at least portions of the base layer surface tacky to promote capture of skin grafts. The base layer is also porous and in this illustrated embodiment a plurality of pores  18  are disposed between the graft capture sites  16 . The pores  18  can be generally circular or elongated in one or more dimensions. Alternatively, the entire surface of the base layer can be porous or can include a network of lines or cross-shaped incisions or openings. Regardless of the shape or size of the pores  18 , the porosity of the base layer  12  should be sufficient to permit fluid migration from a skin segment through the base layer  12  for absorption by the substrate  10 . 
         [0046]      FIG. 2  depicts the absorbent substrate  10  of  FIG. 1 , viewed from the top, showing a sealing member  20  and a second peripheral removable backing  22  for handling purposes.  FIG. 3  is a side view of the same absorbent substrate  10 , showing the base layer  12 , sealing member  20 , and the first (bottom) and second (top) removable backings  14  and  22 , respectively. 
         [0047]      FIG. 4  is a partial cross-sectional view of an absorbent substrate showing one embodiment of the internal structure. Base layer  16  and sealing member  20  define an enclosure for an absorbent material  24 . The figure also schematically shows a plurality of micrografts  4  carried on a bottom surface (e.g., a skin-contacting surface) of the base layer  12 . A plurality of pores  18  in the base layer permit fluid ingress and provide passageways to the absorbent material  24 . Optionally, one or more wicking layers can be utilized to distribute captured fluids to different portions of the absorbent material. In the illustrated embodiment, a first wicking layer  26  is disposed in proximity to the base layer and a second wicking layer  28  is disposed in proximity to the sealing member  28 . Alternatively, wicking material can form alternating layers with absorbent material layers (sandwich style) or wicking material can be distributed throughout or otherwise dispersed within the absorbent material. In the illustrated embodiment, the first and second wicking layers  26 ,  28 , respectively, can be joined together at the periphery to form a seal  30  that completely or substantially encloses the absorbent material. 
         [0048]    Additionally,  FIG. 4  shows the substrate  10  in use applied to a skin graft transplantation site on a surface of a patient&#39;s skin  2  in need of grafting. On the bottom surface of the base layer  12  are a plurality of captured skin grafts  4 , which can be placed in contact with the skin  2  as the substrate  10  is applied. Fluid migration from the transplant site and extraction into the absorbent material  24  is illustrated by the dotted lines. 
         [0049]      FIG. 6  shows another embodiment of an absorbent substrate, having the base layer  12 , sealing member  20  and a port  40  for coupling to a source of negative pressure  46  and/or a fluid extraction receptacle  48 . The port  40  can further include a conduit  42 , one or more filters  47  and/or a check valve  44  to permit fluid extraction (and, optionally, one-way flow) from the absorbent material, e.g., in instances where the absorbent material reaches or nears a saturated state to an external fluid receptacle or a waste disposal site. 
         [0050]      FIG. 7  is a schematic view of a skin graft harvester  50  for use with an absorbent substrate in accordance with various aspects of the present teachings. In this illustrative embodiment, the harvest  50  includes a detachable head portion  52  and harvester body  54 . The harvester body  54  is adapted for placement on a patient&#39;s skin at a donor site where skin grafts are to be obtained, e.g., on the inner thigh, and secured in place, for example, with strap  56  (shown in phantom). The head  52  can further include a heater (not shown) powered via a coupler  60  adapted to couple with a power source in a base unit (not shown). The head  52  further includes a seal  63  which permits a reduced pressure chamber to be formed when the head  52  and body  54  are joined together and the harvester  50  is coupled to a vacuum pump or other source of reduced pressure, e.g., via coupler  60  connecting the harvester  50  to its base unit. The head  52  can further include one or more windows  58  for observation of skin blisters being formed within the chamber by application of reduced pressure, heat or both. Once the blisters have been formed, the head  52  can be removed, e.g., by deactivating the source of reduced pressure and by actuation of release levers  62 , which break the seal  63  and allow the head  52  to be lifted off the harvester body  54 . 
         [0051]    Additional details on harvesters useful in connection with the present invention can be found in U.S. patent application Ser. No. 13/839,518 filed Mar. 15, 2013; U.S. patent application Ser. No. 13/346,329 filed Jan. 9, 2012; U.S. patent application Ser. No. 13/436,318 also filed Jan. 9, 2012; U.S. patent application Ser. No. 13/014,737 filed Jan. 27, 2011; U.S. patent application Ser. No. 12/851,656 filed Aug. 6, 2010; U.S. patent application Ser. No. 12/851,621 filed Aug. 6, 2010; U.S. patent application Ser. No. 12/851,703 filed Aug. 6, 2010; and U.S. patent application Ser. No. 12/851,682 filed Aug. 6, 2010. The contents of each of the above-referenced related applications are herein incorporated by reference in their entireties. 
         [0052]      FIG. 8  is a schematic view of the skin graft harvester  50  of  FIG. 7  with the head  52  removed and the cutting mechanism  74  exposed. The harvester body  54  can include a base portion  70 , a sled  72 , and actuator handle  80 . The cutting mechanism  74  can include a plurality of plates with initially aligned holes through which skin blisters are drawn by heat and/or application of suction when the head  52  is joined to the harvester body  54  and activated. Once the blisters are formed, they can be cleaved by the cutting mechanism  74 . For example, below the top plate depicted in  FIG. 8 , one or more additional plates, e.g., a cutter plate and a bottom plate can be deployed with aligned holes. By actuation (e.g., pulling up) of handle  80 , the sled  72  is caused to move horizontally such that one of the plates below the top plate, e.g., the “cutter plate” (not shown) also moves (because of its linkage to the sled  72 ), thereby occluding the alignment of holes  78  and cleaving the raised blisters from the donor&#39;s skin. 
         [0053]      FIG. 9  is a schematic view of the skin graft harvester  50  of  FIG. 7  with an absorbent substrate  10  according to the invention deployed in the harvester body  54  to capture skin grafts. In the illustrated embodiment, the user (e.g., clinician) places the substrate  10  in the harvester holding the backing  22  with the sealing member  20  upwards and the base layer (not visible) in contact with the top plate of cutter mechanism (as shown in  FIG. 8 ). By so placing the substrate, the base layer will also come into contact with the skin blisters. In one preferred embodiment, the substrate is so situated before the cutter mechanism is actuated to cleave the blisters into skin grafts (as described above). In other embodiments, the substrate can be placed onto the harvester after cleavage to capture grafts that have already been cleaved from the skin. In either event the substrate can then be removed from the harvester body  54  and applied to a recipient site, as illustrated in  FIG. 4 . 
         [0054]    Returning to  FIGS. 1-4 , the base layer  12  may have a periphery surrounding a central portion and a plurality of pores  18  disposed through the periphery and/or the central portion. The pores  18  in the base layer  12  may have any shape, such as, for example, circles, squares, stars, ovals, polygons, slits, complex curves, rectilinear shapes, triangles, or other shapes. The pores  18  may have a uniform pattern or may be randomly distributed on the base layer  12 . Each pore  18  has a diameter. In certain embodiments, the average diameter of each of the pores  18  can be between about 6 mm to about 50 mm. The pores  18  may also be sized to enhance the Moisture Vapor Transfer Rate (MVTR) of the dressing  124 , described further below. 
         [0055]    Alternatively, the base layer can be fluid permeable, e.g., composed of a material that has high water permeability in either liquid or vapor form, such as polyurethanes, polyesters, polyvinyl chlorides, copolymers of vinyl chloride and vinyl acetate or vinyl chloride and ethylene, polyolefins, polyamides, polyethylene, polypropylene, silicone or polystyrenes, polyacrylics, polyacrylates, polyvinyl alcohol, and copolymers thereof. Examples of water permeable materials include polyurethane films, such as Ensure-IT dressing (Deseret Medical, Inc.) and POLYSKIN® transparent dressing (Kendall Company, Boston, Mass.). 
         [0056]    Various techniques can be employed to increase the permeability or porosity of polymer films, such as the incorporation of inclusions during polymer processing that are subsequently dissolved or evaporated away to leave micro-sized or nano-sized voids, e.g., voids having an average width ranging from about 0.1 nanometers to about 1 millimeter or from about 1 nanometer to about 100 micrometers. In certain embodiments, micro-pores having an average width of about 0.1 micrometers to about 1 millimeter can be desired. In other embodiments, nano-pores having an average size of about 0.1 to about 100 nanometers, preferably about 1 to about 100 nanometers or about 1 to about 10 nanometers, can be advantageous. 
         [0057]    In another embodiment, a permeable or porous base layer can be formed from woven or non-woven (e.g., matted) fibers. The fibrous base layer can include microfibers and/or nanofibers. In certain embodiments, microfibers having an average diameter of about 0.1 to about 10 micrometers can be desired. In other embodiments, nanofibers having an average diameter of about 1 to about 100 nanometers, preferably about 20 to about 80 nanometers, although in some instances, fibers with diameters about 1 to about 20 nanometers, can also be advantageous. 
         [0058]    In yet another embodiment, the base layer can be a foamed material, such as an open cell polymer foam that is formed through the use of chemical and/or physical blowing agents during polymer processing. The foamed base layer should have a sufficiently open cell structure such that fluid passageways are formed through the full thickness of the foamed base layer to facilitate fluid extraction. The average width of the open cells can range from about 1 nanometer to about 1 millimeter, preferably in some instances from about 10 nanometer to about 100 micrometers, or from about 100 nanometers to 10 micrometers. Foam polymers can have from about 1 to 1000 pores per square inch, preferably in some instances, from about 10 to 100 pores per square inch. Preferred foam polymers can void fractions from about 10 to about 90 percent, or void fractions greater than 20 percent, 30 percent, 40 percent or 50 percent. 
         [0059]    The terms “porous” as used herein is intended to encompass not only apertures or holes but also permeable and open cell foam structures as described above. 
         [0060]    The base layer  12 , in certain embodiments, is preferably a soft material suitable for providing a fluid seal with the skin graft transplantation site as described herein. For example, the base layer  12  may comprise a silicone gel, a soft silicone, hydrocolloid, hydrogel, polyurethane gel, polyolefin gel, hydrogenated styrenic copolymer gels, a foamed gel, a soft closed cell foam such as polyurethanes and polyolefins, polyurethane, polyolefin, or hydrogenated styrenic copolymers coated with an adhesive described below. The base layer  12  can have a thickness between about 500 microns (μm) and about 1000 microns (μm). In one embodiment, the base layer  12  has a stiffness between about 5 Shore OO and about 80 Shore OO. The base layer  12  can include hydrophobic or hydrophilic materials. 
         [0061]    In some embodiments, the base layer  12  may be a hydrophobic-coated material. For example, the base layer  12  may be formed by coating a mesh or porous material, such as, for example, woven, nonwoven, molded, or extruded mesh with a hydrophobic material. The hydrophobic material for the coating may be a soft silicone, for example. 
         [0062]    The adhesive  17  used to capture skin grafts and/or adhere the substrate  10  to a patient at the transplantation site may be any medically-acceptable adhesive. For example, the adhesive  17  may comprise an acrylic adhesive, rubber adhesive, high-tack silicone adhesive, polyurethane, or other adhesive substance. In some embodiments, the adhesive  17  may be a pressure-sensitive adhesive comprising an acrylic adhesive with a coating weight of 15 grams/m 2  (gsm) to 70 grams/m 2  (gsm). The adhesive  17  may be a continuous or a discontinuous layer of material. Discontinuities in the adhesive  17  may be provided by pores  18  in the base layer  12 . The apertures in the adhesive may be formed after application of the adhesive to the base layer or by coating the adhesive  17  in patterns on the base layer. 
         [0063]    Factors that may be utilized to control the adhesion strength of the substrate  10  may include the diameter and number of the pores  18  in the base layer  12 , the thickness of the base layer  12 , the thickness and amount of the adhesive  17 , and the tackiness of the adhesive  17 . An increase in the amount of the adhesive  17  generally corresponds to an increase in the adhesion strength of the substrate  10 . Thus, the size and configuration of the adhesive coated portions of the base layer  12 , the thickness of the base layer  12 , and the amount and tackiness of the adhesive utilized may be varied to provide a desired adhesion strength for the substrate  10 . For example, the thickness of the base layer  12  may be about 200 microns, and the adhesive layer  17  may have a thickness of about 30 microns and a tackiness of 2000 grams per 25 centimeter wide strip. 
         [0064]    Continuing with  FIGS. 1-4 , the sealing member  20  has a periphery and a central portion. The periphery of the sealing member  20  may be positioned proximate the periphery of the base layer  12  such that the central portion of the sealing member  20  and the central portion of the base layer  12  define an enclosure. 
         [0065]    The sealing member  20  may cover the tissue site  6  to provide a fluid seal and a sealed space between the tissue site  6  and the sealing member  20  of the substrate  10 . Further, the sealing member  20  may cover tissue, such as a portion of the epidermis  106 , surrounding the tissue site  6  to provide the fluid seal. 
         [0066]    The sealing member  20  may be formed from any material that allows for a fluid seal. A fluid seal is a seal adequate to maintain reduced pressure at a desired site given the particular reduced pressure source or system involved. The sealing member  20  may comprise, for example, one or more of the following materials: hydrophilic polyurethane; cellulosics; hydrophilic polyamides; polyvinyl alcohol; polyvinyl pyrrolidone; hydrophilic acrylics; hydrophilic silicone elastomers; an INSPIRE  2301  material from Expopack Advanced Coatings of Wrexham, United Kingdom having, for example, an MVTR (inverted cup technique) of 14400 g/m 2 /24 hours and a thickness of about 30 microns; a thin, uncoated polymer drape; natural rubbers; polyisoprene; styrene butadiene rubber; chloroprene rubber; polybutadiene; nitrile rubber; butyl rubber; ethylene propylene rubber; ethylene propylene diene monomer; chlorosulfonated polyethylene; polysulfide rubber; polyurethane (PU); EVA film; co-polyester; silicones; a silicone drape; a 3M Tegaderm® drape; a polyurethane (PU) drape such as one available from Avery Dennison Corporation of Pasadena, Calif.; polyether block polyamide copolymer (PEBAX), for example, from Arkema, France; or other appropriate material. 
         [0067]    The sealing member  20  may allow vapor to exit while inhibiting liquids from exiting the sealed space provided by the substrate  10 . The sealing member  20  may be a flexible, breathable film having a high MVTR of, for example, at least about 300 g/m 2  per 24 hours. The sealing member  20  may comprise a range of medically suitable films having a thickness between about 15 microns (μm) to about 50 microns (μm). In other embodiments, a low or no vapor transfer drape can be used as the sealing member. 
         [0068]    The fluid management assembly may be disposed in the enclosure  31  and may include a first wicking layer  26 , a second wicking layer  28 , and an absorbent layer  24 . The absorbent layer  24  may be positioned in fluid communication between the first wicking layer  26  and the second wicking layer  28 . The first wicking layer  26  may have a grain structure (not shown) adapted to wick fluid along a surface of the first wicking layer  26 . Similarly, the second wicking layer  28  may have a grain structure (not shown) adapted to wick fluid along a surface of the second wicking layer  28 . For example, the first and the second wicking layer  26 ,  28  may wick or otherwise transport fluid in a lateral direction along the surfaces of the first and the second wicking layer  26 ,  28 , respectively. The surfaces of the first and the second wicking layer  26 ,  28  may be normal relative to the thickness of each of the first and the second wicking layer  26 ,  28 . The wicking of fluid along the first and the second wicking layers  26 ,  28  may enhance the distribution of the fluid over a surface area of the absorbent layer  24  that may increase absorbent efficiency and resist fluid blockages. Fluid blockages may be caused, for example, by fluid pooling in particular location in the absorbent layer  24  rather than being distributed more uniformly across the absorbent layer  24 . The laminate combination of the first and the second wicking layer  26 ,  28  and the absorbent layer  24  may be adapted as described above to maintain an open structure, resistant to blockage, that can maintain fluid communication with, for example, the tissue site  6 . 
         [0069]    The substrate  10  may include, without limitation, any number of wicking layers and absorbent layers as desired for treating a particular tissue site. For example, the absorbent layer  24  may be a plurality of absorbent layers  24  positioned in fluid communication between the first wicking layer  26  and the second wicking layer  28  as described above. Further, at least one intermediate wicking layer may be disposed in fluid communication between the plurality of absorbent layers  24 . Similar to the absorbent layer  24  described above, the plurality of absorbent layers  24  and the at least one intermediate wicking layer may be positioned within the wicking layer enclosure. 
         [0070]    In one embodiment, the absorbent material or layer  24  may be a hydrophilic material adapted to absorb fluid from, for example, the tissue site  6 . Materials suitable for the absorbent layer  184  may include Luquafleece® material, Texus FP2326, BASF 402c, Technical Absorbents 2317 available from Technical Absorbents (www.techabsorbents.com), sodium polyacrylate super absorbers, cellulosics (carboxy methyl cellulose and salts such as sodium CMC), or alginates. Materials suitable for the first and second wicking layers  26 ,  28  may include any material having a grain structure capable of wicking fluid as described herein, such as, for example, Libeltex TDL2 80 gsm. 
         [0071]    The substrate  10  can be a pre-laminated structure manufactured at a single location or simply individual layers of material stacked upon one another as described above. Individual layers of the substrate  10  may be bonded or otherwise secured to one another without adversely affecting fluid management by, for example, utilizing a solvent or non-solvent adhesive, or by thermal welding. 
         [0072]    In one embodiment, the enclosure  31  defined by the base layer  12  and the sealing member  20  may include an anti-microbial layer. The addition of the anti-microbial agent may reduce the probability of excessive bacterial growth within the dressing  10  to permit the dressing  10  to remain in place for an extended period. The anti-microbial material may be, for example, an additional layer included as a part of the substrate  10  as depicted in  FIGS. 1-4 , or a coating of an anti-microbial agent disposed in any suitable location within the substrate  10 . The anti-microbial material may include elemental silver or similar compounds, for example. 
         [0073]    In an alternative embodiment, the base layer and absorbent material can be replaced by a single, unitary element (with or without wicking material) that provides both the skin-contacting and the fluid absorbing functions.  FIGS. 5A and 5B  illustrates such a substrate  10 A including absorbent base  24 A, an optional upper wicking layer and a sealing member  20 . The substrate can further include a peripheral portion that does not have a foamed or sponge surface for securing the substrate to a patient&#39;s skin and/or a harvester for collection of skin grafts. As discussed above a backing can be disposed over this peripheral portion for handling purposes. The absorbent base can be a foamed polymer (e.g., a sponge) capable of attracting and absorbing fluids when placed in contact with the skin and optionally can further include a port  40  (shown in phantom) for coupling to a source of a source of reduced pressure as discussed above. 
         [0074]    In this context the absorbent material having a skin-contacting surface in lieu of a separate base layer can be a sponge material, e.g., an elastic open pore structured polymer such as cellulose, collagen, gelatin/alginate, polyesters, polyethers, polyvinyl acetates, polyvinyl acetals, polyurethanes, gelatin/hyaluronates or chitosan/hyaluronates, polyvinyl alcohol, and polyacrylates. The sponge material can have a network of interconnected pores for fluid transport, the average cross-sectional width of which can range from 0.05 millimeters (mm) to about 5 millimeters (mm), more preferably from about 0.1 mm to about 1 mm in certain embodiments. 
         [0075]      FIG. 5B  also shows another feature of the invention that can be used in any of the illustrated embodiments, namely a split lower backing. An inner portion of the backing  14 A can be removed when the substrate is applied to the harvester to capture skin grafts. The surface beneath the backing  14 A can include an adhesive for coupling to the harvester and a second portion of the backing  14 B can remain in place until transplantation, at which time it can be removed exposing another portion of the surface (and optionally an adhesive) for contact with the skin. The function (or position) of backings  14 A and  14 B can, of course, be reversed. Various other arrangements can likewise be employed to permit securement of the substrate both at the time of harvesting and at transplantation. 
         [0076]    Referring now to  FIG. 6 , the port  40  for coupling to a source of reduced pressure can be positioned proximate to the sealing member  20  and in fluid communication with the absorbent material  24  through an aperture (not shown) in the sealing member  20  to provide reduced pressure from the reduced-pressure source  46  to the substrate  10 . The port  40  may comprise a medical-grade, soft polymer or other pliable material. As non-limiting examples, the port  40  may be formed from polyurethane, polyethylene, polyvinyl chloride (PVC), fluorosilicone, or ethylene-propylene, etc. In one illustrative, non-limiting embodiment, port  40  may be molded from DEHP-free PVC. The port  40  may be formed in any suitable manner such as by molding, casting, machining, or extruding. Further, the port  40  may be formed as an integral unit or as individual components and may be coupled to the substrate  10  by, for example, adhesive, welding or mechanical coupling. 
         [0077]    The port  40  can also include one or more filters  47 , e.g., an odor filter to inhibit the passage of odors from the tissue site  6  out of the sealed substrate  10 , or a hydrophobic filter. The filter  47  can be disposed in the conduit  42  or other suitable location such that fluid communication between the reduced-pressure source  46  and the substrate is provided through the filter  47 . In another embodiment, the filters  47  can be positioned in any exit location in the substrate  10 , such as an aperture (not shown), that is in fluid communication with the atmosphere or with the reduced-pressure source  46 . The filter  47  may also be positioned in any suitable location in the substrate that is in fluid communication with the graft transplantation site  6 . 
         [0078]    For example, an odor filter  47  may include a carbon material in the form of a layer or particulate, such as a woven carbon cloth filter such as those manufactured by Chemviron Carbon, Ltd. of Lancashire, United Kingdom (www.chemvironcarbon.com). A hydrophobic filter  47  may be comprised of a material that is liquid impermeable and vapor permeable, such as a material manufactured under the designation MMT-314 by W.L. Gore &amp; Associates, Inc. of Newark, Del., United States, or similar materials. 
         [0079]    Continuing with  FIG. 6 , the reduced-pressure source  46  provides reduced pressure to the substrate  10  and the sealed space  31 . The reduced-pressure source  46  may be any suitable device for providing reduced pressure as described herein, such as, for example, a vacuum pump, wall suction, or other source. Additional details on reduced pressure sources can be found, for example, in U.S. patent application Ser. No. 11/646,918 filed Dec. 28, 2006, U.S. patent application Ser. No. 11/810,027 filed Jun. 4, 2007; U.S. patent application Ser. No. 12/661,293 filed Mar. 15, 2010; and U.S. patent application Ser. No. 13/052,873 filed Mar. 21, 2011. The disclosures of each of these patent applications are incorporated by reference in their entireties. 
         [0080]    As used herein, “reduced pressure” generally refers to a pressure less than the ambient pressure at a tissue site being subjected to treatment. Typically, this reduced pressure will be less than the atmospheric pressure. The reduced pressure may also be less than a hydrostatic pressure at a tissue site. Unless otherwise indicated, values of pressure stated herein are gauge pressures. While the amount and nature of reduced pressure applied to a tissue site will typically vary according to the application, the reduced pressure will typically be between −5 mmHg and −500 mmHg, and more typically in a therapeutic range between −100 mmHg and −200 mmHg. 
         [0081]    The reduced pressure delivered may be constant or varied (e.g., patterned or random) and may be delivered continuously or intermittently. Although the terms “vacuum” and “negative pressure” may be used to describe the pressure applied to the tissue site, the actual pressure applied to the tissue site may be more than the pressure normally associated with a complete vacuum. Consistent with the use herein, an increase in reduced pressure or vacuum pressure typically refers to a relative reduction in absolute pressure. An increase in reduced pressure corresponds to a reduction in pressure (more negative relative to ambient pressure) and a decrease in reduced pressure corresponds to an increase in pressure (less negative relative to ambient pressure). 
         [0082]    A conduit  42  having an internal lumen may be coupled in fluid communication between the reduced-pressure source  46  and the substrate  10 . The conduit interface  43  may be coupled in fluid communication with the dressing and adapted to connect between the conduit  42  and the substrate  10  for providing fluid communication with the reduced-pressure source  46 . The conduit interface  43  may be fluidly coupled to the conduit  42  in any suitable manner, such as, for example, by an adhesive, solvent or non-solvent bonding, welding, or interference fit. An aperture (not shown) in the sealing member  20  may provide fluid communication between the substrate and the conduit interface  43 . In one embodiment, the conduit  42  may be inserted into the substrate  10  through an aperture (not shown) in the sealing member  20  to provide fluid communication with the reduced-pressure source  46  without utilization of the conduit interface  43 . The reduced-pressure source  46  may also be directly coupled in fluid communication with the substrate  10  and/or the sealing member  20 . The conduit  42  may be, for example, a flexible polymer tube. A distal end of the conduit  42  may include any one of known couplings for attachment to the reduced-pressure source  46 . 
         [0083]    Although this specification discloses advantages in the context of certain illustrative, non-limiting embodiments, various changes, substitutions, permutations, and alterations may be made without departing from the scope of the specification as defined by the appended claims. Further, any feature described in connection with any one embodiment may also be applicable to any other embodiment.