Abstract:
An apparatus and method for treating a wound can include a flexible sheet for sealingly engaging a patient&#39;s skin around a wound bed, a drain to be located in the wound bed, a vacuum source connected to the drain, and a flow restriction device to control the flow of fluid through an aperture of the flexible sheet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/360,834, filed on Jul. 1, 2010. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to the field of negative pressure wound therapy dressings. In particular, the present invention is directed to a method and apparatus for establishing a pressure differential between a wound bed and a suction apparatus during the application of negative pressure wound therapy. 
     2. Brief Description of Related Art 
     Negative pressure can be used to treat various types of wounds. In some applications, the negative pressure can be applied to the wound via a vacuum source and a dressing. The negative pressure can be used to drain fluid and exudates from the wound, and can also stimulate blood flow in the tissue of the wound bed, thus promoting healing. 
     Improper flow from the wound bed toward the vacuum source can make the negative pressure less effective. For example, if the flow rate is too high, a vacuum source, such as a pump, must run more often, which can lead to premature pump failure. If the flow is too low, then fluids located in the wound bed can stagnate in the wound bed rather than being drained. Therefore, it is desirable to control the flow of fluid from the wound bed. 
     SUMMARY OF THE INVENTION 
     In one embodiment of an apparatus for treating wounds using negative pressure can include a drain, a flexible sheet for covering a wound bed, the flexible sheet having a dressing aperture. The flexible sheet can sealingly engage tissue proximate to the wound bed while the drain is located between the flexible sheet and the wound bed. The dressing can also include a tube in communication with the drain and a flow restriction device covering the dressing aperture. The flow restriction device can reduce the flow of vapor and substantially prevent the flow of liquid through the aperture. 
     In one embodiment, the transmissive patch can have a higher rate of vapor transmission than the flexible sheet. The flexible sheet can be semi-permeable to vapor and generally impermeable to liquid. A vacuum source can be connected to and in communication with the tube. The vacuum source can create a negative pressure at the vacuum source, the negative pressure can be communicated through the tube and drain to the wound bed, and the pressure in the wound bed can thus be greater than the pressure at the vacuum source. 
     In one embodiment, the flow restriction device can include high-density polyethylene fibers. Indeed, the flow restriction device can have a predetermined vapor transmission rate at a given pressure. The flow restriction device can include a semi-permeable barrier and an adhesive, the adhesive forming a seal between the flow restriction device and the flexible sheet. The flow restriction device can include a semi-permeable barrier located between a first layer having a first aperture and a second layer having a second aperture, and at least a portion of the second layer can sealingly engage the flexible sheet and the second aperture can be in communication with the aperture. 
     In one embodiment, a method for treating a wound is described. The method can include the steps of placing a drain tube in a wound bed; covering the drain tube and wound bed with a flexible sheet, the space between the flexible sheet and the wound bed defining a dressing space; creating a dressing aperture in the flexible sheet; covering the dressing aperture with a flow restriction device; connecting a vacuum source to the drain tube; creating negative pressure at the vacuum source; and creating a pressure differential between the negative pressure at the vacuum source and the wound bed to allow a vapor to flow from the dressing space to the vacuum source. 
     In one embodiment of the method, the vacuum source can include a pump, and the method can include the step cycling the pump on and off and maintaining the negative pressure at the vacuum source for at least a predetermined amount of time while the pump is off. The pressure differential can be sufficient to move fluids from the dressing space toward the vacuum pump. The flow restriction device can include a semi-permeable layer located between a first layer having a first aperture and a second layer having a second aperture, wherein at least a portion of the second layer sealingly engages the flexible sheet and the second aperture is in communication with the dressing aperture, and including the step of flowing a vapor through the first aperture, through the semi-permeable layer, through the second aperture, and through the dressing aperture. A flow rate through the flow restriction device, at a given level of negative pressure, can be determined by a lateral distance between the first aperture and the second aperture. The pump can include a low-flow alarm, and the flow through the flow restriction device can be sufficiently high to prevent actuation of the low-flow alarm during normal operation. One step can include perforating the flexible sheet to create the dressing aperture after the flexible sheet is covering the wound bed and another step can include adhering a vent patch having a semi-permeable membrane and an adhesive, to the flexible sheet such that the semi-permeable membrane covers the dressing aperture. In one embodiment, the vapor flow rate through the flow restriction device is greater than the flow rate through the flexible sheet. 
     In one embodiment, a system for treating a wound is described. The system can include a flexible sheet that can cover a wound bed and sealingly engage the skin adjacent to the wound bed. The space between the flexible sheet and the wound bed is defined as a dressing space. The system can also include an aperture through the flexible sheet, a flow restriction device covering the aperture and sealingly engaging the flexible sheet, the flow restriction device permitting vapor to pass through it while preventing liquid from passing through it, a drain located within dressing space, packing located within dressing space, a tube in communication with the drain, and a vacuum source in communication with the drain. In one embodiment, the flow restriction device can include a membrane and an adhesive, the adhesive preventing liquid from traveling past a perimeter of the membrane. In one embodiment, the flow rate of vapor passing through the flow restriction device can be greater than a flow rate of vapor passing through the flexible sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the invention and is therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a sectional side-view of an exemplary embodiment of a negative pressure wound therapy apparatus having a transmissive patch. 
         FIG. 2  is a partial cut-away top view of the negative pressure wound therapy apparatus of claim  1 . 
         FIG. 3  is a detailed top-view of an embodiment of the transmissive patch of the negative pressure wound therapy apparatus of  FIG. 1 . 
         FIG. 4  is a top-view of an alternative embodiment of the transmissive patch of the negative pressure wound therapy apparatus of  FIG. 1 . 
         FIG. 5  is a top-view of another alternative embodiment of the transmissive patch of the negative pressure wound therapy apparatus of  FIG. 1 . 
         FIG. 6  is a top-view of another alternative embodiment of the transmissive patch of the negative pressure wound therapy apparatus of  FIG. 1 . 
         FIG. 7  is a top-view of another alternative embodiment of the transmissive patch of the negative pressure wound therapy apparatus of  FIG. 1 . 
         FIG. 8  is a top-view of another alternative embodiment of the transmissive patch of the negative pressure wound therapy apparatus of  FIG. 1 . 
         FIG. 9  is a sectional view of the embodiment of the transmissive patch of  FIG. 8 , taken along the  9 - 9  line. 
         FIG. 10  is a top-view of another alternative embodiment of the transmissive patch of the negative pressure wound therapy apparatus of  FIG. 1 . 
         FIG. 11  is a sectional view of the embodiment of the transmissive patch of  FIG. 10 , taken along the  11 - 11  line. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and the prime notation, if used, indicates similar elements in alternative embodiments. 
     Referring to  FIGS. 1 and 2 , negative pressure wound therapy involves the application of negative pressure to a wound to facilitate wound closure. The wound  100  can be any open wound such as an acute or chronic wound on the surface of the skin, a fistulae, an incision, and the like. The negative pressure at the wound  100  can remove exudates and bacteria and can also promote the flow of blood in capillaries near the wound. Negative pressure wound therapy (“NPWT”) treatment can include a negative pressure wound therapy dressing (“dressing”)  102  over a wound  100  and a vacuum source  104  for maintaining negative pressure. In one embodiment, vacuum source  104  is connected to dressing  102 , via vacuum tube  106 , to create the negative pressure. The vacuum source  104  can be, for example, any type of vacuum pump or facility vacuum line, including, for example, an electric pump, a battery powered pump, a mechanical suction device, a wall suction line in a hospital, and the like. Vacuum source  104  may operate continuously, cycle between high and low levels of negative pressure (variable), or cycle on and off (intermittent) while applying negative pressure. Negative pressure could be any level of negative pressure, including, for example, between about 0.1 mmHg and 200 mmHg. In one embodiment, the negative pressure is maintained at a set point between about 20 mmHg and 125 mmHg. In one embodiment, the negative pressure is maintained at a point between about 40 mmHg and 80 mmHg. In an embodiment having variable negative pressure, the negative pressure can oscillate between, for example, 40 mmHg and 80 mmHg. In some embodiments, vacuum source  104  is able to detect flow, or the lack thereof, of air and other fluids moving through vacuum source  104 . A lack of flow, or “no-flow” condition, can be caused by a variety of conditions, including, for example, a kinked or blocked vacuum tube  106 , exudates in drain  114 , a full collection canister  108 , an occluded filter (not shown), or stagnant flow wherein there is insufficient pressure differential to draw the exudates through the tube  106  to collection canister  108 . In some embodiments, vacuum source  104  can stop the application of negative pressure, such as by stopping the pump motor, if flow is inadequate. 
     Collection canister  108  can be located between the vacuum source  104  and dressing  102  for collecting fluids drawn from the wound bed  100 . In some embodiments, collection canister  108  has one or more devices (not shown) for preventing fluids from overflowing the canister and contaminating vacuum source. For example, a sensor on canister  108  or proximate to the canister may stop vacuum source  104  when fluid reaches a predetermined level in collection canister. Alternatively, a hydrophobic filter may be located in collection canister or in tubing between collection canister and vacuum source such that all air drawn by vacuum source is pulled through hydrophobic filter. If fluid contacts the hydrophobic filter, the filter becomes occluded and blocks flow from the canister. A sensor on vacuum source  104  may stop vacuum source  104 , such as by turning off a pump, when the filter becomes occluded. Some embodiments may have a float valve that closes when fluid reaches a predetermined level in canister  108  and thus prevent fluids from overflowing canister. 
     Dressing  102  is placed over wound bed  100 . Dressing  102  can include a wound contact layer, or interface layer, such as, for example, packing  112  (packing is also known as wound filler), drain  114  connected to tube  106 , thin flexible sheet  116  to provide a seal, and a flow restriction device such as transmissive patch  118 . In an exemplary embodiment, the packing  112  used to pack the wound can be, for example, fluffed gauze, foam, natural or synthetic polymers, and the like. Packing  112  may promote a more even distribution of negative pressure across wound bed  100 . Other materials, such as hydrophobic polymers (not shown), hydrophobic synthetic polymers (not shown), or anti-bacterial dressings, such as, for example, silver impregnated dressings, may be placed above or below packing  112  to reduce bacterial growth within the wound or to promote a more even pressure distribution in the vicinity of the wound. 
     Drain  114  may be placed in or above wound bed  100 . In an exemplary embodiment, drain  114  is placed over a layer of packing  112 . Drain  114  may be tubing having a plurality of apertures  120 . Alternatively, drain  114  may be tubing having one or more channels  121  ( FIG. 2 ) in an outer diameter surface running in an axial direction along all or a portion of the length of drain  114 . Drain  114  may be in communication with vacuum tube  106  and thus be in communication with collection canister  108  and vacuum source  104 . In some embodiments, additional packing  112  may be placed above drain  114 . 
     Referring to  FIG. 2 , flexible sheet  116  is placed over wound bed  100  to form a seal. In a preferred embodiment, flexible sheet  116  is transparent, thus allowing caregiver to see the condition of dressing and wound bed. Flexible sheet  116  may be sealed to tissue, such as skin  119 , surrounding or proximate to the wound bed  100 . In some embodiments, flexible sheet  116  has an adhesive coating (not shown) on one side, or around the perimeter of one side, for forming a seal against skin  119 . Other techniques for forming a seal between flexible sheet  116  and skin  119  can be used. 
     Flexible sheet  116  can be a semi-permeable material. A semi-permeable material allows some gas to pass through the material but substantially prevents liquids from passing through. In some embodiments, semi-permeable sheets allow vapor to pass through at a moisture vapor transmission rate (“MVTR”) of roughly 22-55, as determined by MVTR Method ASTM E96 Method E. Examples of suitable flexible sheets  116  include Bioclusive™, Tegaderm™, Opsite®, and Transeal™. Other types of flexible sheets  116  may be used and higher or lower MVTRs may be used. Alternatively, an impermeable or generally impermeable flexible sheet  116  can be used. 
     Referring again to  FIG. 1 , when negative pressure is created by vacuum source  104  via tubing  106 , the negative pressure is communicated to the space between flexible sheet  116  and wound bed  100 . This space is identified as dressing space  122 . Dressing space  122  is all or partially occupied by packing  112 , drain  114 , and at times, wound exudate. It is desirable for a pressure differential to exist between dressing space  122  and vacuum source  104 , wherein any fluids present in dressing space  122  are drawn, as a result of negative pressure at vacuum source  104 , toward collection canister  108 . If air or liquid fluids are not introduced into dressing space  122 , then exudates and other fluids may not move away from dressing space  122  toward the vacuum source  104 , thus resulting in pooling, or a buildup of fluids, within dressing space  122 . 
     If the surface area of the flexible sheet  116  is sufficiently large, a small amount of air, or other gas, may be able to pass through flexible sheet  116  into dressing space  122 . Air flow into dressing space  122  causes a slight pressure differential. The pressure differential promotes the movement of air and fluids from dressing space  122  toward vacuum source, wherein fluids can be collected by collection canister  108 . In some situations, the flow through semi-permeable flexible sheet  116  is less than desired. This can occur when, for example, the surface area of flexible sheet  116  is too small and the wound drainage is not sufficient to introduce a significant amount of fluid into the wound bed. It can also occur if the MVTR of the flexible sheet  116  is less than desired. Inadequate flow may result in stagnation of fluids and exudates in dressing space  122 , tubing  106 , or in drain  114 . Furthermore, lack of flow may cause vacuum source  104  to generate an alarm, such as an occluded line, full-canister, or any other type of low/no-flow alarm. 
     Dressing aperture  123  may be used to admit air or other gases into dressing space  122 . Dressing aperture  123  can be a hole or opening through flexible sheet  116 . Dressing aperture may be a simple pin-hole, a slit, or may be a larger opening in flexible sheet  116 . In one embodiment, dressing aperture  123  can be approximately 1-3 mm in diameter. Preferably, dressing aperture  123  in flexible sheet  116  is smaller than transmissive patch  118 . An opening in flexible sheet  116  that is not covered by a flow restriction device may admit too much air into dressing space  122 . A flow restriction device, such as, for example, transmissive patch  118 , may be used to reduce the flow of vapor and substantially prevent the flow of liquid through dressing aperture  123 . Indeed, the flow restriction device can restrict the types and volumes of material passing through dressing aperture  123  while still passing gas or vapor at a higher rate than would otherwise pass through flexible sheet  116 . 
     Referring to  FIG. 2 , transmissive patch  118  can have a semi-permeable barrier  124  and can have cover layer  126 . Cover layer  126  may be used to hold and seal semi-permeable barrier  124  in place. Semi-permeable barrier  124  can be considered semi-permeable, in that in some embodiments, it can pass gas and other vapor at a restrictive rate, but does not allow liquids, bacteria, or germs to pass through. In one embodiment, liquids, bacteria, or germs are not able to pass through semi-permeable barrier  124  in either direction. In an exemplary embodiment, semi-permeable barrier  124  may be made out of a “breathable” material such as flashspun high-density polyethylene fibers. A commercial embodiment of a suitable material is sold under the brand name Tyvek®. Other materials that block liquid and germ transmission but allow gas and vapor transmission may be used for semi-permeable barrier  124 . Preferably, the transmission rates of semi-permeable barrier  124  are higher than the transmission rates of flexible sheet  116 . Semi-permeable barrier  124  may be any shape including, for example, round, square, or rectangular. Semi-permeable barrier  124  may be any size. In an exemplary embodiment, semi-permeable barrier  124  is roughly 2-3 mm in diameter, but may be larger or smaller to allow more or less flow through semi-permeable barrier  124 . Preferably, the size of semi-permeable barrier  124  is smaller than the size of flexible sheet  116 . Semi-permeable barrier  124  can be any thickness. In one embodiment, the thickness of semi-permeable barrier  124  is selected to facilitate a particular rate of vapor transmission. In this embodiment, a thicker layer of semi-permeable barrier  124  can be selected to cause a lower rate of vapor transmission. 
     The rate at which air passes through semi-permeable barrier  124  is determined by the type and density of the semi-permeable barrier material, the thickness of the semi-permeable barrier material, and the surface area of the semi-permeable barrier material. In an exemplary embodiment, the semi-permeable barrier  124  is selected to permit a predetermined amount of air to pass through at a given level of negative pressure. Preferably, semi-permeable barrier  124  passes enough air to create an appropriate pressure differential between dressing space  122  and vacuum source  104 . An appropriate pressure differential is one that is sufficient to cause fluids to move from the dressing space  122  toward vacuum source  104 . Preferably, semi-permeable barrier  124  also provides sufficient flow restriction to maintain negative pressure within dressing space  122 . Such sufficient flow conditions can reduce the occurrence of a low-flow or “check dressing seal” alarm. 
     In some embodiments, vacuum source  104  is a pump unit wherein the pump motor cycles on when the negative pressure level drops below a certain level. If flow through dressing aperture  123  is too high, the pump motor would have to run constantly or at least frequently, which could lead to premature pump failure. In a preferred embodiment, flow through semi-permeable barrier  124  is low enough to maintain negative pressure in dressing space  122  for a period of time while pump motor is cycled off. In other words, pump motor does not have to run constantly to maintain a level of negative pressure. Furthermore, if too much air flows through dressing space  122 , the wound bed or dressing components, such as packing  112 , may become too dry. The flow through semi-permeable barrier  124  is low enough to prevent the wound bed and dressing components from drying out. 
     Cover layer  126  can be used to secure semi-permeable barrier  124  to flexible sheet  116 . In an exemplary embodiment, cover layer  126  is an adhesive, flexible cover made of a semi-permeable material such as Bioclusive™, Tegaderm™, Opsite®, or Transeal™. Cover layer  126  can be a permeable, semi-permeable, or impermeable material. 
     In an exemplary embodiment, cover layer  126  is larger than semi-permeable barrier  124  and thus a perimeter of cover layer  126  extends beyond semi-permeable barrier  124 . The adhesive coating of cover layer  126  can sealingly engage cover layer  126  to flexible sheet  116 , thus preventing fluids from leaking past transmissive patch  118 . Cover layer  126  can have one or more openings  128  to permit ambient air to directly contact the surface of semi-permeable barrier  124 . Because air is able to pass through openings  128  of cover layer  126 , the permeability of cover layer  126  does not significantly impact the overall permeability of transmissive patch  118 . As shown in  FIG. 3 , cover layer  126  may be an annular ring with a central opening  128 , the central opening  128  having diameter  130  that is smaller than diameter  132  of semi-permeable barrier  124 . In this embodiment, semi-permeable barrier  124  is a round disc, with a diameter  132  that is larger than central opening  128  but smaller than the outer diameter of cover layer  126 . 
     Referring to  FIG. 4 , in an alternative embodiment, cover layer  136  is a disc having a diameter greater than the diameter of disc-shaped semi-permeable barrier  124 . Cover layer  136  is perforated with multiple apertures  138  for passing air through cover layer  136  to the surface of semi-permeable barrier  124 . Apertures  138  may be round, square, or any other shape. 
     Referring to  FIG. 5 , in another alternative embodiment, cover layer  140  and semi-permeable barrier  142  each have a rectangular shape. The edges of cover layer  140  extend beyond the edges of semi-permeable barrier  142 . Cutouts  144  on cover layer  140  expose the surface of semi-permeable barrier  142 . Cutouts  144  may be round, square, rectangular, or any other shape. 
     Referring to the alternative embodiment of  FIG. 6 , cover layer  146  has a single opening  148 . Semi-permeable barrier  150  is a round disc having a 2-3 mm diameter and is centered on the single opening  148  of cover layer  146 . 
     Referring to  FIG. 7 , in another alternative embodiment, vent patch  154  can be a semi-permeable barrier  156  without a cover layer. Adhesive  158  can be located around the outer edge and used to adhere vent patch  154  to flexible sheet  116  ( FIG. 1 ). 
     Referring to  FIGS. 8 and 9 , vent patch  170  can include a semi-permeable barrier  172  that is sandwiched between cover layer  174  and adhesive layer  176 . One side of cover layer  174  can have an adhesive for adhering to semi-permeable barrier  172  and adhesive layer  176 . Similarly, adhesive layer  176  can have an adhesive coating on one side for adhering to flexible sheet  116  ( FIG. 1 ). Either or both of cover layer  174  and adhesive layer  176  can have adhesive coatings. Vent patch  170  can be any shape including, for example, round, rectangular, or elliptical. 
     Inlet aperture  180  can be a hole or other perforation through cover layer  174 . Outlet aperture  182  can be a hole or other perforation through adhesive layer  176 . In one embodiment, the adhesive of adhesive layer  176  is used to adhere vent patch  170  onto flexible sheet  116 . In a preferred embodiment, outlet aperture  182  is located over dressing aperture  123 . Inlet and outlet apertures  180 ,  182  can each be any size. They can be, for example, 1 mm in diameter, a pin-hole, or any other size or shape. In one embodiment, outlet aperture  182  is larger than inlet aperture  180  which can make it easier to align outlet aperture  182  with dressing aperture  123 . 
     When suction is applied to dressing space  122  ( FIG. 1 ), flow path  185  is created, wherein air or other fluid located outside of vent patch  170  passes through inlet aperture  180 , through semi-permeable barrier  172 , through outlet aperture  182 , through dressing aperture  123  ( FIG. 1 ), and into dressing space  122  ( FIG. 1 ). The rate of fluid flow is controlled by the thickness of semi-permeable barrier  170 , the lateral distance between inlet aperture  180  and outlet aperture  182 , and the level of negative suction. 
     Referring to  FIGS. 10 and 11 , in another embodiment, wherein vent patch  186  can include a semi-permeable barrier  188  that is sandwiched between cover layer  190  and bottom layer  192 . In this embodiment, cover layer  190  can extend laterally beyond bottom layer  192 , such that adhesive surface  193  can adhere to flexible sheet  116  ( FIG. 1 ), as well as bottom layer  192  and semi-permeable barrier  188 . The seal formed between cover layer  190  and flexible sheet  116  ( FIG. 1 ) is generally airtight such that it prevents fluids such as air or liquids from passing between cover layer  190  and flexible sheet  116  ( FIG. 1 ). Vent patch  186  can be any shape including, for example, round, rectangular, or elliptical. 
     Inlet aperture  194  can be a hole or other perforation through cover layer  190 . Outlet aperture  196  can be a hole or other perforation through adhesive layer  192 . In a preferred embodiment, outlet aperture  196  is located over dressing aperture  123  ( FIG. 1 ). Inlet and outlet apertures  194 ,  196  can each be any size. They can be, for example, 1 mm in diameter, a pin-hole, or any other size or shape. In one embodiment, outlet aperture  196  is larger than inlet aperture  194  which can make it easier to align outlet aperture  196  with dressing aperture  123 . 
     When suction is applied to dressing space  122  ( FIG. 1 ), flow path  198  is created, wherein air or other fluid located outside of vent patch  186  passes through inlet aperture  194 , through semi-permeable barrier  188 , through outlet aperture  196 , through dressing aperture  123  ( FIG. 1 ), and into dressing space  122  ( FIG. 1 ). The rate of fluid flow is controlled by the thickness of semi-permeable barrier  186 , the lateral distance between inlet aperture  194  and outlet aperture  196 , and the level of negative suction. 
     Referring back to  FIGS. 1 and 2 , a health-care provider applies packing  112 , drain  114 , and flexible sheet  116  over wound bed  100 . In one embodiment, the health-care provider connects tube  106  to collection canister  108 , which is connected to vacuum source  104 . The health-care provider can activate vacuum source  104  to confirm that dressing  102  has proper seal integrity including, for example, having an adequate seal against skin  119 . The health-care provider can then create dressing aperture  123  in flexible sheet  116 , or locate dressing aperture  123  if dressing aperture  123  already exists in flexible sheet  116 . Dressing aperture  123  may be a pin hole, a small incision, or any other type of opening. Preferably, the size of dressing aperture  123  is smaller than the size of semi-permeable barrier  124 , such that semi-permeable barrier  124  is able to completely cover dressing aperture  123 . Dressing aperture  123  can be located at any location on flexible sheet  116 . Preferably, dressing aperture  123  is located over wound bed  100 . 
     The caregiver then applies transmissive patch  118  over dressing aperture  123 . Transmissive patch  118  may be preassembled, with semi-permeable barrier  124  already adhered to cover layer  126 , or care giver may individually apply semi-permeable barrier  124  and cover layer  126  over dressing aperture  123 . Cover layer  126  adheres to flexible sheet  116  and to semi-permeable barrier  124 . In one embodiment, transmissive patch  118  can be attached to flexible sheet before it reaches the care giver. For example the manufacturer can attach transmissive patch  118  to flexible sheet  116  at the manufacturing facility, or a technician can attach transmissive patch  118  to flexible sheet  116  in a preparation area prior to distributing the pre-assembled unit to the care giver. 
     Cover layer  126  forms a seal around outer edge of semi-permeable barrier  124 , thus preventing air or fluids from passing between the surface of flexible sheet  116  and the edge of semi-permeable barrier  124 . Air is able to pass through openings  128  of cover layer  126  and then permeate through semi-permeable barrier  124  into dressing space  122 . The transmission rate of semi-permeable barrier may work to reduce the volume and velocity of air passing through dressing aperture  123 . Various sizes of dressing apertures  123 , combined with various materials, thicknesses, and sizes of semi-permeable barrier  124  may be used to permit a desired flow rate through dressing aperture  123 . The permeation of air through semi-permeable barrier  124  and dressing aperture  123  into the dressing space  122  can create the desired pressure differential to allow air and other fluids to move from dressing space  122  toward the vacuum source  104 . 
     Semi-permeable barrier  124  can prevent fluids, bacteria, or germs originating within dressing space  122  from leaking out of dressing  102 . Any fluids or germs that are able to move through dressing aperture  123  are blocked by semi-permeable barrier  124 . Cover layer  126  prevents fluids and germs from passing between semi-permeable barrier  124  and flexible sheet  116 . Furthermore, bacteria and germs originating outside of dressing  102  are not able to pass through semi-permeable barrier  124  into dressing space  122 . 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention. 
     Furthermore, recitation of the term about and approximately with respect to a range of values should be interpreted to include both the upper and lower end of the recited range. As used herein, the terms first, second, third and the like should be interpreted to uniquely identify elements and do not imply or restrict to any particular sequencing of elements or steps. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereupon without departing from the principle and scope of the invention. Accordingly, the scope of the present invention should be determined by the following claims and their appropriate legal equivalents. 
     The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
     Optional or optionally indicates that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
     Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range. 
     In the drawings and specification, there have been disclosed a typical preferred embodiment of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The invention has been described in considerable detail with specific reference to these various illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification and as defined in the appended claims.