Patent Publication Number: US-2020289727-A1

Title: Wound dressing with negative pressure retaining valve

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Application No. 62/595,289, filed on Dec. 6, 2017, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to a wound dressing and more particularly to a wound dressing for use as part of a negative pressure wound therapy system. 
     Negative pressure wound therapy (NPWT) is a type of wound therapy that involves applying a negative pressure to a wound site to promote wound healing. Some wound treatment systems apply negative pressure to a wound site using a pneumatic pump to generate the negative pressure and flow required. However, continuous regulated negative pressure typically requires the pump to remain tethered to the wound site. It would be desirable to provide a wound therapy system and/or a wound dressing that permits additional functionality compared with conventional NPWT systems. 
     SUMMARY 
     One implementation of the present disclosure is a wound dressing including a cover layer, a tube, and a one-way valve. The cover layer is sealable to skin surrounding a wound site and includes a port extending through the cover layer. The tube includes a first end coupled to the cover layer via the port and a second end including an in-line connector configured to releasably attach the tube to a negative pressure source and detach the tube from the negative pressure source. The one-way valve is located along the tube between the first end and the second end and configured to allow fluid flow through the tube in a first direction from the first end to the second end and prevent fluid flow through the tube in a second direction from the second end to the first end. 
     In some embodiments, the one-way valve is located within the in-line connector at the second end of the tube. In some embodiments, the one-way valve is configured to maintain negative pressure within the tube when the tube is detached from the negative pressure source. 
     In some embodiments, the wound dressing includes a filter configured to prevent liquid within the tube from reaching the one-way valve. In some embodiments, the filter is located within the in-line connector at the second end of the tube. 
     In some embodiments, the wound dressing includes a hydrophobic filter configured to retain liquid within the wound dressing. In some embodiments, the fluid flow through the tube is airflow. 
     In some embodiments, the wound dressing includes a pressure indicator configured to measure a pressure within the tube or at the wound site. In some embodiments, the pressure indicator is a mechanical pressure indicator. 
     In some embodiments, the negative pressure source includes at least one of a motorized pump or a manually-operable pump. 
     In some embodiments, an internal volume of the tube between the one way valve and the cover layer defines a negative pressure reservoir fluidly connected with the wound site and configured to stabilize changes in pressure at the wound site. In some embodiments, the negative pressure reservoir within the tube has a volume of at least 5,000 mm 3 . In some embodiments, the negative pressure reservoir within the tube has a volume of at least 10,000 mm 3 . 
     In some embodiments, the negative pressure reservoir is configured to maintain pressure at the wound site at least 50 mmHg below atmospheric pressure when the tube is detached from the negative pressure source. In some embodiments, the negative pressure reservoir is configured to maintain pressure at the wound site at least 50 mmHg below atmospheric pressure for at least 24 hours when the tube is detached from the negative pressure source. In some embodiments, the negative pressure reservoir is configured to prevent pressure at the wound site from changing by more than 75 mmHg within a 12 hour period when the tube is detached from the negative pressure source. 
     In some embodiments, the tube has a length of at least 1 m. In some embodiments, the tube has an inner diameter between 2 mm and 3 mm. In some embodiments, the tube has an inner diameter between 3 mm and 4 mm. In some embodiments, the tube has an outer diameter of at least 4 mm and a wall thickness of at least 0.75 mm. 
     In some embodiments, the tube is configured to configured to retract to form a substantially planar coil when not connected to the negative pressure source. In some embodiments, the tube is pre-coiled such that the tube forms a substantially planar coil in the absence of external force applied to the tube. 
     In some embodiments, a cross-section of the tube is substantially rectangular. In some embodiments, the tube has a wall thickness that provides sufficient rigidity to prevent collapse of the tube when a pressure within the tube is at least 100 mmHg below atmospheric pressure. 
     Another implementation of the present disclosure is a negative pressure wound therapy (NPWT) system including a negative pressure source, a wound dressing, a tube, and a one-way valve. The wound dressing is sealable to skin surrounding a wound site. The tube includes a first end coupled to the wound dressing and a second end including an in-line connector configured to releasably attach the tube to the negative pressure source and detach the tube from the negative pressure source. The one-way valve is located along the tube and configured to prevent fluid flow through the tube from the second end to the first end. 
     In some embodiments, the one-way valve is located within the in-line connector at the second end of the tube. In some embodiments, the one-way valve is configured to maintain negative pressure within the tube when the tube is detached from the negative pressure source. 
     In some embodiments, the NPWT system includes a filter configured to prevent liquid within the tube from reaching the one-way valve. In some embodiments, the filter is located within the in-line connector at the second end of the tube. 
     In some embodiments, the wound dressing includes a hydrophobic filter configured to retain liquid within the wound dressing. In some embodiments, the fluid flow through the tube is airflow. 
     In some embodiments, the NPWT system includes a pressure indicator configured to measure a pressure within the tube or at the wound site. In some embodiments, the pressure indicator is a mechanical pressure indicator. 
     In some embodiments, the negative pressure source includes at least one of a motorized pump or a manually-operable pump. 
     In some embodiments, an internal volume of the tube between the one way valve and the wound dressing defines a negative pressure reservoir fluidly connected with the wound site and configured to stabilize changes in pressure at the wound site. 
     In some embodiments, the negative pressure reservoir within the tube has a volume of at least 5,000 mm 3 . In some embodiments, the negative pressure reservoir within the tube has a volume of at least 10,000 mm 3 . 
     In some embodiments, the negative pressure reservoir is configured to maintain pressure at the wound site at least 50 mmHg below atmospheric pressure when the tube is detached from the negative pressure source. In some embodiments, the negative pressure reservoir is configured to maintain pressure at the wound site at least 50 mmHg below atmospheric pressure for at least 24 hours when the tube is detached from the negative pressure source. In some embodiments, the negative pressure reservoir is configured to prevent pressure at the wound site from changing by more than 75 mmHg within a 12 hour period when the tube is detached from the negative pressure source. 
     In some embodiments, the tube has a length of at least 1 m. In some embodiments, the tube has an inner diameter between 2 mm and 3 mm. In some embodiments, the tube has an inner diameter between 3 mm and 4 mm. In some embodiments, the tube has an outer diameter of at least 4 mm and a wall thickness of at least 0.75 mm. 
     In some embodiments, the tube is configured to configured to retract to form a substantially planar coil when not connected to the negative pressure source. In some embodiments, the tube is pre-coiled such that the tube forms a substantially planar coil in the absence of external force applied to the tube. 
     In some embodiments, a cross-section of the tube is substantially rectangular. In some embodiments, the tube has a wall thickness that provides sufficient rigidity to prevent collapse of the tube when a pressure within the tube is at least 100 mmHg below atmospheric pressure. 
     Another implementation of the present disclosure is a wound dressing including a cover layer, a tube, a one-way valve, a filter, and a pressure indicator. The cover layer is sealable to skin surrounding a wound site and includes a port extending through the cover layer. The tube includes a first end coupled to the cover layer via the port and a second end including an in-line connector configured to releasably attach the tube to a negative pressure source and detach the tube from the negative pressure source. The one-way valve is located within the in-line connector and configured to prevent fluid flow into the tube via the second end. The filter is located within the in-line connector and configured to prevent liquid within the tube from reaching the one-way valve. The pressure indicator is located within the in-line connector and configured to measure a pressure at the second end of the tube. 
     Those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the devices and/or processes described herein, as defined solely by the claims, will become apparent in the detailed description set forth herein and taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a negative pressure wound therapy (NPWT) system including a wound dressing, a tube, and a pump unit, according to an exemplary embodiment. 
         FIG. 2  is a perspective view of the pump unit of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 3  is a top perspective view of the wound dressing of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 4  is another top view of the wound dressing of  FIG. 1  illustrating the tube retracted into a coil, according to an exemplary embodiment. 
         FIG. 5  is a cross-sectional view of the tube of  FIG. 1  illustrating an embodiment in which the tube has a circular cross-section, according to an exemplary embodiment. 
         FIG. 6  is a cross-sectional view of the tube of  FIG. 1  illustrating an embodiment in which the tube has a rectangular cross-section, according to an exemplary embodiment. 
         FIG. 7  is a block diagram of the NPWT system of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 8  is a graph illustrating results of a dry test experiment performed using the wound dressing of  FIG. 1 , according to an exemplary embodiment. 
         FIG. 9  is a graph illustrating results of a wet test experiment performed using the wound dressing of  FIG. 1 , according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     Referring generally to the FIGURES, a wound dressing and negative pressure wound therapy system are shown, according to various exemplary embodiments. Negative pressure wound therapy (NPWT) is a type of wound therapy that involves applying a negative pressure to a wound site (relative to atmospheric pressure) to promote wound healing. The NPWT system includes a negative pressure source (e.g., a manually-operable or motorized pump), a wound dressing, and a tube connecting the negative pressure source to the wound dressing. The wound dressing is sealable to a patient&#39;s skin surrounding a wound site. The pump operates to create negative pressure at the wound site by removing air from the wound site via the tube. 
     The tube includes a first end coupled to the wound dressing and a second end opposite the first end. The second end of the tube includes an in-line connector configured to releasably attach the tube to the negative pressure source and detach the tube from the negative pressure source. The in-line connector includes a one-way valve configured to prevent fluid flow through the tube from the second end to the first end. Accordingly, the one-way valve allows the tube to be disconnected from the negative pressure source without losing the negative pressure within the tube and/or at the wound site. The internal volume of the tube also acts as a negative pressure reservoir to stabilize changes in pressure at the wound site. These and other features of the wound dressing and NPWT system are described in greater detail below. 
     Negative Pressure Wound Therapy (NPWT) System &amp; Wound Dressing 
     Referring now to  FIG. 1 , a negative pressure wound therapy system  100  is shown, according to an exemplary embodiment. System  100  is shown to include a wound dressing  110 , pump unit  106 , and a tube  104  connecting wound dressing  110  with pump unit  106 . Wound dressing  110  can be sealed to a patient&#39;s skin surrounding a wound site and may provide an airtight seal over the wound site. Pump unit  106  can be configured to draw air or other fluids from the wound site via tube  104  such that the wound site is maintained at negative pressure relative to atmospheric pressure. In various embodiments, pump unit  106  can be a manually-operable pump, a motorized pump, or any other device that functions as a negative pressure source. 
     Wound dressing  110  can be formed as a substantially flat sheet for topical application to wounds or contoured for application to body surfaces having high curvature. The size of wound dressing  110  can vary depending on the size of the wound to be dressed. For example, it is contemplated that the size of wound dressing  110  can range from 1 cm 2  to 200 cm 2 , and more preferably from 4 cm 2  to 100 cm 2 . However, other shapes and sizes of wound dressing  110  are also possible depending on the intended use. Wound dressing  110  is shown to include a cover layer  120 , an upper pressure distribution layer  118 , an absorbent layer  116 , a lower pressure distribution layer  114 , and a wound interface layer  112 . 
     Cover layer  120  can be configured to seal to skin surrounding a wound site. In some embodiments, cover layer  120  is made of a material that prevents or greatly reduces the permeation of air or other fluids through cover layer  120 . For example, cover layer  120  may be made of polyurethane or other suitable polymeric materials and may include an elastomeric film or membrane that can provide a seal around the wound site. In some embodiments, cover layer  120  provides a barrier to microbes, a barrier to external contamination, and protection from physical trauma. For example, cover layer  120  may be constructed from a material that can reduce pressure losses and provide a fluid seal between two components or two environments, such as between a therapeutic environment between cover layer  120  and the wound and a local external environment. 
     In some embodiments, cover layer  120  is coated with an acrylic or other adhesive. The adhesive applied to cover layer  120  ensures that wound dressing  110  adheres to the patient&#39;s skin and that wound dressing  110  remains in place throughout the wear time. In some embodiments, the perimeter of cover layer  120  extends beyond (e.g., circumscribes) the perimeters of upper pressure distribution layer  118 , absorbent layer  116 , lower pressure distribution layer  114 , and wound interface layer  112  to provide an adhesive-coated margin for adhering wound dressing  110  to the skin of a patient adjacent to the wound being treated. The adhesive-coated margin may extend around all sides of layers  112 - 118  such that wound dressing  110  is a so-called island dressing. In other embodiments, the adhesive-coated margin can be eliminated and wound dressing  110  can be adhered to the patient&#39;s skin using other techniques. 
     In some embodiments, cover layer  120  includes a connector pad  122  and a port  124  extending through cover layer  120 . Connector pad  122  can be coupled to a first end of tube  104  and may secure the first end of tube  104  to cover layer  120 . Tube  104  may extend through cover layer  120  and connector pad  122  via port  124  such that tube  104  is fluidly connected to a space between cover layer  120  and the wound. This allows pump unit  106  to remove air or other fluids from the wound site via port  124  and tube  104  to maintain the space within wound dressing  110  at negative pressure. Upper pressure distribution layer  118  may be located adjacent to cover layer  120  (opposite connector pad  122 ) and can be configured to distribute the negative pressure across absorbent layer  116 . Similarly, lower pressure distribution layer  114  can be configured to distribute the negative pressure across wound interface layer  112  and ultimately the surface of the wound. 
     Absorbent layer  116  may be located between pressure distribution layers  114  and  118  and can be configure to absorb wound exudate or other fluids at the wound site. In some embodiments, absorbent layer  116  includes a superabsorbent material such as a hydrogel or hydrogel composition. Several examples of hydrogels and hydrogel compositions which can be used to form absorbent layer  116  are described in detail in U.S. Pat. No. 8,097,272 issued Jan. 17, 2012, U.S. Pat. No. 8,664,464 issued Mar. 4, 2014, and U.S. Pat. No. 8,058,499 issued Nov. 15, 2011. The entire disclosure of each of these patents is incorporated by reference herein. 
     The expressions “hydrogel” and “hydrogel compositions” used herein are not to be considered as limited to gels which contain water, but extend generally to all hydrophilic gels and gel compositions, including those containing organic non-polymeric components in the absence of water. For example, absorbent layer  116  may be formed from a polyurethane that entraps water to form a gel. In some embodiments, absorbent layer  116  is substantially continuous and/or substantially non-porous or non-foamed. Absorbent layer  116  may include a flexible plasticized hydrophilic polymer matrix having a substantially continuous internal structure. The density of absorbent layer  116  may be greater than 0.5 g/cm 3 , more preferably greater than 0.8 g/cm 3 , and most preferably from 0.9 to 1.1 g/cm 3 . In some embodiments, the thickness of absorbent layer  116  is from 1 mm to 10 mm, more preferably from 2 mm to 5 mm. 
     In some embodiments, absorbent layer  116  is cross-linked and preferably it is substantially insoluble in water at ambient temperature. However, the structure of absorbent layer  116  absorbs and entraps liquid to provide a highly hydrated gel structure in contrast to the porous foam structure absorbent layer  116 . Preferably, the gel can absorb 1 to 10 g/g of physiological saline at 20°, more preferably 2 to 5 g/g. 
     In some embodiments, the dry weight of absorbent layer  116  is from 1000 to 5000 g/m 2 , more preferably from 2000 to 4000 g/m 2 . In some embodiments, absorbent layer  116  includes from 1% to 30% of water, more preferably from 10% to 20% by weight of water before use. In some embodiments, absorbent layer  116  contains from 1% to 40%, more preferably from 5 to 15%, by weight of one or more humectants, preferably selected from the group consisting of glycerol, propylene glycol, sorbitol, mannitol, polydextrose, sodium pyrrolidine carboxylic acid (NaPCA), hyaluronic acid, aloe, jojoba, lactic acid, urea, gelatin, lecithin and mixtures thereof. The entrapped water and optional humectants give the hydrogel a soft, moist wound-friendly surface for contacting the wound. 
     In some embodiments, absorbent layer  116  includes a hydrophilic foam. The hydrophilic foam can be laminated or otherwise coupled to the superabsorbent material via a fusible fiber. The hydrophilic foam may include a polyurethane foam and/or a flexible plasticized hydrophilic polymer matrix having an internal cellular structure. Several examples of hydrophilic foams which can be used to in absorbent layer  116  are described in detail in U.S. Pat. No. 8,097,272 issued Jan. 17, 2012, U.S. Pat. No. 8,664,464 issued Mar. 4, 2014, and U.S. Pat. No. 8,058,499 issued Nov. 15, 2011. The entire disclosure of each of these patents is incorporated by reference herein. 
     Advantageously, the hydrophilic foam provides enhanced absorbency for liquid exudate. This is because the initial substantially anhydrous condition and porous structure of the hydrophilic foam enables it to absorb a larger amount of water by both chemical and physical absorption that is the case for the corresponding hydrogel material. Furthermore, the porous structure of the foam provides for rapid uptake of liquid exudate, in contrast to pure hydrogel dressings. 
     In some embodiments, the hydrophilic foam layer has a thickness of from 1 to 20 mm, more preferably from 1.5 to 5 mm. In some embodiments, the hydrophilic foam has a density of from 0.28 g/cm 3  to 0.5 g/cm 3 , and more preferably from 0.32 g/cm 3  to 0.48 g/cm 3 . Preferably, the hydrophilic foam has an elongation to break of at least 150%, more preferably from 500% to 1000%. The hydrophilic foam can absorb aqueous fluids such as wound exudate with swelling. The hydrophilic foam may be highly cross-linked and substantially insoluble in water. 
     In some embodiments, the hydrophilic foam has an absorbency of at least 3 grams of saline per gram of foam, and preferably a swellability in water of at least 200%. In some embodiments, the hydrophilic foam is constructed using the foam as described in European Patent No. 0541391 issued Jun. 10, 1998, the entire disclosure of which is incorporated by reference herein. In some embodiments, the hydrophilic foam includes less than 10% water prior to use as an absorbent, more preferably less than 5% water, and even more preferably it contains less than 2% of water before use. 
     Wound interface layer  112  may form a wound-contacting surface of wound dressing  110 . In some embodiments, wound interface layer  112  is made of silicone or other non-adherent materials that provides an effective seal for negative pressure, yet enable easy repositioning or removal, minimising trauma to periwound skin. Wound interface layer  112  can be configured to reduce potential adherence of absorbent layer  116  or lower pressure distribution layer  114  to the wound or tissue site, to enable fluid to be effectively drawn away from the wound via wound interface layer  112 , absorbent layer  116 , or both. In some embodiments, wound interface layer  112  is made of a hydrophobic material such as polyethylene (PE) or other hydrophobic polymers. The use of a hydrophobic material for wound interface layer  112  may be particularly advantageous to prevent the attachment of bacteria to the wound or tissue site. In some embodiments, wound interface layer  112  is perforated for increased fluid flow. 
     In various embodiments, wound interface layer  112  may include at least one of an alkyl acrylate polymer (e.g., a methyl acrylate polymer, an ethyl acrylate polymer, or the like) an alkacrylate polymer (e.g., a methacrylate polymer, an ethacrylate polymer, or the like) and/or an alkyl alkacrylate polymer (e.g., a methyl methacrylate polymer, an ethyl methacrylate polymer, a methyl ethacrylate polymer, an ethyl ethacrylate polymer, or the like). Such (alk)acrylate polymers may be homopolymers but are more often copolymers, for example, with olefin comonomers. In some embodiments, wound interface layer  112  includes anethylene-methyl acrylate copolymer, such as used in TIELLE dressings and SILVERCEL non-adherent dressings available from Systagenix Wound Management, Limited. In various embodiments, wound interface layer  112  may include a silicone or polysiloxane polymer or copolymer. 
     In some embodiments, wound dressing  110  includes a hydrophobic filter configured to retain liquids within wound dressing  110 . Such liquids may include, for example, wound exudate, water, condensed fluid, and/or other liquids. The hydrophobic filter may prevent any liquids within wound dressing  110  from leaving wound dressing  110  and entering tube  104 . Accordingly, any fluid flow through tube  104  may be limited to airflow (or other gasses) in some embodiments. 
     Pump Unit 
     Referring now to  FIG. 2 , pump unit  106  is shown in greater detail, according to an exemplary embodiment. Pump unit  106  is shown as a manually-operable pump having a plunger  126  and a shell  128 . Plunger  126  can be aligned with a central axis of shell  128  and configured to move axially relative to shell  128 . A user can press the top surface of plunger  126  toward shell  128  to cause plunger  126  to retract into shell  128 . In some embodiments, plunger  126  is coupled to a spring within shell  128  that causes plunger  126  to return to an extended position (as shown in  FIG. 2 ) in the absence of external force. 
     Plunger  126  and shell  128  may house an internal pneumatic chamber that decreases in volume when plunger  126  is depressed and increases in volume when plunger  126  is extended. The pneumatic chamber may be pneumatically connected to the atmosphere outside of pump unit  106  via a one-way valve that allows air to exit the pneumatic chamber but prevents air from entering the pneumatic chamber via the one-way valve. Accordingly, when plunger  126  is depressed, the air within the pneumatic chamber may be forced through the one-way valve and discharged to the atmosphere outside pump unit  106 . When plunger  126  is released, air from tube  130  may be drawn into pump unit  106 . A first end of tube  130  may connect to plunger  126 , whereas a second end of tube  130  may include a connector  132 . Connector  132  can be configured to attach to an in-line connector of tube  104  (described in greater detail below) to couple pump unit  106  to wound dressing  110 . Accordingly, pump unit  106  can be operated to draw air from wound dressing  110  to provide negative pressure within wound dressing  110 . 
     Although pump unit  106  is shown as a manually-operable pump, it should be understood that any other types of pump can be used to provide a similar effect. For example, pump unit  106  may be a motorized pump or any other type of device that functions as a negative pressure source. A “negative pressure source” is any type of pump or other device that operates to create a negative pressure zone or space relative to the pressure of the local environment (e.g., atmospheric pressure) around surrounding wound dressing  110 . The negative pressure source can be coupled to wound dressing  110  via tube  104  to maintain the wound site negative pressure, thereby providing negative pressure wound therapy at the wound site. 
     Tube with in-Line Connector 
     Referring now to  FIGS. 3-4 , tube  104  is shown in greater detail, according to an exemplary embodiment. A first end of tube  104  can be coupled to wound dressing  110  via connector pad  122  and/or cover layer  120 . A second end of tube  104  includes an in-line connector  140 . In-line connector  140  is configured to releasably attach tube  104  to pump unit  106  (or another negative pressure source) and detach tube  104  from pump unit  106  (or another negative pressure source). Advantageously, in-line connector  140  may include an internal one-way valve  146  (shown in  FIG. 7 ) configured to allow air to exit tube  104  but prevent air from entering tube  104 . Accordingly, the negative pressure within tube  104  may be maintained when tube  104  is detached from the negative pressure source. 
     In some embodiments, tube  104  includes a wide portion  134  and a narrow portion  136  linked by a reduction component  138 . Wide portion  134  may have a relatively larger diameter and/or cross-sectional area than narrow portion  136 . In some embodiments, in-line connector  140  is located at a free end of wide portion  134 , whereas narrow portion  136  is coupled to wound dressing  110 . In some embodiments, tube  104  has a length of at least one meter. However, it is contemplated that tube  104  can have any other length in various other embodiments. 
     Referring particularly to  FIG. 4 , in-line connector  140  is shown to include a pressure indicator  142 . Pressure indicator  142  can be configured to measure air pressure within tube  104  at the location of in-line connector  140 . The pressure measured by pressure indicator  142  may also be the pressure at the wound site due to the pneumatic coupling provided by tube  104 . In some embodiments, pressure indicator  142  is a mechanical pressure indicator. For example, pressure indicator  142  may include a sealed chamber configured to expand and contract responsive to the pressure within tube  104 . The sealed chamber can be coupled to a colored slider or other visual indicator that moves over a window when the sealed chamber expands and contracts. Accordingly, the portion of the colored slider or other indicator visible through the window may indicate the pressure within tube  104 . In other embodiments, pressure indicator  142  may be an electronic pressure sensor or any other type of pressure indicator. 
     Tube  104  can be configured to configured to retract to form a substantially planar coil when not connected to the negative pressure source. This allows tube  104  to retract into a compact arrangement when disconnected from the negative pressure source. Applying an external force to tube  104  may cause tube  104  to extend from the compact arrangement shown in  FIG. 4  to allow in-line connector  140  to reach a negative pressure source. In some embodiments, tube  104  is pre-coiled (e.g., via a heat treatment) such that tube  104  forms a substantially planar coil in the absence of the external force. 
     Referring now to  FIGS. 5-6 , tube  104  can have a variety of different cross-sectional shapes and sizes.  FIG. 5  illustrates an embodiment in which tube  104  has a substantially circular cross-section. In some embodiments, tube  104  has an inner diameter (D i ) between 2 mm and 3 mm. In other embodiments, tube  104  has an inner diameter (D i ) between 3 mm and 4 mm. In some embodiments, tube  104  has an outer diameter (D 0 ) of at least 4 mm. Tube  104  may have a wall thickness (t) that provides sufficient rigidity to prevent the collapse of tube  104  when the pressure within tube  104  is at least 100 mmHg below atmospheric pressure. For example, tube  104  may have a wall thickness (t) of at least 0.75 mm. However, the wall thickness of tube  104  may vary depending on the material used to form tube  104 . 
       FIG. 6  illustrates an embodiment in which tube  104  has a substantially rectangular cross-section. In some embodiments, tube  104  has an inner width (W i ) between 2 mm and 3 mm. In other embodiments, tube  104  has an inner width (W i ) between 3 mm and 4 mm. In some embodiments, tube  104  has an outer width (W o ) of at least 4 mm. When tube  104  has a rectangular cross-section, tube  104  may have a wall thickness (t) that provides sufficient rigidity to prevent the collapse of tube  104  when the pressure within tube  104  is at least 100 mmHg below atmospheric pressure. For example, tube  104  may have a wall thickness (t) of at least 0.75 mm. However, the wall thickness of tube  104  may vary depending on the material used to form tube  104 . 
     Referring now to  FIG. 7 , a block diagram illustrating several components of NPWT system  100  in greater detail is shown, according to an exemplary embodiment. As discussed above, wound dressing  110  may include several dressing layers  112 - 120  and a connector pad  122 . Wound dressing  110  can be configured to seal to a patient&#39;s skin surrounding wound site  150 . A first end of tube  104  may be coupled to wound dressing  110  via connector pad  122 . In-line connector  140  may be located at a second end of tube  104  opposite the first end. 
     In-line connector  140  is shown to include a pressure indicator  142 , a filter  144 , and a one-way valve  146 . In some embodiments, pressure indicator  142 , filter  144 , and one-way valve  146  are located within in-line connector  140 . Pressure indicator  142  may be a mechanical pressure indicator, an electronic pressure sensor, or any other type of pressure sensing device, as previously described. Filter  144  can be configured to prevent any liquid within tube  104  from reaching one way valve  146 . For example, wound exudate from wound site  150  may be drawn into tube  104  when pump unit  106  operates to draw a negative pressure within tube  104 . Filter  144  may be positioned between one-way valve  146  and wound dressing  110  such that any liquid within tube  104  does not reach one-way valve  146  and/or pump unit  106 . 
     Advantageously, one-way valve  146  may permit airflow through one-way valve  146  in the direction of the arrows in  FIG. 7  (i.e., from tube  104  to pump unit  106 ) but may prevent airflow in the opposite direction (i.e., airflow into tube  104  via in-line connector  140 ). This allows pump unit  106  to remove air from tube  104  when in-line connector  140  is connected to pump unit  106 , thereby creating negative pressure within tube  104 . However, one-way valve  146  prevents airflow into tube  104 , thereby maintaining the negative pressure within tube  104 , even when in-line connector  140  disconnected from pump unit  106 . 
     In some embodiments, an internal volume of tube  104  acts as a negative pressure reservoir  148  for wound dressing  110 . Negative pressure reservoir  148  may be defined as the volume within tube  104  that can be occupied by air or other fluids and may extend between wound dressing  110  and pump unit  106 . Negative pressure reservoir  148  may be maintained at a negative pressure relative to the pressure of the local environment (e.g., atmospheric pressure) outside wound dressing  110 . Negative pressure reservoir  148  may be fluidly connected with wound site  150  and may be maintained at the same pressure as wound site  150 . 
     Advantageously, negative pressure reservoir  148  may act to stabilize changes in pressure at wound site  150 . For example, wound site  150  may exude liquid over time that occupies some of the open volume within wound dressing  110 . If wound dressing  110  were not connected to negative pressure reservoir  148 , the decrease in open volume would significantly increase the pressure at wound site  150  and would lessen the therapeutic effects of negative pressure wound therapy. However, because wound dressing  110  is fluidly connected to negative pressure reservoir  148 , any loss in open volume within wound dressing  110  may be insignificant relative to the volume of negative pressure reservoir  148 . Accordingly, the decrease in open volume within wound dressing  110  may not significantly increase the pressure at wound site  150 . Negative pressure reservoir  148  may also stabilize changes in pressure caused by air leakage into wound dressing  110  and/or tube  104  in a similar manner. 
     The degree of pressure stabilization provided by negative pressure reservoir  148  may depend on the volume of negative pressure reservoir  148 . In some embodiments, negative pressure reservoir  148  has a volume of at least 5,000 mm 3  or at least 10,000 mm 3 . In some embodiments, negative pressure reservoir  148  is configured to maintain pressure at wound site  150  at least 50 mmHg below atmospheric pressure when tube  104  is detached from the negative pressure source. In some embodiments, negative pressure reservoir  148  maintains the pressure at wound site  150  at least 50 mmHg below atmospheric pressure for at least 24 hours when tube  104  is detached from the negative pressure source. In some embodiments, negative pressure reservoir  148  is configured to prevent pressure at wound site  150  from changing by more than 75 mmHg within a twelve hour period when tube  104  is detached from the negative pressure source. 
     Experimental Test Results 
     Referring now to  FIGS. 8-9 , a pair of graphs  160  and  170  illustrating experimental test results for NPWT system  100  and wound dressing  110  are shown, according to an exemplary embodiment. Both graphs  160  and  170  illustrate the ability of wound dressing  110  to maintain negative pressure under typical wound treatment conditions. In both experiments, wound dressing  110  was sealed to a surface and tube  104  was attached to pump unit  106 . Pump unit  106  was operated to draw air out of wound dressing  110  and tube  104 , thereby applying a negative pressure of approximately 130-140 mmHg relative to the pressure of the local environment (e.g., the atmosphere) outside wound dressing  110 . Tube  104  was then disconnected from pump unit  106  and the pressure within wound dressing  110  was monitored to determine how well wound dressing  110  holds the negative pressure. 
     Graph  160  plots the results of a dry experiment in which no fluids were instilled to wound dressing  110 . Line  162  illustrates the results of a first test, whereas line  164  illustrates the results of a second test. In the first test, the negative pressure within wound dressing  110  dropped from approximately 140 mmHg to approximately 120 mmHg over a time period of almost six hours. In the second test, the negative pressure within wound dressing  110  dropped from approximately 130 mmHg to approximately 110 mmHg over a time period of almost six hours. At this rate of decline, it would take significantly longer than twelve hours (e.g., 20-24 hours) for the negative pressure within wound dressing  110  to drop below 50 mmHg, which is considered therapeutic for NPWT. Accordingly, wound dressing  110  would only need to be changed or recharged (i.e., by reconnecting pump unit  106  and removing more air) once or twice per day to maintain the negative pressure at a therapeutic level. 
     Graph  170  plots the results of a wet experiment in which fluid was instilled to wound dressing  110  at a rate of 0.833 mL/hr. Line  172  illustrates the results of a first test, whereas line  174  illustrates the results of a second test. In the first test, the negative pressure within wound dressing  110  dropped from approximately 140 mmHg to approximately 50 mmHg after approximately 34 mL of fluid was instilled over a time period of approximately 40 hours. In the second test, the negative pressure within wound dressing  110  dropped from approximately 130 mmHg to approximately 50 mmHg after approximately 20 mL of fluid was instilled over a time period of approximately 24 hours. In both tests, the negative pressure within wound dressing  110  remained above 50 mmHg for at least 24 hours. Accordingly, wound dressing  110  would only need to be changed or recharged (i.e., by reconnecting pump unit  106  and removing more air) once per day to maintain the negative pressure at a therapeutic level. 
     Configuration of Exemplary Embodiments 
     The construction and arrangement of the elements of the wound dressing and negative pressure wound therapy system as shown in the exemplary embodiments are illustrative only. Although only a few embodiments of the present disclosure have been described in detail, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements. 
     The elements and assemblies may be constructed from any of a wide variety of materials that provide sufficient strength or durability, in any of a wide variety of colors, textures, and combinations. Additionally, in the subject description, the word “exemplary” is used to mean serving as an example, instance, or illustration. Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. Rather, use of the word “exemplary” is intended to present concepts in a concrete manner. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the preferred and other exemplary embodiments without departing from the scope of the appended claims. 
     The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Any means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes and omissions may be made in the design, operating configuration, and arrangement of the preferred and other exemplary embodiments without departing from the scope of the appended claims.