Patent Abstract:
An apparatus includes a pressure housing configured to be coupled to a vacuum source and a diaphragm sealingly coupled to the pressure housing. The diaphragm is configured to move in response to the vacuum source. A magnet is coupled to the diaphragm. A magnetic switch is disposed opposite the magnet and is configured to be actuated by the magnet when the magnet is a predetermined distance from the magnetic switch. The magnetic switch is configured to selectively actuate the vacuum source.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/350,089, entitled “Wound Irrigation Device Pressure Monitoring and Control System,” filed Sep. 2, 2006, which is a continuation-in-part of U.S. patent application Ser. No. 11/237,880, entitled “Wound Irrigation Device,” filed on Sep. 29, 2005, which is a continuation of U.S. patent application Ser. No. 11/198,148, entitled “Wound Irrigation Device,” filed on Aug. 8, 2005, each of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The invention is generally directed to a method and apparatus for the promotion of wound healing. More particularly, the present invention relates to providing fluid irrigation and vacuum drainage of a wound. 
         [0003]    Negative pressure wound therapy, also known as vacuum drainage or closed-suction drainage is known. A vacuum source is connected to a semi-occluded or occluded wound dressing. Various porous dressings comprising gauze, felts, foams, beads and/or fibers can be used in conjunction with an occlusive semi-permeable cover and a controlled vacuum source. 
         [0004]    In addition to using negative pressure wound therapy, many devices employ concomitant wound irrigation. For example, a known wound healing apparatus includes a porous dressing made of polyurethane foam placed adjacent a wound and covered by a semi-permeable and flexible plastic sheet. The dressing further includes fluid supply and fluid drainage connections in communication with the cavity formed by the cover and foam. The fluid supply is connected to a fluid source that can include an aqueous topical antibiotic solution or isotonic saline for use in providing therapy to the wound. The fluid drainage can be connected to a vacuum source where fluid can be removed from the cavity and subatmospheric pressures can be maintained inside the cavity. The wound irrigation apparatus, although able to provide efficacious therapy, is somewhat cumbersome, difficult to use, and generally impractical. Such a device does not address various factors concerning patients, specifically ease of use, portability and the ability to provide therapy with a minimum amount of unwanted mechanical noise. 
         [0005]    Other devices use vacuum sealing of wound dressings consisting of polyvinyl alcohol foam cut to size and stapled to the margins of the wound. The dressings are covered by a semi-permeable membrane while suction and fluid connections are provided by small plastic tubes introduced subcutaneously into the cavity formed by the foam and cover. Such devices alternate in time between vacuum drainage and the introduction of aqueous medicaments to the wound site. Such devices also fail to address portability, ease of use and noise reduction. 
         [0006]    Therapeutic negative pressure wound healing devices or vacuum assisted continuous wound irrigation devices require a control mechanism to maintain vacuum at a desired predetermined level. Typically, these control systems rely on a (negative) pressure sensor of some type that converts the measured pressure to an electrical signal that can be utilized by control circuits to maintain a preset level. Many sensors use an electrical strain-gauge technology that produces a voltage signal in proportion to applied vacuum. Other sensors are electromechanical in nature and produce a changing resistance in proportion to applied vacuum. Still other sensors are mechanical switches that are off when vacuum is above a predetermined level, and on when vacuum is below a predetermined level. In any case, in order to efficiently maintain the vacuum of a suction wound therapy device, some type of electrical or mechanical sensor is necessary as part of a control loop. 
         [0007]    The cost of the pressure sensor can be a significant percentage of the overall cost of the product. While these sensors are readily available and well known, they are also relatively expensive. Typically electronic sensors such as the Motorola MPX5050 cost approximately $15 in single piece quantities. Similarly, purely mechanical pressure switches, such as those available from AirLogic, cost between $18 and $25 in single piece quantities. 
       SUMMARY OF THE INVENTION 
       [0008]    An embodiment of the invention includes a pressure housing configured to be coupled to a vacuum source and a diaphragm sealingly coupled to the pressure housing. The diaphragm is configured to move in response to the vacuum source. A magnet is coupled to the diaphragm. A magnetic switch is disposed opposite the magnet and is configured to be actuated by the magnet when the magnet is a predetermined distance from the magnetic switch. The magnetic switch is configured to selectively actuate the vacuum source. 
         [0009]    One embodiment of the invention is directed to a wound irrigation system using an electromechanical vacuum apparatus that includes a microprocessor-based device having stored thereon software configured to control the electromechanical vacuum apparatus. A first vacuum pump is electrically associated with the microprocessor and is capable of generating a vacuum. An optional second vacuum pump is electrically associated with the microprocessor and is capable of maintaining a predetermined vacuum level. A first electronic vacuum-pressure sensor is operably associated with the vacuum pump(s) and said microprocessor for monitoring vacuum level. A fluid-tight collection canister includes an integrated barrier to prevent contents from escaping the canister. Canulated tubing is associated with the canister and vacuum pump(s) for communicating vacuum pressure therefrom. A second electronic vacuum-pressure sensor is operably associated with the canister and the microprocessor for monitoring canister vacuum. A dressing includes of a porous material and semi-permeable flexible cover, Canulated tubing is associated with the dressing and the canister to communicate vacuum pressure therefrom. An irrigation vessel contains a fluid to be used in irrigating the wound. Canulated tubing is associated with the irrigation vessel and the dressing to communicate fluid thereto. The electromechanical vacuum apparatus has an integrated compartment that can hold the irrigation vessel. The electromechanical vacuum apparatus may optionally include a device for regulating the quantity of fluid flowing from said irrigation vessel to said dressing. The electromechanical vacuum apparatus may include batteries enabling portable operation thereof. 
         [0010]    An embodiment of the invention includes a method for improving the generation and control of a therapeutic vacuum. In this embodiment, a multi-modal algorithm monitors pressure signals from a first electronic vacuum-pressure sensor associated with a vacuum pump and capable of measuring the output pressure from the pump. The algorithm further monitors pressure signals from a second electronic vacuum-pressure sensor associated with a collection canister and capable of measuring the subatmospheric pressure inside the canister. The canister is connected to the vacuum pump by a canulated tube that communicates subatmospheric pressure therefrom. The canister is connected to a suitable dressing by a canulated tube that communicates subatmospheric pressure thereto. At the start of therapy, both the first and second electronic vacuum-pressure sensors indicate the system is equilibrated at atmospheric pressure. A first-mode control algorithm is employed to remove rapidly the air in the canister and dressing, and thus create a vacuum. The first-mode implemented by the control algorithm is subsequently referred to herein as the “draw down” mode. Once the subatmospheric pressure in the canister and dressing have reached a preset threshold as indicated by the first and second electronic vacuum-pressure sensors respectively, the algorithm employs a second-mode that maintains the desired level of subatmospheric pressure in both the canister and the dressing for the duration of the therapy. The second-mode implemented by the control algorithm is subsequently referred to herein as the “maintenance” mode. The second-mode control algorithm is configured to operate the vacuum pump at a reduced speed thus minimizing unwanted mechanical noise. In an alternative embodiment, a second vacuum pump can be used for the maintenance mode, which has a reduced capacity, is smaller, and produces significantly lower levels of unwanted mechanical noise. The second-mode control algorithm is configured to permit the maintenance of vacuum in the presence of small leaks, which invariably occur at the various system interfaces and connection points. The method can be performed by, for example, a microprocessor-based device. 
         [0011]    In another embodiment application-specific dressings are configured according to the individual needs of varying wound types. A myriad of new materials that broadly fall into the categories of antibacterial, biodegradable, and bioactive can be used to create highly efficacious wound dressings. For a material to function with a wound irrigation and vacuum drainage system, the dressing composition can be porous enough to permit the uniform distribution of subatmospheric pressure throughout the dressing and subsequently to facilitate the removal of fluids therethrough. In addition, the dressings possess various mechanical properties that can create the proper macro-strain and micro-strain on the wound bed believed to contribute to the production of growth factors and other cytokines that promote wound healing. Accordingly, some embodiments include several dressing arrangements that use, for example, the aforementioned materials to produce dressings for specific wound types. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic block diagram of an embodiment of the invention for providing wound irrigation and vacuum drainage. 
           [0013]      FIG. 2  is a flow diagram for a method according to an embodiment of the invention. 
           [0014]      FIG. 3  is an illustration of a maintenance-mode control circuit according to an embodiment of the invention. 
           [0015]      FIG. 4  is an illustration of a maintenance-mode control circuit according to another embodiment of the present invention. 
           [0016]      FIGS. 5A-5C  are illustrations of a device according to an embodiment of the invention for providing portable wound irrigation and vacuum drainage. 
           [0017]      FIG. 6  is a second illustration of a device according to an embodiment of the invention for providing portable wound irrigation and vacuum drainage. 
           [0018]      FIG. 7  is a third illustration of a device according to an embodiment of the invention for providing portable wound irrigation and vacuum drainage. 
           [0019]      FIG. 8  is an illustration of an application-specific dressing according to an embodiment of the invention incorporating an antibiotic silver mesh between the dressing substrate and wound. 
           [0020]      FIG. 9  is an illustration of an application-specific dressing according to an embodiment of the invention incorporating biodegradable materials in the dressing. 
           [0021]      FIG. 10  is an illustration of an application-specific dressing according to an embodiment of the invention incorporating bioactive materials in the dressing. 
           [0022]      FIG. 11  is a schematic illustration of a method and control system for maintaining a desired preset vacuum level in a medical device according to an embodiment of the invention. 
           [0023]      FIG. 12  is an example of a graphical representation of the relationship between pump motor current, pump air flow and vacuum level of the system illustrated in  FIG. 11 . 
           [0024]      FIG. 13  is a schematic representation of a control system switch according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Although those of ordinary skill in the art will readily recognize many alternative embodiments, especially in light of the illustrations provided herein, this detailed description is of an embodiment of the invention, the scope of which is defined only by the claims appended hereto. 
         [0026]    As illustrated in  FIG. 1 , a wound irrigation and vacuum drainage system is referred to by the numeral  100  and generally includes a microcontroller  101  having an embedded microprocessor  102 , Random Access Memory (RAM)  103  and Read Only Memory (ROM)  104 . ROM  104  contains the programming instructions for a control algorithm  150  (see  FIG. 2 ). ROM  104  is non-volatile and retains its programming when the power is terminated. RAM  103  is utilized by the control algorithm for storing variables such as pressure measurements, alarm counts and the like, which the control algorithm  150  uses while generating and maintaining the vacuum. A membrane keypad and display  160  is electrically associated with microcontroller  101  through communication cable  164 . Membrane switches  161  provide power control and membrane switches  162  are used to preset the desired vacuum levels. Light emitting diodes (LEDs)  163  are provided to indicate alarm conditions associated with canister fluid level and dressing leaks. 
         [0027]    Microcontroller  101  is electrically associated with, and controls the operation of, a first vacuum pump  105  and an optional second vacuum pump  107  through electrical cables  106  and  108  respectively. First vacuum pump  105  and optional second vacuum pump  107  can be one of many types including, for example, the pumps sold under the trademarks Hargraves® and Thomas®. Vacuum pumps  105  and  107  can use, for example, a reciprocating diaphragm or piston to create vacuum and are typically powered by a D.C. motor that can also optionally use a brushless commutator for increased reliability and longevity. Vacuum pumps  105  and  107  are pneumatically associated with an exudate collection canister  114  through a single-lumen tube  115 . In one embodiment, canister  114  has a volume which does not exceed 1000 ml. This can prevent accidental exsanguination of a patient in the event hemostasis has not yet been achieved at the woundsite. Canister  114  can be of a custom design or one available off-the-shelf and sold under the trademark Medi-VAC®. In addition, a fluid barrier  129  is associated with canister  114  and is configured to prevent fluids collected in canister  114  from escaping into tubing  115  and fouling the vacuum return path. Barrier  129  can be of a mechanical float design or may have one or more membranes of hydrophobic material such as those available under the trademark GoreTex™. A secondary barrier  113  using a hydrophobic membrane is inserted inline with pneumatic tubing  115  to prevent fluid ingress into the system in the event barrier  129  fails to operate as intended. Pneumatic tubing  115  connects to first vacuum pump  105  and optional second vacuum pump  107  through “T” connectors  111  and  112  respectively. 
         [0028]    Vacuum-pressure sensor  109  is pneumatically associated with first vacuum pump  105  and optional vacuum pump  107  and electrically associated with microcontroller  101  through electrical cable  110 . Pressure sensor  109  provides a vacuum-pressure signal to the microprocessor  102  enabling control algorithm  150  to monitor vacuum pressure at the outlet of the vacuum pumps  105  and  107 . An acoustic muffler  128  is pneumatically associated with the exhaust ports of vacuum pumps  105  and  107  and is configured to reduce induction noise produced by the pumps during operation. In normal operation of irrigation system  100 , first vacuum pump  105  is used to generate the initial or “draw-down” vacuum while optional second vacuum pump  107  can be used to maintain a desired vacuum within the system compensating for any leaks or pressure fluctuations. Vacuum pump  107  can be smaller and quieter than vacuum pump  105  providing a means to maintain desired pressure without disturbing the patient. 
         [0029]    A battery  127  is optionally provided to permit portable operation of the wound irrigation system  100 . Battery  127 , which can be Nickel-Metal-Hydride (NiMH), Nickel-Cadmium, (NiCd) or their equivalent, is electrically associated with microcontroller  101  through electrical cables  136  and  137 . Battery  127  is charged by circuits related with microcontroller  101  while an external source of power is available. When an external source of power is not available and the unit is to operate in a portable mode, battery  127  supplies power to the wound irrigation system  100 . 
         [0030]    A second pressure sensor  116  is pneumatically associated with canister  114  through a single-lumen tube  119 . Pressure sensor  116  is also electrically associated with microcontroller  101  and provides a vacuum-pressure signal to microprocessor  102  enabling control algorithm  150  to monitor vacuum pressure inside canister  114  and dressing  123 . A “T” connector  118  is connected pneumatic tube  119  to pressure sensor  116  and a vacuum-pressure relief solenoid  120  configured to relieve pressure in the canister  114  and dressing  123  in the event of an alarm condition, or if power is turned off. Solenoid  120 , can be, for example, one available under the trademark Pneutronics®; Solenoid  120  is electrically associated with, and controlled by, microprocessor  101  through electrical cable  130 . Solenoid  120  is configured to vent vacuum pressure to atmosphere when the electrical coil is de-energized as would be the case if the power is turned off. An orifice restrictor  121  is provided inline with solenoid  120  and pneumatic tube  119  to regulate the rate at which vacuum is relieved to atmospheric pressure when solenoid  120  is de-energized. Orifice restrictor  121  is, for example, available under the trademark AirLogic®. 
         [0031]    A wound dressing  123  includes a sterile porous substrate  131 , which can be a polyurethane foam, polyvinyl alcohol foam, gauze, felt or other suitable material, a semi-permeable adhesive cover  132  such as that sold under the trademark Avery Denison®, an inlet port  134  and a suction port  135 . Dressing substrate  131  is configured to distribute evenly the vacuum pressure throughout the entire wound bed and has mechanical properties suitable for promoting the formation of granular tissue. In addition, when vacuum is applied to dressing  123 , substrate  131  creates micro- and macro-strain at the cellular level of the wound stimulating the production of various growth factors and other cytokines and promoting cell proliferation. Dressing  123  is fluidically associated with canister  114  through a single-lumen tube  122 . The vacuum pressure in the cavity formed by substrate  131  of dressing  123  is largely the same as the vacuum pressure inside canister  114  minus the weight of any standing fluid inside tubing  122 . A fluid vessel  124 , which can be a standard I.V. bag, contains medicinal fluids such as aqueous topical antibiotics, physiologic bleaches, or isotonic saline. Fluid vessel  124  is removably connected to dressing  132  though port  134  and single-lumen tube  125 . An optional flow control device  126  can be placed inline with tubing  125  to permit accurate regulation of the fluid flow from vessel  124  to dressing  123 . In normal operation, continuous woundsite irrigation is provided as treatment fluids move from vessel  124  through dressing  123  and into collection canister  114 . This continuous irrigation keeps the wound clean and helps to manage infection. In addition, effluent produced at the woundsite and collected by substrate  131  will be removed to canister  114  when the system is under vacuum. 
         [0032]    Referring to  FIG. 2 , an example of the general processing steps of algorithm  150  are illustrated. Algorithm  150  includes a continuously executing “Main Loop”  270  having six functional software modules: Initialization module  210 , Check Membrane Switches module  220 , Update Display module  230 , Update Vacuum Control module  240 , Check for Alarms (full canister, leak, internal) module  250 , and Reset Watchdog Timer module  260 . 
         [0033]    At initialization step  210 , all the variables associated with the operation of the control algorithm  150  are reset. The initialization step  210  can execute, for example, when power is applied to the system. The variables that can be reset include, for example, alarm flags, alarm time counters, pressure targets, pressure limits and internal variables used for storing mathematical calculations. 
         [0034]    At step  220 , the algorithm  150  checks for any user input via the membrane keypad. At step  221 , any keypresses are checked. At step  222 , all therapy-related parameters are updated. For example, a user may press the vacuum-level-preset switch  162  which would be detected at step  221 . The new target pressure selected by the user would then be stored as a therapy parameter in step  222 . If no keys are pressed, or once the therapy parameters have been updated subsequent any key press, algorithm  150  updates the display at step  230 . 
         [0035]    At step  230 , all status LED&#39;s are updated including any alarm indications that may have been identified in the previous pass through the main loop  270 . 
         [0036]    At step  240 , algorithm  150  monitors and updates control of the vacuum pump(s)  105  and  107 , and vent solenoid  120 . At step  241 , the actual pressure at the pump(s)  105  and  107  and the canister  114  is read via electronic vacuum-pressure sensors  109  and  116 , respectively. These analog readings are digitized and stored for use on the next pass through main loop  270 . At step  242 , vacuum limits and targets are selected based on the pre-determined therapy parameters identified in step  220 . At step  243 , a decision is made regarding in which mode the pump(s) will be operated. If the first-mode is selected at step  243 , algorithm  150  will operate vacuum pump  105  at full-power minimizing the time to remove the air from canister  114  and dressing  132 . If the second-mode is selected at step  243 , algorithm  150  will operate vacuum pump  105  at partial-power providing just enough airflow to keep up with any leaks in the system as described in detail earlier. In this mode, pump  105  operates very quietly and would not disturb the patient. Alternatively, and described in more detail hereinbelow, an optional pump  107  can be utilized in conjunction with pump  105  during second-mode operation. In this embodiment, pump  107  is smaller and quieter than pump  105  and has reduced airflow capacity. Pump  107  is configured to provide just enough airflow to compensate for system leaks or other loss of vacuum. 
         [0037]    Once the mode is selected at step  243 , algorithm  150  produces electronic control signals that turn the vacuum pump(s)  105  and  107  on or off at step  244 . In addition, and as described in detail hereinabove, a solenoid valve  120  vents vacuum pressure to atmosphere when power is terminated, or in the event vacuum pressure exceeds the preset limits established at step  242 . At step  245 , the control signals are provided and are based on comparisons between actual pressure, target pressure and the preset high-pressure limit. Mode determination, vacuum pump control, and vent control are all based on comparisons between the pre-selected target pressure levels and actual pressure readings obtained at steps  241  and  242 , respectively. 
         [0038]    After pressure adjustments are made and the actual pressure readings obtained at step  240 , the algorithm  150  checks for alarm conditions at step  250 . At step  251 , leak conditions, which are readily identified by analyzing the readings from pressure sensors  109  and  116 , are identified. If a leak condition is detected at step  251 , the algorithm  150  waits three minutes before flagging the leak alarm and alerting the user at step  230  during the next pass through main loop  270 . At step  252 , a full canister condition is checked, again easily identified by analyzing the readings from pressure sensors  109  and  116 . If a full canister condition is detected at step  252 , the algorithm  150  waits one minute before flagging the full canister alarm and alerting the user at step  230  during the next pass through main loop  270 . At step  253 , the readings from pressure sensors  109  and  116  are examined to determine if any internal errors exist. An internal error would occur if one pressure sensor indicated a pressure reading, for example, 30 mmHg higher or lower than the other sensor. Again, if the internal error condition is detected at step  253 , the algorithm  150  waits two minutes before flagging the internal error alarm and alerting the user at step  230  during the next pass through main loop  270 . 
         [0039]    After completion of steps  220 ,  230 ,  240  and  250 , algorithm  150  will reset the watchdog timer at step  260 . The watchdog timer is provided as a safety feature in the event of an unanticipated software glitch and is incorporated within embedded microprocessor  102 . In the event control algorithm  150  “locks up”, main loop  270  would no longer function. When main loop  270  ceases to function, the hardware watchdog timer would not be reset at step  260  and would therefore timeout. Once the watchdog timer has timed-out, it will automatically reset embedded microprocessor  102  and algorithm  150  will re-initialize all variables and parameters at step  210 . Subsequent to the re-initialization, algorithm  150  would again sequentially execute the modules as described above via main loop  270 . 
         [0040]    Referring now particularly to  FIG. 3 , an example of a linear control circuit associated with vacuum pump(s)  105  and  107  includes a control input  301 , which is a digital signal provided by microcontroller  101 . Digital control input  301  is associated with the second-mode described above. When digital control input  301  is in its low or off state, diode  304  becomes forward biased and subsequently discharges capacitor  303 . After a short period of time, the voltage across capacitor  303  trends towards zero and the capacitor is substantially fully discharged. When digital control input  301  is in its high or on state, diode  304  becomes reverse biased and is effectively removed from the circuit. In this case, with said second-mode activated, resistor  302 , which is in series with capacitor  303 , will begin to charge capacitor  303  at a rate determined by the values of both components and proportional to 1/R*C. After approximately 1/R*C seconds have elapsed, capacitor  303  becomes fully charged and no additional current will flow through resistor  302 . The voltage across capacitor  303  will be approximately equal to the magnitude of the digital control input  301  voltage. The junction of resistor  302  and capacitor  303  is connected to the base terminal of an NPN bi-junction transistor  305 . Transistor  305  can be, for example, a TIP-32C. Transistor  305  is configured as an emitter follower and in this arrangement will provide current amplification. The positive terminal of vacuum pump(s)  105  and  107  is connected to the emitter terminal of transistor  305  while the collector terminal of transistor  305  is connected directly to the 12-volt power supply  307 . An additional capacitor  306  is provided to prevent unwanted transients on the power supply caused by the inductive loading of vacuum pump(s)  105  and  107 . The negative terminal of vacuum pump(s)  105  and  107  and the negative terminal of capacitor  303  are connected to the common ground reference point  308 . 
         [0041]    When the digital control input  301  transitions from its low-to-high state, the voltage across capacitor  303  begins to ramp-up slowly until reaching a maximum 1/R*C seconds later. Because of the configuration of transistor  305 , the voltage rise at the emitter terminal will mirror the voltage rise at the base terminal, thus the voltage supplied to vacuum pump(s)  105  and  107  will also slowly ramp-up until reaching a maximum 1/R*C seconds later. As the voltage supplied to the pump(s) increases, the pump(s) will operate faster and thus produce more outflow and increased vacuum. Since the time constant is selectable by choosing appropriate values for resistor  302  and capacitor  303 , the rate at which the pumps begin to increase speed can be pre-selected and can permit operation at a slower and quieter speed for an extended period of time. As the pump(s)  105  and  107  begin to increase their outflow, vacuum in the system  100  is increased. This increase is measured by algorithm  150 , which subsequently changes the state of digital control input  301  in response thereto. As described in detail above, once target pressure has been re-established, the pump(s)  105  and  107  will be shut off. As the digital control input  301  transitions from its high-to-low state after target pressure is met, diode  304  rapidly discharges capacitor  303  as described earlier, and the voltage supplied to pump(s)  105  and  107  is effectively removed turning the pump(s) off. 
         [0042]    Referring now particularly to  FIG. 4 , an example of a Pulse Width Modulation (PWM) control circuit  400  associated with vacuum pump(s)  105  and  107  includes an a stable multivibrator circuit  401  configured with a duty-cycle that can be varied from approximately 10 to 90 percent. Multivibrator circuit  401  can be, for example, an LM555, and is referred to further herein as “Timer”  401 . A 12-volt power supply  417  provides electrical power to timer  401  and vacuum pump(s)  105  and  107 . Capacitor  414  is connected between the power supply  417  and the common ground point  414 . Capacitor  414  functions to remove transients from the power supply  417  due to inductive loading produced by the operation of pump(s)  105  and  107 . In some embodiments of the invention, vacuum pump(s)  105  and  107  have three terminals—a positive and negative terminal for power, and a third terminal  416  that is the PWM control input. The positive terminal of pump(s)  105  and  107  connects to the power supply  417 . The negative terminal connects to the drain lead of a MOSFET  402 , such as, for example, an IRF  510 , commonly available and sold under the trademark International Rectifier®. The source lead of MOSFET  402  connects to the common ground point  414 . MOSFET  402  switches the power on and off to pump(s)  105  and  107  in response to a control input  412 . The signal from control input  412  is provided by microcontroller  101  and acts in conjunction with mode-select signal  411 . A resistor  413  is connected between the gate of MOSFET  402  and common ground point  414  and provides ground reference for the gate of MOSFET  402  and drive impedance for control input  412 . 
         [0043]    Timer  401  has several peripheral components that control the frequency of operation and the duty-cycle of the output waveform. Capacitor  408  stabilizes an internal voltage reference and keeps the output frequency constant. Diodes  405  and  406  charge and discharge capacitor  407  through resistors  403  and  404 . Resistor  404  and capacitor  407  determine the output frequency while variable resistor  403  determines the duty-cycle and can be adjusted from 10 to 90 percent. Typically the output frequency would be between 10 kilohertz and 20 kilohertz to minimize switching noise as these frequencies are above the nominal range of human hearing. The output of timer  401  is used as the PWM input  416  and varies the motor speed of pump(s)  105  and  107  in proportion to duty-cycle. A high duty-cycle causes the pump motor to run faster and produce greater outflow while a low duty-cycle causes the pump motor to run slower and quieter with an associated reduction in outflow. 
         [0044]    A digital-mode signal from mode select  411  indicating the second mode, which enables selection of said first-mode or said second-mode, is provided to capacitor  407  through diode  409 . When the mode-select signal from mode select  411  transitions from a high to low state, diode  409  is forward biased and rapidly discharges capacitor  407 . When capacitor  407  is in its discharged state, the PWM signal  416  generated by timer  401  is forced high. A constant, high PWM is equivalent to a 100% duty-cycle and thus pump(s)  105  and  107  run at maximum in this configuration. As mode-select signal from mode select  411  transitions from a low to high state, diode  409  is reverse biased and therefore effectively removed from the circuit. Timer  401  then operates in an astable mode producing a reduced duty-cycle PWM signal  416 . Resistor  410  is connected between mode select input  411  and common ground point  414  to provide drive impedance for microcontroller  101 . 
         [0045]    When control algorithm  150  determines that the first-mode (draw-down) is required such as when the system is initializing and drawing-down the dressing, mode select signal from mode select  411  will be in a low state while control-input signal from control input  412  will be in a high state. This configuration will cause vacuum pump(s)  105  and  107  to produce the greatest amount of outflow. Likewise when control algorithm  150  determines that said second-mode (maintenance) is required such as when the measured therapeutic vacuum level dips below the predetermined low-limit, mode-select signal from mode select  411  will be in a high state while control-input signal from control input  412  will be in a high state. This configuration will cause vacuum pump(s)  105  and  107  to operate at a slower speed producing reduced outflow and reduced unwanted mechanical noise while simultaneously restoring therapeutic vacuum to the target level. If control-input signal from control input  412  is in a low state, the pump(s) are disabled and do not operate at all. This acts as a safety feature in the event of a component failure that causes pump(s)  105  and  107  to latch in an on-state. 
         [0046]    Referring now particularly to  FIG. 5A , another embodiment of a portable system  500  for providing therapeutic wound irrigation and vacuum drainage is illustrated. System  500  includes a self-contained plastic housing  501  configured to be worn around the waist or carried in a pouch over the shoulder for patients who are ambulatory, and hung from the footboard or headboard of a bed for patients who are non-ambulatory. A membrane keypad and display  504  is provided to enable the adjustment of therapeutic parameters and to turn the unit on and off. Depressing membrane switch  505  will turn the power to system  500  on while depressing membrane switch  506  will turn the power off. Membrane switch  509  adjusts the target therapeutic pressure up and likewise membrane switch  510  adjusts the target therapeutic pressure down. In some embodiments of the invention, system  500  has three pressure settings LOW, MEDIUM and HIGH which generally correspond to, for example, 70 mmHg, 120 mmHg and 150 mmHg, respectively. Although these three pressure settings are provided by way of example, they are not intended to be limiting because other pressures can be utilized for wound-type specific applications. Membrane LEDs LOW  522 , MEDIUM  523  and HIGH  524 , indicate the current target therapeutic setting of the unit. LED  507  indicates a leak alarm and LED  508  indicates a full-canister alarm. When either alarm condition is detected, these LEDs will light in conjunction with an audible chime. Housing  501  incorporates a compartment  502  that is configured in such a way as to receive and store a standard IV bag  503 . IV bag  503  may contain an aqueous topical wound treatment fluid that is utilized by system  500  to provide continuous irrigation. In some embodiments, the wound treatment fluid can be introduced directly into compartment  502 . Additionally, the IV bag  503  can be externally coupled to the device. As shown in  FIG. 5B , a belt clip  514  is provided for attaching to a patient&#39;s belt and an optional waist strap or shoulder strap is provided for patient&#39;s who do not wear belts. 
         [0047]    As shown in  FIG. 5C , an exudate collection canister  511  comprises a vacuum-sealing means  517  and associated hydrophobic filter  520  (not shown), vacuum sensor port  518  and associated hydrophobic filter  519  (not shown), frosted translucent body  521 , clear graduated measurement window  522 , locking means  523  and multilumen tubing  512 . Collection canister  511  typically has a volume less than 1000 ml to prevent accidental exsanguination of a patient. Vacuum sealing means  517  mates with a corresponding sealing means  516  that is incorporated in housing  501 . In addition, locking means  523  has corresponding mating components within said housing. Hydrophobic filters  519  and  520  can be, for example, those sold under the trademark GoreTex® and are ensured the contents of canister  511  do not inadvertently ingress housing  501  and subsequently cause contamination of the therapy device  500 . Vacuum sensor port  518  enables microcontroller  101  to measure the pressure within the canister  511  as a proxy for the therapeutic vacuum pressure under the dressing  131 . Multilumen tubing  512  provides one conduit for the irrigation fluid to travel to dressing  131  and another conduit for the vacuum drainage. Thus, IV bag  503 , tubing  512 , dressing  131  and canister  511  provide a closed fluid pathway. In this embodiment, canister  511  would be single-use disposable and may be filled with a gelling substance to enable the contents to solidify prior to disposal. Gelling agents are available, for example, under the trademark Isolyzer®. 
         [0048]    As shown in  FIG. 5A , at the termination of tubing  512 , a self-adhesive dressing connector  515  is provided for attaching the tubing to drape  132  with substantially air-tight seal. Dressing connector  515  can have an annular pressure-sensitive adhesive ring with a release liner that is removed prior to application. In actual use, a small hole  530  can be cut in drape  132  and dressing connector  515  would be positioned in alignment with said hole. This enables irrigation fluid to both enter and leave the dressing through a single port. In an alternative embodiment, tube  512  bifurcates at the terminus and connects to two dressing connectors  515  which allow the irrigation port to be physically separated from the vacuum drainage port thus forcing irrigation fluid to flow though the entire length of the dressing if it is so desired. 
         [0049]    Referring now to  FIG. 6 , and according to a further embodiment of the invention, a dressing system  600  for providing therapeutic wound irrigation and vacuum drainage is illustrated. Dressing system  600  includes a sterile porous substrate  131 , which can be fabricated from polyurethane foam, polyvinyl alcohol foam, gauze, felt or other suitable material; a semi-permeable adhesive cover  132  such as that sold under the trademark Avery Denison®; a single lumen drainage tube  122  for the application of vacuum and removal of fluids from the woundsite; and a pliable fluid vessel  601  situated between the semi-permeable cover  132  and the porous substrate  131 . Fluid vessel  601  comprises a self-sealing needle port  603  situated on the superior aspect of the vessel and a regulated drip port  602  situated on the inferior aspect of the vessel. Needle port  603 , permits the introduction of a hypodermic needle  604  for the administration of aqueous topical wound treatment fluids. These aqueous topical fluids can include antibiotics such as Bacitracin or Sulfamide-Acetate; physiologic bleach such as Chlorpactin or Dakins solution; and antiseptics such as Lavasept or Octenisept. Regulated drip port  602  permits fluid within vessel  601  to egress slowly and continuously into porous substrate  131  whereupon the therapeutic benefits can be imparted to the woundsite. Single-lumen drainage tube  122  provides enough vacuum to keep the dressing  600  at sub-atmospheric pressure and to remove fluids, which include the irrigation fluid and wound exudate. The advantage of dressing system  600  is the incorporation into the dressing of vessel  601  thus eliminating the need for an external fluid vessel and associated tubing and connectors making the dressing more user friendly for patient and clinician alike. 
         [0050]    In normal clinical use, dressing  600  is applied to the wound site by first cutting porous substrate  131  to fit the margins of the wound. Next, semi-permeable drape  132  with integrated (and empty) fluid vessel  601  is attached positioning drip port  602  central to the porous substrate  131 . Once the drape  132  is properly sealed around the periwound, a properly prepared hypodermic needle  604  can be inserted in self-sealing needle port  603 , and fluid vessel  601  subsequently can fill with the desired aqueous topical wound treatment solution. 
         [0051]    Referring now particularly to  FIG. 7 , and according to another embodiment of the invention, a dressing system  700  for therapeutic wound irrigation and vacuum drainage is illustrated. The system  700  includes a sterile porous substrate  131 , which can be fabricated from polyurethane foam, polyvinyl alcohol foam, gauze, felt or other suitable material; a semi-permeable adhesive cover  132  such as that sold under the trademark Avery Denison®; a single lumen drainage tube  122  for the application of vacuum and removal of fluids from the woundsite; and a pliable fluid vessel  601  situated outside and superior to said semi-permeable cover  132 . Fluid vessel  601  comprises a self-sealing needle port  603  situated on the superior aspect of the vessel and a regulated drip port  602  situated on the inferior aspect of the vessel. In addition, an annular adhesive ring is provided on the inferior aspect of vessel  601  surrounding regulated drip port  602  for subsequent attachment to drape  132 . Needle port  603  permits the introduction of a hypodermic needle  604  for the administration of aqueous topical wound treatment fluids. These aqueous topical fluids can include antibiotics such as Bacitracin or Sulfamide-Acetate; physiologic bleach such as Chlorpactin or Dakins solution; and antiseptics such as Lavasept or Octenisept. Regulated drip port  602  permits fluid within vessel  601  to egress slowly and continuously into porous substrate  131  through a hole in drape  132  whereupon the therapeutic benefits can be imparted to the woundsite. Single-lumen drainage tube  122  provides enough vacuum to keep the dressing  600  at sub-atmospheric pressure and to remove fluids which include the irrigation fluid and wound exudate. 
         [0052]    In normal clinical use, dressing  700  is applied to the wound site by first cutting porous substrate  131  to fit the margins of the wound. Next, semi-permeable drape  132  is applied over the woundsite covering the substrate  131  well into the periwound area. A hole approximately ¼″ diameter is made in drape  132  central to porous substrate  131 . Lastly, fluid vessel  601  is attached by adhesive annular ring  605  with drip port  602  aligned with the hole previously cut in drape  132 . Once the fluid vessel  601  is properly sealed to the drape  132 , a properly prepared hypodermic needle  604  is inserted in self-sealing needle port  603  and fluid vessel  601  subsequently filled with the desired aqueous topical wound treatment solution. 
         [0053]    Referring now particularly to  FIG. 8 , an embodiment of an application-specific dressing  800  of the invention is illustrated. The dressing  800  includes a sterile porous substrate  131 , which can be fabricated from polyurethane foam, polyvinyl alcohol foam, gauze, felt or other suitable material; a semi-permeable adhesive cover  132  such as that sold under the trademark Avery Denison®; a single lumen drainage tube  122  for the application of vacuum and removal of fluids from the woundsite; single lumen irrigation tube  125  to facilitate the application of aqueous topical wound fluids to a wound bed  801 ; and a perforated woven cloth impregnated with metallic silver  810  and bonded to porous substrate  131 , for providing an antibiotic action within the wound. Alternatively, and as depicted in  FIG. 8 , an integrated dressing connector  515  can be used with multi-lumen tubing  512  permitting the wound irrigation and vacuum drainage system to fluidically communicate with dressing  800 . 
         [0054]    Antibiotic silver layer  810  is fenestrated to permit the unimpeded removal of fluids from the wound bed  801  through the substrate  131  and subsequently through vacuum drainage tubing  122  or  512 . In addition, fenestrations in layer  810  permit the even distribution of sub-atmospheric pressure across the wound bed  801  and permit granular tissue formation. Use of silver in a wound as part of a wound dressing is available to the clinician under the trademark(s) Acticoat™ and Silvadene™ and others. Silver can be utilized specifically for burns, sternotomy, radiated fistulas, traumas, and open fractures. Silver is utilized in treating multiple resistant staph aureus (MRSA), preventing odor, reducing incidence of infection and to promote general healing. This embodiment combines the use of silver with wound irrigation and vacuum drainage to provide therapy to the specific wound types identified hereinabove. Antibiotic silver layer  810  can be made of a silver coated woven nylon such as that commercially available under the trademark SilverIon® from Argentum Medical. The material can be fabricated from woven nylon coated with 99.9% pure metallic silver utilizing a proprietary autocatalytic electroless chemical (reduction-oxidation) plating technology. Alternatively, a non-woven material such as ActiCoat® Foam from Smith and Nephew, uses two rayon/polyester non-woven inner cores laminated between three layers of High Density Polyethylene (HDPE) Mesh. This material, like the Silverlon® material, can also be fenestrated and used with dressing  800 . The antibiotic layer  810  is bonded to porous substrate  131  using a number of available techniques including: in-mold binding, adhesives (such as methyl methacrylate—based bonding agents), and RF or Ultrasonic welding. 
         [0055]    Dressing  800  is applied to the wound as described in detail hereinabove. Because of the potential chemical interactions between the various materials used in the construction of dressing  800 , attention can be paid to the types of aqueous topical wound fluids used to ensure compatibility. 
         [0056]    Referring now particularly to  FIG. 9 , another embodiment of an application-specific dressing  900  is illustrated. The dressing  900  includes a sterile porous substrate  910 , which can be fabricated from polyurethane foam, polyvinyl alcohol foam, gauze, felt or other suitable material; a semi-permeable adhesive cover  132  such as that sold under the trademark Avery Denison®; a single-lumen drainage tube  122  for the application of vacuum and removal of fluids from the woundsite; single-lumen irrigation tube  125  to facilitate the application of aqueous topical wound fluids to a wound bed  801 ; and a sterile porous layer of biodegradable material  910  bonded to porous substrate  920 , for providing an inducement to healing within the wound. Biodegradable layer  910  is placed substantially within the wound site and is in intimate contact with wound bed  801 . Biodegradable layer  910  can be made from myriad materials such as polylactide-co-glycolic acid (PLGA). Alternatively, and as depicted in  FIG. 9 , an integrated dressing connector  515  can be used with multi-lumen tubing  512  permitting the wound irrigation and vacuum drainage system to fluidically communicate with dressing  900 . 
         [0057]    Biodegradable layer  910  is porous with similar mechanical characteristics to substrate  920  to permit the unimpeded removal of fluids from the wound bed  801  through the substrate  920  and subsequently through vacuum drainage tubing  122  or  512 . In addition, porosity in layer  910  permits the even distribution of sub-atmospheric pressure across the wound bed  801  and encourages granular tissue formation into layer  910 . Biodegradable layer  910  is bonded to substrate  920  in such a way that it will readily release from substrate  920  when the dressing is removed from the wound so that the biodegradable layer  910  remains in place and provides a matrix through which tissue growth can occur. The adhesives for removably bonding layers  910  and  920  include, for example, cured silicones, hydrogels and/or acrylics. The thickness of layer  910  can be selected such that ingrowth, which can be as much as 1 mm per day for a typical wound, will not entirely infiltrate layer  910  and invade the removable substrate  920 . Alternatively, biodegradable layer  910  can be made up of a matrix of beads adhered together with the same kinds of releasable bonding agents discussed in detail above. 
         [0058]    Dressing  900  is suited for wound types that have large defects or voids, which require rapid filling of tissue to provide a foundation for re-epithelialization in the final stages of healing. These application-specific wounds include necrotizing fasciitis, trauma, and iatrogenic wounds such as would occur with certain oncological procedures. In addition to addressing soft tissue repairs, dressing  900  can be configured to heal large bone defects such as those that result from surgical treatment of osteocarcinoma, and trauma where significant bone loss occurs. For these types of wounds, biodegradable layer  910  would be made of a rigid material that would serve as a matrix to encourage osteoblast invasion and bone growth into the defect. As described above, the material that makes up layer  910  would remain in the wound after the dressing is removed. 
         [0059]    Dressing  900  can be applied as described above in the previous embodiments; the only significant difference being that during dressing changes, the biodegradable portion, layer  910 , would remain in the wound. With a conventional dressing change, typically all the dressing material and debris would be removed to prevent possibility of foreign body reaction and infection. Here, subsequent dressing would be applied over the previous dressing&#39;s biodegradable layer  910  facilitating tissue grown therein. Once a suitable foundation of granular tissue has formed in the wound, the clinician would discontinue use of the biodegradable dressing substituting instead one of the other dressing materials and configurations disclosed hereinabove until the wound was completely healed. 
         [0060]    Referring now particularly to  FIG. 10 , an embodiment of an application-specific dressing  1000  is illustrated. The dressing  1000  includes a sterile porous substrate  1030 , which can be fabricated from polyurethane foam, polyvinyl alcohol foam, gauze, felt or other suitable material; a semi-permeable adhesive cover  132  such as that sold under the trademark Avery Denison®; a single-lumen drainage tube  122  for the application of vacuum and removal of fluids from the woundsite; single-lumen irrigation tube  125  to facilitate the application of aqueous topical wound fluids to a wound bed  801 ; a sterile porous layer of biocompatible material  1020  releasably bonded to porous substrate  1030 ; and an autologous graft layer  1010  integrated with biocompatible material  1020  for stimulating a healing response in a wound. Biocompatible layer  1020  and autologous graft layer  1010  are placed substantially within the wound site with autologous graft layer  1010  in intimate contact with wound bed  801 . Alternatively, and as depicted in  FIG. 10 , an integrated dressing connector  515  can be used with multi-lumen tubing  512  permitting the wound irrigation and vacuum drainage system to fluidically communicate with dressing  1000 . 
         [0061]    Biocompatible layer  1020  can be an acellular dermal matrix manufactured from donated human skin tissue, which is available under the trademark AlloDerm® from LifeCell Inc. This dermal matrix has been processed to remove all the cells that lead to tissue rejection while retaining the original biological framework. Cells taken from the patient or other molecules can subsequently be seeded into this matrix forming layer  1010 . These cells or molecules can include but are not limited to: fibroblasts, platelet derived growth factor (PDGF), Transforming Growth Factor Alpha (TGF-α), Transforming Growth Factor Beta (TGF-β) and other cytokines. PDGF is a polypeptide hormone derived from platelets, which stimulate fibroblasts to migrate and lay down collagen and fibronectin thereby initiating wound repair. If targeted cells are taken from the patient and seeded into biocompatible layer  1020  forming layer  1010 , the body will not reject them. In addition to seeding the inferior aspect of layer  1020  with the above described autologous cells or molecules, the superior aspect of layer  1020  can be seeded with live dermal cells taken from the patient using a mesh graft or micrografting technique. The configuration of two graft layers  1010  enclosing a biocompatible layer  1020  permits intrinsic tissue regeneration in such a way as to minimize the formation of scar tissue and maintain original structure. 
         [0062]    Dressing  1000  is designed for wound types that require reconstruction where the newly regenerated tissue has cellular structure similar to the original tissue. These application-specific wounds include surgical dehiscence, burns, and diabetic ulcers. 
         [0063]    In normal clinical use, the dressing  1000  would be prepared on a patient-by-patient basis first by harvesting the requisite cells from donor sites followed by processing (when necessary to derive bioactive components) then seeding the cells or cytokines into the biocompatible layer  1020 . Special care and handling can be used in the preparation of dressing  1000  to promote preservation of the bioactive components and maintenance of the sterility of the dressing. Once the dressing has been properly configured for the patient, it is applied as described in detail hereinabove. When dressing changes occur, biocompatible layer  1020  and autologous graft layer  1010  will remain in the wound much like the biodegradable dressing  900  also described in detail above. 
         [0064]    Referring now to  FIG. 11 , a pressure monitoring and control system  1100  comprises a direct current (D.C.) power supply  1101  (e.g., 12 Volts), a diaphragm-type D.C. vacuum pump  1102 , an electronic switching control element  1104 , a shunt resistor  1105 , which provides an indication of pump motor current draw, and a return path contact point  1103 . System  1100  also includes an analog amplifier  1106 , A/D converter  1107  and CPU  1108 . CPU  1108  acquires and stores the pressure signal  1109  and provides a control signal  1110  which turns pump motor  1102  on and off to maintain a preset pressure level. 
         [0065]    Control element  1104  can be, for example, a Field Effect Transistor (FET) switch or the like such as the ZVN-4306A available under the trademark ZETEX. This device turns the pump motor  1102  on and off in response to a control signal  1110 , which connects to the GATE terminal of control element  1104 . When control element  1104  is turned on, current begins to flow through pump motor  1102 . This current relates to the amount of work pump motor  1102  is performing with respect to the required negative pressure setting of the therapy unit. As vacuum level increases, the amount of work pump motor  1102  is performing also increases and the required current draw increases. 
         [0066]    Referring now to  FIG. 12 , a graphical representation of the relationship between pump motor current, pump air flow and vacuum level, a pump motor current curve  1111  is shown as well as two reference points  1112  and  1113 . Reference point  1112  represents the current flow when the vacuum pressure is 0 in Hg (0 mmHg) and reference point  1113  represents the current flow when the vacuum pressure is 4.72 in Hg (120 mmHg). While the pump-motor current curve  1111  is non-linear over its entire range, it is relatively linear between points  1112  and  1113 , which represents a typical therapy range. In this range, the curve  1111  is said to be ‘piecewise linear’ and a direct relationship exists between the vacuum pressure produced and the pump motor current required to produce it. In this case, for this particular vacuum pump, available from Hargraves Technical Corp., the current draw at 0 mmHg (when the pump is first turned on for example) is 150 milliamps. When the vacuum pressure reaches 4.72 in Hg (120 mmHg), the current draw is approximately 224 milliamps. Between these two pressure levels, the current draw of the vacuum pump motor  1102  varies linearly between 150 milliamps and 224 milliamps. This current measurement thus serves as a proxy for actual vacuum pump pressure and can be used in place of a pressure sensor to determine the system&#39;s therapeutic pressure. 
         [0067]    Referring again to  FIG. 11 , a shunt resistor  1105  is provided in series with the FET  1104  and vacuum pump motor  1102  to transform the pump motor  1102  current draw to a voltage. Resistor  1105  can be any desired value typically between 0.1 and 1 ohms and can be an off-the-shelf type. According to Ohm&#39;s law, the voltage across a resistor is equal to the current flowing through the resistor multiplied by the resistance. In this case, resistor  1105  has a resistance of 1 ohm. Thus if one amp were to flow through resistor  1105 , the resulting voltage across resistor  1105  would be 1 volt. Likewise, a current flow of 150 milliamps through resistor  1105  will produce 150 millivolts across it while a current flow of 224 milliamps will produce 224 millivolts. Amplifier  1106  is provided to enlarge the voltage across resistor  1105  to levels more suitable for digital conversion and analysis. In this case, a gain of 10 would cause the voltage signal from resistor  1105  to swing from 1.5 to 2.24 volts. The 1.5 volts output would correspond to a vacuum pump motor current draw of 150 milliamps (0 mmHg) and the 2.24 volt output would correspond to a vacuum pump motor current draw of 224 milliamps (120 mmHg). Thus, the pressure level of the system can be ascertained by the linear relationship between the output signal of amplifier  1106  and the pressure. A/D converter  1107  digitizes the pressure signal and transmits this digital representation to CPU  1108 . In many CPUs available off-the-shelf, the A/D converter is an integral part of the CPU and would not need to be implemented with external components. 
         [0068]    One or more control algorithms can be implemented in CPU  1108  to analyze the pressure signal  1109  and provide control output signal  1110  therefrom. A simple example of a control method could include measuring signal input  1109  and comparing it with a predetermined level such as 2.24. When the signal  1109  is lower than 2.24, the output signal  1110  switches to a high logic state turning on FET  1104  and pump motor  1102 . As vacuum in the system increases, the current draw from pump motor  1102  increases and the signal input  1109  increases. Once signal  1109  reaches 2.24 (indicating 120 mmHg), the output signal  1110  switches to a low logic level turning off FET  1104  and pump motor  1102 . Thus with this simple control method, the pressure in the system could be maintained at 120 mmHg. The predetermined desired vacuum level could easily be selected by varying the comparison threshold to a value representative of the required negative therapeutic pressure. 
         [0069]    Referring now to  FIG. 13 , an inexpensive, adjustable, low-hysteresis vacuum switch  1200  comprises a direct current (D.C.) power supply  1101 , a diaphragm-type D.C. vacuum pump  1102 , a magnetic reed switch  1209 , and a return path contact point  1103 . System  1200  also includes an air-tight cylindrical housing  1204 , hose barb  1205 , diaphragm  1206 , rare-earth magnet  1207  and adjustable bracket  1208  for turning pump motor  1102  on and off as required to maintain a preset pressure level. 
         [0070]    Switch  1209  is of a magnetic-reed design, which is normally open and closes (makes contact) when exposed to a magnetic field of sufficient strength. Magnetic reed switch  1209  is inserted in series with the pump motor  1102  and return path contact point  1103 , via wires  1210  completing the circuit and regulating the operation of pump motor  1102 . Switch  1209  functions similarly to the FET switch  1104  as depicted in  FIG. 11 , and described further hereinabove, and is a control element of system  1200 . A magnet  1207  is provided to actuate switch  1209  as well as an adjustable bracket  1208  to set the “open/close” thresholds for switch  1209 . Magnet  1207  is further integrally attached to a flexible diaphragm  1206 , which is likewise sealed to an air-tight cylindrical housing  1204 . A hose barb  1205  is provided to facilitate the communication of vacuum to cylindrical housing  1204 . As the vacuum pressure is varied within cylindrical housing  1204 , flexible diaphragm  1206  changes its geometry to a concave shape. Because of the elastic properties of diaphragm  1206 , which can be fabricated from a polymer(s) such as polyurethane (PU) and polyethylene (PE), a linear relationship exists between the diaphragm&#39;s concavity and the vacuum level of the cylindrical housing  1204 . Magnet  1207 , which is centrally attached to diaphragm  1206 , moves up and down in the vertical dimension with relation to the concavity of diaphragm  1206 . A bracket  1208  is adjustably attached to housing  1204  and switch  1209  is removably attached to bracket  1208 . The primary function of bracket  1208  is to hold magnetic reed switch  1209  in a fixed position relative to the magnet  1207 . 
         [0071]    When pressure within the cylindrical housing  1204  is below its predetermined therapeutic level (e.g., 120 mmHg), such as would be the case if the pressure within the vessel was 0 mmHg, diaphragm  1206  is minimally concave and the distance between magnet  1207  and magnetic reed switch  1209  relatively close. At this point, switch  1209  will closes and energizes the circuit causing vacuum pump motor  1102  to turn on. Vacuum pump  1102  subsequently reduces pressure within the cylindrical housing  1204 , which causes diaphragm  1206  to become more concave. As diaphragm  1206  increases in concavity, magnet  1207  moves farther away from magnetic reed switch  1209 . At a critical set-point, adjustable by moving bracket  1208 , the switch  1209  will open causing the vacuum pump motor  1102  to turn off. This “turn off” point will correspond to the desired pressure operating point of the therapy system. As the system slowly leaks and air bleeds back into the system reducing vacuum, the cycle repeats itself thus maintaining the desired therapeutic vacuum level. This level is predetermined and preset by adjusting bracket  1208  to produce the desired results. 
         [0072]    The above described embodiments are set forth by way of example and are not limiting. It will be readily apparent that obvious modifications, derivations and variations can be made to the embodiments. For example, the vacuum pump(s)  1105  and  1107  described hereinabove as either a diaphragm or piston-type could also be one of a syringe based system, bellows, or even an oscillating linear pump. Similarly, the vacuum-control algorithm described in detail above as multi-modal could be one of many other algorithms. Likewise, use of PLGA as a biodegradable substance for a component of dressing could be one of many different types of biodegradable materials commonly used for implantable medical devices.

Technology Classification (CPC): 0