Abstract:
A method of removing sterilization gas from a load disposed in a chamber. In accordance with the method, the chamber is evacuated to a subatmospheric pressure. A diffusion gas comprising helium is provided. An amount of the diffusion gas is introduced into the chamber effective to create a superatmospheric diffusion pressure in the chamber. The diffusion gas is allowed to diffuse throughout the chamber to displace sterilization gas from the load. The chamber is evacuated to remove portions of the diffusion gas and the sterilization gas from the chamber.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to sterilization in general and, more particularly, to the removal of sterilant from objects subjected to gaseous sterilization. 
     Gaseous sterilization is an attractive alternative to other methods of sterilization, such as steam sterilization, plasma sterilization, and radiation sterilization, because gaseous sterilization does not utilize high temperatures, corrosive chemicals, or high radiation levels, which can damage objects being sterilized. Because of these favorable qualities, gaseous sterilization is commonly used in hospitals to sterilize medical devices. 
     In gaseous sterilization, objects to be sterilized are contacted with a gaseous sterilant having good microbiocidal properties. Ethylene oxide (ETO) is the most commonly used gaseous sterilant. ETO has excellent microbiocidal properties, but is extremely volatile and flammable. The National Fire Protection Association (NFPA) has given ETO the highest possible flammability hazard rating under NFPA 704. Since ETO is so volatile and flammable, an inert gas is often mixed with ETO to suppress its flammability. Inert gases that are often mixed with ETO include: carbon dioxide (CO 2 ); nitrogen (N 2 ); chlorofluorocarbons (CFCs), such as dichlorodifluoromethane (CFC-12); hydrochlorofluorocarbons (HCFCs), such as chlorodifluoromethane (HCFC-22), and monochloro-tetrafluorethane, which exists in two isomeric forms, 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124), 1-chloro-1,1,2,2-tetrafluoroethane (HCFC-124a); and mixtures of the foregoing. 
     For many years, the most commonly used flammability suppressed ETO mixture was a mixture of 12% ETO and 88% CFC-12 (commonly referred to as the “12/88 mixture”). Due to environmental concerns, however, the use of CFCs is being phased out under the Montreal Protocol. Accordingly, flammability suppressed ETO mixtures using HCFCs are becoming more predominant. An example of such a flammability suppressed ETO mixture using HCFCs is disclosed in U.S. Pat. No. 5,376,333 to Shankland et al., which is incorporated herein by reference. Shankland discloses a suppressed ETO mixture comprising 3 to 13 weight percent ETO and 87 to 97 weight percent of monochlorotetrafluorethane. Another example of a flammability suppressed ETO mixture includes a mixture comprising about 10 weight percent ETO and about 90 weight percent of a mixture of HCFC-124and HCFC-22. 
     In a typical gaseous sterilization process utilizing ETO or an ETO mixture as the sterilant, a load to be sterilized is first placed in a sterilization chamber. The chamber is hermetically sealed and a vacuum is drawn to remove air from the chamber. The chamber is heated and water vapor is introduced into the chamber, as needed, to bring the chamber to an optimal relative humidity. The sterilant is then introduced into the chamber. The load is exposed to the sterilant for a sterilization period of time, which is typically between 1 and 6 hours, depending on the concentration of sterilant and the temperature of the chamber. 
     After the sterilization period of time, the load is aerated to remove the sterilant therefrom. Depending on the construction and capabilities of the sterilizer, the load is either aerated in the chamber or in a separate aerator. If the load is composed of a porous material, such as plastic, or ceramic, the load must be aerated for a prolonged detoxification or aeration period of time. With a material such as polyvinylchloride (PVC), the aeration period of time with current technology is typically between 8 and 24 hours, depending on the intended use of the load. As can be appreciated, such a long period of time is undesirable because the sterilizer and the load cannot be re-used during that period of time. 
     Methods have been developed to reduce the aeration period of time in ETO sterilization processes. An example of such a method is disclosed in U.S. Pat. No. 4,770,851 to Joslyn, which is incorporated herein by reference. In the Joslyn aeration method, a sterilization chamber containing a load is evacuated to a subatmospheric pressure after a sterilization cycle is complete. Steam is then flushed through the chamber, while the subatmospheric pressure is maintained in the chamber. The chamber is then pressurized with heated air, thereby causing some of the steam to condense on interstices of the load. The chamber is then evacuated again to the subatmospheric pressure, thereby causing the steam to vaporize and carry away residual sterilant from the load. 
     The Joslyn aeration method is a substantial improvement over conventional aeration methods. Typically, the Joslyn aeration method reduces the aeration period of time for PVC to between 4 and 8 hours, depending on the intended use of the load. This period of time, however, is still substantial, and certain types of materials may be damaged by condensing steam on their interstices. 
     Based upon the foregoing, there is a need in the art for an improved method of removing sterilization gas from a load. The present invention is directed to such a method. 
     SUMMARY OF THE INVENTION 
     It therefore would be desirable, and is an advantage of the present invention, to provide an improved method of removing sterilization gas from a load disposed in a chamber. In accordance with the method, the chamber is evacuated to a subatmospheric pressure. A diffusion gas is provided. An amount of the diffusion gas is introduced into the sterilization chamber. The amount of the diffusion gas is allowed to diffuse throughout the sterilization chamber, thereby causing sterilization gas to diffuse away from the load. Portions of the diffusion gas and sterilization gas are then removed from the sterilization chamber. 
     In one embodiment of the present invention, the diffusion gas is non-flammable and non-condensible at 0-32 psig. In another embodiment, the diffusion gas comprises helium and the steps of introducing the diffusion gas through removing the diffusion gas are repeated until an acceptable residue level of sterilization gas on the load is attained. 
     Also provided in accordance with the present invention is a method of sterilizing a load in a sterilization chamber using sterilization gas. In accordance with the method, the load is placed in the sterilization chamber and is exposed to sterilization gas. The sterilization chamber is evacuated to a subatmospheric pressure, thereby removing sterilization gas. A diffusion gas is provided that is selected from the group consisting of helium, hydrogen, nitrogen, argon, and carbon dioxide and mixtures thereof. The diffusion gas is introduced into the sterilization chamber in an amount effective to create a superatmospheric diffusion pressure in the sterilization chamber. The amount of the diffusion gas is allowed to diffuse throughout the sterilization chamber, thereby causing sterilization gas to diffuse away from the load. Portions of the diffusion gas and sterilization gas are then removed from the sterilization chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 shows a schematic representation of a sterilization system having an interior chamber; and 
     FIG. 2 shows a graphic representation of a process for removing sterilant from a load, with time on the y-axis and pressure of the interior chamber on the x-axis. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It should be noted that in the detailed description which follows, parts are parts by weight and percents are weight percents unless otherwise indicated or apparent. When a preferred range such as 5-25 is given, this means preferably at least 5 and preferably not more than 25. It should also be noted that in order to clearly and concisely disclose the present invention, the drawings may not necessarily be to scale and certain features of the invention may be shown in somewhat schematic form. 
     Referring now to FIG. 1, there is shown a sterilization system  10  wherein the present invention may be practiced. The sterilization system  10  may be a commercially available ETO sterilization system that has been modified to practice the present invention. The sterilization system  10  generally includes a sterilization vessel  12 , a sterilant source  14 , a sterilant supply system  16 , a sterilant removal system  18 , and a gas diffusion system  20 . The sterilization system  10  may also optionally include a steam supply system  22 . 
     The sterilization vessel  12  is preferably composed of stainless steel and defines an interior chamber  24  having an open end. A door  26  is pivotably mounted to the sterilization vessel  12  and is pivotable between an open position, wherein the door  26  is spaced from the open end, and a closed position, wherein the door  26  covers the open end  24 . A conventional lock assembly (not shown) is provided to lock the door  26  in the closed position. The door  26  and the sterilization vessel  12  are provided with seals  27 , which cooperate to hermetically seal the open end when the door  26  is locked in the closed position. A circulation fan  28  may be mounted to the sterilization vessel  12  to provide circulation and uniform environmental conditions in the interior chamber  24 . A pressure transducer  29  may be connected to the sterilization vessel by a conduit  30 . 
     A heating system is provided to heat the interior chamber  24 . The heating system includes a heating device  31  disposed around the exterior of the sterilization vessel  12 , and a control device (not shown). The heating device  31  may be an electric resistance heating coil, or other type of heating means, such as a hot water or steam jacket. The control device regulates the flow of electricity or hot water or steam through the heating device  31 , or otherwise controls the heating device  31 , so as to maintain the interior chamber  24  at a selected temperature. 
     The sterilant source  14  preferably includes a pair of pressurized tanks  32  for holding a sterilant under pressure. The sterilant may be 100% ETO, the 12/88 mixture, or a mixture of about 8-12% ETO and about 88-92% CO 2 , or N 2 . More preferably, the sterilant is a mixture of about 3-13% ETO and about 87-97% HCFC. Still more preferably, the sterilant is a mixture of about 9-12% ETO and about 88-91% monochlorotetrafluoroethane (the “ETO/HCFC-124mixture”). Still more preferably, the sterilant is a mixture of about 10 weight percent ETO and about 90 weight percent of a mixture of HCFC-124and HCFC-22 (the “ETO/HCFC-124/HCFC-22mixture”), which is available from Allied Signal under the name OXYFUME 2002, and from the Pennsylvania Engineering Company under the name PENNGASS 2. 
     The tanks  32  are preferably disposed in a sealed enclosure  34  connected to a ventilation system (not shown) that maintains the enclosure  34  at a slightly negative pressure. The tanks  32  are pressurized to maintain the sterilant in liquid form. If the sterilant is the ETO/HCFC-124/HCFC-22mixture, the tanks  32  are pressurized to about 60 psig. Eductor tubes (not shown) are disposed in the tanks  32  to conduct the sterilant from the bottoms of the tanks  32  to a header assembly  36 , which connects the tanks  32  to the sterilant supply system  16 . The sterilant is supplied from the tanks  32  sequentially such that the sterilant is supplied from only one of the tanks  32  at a time. Preferably, the header assembly  36  is provided with an automatic transfer feature that automatically switches from an exhausted one of the tanks  32  to a filled one of the tanks  32 , without interrupting the supply of sterilant. 
     The sterilant supply system  16  supplies the sterilant from the sterilant source  14  to the sterilization vessel  12 . The sterilant supply system  16  includes a vaporizer  38  having an inlet connected to the header assembly  36  by piping  40 , and an outlet connected to the sterilization chamber by piping  41 . A solenoid valve  42  is disposed in the piping  40  and is operable to control the supply of sterilant to the vaporizer  38  and, thus, the sterilization vessel  12 . The vaporizer  38  reduces the pressure of the sterilant and heats the sterilant, thereby causing the sterilant to vaporize into a gas. The vaporizer  38  is controlled such that the temperature of the sterilant gas entering the sterilization vessel  12  is at a predetermined temperature that will not exceed the temperature limit of the load being sterilized. 
     The steam supply system  22  may be provided to supply steam to the sterilization vessel  12  prior to sterilization in order to raise the humidity in the interior chamber  24  and hydrate microorganisms on the load disposed therein. If the sterilization system  10  is based on an ETO sterilization system obtained from or modified by the Joslyn Sterilizer Corporation, the steam supply system  22  may also be used to remove air from the interior chamber  24  pursuant to an air removal method disclosed in U.S. Pat. No. 4,7,70,851 to Joslyn, referenced earlier. The air removal method of Josyln is similar to the Josyln aeration method described above and utilizes a plurality of alternating steam and pressurized air pulses. 
     In lieu of using steam to raise the humidity in the interior chamber  24 , other conventional humidifying means may be employed. For example, a moisture-releasing device may be placed in the interior chamber  24  along with the load to be sterilized. An example of such a moisture-releasing device is disclosed in U.S. Pat. No. 5,135,715 to Andersen, which is incorporated herein by reference. 
     Gaseous sterilization with ETO is more effective in killing microorganisms if the microorganisms are hydrated and if the sterilization process is carried out in an atmosphere having at least 30% relative humidity. Thus, the humidifying means chosen should be able to maintain the interior chamber  24  at a relative humidity of at least 30%. 
     The gas removal system  18  removes gas from the interior chamber  24 . The gas removal system  18  includes a vacuum pump  44  having an inlet connected to the sterilization vessel  12  by piping  46 , and an outlet connected to a vent  48  by piping  50 . A solenoid valve  52  is disposed in the piping  46  and is operable to control the removal of gas from the interior chamber  24 . The vacuum pump  44  may be an open water sealed vacuum pump, or more preferably, a dry vacuum pump, or a recycled sealing fluid vacuum pump. Preferably, the operation of the vacuum pump  44  and the solenoid valve  52  is controlled by a programmable controller  54 . 
     In accordance with the present invention, the gas diffusion system  20  cooperates with the gas removal system  18  to remove gaseous sterilant from the interior chamber  24  after sterilization. The gas diffusion system  20  includes at least one tank  56 , or more preferably, a pair of tanks  56  of a compressed diffusion gas. 
     As will become more apparent below, it is desirable for the diffusion gas to have a fast rate of diffusion. The rate of diffusion of a gas is inversely proportional to the square root of its molecular weight. In addition, the rate of diffusion of a gas is proportional to temperature and the negative gradient of the density of the gas. Thus, it is desirable for the diffusion gas to be light, i.e., have as low a molecular weight as possible, and to introduce the diffusion gas at an increased temperature and pressure. It is also preferable if the diffusion gas is not an oxidizer, is unable to support combustion, and is non-flammable. It is further preferable if the diffusion gas is non-condensible at 0-32 psig, more preferably 0-50 psig, and is inert. Since helium is the lightest nonflammable gas, is non-condensible at 0-50 psig, and is inert, the diffusion gas is preferably about 100 percent helium, more preferably 100% United States Pharmacopeia (USP) helium, i.e., medical grade helium. Less preferably, the diffusion gas is a mixture of about 80-90 percent helium and about 10-20 percent hydrogen (H 2 ). Less preferably, the diffusion gas is a mixture of about 51-99 percent helium and about 1-49 percent of a gas selected from the group consisting of hydrogen, nitrogen (N 2 ), argon (Ar), carbon dioxide (Co 2 ), air, and mixtures thereof, wherein the amount of hydrogen present is insufficient to make the diffusion gas flammable, which is about 20 percent or less. Less preferably, the diffusion gas is a mixture of about 1-50 percent helium and about 50-99 percent of a gas selected from the group consisting of hydrogen, nitrogen, argon, carbon dioxide, air, and mixtures thereof, wherein the amount of hydrogen present is insufficient to make the diffusion gas flammable. Less preferably, the diffusion gas is about 50-100 percent nitrogen and about 0-50 percent of a gas selected from the group consisting of hydrogen, argon, carbon dioxide, air, and mixtures thereof, wherein the amount of hydrogen present is insufficient to make the diffusion gas flammable. Less preferably the diffusion gas is a mixture of about 50-100 percent air and about 0-50 percent of a gas selected from the group consisting of hydrogen, argon, carbon dioxide, and mixtures thereof, wherein the amount of hydrogen present is insufficient to make the diffusion gas flammable. Less preferably, the diffusion gas is a mixture of about 50-100 percent C0 2  and about 0-50 percent of a gas selected from the group consisting of hydrogen, argon, air, and mixtures thereof, wherein the amount of hydrogen present is insufficient to make the diffusion gas flammable. 
     If the diffusion gas is helium, the tanks  56  are pressurized to about 2,200 psig. The tanks  56  have conventional outlet assemblies that include shutoff valves  58 . The outlet assemblies of the tanks  56  are connected to a header assembly  60  having pressure reducing devices  62 . The diffusion gas is supplied from the tanks  56  sequentially such that the diffusion gas is supplied from only one of the tanks  56  at a time. Preferably, the header assembly  60  has an automatic transfer feature that automatically switches from an exhausted one of the tanks  56  to a filled one of the tanks  56 , without interrupting the supply of diffusion gas. 
     The header assembly  60  may be connected by piping  66  to a bioretentive filter  63  that removes any bacteria, viruses, or fungi that may be present in the diffusion gas. An inlet of a heater  64  is connected to the filter  63  by piping  65 . An outlet of the heater  64  is connected to the sterilization vessel  12  by piping  68 . A solenoid valve  70  is disposed in the piping  66  and is operable to control the flow of the diffusion gas to the filter  63  and, thus, the heater  64  and the interior chamber  24 . The operation of the solenoid valve  70  is preferably controlled by the programmable controller  54 . 
     In order to sterilize a load  72  using the sterilization system  10 , the load  72  is preferably wrapped, packaged, or otherwise covered with a protective material  74  to form a protected space  76  within which the load  72  is disposed. The protective material  74  may be muslin, paper, plastic or other material specially designed to maintain or preserve the sterility of the load  72  after removal from the sterilization system  10 . The load  72  may be pre-packaged with the protective material  74  by the manufacturer of the load  72 , or the load  72  may be wrapped or packaged with the protective material  74  at the site where the sterilization system  10  is located. 
     The door  26  is moved to the open position and the packaged load  72  is placed in the interior chamber  24 . The door  26  is then closed and locked. The heating device  31  is activated to heat the interior chamber  24  to a sterilization temperature in a range from about 20° C. to about 100° C. depending upon the nature of the load  72 . Preferably, the sterilization temperature is in a range from about 30° C. to about 60° C. more preferably, about 54° C. (130° F.). The relative humidity of the interior chamber  24  is raised above 30 percent, more preferably above  65  percent. Air is removed from the interior chamber  24  before, during or after the humidification. The air is removed by opening the solenoid valve  52  and actuating the vacuum pump  44  until the interior chamber  24  has a vacuum or negative gauge pressure P 1  of about 20-25 inches of mercury. If the sterilization system  10  is based on an ETO sterilization system obtained from the Joslyn Sterilizer Corporation, the alternating steam and pressurized air pulses of the Joslyn air removal method may also be employed to remove the air and humidify the interior chamber  24 . 
     The solenoid valve  42  is actuated to cause the sterilant to flow through the piping  40  to the vaporizer  38 , where the sterilant is vaporized and heated to the sterilization temperature, which, as set forth above, is preferably about 54° C. The gaseous sterilant flows from the vaporizer  38  into the interior chamber  24  through the piping  41 . The gaseous sterilant is admitted into the interior chamber  24  in an amount that preferably produces a pressure P 2  in the interior chamber  24 , which is greater than atmospheric pressure, i.e., is superatmospheric. Preferably, the pressure P 2  is in a range from about 0.1 psig to about 32 psig, more preferably about 12 psig. The load  72  is maintained in the interior chamber  24  for a sterilization period of time, which may be between 1 and 6 hours. 
     At the conclusion of the sterilization period of time, the sterilant removal method of the present invention is performed to remove the sterilant from the interior chamber  24  and the load  72 . Preferably, the sterilant removal method is controlled by the programmable controller  54 . 
     Referring now to FIG. 2, the interior chamber  24  is evacuated to the pressure P 1  by opening the solenoid valve  52  and actuating the vacuum pump  44 . This evacuation is maintained between  80  and  82  and removes most of the gaseous sterilant from the interior chamber  24 . At  82 , the solenoid valve  52  is closed and the vacuum pump  44  is deactivated. 
     Referring back to FIG. 1, the solenoid valve  70  is actuated to cause the diffusion gas to flow through the piping  66  to the filter  63  and thence the heater  64 , which is activated to heat the diffusion gas to a diffusion temperature T 2 . Preferably, T 2  is above room temperature so as to speed up the diffusion rate of the diffusion gas. Preferably T 2  is in a range from about 30° C. to about 70° C., more preferably, about 60° C. (140° F.). The diffusion gas flows from the heater  64  into the interior chamber  24  through the piping  68 . The diffusion gas is admitted into the interior chamber  24  until the pressure in the interior chamber  24  reaches a diffusion pressure P 3  as shown at  84  of FIG.  2 . Preferably, P 3  is superatmospheric so as to increase the diffusion rate of the diffusion gas, and is greater than the sterilization pressure P 2 . Preferably, P 3  is in a range from about 0.1 psig to about 50 psig, more preferably in a range from about 5 psig to about 32 psig, more preferably in a range from greater than 12 psig to about 25 psig. Preferably, the heating device  31  is manipulated to maintain the temperature of the interior chamber  24  at the diffusion temperature T 2 . 
     The diffusion gas is allowed to diffuse throughout the interior chamber  24 . Since the diffusion gas is preferably a light gas, the diffusion gas readily passes through the protective material  74 , even if it is composed of plastic. The diffusion gas enters the protected space  76  within which the load  72  is located or disposed and moves into the interstices of the load  72 . If permitted, the diffusion gas will continue to diffuse into the protected space  76  and the interstices of the load  72  until the concentration of the diffusion gas is uniform throughout the protected space  76 , the interstices of the load  72 , and the rest of the interior chamber  24 . 
     In order to maintain a uniform gas concentration throughout the interior chamber  24 , the diffusion of the diffusion gas into the protected space  76  and the interstices of the load  72  is accompanied by the diffusion of sterilant out of the interstices of the load  72  and the protected space  76  and into the remaining portion of the interior chamber  24 . If permitted, this opposing diffusion of sterilant will continue until the concentration of sterilant is uniform throughout the interior chamber  24 , at which point a substantial portion of the sterilant will have been removed from the protected space  76  and the interstices of the load  72 . 
     Referring back to FIG. 2, the foregoing diffusion period is maintained between  84  and  86  to allow the diffusion gas to diffuse into the protected space  76  and the interstices and thereby displace sterilant. At the conclusion of the diffusion period, the solenoid valve  52  is opened and the vacuum pump  44  is actuated to evacuate the interior chamber  24  to the pressure P 1  for a period between  88  and  90 . The evacuation of the interior chamber  24  removes most of the helium and remaining sterilant from the interior chamber  24 . 
     The process from  82  to  90  is repeated if and until an acceptable sterilant residue level is attained as shown at  92 . The duration and number of repetitions of the process may vary based on the use of the load  72  because the acceptable sterilant residue level is dependent upon the use of the load  72 . If the load  72  is for contact with skin or mucosa, the acceptable sterilant residue level is about 250 ppm, whereas if the load  72  is for implantation or contact with blood or tissue, the acceptable sterilant residue level is about 25 ppm. 
     The duration and number of repetitions of the process from  82  to  90  may also vary based on the composition of the load  72  and the protective material  74 . The rate of diffusion of a gas is much slower through materials such as plastic (and in particular, PVC) than it is through materials, such as cloth and paper. 
     When the acceptable sterilant residue level is attained, the pressure of the interior chamber  24  is vented to atmospheric pressure as shown at  94 . The load  72  is then removed from the interior chamber  24  and used as needed. 
     While the invention has been shown and described with respect to a particular embodiment thereof, this embodiment is for the purpose of illustration rather than limitation, and other variations and modifications of the specific embodiment herein described will be apparent to those skilled in the art, all within the intended spirit and scope of the invention. For example, the removal of the residual sterilant from the load  72  using the diffusion gas may be performed in a separate aerator rather than in the sterilization vessel  12 . 
     Accordingly, the invention is not to be limited in scope and effect to the specific embodiment herein described, nor in any other way that is inconsistent with the extent to which the progress in the art has been advanced by the invention.