Background

Monitoring of sterilization parameters is essential to ensure that optimum sterilizing conditions during a steam or chemical sterilization cycle are met. Environmental conditions in the chamber are frequently measured by various sensors, such as temperature, pressure, or sterilant concentration sensors, positioned in strategic places, such as a chamber wall or a drain line. The sensors, in turn, may be connected by various methods (e.g., electrical, radio transmitter, etc.) to an integral or remote microprocessor controller programmed to monitor and respond to the sensor readings and provide control of critical cycle parameters in the chamber, such as temperature, pressure, relative humidity, sterilant concentration and time during the cycle. Control of cycle parameters in the chamber, however, does not guarantee that sterilization conditions have been met within the load to be sterilized. Systems have been developed employing temperature and pressure sensors placed within an actual load or in standardized devices simulating a load. Each of these prior systems has disadvantages. For example, a sensor placed in an actual load monitors a condition only at the sensor location and does not necessarily reflect the condition elsewhere in the load. Load simulation devices, such as those containing a heat sink to detect the presence of air or superheated steam or those containing sensors to monitor and record time, temperature pressure and/or moisture, have the disadvantage that the load-simulation devices are not integrated with the sterilizer control system and are monitors

only. In some, information is available only after the sterilization cycle, when the device is removed from the chamber and the record of a parameter is interpreted visually (e.g. a color change) by the operator. In others, the monitored information is transmitted to an external stand - alone control and display unit adding to the expense of a sterilization system. Neither approach provides the capability of real-time monitoring of critical load parameters with direct and simultaneous conveyance of the information to the sterilizer control system allowing real-time control of critical sterilization parameter levels within the load. Further, prior load-simulation devices monitor only such parameters as temperature, pressure, time, moisture or the presence of a sterilant . They do not provide the capability of also directly monitoring the concentration of a chemical sterilant, such as ethylene oxide gas or hydrogen peroxide liquid or vapor, in a load, or of directly conveying the results to the sterilization control for real-time control of the sterilant concentration in the load. Currently, the Association for Advancement of

Medical Instrumentation (AAMI) guidelines recommend that chemical integrators and biological indicators be used to verify that process parameters critical for sterilization have been achieved. Chemical integrators provide a visual indication (e.g., a color change) that predetermined sterilization parameters were presumably achieved; for example, in the case of steam or ethylene oxide sterilization, a chemical integrator might indicate that a given temperature with the presence of moisture was achieved for a given time. Chemical integrators, however, are not sophisticated enough to monitor critical cycle parameters (e.g., temperature, pressure, sterilant concentration) to a confidence level that would assure that sterilization has occurred and to allow release of the load for use based on the indicator results alone. Therefore, biological indicators are additionally employed. Presumably, if proper conditions in the chamber with respect to time, temperature, pressure and/or sterilant concentration are achieved and maintained for the required

exposure period, the biological agent in the indicator will be killed, and thereby indicate cycle efficacy. However, the requirement fora sometimes lengthy incubation of the biological indicator to assure confirmation of sterility can result in an undesirable time delay after cycle completion before the sterilization efficacy is known This delay can significantly affect productivity and, therefore, the cost of processing goods through the sterilization system, in addition to the inconvenience of delayed turnaround of critical medical oriental instruments.

Recently, the concept of parametric release has been described for moist heat sterilization, and seeks to provide a more efficient means for monitoring a steam sterilization process. Parametric release is based on the physical monitoring in the chamber of the parameters of pressure, temperature and rate of change of temperature and pressure during the moist heat sterilization cycle. The chamber control is set for a predetermined cycle, to achieve and maintain predetermined critical parameter levels for a given period of time. The chamber parameters are monitored throughout the cycle. If the monitoring indicates a difference between a set and measured parameter value that exceeds specified limits, a warning is given to the cycle operator. If the monitoring indicates that the critical levels in the chamber are achieved and maintained for the time required to achieve a given sterility assurance level, the cycle is considered efficacious and the load is released for use. Therefore, parametric release systems are designed to provide monitoring and notification only of achieved parameters in the chamber. They do not suggest providing real-time sensing data to the sterilizer control system to enable the sterilizer control to react to changes in the critical parameters and adjust them in order to avoid unsuccessful cycles. Rather, current International Organization For Standardization (ISO) and European Committee for Standardization (CEN) standards require that the monitoring system for parametric release be separate from the sterilizer control system. Further, the process is described

only for control of parameters in the chamber and does not address the monitoring and control of the critical parameter levels in the load itself.

A need exists, therefore, for a sterilization system that provides both real-time monitoring and real-time control of critical sterilization parameters in the load, to a sterility assurance level that eliminates the need for chemical and biological indicators . There is a further need for a device that provides real-time monitoring of critical sterilization parameters in the load, and is also integrated with the sterilizer control system to enable the control to react to monitored changes in the critical parameter levels and adjust them in real-time in order to avoid unsuccessful cycles. There is a further need for a device that reproducibly simulates a standard challenge load undergoing sterilization and that contains critical parameter sensors that are directly integrated into the sterilizer control system. There is further a need for a sterilization system that provides for the release of a load when the critical values of sterilization parameters in the load are shown to have been met .

Summary of the Invention

In accordance with one aspect of the present invention, a challenge load-simulating device is provided. A housing defines a sterilant or disinfectant receiving opening. A baffle which is resistant to penetration of a sterilant or disinfectant is disposed adjacent the sterilant or disinfectant receiving opening. A receiving area receives the sterilant or disinfectant that penetrates past the baffle. A sensor probe is positioned at the receiving area for real-time sensing during a sterilization or disinfection cycle of the received sterilant or disinfectant to determine a sterilization or disinfection parameter value therefrom. A transmitting means electronically transmits the sterilization or disinfection parameter value from the device for use by a control system which provides real-time control of the

- 5 - sterilization or disinfection cycle in accordance with the transmitted parameter value.

In accordance with another aspect of the present invention, a method of monitoring and controlling a sterilization or disinfection process in a sterilization or disinfection chamber is provided. A challenge load-simulating device that has a baffle that is resistant to penetration by a sterilant or disinfectant and a receiving area for receiving sterilant and disinfectant that penetrates the baffle is placed in the chamber. A sterilization or disinfection cycle is conducted in the chamber. During the sterilization or disinfection cycle, a physical property at the receiving area is sensed and a corresponding electronic parameter value is generated. The sensed parameter value is transmitted to a control system. The sterilization or disinfection cycle is controlled in real-time in response to the transmitted, sensed parameter value.

One advantage of the present invention is that it enables a sterilization or disinfection cycle to be monitored and controlled in real-time.

Another advantage of the present invention is that it enables a determination to be made while a sterilization cycle is in progress whether the necessary conditions for sterilization or disinfection have been achieved. Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.

Brief Description of the Drawings The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. FIGURE IA is a schematic illustration of the present invention, including a load-simulating device connected to a sterilization chamber and sensor probes integrated into the

sterilizer control system and a parametric release monitoring system.

FIGURE IB is a schematic illustration of a load simulating device positioned within a sterilizer drain line. FIGURE 2 illustrates an embodiment of a sensor fitting in accordance with the invention.

FIGURES 3A, 3B and 3C are schematic illustrations of the load-simulating device of the invention.

FIGURES 4A and 4B illustrate an embodiment of a load simulating device of the invention in a closed configuration and an exploded view, respectively.

FIGURES 5A, 5B and 5C schematically illustrate another embodiment of a load-simulating device of the invention. FIGURE 6 illustrates an example of the pre-exposure phase of a steam sterilization cycle which may be employed in the invention.

FIGURE 7 illustrates an example of the exposure phase of a steam sterilization cycle which may be employed in the invention.

FIGURE 8 illustrates an example of a timing cycle which may be employed in the invention.

Detailed Description

The present invention is concerned with the real-time control of sterilization cycle parameters within a load simulation device that simulates the same conditions as those within an acceptable standard challenge load to be sterilized. Integration of such a load-simulating device into a sterilization chamber parameter sensing system allows real- time monitoring and transmission of cycle parameter values from the load-simulating device to the sterilizer control system. If the parameter values fall outside the range of acceptable values, the sterilizer control system directs the operation of control means, such as heaters, valves, pumps, timers, etc. in real time to bring the parameter values into an acceptable range within the load-simulating device. Thus, optimum and efficacious sterilization conditions can be

achieved and maintained within the load (as measured by the conditions sensed within the load-simulating device) resulting in a significant reduction in the number of unsuccessful cycles. Moreover, when acceptable sterilization parameters are shown to have been met, the sterile load is automatically released for use immediately upon completion of the cycle. Thus, the need for biological indicators and chemical integrators is eliminated.

The invention may be used with any sterilization process in which a successful outcome depends on achieving and maintaining controllable sterilization parameters for a given time. Such sterilization processes include, but are not limited to, sterilization with steam, ethylene oxide gas, liquid and vaporized hydrogen peroxide, liquid and vaporized formaldehyde, liquid and vaporized peroxy compounds, ozone, ionized gases, plasmas, and combinations thereof.

The load-simulating device is integrated into the sterilizer parameter sensing and control system and employs one or more resistance barriers to penetration of the sterilant, in the form of a tortuous path, similar to the barrier encountered by a sterilant penetrating a load of wrapped goods or goods in a sealed pouch. The acceptable standard challenge load simulated by the load simulating device reflects a "worst-case" load to be sterilized. Therefore, each type of resistance barrier in the load-simulating device is specifically designed for the particular sterilant to be employed in order to accurately simulate load conditions, or worst-case conditions, using the specified sterilant. For example, for sterilants such as hydrogen peroxide vapor a sufficient resistance barrier may comprise a tortuous path for entrance of the sterilant into the device. For other sterilants, such as ethylene oxide gas, the resistance barrier may additionally or alternatively comprise another tortuous path within the interior of the device, such as a packed material or a baffle or series of baffles. The resistance barrier may be purely a physical barrier and/or may also comprise a physical or chemical material which is slightly absorptive of the sterilant.

Suitable resistance barrier materials may include, but are not limited to, cellulosic materials for steam and/or ethylene oxide sterilants, teflon, silicon, polypropylene and polycarbonate materials for ethylene oxide and/or hydrogen peroxide sterilants, and combinations thereof. Effective resistance barrier materials for other sterilants, such as formaldehyde, ozone, or ionized gases and plasmas, are known to persons skilled in the art of sterilization.

Turning now to the drawing figures, the invention will be described in detail. As illustrated in FIGURE IA and IB, the system includes a sterilization chamber 1 having a sterilant inlet 2 and sterilant inlet valve 3 and a chamber drain line or exhaust outlet 4 and chamber outlet valve 5. A load-simulating device 6 is located within the chamber 1 and is removably connected to a chamber wall 7 or chamber drain wall 8 as described herein below. If connected to a chamber wall 7, the load-simulating device 6 is preferably located in a recessed portion 9 of the chamber wall 7, such that the load-simulating device 6 does not interfere with loading and unloading of goods in the chamber 1. When the device 6 is employed in a steam sterilization chamber, it is more preferable to locate the device 6 close to or within the drain line 4 in order to detect more readily the presence of unwanted air that will tend to settle there, as known in the art, and allow for correction of the problem, as described herein below.

In the embodiment illustrated in FIGURES IA and IB, and shown schematically in FIGURES 3A and 3B, the load simulating device 6 comprises a housing 10 for a resistance barrier 11 to penetration of a sterilant and a receiving area 12 for sterilant that successfully penetrates the resistance barrier 11. At least one sensor probe 13 is positioned in the receiving area 12 of the load simulating device 6 for real-time sensing and monitoring of at least one sterilization parameter value during a sterilization cycle. A connection flange or a weld fillet 15 (more clearly shown in FIGURE 2) connects the device 6 to the sterilization chamber wall 7 or drain line wall 8 at or near the location of the sensor probe

13. An optional use indicator 50 is preferably positioned on a surface of the load-simulating device 6 in contact with the chamber environment . The use indicator 50 serves only to indicate, preferably by a visible color change, that the load-simulating device 6 has been exposed to a sterilant. The use indicator 50 is not intended to serve as a chemical integrator.

In the embodiment illustrated in FIGURES IA and IB, the housing 10 of the device 6 is constructed with a small opening 14 at one end, to allow a liquid, gas or vapor sterilant to enter into the interior of the device 6 . The illustrated opening in the housing is used in conjunction with the interior resistance barrier 11 that provides a tortuous path for the sterilant. However, the sterilant may alternatively enter the device by another route, preferably one that provides a tortuous path/resistance barrier, such as through a seam in the housing, or penetration by wetting through material comprising the housing wall. In this case, the additional resistance barrier 11 in the interior of the device may also be included or be optional. Therefore, the device is intended to provide one or more resistance barriers/tortuous paths, depending on the characteristics of the sterilant employed.

Regardless of the route of sterilant entry, the load-simulating device 6 itself, as defined by the housing 10, is preferably shaped to simulate a dead-ended lumen, known to be difficult to sterilize because of the known difficulty of sterilant penetration into lumens in general. Therefore, in a preferred embodiment, the device 6 itself, by virtue of simulating a dead-ended lumen, comprises a resistance barrier to penetration of the sterilant.

During a sterilization cycle, the liquid, gas or vapor sterilant in the sterilization chamber 1 enters the load-simulating device 6 and is constrained to follow a prescribed path. The sterilant passes through the optional resistance barrier 11 and sterilant that penetrates the barrier 11 reaches the receiving area 12 where it come into contact with the sensor probe 13. Therefore, there is a fluid

connection between the sterilization chamber 1 and the sensor probe 13.

The sensor probe 13 may be present as a single probe or a plurality of probes or sensing elements. Parameters which may be sensed by the sensor probe or probes 13 include, but are not limited to, temperature, pressure, concentration of sterilant, relative humidity and multiples and combinations of these. For example, a set of sensor probes may contain two or more pressure sensors (Pl, P2) and two or more temperature sensors (Tl, T2) and two or more chemical sterilant concentration sensors (Cl, C2) ; and each parameter may be sensed by two or more separate sensing probes or by a single probe housing two or more sensing elements. As described herein below, multiple probes, preferably dual probes, for sensing a particular parameter are employed to comply with ISO and/or CEN standards requiring a separate set of sensing probes for parameter control and for parametric release of the load. Multiple probes, for example, an array of concentration-sensing elements, may be necessary in order to determine the concentration of certain chemical sterilants, such as multi-component sterilants.

A transmitting means 16 is connected to each sensor probe or sensing element 13 for transmitting a sensed parameter value from the sensor probe 13 to a receiving means 19, such as the sterilizer control system 17 or a parametric release monitoring system 18. The transmitting means 16 may comprise any means that is capable of transmitting the sensor data to the receiving means 19 including, but not limited to, electrical connection of the sensor probe 13 to the receiving means 19 and electronic or radio frequency transmission of the sensor data to the receiving means 19.

The sterilizer control system 17 may be any system including, but not limited to, a microprocessor or a logic circuit that is programmed to receive the sensed parameter value and also to control the value of the parameter in real¬ time during the sterilization cycle by governing a plurality of parameter control means 100 which operate valves, pumps, timers, heaters, etc. The sterilizer control system 17 is

also programmed to store a predetermined reference sterilization parameter range and to compare the received sensed parameter value to the reference parameter range. If the sensed parameter value falls within the reference parameter range, acceptable sterilization conditions are indicated, and the cycle continues. If the sensed parameter value falls outside of the reference parameter range, the sterilizer control system 17 is programmed to signal the parameter control means 100 to operate until the value of the sensed parameter falls within the reference parameter range. Thus, if the sensed temperature reading in the load simulating device 6 is below an acceptable limit, the sterilizer control system 17 signals the parameter control means 100 to operate a chamber heating means until the temperature reading of the temperature-sensing probe 13 in the load-simulating device falls within the range that is acceptable for sterilization. If a sensed sterilant concentration in the load-simulating device 6 is below The acceptable limit, the sterilizer control system 17 signals the parameter control means 100 to control the operation of a sterilant injector to increase the concentration of sterilant injected into the chamber 1, until the concentration of sterilant sensed by the concentration-sensing probe 13 is at an acceptable value, or within a range of acceptable values . In each of these examples, the sterilizer control system 17 also signals a timer to be reset to compensate for the time during which the sterilization cycle experienced unacceptable conditions. In many sterilization cycles, critical parameters are interdependent . For example, in a vapor hydrogen peroxide sterilization system, the concentration of the vapor that is allowable, i.e., does not exceed the dew point concentration, in the load at any given time is dependent on the temperature, pressure, and/or relative humidity in the load at that time. Therefore, in systems such as these, the sterilizer control system is programmed to monitor more than one parameter and analyze the data to determine whether or not the Environmental conditions are within the acceptable range of values.

In each embodiment of the invention, a redundant set or sets of temperature, pressure or other sensors 13, such as relative humidity or chemical sterilant concentration sensors may be incorporated. For example, in a preferred embodiment as illustrated in FIGURE IA, one set of sensor probes (T2, P2, C2) is used only as a parametric release monitoring system 18, for monitoring sterilization parameters to determine if acceptable sterilization conditions in the load-simulating device 6 have been achieved and the load may be released as sterilized. Another set of sensors probes (Tl, Pl, Cl) in the load-simulating device 6 transmits readings of temperature, pressure or other parameter levels to the sterilizer control system 17 for controlling the process parameters by the parameter control means 100. In this embodiment, the release monitoring sensors (T2, P2 , C2) are preferably connected to a user interface (display and/or printout) circuit which is separate from the circuit that connects the sterilizer control system 17 and the sensors (Tl, Pl, Cl) that provide data for process control. This feature addresses the concern outlined in current CEN and ISO standards for the need to keep the parametric release system independent of the system that controls the sterilizer cycle. The independent release monitoring sensors act as a back-up and redundant system to the sensors integrated into the sterilizer control system. Thus, erroneous release of a load that is not sterilized because sensors used for control purposes falsely indicate

(e.g., due to being out of calibration or subject to a component or electrical failure) that sterilization conditions are being achieved, is virtually prevented. FIGURE 2 illustrates an embodiment of one possible sensor fitting 20 for use in the present invention, for containing at least one 22, and preferably a plurality of sensor probes 22, 23, 24, and for attaching the sensor probe or probes to the load-simulating device 6, and to the chamber wall 7 or drain wall 8. It is envisioned that any sensor fitting which is capable of accommodating the sensor probes and load-simulating device and accomplishing the objectives of the invention, may be alternatively used in the practice of

the invention. In the illustrated embodiment, at least one each of a temperature sensor probe 22, a pressure sensor probe 23 and a chemical sterilant concentration sensor probe 24 are used in the practice of the invention. However, these probes are meant to be representative only, and are interchangeable with probes measuring other parameters, such as relative humidity. They may also represent a plurality of one or more types of probes, such as a plurality of concentration-sensing probes for different components of a multi-component chemical sterilant, or a plurality of temperature or pressure sensing probes.

As illustrated in FIGURE 2, this embodiment of the sensor fitting 20 comprises a housing 25 having an outer wall and an interior wall which defines a hollow interior having a first end 27 and a second end 29 and side walls 31. The first end 27 of the sensor fitting 20 is shaped to protrude into the interior of the chamber 1 through a complementary opening in the chamber wall 7 or chamber drain wall 8. The outer wall of the housing 25 is secured to the chamber wall 7 or drain wall 8 by means of a connection flange or a weld fillet 15 that provides a seal between the sensor fitting 20 and the chamber wall 7 or drain wall 8. The second end 29 of the sensor fitting 20 extends exteriorly from the chamber wall 7 or drain wall 8. The first end 27 and second end 29 and the sidewalls 31 of the sensor fitting 20 comprise openings 30 for receiving a sensor probe or plurality of sensor probes (see below) . As illustrated in this embodiment, a temperature sensor probe 22 extends through the hollow interior of the length of the sensor fitting 20 and comprises a tip portion 21 which protrudes beyond the open first end 27 of the sensor fitting 20, a middle portion 26 contained within the hollow interior of the sensor fitting 20, and a base portion 28 which extends beyond the open second end of the sensor fitting 20. The position of the temperature probe 22 within the hollow sensor fitting 20 may be optionally stabilized by means of a support flange 32, connected to an interior wall of the housing 25, containing a plurality of openings 34 sufficient to ensure

that a fluid environment is maintained throughout the hollow interior of the sensor fitting 20.

As described herein above, the housing 25 of the sensor fitting 20 comprises an opening or a plurality of other openings 30 for receiving other sensor probes. The probes illustrated in FIGURE 2 include, but are not limited to, a pressure sensing probe 23 and/or a chemical sterilant concentration sensing probe 24. Each of the sensor probes 23, 24 is in fluid connection with the hollow interior of the sensor fitting 20 and is engaged, preferably threadably engaged, to the housing 25 to form a seal between the sensor probe 23, 24 and the sensor fitting 20. Each of the sensor probes 22, 23, 24 terminates in a separate transmission means 16, extending from each probe and external to the sensor fitting 20 for transmitting sensed data to the receiving means 19.

The base portion 28 of the temperature probe 22, including the transmitting means 16, further extends through a compression fitting 35 comprising a housing 36 defining an anterior opening 37 containing a flexible ring member 38, preferably a ferrule, that encircles the base portion 28 of the probe 22 and a space 39 surrounding the ring member 38, and a posterior opening 40 for affording the passage of the base portion 28 of the temperature probe 22 therethrough, the transmission means 16 extending exteriorly from a posterior opening 40. The compression fitting 35 is removably engagable to the second end 29 of the sensor fitting 20. A pressure-tight seal between the compression fitting 35 and the sensor fitting 20 is achieved when the second end 29 of the sensor fitting 20 threadably engages the anterior opening 37 of the compression fitting 35, occupies the space 39 between the housing 36 and the ring member 38 and thereby, sealably compresses the ring member 38 around the temperature probe 22.

As described herein above, the first end 27 of the sensor fitting 20 is shaped to protrude into the interior of the chamber 1 through a complementary opening in the chamber wall 7 or chamber drain wall 8. The first end 27 of the

sensor fitting 20 is also removably and sealably connectable, preferably threadably connectable, to the load simulating device 6 within the chamber 1. As described herein above, the tip portion 21 of the temperature probe 22 extends beyond the first end 27 of the sensor fitting 20. In a preferred embodiment, when the sensor fitting 20 is connected to the load-simulating device 6, the top portion 21 of the temperature probe 22 extends into the receiving area 12 of the load-simulating device 6 but does not contact or extend into the resistance barrier 11.

When a sensor fitting 20 such as that described in FIGURE 2 is employed, there are a number of possible embodiments for the inter-connection of the sensor probes 13, the load-simulating device 6, and the transmitting means 16. For example, in one embodiment schematically illustrated in FIGURE IA, the sensor probes 13 including the transmission means 16 connecting the probes 13 to the sterilizer control system 17 are preconnected and premounted via the sensor fitting 20 of FIGURE 2 to the chamber wall 7 or drain line 8. Thus, the load-simulating device 6 is removably connected to the premounted sensor fitting 20 inside the chamber wall 7 or drain wall 8 at the location of the probes 13 in the manner illustrated in the embodiment of FIGURE 2. In this embodiment, the load-simulating device may be, and preferably is, disposable. Alternatively the device may be reusable if, for example, it is recharged or dried out (in the case of a sterilization cycle involving moisture) . The sensor probes may be permanently or temporarily mounted to the chamber, as desired. In another embodiment, schematically illustrated in

FIGURE 3C, sensor probes 48 are removably connectable to the load-simulating device as illustrated in FIGURE 2 and described herein above. However, in this embodiment, a sensor connector 41, which may extend into the interior of the chamber, has a electrical connector portion 42 connected to the chamber wall 7 or drain line 8, and a signal transmission means portion 44 connectable to a signal receiver (not shown) . The sensor probes 48 terminate in one or more complementary

electrical interfaces 46. Thus, in this embodiment, the sensor probes 48 are preconnected to a load-simulating device and then interfaced to the sterilizer control via an electrical connection inside the chamber. This embodiment, the sensor probes are also reusable and/or disposable.

FIGURES 4A, 4B, 5A, 5B, and 5C illustrate embodiments of a load simulating device which may be employed in the present invention. The precise nature of the load-simulating device to be used for a given sterilization cycle depends on the nature of the sterilant and the sterilization parameters to be monitored. For example, a load-simulating device for a steam sterilization cycle, a hydrogen peroxide vapor sterilization cycle and an ethylene oxide sterilization cycle, etc. may be different from each other, because of different critical sterilization parameters and sterilant properties. Thus, for steam, the resistance barrier in the load-simulating device preferably comprises a barrier material, such as a cellulosic, that absorbs heat, and the sensors within the device preferably monitor and provide for control of both temperature and pressure within the device. For an ethylene oxide sterilant, a tortuous path for penetration of the sterilant into and through the device preferably comprises a physical barrier to the flow of the gas. The materials selected for the barrier are determined by the solubility and diffusion rate of the ethylene oxide in the material and the thickness of the barrier. For example, ethylene oxide has a higher diffusion rate through silicon than through polyethylene, so polyethylene is preferable to silicon as a barrier material. The sensor probes employed for an ethylene oxide cycle preferably monitor and provide for control of temperature, pressure, relative humidity and concentration of the sterilant within the load-simulating device. A preferable load simulating device for hydrogen peroxide liquid or vapor sterilization includes a dead-ended device and a resistance barrier comprising a physical restriction of the flow of the sterilant (e.g., through a restricted orifice or orifices) and/or requiring changes in direction of flow (e.g. , around baffles) .

The preferred materials of construction of the resistance barrier comprise those which inhibit gas penetration and do not substantially absorb the sterilant. Thus, for hydrogen peroxide sterilization, polyethylene, polypropylene, teflon, silicon, and polycarbonate are preferred materials. The sensor probes employed for a vapor hydrogen peroxide cycle preferably monitor and provide for control of temperature, pressure, relative humidity and concentration of the sterilant within the load-simulating device.

The preferred load-simulating devices generally include a housing defining one or more resistance barriers to the passage of a sterilant and a receiving area where sterilant which has penetrated the resistance barrier(s) come into contact with one or more sensor probes . As described herein above, one of the resistance barriers may be a dead ended lumen defined by the housing.

The load-simulating device shown in FIGURE 4A in a closed configuration and in FIGURE 4B in an exploded view is illustrative of a housing comprising a tortuous path for entry of a sterilant into the interior of the device . A typical exterior housing for use in a steam or ethylene oxide sterilization cycle is disclosed in commonly owned U.S. Patents 4,839,291 and 4,914,034. In brief, the housing 54 of a canister 52 comprises a central tubular portion 56, a first tubular end portion 58 and a second tubular end portion 60. The central tubular portion 56 has two open ends. Each of the tubular end portions 58, 60 includes an outer member 62 having a closed end, and an inner member 64 having an open end. The outer member 62 of tubular end portion 58 further has a hole or opening 68 in its closed end that is covered with an adhesive backed tab 70. The tab 70 permits the optional opening or closure of hole 68. The inner members 64 of each of the end portions 58, 60 telescope into the central tubular portion 56 of the housing 54 allowing each of the outer members 62 to abut the central tubular portion 56 and form a seam or gap 72 between the central tubular portion and the outer members 62 of the tubular end portions 58, 60. The seam

or gap 72 forms a tortuous path for entry of the sterilant into the interior of the canister 52. Further, the seam or gap 72 may optionally be covered by a sterilant-permeable layer (not shown) , such as medical grade paper, to form a further tortuous path for entry of the sterilant into the interior. Another tortuous path for entry of the sterilant is defined by the close tolerance between the telescoping surfaces of the central tubular portion 56 and inner members 64 of the end portions 58, 60 of the housing 54. As practiced in the present invention and described herein previously, the device may optionally contain a further resistance barrier

(not shown) to sterilant passage, such as a packed material or a baffle or series of baffles, within the interior of the device. Preferably, such an internal resistance barrier is employed when the sterilant enters the canister through opening 68 when tab 70 is removed.

The central tubular portion 56 of the canister 52 illustrated in FIGURES 4A and 4B includes a connection fitting 74 that is removably connectable to a sensor fitting, as illustrated in FIGURE 2. The device optionally has a use indicator 50 positioned on an exterior surface.

FIGURES 5A, 5B, and 5C illustrate another embodiment of a load-simulating device which incorporates a tortuous path in the interior of the device . A typical device may include a tortuous path as described for a steam sterilization cycle disclosed in commonly owned U.S. Patent 4,594,223. However, the tortuous path may be different from the disclosed device

(e.g., baffles, etc.) depending on the sterilant employed, as described hereinabove. Briefly, the device 80 comprises a resistance barrier 82 within a canister housing 84 and a use indicator 50 on the exterior of the canister. One end of the housing 84 is in fluid communication with the chamber environment and has an opening 86 for the passage of a sterilant into and through the length of the device. The receiving area 88 for sterilant fluidly penetrating the resistance barrier 82 has a connection fitting 90 at the opposite end for removable attachment to a sensor fitting as shown in FIGURE 2. As shown in cross-section in FIGURE 5C,

the receiving area 88 is constricted to prevent resistance barrier material from entering the receiving area. As more fully described in U.S. Patent 4,594,223, if steam sterilization is employed the constriction also serves as a collection area for any unwanted air mixed with the steam, the air being in fluid contact with the sensors, to allow control of the cycle for correction of the problem or aborting of the cycle.

The materials from which the housing of the load simulating device and/or any internal resistance barrier are manufactured may be different from each other and are selected to be compatible with the sterilant employed. The housing material may be slightly absorptive of the sterilant, but may not be so absorptive as to affect the concentration level of sterilant in the chamber in the area surrounding the device or to result in high levels of residual sterilant which may be difficult to remove at the completion of the sterilization cycle. Suitable, and preferred, housing and/or resistance barrier materials may include, but are not limited to, cellulosic materials for steam and/or ethylene oxide sterilants; teflon, silicon, polypropylene and polycarbonate materials for ethylene oxide and hydrogen peroxide sterilants; and combinations thereof.

FIGURES 6, 7, and 8, illustrate the method of the invention in a typical steam sterilization cycle employing the real-time monitoring and control of cycle parameters within the load-simulating device and parametric release of the load for use when the parameters are met . Although a steam sterilization cycle is illustrated, the method of the invention can be modified by one skilled in the art to accommodate any sterilant such as ethylene oxide, hydrogen peroxide, formaldehyde, ozone, peroxy compounds, and the like. For a steam sterilization cycle, the parameters of temperature, pressure and time are preferably monitored. For an ethylene oxide cycle, the parameters of temperature, pressure, relative humidity, time, and the concentration of ethylene oxide are preferably monitored. For a chemical sterilant, such as liquid or vaporized formaldehyde or liquid

or vaporized hydrogen peroxide, the parameters of temperature, pressure, relative humidity, time, and the concentration of the sterilant are preferably monitored. A s illustrated in FIGURE 6, the method of the invention begins with a pre-exposure phase pulse (number "i") 101 which, for a pre-vacuum type sterilizer is typically a vacuum pull and steam charge, and for a gravity type sterilizer is typically a steam flush with an open drain line. Following "Pulse i", the pressure in the load-simulating device (test device) is sensed by the pressure probe. If, for example due to air in the device, the sensed pressure (P _test + _. Z p _sia ) does not fall within an acceptable predetermined set point pressure (P _set + 2 P _s i _a ) range, the sterilizer control system signals the parameter control means 100 to produce another pulse (i+1) 103. Extra pulses continue only until the pressure sensor indicates that P _Cest = P _Sec 102. The number of extra pulses is limited (to six or less, in the illustration) 104 or the cycle is aborted 105 in order to prevent an infinite cycle which could occur, for example, in the event of a chamber air leak. When P _test = P _set 102, a pass situation is indicated, and the temperature is sensed by the temperature probe. If the sensed temperature (T _Cest ± Y°C) does not fall within an acceptable predetermined set point temperature (T _set + Y°C) range, for example due to the presence of air in the device, the sterilizer control system signals the parameter control means

100 to produce another pulse. When T _teat = T _set 106, a pass situation is indicated and the cycle enters the exposure phase

107 illustrated in FIGURE 7 and starts the exposure timer 108.

The same principles of pass/fail apply to the sensed pressure and temperature in the load-simulating device during this exposure phase. Whenever the sensed pressures or temperatures are not acceptable, the exposure timer is stopped 120 for the time required to bring the parameters into the acceptable range, as illustrated in FIGURE 8. If the elapsed time (FT _elapse ) 121 exceeds a certain set point (900 seconds, in the example) 122, the cycle is aborted 123, in order to avoid an infinite cycle.

Throughout the cycle, the monitoring sensors in the load-simulating device transmit data to a receiving means comprising a parametric release sensing system. When the data indicate that the critical parameters of the cycle have been achieved in the load-simulating device, the parametric release system presumes the load has been sterilized. The parametric release monitoring system thus allows release of the load for use when the monitored sterilization parameter levels indicate sterilization cycle efficacy within the load-simulating device. While the invention has been described herein with reference to the preferred embodiments, it is to be understood that it is not intended to limit the invention to the specific forms disclosed. On the contrary, it is intended to cover all modifications and alternative forms falling within the spirit and scope of the invention.