MONITORING AND CONTROL OF STERILIZATION PROCESSES WITH SEMICONDUCTOR SENSOR MODULES

Abstract: A semiconductor sensing element (124) is mounted in a sterilization chamber (1). The composition of the sensor is selected such that one of its electrical characteristics, for example, capacitance or resistance, changes with concentration of a chemical sterilant within the chamber in real-time. A receiving unit (126) monitors the solid-state sensing element and stores a baseline electrical characteristic value corresponding to a baseline concentration of the sterilant at a first set of values of environmental parameters, for example, temperature, pressure, relative humidity, air-flow velocity, an oxidizing substance concentration, and combinations of these. The receiving unit compares changes in the real-time semiconductor sensing element electrical characteristic with the baseline characteristic value to provide a real-time indication of sterilant concentration. A sterilizer control (12) controls a concentration of sterilant in the sterilization chamber and other sterilization system parameters in accordance with the real-time sterilant concentration from the receiving unit. When one or more of the environmental parameters change, the sterilizer control causes the receiving unit to change the baseline electrical characteristic value to one corresponding to a new baseline sterilant concentration value for the new set of environmental parameters.

Description: MONITORING AND CONTROL OF STERILIZATION PROCESSES WITH SEMICONDUCTOR SENSOR MODULES Background of the Invention In systems for sterilization and decontamination, certain critical process parameters must be maintained within acceptable limits for a given period of time in order to achieve target sterility assurance levels. Real time monitoring and control methods for such critical parameters as temperature, pressure and time are widely known and practiced. However, in liquid and gas chemical sterilization and decontamination systems, the critical parameter of sterilant concentration must also be monitored and controlled. Until the present invention, there has been no practical method for directly measuring the sterilant concentration in real time that is economically feasible for use in all types of sterilization and decontamination systems. Indirect methods to measure the concentration of chemical sterilants, such as ethylene oxide, formaldehyde, hydrogen peroxide and peroxygen compounds are commonly employed. For example, the theoretical concentration of vaporized hydrogen peroxide in a sterilization chamber at any given time may be calculated by correlating the consumption of liquid peroxide supplied to the vaporization system with such system parameters as temperature, pressure, relative humidity, dew point and, if applicable, air flow rate and chamber volume. Based on this indirect calculation, the sterilizer control systems may then operate various sterilizer components, such as pumps, valves, etc., to control the amount of liquid hydrogen peroxide delivered to the vaporizer in order to maintain the desired theoretical concentration of sterilant gas/vapor in the chamber, while avoiding condensation due to vapor saturation. It is known, however, that for most chemical sterilant systems there may be a disparity between the theoretical sterilant concentration and the actual sterilant concentration due to variable environmental factors, including decomposition, absorption and/or adsorption of the sterilant gas/vapor upon contact with various surfaces within the system and the load being processed. The effect of these factors on the actual sterilant concentration s unpredictable and varies from load to load and system to system such that adjusting the supply of liquid chemical sterilant to account for these effects s impractical and inefficient . To assure that target sterility assurance levels have been met, therefore, a system for direct, real time measurement of the actual chemical sterilant concentration int he chamber at any given time is preferred. Recently, developments in near infrared spectrophotometry have allowed the use of this technology to directly measure and control the concentration of hydrogen peroxide vapor in a given sterilization or decontamination environment in real time. This approach, while demonstrated to be effective, is costly and may appreciably add to the overall cost of sterilization and decontamination systems. In addition, the large overall size of the near infrared system, as currently configured, precludes easy integration into small sterilizer or decontamination systems, such as small endoscope or dental instrument sterilizers. There remains a need for a practical and reliable system that provides real-time monitoring and control of the actual concentration of liquid or gas chemical sterilants during a sterilization or decontamination cycle and that can be inexpensively and easily integrated into virtually any system. There is also a need for a system that is highly selective for and sensitive to the chemical sterilant of choice . A known and relatively inexpensive technology for selectively measuring concentrations of gases and/or liquid employs semiconductor-based sensors. Semiconductor technology has been used for detecting and warning of toxic and/or explosive gases in domestic and industrial environments. For example, semiconductor-based sensors have been used to detect gases in the cabs of motor vehicles, and to produce signals or control outputs for the switching ON or OFF of air filtering systems or ventilation systems if the gas concentration reaches a predetermined se -point concentration. Semiconductor monitoring systems have been used for detection of ethylene oxide in ears around sterilizers or on a continuous, 24 -hour air sampling basis. Semiconductor sensor probes have also been used to detect the constituents and concentrations of gases, such as carbon monoxide, oxides of nitrogen, and hydrocarbons in exhaust gases from internal combustion engines. Other such sensors have variously been used to sense the presence and/or concentrations of gases and liquids, such as ethanol, carbon dioxide, glutaraldehyde and ammonia. A known problem with semiconductor-based systems is their sensitivity to environmental conditions. For example, variations in temperature, relative humidity, and/or flow velocities of air currents that are to be monitored can affect the stability and accuracy of the semiconductor sensor signal and produce erroneous gas concentration readings. Moreover, semiconductor sensors may demonstrate changes in electrical conductivity in the presence of oxygen and oxidizing gases other than the gas being measured. Variations in these environmental conditions during a sterilization cycle and between cycles, often in combination with variations in pressure, are present in virtually all sterilization and decontamination systems employing liquids and gases, such as ethylene oxide, peroxygen compounds, formaldehyde, hydrogen peroxide, peracetic acid, and the like. Thus, until the present invention, semiconductor-based sensors have not been described for real-time monitoring of the concentration of sterilant liquid or gas within a sterilization chamber during a decontamination or sterilization cycle. Summary of the Invention In accordance with one aspect of the present invention, a sterilization system is provided which includes a sterilization chamber, a means for admitting a chemical sterilant into the chamber, a means for removing sterilant from the chamber, and a process controller for controlling the sterilant admitting means and the sterilant removing means. A sensing element is positioned in the chamber, which sensing element has an electrical characteristic which varies with a concentration of the sterilant . A receiving unit is connected to the sensing element . The receiving unit stores a baseline electrical characteristic value corresponding to a baseline concentration of the sterilant at a first set of environmental parameters and detects a real-time change in the electrical characteristic of the sensing element relative to the baseline electrical characteristic value as an indication of a realtime concentration of the sterilant at a first set of environmental parameter values. The sterilizer control compares the real-time concentration value with reference sterilant concentration ranges and controls a concentration of the sterilant in the chamber in accordance with the comparison . In accordance with a first more limited aspect of the present invention, the sensing element is a semiconductor element . In accordance with another more limited aspect of the present invention, the control system monitors environmental parameters during a sterilization cycle. In response to a change in the value of an environmental parameter, the control system causes the receiving unit to store a new baseline electrical characteristic value corresponding to the new set of environmental parameter values. In accordance with another aspect of the present invention, a method of sterilization is provided in which a liquid or gaseous chemical sterilant is admitted into and removed from a sterilization chamber. A concentration of the sterilant is sensed with a sensing element having an electrical characteristic that varies with a concentration of the chemical sterilant. A baseline concentration of the sterilant is established by storing a baseline electrical characteristic value at a first set of environmental parameter values. A change in the electrical characteristic is monitored relative to the baseline characteristic value in real-time to determine a real-time concentration of the sterilant. The sterilant concentration in the chamber is controlled in real-time in accordance with the monitored real-time concentration of the sterilant. Still further advantages of the present invention will become apparent to those of ordinary skill in the art on 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 1 is a schematic representation of a typical sterilization system employing the semiconductor sensor modules of the invention; and, FIGURE 2 is a schematic representation of a semiconductor module employed in the invention. Detailed Description of the Preferred Embodiments The present invention is concerned with the real-time monitoring and control of the sterilant concentration during all phases of a sterilization cycle in order to maintain the concentration levels within acceptable ranges for given time periods to assure that critical concentration parameter values have been met, sterilization efficacy has been achieved, and the chamber has been properly exhausted at the end of the cycle. The concentration of liquid and/or gas sterilants during the cycle is measured by a semiconductor-based sensor in fluid communication with the sterilant . The sensor module has a sensing element and integrated electronics that react to changes in the concentration of certain chemicals to which the element is exposed. The electronics and the software associated with the module are configured to react to a target chemical compound. Any semiconductor-based sensor module, and its associated software, that is selective for and sensitive to the particular liquid or gas used as the sterilant may be adapted for use in the invention. For example, a suitable sensor module for detecting and measuring concentrations of ethylene oxide gas is the EtO Model 300 Gas Monitoring System in combination with Intellisense software, "Matrix Fingerprinting Technology" , both manufactured by Air Purification Laboratories, Lexington, Kentucky, U.S.A. Sensor modules and associated software for glutaraldehyde , ammonia, carbon monoxide, carbon dioxide, hydrogen, alkanes, alcohols and liquid hydrogen peroxide are also available from Air Purification Laboratories. However, these and other similar gas or liquid detection sensor systems are typically manufactured for use as environmental air or liquid sampling devices and, therefore, must be adapted for use in the sterilization systems according to the system and method of the present invention. The stability and sensitivity, and thus the accuracy, of the sensor sterilant concentration measurements are optimized for differing environmental conditions encountered in different sterilization systems. For example, sterilization systems employing ethylene oxide gas or hydrogen peroxide vapor involve different temperatures, pressures, relative humidities, and/or air flow velocities. In addition, some sterilization systems may include the presence of oxidizing substances, other than the sterilant to be measured, that may affect the electrical conductivity or resistance of the probe. Further, environmental conditions, such as relative humidity, temperature and pressure, may change dramatically during a single sterilization cycle. Such sterilization cycles include, but are not limited to, those employing deep vacuums with or without admission of air during the cycle, flow-through systems at near atmospheric pressure, and combinations of these, and other systems that are known in the art. Virtually every sterilization system also employs a cycle phase wherein the sterilant is exhausted from the sterilizer after the exposure phase and may include aeration of the sterilizer. To assure the accuracy of the sensor in measuring the concentration of the sterilant during all sterilization cycle phases, the sensor is preferably initially calibrated for the sterilization application for which it is to be used according to the initial cycle parameters in the sterilization chamber including, but not limited to, temperature, pressure, relative humidity and, if applicable, the presence and/or flow of air or the presence of other oxidizing gases in the system. Thus, the sensor-based monitoring and control system may be used, and calibrated for, virtually every sterilization cycle involving liquid and gas sterilants. To assure the accuracy of the sterilant concentration measurement throughout changing environmental conditions during a sterilization cycle, the sterilizer control system is programmed to monitor the environmental parameters and, if one or more of the environmental parameters changes, to signal the sensor module electronics and software to recognize and program a new baseline value for the sterilant concentration at the new condition. As discussed below, the baseline value is preferably readjusted in response to pressure changes within the sterilization system. As illustrated in FIGURE 1, a typical sterilization system employing vapor hydrogen peroxide or ethylene oxide gas sterilant comprises a sterilization or decontamination enclosure or chamber 1 having a sealable door 2, a sterilant supply inlet line 3, a sterilant inlet valve 4, a chamber drain line or exhaust outlet line 5, and a chamber outlet valve 6 typically connected to a vacuum pump 7. Although the schematic represents an ethylene oxide or hydrogen peroxide gas sterilization system, a liquid or other gas sterilization system could be represented differently without departing from the spirit of the invention. A semiconductor-based sensor module 8A is located within the chamber or removably connected to a chamber wall 9 or chamber dram/exhaust outlet wall 10 with the sensor in fluid contact with sterilant in the chamber 1 to measure the concentration of the sterilant in the chamber. Sensor modules 8B, 8C, and others not shown are additionally or alternatively located in another area of the sterilization system, such as m a line 3 leading to the sterilant supply inlet valve 4 or m a line 18 leading from the outlet valve 6. A single semiconductor module m fluid communication with the sterilant is sufficient to provide real time sterilant concentration values As described below, it is sufficient that the sensing element alone be in fluid communication with the sterilant, such that the remainder of the sensor module may be interior or exterior to the chamber. The sterilant concentration sensing element of the sensor module may be located m a load-simulation device for real time monitoring and control of the concentration of a sterilant in a load and for parametric release of the load when the sterilization parameters, including sterilant concentration have been met . When employing liquid sterilants, such as aqueous hydrogen peroxide, as a source of a gas sterilant, it may also be desirable to employ a sensor module to determine the actual concentration of the liquid sterilant being delivered to the means for generating the gas sterilant Preferably, an additional semiconductor- junction sensor module 8C is located exteriorly to the sterilization system to sense the presence of and concentration of a gas sterilant in the environment around the sterilization system, and to provide for audible and visual alarms if predetermined gas limits are exceeded in this area As described below, the external sensor module 8C is in electronic communication with a sterilizer control 12 which signals a process control unit 14 to operate valves, pumps, etc to shut off the sterilant delivery line 3 and/or exhaust sterilant from the sterilization system if predetermined gas limits are exceeded. The sterilization system illustrated in FIGURE 1 also includes sensors (not shown) for measurement of system environmental parameters, such as resistance temperature detectors (RTDs) for temperature monitoring, pressure transducers for pressure monitoring, humidity sensors for monitoring relative humidity and air velocity meters for measuring air flow rates. The preferred locations for the environmental parameter sensors and sterilant concentration sensor modules in any particular sterilization system are dependent on the sterilization system employed and are known to those skilled in the art. As illustrated in FIGURES 1 and 2, each semiconductor-based sensor module 8 has a sensing end 120 that comprises a sensing element 124 of a chemical composition that is selected based on the particular sterilant liquid or gas to be detected in a particular sterilization system. When in fluid contact with the sterilant under a given set of environmental conditions (i.e., the parameters of temperature, pressure, relative humidity, air flow, etc.), the sensing element 124 exhibits a particular electrical characteristic, such as conductivity, capacitance or resistance, that is measurable. Assuming that the environmental parameters do not change, a change in the electrical characteristic of the sensing element indicates a concomitant, reliably measurable change in sterilant concentration. The sensor module 8 also comprises a receiving unit 126 electronically connected to the sensing element 124 for monitoring the electrical characteristic value, such as capacitance, resistance, or other ampermetric and potentiostatic values, and for detecting in real time a change in the electrical characteristic value as an indication of a proportional change in sterilant concentration. The receiving unit 126 comprises electronics and software to receive and analyze an electronic signal from the sensing element 124 and to convert the signal into an analog signal for transmission, via a transmitting unit 122, to the sterilizer control system 12. Thus, the electrical signal from the sensor module 8 that is indicative of the sterilant concentration under a given set of environmental conditions is transmitted to the sterilizer control system 12 as an analog output signal, which may be in milliamperes or volts. The transmitted signal is read by the sterilizer control system 12 which then sends an input signal to a process control system 14. Based on this input signal, the process control system 14 provides output instructions to operate electrically operated valves, pumps, etc., which in turn control various phases of the sterilization cycle, including delivery of liquid or gas sterilant during the exposure phases and/or removal of the sterilant during exhaust or purge phases. In the case of the environmental sensor monitor 8C, the process control system 14 is preferably programmed to control various air handling and ventilation systems to clear the environment of the noxious gases and, more preferably, to respond by preventing further delivery of sterilant to the system and/or by exhausting sterilant from the system. The sterilizer control system 12 may be any system including, but not limited to, a microprocessor or a logic circuit that is programmed to receive the sensed sterilant concentration value and also to control the value of the sterilant concentration in real time during the sterilization cycle by governing the process control unit 14. Thus, predetermined target processing conditions inside the sterilization system may be maintained during all phases of the sterilization cycle, including the exposure to sterilant phase and the exhaust/purge phase. In a typical sterilization cycle employing the sterilant concentration real time monitoring and control system of the invention, the sensing element 124 is positioned in a sterilizer 1 for sensing a baseline concentration of the sterilant at a first set of environmental parameter values. The sensing element 124 has a baseline electrical characteristic value at the baseline concentration. The receiving unit 126, electronically connected to the sensing element 124 is programmed to store the baseline electrical characteristic value and to detect m real time a change in that value, as the sterilization cycle proceeds, as an indication of a new sterilant concentration at the first set of environmental parameter values. The new value of the sterilant concentration is transmitted in real time to the sterilizer control system 12 via the transmitting unit 122 The sterilizer control system 12 is programmed to store a predetermined reference sterilant concentration range at the first set of environmental parameter values. The sterilizer control system is further programmed to receive the new sensed concentration value and to compare that value to the stored reference concentration range, and to indicate acceptable sterilization conditions when the new value falls within the reference range, and to signal the process control unit 14 to change the sterilant concentration when the new value falls outside the reference concentration range. As described above, when an environmental condition m the sterilization system changes during a sterilization cycle, the sensor module 8 is adjusted to assure the accuracy of the sterilant concentration reading. Thus, the sterilizer control system 12 is preferably further programmed to monitor the set of environmental parameter values during the cycle and, when a parameter value changes, to signal the receiving unit 126 to adjust the sensor system by recognizing and storing a new baseline sterilant concentration value at the new set of environmental parameter values For example, in a sterilization system employing vapor hydrogen peroxide as the sterilant in a vacuum chamber, the sensor system is initially set with respect to the temperature, pressure and relative humidity of the chamber at the beginning of a cycle. Hydrogen peroxide vapor is introduced into the chamber, producing a slight rise in pressure. A real time measurement of the concentration of the vapor is determined and transmitted to the sterilizer control system After a hold period, air is introduced into the chamber and the pressure in the chamber rises. Pressure transducers m the chamber transmit the rise in pressure value to the sterilizer control system which then sends a signal to the receiving unit 126 to establish a new baseline for the sterilant concentration reading at the new pressure level. Preferably, the sterilant concentration is monitored and controlled in real time during the sterilization cycle to maintain a concentration that falls within the predetermined sterilant concentration range for a period of time sufficient to achieve sterilization. During the exhaust/aeration phase of the cycle, the sensor module 8 continues to provide real time monitoring of the sterilant concentration, and the sterilizer control system 12 continues to signal the process control unit 14 to operate valves, pumps, etc., to exhaust and/or aerate the system until all the sterilant is removed from the sterilizer or is at an acceptable level to end the sterilization process. During the aeration phase of the sterilization cycle, the pressure in the chamber typically alternates between deep vacuum and pressure rises due to admission of air. To obtain a real time sterilant concentration reading under these conditions requires frequent adjustment of the sensor system baseline reading . To monitor and control the concentration of the sterilant in the immediate environment exterior to the sterilizing system, an additional exterior sensor probe 8C is provided. The sensing element of this probe is in electronic communication with the receiving unit and the external sterilant concentration is transmitted to the sterilizer control system through the transmitting unit . The sterilizer control system is programmed to store a predetermined sterilant concentration limit value and to signal the process control system to stop the sterilization cycle and/or exhaust sterilant from the sterilizer when the external concentration value exceeds the predetermined limit value. In another embodiment, the semiconductor sensor module transmitting unit 122 transmits an analog signal representing the sterilant concentration value in real time to an external microprocessor 16 intermediate to the sterilizer control system 12. Such an intermediate processor 16 preferably has the capability of data storage, data display, and alarm capabilities separate from the sterilizer control system 12. Data from the intermediate processor may then be sent to the sterilizer control system and also may be retained as a permanent record of sterilization parameters throughout the sterilization process. The method of the invention proceeds by the steps of exposing the sensing element of the sensor probe to the sterilant during a sterilization cycle; sensing a baseline concentration of the sterilant at a first set of environmental parameter values, detecting a real time change in the electrical characteristic of the sensing element as an indication of a new concentration of the sterilant at the first set of environmental parameter values; transmitting the value of the new concentration to the sterilizer control, and controlling the sterilant concentration in real time during the sterilization cycle in response to a signal from the sterilizer control system. The method preferably includes the step of establishing a new baseline sterilant concentration value in response to a change m value of any one of the first set of environmental parameters during the sterilization cycle The method further preferably includes the steps of maintaining the sterilant concentration value within the reference sterilant concentration range fot" a period of time sufficient to achieve sterilization and exhausting the sterilant from the sterilizer when sterilization is achieved The method preferably further includes the steps of providing a sensor probe external to the sterilizer, programming the sterilizer control system to store a predetermined sterilant concentration limit value, and stopping the admitting of sterilant into the sterilizer and/or exhausting the sterilant from the sterilizer when the external sterilant concentration exceeds the predetermined concentration limit value.

Claims: Having thus described the preferred embodiment, the invention is now claimed to be: 1. A sterilization system including a sterilization chamber, a means for admitting a chemical sterilant into the chamber, a means for removing sterilant from the chamber, and a process controller for controlling the sterilant admitting means and the sterilant removing means, characterized by: a sensing element positioned in the chamber, the sensing element having an electrical characteristic which varies in accordance with a concentration of the sterilant; a receiving unit connected to the sensing element, the receiving unit storing a baseline electrical characteristic value corresponding to a baseline concentration of the sterilant at a first set of environmental parameter values and detecting in real-time a change in the electrical characteristic of the sensing element relative to the baseline electrical characteristic value as an indication of a real-time concentration of the sterilant at the first set of environmental parameter values; the sterilizer control comparing the real-time concentration value with reference sterilant concentration ranges and controlling a concentration of sterilant in the chamber in accordance with the comparison. 2. The system as set forth in any of preceding claims 1-5, further characterized by: the sensing element being a semiconductor element . 3. The system as set forth in either of preceding claims 1 and 2 further characterized by: the control system monitoring environmental parameters of the set of environmental parameter values during a sterilization cycle and in response to a change in a value of a monitored environmental parameter, causes the receiving unit to store a new baseline electrical characteristic value corresponding to the changed set of environmental parameter values . 4. The system as set forth in any of preceding claims 1-3, further characterized by: the electrical characteristic including one of capacitance and resistance. 5. The system as set forth in any of preceding claims 1-4, further characterized by: the sterilant being selected from the group consisting of ethylene oxide gas, liquid hydrogen peroxide, hydrogen peroxide gas, liquid formaldehyde, formaldehyde gas, liquid peroxygen compounds, gaseous peroxygen compounds, ozone, alcohol, glutaraldehyde , ammonia, and mixtures thereof. 6. The system as set forth in any of preceding claims 1-5, further characterized by: a second sensing element positioned externally to the chamber for sensing sterilant concentrations outside of the chamber, the second sensing element being connected with the sterilizer control. 7. The system as set forth in any of preceding claims 1-6, further characterized by: the sensing element being removable from the sterilizing chamber to facilitate replacement thereof . 8. A method of sterilization in which a liquid or gaseous chemical sterilant is admitted into and removed from a sterilization chamber, characterized by: sensing a concentration of the sterilant with a sensing element having an electrical characteristic that varies with a concentration of the chemical sterilant; establishing a baseline concentration of the sterilant by storing a baseline electrical characteristic value at a first set of environmental parameter values; monitoring a change in the electrical characteristic relative to the baseline characteristic value in real-time to determine a real-time concentration of the sterilant; controlling the sterilant concentration in the chamber in real-time in accordance with the monitored real-time concentration of the sterilant. 9. The method as set forth in claim 8, further characterized by: the environmental parameter values including at least one of temperature, pressure, relative humidity, air flow velocity, an oxidizing substance concentration, and combinations thereof. 10. The method as set forth in either of preceding claims 8 and 9, further characterized by: the sterilant being selected from the group consisting essentially of ethylene oxide gas, liquid hydrogen peroxide, hydrogen peroxide gas, liquid formaldehyde, formaldehyde gas, liquid peroxygen compounds, gaseous peroxide compounds, ozone, alcohol, glutaraldehyde , ammonia, and mixtures thereof. 11. The method as set forth in any of preceding claims 8, 9, and 10, further characterized by: the electrical characteristic being a potentiostatic characteristic . 12. The method as set forth in any of preceding claims 8, 9, 10, and 11, further characterized by: monitoring for at least one of the environmental parameters ,- in response to a change in one of the monitored environmental parameters, establishing a new baseline electrical characteristic.

Inventors: STEWART BONNIE, ZELL PETER E, STEWART, BONNIE, ZELL, PETER, E.
Applicants: AMERICAN STERILIZER CO, AMERICAN STERILIZER COMPANY
Application year: 1997
Main classification: A61L2
IPC codes: A61L2, G01N27