US3598516A

US3598516A - Method of sterilizing

Info

Publication number: US3598516A
Application number: US860447*A
Authority: US
Inventors: James J Shull; Robert S Lloyd
Original assignee: Individual
Current assignee: JAMES J SHULL; ROBERT S LLOYD
Priority date: 1969-01-02
Filing date: 1969-01-02
Publication date: 1971-08-10
Anticipated expiration: 1988-08-10

Abstract

THE STERILIZING PROCESS IS CARRIED OUT IN A SCALED CONTAINER OR AUTOCLAVE WHEREIN AIR IN THE CHAMBER IS DISPLACED WITH STEAM WHILE THE CHAMBER IS MAINTAINED AT VACUUM CONDITION. THE STEAM IS FED IN AT A RATE NO GREATER THAN THE CAPACITY OF THE VACUUM PUMP SO THAT THE PUMP WILL MAINTAIN THE DESIRED PRESSURE IN THE CHAMBER. STEAM THEREBY HEATS THE LOAD TO THE DESIRED TEMPERATURE IN A SHORT TIME AND THEREBY CREATES A MORE EFFICIENT AN SHORTER OVERALL TIME FOR THE STERILIZING CYCLE. A CONTROL IS USED, MADE UP OF A NEEDLE VALVE IN THE STEAM LINE WHICH IS USED TO SET THE APPROXIMATE FLOW RATE. ACTUALLY, THIS FLOW RATE IS SLIGHTLY GREATER THAN THE FLOW RATE REQUIRED. AN ADDITIONAL SOLENOID VALVE IS PLACED IN THE STEAM LINE IN SERIES WITH THE NEEDLE VALVE AND A THERMOSTATIC CONTROL IS PLACED IN THE CHAMBER DRAIN LINE TO TURN THE SOLENOID VALVE ON AND OFF TO MAINTAIN THE DESIRED LOAD TEMPERATURE AT APPROXIMATELY 130*F. THE TURNING ON AND OFF OF THE SOLENOID VALVE TO MODULATE THE DRAIN LINE TEMPERATURE SETS UP A PLUSING FLOW OF STEAM AND STEAM PRESSURE CONDITION OF APPROXIMATELY 25 TO 30 IN. HG WHICH HELPS TO PURGE THE LOAD OF ENTRAINED AIR THEREBY HEATING THE LOAD MORE RAPIDLY. FOLLOWING THE SIMULTANEOUS APPLICATION OF VACUUM AND STEAM, A MICROBIOCIDAL CHEMICAL STERILIZING GAS IS ADMITTED TO THE STERILIZING CHAMBER.

Description

Aug. 10, 1971 J. J. SHULL ETAL 3,598,516

METHOD OF STERILIZING Filed Jan. 2, 1969 2 Sheets-Sheet 1 l0 ETHYLENE OXIDE STEAM /MO!STURE I HR 5! I l 8? TEMQ l ATMOS w g I PRESSuRE V) I 51 Q 0 o n. MIN. TIME 70 E Fig. 2

DR JAMES J. SHULL ROBERT S. LLOYD United States Patent US. CI. 2157 5 Claims ABSTRACT OF THE DISCLOSURE The sterilizing process is carried out in a sealed container or autoclave wherein air in the chamber is displaced with steam while the chamber is maintained at vacuum condition. The steam is fed in at a rate no greater than the capacity of the vacuum pump so that the pump will maintain the desired pressure in the chamber. Steam thereby heats the load to the desired temperature in a short time and thereby creates a more eflicient and shorter over all time for the sterilizing cycle. A control is used, made up of a needle valve in the steam line which is used to set the approximate flow rate. Actually, this flow rate is slightly greater than the flow rate required. An additional solenoid valve is placed in the steam line in series with the needle valve and a thermostatic control is placed in the chamber drain line to turn the solenoid valve on and off to maintain the desired load temperature at approximately 130 F.

The turning on and off of the solenoid valve to modulate the drain line temperature sets up a pulsing flow of steam and steam pressure condition of approximately 25 to 30 in. Hg which helps to purge the load of entrained air thereby heating the load more rapidly. Following the simultaneous application of vacuum and steam, a microbiocidal chemical sterilizing gas is admitted to the sterilizing chamber.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of patent application, Ser. No. 414,721, filed Nov. 30, 1964 and now abandoned.

FIELD OF INVENTION This invention relates to sterilizing methods and, more particularly, to a method of sterilizing with a gaseous agent, requiring moisture for its activity whereby said method also insures rapid heating as well as moisturization of materials to be sterilized, thereby permitting shorter sterilizing times than have heretofore been possible in commercially available equipment.

DESCRIPTION OF PRIOR ART The Merriam Pat. 2,080,179 discloses a sterilizing cycle wherein steam is introduced to the chamber after the chamber has been evacuated and prior to the introduction of gas. Merriam suggests dry steam at atmospheric temperature and suggests that the product may be heated before putting it into the chamber.

The Hickey Pat. 3,035,886 discloses a sterilizing process wherein the air from the chamber is purged with steam at atmospheric pressure and thereafter, ethylene oxide gas is introduced.

SUMMARY OF INVENTION It is the purpose of this invention to provide a method and apparatus whereby the sterilizing cycle time is substantially shorter than conventional cycles now in use, and whereby the protective effect on the bacteria by contaminating debris or salts is minimized or overcome.

3,598,516 Patented Aug. 10, 1971 It is another object of the invention to limit the necessity for humidity measuring and control systems on gas sterilizers.

These objects are accomplished by two unique innovations in the method of heating and moisturizing the material within the sterilizer chamber.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagrammatic view of the sterilizer chamber and functioning components according to the invention.

FIG. 2 is a graph showing load temperature, chamber relative humidity and chamber pressure during the course of a typical sterilizing cycle according to the invention.

FIG. 3 is a diagram which shows the advantage of time saving over a known sterilizing cycle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT It is known in the art that the time required to sterilize with ethylene oxide is inversely proportional to the temperature of the most remote part of the materials to be sterilized with ethylene oxide. Laboratory investigations revealed that injection of steam into the chamber at low absolute pressures was the most rapid way practical to heat materials contained within said chamber. It was discovered that to carry out such a process most efficiently a chamber containing materials to be sterilized may be evacuated and steam may be injected into the chamber simultaneously with the evacuation. Such operation may be continued for a sufficient time to allow the steam to displace substantially all air from the materials in the chamber and to establish the desired temperature in the goods.

Because steam moisturizes materials upon which it condenses during the heating process, near complete humidification of the materials results. Both the heating and the humidification process, however, are dependent upon substantially complete elimination of air from the chamber and the materials contained therein. Experimental studies showed that air admitted to the sterilizer together with the injected steam at low pressure interfered with sterilization of bacterial spore control.

When the vacuum-steam portion of the cycle has been continued for a suificient time for the materials to achieve a suitable temperature, for example, 130 F. plus or minus 5 F. throughout the materials being preferred, the vacuum line is closed and the sterilizing gas is admitted to the chamber until a suitable gas concentration is achieved. The gas admission is then terminated, gas concentration being maintained, however, by adding make up gas to the chamber as required. Experiments have shown that steam, together with the gas in addition to the humidifying initially, was advantageous to obtain a sterility of spore test preparations in which the spores were protected by salts or cellular and organic materials. It is therefore desirable to add steam during gas change.

In carrying out the invention, a sterilizing chamber 10 was outfitted with a vacuum pump 12 connected to the drain line 13 in the bottom of the chamber. A heat exchanger 14 was provided and steam at, for example, 50 to p.s.i.g. connected as indicated. To heat, steam is fed to the sterilizer chamber through the charging line 5. Steam was admitted to the chamber through the heat exchanger through the metering valve 17 and the solenoid valve 16. The ethylene oxide was admitted through valve 18 and through the heat exchanger 14 to the sterilizer 10. The walls of the sterilizer chamber were preheated by a steam jacket (not shown) to the desired temperature.

A thermostat 23 in the drain line controlled the solenoid valve in the steam line so that the temperature of the drain line remained at approximately F. The metering valve 17 was set to give a flow slightly above that required to maintain the desired temperature in the chamber, the proper amount of steam being controlled by solenoid valve 16 actuated by thermostat 23. The gas may be flushed from the chamber by filtered air by way of valve 21 and filter 22. All pressures referred to in this specification are absolute pressures unless otherwise stated.

The following are examples of operating cycles:

EXAMPLE I 1) Load the chamber with materials to be sterilized.

(2) Start vacuum pump 12.

(3) Open valve 19 and valve 17 and valve 16 to admit steam through the heat exchanger 14 at a rate that is 'slightly greater than that necessary to maintain the temperature of approximately 130 F. in the load.

Continue evacuating and injecting steam for a period of time necessary for the temperature of the load to reach 130 F. throughout.

(4) Close valve 19 and open valve 18 to admit sterilizing gas leaving valve 16 open to admit sterilizing gas and steam simultaneously to chamber 10. Continue admission of gas and steam until the chamber pressure reaches 8 p.s.i.g.

(5) Close valve 17 and valve 18. Maintain pressure for one half hour by adding ethylene oxide mixture as necessary through valve 18.

(6) Open valve 19 and evacuate chamber to 26 inches Hg.

(7) Close valve 19 and open valve 21 to admit sterile air through filter 22.

Employing the method and apparatus just described, the cycle time required to sterilize materials and bacterial spores Within the chamber was reduced to less than 60 minutes from the 4 to 6 hour cycle common to conventional systems. Furthermore, this improved method is capable of destroying spores encapsulated within salt crystals and embedded in organic materials within 1 or 2 hours as opposed to the inability of conventional systems to sterilize such preparations regardless of time employed within reasonable limits.

EXAMPLE II The chamber was loaded with a cardboard carton containing as many cardboard inserts as the carton would hold with sterility predicated upon the sterilization of a number of accepted sterility contols inserted in the load. The cardboard carton was 12 x 12 x 22 inches in dimension. Thermocouple lead wires were connected to a potentiometer for measuring temperature rise within the cardboard carbon placed in various locations. A humidity sensing element for measuring the moisture content of the load was inserted in the cardboard carton.

Sterility controls consisting of glassine envelopes containing strips of filter paper impregnated with bacterial spores and dried at standard temperatures and pressures and spore-containing ceramic tiles, also packed in glassine envelopes, were placed in select locations inside the cardboard cartons. After placing the prepared and sealed cardboard cartons into the sterilizing chamber and activating the temperature recording potentiometer and humidity detecting equipment, the gas cycle as described above was initiated, using the following steps:

(1) Evacuate the chamber to 20 in. Hg and introduce steam during the time of evacuation at a rate equivalent to that required to maintain a load temperature of 130 F. with the vacuum system in operation.

(2) Maintain these conditions for a period of 20 minutes.

(3) Introduce an ethylene oxide-Freon gas sterilizing mixture to a chamber pressure of 8 p.s.i.g. at a rate of .50 lb./minute.

(4) Expose material load at 8 p.s.i.g. for a period of 60 minutes.

(5) Exhaust chamber to atmosphere and evacuate to 26 in. Hg and flush chamber with filtered air to atmospheric pressure.

(6) Repeat step 5.

7) Remove material load and test sterility controls for sterility using accepted bacteriological techniques.

Results: Sterility results were satisfactory.

EXAMPLE III The chamber was loaded with packaged articles representative of those commonly used in a typical hospital load as predicated upon the sterilization of a number of accepted sterility controls inserted in the packaged articles and using the gas cycle as described herein.

In this example, the steps were exactly as in Example II with the exception that the exposure period was 35 minutes.

Results: Load temperatures, humidity readings and chamber temperatures were comparable and closely similar to those illustrated in Example II. Sterility results were satisfactory.

EXAMPLE IV A typical load of hospital packaged articles with sterility predicated upon the sterilization of accepted sterility controls inserted in the packaged articles was placed in the chamber. The materials were made up of hospital articles consisting of packaged rubber cathethers, packaged anesthesia equipment and packaged surgical dressings placed in metal trays to simulate a typical hospital load.

The steps were carried out as in Examples II and III. The exposure period used was 40 minutes.

Results: The load temperature, humidity and chamber temperatures were closely comparable to those in the prior examples. Sterility results were satisfactory.

EXAMPLE V A load of packaged hospital materials typical of materials used in hospitals and predicating sterility upon the sterilization of accepted sterility controls inserted in the packaged articles and using the gas cycle as described in Example II.

Materials consisted of such articles as packaged plastic boxes, suction catheters, plastic tubes and items of small glassware were used to make up the test load.

The cycle was carried out as described in Example II allowing for exposure time of 45 minutes.

Results: The results were satisfactory.

EXAMPLES OF TESTS WHEREIN THE RESULTS WERE UNSATISFACTORY That is, non-sterility of the load resulted.

In contrast to the tests described above and to illustrate the ineffectiveness of gas cycles which are somewhat similar to that described but which differ in certain preconditioning techniques, the following examples are presented. To illustrate the manner in which the gas cycles differ from those of the original cycle, a comparison of the cycle variations with the patent application cycle was made as follows:

EXAMPLE I(A) Comparison of cycles A chamber was loaded with a number of sterility controls comprising spore contaminated strips of filter paper packaged in glassine, paper, muslin and plastic film envelopes, glass beads contaminated with water, saline and blood serum spore suspensions, and also packaged in similar envelopes and cotton-plugged test tubes containing spore strips and spore contaminated glass beads were placed in the material load comprising 6 cardboard cartons with cardboard inserts to illustrate a full hospital load of porous materials. The load was placed in the sterilizer and exposed to the inoperable cycle for a period of 15 minutes.

Patent application cycle Cycle variation Results satisfactory Results unsatisfactory EXAMPLE II (A) Cycle variation Patent application cycle (a) Chamber vacuum with steam (a) Chamber vacuum with steam injection for 20 minutes (preinjection for minutes. conditioning (b) Gas charge with steam to 8.0 (b) Gas charge with steam to 8.0

p.s.i.g. in 2-3.0 minutes. p.s.i.g. in 2-3.0 min.

(0) Exposure period of 60 minutes (0) Exposure period of 30 minutes.

((1) Cliamlggr temperature: 130 ((1) Cliamg er temperature: 130

Results satisfactory Results unsatisfactory Material load.A material load was prepared for this test comprising three cardboard cartons (9 /2 x 9 /2 x /2 inches) filled with cardboard inserts.

Sterility test results are shown in Example II(A).

EXAMPLE III(A) Patent application cycle Cycle variation (a) Chamber vacuum with steam (a) Chamber vacuum with no injection for minutes (presteam injection. conditioning).

(b) Gas charge with steam to 8.0 (b) Gas charge with steam to 8.0

p.s.i.g. in 2-3.0 minutes. p.s.i.g. in 2-3.0 min.

(0) Exposure period of 60 minutes (0) Exposure period of 60 min.

(d) Cgflmbgl temperature: 130 (d) Cllr amg er temperature: 130

Results satisfactory Results unsatisfactory Material load-The material load for this test was identical to that described for Example 11.

Sterility controls.-The sterility controls were identical to those listed for Example II and were inserted inside the three cardboard cartons in approximately the same areas as those employed in Example II.

EXAMPLE IV(A) Cycle variation (unsuccessful cycle) Patent application cycle (successful cycle) (:1) Chamber vacuum with steam (a) Chamber vacuum with steam injection for 20 minutes (preinjection ior 10 minutes. conditioning).

(b) Gas charge with steam to 8.0 (b) Continuation of vacuum with p.s.i.g. in 2-3.0 minutes. steam air mixture for 10 minutes.

(0) Exposure period of 60 minutes (0) Gas charge with steam to 8.0

p.s.i.g. in 2-3.0 min.

(d) Cliambsr temperature: 130 (d) Exposure period of 60 min.

(e) Chamber temperature: 130

Results satisfactory Results unsatisfactory EXAMPLE V(A) Cycle variation Patent application cycle (a) Chamber vacuum with steam (a) Chamber vacuumsteam injection for 20 minutes (preinjection for 8.0 min. conditioning) 0)) Gas charge with steam to 8.0 (b) Continuation of chamber p.s.i.g. in 2-3.0 minutes. vacuum with steam and air injection for 10 min.

(c) Exposure period of 60 minutes (0) Gigs charge to 8.0 p.s.i.g. in

3.0 minutes. ((1) Chamber temperature: 120 F. (d) Exposure period of 120 minutes (e) Chamber temperature: 130

Material load.The material load for this test was identical to that described for Example II.

Sterility control.The sterility controls were similar to those listed for Example II and were inserted inside the three cardboard cartons in approximately the same areas as those employed in Example H.

Results.Sterility of the sterility controls was not achieved in this test due to the irregular conditions employed in the preconditioning phase.

EXAMPLE VI(A) Patent application cycle Cycle variation (a) Chamber vacuum with injec- (a) Chamber vacuum with steam gon for)20 minutes (precondiinjection for 8 minutes.

oning (b) Gas charge with steam to 8.0 (0) Gas charge to 8.0 p.s.i.g. in

p.s.i.g. in 2-3.0 minutes. 2-3.0 mi

(c) Exposure period of 60 minutes" (c) Exposure period of 30 minutes.

((1) Chamber temperature: ((1) Chamber temperature Results satisfactory Results unsatisfactory Material Load.-A material load was prepared for this test comprising a typical hospital load of a variety of typical hospital items such as surgical instruments, face masks, rubber gloves, and catheters, wrapped in paper and polyethylene film (2.0 mils thickness).

CONCLUSIONS It was concluded that the overall results of the studies conducted showed that a properly synchronized preconditioning period was necessary in order to achieve sterility of the load materials. This is illustrated by the manner in which a controlled flow of steam is introduced into the chamber during the initial vacuum resulting in the maintenance of a specific range of absolute pressure within the chamber and the continuous flow of a time period sufiicient for the goods to equilibrate in temperature and become moisturized.

The attached examples show that under these conditions, the material load is heated and moisturized to a condition in which sterility is attained within a period shorter than many of the presently used ethylene oxide cycles.

In contrast, when the preconditioning factors are changed or modified to the extent that the desired chamber absolute pressure is not attained or, if attained, is not maintained for a sufficient period of time resulting in an unheated condition of the goods and non-moisturized goods, sterility is not achieved even though the actual exposure times of the material load to the sterilizing agent are exactly the same.

Graph lines, FIG. 3

FIG. 3 shows the relationship of load temperatures occurring in a gas cycle utilizing the steam preconditioning phase described herein and in a gas cycle where the preconditioning phase is Omitted.

Line ACD shows a pressure-time curve in a sterilizing cycle where gas is admitted immediately after vacuum is established. Line HI is a load temperature curve which occurs under the conditions designated by line ACD. Line AKI shows a pressure-time curve in a sterilizing cycle, such as described in the present disclosure; in which steam is introduced for conditioning at point B. Line EFG is a temperature-time curve which occurs wherein steam is injected at the beginning of the Nacuum or conditioning cycle in accordance with the method disclosed herein. This line shows that the load being sterilized has reached the desired operating temperature prior to the time the gas is admitted at point K.

The graph shows that in the cycle disclosed herein the load temperate reaches the desired value throughout in about 10 minutes. In the same time, using a cycle in which steam is not admitted according to the disclosed method, the load temperature had reached only about 27 C. (80 F.) at the time gas was admitted. This temperature is far below the desired operating temperature.

Since gas sterilization takes place at a very slow rate at 27 C. (80 F.), this particular cycle had to be a great deal longer in time than the cycle disclosed.

By contrast, with a cycle shown by curve EFG, the temperature of the center of the load had reached the desired 54 C. (130 F.) in a much shorter time thus allowing for a much shorter sterilizing time. Thus, it is clear that with the cycle Where steam is injected during the time the chamber is being evacuated, the load temperature reaches the desired 54 C. (130 F.) before gas admission. Therefore, FIG. 3 shows that the total cycle time with steam injection in accordance with the examples disclosed herein will be shorter than with prior cycles.

In summary, rise of load temperature is very slow, particularly when the preconditioning cycle is omitted and the ethylene oxide mixture is introduced into the chamber immediately after the desired initial vacuum is obtained. Since it has been established that the load of materials is, no doubt, moisturized during the preconditioning phase, although the actual concentration of moisture is not intentionally measured or even maintained at a selected level, humidity curves have been omitted from the graph. As indicated in FIG. 3, load temperatures occurring during the disclosed cycle using no conditioning are very slow in rising to the desired operating temperature, 54C. (130 F.). Significant temperature rise occurs only after the chamber has been charged and the exposure period has been initiated. This is in direct contrast to the shown in the curve EFG where load temperatures rise during the steam-vacuum conditioning phase and reach the desired operating temperature before the chamber is charged with gas.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of sterilizing comprising loading a sealed chamber with articles to be sterilized,

applying a vacuum means to said chamber thereby reducing the pressure below atmospheric and simultaneously admitting steam to said chamber at a rate that will maintain the temperature of said chamber at a desired temperature, and continuing said simultaneous vacuum application and steam admission for a predetermined period,

and stopping said vacuum means and stopping said steam means to said chamber and admitting a microbiocidal chemical sterilizing gas to said chamber, and maintaining said sterilizing gas in said chamber for a predetermined time period for sterilization of the articles disposed therein. 2. The method recited in claim 1 wherein said vacuum means maintains said chamber at approximately 20 inches Hg absolute.

3. The method recited in claim 1 wherein said sterilizing gas is ethylene oxide and said sterilizing gas is admitted at the rate of approximately 0.5 pound per minute.

4. The method recited in claim 3 wherein said steam and said vacuum are maintained in said chamber for a period as determined by the size of the load to ensure maintenance of said desired temperature. 5. A method of sterilizing comprising loading a sealed chamber with articles to be sterilized, applying a vacuum means to said chamber thereby reducing the pressure in the chamber below atmospheric and simultaneously admitting steam to said chamber at a rate that will maintain the temperature of said chamber at a desired temperature, and continuing said simultaneous vacuum application and steam admission for a predetermined period,

and stopping said vacuum means and stopping said steam means to said chamber and admitting a mixture of microbiocidal chemical sterilizing gas and steam to said chamber to bring the pressure in said chamber to a desired pressure and maintaining said mixture of sterilizing gas and steam in said chambe for a predetermined time period for sterilization a the articles disposed therein.

References Cited UNITED STATES PATENTS 2,080,179 5/1937 Merriam et al. 21DIG.-4 3,035,886 5/1962 Hickey 21DIG.4 3,042,533 7/1962 McConnell et al. 2l-DIG.-4 3,068,064 12/1962 McDonald 2l-DIG.4 3,372,980 3/1968 Satas 2lDIG.-4 3,409,389 11/1968 Bjork 21--56 FOREIGN PATENTS 656,699 1/ 1963 Canada 21-DIG.-4

1,148,704 5/1963 Germany 21-DIG.4

JOSEPH SCOVRONEK, Primary Examiner B. S. RICHMAN, Assistant Examiner US. Cl. X.R. 21--58, DIG.4