CA1165188A - Steam sterilization indicator - Google Patents

Abstract

IMPROVED STEAM STERILIZATION INDICATOR

ABSTRACT

A steam sterilization indicator comprising a tablet including a fusible material and an amount of binder, with a steam permeable membrane covering at least a part of the indicator. The fusible material is meltable at and above a predetermined temperature in the presence of substantially saturated steam. The melting point of the fusible material is substantially lower in the presence of saturated steam than when dry.
A wicking strip having one end in close proximity to the tablet is provided whereby upon the attainment of the predetermined temperature, or above, the fusible material melts and moves along the strip at a rate proportional to the temperature of the saturated steam.
The binder holds the fusible material in tablet form prior to the attainment of the predetermined temperature or above and further provides a mechanism for altering the rate movement of the fusible material along the strip as a function of the amount of binder used for certain temperature ranges, such rate of movement being similar to spore death kinetics.

Description

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This is a division of Canadian patent applica-tion Serial ~o. 344,525 filed January 28, 1980.
This invention relates to steam sterilization indicators. More particularly, it relates to steam sterilization indicators which may be variable and/or adjustable in rate of indication at diferent steriliza-tion temperatures.
Hospital utensils, such as surgical instruments, undergo sterilization for each use. In most instances, an autoclave is used to expose the utensils to live steam at various temperatures, usually betwean 250F and 275F, although other temperatures are also used. The purposeJ:for providing such sterilization is to destroy, with a high probability of success or safety factor, the microbial contamination which may be contained on these utensils. It is important to gauge the sterilization process so ~at the user may be assured that the utensils have, in fact, been subjected to those well defined conditions necessary to render the material free of living organisms with a high probability of success.
Several devices and techniques have been used to provide for such indication.
Of course, the materials which have been processed through the sterilizer could be biologically sampled to determine biological activity. However, this - technique, while highly accurate, obviously would be very costly and impractical.
One very reliable method for providing indica-tion of sterility is to utilize challenge spores which are placed in the autoclave during sterilization and then examined for their biological activity afterwards. For 1~6S~

steam sterilizations, these challenge spores are usually Bacillus stearothermophilus and are used because they have a very high resistance to steam sterilization, thus giving a large safety factor. One example of this technique is set forth in U.S. Patent No. 3,440,144 of April 22, 1969 to Andersen which provides a device for conducting such a test without the need to worry about subsequent contamination after the sterilization process is completed.
Another means to indicate sterilization is the use of sterilizer temperature recorder and gauges. These devices are usually attached to the sterilizer and measure the temperature in the sterilizer's exhaust line. While they are able to detect most malfunctions of the sterilizer, they cannot measure the condition at the place where the instruments were being sterilized.
A means for measuring the pxesence of steam, which is critical for steam sterilization, is an auto-clave indicating tape. An example of such indicator tape is set forth in U.S. Patent No. 2,889,799 of June 9, 1959, to Korpman. A pressure-sensitive adhesive tape is used which includes a heat modifiable dye stuff impregnated thereon changing color at predetermined temperatures.
However, these indicator tapes do not ta~e into account the time that the instruments have been e~posed to sterilizing temperature, and furthermore, are susceptible to prematurely changing color at low temperatures~

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Another test which has been utilized is a so-called Bowie and Dick test. This test measures the uniformity of steam concentration in dressing packs.
The test consists of several strips of autoclave indicating tape on a sheet of paper which is placed in the test pack.
T~e tape on the paper is measured for uniformity of color change. One of the major limitations of this test is its failure to distinguish between high temperatures ~or a short period of time or low temperatures for a long period of time.
More recently, steam sterility indicators have been provided which integrate time, temperature and steam presence. Such a device is shown in U.S. Patent No. 3,981,683 of September 21, 1976, to Larsson et al.
This device utilized a chemical such as 2-ethoxybenzamide or salicylamide as a fusible material. The melting points of these compounds are depressed by the presence of steam.
A wicking strip is provided in close proximity to the chemical so that upon melt the chemical will slowly travel up the wick at a rate proportional to the sterilization temperature and time of exposure to such temperature, as well as the presenGe of steam. The device includes a cover strip which is a polymeric rate controlling film permitting water vapor (steam) to pass through thus depressing the melting point of the chemical. The strip cover and the wick are adhered to a backing by the use of an adhesive such as a silicone.

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The device set forth in U.S. Patent 3,981,6~3 is particularly useful where the exact temperature in the sterilization process is unknown. If it were known that the apparatus to be sterilized was an exact tempera-ture, for example 250F, then the sterilizer could be run for exact amount of time so that the user could be assured of sterilization within a certain safety factor.
However, without fitting the autoclave with some highly sophisticated and accurate monitoring equipment, it is impossible to know whether all areas of the autoclave are uniform at the same temperature. It is well known that the temperature of items being sterilized can vary due to many variables such as air entrapment, penetration of steam through packing material and position within the autoclave. Therefore, due to this unknown variable of temperature, it is a common practice for the micro-biologist to investigate how a controlled change of temperature will affect the kill oE the microorganism.
He would do this by repeating the microbial death rate experiment at temperatures other than 250F. After completing these experiments at other temperaturesj a relationship can be obtained where the amount of time required to produce say 10 5 probability of surviving microorganisms, since this or some other safety factor producing a non-sterile item can be calculated.
Accordingly, it is one object of this invention to provide an improved steam sterilization indicator.
It is another object of this invention to provide an improved indicator which integrates time and temperature in the presence of steam.

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It is a further object of this invention to provide a steam sterilization indicator which may be made variable in rate indication for tracking the kill time of various microorganisms at various temperatures.
It is still another object of this invention to provide a steam sterilization indicator which uses less materials than many prior art indicators.
It is another object of this invention to provide a steam sterilization indicator which is easily and cheaply manufactured~
It is a further object of this invention to provide a steam sterilization indicator which is adapted to 510wly integrate time and temperature in the presence of steam so that the device may be made shorter, thus using fewer materials.
It is still another object of this invention to provide a sterilization indicator with an additional safety margin but still closely tracks the kill time of microbes at various temperatures.
In accordance with one form o~ this invention there is provided a steam sterilization indicator which includes a tablet made of a fusible material and an amount of a binder. The ~usible material is meltable at and above a predetermined temperature in the presence of substantially saturatedcsteam. A wicking strip having one end in close proximity to the tablet is mounted on a backing. The backing also receives the tablet. In a steam environment, when predetermined temperature of melt is reached for a tablet, the fusible material moves along the strip at a rate proportional to the integration - - , is~

of time and temperature. The binder holds the fusible material in tablet form and further provides a mechanism for alterin~ the rate of movement of the fusible mate-rial along the strip in proportion to the amount of binder used. An acrylic adhesive may be used to adhere the strip to the backing, as well as a transparent cover layer to the b~cking. The acrylic will further alter the movement of the fusible material along the strip.
In accordance with a further broad aspect of the present inve~ntion there is provided a steam sterili-zation indicator comprising a tablet including a fusible material and an amount of binder, with a steam permeable membrane covering at least a part of the indicator. The fusible material is meltable at and above a predetermined temperature in the presence of substantially saturated steam. The melting point of the fusible material is substantially lower in the presence of satura~ged steam than when dry. A wicking strip having one end in close proximity to the tablet is provided whereby upon the attainment of the predetermined temperature, or above, the fusible material melts and moves along the strip at a rate proportional to the temperature of the saturated steam. The binder holds the fusible material in tablet form prior to the attainment of the predetermined tempe-rature or above and further provides a mechanism for altering the rate movement of the fuxible material along the strip as a function of the amount of binder used for certain temperature ranges, such rate of move-ment being similar to spore death kinetics.

The subject matter which is regarded as the invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, may be better understood with reference to the following description taken in conjunc-tion with the accompanying drawings in which:
FIGURE 1 is a plan view of the indicator incorporating the present invention with a portion of the cover peeled back;
FIGURE 2 is a side view of the tablet which is utilized in Figure 1, FIGURE 3 is a graph of the death curve of the microbe C~sporogenes at various temperatures showing how the Z value is calculated, FIGURE 4 is a graph depicting the effect o-f different Z values on thermal death curves of microbes, FIGURE 5 is a graph showing the running time of the indicator shown in Figure ]. at several tempera-tures with various amounts of bincler in the tablet, FIGURE 6 depicts the run time of one of the devices of the subject invention at various temperatures compared with a;prior art device and the thermal death curve of a microbe.

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Referrlng first to Figures 3 and 4, Figure 3 shows an example of what the relationship of kill time vs. temperature might look like. The slope of the line is typical for microbial death rates, and, as can be seen, it is highly temperature sensitive. The death rate might be slowed down by a factor of 10 with a decrease of only 18F. Conversely, an increase in temperature of only 18F will require only 1/10 the sterilization time. In other words, sterilizing to a probability of 10 5 in this example in Figure 2 requires llQI~inubes~iat 232F, 11 minutes at 250F and 1.1 minute at 268F. This value of 18F has been called the Z value and is defined as the number of degrees that are required to traverse a thermal death rate curve by one log~ Thus this Z value becomes important when estimating spore death at different temperatures. This relationship has been defined mathe-matically through the following equation:
t = (F ) x 1o[(250 - T)/z]
where t = the amount of time required at the actual process temperature (T). :
In other words, it would require t minutes at temperature T in order to do the equivalent amount of sterilization as Fo minutes at 250F (the reference temperature for steam sterilization). While a Z value of 18F is typical, it may vary quite often from between 16F to 23F, and other values, depending on the type of microorganism, the pH
and salt concentration as well as other variables. There-fore, if an adequate sterilization process is to be described;, you must not only know the relative resistance at 250F but also the relative resistance at other temperatures. Thus the Z values must be known.

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The graph in Figure 4 graphically illustrates how a changein Z value can affect the sterilization times required at temperatures other than 250F. Notice the different slopes of the lines for the various Z values.
By using the standard sterilization equation set forth above, you can calculate that if Z is e~ual to 23F, a time of 81.5 minutes is required at 230F to obtain an Fo of 11 minutes. Conversely, for the same Fo, the time would havb to be increased 195.5 minutes at 230F for a Z value equal to 16F. Thus, it may be seen that, at temperatures lower than 250F, as the Z value decreases, the kill time at predetermined temperatures increases.
The device set forth in the aforesaid U.S. Patent No. 3,981,683 provides an indicator which is useful in a steam sterility process for spores having a Z value of 18F, but it is not very flexible in terms of measuring sterilization of devices contaminated with spores with other Z values.
Furthermore, it is a rather long device, thus materials are wasted.
Referring now more particularly to Figure 1, there is provided steam sterilization indicator 1 ~hich indicates that an environment or utensils in close proximity to it, has undergone proper steam sterilization. The device includes backing 2 which may be made of a metallic material acting as a good moisture barrier. In the preferred embodi-ment, the backing 2 is constructed of 3 mil thickness dead soft aluminum. The backing acts as a carrier or substrate for tablet 4 and wicking strip 5. The aluminum backing includes embossment or depression 3 near one of its ends for receiving the temperature and moisture sensitive tablet 4O Tablet 4 is made primarily from a chemical ~A~fi~

which melts or fuses at a predetermined temperature and above. However, its melt temperature is depressed somewhat in the presence of saturated steam. In the preferred embodiment, the chemical is salicylamide.
In order to increase manufacturing efficiency, i.e., placing the chemical in the embossment in the backing, it is desirable to maintain the chemical in tablet form. To do this and to provide the surprising results as will be described below, an amount of a binder is added to the temperature and moisture sensitive chemical. In the preferred embodiment of this invention the binder utilized is polyvinylpyrrolidine (PVP). The tablet also includes other constituents which are Xnown to those skilled in the art of manufacturing tablets and may include such materials as talc and "Syloid" (trade mark).
In order to provide a color indication on strip 5 which will be described below, tablet 4 may also include a heat stable soluble dye which wicks onto and moves along strip 5 so long as the temperature ancl moisture content are sufficient to maintain the fusible chemical in its liquid state. Dyes such as Spirit Soluble Fast Black RE and Spirit Soluble Orange RR, both available from BASF
Wyandotte Company, are suitable dyes.
The device is covered by clear plastic cover layer 6 which in this embodiment is a 2 mil thickness unoriented polypropylene film, one form of which is available from the Exxon Corporation as Extrel 50 ~ This clear layer enables one to see the position of the color front along wick 5. It also provides a controlled exposure of the temperature and moisture sensitive chemical to the steam since the polypropylene is slowly permeable to moisture transmission. The cover layer 6 as well as indicator ~, 6s~

strip 5 is adhered to the backing 2 by an adhesive 7, which in the preferred embodiment is an acrylic adhesive sold under the trade mark "3M 467", by Minnesota Mining Manufacturing Company. The importance of the use of the acrylic adhesive as well as the binder in the tablet will be made more clear below.
Wicking strip 5 is normally a porous material capable of wicking a liquid by capillary action. In this embodiment, the wick was made of "Whatman 1 Chrome" ~trade mark, available from the Whatman Company). It is placed either in contact with or nearly in contact with tablet 4 such that one end of the strip is within embossment 3. The wicking strip absorbs the melted chemical and carries the dye down the strip so long as the temperature is high enough and steam is present in a sufficient density. The rate of movement of the color front as well as the kill rate of microbes results from an integration of time and temperature so that this device is useful at various temperatures. That is, the time récIuired for the color front to move a certain fixed distance is very temperature dependent. The same is ~
true for the kill time of microbes. Then the color front on the strip reaches a certain position on the indicator, such as that indicated at position 8, it is assumed that the environment has undergone proper sterilization, i.e., the probability that all of the microbes present have been killed is, say, .99999. The device may be covered on the outside of the clear plastic covering with another sheet of paper (not shown) having an elongated slot which provides for a visual indication of the strip. This paper may have various indicia thereon.

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The device thus described operates in a similar manner with the device described in the aforesaid U.S.
Patent No. 3,981,683. However, the present device includes at least two important differences in composition and several important differences in operation. One of the differences in composition between the device of the subject invention and the previously patented device is that the temperature and moisture dependent chemical is in tablet form rather than just a glob of material. An acceptable tablet is shown in Figure 2. It is much easier to deposit a solid tablet into the embossment 3 during the manufacturing of the device. Furthermore, the exact quantity of chemical may thus be inserted easily into the embossment.
As stated previously, a binder such as PVP was utilized to hold the chemical in tablet form in order to provide this improved manufacturing process. In doing so, a surprising result occurred. It was found that by the use of this binder a programmable device could be manufactured which has a color movement rate proportional to the percentage of binder contained in the tablet. Furthermore, an indicatQr which follows the rate of kill of micro~es having various Z
values could be obtained by varying the percentage of binder.
The following table shows a comparison of kill times or a microbe having a Z value of 20.5 and a Z value of 18 and the run times along a strip 11.2 mm long utilizing a device incorporating the subject invention having salicylamide chemical with varying percentages of PVP binders.

TABLE I
Kill Times (Mins.) r Times ~Mins) Temp.Z = 20.5 Z = 18 0% 1/O 2% ~`3O~o~P~P
230113.5155.0 93.7 133.0174 240 23564.781.8 5871.5 85 111 24036.943.1 3538.5 44 48 24521.022.7 21.7 21.523.5 26 25012.012.0 13.0 12.513.0 13 2556.84 6.3 8.0 7.5 q.7 7 2603.903.34 4.9 4.8 4.7 4.4 2652.231.76 2.9 2.9 3.0 3.2 2701.270.93 1.8 1.9 2.1 2.6 Table I shows the indicator times of the device of Figure 1 using from 0/O PVP binder to 3% binder at various temperatures. As can be seen from the chart in Table I, the indicator times may be increased by increasing the amount of binder in the tablet so as to conform with the Z value o~ the particular microbe which is to be killed.
A gxaphic example of how the Z value of the device may be altered by adding or changing the binder content may be seen from the graph in Figure 5 which again shows three devices each having binders from 0 to 3%. The graph shows that the time for the device to run to completion increases, particularly at the lower temperatures, as the percentage of binder increases.
As it further may be seen from Figure 5, the changes in the binder content affects the range between 230F and 250F and the range of 260F to 270F, much more than at the mid-range.
Further, as can be seen, adding a binder intro-duces a safety factor in the operation of the indicator.
One may notice from Table I that a device with 0% binder to monitor a sterilization process with Z value of 20F
will run in 93.7 minutes, while the kill time of the particular microbe is 113.5 minutes, thus giving a false and potentially dangerous indication of kill. However, it should be noted that by adding 1% binder the indicator times are lengthened such that there is a slight margin of safety at all temperatures, and therefore this device will never indicate sterilization prematurely. For the bacteria which has a Z of 18F, a 2% binder would be utilized. It should be recognized from Figure 5 and from Table 1 that at temperatures above 260F the lines of the various percentage binders tend to come together and substantially flatten out. However, it should be noted that the lines are always curving upwardly and on the safety side of any bacteria with such a Z curve and these should never provide premature indication of sterilization.
Figure 6 shows a graph of a comparison of the device of the subject invention utilizing a 2% binder in comparison with the thermal death curve of B stearo-thermophilus and a prior art device as set forth in U.S.
patent 3,981,683. As can be seen, the device which utilizes a 2% binder PVP in salicylamide substantially tracks the death curve of B stearothermophilus, but on the high safe side. However, as can be seen~ the prior art device particularly at low temperature tracks the bacteria death curve on the lower unsafe side, and also shows a low "knee" at some high temperatures.
It has further been found that one may lower the temperature dependent reaction rate by utilizing an acrylic adhesive in the place of a silicone pressure sensitive adhesive which was used in the device set forth in U.S. patent 3,981,683. The adhesive is used to hold the wick and the cover onto the backing. It has also been found that there was a gross change in reaction rate of the device whereby its equivalent Z

value was decreased from 26F to 20F by replacing the silicone base pressure sensitive adhesive with an acrylic.
Table II below shows the actual time required for the device of the subject invention to indicate sterilization using a silicone adhesive, namely DensilTM 207~, provided by Dennison Inc. and two brands of acrylics, namely Dencryl 410, also provided by Dennison, Inc., and TackmasterTM 535, provided by the National Starch and Chemical Corporation.
TAB~E II

Indicator Times ~Min~.~
~E~ Silicon_ Acrylic IAcrylic II

260F 5.1 4.7 3.6 270F 3.1 ~1.85 1.9 _ Approx.26.0F 20.9F 20.0F
equivalent Z Values Thus it may seèm-that the indicator times are substantially lengthened and it may be seen that the equivalent Z value was decreased by approximately 5 to 6 by utilizing an acrylic in place of the silicone adhesive. It is not understood exactly what phenomenon is causing these changes in equivalent Z value and thus in indicator times, however, it is quite possible that the acrylic may be reacting with the salicylamide in some way to slow down the movement of the color front or the acrylic may be acting as a better seal as compared to the silicone to prevent some of the steam from penetrat-ing into the chemical, thus affecting the depression in the melt point.

Claims (9)

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

1. A steam sterilization indicator comprising: a tablet including a fusible material and an amount of binder, a steam permeable membrane covering at least a part of said indicator; said fusible material meltable at and above a predetermined temperature in the presence of substantially saturated steam, the melting point of said fusible material being substantially lower in the presence of saturated steam than when dry; a wicking strip having one end in close proximity to said tablet whereby upon the attainment of said predetermined temperature, or above, said fusible material melts and moves along said strip at a rate proportional to the temperature of the saturated steam; said binder holding said fusible material in tablet form prior to the attainment of said pre-determined temperature or above and further providing a mechanism for altering the rate of movement of said fusible material along said strip as a function of the amount of binder used for certain temperature ranges, such rate of movement being similar to spore death kinetics.

2. The indicator as set forth in claim 1 wherein said fusible material is salicylamide.

3. The indicator as set forth in claim 1 wherein said binder is polyvinylpyrrolidine.

4. The indicator as set forth in claim 1 further including a dye in said tablet for providing a moving color front on said strip.

5. An indicator as set forth in claim 4 wherein said dye is taken from the group consisting of Spirit Soluble Fast Black RE and Spirit Soluble Orange RR.

6. An indicator as set forth in claim 1 further including a backing, said steam permeable membrane being a cover; at least a portion of said cover being trans-parent, said backing and said cover enclosing said tablet and said strip' an adhesive for holding said cover and said strip to said backing.

7. An indicator as set forth in claim 6 wherein said adhesive is an acrylic whereby said acrylic adhesive acts to further alter the rate of movement of said fusible material along said strip.

8. The device as set forth in claim 1 wherein said amount of binder is no greater than 3% by weight of said tablet.

9, The device as set forth in claim 1 wherein the amount Of said binder is within the range of from 1% to 3% by weight of said tablet.