WO1995024622A1

WO1995024622A1 - Steam sterilization indicator

Info

Publication number: WO1995024622A1
Application number: PCT/US1995/002341
Authority: WO
Inventors: Frank E. Platko
Original assignee: Propper Manufacturing Company, Inc.
Priority date: 1994-03-11
Filing date: 1995-02-27
Publication date: 1995-09-14
Also published as: AU1882795A

Abstract

A steam sterilization indicator comprises: (a) a foil backing (1) having a thickness of at least 5 mil; (b) a wick (5) disposed on the foil backing (1); (c) a meltable chemical (4) disposed on the foil backing (1) in a position relative to the wick (5) such that upon melting the melted chemical will be transported through the wick (5), said meltable chemical (4) being melted at or below a target sterilization temperature in the presence of saturated steam, and at a temperature above the target sterilization temperature in the abscence of saturated steam; (d) a steam permeable cover (6) disposed over wick (5) and the meltable chemical (4); and (e) an aggressive high temperature silicone adhesive (2), said adhesive (2) adhering the foil backing (1) to the steam permeable cover (6) whereby a substantially sealed envelope surrounding the wick (5) and the meltable chemical (4) is formed from the foil backing (1), the steam permeable cover (6) and the adhesive (2). A suitable adhesive for use in this device is the type of adhesive employed in Densil® 2078, a 2 mil silicone adhesive film.

Description

STEAM STERILIZATION INDICATOR

BACKGROUND OF THE INVENTION This application relates to an indicator for use in monitoring the effectiveness of a steam sterilization process. More specifically, the application relates to a novel steam sterilization indicator which integrates the effects of time and temperature and saturated steam.

Steam sterilization integrators have been described in U.S. Patents Nos. 3,324,723; 3,981,683; 4,448,548 and 5,158,363 each of which is incorporated herein be reference. In general, an integrator device is formed of an envelope which encloses a wick and meltable chemical. The meltable chemical is selected from among chemicals having a normal melting point above a target sterilization temperature which is depressed to below the target sterilization temperature when contacted with sat- urated steam. When the indicator is exposed to saturated steam at a sufficiently high temperature, the chemical melts and travels along the wick, thus providing a visual indication of the time period over which conditions of sufficient temperature and steam exposure are maintained. The envelope is selected to fulfill two functions: to prevent leakage of the chemical from the indicator, and to control the rate at which steam from the sterilizer can contact the meltable chemical. As is apparent from U.S. 3,981,683, 4,448,548 and 5,158,363 and various commercial products sold under the trademarks SteriGage®, SteamLine®, Vapor Line™ and Therma-Log®, a common approach to the formation of the envelope is two utilize a foil backing to which a steam permeable cover is glued. In seeking an ideal adhesive for use in this application, the present inventor has formulated criteria that the adhesive should meet. First, it must be stable at the temperatures which occur during sterilization, i.e., approximately 121 to 134°C so that the indicator does not simply fall apart. Second, it must form a good bond between the foil backing and the cover to minimize steam leakage through gaps in the adhesive, which could negate the rate controlling effect of the cover. Finally, it must be able to maintain adhesion through the torsion- al stresses on the indicator which result from the vacuum and steam pulses of the sterilizer. It is the object of this invention to provide a steam sterilization indicator having an adhesive which meets these requirements.

SUMMARY OF THE INVENTION This and other objectives are achieved by a steam sterilization indicator in accordance with the invention comprising: (a) a foil backing having a thickness of at least 5 mil;

(b) a wick disposed on the foil backing;

(c) a meltable chemical disposed on the foil backing in a position relative to the wick such that upon melting the melted chemical will be transported through the wick, said meltable chemical being melted at or below a target sterilization temperature in the presence of saturated steam, and at a temperature above the target sterilization temperature in the absence of saturated steam;

(d) a steam permeable cover disposed over wick and the meltable chemical; and

(e) an aggressive high temperature silicone adhesive, said adhesive adhering the foil backing to the steam permeable cover whereby a substantially sealed envelope surrounding the wick and the meltable chemical is formed from the foil backing the steam permeable cover and the adhesive. A preferred adhesive for use in this device is the type of adhesive employed in Densil® 2078, a 2 mil silicone adhesive film. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 shows an exploded view of an indicator in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows an exploded view of an indicator in accordance with the invention. The indicator is formed from a metal foil backing 1 to which is applied a layer of a high performance silicone adhesive 2. Preferably, an embossment 3 is formed in the foil backing 1 to receive a tablet of meltable chemical 4. Wick 5 is dis¬ posed on the foil backing 1 in contact with or in close proximity to the tablet of meltable chemical 4. A steam permeable cover 6 is placed over the wick 5 and the tablet of meltable chemical 4. The steam permeable cover 6, the adhesive layer 2 and the foil backing 1 taken together form a substantially sealed envelope surrounding wick 5 and the tablet of meltable chemical 4. Finally, an optional paper cover plate 7 having a window 8 therein is affixed over the steam permeable cover 6.

A suitable metal foil for use as the foil backing 1 is 5 mil aluminum foil Grade: 1100-0 MF, "A" wettable available from Metal Foils, Inc., Willoughby, Ohio. Other metal foils of comparable thickness and rigidity may also be employed provided they are sufficiently malleable to permit the formation of the embossment 3.

The meltable chemical 4 may be in the form of a regularly shaped tablet or pellet as shown in Fig. 1, or in the form of an irregularly-shaped, amorphous mass. In general, the tablet may by any chemical which melts at or below a target sterilization temperature in the presence of saturated steam, and at a temperature above the target sterilization temperature in the absence of saturated steam. A preferred chemical for use in steam sterilizers having a target temperature of 121-134°C is salicylamide. A binder may be added to the tablets as described in U.S. Patent No. 4,448,548, but is not required. When the meltable chemical selected is itself colored, no additional materials are required within the tablet 4. In the case of colorless or substantially colorless compounds, a dye may be added to facilitate easy observation of the advance of the meltable chemical along the wick. Suitable dyes include Orasol Brilliant Blue, Spirit Soluble Fast Black (also called Zapon Black X-50) , and Spirit Soluble Orange RR.

The wick is advantageously a strip of filter paper through which the melted chemical will migrate. The specific grade of filter paper is selected to yield a device having an appropriate migration distance for the melted chemical at the sterilization temperature in the normal sterilization cycle time. Specific suitable papers include Whatman No. 1 Chrome and Ahlstrom Grade 602-025.

The steam permeable cover may be made from various materials which are permeable to steam at the target sterilization temperature. A preferred material for use as the steam permeable cover is 1.2 mil transverse oriented polypropylene. In general, films of about 0.5 to 2 mil can be used depending on the degree of steam transmission required.

The optional paper cover 7 is applied to facilitate marking of the indicator with instructions, a line to indicate a positive or negative test result and indica¬ tions of origin. It should be substantially lint free and capable of being printed. The paper cover is attached to the outer surface of the steam permeable ^' cover by an adhesive such as a pressure sensitive latex emulsion. Since the performance of the indicator has been observed to be the same whether or not the paper cover is included, however, the specific identities of the paper and the adhesive holding it to the steam permeable cover are not critical.

The key element in the present invention is the use of a high performance silicone adhesive such as the adhesives used in Densil® 2078 (available from Flexcon Corp., Spencer, Massachusetts), or CW-14HT (available from Specialty Tapes, a Division of RSW, Inc., Racine, Wisconsin) , and the use of this adhesive in combination with the 5 mil or greater foil backing. The silicone adhesive is preferably applied as a film to either the foil backing or the steam permeable cover. In the case of Densil® 2078, this film to 2 mil thick. Thinner films, for example the 1 mil film sold as Densil® 1078 can also be employed, as can thicker films, although in the latter case the cost may be prohibitive.

The adhesive bonds the steam permeable cover to the foil backing to form a substantially sealed envelope around the wick and the tablet of meltable chemical. As used herein, the term "substantially sealed" means that a seal is formed which is free of distinct channels or holes through which gases, including steam, may pass in an unobstructed manner. It will be understood, however, that the adhesive may possess some degree of permeability to steam or other gases, which may be greater than, equal to or lesser than the permeability of the steam permeable cover, and thus that some portion of the steam probably enters the device through the adhesive rather than through the steam permeable film. The combination of the high performance silicone adhesive and the thicker foil backing makes it possible to achieve an indicator of small size (an important feature for minimizing material costs) that has excellent performance characteristics, including a low failure rate, high indicator-to-indicator reproducibility, and low indicator distortion resulting in improved indicator readability. This achievement of a highly successful indicator of small size using this combination of mater¬ ials is surprising in view of the knowledge available in the prior art. U.S. Patent No. 3,981,683 discloses an indicator in which a silicone adhesive is used, but the indicators made using this technology are of larger size, i.e., about 100 mm by 21 mm. U.S. Patent No. 4,448,548 discloses smaller size indicators, but suggests that silicone adhesives are not suitable for such devices. Indeed, U.S. Patent No. 4,448,548 uses Densil® 2078 and a 3 mil foil backing as a comparative example and suggests that the performance is defective. The '548 patent hypothesizes that this may be due to the formation of a seal by the acrylic adhesive which is better than the seal achieved by the silicone adhesive. Although nowhere reflected in the prior art, applicant's experience has been that indicators made using Densil 2078® adhesive films and 3 mil foil, i.e., an indicator made in accordance with the '548 patent, undergo significant physical twisting and deformation during a sterilization cycle. While not intending to be bound by any specific theory, this deformation may result in the formation of holes through which non-selective transport of gases, including steam, is possible. This would result in an increase in the exposure of the inter- ior of the indicator to steam, and thus in the potential for false positive results.

Contrary to the '548 patents hypothesis of poor adhesion by the silicone, it is applicant's belief that this deformation occurs because the high performance adhesive is actually too good an adhesive, and fails to allow for the stresses induced by the vacuum and steam pulses of the sterilizer. The use of a thicker foil backing overcomes this problem, which is wholly unrecog¬ nized in the art, and produces a useful indicator of very good performance characteristics.

The invention will now be further described by way of the following non-limiting examples.

EXAMPLE 1 Sterilization indicators were prepared in the manner depicted in Fig. 1 using 5 mil aluminum foil Grade: 1100- 0 MF, "A" wettable as the foil backing, a tablet consisting of salicylamide and a dye, an Ahlstrom 602-025 paper wick, Densil® 2078 laminating adhesive, a 1.2 mil polypropylene film and a paper cover. The indicators were run in several batches in a standard QC cycle in a pre-vacuum sterilizer at 272°F for varying periods of time, and the distance that the melted chemical migrated in the wick was measured. The results are summarized in Table 1.

TABLE 1

SAMPLE SAMPLE EXPOSURE TEMP Length of Migration (mm) SIZE TIME (°F)

HIGH LOW AVG

1 8 45 sec 272 9 8 8.5

2A 2 1.5 min 272 14 14 14

2B 5 1.5 min 272 14 12 13

3A 2 2 min 272 16 16 16

3B 5 2 min 272 15.5 15.5 15.5

4 8 4 min 272 29 28 28.8

EXAMPLE 2 Indicators of the same type used in Example 1 were tested in a gravity-type sterilizer operated at 250°F. The results are summarized in Table 2. In Table 2, the notation "max" reflects that the chemical migrated the full length of the indicator strip.

TABLE 2

SAMPLE SAMPLE EXPOSURE TEMP Length of Migration (mm) SIZE TIME (°F)

HIGH LOW AVG

5 8 5 min 250 13 10 10.5

6A 2 10 min 250 20 20 20

6B 4 10 min 250 19 17 18.3

6C 5 10 min 250 22 20 21

7 8 20 min 250 max max max

8 8 30 min 250 max max max

Example 3 Indicators were made as described in Example 1, except that 1.0 mil polypropylene film was used. The indicators were tested at 250 and 272°F. The results are summarized in Table 3 as samples 9, 10 and 11 together with results for 1.2 mil film for comparison.

TABLE 3

SAMPLE SAMPLE EXPOSURE TEMP Length of Migration (mm) SIZE TIME (°F)

HIGH LOW AVG

9 8 45 sec 272 11 10 10.5

1 8 45 sec 272 9 8 8.5

10 8 4 min 272 max 29 31 (32)

4 8 4 min 272 29 28 28.8

11 7 5 min 250 13 10 11.4

5 8 5 min 250 13 10 10.5

In the case of sample 11, one indicator was excluded from the results in the table because of uncharacteristic behavior. This indicator migrated 23 mm. Upon microscopic examination of the indicator, however, a crease was observed in the foil. It is believed that this crease caused a leakage of steam and that this resulted in the unusually long migration distance.

Comparative Example

Indicators were made using 3 mil foil and 1.0 mil or

1.2 mil polypropylene film and tested at 250 or 270°F.

All samples were tested in the same sterilizer used in the preceding three examples, except samples 13 and 15 which were run in an Amsco Eagle sterilizer. The results are summarized in Table 4. TABLE 4

SAMPLE SAMPLE EXPOSURE TEMP Length of Migration (mm) SIZE TIME (°F)

HIGH LOW AVG

12 A 4 45 sec 272 19 17 18 (1 mil)

12 B 4 45 sec 272 15 13 13.8 (1 mil)

13 5 1 min 272 21 15 14.6

(1 mil)

14 A 4 4 min 272 max max max (1 mil)

14 B 12 4 min 272 max 24 29 (1 mil)

15 4 5 min 250 20 9 12.8

(1 mil)

16 4 20 min 250 max 20 24

(1 mil)

17 4 20 min 250 18 12 14.5

(1 mil)

18 (1.2 4 20 min 250 10 8 9 mil)

These results show that the indicators using the three mil foils were erratic in the length of migration observed. Such erratic behavior is unacceptable in a steam sterilization where the migration distance is compared to a standard mark to determine whether to pass or fail the sterilizer contents. During the testing of the indicators using the 3 mil foil, some physical distortion of the indicators was observed in most instances. This distortion sometimes caused lifting of the steam permeable cover from the foil, and made the indicator prone to leakage of the steam into the indicator (which could cause erratic movement of the chemical and dye) and leakage of composite out of the indicator. In contrast, little or no distortion of the indicators with a 5 mil foil backing was observed.

EXAMPLE 5 A side by side comparison was performed between indicators made using 3 mil foil backing and 5 mil foil backing. The test was run in an Amsco Eagle sterilizer operating at 272°F with an exposure time of 1.5 minutes. As shown in Table 5, in this particular test, any differ¬ ences in the performance of the two types of indicators was minor. Nevertheless, based on the overall pattern of behavior of the 3 mil indicators, it is believed that this was an artifact of the small sample size rather than a true indication of the comparative reliability of the indicators.

TABLE 5

SAMPLE SAMPLE EXPOSURE TEMP Length of Migration (mm) SIZE TIME (°F)

HIGH LOW AVG

19 12 1.5 min 272 15 12 13.5

(3 mil)

20 12 1.5 min 272 15 14 14.3

(5 mil)

Claims

I Claim:

1. A steam sterilization indicator comprising: (a) a foil backing (1) having a thickness of at least 5 mil; (b) a wick (5) disposed on the foil backing (1) ; (c) a meltable chemical (4) disposed on the foil backing (1) in a position relative to the wick (5) such that upon melting the melted chemical (4) will be transported through the wick (5) , said meltable chemical (4) being melted at or below a target sterilization temperature in the presence of saturated steam, and at a temperature above the target sterilization temperature in the absence of saturated steam; (d) a steam permeable cover (6) disposed over wick (5) and the meltable chemical (4) ; and (e) an aggressive high temperature silicone adhesive (2) , said adhesive adhering the foil backing (1) to the steam permeable cover (6) whereby a substantially sealed envelope surrounding the wick (5) and the meltable chemical (4) is formed from the foil backing (1) , the steam permeable cover (6) and the adhesive (2) .

2. A steam sterilization indicator according to claim 1, wherein the adhesive (2) is the adhesive employed in Densil® 2078.

3. A steam sterilization indicator according to claim 1 or 2, wherein the adhesive (2) is applied to the foil backing (1) as a film.

4. A steam sterilization indicator according to claim 1 or 2, wherein the adhesive (2) is applied to the steam permeable cover layer (6) as a film.

5. A steam sterilization indicator according to any of claims 1-4, wherein the meltable chemical (4) comprises salicylamide and a dye.

6. A steam sterilization indicator according to any of claims 1-5, wherein the steam permeable film (6) is polypropylene.