Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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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
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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
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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
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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
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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
230F 110 137 154
240F 29 51 53
250F 12 12 12
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.
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