Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TEST PACKS FOR ST~:R~,T7,1i~ s
The present invention relates to test packs and testing systems for
deterrnining the efficacy of sterilization cycles in sterilizers.
A sterilization process used to sterilize met1ic~l and hospital equipment is
only effective if a certain combination of environment~l conditions is achieved
within the sterilization chamber of the sterilizer. For example, when steam is used
as a sterilant, the object of the sterilization process is to bring steam of a suitable
quality, and at an applupliate temperature into contact with all surfaces ofthe
articles being sterilized for a correct length of time.
In some steam sterilizers the process of sterilization is typically con-1uctecl in
three main phases. In the first phase, air trapped within the load being processed is
removed. The second phase is a sterilizing stage, in which the load is subjected to
steam under pressure for a recognized combination of time and temperature, whichis known to effect sterilization. The third phase is a drying phase in which
condensate formed during the first two phases is removed by ev~cu~ting the
chamber.
Air removal from the sterilization chamber may be achieved in a number of
ways. For example, in a gravity steam sterilizer, the principle of gravity
displacement is lltili7e~l, in which steam entering at the top of the chamber displaces
the air through a valve in the base of the chamber. In a prevacuum steam sterilizer,
on the other hand, air is removed forcibly by deep evacuation of the chamber or by
a combination of evacuation and steam injection at either subatmospheric and/or
superatmospheric pressures.
Any air which is not removed from the sterilization chamber during the air
removal phase of the cycle or which leaks into the chamber during a subatmospheric
pressure stage due to faulty gaskets, valves or seals, may forrn air pockets within
the load that is being sterilized. Likewise, any non-condensable gases (which, in
this context, means gases having a boiling point below that of the sterilant) that are
present in the sterilization chamber or are carried within steam supplied to thechamber may form gas pockets within the load. These air or gas pockets will create
a barrier to steam penetration, thereby preventing adequate sterilizing conditions
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being achieved for all surfaces of the load. This is particularly true when porous
materials such as hospital linens or fabrics are being sterilized since the air or gas
pockets prohibit the steam from penetrating to the interior layers of such materials.
As a result, sterilization may not occur. Therefore, there is a need to be able to
determine the efficacy of sterilization cycles and in particular, to dGLe~ ine whethe
there has been sllffir;ent steam penetration. Similarly, when a steril~nt other than
steam is used, there is a need to be able to determine that the sterilant has
penetrated a load sufficiently for sterilization to take place.
One commonly-used procedure for eV~ ting the effectiveness of air
removal during the air removal phase of a porous load steam sterilization cycle
and/or for testing for the presence of non-con~çn~ble gases is known as the
Bowie-Dick test. The typical Bowie-Dick test pack ~sc~nti~lly consists of stack of
freshly }aundered towels folded to a specific size, with a chemical indicator sheet
placed in the centre of the pack. Chemical indicator test sheets undergo a visible
change from one distinct colour to another, for example, from an initial white to a
finat black colour, upon exposure to the sterilization process. ~f the air removal
within the sterilizer is insufficient, or if non-conclen~hle gases are present during
the process in sufficient quantity, an air/gas pocket will form in the centre of the
pack thereby preventing steam from con~ctin$ the steam sensitive ~h~micsll
indicator sheet. The consequence of inadequate steam penetration is a non-uniform
colour development across the surface of the chemical indicator test sheet: thus,
the presence of the air/gas pocket will be recorded by the failure of the indicator to
undergo the complete or uniform colour change indicative of adequate steam
penetration.
Biological indicators can also be used to provide h~,l"~lion on the
adequacy of a sterilization cycle. Biological indicator test systems typically employ
living spores which are subjected to a sterilization cycle. After the cycle, the spores
are incllb~ted and the system detects if there is any growth. If there is no growth, it
indic~tes that the sterilization process has been effective. Thus, biological indicators
can determine whether conditions for sterilization were present, but the length of
time to obtain results due to the incubation period is often at least 24 hours.
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Th~rerol-e, biological indicator systems are often used in conjunction with chPmic~l
inrlic~tors because the colour change of the f.h~miç~l indictors provides an instant
result. Further, by using both ~h~miç~l and biological in-lic~tors, i,.ro.,l,~ion on
both the ~eqn~cy ofthe air removal stage and the sterilization stage is provided.
Para",ellic monitoring has also been used to either monitor or control a
sterilization cycle to ensure proper sterilization conditions are ~tt~inecl For
example, in U.S. Patent No. 4,865,814 to Childress, an ~lltQm~ltic sterilizer isdisclosed which includes a microprocessor which monitors both the temperature
and pl es~ule levels inside the sterilization chamber and controls a heater to allow
both pressure and temperature to reach predetermined levels before starting a timer.
Once the timer is started, it is stopped if the pressure or temperature levels drop
below a predetermined minimllm ~ince it is known that the pressure and
temperature variables of saturated steam are dependent variables when saturated
steam is enclosed in a sealed chamber, monitoring of these two variables can ensure
that proper conditions are m~int~ined during the sterilization cycle.
Although it is desirable to monitor environment~l conditions within the
sterilization chamber itself, it is generally considered more desirable to be able to
monitor the environment~l conditions within an actual load being sterilized or within
a test pack (such as the Bowie-Dick test pack) that represents such a load.
Although the typical Bowie-Dick test pack is generally recognized as adequate for
use in determining the efflcacy of the air removal stage of prevacuum sterilizers, it
still presents many disadvantages. Since the test pack is not preassembled, it must
be constructed every time the procedure is used to monitor sterilizer performance.
The plepa-~lion, assembly and use of the towel pack is time con~ ~min~ and
cumbersome and, moreover, varying factors, such as }aundering, pr~hl-mi~lification,
towel thickness and wear, and the number of towels used, alter the test results.Therefore, alternative Bowie-Dick test packs have been developed to overcome
these limitations.
An examp}e of an alternative Bowie-Dick test pack for steam or gas
sterilizers is described in EP-A-0419282. That test pack incl~ldçs a container having
top and bottom walls with a porous packing material disposed within the container.
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The p~L in~ material rh~lenges the penetration of the sterilant by providing a
restricted pathway which acts to impede the flow of the sterilant through the test
pack. A removable lid seals the bottom end of the container, while a hole in the top
wall of the container allows for the downward ingress of steann into the p~qcl~ine
material within the container. The test pack inclucles a chPmic~l and/or a biological
in~ic~tor for detecting sterilant penetration. In the case of a ch~mic~l in~ tQr, if
sterilant s~cces.cfi~lly penel~ ~es the packing material of the test pack, the chçrnie~l
indicator sheet will undergo a complete colour change. If the sterilant does notsufficiently penetrate the packing material, the chemical indicator will not undergo a
complete uniform colour change, thereby indicating inadequate air removal or thepresence of non-cond~n.~hle gas, or in other words, a Bowie-Dick test failure.
Other test packs for use in steam or gas sterilizers are described in EP-A-
0421 760; US-A-5 066 464; WO 93/21964 and US-A-5 270 217. In each ofthose
test packs, sterilant from the sterilization chamber must cross some form of physical
barrier before it reaches a sterilant sensor within the test pack.
Another known arrangement for challenging the penetration of sterilant to a
particular location within a test pack comprises a very long (typically, ~.Sm)
stainless steel tube with a narrow bore (typically, 2.0 mm) which provides the only
access for sterilant to the predetermined location. US-A-4 115 068 describes an
intlicator device which comprises an upright tube cont~ining a temperature sensitive
indicator strip. The tube is forrned from a thermally inc~ ting material and is lined
with a thermally conr~ucting material.
The problem with which the present invention is concerned is that of
providing, for sterilizer testing systems, a test pack with is col..pal ~ ely simple and
2~ inexpensive to m~m~f~cture but which will function reliably to enable in~l;ve
sterilization cycles to be identified.
The present invention provides a test pack for use in determining the
efficacy of a sterilization cycle in a sterilization chamber, the test pack comprising:
a housing made from a material which is impermeable to gas and
liquid, the housing dçfinin~ an enclosure having an opening for the entry of -~
sterilant;
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a sensor positioned to detect the presence of sterilant within the
enclosure at a location remote from the opening; and
particulate material contained within the enclosure to ~ en~e the
penetration of sterilant to the said location. It is pl ~rel led, regardless of
particle size, that the particulate material comprises at least 100 particles.
The present invention also provides a test pack for use in determining the
efficacy of a sterilization cycle in a sterilization chamber, the test pack comprising:
a housing made from a material which is imperrneable to gas and
liquid, the housing d~fining an enclosure having an opening for the entry of
sterilant;
a sensor positioned to detect the presence of sterilant within the
enclosure at a location remote from the opening; and
sintered material contained within the enclosure to challenge the
penetration of sterilant to the said location.
Preferably, the dimensions of the enclosure are such that the ratio of the
length of the enclosure to the diameter at the wider end is within the range of from
0.5:1 to 3:1.
Preferably, the housing has a ther~nal conductivity within the range of from
0.1 5 to 1.00 W/m. ~K, and a thermal capacity within the range of from 0.5 to 2.5
W/kg. ~K. The ho~l~ing may comprise a po}ymeric material, for example
polysulfone; polyphenylsulfone; polyethersulfone; po}yetheretherketone (PEEK);
polytetrafluorethylene; poly (etherketoneetherketoneketone) ~ KK); or
polymethylpentene.
The present invention further provides a test pack for use in determining the
efficacy of a sterilization cycle in a sterilization chamber, the test pack Co~ ;sillg:
a housing made from a material which is impermeable to gas and
liquid
a generally-conical enclosure within the housing;
an opening for the entry of sterilant in the wider end of the
enclosure, and a sensor positioned to detect the presence of sterilant within
the enclosure at the narrower end thereof; and
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a ch~llPn~e m~-litlm contained within the enclosure to ch~llen~e the
penetration of sterilant to the narrower end of the enclosure,
wherein the housing has a thermal conductivity within the range of
from ~.lS to 1.00 W/m. ~K and a thermal capacity within the range offrom
S 0.5 to 2.5kJ/kg. ~K, and the t1imPn~ions ofthe enclosure are such that the
ratio for the length of the enclosure to the ~ ler of the wider end is
within the range of from O.S :1 to 3 :1. The housing material may be a
polymeric material. Preferably, the housing material is polysulfone;
polyphenylsulfone; polyethersulfone; polyetheretherketone (PEEK);
polytetrafiuorethylene; poly (etherketoneetherketoneketone) (p~.Kl~K); or
polymethylpentene.
In a test pack in accordance with any aspect of the present invention, the
sensor may be a temperature sensor positioned to measure the temperature of the
said location. Alternatively, the sensor may be a biological or a chemical indicator.
By way of example only, embo-lim~nP of the invention will now be
described with reference to the acco"-pal,ying drawings, in which:
Fig. 1 is a perspective view of a test pack in acco~ dance with the invention;
Fig. 2 shows a longit-l~in~t cross-section ofthe device of Fig. l;
Figs. 3 to 6 show dia~- ammatic cross sections of alternative test pack
housings; and
Figs. 7 and 8 show diagrammatic cross-sections of other test packs in
accordance with the invention.
Figs. 1 and 2 show a test pack 1 suitable for use in a system for testing the
efficacy of a sterilization cycle, either in a steam sterilizer, or in a low temperature
gas sterilizer in which sterilization is carried out using a microbiocidal agent (for
example, formaldehyde) in the presence of moisture. The test pack 1 is intt-n~ecl to
be located in the sterilization chamber of the sterilizer to provide a challenge path
along which sterilant (for example, steam) from within the chamber must pass
before it can be detected by a sensor at a predetermined location within the device.
If the presence of sterilant at the predeterrnined location is not detected by the
sensor during a sterilization cycle (indicating that the conditions within.the
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sterilization chamber have not enabled sterilant to penetrate the challenge path), the
sterilization cycle is judged to be in~ff~ctive.
The test pack 1 shown in Figs. 1 and 2 comprises a hollow housing forrned
by an open-ended body portion 2 which is closed by an end cap 3. The body
portion 2 defines an enclosure 4 of generally conical shape, the wide end 4a of
which is the open end of the body portion. Externally, the body position 2 has the
form of a cylinder with a threaded annular flange 5 at the open end, to which the
end cap 3 is secured. The body portion 2 and the end cap 3 are formed from a
material which is impermeable to gas and liquid and has selected thermal
characteristics, described below.
The end cap 3 is formed with apertures 6 through which sterilant can enter
the enclosure 4 during a sterilization cycle as described above. The enclosure 4 is at
least partially filled with a particulate challenge me~ m 7, described in greater
detail below, the function of which is to challenge the penetration of sterilant into
the enclosure during the sterilization cycle. A sterilant sensor 8, of any suitable
type, is located at the narrow, closed, end 4b of the enclosure 4.
The test pack 1 is preferably used in the orientation shown in Figs. 1 and 2
that is, with the end cap 3 at the bottom. To prevent the escape of particulate 7
through the apertures 6, one or more layers of a ret~ining material 9 are placed over
the open end of the enclosure 4. The ret~inin~ material should, of course, not
prevent sterilant from entering the enclosure 4 through the apertures 6.
When the test pack ~ is subjected to a sterilization cycle within a
sterilization chamber, the particulate 7 impedes the flow of sterilant into the
enclosure 4 so that pockets of air/non-con~Pn~kle gases will tend to remain within
the enclosure, particularly towards the closed end 4b. The size of the air/gas
pockets is indicative of the efficiency of the sterilization cycle, being larger when the
air removal phase ofthe cycle is less adequate. By an applopliate selection ofthe
particulate 7, and the therrnal properties of the test pack housing 2, 3, it can be
arranged that sterilant will not penetrate, or not penetrate fi~lly, to the narrow end
4b of the enclosure 4 when the environmental conditions in the sterilization chamber
do not satisfy the requirements for effective steril;zation. Detection of the presence
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of sterilant by the sensor 8 is then an indication that a sterilization cycle has been
e~ective, while non-detection ~or partial detection) is an indication that a
sterilization cycle has failed to meet requirements.
The test pack 1 can be of any suitable size and, p, ~r~-~bly, is as small as r
S possible. Typically, the length of the enclosure 4 (i.e. from the wide, open, end 4a
to the narrow, closed, end 4b) is within the range 3 to 30 cm and the ~i~m.oter Of
the wide end is within the range 2 to 15 cm although ~lim~nsions outside those
ranges are not ~cl~cle~l The length/diameter ratio is preferably in the range 0.5/1
to 3/1 and, for the test pack shown in Figs. 1 and 2, is about 1/1. The material from
which the body portion 2 is formed should have a low thermal conductivity in
combination with a thermal capacity which results in some condensation of moisture
within the enclosure 4 during a sterilization cycle, leading to the enLI ~p~n~.nt within
the enclosure of non-condensable gases such as air. On the other hand, the thermal
capacity of the body portion 2 should not be high that the particulate 7 becomesclogged with moisture. Preferably, the material from which the body portion 2 isformed has a thermal conductivity in the range 0.15 to 1.00W/m. ~K and a thermalcapacity in the range 0.5 to 2.5 kJ/kg. ~K. Suitable materials for the body portion
include polysulfone, polyphenylsulfone, polyethersulfone, polyetheretherketone
OE~EK), poly (etherketoneetherketoneketone) (Pl~ t~ K), polytetrafiouorethylene
and polymethylpentene.
The particulate 7 within the enclosure 4 should be s~lected to ensure that
e~ective sterilization cycles can be ~ ting~ hed ~om non-effective cycles in a
reproducible and predictable manner. The feature of the particulate which enablethat to be accomplished include the nature and amount of the material, the particle
size, and the range of particle sizes. The particulate may be a glass, a ceramic, or a
ploymer, or combinations thereof. The enclosure 4 need not be filled completely by
the particulate: depending on the particulate it can, for example, be only 50% full in
which case some form of porous barrier is required within the enclosure to contain
the particulate at the narrower end, ad~acent the sensor 8. the particle size may be
as small as 7,um or as large as 5 mm, but when various sizes are present they
preferably do no differ by more than 20% from the average particle size.
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Regardless of the particle size, there are preferably at least 100 particles in the
enclosure 4. The particles may be either solid or hollow.
Suitable particulates include:
(i) solid glass beads or spheres (for elr~mI~le, 46~1m beads and 7.811m beads
both available, under the trade desi~n~tions "7eesospheres, 850" and
"Zeeospheres, 600", from 3M 7~eelan Industries, of St. Paul, Minnesota,
U.S.A.; and 4 mm spheres available, under the trade de~ign~tion
"G0300/53", from Fisher Scientific of Loughborough, Leicestershire,
Fngl~ntl);
(ii) hollow ceramic spheres having a ~i~m~ter within the range 50~1m to 4
mm (for example, 150-300 ~lm hollow spheres available, under the trade
de~i~n~tion "Z-Light", from 3M Zeelan Industries, of St. Paul, Minnesota,
U. S .A.);
(iii) polypropylene granules (for example, 4 mm granules available, under
the trade design~tion "Novolen 1142 N", from BASF of Ludwigshaven,
C~- Illally).
A p,erelled particulate comprises hollow ceramic spheres as in (ii) above,
selected so that the sizes of the spheres do not differ by more than 20% from the
average size.
The ret~ining material 9 may be spun bond polypropylene or any similar
material.
Although the enclosure 4 of the test pack 1 has a generally-conical form, it
need not have linear sides but could, for example, have stepped or curved sides
while still being generally conical. Moreover, although the generally-conical from is
plerelled, the enclosure could have any other suitable shape: it could, for example,
have some other tapering shape such as pyramidal, or it could be cylindrical.
Some alternative shapes for the body portion 2 of the test pack are shown in
Figs. 3 to 6. The body portion shown in Fig. 3 defines an enclosure 4 of the same
shape as that of Fig. 2 but, in this case, the side walls 30 of the enclosure are of
constant thickn~ss so that the body portion comprises a conical portion 31 and, at
-
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JiO
the narrow end 4b of the enclosure, a cylindrical portion 32 corresponding to the
upper end of the body portion 2 in Fig. 2.
The body portion shown in Fig. 4 defines an enc}osure 4 which is also
generally conical but shorter than that of Fig. 2 and with a larger .lin~l~el~l- open ~nd
4a so that the length/~i~meter ratio in this case is about 0.5/1. The body portion
shown in Fig. 5, on the other hand, defines a conical enclosure which is longer than
that of Fig. 2 and has a smaller diameter open end 4a so that the length/ li~metçr
ratio is about 3/1.
Fig. 6 shows a body portion 2 which is similar to that shown in Fig. 3 except
that the cylindrical upper end 32 is replaced by a tubular portion 33 which opens at
one end into the enclosure 4 and is closed at the other end. A plurality of sterilant
sensors 34, one at the closed end of the tubular portion 33 and the others spaced
along the length of the tubular portion, replace the single sensor 8 of Fig. 3 .:~ach of the alte i,aLi~e body portions illustrated in Figs. 3 to 6 preferably
has similar thermal characteristics to that of Figs. 1 and 2, and may be formed from
similar materials.
The sensors 8, 34 of Figs. 2 to 6 are temperature sensors, connecte~ to
provide an electrical signal indicative of the temperature in the ~qdj~nt region o~
the enclosure 4. When such a test pack is subjected to a sterilization cycle, the
particulate 7 prevents the sensor(s) 8, 34 from being exposed to the full effect of ~he
sterilant in the chamber, giving rise to a difference between the temperature sensed
by the sensor and the environment~l temperature in the chamber. l~etection of that
temperature difference enables the efficacy of the sterilization cycle to be
delel...,lled.
It is however, not ~s~sçnti~l for temperature sensors to be employed in the
test packs described above with reference to Figs. 1 to 6. Sensors which detect a
different environmenta} parameter, for example hllmi~lity, could be used incte~t~
alternatively, the upper part of the body portion 2 of a test pack could be modified
to incorporate a chemical or a biological indicator to detect the presence of sterilant
at the closed end of the enclosure 4. If required, several sensors could be
employed. For example, a chemical indicator could be used in combination with a
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biological indicator, or sensors could be used to detect several envho."
parameters (e.g. temperature, hurnidity and ~res:iul-e).
Fig. 7 illustrates an electronic test unit 10 which is a development of the testpack shown in Figs. 1 and 2. The test unit 10 is a self-contained unit which can be
placed in a sterilization chamber to determine the efficacy of a sterilization cycle.
As described below, the test unit 10 functions, during a sterilization cycle, tomeasure the temperature at two locations, one being at the narrow end of the
enclosure 4 and the other being at a reference point within the sterilization chan~be
itself. those k;,-,pe,~ re measurements are then used to determine whether or not
the sterilization cycle, in particular the air removal phase of the cycle, was effective
(i.e. met certain prescribed requi.el,lenls3.
In the test unit shown in Fig. 7, the end wall of the bod~ portion 2 is
hollowed out to form a housing 1 1 which contains the electronic components of the
test unit. Those components will be described below. The electronics housing 11
has a removable end cap 12 and is positioned within an outer housing 13 to which it
secured, for example by screws. When secured, the outer housing 13 holds the endcap 12 to the electronics housing 11 so that the latter is sealed. Outer housing 13 is
constructed of a structurally rigid material, such that when stressed, it returns to its
original shape. For example, any type of metal, as well as glass fiber or carbon fiber
reil~or~ied plastic with softening temperatures higher than 150~C can be used for
outer housing 13.
The components housed inside electronics housing 11 may be protected
from the extreme heat within the sterilization chamber by a vacuum within the
housing. To that end, electronics housing 11 includes one-way valve 14 which
opens when the pressure external to the housing 11 falls below a predetermined
value. Then, when a vacuum is pulled within a sterilization chamber with test unit
10 placed inside, valve 14 opens to allow a vacuum also to be pulled within
electronics housing 11~ Electronics housing 11 contains the temperature sensor 8together with a second temperature sensor 15. Temperature sensors 8 and 15 may
~ 30 be any suitable type of temperature tr~nscl~lcçr, for example, thermocouples or
thermistors. Temperature sensor 8 as already described with reference to Fig. 2 is
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t~2
positioned such that is measures the temperature at the end 4b of the enclosure 4.
Temperature sensor 15, on the other hand, measures the external tt;mpt~ re.
Thus, when unit 10 is placed within a sterilization chamber, temperature sensor 15
measures the chamber temperature.
Housing 11 also contains a circuit board 16, mounted so that it is thermally
isolated from the walls of the housing to prevent conduction of external head to the
~lC~LI OlUCS mounted on the board, which include a microprocessor and a memory,
preferably an electrically erasable prog~nllnable read-only memory (EEPROM).
Surface mounted chips 17, batteries 18, the temperature sensors 8 and 15, a light
~ diode 19 and a pressure sensor 20 are all electrically connPcted to circuit
board 16.
As te-~lpe.aL.Ire sensors 8 and 15 measure temperatures, the temperature
readings are stored in the test pack memory together with time data from the
microprocessor. Once the microprocessor determines that a sterilization cycle iscomplete, it then determines (from the stored temperature readings) whether the
sterilization cycle is s~ti~f~ctQry, in other words, that the sterilant has adequately
penetrated the enclosure 4 of the test unit 10.
If the microprocessor determines that the sterilization cycle was s~ti~f~ctory,
light çmitt;n~ diode (LED) 19 emits light. In a completely self-contained electronic
test paclc, only a single LED is necPcc~ry to indicate whether the cycle has passed.
With a single LED, the ~ED may continuously burn to indicate a pass cycle and
may flash to indicate a fail cycle. Alternatively, two LEDs may be used, to indicate
a pass cycle and fail cycle respectively. If the sterilization cycle has passed, one
LED emits a green light. If the microprocessor determines that the sterilizationcycle has failed, the other LFn emits a red light.
In some situations, it is desirable to transfer the data stored in the memory
of the unit to an outside processor or memory or a printer. Data transfer may beiniti~ted by actuating a m~gnetic~lly ach~ted switch (not shown), preferably a reed
switch.
The manner in which the unit 10 determines the efficiency of a sterilization
cycle is, briefly, as follows. As already described with reference to Figure 1, the
-
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13
çh~ n~e medillm 7 prevents the sensor 8 from being exposed to the full effects of
sterilant, thus giving rise to a difference between the ten~l)e, ~Lure at the sensor 8
and the temperature at the sensor l 5. The unit lO determines if that temperature
di~.ence exceeds a predete-",ined value at a predetermined point within the
sterilization cyc}e and, if so, the cycle is judged to be l-n.~ti~fActory. This
predetermined temperature difference is determined by validation ~velinl~,.lL~ in
which the performance of the electronic test unit lO is co-npaled with that of astandard ~3owie-Dick textile test pack according to recognized International,
European or National standards. For example, the test unit lO could be pre-
programmed so that, if the temperature difference is greater than 2~C in a 2 minute
and 40 seconds period after the ch~mber te",pel ~L.Ire reaches a sterilization hold
temperature of 134~C, the cycle is considered lln~tief~ctory. Further, the chamber
temperature must remain above an adequate sterilization temperature for
sterilization to occur.
l 5 While the ex~min~tion of the temperature difference between the external
and internal temperature (as just described) provides direct hlro~ alion on the
penetration of head to the sensor 8 located within the test unit l 0, it does not
directly reflect penetration of sterilant to the sensor (although, by inference, rapid
equilibrium between the sensing point within the enclosure 4 and the sterilization
chamber indicates the absence of an in~ ting air/gas pocket in the enclosure). In
the case of a steam sterilizer, it is possible, however, to measure directly themoisture penetration to the sensing point within the ch~llen~e device. To that end,
a moisture sensor, such as a conductivity sensor or a relative humidity sensor, can
be used instead of or in addition to, the temperature sensor 8 to determine adeqll~te
moisture penetration to the sensing point within the challenge device and therefore,
by inference, steam. The temperature sensor l 5 measuring the sterilization chambe
temperature remains the same.
The test unit shown in Fig. 7 can be modified to incorporate a chernical or
biological indicator, rather than the temperature sensors 8, 15 and associated
electronics. The indicator is located in the housing 1 l so that it is exposed to the
conditions at the narrow end 4b of the enclosure 4. To that end, the end wall of the
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14
enclosure 4 is apertured to permit the passage of sterilant into the housing 11 and
covered with a porous barrier to prevent the particulate 7 from entering the housing
11. Biological and chemical inriic~tQrs are both well know:
a suitable biological indicator is available, under the trade desi.~n~tir)n
S "ATTEST", from Minnesota Mining and M~nl-f~ct-lring Co~ a~ of St.
Paul, Minnesota, U.S.A.; and
a suitable chemical in~ic~tor is available, under the trade de~i~n~tion
"Comply 1250", also from Minnesota Mining and M~mlf~ct~lring Conlpa..y
of St. Paul, Minnesota, U.S.A.
The indicator, whether biological or chemical, is located in the housing 11 of
the test unit prior to a test of a sterilization cycle and is removed when the cycle has
been completed so that it can be e7c~mine~ to deterrnine whether or not the cycle
was effective. A replacement indicator is then put into the housing 11 of the test
unit so that the unit can be re-used.
Fig. 8 shows another test pack, similar to that shown in Figs. 1 and 2 except
that the particulate 7 is replaced by a porous plug 80 of sintered plastics or ceramic
material, for example a sintered HD (high density~ polyethylene material such asthat available, under the trade design~tion "SIPERM ~", from Thyssen
~r~gn~tterhnik Gmb~ of Dortmund, Germany. The porous plug 80 is p-Gr~----ed
by sintering (i.e. heating particulate material to cause it to coalesce into a solid) in a
mould having the same dimensions as the enclosure 4 of the test pack. More
sperifir.~lly, the particulate material for the plug 80 is mixed with a binder material
which is volatile or easily melted. The mixture is then in)ected into, or placed in, the
mould and sintered. During the sintering process, the binder material evaporates or
melts rapidly, leaving a porous plug. Alternatively, it is possible to use other binder
materials which are not removed during the sil.Lelhlg process but are removed after
sintering (for example, using chemical solvents) to leave a porous plug. The
porosity of the plug 80, produced by the sintering process, can be varied by
çh~ngin~ the shape, si~e and distributions of the particles in the mixture that is
loaded into the mould.
CA 02238~22 1998-0~-26
WO 97/19709 1 5 PCT/US96/18599
It is not e~nti~l for the plug 80 to fill the entire length of the enclosure 4 as
shown in Fig. 8 although it should always contact the wal1s of the enclosure and be
located at the nalluv~/er end, ~dj~c~nt the sensor 8. In that case, the plug would be
formed in a shorter mold.
S It will be appreciated that a challenge medillm formed from a sinteredmaterial as shown in Fig. 8 could be used in any of the body portions shown in Figs.
3 to 6 or in ;the test unit shown in Fig. 7. ~oreover, since it is possible to produce
plugs of sintered materials in other shapes, the use of S;l~Lt~l ed material as a
çh~ .nge medillm is not restricted to test packs, such as those shown in Figs. 1 to
8, in which a conical plug is required. It would, for example, be possible to use
cylindrical plugs of sintered material as a challenge medium in test packs of the
shape described in EP-A-0 419 282.
The challenge media described above for the test packs/units shown in Figs.
1 to 8 are intPn~ler1 to be re-usable (i.e. the test packs/units would not be disposed
of after a single use~. It will be appreciated, however, that both the particulate
material shown in Figs. 2 and 7 and the sintered material shown in Fig. 8 could be
replaced when necessary, although the rest of the test pack/unit would be lelailled
and re-used. Alternatively, both forms of challenge medium could be used in
disposable test packs ofthe type described in EP-A-0 419 282.
It will also be appreciated that the use of particulate or si-llel ed material as
the challenge medium in the test packs shown in Figs. 1 to 8, although plefelled, is
not esse~ti~l Other forms of challenge me~ m, for c.~al~lple a foam material or any
ofthe packing materials described in the above-mentioned EP-A-0 419 282, could
be used in a test pack having the construction and thermal characteristics described
above with reference to Figs. 1 to 8 (i.e. a test pack in which the challenge me~ m
is contained within a generally conical enclosure formed in a housing having a
thermal conductivity in the range 0.15 to l.OOW/m.~K and a therrnal capacity in the
range 0.5 to 2.5 W/kg.~K, with the dimensions of the enclosure being such that the
ratio of the length of the enclosure to the diameter of its wider end is in the range
0.5:1 to 3:1).
