Methods for sterilization of materials by chemical sterilants

Methods for sterilization wherein air is purged from the load to be sterilized prior to the addition of the sterilant and wherein residue chemical sterilants, such as ethylene oxide, are purged post-sterilization by introducing air and steam at pressures which cause the steam to condense on the interstices of the load and then vaporize to provide a carrier for trapped air (prior to sterilization) and residue chemical sterilant (post-sterilization).

DESCRIPTION 
This invention relates to methods for the introduction and removal of 
chemical sterilants into and out of various materials. 
The invention is especially suitable for sterilization of loads, such as 
towels, sheets and tubing used by hospitals, that contain voids or spaces. 
A major concern in sterilization with chemical sterilants is the inability 
of humidity and sterilant to reach and then be removed from the 
interstices of materials being processed. 
It has been discovered, in accordance with the invention, that one key to 
an effective sterilization process is to remove non-condensible gas, such 
as air, from the materials and substitute a condensing gas, such as steam, 
to provide humidity and allow more effective sterilant penetration. During 
air removal, the pressure of the condensing gas is raised. This drives the 
gas into the interstices of the load. The gas, which dew points on the 
load, can be revaporized and will carry any non-condensing gas trapped in 
the interstices of the load out with it. Thus, when the chemical sterilant 
is admitted to the sterilizing chamber, the condensible gas again dew 
points on the surface of the material and allows the chemical sterilant to 
penetrate into the voided interstices of the materials being processed. 
Another key to promote effective sterilization, which has been discovered 
in accordance with the invention, is to remove residue sterilant from the 
insterstices of the sterilized load. This can be accomplished by 
introducing a condensing gas, such as steam, into the interstices of the 
load. Pressurization of the condensing gas causes the steam to dew point 
on the load. The steam then can be revaporized and will carry the residue 
chemical sterilant in the interstices of the load out with it. The 
sterilized materials, therefore, are ready for immediate use. 
In current chemical sterilization processes, non-condensible gases, such as 
air, are removed prior to introduction of a chemical sterilant by 
evacuating the sterilizing chamber and admitting steam at subatmospheric 
pressure, and then purging the chamber with steam at subatmospheric 
pressure. In some processes, small pressure pulses in the order of 1 psi 
are incorporated in the steam flush to enhance the removal of air from the 
interstices of the load. Because of the small pressure pulse which can be 
applied with steam before the chamber environment would exceed the 
allowable temperature for materials being processed, the removal of air 
from the inside of materials is very inefficient. 
A second major concern is that chemical sterilants, such as ethylene oxide 
or formaldehyde, remain both in the sterilizing chamber and the product 
following sterilization. When a sterilizer is opened following 
sterilization, residue sterilant in the sterlizing chamber enters the 
working environment posing a potential health hazard to the sterilizer 
operator. Residue sterilant remaining in the product also poses a 
potential health hazard if there is physical contact with the material. 
Several equipment design methods have been employed to minimize chemical 
sterilant residues. Post cycle pressure pulsing with air to dilute the 
sterilizing chamber sterilant concentration or flushing the sterilizing 
chamber with air are typical of these methods. This procedure does not 
readily remove the sterilant inside packaging materials or sterilant 
absorbed by the materials. Aeration (ventilated air washing) periods are 
required before handling or using the materials. Aeration of such 
materials must be performed in well vented areas to prevent the build up 
of sterilant in the working environment. This results in material handling 
logistics involving quarantine facilities as well as the necessity of 
environmental monitoring and control. 
It is the object of the present invention to provide improved sterilization 
methods and apparatus for removal of air prior to sterilization, efficient 
humidification of the load during sterilization, and removal of the 
chemical sterilant from both the materials being sterilized and the 
sterlizing chamber in a chemical sterilization process. 
It is a feature of the invention to provide a plurality of pressure pulses 
and steam flush periods to (a) remove non-condensible gases, such as air 
or chemical sterilants, from the sterilizing chamber, (b) drive a 
condensing gas, such as steam, into the materials to be or being 
sterilized to produce condensation on the interstices of the load, (c) 
revaporize the condensate to humidify the materials and to transport the 
air or chemical sterilant from the interstices of the load to the 
sterilizing environment and (d) dilute the air or chemical sterilants from 
the sterilizing chamber. 
It is a further feature of the invention to provide improved sterlization 
methods and apparatus which use the condensing gas, as a dilutent-carrier 
substance, at a temperature below that which would degrade the materials 
being processed. 
An advantage of this invention is the decreased risk of exposure to 
chemical sterilants upon completion of sterilization.

Referring to FIG. 1, the sterlizer comprises a chamber structure indicated 
generally at 10 which consists of an open ended vessel 12, an end closure 
door 14 and a sterilizing chamber 16. Vessel 12 is combined with a flange 
20. The door 14 can be hinged or otherwise mounted to the flange 20 to 
facilitate opening and closing of the chamber 16. A conventional lock 
assembly can be used with the door 14 enabling the door to be firmly 
locked to seal the chamber 16 during the sterlization and detoxification 
process. The chamber temperature is controlled by a heating strip 22, such 
as an electric heating strip or any other controlled heating means, which 
can be set to control the vessel 12 at a predetermined temperature, the 
wall of the vessel 12. Conduits and controls to perform the sterilization 
process and the load to be sterilized (e.g., sheets, towels or tubing) are 
not shown. 
A drain line 32 is connected to a vacuum pump 33 through a control valve 
34. The output of the vacuum pump is discharged to a vent 35. Valve 34 
controls the discharge of fluids through the vacuum pump 33. 
Conduit 42 allows a non-condensing gas, such as air or a chemical 
sterilant, to vent into the sterilizing chamber through a bioretentive 
filter 60, a control valve 41 and a heat exchanger 36. The gas may be 
transported from a pressurized source 39 or simply atmospheric air, where 
appropriate. The heat exchanger 36 is controlled such that the temperature 
of gas entering the vessel 16 is at a predetermined temperature which will 
not exceed the temperature limits of the materials being processed. 
Conduit 51 allows a condensing gas, such as steam, into the vessel 16 from 
a steam supply 38 through a control valve 40. A circulation blower 58 is 
attached to the vessel 12 to provide circulation and uniform environmental 
conditions in the chamber 16. A pressure transducer 45 is connected to the 
vessel 12 through conduit 37. 
Operation of the sterilization apparatus to remove air from the load and 
the sterilizing chamber will be apparent from a review of FIG. 2 which 
diagrams the pressure in the chamber during the sterilization process. 
At the beginning of a sterilizing process, the sterlizing chamber 
environment is room air at substantially atmospheric pressure as shown 
generally at 101. To initiate the process, the sterilizing chamber is 
evacuated to an initial vacuum level P1 which is shown at 102. Referring 
to FIG. 1, this is accomplished by turning on the vacuum pump 33 and 
opening the vacuum control valve 34 with all other valves closed. This 
removes the gross quantities of air in the sterilizing chamber 16. 
When the pressure P1 is attained, a condensing gas, such as steam, is 
flushed through the sterilizing chamber 16 while maintaining substantially 
a constant subatmospheric pressure P1. This is accomplished by modulating 
the steam control valve 40 on and off relative to the selected pressure 
P1. The vacuum level P1 is selected such that steam entering the 
sterilizing chamber will not exceed temperatures the materials can 
withstand without damage (e.g., searing or melting). 
On completion of the timed interval for the flush period shown generally at 
103 in FIG. 2, the sterilizing chamber environment is substantially steam 
and some of the steam penetrates the periphery of the materials. Next, the 
sterilizing chamber 16 is pressurized with heated air, which has passed 
through a bioretentive filter 60, or with additional steam to a given 
presure level P2 as shown generally at 104 of FIG. 2. Air addition is 
achieved by closing the steam and vacuum control valves 40 and 34 and 
opening the air in control valve 41 in FIG. 1. If steam is used, steam 
control valve 40 is modulated until pressure level P2 is reached. Care is 
taken to insure that the temperature at which damage to the material 
occurs is not exceeded. In either case, steam is driven into the load. The 
high pressure compresses steam into the interstices of the load and some 
of the steam condenses on these surfaces. The chamber is subsequently 
evacuated to the predetermined pressure P1 as shown at 106 of FIG. 2. 
During the evacuation period between 104 and 106, excess air and steam in 
the chamber 16 are removed. In addition, the condensate begins to 
revaporize and air and steam are pulled out of the materials into the 
sterilizing chamber. Evacuation of the chamber 16 is accomplished by 
opening the vacuum control valve 34 and closing control valves 41 and 40. 
Referring to FIG. 4, it was found that there is a time constant between 
when the excess gases (air and steam) begin to be pulled out of the 
materials and when the condensate on the materials picks up enough heat 
energy to be revaporized and begins to be pulled out of the material. This 
time lag between removal of excess gases and when the material picks up 
sufficient heat energy, as chamber pressure decreases, to revaporize 
creates a concentration gradient through the load which allows the 
vaporized condensate to act as a transport mechanism to carry the air out 
of the interstices of the materials. 
Referring generally to 107 of FIG. 2, the air is flushed from the 
sterilizing chamber 16 of FIG. 1 in the same way as described for the 
steam flush configuration shown at 102 to 103 of FIG. 2. The procedure 
producing the process configuration from 103 to 107 is repeated until air 
is removed from the system as shown generally at 109. 
Following the air removal-humidification process of the present invention, 
the sterilizing chamber is then pressurized with a sterilant gas, such as 
ethylene oxide, steam or formaldehyde, to initiate the microbial 
destruction as shown at 111 of FIG. 2. Because all of the air is removed 
from the materials and the sterilizing chamber, the sterilant pressure 
compresses the steam present at 109 in the load and the steam condenses on 
the surface of the materials. This allows the sterilant to penetrate with 
the steam into remote areas and interstices of the load which would not 
ordinarily be accessible. 
Table 1 shows a comparison of the effectiveness of conventional 
humidification processes versus the present process in delivering humidity 
and sterilant to the inside of tubing (load). The results demonstrate that 
humidity (Rh) does not penetrate the tubing in a conventional process, but 
that it does with present invention. Table 1 also shows that chemical 
sterilant does not penetrate the tubing in the conventional process where 
it does in the present process, which penetration occurs within 30 
minutes. 
TABLE 1 
______________________________________ 
Conventional Ethylene 
Humidification 
Oxide Sterilization 
Present Invention 
Time Process Penetration 
Penetration 
in Minutes Humidity Sterilant Humidity 
Sterilant 
______________________________________ 
30 none none 40% Rh yes 
60 none none 
90 none none 
120 none none 
______________________________________ 
Referring to FIG. 3, at the end of a sterilizing process, the chamber 16 is 
pressurized with the sterilant as shown generally at 111. If steam is the 
sterilant, the pressure is released by opening valve 34 to atmospheric, 
and the load is allowed to cool before removal from the chamber. To 
initiate the detoxification process when a chemical sterilant is used, the 
sterilizing chamber is evacuated to an initial vacuum level P1 which is 
shown at 112. 
Referring to FIG. 1, this is accomplished by turning on the vacuum pump 33 
and opening the vacuum control valve 34 with all of the other valves 
closed. This removes the gross quantities of sterilant in the sterilizing 
chamber 16. 
When the pressure P1 is attained, a condensing gas, such as steam, is 
flushed through the sterilizing chamber 16 while mainaining substantially 
a constant pressure P1. This is accomplished by modulating the steam 
control valve 40 on and off relative to the selected pressure P1. 
Referring to FIG. 3, this flushing period is maintained between 112 and 113 
to allow more of the sterilant to be diluted in the sterilizing chamber 
16. Tbe vacuum level P1 is selected such that steam entering the 
sterilizing chamber is superheated above the dew point on the surface and 
on the interstaces of the materials which have been sterilized and the 
temperature will not exceed temperatures the materials can withstand 
without damage (e.g., searing or melting). 
On completion of the timed interval for the flush period shown generally at 
112 to 113 in FIG. 3, the sterilizing chamber environment is substantially 
steam and some of the steam penetrates the periphery of the materials. 
Next, the sterilizing chamber 16 is pressurized with heated air, which has 
passed through a bioretentive filte 60, or with additional steam to a 
given level P3 as shown generally at 114 of FIG. 3. Air addition is 
achieved by closing the steam and vacuum control valves 34 and 40 and 
opening the air in control valve 41 shown in FIG. 1. If steam is used, 
control valve 40 is modulated until pressure R3 is reached. Care is taken 
to insure that the temperature at which damage to the material occurs is 
not exceeded. In either case, steam is driven into the load. The high 
pressure compresses the steam in the load and some of the steam condenses 
on these surfaces. This condensate extracts water soluble sterilants, such 
as ethylene oxide or formaldehyde, from the materials by producing a low 
concentration gradient at the surface and the water has a greater 
attractive force than the material for the sterilant. 
For plastic materials, the conpensate is allowed to remain on the surface 
of the materials for an interval (dwell period) as shown generally at 115 
in FIG. 3. After this predetermined dwell period, the chamber is evacuated 
to the predetermined pressure P1. The extraction dwell period 115 is 
selected based upon the rate at which the sterilant is extracted from the 
materials being processed. Paper or cotton materials may require almost no 
dwell period while plastics may require a 10 minute dwell. 
During the evacuation period between 115 and 116, excess air, sterilant and 
steam are pulled out of the materials into the sterilizing chamber. 
Evacuation is accomplished by opening the vacuum control valve 34 and 
closing control valves 41 and 40. 
Referring to FIG. 4, it also was found that there is a time constant 
between when the sterilant gases are pulled out of the materials and when 
the condensate on the materials picks up enough heat energy to be 
revaporized and begin to be pulled out of the material. This time lag 
between removal of excess gases and when the material picks up sufficient 
heat energy, as chamber pressure decreases, to revaporize creates a 
concentration gradient through the load which allows the vaporized 
condensate to carry most of the air (or steam) and sterilant out of the 
interstices of the materials. 
Referring again to 117 of FIG. 3, the air (or steam) and sterilant are 
flushed from the sterilizing chamber during the period 117 in the same way 
as described for the steam flush configuration shown at 107 in FIG. 2. The 
process from 113 to 117 is then repeated until an acceptable sterilant 
residue level is attained as shown at 119. Two cycles are used in this 
example, but the duration and number of repetitions may differ because 
they are related to the type of material being sterilized and its destined 
usage (e.g., implants, such as pacemakers, may require a higher number of 
repetitions and longer cycles than other materials, such as towels). The 
sterilizing chamber is then vented to atmospheric pressure as shown at 121 
and the load may be removed. This is accomplished by closing control 
valves 31 and 40 and opening control valve 41. 
Table 2 shows that removal of chemical sterilants by the process ("steam 
detoxification") of this invention results in much lower residues of 
sterilant in load then can be safely used. Ambient aeration is described 
above. Mechanical aeration is carried out by flushing heated, filtered air 
through the environment and around the materials which have been 
sterilized. Concentration of residue chemical sterilant was measured by 
total extraction, gas chromatographic evaluation. 
Variations and modifications of the described methods and apparatus, within 
the scope of the invention, may suggest themselves to those skilled in the 
art. Accordingly, the foregoing description should be taken as 
illustrative and not in a limiting sense. 
TABLE 2 
__________________________________________________________________________ 
Ethylene Oxide residue after two hours sterilization at 130.degree. F., 
650 
mg/1 EO & 50% Rh for various packaging materials. 
CONCENTRATION 
AMBIENT 
MECHANICAL 
STEAM 
FOLLOWING AERATION 
AERATION DETOXIFICATION 
MATERIAL 
STERILIZATION 
70-75.degree. F. 
130.degree. F. 
130-132.degree. F. 
__________________________________________________________________________ 
Brown 4800 ug/g 24 Hr. Not Tested 
30 min. 
Kraft 3800 ug/g (ND) 
Paper 
Glassine 
4700 ug/g 24 Hr. Not Tested 
30 min. 
3100 ug/g (ND) 
Clay 4200 ug/g 1100 ug/g 
Not Tested 
30 min. 
Board (ND) 
48 Muslin 
Not Tested Not Tested 
Not Tested 
30 min. 
Towels (ND) 
(pack) 
PVC 11,000 ug/g 
24 Hr. 1.5 Hr. 1.5 Hr. 
Tubing 1074 ug/g 
3166 ug/g 
986 ug/g 
2 mm 2.5 Hr. 
Wall 624 ug/g 
__________________________________________________________________________ 
ND (EO not detected)