Solids:
Liquids:
Gases:
Ethylene oxide. This highly reactive gas (C2H4O) is flammable, toxic, and a
strong mucosal irritant. Ethylene oxide can be used for sterilization at low
temperatures (20–60 8C). The gas has a high penetration capacity and can
even get through some plastic foils. One drawback is that this gas cannot
kill dried microorganisms and requires a relative humidity level of 40–
90% in the sterilizing chamber. Ethylene oxide goes into solution in plastics,
rubber, and similar materials, therefore sterilized items must be allowed to
stand for a longer period to ensure complete desorption.
Aldehydes. Formaldehyde (HCHO) is the most important aldehyde. It can be
used in a special apparatus for gas sterilization. Its main use, however, is in
disinfection. Formaldehyde is a water-soluble gas. Formalin is a 35% solution
of this gas inwater. Formaldehyde irritatesmucosa; skin contactmay result in
inflammations or allergic eczemas. Formaldehyde is a broad-spectrum ger-
micide for bacteria, fungi, and viruses. At higher concentrations, spores
are killed as well. This substance is used to disinfect surfaces and objects
in 0.5–5% solutions. In the past, it was commonly used in gaseous form to
disinfect the air inside rooms (5 g/m3). The mechanism of action of formal-
dehyde is based on protein denaturation.
Another aldehyde used for disinfection purposes is glutaraldehyde.
Alcohols. The types of alcohol used in disinfection are ethanol (80%), propanol
(60%), and isopropanol (70%). Alcohols are quite effective against bacteria and
fungi, less so against viruses. They do not kill bacterial spores. Due to their
rapid action and good skin penetration, the main areas of application of al-
cohols are surgical and hygienic disinfection of the skin and hands. One dis-
advantage is that their effect is not long-lasting (no depot effect). Alcohols
denature proteins.
Phenols. Lister was the first to use phenol (carbolic acid) in medical applica-
tions. Today, phenol derivatives substituted with organic groups and/or halo-
gens (alkylated, arylated, and halogenated phenols), are widely used. One
common feature of phenolic substances is their weak performance against
spores and viruses. Phenols denature proteins. They bind to organicmaterials
to a moderate degree only, making them suitable for disinfection of excreted
materials.
Halogens. Chlorine, iodine, and derivatives of these halogens are suitable for
use as disinfectants. Chlorine and iodine show a generalized microbicidal ef-
fect and also kill spores.
Chlorine denatures proteins by binding to free amino groups; hypochlo-
rous acid (HOCl), on the other hand, is produced in aqueous solutions, then
disintegrates into HCl and 1/2 O2 and thus acts as a powerful oxidant. Chlorine
is used to disinfect drinkingwater and swimming-poolwater (up to 0.5mg/l).
Calcium hypochlorite (chlorinated lime) can be used in nonspecific disinfec-
tion of excretions. Chloramines are organic chlorine compounds that split off
chlorine in aqueous solutions. They are used in cleaning and washing pro-
ducts and to disinfect excretions.
Iodine has qualities similar to those of chlorine. Themost important iodine
preparations are the solutions of iodine and potassiumiodide in alcohol (tinc-
ture of iodine) used to disinfect skin and small wounds. Iodophores are com-
plexes of iodine and surfactants (e.g., polyvinyl pyrrolidone). While iodo-
phores are less irritant to the skin than pure iodine, they are also less effective
as germicides.
Oxidants. This group includes ozone, hydrogen peroxide, potassiumperman-
ganate, and peracetic acid. Their relevant chemical activity is based on the
splitting off of oxygen. Most are used as mild antiseptics to disinfect mucosa,
skin, or wounds.
Surfactants. These substances (also known as surface-active agents, tensides,
or detergents) include anionic, cationic, amphoteric, and nonionic detergent
compounds, of which the cationic and amphoteric types are the most effec-
tive (Fig. 1.8).
The bactericidal effect of these substances is onlymoderate. They have no
effect at all on tuberculosis bacteria (with the exception of amphotensides),
spores, or nonencapsulated viruses. Their efficacy is good against Gram-pos-
itive bacteria, but less so against Gram-negative rods. Their advantages in-
clude low toxicity levels, lack of odor, good skin tolerance, and a cleaning ef-
fect.
Chemical Sterilization
Although Ethylene Oxide is the most commonly used chemical for sterilization of devices, other chemicals are also used, and novel methodologies are being developed.
Where is Chemical Sterilization Appropriate?
Chemical sterilization is typically used for devices that would be sensitive to the high heat used in steam sterilization, and for devices that may be damaged by irradiation (rubbers and plastics can become more brittle after irradiation.)
Often chemical sterilizers function by using low temperature, highly reactive gases that come into direct contact with the test article (often through a semi-porous membrane or package.) Liquids – for example, bleach – are also used for sterilization.
Considerations in Chemical Sterilization
A primary concern in using chemical sterilization is ensuring that the item to be sterilized is compatible with the sterilant. Some sterilants can be chemically damaging to certain materials; you may wish to consult with your materials manufacturer for more information.
Other concerns regarding chemical sterilization include the potential harm to humans exposed to the sterilization chemicals or residuals from the sterilization process. The sterilization process must be monitored to ensure the safety of workers performing the sterilization.
Chemical Use in Disinfection
For reusable devices (such as those used in hospitals) chemicals are often used for repeated disinfection after each use (disinfection is a different process from sterilization). The chemical(s) and procedure used must be validated. In this case it is important to perform a reusable device cleaning and disinfection validation.
Chemicals Used for Sterilization or Disinfection
Ethylene Oxide
Ozone
Bleach
Glutaraldehyde and Formaldehyde
Phthalaldehyde
Hydrogen Peroxide
Peracetic Acid
Silver
Chlorine dioxide gas
Another alternative for chemical sterilization is chlorine dioxide gas (ClO2), an oxidative gas, which is most efficient at temperatures ranging from 25°C to 30°C (Kowalski and Morrissey, 2004). Chlorine dioxide possesses the bactericidal, virucidal and sporicidal properties of chlorine, but, unlike chlorine, does not lead to the formation of trihalomethanes or combine with ammonia to form chlorinated organic products (chloramines). It is also not mutagenic or carcinogenic in humans. It is commonly used for decontaminating surfaces and equipment. The use concentration is usually between 10 and 30 mg/L. The process has been shown to be effective for the sterilization of medical products, is relatively rapid (1.5–3 h) in duration and there is little or no need for post-sterilization. However, this strong oxidative gas also requires pre-humidification and may corrode some materials. Although this technology was first developed in the late 1980s (Jeng and Woodworth, 1990), it has still not been FDA cleared yet (Rutala, 2008), raising questions regarding its efficacy or safety.
Chemical sterilization
Chemical sterilization has been used as a reliable method, but it presents its own set of challenges. The major concerns are (1) the possibility that the sterilant will react with the polymer material being sterilized; (2) the toxic effect of residual chemicals left on the product; and (3) operator safety associated with the exposure to a sterilant.
Ethylene oxide (EO) sterilization
While chemical methods, such as EO, have proven to be effective at much lower temperatures than thermal sterilization, there is a concern that a certain level of humidity is required to work in concert with the gas to achieve the desired SAL. This is a disadvantage of this method because, for absorbable polymers, moisture can accelerate chain degradation and adversely affect the mechanical properties of the material. Another disadvantage is that EO is known to be a carcinogen and highly explosive, which introduces many safety concerns for operators. In addition, the gas can react with the polymer being sterilized. Incomplete removal of EO and hence residual gas and its reaction by-products (such as ethylene chlorohydrin) in implanted materials are of great concern. Accordingly, extra time is required for aeration of the materials to ensure that the residual amounts are brought down to safe levels.3 It is also worth noting that EO is essentially a surface sterilant and its diffusion into the bulk of a device is limited. Penetration depends on the amount of surface area that needs to be exposed to the gas and the path that the gas needs to take to reach all areas of the device. For example, it is more difficult to sterilize the inside of a long narrow tube than it is to sterilize the outside surface of a device.
Formaldehyde sterilization
Formaldehyde has been used as a sterilizing agent for a long time. Although this method is relatively inexpensive, however, it has a number of drawbacks which also apply to EO sterilization. In addition, it is difficult to generate and distribute formaldehyde gas and there is a potential for the polymerization of the gaseous monomer.6 For most practical purposes, formaldehyde is also a surface sterilant as discussed above for EO.
Liquids:
Gases:
Physical and Chemical Method of Sterilization
Physical and Chemical Method of Sterilization
What is Sterilization?
Sterilization is a process of destruction of all forms of living microorganisms from a substance.
The Methods of Sterilization
- Physical methods
- Chemical methods
Physical Methods of Sterilization:
- Heat method of sterilization
- Radiation
- Filtration
Heat Method of Sterilization
This is the most common method of sterilization. The heat used kills the microbes in the substance. The temperature of the heat and duration of heating are the factors that affect the extent of sterilization.
In heat sterilization process, the longer the exposure to heat the better is the sterilization at a given temperature. As the temperature of heat raises the timespan required for sterilization decreases.
Further, the sterilization time increases with a decrease in temperature and vice-versa. But one needs to maintain minimum sterilization time or minimum contact time for the heat to be in touch with microbes or bacteria and thereby kill them.
The heat method of sterilization is again of two types based on the type of heat used.
- A) Moist heat methods
- B) Dry heat methods
a) Moist heat method of sterilization:
Here heat is applied in the form of steam or just boiling.
This method includes techniques like:
- Boiling
- Pasteurization
- By use of steam (Autoclave)
- Boiling is preferred for metallic devices like surgical scissors, scalpels, needles, etc. Here substances are boiled to sterilize them.
It is preferred for metallic devices like surgical scissors, scalpels, needles, etc. Here substances are boiled to sterilize them.
- Pasteurization is the process of heating the milk at a temperature of 6o degrees or 72 degrees 3 to four times. Here alternative heating and cooling kills all the microbes and molds without boiling the milk.
- Using Steam (autoclaving) Here the substances are subjected to sterilization in an autoclave a steam sterilization equipment. The process is carried out at a temperature of 115 degrees for 60 min or 121 degrees for 20 min at 15psi pressure.
This method is used for sterilization of;
- Flammable substances
- Culture media
- Solutions
- Equipment
- Glass wares
Bacterial spores are the forms of bacteria which are inert. They form a rigid cover over the cell wall during harsh climate. This cover prevents any damage to cell and drying of the cell. By steam sterilization, these forms of bacteria are also killed as steam destroys the cell wall.
b) Dry heat methods:
Following methods are subjected for dry heat sterilization:
- Flaming
- Incineration
- Hot air oven.
- Radiation sterilization
- Flaming is the process of exposing metallic device like the needle, scalpels, and scissors to flame for few minutes. The fire burns the microbes and other dust on the instrument directly.
- Incineration is done especially for inoculating loops used in microbe cultures. The metallic end of the loop is heated to red hot on the flame. This exposure kills all the germs.
- Hot air oven is suitable for het sterilization of dry material like powders, metal devices, glassware, and other such laboratory stocks. Dry heat destroys microorganisms by dehydration and oxidation or even incineration.
Radiation
This method involves exposing the packed materials to radiation for sterilization. There are two types of radiations available for sterilization i.e.
a) non-ionic and
b) ionic radiation.
a) Non-ionic radiations are safe to the operator of sterilization, and they are like Ultra Violet radiations, they can be used even at the door entrances to prevent entry of live microbes through the air.
b) Ionizing radiation sterilization. They are powerful radiation and very useful for sterilization. The operator needs to protect himself from exposure from these radiations by use of special clothing. Ex: X-rays, γ-rays, etc.
Filtration
In this method, liquids are filtered through bacterial filters to remove any microbes present. This method is very effective for sterilization of heat sensitive liquids. The chances of clogging and long time duration for the process to happen are drawbacks.
For sterilization three types of filters are used:
- A) Membrane filters: These are thin filters which are made of cellulose. They can be employed for online sterilization during injection by placing the membrane between the syringe and needle. Used for sterilization of solvents, gasses.
- B) Seitz filters: These are made of asbestos or other material. They are pad like and thicker than membrane filters. They do not rupture during filtration. But the solution might get absorbed by the filter pad itself.
- Sintered glass filters: These are made of glass and hence do not absorb liquids during filtration. The disadvantage is that they are very brittle and break easily.
- c) Candle filters: These are made of clay like diatomous mud. This special mud has minute pores made by algae. The filters have many minute lengthy pores. The microbes get stuck during their travel through the pore in the candle.
Chemical Methods of Sterilization:
The articles are subjected to sterilization by using toxic gasses. The gas penetrates quickly into the material like steam so, the sterilization is effective. But the chances of explosion and cost factors are to be considered.
Gasses used for sterilization are very poisonous. The commonly used gas is ethylene oxide with a combination of carbon-dioxide. Carbon dioxide is added to minimize the chances of an explosion.
Hydrogen Peroxide Sterilization, also known as hydrogen peroxide gas sterilization, is a low temperature sterilization process commonly used to sterilize heat-sensitive devices. A hydrogen peroxide sterilization process involves H2O2 vapor filling the sterilizer chamber, contacting and sterilizing exposed device surfaces. Once the sterilization cycle has completed, the vapor is vacuumed from the chamber and converted to water and oxygen.
So of the available methods,
- Methods of sterilization of surgical instruments are Boiling, Incineration, and Autoclave.
- Methods of sterilization of glass ware are autoclave, boilingand also the hot-air oven.
- Methods of sterilization of water we use filtration and for other moist liquid material autoclave.4. For powders and other dry forms, it is hot air oven if thermo stable or gaseous methods and radiation.5. Methods of Sterilization in hospitals are for surgical metallic instruments boiling, autoclave, incineration can be done. To prevent microbial contamination due to air. UV radiation lamps for sterilization can be arranged at the doors.
Chemical Sterilization
Although Ethylene Oxide is the most commonly used chemical for sterilization of devices, other chemicals are also used, and novel methodologies are being developed.
When is Chemical Sterilization Appropriate?
Chemical sterilization is typically used for devices that would be sensitive to the high heat used in steam sterilization, and for devices that may be damaged by irradiation (rubbers and plastics can become more brittle after irradiation.)
Often chemical sterilizers function by using low temperature, highly reactive gases that come into direct contact with the test article (often through a semi-porous membrane or package.) Liquids – for example, bleach – are also used for sterilization.
Considerations in Chemical Sterilization
A primary concern in using chemical sterilization is ensuring that the item to be sterilized is compatible with the sterilant. Some sterilants can be chemically damaging to certain materials; you may wish to consult with your materials manufacturer for more information.
Other concerns regarding chemical sterilization include the potential harm to humans exposed to the sterilization chemicals or residuals from the sterilization process. The sterilization process must be monitored to ensure the safety of workers performing the sterilization.
Chemical Use in Disinfection
For reusable devices (such as those used in hospitals) chemicals are often used for repeated disinfection after each use (disinfection is a different process from sterilization). The chemical(s) and procedure used must be validated. In this case it is important to perform a reusable device cleaning and disinfection validation.
Chemicals Used for Sterilization or Disinfection
- Ethylene Oxide
- Ozone
- Bleach
- Glutaraldehyde and Formaldehyde
- Phthalaldehyde
- Hydrogen Peroxide
- Peracetic Acid
- Silver
Ethylene oxide. This highly reactive gas (C2H4O) is flammable, toxic, and a
strong mucosal irritant. Ethylene oxide can be used for sterilization at low
temperatures (20–60 8C). The gas has a high penetration capacity and can
even get through some plastic foils. One drawback is that this gas cannot
kill dried microorganisms and requires a relative humidity level of 40–
90% in the sterilizing chamber. Ethylene oxide goes into solution in plastics,
rubber, and similar materials, therefore sterilized items must be allowed to
stand for a longer period to ensure complete desorption.
Aldehydes. Formaldehyde (HCHO) is the most important aldehyde. It can be
used in a special apparatus for gas sterilization. Its main use, however, is in
disinfection. Formaldehyde is a water-soluble gas. Formalin is a 35% solution
of this gas inwater. Formaldehyde irritatesmucosa; skin contactmay result in
inflammations or allergic eczemas. Formaldehyde is a broad-spectrum ger-
micide for bacteria, fungi, and viruses. At higher concentrations, spores
are killed as well. This substance is used to disinfect surfaces and objects
in 0.5–5% solutions. In the past, it was commonly used in gaseous form to
disinfect the air inside rooms (5 g/m3). The mechanism of action of formal-
dehyde is based on protein denaturation.
Another aldehyde used for disinfection purposes is glutaraldehyde.
Alcohols. The types of alcohol used in disinfection are ethanol (80%), propanol
(60%), and isopropanol (70%). Alcohols are quite effective against bacteria and
fungi, less so against viruses. They do not kill bacterial spores. Due to their
rapid action and good skin penetration, the main areas of application of al-
cohols are surgical and hygienic disinfection of the skin and hands. One dis-
advantage is that their effect is not long-lasting (no depot effect). Alcohols
denature proteins.
Phenols. Lister was the first to use phenol (carbolic acid) in medical applica-
tions. Today, phenol derivatives substituted with organic groups and/or halo-
gens (alkylated, arylated, and halogenated phenols), are widely used. One
common feature of phenolic substances is their weak performance against
spores and viruses. Phenols denature proteins. They bind to organicmaterials
to a moderate degree only, making them suitable for disinfection of excreted
materials.
Halogens. Chlorine, iodine, and derivatives of these halogens are suitable for
use as disinfectants. Chlorine and iodine show a generalized microbicidal ef-
fect and also kill spores.
Chlorine denatures proteins by binding to free amino groups; hypochlo-
rous acid (HOCl), on the other hand, is produced in aqueous solutions, then
disintegrates into HCl and 1/2 O2 and thus acts as a powerful oxidant. Chlorine
is used to disinfect drinkingwater and swimming-poolwater (up to 0.5mg/l).
Calcium hypochlorite (chlorinated lime) can be used in nonspecific disinfec-
tion of excretions. Chloramines are organic chlorine compounds that split off
chlorine in aqueous solutions. They are used in cleaning and washing pro-
ducts and to disinfect excretions.
Iodine has qualities similar to those of chlorine. Themost important iodine
preparations are the solutions of iodine and potassiumiodide in alcohol (tinc-
ture of iodine) used to disinfect skin and small wounds. Iodophores are com-
plexes of iodine and surfactants (e.g., polyvinyl pyrrolidone). While iodo-
phores are less irritant to the skin than pure iodine, they are also less effective
as germicides.
Oxidants. This group includes ozone, hydrogen peroxide, potassiumperman-
ganate, and peracetic acid. Their relevant chemical activity is based on the
splitting off of oxygen. Most are used as mild antiseptics to disinfect mucosa,
skin, or wounds.
Surfactants. These substances (also known as surface-active agents, tensides,
or detergents) include anionic, cationic, amphoteric, and nonionic detergent
compounds, of which the cationic and amphoteric types are the most effec-
tive (Fig. 1.8).
The bactericidal effect of these substances is onlymoderate. They have no
effect at all on tuberculosis bacteria (with the exception of amphotensides),
spores, or nonencapsulated viruses. Their efficacy is good against Gram-pos-
itive bacteria, but less so against Gram-negative rods. Their advantages in-
clude low toxicity levels, lack of odor, good skin tolerance, and a cleaning ef-
fect.
1. Chemical Disinfectant Groups
a. Aldehydes: (Formaldehyde, Paraformaldehyde, Glutaraldehyde)
Formaldehyde – and its polymerized solid paraformaldehyde have broad-spectrum biocidal activity and are both effective for surface and space decontamination. As a liquid (5% concentration), formaldehyde is an effective liquid decontaminant. Its biocidal action is through alkylation of carboxyl, hydroxyl and sulfhydryl groups on proteins and the ring nitrogen atoms of purine bases. Formaldehyde’s drawbacks are reduction in efficacy at refrigeration temperature, its pungent, irritating odor, and several safety concerns. Formaldehyde is presently considered to be a carcinogen or a cancer-suspect agent according to several regulatory agencies. The OSHA 8-hour time-weighted exposure limit is 0.75 ppm.
Paraformaldehyde – is a solid polymer of formaldehyde. Paraformaldehyde generates formaldehyde gas when it is depolymerized by heating to 232 to 246°C (450 to 475°F); the depolymerized material reacts with the moisture in the air to form formaldehyde gas. This process is used for the decontamination of large spaced and laminar-flow biological safety cabinets when maintenance work or filter changes require access to the sealed portion of the cabinet. A neutralization step, heating ammonium carbonate, is required prior to ventilation of the space. Formaldehyde gas can react violently or explosively (7.0 – 73% v/v in air), when exposed to incompatibles, therefore, only individuals that have specific training and have been approved by the Dept. of Environmental Health & Safety are permitted to use this gas.
Glutaraldehyde – is a colorless liquid and has the sharp, pungent odor typical of all aldehydes, with an odor threshold of 0.04 parts per million (ppm). It is capable of sterilizing equipment, though to effect sterilization often requires many hours of exposure. Two percent solutions of glutaraldehyde exhibit very good activity against vegetative bacteria, spores and viruses. It is ten times more effective than formaldehyde and less toxic. However, it must be limited and controlled because of its toxic properties and hazards. It is important to avoid skin contact with glutaraldehyde as it has been documented to cause skin sensitization. Glutaraldehyde is also an inhalation hazard. The NIOSH ceiling threshold limit value is 0.2 ppm.
Cidex, a commercially prepared glutaraldehyde disinfectant is used routinely for cold surface sterilization of clinical instruments. Glutaraldehyde disinfectants should always be used in accordance with the manufacturer’s directions.
b. Halogen-Based Biocides: (Chlorine Compounds and Iodophores)
1. Chlorine Compounds
Chlorine compounds are good disinfectants on clean surfaces, but are quickly inactivated by organic matter and thus reducing the biocidal activity. They have a broad spectrum of antimicrobial activity and are inexpensive and fast acting. Hypochlorites, the most widely used of the chlorine disinfectants, are available in liquid (e.g., Sodium hypochlorite), household bleach and solid (e.g., calcium hypochlorite, sodium dichloroisocyanurate) forms. Household bleach has an available chlorine content of 5.25%, or 52,500 ppm. Because of its oxidizing power, it loses potency quickly and should be made fresh and used within the same day it is prepared. The free available chlorine levels of hypochlorite solutions in both opened and closed polyethylene containers are reduced to 40% to 50% of the original concentration over a period of one month at room temperature.
There are two potential occupational exposure hazards when using hypochlorite solutions. The first is the production of the carcinogen bis-chloromethyl ether when hypochlorite solutions come in contact with formaldehyde. The second is the rapid production of chlorine gas when hypochlorite solutions are mixed with an acid. Care must also be exercised in using chlorine – based disinfectants which can corrode or damage metal, rubber, and other susceptible surfaces. Bleached articles should never be autoclaved without reducing the bleach with sodium thiosulfate or sodium bisulfate.
Chloramine T which is prepared from sodium hypochlorite and p-toluenesulfonamide is a more stable, odorless, less corrosive form of chlorine but has decreased biocidal activity in comparison to bleach.
2. Iodophors
Iodophors are used both as antiseptics and disinfectants. An iodophor is a combination of iodine and a solubilizing agent or carrier; the resulting complex provides a sustained-release reservoir of iodine and releases small amounts of free iodine in aqueous solution. Antiseptic iodophors are not suitable for use as hard-surface disinfectants because they contain significantly less free iodine than do those formulated as disinfectants.
Wescodyne, Betadyne, Povidone-Iodine and other iodophors are commercially available Iodine-based disinfectants, which give good control when the manufacturer’s instructions for formulation and application are followed. Both bleach and iodophors should be made up in cold water in order to prevent breakdown of the disinfectant.
c. Quaternary Ammonium Compounds: (Zephirin, CDQ, A-3)
Quaternary ammonium compounds are generally odorless, colorless, nonirritating, and deodorizing. They also have some detergent action, and they are good disinfectants. However, some quaternary ammonium compounds activity is reduced in the presence of some soaps or soap residues, detergents, acids and heavy organic matter loads. They are generally ineffective against viruses, spores and Mycobacterium tuberculosis. Basically these compounds are not suitable for any type of terminal disinfection.
The mode of action of these compounds is through inactivation of energy producing enzymes, denaturation of essential cell proteins, and disruption of the cell membrane. Many of these compounds are better used in water baths, incubators, and other applications where halide or phenolic residues are not desired.
d. Phenolics: (O-phenophenoate-base Compounds)
Phenolics are phenol (carbolic acid) derivatives. These biocides act through membrane damage and are effective against enveloped viruses, rickettsiae, fungi and vegetative bacteria. They also retain more activity in the presence of organic material than other disinfectants. Cresols, hexachlorophene, alkyl- and chloro derivatives and diphenyls are more active than phenol itself. Available commercial products are Lysol, Pine-Sol, Amphyl, O-syl, Tergisyl, Vesphene, L- Phaseand Expose.
e. Acids/Alkalis:
Strong mineral acids and alkalis have disinfectant properties proportional to the extent of their dissociation in solution. Some hydroxides are more effective than would be predicted from their values. In general acids are better disinfectants than alkalis. Mode of action is attributed to an increase of H+ and OH– species in solutions which interfere with certain microbial functions, however the total effect is not only dependent on pH alone. Weak organic acids are more potent than inorganic acids despite low dissociation rates in solution. Action is attributed to the disruption of 2° and 3° conformation of enzymes and structural proteins.
f. Heavy Metals:
Soluble salts of mercury, silver lactate, mercuric chloride and mercurous chloride are efficient bactericidal agents. Silver nitrate and mercuric chloride are commonly used as 1:1000 aqueous solutions. Action is through attack on protein sulfhydryl groups and disruption of enzyme functions. Organic matter can reverse the disinfectant properties of mercurials.
Caution: Please consult with EH&S’s Hazardous Materials group prior to using heavy metals because many of these must be disposed of as a hazardous waste. Specifically, disposal of elemental mercury and salts of mercury are very costly.
g. Alcohols:
Alcohols work through the disruption of cellular membranes, solubilization of lipids, and denaturation of proteins by acting directly on S-H functional groups. Ethyl and isopropyl alcohols are the two most widely used alcohols for their biocidal activity. These alcohols are effective against lipid-containing viruses and a broad spectrum of bacterial species, but ineffective against spore-forming bacteria. They evaporate rapidly, which makes extended contact times difficult to achieve unless the items are immersed.
The optimum bactericidal concentration for ethanol and isopropanol is in the range of 60% to 90% by volume. Their cidal activity drops sharply when diluted below 50% concentration. Absolute alcohol is also not very effective. They are used to clean instruments and wipe down interior of Biological Safety Cabinets and bottles, etc. to be put into Biological Safety Cabinets. Alcohols are generally regarded as being non-corrosive.
Although Ethylene Oxide is the most commonly used chemical for sterilization of devices, other chemicals are also used, and novel methodologies are being developed.
Where is Chemical Sterilization Appropriate?
Chemical sterilization is typically used for devices that would be sensitive to the high heat used in steam sterilization, and for devices that may be damaged by irradiation (rubbers and plastics can become more brittle after irradiation.)
Often chemical sterilizers function by using low temperature, highly reactive gases that come into direct contact with the test article (often through a semi-porous membrane or package.) Liquids – for example, bleach – are also used for sterilization.
Considerations in Chemical Sterilization
A primary concern in using chemical sterilization is ensuring that the item to be sterilized is compatible with the sterilant. Some sterilants can be chemically damaging to certain materials; you may wish to consult with your materials manufacturer for more information.
Other concerns regarding chemical sterilization include the potential harm to humans exposed to the sterilization chemicals or residuals from the sterilization process. The sterilization process must be monitored to ensure the safety of workers performing the sterilization.
Chemical Use in Disinfection
For reusable devices (such as those used in hospitals) chemicals are often used for repeated disinfection after each use (disinfection is a different process from sterilization). The chemical(s) and procedure used must be validated. In this case it is important to perform a reusable device cleaning and disinfection validation.
Chemicals Used for Sterilization or Disinfection
Ethylene Oxide
Ozone
Bleach
Glutaraldehyde and Formaldehyde
Phthalaldehyde
Hydrogen Peroxide
Peracetic Acid
Silver
Chlorine dioxide gas
Another alternative for chemical sterilization is chlorine dioxide gas (ClO2), an oxidative gas, which is most efficient at temperatures ranging from 25°C to 30°C (Kowalski and Morrissey, 2004). Chlorine dioxide possesses the bactericidal, virucidal and sporicidal properties of chlorine, but, unlike chlorine, does not lead to the formation of trihalomethanes or combine with ammonia to form chlorinated organic products (chloramines). It is also not mutagenic or carcinogenic in humans. It is commonly used for decontaminating surfaces and equipment. The use concentration is usually between 10 and 30 mg/L. The process has been shown to be effective for the sterilization of medical products, is relatively rapid (1.5–3 h) in duration and there is little or no need for post-sterilization. However, this strong oxidative gas also requires pre-humidification and may corrode some materials. Although this technology was first developed in the late 1980s (Jeng and Woodworth, 1990), it has still not been FDA cleared yet (Rutala, 2008), raising questions regarding its efficacy or safety.
Chemical sterilization
Chemical sterilization has been used as a reliable method, but it presents its own set of challenges. The major concerns are (1) the possibility that the sterilant will react with the polymer material being sterilized; (2) the toxic effect of residual chemicals left on the product; and (3) operator safety associated with the exposure to a sterilant.
Ethylene oxide (EO) sterilization
While chemical methods, such as EO, have proven to be effective at much lower temperatures than thermal sterilization, there is a concern that a certain level of humidity is required to work in concert with the gas to achieve the desired SAL. This is a disadvantage of this method because, for absorbable polymers, moisture can accelerate chain degradation and adversely affect the mechanical properties of the material. Another disadvantage is that EO is known to be a carcinogen and highly explosive, which introduces many safety concerns for operators. In addition, the gas can react with the polymer being sterilized. Incomplete removal of EO and hence residual gas and its reaction by-products (such as ethylene chlorohydrin) in implanted materials are of great concern. Accordingly, extra time is required for aeration of the materials to ensure that the residual amounts are brought down to safe levels.3 It is also worth noting that EO is essentially a surface sterilant and its diffusion into the bulk of a device is limited. Penetration depends on the amount of surface area that needs to be exposed to the gas and the path that the gas needs to take to reach all areas of the device. For example, it is more difficult to sterilize the inside of a long narrow tube than it is to sterilize the outside surface of a device.
Formaldehyde sterilization
Formaldehyde has been used as a sterilizing agent for a long time. Although this method is relatively inexpensive, however, it has a number of drawbacks which also apply to EO sterilization. In addition, it is difficult to generate and distribute formaldehyde gas and there is a potential for the polymerization of the gaseous monomer.6 For most practical purposes, formaldehyde is also a surface sterilant as discussed above for EO.