Application of Moist Heat Sterilisation

            An autoclave is the apparatus used in order to utilise moist heat sterilisation, a method of sterilisation which destroys microbes through the action of steam at high pressures. Moist heat sterilisation allows accurate control and monitoring of the sterilisation process, which means it is energy efficient. Coupled with the fact that moist heat sterilisation exhibits rapid heat transfer from the environment to the items being sterilised, mo ist heat sterilisation is highly cost effective.

            At around 121℃, moist heat sterilisation can be used to sterilise aqueous solutions and suspensions such as injections, ophthalmic solutions, irrigations, dialysis and fluids that dry heat sterilisation cannot. It is even more appropriateIt to use when there are a lot of solutions to be sterilised as the energy carried by the steam flows quickly around every article in the load. This is helped by the fact that as the water condenses it creates a pocket of low pressure into which the much higher pressure steam will readily move. 

            Surgical dressings can only be sterilised by moist heat sterilisation because the porous nature of the dressing means the moisture penetrates it quickly, bringing the dressings up to the required temperature faster than dry heat and preventing it from burning. For this purpose, a typical autoclave would be set at around 134℃ for approximately 3 minutes. Metal instruments e.g. scalpels, can be sterilised in moist heat because the lethal action of water plus heat, destroys micro-organisms more efficiently therefore needing a shorter exposure time at a lower temperature. Immediate drying following sterilisation is needed with these metal instruments to avoid corrosion as the oxidation process is sped up because of the heat.

             Rubber or plastic items (if sterilised separately from the containers) are best sterilised under moist heat because of their thermolability  (destruction or degradation under high temperature).  Moist heat kills micro-organisms by coagulating and denaturing the enzymes and structural protein, but cannot kill pyrogens like dry heat.

            As a method of testing that the required sterilising conditions have been met, it is common for a sealed sample of a thermophilic bacteria e.g. Geobacillus stearothermophillis, to be routinely placed in the autoclave along with the actual items to be sterilised. Following sterilisation, the sample is incubated and tested for any growth. If any growth is noted, it can be said that suitable sterilisation was not achieved and the items should be sterilised again

  

Application of Dry Heat Sterilisation

             Dry heat sterilisation is used to sterilise heat safe items that cannot be disassembled or may be affected by rusting if moisture was introduced. It is used to ensure that items are free from harmful pathogens and deemed sterile to be used under sterile conditions. Equipment to be sterilised by the dry heat method must be exposed to specific temperatures for specific time durations, depending on the item, the possibility of damage to it and the type of microbe that it is could be contaminated with.

180°C	For no less than 30 minutes
170°C	For no less than 1 hour
160°C	For no less than 2 hours

There are two different types of dry heat sterilisers available:

·       Oven type steriliser - this involves coils at the bottom of the oven unit heating to the desired temperature, this then causes the hot air to rise inside the oven via gravity convection. This method is slower as a longer period of time is required to reach sterilising temperature.

·       Mechanical convection – a motor-driven blower circulates the heated air throughout the chamber at a high velocity and allows for a more rapid transfer of heat from the air to the instruments.

            

A Typical Dry Heat Steriliser

Dry heat is used to sterilise numerous types of equipment and materials. Before sterilisation, all equipment must be free from of all signs of visible dirt and be dry.

Glassware – items such as beakers, syringe s, petri dishes and test tubes can be sterilised using dry heat, the items must be cleaned first though with apyrogenic water. This water is free from pyrogens and is essentially water for injections.

Porcelain and metal – items such as pestle and mortars, scalpels and needles

Oils and fats – anhydrous oils (oils which contain no water) and fats

Powders – items such as talcum powder and sulphonamides

  

Summary of Moist/Dry Heat Sterilisation

Moist heat		Dry heat
Advantages	Disadvantages	Advantages	Disadvantages
A high temperature ensures rapid heat transfer	Unsuitable for products which are anhydrous: such as powder and oils	Suitable for products that would be harmed if exposed to moisture	Low heat transfer between items
Destroys microorganisms more effectively than dry heat	It does not destroy pyrogens	Less damage caused to glass and metal than moist heat (except for sharp instruments)	Unsuitable for use on most rubbers and plastics – they are too thermolabile for this method of sterilisation
Suitable for use for a wide variety of injections, solutions, irritants, dialysis fluid.		Suitable for sterilising glass containers and able to destroy pyrogens	Unsuitable for use on aqueous products
Suitable for sterilising surgical dressings and materials due to the moistures ability to rapidly penetrate porous material.		Materials not contaminated with toxic substances	Unsuitable for use on surgical dressings
Suitable for sharp instruments			Accurate control of the process parameters is more difficult than moist sterilisation.
Accurate control of the process parameters is possible
No toxic contaminants are left in the materials that have been sterilised

The Science of Sterilisation by Radiation by Adam Clough and Stacey Hasprey

Introduction

Radiations used in Sterilisation can be divided up into two groups, electro-magnetic waves and streams of particulate matter. The first group encompasses ultraviolet light and gamma rays and the second includes alpha and beta radiations.

UV light and gamma rays can both damage molecules that are vital to all living cells. As the waves pass through the matter they excite the electrons surrounding the nuclei. The electrons can then gather enough energy to escape from the atoms resulting in ionization. This then gives the effect of Sterilisation.

Ultraviolet light

The main method of using UV light in Sterilisation is by passing a low current at high voltages through mercury vapour in an evacuated tube made of borosilicate glass. The wavelength created is between 220-280nm but the vital wavelength required is 265nm which is thought to be most effective in killing microorganisms and will reduce populations of vegetative cells by up to 90%.

The intensity of UV radiation is shown as energy in an area, so microwatts/mm² is used. The intensity of UV radiation should be between 10-60microwatts/mm²

As a general rule UV light should not be relied on for Sterilisation of appliances alone, it should be done with other forms of sterilisation.

Ionizing radiations

This includes both high-speed electrons and gamma rays. The process of Sterilisation for commercial equipment must have certain factors associated with it to be a useful sterilisation method. It must:

·         Have good penetrating power

·         High sterilising efficiency

·         Have little to no damaging effect on materials

·         Be able to be produced efficiently

High-speed electrons are generated by using a machine known as a van de Graaff accelerator. The beam of high-speed electrons which is fired out of an highly evacuated tube is narrow and intense and it is used to irradiate equipment on a conveyor belt. The equipment that has become irradiated is now sterilized.

Cobolt is used as the source of gamma radiation. The equipment that is treated using gamma rays will have two types of gamma rays in succession because of the nature of cobolt. The energies produced from the two gamma rays are 1.33 MeV and 1.17 MeV.

Mode of Action

Ionizing radiations causes excitations, ionizations and free radical formations – where water is present. These are powerful oxidizing and reducing agents which damage essential molecules in living cells.

                  http://www.bbc.co.uk/schools/gcsebitesize/science/images/addgateway_sterilisation.gif

Steriliaing Dose

The dose used is measure in kGy. The gray (Gy) is the SI equivalent to 100 rad. The recognised sterilising dose is 25kGy. The choice of the dose was based on an experiment whereby paper discs or pieces of plastic or foil were heavily contaminated with pathogens, non-pathogens, vegetable bacteria and spores. These were then exposed to different variations of radiation doses.

Within radiation the D10 value (decimal reduction dose) is the dose (measured in Gy) that reduces the number of viable organisms.

Sterilising Time

Depending on the size and density of the material which is to be irradiated, the beams or pulses of electrons from accelerators is able to deliver a dose in fractions that vary between 1 second to a few seconds. This differs from isotope sources, due to their diffuse and penetrating nature of their emissions; therefore the dose has to accumulate over several days.

 Advantages

The advantages of this type of Sterilisation are:

·         There is no significant temperature rise

·         There can be a continuous process because exposure is either short (machine generation) or large (isotope generation)

·         No aseptic handling is required

·         The method of Sterilisation is accurate and reliable

Undesirable effects and Disadvantages

The Disadvantages of radiation Sterilisation include:

·         Chemical decomposition (this can be immediate or can occur after storage)

·         Costs for capital and/or replacements are high

·         The precautions which have to take place in order to protect operators from harmful effects of ionizing radiation are elaborate and expensive

·         Alterations in colour, texture and solubility

Repeated irradiation can cause degradation with certain dressings and plastics

The Science of Gaseous Sterilisation by Salma, Shorif and Ann

Introduction

Gas is used to sterilise large areas in industrial and medical environments  examples of where they are used includes using gas to sterilise a production area after sterility has been broken this may be due to maintenance work being carried out in the area. It is also used to sterilise equipment this can include equipment that is used for surgical procedures and equipment that is used in pharmaceutical sterile environments. Chemicals such as formaldehyde and ethylene oxide are used the later being the most successfully used gas used on a large scale due to the ability to sterilise without causing damage to equipment and having the ability to defuse out of the equipment when given the correct airing during the sterilisation cycle.

Ethylene oxide is effective in killing all microorganisms and during the validation process of ethylene oxide sterilising cycle’s spores of bacillus subtilis var.niger are used as this is the most resistant organism to act against ethylene oxide. To ensure effective sterilising certain conditions must be met this includes-:

·         The temperature of sterilisation

·         The concentration of ethylene oxide

·         The exposure time

·         The penetration of the load

·         The humidity of the area

Health and safety

When using or coming into contact with ethylene oxide health and safety precautions must be considered, equipment must be worn to protect the eyes, skin and the respiratory passage.

·         gloves must be worn to protect skin on hands

·         protective clothing must be worn to protect skin

·         a full face mask with breathing equipment must be worn to protect eyes, face and for protect from gas inhalation

The gas vapour is irritating to the eyes and the skin and in liquid form can cause burns. If low doses are inhaled it can cause delayed nausea and high concentrations can cause narcotic and neurotoxic effect followed by vomiting, coughing and irritation to the respiratory passage. This can lead to emphysema and pulmonary edema.

  

Relative humidity of sterilising atmosphere

The relative humidity must be increased to between 40 and 50% to produce a suitable force that will provide moisture across the load to create optimum conditions at the surface of the microorganisms and increase the rate of diffusion through to the wrapped equipment. Moisture in the sterilisation cycle is required during the pre-vacuum stage to prevent the dry spores being dehydrated and becoming resistant to the action of ethylene oxide.

  

Temperature of the sterilisation cycle

The temperature of a sterilisation cycle can affect the time it takes for the process to complete where a cycle at room temperature can be achieved over a longer period of time and a high temperature of 60oC will decrease the cycle time.

Concentration of ethylene oxide and time of exposure

Manufactures of ethylene oxide sterilises recommend exposure to 850-900mg/litre for 3 hours or 450mg/litre for 5 hours at 64oC

 Penetrability of ethylene oxide through the load

Ethylene oxide is able to penetrate through paper, fabrics, different types of plastics and rubber. This allows materials to be sterilised suitably packed in appropriate containers, it is still important to make sure articles of gaseous sterilisation are clean. The efficiency of the process is reduced by organic matter however it does not prevent it. Diffusion of moisture is prevented by organisms within crystals. Spores protected in this way are capable of resisting sterilisation due to contact with steam, ethylene oxide, or dry heat these conditions effect sterilisation. Therefore it is important that care is taken to prevent physical protection of microorganisms in gas impermeable deposits. Sterilised articles should not be used until the absorbed gas has escaped because ethylene oxide is absorbed by many substances.

Ethylene oxide sterilisers

Design

The features of suitable equipment include:

·         An exposure chamber that is gas tight and able to withstand high pressure and vacuum.

·         A means of heating the chamber e.g. steam, or hot water jacket

·         A baffled inlet of the gas mixture

·         A method of completely vaporising the gas mixture and warming it to the sterilising temperature.

·         A means of extracting air before, and the gas mixture after, sterilisation.

·         A system for adding water to provide the right humidity.

·         Provision for the admission of sterile air at end of the process.

·         Safety valve and suitable indicators and recorders of temperature and pressure

Method of use

·         The chamber is loaded

·         Water is introduced to prevent vacuum dehydration of microorganisms

·         Door is closed and temperature raised to sterilisation level.

·         The heat exchanger is raised to high temperature (about 100^oC)

·         A high efficiency vacuum pump is used to reduce the air pressure in the chamber to about 1.5kPa.

·         More water is added if necessary

·         Warm gas mixture is added until correct pressure is reached

·         Exposure time is allowed

·         Gas is deabsorbed.

Control of the process

Similar method to steam sterilisation have been used – physical, chemical and biological monitoring of the process. The temperature and pressure should be recorded throughout the sterilisation cycle. Conventional physical and chemical monitoring is not able to provide assurance that each item of the load has been subjected to the predetermined conditions and gas concentration. Biological monitoring is the most important, either paper or aluminium foil strips coated with known concentrations of Bacillus subtilis var niger spores after exposure they are tested for sterility.

Application of ethylene oxide sterilisation

Ethylene oxide is suitable for sterilising where it is known that the microorganisms are on the surface of the particles and not embedded inside them. Ethylene oxide can be used to sterilise equipment, instruments and articles that are made from plastic, rubber, metal and other materials without causing damage. Catheters and syringes can also be sterilised. Other articles such as intravenous sets, prostheses, blood oxygenators, bottles and vials are also suitable. Some plastics may become damaged however the damage is not caused with pure ethylene oxide or mixtures with carbon dioxide. Contact with liquid ethylene oxide must always be avoided. Fragile rubber articles survive more treatments with ethylene oxide than steam. Equipment such as cystoscopes, bronchoscopes, opthalmoscopes and Geiger-Müller counters can also be sterilised with ethylene oxide.

DRY HEAT STERILISATION By Charlotte, Michelle and Rebecca

Dry heat sterilisation is most commonly used for non aqueous preparations that are stable at high heats.  This includes powders, some types of containers and certain impregnated dressings. 

Dry heat sterilisation makes use of hot air free from (or with minimal) water vapour, which plays no role in this sterilisation process.  It is usually carried out in a hot air oven making use of radiation, convections and conduction.  Heat is passed from the source to the load, and ‘blown’ about the chamber to achieve uniform temperatures in all areas.  Heat is then conducted from the outer layer to inner layers of the item.  Items must be dry prior to sterilisation to prevent water interfering with the process.

Published evidence shows the following temperature-time exposures needed to kill certain pathogens by dry heat.

Time	Temperature	Microbe type
90 minutes	110°C	Vegetative bacteria and most viruses
3 hours	140°C	Resistant bacterial spores and resistant viruses
90 minutes	115°C	Mould spores

Recommended time – temperature combinations

Different products require different combinations of time and temperature.  Three cycles listed in the BP (1993) include:

• Minimum of 180 °C for at least 30 minutes
• Minimum of 170°C for at least 1 hour
• Minimum of 160°C for at least 2 hours

For glass containers used for large volumes of dosage forms for injection, a cycle of 250°C for 45 minutes is most appropriate as this is considered sufficient for denaturing pyrogens that may be absorbed into the surface of the glassware.

During a cycle it is imperative that the entire contents of each container is maintained at a certain temperature for the specific length of time, allowing for temperature variations in hot-air ovens, to ensure effective sterilisation.

Design and operation of the dry heat steriliser

Ovens for dry heat sterilization are designed and equipped with forced air circulation, these ovens are stainless steel with a high quality argon welding and have curved joints for cleaning and hygiene purposes.  They have structure reinforcement for chambers to prevent thermal shocks and vibration and can sterilise glassware and metal material and reduce the reduction of bacteria by sterilising and de-pyrogenation. This can reach a maximum temperature of up to 250 degrees. The ovens for dry heat also have duly insulated doors with a silicon gasket for positive sealing and have positive door interlocking with pneumatically operated cylinders which prevent simultaneous opening of both doors.

Dry heat ovens are recommended to be maintained at a positive pressure and all air entering the chamber should be via a bacteria proof filter. The efficiency of dry heat process depends on velocity and the designed circulation pattern. Only a small amount of heat is transferred from the heat source to the articles in a hot air oven by conduction because, the limited pathways and small areas of the contact. Radiation is the chief form of heat transfer so the heaters need to be arranged all round the chamber. It is useful to have ovens fitted with the facility for automatic boost heating to give minimum heat up times but it is essential to have accurate temperature control by easily set regulators. A loaded oven temperature variation should not exceed 5 degrees once the sterilising temperature is reached. The temperature variation is normally measured as the difference between the temperature at the centre and any other point.

When loading the dry heat ovens it is best to load it will the same material of the same size. it is also beneficial not to have dull or blackened equipment as it does not reflect heat, If using glassware it must be cleaned thoroughly because heat transfer will be impaired if the surface is coated with a greasy film.

           

                                             

Infrared conveyor oven

This is one type of oven that can be used for dry heat sterilisation. This works by placing the items (generally smaller items such as glass syringes and other glass wear) on the conveyor belt and allowing to pass through the tunnel under the heated infrared radiators at about 180 oC. Infrared is a thermal type of radiations that absorbs the energy and converts it to heat. The oven is used for a continual process as it is highly effective constantly carrying out heat transfer from the source. It is unaffected by the thermal resistances of the static surface air films.

                                                                                  

Heating with bactericide

This is a method used for sterilizing aqueous solutions that are unable to handle normal autoclaving conditions, this sterilizes at a temperature range 98 to 100 for 30 minutes with the presents of a bactericidal substance to help kill or inactive the bacteria contaminating that certain product. The chose of bactericidal all depends on the compatible with the product its container and the closure chosen. This is to prevent any toxins being exposed which may cause potential harm to the patient. This method of sterilisation is no longer recognized by the BP (1993) due to the risks it carries.