Steam Sterilization

4.2.1 Steam Sterilization

Steam sterilization (also called autoclaving) is conducted in an autoclave which is a container that can withstand high pressure and temperature.¹¹ The autoclave is filled with the products and devices that need to be sterilized. The product or device comes into direct contact with steam at high temperature and pressure for a specified period of time. Pressure serves as a means to obtain the high temperatures necessary to quickly kill microorganisms. Moist heat destroys microorganisms by the irreversible coagulation and denaturation of enzymes and structural proteins. After the required time has passed, the steam is released and the sterilized objects are removed. The entire batch processing cycle can take from 10 to 60 min. The four parameters associated with steam sterilization are steam, pressure, temperature, and time. Steam sterilization is inexpensive and has a high sporicidal effect with short application times. Steam sterilization is typically conducted at the hospital or clinical for reusable devices. Metal surgical instruments and glass products can withstand several sterilization cycles and can thus be reused for procedures several times. Plastics on the other hand may only be able to withstand anywhere from 1 to 2 cycles to serval hundred depending upon the material’s hydrolytic stability and temperature resistance.

It is important to remove all the air from the autoclave before introducing steam because air is heavier than steam and will reduce the steam concentration (and hence the effectiveness) of the sterilization. High-speed steam sterilization is conducted at higher temperatures (134°C/273°F) and shorter cycle times (between 3 and 10 min). High temperatures, along with moisture, will kill microorganisms. High-pressure steam first condenses when it comes in contact with the part while continuing to heat it. Appropriate time/temperature cycles are developed based on the type and the amount of load in the chamber to ensure complete sterilization and destruction of microorganisms. Steam should penetrate and reach all surfaces of the product for proper sterilization efficacy. Poor cleaning, improper moisture, impermeable packaging, or overpacking the autoclave chamber can reduce the effectiveness of steam sterilization.

The critical factors in ensuring the reliability of steam sterilization are (1) the right temperature and time, and (2) the complete replacement of air with steam (i.e., no entrapment of air). The steam cycle is monitored by mechanical, chemical, and biological monitors. Steam sterilizers usually are monitored by measuring and controlling the temperature, the time at the target temperature, and the pressure using chemical indicators. The use of appropriate biological indicators at locations throughout the autoclave is considered as the best indicator of sterilization. The effectiveness of steam sterilization is monitored with a biological indicator containing spores of Geobacillus stearothermophilus (formerly Bacillus stearothermophilus). More recently, parametric release methods have been used to evaluate the sterility of devices.¹²

Sometimes products that have a higher softening temperature than the autoclaving temperature can warp or distort due to the release of molded-in stress.¹³ Molded-in stress is caused by the rapid cooling or improper design of the part. Heating the part relieves the molded-in stress, causing differential stress and hence deformation. Where autoclaving is to be used, the effect of multiple sterilization cycles needs to be considered to prevent cumulative effects of the treatment on the plastic. If the devices are to be packaged before autoclaving, then the packaging material and packaging method needs to be chosen carefully. The suitability of a package for autoclaving will depend on the material, the size of the package, the wall thickness of the package, and the contents. Autoclaving is used significantly in hospitals for the sterilization of multiple-use articles. It is not the predominant method in the commercial sterilization of medical devices because of the difficulties involved with autoclaving packaged products.

Most plastics will survive 1–5 cycles of steam sterilization. For those reusable devices that need up to 100 sterilization cycles, polysulfones, polyether sulfones, polyetherimides, polyether ether ketone (PEEK), and liquid crystal polymers (LCPs) are generally used. For applications that require more than 100 cycles, polyphenylsulfones, PEEK, and LCPs can be used. Polyphenylene sulfones can be used for up to 1000 cycles of steam sterilization.

The standard that governs the requirements for steam sterilization is ISO 17665-1.¹⁴

8.5.2.1 Steam

Steam sterilization uses thermal energy from high-pressure saturated steam to denature DNA and destroy enzymes to kill microbes on the surfaces of devices. A packaged device is placed into a chamber (autoclave) and pressurized steam is introduced at 121°C for a specific amount of time. This process involves high temperature and humidity and is incompatible with polymeric materials with melting or softening points, or glass transition temperatures below or near the temperatures involved. Glass, most metals, paper, and some polymers such as silicones, polyurethanes, and polycarbonates are compatible (i.e., physical properties are not affected). Steam sterilization can result in corrosion of some metals.

For steam sterilization to be effective, moisture and heat must be able to penetrate packaging materials and contact and transfer thermal energy to all surfaces of a medical device. This requires the use of porous packaging (made from paper, Tyvek®, or similar materials) that allows water vapor to enter and exit but prevents the same for microorganisms. Barriers to contact (e.g., air pockets within the device), surface contaminants (e.g., dirt, grease, lubricants, other manufacturing residues), and design features (e.g., O-rings that seal surfaces, tortuous paths that impede the diffusion of steam to parts of the device, nooks and crannies that hide and protect microorganisms) can interfere with effective steam sterilization. These barriers can be overcome through proper design of the device. Devices with higher masses require more time to reach effective sterilization temperatures. However, materials with high thermal conductivities will accelerate heat transfer.

Steam sterilisation

Steam sterilisation has limited industrial application but is very commonly used in healthcare facilities. A steam steriliser, also known as an ‘autoclave’, uses saturated steam at 121–132 °C. A typical standard for steam sterilisation is achieved after 15 to 30 minutes under a pressure of 106 kPa (1 atm) once all surfaces have reached a temperature of 121 °C (Block, 2000). To insure reliability of this sterilisation method, the critical factors are: (i) proper temperature and time; and (ii) the complete replacement of the air with steam (i.e. no entrapment of air). Some autoclaves use a pre-cycle vacuum to remove air prior to steam introduction. Others utilise a steam activated exhaust valve that remains open during the replacement of air by live steam until the steam triggers the valve to close.

Moist heat sterilisation kills microorganisms by destroying structural and metabolic components essential to their replication. The coagulation of essential enzymes and the disruption of proteins and lipids are the main lethal events (Kowalski and Morrissey, 2004). The advantage of wet heat is a better heat transfer to, and into, the cell, resulting in needing an overall shorter exposure time and a lower temperature (Block, 2000).

Steam sterilisation has many advantages. It is a simple, rapid, effective, safe, environment-friendly and low-cost sterilisation method. It yields little waste (entropy is its only by-product). Monitoring physical parameters (moisture, temperature, time, etc.) can be used to ensure efficiency of sterilisation, although biological indicators are still commonly used in several countries. It can also sterilise liquids. It is therefore commonly used in healthcare centres for the sterilisation of reusable metallic devices and instruments, hospital linen, various solutions, etc.

Its main limitation is clearly that it is incompatible with many materials. Steam sterilisation damages most polymers. It can also cause corrosion of some metallic devices, in particular high carbon steels used for surgical and dental instruments, and cause unprotected cutting edges to dull. Moisture also can adversely affect electronics. To avoid this, it is of utmost importance to clean and thoroughly dry the instruments before sterilising by autoclave. One way to reduce progressive corrosion of carbon steel instruments is to dip them in an anticorrosive solution prior to autoclaving (Stach et al., 1995; Holmlund, 1965). In surgical trays, contact between instruments of dissimilar metals should be avoided to prevent galvanic corrosion.

Damage on polymers can vary from a little oxidation to complete distortion and melting, depending on polymer composition and properties. It is not possible here to do a complete review of polymers regarding steam sterilisation. More information can be found in handbooks. It is important to mention that some polymers can be safely sterilised by steam. This is the case of polypropylene (PP), PTFE, aromatic polyurethanes, nylons, Tyvec, polycarbonate, etc. Others will undergo mild to severe changes. Steam sterilisation of certain polyurethanes may result in a toxic hydrolytic byproduct, dimethyl aniline (Shintani, 1995). The number of plastic materials capable of being steam sterilised will vary considerably with the selected temperature of sterilisation. To accommodate more heat-sensitive polymers, so-called ‘low-temperature’ steam sterilisation is sometimes used, but is subject to controversy. The process then takes place at 110–115 °C, but during 35–40 minutes instead of 10–15 minutes once temperature is obtained. In contrast, fewer materials can be sterilised by flash sterilisation, which uses higher temperatures (134 °C, 3–6 min in vacuum with steam pulses). Flash sterilisation is used in a clinical setting when an instrument or device is needed urgently (Carlo, 2007).

8.3.2 Steam sterilisation

Steam sterilisation is most commonly used for medical devices such as surgical instrumentation and is unlikely to be the method of choice for implantable sensor systems. Using steam is one of the most reliable sterilisation methods, but it damages many plastics, electronics, fibre optics and biological materials. Therefore, only a brief overview of steam sterilisation is presented here.

Autoclaves are widely used for heat sterilisation and commonly use steam heated to 121–134 °C (250–273 °F) with a holding time of at least 15 minutes at 121 °C or 3 minutes at 134 °C, longer for liquids and surgical instruments packed in layers of cloth. Treatment inactivates all fungi, bacteria, viruses and also bacterial spores, which can be quite resistant to some methods. The most common, and historic, steam sterilisation cycles used in the medical device industry are gravity-displacement and dynamic air removal (Perkins, 1982). In a gravity-displacement system, steam enters the sterilisation chamber and displaces the residual air through an open vent (hot air rises). However, dynamic air removal has been shown to be more efficient because the machine can pump in conditioning air (humid and warm typically), then forcefully discharge this environment and replace with subsequent cycles.

A draw down vacuum is used to remove the conditioning air cycles from the packaged product and the chamber. Ambient air is removed from the chamber in what is known as a ‘pre-vac’ cycle, which is typically a series of pressure and vacuum excursions. These serial staged cycles provide the time and conditions necessary to ensure the entrapped chamber atmosphere can be withdrawn from within the package and from within the product itself.

Another method, flash sterilisation, involves much higher temperatures being applied for a shorter time and is suitable for devices for immediate use, such as surgical instruments. Usually ‘flash’ refers to an open batch sterilisation cycle where instruments are only lightly wrapped, if at all, only high temperature steam is injected into the chamber, and little or no vacuum cycling is involved. Flash sterilisation is unlikely to have any utility for sterilising implantable sensor systems since the high temperature and humidity could harm the materials and electronics. Even if the sensors withstand the cycle, the conditions cannot force sterilant into the sensor elements to ensure sterilisation of any accessible monitoring chambers.

Porous load sterilizer

Steam sterilization and most sterilization processes inactivate all organisms by 6-log reduction, but cannot destroy prions and endotoxins effectively [81].

Moist heat sterilization is carried out using clean dry saturated steam at high temperature and pressure for a specific time in the absence of air. This is the most practical and commonly used method to sterilize surgical instruments tolerant to heat and moisture. The steam sterilization is the preferred method because it is effective, reliable, nontoxic, noncorrosive, easy to use, and has the ability to be monitored and validated [42, 47, 51, 81]. Monitoring and control of the process is carried out by observing the attainment of physical parameters. Biological indicator is not normally required for validation and routine monitoring.

BS EN 285 [81] and BS EN 61010-2-040 [64] give a specification for a porous load sterilizer and engineering services. The guidance (e.g., HTM 01-01, SHTM 01-01, WHTM 01-01) provides operational guidance [42, 47, 51].

Porous load sterilizer also has a drying function following sterilization [81].

13.4.2 Steam sterilization (autoclave)

Gravity displacement and high-speed prevacuum sterilizers are the two major types of autoclaves. The sterilizer works by introducing steam to both the top and sides of the chamber. Gravity displacement autoclave is mostly used in the laboratory to process media, medical waste, water, pharmaceutical products, and nonporous devices. The penetration time for this type of sterilizer is prolonged as a result of incomplete air elimination. This point is illustrated by the decontamination of 10 lbs. of microbiological waste, which takes at least 45 minutes at 121°C because the air remaining in the waste load significantly reduces steam permeation and heating efficiency [18].

High-speed prevacuum sterilizers are similar to gravity displacement sterilizers except that they are fitted with a vacuum pump (or ejector) to ensure that the air is removed from the sterilizing chamber and loaded before the steam is allowed. The main benefit of utilizing a vacuum pump is that the steam penetration into porous loads is almost instantaneous. The Bowie-Dick test is used to detect air in the chamber and inadequate air removal and consists of folded 100% cotton surgical towels that are clean and conditioned.

Steam sterilization is a steam pulsing processing design, which eliminates air quickly by constantly rotating a steam pulse and a pressure pulse above atmospheric pressure. Air is quickly removed from the load as it is with the prevacuum sterilizer, but air leakage does not affect this process because the steam in the sterilizing chamber is always above atmospheric pressure. Typical sterilization temperatures and times are between 132°C and 135°C with an exposure time of 3–4 minutes for porous loads and instruments [19]. As with other sterilization devices, the steam cycle is controlled by mechanical, chemical, and biological detectors. Steam sterilizers are usually monitored utilizing a printout or graphically by measuring temperature, temperature time, and pressure. Usually, chemical indicators are added to the exterior and inserted into the pack to track temperature or time and temperature. The efficacy of steam sterilization is controlled by a biological indicator containing Geobacillus stearothermophilus spores. Portable (tabletop) steam sterilizers are used in outpatient, dental, and rural clinics. These sterilizers are designed for small instruments, such as hypodermic syringes and needles and dental instruments. The ability of the sterilizer to reach physical criteria necessary to achieve sterilization should be monitored by mechanical, chemical, and biological indicators.

Typical Cycle Curve

The data thus obtained allow calculating the equivalent time; the F_o-value; and the differences between the maximum and center temperatures, between lower and center temperatures, and between lower and maxim temperatures. The analysis of these data enables the assessment of the performance of the cycle.

To create specific parameters for different types of loads, it is necessary to use the concept of F_o-value (i.e., the lethality of the cycle expressed in terms of the equivalent time), in minutes, at a temperature of 121C with reference to microorganisms possessing a Z-value (see Glossary) of 10°C.

2.12.6 Validation of Sterilization

The validation of a steam sterilization process requires careful planning. It is possible to validate in two ways: either directly, using media, or indirectly, using temperature and pressure requirements. Each has certain advantages. Validating sterilization directly involves sterilizing the equipment, followed by sterile filling with a standard medium such as tryptone soy broth (TSB), and then incubating this for 7–14 days. If the TSB is sterile at the end of this period, the initial vessel sterilization can be said to have been successful.

The number of external thermocouples to be used should be appropriate to the size and complexity of the item being sterilized. The number of these thermocouples is best kept to a minimum, as they all must be calibrated before and after the validation. The number of points using the surface contact thermometer can be totally flexible. The surface contact thermometer used must be the fast-response type, otherwise good temperature measurements cannot be obtained. A rule of thumb for external temperatures is that if the required internal temperature is 121 °C, the external temperature should be greater than 115 °C. However, it is advisable to do some tests on typical areas of the plant to check this. Insulation of the line affects the differential. The pressure gauge, if fitted, provides a useful cross check to the temperature data [16, 17].

All equipment used for validation should be calibrated to a traceable standard before and after the validation exercise, and these calibration certificates should be enclosed with the validation report. The general acceptance criteria include internal temperature greater than 121 °C; external temperature greater than 115 °C; and pressure greater than 1.1 atm. These figures must be maintained for the whole of the sterilization period, and no drop must be seen [7, 10, 12].

Validation of a sterilizing filtration process is critical since it is impossible with the currently available technology to measure the sterility of each filled container; therefore, sterility assurance of the filtered product must be achieved through validation of the filtration process. Validating a pharmaceutical sterile filtration process involves three things: determining the effect of the liquid on the filter, determining the effect of the filter on the liquid, and demonstrating that the filter removes all microorganisms from the liquid under actual processing conditions [38].

10.4 Sterilization specifications⁶

For sterilizing, a number of physical and chemical processes can be used, such as steam sterilization, dry heat sterilization, and chemical sterilization. It is also important to distinguish between sterilization and cleaning. Without a cleaning process, sterilization cannot be achieved.

Factors such as packaging, transportation, and storage conditions will be affected based on the method of sterilization. Sterilization is mostly used to remove pathogens.

Other Terminal Sterilization Technologies

Moist heat, i.e., steam, sterilization has long been a workhorse technology for reusable devices in hospitals and certain industrial applications. The high temperatures limit its use for most devices with plastic materials, including most single-use devices. Moist heat sterilization, like other major industrial and hospital processes, is well-characterized in national and international sterilization standards (see Table III.1.2.1). Gas phase hydrogen peroxide sterilization has, in the last decade, come into broad application in hospitals, but has limited industrial applications. This was the first technology to drive the generic ISO 14937 “General Criteria” sterilization standard (see Table III.1.2.1). A number of additional gas chemical sterilization technologies have been developed, but have not found significant industrial terminal sterilization application, including chlorine dioxide, ozone, nitrogen dioxide, supercritical carbon dioxide, and propylene oxide. In addition, dry heat sterilization has limited applications.

Liquid chemical sterilization is sometimes used for sterilization, in applications either with animal or human tissues that are not compatible with terminal sterilization options or as a high level disinfection for reuse of certain devices, such as blood dializers. The process involves the immersion of the device into formulations of either an aldehyde or an oxidizing agent (e.g., gluteraldehyde, hydrogen peroxide or peracetic acid), sometimes with buffers, anti-corrosive agents, and detergents. The method does not provide the process control or sterility assurance levels of terminal sterilization processes (Chamberlain et al., 1999). Automation of the process, however, has led to significant success in providing safe tissue product.