Considerations When Choosing an Option
Sterilization is a critical step on the path to getting your medical device to the patient. The sterilization process uses toxic gases, rays, or chemicals which can be harmful to the environment, so it is important to choose judiciously. Choosing an appropriate sterilization method requires the consideration of potential bioburden, cost, device complexity, material compatibility, process, time requirements, and environmental and occupational impacts.
When developing a device, it is important to think about sterilization options from the design phase as some materials are not compatible with all methods. The sterilization method chosen can affect product and packaging material choices. For example, Gamma Irradiation (GI) sterilization has been found to have adverse effects on certain materials including plastics commonly used for packaging. Additionally, devices that contain Teflon are not recommended for GI sterilization. Electron Beam (E-Beam) sterilization is slightly more forgiving of materials as 70% of products have been found to be compatible with this sterilization method. However, E-Beam can be harmful to batteries or electronic components and can degrade some grades of rubber and polypropylene. Ethylene oxide (EO) can be absorbed into certain plastics, thereby making EO not a viable option for certain devices. However, stainless steel and other metals do not absorb a significant amount of EO.
Design and Use
Medical device sterilization requirements vary with the use and complexity of the device. Devices that have minimal patient contact may not require any sterilization. Complex devices that are implanted may require multiple sterilizations including separate sterilization of various parts prior to assembly. Therefore, not all methods will work on all medical devices. The more complex a device is, the more important the sterilization method choice will be. Improving the design of the device to minimize complexity, limit handling, and incorporate biomedical materials and safe microbial agents can make sterilization easier and more efficient.
Workers at radiation facilities are at risk of exposure to radiation and, GI and E-Beam produce ozone. Ozone is an irritant to mucous membranes and facilities must be ventilated to minimize exposure and possibly treat ozone prior to exhausting. Cobalt 60 has a useful life of 20 years, but takes 175 years to totally decay to stable nickel during which time it is giving off gamma radiation. The radioactive cobalt must be safely and securely stored for 200 years.
Dense Phase Carbon Dioxide (CO2)
When gaseous or liquid CO2 is heated and compressed above 31oC and 73 atmospheres it becomes a supercritical fluid (SCF) that demonstrates the properties of both a liquid and a gas. Substances in the SCF state have better solvating ability than the same substances in a liquid state.1 SCF have viscosities similar to a gas allowing them to penetrate small orifices. Its lower operating temperatures make it viable for sensitive devices and tissues. SCF CO2 is non-flammable, less toxic, odorless, and less polluting than other forms of sterilization, such as EO or GI making it an environmentally preferable method. SCF CO2 sterilization process does not contribute to the greenhouse gas effect because it uses by-product carbon dioxide that is captured, purified, and liquefied from other manufacturing processes2. It is commercially available from Novasterilis.
Dry heat sterilization is conducted in electrically heated ovens at 160-180°C. It can be used for materials not permeable to steam, such as solids, or those that can be corroded by moisture, such as metals. Dry heat cannot be used for heat sensitive materials, such as plastics. Dry heat sterilization methods do not use toxics or leave residues, making it an effective and environmentally preferable method, particularly when an energy efficient oven is used.
E-Beam sterilization methods use a high-energy stream of electrons to inactivate microorganisms. It has low penetration contributing to less product degradation and is not recommended for dense products. E-beam is a good choice for products manufactured under aseptic conditions whose packaging then needs sterilization. It is a quick process, taking only seconds. Certain polymers can degrade with E-Beam sterilization and should be avoided. However some polymers benefit from the E-beam sterilization process; certain polymers crosslink resulting in increased strength and reduced deformation. Using E-Beam as a sterilization method can also eliminate the need for the use of a porous material, such as Tyvek, as a sterile barrier resulting in packaging cost and waste savings. E-Beam is directional and may not be adequate for complex devices. This may be addressed by sterilizing these devices twice, rotating them in between sterilization sessions.
Ethylene Oxide (EO)
EO is one of the most commonly used sterilization methods for medical devices, as many devices are impacted by high temperature steam sterilization. EO is a colorless, flammable, toxic, highly reactive compound with a low boiling point, which allows it to behave as a gas at room temperature and pressure. Sterilization using EO can be achieved in as little as two to three hours. During the sterilization process, EO is absorbed by materials in the device. Due to EO toxicity, the sterilized items must be off-gassed, a process that can take from several hours to a few days.
EO has been proven to be an effective sterilization method; however, there are concerns associated its use. EO is flammable as a liquid and explosive as a vapor. It is also a carcinogen and can cause blindness, liver and kidney damage, and severe burns to skin. Due to these risks, EO sterilization process requires the use of additional ventilation, personal protective equipment, and in some cases, pollution control equipment to treat residuals.
Gamma Irradiation (GI)
GI uses gamma rays to ionize electrons and create free radicals in the genetic material of living organisms. Gamma rays are generally produced from radioactive sources, such as Cobalt 60 or Cesium 137, although non-radioactive sources are being investigated. GI is able to penetrate packaging and higher density components. This sterilization method can be performed at low temperatures and low humidity levels, and readily allows for more flexibility in packaging materials, including the use of bio-plastics which are not compatible with many other sterilization methods. Additionally, sterilization with GI is a much faster process than EO sterilization. It does not require preconditioning or aeration procedures and can be completed within 90 minutes.
Unlike EO sterilization, GI does not leave any harmful residue and there is no toxicity danger to device recipients. However, GI can cause chemical and structural changes to polymers that weaken or degrade them. As of late 2009, GI is not readily available to Minnesota’s medical device manufacturers; the closest facility is near Chicago, Illinois. The additional costs associated with shipping can be offset with sterilizing packaged devices and shipping directly to the user.
Hydrogen Peroxide Plasma
Another sterilization process that is more environmentally benign and less toxic than EO is hydrogen peroxide plasma sterilization. This is a low temperature process that uses 58% hydrogen peroxide, which is vaporized in a sterilization chamber. The vapor reaches and sterilizes all surfaces. Then, using radio frequency energy, the vapor is converted to plasma that consists of highly charged particles and free radicals that sterilizes any remaining areas of the device. The entire process takes about an hour and byproducts of this method are water vapor and oxygen.
This sterilization process is ideal for in-house, just-in-time sterilization, but not large-batch sterilization due to limitations on chamber sizes. It can also be useful for low-volume, high-value devices such as pacemakers and implants. Additionally, cost is a consideration as performing this method is inexpensive, but companies should have the equipment on-site, which can involve substantial capital costs.
Steam sterilization uses water saturated steam at temperatures between 121 to 134°C which is most effective against bacterial spores. Steam sterilization will sterilize permeable materials and is a good choice for solids that can withstand heat and pressure. In terms of toxics used and released, steam and dry heat are preferable to other methods. While steam can be energy intensive, in some instances requiring the use of a separate boiler, new steam systems can run on electricity, increasing efficiency and reducing environmental impacts.
1 Matthews, M. et al, Exploring the Feasibility of Using Dense Phase Carbon Dioxide for Sterilization, Medical Device Link.com, www.devicelink.com. Accessed 11/14/08.
2 Matthews, M. et al, Exploring the Feasibility of Using Dense Phase Carbon Dioxide for Sterilization, Medical Device Link.com, www.devicelink.com. Accessed 11/14/08.
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