Methods of Sterilizing Medical Devices: an Overview
Many medical devices are sterilized by the manufacturer and packaged and shipped in a way to maintain the device sterility until it is ready for use in the hospital or clinic. These sterilization processes help mitigate the risk of exposing the patient to pathogens which may result in infection. Although many methods of sterilizing medical devices exist, we will focus on three of the most common methods used by medical device manufacturers and review some of the trade-offs and key considerations which inform the selection of a suitable sterilization process for a medical device.
Ethylene Oxide Gas (EtO)
How it works: The device is typically packaged in a pouch or tray which incorporates a semi-permeable film (e.g. Tyvek). This film is breathable to allow the permeation of ethylene oxide into the device while preventing permeation of pathogens. A batch of product is placed in a EtO sterilization chamber and undergoes several cycles of humidification, evacuation, gas introduction, and aeration. The ethylene oxide gas chemically reacts with amino acids, proteins, and DNA to prevent microbial reproduction. Ethylene Oxide sterilization is governed by the internationally recognized consensus standard ISO 11135.
Benefits:
Due to its relatively low temperature processing conditions (e.g. +30 to +60C) and no impact to erasable programmable read only memory (EPROM), EtO is a preferable sterilization process for devices with embedded electronics. However, EtO gas is highly flammable and special considerations must be taken to ensure safety when processing products with embedded batteries (e.g. battery isolation).
Drawbacks:
Ethylene oxide gas is a carcinogen and has come under increased scrutiny by the EPA in recent years, including the abrupt closure of a Sterigenics EtO processing facility in Woodridge, IL.
Devices must undergo an off-gassing period after sterilization to allow the EtO gas to dissipate and avoid unacceptable exposure for the patient. These “EtO residuals” are monitored on an ongoing basis by the manufacturer.
The temperature and humidity cycling of the EtO sterilization process can impact hygroscopic polymers common to products such as catheters and result in dimensional changes and adhesive delamination.
vacuum cycle can affect embedded batteries.
Gamma Ray
How it works: The product is placed on a conveyer and transported through an area of intense gamma irradiation. The gamma irradiation is commonly generated by the decay of the radioisotope Cobalt 60. The ionizing radiation damages the DNA of any pathogens on the device and generates free radicals which disrupt chemical bonds within DNA. The product is exposed to a prespecified dosage of radiation (expressed in units of kiloGray). Gamma sterilization is governed by the internationally recognized consensus standard ISO 11137.
Benefits:
Gamma sterilization has a relatively fast processing time and takes place at normal atmospheric pressures.
There is no exposure to moisture/humidity and aeration is not required.
Gamma radiation penetrates deep into the packaged product without concern for packaging or product density.
Drawbacks:
Gamma irradiation can negatively affect many plastics commonly used in medical devices, resulting in yellowing, crazing, and embrittlement.
Gamma irradiation can impact erasable programmable read only memory (EPROM) which can result in data loss or device failure of products with preloaded software or firmware.
Electron Beam
How it works: E-beam sterilization processing is very similar to Gamma. The product is placed on a conveyer and transported through an area of radiation exposure by accelerated electrons. The electron beam irradiation generates free radicals which disrupt chemical bonds within DNA which leads to cell death. The product is exposed to a prespecified dosage of radiation (expressed in units of kiloGray). E-beam sterilization is governed by the internationally recognized consensus standard ISO 11137.
Benefits:
E-beam sterilization has a relatively fast processing time and takes place at normal atmospheric pressures.
There is no exposure to moisture/humidity and aeration is not required.
E-beam sterilization typically has less detrimental effects on polymers when compared to Gamma.
Drawbacks:
E-beam sterilization is less penetrating than Gamma and more dependent on packaging and product density. Additional considerations must be made to ensure consistent dosage of product batches.
Though typically less harsh than Gamma, E-beam can also negatively affect many plastics commonly used in medical devices, resulting in yellowing, crazing, and embrittlement.
E-beam irradiation can impact erasable programmable read only memory (EPROM) which can result in data loss or device failure of products with preloaded software or firmware.
The various methods of device sterilization used by medical device manufacturers each carry different benefits and risks which must be weighed during the design and development process to ensure a suitable sterilization method is selected.
