Medical devices that come into contact with sterile body tissues or fluids undergo sterilization processes to kill all the microorganisms on their surfaces. Unfortunately, the same processes intended to kill microorganisms can also “kill” a poorly designed device.
Many of these devices are electromechanical. They include surgical power tools; implantable electronics, such as stimulators and monitors; and single-use plastic devices containing electronics for authentication.
Three of the most common types of sterilization processes are autoclave, ethylene oxide (EtO), and ionizing radiation. Below, we describe each of these processes, as well as the design mitigation techniques to maximize device reliability and compliance for each.
Autoclave (Pressurized Steam) Sterilization
The application of pressurized steam is the most widely used sterilization process. Because it occurs inside an autoclave machine, the name of the machine has become synonymous with the process. The primary benefit of the autoclave process is time — it typically takes just 15 minutes for steam at 121°C (250°F) and only three minutes at 134°C (273°F).
This rapid cycle is ideal for surgical instruments that might be reprocessed multiple times a day. Unfortunately, the high temperatures and pressures can be destructive. The most likely failure modes induced by pressurized steam include plastic deformation, electrical shorting or disconnections, accelerated aging of semiconductors, and electrochemical damage to energy storage devices such as batteries and capacitors.
Design for Autoclave Sterilization: Various design mitigations are possible to prevent these failure modes. The first is to use plastic materials that are capable of handling these high temperatures. Commonly used autoclavable plastics include polypropylene, Radel (polyphenylsulfone), Ultem (polyetherimide), Polysulfones, and many fluoropolymers and silicone rubbers. All have very high melting points.
In order to insulate the internal components from environmental stress, material selection and sealing against ingress are critical. Silicone or epoxy potting, and conformal coating are two means of protecting electronic components and PC boards. Gasketing of sterilized housing assemblies is often recommended by means of separate rubber gaskets and O-rings, or 2-shot molded parts with integrated sealing TPE gasketing.
When sealing is not possible, it’s important for the design to avoid the possibility of condensed fluids getting trapped. In this case, employ well-placed drainage holes and vents.
Battery packs are particularly vulnerable because the internal cells will become damaged at much lower temperatures (60°C) than the autoclave environment (up to 134°C). A common mitigation is to install thermal insulators and sensors inside the battery pack and specify a maximum duration for the autoclave process to ensure cells are kept within their specified limits.
Component selection is also critical to prevent electronic failures — mechanical switches and buttons should be replaced with non-contact Hall effect sensors and magnets. Electrolytic capacitors should also be avoided if possible, as their lifetime can be significantly reduced with elevated temperatures. Finally, conformal coating of the circuit board is recommended to further protect its components.
Design compliance with autoclave sterilization is evaluated through tests against industry standards such as ISO 17665, AAMI TIR12, and AAMI ST79.
Ethylene Oxide (EtO) Sterilization
Ethylene Oxide (EtO) is a poisonous gas intended to kill microorganisms on the surface of medical devices. It’s widely used for sterilization of single-use plastic components due to its lower temperature levels and material compatibility. The process applies the gas at temperatures up to 63°C and durations of up to six hours. The primary risk to the device is residual gas trapped within the enclosure which may be harmful to patients and other users.
Design for Ethylene Oxide (EtO) Sterilization: Design mitigations are recommended to address the risk of residual EtO gas trapped in the device. Careful selection of materials and design of the enclosure and its sealing should prevent cavities where the gas may be trapped. Most plastics that are commonly used in the design of medical products are capable of withstanding the conditions of EtO sterilization — even materials like polypropylenes and PC materials often used for low-cost disposables. In addition, circuit boards should be conformally coated to eliminate access to pockets for residual gas.
The flammable nature of EtO gas requires extra care in design to make sure electronics cannot create a spark. For a sterilizable product that includes a permanent battery or power source, it’s a good idea to design in redundancy in the power cutoff circuit, such as an on-off switch and a pull tab, and to implement design limits on power density and current.
Design compliance with EtO sterilization is evaluated through tests against industry standards such as ISO 10993-7 and ISO 11135.
Ionizing Radiation Sterilization
Ionizing Radiation inactivates microorganisms within medical devices. It is most used for sterilization of pre-packaged single-use plastic components due to its ability to penetrate beneath the surface. There are two main forms of ionizing radiation sterilization — gamma ray and electron beams. These processes have different levels of penetration, where gamma rays penetrate much deeper than e-beams and are considered here.
Design for Ionizing Radiation Sterilization: The primary risk to devices caused by gamma radiation is degradation, especially for polymers. Gamma radiation in high doses also has measurable degradation in semiconductors. Material selection is the primary mitigation for sterilization damage. Most polymers commonly used in medical products, such as PC, ABS, polyurethanes, some polyolefins and most elastomers fare well under gamma irradiation — but care should be taken to select appropriate radiation-stable grades.
Shielding may help electronics in the presence of low dose radiation. In general, however, electronic devices are not suitable for gamma radiation due to potential damage in semiconductors and erasing of non-volatile memory. Proper validation is necessary to evaluate this risk.
Design compliance with gamma sterilization is evaluated through tests against industry standards such as ISO 11137 and ISO 13004.
The Importance of Labeling
Medical devices must provide sufficient information in their labeling to identify appropriate sterilization processes, levels, and durations. ISO 17664 and AAMI TIR12 specify the requirements for the information to be provided by the medical device manufacturer for the processing of a medical device prior to use or reuse. Proper labeling is an essential mitigation for the risks of sterilization damage to a device.
In summary, designers have several tools available to prevent sterilization damage. These include careful materials specification, electromechanical design, components selection, and labeling. The appropriate mitigations vary by sterilization process. These efforts can dramatically increase device reliability, minimize compliance failures, and most importantly, improve patient outcomes.
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