Navigating Sterilization Options for Silicone Medical Implants
Medical device manufacturers are on the cutting edge of developing technology that’s critical for changing the lives of millions around the globe. Whether the device provides an innovative cancer therapy or pioneering diabetes care, device designers often rely on silicone as a material that can enhance performance capabilities or increase manufacturing efficiencies.
For more than 60 years, medical-grade silicone has established a proven track record for biocompatibility and safety, making it an ideal material choice for medical devices, including long-term implants. Manufacturers also choose this inert polymer because of performance attributes such as gas permeability, chemical and thermal stability, hydrophobicity and low surface tension. Its versatility is a significant advantage, too. Silicones can be used in a range of applications, from adhesives that join or seal components to extruded parts, such as tubing.
Silicone’s unique attributes make it an ideal material choice for medical implants, including shunts and drug-eluting devices. Sterilization removes microbial contamination from a device to reduce the risk of infection or irritation to the patient. In general, medical-grade silicone is well suited for sterilization, whether the process occurs in a clinical facility or as a final step of device assembly and packaging.
Understanding the advantages and disadvantages of each sterilization method can help manufacturers choose a sterilization method that is the most appropriate for their medical devices with silicone, as well as the device’s unique components, design features and construction method.
Considerations for Choosing a Sterilization Method
Manufacturers have a range of processes to choose from, including:
Ethylene oxide (EtO): This long-established and widely used chemical method is very effective and safe to use with medical devices including pacemakers and cochlear implants that are sensitive to temperature or moisture or contain delicate electronics. EtO has proven effective for sterilizing single-use and reusable devices. Importantly, EtO has been reliably shown to permeate and diffuse through medical device packaging. This attribute makes the process an effective last step prior to shipping when manufacturing medical devices.
One disadvantage to this process is that full sterilization can take from 16 to 48 hours of exposure at specified levels. Also, the process’s byproducts, including EtO residues and reaction byproducts, must be removed to ensure the device is safe for use. While EtO has repeatedly proven effective and efficient, recent concerns about pollution and potential human harm have taken root with regulatory bodies such as the United States Environmental Protection Agency (EPA).[1]
Vaporized hydrogen peroxide (VHP): The FDA announced in 2024 this process is considered an Established Category A method of medical device sterilization. VHP, which has typically been used to sterilize reusable medical devices for patient care, relies on hydrogen peroxide vapor under a vacuum for safe, rapid sterilization. Hydrogen peroxide produces no toxic byproducts because it breaks down into water and oxygen. Along with lowering the risk for toxicity, this factor also reduces aeration time.[2] While not suitable to sterilize large, dense packaging, VHP is ideal for surface sterilization or applications that don’t require deep penetration.[3]
Electron beam: This method uses high-energy electrons to sterilize an object by stripping electrons from the atoms of the exposed surface. Effective and safe, the e-beam process is ideal for sterilizing heat-sensitive products and devices that are fully sealed or otherwise impermeable to air exchange. In addition, it processes in less time than EtO and leaves no chemical residue. However, e-beam is not recommended for sterilization of delicate electronic components. In addition, higher doses and repeated e-beam exposure may impact silicone’s physical or mechanical properties, such as increasing the material’s durometer and modulus.
Gamma radiation: Like the e-beam process, gamma radiation exposes medical devices to a predetermined dose of gamma radiation to kill microbes on the device’s surface. Gamma sterilization, which leaves no residue, is well suited for heat-sensitive products and devices sealed against air exchange. While it takes longer than e-beam sterilization, gamma rays better penetrate high-density materials.
Repeated or higher-dose gamma beam exposure may negatively impact silicone properties, such as durometer and tensile strength. When the material molecular weight changes, the silicone can become discolored or brittle. As a result, gamma beam sterilization is better suited for single-use devices that are not exposed to multiple or repeat doses or permanent medical implants that have been exposed to a predetermined dose of gamma radiation.
Steam autoclave: The most common method for hospital sterilization processes, steam autoclave is a cost-effective method that has little to no impact on silicone’s elastomeric properties, making it ideal for medical device applications or repeated sterilization on reusable devices. This sterilization method should not be used with any heat- or moisture-sensitive medical devices, including devices that include electronics. In addition, steam autoclave is not recommended for use with specific design features. For example, the process can cause thermal expansion or distortion in medical devices with a silicone shell that’s tightly joined with other materials.
Dry heat: This long-established process uses high temperatures to remove microbial contamination. Widely used in hospital and clinical settings, dry heat is primarily recommended for materials that cannot be safely sterilized in steam under pressure. Use dry heat only on materials that can withstand high levels of heat. And, as with steam autoclaves, this method can cause thermal expansion. Manufacturers should consider other sterilization process, such as EtO, e-beam or gamma beam, for devices with joints or other expansion-sensitive elements. Repeated dry heat exposure can impact silicone properties, causing decreased elongation and increased durometer, as well as potential discoloration.
A Novel Device Technology
In situ cure technology allows the development of implantable medical devices that form and cure within the body. A novel dual-cartridge prefilled dispensing system allows the uncured silicone to be sterilized. Each barrel features a gas-permeable seal that allows sterilant gas to permeate the seal, sterilizing the silicone in the cartridge. This allows medical professionals to inject the silicone into the body, where it cures to create a customized implanted device.
Best Practices for Choosing Silicones for Medical Devices
Understanding common best practices around silicone use in medical devices can help ensure the right choice of medical-grade material and save time when taking the final product to market. While it’s common to consider sterilization methods during device design stages, it is also important to consider how effectively a particular sterilization method can render a device sterile without detrimental effects to material composition and patient safety.
When designing medical devices that use silicone, it’s essential to work with an experienced supplier capable of supplying high-purity off-the-shelf or custom formulations that meet performance and manufacturing needs at every stage of the workflow. The right silicone supplier can provide guidance around silicones and sterilization, as well as necessary documentation for regulatory bodies like the U.S. FDA.
Medical device innovation is key to helping patients around the world. By understanding silicone, its use in medical devices, and how sterilization impacts the material property, medical device manufacturers can make the right selection to improve manufacturing processes and patients’ lives.
References
[1], [2] MDDI Staff (January 9, 2024). Vaporized Hydrogen Peroxide Sterilization Gets FDA Established Category A Status. MD+DI. Retrieved from https://www.mddionline.com/sterilization/vaporized-hydrogen-peroxide-sterilization-gets-fda-established-category-a-status.
[3] S. Martin, E. Duncan (2013). Sterilisation considerations for implantable sensor systems. Implantable Sensor Systems for Medical Applications. Retrieved from https://www.sciencedirect.com/topics/immunology-and-microbiology/vaporized-hydrogen-peroxide.
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