Choosing the Best Method for Sterilizing Medical Device Silicones

Silicone has a well-established history of successful use in medical devices, and its use continues to grow. For more than 50 years, it has been used due to its proven safety and biocompatibility, as well as its versatility in supporting the efficient design and manufacturing of a broad range of products, including device components, tubing, lubricant, and in many other functional areas. Silicone’s biocompatibility makes it an excellent material for both short-term implantation in the body (<29 days), such as wearable insulin pumps, and long-term implantation, such as pacemakers.

Regardless of silicone’s safety and biocompatibility, it is still necessary to fully and safely sterilize medical devices that use silicone materials prior to implantation. Sterilization removes unwanted microbial contamination from silicones used on and in a device, reducing the risk of infection or irritation to the patient. There is a range of sterilization methods now commonly used in the industry, each method having both advantages and disadvantages. Understanding and weighing these factors can help medical device manufacturers and clinicians select the best solution for a given device or clinical use.

Understanding Silicone Biocompatibility

To make effective sterilization choices, it’s helpful to understand the attention paid to the science underlying the biocompatibility of silicone, which stems from the necessity to protect the safety of a patient who has short- or long-term contact with an implantable medical device made of or including silicone.

Silicone’s useful properties include chemical and thermal stability, low surface tension, hydrophobicity, and gas permeability. As a result, as silicone manufacturers have worked to improve its value and usefulness for implantation, they have also invested in testing and documenting the quality and purity of their products and processes to make it easier for medical device manufacturers to use it in more and more devices.

Generally speaking, a material is biocompatible if, when in contact with the body or tissues, no adverse reaction or biological response occurs; the material is inert with regard to how the body reacts. It’s also important to understand medical-grade silicones are not subject to FDA approval—it is the medical devices themselves that must be approved by the FDA. The FDA uses the International Standard ISO 10993 to approve devices, which entails a series of standards for evaluating the biocompatibility of medical devices to manage biological risk.

Choosing the Optimal Sterilization Method

In most cases, sterilization is a final step of the device assembly and packaging process. There are some devices that, due to the way they are handled in an implantation procedure, need to be sterilized in the clinical facility, often using systems like autoclaves that are also used to sterilize instruments and other items.

Selecting the right sterilization process requires an understanding of the inherent advantages and potential disadvantages of each process and aligning those decisions with a device manufacturer’s production and packaging processes, as well as a device’s unique components, method of construction, and design features.

There are currently five commonly used processes for sterilizing medical devices with silicone components: ethylene oxide, electron beam, gamma radiation, steam autoclave, and dry heat.

Ethylene oxide (EtO): EtO sterilization is a low-temperature process (typically between 37 and 63 degrees Celsius) that uses ethylene oxide gas to reduce the level of infectious agents. EtO is used in gas form at 100% concentration or as a gas mixture with CO2.

EtO is very effective and safe to use with medical devices sensitive to moisture or high temperatures, such as pacemakers and cochlear implants, which contain delicate electronics. EtO is also proven effective for sterilizing single-use or reusable devices. Most importantly, it has been reliably shown to permeate and diffuse through medical device packaging.

Thus, EtO provides an effective “final step” in the manufacturing of medical devices prior to shipping. The devices are fully packaged and will not be exposed to outside microbial contaminants until the device is removed from its packaging. To use it in this way, the packaging needs to be permeable to EtO, but impermeable to microbes.

There are few drawbacks to EtO. Most EtO sterilization processes can take from 16 to 48 hours of exposure at specified levels to fully sterilize the devices. Also, ethylene oxide residues and reaction byproducts such as ethylene glycol and ethylene chlorohydrin must be removed to maintain safety of the device. Regardless of these limitations, the proven effectiveness and efficiency offered by EtO sterilization has been repeatedly demonstrated.

Electron beam: Electron beam (e-beam) sterilization uses high-energy electrons to sterilize an object. It is similar to X-rays and gamma radiation in that each form of radiation ionizes the material it strikes by stripping electrons from the atoms of the exposed surface, thus sterilizing the device.

E-beam sterilization is an effective and safe method for sterilizing medical devices sensitive to high heat levels. It is also useful for devices fully sealed and/or otherwise impermeable to air or gases. It also offers a much shorter processing time compared to EtO. Packaged devices of light to medium density can be transported via conveyor or cart at predetermined speed to obtain the desired e-beam dosage.

It is not recommended for use with delicate electronic components. In addition, higher doses and repeated exposure to the e-beam may impact silicone physical or mechanical properties, giving it, for example, increased durometer and modulus, and decreased elongation.

Gamma radiation: In a process similar to e-beam, medical devices are exposed to a predetermined dose of gamma radiation, which irradiates and kills microbes on the surface of a device. Also similar to e-beam, gamma sterilization is well suited for heat-sensitive products and devices sealed against air exchange.

Gamma sterilization does take longer than the e-beam process, but the gamma rays better penetrate materials of higher density. Gamma sterilization throughput is still significantly higher than that of EtO. Another advantage of gamma and e-beam sterilization is these methods do not leave any chemical residues, which is often seen with EtO sterilization and steam autoclave.

Higher gamma beam doses and repeated exposure may impact silicone properties such as durometer and tensile strength; in extreme cases, discoloration and embrittlement of the silicone can occur as the material molecular weight begins to change. As a result, gamma beam sterilization may best be used with single-use devices or those permanently implanted in a patient.

Steam autoclave: Steam autoclave is the most commonly used hospital and clinical sterilization process compared to other methods often performed as part of larger-scale assembly and packaging operations.

Its advantages include little to no impact on silicone properties, making it well suited for medical devices that incorporate silicone and are used repeatedly in medical settings. It is also more cost-effective for hospitals and clinical practices to use autoclaves as part of their sterilization tool set.

Steam autoclaves should not be used with any medical device sensitive to heat or moisture, including any device incorporating electronics. There may also be design factors to consider. For example, if a medical device has a silicone shell that’s tightly joined with other materials, steam autoclaves can cause thermal expansion and distortion of the shell.

Dry heat: Like steam autoclaves, sterilization occurs through high temperatures, which kill microbial contamination. Dry heat is also widely used in hospital and clinical settings and is mainly recommended for materials that cannot be safely sterilized in steam under pressure.

It is important to note it should only be used for materials that can withstand high levels of heat. And, as with steam autoclaves, the device design needs to take into consideration thermal expansion due to repeated exposure to high temperatures. If the device has joints or other elements that should not undergo expansion, a method such as EtO, e-beam, or gamma beam sterilization is recommended.
In addition, repeated dry heat sterilization may impact silicone properties, leading to increased durometer and decreased elongation, as well as potential discoloration over time.

Leveraging Sterilization Innovation

Both medical device manufacturers and the silicone manufacturers with which they work continue to develop new ways to advance medical-grade silicone applications while ensuring patient safety and serving patient needs. A recent example is in-situ cure technology.

This technology makes it possible to create implantable medical devices that form and cure within the body, offering medical device manufacturers opportunities to create new therapeutic solutions. Its innovative packaging solution features a dual-cartridge prefilled dispensing system that allows the silicone to be sterilized in its uncured form.

Each barrel has a gas-permeable plunger seal that allows EtO sterilant gas to permeate through the plunger seal to sterilize the contents of the cartridge. The sterilized two-part silicone can then be injected into a location in the body where it then cures to customize the fit of the implanted device.

Conclusion

Each medical device has its unique physical and functional characteristics that, combined with manufacturing processes and packaging, need to be addressed to select the optimum sterilization process. Experienced silicone manufacturers have deep insights into the ways different silicone materials respond to each sterilization method; they can be a valuable source of support through the evaluation, qualification, and approval process.