Introduction
Silicone sterilization is the critical step in any medical LSR application, especially for skin-contact and implantable applications. Liquid silicone rubber is a preferred material option in medical, health-care, baby-care industries. LSR is common applied for sealing applications in long-term, due to its constant properties in a wide range of temperatures.
Liquid silicone rubber has excellent low compression set, it can return to original shape with repeatable compression, this quality make it suitable for pumping and pressure sensing applications. Furthermore, LSR is inertness, it is specified for products subjected to cleaning solutions and chemicals, and hygienic products with sterilization.
Once we sterilize the liquid silicone rubber products, there are several options and factors we should consider carefully. In order to ensure medical devices safety and quality.
Importance of Sterilization
Sterilization is essential for medical devices in normal application, the medical industry has strict regulations and stringent guidelines. In reason of the critical nature of medical devices and their interaction with biological matter, any medical devices with body tissues or fluids contacting are critical items. All these devices must be sterile when used, because any microbial contamination will result in disease transmission. Therefore, LSR products in medical applications require repeatable and regular sterilization by chemical vapor or high-level energy, in order to remove any bacterial surface contamination.
Normally, the sterilization process is in temperature from 110℃ to 190℃, in dry or steam environment, and sometime with pressure. Therefore, sterilization requirement and methods are important considerations in material selection for medical parts. Most elastomeric materials like thermoplastic elastomer (TPE) will fail and age once exposed to extreme temperature and chemicals in sterilization. However, liquid silicone rubber with natural inertness can withstand a wide range of temperature from -50℃ to 200℃, it can be sterilized for compliance with health regulations.
Sterilization Effect on LSR
LSR medical devices’ manufacturer will sterilize finished parts before shipping. In addition, we must design LSR parts to ensure the sterilization in re-usage application. As common sterilization methods including heat, chemical and radiation.
Dry heat or steam autoclaveing will have little effect on LSR physical properties, in reason of its moisture and heart resistance. Steam autoclave is the most popular way to sterilize reusable devices. However, this method is not suitable for medical grade adhesives, because many substrates and adhesive cannot withstand the high-pressure steam environment.
Eto can diffuse through the silicone polymer network, so we must ensure all Eto are removed from silicone before usage, this usually takes 24 hours.
Radiation sterilization methods may damage liquid silicone rubber structure in molecular level. This result in mechanical properties changes, such as loss of flexibility, decrease in tensile strength and elongation, and increase in durometer. Therefore, we must ensure all these losses within specification limits, and the minor loss in physical properties never affect ultimate function of medical devices.
Sterilization Methods for Medical Devices
There are a variety of sterilization methods for LSR medical devices, except the traditional heat methods, LSR products also can be sterilized by radiation, chemical or alcohol.
Dry Heat
Dry heat is actually the first sterilization method in development without water. Dry heat sterilization normally involves the items exposing to a temperature of 170℃ in normal air pressure for 1 hour. In order to ensure the most resistant spores killed by oxidation of cellular components. Dry heat has similar result as steam sterilization, but with less efficiency. It is more suitable for specific situations.
Sterilization Conditions
Heat conduction from exterior surface to interior
Normally 170℃ for 1 hour or 160℃ for 2 hours. Timing starts once achieve desired temperature
Thermal expansion of silicone contents in processing
Advantages
Special for materials without safe sterilization with steam under pressure
Disadvantages
Harmful for heat sensitive LSR components
Cannot be applied with temperature sensitive devices
LSR decomposition temperature at almost 260℃
Steam Autoclave
Steam autoclave is a more reliable sterilization method than open flaming. The autoclave is a strong and heated container with high temperatures and pressures. We can apply autoclaving to sterilize laboratory instruments, pharmaceutical items, and surgical equipment. Steam autoclave is nontoxic, it is a more economical solution with quickly heat and penetration.
Autoclaves have different shapes and sizes for different functions. Autoclave sterilization is not suitable for all universal applications. Its size is the main limiting factor, it is difficult to use in small spaces.
Sterilization Conditions
Process temperature: 126℃/20 min
Flash autoclave: 134℃/10 min
Thermal expansion of LSR contents in processing
Advantages
The most common method of hospital sterilization
Non-toxic, environmental friendly, economical
Litter effect on LSR elastomer
Disadvantages
Cause combustion of lubricants in LSR
Reduce LSR ability of light transmitting
The combination of steam and 121℃ to 134℃ temperature can degrade plastic materials
Detrimental for heat sensitive devices
Not suitable for most silicone adhesives
Ethylene Oxide (ETO)
Ethylene oxide (ETO) sterilization is a method to disinfect medical devices by a gaseous chemical. ETO is a colorless chemical gas, it is explosive and flammable. The four essential parameters of ETO are gaseous concentration, temperature, relative humidity, and exposure time. These elements work together to create the effectiveness of ETO sterilization. ETO is effective at removing bacteria, but it has potential toxicological issues once the gas is absorbed and released into human tissues.
Sterilization Conditions
2 to 12 hours exposure at 25℃
Aeration step to remove residual ETO for 24 hours
Advantages
Compatible with moisture or heat-sensitive medical devices and equipment without any detriment to LSR material function
ETO sterilization will lead to improvement in tensile strength and elongation of LSR
Permeates and diffused through LSR polymer networks
Disadvantages
Toxic, environmental threat
Slow processing time
Not recommended for silicone fluids
Critical to ensure full removal of ETO
Isopropyl Alcohol (IPA) Rinsing
ISopropyl Alcohol (IPA) rinsing is the easiest sterilization method for medical devices. IPA is a high effective cleaning solvent. This sterilization method can penetrate organism cell wall, coagulate proteins and kill the organism. Furthermore, different concentrations, solutions and purity grades will give rise to different results. This method is widely applied for disinfecting medical devices in hospitals, electronics, pharmaceuticals and clean rooms.
Sterilization Conditions
Apply IPA even to part surface
Sit parts at least 30 seconds
Advantages
Never alter LSR properties
Dry off without residue left
Sterilizing parts and clean parts surface at the same time
Disadvantages
Limited sterilization efficiency
Gamma Radiation
Gamma radiation is the most favorite sterilization method of silicone tubing. It is easy to use once LSR parts are already packaged in final form. Gama radiation sterilization has high penetration power, reliability and accuracy. Since it can kill bacteria by DNA breaking down, and prevent division and multiplication. This sterilization method never result in contamination or left any toxic residual. After gamma sterilization, cross-linking in silicone will increase. This gives rise to hardness and shape memory increase.
Sterilization Conditions
Common dosages of 2.5-3.0 Mrad
Normal ambient temperature
Advantages
Excellent for heat sensitive LSR products
Suitable for products impermeable to air change
No residues left in sterilization process
Short processing time
Easy validation
Disadvantages
Not suitable for gels
Cause minor increase in modulus and durometer
High doses and repeatable exposing result in further properties changes
In extreme case, give rise to LSR decomposition of discoloration and brittleness
Electron Beam
Electron beam is another common radiation sterilization method for LSR medical devices. It is an alternative to gamma rays sterilization, and has similar physical effects on LSR materials. However, its affection on physical properties is less than that with gamma irradiation.
Sterilization Conditions
Ionization energy utilization
Silicone penetration energy: 2-3 Mrad and up
Normal ambient temperature
Advantages
Excellent for heat sensitive LSR products
Suitable for products impermeable to air change
No residues left in sterilization process
Short processing time
Disadvantages
High setup cost
Vacuum-like package to avoid inadvertent inflation
Cause minor increase in modulus and durometer
High doses and repeatable exposing result in further properties changes
In extreme case, give rise to LSR decomposition of discoloration and brittleness
LSR vs TPE
It is important to understand the difference between LSR and TPE, once we select the right materials for medical applications
LSR: LSR is the best material for implantable device application, in reason of its heat resistance, stable properties, and fewer diverse reactions with skin contact.
TPE: Thermoplastic elastomers is great for permeability related applications, it provides high fatigue resistance, low compression set, abrasion resistance and high elasticity.
LSR has better performance than TPE in both high and low temperatures for medical and healthcare applications. Many medical and healthcare devices require materials with excellent chemical resistance to withstand sterilization, and biocompatibility for skin contact applications. Furthermore, liquid silicone rubber can also satisfy micro molding requirements for tiny critical components.
Miwo Medical Devices
In Miwo, we provide LSR molding and LSR 2-shot molding solutions for our customers with specific silicone sterilization requirement. Our engineering team will work closely with our customer from the onset, in order to understand the products thoroughly and their detail functions. Why not contact us now for your new LSR project？
