Several polymers have been adopted for widespread use in medical devices, trays, instruments, and other applications used in hospitals, ambulatory surgery centers, as well as medical, dental, and veterinary clinics. These materials can vary greatly in their tolerance to specific sterilization methods. The challenge for medical device manufacturers is to ensure the materials they specify are compatible with both the sterilization processes used by their medical facility clients, and the number of sterilization cycles each finished product is expected to withstand in normal use.
Table 1 provides a general summary of how some of the major medical plastics withstand repeated cycles in different sterilization methods. The more detailed description of each medical polymer following the table provides additional details on their properties and sterilization tolerance that may be helpful in narrowing material candidates for medical device applications.
Table 1: Compatibility of Medical Plastics with Sterilization Methods.*
| Performance key: | Highly resistant, widely used |
Good resistance with limited cycles |
Minimal or no resistance |
|
Plastics’ Performance: Multiple Sterilization Cycles
|
|||||||
|---|---|---|---|---|---|---|---|
|
Procedure:
|
PEEK
|
Radel PPSU
|
Ultem PEI
|
Udel PSU
|
PP-H
|
PC
|
POM-C
|
| Steam 250oF/121oC | |||||||
| Steam 273oF/134oC | |||||||
| ETO 140oF/60oC | |||||||
| Gamma 73oF/23oC | |||||||
| Plasma 113oF/45oC | |||||||
| Hot Air 356oF/180oC | |||||||
| Hydrogen Peroxide | |||||||
*Number of cycles, part design and polymer grade can affect results. Always test finished units.
PEEK – Polyetheretherketone
The reason behind the many PEEK specifications for medical devices is the simple fact that it tolerates all sterilization processes extremely well. It can withstand steam, ethylene oxide gas, vaporized hydrogen peroxide and gamma radiation. It also tolerates repeated sterilization cycles without significant changes to the material’s structure. For example, it retains its properties very well even after more than 1,000 steam sterilization cycles. While the material’s initial cost is typically higher than other polymers, it usually offsets that with the advantage of lending exceptionally long lifespan to frequently sterilized medical devices.
Radel PPSU – Polyphenylsulfone
PPSU, available under the Radel tradename, also possesses excellent toughness, durability, thermal properties and chemical resistance. It is well-suited for multiple-use instruments that see autoclaving, as it withstands exposure to multiple cycles without sustaining damage, even when harsh disinfectants are present. Radel PPSU is also compatible with chemical and gamma sterilization. Its colorability and color stability have made it a frequent choice for instruments that are color coded for procedures such as joint replacement trials and handles for instruments in procedure-specific kits. It has replaced POM-C (copolymer acetal) in many of these applications because of its superior color stability and property retention after repeated sterilization cycles. As with PEEK, its higher initial cost is typically offset by the longevity it provides medical devices through many sterilization cycles.
Its broadly robust physical properties make Radel PPSU a common choice for medical instruments and device housings. Its toxicological inertness also makes the polymer a viable option for some Class II and III devices.
Radel PPSU, like PEEK, stands out for its stability during sterilization, though it does not withstand the high number of sterilization typical with PEEK. PPSU retains its material properties after several hundred autoclave cycles, and significant material changes aren’t typically seen until the 800-cycle mark. It is also compatible with gamma, ethylene oxide and hydrogen peroxide sterilization. Discoloration may be seen earlier –between 200 and 500 cycles.
Ultem PEI – Polyetherimide
PEI – polyetherimide – is most often referred to by its trade name Ultem. PEI is a high-performance polymer with excellent chemical resistance (including resistance to disinfectants and detergents) and dimensional stability. It also possesses excellent strength at elevated temperatures.
Ultem is comparable to Radel and PEEK in sterilization resistance. It can be exposed to hundreds of autoclave cycles – in fact, it’s incorporated into some autoclave devices. It also withstands gamma and chemical sterilization well. With its excellent sterilization resistance, Ultem is also featured in respiratory devices, infusion pumps and structural components for medical equipment.
PSU – Polysulfone
PSU, or polysulfone, withstands most forms of sterilization, including ethylene gas, vaporized hydrogen peroxide, gamma radiation and autoclaving. PSU may exhibit stiffening and possible embrittlement following repeated autoclaving cycles, which have an annealing effect on the material.
PSU’s attributes include transparency, impact resistance and rigidity. Commercialized under the Udel polysulfone tradename, it has been used in healthcare applications for many decades.
When converted into sheets, PSU is used to thermoform anesthesia masks and instrument trays. It is also injection molded into dental instrument components and heart valve sizers, among other applications. PSU is another thermoplastic that can withstand hundreds of autoclave cycles up to 250oF. It’s considered an alternative to polycarbonate in applications where repeated steam sterilization is expected. It is, however, not as robust as PEEK, Radel and Ultem in terms of gamma or chemical resistance.
POM-C – Polyoxymethylene (Copolymer Acetal)
POM-C, more commonly known as copolymer acetal or by the brand Celcon, is another durable, stiff medical plastic. It does not readily absorb moisture, so it’s an appropriate choice for repeated steam sterilization cycles up to 250oF. Its sterilization life is usually considered to be several hundred autoclave cycles, and it will discolor. This is seen as a disadvantage for sterilized devices that must retain their color for identity during a surgical procedure, such as joint replacement trials. In those cases, Radel is gaining much broader use because of its greater color stability. POM can withstand ethylene oxide and hydrogen peroxide sterilization, but it is not a viable option for gamma irradiation. Exposure to gamma sterilization will cause POM to degrade in structural properties and toughness.
However, POM’s overall combination of strength, biocompatibility and its degree of sterilization tolerance has made it a viable material option for medical applications such as insulin pens that require less rigorous sterilization practices.
PC – Polycarbonate
Polycarbonate is a common replacement for glass in medicine, as it provides excellent optical clarity, biocompatibility and strength. These properties make PC a good choice for arterial filters, blood oxygenators and other components that contain or facilitate the flow of bodily fluids. Because of its transparency, fluids are also highly visible within devices made from polycarbonate.
Polycarbonate can also be sterilized using gamma radiation, ethylene oxide gas or vaporized hydrogen peroxide. It is not, however, compatible with steam sterilization. Following more than 10 cycles, PC’s tensile elongation and overall resilience or toughness are diminished below acceptable levels.
PP-H – Polypropylene homopolymer
Polypropylene homopolymer, or PP-H, is a widely used plastic for medical applications and offers a combination of broad chemical resistance, low moisture absorption and hydrolysis resistance. Together, this means PP-H is a good fit for autoclave sterilization, and the material can withstand hundreds of steam sterilization cycles. Specifically, PP-H is stable up to 800 steam sterilization cycles, but discoloration and color changing are seen much earlier – typically around 200 cycles. It can be chemically sterilized and is compatible with ethylene oxide gas methods but is not appropriate for gamma sterilization processes.
With its excellent resistance to hydrolysis and heat, PP-H is a frontline choice for reusable containers and test equipment. It’s also used to make syringes, oxygenator components and medical fittings.
How Medical Plastics Converters Can Help Narrow Material Selection Options
Two factors should be considered when evaluating options for medical plastics regarding their sterilization compatibility: their inherent resistance to the sterilization medium, and their longevity or number of cycles of useful life in different sterilization methods. This is where initially high-cost materials such as PEEK and Radel vs. POM and PC prove their worth.
Analyzing the physical properties as well as longevity in different sterilization methods can make the selection of plastics candidates complex for a device manufacturer. It’s also an area where an experienced medical plastics injection molder or shapes extruder can help. A processor with deep expertise in medical grade polymers and their application history can narrow the range of options and help speed the product design and development process.