Sterilization considerations when selecting a medical plastic

Plastics react differently to various sterilization methods. Therefore, when selecting a material for a new medical device, you should ask yourself if the component will undergo sterilization and if so, which method of sterilization will be used? And Also, how many times will it be sterilised?

The most common sterilization methods are ethylene oxide (EtO), irradiation (gamma/e-beam) and steam autoclaving. There are also other alternatives such as STERRAD and vaporized hydrogen peroxide (VHP).

The majority of thermoplastic polymers can withstand exposure to EtO without displaying any significant changes to their properties or color, even if some dimensional changes are possible, as EtO is typically done in presence of humidity at around 55°C. It should be noted that the device undergoing EtO sterilization should have a gas permeable packaging allowing the gas to go through it and therefore reach the device to efficiently sterilize it. 

However, gamma/e-beam sterilization in the majority of cases will alter the optical appearance of the polymer – mostly a color shift towards yellow: In addition, it may change the mechanical properties of the material such as tensile strength, impact strength and elongation.

For instance, standard polypropylene (PP) grades may not be suited for radiation sterilization due to their loss of physical properties and discoloration that takes place immediately after sterilization and can worsen over time. To overcome this issue, many polyolefin producers such as Repsol, have developed grades stabilized for gamma radiation.

A color shift is not always an indication of loss of mechanical properties. In terms of mechanical performance, polycarbonate (PC) is generally resistant to radiation, but it will turn yellow when sterilized. A common trick among polymer producers is to add a small amount of blue/violet pigment where the color shift then results in a more pleasing ‘gray’ color. Also the color shift is generally proportionate to the gamma dosage. If the part is sterilized at 50 or even 75kGy, the discoloration will be more pronounced than at 25kGy. Polycarbonate producers, such as Trinseo, have therefore developed grades with different tints to help compensate for color changes after sterilization at various radiation doses. Also the yellowness diminishes with time. After exposure to gamma radiation, the transparent PC parts equilibrate back toward their original color prior to irradiation but this can take several weeks.

The most challenging sterilization method remains steam sterilization/autoclaving as many thermoplastic polymers are sensitive to heat and hydrolysis. Some materials will lose structural integrity when exposed to high temperatures of 121°C to 134°C. Even parts using plastic materials with a softening temperature higher than the autoclaving temperature can suffer from the release of molded-in stresses resulting in dimensional instability and/or warpage. Therefore, devices intended to undergo steam sterilization should be made of heat-resistant materials and a special care should be taken regarding the molding in order to avoid residual stress.

Another precaution is to consider the chemistry of the thermoplastic polymer – under high temperature and pressure in the autoclave - what can be potentially released from the thermoplastic part in terms of additives or what chemical changes can occur in the polymer structure itself. It is vital to know if there is a risk of formation or release of potentially hazardous substances (eg. aromatic TPUs can form MDA-methylene dianiline – a known carcinogen and so are not recommended for steam sterilization).

Materials such as PP, PC and polyarylamide (PARA) can be used, but special care is required in regards to how many cycles they will be exposed to. PP can typically withstand autoclave when correctly molded but is not recommended for repeated sterilization. PARA is also not recommended for steam autoclaving for more than a few cycles as the material quickly loses tensile strength. General recommended conditions for steam autoclave sterilization for PC are 1 to 5 cycles at 134°C. After 10 cycles, the instrumented dart impact begins to decrease and the tensile elongation is significantly affected.

Multiple steam sterilization of cyclic olefin copolymers (COC) requires a special process to avoid haze. The high temperature and pressurized steam drives water vapor into the matrix of the plastic part. At the end of the sterilization cycle, steam is vented and the cooling phase begins. Since COC is an outstanding water vapor barrier material, water vapor will become trapped in the plastic matrix if temperature rapidly falls. Water vapor condenses forming haze. This effect can be reduced by replacing steam with dry air at elevated temperatures for some time, which allows trapped water vapor to escape and haze to clear.

If the product is not a single-use device, it will most probably be subjected to multiple sterilizations before being discarded, so plastics which possess superior toughness and heat resistance are better choices. Certain blends of COC can for instance withstand exposure to more than one thousand cycles of steam sterilization at 134°C without losing their mechanical and thermal properties. Thus, these products can be considered as potential material solutions for trays, containers as well as handles of reusable surgical instruments which undergo multiple steam sterilization cycles.

Before selecting a material for a new device, it is necessary to know upfront which sterilization method will be used, as thermoplastics are very different from one another in terms of chemistry and therefore react differently to radiation or heat and moisture. If the selected material is not compatible with the sterilization method, this could lead to failure. Having a close relationship with your qualified polymer supplier will allow you to get the best advice up front in order to avoid wasting time and money.