It's not hard to see trouble coming when a protective sheath over a hypodermic needle fits so snugly that ethylene-oxide gas (EtO), a sterilant, cannot penetrate the sealing surfaces. Or consider a two-part, water-tight float in a medical device. If EtO is selected, the float would have to be separated for proper exposure to the gas. That will require extra steps that add cost.

Materials sterilized by radiation present other design headaches. Select the wrong material and products discolor, or worse, become brittle. To avoid such problems, we asked several experts for guidelines that help design for sterilization.

Start with standards

Sterilization, of course, really begins in the design department. It starts with selecting materials that will withstand the high temperatures of steam, or not discolor and become brittle from radiation. “There should be a lot of up front work deciding what sterilization method works best on a new product,” says David Parente, senior consultant and manager of Namsa Advisory Service, Atlanta (namsa.com).

For instance, “review the appropriate standards before starting the design and its packaging,” suggests Betty Howard, Gamma Technology Center Manager at Steris Isomedix Services, Morton Grove, Ill. (isomedix.com). Several documents provide guidelines for sterilizing medical products. In the U.S., they are published by Association for the Advancement of Medical Instrumentation (AAMI). “Many AAMI standards are also ISO so following them meets international standards as well.”

For example, ANSI/ISO/AAMI 11137 covers gamma and ebeam sterilization. “It carries a lot of information including how to validate that the sterilization was effective,” says Howard. “Another for gamma, TIR27 (technical information report 27), presents slightly different methods for validating gamma sterilization, but at different dosages. The standard can simplify the process, but puts some restrictions on when it can be used. Older documents such as AAMI 13409 also provide guidance.”

TIR29 also covers gamma-radiation sterilization. It qualifies plants and facilities that deliver the dosages.

AAMI 11135 covers EtO. “There are numerous guidelines for optimizing each parameter in an EtO method, such as for how much gas, concentration, time, and humidity. Each is critical and must be worked out,” says contract sales manager Brenda Sparks at Centurion Sterilization, Howell, Mich. (centurionsterile.com).

A second guideline for designers is know your materials. “Look up information on the anticipated materials and their tolerances to the selected sterilization method,” she says. “If polypropylene is the candidate, look up references that describe what happens to that material when it's gamma radiated or steamed.” It sounds obvious, but a common mistake is not considering the production material. The problem is this: if the material has a narrow tolerance to the sterilant, the product may be impossible to sterilize.

“Get an idea of a material's tolerance to sterilization when building prototypes,” says Howard. “When you consider any method — gamma, EtO, steam, or heat — the product may already have a problem. If you wait too long to solve it, you may have fewer options when change is needed. For instance, you may be forced into a sterilization method that tolerates a narrow parameter range, and that usually means more cost,” she says.

“Designers choose materials they are most familiar with and then they look for a way to sterilize them,” she adds. “Think of the material when designing. Or you could end up with a great device that cannot be sterilized.”

“EtO is the friendliest form of sterilization because it can be used with a wide range of materials,” adds Sparks. But even it has caveats. “Silicone tends to hang on to residual EtO gas. Although that does not disqualify the product from EtO, it may require adjusting the parameters to avoid additional aeration.”

Material selection becomes more of an issue with radiation. “Two issues surround radiation sterilization,” says Parente. One is color change. It's easily addressed because suppliers can put additives or bluing into the resin. Colors in the material keep it from significantly changing. The other issue is embrittlement. The material gets stiffer. Tests after irradiation usually show lower tensile strength and less elongation. The loss of flexing strength means products break easily. “Beware of teflon, polypropylene, and many acetyl compounds. They don't do well with radiation,” he says.

Radiation sterilization requires qualifying a material for its immediate use and over longer periods. “Radiation effects usually worsen over time,” adds Parente. A part might look fine at first but degrade over the next few years. “And although a vendor may insist a material resists radiation, in my opinion, nothing is unaffected by radiation,” he says.