Gamma radiation is the medical device/equipment industry’s near-ideal answer to the problem of sterilization. It is faster and more thorough than its nearest competitive process ethylene oxide (EtO), causes negligible temperature rise in molded products, and unlike EtO, leaves no residual gases that have to be removed after sterilization.

However, gamma radiation has one major limitation that keeps it from becoming the universal sterilization process for devices that are coated to lower friction. Because it is high-energy radiation that completely penetrates objects, it affects the cell or polymer structure of the object or coating. For instance, some bone implant bearing pads lose tensile strength, and a long list of engineering polymers is degraded to the point of being worthless.

Unfortunately, two of these polymers are the preferred materials for solving tribological problems on biomedical components. Where low friction is needed for smooth motion or for ergonomic reasons, PTFE (polytetrafluoroethylene) and FEP(fluorinated ethylene propylene), coatings are the mainstays for designs where surfaces slide, twist, or roll over each other.

Neither polymer fares well when exposed to gamma sterilization, however. As high-energy photons zip through them, free radicals scission some of the fluorinecarbon links. Fluorine combines with hydrogen to form HFl and various salts. The greater the dosage of radiation, the greater the damage. At the normal sterilization level of 25 kGy, PTFE essentially turns to wax, and FEP is similarly degraded. In regions of stress, the negative effects of gamma radiation are even greater.

Loss of strength is not the only drawback when these coatings are irradiated. Biomed OEMs have had to contend with color shifting (yellowing) and rank odors emitted from gamma sterilized products. Limiting gamma dosage and careful sterility sampling minimizes problems, but this is always a balancing act between complete sterilization and degree of degradation. And it slows down production.

Familiar barrier

Surprisingly, one of the older commonplace plastics –polyethylene– is not only unharmed by gamma, it tends to become stronger. Unlike many engineering polymers, polyethylene cross links when exposed to gamma. Energy passes through it and is absorbed without creating free radicals and uncoupling chains as is the case with PTFE and FEP.

Historically, PE has taken a back seat to PTFE and FEP because they have slightly lower coefficients of friction and better release, and was ignored for biomedical applications until engineers in the food industry noticed that PE packaging went through radiation cycles without damage.

While not as spectacular as the fluoropolymers, PE coatings give good friction and wear properties (stronger than PTFE). For these reasons and because they are gamma stable, PE coatings usage is growing among OEMs who prefer the speed, simplicity, and thoroughness of gamma sterilization.

Surface Solutions Group of Chicago formulates PE into a coating matrix, using slightly larger than nano-size particles of the highdensity resin, resulting in its Plasti Glide series of coatings. These formulations— specifically Plasti Glide® ON2-115—can be applied via several processes, in films as thin as 0.0001- in. The track record is good. Valves, tubes, blades, wires, and cannula coated with a thin coat of polyethylene have been successfully coated. It is even applied to tubing with internal diameters of 0.008 in.

Wear resistance and chemical resistance are good. Tests show it to be 8 times better than thin-film PTFE. Properties include superb adhesion and good electrical resistance. Coatings are available in clear and basic colors, all with a dynamic coefficient of friction of 0.012 vs PTFE at 0.010 (ASTM D 3702 @50 fpm).

These coatings are applied to not only metals such as stainless steel, but also to the commonly used glass filled and rigid polymers like Kynar and many more tough, but high-friction engineering plastics. An added bonus is that the bonding temperature of ON2-115 is less than 350°F, half that of PTFE. This allows a wide range of polymer-based products to now have a low-friction surface without being thermally degradation. Flexible substrates of rubber including Pebax, can have ON2-115 bonded to them, imparting similar low friction “skins” on these flexible polymers.

Other benefits include antimicrobial protection and coverage of complex shapes. Several levels of antimicrobial versions of ON2-115 have been tested successfully, providing low friction and also biopathogenkilling properties.

And successes with coating many surfaces, including internal surfaces of tubes, valves, or guides—as well as all exterior surfaces have been accomplished at Surface Solutions Group.