DSM has conducted sterilization tests for the 3D printed materials developed for healthcare purposes. These studies give indications on material changes that can occur during the sterilization process. It remains the responsibility of the device manufacturers to determine the fitness of device in their application.
The most commonly used methods of sterilizing a medical device are steam autoclave, irradiation, ethylene oxide and hydrogen peroxide gas plasma.
Steam autoclaving is a widely used, and available as sterilization technique for dental, and medical devices. It is fast, reliable, inexpensive and all healthcare agencies recommend using steam autoclaving whenever possible.
Disadvantage of this sterilization method in combination with polymer materials is the high temperature, and moisture levels that can lead to deformations during the process. Changes in mechanical properties occur during, and right after the process. Mechanical properties are regained to about 80% in 2hrs. Parts should also be allowed to cool to room temperature, without adding any forces on it after the autoclave cycle to avoid accidentally bending or breaking the part. Thin walls or unsupported areas can deform under its own weight, thicker parts are more stable. In the designs parts can be optimized to prevent these deformations.
Cycle times necessary to achieve sterility vary depending on the items that are being sterilized and the sterilization vessel being used. Generally, cycle times for flash cycles is 3-5 min or 15 to 30 minutes for standard cycles at temperatures ranging between 121°to 135°Celsius.
Moisture uptake might take place during sterilization, therefore a post-oven treatment under low temperature or keeping the part in a well ventilated room is advised.
Sterilization with autoclave is known to influence biocompatibility of the material extra biocompatibility testing after sterilization is recommended
Low-temperature hydrogen peroxide gas plasma
This low-temperature sterilization process utilizes a concentrated solution of hydrogen peroxide that is injected into a chamber under vacuum. The gas can diffuse throughout the chamber for a period before an electromagnetic field is introduced that creates a gas plasma. The gas plasma breaks down hydrogen peroxide to form free radicals, as well as UV-energy. The free radicals and UV-energy destroy pathogens on the device. Once the plasma begins to lose energy, oxygen and water are formed as by-products. The process can be repeated as necessary to achieve desired sterility on a given device. After the final cycle, the chamber is vented to remove oxygen and moisture.
It should be noted that this type of sterilization may have limitations with certain metals and smaller diameter lumens. Materials must also be dry for this process and must be packaged in gas-permeable packages.
Hydrogen peroxide gas plasma sterilization can be used with some polymer materials due to the lower temperatures than autoclave.
Irradiation is the use of either an electron beam or gamma rays to sterilize materials. It is especially useful for sterilizing large, bulk-quantities of disposable items. Dosage or exposure commonly used for irradiation ranges between 20 to 55 kiloGrays (kGy). This method is suitable for some 3D printed polymers, although mechanical properties and color can change during the process.
Color change is a very common occurrence, but will not necessarily cause problems for the final application. It has been found that even low dose irradiation can cause significant color changes. Clear 3d printed materials tend to change color from clear to green or pink, while white materials tend to change from white to tan or brown.
Mechanical property changes are common for plastics during irradiation. Often the changes are negligible and sometimes beneficial. Irradiation tends to increase both Young’s Modulus and percent elongation at break, while keeping Izod impact unchanged and in some cases increasing impact strength.
Polymers can be sensitive to heat, and high moisture environments. Ethylene oxide (EO) is often used to sterilize these materials. EO is an extremely toxic gas and alkylating agent. It works to sterilize materials by disrupting the proteins, DNA and RNA of pathogens, destroying them or interfering with their ability to replicate, and can inactivate all microorganisms.
Items to be sterilized by EO must be completely dry. EO sterilization may require about 12 hours of cycle time, and materials must be packaged in gas-permeable packages. ETO is absorbed by many materials, finding the right aeration parameters is crucial, and depends on material, and geometry of the parts.
Mechanical properties can be moderately affected. The surface of some materials can react with the EO gas leading to toxic residues, therefore careful testing to get the right setting is needed.
If EO sterilization is used, it is recommended that the parts be thermally post-treated, coated or dyed to create a surface barrier to the reactive EO environment.
ISO 10993-5 has been passed using the following sterilization method: