October 2016

Radiation sterilization of custom-colored medical resins

To help designers achieve personalized (visual) aesthetics while retaining functionality of their medical devices, Makrolon® polycarbonate medical-grade resins are available in a wide variety of clear tints or opaque colors.

Covestro’s resins and color recipes are carefully formulated to preserve as much of the inherent properties of polycarbonate as possible, and ensure that the formulated resins meet ISO 10993 biocompatibility requirements for 30-day direct patient contact, even after sterilization.

For devices that are sterilized by eBeam or gamma radiation, Covestro offers several specialty medical-grade Makrolon® polycarbonate resins. These specially stabilized grades are formulated to reduce the color shift that occurs after irradiation. Makrolon® Rx1805 and Rx2530 are two grades which are stabilized for radiation-sterilization and are offered in a broad range of tints and opaque colors.

The reactions which take place after the irradiation of polycarbonate have been extensively studied over several decades1. Even with noticeable color shift post sterilization, polycarbonate can demonstrate better property retention than resins that do not show a color shift. From the perspective of “built-in” features, the post-sterilization color shift is proportional to dosage and can serve as a visual confirmation that the sterilization was correctly performed. It can also help confirm that the correct material was used to manufacture the device. Previously published articles have demonstrated that despite this color shift, tinted polycarbonate retains its physical properties and displays excellent impact strength after radiation sterilization. Several technical articles on this topic are available to you from your Covestro representative.

Though medical applications typically require transparent materials in order to detect bubbles, fluid levels or impurities, opaque materials can also be integral in designs. In this newsletter, we examine opaque white Makrolon® and examine the color shift and impact performance after sterilization. For this study, the color and impact performance of Makrolon® 2458-013771 polycarbonate and Makrolon® Rx1805-013771 polycarbonate (both the same white starting color) were examined.

Color Changes After Sterilization

Typically after sterilization, in the presence of air and light, the color recovery of 3 mm-thick parts is complete after two weeks. Color measurements were taken after the parts were allowed to complete their color recovery. The color changes, measured in CieLab color coordinates, after eBeam sterilization (30 kGy) are shown in the figure below.

Change in color coordinates for opaque/white polycarbonate samples as determined by the comparison of as molded and after sterilization (+30kGy) color chips.

The color shift after sterilization was predominantly in the +/-b color space (yellow-blue axis) which shows that the resin increased in yellowness. Though a slightly larger color shift was observed for Makrolon® 2458-013771 compared with Rx1805-013771, it is important to note that Rx1805 contains a proprietary stabilizer which reduces the color shift after irradiation. However, it is also important to point out that the calculated ΔE value between the two resins is 2.72, which can be considered to be a subtle difference and is more difficult to discern that with clear tints.

Impact Property Retention

The impact properties of both products after sterilization were also examined. A subset of multiaxial impact properties measured at -30°C are shown in the Figure below. As a reference, results from a clear-tint Makrolon® 2458 polycarbonate sample (in tint 550115) are also shown.

Multiaxial impact properties (ASTM 3763B) at -30°C before (left) and after (right) eBeam sterilization (+30kGy).

The total energy absorbed shows a very modest change after sterilization. While Makrolon® Rx1805 polycarbonate showed the overall best retention in total energy, it is worth mentioning that it also is the highest performing polycarbonate that benefits from a high molecular weight. Further, despite the color shift and the slight decrease in total energy after impact, the outstanding ductility of all products evaluated is retained. A representative picture of the white-colored materials tested for impact properties is shown below.

Photos of Rx1805-013771 and Makrolon® 2458-013771 impacted at -30°C, after sterilization (30 kGy).