Suitability of Alternative Sterilization Techniques For Bioabsorbable Composite Implants : In vitro feasibility study

Abstract

Bioabsorbable implants are increasingly favored in the medical field for their ability to degrade naturally in the body, eliminating the need for invasive removal procedures. Typically composed of materials such as polylactide (PLA), these implants offer excellent biocompatibility and predictable degradation behavior, making them a reliable choice for many medical applications. However, due to their sensitive nature, sterilization of these materials can be challenging, and the conventional sterilization methods, such as ethylene oxide and gamma irradiation, are known to sometimes alter their properties in ways that could potentially impact the overall safety and performance of the device. Coupled with recent concerns related to the use of ethylene oxide, a need for safe and more compatible sterilization methods is growing rapidly. Although some emerging low-temperature sterilization methods have shown potential as alternatives, comprehensive comparative studies evaluating their effects on bioabsorbable materials are still limited.

Hence, this work focuses on identifying and evaluating the suitability of alternative sterilization methods that could be used for bioabsorbable PLA-based polymers and composites. Based on an extensive literature review, four methods, namely vaporized hydrogen peroxide (VHP), nitrogen dioxide (NO2), supercritical carbon dioxide (scCO2), and chlorine dioxide (ClO2), were identified and selected for further evaluation through in vitro studies, where their material compatibility was assessed by comparing their post-sterilization properties and degradation behavior to nonsterile products. Two PLA-based composite implants, orthopedic screw and pin, were used as test samples, differing in terms of fiber content, composition, and manufacturing method. The effects of sterilization on material properties were assessed over a nine-week in vitro period through mechanical testing, dimension measurements, surface imaging with scanning electron microscopy (SEM), and molecular characterization using differential scanning calorimetry (DSC), inherent viscosity (IV), and gas chromatography (GC).

The results showed that VHP, NO2, and ClO2 sterilization all show excellent material compatibility with only a limited influence on the implant properties. ClO2 sterilization caused the least changes, whereas the most significant effect of VHP was a slight surface dulling, possibly related to minor surface crystallization or oxidation. NO2 again resulted in minor yellowing and slightly greater changes in screws compared to pins, but none of the changes were significant enough to compromise the functionality of the implant. In contrast, scCO2 significantly altered the properties of the implants, inducing changes, such as increased crystallinity and dimensional instability, limiting its suitability under the tested conditions. Notably, screws were more susceptible to changes than pins, likely due to the differences in polymer content, fiber type, and geometry.

The findings of the study emphasize the importance of tailoring the sterilization process to the specific characteristics of the device. VHP emerged as the most balanced option considering both compatibility and feasibility, while NO2 and ClO2 also showed great potential particularly for sensitive materials. Even though scCO2 will require further optimization, its unique properties give promise for future applications. Overall, the study demonstrates the capabilities of alternative sterilization methods for sterilizing bioabsorbable implants and their potential to challenge the reliance on conventional techniques.