Constitutive Modeling of Degradation-Induced Mechanical Behavior in Commercial Polydioxanone for Implantable Devices

In this study, the mechanical behavior of a commercially available polydioxanone (RESOMER X 206S, Evonik) and its evolution under in vitro degradation were investigated through combined experimental and constitutive modeling approaches. Tensile tests were performed on injection-molded dog-bone specimens at multiple time points during immersion in phosphate-buffered saline (PBS) at 37°C for up to 4 weeks. Experimentally, the material exhibited a distinct transition from ductile to brittle behavior after approximately 3 weeks of degradation. Based on these findings, an extension of the Three Network Model (TNM), termed TNM for Degradation (TNMfD), was developed and validated to predict the degradation-dependent mechanical response of biodegradable polymers under nearly incompressible conditions. The model parameters of the TNM were initially identified to accurately reproduce the mechanical response of PDO before degradation, establishing baseline values for degradation-insensitive parameters and reference points for degradation-dependent ones. Subsequently, two formulations of the hydrolytic damage parameter were evaluated within the TNMfD framework. Both formulations successfully captured the degradation-induced changes in mechanical behavior, demonstrating good agreement with experimental data. The proposed TNMfD provides a robust, efficient numerical tool for the in silico assessment and design of biodegradable polymer-based medical devices.