The bioabsorbable thermoplastic Poly(lactic acid), PLA, is extensively used in many medical applications including surgical sutures, drug delivery systems, internal fixation devices, tissue engineering scaffolds, and drug eluting stents. Frequently, a PLA component is required to withstand mechanical loading for a desired amount of time and then degrade via hydrolysis. In its raw, undegraded form, PLA exhibits a non-linear, viscoplastic mechanical response. Through the degradation process, the modulus, yield stress and flow behavior of PLA changes. Accurate simulations of bioabsorbable implants require a constitutive model that accounts for the viscoplastic nature of the material and its evolution over time. In this work we present the development of a new material model framework for predicting the time-dependent viscoplastic response of PLA. The proposed material model also captures the change in mechanical behavior over time due to hydrolysis. The details of the proposed model are presented, and the model predictions are compared to preliminary experimental data.

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