The Influence of Mechanical Loading on the Hydrolysis of Biodegradable Polymer Implants

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This grant will explore the time-dependent changes in the physical properties and the size and shape of biodegradable polymers under combined fluid sorption, mechanical loading, hydrolysis, and erosion, which are driven by changes in the polymer macromolecular structures. Biodegradable polymers are promising materials for temporary biomedical implants that can provide mechanical support for damaged tissues until they heal. Using biodegradable polymers enables tailoring the specific functionality of the implants to specific patients. These materials also allow for tissue regeneration while avoiding the need for subsequent surgery to remove the implants at the end of their functional life. The degradation in biodegradable polymers is due to a hydrolytic process in which fluid diffuses into the polymers, breaking the polymer chains and forming monomers that eventually diffuse out of the polymer (erosion). A key obstacle in the development of effective biodegradable polymer implants involves a knowledge gap in understanding how mechanical loading and hydrolytic processes intertwine and their resulting influence on the time-dependent structural integrity and load-carrying ability of the implant. This project will implement a synergetic experimental and modeling strategy to address this knowledge gap. The project also provides students with technical training while promoting the retention of underrepresented students and military veterans. Additionally, virtual reality (VR)- and augmented reality (AR)-based learning modules aimed at understanding the interplay among various stimuli on polymer degradation will be developed and shared for the general public to use. Specifically, this project will investigate degradation in poly-lactic glycolic acid (PLGA) polymers with different initial macromolecular structures, ranging from being fully amorphous to highly crystalline, which will provide variations in the physical properties and degradation behaviors. The PLGA specimens, of different shapes and sizes, will be immersed in saline water at 37oC while being subjected to various mechanical loading. Nonlinear time-dependent constitutive models that incorporate changes in the macromolecular structures and mass of the polymers will be formulated to describe the intertwining mechanisms in the mechano-hydrolytic process and to predict the changes in the geometry and size of the specimens during degradation. The goal is to address 1) whether the hydrolytic scission is purely a chemical reaction between fluid and polymer molecules, or whether the scission induced by mechanical loading leads to shorter polymer chains and therefore accelerates the hydrolytic process; 2) how different histories of mechanical loading alter the erosion process, thereby gradually changing the shape and size of the implant.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.