Effects of gamma sterilisation on dissipation mechanisms in polyethylene glycol hydrogels - Novel insights revealed by nanoindentation and modelling.

Polyethylene glycol (PEG) hydrogels, renowned for their hydrophilicity, biocompatibility, and biodegradability, play a crucial role in a range of biomedical applications. Optimising their performance requires a quantitative and mechanistic understanding of time-dependent mechanical behaviour. This study investigates the effect of a clinically relevant gamma-sterilisation (GS) procedure on the time-dependent mechanical behaviour of PEG hydrogels. Multi-rate nanoindentation loading-creep tests were performed on non-irradiated and irradiated samples. An inverse poro-visco-hyperelastic finite element model, coupled with global optimisation, was used to simultaneously fit load-displacement and creep curves and to extract a consistent set of equivalent elastic, viscoelastic and poroelastic parameters. The results show that γ-irradiation not only increases Young's modulus but also shortens relaxation times and suppresses long-time poroelastic flow, suggesting an effective shift in the time-dependent response from fluid-flow effects towards solid-network mechanics over the investigated time scales. These findings clarify the role of GS in altering the time-dependent mechanical response of hydrogels and establish a general approach for the mechanical characterisation when designing PEG-based hydrogels for biomedical applications.