Molecular Simulation of Hopping Mechanisms of Nanoparticles in Regular Cross-Linked Polymer Networks

Mechano-diffusion of particles in stretchable hydrogels

Precise control over particle diffusion is promising for diverse modern technologies. Traditionally, particle diffusion is governed by the inherent properties of a liquid medium, limiting versatility and controllability. Here, we report a mechano-diffusion mechanism that harnesses mechanical deformation to control particle diffusion in stretchable hydrogels with a significantly enlarged tuning ratio and a highly expanded tuning freedom. The working principle is to leverage the mechanical deformation of stretchable hydrogels for modulating the polymer network's geometric transformation and the polymer chain's energy modulation, which synergistically tunes the energy barrier for particle diffusion. Using a model particle-hydrogel material system and a customized mechano-diffusion characterization platform, we demonstrate that tension loads can enhance the diffusivity of gold nanoparticles up to 22 times, far exceeding that in traditional liquid medium and by external fields. Additionally, we show particle diffusion in hydrogels can be manipulated spatiotemporally by controlling the hydrogels' stress state and loading rate. To further push the limit of the mechano-diffusion, we use experiment, theory, and simulation to explore particle diffusion in biaxially stretched hydrogels, simultaneously expanding the mesh size and reducing the energy barrier. The enlarged tuning ratio and expanded tuning freedom enable a model-guided drug delivery system for pressure-controlled release of drug molecules. Understanding this spatiotemporal mechano-diffusion mechanism will provide insights pertinent to a broad range of biological and synthetic soft materials.

Coarse-Grained Simulations of Release of Drugs Housed in Flexible Nanogels: New Insights into Kinetic Parameters

The diffusion-controlled release of drugs housed in flexible nanogels has been simulated with the help of a coarse-grained model that explicitly considers polymer chains. In these in silico experiments, the effect of its flexibility is assessed by comparing it with data obtained for a rigid nanogel with the same volume fraction and topology. Our results show that the initial distribution of the drug can exert a great influence on the release kinetics. This work also reveals that certain surface phenomena driven by steric interactions can lead to apparently counterintuitive behaviors. Such phenomena are not usually included in many theoretical treatments used for the analysis of experimental release kinetics. Therefore, one should be very careful in drawing conclusions from these formalisms. In fact, our results suggest that the interpretation of drug release curves in terms of kinetic exponents (obtained from the Ritger-Peppas Equation) is a tricky question. However, such curves can provide a first estimate of the drug diffusion coefficient.