Coarse-Grained Simulations of Solute Diffusion in Crosslinked Flexible Hydrogels

The Influence of Local Constraints on Solvent Motion in Polymer Materials

The influence of obstacles in the form of polymer chains on the diffusion of a low-molecular-weight solvent was the subject of this research. Studies were performed by computer simulations. A Monte Carlo model-the Dynamic Lattice Liquid algorithm-based on the idea of cooperative movements was used. The tested materials were polymer networks with an ideal structure (with a uniform mesh size) and real, irregular networks (with a non-uniform mesh size) obtained numerically by copolymerization. The diffusion of the solvent was analyzed in systems with a polymer concentration that did not exceed 16%. The influence of the polymer concentration and macromolecular architecture structure on the mobility and character of the motion of the solvent was discussed. The influence of irregular network morphology on solvent dynamics appeared to be significantly stronger than that of regular networks and star-like polymers.

Magnetic nanocomposites for magneto-promoted osteogenesis: from simulation auxiliary design to experimental validation

Magnetically actuated mechanical stimulation, as a novel form of intelligent responsive force stimulation, has a great potential for remote spatiotemporal regulation of a variety of life processes. Hence, the optimal design of magnetic nanomaterials for generating magneto-mechanical stimuli becomes an important driving force in the development of magneto-controlled biotherapy. This study aims to clarify the general rule that the surface modification amount of magnetic nanoparticles (NPs) affects the biological behavior (e.g., cell adhesion, proliferation and differentiation) of pre-osteoblast cells. First of all, course-grained molecular dynamics simulations predict that 23.3% graft modification of the NPs can maximize the heterogeneity of the dynamics of the polymer matrix, which may generate enhanced mechanical stimuli. Then, experimentally, iron oxide (IO) NPs grafted with different amounts of poly(γ-benzyl-L-glutamate) (PBLG) were prepared to obtain homogeneous magnetic nanocomposites with improved mechanical properties. Further in vitro cell experiments demonstrate that the grafting amounts of 21.46% and 32.34% of PBLG on IO NPs are the most beneficial for the adhesion and osteogenic differentiation of cells. Simultaneously, the maximized upregulation of the Piezo1 gene indicates that the cells receive the strongest magneto-mechanical stimuli. The consistent conclusion of the experiments and simulations indicates that 20-30% PBLG grafted on the IO surface could maximize the ability of magnetic stimuli to regulate the biological behavior of the cells, which validates the feasibility of simulation auxiliary material design and is of great importance for promoting the application of magneto-controlled biotherapy in bioengineering and biomedicine.

Dynamics of a spherical self-propelled tracer in a polymeric medium: interplay of self-propulsion, stickiness, and crowding

We employ computer simulations to study the dynamics of a self-propelled spherical tracer particle in a viscoelastic medium, made of a long polymer chain. Here, the interplay between viscoelasticity, stickiness, and activity (self-propulsion) brings additional complexity to the tracer dynamics. Our simulations show that on increasing the stickiness of the tracer particle to the polymer beads, the dynamics of the tracer particle slows down as it gets stuck to the polymer chain and moves along with it. But with increasing self-propulsion velocity, the dynamics gets enhanced. In the case of increasing stickiness as well as activity, the non-Gaussian parameter (NGP) exhibits non-monotonic behavior, which also shows up in the re-scaled self part of the van-Hove function. Non-Gaussianity results owing to the enhanced binding events and the sticky motion of the tracer along with the chain with increasing stickiness. On the other hand, with increasing activity, initially non-Gaussianity increases as the tracer moves through the heterogeneous polymeric environment but for higher activity, the tracer escapes resulting in a negative NGP. For higher values of stickiness, the trapping time distributions of the passive tracer particle broaden and have long tails. On the other hand, for a given stickiness with increasing self-propulsion force, the trapping time distributions become narrower and have short tails. We believe that our current simulation study will be helpful in elucidating the complex motion of activity-driven probes in viscoelastic media.

Synergic Thermo- and pH-Sensitive Hybrid Microgels Loaded with Fluorescent Dyes and Ultrasmall Gold Nanoparticles for Photoacoustic Imaging and Photothermal Therapy

Smart microgels (μGels) made of polymeric particles doped with inorganic nanoparticles have emerged recently as promising multifunctional materials for nanomedicine applications. However, the synthesis of these hybrid materials is still a challenging task with the necessity to control several features, such as particle sizes and doping levels, in order to tailor their final properties in relation to the targeted application. We report herein an innovative modular strategy to achieve the rational design of well-defined and densely filled hybrid particles. It is based on the assembly of the different building blocks, i.e., μGels, dyes, and small gold nanoparticles (<4 nm), and the tuning of nanoparticle loading within the polymer matrix through successive incubation steps. The characterization of the final hybrid networks using UV-vis absorption, fluorescence, transmission electron microscopy, dynamic light scattering, and small-angle X-ray scattering revealed that they uniquely combine the properties of hydrogel particles, including high loading capacity and stimuli-responsive behavior, the photoluminescent properties of dyes (rhodamine 6G, methylene blue and cyanine 7.5), and the features of gold nanoparticle assembly. Interestingly, in response to pH and temperature stimuli, the smart hybrid μGels can shrink, leading to the aggregation of the gold nanoparticles trapped inside the polymer matrix. This stimuli-responsive behavior results in plasmon band broadening and red shift toward the near-infrared region (NIR), opening promising prospects in biomedical science. Particularly, the potential of these smart hybrid nanoplatforms for photoactivated hyperthermia, photoacoustic imaging, cellular internalization, intracellular imaging, and photothermal therapy was assessed, demonstrating well controlled multimodal opportunities for theranostics.

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.

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

We use coarse-grained simulations to explore the diffusion mechanism of nanoparticles with different sizes at various nanoparticle-polymer interactions in regular cross-linked polymer networks. The long time diffusivities of nanoparticles show a non-monotonic tendency at various nanoparticle-polymer interactions, due to the intermittent hopping of nanoparticles through network cells. The preferred locations of small nanoparticles switch from the cell centers to the corner of cells as they interact with network more strongly, which results in the hopping energy barrier between different cells switching from cell center localization to adsorption on networks. Steric hindrance seriously hampers large nanoparticles from hopping to neighboring network cells, the interactions between nanoparticle and network enhance the network deformability and also affect the hopping of nanoparticles. The multiple constraint mechanisms result in the non-monotonic diffusivities of nanoparticles with different interactions and non-Brownian motions at different time scales. Our work illustrates the hopping mechanisms of nanoparticles in polymer networks from thermodynamic and dynamic points of view.