Prediction of the structure and mechanical properties of polycaprolactone-silica nanocomposites and the interphase region by molecular dynamics simulations: the effect of PEGylation.

Polymer/silica (PS) nanocomposites are, among numerous combinations of inorganic/organic nanocomposites, one of the most important materials reported in the literature and have been employed in a wide variety of applications. Due to this great interest in the scientific and industry community, knowledge about their physiochemistry allows for a better understanding of their development and improvement. One area of interest found in biopolymers is silica, where silica nanoparticles can be used to increase their mechanical properties and give them higher opportunities to replace synthetic plastics. With this aim in mind, molecular dynamics (MD) simulations were used to predict the structure and mechanical properties of the interphase region and nanocomposite systems of polycaprolactone (PCL), a common poly(hydroxy acid) type biopolymer, reinforced with silica nanoparticles. Two types of nanoparticles were studied to assess the effect of PEGylation: hydroxyl (ungrafted) and polyethylene glycol (PEG) (grafted or PEGylated) functionalized silica. The interaction energy between the nanoparticle and the polymeric matrix was determined, showing an increase of the affinity between each component due to the PEGylation of the nanoparticle. Through the analysis of polymer density profiles, the structure and thickness of the interphase region were determined, and it was observed that PEGylation increased the interphase thickness from 10.80 Å to 13.04 Å while it decreased the peak and average polymer density of the interphase region. Using compressed and expanded molecular models of the neat PCL polymer, the mechanical properties of the interphase region were related to its density through an interpolation model, and mechanical property profiles were obtained, from which the average values of the Young's modulus, Poisson's ratio and shear modulus of the interphase region were calculated. Finally, the mechanical properties of the nanocomposites were determined by molecular mechanics simulations, showing that the silica nanoparticles increased the stiffness of the composite system to about 7-8% with respect to that of the neat polymer, having a 2.09% weight of bare silica or 2.82% weight of PEGylated silica. PEGylation did not show an additional effect on the overall mechanical properties. A mean field micromechanics model (Mori-Tanaka) corroborated the properties calculated for the interphase region using MD simulations. It was concluded that the PEGylation of the nanoparticle improved the affinity, and thus the dispersion, of the silica nanoparticles towards the PCL matrix, but with no further increase in the mechanical properties of the composite.

The use of mixed collagen-Matrigel matrices of increasing complexity recapitulates the biphasic role of cell adhesion in cancer cell migration: ECM sensing, remodeling and forces at the leading edge of cancer invasion.

The migration of cancer cells is highly regulated by the biomechanical properties of their local microenvironment. Using 3D scaffolds of simple composition, several aspects of cancer cell mechanosensing (signal transduction, EMC remodeling, traction forces) have been separately analyzed in the context of cell migration. However, a combined study of these factors in 3D scaffolds that more closely resemble the complex microenvironment of the cancer ECM is still missing. Here, we present a comprehensive, quantitative analysis of the role of cell-ECM interactions in cancer cell migration within a highly physiological environment consisting of mixed Matrigel-collagen hydrogel scaffolds of increasing complexity that mimic the tumor microenvironment at the leading edge of cancer invasion. We quantitatively show that the presence of Matrigel increases hydrogel stiffness, which promotes ß1 integrin expression and metalloproteinase activity in H1299 lung cancer cells. Then, we show that ECM remodeling activity causes matrix alignment and compaction that favors higher tractions exerted by the cells. However, these traction forces do not linearly translate into increased motility due to a biphasic role of cell adhesions in cell migration: at low concentration Matrigel promotes migration-effective tractions exerted through a high number of small sized focal adhesions. However, at high Matrigel concentration, traction forces are exerted through fewer, but larger focal adhesions that favor attachment yielding lower cell motility.