Form follows the ion: The morphogenesis of fluorapatite–gelatine nanocomposites is studied from both electron microscopy and atomistic simulation revealing ion impregnation effects on the growth mechanisms. Depending on the ionic species used for pre‐treatment of the organic component dramatic changes of the mesoscopic structuring are observed.

We report on atomistic simulations related to the nucleation of zinc oxide nanocrystals from ethanolic solution. The underlying mechanisms are explored from the very initial stage of Zn2+ and OH(-) ion association to the formation of nanometer-sized aggregates counting up to 250 ions. The embryonic aggregates consist of zinc and hydroxide ions, only. At later stages of aggregate growth, proton transfer reactions at the aggregate-solvent interface account for the formation of O(2-) ions and induce the precipitation of zinc oxide. After the association of around 150 ions, ZnO domains were found to nucleate in the central region of the [Znx(OH)yOz]2x-y-2z) aggregates. In the course of further ion association and condensation reactions, progressive self-organization leads to an extended core in which the ions are arranged according to the wurtzite structure.

Apatite for the structure: Nucleation mechanisms of apatite–collagen composites at the atomistic level of detail have been investigated. Peculiar motifs of the apatite crystal structure were identified in which formation is promoted by ion association to the biomolecule. Apart from acting as a nucleation seed for ionic ordering, collagen triple helices (see picture; red bands) also induce orientation control in these motifs.

Cells use their dynamic actin network to control their mechanics and motility. These networks are made of branched actin filaments generated by the Arp2/3 complex. Here we study under which conditions the microscopic organization of branched actin networks builds up a sufficient stress to trigger sustained motility. In our experimental setup, dynamic actin networks or "gels" are grown on a hard bead in a controlled minimal protein system containing actin monomers, profilin, the Arp2/3 complex and capping protein.We vary protein concentrations and follow experimentally and through simulations the shape and mechanical properties of the actin gel growing around beads. Actin gel morphology is controlled by elementary steps including "primer" contact, growth of the network, entanglement, mechanical interaction and force production. We show that varying the biochemical orchestration of these steps can lead to the loss of network cohesion and the lack of effective force production. We propose a predictive phase diagram of actin gel fate as a function of protein concentrations. This work unveils how, in growing actin networks, a tight biochemical and physical coupling smoothens initial primer-caused heterogeneities and governs force buildup and cell motility.actin force generation | modeling | symmetry breaking I n eukaryotic cells, actin network formation and self-organization drive a variety of cellular processes including cell polarization, cell motility, and morphogenesis. Motile cells can change their speed and mechanical properties by controlling the biochemistry of network assembly. Polymerization of actin monomers into a branched network of filaments generates forces that are sufficient for lamellipodium formation and cell migration. In lamellipodia of crawling cells, filament nucleation and branching is triggered through the activation of the Arp2/3 complex on the side of a preexisting filament (the "primer") by nucleation promoting factors (NPFs) such as proteins from the WASP family (1-3). This process of branching off filaments repeats itself, leading to the auto-catalytic formation of a network of entangled filaments (4). However, it is not clear how the microscopic structure, in particular heterogeneities in actin network, impacts the mechanical properties during the production of force at the onset of motility.A major progress in understanding actin-based motility came with the introduction of reconstituted biomimetic systems inspired by motile pathogens such as Listeria monocytogenes (5, 6). These in vitro systems provided evidence for actin-driven force generation and paved the way to biophysical modeling. Over the last decade, several models have been proposed, each of them addressing phenomena on a different scale. One class of models describes actin networks at a macroscopic scale as a continuous elastic gel (6-9) that deforms due to the accumulation of an internal stress generated by actin polymerization. These macroscopic continuous approaches offer valuable insights into actindriven force generati...