Collagen intrafibrillar mineralization as a result of the balance between osmotic equilibrium and electroneutrality.

Mineralization of fibrillar collagen with biomimetic process-directing agents has enabled scientists to gain insight into the potential mechanisms involved in intrafibrillar mineralization. Here, by using polycation- and polyanion-directed intrafibrillar mineralization, we challenge the popular paradigm that electrostatic attraction is solely responsible for polyelectrolyte-directed intrafibrillar mineralization. As there is no difference when a polycationic or a polyanionic electrolyte is used to direct collagen mineralization, we argue that additional types of long-range non-electrostatic interaction are responsible for intrafibrillar mineralization. Molecular dynamics simulations of collagen structures in the presence of extrafibrillar polyelectrolytes show that the outward movement of ions and intrafibrillar water through the collagen surface occurs irrespective of the charges of polyelectrolytes, resulting in the experimentally verifiable contraction of the collagen structures. The need to balance electroneutrality and osmotic equilibrium simultaneously to establish Gibbs-Donnan equilibrium in a polyelectrolyte-directed mineralization system establishes a new model for collagen intrafibrillar mineralization that supplements existing collagen mineralization mechanisms.

Carbon dioxide and methane transport in DDR zeolite: insights from molecular simulations into carbon dioxide separations in small pore zeolites.

The silica zeolite DDR is a strong candidate for separations of CO(2)/CH(4) because of the narrow windows that control molecular transport inside the material's pores. We have used molecular simulations to describe diffusion of CO(2) and CH(4) inside DDR pores. Our simulations introduce a new force-field for this system that for the first time gives results that are consistent with experimental measurements of single-component adsorption and diffusion. Diffusivities obtained from previous simulations greatly overestimated the transport rates of CH(4) and, to a lesser extent, CO(2). Because CH(4) diffuses extremely slowly in DDR, we applied a transition state theory-based kinetic Monte Carlo scheme to accurately describe this diffusion. The most important observation from our calculations is that the characteristics of CO(2)/CH(4) diffusion in DDR are very different from the usual situation in nanoporous materials, where the presence of a slowly diffusing species retards transport rates of a more rapidly diffusing species. In DDR, we show that CO(2) diffusion rates are only weakly affected by the presence of CH(4), despite the very slow diffusion of the latter molecules. The physical origins of this unusual behavior are explained by analyzing the adsorption sites and diffusion mechanism for each species. Our finding suggests DDR membranes are favorable for CO(2)/CH(4) separations and that similar properties may exist for other 8MR zeolites.

Investigation of ethanol infiltration into demineralized dentin collagen fibrils using molecular dynamics simulations.

The purpose of this study is to investigate the interaction of neat ethanol with bound and non-bound water in completely demineralized dentin that is fully hydrated, using molecular dynamics (MD) simulation method. The key to creating ideal resin-dentin bonds is the removal of residual free water layers and its replacement by ethanol solvent in which resin monomers are soluble, using the ethanol wet-bonding technique. The test null hypotheses were that ethanol cannot remove any collagen-bound water, and that ethanol cannot infiltrate into the spacing between collagen triple helix due to narrow interlayer spacing. Collagen fibrillar structures of overlap and gap regions were constructed by aligning the collagen triple helix of infinite length in hexagonal packing. Three layers of the water molecules were specified as the layers of 0.15-0.22nm, 0.22-0.43nm and 0.43-0.63nm from collagen atoms by investigating the water distribution surrounding collagen molecules. Our simulation results show that ethanol molecules infiltrated into the intermolecular spacing in the gap region, which increased due to the lateral shrinkage of the collagen structures in contact with ethanol solution, while there was no ethanol infiltration observed in the overlap region. Infiltrated ethanol molecules in the gap region removed residual water molecules via modifying mostly the third water layer (50% decrease), which would be considered as a loosely-bound water layer. The first and second hydration layers, which would be considered as tightly bound water layers, were not removed by the ethanol molecules, thus maintaining the helical structures of the collagen molecules.

Fabrication of flexible carbon nanotube field emitter arrays by direct microwave irradiation on organic polymer substrate.

Carbon nanotubes (CNTs) were directly synthesized on flexible polymer substrates without damage of polymer by microwave irradiation. Cobalt was used as the catalysts, and the synthesis was done in the atmospheric pressure with an acetylene carbon source. Only 5 s was required for the synthesis of well-graphitized CNTs. Field emission measurements revealed that this flexible CNT field emitter array has a great potential for the flexible field emission displays (FEDs).