Chemical ordering phenomena in nanostructured FePt: Monte Carlo simulations.

Free-surface-induced L10 chemical long-range ordering phenomena in a nanolayer, a nanowire and a cubic nanoparticle of FePt were studied by means of Monte Carlo simulations. The system was modeled with nearest-neighbor and next-nearest-neighbor interatomic pair interactions deduced from ab initio calculations. The generated samples, the dimensionality of which was determined by appropriate periodic boundary conditions imposed upon the generated supercells, were initially either perfectly ordered in the c-variant L10 superstructure ((001)-oriented monatomic planes), or completely disordered in the fcc crystalline structure. Vacancy-mediated creation of equilibrium atomic configurations was modelled by relaxing the systems at temperatures below the 'order-disorder' transition point using the Glauber algorithm implemented with the vacancy mechanism of atomic migration. The (100)-type-surface-induced heterogeneous nucleation of L10-order domains was observed and quantified by means of an original parameterization enabling selective determination of volume fractions of particular L10-variants. Due to the specific competition between the three kinds of (100)-type free surfaces, the initial c-L10 variant long-range order appeared to be the most stable in the cubic nanoparticle. The initially disordered samples were transformed by the creation of a specific L10 domain structure with a mosaic of particular L10-variant domains at the surfaces and almost homogeneous long-range order in the inner volume. The analysis of correlation effects revealed that chemical ordering was initiated at the free surfaces.

Oxygen diffusion in marine-derived tissue engineering scaffolds.

This paper addresses the computation of the effective diffusivity in new bioactive glass (BG) based tissue engineering scaffolds. High diffusivities facilitate the supply of oxygen and nutrients to grown tissue as well as the rapid disposal of toxic waste products. The present study addresses required novel types of bone tissue engineering BG scaffolds that are derived from natural marine sponges. Using the foam replication method, the scaffold geometry is defined by the porous structure of Spongia Agaricina and Spongia Lamella. These sponges present the advantage of attaining scaffolds with higher mechanical properties (2-4 MPa) due to a decrease in porosity (68-76%). The effective diffusivities of these structures are compared with that of conventional scaffolds based on polyurethane (PU) foam templates, characterised by high porosity (>90%) and lower mechanical properties (>0.05 MPa). Both the spatial and directional variations of diffusivity are investigated. Furthermore, the effect of scaffold decomposition due to immersion in simulated body fluid (SBF) on the diffusivity is addressed. Scaffolds based on natural marine sponges are characterised by lower oxygen diffusivity due to their lower porosity compared with the PU replica foams, which should enable the best oxygen supply to newly formed bone according the numerical results. The oxygen diffusivity of these new BG scaffolds increases over time as a consequence of the degradation in SBF.

A comparative study of oxygen diffusion in tissue engineering scaffolds.

Tissue engineering scaffolds are designed to support tissue self-healing within physiological environments by promoting the attachment, growth and differentiation of relevant cells. Newly formed tissue must be supplied with sufficient levels of oxygen to prevent necrosis. Oxygen diffusion is the major transport mechanism before vascularization is completed and oxygen is predominantly supplied via blood vessels. The present study compares different designs for scaffolds in the context of their oxygen diffusion ability. In all cases, oxygen diffusion is confined to the scaffold pores that are assumed to be completely occupied by newly formed tissue. The solid phase of the scaffolds acts as diffusion barrier that locally inhibits oxygen diffusion, i.e. no oxygen passes through the scaffold material. As a result, the oxygen diffusivity is determined by the scaffold porosity and pore architecture. Lattice Monte Carlo simulations are performed to compare the normalized oxygen diffusivities in scaffolds obtained by the foam replication (FR) method, robocasting and sol-gel foaming. Scaffolds made by the FR method were found to have the highest oxygen diffusivity due to their high porosity and interconnected pores. These structures enable the best oxygen supply for newly formed tissue among the scaffold types considered according to the present numerical predictions.