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.

Directional and temporal variation of the mechanical properties of robocast scaffold during resorption.

This paper addresses the mechanical behavior of robocast PCL-Bioglass(®) scaffolds. These structures can be used as 3rd generation implants in tissue engineering to support the regrowth of damaged tissue, in particular bone. After successful tissue regeneration the scaffolds slowly dissolve leaving no foreign material permanently inside the body. However, to avoid mechanical separation from surrounding tissue they must exhibit similar mechanical properties. The present study introduces a detailed numerical study focusing on the determination of effective mechanical material properties, their anisotropy, and mechanical degradation due to scaffold resorption. In order to accurately capture the complex scaffold geometry, micro-computed tomography scans of actual samples are conducted. The resulting three-dimensional data are directly converted into finite element calculation models. Numerical compressive tests of these unmodified models are repeated for three perpendicular directions to investigate mechanical anisotropy, after which the effect of scaffold degradation due to exposure to body fluid is simulated. To this end, two different resorption models, namely surface erosion and bulk degradation, are applied to the micro-computed tomography data. The modified geometry data are then converted into calculation models and numerical compression tests then allow the prediction of the mechanical properties of partially resorbed scaffolds.

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.

Structural stability and energy of a Pd2Ni nanofilm: ab initio calculations.

Recently, using molecular dynamics simulation in conjunction with an embedded-atom method potential, we have predicted Pd2Ni surface-sandwich ordering at the nanoscale. These findings open up a range of opportunities for the synthesis of new kinds of Pd-Ni nanostructures such as a five-layer Pd2Ni nanofilm from which a Pd2Ni nanotube might be fabricated. In this paper, we report on an ab initio spatial optimization and structural energy calculation of a five-layer Pd2Ni nanofilm, which are performed using plane-wave pseudopotential total energy calculations in the generalized gradient approximation of density functional theory. The results of the ab-initio calculations show that the five-layer Pd2Ni nanofilm is structurally stable and its energy is approximately 0.4 eV higher than the energy of a bulk crystal alloy having the same composition.

On the mechanical properties of PLC-bioactive glass scaffolds fabricated via BioExtrusion.

This paper addresses the mechanical characterization of polycaprolactone (PCL)-bioglass (FastOs®BG) composites and scaffolds intended for use in tissue engineering. Tissue engineering scaffolds support the self-healing mechanism of the human body and promote the regrowth of damaged tissue. These implants can dissolve after successful tissue regeneration minimising the immune reaction and the need for revision surgery. However, their mechanical properties should match surrounding tissue in order to avoid strain concentration and possible separation at the interface. Therefore, an extensive experimental testing programme of this advanced material using uni-axial compressive testing was conducted. Tests were performed at low strain rates corresponding to quasi-static loading conditions. The initial elastic gradient, plateau stress and densification strain were obtained. Tested specimens varied according to their average density and material composition. In total, four groups of solid and robocast porous PCL samples containing 0, 20, 30, and 35% bioglass, respectively were tested. The addition of bioglass was found to slightly decrease the initial elastic gradient and the plateau stress of the biomaterial scaffolds.