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Background Finite element models have been widely used to predict biomechanical parameters of the cervical spine. Previous studies investigated the influence of position of rotational centers of prostheses on cervical biomechanical parameters after 1-level total disc replacement. The purpose of this study was to explore the effects of axial position of rotational centers of prostheses on cervical biomechanics after 2-level total disc replacement. Methods A validated finite element model of C3-C7 segments and 2 prostheses, including the rotational center located at the superior endplate (SE) and inferior endplate (IE), was developed. Four total disc replacement models were used: 1) IE inserted at C4-C5 disc space and IE inserted at C5-C6 disc space (IE-IE), 2) IE-SE, 3) SE-IE, and 4) SE-SE. All models were subjected to displacement control combined with a 50 N follower load to simulate flexion and extension motions in the sagittal plane. For each case, biomechanical parameters, including predicted moments, range of rotation at each level, facet joint stress, and von Mises stress on the ultra-high-molecular-weight polyethylene core of the prostheses, were calculated. Results The SE-IE model resulted in significantly lower stress at the cartilage level during extension and at the ultra-high-molecular-weight polyethylene cores when compared with the SE-SE construct and did not generate hypermotion at the C4-C5 level compared with the IE-SE and IE-IE constructs. Conclusions Based on the present analysis, the SE-IE construct is recommended for treating cervical disease at the C4-C6 level. This study may provide a useful model to inform clinical operations. [ABSTRACT FROM AUTHOR]

Twin boundaries (TBs) are ubiquitous interfaces with specific structure in metallic materials. They possess special interactions with dislocations and the fatigue cracking behaviors are distinctive from that of conventional high-angle grain boundaries. A profound understanding on the fatigue cracking mechanisms of various TBs achieved over past decades is reviewed here for the first time. The dislocation slips in the matrix and twin grains determined by grain orientations are closely related to the inclinations of coherent and incoherent TBs. The variable TB-dislocation interactions generate tunable fatigue cracking behaviors of TBs. Besides the grain orientations, the stacking fault energy (SFE) also alters the piling-up of dislocations at TBs by influencing dislocation dissociation. Both factors synergistically affect the fatigue cracking behaviors of TBs with a linear relationship between the difference in Schmid factors and SFE at the threshold of TB cracking. Moreover, the TBs produced by deformation twins in face-centered cubic metals are strong to resist fatigue cracking by promoting deformation homogeneity while those linked with deformation twins in hexagonal-close-packed or body-centered-cubic metals are preferential sites for fatigue cracking with strain localization and stress concentration. These fundamental knowledges of TB fatigue cracking provide important guidance in interfacial design to enhance materials fatigue performance. [ABSTRACT FROM AUTHOR]

Magnetic Ni nanoclusters were synthesized by electron beam deposition utilizing CVD graphene as a scaffold. The subsequent clusters were subjected to high vacuum (5−8 x10 −7 torr) annealing between 300 and 600 °C. The chemical stability, optical and morphological changes were characterized by X-ray photoemission microscopy, Raman spectroscopy, atomic force microscopy and magnetic measurement. Under ambient exposure, nickel nanoparticles were observed to be oxidized quickly, forming antiferromagnetic nickel oxide. Here, we report that the majority of the oxidized nickel is in non-stoichiometric form and can be reduced under high vacuum at temperature as low as 300 °C. Importantly, the resulting annealed clusters were relatively stable and no further oxidation was detectable after three weeks of air exposure at room temperature. [ABSTRACT FROM AUTHOR]

The generalized gradient approximation based on density functional theory is used to study the geometric structures, electronic properties and hydrogen storage abilities of alkali metal atom decorated C 6 M 6 (M = Li, Na). The most stable site for the Li and Na atoms is above the bridge site of the C–C bond of the C 6 ring. It is calculated that C 6 Li 6 and C 6 Na 6 can most adsorb 24 and 12 H 2 molecules respectively. The average adsorption energy for H 2 of C 6 (M– n H 2 ) 6 (M = Li, Na) are from 0.15 eV to 0.6 eV, which are between physical adsorption and chemical adsorption (0.1–0.8 eV), so they can realize the reversible adsorption of hydrogen. However, the average adsorption energy of H 2 and the hydrogen gravimetric density of C 6 (Li–4H 2 ) 6 are larger than those of C 6 (Na–2H 2 ) 6 . Therefore, C 6 Li 6 exhibits better hydrogen storage ability than that of C 6 Na 6 . The atomic orbital hybridization mechanism dominates the adsorption of H 2 by C 6 M 6 (M = Li, Na). However, the desorption of H 2 by C 6 (Li–4H 2 ) 6 can be realized easier than that by C 6 (Na–2H 2 ) 6 at the room temperature. The dynamic simulation, electronegativity, hardness, electrophilicity, and aromaticity of all the structures explore that C 6 M 6 (M = Li, Na) and their hydrogenation structures have considerable stabilities. [ABSTRACT FROM AUTHOR]