Planar defect nucleation and annihilation mechanisms in nanocontact plasticity of metal surfaces.

The incipient contact plasticity of metallic surfaces involves nucleation of crystalline defects. The present molecular dynamics simulations and nanoindentation experiments demonstrate that the current notion of nanocontact plasticity in fcc metals does not apply to high-strength bcc metals. We show that nanocontact plasticity in Ta-a model bcc metal-is triggered by thermal and loading-rate dependent (dynamic) nucleation of planar defects such as twins and unique {011} stacking fault bands. Nucleation of different planar defects depending on surface orientation leads to distinct signatures (pop ins) in the nanoindentation curves. Nanoscale plasticity is then ruled by an outstanding dynamical mechanism governing twin annihilation and subsequent emission of linear defects (full dislocations). While this investigation concerns Ta crystals, the present are landmark findings for other model bcc metals.

Micromechanics and ultrastructure of pyrolysed softwood cell walls.

Pyrolytic conversion causes severe changes in the microstructure of the wood cell wall. Pine wood pyrolysed up to 325 °C was investigated by transmission electron microscopy, atomic force microscopy and nanoindentation measurements to monitor changes in structure and mechanical properties. Latewood cell walls were tested in the axial, radial and tangential directions at different temperatures of pyrolysis. A strong anisotropy of elastic properties in the native cell wall was found. Loss of the hierarchical structure of the cell wall due to pyrolysis resulted in elastic isotropy at 300 °C. The development of the mechanical properties with increasing temperature can be explained by alterations in the structure and it was found that the elastic properties were clearly related to length and orientation of the microfibrils.

Cell-based resurfacing of large cartilage defects: long-term evaluation of grafts from autologous transgene-activated periosteal cells in a porcine model of osteoarthritis.

OBJECTIVE: To investigate the potential of transgene-activated periosteal cells for permanently resurfacing large partial-thickness cartilage defects. METHODS: In miniature pigs, autologous periosteal cells stimulated ex vivo by bone morphogenetic protein 2 gene transfer, using liposomes or a combination of adeno-associated virus (AAV) and adenovirus (Ad) vectors, were applied on a bioresorbable scaffold to chondral lesions comprising the entire medial half of the patella. The resulting repair tissue was assessed, 6 and 26 weeks after transplantation, by histochemical and immunohistochemical methods. The biomechanical properties of the repair tissue were characterized by nanoindentation measurements. Implants of unstimulated cells and untreated lesions served as controls. RESULTS: All grafts showed satisfactory integration into the preexisting cartilage. Six weeks after transplantation, AAV/Ad-stimulated periosteal cells had adopted a chondrocyte-like phenotype in all layers; the newly formed matrix was rich in proteoglycans and type II collagen, and its contact stiffness was close to that of healthy hyaline cartilage. Unstimulated periosteal cells and cells activated by liposomal gene transfer formed only fibrocartilaginous repair tissue with minor contact stiffness. However, within 6 months following transplantation, the AAV/Ad-stimulated cells in the superficial zone tended to dedifferentiate, as indicated by a switch from type II to type I collagen synthesis and reduced contact stiffness. In deeper zones, these cells retained their chondrocytic phenotype, coinciding with positive staining for type II collagen in the matrix. CONCLUSION: Large partial-thickness cartilage defects can be resurfaced efficiently with hyaline-like cartilage formed by transgene-activated periosteal cells. The long-term stability of the cartilage seems to depend on physicobiochemical factors that are active only in deeper zones of the cartilaginous tissue.

Five-coordinate complexes [FeX(depe)(2)]BPh(4), X = Cl, Br: electronic structure and spin-forbidden reaction with N(2).

The bonding of N(2) to the five-coordinate complexes [FeX(depe)(2)](+), X = Cl (1a) and Br (1b), has been investigated with the help of X-ray crystallography, spectroscopy, and quantum-chemical calculations. Complexes 1a and 1b are found to have an XP(4) coordination that is intermediate between square-pyramidal and trigonal-bipyramidal. Mössbauer and optical absorption spectroscopy coupled with angular overlap model (AOM) calculations reveal that 1a and 1b have (3)B(1) ground states deriving from a (xz)(1)(z(2))(1) configuration. The zero-field splitting for this state is found to be 30-35 cm(-1). In contrast, the analogous dinitrogen complexes [FeX(N(2))(depe)(2)](+), X = Cl (2a) and Br (2b), characterized earlier are low-spin (S = 0; Wiesler, B. E.; Lehnert, N.; Tuczek, F.; Neuhausen, J.; Tremel, W. Angew. Chem, Int. Ed. 1998, 37, 815-817). N(2) bonding and release in these systems are thus spin-forbidden. It is shown by density functional theory (DFT) calculations of the chloro complex that the crossing from the singlet state (ground state of 2a) to the triplet state (ground state of 1a) along the Fe-N coordinate occurs at r(C) = 2.4 A. Importantly, this intersystem crossing lowers the enthalpy calculated for N(2) release by 10-18 kcal/mol. The free reaction enthalpy Delta G degrees for this process is calculated to be 4.7 kcal/mol, which explains the thermal instability of N(2) complex 2a with respect to the loss of N(2). The differences in reactivity of analogous trans hydrido systems are discussed.