Gel-cast glass-ceramic tissue scaffolds of controlled architecture produced via stereolithography of moulds.

Two glass-ceramic scaffolds with a simple cubic structure of 500 µm square ligaments and square channels of width 400 or 600 µm have been fabricated by gel-casting into moulds produced by stereolithography, followed by mould removal, polymer burnout and sintering. The scaffolds have crushing strengths of 41 ± 14 and 17 ± 5 Mpa, respectively. Using a method of assembling discrete slices of scaffold, we are able to study cell behaviour within a scaffold by disassembly. Both scaffold structures were seeded with primary human osteoblasts and these penetrate, adhere, spread and proliferate on the scaffold structure. The larger channel diameter scaffold shows a greater cell population (despite its smaller surface area) and more pronounced production of ECM components (collagen and mineralization) with increased time in culture. Studies of sectioned scaffolds show that cell density and ECM production decrease with depth and that the difference between the two scaffold architectures is maintained.

Lattice strains and load partitioning in bovine trabecular bone.

Microdamage and failure mechanisms have been well characterized in bovine trabecular bone. However, little is known about how elastic strains develop in the apatite crystals of the trabecular struts and their relationship with different deformation mechanisms. In this study, wide-angle high-energy synchrotron X-ray diffraction has been used to determine bulk elastic strains under in situ compression. Dehydrated bone is compared to hydrated bone in terms of their response to load. During compression, load is initially borne by trabeculae aligned parallel to loading direction with non-parallel trabeculae deforming by bending. Ineffective load partitioning is noted in dehydrated bone whereas hydrated bone behaves like a plastically yielding foam.

Nanoindentation of histological specimens: Mapping the elastic properties of soft tissues.

Although alterations in the gross mechanical properties of dynamic and compliant tissues have a major impact on human health and morbidity, there are no well-established techniques to characterize the micromechanical properties of tissues such as blood vessels and lungs. We have used nanoindentation to spatially map the micromechanical properties of 5-mum-thick sections of ferret aorta and vena cava and to relate these mechanical properties to the histological distribution of fluorescent elastic fibers. To decouple the effect of the glass substrate on our analysis of the nanoindentation data, we have used the extended Oliver and Pharr method. The elastic modulus of the aorta decreased progressively from 35 MPa in the adventitial (outermost) layer to 8 MPa at the intimal (innermost) layer. In contrast, the vena cava was relatively stiff, with an elastic modulus >30 MPa in both the extracellular matrix-rich adventitial and intimal regions of the vessel. The central, highly cellularized, medial layer of the vena cava, however, had an invariant elastic modulus of ~20 MPa. In extracellular matrix-rich regions of the tissue, the elastic modulus, as determined by nanoindentation, was inversely correlated with elastic fiber density. Thus, we show it is possible to distinguish and spatially resolve differences in the micromechanical properties of large arteries and veins, which are related to the tissue microstructure.

Elastic strains in antler trabecular bone determined by synchrotron X-ray diffraction.

The microstructure and associated mechanical properties of antler trabecular bone have been studied using a variety of techniques. The local trabeculae properties, as well as the three-dimensional architecture were characterized using nanoindentation and X-ray microtomography, respectively. An elastic modulus of 10.9+/-1.1 GPa is reported for dry bone, compared with 5.4+/-0.9 GPa for fully hydrated bone. Trabeculae thickness and separation were found to be comparable to those of bovine trabecular bone. Uniaxial compression conducted in situ during X-ray microtomography showed that antler can undergo significant architectural rearrangement, dominated by trabeculae bending and buckling, due to its low mineral content. High-energy synchrotron X-ray diffraction was used to measure elastic strains in the apatite crystals of the trabeculae, also under in situ uniaxial compression. During elastic loading, strain was found to be accommodated largely by trabeculae aligned parallel to the loading direction. Prior to the macroscopic yield point, internal strains increased as trabeculae deformed by bending, and load was also found to be redistributed to trabeculae aligned non-parallel to the loading direction. Significant bending of trabecular walls resulted in tensile strains developing in trabeculae aligned along the loading direction.

Characterization of the three-dimensional structure of a metallic foam during compressive deformation.

X-ray microtomography has been employed to collect three-dimensional images of aluminium closed-cell foam, enabling the internal structure to be characterized in three dimensions. An experimental technique and image analysis approach has been developed, and is described, in terms of the labelling of cells and the extraction of quantitative data such as the cell volume and cell compression. An in situ compressive deformation experiment has been performed on a single sample in order to illustrate the approach. The effect of the three-dimensional cellular structure on the mechanisms of deformation suggests not only the position of large cell volumes to be very important in the local concentration of stress, but also the distribution of cell volumes of immediate neighbours.

Fluorapatite-mullite glass sputter coated Ti6Al4V for biomedical applications.

A number of bioactive ceramics have been researched since the development of Bioglass in the 1970's. Fluorapatite mullite has been developed from the dental glass-ceramics used for more general hard tissue replacement. Being brittle in nature, glass-ceramics are currently used mainly as coatings. This paper shows that fluorapatite glass LG112 can be used as a sputtered glass coating on roughened surfaces of Ti6Al4V for possible future use for medical implants. An AFM was used to measure the roughness of the surface before and after coating to determine the change in the topography due to the coating process as this greatly affects cell attachment. The sputter coating partially filled in the artificially roughened surface, changing the prepared topography. Osteoblasts have been successfully grown on the surface of these coatings, showing biocompatibility with bone tissue and therefore potential use in hard tissue repair.

Full-field strain mapping by optical correlation of micrographs acquired during deformation.

Optical correlation is an emerging strain-mapping technique that allows full-field surface strain mapping by comparing the images of the same region before, during and after deformation. The fundamental aspects of optical correlation are presented, with emphasis on the applicability of the technique to the analysis of micrographs obtained during in situ deformation studies. Without considering specific algorithms, this paper discusses important practical issues such as accuracy and spatial resolution and how these are affected by image quality and other experimental difficulties. The technique was used to analyse image sequences obtained during in situ deformation tensile tests on two very different materials: antler bone and ferritic steel. As the technique does not require patterns or coatings to be applied on the surface of interest, the strain maps obtained could be used to relate strain heterogeneity to the underlying microstructure.

X-ray tomographic imaging of Ti/SiC composites.

In this paper, high-resolution tomographic synchrotron X-ray imaging is applied to study the occurrence and evolution of damage in Ti-6Al-4V/SCS6 SiC fibre composite materials. Three composite morphologies of increasing complexity have been studied, namely single fibre, single-ply and multi-ply composites. The single fibre composite was strained to full fibre fragmentation and the progressive introduction of damage monitored. For the single-ply composite, damage was introduced deliberately by laser drilling to establish the effect of damaged fibres on their neighbours, whereas for the multi-ply composite the morphology of a fibre bridging fatigue crack was studied. In addition to traditional mode I fibre fractures, subsequent fibre wedge cracks were observed presumably nucleating from damage introduced into the fibre surface by the first fracture event. In addition to these crack morphologies, spiral defects were observed for the single ply during failure. Finally, for the multi-ply composite, the matrix crack front showed a number of characteristic features, including advancement in fibre-free regions, crack bifurcation near fibres and different crack plane heights either side of a fibre.