The high degree of modification through chemical substitution afforded by the perovskite crystal structure and its related counterparts allows a systematic study of structure-property relationships critical to understand the wide variety of exotic phenomena observed in these materials where the spin, charge, orbital, and lattice degrees of freedom are highly correlated. From the multiple phenomena observed in these materials, which includes multiferroicity, catalytic activity, and high temperature superconductivity, this study is concerned with a material that displays colossal magnetoresistance (CMR), La₂₋₂xSr₁₊₂xMn₂O₇; this is a naturally bilayered manganite that exhibits CMR at a paramagnetic to ferromagnetic transition that coincides with an insulator to metal transition. The strong correlation between different degrees of freedom in the material leads to considerable variation in its magnetic properties due to doping even in the small range studied of 0.32 [less than or equal to] x [less than or equal to] 0.4, where the easy axis of magnetization changes from the c-axis to the ab plane. Magnetic force microscopy (MFM) was used for this part of the work, to visualize the local variation of the out of plane (c-axis) magnetization or magnetic microstructure of La₂₋₂xSr₁₊₂xMn₂O₇ for 0.32 [less than or equal to] x [less than or equal to] 0.4 at the exposed ab surface and its evolution due to an applied magnetic field at 4 K. For the x = 0.32 composition, which is close to the out of plane to in plane magnetization transition, a strong preferred magnetization direction within the ab plane or magnetocrystalline anisotropy was observed. The stray magnetic field of the MFM tip perturbs the magnetic microstructure of low coercivity materials like diluted magnetic semiconductors, making it unsuitable for the study of such materials. For this reason, as part of this project a scanning Hall probe microscope (SHPM), a magnetic imaging technique complementary to MFM that uses a Hall sensor that provides a magnetically non-invasive calibrated measurement of the stray fields at the surface of a sample with good resolution (~ 1 [micrometer]), was designed. The construction of a compact cryogenic variable-temperature (77 - 300 K) SHPM, highlighting its features, is described. / text

Low velocity impact behaviour of bi-layered laminates of acrylic and polycarbonate was investigated using a combination of drop tower impact experiments and explicit finite element analysis in LS-DYNA. Material characterisation tests were conducted in tension and in compression to obtain material properties for input to the material model in the numerical analysis. Quasistatic plate bending tests were conducted at different loading rates to compare the quasistatic response of the materials to the impact behaviour. Impact tests on circular plates of monolithic acrylic and polycarbonate were carried out using an instrumented drop weight impact tester. The impact force histories were recorded and a multiparameter approach was used to determine critical energy. Acrylic exhibited radial cracking, spalling and pene- tration while polycarbonate underwent large deformation and failed by dishing and plugging. The damage caused by impact in the bilayered laminate included partial or full delamination at the interface and radial cracks in the acrylic layer. The low velocity impact responses were simulated using 8-noded solid elements in LS- DYNA. A node-splitting technique based on maximum tensile stress failure criterion and an erosion approach based on maximum principal stress criteria was used to model the failure of acrylic. A material model that takes into account the asym- metric behaviour in tension and compression was investigated. The delamination between the acrylic and polycarbonate plate was modelled by a tiebreak contact with a shear strength based failure. The results of the finite element simulations are in good agreement with the experimental data.

Skin is an organ with a complex structure which plays a crucial role in thebody’s defence against external threats and in maintaining major homeostatic functions. The need for in vitro models that mimic the in vivo milieu is therefore high and relevant with various applications including, among others, penetration, absorption, and toxicity studies. In this context, the choice of the biomaterial that will provide a 3D scaffold to the cultured cells is defining the model’s success. The FN-4RepCT silk is here suggested as a potent biomaterial for skin tissue engineering applications. This recombinantly produced spider silk protein (FN-4RepCT), which can self-assemble into fibrils, creates a robust and elastic matrice with high bioactivity, due to its functionalization with the fibronectin derived RGD-containing peptide. Hence it overcomes the drawbacks of other available biomaterials either synthetic or based on animal derived proteins. Additionally, the FN-4RepCT silk protein can be cast in various 3D formats, two of which are utilized within this project. We herein present a bilayered skin tissue equivalent supported by the FN-4RepCT silk. This is constructed by the combination of a foam format, integrated with dermal fibroblasts and endothelial cells, and a membrane format supporting epidermal keratinocytes. As a result, a vascularized dermal layer that contains ECM components (Collagen I, Collagen III, and Elastin) is constructed and attached to an epidermal layer of differentiated keratinocytes.The protocol presented in this project offers a successful method of evenly integrating cells in the FN-4RepCT silk scaffold, while preserving their ability to resume some of their major in vivo functions like proliferation, ECM secretion, construction of vascular networks, and differentiation. The obtained results were evaluated with immunofluorescence stainings of various markers of interest and further analysed, when necessary, with image processing tools. The results that ensued from the herein presented protocol strongly suggest that the FN-4RepCT silk is a promising biomaterial for skin tissue engineering applications.