Abstract
Purified elastin was examined by means of dynamic and static mechanical testing, scanning and transmission electron microscopy, and a study performed of its swelling characteristics and density in various solvents. Purification of the material was effected by a combination of auto-claving and solvent extraction and the method investigated for suitability by gravimetric analysis, chemical testing and scanning electron microscopy. Investigation of previously reported mechanocalorimetric experiments on elastin (one of the major foundations of the globular theory of elastin structure) revealed a misinterpretation in the results and a re-interpretation could be made which fully supported the concept of elastin as an amorphous elastomer. This re-interpretation was backed by experimental results of swelling studies on the material. Dynamic investigation of the temperature dependence of the shear modulus using a free oscillating inverted torsion pendulum between 0°C and 70°C revealed water and ethanol/water (up to 50 vol % ethanol) saturated elastin to display modulus/temperature characteristics corresponding with those of a rubber at the higher temperature end of its glass transition region. Formamide saturated elastin displayed the linear positive shear modulus relationship with temperature associated with equilibrium elastic rubbers. The effect of varying saturation was also examined by the method. Decreasing saturation was found to increase the glass transition temperature of the water/elastin composite. The glass transition temperatures and secondary relaxations of fully saturated elastin/water and elastin/formamide were investigated by measurement of logarithmic decrement at below 0°C. Further studies of the mechanical states of the various elastin/liquid systems were carried out using apparatus based on the 'Intron' tensile testing machine. The stress responses to step and steady state strains were monitored over varying temperatures. 'Constant swelling' elastin solvents of 20:80 v/v ethanol/water, 30:70 v/v ethanediol/water, and 40:60 v/v glycerol/water were used as well as water and formamide alone. The results confirmed the viscous element of the transition region state of the elastin/water and elastin/solvent/water systems and the equilibrium rubber elasticity of the elastin/formamide system. Further steady-state strain tests on the elastin/water system at varying elongations yielded data which could be analysed to provide an assessment of the energy contribution to the retractive force of the material (fe/f). This ratio appeared to lie in the region 0.0 to 0.1, a value typical of amorphous rubbers and one which re-affirms the random coil concept of elastin structure. Structural examination was performed by scanning and transmission electron microscopy. Elastin from both elastin/water and elastin/formamide systems was used from both purified bovine ligamentum nuchae and porcine aorta. For high resolution transmission electron microscopy the technique of negative staining was employed to enhance contrast. Specimen preparation was by a method of fragmentation followed by air or freeze drying. Results indicated that previously observed fibrillar structures within the material were probably an artefact caused by the preparation process and not typical of elastin in its rubbery, in vivo condition. The overall conclusion of the work was that the structure of solvated elastin conforms to the classical random coil model and not to the globular, fibrillar or 'oiled-coil' concepts suggested in the literature.