Browsing by Author "Schaefer, Arne"

We report an atomic force microscopy (AFM) study on thick multilamellar stacks of approximately 10 mu m thickness (about 1500 stacked membranes) of 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine deposited on silicon wafers. These thick stacks could be stabilized for measurements under excess water or solution. From the force curves we determine the compressional modulus B and the rupture force F(r) of the bilayers in the gel (ripple), in the fluid phase, and in the range of critical swelling close to the main transition. We observe pronounced ripples on the top layer in the P(beta') (ripple) phase and find an increasing ripple period Lambda(r) when approaching the temperature of the main phase transition into the fluid L(alpha) phase at about 24 degrees C. Metastable ripples with 2 Lambda(r) are observed. Lambda(r) also increases with increasing osmotic pressure, i.e., for different concentrations of polyethylene glycol.

We have investigated the mechanical properties of spider dragline fibers of three Nephila species under varied relative humidity. Force maps have been collected by atomic force microscopy. The Young's modulus E was derived from the indentation curves of each pixel by the modified Hertz model. An average decrease in E by an order of magnitude was observed upon immersion of the fiber in water. Single fiber stretching experiments were carried out for comparison, and also showed a strong dependence on relative humidity. However, the absolute values of E are significantly higher than those obtained by indentation. The results of this work thus show that the elastic properties of spider silk are highly anisotropic, and that the silk softens significantly for both tensile and compressional strain (indentation) upon water uptake. In addition, the force maps indicate a surface structure on the sub-micron scale.