Structure and Nanomechanics of Dry and Hydrated Intermediate Filament Films and Fibers Produced from Hagfish Slime Fibers.

Intermediate filaments (IFs) are known for their extensibility, flexibility, toughness, and their ability to hydrate. Using keratin-like IFs obtained from slime fibers from the invertebrate Atlantic hagfish ( Myxine glutinosa), films were produced by drop-casting and coagulation on the surface of a MgCl2 buffer. Drop-casting produced self-supporting, smooth, and dense films rich in ß-sheets (61%), whereas coagulation formed thin, porous films with a nanorough surface and a lower ß-sheet content (51%). The films hydrated and swelled immediately when immersed in water and did not dissolve. X-ray diffraction showed that the ß-crystallites remained stable upon hydration, that swelling presumably happens in the amorphous C-terminal tail-domains of the IFs, and that high salt conditions caused a denser network mesh size, suggesting polyelectrolyte behavior. Hydration resulted in a roughly 1000-fold decrease in apparent Young's modulus from 109 to 106 Pa as revealed by atomic force microscopy nanoindentation. Nanoindentation-based power-law rheology and stress-relaxation measurements indicated viscoelasticity and a soft-solid hydrogel character for hydrated films, where roughly 80% of energy is elastically stored and 20% is dissipated. By pulling coagulation films from the buffer interface, macroscopic fibers with highly aligned IF ß-crystals similar to natural hagfish fibers were produced. We propose that viscoelasticity and strong hydrogen bonding interactions with the buffer interface are crucial for the production of such long biomimetic fibers with aligned ß-sheets. This study demonstrates that hagfish fiber IFs can be reconstituted into functional biomimetic materials that are stiff when dry and retain the ability to hydrate to become soft and viscoelastic when in water.

The effect of milk processing on the microstructure of the milk fat globule and rennet induced gel observed using confocal laser scanning microscopy.

Confocal laser scanning microscopy (CLSM) was successfully used to observe the effect of milk processing on the size and the morphology of the milk fat globule in raw milk, raw ultrafiltered milk, and standardized and pasteurized milk prepared for cheese manufacture (cheese-milk) and commercial pasteurized and homogenized milk. Fat globule size distributions for the milk preparations were analyzed using both image analysis and light scattering and both measurements produced similar data trends. Changes to the native milk fat globule membrane (MFGM) were tracked using a MFGM specific fluorescent stain that allowed MFGM proteins and adsorbed proteins to be differentiated on the fat globule surface. Sodium dodecyl sulfate polyacrylamide gel electrophoresis confirmed the identity of native MFGM proteins isolated from the surface of fat globules within raw, UF retentate, and cheese-milk preparations, whereas only casein was detected on the surface of fat globules in homogenized milk. The microstructure, porosity, and gel strength of the rennet induced gel made from raw milk and cheese-milk was also found to be comparable and significantly different to that made from homogenized milk. Our results highlight the potential use of CLSM as a tool to observe the structural details of the fat globule and associated membrane close to its native environment.

Forces between a rigid probe particle and a liquid interface. II. The general case.

The semianalytic theory developed previously (Chan, D. Y. C., Dagastine, R. R., and White, L. R., J. Colloid Interface Sci. 236, 141 (2001)) to predict the force curve of an AFM measurement at a liquid interface using a colloidal probe has been expanded to incorporate a general force law with both attractive and repulsive forces. Expressions for the gradient of the force curve are developed to calculate the point at which the probe particle on the cantilever will spontaneously jump in toward the liquid interface. The calculation of the jump instability is reduced to a straightforward embroidery of the simple algorithms presented in Chan et al. In a variety of sample calculations using force laws including van der Waals, electrostatic, and hydrophobic forces for both oil/water and bubble/water interfaces, we have duplicated the general behaviors observed in several AFM investigations at liquid interfaces. The behavior of the drop as a Hookean spring and the numerical difficulties of a full numerical calculation of F(deltaX) are also discussed.

Forces between a Rigid Probe Particle and a Liquid Interface.

The effect of disjoining pressure between a rigid spherical probe particle (attached to an AFM cantilever) and a liquid interface (e.g., oil/water or air/water) is treated in an analytic manner to describe the total force F exerted on the probe as a function of the distance X of the probe from the rigid substrate (AFM stage) on which the liquid interface resides. Two cases (i) a flat interface under gravity and (ii) a drop whose size is sufficiently small that gravity can be neglected have been examined. A simple numerical algorithm is given for computing F(X) (the AFM observable) from a given form for the disjoining pressure. Numerical results are displayed for electrostatic probe/interface interactions which reveal the linear compliance regime experimentally observed in AFM experiments on these systems. The slope of the linear compliance regime is shown to be a function of the properties of the interface (capillary length, particle radius, drop size, contact angle of drop on rigid substrate etc.). Copyright 2001 Academic Press.