Biomedical subjects

Nianhuan Chen

Publications and source records attributed to Nianhuan Chen.

9 recordsLinked to original sources

PubMed2006

AFM studies of environmental effects on nanomechanical properties and cellular structure of human hair.

Characterization of cellular structure and physical and mechanical properties of hair are essential to develop better cosmetic products and advance biological and cosmetic science. Although the morphology of the cellular structure of human hair has been traditionally investigated using scanning electron microscopy and transmission electron microscopy, these techniques provide limited capability to in situ study of the physical and mechanical properties of human hair in various environments. Atomic force microscopy (AFM) overcomes these problems and can be used for characterization in ambient conditions without requiring specific sample preparations and surface treatment. In this study, film thickness, adhesive forces and effective Young's modulus of various hair surfaces were measured at different environments (humidity and temperature) using force calibration plot technique with an AFM. Torsional resonance mode phase contrast images were also taken in order to characterize the morphology and cellular structure changes of human hair at different humidity. The correlation between the nanomechanical properties and the cellular structure of hair is discussed.

Hair↗

PubMed2005

Crystallization in thin liquid films induced by shear.

It is known that the thin-film structure of confined fluids and solids can be changed when the confining surfaces are sheared. Positional and orientational short- or long-range reordering can occur that often have no bulk counterparts. These multilayer, monolayer, or even sub-monolayer effects are important for understanding adhesion and friction processes, but they have proved difficult to measure, partly due to a lack of experimental techniques and partly to their apparent subtle dependence on many experimental parameters. Here we report the use of shear measurements and "optical absorption spectroscopy" in the surface forces apparatus to measure a shear-induced phase transition of an anisotropic (dye) molecule confined between two shearing mica surfaces in aqueous solution. Our studies on the shear-induced ordering and friction forces of highly anisotropic cyanine dye molecules in thin water films show only a weak effect of molecular anisotropy on shear-induced ordering, friction forces, and the onset of shear-induced crystallization, although dramatic changes do occur when the confined molecules ultimately crystallize.

Journal Article↗

PubMed2005

Static forces, structure and flow properties of complex fluids in highly confined geometries.

The Surface Forces Apparatus has been successfully used to measure the static and dynamic forces between surfaces across ultra-thin films of water and aqueous electrolyte solutions, and--more recently--polyelectrolyte-coated and articular cartilage surfaces in various solutions including hyaluronan, lubricin, and synovial fluid. The results give new insights into the lubricating action of biological lubricants such as synovial fluid and hyaluronan (a polysaccharide in synovial fluid), and biological surfaces such as phospholipid bilayers and cartilage surfaces. Contrary to earlier indications of long-range water-structuring at biological surfaces, more recent measurements clearly show that the viscosity of physiologically concentrated water (saline) is bulk-like beyond the first 1 or 2 layers from a single surface, and beyond 4-6 layers in thin films between two surfaces (the structure and forces may, however, be affected to larger distances). This implies that most structural, interaction force, and viscosity-related phenomena are determined--not only by the properties of the solvent (water)per se--but also by the surfaces and the water, ions, solutes, and macromolecules (proteins, polymers) exposed or adsorbed at the surfaces and, to a lesser degree, dissolved in the solvent. However, sometimes it is difficult to make a clear differentiation, e.g., one could consider hydration or surface-bound 'structured' water as part of the surface or as part of the intervening water between the two surfaces.

Animals↗

PubMed2004

Lubrication and wear properties of grafted polyelectrolytes, hyaluronan and hylan, measured in the surface forces apparatus.

Hyaluronan is believed to have an important function in the boundary biolubrication of articular cartilage. Using a Surface Forces Apparatus, we tested the tribological properties of surface bound, rather than "free" hyaluronan. The grafting process of the polyelectrolyte included either a biological route via an HA-binding protein or a chemical reaction to covalently bind the polymer to a lipid bilayer coated surface. In another reaction, we constructed a surface with covalently grafted hylan (crosslinked hyaluronan). We studied the normal and shear forces between these surfaces. None of the systems demonstrated comparable lubrication to that found between cartilage surfaces except at very low loads. Both grafted hyaluronan and hylan generated coefficients of friction between 0.15 and 0.3. Thus, the polysaccharide, which is a constituent of the lamina splendens (outermost cartilage layer), is not expected to be the responsible molecule for the great lubricity of cartilage; however, it may contribute to the load bearing and wear protection of these surfaces. This was concluded from the results with hylan, where a thin gel layer was sufficient to shield the underlying surfaces from damage even at applied pressures of over 200 atmospheres during shear. Our study shows that a low coefficient of friction is not a requirement for, or necessarily a measure of, wear protection.

Aluminum Silicates↗

PubMed2004

Large deformations during the coalescence of fluid interfaces.

Surface forces and shape changes were simultaneously measured during the approach and coalescence of two liquid-liquid and liquid-air interfaces. Large normal and lateral deformations were observed that are nevertheless consistent with a simple theoretical analysis of the long-range effects of short-range attractive van der Waals forces. The results imply that two fluidlike structures such as liquid droplets and soft biological cells can sense each other at much larger separations than previously assumed based on criteria taken from the interactions of hard particles.

Journal Article↗

PubMed2004

Correlation of AFM and SFA measurements concerning the stability of supported lipid bilayers.

Phospholipid bilayers were studied by means of atomic force microscopy (AFM) and a surface force apparatus (SFA). The stability of the supported bilayers was described by the amount of irregularities in the topography of the membrane by means of AFM and by the occurrence of hemifusion in the SFA, which is an indicator of defective bilayers. The bilayers, composed of lipids having the same headgroup but different chain lengths in the two leaflets, were prepared by Langmuir-Blodgett deposition and transferred at different surface pressures. The topography of the supported bilayers in aqueous solution, as imaged by AFM, revealed an increasing number of defects in the supported lipid membranes with decreased deposition pressure of the outer lipid layer. These defects, which appeared in the form of monolayer and bilayer (self-assembled) thick holes within the membrane, were energetically favorable over an evenly depleted bilayer. We found that the quantity of these defects (holes of </=0.5 micro m diameter and covering up to 30% of the surface area) correlated well with the stability of the bilayers as measured by SFA, a truly complementary instrument.

Elasticity↗

PubMed2003

Thickness and refractive index measurements using multiple beam interference fringes (FECO).

We report on the use of optical interferometry employing fringes of equal chromatic order (FECO) in a surface force apparatus (SFA) to determine film thicknesses and refractive indices of confined media for a wide range of separations. In particular, we show how to calculate the surface separation (film thickness) based on two fringes whose contact position was not measured. We discuss the measurement accuracy, and though the theoretical accuracy is 1 A for all separations, we show that in practice, for large separations, it is very hard to get to this accuracy.

Journal Article↗

PubMed2002

Thin film rheology and lubricity of hyaluronic acid solutions at a normal physiological concentration.

Using a surface forces apparatus to measure forces, and optical (multiple beam) interferometry to measure surface shapes and separations (to +/-1 A), the normal, viscous, and shear (lubrication) forces between smooth mica surfaces in aqueous hyaluronic acid (HA) solutions were measured. The experimental conditions of loading pressures, pH, and HA concentration were set to closely correspond to physiological human knee-joint conditions. From the force and optical (refractive index) measurements, it was concluded that, like other negatively charged polyelectrolytes, HA does not naturally adsorb on the mica surface which is hydrophilic and weakly negatively charged at physiological conditions: the polymer solution exhibits the bulk viscosity (22.5 +/- 1.5 cP) for films thicker than about 0.4 miccrom of the polymer, whereas for thinner films, the viscosity decreases monotonically toward the value of the pure electrolyte solution (1 cP) as HA is extruded from between the surfaces. This is indicative of a repulsive "depletion" interaction of HA with each mica surface and to a weakly attractive polymer-mediated force between the two surfaces. Thus, free HA in synovial fluid is not expected to act as a good "boundary lubricant." Relaxation measurements on approaching and receding surfaces in HA solutions were also performed, and it is shown that the presence of HA in the bulk solution can improve "hydrodynamic" modes of lubrication, for example, by assuaging the compression stroke. The study includes information that is beneficial to researchers working with biomaterials viscosupplementation devices.

Adsorption↗

PubMed2002

Adhesion and friction mechanisms of polymer-on-polymer surfaces.

The adhesion and friction of smooth polymer surfaces were studied below the glass transition temperature by use of a surface forces apparatus. The friction force of a crosslinked polymer was orders of magnitude less than that of an uncrosslinked polymer. In contrast, after chain scission of the outermost layers, the adhesion hysteresis and friction forces increase substantially. These results show that polymer-polymer adhesion hysteresis and friction depend on the dynamic rearrangement of the outermost polymer segments at shearing interfaces, and that both increase as a transition is made from crosslinked surfaces to surfaces with long chains to surfaces with quasi-free ends. The results suggest new ways for manipulating the adhesion and friction of polymer surfaces by adjusting the state of the surface chains.

Journal Article↗