Biorheology

We present a theoretical analysis of fluid flow and particle interactions in the cone‐plate viscometer under conditions typically applied in biological studies. The analysis demonstrates that at higher shear rates, besides linear primary flow in the rotational direction, prominent non‐linear secondary flow causes additional fluid circulation in the radial direction. Two parameters, the cone angle and Reynolds number, characterize flow in the viscometer over all ranges of shear rate. Our results indicate that secondary flow causes positional variations in: (i) the velocity gradient, (ii) the direction and magnitude of the wall shear stress at the plate surface, (iii) inter‐particle collision frequency, (iv) magnitude and periodicity of normal and shear forces applied during particle–particle interactions, and (v) inter‐particle attachment times. Thus, secondary flow may significantly influence cellular aggregation, platelet activation and endothelial cell mechanotransduction measurements. Besides cone‐plate viscometers, this analysis methodology can also be extended to other experimental systems with complex non‐linear flows.

A theoretical model is developed to predict the elastic properties of very soft tissues such as glands, tumors and brain. Tissues are represented as regular arrays of polyhedral (cubic or tetrakaidecahedral) cells, surrounded by extracellular spaces of uniform width. Cells are assumed to be incompressible, with very low resistance to shear deformation. Tissue shear rigidity is assumed to result mainly from the extracellular matrix, which is treated as a compressible elastic mesh of interconnected fibers. Small‐strain elastic properties of tissue are predicted using a finite‐element method and an analytical method. The model can be used to estimate the compressibility of a very soft tissue based on its Young's modulus and extracellular volume fraction.

The thixotropic (shear‐thinning) effect of the synovial fluid in squeeze‐film lubrication of the human hip joint is evaluated, taking into account filtration of the squeezed synovial film by biphasic articular cartilage. A porous, homogeneous, elastic cartilage matrix filled with the interstitial ideal fluid, with the intact superficial zone (of lower permeability and stiffness in compression) already disrupted or worn away, models an early stage of arthritis. Due to a high viscosity of the normal synovial fluid at very low shear rates, the squeezed synovial film at a fixed time after the application of a steady load is found to be much thicker in a small central part of the lubricated contact area. In the remaining part, the film is thin as it corresponds to the Newtonian fluid with the same high‐shear‐rate viscosity. Filtration is lower for the normal cartilage with the intact superficial zone due to its lower permeability and compression stiffness. But even in the fictitious case of zero filtration, calculations show that the effect of thixotropy on the increase of the minimum synovial film thickness would manifest itself as late as after several tens of seconds since the physiologic load application. At that time, this thickness would be as low as about 0.3 μm. It follows that thixotropy of the normal synovial fluid (and so much more of the inflammatory fluid) is irrelevant in squeeze‐film lubrication of both the normal and arthritic human hip joints.

In this paper, some experimental measurements of the behaviour of bovine brain tissue under large shear strains in vitro are reported, and a constitutive model which is consistent with the data is developed. It was determined that brain tissue is not strain‐time separable, showing slower relaxation at higher strains, and that the stresses in shear are not linear with increasing shear strain. The new constitutive model is a differential model, including both an “elastic” term, of the Mooney type and a nonlinear viscoelastic term. The latter allows for the change in relaxation behaviour with strain, by modifying an upper convected multimode Maxwell model with a damping function. The model shows good agreement with the experimental shear results and could be used to describe other types of data.

The effects of shear stress on interleukin 8 (IL‐8) production by human umbilical vein endothelial cells (HUVEC) were studied by subjecting the HUVEC to a steady flow laminar shear stress of up to 0.7 N/m² in a parallel plate flow chamber. Shear stress decreased IL‐8 mRNA expression in a dose and time‐dependent fashion. High glucose concentrations increased IL‐8 mRNA levels in a MAPK‐p38‐dependent manner, which was suppressed by shear stress. Measurement of IL‐8 protein in HUVEC culture media by ELISA demonstrated that IL‐8 secretion was also increased by high glucose and suppressed by shear stress. These results suggest that the anti‐atherogenic effect of shear stress arises partly from the suppression of the production of IL‐8 which has been shown to trigger the adhesion of monocytes to a vascular endothelium and also acts as a mitogen and chemoattractant for vascular smooth muscle cells.