Biorheology

In this study, we measured the specific hydraulic conductivity (K) of Matrigel™ at 1% and 2% concentrations as a function of perfusion pressure (0 to 100 mmHg) and compared the results to predictions from two models: a fiber matrix model that predicted K of the gel based upon its composition, and a biphasic model that predicted changes in K caused by pressure induced compaction of the gels. The extent of gel compaction as a function of perfusion pressure was also assessed, allowing us to estimate the stiffness of the gels. As expected, 2% Matrigel™ had a lower K and a higher stiffness than did 1% Matrigel™. Measured values of K of both 1% and 2% Matrigel™ samples showed good agreement with the predictions of the fiber matrix model. Pressure-induced changes in K were better described by the biphasic model than a model in which uniform compression of the gel was assumed. We conclude that K of multi-component gels, such as Matrigel™can be well characterized by fiber matrix models, and that pressure-induced changes in K of such gels can be well characterized by biphasic models.

Sheets of normal human epidermal keratinocytes (NHEKs) were reconstituted in vitro on tensed but highly compliant, freestanding polydimethylsiloxane (PDMS) membranes, 5.0 mm in diameter and 10 μm thick. NHEK-PDMS composite diaphragm (CD) specimens were then subjected to cyclical axisymmetric inflation tests to investigate epithelial sheet rheology under conditions of physiologically severe deformations (∼50% nominal polar biaxial strains). Because the compliance of the specially formulated PDMS membrane was greater than that of the attached cell layer, the finite load-deformation behavior (mechanical response) of the living NHEK sheet was inferred from differences between the mechanical behavior of the CD specimen and the response of the underlying PDMS membrane measured prior to cell culture. In these composite diaphragm inflation (CDI) experiments, interconnected NHEKs exhibited rheological behaviors that were suggestive of a viscoelastic–plastic stress response. Remarkably, specimens returned to quiescent culture following a sequence of inflation tests recovered at least 80% of their original ability to store elastic strain energy, evidence of biological adaptation and recovery or restitutio ad integrum. Unlike methodologies that assay the morphological or biochemical response of cultured cells to an applied mechanostimulus, CDI experiments can be used to probe the load-bearing functions of desmosomes and adherens junctions within a living epithelial sheet, as well as to assess the rheological behaviors of the intermediate filament and microfilament networks that these cell–cell junctions serve to interconnect.

Shear stress can modulate endothelial cell function by regulating gene expression. We have previously demonstrated that low shear stress (4.2 dyn/cm²) induces the expression of interleukin-8 (IL-8) gene in endothelial cells. The present study was undertaken to further investigate both the effects of shear stress on IL-8 expression and the mechanisms controlling IL-8 mRNA up-regulation in human umbilical vein endothelial cells (HUVEC). We show that shear stress (from 2.23 to 19.29 dyn/cm²) induces the IL-8 expression at both the mRNA and protein levels by stimulating transcription. In order to determine the possible contribution of G protein, HUVEC were pretreated with an inhibitor of G-protein activation, GDPβS, which abrogated the low shear stress-induced IL-8 gene expression. Such gene expression was also partially inhibited by the tyrosine kinase inhibitor (tyrphostin-25) and in addition by EGTA, BATPA/AM (the intracellular Ca²⁺ chelator), Verapamil (a Ca²⁺ channel blocker), cAMP-dependent protein kinase inhibitor (KT5720) and phospholipase C inhibitor (neomycin). However, the cGMP-dependent protein kinase inhibitor, KT5823, had no effect on such expression. These findings therefore demonstrate the involvement of several signaling molecules, including tyrosine kinase, G protein, calcium, phospholipase C, and cAMP-dependent protein kinase, in the low shear stress-induced IL-8 gene expression.

Prior reports describing the effects of lanthanum (La³⁺) on red blood cells (RBC) have focused on the effects of this lanthanide on cell fusion or on membrane characteristics (e.g., ion movement across membrane, membrane protein aggregation); the present study explores its rheological and biophysical effects. Normal human RBC were exposed to La³⁺ levels up to 200 μM then tested for: (1) cellular deformability using a laser-based ektacytometer and an optical-based rheoscope; (2) membrane viscoelastic behavior via micropipettes; (3) surface charge via micro electrophoresis. La³⁺ concentrations of 12.5 to 200 μM caused dose-dependent decreases of deformability that were greatest at low stresses: these rheological changes were completely reversible upon removing La³⁺ from the media either by washing with La³⁺-free buffer or by suspending La³⁺-exposed cells in La³⁺-free media (i.e., viscous dextran solution). Both membrane shear elastic modulus and membrane surface viscosity were increased by 25–30% at 100 or 200 μM. As expected, La³⁺ decreased RBC electrophoretic mobility (EPM), with EPM inversely but not linearly associated with deformability; changes of EPM were also completely reversible. These results thus indicate novel aspects of RBC cellular and membrane rheological behavior yet raise questions regarding specific mechanisms responsible for La³⁺-induced alterations.

Understanding the mathematical relationships of volume blood flow and wall shear stress with respect to microvessel diameter is necessary for the study of vascular design. Here, for the first time, volume flow and wall shear stress were quantified from axial red blood cell velocity measurements in 104 conjunctival microvessels of 17 normal human volunteers. Measurements were taken with a slit lamp based imaging system from the post capillary side of the bulbar conjunctiva in microvessel diameters ranging from 4 to 24 micrometers. The variation of the velocity profile with diameter was taken into account by using a profile factor function. Volume flow ranged from 5 to 462 pl/s with a mean value of 102 pl/s and gave a second power law best fitting line (r=0.97) deviating significantly from the third power law relation with diameter. The estimated wall shear stress declined hyperbolically (r=0.93) from a maximum of 9.55 N/m² at the smallest capillaries, down to a minimum of 0.28 N/m² at the higher diameter post capillary venules. The mean wall shear stress value for all microvessels was 1.54 N/m².

The viscoelastic properties of Aeromonas (A) gum in water were investigated by using the Rheometric Scientific ARES controlled strain rheometer. An intrinsic viscosity of 8336 ml/g was obtained according to the Fuoss–Straus equation. The effect of salt concentration on intrinsic viscosity revealed that the A gum exists as semiflexible chain. Typical shear-thinning (pseudoplastic) behavior was observed at concentrations higher than 0.52%. The zero shear viscosity (η₀) increased with increasing polysaccharide concentration (c) showing a gradient of approximately 1.0, 2.9 and 4.8 in different concentration domains. The critical concentrations c^* and c^**, at which the transitions from a dilute solution of independently moving chains to semidilute and then concentrated domains occurred, were determined roughly to be 1.2% and 3.5%. The results from dynamic experiments revealed that the A gum solution shows characteristics of polymer solutions without any evidence of gel-like character. All the results from steady and dynamic tests suggest that the A gum is a non-gelling polysaccharide. The temperature dependence of apparent viscosity was described by Arrhenius equation and the flow activation energy was estimated to be 45.2 kJ/mol, which is independent on polymer concentration.

Hyaluronic acid (HA) is a polysaccharide widely used in biomedical applications, due to its elevated biocompatibility and the peculiar viscoelastic properties of its solutions. Although the viscoelastic behaviour of HA solutions has been extensively studied in the literature it has been often reported in the range of low frequency (1–100 Hz) and high salt concentration, whereas the main rheological peculiarities of this molecule are expected at high frequency (>100 Hz) and low salt concentration. In this work we studied the viscoelastic properties of low molecular weight HA (155 kDa) in wide range of concentrations (0.01–20 mg/ml) at low ionic strength and over an extended frequency range (0.1–1000 Hz) using both optical tweezers and conventional rheometry. Good agreement between the high frequency dynamic behaviour (optical tweezers) and the viscoelastic properties at low frequency (rheometry) was found. We also found that, in apparent contradiction with polyelectrolyte solution theory, HA solution behaves as liquid-like viscoelastic fluid (G″>G′) even at concentrations higher than the entanglement concentration where a weak-gel behavior should be expected.