Background: The venous response to elevated blood pressure (BP) is of major importance because it is closely related to the etiology of venous diseases and the competency of vein grafts. In vitro culture experiments may provide useful information on the function of vein grafts because it is easier to separate mechanical and hemodynamic effects from other systemic influences compared to in vivo experiments.
Objective: To study the effects of BP elevation on wall dimensions and mechanical properties of in vitro cultured veins.
Methods: Rabbit femoral veins were cultured in vitro under internal pressures of 1 to 50 mmHg for 1 week, and their wall dimensions, biomechanical properties, and histology were determined.
Results: No significant differences were observed in internal vein diameter and wall thickness among vessels cultured at 10-50 mmHg compared to non-cultured control vessels. For an internal pressure of 10 mmHg applied to vessels during culture (equivalent to in vivo working BP), wall circumferential stress was maintained within control levels. There were no significant effects of pressure on basal tone and contractility of vascular smooth muscle and vascular compliance.
Conclusions: The in vitro results were essentially similar to those obtained from previous in vivo animal experiments, indicating that in vitro tissue culture techniques are applicable to studies of venous remodeling.
Background: Previous numerical modeling studies on red blood cell (RBC) aggregation have elucidated the inverse relationship between shear rate and RBC aggregation under steady flow. However, information on the cyclic variation in RBC aggregation under pulsatile flow remains lacking.
Objective: RBC aggregation was simulated to investigate the complex interrelationships among the parameters of RBC motion under pulsatile flow.
Methods: A two-dimensional particle model was used to simulate RBC motion driven by hydrodynamic, aggregation, and elastic forces in a sinusoidal pulsatile flow field. The kinetics of RBCs motion was simulated on the basis of the depletion model.
Results: The simulation results corresponded with previously obtained experimental results for the formation and destruction of RBC aggregates with a parabolic radial distribution during a pulsatile cycle. In addition, the results demonstrated that the cyclic variation in the mean aggregate size of RBCs increased as velocity amplitude increased from 1 cm/s to 3 cm/s under a mean steady flow of 2 cm/s, as mean steady flow velocity decreased from 6 cm/s to 2 cm/s under a velocity amplitude of 1.5 cm/s, and as stroke rate decreased from 180 beats per minute (bpm) to 60 bpm.
Conclusions: The present simulation results verified previous experimental results and improved the current understanding of the complex spatiotemporal changes experienced by RBC aggregates during a sinusoidal pulsatile cycle.
Background: It is generally unknown if taste sensitivity is dependent upon saliva viscosity. The rheological properties of saliva result from many factors and it has been shown to behave as a non-Newtonian fluid whose viscosity decreases with increasing shear rate. Taste sensitivity may be quantitatively assayed by electro-gustometry.
Objectives: The aim of this work was to compare saliva rheological properties, obtained with a rotary-oscillating rheometer, to quantitative measures of taste sensitivity, using electro-gustometry.
Methods: Saliva samples were taken from 27 healthy non-smoking donors - 7 men and 20 women aged 18-65 years (mean age - 37 years). After thresholds of taste sensation were measured, the saliva samples were taken and characterized in terms of their rheological properties and pH. Saliva viscosity was measured in the order of decreasing shear rate in the range 100-0.01 s-1. Viscoelastic properties were examined under constant frequency oscillations (with f = 0.5 Hz) and with decreasing shear effective amplitude γeff'.
Results and conclusions: Saliva viscosity was found to decrease with increasing shear rate and varied with time. Analysis of the dependence of the viscosity values of saliva and components of complex viscosity did not show a significant correlation with taste sensation. A dependency of taste sensation on pH could not be discerned due to the narrow range of naturally occurring pH.
Background: Prediction of thrombus formation at intact arterial walls under low shear flow conditions is clinically important particularly for better prognoses of embolisation in cerebral aneurysms. Although a new mathematical model for this purpose is necessary, little quantitative information has been known about platelet adhesion to intact endothelial cells.
Objective: The objective of this study is to measure the number of platelets adhering to intact endothelial cells with a focus upon the influence of the shear rate.
Methods: Endothelial cells disseminated in μ-slides were exposed to swine whole blood at different shear rates. Adenosine diphosphate (ADP) was used as an agonist. Adherent platelets were counted by means of scanning electron microscopy.
Results: At an ADP concentration of 1 µM, 20.8 ± 3.1 platelets per 900 µm2 were observed after 30-minute perfusion at a shear rate of 0.8 s-1 whereas only 3.0 ± 1.4 per 900 µm2 at 16.8 s-1.
Conclusions: The number of adherent platelets is determined by a balance between the shear and the degree of stimulation by the agonist. At an ADP concentration of 1 µM, a limit to the shear rate at which platelets can adhere to intact endothelial cells is considered to be slightly higher than 16.8 s-1.
Background: Cartilage surface contact geometry influences the deformational behavior and stress distribution throughout the extracellular matrix (ECM) under load.
Objective: To test the correlation between the mechanical and cellular response of articular cartilage when loaded with two different-sized spherical indenters under dynamic reciprocating sliding motion.
Methods: Articular cartilage explants were subjected to a reciprocating sliding load using a 17.6 mm or 30.2 mm spherical ball for 2000 cycles at 10 mm/s and 4 kg axial load. Deformation of the cartilage was recorded and contact parameters were calculated according to Hertzian theory. After mechanical loading cartilage samples were collected and analyzed for ECM collagen damage, gene regulation and proteoglycan (PG) loss.
Results: Significantly higher ECM deformation and strain and lower dynamic effective modulus were found for explants loaded with the smaller diameter indenter whereas contact radius and stress remained unaffected. Also, the 17.6 mm indenter increased PG loss and significantly upregulated genes for ECM proteins and enzymes as compared to the 30.2 mm indenter.
Conclusion: Sliding loads that increase ECM deformation/strain were found to induce enzyme-mediated catabolic processes in articular cartilage explants. These observations provide further understanding of how changes in cartilage contact mechanics under dynamic conditions can affect the cellular response.
Background: Red blood cell (RBC) deformability may increase, or decrease, following application of shear stress ("shear conditioning"), depending upon the specific magnitude and duration of exposure. However, the time course of altered RBC deformability following shear remains unresolved.
Objective: We utilised shear conditioning known to increase (10 Pa) or decrease (64 Pa) RBC deformability and subsequently rested the cells; serial measurements of deformability during the rest period facilitated defining the time course of recoverability. A second experiment repeated the shear conditioning and recovery period to explore whether multiple duty-cycles augmented the response following the initial exposure.
Methods: Shear conditioning was performed for 300 s at the desired shear stress. Ektacytometry was used to quantify human RBC deformability immediately and during rest (3, 5, 60, 120, 240, 300 s) using discrete samples. RBC were shear conditioned twice in a separate experiment, with 300 s rest separating the conditioning.
Results: Shear conditioning at 10 Pa induced increased cell deformability by 19.5 ± 0.3%, which reduced to 7.2 ± 0.4% after 300 s of rest. Shear conditioning at 64 Pa decreased cell deformability by 30.5 ± 13.9%, and after 300 s rest, remained decreased (19.3 ± 9.4%) compared with baseline. The second duty-cycle augmented initial responses induced by shear conditioning.
Conclusion: Specific shear conditioning results in either temporarily increased cell deformability, or a less reversible decrease of RBC deformability.
Background: The rheological properties of sputum may influence lung function and become modified in disease.
Objective: This study aimed to correlate the viscoelastic properties of sputum with clinical data on the severity of disease in patients with chronic obstructive pulmonary disease (COPD).
Methods: Sputum samples from COPD patients were investigated using rheology, simple mathematical modelling and Scanning Electron Microscopy (SEM). The samples were all collected from patients within two days of their admission to Prince Philip Hospital due to an exacerbation of their COPD. Oscillatory and creep rheological techniques were used to measure changes in viscoelastic properties at different frequencies over time.
Results: COPD sputum was observed to behave as a viscoelastic solid at all frequencies studied. Comparing the rheology of exacerbated COPD sputum with healthy sputum (not diagnosed with a respiratory disease) revealed significant differences in response to oscillatory shear and creep-recovery experiments, which highlights the potential clinical benefits of better understanding sputum viscoelasticity. A common power law model G(t)=G0(tτ0)-m was successfully fitted to experimental rheology data over the range of frequencies studied.
Conclusions: A comparison between clinical data and the power law index m obtained from rheology, suggested that an important possible future application of this parameter is as a potential biomarker for COPD severity.