Biorheology - Volume 40, issue 4

Authors: Zuidema, P. | Govaert, L.E. | Baaijens, F.P.T. | Ackermans, P.A.J. | Asvadi, S.

Article Type: Research Article

Abstract: The possibility of curling hair is attributed to the fact that the mechanical properties of hair fibres depend on time, temperature and humidity. In this study the dependence of the viscoelastic behaviour of human hair fibres on humidity is characterised in a bending deformation. An experimental set‐up was used to perform bending relaxation measurements at different humidities. The relaxation data were fitted using a so‐called “stretched exponential equation”. The humidity dependence was incorporated by applying time–humidity superposition. Also, the influence of humidity on the initial E‐modulus was found. The influence of humidity on the behaviour of human hair fibres could …be modelled by using a general characterisation of the behaviour of human hair fibres. The general characterisation is used to predict the recovery in a human hair fibre after curling. The predicted recovery appeared to agree reasonably well with experimentally determined values. Show more

Keywords: Human hair, viscoelasticity, recovery, humidity dependence

Citation: Biorheology, vol. 40, no. 4, pp. 431-439, 2003

Authors: Manley, Eugene | Provenzano, Paolo P. | Heisey, Dennis | Lakes, Roderic | Vanderby, Ray

Article Type: Research Article

Abstract: The goal of this study was to determine the duration of time that ligaments from a study group need to be loaded in order to adequately determine their collective viscoelastic behavior. Rat ligaments were subjected either to creep or stress relaxation for 1,000 s or stress relaxation for 10,000 s to compare estimates of viscoelastic behavior for different test durations. Stresses versus time (relaxation) or strains versus time (creep) were fit with power law models (tβ where β is the rate of creep or relaxation on a log–log scale). Time intervals were separated by logarithmic decade and analyzed using …a Random Coefficients approach to compute residual specimen error as a function of the number of decades of data analyzed. Standard Regression was also used for comparison. Results show that by testing for ≤100 s (i.e. two logarithmic decades of time) offers 1% less accuracy than testing for 1,000 seconds (i.e. three decades) when estimating the viscoelastic behavior of a specimen. These 100 s power law estimates are far more accurate than the between specimen dispersion of viscoelastic properties. Hence, a better way to compare viscoelastic behavior between study groups is to test more specimens for shorter durations. This reduces experimental time per sample and therefore increases efficiency. Show more

Keywords: Stress relaxation, creep, ligament, random coefficients, standard regression

Citation: Biorheology, vol. 40, no. 4, pp. 441-450, 2003

Authors: Banerjee, Rupak K. | Back, Lloyd H. | Back, Martin R. | Cho, Young I.

Article Type: Research Article

Abstract: To evaluate the local hemodynamics in flow limiting coronary lesions, computational hemodynamics was applied to a group of patients previously reported by Wilson et al. (1988) with representative pre‐angioplasty stenosis geometry (minimal lesion size dm =0.95 mm; 68% mean diameter stenosis) and with measured values of coronary flow reserve (CFR) in the abnormal range (2.3±0.1). The computations were at mean flow rates ($\widetilde{Q}$ ) of 50, 75 and 100 ml/min (the limit of our converged calculations). Computed mean pressure drops Δ$\tilde{p}$ were ∼9 mmHg for basal flow (50 ml/min), ∼27 mmHg for elevated flow (100 ml/min) and …increased to an extrapolated value of ∼34 mmHg for hyperemic flow (115 ml/min), which led to a distal mean coronary pressure $\tilde{p}$ rh of ∼55 mmHg, a level known to cause ischemia in the subendocardium (Brown et al., 1984), and consistent with the occurrence of angina in the patients. Relatively high levels of wall shear stress were computed in the narrow throat region and ranged from about 600 to 1500 dyn/cm2 , with periodic (phase shifted) peak systolic values of about 3500 dyn/cm2 . In the distal vessel, the interaction between the separated shear layer wave, convected downstream by the core flow, and the wall shear layer flow, led to the formation of vortical flow cells along the distal vessel wall during the systolic phase where Reynolds numbers Ree (t) were higher. During the phasic vortical mode observed at both basal and elevated mean flow rates, wide variations in distal wall shear stress occurred, distal transmural pressures were depressed below throat levels, and pressure recovery was larger farther along the distal vessel. Along the constriction (convergent) and throat segments of the lesion the pulsatile flow field was principally quasi‐steady before flow separation occurred. The flow regimes were complex in the narrow mean flow Reynolds number range $\widetilde{\mathrm{R}}$ ee =100–230 and a frequency parameter of αe =2.25. The shear layer flow disturbances diminished in strength due to viscous damping along the distal vessel at these relatively low values of $\widetilde{\mathrm{R}}$ ee , typical of flow through diseased epicardial coronary vessels. The distal hyperemic flow field was likely to be in an early stage of turbulent flow development during the peak systolic phase. Show more

Citation: Biorheology, vol. 40, no. 4, pp. 451-476, 2003

Authors: Neu, Björn | Armstrong, Jonathan K. | Fisher, Timothy C. | Meiselman, Herbert J.

Article Type: Research Article

Abstract: Recent studies have shown that the covalent attachment of poly(ethylene glycol), abbreviated as PEG, to the surface of human red blood cells (RBC) leads to masking of membrane antigenic sites and inhibition of RBC aggregation. The effects of PEG coating on the regions near the RBC glycocalyx were thus explored using cell micro‐electrophoresis. Both linear (3.35, 18.5, 35.0) and an 8‐arm 35.9 kDa reactive PEG were used; in one series, thick cross‐linked coats were obtained using a branched PEG amine as a cross‐linker. The results indicate marked decreases of RBC mobility (up to 90%) which were affected by polymer molecular …mass and geometry. Since PEG is neutral and its covalent attachment is predominantly to primary amine groups, such decreases of mobility most likely reflect structural changes near and within the RBC glycocalyx rather than decreased surface charge density. Experimental data were analyzed using a theoretical approach which allows calculation of the thickness and friction of the polymer layers: (1) for linear PEGs, thickness increased and friction decreased with polymer mass; (2) compared to linear PEGs of similar molecular mass, thickness was less and friction was greater for the branched PEG; (3) cross‐linked PEG coatings were more than 50 nm thick and were insensitive to changes of ionic strength. These observations are consistent with the aggregation behavior of PEG‐coated RBC and indicate the usefulness of micro‐electrophoresis methods for studies of covalently‐attached polymers: the resulting calculated thickness and friction factors should be of value in achieving desired cellular surface characteristics or levels of cell–cell interaction. Show more

Keywords: Micro‐electrophoresis, poly(ethylene glycol), red blood cell, polymer‐coating

Citation: Biorheology, vol. 40, no. 4, pp. 477-487, 2003

Authors: Wu, Ze‐Zhi | Li, Ping | Huang, Qi‐Ping | Qin, Jian | Xiao, Guo‐Hui | Cai, Shao‐Xi

Article Type: Research Article

Abstract: A micropipette technique was used to investigate the effects of four synthetic peptides, YIGSR, CDPGYIGSR, RGDS and GRGDTP, on the adhesion of hepatocellular carcinoma (HCC) cells onto type IV collagen/laminin/fibronectin coated surfaces. Adhesion of HCC cells to laminin was found to be YIGSR‐ or CDPGYIGSR‐dependent while that to fibronectin and type IV collagen was RGDS‐ or GRGDTP‐dependent. The reduction in adhesion strengths of HCC cells was slight to moderate (up to 55%), and was dependent on the peptide concentration. The decrease in adhesion strengths was reversed by an increase in ligand coating concentration and was compromised by prolonged interaction of …the cells with the surfaces. These results suggested that the inhibition was due to competitive retardation rather than to a blockade of adhesion strengthening. A simple asymptotic function was adopted to fit the correlation between the mean of cell adhesion strengths and peptide concentration within defined concentration ranges. Regression analysis showed that cell adhesion strengths appeared to approach a plateau with increasing concentration of the inhibitory peptides, which was not always uniform over the entire concentration range tested. Further reduction in adhesion strengths was observed at higher peptide concentrations. It is suggested that the constants obtained by fitting over a low peptide concentration range might be kinetically representative of the inhibition during early events of adhesion or attachment. Show more

Keywords: Type IV collagen, laminin, fibronectin, adhesive recognition sequence, micropipette

Citation: Biorheology, vol. 40, no. 4, pp. 489-502, 2003