Biorheology

Our aim was to study the effect of an axially directed blood plasma flow on the dissolution rate of cylindrical non-occlusive blood clots in an in vitro flow system and to derive a mathematical model for the process. The model was based on the hypothesis that clot dissolution dynamics is proportional not only to the biochemical proteolysis of fibrin but also to the power of the flowing blood plasma dissipated along the clot. The predicted rate of thrombolysis is then proportional to the square of the average blood plasma velocity for laminar flow and to the third power of the average velocity for turbulent flow. To verify the model, the time dependence of the clot cross-sectional area was measured by dynamic magnetic resonance microscopy during fast (turbulent) and slow (laminar) flow of plasma through an axially directed channel along the clot. The flowing plasma contained a magnetic resonance imaging contrast agent (Gd-DTPA) and a thrombolytic agent (recombinant tissue-type plasminogen activator). The experimental data fitted well to the model, and confirmed the predicted increase in the dissolution rate when blood flow changed from a laminar to a turbulent flow regime.

The influence of the molecular weight and the type of gelatin (A or B), as well as the molecular weight of poly (γ-glutamic acid) (γ-PGA), on the properties of gelatin/γ-PGA mixed bioadhesives were studied. The gelation of the system was enhanced by a crosslinker, 1-(3-dimethylaminopropyl)-3-(ethylcarbodiimide) hydrochloride (EDC). The gelation time of the bioadhesives was analyzed using rheological measurements. The results indicated that the type of gelatin was a critical factor in determining the gelation time of the biological glues. The mixed glues had greater bonding strength and smaller gelation times as the molecular weight of γ-PGA or gelatin increased. The swelling ratio decreased and the denaturation temperature increased upon raising the EDC concentration, indicating a greater degree of crosslinking at higher EDC concentrations. The mixed glues crosslinked with various concentrations of EDC (1.7–2.5%) showed no cytotoxicity to fibroblasts. In addition, no significant inflammatory response was observed in the rat subcutaneous implantation. The bioadhesives based on gelatin/γ-PGA remained at the site for 7 days while the fibrin glue had almost completely degraded. By choosing the appropriate gelatin type and higher molecular weight γ-PGA in the mixtures, the gelatin/γ-PGA biological glues could serve as soft tissue adhesives. Rheological characterization was essential in the evaluation of biological glues.

Although the study of red blood cell (RBC) aggregation continues to be of basic science and clinical interest, aggregation standards for calibration do not exist, and most aggregation studies report data in terms of arbitrary units: quantitative comparisons between studies are thus essentially precluded. However, use of low shear viscometry plus the Casson equation provides a yield shear stress that has defined units and is known to reflect RBC aggregation. Employing human RBC–plasma suspensions exhibiting a wide range of aggregation, the present study examined relations between yield shear stress values and aggregation indices obtained using the Myrenne aggregometer: the latter approach uses a light-transmission technique and provides an “M” index at stasis and an “M1” at very low shear. Our results for normal controls and for angina patients without coronary artery disease indicate highly significant correlations (p<0.001) between the yield stress and both M and M1. Thus, within the range of aggregation studied, these findings lend support to the rheological validity of the Myrenne approach; extension of our findings to intensely aggregating RBC suspensions may require additional validation studies.

Changes in the mechanical and adhesive properties of neutrophils may modify perfusion of the microcirculation in cooled tissue. We tested how integrin-mediated adhesion of isolated human neutrophils was altered by cooling, or cooling and rewarming. First, adhesion was tested in a static assay. In the presence or absence of integrin-activating agents (formyl peptide, fMLP or Mn⁺⁺), there were significant reductions in adhesion to immobilised albumin at 10°C or 0°C compared to 37°C, although a slight increase in adhesion was induced by fMLP or Mn⁺⁺ at 10°C or 0°C. If cells were cooled for 5 or 20 min at 10°C and rewarmed (in the absence of activators) there was >100% increase in adhesion compared to cells held at 37°C. In a flow assay, neutrophils perfused over P-selectin at 37°C formed rolling attachments, but if neutrophils were cooled to 10°C and rewarmed for 1 or 5 min, there was transformation to stationary adhesion, which was reversed by antibody against CD18. After 20 minutes of rewarming, rolling was restored. Cooling and rewarming did not cause de novo expression of CD11b/CD18, and so appears to transiently activate constitutively-expressed integrin. Thus, integrin-mediated adhesion may be impaired in cold tissue but on return to normal temperature, neutrophils may transiently adhere locally or in remote vessels.

Since the early seventies, the material properties of brain tissue have been studied using a variety of testing techniques. However, data reported in literature show large discrepancies even in the linear viscoelastic regime. In the current study, the effect of the sample preparation procedure and of post-mortem time on the mechanical response of porcine brain tissue is examined. Samples from the thalamus region were prepared with different techniques and were tested for different loading histories. Each sample was tested in oscillatory shear tests (1% strain amplitude, 1–10 Hz frequencies) followed by sequences of 5% strain loading-unloading cycles. The stress response to the loading-unloading cycles showed a clear dependency on post-mortem time, becoming more stiff with increasing time. This dependency was affected by the mechanical history induced by the preparation procedure.