Aggregation efficiency of activated normal or fixed platelets in a simple shear field: effect of shear and fibrinogen occupancy

Shear rate can affect protein adsorption and platelet aggregation by regulating both the collision frequency and the capture efficiency (alpha). These effects were evaluated in well defined shear field in a micro-couette for shear rate G = 10 - 1000 s-1. The rate of protein binding was independent of G, shown for adsorption of albumin to latex beads and PAC1 to activated platelets. The initial aggregation rate for ADP-activated platelets in citrated platelet-rich plasma followed second order kinetics at the initial platelet concentrations between 20,000 and 60,000/microliters. alpha values, which dropped nearly fivefold for a 10-fold increase in G, were approximately proportional to G-1, contrary to a minor drop predicted by the theory that includes protein cross-bridging. Varying ADP concentration did not change alpha of maximally activated platelet subpopulations, suggesting that aggregation between unactivated and activated platelets is negligible. Directly blocking the unoccupied but activated GPIIb-IIIa receptors without affecting pre-bound Fg on "RGD"-activated, fixed platelets (AFP) by GRGDSP or Ro 43-5054 eliminated aggregation, suggesting that cross-bridging of GPIIb-IIIa on adjacent platelets by fibrinogen mediates aggregation. Alpha for AFP remained maximal (approximately 0.24) over 25-75% Fg occupancy, otherwise decreasing rapidly, with a half-maximum occurring at around 2% occupancy, suggesting that very few bound Fg were required to cause significant aggregation.     

Misfit accommodation in a quenched

Quenched and aged specimens of the Al-1.3 at. pct Cu-19.1 at. pct Si alloy were studied by X-ray diffraction. Since this alloy contains a high volume percentage (~20 vol pct) of second- phase Si particles, it is regarded as a model for a metal matrix composite (MMC). During isothermal aging of the solid-quenched Al-1.3 at. pct Cu-19.1 at. pct Si alloy, the Cu and Si atoms precipitate. This causes the Al-rich phase lattice parameter to increase from a value lower to a value higher than the lattice parameter of pure unstrained aluminum. Due to thermal misfit after quenching from heat-treatment temperatures, all lattice parameters are influenced by re- sidual stresses. A model describing the elastic/plastic accommodation of a misfitting spherical inclusion in an infinite matrix is adapted for the case of misfitting inclusions in a finite matrix. This model describes the measured lattice parameter shifts of the Si phase reasonably well. Comparison of the model for elastic accommodation and the model for elastic/plastic accom- modation with measured stresses shows significant discrepancies for the low-temperature range (ΔT < 200 K). These discrepancies may be related to the volume effect of defects (dislocations, vacancies) created in the plastic zone.