Surface Shear Rate Research Articles

Platelet aggregation in flowing blood in vitro. II. Dependence of aggregate growth rate on ADP concentration and shear rate

Experiments were carried out with citrated dog blood using the new experimental system described in the companion paper (E. F. Grabowski, 1978, Microvas. Res. 16, 159–182). In this system, platelet aggregates form at and adhere to a specified, microscopic site on Cuprophan (PT 150, Enka Glanzstoff) membrane. Important features include controlled surface shear rate, controlled ADP diffusion through the membrane and into the flowing blood at the microscopic site, and quantification of aggregate growth rates by videodensitometry. In early trials it was learned that prior platelet adhesion markedly enhanced aggregation in response to ADP; the degree of prior adhesion was therefore standardized for subsequent experiments. When rate of entry of ADP was increased logarithmically, the rate of change of mean thickness of platelet aggregates adherent to the membrane increased monotonically. With an entry rate leading to ADP in micromolal concentrations at the membrane-blood interface, however, growth rate prior to embolization passed through a maximum with respect to surface shear rate (varied from 99 to 986 sec −1). The maximum occurred between 394 and 635 sec −1, the growth rate at 394 sec −1 being, for example, 3.1 ± μm/min (mean ± SD for three animals). On the other hand, aggregate growth rate for an entry rate leading to millimolal ADP increased monotonically with increasing shear rate over the same range. Aggregate growth rates appear therefore to be limited at low shear rate by platelet convection, but at higher shear rates by one or more of (1) dilution of ADP, (2) finite platelet-ADP interaction times, and (3) surface shear stress. For the range of shear rates studied, dilution of ADP seems likely to be the most important of these last factors.

Platelet aggregation in flowing blood in vitro. I. Production by controlled ADP convective diffusion and quantification by videodensitometry

The overall objective of this work has been to quantify and to control the effects of flow and diffusion of adenosine diphosphate (ADP) on platelet adhesion and aggregation in whole blood. Flow conditions were chosen such that the study has maximal relevance to arterial thrombosis, where platelets are believed to play a prominent role. Toward this end, a blood flow chamber was designed and constructed wtihin which platelet aggregates form at and adhere to a specified, microscopic site on Cuprophan (PT 150, Enka Glanzstoff) membrane. Important features include controlled surface shear rate, controlled ADP diffusion (assessed by separate sutdies with [ 3H]ADP) through the membrane and into the flowing blood at the microscopic site, and a design permitting continuous in situ observation and quantification (e.g., aggregate mean thicknesses and thickness growth rates) using videomicroscopy and dual window densitometry of video recordings. Video calibration is carried out using differing, but known thicknesses of blood film between the light source and microscope objective. Although the wavelength of the incident light is 800 ± 90 nm (mean ± half-bandwidth for 50% peak intensity), the Lambert-Beer law is applicable to the present system, as shown by separate determination of the relative constancy of the absorptivities of both whole blood and packed platelets over the range from 700 to 900 nm. The chamber, videomicroscope, and videodensitometer constitute a whole blood platelet aggregometer, one with which shear rate (99 to 986 sec −1 or more) and entry rate of ADP into citrated or heparinized blood (leading to membrane-blood interface concentrations of ADP of micromolal to millimolal in present experiments) may be varied independently. The calibrated output of the device is a recording of the mean thickness of adherent aggregates within a standardized electronic “window” as a function of time. From such a recording, which may be correlated with simultaneous morphologic observations by videomicroscopy, estimates may be made of time to embolization, maximum mean aggregate thickness, and rate of increase of mean aggregate thickness.

Traction and Lubricant Film Temperature as Related to the Glass Transition Temperature and Solidification

Does a traction fluid have to be a glass or solid under operating conditions. Infrared spectra on dynamic EHD contacts of several types of fluid were used to determine the surface and oil-film temperatures. Polarized spectral runs were made to study molecular alignment. Static glass transition pressures at appropriate temperatures were between 0.1 and 2.0 GPa, with the traction fluid showing the highest. In the EHD contact region, the traction fluid showed both the highest film temperatures as well as the greatest degree of molecular alignment. A plot of the difference between the film and surface temperatures vs shear rate resulted in a master plot valid for all the fluids. From this work, the authors propose a model of 'fluid' traction, where friction between parallel rough molecules provides the traction.

Platelet adhesion to foreign surfaces under controlled conditions of whole blood flow: human vs rabbit, dog, calf, sheep, pig, macaque, and baboon.

Our results reveal significant differences between mammalian species with respect to platelet adhesion to foreign surfaces exposed to heparinized, flowing blood. In particular, we have demonstrated the following: 1) At a surface shear rate characteristic of shear rates in mammalian arteries, human platelet adhesion (and that of calf, baboon, macaque, hog or sheep) is negligible in comparison to dog or rabbit platelet adhesion after 10 mins of blood flow. 2) The species differences are biomaterial dependent: the human-dog difference is present with Cuprophan or Avcothane, but absent with compressed Gore-Tex or fluorinated ethylcellulose. (The platelets of humans and dogs adhere to comparable degrees on the latter 2 biomaterials.) 3) With Cuprophan and recirculated blood, the human-dog difference persists at 30 and 180 mins. 4) By means of videomicroscopy, the species differences are unlikely only to be an artifact of the possible formation and embolization of surface-adherent aggregates. Tests for the thromboresistance of candidate biomaterials and for platelet adhesion under controlled flow conditions must therefore begin to take into account differences between humans and other species.

Surface viscosity of monomolecular films of n-alkoxy ethanols at various temperatures and pressures

Abstract The surface viscosities of monomolecular films of n-alkoxy ethanols CH3(CH3)n−1OCH2CH2OH designated as CnOC2H4OH (n = 16, 18, 20, 22) were studied as a function of surface pressure (2–40 dynes/cm), temperature (15°–30°C), and shear rate (0.12–1.8 sec−1) using a rotational viscometer. The transition pressures observed for two-dimensional phase changes, i.e., from a liquid condensed (Lc) to an intermediate (I) or to a liquid-solid (LS) state agree fairly well with those found in π-A isotherms. For the C2OC2H4OH film the LS → S transformation has been obtained at 34 dynes/cm; this is higher by 2 dynes/cm than that observed by Deo et al. in π-A isotherms. The LS phase is found to be stable for a particular compound and at a particular temperature. The supermobile character of the film may be attributed to the strong hydrogen bonding between the film molecules themselves and a weaker interaction with the water subphase. Activation energy for viscous flow and relaxation time have been determined for all alkoxy ethanols at Lc → I or Lc → LS transition pressures and at different temperatures.