دانلود رایگان مقاله انگلیسی جریان خون و جاذبه بسیار کم - الزویر 2018

abstract

The absence of gravity during space flight can alter cardio-vascular functions partially due to reduced physical activity. This affects the overall hemodynamics, and in particular the level of shear stresses to which blood vessels are submitted. Long-term exposure to space environment is thus susceptible to induce vascular remodeling through a mechanotransduction cascade that couples vessel shape and function with the mechanical cues exerted by the circulating cells on the vessel walls. Central to such processes, the glycocalyx – i.e. the micron-thick layer of biomacromolecules that lines the lumen of blood vessels and is directly exposed to blood flow – is a major actor in the regulation of biochemical and mechanical interactions. We discuss in this article several experiments performed under microgravity, such as the determination of lift force and collective motion in blood flow, and some preliminary results obtained in artificial microfluidic circuits functionalized with endothelium that offer interesting perspectives for the study of the interactions between blood and endothelium in healthy condition as well as by mimicking the degradation of glycocalyx caused by long space missions. A direct comparison between experiments and simulations is discussed.

4. Flows in brush-coated and endothelium-bearing microchannels

It is recognized that, beyond its biochemical and mechanotransduction functions, the endothelial glycocalyx plays a hydrodynamic role and affects the resistance to blood flow, in particular in microvessels whose dimensions are comparable with the size of RBCs. However, in vivo experiments are extremely challenging to perform, and control over the flow parameters and the actual state and thickness of the glycocalyx is limited [43]. This calls for in vitro fundamental studies, in which the hydrodynamic effect of a surface-bound soft and deformable macromolecular layer can be more easily and systematically studied.

In this spirit, we have elaborated microchannels made of cylindrical glass capillaries with an inner diameter of 10 μm, in which we have grown, using the so-called grafting-from method, polymer brushes that are swollen in aqueous media and mimic the presence of the glycocalyx [6]. This strategy allows us to have a good control over the imposed flow conditions as well as over the thickness of the surface-bound layer.

We have thus shown that, in the presence of a polymer brush, the hydraulic resistance of a microchannel increases, i.e. the flow velocity is reduced, all the more so that the brush thickness is important, as illustrated in Fig. 4a. Interestingly, we find that for brush thicknesses comparable with that of an actual glycocalyx (several hundreds of nanometers), the observed velocity reduction, in the order of 30%, closely compares with what has been deduced from in vivo measurements [43]. This suggests that our bio-inspired microchannels do allow us to reproduce correctly the hydrodynamic effect attributed to the glycocalyx. Still, we note that the magnitude of this effect is systematically larger than expected from a mere reduction of the inner channel diameter due to the presence of the brushes (see Fig. 4a). Although we cannot yet account quantitatively for such an unexpectedly large impact of the wall-bound brushes, recent numerical simulations provide evidence for the development of non-trivial near-wall backflows and for the build-up of surface waves at the brush/fluid interface [44], which might be at the origin of our observations. This points to the existence of an extremely rich phenomenology in such problems of flow with soft boundaries.