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Human Blood Plasma in Capillary-size Flow: Revealing Hidden Elasticity and Scale Dependence

Abstract : The dynamical mechanical analysis of blood generally uses models inspired by conventional flows, assuming scale-independent homogeneous flows and without considering fluid-surface boundary interactions. The present experimental study highlights the pertinence of a non-conventional approach to identify dynamic properties of human blood plasma. A finite shear elastic response (solid-like property) is identified in nearly static conditions, which also depends on the scale (being reinforced at small scales). Therefore, blood plasma is a scale dependent viscoelastic solid that flows over a weak stress threshold. This finding opens new routes for medical diagnosis and device fabrication. The transport mechanisms of blood in the vasculature face multiple biological, physical, and biochemical parameters that make their flow laws complex, specific, and fascinating. Ninety years after the first works from Fahraeus and Lindqvist [1] showing the decrease of blood viscosity with decreasing tube diameter, numerous in-silico [2-4], ex-vivo [5-8], and in-vivo studies [9-12] shed light on the behavior of blood cells in flow. Bulk blood viscosity results from such behavior through red blood cell (RBC) alignment in the streamlines and mutual elastic cell-cell coupling [13]. The effect of blood viscosity on vascular resistance includes several mechanisms; on one hand blood itself generates processes in the vessel wall. Its flow velocity and the degree of the cell-free layer along the wall determine the endothelial wall shear stress [14,15] needed to adjust the vessel diameter through mechanotransduction [16,17]. Flowing RBCs rhythmically discharge ATP into blood plasma, being a signal for NO production in endothelial cells [18]. RBCs themselves release their NO into the vessel lumen [19], and communicate the metabolic demand of tissues [20,21]. On the other hand, the vascular geometry influences blood properties by the RBC distribution in the vessel cross-section, by the induction of lift forces, and by the addition of pulsations into blood flow along the elastic fibers in the vessel media [4,13,22]. Therefore, blood and vessels must be recognized as a functional unit to regulate the vascular resistance [23]. Their interaction anticipates a well-designed interface. Considering the huge information available on the molecular interactions between blood and vessel wall [24], it is surprising that the dynamic of the interface is less explored, even though an immobile plasma portion was postulated already 50 years ago [25]. Blood cells do not shear directly along the endothelial cell surface, as there is a fluid layer of significant length in between, which resists compression and local shear stresses [26]. The fluidic blood-wall interface starts with hydrated proteoglycans, hyaluronan, and glycoproteins on the endothelial surface and extends towards the vessel lumen through adhesion of plasma proteins to form the endothelial surface layer (ESL) [26,27]. This 0.5-3.0 µm layer is fragile and its characterization is difficult being tributary of the technique that is used to analyze it [28,29]. It allows passage of leucocytes through it, but rearranges quickly to its original thickness, once, the cell has left the layer. It influences the blood flow profile [30] and the RBC-wall interactions [31], often by minimizing the amount of leucocytes, lipoproteins and fibrinogen [32] at close proximity to the endothelium. By mechanical means, the protective role of the ESL is the result of its thickness and viscoelasticity. Pries and coworkers [33] postulated structures forming the layer. But where would these elastic structures come from and how stabile are they? A classical approach to identify viscoelasticity of liquids is rheology. The current state of the art in conventional rheometry consists in measuring the resistance of the fluid to a shear strain while the sample is placed between two metallic surfaces. One surface is animated of an oscillatory motion while the other one is linked to a sensor to measure the shear stress dynamically transmitted from one substrate to the other one by the sample. It is often forgotten that this technique is entirely dependent on the interaction between the sample and the substrate since a good adhesion of the sample on the substrate is needed to transmit the shear forces. Metallic surfaces are highly unphysiological and might not provide the best anchor for blood. Classical rheometry (gap width of 1 mm) also underestimates the importance of the scale. A testing on macroscopic scales will not capture the quality of interaction between the fluid and its boundaries, especially if the elastic layer is thin. We therefore adapted the dynamic characterization of blood plasma to access its true properties. The methodology includes an optimized stress transfer through the liquid by enhancing the force of interaction of its first molecular layers to the two surfaces in the rheometer. Excellent wetting is a prerequisite to optimize the no-slippage condition [34]. The degree of wetting can be characterized by the method of the contact angle θ (Fig. 1). Different approaches have shown that the slippage propensity is reduced when the contact angle is reduced [34]. Low strain and the reinforcement of the fluid-substrate cohesion enable to access higher viscoelastic moduli, and at small scale revealed the elastic character of ordinary liquids and fluids [35,36]. The improved wetting method combined to a conventional rheometer showed that ordinary liquids (classified Newtonian by classical rheometry) exhibited a solid-like response below an elastic threshold [37,42]. We felt
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https://hal.archives-ouvertes.fr/hal-02513618
Contributor : Laurence Noirez <>
Submitted on : Friday, March 20, 2020 - 6:27:05 PM
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U Windberger, Laurence Noirez. Human Blood Plasma in Capillary-size Flow: Revealing Hidden Elasticity and Scale Dependence. 2020. ⟨hal-02513618⟩

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