Role of red cells and plasma composition on blood sessile droplet evaporation

The morphology of dried blood droplets derives from the deposition of red cells, the main components of their solute phase. Up to now, evaporation-induced convective flows were supposed to be at the base of red cell distribution in blood samples. Here, we present a direct visualization by videomicroscopy of the internal dynamics in desiccating blood droplets, focusing on the role of cell concentration and plasma composition. We show that in diluted suspensions, the convection is promoted by the rich molecular composition of plasma, whereas it is replaced by an outward red blood cell displacement front at higher hematocrits. We also evaluate by ultrasounds the effect of red cell deposition on the temporal evolution of sample rigidity and adhesiveness.

Figure 1

Schematic representation of the experimental setup used for the investigation of the drying of blood sessile droplets by ultrasounds.

Figure 2

Shear reflection coefficient of ultrasounds penetrating through an evaporating droplet of RBCs (red circles), HPs (green triangles), and HRBCs (violet stars) in plasma ($\text{Ht}=45\%$) as a function of the normalized time ($t/t_{\text{dry}}$, where $t_{\text{dry}}=26.5\pm 2.5$ min for RBCs, $\approx 60$ min for HPs, and $34\pm 2$ min for HRBCs). The three regions correspond to the main stages of the drying process: solvent evaporation (I), gel-sol transition (II), and delamination (III). The insets in the plot illustrate the characteristics of RBC droplets at different instants of the evaporation. Parts (a), (b), and (c) show the final morphology of RBC, HP, and HRBC samples, respectively (scale bar, 1 mm).

Figure 3

Estimation of RBC displacement near droplet edges by PIV at (a) $t=0.05t_{\text{dry}}$ and (b) $t=0.45t_{\text{dry}}$ in a plasma suspension ($\text{Ht}=45\%$). Colored scale bars refer to cell velocity, going from its maximum (red) to zero (violet). Velocity fields are superposed to images acquired in BF to highlight the development of corona rings with time (scale bars, $200\mu \mathrm{m}$).

Figure 4

(a) and (b) Convective flows observed in a droplet of RBCs suspended in plasma in dilute conditions ($\text{Ht}=1\%$) at, respectively, $t=0.1t_{\text{dry}}$ and $0.4t_{\text{dry}}$. The segments represent cell trajectories over a temporal range of $\approx 10\mathrm{s}$. (c) Measurement of RBC displacement for $t=0.6t_{\text{dry}}$. The intensity of the velocity ranges between 0.4 (red) and 0 (violet) $\mu \mathrm{m}/\mathrm{s}$ (scale bars, $200\mu \mathrm{m}$).

Figure 5

(a) Evolution of the shear reflection coefficient with time for droplets of plasma and RBC-plasma suspensions with $\text{Ht}=22\%$ and 45%. In the inset, $r_{\text{min}}$ and the drying time ($t_{\text{dry}}$) are plotted as a function of the hematocrit. Images referring to droplets of pure plasma and 22% RBC suspensions are acquired by BF when the sample is solidified ($t>1.0t_{\text{dry}}$) (scale bars, 1 mm). (b) Images in fluorescence of dry patterns of RBCs characterizing droplets with Ht equal, respectively, to 10%, 22%, and 45% (scale bars, 1 mm).

Figure 6

Temporal evolution of the shear reflection coefficient for droplets of PBS-BSA solution ($t_{\text{dry}}\approx 35$ min) and washed blood with $\text{Ht}=10$ ($t_{\text{dry}}\approx 23.5$ min) and 45% ($t_{\text{dry}}\approx 24$ min) compared to the behavior of drops of RBCs in plasma at physiological Ht. The images on the diagram are acquired after the complete desiccation of the samples (scale bar, 1 mm).

Figure 7

Schematic description of the impact of suspending media composition on the dynamics occurring during the evaporation of blood droplets in diluted and dense conditions. Light blue arrows refer to stress distribution at the air-liquid interface, whereas yellow ones represent RBC motion inside the droplet. Here $t_0$ is the deposition time.