Capillary flow of whole blood in microsystems: non-Newtonian blood behavior and substrate reagent-coating effect

J. Berthier, D. Gosselin, M. Huet, G. Sabatte, M-L. Cosnier, P. Pouteau, M. Cubizolles
CEA-Leti, University Grenoble-Alpes,
France

Keywords: whole blood, capillarity, rheology, rouleaux, spreading, dynamic contact angle

Summary:

Introduction: The use of whole blood is a preferred solution for portable diagnostic and monitoring systems. We analyze here two particularities of microsystems for blood analysis: the blood non-Newtonian behavior, and the capillary flow in reagent-coated channels. Most of the time, such systems use capillary forces to move the samples. It is shown first that the capillary flow of blood does not follow the Lucas-Washburn-Rideal law when the capillary flow velocity is small, due to its non-Newtonian rheology and to the formation of rouleaux of RBCs. In a second step, we investigate the capillary flow of blood on reagent-coated surfaces; first experimentally by observing the spreading of a droplet of blood on different reagent-coated substrates (IgM, dye, etc.); second theoretically and numerically using the general law for spontaneous capillary flows and the Evolver numerical program. Spontaneous capillary flow of whole blood: Whole blood rheology is complex. It depends on many parameters, such as hematocrit and fibrinogen levels. An average law for viscosity as a function of the shear rate is the Casson law (figure A). When the shear rate is small, rouleaux of RBCs form, giving the blood its strong non-Newtonian, yield stress behavior (figure B). Hence the capillary flow of whole blood departs from the LWR law for large penetration distances (figure C). Spreading of whole blood on reagent-coated substrates: Experiments have been conducted to observe the spreading of whole blood on different substrates: non coated COP and COP coated with IgM, dye, dyewith Tween 10, etc (figure D). We check that the Hoffmann-Voinov-Tanner law is approximately respected. Assuming no instantaneous dissolution, we determine the dynamic and static contact angle on such surfaces. Capillary flow in coated channels: It is observed that the reagent-coating of microchannels often occurs in corners and wedges (figure E). Assuming different morphologies of these coatings, and using the contact angles determined in the preceding approach, the SCF conditions are determined for each different configuration (figure F). Conclusion: The design of portable point-of-care and home-care systems for blood analysis relies on the capillary flow of whole blood. This work aimed to the comprehension of the behavior of whole blood in such geometries.