Résumé
Blood aggregation occurs when erythrocytes, red blood cells (RBC), agglutinate together into compact stacks of cells called rouleaux. Blood aggregation can then be compared to a liquid-gel transition that appears at low flow regime, and is responsible of the high value of the blood viscosity at low shear rate. This paper will present a physical device that can detect the appearing of this liquid-gel transition using the transmission and reflection properties of ultrasonic waves. In order to detect the liquid-gel transition in resting blood we use the property of an ultrasonic shear wave (5 MHz) to be transmitted or reflected at a glass-liquid interface that is populated with RBC. The setup, as sketched in Figure1, is made of a piezoelectric transducer, a PZT stack that is bonded on the bottom face of a glass cylinder that acts as an acoustic delay line [1]. A burst of signal (2µs) is sent by the PZT toward the glass coverslip, travels through the delay line, bounces at the interfaces and returns with a delay to the PZT where it is detected. Amplitudes of the echo is then compared to input signal in order to extract a reflexion coefficient (r). Experiments are performed both on droplets of healthy blood deposited on glass coverslips and in PDMS microfluidic circuits at low flow strength and at rest. In particular, a spiral shaped microchip, as can be seen in Figure 2, is used both to mimic a vascular network (160x150µm section channels) and to expose a large surface (about 25 mm2) of the sample to the ultrasonic signal [2]. The detection of r over time is coupled with observations of RBCs behaviour during the sample desiccation by optical microscopy to elucidate the dynamics of the drying process of blood. All measurements are carried out at controlled temperature (25 °C) and relative humidity (30-60%). Note that the acoustic power (<1pJ) sent to the blood is small and does not interact with cells through acoustophoresis mechanisms. (a) (b) Figure 1: (a) Schematic (not to scale) of the ultrasound pulse echo setup for the analysis of liquid gel transition in steady blood flow due to red blood cells aggregation. (b) Photograph of the spiral shaped microfluidic circuit bonded on a microscope glass blade a coupled to an ultrasonic pulse echo system. The spiral channel is 160μm wide, 150μm for a total diameter of diameter of 6mm.