Acoustic Levitation: A Novel Approach to Study the Physical Properties of Blood in Normal Subjects and in Patients with Sickle Cell Disease

Acoustic levitation is a novel technique which we have developed for the purpose of measuring physical properties of blood, plasma and red cells in a non-contact setting, which eliminates issues and concerns related to the interaction of blood with non-physiological surfaces. Figure 1 shows a levitated blood drop between the ultrasound transducer and reflector. By exciting drop shape oscillation, we can study its vibrational behavior and infer its viscoelastic properties.

Whole blood samples were collected from volunteers in tubes containing Ethylenediaminetetraacetic acid (EDTA) as anticoagulant. Patients' samples were studied using left-over material after a clinically-ordered CBC had been performed. Samples with different hematocrit (Hct) were obtained in the same subject, by removing or adding autologous plasma after centrifugation.

To calibrate the device, we compared the results obtained by our instrument with the blood viscosity determination provided by a cone-plate rotational rheometer (HR-2, Hybrid rheometer - TA Instrument). Figure 2 (Left) shows the results from levitator and rotational rheometer.

We also compared normal samples with samples of patients with sickle cell disease (SCD) over a wide range of Hct values, obtained by addition or removal of autologous plasma. As shown in Figure 2 (Right), for any given Hct the viscosity measurements obtained by the levitation device were greater in blood of patients with sickle cell disease compared with normal controls.

For each sample of blood from patients with sickle cell disease, we assessed viscosity with the levitation device at different values of Hct (autologous plasma addition or removal) and were able to determine the slope of this regression line. We also compared the viscosity value determined at an arbitrary Hct value of 55% among all the clinical samples collected from patients with SCD. As shown in Figure 3, when these data were analyzed as a function of the fetal Hb content of each blood sample (obtained from the total Hb value of the unmodified clinical sample and the % of Hb F determined by HPLC), a clear negative correlation emerged between total Hb F values (in g/dl) and viscosity, measured as either the slope of the regression line or the value at a fixed Hct of 55%. The clinical samples included several patients treated with hydroxycarbamide, showing an important correlation between higher values of Hb F and lower blood viscosity.

These studies demonstrate that acoustic levitation is a non-contact technique which can be successfully used to measure physical properties of human blood. We show that viscosity measurements obtained with this new device compare very favorably with traditional rotational viscometer. Furthermore, this new technology can reproducibly measure viscosity over a wide range of Hct values and for any given Hct, it shows that the viscosity of samples from patients with SCD is greater than normal controls. We also show that our measurement is sensitive enough to detect variation in viscosity associated with different absolute values of fetal Hb in blood, and more specifically showing lower viscosity with higher Hb F values, opening the possibility that this technique could be used to determine changes in viscosity associated with hydroxycarbamide therapy in patients with sickle cell disease.

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