Concentration-Dependent Viscoelasticity of Poloxamer-Shelled Microbubbles

The oscillation of shelled microbubbles during exposure to ultrasound is influenced by the mechanical properties of the shell components. The oscillation behavior of bubbles coated with various phospholipids and other amphiphiles has been studied. However, there have been few investigations of how the adsorption conditions of the shell molecules relate to the viscoelastic properties of the shell and influence the oscillation behavior of the bubbles. In the present study, we investigated the oscillation characteristics of microbubbles coated with a poloxamer surfactant, that is, Pluronic F-68, at several concentrations after the adsorption kinetics of the surfactant at the gas-water interface had reached equilibrium. The dilatational viscoelasticity of the shell during exposure to ultrasound was analyzed in the frequency domain from the attenuation characteristics of the acoustic pulses propagated in the bubble suspension. At Pluronic F-68 concentrations lower than 2.0 x 10-2 mol L-1, the attenuation characteristics typically exhibited a sharp peak. At concentrations higher than 2.0 x 10-2 mol L-1, the peak flattened. The dilatational elasticity and viscosity of the shell were estimated by fitting the theoretical model to the experimental values, which revealed that both the elasticity and viscosity increased markedly at approximately 2.0 x 10-2 mol L-1. This suggests that the adsorption properties of Pluronic F-68 strongly affect the oscillation characteristics of microbubbles of a size suitable for medical ultrasound diagnostics.

Automated summary

This study investigated the relationship between the adsorption conditions of Pluronic F-68 and the viscoelastic properties as well as oscillation characteristics of microbubbles. The experimental results showed that when the concentration of Pluronic F-68 was below 2.0 x 10-2 mol L-1, the oscillation characteristics exhibited a sharp peak, while above this concentration, the peak flattened. By fitting the experimental data, it was found that the elasticity and viscosity of the shell significantly increased at approximately 2.0 x 10-2 mol L-1. This suggests that the adsorption properties of Pluronic F-68 strongly influence the oscillation characteristics of microbubbles suitable for medical ultrasound diagnostics.