Vibration Characteristics and Persistence of Poloxamer- or Phospholipid-Coated Single Microbubbles under Ultrasound Irradiation

Quantitative estimation of phospholipid molecules desorbed from a microbubble surface under ultrasound irradiation

Microbubbles have potential applications as drug and gene carriers, and drug release can be triggered by externally applied ultrasound irradiation while inside blood vessels. Desorption of molecules forming the microbubble shell can be observed under ultrasound irradiation of a single isolated microbubble, and the volume of desorbed molecules can be quantitatively estimated from the contact angle between the bubble and a glass plate. Microbubbles composed of a 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) shell and a poorly-soluble gas are created. When the microbubbles are exposed to a pulsed ultrasound, the contact angles increase dramatically; the percentage of DMPC molecules desorbed from the bubble surface reaches 70%. Vibration of a single bubble in the radial direction is measured using a laser Doppler vibrometer. The relationship between the vibrational characteristics and the amount of molecular desorption reveals that a larger vibrational amplitude of the bubble around the resonance size induces a larger amount of molecular desorption. These results support the possibility of controlling molecular desorption with pulsed ultrasound.

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 × 10^-2 mol L^-1, the attenuation characteristics typically exhibited a sharp peak. At concentrations higher than 2.0 × 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 × 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.