Concentration-Dependent Viscoelasticity of Poloxamer-Shelled Microbubbles

Abstract

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.

Controlled Tempering of Lipid Concentration and Microbubble Shrinkage as a Possible Mechanism for Fine-Tuning Microbubble Size and Shell Properties

“…With the MB size distribution known, the MB shell stiffness ( S p in N/m) and shell friction ( S f in kg/s) are estimated by fitting a linearized encapsulated MB model proposed by de Jong et al , to the measured attenuation curves. The linearized model has been used extensively in previous literature studies; ,− it neglects multiple scattering effects and assumes low amplitude bubble oscillations (both of which are reasonable considering the high MB dilution ratios and low acoustic pressure employed in our experiments). For more information about the model, one can refer to previously published literature. , The size distribution of MBs is directly used to obtain the estimated attenuation per unit distance using the following equation α normale normals normalt normali normalm normala normalt normale normald ( r , f ) = 10 ln ( 10 ) ∑ r n ( r ) σ s ( r , f ) δ normalt normalo normalt ( r , f ) δ normalr normala normald ( r , f ) Here, n is the MB number density from measurements obtained using Coulter counter, σ s is the scattering cross sections of the MBs, and δ tot is the total damping which is the sum of the radiation (δ rad ), viscous (δ vis ), and shell friction (δ sh ) terms.…”

“…The ultrasound attenuation of the monodisperse lipid-coated MBs is measured through the transmission and reception method. This is a widely accepted method for characterization of the shell properties of MBs. ,− The schematic representation of our experimental setup is shown in Figure . In our experiments, we either use one pair of 2.25 MHz unfocused transducers (C304-SU, 2.25 MHz, 1″ diameter, Olympus NDT, USA) or one pair of 1 MHz unfocused transducers (C302-SU, 1 MHz, 1″ diameter, Olympus NDT, USA).…”

“…The MB sample is interrogated using trains of broadband pulses with a pulse repetition frequency of 100 Hz and averaged over 64 signals. By using a broadband transmission similar to what other researchers in the field have used, ,− the response of the MBs over a range of frequencies can be obtained in a short time. The pulse lengths of the 1 and 2.25 MHz center frequency transducers were approximately 2.6 and 1.4 μs, respectively.…”

“…An attenuator (50B-001 BNC, JFW Industries Inc., Indianapolis, IN, USA) is used to minimize potential nonlinear effects ,, by keeping the peak negative pressure sufficiently small. Low pressure sonication in a linear regime has also been recently applied by Tabata et al…”

“…An attenuator (50B-001 BNC, JFW Industries Inc., Indianapolis, IN, USA) is used to minimize potential nonlinear effects27,47,48 by keeping the peak negative pressure sufficiently small. Low pressure sonication in a linear regime has also been recently applied by Tabata et al44 Shell Property Estimates. With the MB size distribution known, the MB shell stiffness (S p in N/m) and shell friction (S f in kg/s) are estimated by fitting a linearized encapsulated MB model proposed by de Jong et al…”