Acoustic investigation of pressure-dependent resonance and shell elasticity of lipid-coated monodisperse microbubbles

In this study, frequency-dependent attenuation was measured acoustically for monodisperse lipid-coated microbubble suspensions as a function of excitation pressure and radius. The resonance frequency was identified from the attenuation spectra and had an inverse relationship with mean microbubble diameter and excitation pressure. A reduction in the estimated shell elasticity constant from 0.50 N/m to 0.29 N/m was observed as the excitation pressure was increased from 25 kPa to 100 kPa, respectively, which suggests a nonlinear relationship exists between lipid shell stiffness and applied strain. These findings support the viewpoint that lipid shells coating microbubbles exist as heterogeneous mixtures that undergo dynamic and rapid variations in mechanical properties under applied strains.