Characterization of phospholipid monolayer viscoelasticity using microbubble acoustic radiation force*

Lipid monolayers (LM) stabilize the gas/water interface of the lung, ear and eye, and serve as model membrane for understanding the properties of bilayer leaflets. However, the nature of viscoelasticity of gel-phase LM for dilatational deformations remains poorly characterized. Microbubbles offer a novel experimental platform to study LM viscoelasticity, and their behavior is directly relevant to medical ultrasonics applications. The acoustic radiation force acting on a microbubble (MB) is maximal at the resonance frequency, which depends on its size and the LM shell viscoelastic properties. Here, we demonstrate a method using multi-gate spectral Doppler with an open source ultrasound scanner to measure the acoustic radiation force displacements of individual resonant MBs as we scan a frequency range between 3 and 7 MHz. The LM shell viscoelastic properties are determined by fitting the measured displacements to a modified Rayleigh-Plesset equation with Sarkar shell terms. Our results show the effects of lipid composition and temperature. These results provide mechanistic insights into LM dilatational viscoelasticity and illustrate rational design principles to engineer MBs for ultrasound imaging and therapy.