Response to ultrasound of two types of lipid-coated microbubbles observed with a high-speed optical camera

Abstract

Microbubbles (MBs) can be coated with different lipids, but exact influences on acoustical responses remain unclear. The distribution of lipids in the coating of homemade MBs is heterogeneous for DSPC and homogeneous for DPPC-based MBs, as observed with 4Pi confocal microscopy. In this study, we investigated whether DSPC and DPPC MBs show a different vibrational response to ultrasound. MBs composed of main lipid DSPC or DPPC (2 C-atoms less) with a C4F10 gas core, were made by sonication. Microbubble spectroscopy was performed by exciting single MBs with 10-cycle sine wave bursts having a frequency from 1 to 4 MHz and a peak negative pressure of 10, 20, and 50 kPa. The vibrational response to ultrasound was recorded with the Brandaris 128 high-speed camera at 15 Mfps. Larger acoustically induced deflation was observed for DPPC MBs. For a given resting diameter, the resonance frequency was higher for DSPC, resulting in higher shell elasticity of 0.26 N/m as compared to 0.06 N/m for DPPC MBs. Shell viscosity was similar (~10-8 kg/s) for both MB types. Non-linear behavior was characterized by the response at the subharmonic and second harmonic frequencies. More DPPC (71%) than DSPC MBs (27%) showed subharmonic response, while the behavior at the second harmonic frequency was comparable. The different acoustic responses of DSPC and DPPC MBs are likely due to the choice of the main lipid and the corresponding spatial distribution in the MB coating.