Lipid distribution and viscosity of coated microbubbles

Ultrasound contrast agents consist of gas-filled coated microbubbles with diameters between 1 and 10 μm. Within an ultrasound field, high differences in responses of similar sized microbubbles have been reported. Heterogeneous coating properties have been suggested to be the underlying cause. Until now, properties of this coating, like viscosity have been studied dynamically using a set-up of vibrating microbubbles in an ultrasound field. This study focuses on determining the viscosity of the coating for lipid-coated microbubbles in a static set-up. The viscosity of the coating was determined by measuring the mobility of a fluorescent lipid using Fluorescence Recovery After Photobleaching (FRAP). We found a surface shear viscosity of 8×10^-6 kg/s that was independent of the microbubble size. In addition, we found that the lipid distribution in the coating was heterogeneous and varied from microbubble to microbubble. In conclusion, this study shows that the static surface shear viscosity of the coating can be determined in an independent way which can now be used in microbubble dynamics models.