On the fine-tuning of the size and resonance frequency of microfluidically-generated monodisperse microbubbles

Monodisperse microbubbles with diameters less than 10 μm are desirable in several ultrasound imaging and therapeutic delivery applications. Microfluidics technology has the unique advantage of generating size-controlled monodisperse microbubbles, and it is now well-established that the diameter of microfluidically-made bubbles can be tuned by varying the liquid flow rate, gas pressure, and the dimensions of the microfluidic channel. It is also observed that once the microbubbles form, the bubbles shrink and eventually stabilize to a final equilibrium diameter.

However, how the lipid solution concentration affects the degree of i) bubble shrinkage, ii) final stable size of microbubbles, iii) microbubble resonance frequency, and iv) microbubble shell viscoelasticity has not been thoroughly examined. Additionally, how the MB shell viscoelasticity and resonance frequency vary with changes in bubble manufacturing method is still unknown.

This thesis is based on the hypothesis that the arrangement of lipids on the gas-liquid interface of microbubbles affects their acoustic behavior. The experiments were carried out to examine approaches for controlling the properties of bubbles, such as their size and the viscoelasticity of their shells. Specifically, the goals of this project are to i) generate monodisperse bubbles using a flow-focusing microfluidic device and observe the effect of gas composition and lipid concentration on the degree of microbubble shrinkage, ii) investigate the influence of microbubble final diameter and lipid concentration in the aqueous phase on the resonance frequency and shell viscoelastic properties of microbubbles, and iii) compare the microbubble resonance frequency and shell viscoelasticity on the linear behavior of microbubbles for microbubbles that have been size-sorted following conventional agitation and microbubbles generated using microfluidics.

Our results indicate that with increasing lipid concentration, the degree of microbubble shrinkage is decreased. We observe no dependency of microbubble shrinkage on gas composition. We also demonstrate that the resonance frequency increases by 180-210% with increasing lipid concentration (from 5.6 to 16.0 mg/mL) for the same bubble diameter. We also determine that microbubble shell viscoelasticity increases with increasing lipid concentration and microbubble final diameter, and the level of change depends on the degree of microbubble shrinkage. Finally, our results demonstrate that microbubbles that have been size-sorted following agitation resonate at higher resonance frequencies in comparison with microbubbles of the same final mean diameter generated using microfluidics. We anticipate that this approach for generating and tuning the size and shell viscoelasticity of monodisperse microbubbles will find utility in biomedical applications, such as contrast-enhanced ultrasound imaging and ultrasound-triggered gene delivery.