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The acoustic response of microbubbles (MBs) depends on their resonance frequency, which is dependent on MB size and shell properties. Monodisperse MBs with tunable shell properties are thus desirable for optimizing and controlling MB behavior in acoustics applications. By utilizing a novel microfluidic method that uses lipid concentration to control MB shrinkage, we generate monodisperse MBs of four different initial diameters at three lipid concentrations (5.6, 10.0, and 16.0 mg/mL) in the aqueous phase. Following shrinkage, we measure MB resonance frequency and determine its shell stiffness and viscosity. The study demonstrates that we can generate monodisperse MBs of specific sizes and tunable shell properties by controlling MB initial diameter and aqueous phase lipid concentration. Our results indicate that the resonance frequency increases by 180-210% with increasing lipid concentration (from 5.6 to 16.0 mg/mL) while bubble diameter is kept constant. Results depict that the resonance frequency increases by ~195% with increasing lipid concentration from 5.6 to 16.0 mg/mL, for ~11 um final diameter MBs. Additionally, we find that the resonance frequency decreases by ~275% with increasing MB final diameter from 5 to 12um, when we use a lipid concentration of 5.6 mg/mL. We also determine that MB shell viscosity and stiffness increase with increasing lipid concentration and MB final diameter, and the level of change depends on the degree of shrinkage experienced by MB. Specifically, we find that by increasing the concentration of lipids from 5.6 to 16.0 mg/mL, the shell stiffness and viscosity of ~11 um final diameter MBs increase by ~400% and ~200 %, respectively. This study demonstrates the feasibility of fine-tuning the MB acoustic response to ultrasound by tailoring MB initial diameter and lipid concentration.

Correct measurement of the shell properties of coated microbubbles (MBs) is essential to understanding and optimizing their response to ultrasound (US) exposure parameters in diagnostic and therapeutic ultrasound. MBs are surrounded by blood; however, the influence of the surrounding medium charges on the MB properties is poorly understood. This study aims to measure the medium charge interactions with MB shells by measuring the frequency-dependent attenuation of the same size MBs in mediums of varying charge density. In-house lipid-coated MBs with C3F8 gas core were made and were isolated to a mean size of 2.35um. MBs were diluted to ~8*10^5 MBs/mL in distilled water (DW), and two different concentrations of phosphate-buffered saline solution (PBS-1x and PBS-10x). The frequency-dependent attenuation of the MBs solutions was measured using an aligned pair of PVDF transducers with a center frequency of 10MHz and 100% bandwidth. The MB shell properties were estimated by fitting the linear equation to experiments. Using a pendant drop tensiometer, the surface tension of mm-size drops was measured inside DW, PBS-1x and PBS-10x. The frequency of the peak attenuation changes at different salinity levels was 13, 7.5 and 6.25MHz in DW, PBS-1x and PBS-10x, respectively. The attenuation peak increased by ~140% with increasing ion density. MBs' estimated shell elasticity decreased by 64% between DW and PBS-1x and 36% between PBS-1x and PBS-10x. Reduction in the shell stiffness is in qualitative agreement with the drop surface tension measurements. The shell viscosity was reduced by ~40% between DW and PBS-1x and 42% between PBS-1x and PBS-10x. The reduction in the stiffness and viscosity is possibly due to the formation of a densely charged layer around the shell, further reducing the effective surface tension on the MBs.

Understanding the pressure dependence of the nonlinear behavior of ultrasonically excited phospholipid (PL)-stabilized NBs is important for optimizing US exposure parameters for implementations of contrast enhanced ultrasound, critical to molecular imaging. The viscoelastic properties of the shell can be controlled by introduction of membrane additives, such as propylene glycol as a membrane softener or glycerol as a membrane stiffener. We report for the first time, the production of high yield NBs with narrow dispersity and different shell properties. Through precise control over size and shell structure, we show how these shell components interact with the phospholipid membrane, change its structure, affect their viscoelastic properties, and consequently change their acoustic response. A two-photon microscopy technique through a polarity-sensitive fluorescent dye, C-laurdan, was utilized to gain insights on the effect of membrane additives to the membrane structure. We report how the shell stiffness of NBs affects the pressure threshold (Pt) for the sudden amplification in the scattered acoustic signal from NBs. For narrow size NBs with 200 nm mean size, we find Pt to be between 123-245 kPa for the NBs with the most flexible membrane as assessed using C-Laurdan, 465-588 kPa for the NBs with intermediate stiffness and 588-710 kPa for the NBs with stiff membranes. Numerical simulations of the NB dynamics are in good agreement with the experimental observations confirming the dependence of acoustic response to shell properties, thereby substantiating further the development in engineering the shell of UCAs. The viscoelastic dependent threshold behavior can be utilized for significantly and selectively enhancing the diagnostic and therapeutic ultrasound applications of potent narrow size NBs.

Simulation involves predicting responses of a physical system. In this article, we simulate opto-acoustic signals generated in a three-dimensional volume due to absorption of an optical pulse. A separable computational model is developed that permits an order-of-magnitude improvement in computational efficiency over a non-separable model. The simulated signals represent acoustic waves, measured by a probe with a linear transducer array, in a rotated and translated coordinate frame. Light is delivered by an optical source that moves with the probe's frame. A spatio-temporal impulse response for rectangular element transducer geometry is derived using a Green's function solution to the acoustic wave equation. The approach permits fast and accurate simulation for a probe with arbitrary trajectory. For a 3D volume of $n^3$ voxels, computation is accelerated by a factor of $n$. This may potentially have application for opto-acoustic imaging, where clinicians visualize structural and functional features of biological tissue for assessment of cancer and other diseases.