Microbubble Shell Research Articles

Nanoparticle-shelled Microbubbles Used for Medical Ultrasound Nonlinear Imaging

The nonlinear property of microbubbles is important for ultrasound contrast imaging. Many technologies have been proposed to enhance the harmonic and subharmonic ability of the microbubbles. In this study, microbubbles with inclusion of different concentration of Fe3O4 nanoparticles in the shell were prepared and their acoustic signals were investigated. Results have shown that the acoustic scattering cross-section of microbubbles would change with the different concentration of Fe3O4 nanoparticles inclusion. When the Fe3O4 concentration is 86.47μg/ml, the microbubbles possess best harmonic acoustic response and maintain better in vitro nonlinear contrast imaging effect and longer life time. Therefore, by optimizing the nanoparticle concentration in the shell of microbubbles can make the microbubbles multifunctioned and at the same time retaining the excellent nonlinear acoustic response property.

Quantitative characterization of viscoelasticity of microbubbles in ultrasound contrast agent

Ultrasound contrast agent (UCA) microbubbles have been commonly used in clinic to enhance the acoustic backscattering signals in ultrasound imaging diagnosis. With increasing demand for the continuous improvement of imaging resolution and sensitivity, new type UCAs (e.g., targeted microbubbles and multifunctional microbubbles) have attracted growing interest in both medical and scientific communities. Many efforts have been made to modify microbubble shell properties, which can strongly affect microbubble dynamic behaviors, so as to enable to create some new functionalities of UCAs. However, accurate characterization of the shell mechanical properties of UCAs has been recognized to be rather challenging. In previous work, microbubble’s mechanical properties are normally estimated by fitting measured dynamic response signals with coated-microbubble models. Inevitable uncertainty will be introduced in fitting results because there are more than one unknown shell parameters are adopted in these dynamic models. In the present paper, a comprehensive approach is developed to quantitatively characterize the visco-elasticity of the encapsulated microbubbles. By combining the techniques of atomic force microscopy (AFM), single particle optical sensing (SPOS), acoustic attenuation measurement, and the coated-bubble dynamics simulation, the size distribution, shell thickness, shell elasticity and viscosity of UCA microbubbles are determined one by one in sequence. To examine the validity of this approach, a kind of albumin-shelled microbubbles with diameters ranging from 1 to 5 μm are fabricated in our lab. Based on AFM technology, the microbubble effective shell stiffness and bulk elasticity modulus are measured to be 0.149±0.012 N/m and 8.31±0.667 MPa, respectively. It is noteworthy that the shell elastic property is shown to be independent of the initial size of microbubbles. Furthermore, the size distribution and acoustic attenuation measurements are also performed of these bubbles. Then, combined with microbubble dynamic model simulations, the UCA shell viscosity is calculated to be 0.374±0.003 Pa·s. Compared with previous estimation method, the current technology can be used as an effective tool to assess UCA shell visco-elasticity with improved accuracy and certainty. It is also shown that the feasibility to optimize the design and fabrication of UCAs can satisfy different requirements in ultrasound diagnostic and therapeutic applications.

Investigation of Microbubble Composition on Ultrasonic Dispersion Properties for Biosensing Applications

Lipid shelled microbubbles are gaining attention as possible biosensors for monitoring the microbubble's in-vivo environment. These novel applications require the microbubble shell and gas components to be readily responsive to environmental changes. Since the ultrasonic properties of the microbubbles, for instance the resonance frequency or attenuation, are inherently related to the material properties of the monolayer shell such as visco-elasticity and thickness as well as on the physical properties of the encapsulated gas, it is important to investigate the influence of the shell composition and gas content on the ultrasonic behavior as well as the change in response after modifications of the microbubble environment. In this study, homemade microbubbles are characterized using ultrasonic through-transmission measurements in the range of 125kHz to 10MHz, thereby providing the dispersion relations of phase velocity and attenuation. Using this approach, the evolution of the dispersion properties of such bubbly media in time has been followed in order to detect changes in microbubble stability. In addition, several microbubble populations have been subjected to thermal changes to investigate their temperature dependence. The experimental observations have been compared to results from a nonlinear least squares fitting procedure with a theoretical model accounting for linear as well as nonlinear bubble behaviour. As such, the model allows to give a semi-quantitative interpretation of the dynamic behaviour and evolution of a microbubble population in the medium.

Perfluoroalkylated poly(oxyethylene) thiols: Synthesis, adsorption dynamics and surface activity at the air/water interface, and bubble stabilization behaviour

New amphiphiles with a poly(oxyethylene) spacer inserted between a thiol function and a F-alkylated chain have been synthesized. The F -alkylated POE thiols adsorb rapidly at the air/water interface and decrease the tension of water to values as low as ∼16 mN m −1 . They form microbubbles in aqueous solutions with diameters smaller than 10 μm. These microbubbles were stable for days when a fluorocarbon gas is present in their gas phase. • New amphiphiles with a poly(oxyethylene) spacer inserted between a thiol function and a F -alkylated chain have been synthesized. • The F -alkylated POE thiols adsorb rapidly at the air/water interface. • They decrease the interfacial tension of water to values as low as ∼16 mN m −1 . • They form microbubbles in aqueous solutions with diameters smaller than 10 μm. • The microbubbles were stable for days when a fluorocarbon gas is present in their gas phase. We report the synthesis of four new perfluoroalkylated thiols, C n F 2 n +1 (CH 2 ) 2 (OCH 2 CH 2 ) y SH ( FnH 2(OE) y with n = 4 and 6; y = 4 and 6, that have a hydrophilic poly(oxyethylene) (POE) segment inserted between the hydrophobic fluorinated chain and the thiol functional group. The dynamics of adsorption and surface activity of these compounds at the air/water interface were studied using bubble profile analysis tensiometry. The adsorption kinetics profiles show that all compounds diffuse rapidly to the air/water interface and decrease the interfacial tension of water to values ranging from 16.5 to 31.6 mN m −1 , which qualifies them as potential components of microbubble shells. Their ability to self-assemble and stabilize gaseous microbubbles in the presence or absence of a fluorocarbon gas (perfluorohexane) was investigated by optical microscopy. Three out of the four compounds (the two F 6 derivatives and F 4 H 2(OE) 6 ) were able to form and stabilize microbubbles. Microbubbles based on both F 6 PEG thiols are more stable than those based on the F 4 derivative. The number of oxyethylene (OE) groups substantially influences bubble stability; the higher the number of OEs, the more stable the microbubbles. Finally, the most important stabilising effect was obtained by introducing an internal fluorocarbon gas, whose presence in the bubble increases bubble stability from 6 h to 5 days for the most promising candidate, F 6 H 2(OE) 6 SH.

Mechanical and dynamic characteristics of encapsulated microbubbles coupled by magnetic nanoparticles as multifunctional imaging and drug delivery agents

Development of magnetic encapsulated microbubble agents that can integrate multiple diagnostic and therapeutic functions is a key focus in both biomedical engineering and nanotechnology and one which will have far-reaching impact on medical diagnosis and therapies. However, properly designing multifunctional agents that can satisfy particular diagnostic/therapeutic requirements has been recognized as rather challenging, because there is a lack of comprehensive understanding of how the integration of magnetic nanoparticles to microbubble encapsulating shells affects their mechanical properties and dynamic performance in ultrasound imaging and drug delivery. Here, a multifunctional imaging contrast and in-situ gene/drug delivery agent was synthesized by coupling super paramagnetic iron oxide nanoparticles (SPIOs) into albumin-shelled microbubbles. Systematical studies were performed to investigate the SPIO-concentration-dependence of microbubble mechanical properties, acoustic scattering response, inertial cavitation activity and ultrasound-facilitated gene transfection effect. These demonstrated that, with the increasing SPIO concentration, the microbubble mean diameter and shell stiffness increased and ultrasound scattering response and inertial cavitation activity could be significantly enhanced. However, an optimized ultrasound-facilitated vascular endothelial growth factor transfection outcome would be achieved by adopting magnetic albumin-shelled microbubbles with an appropriate SPIO concentration of 114.7 µg ml−1. The current results would provide helpful guidance for future development of multifunctional agents and further optimization of their diagnostic/therapeutic performance in clinic.

Characterizing the pressure field in a modified flow cytometer quartz flow cell: A combined measurement and model approach to validate the internal pressure

We incorporated an ultrasound transducer into a flow cytometer to activate microbubbles passing the laser interrogation zone (J. Acoust. Soc. Am. 126, 2954–2962, (2009)). This system allows high throughput recording of the volume oscillations of microbubbles, and has led to a new bubble dynamics model that incorporates shear thinning (Phys. Med. Biol. 58, 985–998 (2013)). Important parameters in the model include the ambient microbubble size, R0, driving pressure, PA, and the shell parameters χ and κ, the shell elasticity and viscosity, respectively. R0 is obtained by calibrating the cytometer. Pressure calibration is difficult because the flow channel width (<200µm) is too small to insert a hydrophone. The objective of this study was to develop a calibration method for a 20-cycle, 1 MHz transient pressure field. The pressure field propagating through the channel and into water was compared to a 3-D FEM model. After validation, the model was used to simulate the driving pressure as input for the bubble dynamics model, leaving only χ and κ variables. This approach was used to determine the mechanical properties for different bubbles (albumin, lipid, and lyzozyme shells). Excellent fits were obtained in many cases, but not all, suggesting heterogeneity in microbubble shell parameters.

Theranostic USPIO-Loaded Microbubbles for Mediating and Monitoring Blood-Brain Barrier Permeation.

Efficient and safe drug delivery across the blood-brain barrier (BBB) remains to be one of the major challenges of biomedical and (nano-) pharmaceutical research. Here, we show that poly(butyl cyanoacrylate)-based microbubbles (MB), carrying ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles within their shell, can be used to mediate and monitor BBB permeation. Upon exposure to transcranial ultrasound pulses, USPIO-MB are destroyed, resulting in acoustic forces inducing vessel permeability. At the same time, USPIO are released from the MB shell, they extravasate across the permeabilized BBB and they accumulate in extravascular brain tissue, thereby providing non-invasive R2*-based magnetic resonance imaging information on the extent of BBB opening. Quantitative changes in R2* relaxometry were in good agreement with 2D and 3D microscopy results on the extravascular deposition of the macromolecular model drug FITC-dextran into the brain. Such theranostic materials and methods are considered to be useful for mediating and monitoring drug delivery across the BBB, and for enabling safe and efficient treatment of CNS disorders.

Integration of surface-active, periodically sequenced peptides into lipid-based microbubbles.

The development of microbubbles toward functional, "theranostic" particles requires the incorporation of constituents with high binding specificity and therapeutic efficacy. Integrating peptides or proteins into the shell of lipid-based microbubbles can provide a means to access both receptor-ligand interactions and therapeutic properties. Simultaneously, peptides or proteins can define the characteristic monolayer mechanics of lipid bubbles and eliminate the need for post-bubble generation modification. The ability to engineer peptide sequences de novo that effectively partition into the bubble monolayer remains parametrically daunting. This work contributes to this effort using two simple amphipathic helical peptides that examine the role of local electrostatics and secondary structure. The two periodically sequenced peptides both have three positive charges, but peptide "K-2.5" spaces those charges 2.5 amino acids apart, while peptide "K-6.0" spaces the charges six amino acids apart. Size populations were determined for bubbles containing each peptide species using light scattering, and a quantitative method was developed to clearly define the fraction of peptides binding onto the microbubble monolayer. The impact of both the initial peptide concentration and the zwitterionic:anionic lipid ratio on peptide binding was also evaluated. Our results indicate that the lipid ratio affected only K-6.0 binding, which appears to be an outcome of the greater ensemble average α-helical population of the K-6.0. These findings provide further insights into the role of charge separation on peptide secondary structure, establishing a simple design metric for peptide binding onto microbubble systems.

PH-controlled microbubble shell formation and stabilization.

We report on microbubbles with a shell self-assembled from an anionic perfluoroalkylated surfactant, perfluorooctyl(ethyl)phosphate (F8H2Phos). Microbubbles were formed and effectively stabilized from aqueous solutions of F8H2Phos at pH 5.6-8.5. This range overlaps the domains of existence of the monosodic and disodic salts. The shell morphology of microbubbles formed spontaneously by heating aqueous solutions of F8H2Phos was monitored during cooling, directly on the microscope's stage. At pH 5.6, the shell collapses through nucleation of folds, as typical for insoluble surfactants. At pH 8.5, no folds were seen during shrinking. At higher pH, the microbubbles rapidly adsorb on the glass. The effect of pH (from 5.6 to 9.7) on adsorption kinetics of F8H2Phos at the air/water interface, and on the elasticity of its Gibbs films, was determined. At low pH, F8H2Phos is highly surface active. The interfacial film undergoes a dilute-to-condensed phase transition and a dramatic increase of elastic module, leading to extremely high values (up to 500 mN m(-1)). At high pH, the surfactant's adsorption is quasi-instantaneous, but interfacial tension lowering is limited, leading to very low elastic module (∼5 mN m(-1)). At pH 5.6 and 8.5, the interfacial tension of F8H2Phos adsorbed on millimetric bubbles and compressed at a rate similar to that exerted on micrometric bubbles during deflation is lower than the equilibrium interfacial tension. Langmuir monolayers of F8H2Phos are highly stable at low pH and feature a liquid expanded/liquid condensed transition; at high pH, they do not withstand compression. Both mono- and disodic F8H2Phos salts are needed to effectively stabilize microbubbles: the rapidly adsorbed disodic salt stabilizes a newly created air/water interface; the more surface active monosodic salt then replaces the more water-soluble disodic salt at the interface. During deflation, the surfactant shell undergoes a transition toward a highly elastic phase, which further contributes to bubble stabilization.

Scaling of the viscoelastic shell properties of phospholipid encapsulated microbubbles with ultrasound frequency

Phospholipid encapsulated microbubbles are widely employed as clinical diagnostic ultrasound contrast agents in the 1–5MHz range, and are increasingly employed at higher ultrasound transmit frequencies. The stiffness and viscosity of the encapsulating “shells” have been shown to play a central role in determining both the linear and nonlinear response of microbubbles to ultrasound. At lower frequencies, recent studies have suggested that shell properties can be frequency dependent. At present, there is only limited knowledge of how the viscoelastic properties of phospholipid shells scale at higher frequencies. In this study, four batches of in-house phospholipid encapsulated microbubbles were fabricated with decreasing volume-weighted mean diameters of 3.20, 2.07, 1.82 and 1.61μm. Attenuation experiments were conducted in order to assess the frequency-dependent response of each batch, resulting in resonant peaks in response at 4.2, 8.9, 12.6 and 19.5MHz, respectively. With knowledge of the size measurements, the attenuation spectra were then fitted with a standard linearized bubble model in order to estimate the microbubble shell stiffness Sp and shell viscosity Sf, resulting in a slight increase in Sp (1.53–1.76N/m) and a substantial decrease in Sf (0.29×10−6–0.08×10−6kg/s) with increasing frequency. These results performed on a single phospholipid agent show that frequency dependent shell properties persist at high frequencies (up to 19.5MHz).

Aggregate Enhanced Trimodal Porphyrin Shell Microbubbles for Ultrasound, Photoacoustic, and Fluorescence Imaging

Microbubbles (MBs) are currently used as ultrasound (US) contrast agents and as delivery vehicles for site-specific US-triggered drug and gene delivery. Multimodal US-based imaging methods have been applied preclinically to assess and validate the effectiveness and fate of MBs in imaging and therapy. Here we present the first intrinsically trimodal MBs by incorporating a dense concentration of porphyrin molecules within a MB shell, enabled by the use of a single porphyrin-lipid component. These MBs possess US, photoacoustic, and fluorescence properties that are demonstrated in solution and in a mouse tumor xenograft model. They also have potential to be extended to other imaging modalities such as magnetic resonance imaging and nuclear imaging.

Development of therapeutic microbubbles for enhancing ultrasound-mediated gene delivery

Ultrasound (US)-mediated gene delivery has emerged as a promising non-viral method for safe and selective gene delivery. When enhanced by the cavitation of microbubbles (MBs), US exposure can induce sonoporation that transiently increases cell membrane permeability for localized delivery of DNA. The present study explores the effect of generalizable MB customizations on MB facilitation of gene transfer compared to Definity®, a clinically available contrast agent. These modifications are 1) increased MB shell acyl chain length (RN18) for elevated stability and 2) addition of positive charge on MB (RC5K) for greater DNA associability. The MB types were compared in their ability to facilitate transfection of luciferase and GFP reporter plasmid DNA in vitro and in vivo under various conditions of US intensity, MB dosage, and pretreatment MB–DNA incubation. The results indicated that both RN18 and RC5K were more efficient than Definity®, and that the cationic RC5K can induce even greater transgene expression by increasing payload capacity with prior DNA incubation without compromising cell viability. These findings could be applied to enhance MB functions in a wide range of therapeutic US/MB gene and drug delivery approach. With further designs, MB customizations have the potential to advance this technology closer to clinical application.

The Delayed Onset of Subharmonic and Ultraharmonic Emissions from a Phospholipid-Shelled Microbubble Contrast Agent

Characterizing the non-linear response of microbubble contrast agents is important for their efficacious use in imaging and therapy. In this article, we report that the subharmonic and ultraharmonic response of lipid-shelled microbubble contrast agents exhibits a strong temporal dependence. We characterized non-linear emissions from Targestar-p microbubbles (Targeson Inc., San Diego, CA, USA) periodically for 60 min, at 10 MHz excitation frequency. The results revealed a considerable increase in the subharmonic and ultraharmonic response (nearly 12–15 and 5–8 dB) after 5–10 min of agent preparation. However, the fundamental and the harmonic response remained almost unchanged in this period. During the next 50 min, the subharmonic, fundamental, ultraharmonic, and harmonic responses decreased steadily by 2–5 dB. The temporal changes in the non-linear behavior of the agent appeared to be primarily mediated by gas-exchange through the microbubble shell; temperature and prior acoustic excitation based mechanisms were ruled out. Further, there was no measurable change in the agent size distribution by static diffusion. We envisage that these findings will help obtain reproducible measurements from agent characterization, non-linear imaging, and fluid-pressure sensing. These findings also suggest the possibility for improving non-linear imaging by careful design of ultrasound contrast agents.

A Novel Microfluidic Chip for Assessing Dynamic Adhesion Behavior of Cell-Targeting Microbubbles

The primary aim of this study was to develop a microfluidic chip to study the dynamic adhesion behavior of cell-targeted microbubbles. The microfluidic device is composed of polydimethylsiloxane and is fabricated using the soft lithography technique. Each chamber of the microfluidic chip comprises eight U-shaped microsieves, by which various flow velocity distributions are generated. LyP-1-conjugated microbubbles were prepared by coating the surface of the phospholipid shell of microbubbles with LyP-1 peptides via biotin-avidin linkage. Under static conditions, the resulting targeted microbubbles are able to bind onto the surface of cells on incubation with breast cancer cells. Under dynamic fluid conditions, the cell targeting efficiency of the microbubbles was assessed at various flow velocity distributions in a chamber. Accumulation of targeted microbubbles was strongly influenced by flow velocity. Better retention of targeted microbubbles on cell surfaces was achieved at low mean flow velocities (<0.03 cm/s), in agreement with our computer simulation results. In conclusion, our results indicate that the microfluidic system is a useful platform for studying the microbubble-cell adhesive interaction.

Mechanical Characterization of Ultrasonically Synthesized Microbubble Shells by Flow Cytometry and AFM

The mechanical properties of the shell of ultrasonically synthesized lysozyme microbubbles, LSMBs, were evaluated by acoustic interrogation and nanoindentation techniques. The Young's modulus of LSMBs was found to be 1.0 ± 0.3 MPa and 0.6 ± 0.1 MPa when analyzed by flow cytometry and AFM, respectively. The shell elasticity and Young's modulus were not affected by the size of the microbubbles (MBs). The hydrogel-like protein shell of LSMBs offers a softer, more elastic and viscous interface compared to lipid-shelled MBs. We show that the acoustic interrogation technique is a real-time, fast, and high-throughput method to characterize the mechanical characteristics of air-filled microbubbles coated by a variety of materials.

Binding and isolation of tumor cells in biological media with perfluorocarbon microbubbles

With the emerging interest in personalized medicine, there is strong demand for new technologies for clinical sample interrogation. Exfoliated tumor cells in variety of pathological samples (e.g., blood, bone marrow, urine) could provide invaluable information for diagnosis and prognosis of cancers. Here we describe a detailed method for capture and isolation of tumor cells in medium, blood, or large issue buffy coat using EpCAM-targeted buoyant microbubbles (MBs). Perflorohexane gas lipid shell MBs were prepared with emulsification method and conjugated with antibody as described by us before [25]. The binding of EpCAM-targeted MBs to A549 (human lung carcinoma) and 4T1 (mouse breast carcinoma) cells spiked into BSA/PBS or blood was more than 90%, which was comparable with commercial anti-EpCAM immunomagnetic beads (DynaBeads). Anti-EpCAM MBs efficiently (75–82%) isolated BxPC3 pancreatic tumor cells spiked into medium, blood or a buffy coat, within 15–30min of incubation. We discuss MB parameters and experimental conditions critical to achieve efficient cells binding and isolation. In conclusion, MB-assisted cell isolation is a promising method for rapid enrichment of cells and biomarkers from biological samples.

Theoretical characterisation of the radial and translational motion of coated microbubbles under acoustic excitation

Ultrasound contrast agents, in the form of coated microbubbles, are a powerful tool in current diagnostic imaging. Given their sensitive dynamic response they also have the potential to be used for quantitative measurements of the properties of the surrounding tissue (e.g. percentage perfusion or blood pressure). For this potential to be realised, however, the theoretical descriptions of bubble behaviour, in particular the constitutive equations for the microbubble shell, need to be improved and a method needs to be developed for the accurate characterisation of individual bubbles. In this paper the first steps are taken towards deriving a complete model for the coupled radial and translational motion of a coated bubble. It is then shown that with this model the bubble can be characterised by a unique set of parameters describing the bubble shell corresponding to its viscous and elastic response. This uniqueness will enable the model to be used to interpret experimental data and quantify these parameters for which accurate values are currently lacking but which are critical to predicting bubble response and hence enabling advanced diagnostic applications.

Mapping microbubble viscosity using fluorescence lifetime imaging of molecular rotors

Encapsulated microbubbles are well established as highly effective contrast agents for ultrasound imaging. There remain, however, some significant challenges to fully realize the potential of microbubbles in advanced applications such as perfusion mapping, targeted drug delivery, and gene therapy. A key requirement is accurate characterization of the viscoelastic surface properties of the microbubbles, but methods for independent, nondestructive quantification and mapping of these properties are currently lacking. We present here a strategy for performing these measurements that uses a small fluorophore termed a "molecular rotor" embedded in the microbubble surface, whose fluorescence lifetime is directly related to the viscosity of its surroundings. We apply fluorescence lifetime imaging to show that shell viscosities vary widely across the population of the microbubbles and are influenced by the shell composition and the manufacturing process. We also demonstrate that heterogeneous viscosity distributions exist within individual microbubble shells even with a single surfactant component.

Influence of Shell Composition on the Resonance Frequency of Microbubble Contrast Agents

The effect of variations in microbubble shell composition on microbubble resonance frequency is revealed through experiment. These variations are achieved by altering the mole fraction and molecular weight of functionalized polyethylene glycol (PEG) in the microbubble phospholipid monolayer shell and measuring the microbubble resonance frequency. The resonance frequency is measured via a chirp pulse and identified as the frequency at which the pressure amplitude loss of the ultrasound wave is the greatest as a result of passing through a population of microbubbles. For the shell compositions used herein, we find that PEG molecular weight has little to no influence on resonance frequency at an overall PEG mole fraction (0.01) corresponding to a mushroom regime and influences the resonance frequency markedly at overall PEG mole fractions (0.050–0.100) corresponding to a brush regime. Specifically, the measured resonance frequency was found to be 8.4, 4.9, 3.3 and 1.4 MHz at PEG molecular weights of 1000, 2000, 3000 and 5000 g/mol, respectively, at an overall PEG mole fraction of 0.075. At an overall PEG mole fraction of just 0.01, on the other hand, resonance frequency exhibited no systematic variation, with values ranging from 5.7 to 4.9 MHz. Experimental results were analyzed using the Sarkar bubble dynamics model. With the dilatational viscosity held constant (10–8 N·s/m) and the elastic modulus used as a fitting parameter, model fits to the pressure amplitude loss data resulted in elastic modulus values of 2.2, 2.4, 1.6 and 1.8 N/m for PEG molecular weights of 1000, 2000, 3000 and 5000 g/mol, respectively, at an overall PEG mole fraction of 0.010 and 4.2, 1.4, 0.5 and 0.0 N/m, respectively, at an overall PEG mole fraction of 0.075. These results are consistent with theory, which predicts that the elastic modulus is constant in the mushroom regime and decreases with PEG molecular weight to the inverse 3/5 power in the brush regime. Additionally, these results are consistent with inertial cavitation studies, which revealed that increasing PEG molecular weight has little to no effect on inethe rtial cavitation threshold in the mushroom regime, but that increasing PEG molecular weight decreases inertial cavitation markedly in the brush regime. We conclude that the design and synthesis of microbubbles with a prescribed resonance frequency is attainable by tuning PEG composition and molecular weight.

Size distribution of microbubbles as a function of shell composition

The effect of modifying the shell composition of a population of microbubbles on their size demonstrated through experiment. Specifically, these variations include altering both the mole fraction and molecular weight of functionalized polymer, polyethylene glycol (PEG) in the microbubble phospholipid monolayer shell (1–15mol% PEG, and 1000–5000g/mole, respectively). The size distribution is measured with an unbiased image segmentation program written in MATLAB which identifies and sizes bubbles from micrographs. For a population of microbubbles with a shell composition of 5mol% PEG2000, the mean diameter is 1.42μm with a variance of 0.244μm. For the remainder of the shell compositions studied herein, we find that the size distributions do not show a statistically significant correlation to either PEG molecular weight or mole fraction. All the measured distributions are nearly Gaussian in shape and have a monomodal peak.