On the interplay of shell structure with low- and high-frequency mechanics of multifunctional magnetic microbubbles

Polymeric materials for ultrasound imaging and therapy

Ultrasound (US) is routinely used for diagnostic imaging and increasingly employed for therapeutic applications. Materials that act as cavitation nuclei can improve the resolution of US imaging, and facilitate therapeutic US procedures by promoting local drug delivery or allowing temporary biological barrier opening at moderate acoustic powers. Polymeric materials offer a high degree of control over physicochemical features concerning responsiveness to US, e.g. via tuning chain composition, length and rigidity. This level of control cannot be achieved by materials made of lipids or proteins. In this perspective, we present key engineered polymeric materials that respond to US, including microbubbles, gas-stabilizing nanocups, microcapsules and gas-releasing nanoparticles, and discuss their formulation aspects as well as their principles of US responsiveness. Focusing on microbubbles as the most common US-responsive polymeric materials, we further evaluate the available chemical toolbox to engineer polymer shell properties and enhance their performance in US imaging and US-mediated drug delivery. Additionally, we summarize emerging applications of polymeric microbubbles in molecular imaging, sonopermeation, and gas and drug delivery, based on refinement of MB shell properties. Altogether, this manuscript provides new perspectives on US-responsive polymeric designs, envisaging their current and future applications in US imaging and therapy.

Freeze-Dried Microfluidic Monodisperse Microbubbles as a New Generation of Ultrasound Contrast Agents

We succeeded in freeze-drying monodisperse microbubbles without degrading their performance, that is, their monodispersity in size and echogenicity. We used microfluidic technology to generate cryoprotected highly monodisperse microbubbles (coefficient of variation [CV] <5%). By using a novel retrieval technique, we were able to freeze-dry the microbubbles and resuspend them without degradation, that is, keeping their size distribution narrow (CV <6%). Acoustic characterization performed in two geometries (a centimetric cell and a millichannel) revealed that the resuspended bubbles conserved the sharpness of the backscattered resonance peak, leading to CVs ranging between 5% and 10%, depending on the geometry. As currently observed with monodisperse bubbles, the peak amplitudes are one order of magnitude higher than those of commercial ultrasound contrast agents. Our work thus solves the question of storage and transportation of highly monodisperse bubbles. This work might open pathways toward novel clinical non-invasive measurements, such as local pressure, impossible to carry out with the existing commercial ultrasound contrast agents.

Contrast-enhanced ultrasound imaging using long-circulating cationic magnetic microbubbles in vitro and in vivo validations

Traditional encapsulated microbubbles are recently used as delivery carriers for drugs and genes, but they have low efficiency. If the local microbubble concentration could be increased, this might be able to improve the therapeutic efficacy of diseases. In this study, we developed novel cationic magnetic microbubbles (MB_M), which could simultaneously realize targeted aggregation under a magnetic field as well as ultrasonographic real-time visualization. Their physicochemical properties, biocompatibility, ultrasonography, magnetic response characteristics, and biodistribution were systematically evaluated. Here, the MB_M were 2.55±0.14µm in size with a positive zeta potential, and had a good biocompatibility. They were able to enhance ultrasonographic contrast both in vitro and in vivo. MB_M could be attracted by an external magnet for directional movement and aggregation in vitro. We confirmed that MB_M also had a great magnetic response in vivo, by means of fluorescence imaging and contrast-enhanced ultrasound imaging. Following intravenous injection into tumor-bearing mice, MB_M showed excellent stability in the internal circulation, and could accumulate in the tumor vasculature through magnetic targeting. With the excellent combination of magnetic response and acoustic properties, cationic magnetic microbubbles (MB_M) have promising potential for use as a new kind of drug/gene carrier for theranostics in the future.

Deriving acoustic properties for perfluoropentane droplets with viscoelastic cellulose nanofiber shell via numerical simulations

Perfluoropentane droplets with cellulose nanofibers (CNF) shells have demonstrated better stability and easier surface modification as ultrasound contrast agents and drug delivery vehicles. This paper presents a theoretical model assuming a four-phase state "inverse antibubble," with the core filled with gas perfluoropentane surrounded by liquid perfluoropentane. A continuous, incompressible, and viscoelastic stabilizing layer separates the core from the surrounding water. A parametric study is performed to predict the frequency-dependent attenuation coefficient, the speed of sound, and the resonance frequency of the droplets which have a mean diameter of 2.47 ± 0.95 μm. Results reveal that the CNF-stabilized perfluoropentane droplets can be modeled in a Rayleigh-Plesset like equation. We conclude that the shell strongly influences the acoustic behavior of the droplets and the resonance frequency largely depends on the initial gas cavity radius. More specifically, the peak attenuation coefficient and peak-to-peak speed of sound decrease with increasing shear modulus, shear viscosity, and shell thickness, while they increase with increasing gas cavity radius and concentration. The resonance frequency increases as shear modulus and shell thickness increase, while it decreases as shear viscosity and gas cavity radius increase. It is worth mentioning that droplet concentration has no effect on the resonance frequency.

On the Development of a Novel Contrast Pulse Sequence for Polymer-Shelled Microbubbles

Contrast agents are routinely used in ultrasound examinations. Nonlinear ultrasound imaging techniques have been developed over decades to enhance the contrast between the tissue and the blood pool after the injection of ultrasound contrast agents (UCAs). In this study, we introduce a new contrast pulse sequence, CPS4. The CPS4 combines pulse inversion (PI), subharmonic (SH), and ultraharmonic (UH) techniques to remove propagation distortion while capturing the unique SH and UH responses from UCAs. The novel CPS4 and conventional PI, SH, and UH techniques were used to detect the presence of a research-grade, thick-shell, polymer microbubble in a tissue-mimicking flow phantom. The contrast-to-tissue ratios (CTRs) obtained from the applications of all techniques were compared. The results show that the highest CTR of approximately 16 dB was obtained using CPS4, which was superior to the individual reference techniques: PI, SH, and UH techniques, in all scenarios considered in this study.

Mechanisms underlying sonoporation: Interaction between microbubbles and cells

The past several decades have witnessed great progress in "smart drug delivery", an advance technology that can deliver genes or drugs into specific locations of patients' body with enhanced delivery efficiency. Ultrasound-activated mechanical force induced by the interactions between microbubbles and cells, which can stimulate so-called "sonoporation" process, has been regarded as one of the most promising candidates to realize spatiotemporal-controllable drug delivery to selected regions. Both experimental and numerical studies were performed to get in-depth understanding on how the microbubbles interact with cells during sonoporation processes, under different impact parameters. The current work gives an overview of the general mechanism underlying microbubble-mediated sonoporation, and the possible impact factors (e.g., the properties of cavitation agents and cells, acoustical driving parameters and bubble/cell micro-environment) that could affect sonoporation outcomes. Finally, current progress and considerations of sonoporation in clinical applications are reviewed also.

Multifunctional Magnetic Resonance Imaging Probes

Magnetic resonance imaging is characterized by high spatial resolution and unsurpassed soft tissue discrimination. Development and characterization of both intrinsic and extrinsic magnetic resonance (MR) imaging probes in the last decade has further strengthened the pivotal role MR imaging holds in the assessment of cancer in preclinical and translational settings. Sophisticated chemical modifications of a variety of nanoparticulate probes hold the potential to deliver valuable multifunctional tools applicable in diagnostics and/or treatment in human oncology. MR imaging suffers from a lack of sensitivity achievable by, e.g., nuclear medicine imaging methods. Advantages of including additional functionality/functionalities in a probe suitable for MR imaging are thus numerous, comprising the addition of fundamentally different imaging information (diagnostics), drug delivery (therapy), or the combination of both (theranostics). In recent years, we have witnessed a plethora of preclinical multimodal or multifunctional imaging probes being published mainly as proof-of-principle studies, yet only a handful are readily applicable in clinical settings. This chapter summarizes recent innovations in the development of multifunctional MR imaging probes and discusses the suitability of these probes for clinical transfer.

Micro/nano-bubble-assisted ultrasound to enhance the EPR effect and potential theranostic applications

Drug delivery for tumor theranostics involves the extensive use of the enhanced permeability and retention (EPR) effect. Previously, various types of nanomedicines have been demonstrated to accumulate in solid tumors via the EPR effect. However, EPR is a highly variable phenomenon because of tumor heterogeneity, resulting in low drug delivery efficacy in clinical trials. Because ultrasonication using micro/nanobubbles as contrast agents can disrupt blood vessels and enhance the specific delivery of drugs, it is an effective approach to improve the EPR effect for the passive targeting of tumors. In this review, the basic thermal effect, acoustic streaming, and cavitation mechanisms of ultrasound, which are characteristics that can be utilized to enhance the EPR effect, are briefly introduced. Second, micro/nanobubble-enhanced ultrasound imaging is discussed to understand the validity and variability of the EPR effect. Third, because the tumor microenvironment is complicated owing to elevated interstitial fluid pressure and the deregulated extracellular matrix components, which may be unfavorable for the EPR effect, few new trends in smart bubble drug delivery systems, which may improve the accuracy of EPR-mediated passive drug targeting, are summarized. Finally, the challenging and major concerns that should be considered in the next generation of micro/nanobubble-contrast-enhanced ultrasound theranostics for EPR-mediated passive drug targeting are also discussed.

Multifunctionalized Microscale Ultrasound Contrast Agents for Precise Theranostics of Malignant Tumors

In ultrasonography, ultrasound contrast agents (UCAs) that possess high acoustic impedance mismatch with the bulk medium are frequently employed to highlight the borders between tissues by enhanced ultrasound scattering in a clinic. Typically, the most common UCA, microbubble, is generally close in size to a red blood cell (<∼10 μm). These microscale UCAs cannot be directly entrapped into the target cells but generate several orders of magnitude stronger echo signals than the nanoscale ones. And their large containment and high ultrasound responsiveness also greatly facilitate to perform combined treatments, e.g., drug delivery and other imaging techniques. So multifunctionalized microscale UCAs appear on this scene and keep growing toward a promising direction for precise theranostics. In this review, we systematically summarize the new advances in the principles and preparations of multifunctionalized microscale UCAs and their medical applications for malignant tumors.

Comparing Strategies for Magnetic Functionalization of Microbubbles

The advancement of ultrasound-mediated therapy has stimulated the development of drug-loaded microbubble agents that can be targeted to a region of interest through an applied magnetic field prior to ultrasound activation. However, the need to incorporate therapeutic molecules while optimizing the responsiveness to both magnetic and acoustic fields and maintaining adequate stability poses a considerable challenge for microbubble synthesis. The aim of this study was to evaluate three different methods for incorporating iron oxide nanoparticles (IONPs) into phospholipid-coated microbubbles using (1) hydrophobic IONPs within an oil layer below the microbubble shell, (2) phospholipid-stabilized IONPs within the shell, or (3) hydrophilic IONPs noncovalently bound to the surface of the microbubble. All microbubbles exhibited similar acoustic response at both 1 and 7 MHz. The half-life of the microbubbles was more than doubled by the addition of IONPs by using both surface and phospholipid-mediated loading methods, provided the lipid used to coat the IONPs was the same as that constituting the microbubble shell. The highest loading of IONPs per microbubble was also achieved with the surface loading method, and these microbubbles were the most responsive to an applied magnetic field, showing a 3-fold increase in the number of retained microbubbles compared to other groups. For the purpose of drug delivery, surface loading of IONPs could restrict the attachment of hydrophilic drugs to the microbubble shell, but hydrophobic drugs could still be incorporated. In contrast, although the incorporation of phospholipid IONPs produced more weakly magnetic microbubbles, it would not interfere with hydrophilic drug loading on the surface of the microbubble.

Long-term physical evolution of an elastomeric ultrasound contrast microbubble

HYPOTHESIS

One of the main assets of crosslinked polymer-shelled microbubbles (MBs) as ultrasound-active theranostic agents is the robustness of the shells, combined with the chemical versatility in modifying the surface with ligands and/or drugs. Despite the long shelf-life, subtle modifications occur in the MB shells involving shifts in acoustic, mechanical and structural properties.

EXPERIMENTS

We carried out a long-term morphological and acoustic evolution analysis on elastomeric polyvinyl-alcohol (PVA)-shelled MBs, a novel platform accomplishing good acoustic and surface performances in one agent. Confocal laser scanning microscopy, acoustic spectroscopy and AFM nanomechanics were integrated to understand the mechanism of PVA MBs ageing. The changes in the MB acoustic properties were framed in terms of shell thickness and viscoelasticity using a linearised oscillation theory, and compared to MB morphology and to nanomechanical analysis.

FINDINGS

We enlightened a novel, intriguing ageing time evolution of the PVA MBs with double behaviour with respect to a crossover time of ∼50 days. Before, significant changes occur in MB stiffness and shell thickness, mainly due to a massive release of entangled PVA chains. Then, the MB resonance frequency increases together with shell thickening and softening. Our benchmark study is of general interest for emerging viscoelastomeric bubbles towards personalised medicine.

Molecular Imaging of a New Multimodal Microbubble for Adhesion Molecule Targeting

Introduction

Inflammation is an important risk-associated component of many diseases and can be diagnosed by molecular imaging of specific molecules. The aim of this study was to evaluate the possibility of targeting adhesion molecules on inflammation-activated endothelial cells and macrophages using an innovative multimodal polyvinyl alcohol-based microbubble (MB) contrast agent developed for diagnostic use in ultrasound, magnetic resonance, and nuclear imaging.

Methods

We assessed the binding efficiency of antibody-conjugated multimodal contrast to inflamed murine or human endothelial cells (ECs), and to peritoneal macrophages isolated from rats with peritonitis, utilizing the fluorescence characteristics of the MBs. Single-photon emission tomography (SPECT) was used to illustrate ^99mTc-labeled MB targeting and distribution in an experimental in vivo model of inflammation.

Results

Flow cytometry and confocal microscopy showed that binding of antibody-targeted MBs to the adhesion molecules ICAM-1, VCAM-1, or E-selectin, expressed on cytokine-stimulated ECs, was up to sixfold higher for human and 12-fold higher for mouse ECs, compared with that of non-targeted MBs. Under flow conditions, both VCAM-1- and E-selectin-targeted MBs adhered more firmly to stimulated human ECs than to untreated cells, while VCAM-1-targeted MBs adhered best to stimulated murine ECs. SPECT imaging showed an approximate doubling of signal intensity from the abdomen of rats with peritonitis, compared with healthy controls, after injection of anti-ICAM-1-MBs.

Conclusions

This novel multilayer contrast agent can specifically target adhesion molecules expressed as a result of inflammatory stimuli in vitro, and has potential for use in disease-specific multimodal diagnostics in vivo using antibodies against targets of interest.

In vivo biological fate of poly(vinylalcohol) microbubbles in mice

Microbubbles (MBs) are used in clinical practice as vascular ultrasound contrast agents, and are gaining popularity as a platform supporting multimodal imaging and targeted therapy, facilitating drug delivery under ultrasound exposure. Here, we report on the in vivo biological impact of newly discovered MBs with promising features as a multimodal theranostic device. The shell of the air-filled MBs is made of the poly(vinyl alcohol) (PVA), a well-established, FDA-approved polymer. Nevertheless, as size, shape and dispersity can significantly impact the biological response of particulate systems, studying their fate after administration is crucial. The safety and the biodistribution of PVA MBs were analysed in vivo and ex vivo by coupling a near infrared (NIR) fluorophore on their shell: MBs accumulated mainly in liver and spleen at 24 hours post-injection with their clearance from the spleen 7 days post-dosing. A possible way of elimination was identified in macrophages ability to engulf MBs both in vitro and in vivo. One month post-dosing, transmission electron microscopy (TEM) highlighted the lack of relevant defects and the elimination of PVA MBs by Kupffer cells. This study is the first successful attempt to fill the lack of knowledge necessary to bring PVA MBs one step closer to their possible clinical use.

Cellular Uptake of Plain and SPION-Modified Microbubbles for Potential Use in Molecular Imaging

Introduction

Both diagnostic ultrasound (US) and magnetic resonance imaging (MRI) accuracy can be improved by using contrast enhancement. For US gas-filled microbubbles (MBs) or silica nanoparticles (SiNPs), and for MRI superparamagnetic or paramagnetic agents, contribute to this. However, interactions of MBs with the vascular wall and cells are not fully known for all contrast media.

Methods

We studied the in vitro interactions between three types of non-targeted air-filled MBs with a polyvinyl-alcohol shell and murine macrophages or endothelial cells. The three MB types were plain MBs and two types that were labelled (internally and externally) with superparamagnetic iron oxide nanoparticles (SPIONs) for US/MRI bimodality. Cells were incubated with MBs and imaged by microscopy to evaluate uptake and adhesion. Interactions were quantified and the MB internalization was confirmed by fluorescence quenching of non-internalized MBs.

Results

Macrophages internalized each MB type within different time frames: plain MBs 6 h, externally labelled MBs 25 min and internally labelled MBs 2 h. An average of 0.14 externally labelled MBs per cell were internalized after 30 min and 1.34 after 2 h; which was 113% more MBs than the number of internalized internally labelled MBs. The macrophages engulfed these three differently modified new MBs at various rate, whereas endothelial cells did not engulf MBs.

Conclusions

Polyvinyl-alcohol MBs are not taken up by endothelial cells. The MB uptake by macrophages is promoted by SPION labelling, in particular external such, which may be important for macrophage targeting.

Multifunctional hard-shelled microbubbles for differentiating imaging, cavitation and drug release by ultrasound

Polymeric microbubbles bearing a hard shell exhibit prominent stability and tunable acoustical properties that serve the purposes of biomedical imaging and ultrasound (US)-triggered cavitations.

Next generation ultrasound platforms for theranostics

Microbubbles are a well-established contrast agent which improves diagnostic ultrasound imaging. During the last decade research has focused on expanding their use to include molecular imaging, targeted therapy and imaging modalities other than ultrasound. However, bioadhesion of targeted microbubbles under physiological flow conditions is still difficult to achieve, the main challenge being connected to the poor stability of lipid microbubbles in the body's circulation system. In this article, we investigate the use of polymeric microbubbles based on a poly (vinyl alcohol) shell as an alternative to lipid microbubbles. In particular, we report on the development of microbubble shell modification, using mild reaction conditions, with the aim of designing a multifunctional platform to enable diagnosis and therapy. Superparamagnetic iron oxide nanoparticles and a near infrared fluorescent probe, indocyanine green, are coupled to the bubbles surface in order to support magnetic resonance and fluorescence imaging. Furthermore, anchoring cyclic arginyl-glycyl-aspartic acid (RGD) peptide, and cyclodextrin molecules, allows targeting and drug loading, respectively. Last but not least, shell topography is provided by atomic force microscopy. These applications and features, together with the high echogenicity of poly (vinyl alcohol) microbubbles, may offer a more stable alternative to lipid microbubbles for the development of a multimodal theranostic platform.

Modeling and parametric investigation of thick encapsulated microbubble's nonspherical oscillations

Numerous studies have been carried out in the past few decades to investigate the radial oscillations of encapsulated microbubbles (MBs). Nonspherical oscillations also have gained attention, being unavoidable in actual applications of these bubbles. The present paper is intended to describe the nature of resonance trends of such spherical and nonspherical modes of a thick encapsulated MB filled with air and suspended in water. The shell material is assumed to be linear viscoelastic and quasi-incompressible. The considered isotropic and spherically isotropic material parametric range is limited to thick polymer shelled MBs. For the case of an isotropic material, shell viscosity has a major influence on the fundamental modes with meridional wave number n = 0, 4, especially for thicker bubbles, unlike for the case of the spherically isotropic material case considered, where the viscosity has very little influence. For most of the parametric range, n = 2, 3 modes are underdamped and their frequency is found to be lower than the n = 0, 4 modes, for both material cases. An interesting case is found for a spherically isotropic quasi-incompressible material case, where the first few nonspherical mode resonances are very close to radial mode resonance frequency.

Magnetic resonance and ultrasound contrast imaging of polymer-shelled microbubbles loaded with iron oxide nanoparticles

Dual-mode contrast agents (CAs) have great potential for improving diagnostics. However, the effectiveness of CAs is strictly related to both the solution adopted to merge the two agents into a single probe unit, and the ratio between the two agents. In this study, two dual-mode CAs for simultaneous magnetic resonance imaging (MRI) and ultrasound imaging (UI) were assessed. For this purpose, different densities of superparamagnetic iron oxide nanoparticles (SPIONs) were anchored to the external surface of polymer-shelled microbubbles (MBs) or were physically entrapped into the shell. In vitro static and dynamic experiments were carried out with a limited concentration of modified MBs (10⁶ bubbles ml^-1) by avoiding destruction during UI (performed at a peak pressure lower than 320 kPa) and by using a low-field MRI system (with a magnetic flux density equal to 0.25 T). Under these conditions, different imaging techniques, set-up parameters and SPION densities were used to achieve satisfactory detection of the CAs by using both UI and MRI. However, when the SPION density was increased, the MRI contrast improved, whereas the UI contrast worsened due to the reduced elasticity of the MB shell. For both UI and MRI, MBs with externally anchored SPIONs provided better performance than MBs with SPIONs entrapped into the shell. In particular, a SPION density of 29% with respect to the mass of the MBs was successfully tested.

Magnetic resonance and ultrasound contrast imaging of polymer-shelled microbubbles loaded with iron oxide nanoparticles

Dual-mode contrast agents (CAs) have great potential for improving diagnostics. However, the effectiveness of CAs is strictly related to both the solution adopted to merge the two agents into a single probe unit, and the ratio between the two agents. In this study, two dual-mode CAs for simultaneous magnetic resonance imaging (MRI) and ultrasound imaging (UI) were assessed. For this purpose, different densities of superparamagnetic iron oxide nanoparticles (SPIONs) were anchored to the external surface of polymer-shelled microbubbles (MBs) or were physically entrapped into the shell. In vitro static and dynamic experiments were carried out with a limited concentration of modified MBs (10⁶ bubbles ml^-1) by avoiding destruction during UI (performed at a peak pressure lower than 320 kPa) and by using a low-field MRI system (with a magnetic flux density equal to 0.25 T). Under these conditions, different imaging techniques, set-up parameters and SPION densities were used to achieve satisfactory detection of the CAs by using both UI and MRI. However, when the SPION density was increased, the MRI contrast improved, whereas the UI contrast worsened due to the reduced elasticity of the MB shell. For both UI and MRI, MBs with externally anchored SPIONs provided better performance than MBs with SPIONs entrapped into the shell. In particular, a SPION density of 29% with respect to the mass of the MBs was successfully tested.

Investigation of polymer-shelled microbubble motions in acoustophoresis

The objective of this paper is to explore the trajectory motion of microsize (typically smaller than a red blood cell) encapsulated polymer-shelled gas bubbles propelled by radiation force in an acoustic standing-wave field and to compare the corresponding movements of solid polymer microbeads. The experimental setup consists of a microfluidic chip coupled to a piezoelectric crystal (PZT) with a resonance frequency of about 2.8MHz. The microfluidic channel consists of a rectangular chamber with a width, w, corresponding to one wavelength of the ultrasound standing wave. It creates one full wave ultrasound of a standing-wave pattern with two pressure nodes at w/4 and 3w/4 and three antinodes at 0, w/2, and w. The peak-to-peak amplitude of the electrical potential over the PZT was varied between 1 and 10V. The study is limited to no-flow condition. From Gor'kov's potential equation, the acoustic contrast factor, Φ, for the polymer-shelled microbubbles was calculated to about -60.7. Experimental results demonstrate that the polymer-shelled microbubbles are translated and accumulated at the pressure antinode planes. This trajectory motion of polymer-shelled microbubbles toward the pressure antinode plane is similar to what has been described for other acoustic contrast particles with a negative Φ. First, primary radiation forces dragged the polymer-shelled microbubbles into proximity with each other at the pressure antinode planes. Then, primary and secondary radiation forces caused them to quickly aggregate at different spots along the channel. The relocation time for polymer-shelled microbubbles was 40 times shorter than that for polymer microbeads, and in contrast to polymer microbeads, the polymer-shelled microbubbles were actuated even at driving voltages (proportional to radiation forces) as low as 1V. In short, the polymer-shelled microbubbles demonstrate the behavior attributed to the negative acoustic contrast factor particles and thus can be trapped at the antinode plane and thereby separated from particles having a positive acoustic contrast factor, such as for example solid particles and cells. This phenomenon could be utilized in exploring future applications, such as bioassay, bioaffinity, and cell interaction studies in vitro in a well-controlled environment.

Highly Uniform Perfluoropropane-Loaded Cerasomal Microbubbles As a Novel Ultrasound Contrast Agent

Microbubbles are widely used as ultrasound contrast agents owing to their excellent echoing characteristics under ultrasound radiation. However, their short sonographic duration and wide size distribution still hinder their application. Herein, we present a hard-template approach to produce perfluoropropane-loaded cerasomal microbubbles (PLCMs) with uniform size and long sonographic duration. The preparation of PLCMs includes deposition of Si-lipids onto functionalized CaCO3 microspheres, removal of their CaCO3 cores and mild infusion of perfluoropropane. In vitro and in vivo experiments showed that PLCMs had excellent echoing characteristics under different ultrasound conditions. More importantly, PLCMs could be imaged for much longer than SonoVue (commercially used microbubbles) under the same ultrasound parameters and concentrations. Our results demonstrated that PLCMs have great potential for use as a novel contrast agent in ultrasound imaging.

AFM Investigation of Liquid-Filled Polymer Microcapsules Elasticity

Elasticity of polymer microcapsules (MCs) filled with a liquid fluorinated core is studied by atomic force microscopy (AFM). Accurately characterized spherical tips are employed to obtain the Young's moduli of MCs having four different shell thicknesses. We show that those moduli are effective ones because the samples are composites. The strong decrease of the effective MC elasticity (from 3.0 to 0.1 GPa) as the shell thickness decreases (from 200 to 10 nm) is analyzed using a novel numerical approach. This model describes the evolution of the elasticity of a coated half-space according to the contact radius, the thickness of the film, and the elastic moduli of bulk materials. This numerical model is consistent with the experimental data and allows simulating the elastic behavior of MCs at high frequencies (5 MHz). While the quasi-static elasticity of the MCs is found to be very dependent on the shell thickness, the high frequency (5 MHz) elastic behavior of the core leads to a stable behavior of the MCs (from 2.5 to 3 GPa according to the shell thickness). Finally, the effect of thermal annealing on the MCs elasticity is investigated. The Young's modulus is found to decrease because of the reduction of the shell thickness due to the loss of the polymer.

Shell thickness determination of polymer-shelled microbubbles using transmission electron microscopy

Intravenously injected microbubbles (MBs) can be utilized as ultrasound contrast agent (CA) resulting in enhanced image quality. A novel CA, consisting of air filled MBs stabilized with a shell of polyvinyl alcohol (PVA) has been developed. These spherical MBs have been decorated with superparamagnetic iron oxide nanoparticles (SPIONs) in order to serve as both ultrasound and magnetic resonance imaging (MRI) CA. In this study, a mathematical model was introduced that determined the shell thickness of two types of SPIONs decorated MBs (Type A and Type B). The shell thickness of MBs is important to determine, as it affects the acoustical properties. In order to investigate the shell thickness, thin sections of plastic embedded MBs were prepared and imaged using transmission electron microscopy (TEM). However, the sections were cut at random distances from the MB center, which affected the observed shell thickness. Hence, the model determined the average shell thickness of the MBs from corrected mean values of the outer and inner radii observed in the TEM sections. The model was validated using simulated slices of MBs with known shell thickness and radius. The average shell thickness of Type A and Type B MBs were 651nm and 637nm, respectively.

Harmonic responses and cavitation activity of encapsulated microbubbles coupled with magnetic nanoparticles

Encapsulated microbubbles coupled with magnetic nanoparticles, one kind of hybrid agents that can integrate both ultrasound and magnetic resonance imaging/therapy functions, have attracted increasing interests in both research and clinic communities. However, there is a lack of comprehensive understanding of their dynamic behaviors generated in diagnostic and therapeutic applications. In the present work, a hybrid agent was synthesized by integrating superparamagnetic iron oxide nanoparticles (SPIOs) into albumin-shelled microbubbles (named as SPIO-albumin microbubbles). Then, both the stable and inertial cavitation thresholds of this hybrid agent were measured at varied SPIO concentrations and ultrasound parameters (e.g., frequency, pressure amplitude, and pulse length). The results show that, at a fixed acoustic driving frequency, both the stable and inertial cavitation thresholds of SPIO-albumin microbubble should decrease with the increasing SPIO concentration and acoustic driving pulse length. The inertial cavitation threshold of SPIO-albumin microbubbles also decreases with the raised driving frequency, while the minimum sub- and ultra-harmonic thresholds appear at twice and two thirds resonance frequency, respectively. It is also noticed that both the stable and inertial cavitation thresholds of SonoVue microbubbles are similar to those measured for hybrid microbubbles with a SPIO concentration of 114.7 μg/ml. The current work could provide better understanding on the impact of the integrated SPIOs on the dynamic responses (especially the cavitation activities) of hybrid microbubbles, and suggest the shell composition of hybrid agents should be appropriately designed to improve their clinical diagnostic and therapeutic performances of hybrid microbubble agents.

Investigation of PDMS based bi-layer elasticity via interpretation of apparent Young's modulus

As the need of new methods for the investigation of thin films on various kinds of substrates becomes greater, a novel approach based on AFM nanoindentation is explored. Substrates of polydimethylsiloxane (PDMS) coated by a layer of hard material are probed with an AFM tip in order to obtain the force profile as a function of the indentation. The equivalent elasticity of those composite systems is interpreted using a new numerical approach, the Coated Half-Space Indentation Model of Elastic Response (CHIMER), in order to extract the thicknesses of the upper layer. Two kinds of coating are investigated. First, chitosan films of known thicknesses between 30 and 200 nm were probed in order to test the model. A second type of samples is produced by oxygen plasma oxidation of the PDMS substrate, which results in the growth of a relatively homogeneous oxide layer. The local nature of this protocol enables measurements at long oxidation time, where the apparition of cracks prevents other kinds of measurements.

Nanoparticle Loaded Polymeric Microbubbles as Contrast Agents for Multimodal Imaging

Ultrasound contrast agents are typically microbubbles (MB) with a gas core that is stabilized by a shell made of lipids, proteins, or polymers. The high impedance mismatch between the gas core and an aqueous environment produces strong contrast in ultrasound (US). Poly(lactic acid) (PLA) MB, previously developed in our laboratory, have been shown to be highly echogenic both in vitro and in vivo. Combining US with other imaging modalities such as fluorescence, magnetic resonance imaging (MRI), or computerized tomography (CT) could improve the accuracy of many US applications and provide more comprehensive diagnostic information. Furthermore, our MB have the capacity to house a drug in the PLA shell and create drug-loaded nanoparticles in situ when passing through an ultrasound beam. To create multimodal contrast agents, we hypothesized that the polymer shell of our PLA MB platform could accommodate additional payloads. In this study, we therefore modified our current MB by encapsulating nanoparticles including aqueous or organic quantum dots (QD), magnetic iron oxide nanoparticles (MNP), or gold nanoparticles (AuNP) to create bimodality platforms in a manner that minimally compromised the performance of each individual imaging technique.

Sonochemical Synthesis of Hydrophilic Drug Loaded Multifunctional Bovine Serum Albumin Nanocapsules

A facile sonochemical approach is designed to fabricate protein nanocapsules for hydrophilic drugs (HDs), and HD-loaded multifunctional bovine serum albumin (BSA) nanocapsules (MBNCs) have been prepared for the first time. The as-synthesized HD-loaded MBNCs have a satisfying size range and an excellent magnetic responsive ability. Moreover, high-dose hydrophilic drugs could be loaded into the MBNCs. As carriers, HD-loaded MBNCs also show attractive redox-responsive controlled release ability for hydrophilic drugs and could be internalized selectively by the tumor cells through the folate-mediated endocytosis.

Sound velocity and attenuation coefficient of hard and hollow microparticle suspensions observed by ultrasound spectroscopy

Size and elastic properties of micro-particles suspended in liquid can be acoustically determined by ultrasound attenuation and velocity measurements with the aid of elastic scattering theories and a dispersion relation. While quantitative evaluation for hard micron-sized spheres using the theories is available in literature, that for hollow particles is not yet achieved. In this study, we show that the shell thickness and the elastic modulus of hollow particles can be quantitatively evaluated by ultrasound spectroscopy. Several kinds of microparticles including polystyrene rigid particles, polydivinylbenzene rigid particles, borosilicate hollow particles, and phenolic-resin hollow particles were examined as a function of the particle concentration.

Controlled assembly of magnetic nanoparticles on microbubbles for multimodal imaging

Magnetic microbubbles (MMBs) consisting of microbubbles (MBs) and magnetic nanoparticles (MNPs) were synthesized for use as novel markers for improving multifunctional biomedical imaging. The MMBs were fabricated by assembling MNPs in different concentrations on the surfaces of MBs. The relationships between the structure, magnetic properties, stability of the MMBs, and their use in magnetic resonance/ultrasound (MR/US) dual imaging applications were determined. The MNPs used were NPs of 3-aminopropyltriethoxysilane (APTS)-functionalized superparamagnetic iron oxide γ-Fe2O3 (SPIO). SPIO was assembled on the surfaces of polymer MBs using a "surface-coating" approach. An analysis of the underlying mechanism showed that the synergistic effects of covalent coupling, electrostatic adsorption, and aggregation of the MNPs allowed them to be unevenly assembled in large amounts on the surfaces of the MBs. With an increase in the MNP loading amount, the magnetic properties of the MMBs improved significantly; in this way, the shell structure and mechanical properties of the MMBs could be modified. For surface densities ranging from 2.45 × 10(-7) μg per MMB to 8.45 × 10(-7) μg per MMB, in vitro MR/US imaging experiments showed that, with an increase in the number of MNPs on the surfaces of the MBs, the MMBs exhibited better T2 MR imaging contrast, as well as an increase in the US contrast for longer durations. In vivo experiments also showed that, by optimizing the structure of the MMBs, enhanced MR/US dual-modality image signals could be obtained for mouse tumors. Therefore, by adjusting the shell composition of MBs through the assembly of MNPs in different concentrations, MMBs with good magnetic and acoustic properties for MR/US dual-modality imaging contrast agents could be obtained.

Ultrasound contrast agent loaded with nitric oxide as a theranostic microdevice

The current study describes novel multifunctional polymer-shelled microbubbles (MBs) loaded with nitric oxide (NO) for integrated therapeutic and diagnostic applications (ie, theranostics) of myocardial ischemia. We used gas-filled MBs with an average diameter of 4 μm stabilized by a biocompatible shell of polyvinyl alcohol. In vitro acoustic tests showed sufficient enhancement of the backscattered power (20 dB) acquired from the MBs' suspension. The values of attenuation coefficient (0.8 dB/cm MHz) and phase velocities (1,517 m/s) were comparable with those reported for the soft tissue. Moreover, polymer MBs demonstrate increased stability compared with clinically approved contrast agents with a fracture threshold of about 900 kPa. In vitro chemiluminescence measurements demonstrated that dry powder of NO-loaded MBs releases its gas content in about 2 hours following an exponential decay profile with an exponential time constant equal to 36 minutes. The application of high-power ultrasound pulse (mechanical index =1.2) on the MBs resuspended in saline decreases the exponential time constant from 55 to 4 minutes in air-saturated solution and from 17 to 10 minutes in degassed solution. Thus, ultrasound-triggered release of NO is achieved. Cytotoxicity tests indicate that phagocytosis of the MBs by macrophages starts within 6-8 hours. This is a suitable time for initial diagnostics, treatment, and monitoring of the therapeutic effect using a single injection of the proposed multifunctional MBs.

Unique pumping-out fracturing mechanism of a polymer-shelled contrast agent: an acoustic characterization and optical visualization

This work describes the fracturing mechanism of air-filled microbubbles (MBs) encapsulated by a cross-linked poly(vinyl alcohol) (PVA) shell. The radial oscillation and fracturing events following the ultrasound exposure were visualized with an ultrahigh-speed camera, and backscattered timedomain signals were acquired with the acoustic setup specific for harmonic detection. No evidence of gas emerging from defects in the shell with the arrival of the first insonation burst was found. In optical recordings, more than one shell defect was noted, and the gas core was drained without any sign of air extrusion when several consecutive bursts of 1 MPa amplitude were applied. In acoustic tests, the backscattered peak-to-peak voltage gradually reached its maximum and exponentially decreased when the PVA-based MB suspension was exposed to approximately 20 consecutive bursts arriving at pulse repetition frequencies of 100 and 500 Hz. Taking into account that the PVA shell is porous and possibly contains large air pockets between the cross-linked PVA chains, the aforementioned acoustic behavior might be attributed to pumping gas from these pockets in combination with gas release from the core through shell defects. We refer to this fracturing mechanism as pumping-out behavior, and this behavior could have potential use for the local delivery of therapeutic gases, such as nitric oxide.

Multifunctional layered magnetic composites

A fabrication method of a multifunctional hybrid material is achieved by using the insoluble organic nacre matrix of the Haliotis laevigata shell infiltrated with gelatin as a confined reaction environment. Inside this organic scaffold magnetite nanoparticles (MNPs) are synthesized. The amount of MNPs can be controlled through the synthesis protocol therefore mineral loadings starting from 15 wt % up to 65 wt % can be realized. The demineralized organic nacre matrix is characterized by small-angle and very-small-angle neutron scattering (SANS and VSANS) showing an unchanged organic matrix structure after demineralization compared to the original mineralized nacre reference. Light microscopy and confocal laser scanning microscopy studies of stained samples show the presence of insoluble proteins at the chitin surface but not between the chitin layers. Successful and homogeneous gelatin infiltration in between the chitin layers can be shown. The hybrid material is characterized by TEM and shows a layered structure filled with MNPs with a size of around 10 nm. Magnetic analysis of the material demonstrates superparamagnetic behavior as characteristic for the particle size. Simulation studies show the potential of collagen and chitin to act as nucleators, where there is a slight preference of chitin over collagen as a nucleator for magnetite. Colloidal-probe AFM measurements demonstrate that introduction of a ferrogel into the chitin matrix leads to a certain increase in the stiffness of the composite material.

Sonochemical fabrication of dual-targeted redox-responsive smart microcarriers

In the present study, the molecular and magnetic dual-targeted redox-responsive folic acid-cysteine-Fe3O4 microcapsules (FA-Cys-Fe3O4 MCs) have been synthesized via the sonochemical technique, and targeting molecule (folic acid) and Fe3O4 magnetic nanoparticles are introduced into the microcapsule shells successfully. The obtained FA-Cys-Fe3O4 MCs show excellent magnetic responsive ability by the oriented motion under an external magnetic field. The hydrophobic fluorescent dye (Coumarin 6) is successfully loaded into the FA-Cys-Fe3O4 MCs, demonstrating that it could be also easily realized to encapsulate hydrophobic drugs into the FA-Cys-Fe3O4 MCs when the drugs are dispersed into the oil phase before sonication. Cellular uptake demonstrates that FA-Cys-Fe3O4 MCs could target selectively the cells via folate-receptor-mediated endocytosis. Moreover, the FA-Cys-Fe3O4 MCs show their potential ability to be an attractive carrier for drug controlled release owing to the redox responsiveness of the disulfide in the microcapsule shells.

Endocardial border delineation capability of a novel multimodal polymer-shelled contrast agent

Background

A novel polymer-shelled contrast agent (CA) with multimodal and target-specific potential was developed recently. To determine its ultrasonic diagnostic features, we evaluated the endocardial border delineation as visualized in a porcine model and the concomitant effect on physiological variables.

Methods

Three doses of the novel polymer-shelled CA (1.5 ml, 3 ml, and 5 ml [5 × 10⁸ microbubbles (MBs)/ml]) and the commercially available CA SonoVue (1.5 ml [2–5 × 10⁸ MBs/ml]) were used. Visual evaluations of ultrasound images of the left ventricle were independently performed by three observers who graded each segment in a 6-segment model as either 0 = not visible, 1 = weakly visible, or 2 = visible. Moreover, the duration of clinically useful contrast enhancement and the left ventricular opacification were determined. During anesthesia, oxygen saturation, heart rate, and arterial pressure were sampled every minute and the effect of injection of CA on these physiological variables was evaluated.

Results

The highest dose of the polymer-shelled CA gave results comparable to SonoVue. Thus, no significant difference in the overall segment score distribution (2-47-95 vs. 1-39-104), time for clinically sufficient contrast enhancement (20–40 s for both) and left ventricular overall opacification was found. In contrast, when comparing the endocardial border delineation capacity for different regions SonoVue showed significantly higher segment scores for base and mid, except for the mid region when injecting 1.5 ml of the polymer-shelled CA. Neither high nor low doses of the polymer-shelled CA significantly affected the investigated physiological variables.

Conclusions

This study demonstrated that the novel polymer-shelled CA can be used in contrast-enhanced diagnostic imaging without influence on major physiological variables.

Direct thiol-ene photocoating of polyorganosiloxane microparticles

This work presents the modification of polyorganosiloxane microparticles by surface-initiated thiol-ene photochemistry. By this photocoating, we prepared different core/shell particles with a polymeric shell within narrow size distributions (PDI = 0.041-0.12). As core particle, we used highly monodisperse spherical polyorganosiloxane particles prepared from (3-mercaptopropyl)trimethoxysilane (MPTMS) with a radius of 0.49 μm. We utilize the high surface coverage of mercaptopropyl functions to generate surface-localized radicals upon irradiation with UVA-light without additional photoinitiator. The continuous generation of radicals was followed by a dye degradation experiment (UV/vis spectroscopy). Surface-localized radicals were used as copolymer anchoring sites ("grafting-onto" deposition of different PB-b-PS diblock copolymers) and polymerization initiators ("grafting-from" polymerization of PS). Photocoated particles were characterized for their morphology (SEM, TEM), size, and size distribution (DLS). For PS-coated particles, the polymer content (up to 24% in 24 h) was controlled by the polymerization time upon UVA exposure. The coating thickness was evaluated by thermogravimetric analysis (TGA) using a simple analytical core/shell model. Raman spectroscopy was applied to directly follow the time-dependent consumption of thiols by photoinitiation.