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This study explores the rigidity of Pickering-stabilised microbubbles subjected to low-amplitude ultrasound. Such microbubbles might be suitable ultrasound contrast agents. Using an adapted Rayleigh–Plesset equation, we modelled the dynamics of microbubbles with a 7.6-N m−1 shell stiffness under 1-MHz, 0.2-MPa sonication. Such dynamics were observed experimentally, too, using high-speed photography. The maximum expansions were agreeing with those predicted for Pickering-stabilised microbubbles. Subjecting microbubbles to multiple time-delayed pulses yielded the same result. We conclude that Pickering-stabilised microbubbles remain very stable at low acoustic amplitudes.

Mechanisms of BBB opening were investigated using a capillary model created by a three-dimensional cell culture technique. The model was made using human umbilical vein endothelial cells (HUVECs) suspended in collagen gel. Incubation with endothelial growth factor for one week resulted in the creation of capillary lumens, and a suspension of microbubbles supplemented with FITC-dextran, SYTOX Blue, and Calcein-AM was introduced inside the lumens. Bubble-cell interaction under exposure to short pulsed ultrasound of 1.0 MHz in center frequency and 0.4 MPa in peak negative pressure was captured using a high-speed camera, and lumen and endothelial cell damage was visualized using fluorescence microscopy. High-speed observation showed that non-uniform contraction of a bubble caused significant local deformation of the lumen beside the bubble, and strong extravasation from the lumen (FITC-dextran) and significant membrane damage of the endothelial cells (SYTOX Blue) were confirmed at this location. Lethal damage was also observed frequently (Calcein). On the other hand, cells on the opposite side of the lumen received stretching force during bubble expansion, and smaller extravasation and cell damage maintained the integrity of endothelial cells. These suggested the presence of different mechanisms of BBB opening.

Hydrophobic particles inherently resist being suspended. Hydrophobic particles might be regarded as tiny solid particles surrounded by a thin gaseous shell. It has been hypothesised that hydrophobic particles act as cavitation nuclei. This cavitation behaviour would explain the translation speeds observed when hydrophobic polystyrene microspheres were driven through a liquid medium by means of ultrasound. These translation speeds corresponded to those observed with gas microbubbles of similar sizes. If hydrophobic particles do have a thin gaseous layer surrounding the solid cores, a sound field of sufficient pressure amplitude might force the gas layer to form and inertial cavity and subsequently fragment during the collapse phase. In this study, we investigated whether hydrophobic particles can be forced to suspend by using ultrasound. Hydrophobic particles of the materials C65 and ZnO can be forced to be suspended in water using ultrasound. The high-speed observations confirm that hydrophobic particles can act as cavitation nuclei. The lack of cavitation after the first pulse indicates that the gas layer surrounding the hydrophobic particle dissolves after inertial cavitation., Comment: 6 pages, 3 figures (7 frames), Submitted to the 40th Symposium on UltraSonic Electronics (USE2019)

An antibubble is a gas bubble with a liquid droplet inside. Antibubbles have a potential role in ultrasonic imaging, ultrasound-guided drug delivery, and hydrocarbon leakage detection. The liquid core droplets inside antibubbles can be stabilised with a process called Pickering stability. Other systems proposed include spoke crystals to suspend the droplet or endoskeletal structures. In this study, the oscillatory response of microscopic endoskeletal antibubbles to ultrasound has been investigated, primarily to determine whether the presence of an endoskeleton attenuates the oscillation amplitude of the antibubble. Three emulsions containing (anti)bubbles were prepared for evaluation. Each emulsion was pipetted into the observation chamber of a high-speed microscopic observation system. A high-speed camera, operating at 10 million frames per second, was couples to the microscope. During camera recording, the materials were subjected to ultrasound pulses from a laboratory-assembled single-element transducer. The presence of an endoskeleton with droplets hampers the oscillation of antibubbles as compared to identical bubbles without endoskeleton and droplet content., Comment: 6 papers, 4 figures, submitted to the 40th Symposium on UltraSonic Electronics (USE2019)

International audience; In this study, we investigated if hydrophobic particles of the materials carbon black and zinc oxide act as cavitation nuclei, using high-speed photography during 1-MHz, low-amplitude sonication. Carbon black particles were observed to lose their gaseous shells and become acoustically inactive, whilst zinc oxide particles were observed to generate bubbles without becoming acoustically inactive. Zinc oxide might be a suitable candidate for an ultrasound contrast agent. Carbon black might be a suitable candidate for ultrasound-guided drug delivery.

International audience; Antibubbles have been under investigation as potential vehicles in ultrasound-guided drug delivery. It is assumed that antibubbles can expand unhampered, but cannot contract beyond the size of their inner core. In this study, this hypothesis was tested on endoskeletal antibubbles and reference bubbles. These were subjected to 3-cycle pulses of 1-MHz ultrasound, whilst being recorded with a high-speed camera operating at 10 million frames per second. At low acoustic amplitudes (200 kPa), antibubbles and bubbles oscillated symmetrically. At high acoustic amplitudes (1.00 MPa), antibubbles and bubbles oscillated asymmetrically, but antibubbles significantly more so than bubbles. Furthermore, fragmentation and core release were observed at these amplitudes. This finding may have implications for ultrasound-guided drug delivery using antibubbles.

Ultrasound contrast agents comprise gas microbubbles surrounded by stabilising elastic or viscoelastic shells. Microbubbles containing liquid or solid cores are referred to as antibubbles. The manufacturing process of long-lived antibubbles involves the adsorption of colloidal particles at the interfaces, a process called Pickering stabilisation. With and without cores present inside, Pickering-stabilised microbubbles generate a harmonic response, even at modest transmission amplitudes. Therefore, Pickering-stabilised ultrasound contrast agents may be of interest in contrast-enhanced ultrasonic imaging. In a previous study, we determined that the presence of a core inside Pickering-stabilised microbubbles slightly hampered the oscillation amplitude compared to identical microbubbles without a core. The purpose of the present study is to determine whether the absence of a core negatively influences the stability of Pickering-stabilised microbubbles under sonication., Comment: 6 pages, 3 figures, submitted to the 42th Symposium on UltraSonic Electronics (USE2021)