New insights into antibubble formation by single drop impact on a same-liquid pool

HYPOTHESIS

Secondary drops (SDs) generated when falling drops impact a same-liquid bath can potentially generate antibubbles. Different mechanisms of antibubble formation can be identified and their size and formation probability (PAb) can be predicted.

EXPERIMENTS

Surfactant solutions were dropped from various heights using a highly stable pulseless microfluidic pump in a same-liquid bath. The impact was recorded using a high-speed camera. The formation of SDs and antibubbles as well as their sizes were evaluated considering the falling-drop height (HFD) and dimensionless parameters.

FINDINGS

This study reports new mechanisms for antibubble formation from SDs. A decrease in the surface tension yielded a thinner central jet, thereby yielding more SDs. Larger values of the HFD, impact velocity (U), and Weber number (We) increased the SD size and decreased the SD count; the increase in size increased the antibubble size. The number of SDs correlated with the formation of two distinct antibubbles or a single (coalesced) antibubble. The plots for PAb versus HFD, U, and We exhibited two distinct peaks. A moderate increase in the surfactant concentration enhanced PAb in the first regime, whereas an excessive concentration limited antibubble formation. Artificial neural modeling can successfully predict antibubble formation. These findings provide valuable insights for the research on controlled antibubble generation.

Preparation of acid-responsive antibubbles from CaCO3-based Pickering emulsions

HYPOTHESIS

Hydrophobized fumed silica particles were previously reported for producing antibubbles that are quite stable in neutral as well as in acidic media. To produce acid-responsive antibubbles (e.g., for gastric drug delivery), the silica nanoparticles must be replaced by suitable particles, e.g., calcium carbonate (CaCO3), which can degrade at low pH to release the encapsulated drug.

EXPERIMENTS

Two variants of CaCO3-stabilized antibubbles were prepared (by using CaCO3 particles pre-coated with stearic acid, or by using native CaCO3 particles in combination with sodium stearoyl lactylate) and drug release was compared with classic antibubbles produced with hydrophobized fumed silica particles.

FINDINGS

CaCO3 particles (pre-coated with stearic acid) can be used to produce stable antibubbles, which provided an entrapment efficiency of a model drug (methylene blue, MB) of around 85%. A burst release of MB (∼60%) from the antibubbles was observed at pH 2 (i.e., the pH of the stomach), which was further increased to 80% during the next 30 min. On the contrary, at neutral pH, about 70% of the drug remained encapsulated for at least 2 h. We further demonstrated that the acidic conditions led to the desorption of CaCO3 particles from the air-liquid interface resulting in the destabilization of the antibubbles and the release of drug-containing cores.

A water-soluble alternative to simethicone for gastrointestinal endoscopy: Results of a clinical trial

During endoscopy, simethicone defoaming agents are commonly used to improve visualization, but they leave residues and impact drying. This clinical trial involved patients undergoing colonoscopy procedures with substantial bubbles that impeded mucosal wall visibility. As an alternative to simethicone, investigators evaluated a water-soluble, ginger-based gastrointestinal supplement (GI-Ease) that did not contain sugars, thickeners, or binding agents. In 112/114 cases (98%), the bubbles were reduced sufficiently to allow visualization of the gastrointestinal tract, with no adverse events.

Advances in antibubble formation and potential applications

Antibubbles are unusual physical objects consisting of a liquid core(s) surrounded by a thin air film/shell while in a bulk liquid. Antibubbles carry two air-liquid interfaces, i.e., one with the inner liquid and the other with the outer liquid. The distinct structure of antibubbles makes them quite attractive for drug and therapeutic delivery, although their potential applications have not been realized so far. The major challenge in this regard is a short-lived span of antibubbles, which is usually in the order of a few minutes to a few hours based on the stabilization mechanism used. We present a critical overview of different techniques that can be used to generate antibubbles. This includes a more commonly applied conventional approach in which the air-film is created through surface entrapment when a liquid jet/drop falls on a bulk liquid. The other available options rely on entirely different mechanisms for antibubble formation, for instance, through drop encapsulation by a submerged air bubble, or through evaporation/sublimation of volatile oil from a W/O/W double emulsion. Furthermore, the mechanisms of antibubble formation and collapse, and the factors affecting their stability have been discussed explicitly; and wherever required, the concept is correlated to other allied physical objects such as bubbles, liquid marbles, etc. Finally, the potential applications, research gaps in the existing knowledge, and some directions for future research are provided towards the end of this article.

Formulation and characterisation of drug-loaded antibubbles for image-guided and ultrasound-triggered drug delivery

The aim of this study was to develop high load-capacity antibubbles that can be visualized using diagnostic ultrasound and the encapsulated drug can be released and delivered using clinically translatable ultrasound. The antibubbles were developed by optimising a silica nanoparticle stabilised double emulsion template. We produced an emulsion with a mean size diameter of 4.23 ± 1.63 µm where 38.9 ± 3.1% of the droplets contained a one or more cores. Following conversion to antibubbles, the mean size decreased to 2.96 ± 1.94 µm where 99% of antibubbles were <10 µm. The antibubbles had a peak attenuation of 4.8 dB/cm at 3.0 MHz at a concentration of 200 × 10³ particles/mL and showed distinct attenuation spikes at frequencies between 5.5 and 13.5 MHz. No increase in subharmonic response was observed for the antibubbles in contrast to SonoVue®. High-speed imaging revealed that antibubbles can release their cores at MIs of 0.6. In vivo imaging indicated that the antibubbles have a long half-life of 68.49 s vs. 40.02 s for SonoVue®. The antibubbles could be visualised using diagnostic ultrasound and could be disrupted at MIs of ≥0.6. The in vitro drug delivery results showed that antibubbles can significantly improve drug delivery (p < 0.0001) and deliver the drug within the antibubbles. In conclusion antibubbles are a viable concept for ultrasound guided drug delivery.