We report on the preparation and release behavior of daunorubicin-loaded antibubbles targeted for intestinal drug release. The antibubbles were prepared from a Pickering-stabilized W1/O/W2 double emulsion, then freeze-dried and rehydrated to create the distinct structure of antibubbles, i.e., water-in-air-in-water (W1/A/W2). Daunorubicin was loaded into the inner aqueous phase, surrounded by two particle-stabilized interfaces separated by an air gap. Fumed silica nanoparticles with varying degrees of hydrophobicity were used to stabilize both interfaces. The antibubbles, when rehydrated, had a mean diameter of approximately 25 μm and a drug entrapment efficiency of >90%. The release profile showed a non-significant release of the drug in both acidic and neutral media. Notably, a burst release of the drug (around 60%) was observed when the antibubbles were exposed to bile salts in simulated intestinal conditions. Hence, the bile salts were a major driving force in dislocating the nanoparticles at the interfaces. We have supported our findings with quantitative drug-release measurements (measured spectrophotometrically) and microscopic observations of structural changes across different media. Furthermore, protection of the encapsulated daunorubicin from degradation was investigated when the antibubbles were exposed to pH 9 or above (at which daunorubicin typically degrades). The antibubbles preserved almost all the drug over 24 h, whereas most of it (∼90%) degraded in the unencapsulated form. Based on these initial results, antibubbles seem promising for delivering drugs to the intestine, as a proof-of-concept for oral chemotherapy.

Microbubbles are unique materials widely used in food formulations, as well as biomedical and pharmaceutical applications. However, their thermodynamic instability limits practical use. To enhance their stability, we studied the production of microbubble powders using Pickering emulsions as templates, in combination with freeze-drying to remove both the aqueous solvent and the oil inside the emulsion droplets. Hydrophobic silica nanoparticles stabilized O/W Pickering emulsions with cyclooctane as a volatile oil phase, while nanoparticles were dispersed either in the water phase alone or in both water and oil phases. After freeze-drying, SEM microscopy revealed two distinct microbubble structures: single-layer microbubbles when nanoparticles were present only in the water phase, and network microbubbles with an internal particle network, when nanoparticles were present in both the aqueous and oil phases. Both microbubble types exhibit long-term stability. The stability of redispersed bubbles was examined in water and protein dispersions. We found that the presence of a particle network inside significantly improved bubble stability. Microscopy showed that SL microbubbles persisted for less than 3 days, whereas NW bubbles remained stable for over 1 week; correspondingly, 50 % air loss occurred after 5 days (SL) and 7 days (NW). For SL bubbles, faster freezing and the presence of protein significantly enhance stability, with many bubbles remaining after 7 days. Our method of using freeze-drying to create microbubble powders from Pickering emulsion templates solves the problems of low production efficiency and scalability that exist when attempting to directly stabilize air-water interfaces using Pickering stabilization.

Double emulsions (DEs) offer unique compartmentalized structures but are inherently unstable, prompting significant scientific and industrial efforts to enhance their stability. One promising strategy is the use of solid particles-known as Pickering stabilization-resulting in Pickering double emulsions (PDEs), which overcome many limitations of conventional low-molecular-weight (LMW) surfactants. However, the term "Pickering" is often misused in the literature to describe any formulation containing particles, regardless of whether the interface is fully stabilized by them. This review aims to clarify the concept of Pickering stabilization, outline the rationale for its application to DEs, and examine preparation mechanisms, interfacial approaches, potential applications, and current challenges. Particles with dual wettability and high desorption energy irreversibly adsorb at interfaces, forming robust mechanical barriers that inhibit coalescence and reduce diffusion or escape of internal droplets. PDEs can be prepared via two-step emulsification, one-step processes, or advanced microfluidic methods. A variety of Pickering approaches have been developed to engineer particles capable of dual interfacial stabilization, enabling sophisticated functions such as (co-)encapsulation, controlled release, and the formation of hierarchical structures like microspheres, colloidosomes, and antibubbles. To unlock the full potential of PDEs for industrial applications, future research should prioritize eliminating surfactant use, developing safe and sustainable particles, and advancing scalable production methods without compromising emulsion stability or performance.

Antibubbles are a novel carrier for precise, ultrasound-triggered delivery of fluid or nanoparticulate payloads. These structures combine the high acoustic contrast and strong acoustic response of conventional microbubbles with a significantly higher payload capacity thanks to their large fluid core. The release characteristics of antibubbles can be precisely tuned during fabrication to enable release across a range of pressures spanning a few kPa to several hundred kPa. Moreover, antibubbles can release these payloads either all at once, or incrementally across multiple ultrasonic pulses. This talk will illustrate how such characteristics make antibubbles unique reporters for ultrasonic pressure, and how the fabrication and design determine the antibubble’s response. We use high-speed microscopy to highlight the relationship between the antibubble and its released payload. Finally, it will be shown that by properly tailoring the payload, formulation, and supporting matrix, spatial maps of complex acoustic pressure fields can be produced and read out optically, providing real-time diagnostic information about spatially structured ultrasonic fields.

Ibuprofen, one of the most widely used nonsteroidal anti-inflammatory drugs, is a poor-tasting and poorly soluble drug. As an alternative approach to overcome these issues, ibuprofen was encapsulated in Pickering antibubbles using two different oils, cyclomethicone and cyclooctane, as processing aids. The amount of the loaded active agent was determined by thermogravimetry (TG), while the analysis of the evolved gases, performed by online coupling of the heating device to an infrared and a mass spectrometer (EGA-FTIR-MS), allowed for describing the drug decomposition mechanism. Although the dissolution profile and zeta potential values were found to be independent of the preparation method, differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), and Raman microscopy confirmed the occurrence of a slight amorphization of the drug inside the antibubbles. The reported results suggest that this relatively simple encapsulation technique might be an alternative for ibuprofen taste masking and targeted delivery.

In this work we present a new encapsulation method that allows for the controlled release of drugs under simulated small intestinal conditions. This method consists of encapsulation within microbubbles and is characterized by an unprecedented combination of excellent barrier properties and fast and complete triggered release. The method was applied to produce a drink containing taste-masked acetaminophen as a model drug. Micronized acetaminophen (paracetamol) was dispersed in cyclohexane containing pharma-approved hydrophobized silica particles and the resulting dispersion was emulsified in an aqueous phase containing dispersed hydrophobized silica particles and dissolved maltodextrin. The resulting solid-in-oil-in-water emulsion was washed to remove unencapsulated acetaminophen and subsequently freeze-dried to remove both the water and the cyclohexane. This produced a dry material that after reconstitution in water created a suspension of microbubbles containing acetaminophen particles, i.e. a solid-in-gas-in-water dispersion. The encapsulation efficiency was well over 90% and hardly any acetaminophen escaped from the microbubbles during storage for 24 h in aqueous solution. Also, encapsulates were stable in the presence of saliva as well as during in vitro incubation with stomach juice. In line with this, sensory tests showed an excellent masking of the taste of the drug. In vitro incubation with simulated intestinal fluid containing bile salts triggered fast and near complete release of the encapsulated acetaminophen, which should assure good bioavailability in vivo. The described encapsulates that are stable for at least days, are thus expected to be suitable for taste-masking or enteric release applications in liquid formulations, including foods.

Pickering stabilization has gained scientific interest over the past few decades, yet its practical application remains restricted. This is likely due to the challenges in characterizing particle behavior at interfaces, making it difficult to relate findings from model systems to practical applications. Here, we applied several explorative techniques such as x-ray powder diffraction, thermal analysis, and quasi-elastic neutron scattering to obtain a better insight into the role of interfacial hydration in the stabilization of Pickering-stabilized systems. As Pickering-stabilized systems, we used bubbles and the so-called antibubbles that were obtained by freeze-drying an emulsion or double emulsion, respectively, template. Silica particles with different hydrophobicities were used to stabilize these (anti)bubbles. Our results demonstrate that increasing particle hydrophobicity enhances silica-water-maltodextrin interactions, modifying the water structure, and significantly influencing the thermal degradation profiles.Practical applications: Pickering-stabilized bubbles are promising for encapsulating probiotics and delivering antioxidants in food systems by forming gas-shell capsules. To scale these systems for commercial use, it is crucial to understand interfacial hydration and particle wettability, as tuning particle hydrophobicity can significantly enhance edible coatings and extend shelf life. Our results show that neutron spectroscopy provides an effective in situ method for assessing the hydrophobicity of silica particles. This approach offers a novel method for investigating wetting behavior.

Nanoparticles have proven to be attractive carriers in therapeutic drug delivery since they can encapsulate, protect and stabilize a plethora of different drugs, thereby improving therapeutic efficacy and reducing side effects. However, specific targeting of drug-loaded nanoparticles to the tissue of interest and a timely and spatially controlled release of drugs on demand still represent a challenge. Recently, gas-filled microparticles, so-called antibubbles, have been developed which can efficiently encapsulate liquid drug droplets. Here, we show that antibubbles are efficiently taken up by macrophages in vitro and are stably maintained for more than 48 h without compromising antibubble integrity and macrophage viability. We show that application of diagnostic ultrasound induces the disintegration of both antibubbles and carrier cells while not affecting non-loaded macrophages. Using 4-hydroxytamoxifen as a model drug, we show ultrasound-mediated drug release upon adoptive transfer of antibubble-loaded macrophages in mice. Together with the ability of macrophages to accumulate in inflamed tissues, antibubble-loaded macrophages represent an attractive tool for targeted delivery of drugs and its ultrasound-mediated spatial and temporal drug release, highlighting the therapeutic perspective of this strategy.

Antibubbles are liquid droplets encapsulated by a gas film that have recently been explored for on-demand ultrasound-triggered drug release. However, their ultrasound imaging capabilities are limited by their stiff shells stabilized with silica nanoparticles. Here, we develop polymeric antibubbles that generate greater ultrasound contrast than silica-based antibubbles, while showing better stability than conventional polymeric microbubbles.

Antibubbles, characterized by a water-in-air-in-water structure, are a novel dispersion system stabilized by the adsorption of nanoparticles (e.g., silica nanoparticles) at the air-liquid interface and produced via the emulsification-sublimation-rehydration technique. Traditionally, shear-based homogenization has been the standard method for creating Pickering double emulsions, which are then converted into particle-stabilized antibubbles. However, efficient drug delivery using antibubbles requires a small size, narrow size distribution, and high active loading, which are challenges typically faced with shear-based methods. This study introduces the formation of antibubbles using premix membrane emulsification (PME), a gentle technique ideal for heat- and shear-sensitive materials. We conducted a thorough investigation, producing primary and double emulsions with both high-shear homogenization (HSH) and PME through Shirasu porous glass (SPG) membranes. Antibubble size distribution and entrapment efficiency (using a model drug) were analyzed in relation to the process parameters of the emulsification techniques. We found that PME, particularly when using a 30 μm SPG membrane in the secondary emulsification stage, yielded antibubbles with smaller sizes (down to 5 μm in diameter) and significantly higher encapsulation efficiency (up to 80%) compared to HSH. These findings highlight PME's potential as a superior method for producing nanoparticle-stabilized antibubbles for drug delivery applications.

Developing carriers capable of efficiently transporting both hydrophilic and lipophilic payloads is a captivating focus within the pharmaceutical and drug delivery research domain. Antibubbles, constituting an innovative encapsulation system designed for drug delivery purposes, have garnered scientific interest thanks to their distinctive water-in-air-in-water (W1/A/W2) structure. However, in contrast to their precursor, i.e., nanoparticle-stabilized W1/O/W2 double emulsion, traditional antibubbles lack the ability to accommodate a lipophilic payload, as the intermediary (volatile) oil layer of the emulsion is replaced by air during the antibubble fabrication process. Therefore, here, we report the fabrication of triple-emulsion-based antibubbles (O1/W1/A/W2), in which the inner aqueous phase was loaded with a nanoemulsion stabilized by various proteins, including whey, soy, or pea protein isolates. As model drugs, we employed the dyes Nile red in the oil phase and methylene blue in the aqueous phase. The produced antibubbles were characterized regarding their size distribution, entrapment efficiency, and stability. The produced antibubbles demonstrated substantial entrapment efficiencies for both lipophilic (ranging from 80% to 90%) and hydrophilic (ranging from 70% to 82%) components while also exhibiting an appreciable degree of stability during an extended rehydration period of two weeks. The observed variations among different antibubble variants were primarily attributed to differences in protein concentration rather than the type of protein used.

Theranostic agents are materials that act both as tracers during diagnostic imaging and as vehicles carrying and releasing therapeutics during treatment. Ultrasoundtriggered theranostic agents comprise shell-encapsulated microbubbles that pulsate during low-amplitude ultrasonic imaging and release their payload upon higher-amplitude sonication whilst simultaneously assisting in the permeation of target tissue. High-amplitude release may be undesirable due to unwanted side effects related to inertial cavitation. However, low-amplitude release from microbubbles typically requires thin encapsulating shells, which in turn may be permanently disrupted under diagnostic imaging conditions. The purpose of this study was to investigate the suitability for theranostic applications of a novel microbubble agent with thick shells composed of calcium carbonate. Hydrophobised calcium carbonate-encapsulated microbubbles of radii between 1.0 μm and 11 μm were subjected to short ultrasound pulses of 1-MHz ultrasound at acoustic amplitudes of 0.5MPa or less, which corresponds to low mechanical indices. During sonication, high-speed video footage was recorded at a frame rate of ten million frames per second. We observed pulsations but no gas release at a 0.1-MPa amplitude and intra-encapsulation fragmentation during sonication at a 0.3-MPa amplitude. At 0.5-MPa amplitude sonication, release was observed from more than 70% of the microbubbles in the field of view. This finding indicates that the microbubbles were stable scatterers during 0.1-MPa sonication, but instable vehicles during 0.5-MPa sonication. The pressures used in this study to observe release were too low to allow for unwanted inertial cavitation. In conclusion, therefore, the microbubbles studied were a promising theranostic agent whose contents could be released at moderate acoustic amplitudes.

Sonochemistry, although established in various fields, is still an emerging field finding new effects of ultrasound on chemical systems and are of particular interest for the biomedical field. This interdisciplinary area of research explores the use of acoustic waves with frequencies ranging from 20 kHz to 1 MHz to induce physical and chemical changes. By subjecting liquids to ultrasonic waves, sonochemistry has demonstrated the ability to accelerate reaction rates, alter chemical reaction pathways, and change physical properties of the system while operating under mild reaction conditions. It has found its way into diverse industries including food processing, pharmaceuticals, material science, and environmental remediation. This review provides an overview of the principles, advancements, and applications of sonochemistry with a particular focus on the domain of (bio-)medicine. Despite the numerous benefits sonochemistry has to offer, most of the research in the (bio-)medical field remains in the laboratory stage. Translation of these systems into clinical practice is complex as parameters used for medical ultrasound are limited and toxic side effects must be minimized in order to meet regulatory approval. However, directing attention towards the applicability of the system in clinical practice from the early stages of research holds significant potential to further amplify the role of sonochemistry in clinical applications.

For paper manufacturing and biofuel production, the controlled deformation of wood pulp is of interest, provided that the integrity of the fibre structure remains intact. Conventional ultrasonic pretreatment in the near-audible range has been observed to cause uncontrolled inertial cavitation damage in wood pulp fibres. To prevent internal damage, we proposed to subject wood pulp mixed with hydrophobic particles to 1-MHz short pulses above the nucleation threshold of the particles but below the Blake threshold, and to observe the interaction of pulsating cavities and wood pulp fibres assisted by high-speed photography. Our 1-MHz results showed the interaction of a collapsing bubble with a wood pulp fibre wall to form a liquid jet hitting the fibre, without apparent destruction of the structure, whilst our 20-kHz controls confirmed previously observed structural destruction. This study shows the feasibility of controlled wood fibre deformation at a high ultrasound frequency.

The benefits of ultrasound are its ease-of-use and its ability to precisely deliver energy in opaque and complex media. However, most materials responsive to ultrasound show a weak response, requiring the use of high powers, which are associated with undesirable streaming, cavitation, or temperature rise. These effects hinder response control and may even cause damage to the medium where the ultrasound is applied. Moreover, materials that are currently in use rely on all-or-nothing effects, limiting the ability to fine-tune the response of the material on the fly. For these reasons, there is a need for materials that can respond to low intensity ultrasound with programmable responses. Here it is demonstrated that antibubbles are a low-intensity-ultrasound-responsive material system that can controllably release a payload using acoustic pressures in the kPa range. Varying their size and composition tunes the release pressure, and the response can be switched between a single release and stepwise release across multiple ultrasound pulses. Observations using confocal and high-speed microscopy revealed different ways that can lead to release. These findings lay the groundwork to design antibubbles that controllably respond to low-intensity ultrasound, opening a wide range of applications ranging from ultrasound-responsive material systems to carriers for targeted delivery.

Nebulization of mRNA therapeutics can be used to directly target the respiratory tract. A promising prospect is that mucosal administration of lipid nanoparticle (LNP)-based mRNA vaccines may lead to a more efficient protection against respiratory viruses. However, the nebulization process can rupture the LNP vehicles and degrade the mRNA molecules inside. Here we present a novel nebulization method able to preserve substantially the integrity of vaccines, as tested with two SARS-CoV-2 mRNA vaccines. We compare the new method with well-known nebulization methods used for medical respiratory applications. We find that a lower energy level in generating LNP droplets using the new nebulization method helps safeguard the integrity of the LNP and vaccine. By comparing nebulization techniques with different energy dissipation levels we find that LNPs and mRNAs can be kept largely intact if the energy dissipation remains below a threshold value, for LNP integrity 5–10 J/g and for mRNA integrity 10–20 J/g for both vaccines.

One approach to ultrasound therapy is to use therapeutic agents that can be activated by focused ultrasound when they reach a specific site in the body. Commonly, such agents are loaded on the surfaces of microbubbles, which respond strongly to ultrasound and shed the payload. However, microbubbles require high pressures and mechanical indices to burst (typically above 200 kPa, MI > 0.2). Moreover, the quantity and types of therapeutic payload that can be delivered by microbubbles are limited because payloads must be attached to the microbubble surface. Here, we show how these limitations can be overcome by using stabilized antibubbles as an ultrasound-responsive carrier. Antibubbles are liquid droplets encased within an air bubble. Because therapeutic payloads can be encapsulated in the core, larger volumes can be carried per antibubble. Additionally, by carrying payloads in the volume rather than on the surface, a wider variety of payloads can be carried. Through experiments we demonstrate that antibubbles respond strongly to ultrasound and can release payloads with pressures below 50 kPa (MI = 0.05) for certain formulations. By modifying the formulation, we show that the release pressure and temporal release profile can be tuned. Finally, we show that the bursting is highly selective in space, demonstrating that antibubbles can be used for precise delivery of payloads using shaped, low-intensity ultrasound fields.

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.

Microcapsules with a liquid core and a solid shell composed of hydrophobic nanoparticles are broadly applied in food, pharmaceutics, and biotechnologies. For example, Pickering emulsions, colloidosomes, or antibubbles (droplets surrounded by air layers in water) enable controlled release of active agents, biocompatibility, and contact‐less liquid transportation. However, producing controlled nanoparticle‐ or polymer‐laden hydrophobic shells at scale is highly challenging, since bulk methods are polydisperse and microfluidic chips are prone to clogging and slow. Here, clog‐free coating of an aqueous jet with silica nanoparticle suspensions with concentrations up to 10% (w/v), as well as high concentrations of polymers (30% (w/v) poly(lactic acid) (PLA)), is demonstrated, enabling continuous generation of microcapsules at flow rates up to 4 mL min −1 . Pickering emulsions are converted into capsules, providing hydrophobic shells consisting of nanoparticles for controlled release. As a highlight, the scalable fabrication of air‐coated capsules (antibubbles) in the sub‐millimeter range is demonstrated. The shell contains an air film that protects the liquid core for days yet enables ultrasound‐induced release within 3 min. By enabling rapid fabrication of controlled Pickering emulsions, colloidosomes, antibubbles, and biodegradable capsules, jetting through a liquid layer (JetALL) provides a versatile platform for advanced applications in food, pharmacy, and life science.