Encapsulation of Emulsion Droplets with Metal Shells for Subsequent Remote, Triggered Release

Current Challenges in Microcapsule Designs and Microencapsulation Processes: A Review

Microencapsulation is an advanced methodology for the protection, preservation, and/or delivery of active materials in a wide range of industrial sectors, such as pharmaceuticals, cosmetics, fragrances, paints, coatings, detergents, food products, and agrochemicals. Polymeric materials have been extensively used as microcapsule shells to provide appropriate barrier properties to achieve controlled release of the encapsulated active ingredient. However, significant limitations are associated with such capsules, including undesired leaching and the nonbiodegradable nature of the typically used polymers. In addition, the energy cost of manufacturing microcapsules is an important factor to be considered when designing microcapsule systems and the corresponding production processes. Recent factors linked to UN sustainability goals are modifying how such microencapsulation systems should be designed in pursuit of "ideal" microcapsules that are efficient, safe, cost-effective and environmentally friendly. This review provides an overview of advances in microencapsulation, with emphasis on sustainable microcapsule designs. The key evaluation techniques to assess the biodegradability of microcapsules, in compliance with recently evolving European Union requirements, are also described. Moreover, the most common methodologies for the fabrication of microcapsules are presented within the framework of their energy demand. Recent promising microcapsule designs are also highlighted for their suitability toward meeting current design requirements and stringent regulations, tackling the ongoing challenges, limitations, and opportunities.

Viability of Free and Alginate-Carrageenan Gum Coated Lactobacillus acidophilus and Lacticaseibacillus casei in Functional Cottage Cheese

The survivability of encapsulated and nonencapsulated probiotics consisting of Lactobacillus acidophilus and Lacticaseibacillus casei and the nutritional, physicochemical, and sensorial features of cottage cheese were investigated under refrigeration storage at 4 °C for 28 days. Microbeads of L. acidophilus and L. casei were developed using 2% sodium alginate, 1.5% sodium alginate and 0.5% carrageenan, and 1% sodium alginate and 1% carrageenan using an encapsulation technique to assess the probiotic viability in cottage cheese under different gastrointestinal conditions (SGF (simulated gastric juice), SIF (simulated intestinal fluid)), and bile salt) and storage conditions. Scanning electron microscopy (SEM) elucidated the stable structure of microbeads, Fourier transform infrared spectroscopy (FTIR) confirmed the presence probiotics in the microcapsules, and X-ray diffraction (XRD) demonstrated the amorphous state of microbeads. Furthermore, the highest encapsulation efficiency was observed for alginate 1% and carrageenan 1% microbeads (T3), i.e., 95%. Likewise, viability was recorded in T3 against SGF, SIF, and bile salt solution, i.e., 8.5, 8.8, and 8.9 log CFU/g at 80 min of exposure, compared to the control. The results of pH showed a significant (p < 0.05) decline that ultimately increased the titratable acidity. Nutritional analysis of cottage cheese revealed the highest levels of ash, protein, and total solids in T3, exhibiting mean values of 3.2, 22, and 43.2 g/100 g, respectively, after 28 days of storage. The sensory evaluation of cottage cheese demonstrated better color, flavor, and textural attributes in T3. Conclusively, synergistic addition of L. acidophilus and L. casei encapsulated with alginate-carrageenan gums was found to be more effective in improving the viability of probiotics in cottage cheese than noncapsulated cells while carrying better magnitudes of ash and protein, lower acidity, and pleasant taste.

Efficient Fabrication of Self-Assembled Polylactic Acid Colloidosomes for Pesticide Encapsulation

Colloidosomes are microcapsules whose shells are composed of cumulated or fused colloidal particles. When colloidosomes are used for in situ encapsulation, it is still a challenge to achieve a high encapsulation efficiency and controllable release by an effective fabrication method. Herein, we present a highly efficient route for the large-scale preparation of colloidosomes. The biodegradable polylactic acid (PLA) nanoparticles (NPs) as shell materials can be synthesized using an antisolvent precipitation method, and the possible formation mechanism was given through the molecular dynamics (MD) simulation. The theoretical values are basically consistent with the experimental results. Through the use of the modified and unmodified PLA NPs, the colloidosomes with controllable shell porosities can be easily constructed using spray drying technology. We also investigate the mechanism of colloidosomes successfully self-assembled by PLA NPs with various factors of inlet temperature, feed rate, and flow rates of compressed air. Furthermore, avermectin (AVM) was used as a model for in situ encapsulation and a controllable release. The spherical modified colloidosomes encapsulating AVM not only achieve a small mean diameter of 1.57 μm but also realize a high encapsulation efficiency of 89.7% and impermeability, which can be further verified by the MD simulation. AVM molecules gather around and clog the shell pores during the evaporation of water molecules. More importantly, the PLA colloidosomes also reveal excellent UV-shielding properties, which can protect AVM from photodegradation.

Advancing Inorganic Microcapsule Fabrication through Frozen-Assisted Interfacial Reactions Utilizing Liquid Marbles

Inorganic microcapsules (IMs) have gained significant attention as versatile platforms for delivering functional agents in various fields. Traditional template-dependent methods employing hard templates often involve complex and harsh template removal processes. Achieving IMs with diverse composition and structure remains challenging with current preparation strategies. Therefore, in this work, we have for the first time demonstrated an extremely facile and efficient liquid-marbles-based template approach for fabricating pure inorganic microcapsules via interfacial reaction in a mild aqueous solution. The water-water reaction interface is created by changing the wettability of the liquid marble (LM) surface through the icing-melting process. The composition and function of the inorganic shell could be easily adjusted by varying the inorganic reagent species of the interfacial reaction, the hydrophobic particle of the shell, and the reaction environment according to the specific requirements of the application field. Such an approach provides a flexible platform for material preparation.

Stability and Survivability of Alginate Gum-Coated Lactobacillus rhamnosus GG in Simulated Gastrointestinal Conditions and Probiotic Juice Development

Survivability of probiotics is severely affected by harsh gastrointestinal conditions. In the present study, microbeads of Lactobacillus rhamnosus GG were formulated using alginate (1.5% w/v) and combination of alginate (1.5% w/v) with xanthan gum (0.5% w/v) through an emulsion technique to improve bacterial viability in low pH orange juice and in gastrointestinal conditions. The microbeads were tested for encapsulation efficiency, survivability in bile salt, SGF (simulated gastric juice), SIF (simulated intestinal fluid), and storage stability. Probiotic orange juice was formulated and tested for physicochemical parameters (pH, titratable acidity, and total sugars) and sensorial properties during storage. Gum-coated alginate microbeads (T3) showed higher encapsulation efficiency, i.e., 95.2% compared to alginate microbeads (T2), i.e., 86.85%. Similarly, T3 showed the highest resistance against bile salt (8.50 log CFU/g), SGF (7.95 log CFU/g), and SIF (8.0 log CFU/g) during 80 min exposure compared to T2 and free cells. The viability of gum-coated alginate beads (T3) remained above 107 CFU/g in gastrointestinal conditions and at the end of 21 days storage (8.3 log CFU/mL). All physicochemical parameters of probiotic juice were significantly ( $p\le 0.05$ ) decreased with respect to storage except acidity. In addition, minimal changes in physicochemical parameters were observed in T3 compared to other treatments. Treatment had no significant impact on the sensory characteristics of juice, but storage had a significant effect ( $p\le 0.05$ ) on the sensory characteristics of juice. The alginate gum microbeads improve the survivability of probiotics for targeted delivery. Hence, encapsulated probiotics can be used for functional beverage development to take advantage of their therapeutic benefits.

Formation mechanism of binary complex based on β-lactoglobulin and propylene glycol alginate with different molecular weights: Structural characterization and delivery of curcumin

The complexation of protein and polysaccharide has shown considerable potential for the encapsulation of functional food components. In this work, propylene glycol alginate (PGA) molecules with different molecular weights (100, 500, and 2,000 kDa) were prepared through H2O2 oxidation, which were further combined with β-lactoglobulin nanoparticles (β-lgNPs) to form PGA-β-lgNPs complexes for the delivery of curcumin (Cur). Results showed that the depolymerization of PGA molecule was resulted from the breakage of glycosidic bonds in the main chain, and the depolymerization rate of PGA molecule depended on the reaction time, temperature, solution pH and H2O2 concentration. As the increasing molecular weight of PGA, the particle size, zeta-potential and turbidity of the complexes were obviously increased. The formation of PGA/β-lgNPs complexes was mainly driven by non-covalent interaction, including electrostatic gravitational interaction, hydrogen bonding and hydrophobic effect. Interestingly, the difference in the molecular weight of PGA also led to significantly differences in the micro-morphology of the complexes, as PGA with a high molecular weight (2,000 kDa) generated the formation of a “fruit-tree” shaped structure, whereas PGA with relatively low molecular weight (100 and 500 kDa) led to spherical particles with a “core-shell” structure. In addition, the incorporation of PGA molecules into β-lgNPs dispersion also contributed to the improvement in the encapsulation efficiency of Cur as well as physicochemical stability of β-lgNPs, and PGA with a higher molecular weight was confirmed with a better effect. Findings in the current work may help to further understand the effect of molecular weight of polysaccharide on the physical and structural properties as well as effectiveness as delivery systems of polysaccharide-protein complexes, providing for the possibility for the design and development of more efficient carriers for bioactive compounds in food system.

Dynamics of Polymer Nanocapsule Buckling and Collapse Revealed by In Situ Liquid-Phase TEM

Nanocapsules are hollow nanoscale shells that have applications in drug delivery, batteries, self-healing materials, and as model systems for naturally occurring shell geometries. In many applications, nanocapsules are designed to release their cargo as they buckle and collapse, but the details of this transient buckling process have not been directly observed. Here, we use in situ liquid-phase transmission electron microscopy to record the electron-irradiation-induced buckling in spherical 60-187 nm polymer capsules with ∼3.5 nm walls. We observe in real time the release of aqueous cargo from these nanocapsules and their buckling into morphologies with single or multiple indentations. The in situ buckling of nanoscale capsules is compared to ex situ measurements of collapsed and micrometer-sized capsules and to Monte Carlo (MC) simulations. The shape and dynamics of the collapsing nanocapsules are consistent with MC simulations, which reveal that the excessive wrinkling of nanocapsules with ultrathin walls results from their large Föppl-von Kármán numbers around 10⁵. Our experiments suggest design rules for nanocapsules with the desired buckling response based on parameters such as capsule radius, wall thickness, and collapse rate.

Deposition of non-porous calcium phosphate shells onto liquid filled microcapsules

The efficient encapsulation of small molecule active ingredients has been a challenge for many decades across many commercial applications. Recently, successful attempts to address this issue have included deposition of thin metal shells onto liquid filled polymer microcapsules or emulsion droplets to provide an impermeable barrier to diffusion. In this work we have developed a novel method to protect small molecule active ingredients by deposition of thin mineral shells. Platinum nanoparticles are used to catalyse and direct growth of a calcium phosphate shell onto liquid filled polymer microcapsules under various reaction conditions. Findings indicate that a non-porous protective shell is formed on the majority of the microcapsule population, with small concentrations of the core material being released only from those microcapsules with defects, over a 7 days period, when conducting forced release studies into a solvent for the core oil. The resulting microcapsules show no significant cell toxicity when exposed to HEK 293 cells for 72 h.

Preparation of Multicore Colloidosomes: Nanoparticle-Assembled Capsules with Adjustable Size, Internal Structure, and Functionalities for Oil Encapsulation

Colloidosomes, also known as Pickering emulsion capsules, have attracted attention for encapsulation of hydrophilic and hydrophobic actives. However, current preparation methods are limited to single core structures and require the use of modified/engineered nanoparticles for forming the shell. Here, we report a fast, simple, and versatile method for producing multi-oil core silica colloidosomes via salt-driven assembly of purely hydrophilic commercial nanoparticles dispersed within an oil-in-water-in-oil (O/W/O) double emulsion template. The internal structure and overall diameter of the capsules can be adjusted by altering the primary and secondary emulsification conditions. With this approach, 7-35 μm diameter multicore colloidosomes containing 0.9-4.2 μm large oil cores were produced. The capsules can easily be functionalized depending on the type of nanoparticles used in the preparation process. Here, metal oxide nanoparticles, such as Fe₃O₄, TiO₂, and ZnO, were successfully incorporated within the structure, conferring specific functional properties (i.e., magnetism and photocatalysis) to the final microcapsules. These capsules can also be ruptured by using ultrasound, enabling easy access to the internal core environments. Therefore, we believe this work offers a promising approach for producing multicore colloidosomes with adjustable structure and functionalities for the encapsulation of hydrophobic actives.

Fabrication of pH-responsive nanoparticles for high efficiency pyraclostrobin delivery and reducing environmental impact

In this work, a pH-responsive pesticide delivery system using mesoporous silica nanoparticles (MSNs) as the porous carriers and coordination complexes of Cu ions and tannic acid (TA-Cu) as the capping agent was established for controlling pyraclostrobin (PYR) release. The results showed the loading capacity of PYR@MSNs-TA-Cu nanoparticles for pyraclostrobin was 15.7 ± 0.5% and the TA-Cu complexes deposited on the MSNs surface could protect pyraclostrobin against photodegradation effectively. The nanoparticles had excellent pH responsive release performance due to the decomposition of TA-Cu complexes under the acid condition, which showed 8.53 ± 0.37%, 82.38 ± 1.67% of the encapsulated pyraclostrobin were released at pH 7.4, pH 4.5 after 7 d respectively. The contact angle and adhesion work of PYR@MSNs-TA-Cu nanoparticles on rice foliage were 86.3° ± 2.7° and 75.8 ± 3.1 mJ/m² after 360 s respectively, indicating that TA on the surface of the nanoparticles could improve deposition efficiency and adhesion ability on crop foliage. The control effect of PYR@MSNs-TA-Cu nanoparticles against Rhizoctonia solani with 400 mg/L of pyraclostrobin was 85.82% after 7 d, while that of the same concentration of pyraclostrobin EC was 53.05%. The PYR@MSNs-TA-Cu nanoparticles did not show any phytotoxicity to the growth of rice plants. Meanwhile, the acute toxicity of PYR@MSNs-TA-Cu nanoparticles to zebrafish was decreased more than 9-fold compared with that of pyraclostrobin EC. Thus, pH-responsive PYR@MSNs-TA-Cu nanoparticles have great potential for enhancing targeting and environmental safety of the active ingredient.

Biocompatible Wax-Based Microcapsules with Hermetic Sealing for Thermally Triggered Release of Actives

We present a microfluidic approach that utilizes temperature-responsive and biocompatible palm oil as the shell material in microcapsules to simultaneously achieve hermetic sealing as well as on-demand temperature-triggered release of the encapsulated actives. Unlike common paraffin waxes (e.g., eicosane), microcapsule shells comprising palm oil do not form pores or cracks during freezing and provide a hermetic seal, a nearly perfect seal that separates the core containing the actives from the surrounding environment over a prolonged period of time. This allows effective isolation and protection of complex cargoes such as small molecules with high diffusivity, strong acids, and cosmetic actives including niacinamide. Moreover, the palm oil shell melts above the defined melting temperature, allowing the on-demand release of the encapsulated actives. Furthermore, palm oil is biocompatible, is edible, and leaves a minimal footprint when used in personal care and cosmetic products, offering new perspectives in the design of microcapsules for cosmetic applications.

Nanoparticles in Polyelectrolyte Multilayer Layer-by-Layer (LbL) Films and Capsules—Key Enabling Components of Hybrid Coatings

Originally regarded as auxiliary additives, nanoparticles have become important constituents of polyelectrolyte multilayers. They represent the key components to enhance mechanical properties, enable activation by laser light or ultrasound, construct anisotropic and multicompartment structures, and facilitate the development of novel sensors and movable particles. Here, we discuss an increasingly important role of inorganic nanoparticles in the layer-by-layer assembly—effectively leading to the construction of the so-called hybrid coatings. The principles of assembly are discussed together with the properties of nanoparticles and layer-by-layer polymeric assembly essential in building hybrid coatings. Applications and emerging trends in development of such novel materials are also identified.

A fluorous biphase drug delivery system triggered by low frequency ultrasound: controlled release from perfluorous discoidal porous silicon particles

Conventional drug delivery systems face unsatisfactory loading efficiency, poor biological bypass, and uncontrollable release, which are great barriers for improving the treatment of many diseases. Herein, a proof-of-concept of a fluorous biphase drug delivery system (FB-DDS) trigged by low frequency ultrasound (LFUS) is proposed for the first time, where promoted incorporation and stabilization of therapeutic agents in nanocarriers was achieved through fluorine-fluorine interactions, and the encapsulated drugs were controllably released by external sources, resulting in minimized nonspecific toxicity and enhanced therapeutic efficacy. The FB-DDS was constructed from monodisperse, discoidal porous silicon particles (PSP) and was functionalized with 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS17) for loading perfluoropentane (PFP) and fluorinated drugs through fluorine-fluorine interactions. This delivery system was demonstrated by utilizing model compounds including a fluorous-tagged fluorescein and a fluorine containing antibiotic ciprofloxacin. Loading of the model molecules into fluorocarbon-coated carriers was facilitated by fluorous interactions, whereas ejection of the model molecules was promoted by applying LFUS to rapidly evaporate PFP. In the in vitro test, these carriers loaded with fluorine containing ciprofloxacin exhibited excellent antimicrobial activity against Pseudomonas aeruginosa biofilm formation. Overall, this innovative stimulus-responsive fluorous biphase drug delivery system will be a promising candidate for practical applications as well as encouraging further investigation of drug delivery and controlled release strategies.

Metal-shell nanocapsules for the delivery of cancer drugs

Cytotoxic drugs tend to have substantial side effects on healthy tissues leading to systemic toxicity, limited tolerated doses and reduced drug efficacy. A prominent research area focuses on encapsulating cytotoxic drugs for targeted delivery to cancer tissues. However, existing carriers suffer from low drug loading levels and high drug leaching both when circulating systemically and when accumulating in non-target organs. These challenges mean that only few encapsulation technologies for delivery of cytotoxic drugs have been adopted for clinical use. Recently, we have demonstrated efficient manufacture of impermeable metal-shell/liquid core microcapsules that permit localised delivery by triggering release with ultrasound. This method has the potential to improve on existing methods for localised drug delivery because it:We demonstrate here the further miniaturization of both the emulsion droplet template and the thickness of the surrounding metal shell to the nanoscale in an attempt to take advantage of the EPR effect and the excretion of nanoparticles by the hepatobiliary system.

Dynamically Reconfigurable, Multifunctional Emulsions with Controllable Structure and Movement

Dynamically reconfigurable oil-in-water (o/w) Pickering emulsions are developed, wherein the assembly of particles (i.e., platinum-on-carbon and iron-on-carbon particles) can be actively controlled by adjusting interfacial tensions. A balanced adsorption of particles and surfactants at the o/w interface allows for the creation of inhomogeneity of the particle distribution on the emulsion surface. Complex Pickering emulsions with highly controllable and reconfigurable morphologies are produced in a single step by exploiting the temperature-sensitive miscibility of hydrocarbon and fluorocarbon liquids. Dynamic adsorption/desorption of (polymer) surfactants afford both shape and configuration transitions of multiple Pickering emulsions and encapsulated core/shell structured can be transformed into a Janus configuration. Finally, to demonstrate the intrinsic catalytic or magnetic properties of the particles provided by carbon bound Pt and Fe nanoparticles, two different systems are investigated. Specifically, the creation of a bimetallic microcapsule with controlled payload release and precise modulation of translational and rotational motions of magnetic emulsions are demonstrated, suggesting potential applications for sensing and smart payload delivery.

Photocontrollable Release with Coumarin-Based Profragrances

The achievement of controllable and lasting scent on a targeted surface is a long-term goal in the field of flavors and fragrances. Herein, we design a novel series of phototriggered coumarin-based profragrances conjugated with volatile carboxylic fragrances via activatable chemical bridge of ester group, thereby achieving the controllable release of volatile fragrances under ambient conditions. Upon exposure to light, the fragile ester group of profragrances allows the slow release of fragrance molecules, building up a new light-sensitive fragrance delivery system. The incorporated coumarin unit of CM-OH as phototrigger is killing two birds with one stone, that is, precise photocontrollable release of fragrance molecules, and unprecedented fluorescence intensity to monitor the releasing process of fragrance molecules with linear relationship (R² > 0.95). In comparison, the light-induced releasing amount from profragrances of CM-O-EA, CM-O-PEA, CM-O-PA, and CM-O-CA is much lower than corresponding free fragrances by 33-, 8.5-, 13-, and 983-fold, respectively. As demonstrated, the coumarin-based profragrances provide a phototriggered platform to realize the controllable release of volatile fragrances, resulting in a long-lasting headspace concentration on the targeted surface of wallpaper.