Long-Term Retention of Small, Volatile Molecular Species within Metallic Microcapsules

Alginate Hydrogel Beads with a Leakproof Gold Shell for Ultrasound-Triggered Release

Background/Objectives: Focused ultrasound has advantages as an external stimulus for drug delivery as it is non-invasive, has high precision and can penetrate deep into tissues. Here, we report a gold-plated alginate (ALG) hydrogel system that retains highly water-soluble small-molecule fluorescein for sharp off/on release after ultrasound exposure. Methods: The ALG is crosslinked into beads with calcium chloride and layered with a polycation to adjust the surface charge for the adsorption of catalytic platinum nanoparticles (Pt NPs). The coated bead is subject to electroless plating, forming a gold shell. Ultrasound is applied to the gold-plated ALG beads and the release of fluorescein with or without ultrasound stimulation is quantified. Results: Polyethylenimine (PEI), not poly-L-lysine (PLL), is able to facilitate Pt NP adsorption. Gold shell thickness is proportional to the duration of electroless plating and can be controlled. Gold-plated ALG beads are impermeable to the fluorescein cargo and have nearly zero leakage. Exposure to focused ultrasound initiated the release of fluorescein with full release achieved after 72 h. Conclusions: The gold-plated ALG hydrogel is a new material platform that can retain highly water-soluble molecules with a sharp off/on release initiated by focused ultrasound.

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.

Microplastic-Free Microcapsules Using Supramolecular Self-Assembly of Bis-Urea Molecules at an Emulsion Interface

Encapsulation technology is well established for entrapping active ingredients within an outer shell for their protection and controlled release. However, many solutions employed industrially use nondegradable cross-linked synthetic polymers for shell formation. To curb rising microplastic pollution, regulatory policies are forcing industries to substitute the use of such intentionally added microplastics with environmentally friendly alternatives. This work demonstrates a one-pot process to make microplastic-free microcapsules using supramolecular self-assembly of bis-ureas. Molecular bis-urea species generated in-situ spontaneously self-assemble at the interface of an oil-in-water emulsion via hydrogen bonding to form a shell held together by noncovalent bonds. In addition, Laponite nanodiscs were introduced in the formulation to restrict aggregation observed during the self-assembly and to reduce the porosity of the shell, leading to well-dispersed microcapsules (mean Sauter diameter d [3,2] ∼ 5 μm) with high encapsulation efficiency (∼99%). Accelerated release tests revealed an increase in characteristic release time of the active by more than an order of magnitude after encapsulation. The mechanical strength parameters of these capsules were comparable to some of the commercial, nondegradable melamine-formaldehyde microcapsules. With mild operating conditions in an aqueous environment, this technology has real potential to offer an industrially viable method for producing microplastic-free microcapsules.

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.

Long term water trapping in Pickering emulsions undergoing compositional ripening

One approach to achieving low-calorie foods is to substitute regions of high-calorie content with water droplets, forming water-in-oil emulsions. However, in complex food systems consisting of multiple species of dispersed phases, compositional ripening may occur in which the emulsified water undergoes mass transfer to droplets filled with a species that is less soluble in the continuous phase, for example sugar. Here we present two model systems and use them to study compositional ripening for water-in-oil Pickering emulsions. Water-in-dodecane and water-in-tributyrin emulsions stabilised by PMMA particles were prepared and combined with similar emulsions that included sugar in the water. We use confocal microscopy as a function of time combined with particle tracking to explore how these systems evolve in time. For dodecane, as the system evolves, the pure water droplets appear to crumple due to the loss of water; in extreme cases, they eventually 'explode'. Simultaneously, the sugar-filled droplets expand and slowly coalesce. Evidently, our interfacial coating of particles is unable to suppress compositional ripening. In contrast, pure water droplets in tributyrin crumple into small stable structures, potentially retaining water. We show that decreasing the concentration of the sugar solution also decreases the rate of change of water droplet size for both oils. Observations of droplet 'explosions' confirm that the driving force can overcome the trapping of the particles at the interface, in contrast to the case of Ostwald ripening. However the crumpled states in the tributyrin system provide some indication that this effect can be overcome.

Surface Morphology-Enhanced Delivery of Bioinspired Eco-Friendly Microcapsules

We report the development of novel mineralized protein microcapsules to address critical challenges in the environmental impact and performance of consumer, pharmaceutical, agrochemical, cosmetic, and paint products. We designed environment-friendly capsules composed of proteins and biominerals as an alternative to solid microplastic particles or core-shell capsules made of nonbiodegradable synthetic polymeric resins. We synthesized mineralized capsule surface morphologies to mimic the features of natural pollens, which dramatically improved the deposition of high value-added fragrance chemicals on target substrates in realistic application conditions. A mechanistic model accurately captures the observed enhanced deposition behavior and shows how surface features generate an adhesive torque that resists shear detachment. Mineralized protein capsule performance is shown to depend both on material selection that determines van der Waals attraction and on capsule-substrate energy landscapes as parameterized by a geometric taxonomy for surface morphologies. These findings have broad implications for engineering multifunctional environmentally friendly delivery systems.

Eco-Friendly Silica Microcapsules with Improved Fragrance Retention

Microcapsules are employed extensively in various applications; however, most are composed of synthetic plastics. Thus, substitution of their component materials is essential to prevent environmental problems associated with primary microplastics. Herein, we report the synthesis of eco-friendly silica core–shell microcapsules for fragrance retention. The silica shell was prepared via oil/water emulsion template synthesis using tetraethyl orthosilicate (TEOS), which was added to the immature silica microcapsules prior to complete formation of primary silica shells to promote seeded growth for further reaction of silica. The thickness of the silica shell increased from 42.29 to 70.03 nm, while the Brunauer–Emmett–Teller surface area and internal pore area decreased from 155.16 and 30.08 m2/g to 92.28 and 5.36 m2/g, respectively. The silica microcapsules with lower surface areas retained fragrance for more than 80 days, even in a harsh environment of 15% sodium dodecyl sulfate at 60 °C, whereas the fragrance compound in those without additional TEOS treatment was completely released within seven days. Practical qualitative evaluation of fragrance was also performed for application in fragrance delivery because of the enhanced long-term fragrance retention ability. Our findings show the widespread potential of microcapsules synthesized from eco-friendly materials in industrial applications.

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.

Incorporation of nanogels within calcite single crystals for the storage, protection and controlled release of active compounds

Nanocarriers have tremendous potential for the encapsulation, storage and delivery of active compounds. However, current formulations often employ open structures that achieve efficient loading of active agents, but that suffer undesired leakage and instability of the payloads over time. Here, a straightforward strategy that overcomes these issues is presented, in which protein nanogels are encapsulated within single crystals of calcite (CaCO3). Demonstrating our approach with bovine serum albumin (BSA) nanogels loaded with (bio)active compounds, including doxorubicin (a chemotherapeutic drug) and lysozyme (an antibacterial enzyme), we show that these nanogels can be occluded within calcite host crystals at levels of up to 45 vol%. Encapsulated within the dense mineral, the active compounds are stable against harsh conditions such as high temperature and pH, and controlled release can be triggered by a simple reduction of the pH. Comparisons with analogous systems - amorphous calcium carbonate, mesoporous vaterite (CaCO3) polycrystals, and calcite crystals containing polymer vesicles - demonstrate the superior encapsulation performance of the nanogel/calcite system. This opens the door to encapsulating a broad range of existing nanocarrier systems within single crystal hosts for the efficient storage, transport and controlled release of various active guest species.

Lipid Nanotubes as an Organic Template for an Electrically Conductive Gold Nanostructure Network

We demonstrate an approach to fabricate a gold nanowire network that presents a macroscopic electrical conductivity based on a lipid nanotube (LNT) template with attached gold nanoparticles. The poor electrical conductivity that we have previously faced was overcome by centrifugation and resuspension of gold nanoparticle solution for removing stabilizing agents, which increased the density of gold nanoparticles on the LNTs. An additional electroless metal plating further enhanced their contacts at nanoscale. Thanks to these procedures, the sheet resistance was improved by 11 orders of magnitude. As a proof of principle, transparent conductive films were fabricated with these gold nanowires, which exhibited sheet resistance of maximum 70 Ω/□ and transmittance of 50-75% in visible light.

Improved volatile cargo retention and mechanical properties of capsules via sediment-free in situ polymerization with cross-linked poly(vinyl alcohol) as an emulsifier

HYPOTHESIS

It is hypothesized that poly(vinyl alcohol) (PVOH) as an emulsifier destabilizes the insoluble molecular aggregates by increasing interparticle interactions and their tendency toward agglomeration into large particle aggregates during the encapsulation process of one-step in situ polymerization. Porosity of capsule shells is expected to decrease with reducing agglomeration tendency to allow dense packing of smaller insoluble aggregates. Cross-linking the polymer network further reduces shell permeability to improve the retention of volatile cargos. PVOH also modifies the short-range order of polymer network to bestow improved mechanical properties in addition to the shell thickening effect at appropriate synthesis conditions.

EXPERIMENTS

PVOH was used to stabilize a heptane-in-water emulsion as a template for producing capsules via one-step in situ polymerization. Shell morphologies at different PVOH concentrations were compared. Physical freeze-thawing and chemical cross-linking were adopted separately to synthesize capsules with a volatile cargo, and its retention was characterized qualitatively by a solvatochromism-based fluorescent method and quantitative payload calculation. Mechanical properties of capsules were tested with micromanipulation. The effect of graphene oxide (GO) impregnation into capsules was studied with various co-emulsifiers.

FINDINGS

When PVOH alone was used as the emulsifier for capsule synthesis, the higher its concentration, the more porous the shell structure was. At very low concentrations, visible pores were eliminated. Freeze-thaw cycles reduced the permeability of capsule shells when visible pores were absent. Chemical cross-linking with poly(acrylic acid) (PAA) significantly improved the retention of volatile cargo heptane. PVOH substantially reduced polymer sediment during capsule synthesis, which eliminated the tedious centrifugation procedure that normally would have followed. Superior mechanical strength of capsules was achieved with PAA cross-linked PVOH at appropriate conditions. The impregnation of aqueously dispersed GO into capsules was also promoted by using PVOH but not hydrocolloid emulsifiers.

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.

Metal Microcapsules Prepared via Electroless Plating at Liquid-Liquid Interface

We report the facile preparation of metal microcapsules via the formation of a Pickering emulsion, stabilized by catalytic palladium nanoparticles (PdNPs), and subsequent electroless plating at an oil-water interface induced by the adsorbed PdNPs. Metal microcapsules with smooth metal shells are formed by simply agitating the mixture of the plating solution and 1,2-dichloroethane containing poly(vinylpyrrolidone)-coated PdNPs, without external heating or electrical sources as energy inputs. We found that the metal microcapsules had thin copper metal shells with a relatively smooth surface and metallic luster. The metal shells were composed of more than 99 atom % copper in the form of Cu and Cu₂O. We believe that this simple metal microcapsule preparation method may be beneficial to produce novel functional microcapsules with metal shells.

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

A two-step method to encapsulate an oil core with an impermeable shell has been developed. A thin metallic shell is deposited on the surface of emulsion droplets stabilized by metal nanoparticles. This thin shell is shown to prevent diffusion of the oil from within the core of the metal-shell microcapsules when placed in a continuous phase that fully dissolves the oil. The stabilizing nanoparticles are sterically stabilized by poly(vinyl pyrrolidone) chains and are here used as a catalyst/nucleation site at the oil-water interface to grow a secondary metal shell on the emulsion droplets via an electroless deposition process. This method provides the simplest scalable route yet to synthesize impermeable microcapsules with the added benefit that the final structure allows for drastically improving the overall volume of the encapsulated core to, in this case, >99% of the total volume. This method also allows for very good control over the microcapsule properties, and here we demonstrate our ability to tailor the final microcapsule density, capsule diameter, and secondary metal film thickness. Importantly, we also demonstrate that such impermeable microcapsule metal shells can be remotely fractured using ultrasound-based devices that are commensurate with technologies currently used in medical applications, which demonstrate the possibility to adapt these microcapsules for the delivery of cytotoxic drugs.

Robust Metallic Microcapsules: A Direct Path to New Multifunctional Materials

Robustness of microcapsule shells determined the service life and application areas of final smart materials including self-healing composites, anticorrosion coatings, smart concretes, and so on. Herein, we designed and synthesized metal microcapsules by conducting electroless plating directly on liquid droplet surfaces, and metal shells showed superior stability in thermal (600 °C) and polar solvents (acetone and N,N-dimethylformamide) environments. More interestingly, the mechanical strength of metal shells was ten times higher than those of all published microcapsules. Besides, the smart epoxy composites remained stable mechanical properties with metal microcapsule concentrations, and this is the first time to report such results. For engineering materials, mechanical properties played an important role in practical applications, and a higher strength usually accompanied with better safety and longer service life. The microcapsules with designable structures could be synthesized by adjusting shell thickness and core fractions for practical requirements. The metal microcapsules had great potentials to be applied in a smart metallic matrix, conductive multifunctional materials, and pH-responsive materials. In addition, the electroless plating technique was also first applied to liquid surfaces pushing the development of novel smart materials.

Free-Standing Metal Films Prepared via Electroless Plating at Liquid-Liquid Interfaces

We report a simple preparation of free-standing metal films via electroless plating (ELP) at the liquid-liquid (L-L) interface between an aqueous electroless plating solution and an organic solvent. The use of ELP does not require any external energy in the form of heating and stirring. We find that the affinity of the organic solvent for the palladium nanoparticles (PdNPs) as catalysts and the vertical position of the organic and aqueous phases in the biphasic system are important considerations for synthesizing a robust copper film. Specifically, 1,2-dichloroethane which has an appropriate affinity for PdNPs and a higher density than water was found to be a good candidate for use as the organic phase in this system. However, a poor-quality copper film was obtained in the system with 1-hexanol as the organic phase. We also controlled the microscale surface structure of the copper films by using different concentrations of the injected PdNP dispersion. A high density of PdNPs caused smaller regions of metal growth, which contributed to the formation of smoother metal films. Moreover, under the optimal synthesis condition, we confirmed the electrical conductivity of the obtained copper film to be 1.16 × 10^-7 Ωm. We believe that this metal film preparation represents a promising way to produce a range of metal film structures through the use of flexible L-L interfaces as templates.

Metal Coated Colloidosomes as Carriers for an Antibiotic

Colloidosomes are polymer shell microcapsules. They are stable and easy to prepare and have been used to encapsulate drugs for release at specific areas in the body. Traditional polymer shell capsules cannot totally seal drugs, since they are porous, and small molecules diffuse through the polymer shell. In this paper, we report a method for encapsulating an antibiotic kanamycin using gold or silver coated colloidosomes. The colloidosomes are impermeable and can be triggered using ultrasound. To investigate the application of the capsules in a biological system, Escherichia Coli (E. coli) was chosen as a model organism. After triggering, the released antibiotic, as well as the metal shell fragments, kill E. coli. Both the silver and gold shells colloidosomes are toxic to this bacterial system and the gold coated colloidosomes can load a higher concentration of kanamycin.

A fabrication method of gold coated colloidosomes and their application as targeted drug carriers

Colloidosomes have attracted considerable attention in recent years because of their potential applications in a range of industries, such as food, bioreactors and medicine. However, traditional polymer shell colloidosomes leak low molecular weight encapsulated materials due to their intrinsic shell permeability. Here, we report aqueous core colloidosomes coated with a gold shell, which make the capsules impermeable. The shells can be ruptured using ultrasound. The gold coated colloidosomes are prepared by making an aqueous core capsule with a polymer shell and then adding HAuCl4, surfactant and l-ascorbic acid to form a second shell. We propose to use the capsules as drug carriers. The gold coated colloidosomes demonstrate a low cytotoxicity and after triggering, both encapsulated doxorubicin and broken gold fragments kill cancer cells. In addition, we set up a targeting model by modifying the gold shell colloidosomes using 4,4'-dithiodibutyric acid and crosslinking them with proteins-rabbit immunoglobulin G (IgG). Label-free surface plasmon resonance was used to test the specific targeting of the functional gold shells with rabbit antigen. The results demonstrate that a new type of functional gold coated colloidosome with non-permeability, ultrasound sensitivity and immunoassay targeting could be applied to many medical applications.

Adsorption of Catalytic Nanoparticles onto Polymer Substrates for Controlled Deposition of Microcapsule Metal Shells

Efficient encapsulation of small chemical molecules and their controlled targeted delivery provides a very important challenge to be overcome for a wide range of industrial applications. Typically rapid diffusion of these actives across capsule walls has so far prevented the development of a versatile widely applicable solution. In an earlier publication, we have shown that thin metal shells are able to permanently retain small molecules. The critical step in the microcapsule synthesis is the formation of a strongly adsorbed, dense monolayer of catalytic nanoparticles on the surface as this affects the secondary metal film quality. Control over Pt-nanoparticle adsorption density and a clear understanding of Pt-nanoparticle adsorption kinetics is therefore paramount. Maximising the density of heterogeneous catalysts on surfaces is generally of interest to a broad range of applications. In this work, transmission electron microscopy (TEM) and quartz crystal microbalance (QCM) are used to demonstrate that the concentration of nanoparticle polymer stabilizer used during particle synthesis and nanoparticle suspension concentration can be used to control nanoparticle surface adsorption density. We demonstrate that excess polymer, which is often used in nanoparticle synthesis but rarely discussed as an important parameter in the literature, can compete with and thus drastically affect the adsorption of the Pt-nanoparticles.

Preparation and application of flavor and fragrance capsules

The preparation methods and applications of flavor and fragrance capsules based on polymeric, inorganic and polymeric–inorganic wall materials are summarized.

A two-step synthesis for preparing metal microcapsules with a biodegradable polymer substrate

Novel method of producing metal microcapsules, using nanoparticles as the stabiliser, allows better control of nanoparticle distribution thus optimisation of metal shells.

One-Step Bulk Fabrication of Polymer-Based Microcapsules with Hard-Soft Bilayer Thick Shells

Microcapsules are important for the protection, transport, and delivery of cargo in a variety of fields but are often too weak to withstand the high mechanical stresses that arise during the preparation and formulation of products. Although thick-shell strong capsules have been developed to circumvent this issue, the microfluidic or multistep methods utilized thus far limit the ease of fabrication and encapsulation throughput. Here, we exploit the phase separation of ternary liquid mixtures to achieve a high-throughput fabrication of strong bilayer microcapsules using a one-step bulk emulsification process. Phase separation is induced by the diffusion of water from the continuous phase into droplets that initially contain a mixture of monomers, cross-linkers, an initiator, and cosolvent γ-butyrolactone. The double emulsions generated via such a phase separation are converted into microcapsules through a polymerization reaction triggered by UV illumination. Surprisingly, the shells of the consolidated capsules exhibit a hard-soft bilayer structure that can be designed to show a resilient eggshell-like fracture behavior. Our method allows for the production of large volumes of microcapsules with such a strong bilayer shell within a time scale of only a few minutes, thus offering an enticing pathway toward the high-throughput fabrication of mechanically robust encapsulation systems.

Silver-Coated Colloidosomes as Carriers for an Anticancer Drug

Small drug molecules are widely developed and used in the pharmaceutical industry. In the past few years, loading and delivering such molecules using polymer-shell colloidosomes has attracted interest. Traditional polymer capsules fail to encapsulate low-molecular-weight materials for long times, since they are inherently porous and permeable for small molecules. In this paper, we report a method for encapsulating an anticancer drug with small molecule weight, for cell viability tests. The silver-coated colloidosomes are prepared by making an aqueous core capsule with a polymer shell and then adding AgNO₃, surfactant, and l-ascorbic acid to form a second shell. The capsules are impermeable and can be triggered using ultrasound. We propose to use the capsules as drug carriers. The silver demonstrates a low cytotoxicity for up to 10 capsules per cell. After the silver shells are triggered by ultrasound, the released doxorubicin, the broken silver fragments, and the doxorubicin loading on the capsule surface all kill cells. The results demonstrate a nonpermeable silver-shell microcapsule with ultrasound sensitivity for potential medical applications.

Formation of titanium dioxide core-shell microcapsules through a binary-phase spray technique

Core-shell microcapsules consisting of a titanium dioxide shell and a hydrophobic solvent core have been prepared with diameters of a few micrometers and a narrow size distribution using a simple and fast airbrush technique. These microcapsules were prepared at room temperature in a single-step process in which an oil with a dissolved titanium alkoxide precursor was forced together with an aqueous solution, containing a surface-active polymer, through a narrow spray nozzle using a nitrogen gas propellant. Several different parameters of chemical, physical, and processing origin were investigated to find an optimal recipe. Two different alkanes, one ketone, and four alcohols were tested and evaluated as core materials, alone or together with the antifungal biocide 2-n-octyl-4-isothiazolin-3-one (OIT). Long-chain alcohols were found suitable as core oil due to their low solubility in water and surface activity. The addition of the surface-active polymers in the water phase was important in aiding the formation and stabilization of the titanium dioxide shell. An impressive loading of 50 wt% of the semi-hydrophobic OIT was possible to encapsulate using this simple and applicable procedure.

Functional Microcapsules with Hybrid Shells Made via Sol-Gel Reaction within Double Emulsions

Microcapsules with organic-inorganic hybrid shells can be used as functionally responsive delivery systems that are attractive for a broad range of applications. Hybrid-shell microcapsules have often been synthesized by the assembly of solid inorganic nanoparticles and polymers. Efforts to extend this approach to microfluidic emulsification have been hampered by problems with clogging and flow instabilities when utilizing dispersions of solid particles. In this work, hybrid shell microcapsules are synthesized through the reaction of liquid precursors, eliminating the use of solid dispersions. Our microfluidic water-oil-water emulsification technique also enables the preparation of hybrid-shell microcapsules with thicker and more robust shells compared to alternative techniques. By utilizing bridged-silane precursors to form the hybrid material, we demonstrate hybrid-shell microcapsules with independently tunable functional and mechanical/barrier properties. This independent tuning of physical and functional properties allows for the production of functional organic-inorganic hybrid shell microcapsules that can be tailored to meet the demands of a wide range of applications.

Foaming Behavior of Polymer-Coated Colloids: The Need for Thick Liquid Films

The current study examined the foaming behavior of poly(vinylpyrrolidone) (PVP)-silica composite nanoparticles. Individually, the two components, PVP and silica nanoparticles, exhibited very little potential to partition at the air-water interface, and as such, stable foams could not be generated. In contrast, combining the two components to form silica-PVP core-shell nanocomposites led to good "foamability" and long-term foam stability. Addition of an electrolyte (Na₂SO₄) was shown to have a marked effect on the foam stability. By varying the concentration of electrolyte between 0 and 0.55 M, three regions of foam stability were observed: rapid foam collapse at low electrolyte concentrations, delayed foam collapse at intermediate concentrations, and long-term stability (∼10 days) at the highest electrolyte concentration. The observed transitions in foam stability were better understood by studying the microstructure and physical and mechanical properties of the particle-laden interface. For rapidly collapsing foams the nanocomposite particles were weakly retained at the air-water interface. The interfaces in this case were characterized as being "liquid-like" and the foams collapsed within 100 min. At an intermediate electrolyte concentration (0.1 M), delayed foam collapse over ∼16 h was observed. The particle-laden interface was shown to be pseudo-solid-like as measured under shear and compression. The increased interfacial rigidity was attributed to adhesion between interpenetrating polymer layers. For the most stable foam (prepared in 0.55 M Na₂SO₄), the ratio of the viscoelastic moduli, G'/G″, was found to be equal to ∼3, confirming a strongly elastic interfacial layer. Using optical microscopy, enhanced foam stability was assessed and attributed to a change in the mechanism of foam collapse. Bubble-bubble coalescence was found to be significantly retarded by the aggregation of nanocomposite particles, with the long-term destabilization being recognized to result from bubble coarsening. For rapidly destabilizing foams, the contribution from bubble-bubble coalescence was shown to be more significant.

Understanding the Mechanisms of Gold Shell Growth onto Polymer Microcapsules to Control Shell Thickness

Polymer microcapsules have been used commercially for decades, however they have an inherent flaw which renders them impractical as a carrier of small, volatile molecules. The porous nature of the polymer shell allows for diffusion of the encapsulated molecules into the bulk. The use of metal shells is an innovative way to prevent undesired loss of small molecules from the core of microcapsules, however it is important, particularly when using expensive metals to ensure that the resulting shell is as thin as possible. Here we investigate the fundamental mechanisms controlling the gold shell thickness when a fragrance oil is encapsulated in a poly(methyl methacrylate) shell. We consider the distribution of the nanoparticles on the capsule surface, and from quantification of the adsorbed nanoparticle (NP) density and resulting shell thickness, we propose mechanisms to describe the gold shell growth for systems with high and low NP surface coverage. We suggest from our observations that the gold grows to fill in the gaps between NPs. At low NP concentrations, thicker metal shells form. We postulate that this is due to the low NP density on the surface, forcing the gold clusters to grow larger before they meet the adjacent ones. Thus, to grow the thinnest possible shells a densely packed monolayer of platinum nanoparticles is required on the capsule surface.