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Micro-encapsulation
Micro-encapsulation
Micro-encapsulation
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Microencapsulation is a process in which tiny particles or droplets are surrounded by a coating to give small capsules, with useful properties.[1][2] In general, it is used to incorporate food ingredients,[3] enzymes, cells or other materials on a micrometric scale. Microencapsulation can also be used to enclose solids, liquids, or gases inside a micrometric wall made of hard or soft soluble film, in order to reduce dosing frequency and prevent the degradation of pharmaceuticals.[4]

In its simplest form, a microcapsule is a small sphere comprising a near-uniform wall enclosing some material. The enclosed material in the microcapsule is referred to as the core, internal phase, or fill, whereas the wall is sometimes called a shell, coating, or membrane. Some materials like lipids and polymers, such as alginate, may be used as a mixture to trap the material of interest inside. Most microcapsules have pores with diameters between a few nanometers and a few micrometers. Materials generally used for coating are:

IUPAC definition[5]

Microcapsule: Hollow microparticle composed of a solid shell surrounding a core-forming space available to permanently or temporarily entrapped substances.

Note: The substances can be flavour compounds, pharmaceuticals, pesticides, dyes, or similar materials.

The definition has been expanded, and includes most foods, where the encapsulation of flavors is the most common.[6] The technique of microencapsulation depends on the physical and chemical properties of the material to be encapsulated.[7]

Many microcapsules however bear little resemblance to these simple spheres. The core may be a crystal, a jagged adsorbent particle, an emulsion, a Pickering emulsion, a suspension of solids, or a suspension of smaller microcapsules. The microcapsule even may have multiple walls.

Techniques of microcapsule manufacture

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Ionotropic gelation occurs when units of uric acid in the chains of the polymer alginate, crosslink with multivalent cations. These may include, calcium, zinc, iron and aluminium.

Coacervation-phase separation consists of three steps carried out under continuous agitation.

  1. Formation of three immiscible chemical phases: liquid manufacturing vehicle phase, core material phase and coating material phase.
  2. Deposition of coating: core material is dispersed in the coating polymer solution. Coating polymer material coated around core. Deposition of liquid polymer coating around core by polymer adsorbed at the interface formed between core material and vehicle phase.
  3. Rigidization of coating: coating material is immiscible in vehicle phase and is made rigid. This is done by thermal, cross-linking, or dissolution techniques.

Interfacial polycondensation

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In interfacial polycondensation, the two reactants in a polycondensation meet at an interface and react rapidly. The basis of this method is the classical Schotten-Baumann reaction between an acid chloride and a compound containing an active hydrogen atom, such as an amine or alcohol, polyesters, polyurea, polyurethane. Under the right conditions, thin flexible walls form rapidly at the interface. A solution of the pesticide and a diacid chloride are emulsified in water and an aqueous solution containing an amine and a polyfunctional isocyanate is added. Base is present to neutralize the acid formed during the reaction. Condensed polymer walls form instantaneously at the interface of the emulsion droplets.

Interfacial cross-linking

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Interfacial cross-linking is derived from interfacial polycondensation, and was developed to avoid the use of toxic diamines, for pharmaceutical or cosmetic applications. In this method, the small bifunctional monomer containing active hydrogen atoms is replaced by a biosourced polymer, like a protein. When the reaction is performed at the interface of an emulsion, the acid chloride reacts with the various functional groups of the protein, leading to the formation of a membrane. The method is very versatile, and the properties of the microcapsules (size, porosity, degradability, mechanical resistance) can be customized. Flow of artificial microcapsules in microfluidic channels:

In situ polymerization

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In a few microencapsulation processes, the direct polymerization of a single monomer is carried out on the particle surface. In one process, e.g. cellulose fibers are encapsulated in polyethylene while immersed in dry toluene. Usual deposition rates are about 0.5μm/min. Coating thickness ranges 0.2–75 μm (0.0079–2.9528 mils). The coating is uniform, even over sharp projections. Protein microcapsules are biocompatible and biodegradable, and the presence of the protein backbone renders the membrane more resistant and elastic than those obtained by interfacial polycondensation.

Matrix polymerization

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In a number of processes, a core material is imbedded in a polymeric matrix during formation of the particles. A simple method of this type is spray-drying, in which the particle is formed by evaporation of the solvent from the matrix material. However, the solidification of the matrix also can be caused by a chemical change.

Release methods and patterns

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Even when the aim of a microencapsulation application is the isolation of the core from its surrounding, the wall must be ruptured at the time of use. Many walls are ruptured easily by pressure or shear stress, as in the case of breaking dye particles during writing to form a copy. Capsule contents may be released by melting the wall, or dissolving it under particular conditions, as in the case of an enteric drug coating.[8] In other systems, the wall is broken by solvent action, enzyme attack, chemical reaction, hydrolysis, or slow disintegration.

Microencapsulation can be used to slow the release of a drug into the body. This may permit one controlled release dose to substitute for several doses of non-encapsulated drug and also may decrease toxic side effects for some drugs by preventing high initial concentrations in the blood. There is usually a certain desired release pattern. In some cases, it is zero-order, i.e. the release rate is constant. In this case, the microcapsules deliver a fixed amount of drug per minute or hour during the period of their effectiveness. This can occur as long as a solid reservoir or dissolving drug is maintained in the microcapsule.

A more typical release pattern is first-order in which the rate decreases exponentially with time until the drug source is exhausted. In this situation, a fixed amount of drug is in solution inside the microcapsule. The concentration difference between the inside and the outside of the capsule decreases continually as the drug diffuses.

Nevertheless, there are some other mechanisms that may take place in the liberation of the encapsulated material. These include, biodegradation, osmotic pressure, diffusion, etc. Each one will depend on the composition of the capsule made and the environment it is in. Therefore, the liberation of the material may be affected by various mechanisms that act simultaneously.[9]

Applications

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Applications of micro-encapsulation are numerous.

  • It is mainly used to increase the stability and life of the product being encapsulated, facilitate the manipulation of the product and provide for the controlled release of the contents.
    • Microencapsulation is used to preserve vitamin A from the deteriorating effects of oxygen.[2]
    • Microencapsulation of edible oil and other hard-to-powder items turn it into a more convenient powder form while protecting it from oxidation. This is most commonly used for instant food. A water-soluble wall material like maltodextrin or gum arabic is used, so that the microcapsule dissolves away when water is added.[10][11] Alcohol powder is another example.
  • In other cases, the objective is not to isolate the core completely but to control the rate at which it releases the contents, as in the controlled-release of drugs:[12]
    • Ferrous sulfate, a nutritional supplement, is microencapsulated in a way that it passes through the stomach but releases in the intestine. In this way, gastric upset is minimized.
    • Aspirin is treated similarly.
  • Some sunscreens are encapsulated in silica gel. One brand is "Eusolex UV-Pearls". These chemicals are potentially hormone disrupting, so the encapsulation protects the user until the time of application.[13]
  • pesticides are dangerous to handle but less so when encapsulated.[14][15] Microencapsulation minimizes leaching or volatilization risks.[16]

The problem may be as simple as masking the taste or odor of the core, or as complex as increasing the selectivity of an adsorption or extraction process.

  • DNA protection from degradation for product tracing[17] and data storage[18]
  • Protection of bioactive compounds that are easily degradated under normal environmental conditions.[19]

See also

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References

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Bibliography

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Micro-encapsulation

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from Grokipedia
Microencapsulation is a versatile technology that involves enclosing active core materials—such as solids, liquids, or gases—within a protective or shell of to form microscopic capsules, typically ranging from 1 to 1000 micrometers in diameter. This process safeguards sensitive compounds from environmental stressors like oxygen, , , and changes, while enabling controlled release of the core at targeted sites or times. Widely applied across industries, microencapsulation enhances stability, , and functionality of encapsulated substances. The core material, often bioactive or functional (e.g., vitamins, , flavors, or pharmaceuticals), is surrounded by an encapsulant such as polymers (e.g., , alginate) or carbohydrates (e.g., ), forming structures like matrix, multi-walled, or reservoir types. Key properties include , morphology, encapsulation (typically 70-99% depending on method), and mechanical strength, which influence release kinetics and . Common production methods encompass (economical and scalable for food applications), complex coacervation (high efficiency for oils), , coating, and emerging techniques like ionic gelation or processing. These approaches allow customization based on the core's and desired release profile, such as immediate, sustained, or stimuli-responsive (e.g., - or temperature-triggered). In the , microencapsulation fortifies products with nutrients like omega-3 fatty acids or antioxidants, masks off-flavors (e.g., in ), and supports viability in or baked goods, thereby extending and improving sensory attributes. Pharmaceutical applications leverage it for , reducing dosing frequency and side effects through controlled release microparticles. Other sectors include (fragrance encapsulation), ( delivery), and . Recent advances emphasize sustainability, with biodegradable shells from bio-based materials like (PLA) or achieving over 60% degradation in 28 days, aligning with environmental regulations such as the EU's microplastic bans. Challenges persist in , leaching prevention, and energy-efficient manufacturing, but innovations like bioinspired inorganic shells (e.g., silica or ) promise reduced ecological impact without compromising performance. Overall, microencapsulation continues to evolve as a cornerstone of advanced material science and product innovation.

Fundamentals

Definition and Principles

Microencapsulation is a process by which micron-sized particles, droplets, or emulsions—collectively referred to as the core material—are enclosed within a continuous or shell to form microcapsules, typically ranging from 1 to 1000 μm in . This encapsulation creates a protective barrier that isolates the core from its surrounding environment, enabling functionalities such as controlled release and enhanced stability. The resulting microcapsules can be spherical or irregular in shape, with the shell material often forming a semi-permeable that regulates the of substances in and out. The underlying principles of microencapsulation revolve around the physical and chemical separation of phases to achieve encapsulation. Core isolation protects sensitive actives from external factors like oxygen, , , or changes, preventing degradation and extending . Controlled release is facilitated through mechanisms such as across the shell, dissolution, or rupture, allowing the core to be delivered at predetermined rates or in response to stimuli. Additional principles include , where immiscible phases are induced to form the shell around the core, and the use of semi-permeable barriers to mask undesirable sensory attributes like or while maintaining core integrity. These principles are achieved via methods like coacervation or , emphasizing the versatility of the technology in tailoring microcapsule performance. Microcapsules are classified into three primary types based on their internal structure: mononuclear, polynuclear, and matrix. Mononuclear microcapsules consist of a single core surrounded by a shell, providing a discrete for the active material. Polynuclear variants feature multiple cores aggregated within a single outer shell, allowing for higher loading capacity in a compact form. In contrast, matrix-type microcapsules involve the core material dispersed homogeneously throughout a continuous shell matrix, without a distinct , which can enhance uniformity but may alter release kinetics. Key benefits of microencapsulation include enhanced stability of the core against environmental stressors, enabling targeted delivery and improved without immediate exposure. This technology also supports precise control over release profiles, from immediate to sustained patterns, and effectively masks sensory properties to improve . Regarding size classification, microencapsulation specifically targets particles in the 1–1000 μm range, distinguishing it from nanoencapsulation, which operates at sub-micron scales below 1 μm for finer applications.

Historical Development

Microencapsulation technology originated in the early at the National Cash Register (NCR) Corporation, where chemists Barrett K. Green and Lowell Schleicher developed a method to enclose ink dyes in microscopic shells for use in pressure-sensitive . This innovation addressed the need for mess-free duplication, replacing traditional by relying on mechanical rupture of the capsules to release the core material. Their work built on earlier chemistry experiments, marking the first practical application of the technique. The process was patented in 1957 as US Patent 2,800,457, describing oil-containing microscopic capsules formed via coacervation. Commercialization followed swiftly, with NCR launching the first product on March 26, 1954, initially under the brand NCR Paper. By the 1960s, the technology expanded beyond paper into adhesives and textiles, where microcapsules enabled controlled release for enhanced performance, such as in pressure-sensitive adhesives and fabric treatments. Influential early adopters included military applications at , which explored multipurpose capsular adhesives. The 1960s saw initial adoption in pharmaceuticals, with sustained-release aspirin microcapsules developed for managing conditions like (e.g., via Wurster coating). In the late 1970s, researchers like Robert Langer at MIT advanced polymer-based controlled systems, including microencapsulation techniques. In the , the embraced the method for flavor encapsulation, protecting volatile compounds and enabling controlled release in products. The 1990s brought biotech advancements, including probiotic microencapsulation to improve viability during processing and gastrointestinal delivery. Post-2000 developments emphasized for precise control and green synthesis methods using renewable resources to reduce environmental impact. This evolution shifted emphasis from initial mechanical coacervation to chemical methods like interfacial polymerization for greater precision, supported by thousands of patents filed globally by 2020.

Materials

Core Materials

Core materials in microencapsulation refer to the active substances enclosed within the protective shell, serving as the functional that imparts the desired properties to the final microcapsule product. These materials can exist in various physical states, including solids such as drugs, enzymes, and pigments; liquids like oils, flavors, and pheromones; gases such as oxygen used in applications; or even dispersions of multiple phases. The choice of core material is pivotal, as it determines the overall and application of the microcapsule, with the core typically comprising 5-90% by weight of the total formulation depending on the intended use. The properties of core materials significantly influence their suitability for encapsulation. plays a key role, with hydrophilic cores like certain enzymes requiring aqueous-compatible shells, while lipophilic ones such as essential oils demand oil-soluble coatings to ensure stability. and chemical sensitivity is another critical factor; for instance, bioactives prone to degradation, like vitamins or , benefit from encapsulation to shield them from environmental stressors. Volatility affects handling and release, particularly for flavors or pheromones that evaporate readily, and bioactivity must be preserved to maintain therapeutic or functional efficacy, as seen in oxygen-loaded particles for where gas retention is essential. Essential oils, for example, are often selected despite their oxidation susceptibility because encapsulation mitigates during storage. Common examples span multiple industries. In pharmaceuticals, active ingredients like ibuprofen are frequently used as solid cores to enable controlled release and mask bitterness, with studies demonstrating high encapsulation efficiency in matrices. Food applications often involve or vitamins as core materials to enhance shelf-life and targeted delivery in the gut, protecting them from gastric acids. In agrochemicals, pesticides such as insecticides or pheromones serve as liquid or solid cores to reduce environmental exposure and enable sustained . Selection of core materials hinges on several factors to optimize performance. Compatibility with the shell is essential to prevent or instability, while the desired release rate—whether immediate or prolonged—influences core and choices. Environmental stability addresses challenges like or exposure, and preventing core leakage requires low-porosity designs to avoid premature . These criteria ensure the core's integrity throughout processing and application. Preparation of core materials typically precedes full encapsulation, involving techniques like emulsification to disperse liquids or solids into fine droplets within a continuous phase, or to form solid particles from solution. These steps create a uniform core phase that can be readily coated, with emulsification being particularly suited for volatile liquids to minimize loss during handling.

Shell Materials

Shell materials, also known as wall or coating materials, form the outer layer of microcapsules, providing a protective barrier around the core while controlling the interaction with the external environment. These materials are selected to ensure stability, controlled release, and compatibility with the intended application, often influencing diffusion rates in release mechanisms. Shell materials are broadly classified into natural, synthetic, and hybrid types. Natural shell materials include proteins such as gelatin, polysaccharides like alginate and maltodextrin, and lipids such as waxes. Gelatin offers excellent film-forming properties and biocompatibility, making it suitable for encapsulating sensitive cores like oils. Alginate provides gelling capabilities and low toxicity, often used in pH-responsive systems. Synthetic shell materials encompass polymers like polyurea, polyurethane, and ethyl cellulose. Polyurea exhibits high chemical stability, acid and alkali resistance, and solvent resistance, enhancing thermal stability of the encapsulated material. Polyurethane provides tunable mechanical properties and a variety of chemical functionalities, allowing for adjustable permeability. Ethyl cellulose is valued for its transparent, flexible films with high strength and controlled drug release behavior. Hybrid materials, such as protein-polysaccharide complexes (e.g., gelatin-alginate), combine the benefits of natural components for improved elasticity and antimicrobial activity. Key properties of shell materials include mechanical strength, permeability, , and biodegradability. Mechanical strength varies; for instance, cross-linked alginate forms elastic gels, while shells offer robust structural integrity under stress. Permeability is governed by and can be tuned for diffusion-controlled release, with factors like (e.g., around 56°C for starch-based shells) affecting response to changes. is prominent in natural polymers like and , which are non-toxic and suitable for biomedical uses. Many natural shells, such as and alginate, are biodegradable, promoting environmental over synthetic options like polyacrylates. Selection criteria for shell materials emphasize core compatibility, environmental responsiveness, and regulatory approval. Compatibility ensures inertness and stabilization of the core, while responsiveness to stimuli like (e.g., ) or guides material choice. Regulatory aspects, such as (GRAS) status, are critical for food applications, applying to materials like and . Common examples illustrate practical use: shells for pharmaceuticals due to , maltodextrin for food flavors owing to GRAS status and protection of volatiles, and polyacrylates for controlled-release drugs via tunable permeability. is frequently employed for its film-forming strength in encapsulating oils. Modifications enhance shell performance; cross-linking, as in , improves mechanical strength and barrier properties, while (e.g., on ) adds functionality like UV resistance. Shell thickness typically ranges from 0.2 to 20 μm, influencing overall capsule stability and release kinetics.

Manufacturing Techniques

Coacervation

Coacervation is a physicochemical microencapsulation technique that involves of a solution into a -rich phase and a -poor phase, enabling the to deposit around core material droplets dispersed in the solution. This process, first described by Bungenberg de Jong and Kruyt in the , relies on controlled changes in solution conditions to induce the separation, forming a shell that encapsulates sensitive cores such as oils or bioactive compounds. The coacervation process typically proceeds in five main steps under continuous agitation to ensure uniform deposition. First, the core material is dispersed as droplets in a solution to form an . Second, a coacervating agent is added or conditions are adjusted—such as shift, salt addition, change, or non-solvent introduction—to trigger . Third, the resulting phase deposits onto the core droplets, forming a liquid shell. Fourth, the shell is hardened through cross-linking (e.g., using or ) or cooling to solidify the microcapsules. Finally, the microcapsules are recovered by or and dried if needed. Coacervation variants include simple and complex types, distinguished by the number of s involved. Simple coacervation uses a single (e.g., or ) and is induced by desolvation agents like salts (e.g., ) or non-solvents (e.g., ), leading to via reduced polymer solubility. Complex coacervation, more commonly applied in microencapsulation, employs two or more oppositely charged s (e.g., positively charged and negatively charged ), where is driven by electrostatic attractions optimized by adjusting pH to near the or . Other induction methods include temperature reduction for -based systems or non-solvent addition for broader compatibility. The underlying phase separation equilibrium in coacervation is often modeled using Flory-Huggins theory, which quantifies the free energy of mixing for solutions as ΔGM/RT=n1ln⁡ϕ1+n2ln⁡ϕ2+χn1ϕ2\Delta G_M / RT = n_1 \ln \phi_1 + n_2 \ln \phi_2 + \chi n_1 \phi_2, where n1n_1 and n2n_2 are the moles of solvent and , ϕ1\phi_1 and ϕ2\phi_2 are their volume fractions, RR is the , TT is , and χ\chi is the Flory-Huggins interaction parameter reflecting -solvent affinity; for complex coacervation, electrostatic contributions are added to favor the coacervate phase. This theoretical framework, extended by Voorn and Overbeek in the 1950s, predicts coacervation when χ>0.5\chi > 0.5, indicating immiscibility. Advantages of coacervation include mild operating conditions (typically at ambient temperature and neutral ), making it ideal for heat-sensitive cores like essential oils, and achieving high encapsulation efficiencies up to 99% with controlled shell thickness. Resulting microcapsules generally range from 10 to 500 μm in diameter, suitable for applications requiring visible but non-gritty particles. Limitations encompass its batch-wise nature, which hinders ; potential aggregation during deposition; and reliance on cross-linking agents that may introduce toxicity concerns, such as residual . A representative example is the encapsulation of flavor oils, such as orange essential oil, using complex coacervation with and as shell materials; the process protects volatile compounds from oxidation while enabling triggered release in matrices.

Spray Drying

Spray drying is a physical microencapsulation technique that involves atomizing a feed containing core and shell materials into a hot air stream, where rapid of the forms dry, encapsulated particles. This method is particularly suited for producing microcapsules with heat-stable cores dispersed in soluble wall materials, such as carbohydrates or proteins, to create a protective matrix around sensitive actives. The process begins with the preparation of a homogeneous feed by dispersing the core material in a solution of the shell material, often followed by homogenization to ensure stability. This is then pumped to an atomizer, where it is broken into fine droplets (typically 10-100 μm in size) via high-pressure nozzles or centrifugal forces. The droplets enter a co-current hot air stream at inlet temperatures of 100-250°C, leading to instantaneous as water evaporates, forming solid microcapsules that are subsequently separated and collected using a cyclone separator. Key parameters include feed flow rate (e.g., 0.4-144 mL/min), which influences droplet size and efficiency, and air flow, which affects in the chamber. Spray dryers typically consist of a feed reservoir, peristaltic pump, atomizer (nozzle types including pressure, two-fluid pneumatic, or centrifugal/rotary), drying chamber, heater for air (co-current or counter-current flow), cyclone for powder collection, and an exhaust system. Pressure nozzles generate droplets under high feed pressure (up to 100 bar), while two-fluid nozzles use compressed air for finer atomization suitable for viscous feeds, and centrifugal atomizers spin the feed to produce uniform particles at high throughput. These components enable scalable production, with advantages including cost-effectiveness (30-50 times cheaper than freeze drying), one-step operation, and high encapsulation efficiency for solid powders, making it ideal for industrial applications. However, limitations arise from thermal exposure, which can degrade heat-sensitive cores like oils or probiotics if inlet temperatures exceed optimal levels, and the requirement for water-soluble shell materials to facilitate drying. Wall material solubility ensures proper film formation, but sticky or low-solids feeds may cause nozzle clogging. Process optimization is often achieved through (RSM), which models interactions between variables like inlet temperature, feed rate, and solids content to maximize yield and minimize defects such as particle cracking. For instance, RSM has been used to balance drying kinetics and encapsulation efficiency in various systems. A representative example is the encapsulation of , such as Lactobacillus casei, using whey protein isolate as the shell material, where at 170°C inlet temperature achieved 97% cell survival, demonstrating the technique's utility for bioactive protection.

Interfacial Polymerization

Interfacial is a chemical microencapsulation technique that involves the formation of a shell at the interface between two immiscible phases, each containing complementary reactive . In this method, one is typically dissolved in an aqueous phase while the other is in an organic phase, leading to specifically at the droplet interface upon mixing, which encapsulates the material dispersed within one of the phases. This process enables the creation of robust, thin-walled microcapsules suitable for protecting sensitive cores from environmental factors. The process begins with the emulsification of the core material, often hydrophobic, in a continuous aqueous phase to form droplets stabilized by emulsifiers. Next, monomers are added: for instance, an amine such as in the aqueous phase and an acid chloride or in the organic phase. Rapid occurs at the oil-water interface, driven by the high local concentrations of reactants, forming an initial that grows as monomers diffuse through it. Finally, the shell hardens through continued reaction or curing, yielding discrete microcapsules, with control over and uniformity achieved by adjusting stirring speed during emulsification. Variants of interfacial polymerization include polycondensation, where monomers like diamines and diacid chlorides react to form polyamides such as nylon shells, and polyaddition, involving reactions between diisocyanates and diamines to produce polyureas. The reaction kinetics generally follow a second-order rate law, expressed as rate=k[monomer1][monomer2]\text{rate} = k [\text{monomer}_1][\text{monomer}_2], reflecting the bimolecular step-growth mechanism without requiring catalysts or elevated temperatures. These variants allow for tailoring shell properties, such as permeability, by selecting monomer functionalities. This technique offers advantages including the production of ultra-thin shells (typically 0.1-1 μm thick), which provide efficient barrier properties while minimizing material use, a rapid reaction time often completed in minutes, and versatility for encapsulating hydrophobic cores like oils or pesticides in aqueous systems. However, limitations include the potential toxicity of such as isocyanates, which require careful handling and removal to ensure product safety, and risks of incomplete reactions leading to porous or uneven shells, mitigated by optimizing stirring speed and ratios. A representative example is the encapsulation of pesticides using shells formed via interfacial of isocyanates and amines, enabling controlled release and reduced environmental exposure; for instance, xylene-based formulations have achieved high loading capacities up to 480 g/L with slower release profiles when using higher-functionality amines.

Emulsion Solvent Evaporation

The technique is a physicochemical method for microencapsulation that involves dissolving a and the core material in a volatile organic , emulsifying the resulting solution in an aqueous phase, and then removing the through to precipitate the as a solid shell around the core. This approach is particularly effective for producing microspheres or microcapsules with uniform structures, leveraging induced by removal. The process consists of several key steps. First, the is dissolved in a volatile such as (DCM), and the core material—often a hydrophobic —is dispersed or dissolved within this organic phase. Second, this organic phase is emulsified into a continuous aqueous phase containing a stabilizer, such as (PVA), to form droplets that prevent aggregation. Third, the is evaporated under continuous stirring, mild heating, or conditions, causing the to precipitate and solidify around the core, forming the microcapsules. Finally, the resulting microcapsules are recovered through washing to remove residual stabilizer and , followed by drying via filtration or . Variants of this technique are tailored to the of the core material. For hydrophobic cores, a single oil-in-water (O/W) is used, where the -core solution forms the dispersed oil phase in water. For hydrophilic cores, such as water-soluble drugs, a water-in-oil-in-water (W/O/W) double is employed: the core is first emulsified in the solution to create a primary water-in-oil , which is then re-emulsified in the aqueous phase before solvent evaporation. These double emulsions are commonly applied in pharmaceutical formulations to encapsulate sensitive biologics. This method offers advantages for pharmaceutical applications, including the production of biodegradable polymer-based microcapsules with controlled particle sizes typically ranging from 1 to 100 μm, achieved through high-shear homogenization during emulsification. It enables high encapsulation efficiencies, often exceeding 90% for suitable polymer-core combinations, and supports for industrial production. However, limitations include the risk of residual organic in the final product, which requires rigorous purification to meet regulatory standards, and challenges in maintaining stability, which can lead to polydisperse particle sizes if not optimized. The rate of is governed by principles, as described by Fick's : the flux J=−D∂C∂xJ = -D \frac{\partial C}{\partial x}, where DD is the coefficient of the through the matrix, and ∂C∂x\frac{\partial C}{\partial x} represents the concentration gradient driving the from the droplets to the external phase. Incomplete can result in porous or collapsed structures, affecting . A representative example is the encapsulation of vitamin D3 in poly(lactic-co-glycolic acid) (PLGA) microspheres using the O/W single variant, yielding particles with mean diameters around 200-300 nm and encapsulation efficiencies up to 85%, demonstrating its utility for improving the stability and of lipophilic nutrients.

Characterization

Particle Size and Morphology

Particle size and morphology are fundamental physical properties of microcapsules that significantly influence their performance in various applications. The typical size range for microcapsules is 1 to 1000 μm, which determines aspects such as flowability, , and controlled release rates of the encapsulated material. Smaller particles generally enhance surface area and dissolution rates, while larger ones improve mechanical stability and handling. Morphology, including shape and surface characteristics, affects particle interactions, dispersion, and stability in suspensions. Several techniques are employed to characterize and morphology. Optical provides initial visualization of particle and basic size estimation, while scanning electron microscopy (SEM) offers high-resolution images revealing detailed surface morphology, such as spherical or irregular forms. (TEM) is used for internal structure analysis, particularly for thinner shells. For size distribution, diffraction is a standard method that measures the median diameter (D50) and polydispersity by analyzing light scattering patterns from particle ensembles. Advanced analysis involves image processing software to quantify parameters like , circularity, and from images, enabling precise morphological classification. measurement assesses electrostatic stability by evaluating surface charge, where values exceeding ±30 mV typically indicate good colloidal dispersion and resistance to aggregation. Factors such as the manufacturing technique and process parameters directly impact these properties; for instance, often produces uniform spherical particles, whereas coacervation may yield more irregular shapes. Stirring speed during emulsification inversely affects , with higher speeds (e.g., 1000–1300 rpm) reducing by increasing shear forces on droplets. Uniform size distribution correlates with higher encapsulation efficiency in subsequent assessments. Standardization is guided by international norms, including ISO 13320 for laser methods, which ensures reproducible measurements across 0.1–3000 μm for suspensions and powders relevant to microencapsulation. ISO 13322-2 specifies dynamic image analysis for and , incorporating morphological descriptors to evaluate non-spherical particles like microcapsules.

Encapsulation and Loading Capacity

Encapsulation efficiency (EE) quantifies the percentage of core material successfully enclosed within the microcapsule shell relative to the initial amount of core introduced during the process. It is calculated as
EE=core encapsulatedcore initial×100%EE = \frac{\text{core encapsulated}}{\text{core initial}} \times 100\%
or, more commonly in practice,
EE=total core−free coretotal core×100%,EE = \frac{\text{total core} - \text{free core}}{\text{total core}} \times 100\% ,
where free core represents the unencapsulated portion separated from the microcapsules. Loading capacity (LC) measures the amount of core material incorporated relative to the total mass of the resulting microcapsules, expressed as
LC=core encapsulatedtotal capsule mass×100%.LC = \frac{\text{core encapsulated}}{\text{total capsule mass}} \times 100\% .
This metric highlights the payload density and is crucial for determining the practical utility of microcapsules in applications requiring high active content. To measure EE and LC, the core material is typically extracted and quantified after separating free core via , , or dialysis. is widely used for cores with UV-visible , such as proteins or dyes, by dissolving the microcapsules in a suitable and measuring at a specific . (HPLC) provides higher specificity for complex cores, involving extraction of the encapsulated material followed by chromatographic separation and detection, often achieving detection limits in the range. For confirmation, assays like quantify leachable core by immersing microcapsules in a that disrupts the shell, measuring the initial rapid release of unencapsulated or surface-bound core via UV or HPLC. (DSC) serves as a , where successful encapsulation is confirmed by the absence or shift of the core's characteristic endothermic peak in the microcapsule thermogram, indicating molecular dispersion within the shell. Key factors influencing and LC include shell permeability, which determines core retention during formation and post-processing, and overall process yield, reflecting losses from aggregation or incomplete coating. Highly permeable shells, often due to thin or porous walls, can lead to core leakage, reducing , while optimized process parameters enhance yield by minimizing waste. In optimized methods like or coacervation, typical values range from 70% to 95%, demonstrating effective enclosure for scalable production. Optimization of EE and LC often involves adjusting emulsifier concentration in emulsion-based techniques, as higher levels stabilize the core-shell interface, reducing leakage and improving retention; for instance, increasing emulsifier from 1% to 5% w/v can boost EE by 20-30% in oil-in-water systems.

Release Mechanisms

Diffusion-Controlled Release

Diffusion-controlled release in microencapsulation occurs when the active core material migrates through the polymeric shell primarily via Fickian diffusion, a process governed by a across the shell without significant shell degradation. In this mechanism, solute transport follows Fick's laws, where the diffusion rate is slower than the polymer chain relaxation rate, leading to a predictable and steady release profile. This approach is particularly suited for sustained release applications, as it allows for prolonged delivery of the encapsulated substance over time. A key mathematical model describing this release is the Higuchi equation, originally derived for drug release from ointment bases but widely applied to microencapsulated systems with insoluble matrices. The equation is: Q=D(2C0−Cs)Cs tQ = \sqrt{D (2C_0 - C_s) C_s \, t}
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