MICROENCAPSULATION
Microencapsulation: Definition: It is the process by which individual particles or dr dro opl plet ets s of so sollid or liliqu quid id ma mate teri rial al (t (the he co core re)) are surrounded or coated with a continuous film of polymeric material (the shell) to produce capsules in the micrometer to millimetre range, known as microcapsules.
Microencapsulation: Definition: It is the process by which individual particles or dr dro opl plet ets s of so sollid or liliqu quid id ma mate teri rial al (t (the he co core re)) are surrounded or coated with a continuous film of polymeric material (the shell) to produce capsules in the micrometer to millimetre range, known as microcapsules.
Microencapsulation Microencapsulati on (Cont.): Morpholog Morpho logy y of Mic Microc rocaps apsule ules: s: The Th e mo morp rpho holo logy gy of mi micr croc ocap apsu sule les s de depe pend nds s ma main inly ly on th the e co core re material and the deposition process of the shell. 1- Mononuclear (core-shell) microcapsules contain the shell around the core. 2- Polynuclear capsules have many cores enclosed within the shell. 3- Ma Matr trix ix en enca caps psul ulat atio ion n in wh whic ich h th the e co core re ma mate teri rial al is di dist stri ribu bute ted d homogeneously into the shell material. - In addition to these three basic morphologies, microcapsules can also be mononuclear with multiple shells, or they may form clusters of
Morphology of Microcapsules (Cont.):
Microencapsulation (Cont.): Coating material properties: •Stabilization of core material. •Inert toward active ingredients. •Controlled release under specific conditions. •Film-forming, pliable, tasteless, stable. •Non-hygroscopic, no high viscosity, economical. •Soluble in an aqueous media or solvent, or melting. •The coating can be flexible, brittle, hard, thin etc.
Microencapsulation (Cont.): Coating materials:
Gums: Gum arabic, sodium alginate, carageenan.
Carbohydrates: Starch, dextran, sucrose
Celluloses: Carboxymethylcellulose, methycellulose.
Lipids: Bees wax, stearic acid, phospholipids.
Proteins: Gelatin, albumin.
Microencapsulation (Cont.): Benefits of Microencapsulation: 1- microorganism and enzyme immobilization. - Enzymes have been encapsulated in cheeses to accelerate ripening and flavor development. The encapsulated enzymes are protected from low pH and high ionic strength in the cheese. • The encapsulation of microorganisms has been used to improve stability of starter cultures.
Benefits of Microencapsulation (Cont.):
2-Protection against UV, heat, oxidation, acids, bases (e.g.colorants and vitamins). e.g. Vitamin A / monosodium glutamate appearance (white) protection (water, T, ligth) 3- Improved shelf life due to preventing degradative reactions (dehydration, oxidation). 4-Masking of taste or odours. 5- Improved processing, texture and less wastage of ingredients. - Control of hygroscopy - enhance flowability and dispersibility - dust free powder - enhance solubility
Benefits of Microencapsulation (Cont.):
6-Handling liquids as solids 7-There is a growing demand for nutritious foods for children which provides them with much needed vitamins and minerals during the growing age. Microencapsulation could deliver the much needed ingredients in children friendly and tasty way. 8- Enhance visual aspect and marketing concept.
Benefits of Microencapsulation (Cont.):
9- Carbonless copy paper was the first marketable product to employ microcapsules. A coating of microencapsulated colorless ink is applied to the top sheet of paper, and a developer is applied to the subsequent sheet. When pressure is applied by writing, the capsules break and the ink reacts with the developer to produce the dark color of the copy.
Benefits of Microencapsulation (Cont.):
10-Today's textile industry makes use of microencapsulated materials to enhance the properties of finished goods. One application increasingly utilized is the incorporation of microencapsulated phase change materials (PCMs). Phase change materials absorb and release heat in response to changes in environmental temperatures. When temperatures rise, the phase change material melts, absorbing excess heat, and feels cool. Conversely, as temperatures fall, the PCM releases heat as it solidifies, and feels warm. This property of microencapsulated phase change materials can be harnessed to increase the comfort level for users of sports equipment, clothing, building materials, etc.
Benefits of Microencapsulation (Cont.):
11- Pesticides are encapsulated to be released over time, allowing farmers to apply the pesticides less amounts than requiring very highly concentrated and toxic initial applications followed by repeated applications to combat the loss of efficacy due to leaching, evaporation, and degradation.
Benefits of Microencapsulation (Cont.): 12- Ingredients in foods are encapsulated for several reasons. Most flavorings are volatile; therefore encapsulation of these components extends the shelf-life of these products. Some ingredients are encapsulated to mask taste, such as nutrients added to fortify a product without compromising the product’s intended taste. Alternatively, flavors are sometimes encapsulated to last longer, as in chewing gum.
Benefits of Microencapsulation (Cont.): 13- Controlled and targetted release of active ingredients. Many varieties of both oral and injected pharmaceutical formulations are microencapsulated to release over longer periods of time or at certain locations in the body. Aspirin, for example, can cause peptic ulcers and bleeding if doses are introduced all at once. Therefore aspirin tablets are often produced by compressing quantities of microcapsules that will gradually release the aspirin through their shells, decreasing risk of stomach damage. 14- Microencapsulation allows mixing of incompatible compounds.
Microencapsulation Technologies
Microencapsulation processes with their relative particle size ranges. Physico - Chemical Processes
Physico - mechanical Processes
Coacervation (2 – 1200 um)
Spray-drying (5 – 5000 um)
Polymer-polymer incompatibility (0.5 – 1000 um)
Fluidized- bed technology (20 – 1500 um)
Solvent evaporation (0.5 – 1000 um)
Pan coating (600 – 5000 um)
Encapsulation by supercritical fluid
Spinning disc (5 – 1500 um)
Encapsulation by Polyelectrolyte multilayer (0.02 – 20 um)
Co-extrusion (250 – 2500 um)
Microencapsulation processes with their relative particle size ranges (cont.). Physico-Chemical Processes (cont.)
Chemical Processes
Hydrogel microsphere
Interfacial polymerization (0.5 – 1000 um)
Phase Inversion (0.5—5.0 um)
In situ polymerization (0.5 – 1100 um)
Hot Melt (1—1000 um)
Microencapsulation Techniques (Cont.): I. Physico-Chemical Processes: 1- Coacervation: - Two methods for coacervation are available, namely simple and complex processes. -In simple coacervation, a desolvation agent is added for phase separation. - Whereas complex coacervation involves complexation between two oppositely charged polymers.
1- Coacervation (Cont.):
Complex coacervation: 1- First the core material (usually an oil) is dispersed into a polymer solution (e.g., a cationic aqueous polymer, gelatin). 2- The second polymer (anionic, water soluble, gum arabic) solution is then added to the prepared dispersion. 3- Deposition of the shell material onto the core particles occurs when the two polymers form a complex. 4-This process is triggered by the addition of salt or by changing the pH, temperature or by dilution of the medium.
1- Coacervation (Cont.): 5- Finally, the prepared microcapsules are stabilized by crosslinking (with formaldehyde), desolvation or thermal treatment. Complex coacervation is used to produce microcapsules containing fragrant oils, liquid crystals, flavors, dyes or inks as the core material.
Microencapsulation Techniques (Cont.): 2- Polymer-polymer incompatibility: - Also called phase separation. 1- This method utilizes two polymers that are soluble in a common solvent, yet do not mix with one another in the solution. 2- The polymers form two separate phases, one rich in the polymer intended to form the capsule walls, the other rich in the incompatible polymer meant to induce the separation of the two phases. The second polymer is not intended to be part of the finished microcapsule wall.
Microencapsulation Techniques (Cont.): 3- Solvent Evaporation: - It is the most extensively used method of microencapsulation. 1-Prepare an aqueous solution of the drug (may contain a viscosity building or stabilizing agent) 2- Then added to an organic phase consisting of the polymer solution in solvents like dichloromethane or chloroform with vigorous stirring to form the primary water in oil emulsion. 3- This emulsion is then added to a large volume of water containing an emulsifier like PVA or PVP to form the multiple emulsion (w/o/w). 4- The double emulsion is then subjected to stirring until most of the organic solvent evaporates, leaving solid microspheres. 5- The microspheres can then be washed and dried.
Microencapsulation Techniques (Cont.): 4- Polymer Encapsulation by Rapid Expansion of Supercritical Fluids: - Supercritical fluids are highly compressed gasses that possess several properties of both liquids and gases. - The most widely used being supercritical CO2 and nitrous oxide (N2O). - A small change in temperature or pressure causes a large change in the density of supercritical fluids .
Polymer Encapsulation by Rapid Expansion of Supercritical Fluids (Cont.): Steps: 1-Supercritical fluid containing the active ingredient and the shell material are maintained at high pressure and then released at atmospheric pressure through a small nozzle. 2-The sudden drop in pressure causes desolvation of the shell material, which is then deposited around the active ingredient (core) and forms a coating layer. -Different core materials such as pesticides, pigments, vitamins, flavors, and dyes are encapsulated using this method. -A wide variety of shell materials e.g. paraffin wax and polyethylene glycol are used for encapsulating core substances. -The disadvantage of this process is that both the active ingredient and the shell material must be very soluble in supercritical fluids.
Microencapsulation by rapid expansion of supercritical solutions
Microencapsulation Techniques (Cont.): 5- Hydrogel microspheres: 1- Microspheres made of gel-type polymers, such as alginate, are produced by dissolving the polymer in an aqueous solution 2-Then, suspending the active ingredient in the mixture 3- Extruding through a precision device, producing micro droplets 4- Then fall into a hardening bath that is slowly stirred. The hardening bath usually contain calcium chloride solution. Advantage: The method involves an ―all-aqueous‖ system and avoids residual solvents in microspheres. The particle size of microspheres can be controlled by: A- using various size extruders or B- by varying the polymer solution flow rates.
Hydrogel microspheres
Microencapsulation Techniques (Cont.): II Physical Processes: 1- Spray-Drying & spray-congealing : Microencapsulation by spray-drying is a low-cost commercial process which is mostly used for the encapsulation of fragrances, oils and flavors. Steps: 1- Core particles are dispersed in a polymer solution and sprayed into a hot chamber. 2- The shell material solidifies onto the core particles as the solvent evaporates. - The microcapsules obtained are of polynuclear or matrix type.
micro-encapsulation by spray-drying.
Spray drying
Microencapsulation Techniques (Cont.): Spray-congealing: - This technique can be accomplished with spray drying equipment when the protective coating is applied as a melt. 1- the core material is dispersed in a coating material melt. 2- Coating solidification (and microencapsulation) is accomplished by spraying the hot mixture into a cool air stream. - e.g. microencapsulation of vitamins with digestable waxes for taste masking.
Microencapsulation Techniques (Cont.): 2- Fluidized-Bed Technology: - Different types of fluid-bed coaters include top spray, bottom spray, and tangential spray. - used for encapsulating solid or liquids absorbed into porous particles. Steps: 1-Solid particles to be encapsulated are suspended on a jet of air and then covered by a spray of liquid coating material. 2- The rapid evaporation of the solvent helps in the formation of an outer layer on the particles. 3- This process is continued until the desired thickness and weight is obtained.
Schematics of a fluid-bed coater. (a) Top spray; (b) bottom spray; (c) tangential spray
Fluid-bed coater
Microencapsulation Techniques (Cont.): 3- Pan coating:
1- Solid particles are mixed with a dry coating material. 2- The temperature is raised so that the coating material melts and encloses the core particles, and then is solidified by cooling . Or, the coating material can be gradually applied to core particles tumbling in a vessel rather than being wholly mixed with the core particles from the start of encapsulation.
Pan coating:
Microencapsulation Techniques (Cont.): 4- Co-Extrusion: 1- A dual fluid stream of liquid core and shell materials is pumped through concentric tubes and forms droplets under the influence of vibration. 2-The shell is then hardened by chemical cross linkings, cooling, or solvent evaporation. - Different types of extrusion nozzles have been developed in order to optimize the process
Schematic presentation of the Co-extrusion process
Co-extrusion Process
Microencapsulation Techniques (Cont.): 5- Spinning Disk: Steps: :
1- Suspensions of core particles in liquid shell material are poured into a rotating disc. 2- Due to the spinning action of the disc, the core particles become coated with the shell material. 3- The coated particles are then cast from the edge of the disc by centrifugal force. 4- After that the shell material is solidified by external means (usually cooling). - This technology is rapid, cost-effective, relatively simple and has high production efficiencies.
