第三章-2011-1

第三章 微囊化技术
Microencapsulation
3.1 INTRODUCTION
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Definition
Immobilization can be defined as any procedure that confines substances or cells inside a given system and limits its free movement. The concept of microencapsulation is related to a special immobilization system, where the biological material is confined inside particles, beads, or hollow spheres.
Application of microencapsulated system
Improving the relatively outdated biotechnological processes in the food industry, wastewater treatment, leaching, and environmental detoxification. In modern biotechnology, it is used in cell culture processes for the production of highvalue substances, such as antibodies, erythropoetin, the anticancer drug taxol, as well as for the production of artificial seeds and cryopreservation.
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In medicine, microencapsulated cells have been used as artificial organs to treat diabetes, as a delivery system for gene therapy, as an intermediate step in cancer therapy, and for the treatment of Partinson’s disease, etc. Furthermore, microencapsulation can serve as a controlled release system for drug in medicine and pesticides in agriculture.
3.2 MICROCAPSULES
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Although there are a variety of encapsulation techniques and suitable materials, the resulting microcapsules can be divided into only three main groups: beads coated beads hollow spheres.
3.2.1 Beads
Beads can be produced by cooling liquid drops of gelling agents under their melting point, thus transforming the polymer solution into a stable cryogel by internal hydrogen bonding, or by chemically or ionically cross-linking polymers. Bead formation is a one-step process, it is relatively easy to develop a large-scale process. The most common bead formation system is alginate cross-linked with calcium ions or with other divalent metal ions.
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Substrates Pore
Gel
Cells
Metabolic products
Diagram of bead
Properties of beads
Advantages:
The fully developed three-dimensional internal network structure enables the beads to withstand extreme mechanical stress. Permeability of hydrogel beads is excellent, and they are often used for culturing cells and microorganisms.
Disadvantage:
their lack of a real barrier on the surface, so cells can expand during cultivation.
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3.2.2 Coated beads
The beads with one or several additional walls are called as the coated beads, which can overcome the problems associated with the open porous structure of bead surfaces. The beads from charged polymers are treated with a diluted solution of an oppositely charged polyelectrolyte, a simplex membrane on the bead surface is formed. The use of several alternatively electrolytes leads to multilayer membranes.
Substrates Coated membrane Gel
Cells
Metabolic products
Diagram of coated bead
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Properties of coated beads
Advantages: The membrane properties can be engineered independently with respect to diffusion, molecular weight cut-off, cell retention, or immuno-protection from the internal bead structure and material.
Application of coated beads
The most well-investigated clinical application of microencapsulation is based on poly-L-lysine-coated calcium alginate beads in the treatment of the diabetes, which has recently been applied to humans.
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3.2.3 Hollow spheres
Hollow Spheres can be produced in a onestep process using two membrane-forming materials that can not penetrate each other. The cross-linking reaction is limited to the interface area of the hollow sphere, forming drops and producing a stable membrane around a liquid core.
Substrates with low MW Substrates with high MW
membrane
Macromolecules
Liquid core Cells Metabolic products with low MW
Diagram of capsule
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Typical hollow spheres
Typical hollow-sphere-forming materials are all polyelectrolyte combinations:
Cellulose sulfate － polydimethyldiallylammonium chloride (CS－ PDMDAAC) CS－polyethylene imine (PEI) Alginate － cellulose sulfate － CaCl2 － polymethylene co-guanidine (Al-CS-CaCl2-PMCG) Alginate－CaCl2－CMC
Method of formation
Another method is the use of two materials so that cross-linking material of the drop can diffuse into the surrounding solution. The membrane is formed first on the surface of the droplets and then proceeds outward. The membrane thickness can be controlled by the polyelectrolyte concentration of the droplet. Hollow spheres can also be produced from a multi step process: the core of the coated beads is solubilized.
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Applications
The controlled release systems in agriculture and pharmaceutical industry. On shear-sensitive substances such as animal cells in particular. Simplex membrane systems are important for medical applications such as artificial liver support and gene therapy.
Progress in hollow spheres
Considerable progress has been made through a U.S. National Aeronautics and Space Administration study:
over a thousand combinations of polyanions and polycations were tested to identify new polymer candidates for the encapsulation of living cells; especially pancreatic islet cells for treatment of diabetes in humans.
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3.3 METHODS IN MICROENCAPSULATION
All microencapsulation processes are followed by
Formation of a liquid drop Gelation Cross-linking reaction Membrane formation
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The liquid drops can be obtained from extrusion of a liquid through a small needle or orifice, or from emulsification of the drop-forming solution in a immiscible solution by dynamic or static mixers. The main parameters in encapsulation are scale-up ability of the process, a uniform microcapsule size and shape.
3.3.1 Dropping methods
A liquid ejected with low velocity from a needle will break into individual drops. If the velocity is increased, drop formation increases until the maximum velocity is reached and the liquid begins to form a jet.
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The maximum velocity can calculated in following equation:
be
? v < 2? γ ? ρ .d ? i? ?
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v：liquid velocity； γ：interfacial tension； ρ：liquid density di：needle inner diameter
(1) Simple dropping
Simple dropping
The main two factors affecting drop size: ?gravity force trying to tear the drop from the needle tip ?resisting product of the interfacial tension and the tip perimeter. The resistance power and inertial force can be neglected in computations.
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The drop mass can be calculated from the equilibrium of the two main forces:
mg = πd eγ
and
m=
ρπd D 3
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m：drop mass g：gravity acceleration constant de：external needle diameter γ：interfacial tension ρ：liquid density dD：drop diameter.
The drop mass must be corrected by a factor of 0.85 to get the true mass, because drops stretch out and leave a small portion behind when dropping from the needle. Even if very thin needles are used, it is very difficult to obtain droplets with a diameter of 1mm or less.
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(2)Dropping with a superposed air jet
The droplet size can be reduced by superposing the drop forming process with an additional air jet. In calculating the droplet mass, it should take into account the drag forces of the air-flow. The viscous drag force is the laminar effect of the fluid, and the kinetic energy dissipation term represents the impact of the turbulence on the drag.
Superposed air jet
air knife
Monoaxial extrusion technologies based on simple dropping
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