Liposome-microcapsule system

In the dual liposome-microcapsule system, two factors control the release of the active substance escape from lipsomes into the microcapsule interior, and diffusion across a rate limiting capsule wall into the external environment. This system can take advantage of the inherent instability of some lipsomes while over-coming many of the problems associated with their use by protecting them from the environment by the capsule. At the same time, a new measure of control over the time at which a microcapsule will commence delivery of the enclosed agent is introduced by careful choice of the liposome composition. By changing the nature of the liposomes or of the encapsulant (e.g. alginate) different release times and patterns can be obtained. 

Microparticulate systems present the advantage, in comparison with single-unit solid systems, to guarantee a wider contact area between the drug and the mucosa. Different microparticulate systems intended for vaginal administration were developed they include liposomes, microcapsules, and microspheres. Such systems can possess intrinsic bioadhesive properties or can be loaded in a vehicle with bioadhesive properties. 

The formation of ordered two- and three-dimensional microstructuies in dispersions and in liquid systems has an influence on a broad range of products and processes. For example, microcapsules, vesicles, and liposomes can be used for controlled drug dehvery, for the contaimnent of inks and adhesives, and for the isolation of toxic wastes. In addition, surfactants continue to be important for enhanced oil recovery, ore beneficiation, and lubrication. Ceramic processing and sol-gel techniques for the fabrication of amorphous or ordered materials with special properties involve a rich variety of colloidal phenomena, ranging from the production of monodispersed particles with controlled surface chemistry to the thermodynamics and dynamics of formation of aggregates and microciystallites. 

A large variety of drug delivery systems are described in the literature, such as liposomes (Torchilin, 2006), micro and nanoparticles (Kumar, 2000), polymeric micelles (Torchilin, 2006), nanocrystals (Muller et al., 2011), among others. Microparticles are usually classified as microcapsules or microspheres (Figure 8). Microspheres are matrix spherical microparticles where the drug may be located on the surface or dissolved into the matrix. Microcapsules are characterized as spherical particles more than Ipm containing a core substance (aqueous or lipid), normally lipid, and are used to deliver poor soluble molecules... 

Antibodies have also been adsorbed or covalently linked to a variety of other carrier systems, such as liposomes [124], erythrocytes [226,227], and microcapsules and nanospheres, to selectively target them to a specific site in the body. 

Encapsulation within an enteric coat (resistant to low pH values) protects the product during stomach transit. Microcapsules/spheres utilized have been made from various polymeric substances, including cellulose, polyvinyl alcohol, polymethylacrylates and polystyrene. Delivery systems based upon the use of liposomes and cyclodextrin-protective coats have also been developed. Included in some such systems also are protease inhibitors, such as aprotinin and ovomucoids. Permeation enhancers employed are usually detergent-based substances, which can enhance absorption through the gastrointestinal lining. 

Herein, various barriers to oral delivery of macromolecular drugs will be discussed in brief, after which carrier-mediated delivery systems on the basis of chemical modification of the drugs and particulate drug carriers (e.g., nanoparticles, microcapsules, and liposomes) will be introduced as representative of promising approaches currently being investigated. 

Membrane-reservoir systems based on solution-diffusion mechanism have been utilized in different forms for the controlled delivery of therapeutic agents. These systems including membrane devices, microcapsules, liposomes, and hollow fibres have been applied to a number of areas ranging from birth control, transdermal delivery, to cancer therapy. Various polymeric materials including silicone rubber, ethylene vinylacetate copolymers, polyurethanes, and hydrogels have been employed in the fabrication of such membrane-reservoir systems (13). 

Suspensions of liposomes, microspheres and microcapsules, and nanospheres and nanocapsules formed from a variety of polymers or proteins, as discussed in section 8.6.3 form a new class of pharmaceutical suspension in which physical stability is paramount. It is important that on injection these carrier systems do not aggregate, as this will change the effective size and the fate of the particles. The exception to this is the deliberate flocculation of latex particles administered to the eye, where aggregation leads to agglomerated... 

Enzymes may be immobilized by encapsulation in nonpermanent (e.g., liposomes) or permanent (e.g., nylon) microcapsules. The enzyme is trapped inside by a semi-permeable membrane, where substrates and products are small enough to freely diffuse across the boundary. While nonpermanent microcapsules are useful in biochemical research, only permanent microencapsulations yield analytically useful systems, because of their mechanical stability. 

Figure 1 Formulation of controlled drug delivery systems using the supercritical fluid-derived RESS process (A) films, (B) microparticles/nanoparticles/ liposomes, (C) coated products including microcapsules, and (D) microporous foams.

Drug Delivery. Philadelphia Taylor Francis. ISSN 1071-7544. Covers basic research, development, and application principles of drug delivery and targeting at molecular, cellular, and higher levels. Topics covered include all delivery systems and modes of entry, such as controlled release systems microcapsules, liposomes, vesicles, and macromolecular conjugates antibody targeting and protein/peptide delivery. Peer-reviewed. 

Other systems which are currently studied as potential drug delivery systems are classified into two groups Firstly carriers which, by nature, form some kind of multi-or unilamellar vesicle, e.g. liposomes, erythrocyte ghosts, synthetic microcapsules, and secondly carriers based on naturally occurring macromolecules such as albumin, DNA, antibodies, Dextran (reviewed in Ref. >). 

Reservoir delivery systems have been developed in a variety of styles, ranging from microcapsules to hollow fibers to liposomes. Hayashi et al. (1994) produced delivery systems by encapsulating proteins and hormones inpoly-L-lactide microspheres by a solvent evaporation method. The release mechanism for hormones entrapped in liposomes was studied by Ho et al (1986). Progesterone and hydrocortisone skin permeation was enhanced by the presence ofthe liposomes no penetration ofthe liposomes was observed. Examples ofthe most common hydrogels employed in reservoir systems are crystalline-rubbery PEG, PAAm, celluloses, PAA, and PHEMA. 

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