Protein release from alginate matrices
Introduction
Alginate is a naturally occurring biopolymer that is finding increasing applications in the biotechnology industry. Alginate has been used successfully for many years in the food and beverage industry as a thickening agent, a gelling agent and a colloidal stabilizer. Alginate also has several unique properties that have enabled it to be used as a matrix for the entrapment and/or delivery of a variety of proteins and cells. These properties include: (i) a relatively inert aqueous environment within the matrix; (ii) a mild room temperature encapsulation process free of organic solvents; (iii) a high gel porosity which allows for high diffusion rates of macromolecules; (iv) the ability to control this porosity with simple coating procedures and (v) dissolution and biodegradation of the system under normal physiological conditions. This review will first describe the preparation, chemical structure and characterization of alginate. The different methods of gel formation and physical properties of the gels will then be discussed. Finally, specific examples of alginate systems and their application to protein delivery, nucleic acid delivery and cell encapsulation will be given.
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Sources of alginate
Commercial alginates are extracted primarily from three species of brown algae (kelp). These include Laminaria hyperborea, Ascophyllum nodosum, and Macrocystis pyrifera. Other sources include Laminaria japonica, Eclonia maxima, Lesonia negrescens and Sargassum species [1]. In all of these algae, alginate is the primary polysaccharide present and it may comprise up to 40% of the dry weight [2]. Alginate is found in the intracellular matrix where it exists as a mixed salt of various cations found
General formation mechanism
Alginate beads can be prepared by extruding a solution of sodium alginate containing the desired protein, as droplets, into a divalent crosslinking solution such as Ca2+, Sr2+, or Ba2+. Monovalent cations and Mg2+ ions do not induce gelation [2]while Ba2+ and Sr2+ ions produce stronger alginate gels than Ca2+ [11]. Other divalent cations such as Pb2+, Cu2+, Cd2+, Co2+, Ni2+, Zn2+ and Mn2+ will also crosslink alginate gels but their use is limited due to their toxicity. The gelation and
Physical properties
The functional and physical properties of cation crosslinked alginate beads are dependent on the composition, sequential structure, and molecular size of the polymers 4, 10. The flexibility of the alginate polymers in solution increases in the order MG>MM>GG (G=α-l-guluronic acid; M=β-d-mannuronic acid) [4]. Beads with the lowest shrinkage, highest mechanical strength, highest porosity, and best stability towards monovalent cations are made from alginate with an α-l-guluronic acid content
Immunogenicity
There are many factors involved in determining the successful application of polymers as drug delivery carriers in humans, with polymer biocompatibility or/and immunogenicity being two of the more important issues. There are numerous reports addressing the fibrotic reaction of implanted alginates 61, 62, 63, 64, 65. Most authors agree that the chemical composition and the mitogenic contaminants found in alginates are the two main contributors to alginate immunogenicity.
Alginates can be readily
Protein encapsulation
Numerous reports have been published on the encapsulation and release of proteins from alginate matrices. These are summarized in Table 1.
DNA encapsulation
With recent advances in the field of gene therapy, new methods to efficiently deliver DNA oligonucleotides are being evaluated. There are two studies to date that report the potential application of alginate as an enteric delivery vehicle for DNA 120, 31. The encapsulation of DNA and its derivatives may be used in enteric targeting of nucleic acids as gene transfer agents, modified oligonucleotides and carriers for DNA-intercalators. In vitro studies showed that DNA can be successfully
Microsphere and liposome encapsulation
Alginate gels have been used to encapsulate other delivery systems including microspheres and liposomes. Ethylcellulose microspheres were dispersed into an aqueous solution of sodium alginate which was subsequently dropped into a CaCl2 solution [18]. The authors suggested that the beads could potentially be useful as an oral delivery system for micro- or nanoparticles.
Liposomes that contained the model proteins BSA or horse-radish peroxidase were incorporated into alginate spheres with a
Summary
The chemistry and relatively mild crosslinking conditions of alginate have enabled this naturally occurring biopolymer to be used for the encapsulation of a wide variety of biologically active agents including proteins, cells and DNA oligonucleotides. By selection of the appropriate alginate type, gelation conditions, added excipients, and coating agents, matrices of various morphologies, pore size, water content and dehydration rates can be fabricated. This high degree of flexibility can
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