RECENT TRENDS IN ALGINATE, CHITOSAN AND ALGINATE-CHITOSAN ANTIMICROBIAL SYSTEMS

Natural polysaccharides alginate and chitosan have been used extensively, separately or in mixtures (systems), in manufacturing of pharmaceutical products (antimicrobial) and not only. Alginates usually serve as basis for antimicrobial systems, while chitosan, in certain proportions, enhances their physicochemical and antimicrobial properties. Focusing on the recent literature (mostly since 2000), this review outlines the main synthetic approaches for the preparation of systems based on both polymers as well as identify potential areas of their application as antimicrobial agents. Various techniques used for systems preparation like microparticles, films, fibers, nanoparticles, sponges, applications and usefulness of these systems as carriers of antimicrobial compounds will also be discussed.


Introduction
Nowadays, new systems with antimicrobial properties, that are more potent, less toxic to humans, with prolonged action and preferentially based on natural compounds, are of high interest.Consequently, antimicrobial systems based on natural polymers, such as alginates and chitosan, which fulfi l the above mentioned properties and are relatively accessible and ecological as they can be obtained from various agro-industrial waste; are being studied increasingly.According to the available data, this review intends to consolidate for the fi rst time the knowledge about the most popular alginate/chitosan antimicrobial systems, though more attention being paid to the alginate-chitosan systems with encapsulated antimicrobial compounds, namely the synthesis techniques, some physicochemical and bioactive properties as well as areas of use.
Due to the opposite charges, alginate (-) and chitosan (+) form polyelectrolyte complexes, that can be moulded in the form of various systems.Based on a literature review of the last fi fteen years of research in this fi eld, it was identifi ed that the most studied alginate-chitosan systems are: a) microparticles; b) fi lms; c) fi bers; d) nanoparticles; e) sponges.

Microparticles
Alginate-chitosan microparticles are especially used in the pharmaceutical, for sustained-release of antimicrobial compounds.Microparticles provide stability, mask an unpleasant taste and odour, and reduce toxic side effects of encapsulated bioactive substances.
Encapsulation of amoxicillin in alginate-chitosan mucoadhesive microcapsules as gastroretentive delivery system, resulted in enhanced stability [48] and controlled release of antimicrobial drug in the simulated gastric fl uid, compared to amoxicillin plain drug.[48,49] Reducing approximately two times and more of concentration of used chitosan (0.5% w/v and lower) to obtain antimicrobial alginate-chitosan microparticles resulted in selective sustainedrelease of active principle (nitrofurantoin [50], polymyxin B [52], rifampicin, isoniazid and pyrazinamide [54,56]) in simulated intestinal fl uid, while the release in simulated gastric fl uid was very slow.Cefaclor release from alginatechitosan microparticles in simulated gastric fl uid is intensifying with increasing of alginate concentration up to 7% w/v (chitosan concentration -0.5% w/v) [53].Similar release profi le was observed for systems containing antituberculosis drug rifampicin and for which obtaining was used 1% alginate and 1.5% chitosan [55].Alginate-chitosan beads with metronidazole were obtained; due to high contents of chitosan (5% w/v) fl oat on gastric juice and consequently are retained in the stomach where they gradually release antibiotic compound [51].In vivo studies have shown that the encapsulation of rifampicin, isoniazid and pyrazinamide in alginate-chitosan microcapsules resulted in an increase of about 13-15 times in their biological half-life compared to non-encapsulated substances, leading to an increased duration of action of the antituberculous compounds [56].
Using tripolyphosphate cross-linking method were obtained metronidazole containing alginate-chitosan microcapsules, characterized by good mucoadhesive property and prolonged release in simulated intestinal fl uid due to ionic cross-link between negatively charged tripolyphosphate and positively charged chitosan molecules [57].
Alginate-chitosan microsystems with sulfathiazole were obtained using the stages: 1. ionotropic gelation of alginate with calcium chloride in presence of chitosan and sulfathiazole; 2. treatment of obtained beads with sodium tripolyphosphate solution; 3. beads irradiation with microwave radiation.Obtained beads showed release-retarding properties in the simulated gastric fl uid, due to alginate-chitosan complexation and alginate cross-linking, especially after microwave treatment [58].

Sponges
Alginate-chitosan sponge represent a tridimensional porous scaffold, prepared by freeze-drying technique of mixture [83] or consecutively deposited one above the other layers of alginate and chitosan [84][85][86].Pore sizes range from <100 μm [85,86], 200-400 μm [83] and approximately 500 μm [84], as a function of various factors such as cross-linking time with Ca 2+ ions -with its increasing pore sizes being reduced [83] (Figure 8).In the same time with increasing of calcium ions concentration the release rate of antimicrobial compound from systems decreases, because of this, systems containing the lowest concentration of calcium ions (5%), in the conditions of the experiment, showed shortest time in reducing of number of viable bacteria cells to zero [85].
All of the studied alginate-chitosan antimicrobial sponges demonstrated sustained release of antimicrobial drugs from 4 hrs [85] to 30 days [84].Due to these properties, alginate-chitosan antimicrobial sponges can be potentially used in wound dressing manufacturing [83,85,86] and in tissue engineering applications [84].

Figure 7 .
Figure 7. Structures of some antimicrobial compounds encapsulated in alginate-chitosan nanoparticles.