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1. Introduction
The use of natural polymers for the design of drug delivery systems has long been the subject of great interest during the past decades. Sodium alginate is a sodium salt of alginic acid, a naturally occurring nontoxic polysaccharide found in marine brown algae. Alginate has been widely used as food and pharmaceutical additives, such as tablet disintegrant, thickening, and suspending agent. It contains two uronic acids, α -L-guluronic (G) and β -D-mannuronic acids (M), and is composed of homopolymeric blocks and blocks with an alternating sequence [1]. This polymer can form a reticulated structure when it contacts with Ca2+ and thus it has been used to produce sustained release particulate systems for various drugs, proteins, and even cells [2–4]. Gelation occurs by an ionic interaction between the calcium ions and the carboxylate anions of G-G blocks as calcium ions diffuse from the external source into the droplet forming a polyanionic microcapsule.
The addition of a polycation (poly-L-lysine or chitosan) to the gelation medium induces the formation of polyanionic-polycationic complexes, which stabilizes the ionic gel network and reduces the alginate permeability [5, 6].
The main advantage of using alginate to encapsulate drugs is that the alginate gelation process occurs under very mild conditions without using high temperatures or chemical crosslinking agents. Another advantage of using alginate is that the alginate gel can also be converted to sol by adding chelating agents, such as Na+ and EDTA. However, the drug releasing properties of Ca-sodium alginate matrices suffer from some serious problems. Firstly, the drugs could be leaked during the gel formation due to the long immersion time, which decreased the encapsulation efficiency. Secondly, the burst release of the drugs from pure Ca-sodium alginate beads is severe due to the quick breakdown of beads in the in vitro release process. Currently, much effort has been made for improving the performance of Ca-sodium alginate beads as drug delivery carriers.
Therefore, many factors are involved in the formulation of alginate microspheres. Some of them are summarized in Table 1.
Table 1
Some of the factors affecting the encapsulation process of drugs into alginate beads.
| Factor | Drug | Effect | Reference | |
|---|---|---|---|---|
| Related with the formulation | ||||
| Gliclazide | ||||
| Alginate concentration | + EE | |||
| + diameter | ||||
| −porosity | [7] | |||
| −release rate | ||||
| + swelling | ||||
| BSA | ||||