Elsevier

European Polymer Journal

Volume 40, Issue 12, December 2004, Pages 2709-2715
European Polymer Journal

Entrapment and release of sodium polystyrene sulfonate (SPS) from calcium alginate gel beads

https://doi.org/10.1016/j.eurpolymj.2004.07.022Get rights and content

Abstract

The release of sodium polystyrene sulfonate (SPS) from calcium alginate hydrogel beads has been studied. It has been shown that the structure of the cross-linked calcium alginate network is of primary importance in the retention and/or release of the SPS. This has been evidenced by studying the influence of Ca2+ concentration, molar masses (Mn) and the ratio of mannuronic acid/guluronic acid components. A minimum in the SPS release is observed in relation with the organization of the network structure. Conditions inducing the organization of a strong gel (e.g. high Ca2+ concentration for example) are not always related to a low release. A good control of release is found when a compromise between a well-structured hydrogel and sterical consideration of SPS is reached.

Introduction

Alginate is a naturally occurring biopolymer extracted primarily from brown algae. It has been used in the biotechnology industry as a thickening agent, a gelling agent and a colloidal stabilizer. Alginate also has a unique capacity to be used as a matrix for the entrapment and/or delivery of a variety of molecules or particles.

Alginate is a linear unbranched polysaccharide composed of two building blocks namely mannuronic acid (M) and guluronic acid (G) units (Fig. 1). These residues are linked in 1,4 and may vary widely in composition and sequence depending of the alga origin. In the presence of divalent cations, alginate shows gelling properties. Addition of calcium ions induces a cooperative effect between G-blocks until a 3D network is formed according to the well-known “egg-box” model (Fig. 2). Clark [1] had studied the influence of the divalent cations on the strength of the gel.

Ca alginate particles are generally prepared using two methods. The first method (called dripping), gives large particle (about 1 and 2 mm) (e.g. [2], [3]). The second method (called emulsification) gives small but irregular particle with a high tendency to clumping (e.g. [3], [4]). The large pore size of this particle (12–16 nm on the surface as reported by Klein [5] permits only to encapsulate and release macromolecules as protein (insulin [6], fibrinogen, gamma globulin [7] or to entrap cells [8]. Small molecules have high diffusion coefficient in Ca alginate beads and can be entrapmented and retained only if beads are covered with a cationic polymer membrane (capsule) [9]. Data have been reported recently by Kikuchi [10] and Favre [11] about the release of polymer from such Ca-alginate hydrogel beads, however many questions still remain asked.

The aim of our work was to relate the macromolecular structure of alginate beads prepared using the dripping technique to study the entrapment properties and release capacities of a model polymer according to a specific application, a sodium polystyrene sulfonate (SPS) from various Ca alginate beads. The choice of this model polymer is attributed to the presence of sulfonate functions and a molecular mass of around 50 000 g/mol. The amount of calcium ions added, the molar weight and the G-blocks content of the alginate (M/G ratio) were particularly considered.

Section snippets

Materials

Na-alginate samples with different M/G ratio and molecular weight were provided by SKW Biosystems (Baupte, France). Sodium polystyrene sulfonate (dry powder) and calcium chloride (anhydrous) were purchased respectively from Aldrich and Merck. For calcium chloride, the percentages of Mg2+ salts and Ca(OH)2 are negligible (<0.005%). The water is treated with Milli Q system (Millipore, Massachusetts, USA).

Molecular weight

The absolute average molar weight and molar weight distribution was determined by coupling

Physicochemical characterization of alginates and SPS

The four alginate samples varying by their M/G ratio and molar masses have been analyzed by both SEC/MALLS/DRI and viscometry. The results are compiled in Table 1. As guluronic groups are involved in the gel formation with calcium ions, we have focused our attention on the lower ratio e.g. 0.5. Three samples of M/G = 0.5 have been studied differing by their number average molar masses (Mn) from 220 000 to 460 000 g/mol respectively from sample A to sample C. The fourth sample (D) has a M/G ratio

Conclusion

Entrapment and release of a low molar mass water-soluble polymer e.g. sodium polystyrene sulfonate (SPS) have been studied. The hydrogel matrix consisted of calcium alginate beads obtained by a dripping method. The ability of the system to encapsulate and control the SPS release has been investigated through the modification of the cross-linked calcium alginate network structure. Different alginates have been studied varying by their molar masses (Mn) and their mannuronic/guluronic acid (M/G)

Acknowledgments

We are grateful to Mr. G. Brigand from SKW BIOSYSTEMS for providing the alginate samples. We are also grateful to Mrs. Rola Adra for conducting some experiments in our team during her master school stage of University of Rouen.

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