Characterization and biodegradation of chitosan–alginate polyelectrolyte complexes

https://doi.org/10.1016/j.polymdegradstab.2008.10.017Get rights and content

Abstract

Polyelectrolyte complexes (PECs) have been the focus of an expanding number of studies for their wide use. This study investigated the characteristics and biodegradation of chitosan–alginate PECs prepared by freeze-drying a precipitate from sufficient mixtures of the two polymers. The analyses of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) suggested that the partial protonated amine groups of chitosan reacted with the carboxylate groups of alginate and thus strong PECs were formed. After incubating in lysozyme solution, the PECs showed high ability of enzyme adsorption, and low degradation rate in spite of different degrees of deacetylation of chitosan, due to the strong interaction between chitosan and alginate and the hindrance of closely adsorbed lysozyme.

Introduction

Chitosan and alginate are biocompatible biopolymers and have been finding widespread applications in tissue engineering [1]. Chitosan, obtained by partial deacetylation of chitin, is a polysaccharide consisting of glucosamine (GA) and N-acetyl-glucosamine (NAc-GA) linked by β-1,4 glucosidic bonds [2], [3]. Alginate is another linear polysaccharide composed of α-l-guluronic and β-d-mannuronic acid residues [4], [5]. The carboxylate moieties on alginate can ionically interact with the protonated amines on chitosan, forming physical cross-linked hydro-gels known as polyelectrolyte complex (PEC) [6], [7]. Compared with the constituent polymers, the PEC reduces tendency of swelling and improves structural strength and mechanical stability [8], and there has been considerable interest in the chitosan–alginate PEC systems used in tissue engineering in the form of scaffolds [9], [10], membranes [11], fibres [12], [13] and microcapsules [14], [15].

As the ultimate goal for the application of chitosan–alginate PEC systems in tissue engineering is the hope that it could disintegrate naturally along the growing tissues, their controlled degradation plays a crucial role to meet the purpose [16], [17] and some works have been performed on this area. For example, Quong et al. [14] evaluated the stability of chitosan membrane formed on alginate beads in the protection of DNA, and their results showed that the membrane was almost totally inert to hydrolytic enzymes in the gastrointestinal transit, with less than 2% of the weight hydrolyzed for three days. In a study of lysozyme biodegradation of chitosan–alginate film, prepared by evaporating the mixed solution of chitosan and alginate in the ratio of 1:1, the lower molecular weight and increased degree of deacetylation (DD) caused an accelerated weight loss [18]. When alginate-coated chitosan membrane was shaken in phosphate buffer system (PBS), the weight was degraded to about 75% from the original after 30 days [11]. Although the degradation behavior of some kinds of the PECs prepared by different methods was studied by these researchers, as there is inadequate information on the characteristics of PECs, the degree of interaction between chitosan and alginate is not clear, as well as its influence on enzyme degradation. In order to clarify these problems, in this paper we prepared a strong binding PEC of chitosan and alginate, characterized the existed interactions in detail by XPS, FTIR, XRD and DSC, and studied the degradation rate in lysozyme solution.

The process of enzyme degradation is known to be a complicated phenomenon, but the protein adsorption on the surface of the material is generally regarded as a primary event before reaction. Both chitosan and alginate contain a number of functional groups such as hydroxyl, amino, and carboxyl, which provide several possibilities for enzyme adsorption. Chitosan and its modifications have been investigated for adsorption and separation of proteins and cells [19], [20], but in the research of chitosan or chitosan complexes biodegradation, little attention has peen paid. Here we also studied the lysozyme adsorption behavior on chitosan–alginate PECs and its influence on biodegradation.

Section snippets

Materials

Chitosan from raw material (Yuhuan, China) was purified and modified with the molecular weight of 230,000 and DD of 95%, and chitosan with DD of 62% was prepared by controlling the amount of acetic anhydride in the process of N-acetylation [21]. Sodium alginate (Qingdao, China) was used as received with a viscosity of 23 centimeter poise (cP) (1 wt% at 25 °C). Crystallized egg white lysozyme (70,000 U/mg) was purchased from Fluka, USA. Micrococcus lysodeikticus (ATCC No.4698) was supplied by

Characterization of the complexes

PECs are usually formed by mixing two polymer solutions carrying opposite charges, depending on the pH value of the media, the ionic strength and temperature. The pKa value of chitosan is about 6.3 [25], and the pKa of mannuronic acid and guluronic acid of alginate are 3.38 and 3.65 [26], respectively. In 1% acetic acid solution (pH value was about 4.8), chitosan dissolved with sufficient number of amine group protonated. When mixing with sodium alginate solution, most of the carboxylic groups

Conclusions

The strong chitosan–alginate PECs were obtained by mixing the two polymers under vigorous stirring followed by washing to remove the unreacted components and crosslinking with CaCl2. The XPS survey scan and FTIR indicated the presence of protonated amine groups of chitosan, which reacted with the carboxylate groups of alginate. DSC measurements further confirmed the existence of interactions between chitosan and alginate. The results of biodegradation showed that chitosan with low deacetylation

Acknowledgments

This work was supported by the National Basic Research Program of China (grant 2007CB714305), the National Natural Science Foundation of China (grant 20736006).

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