Preparation and characterization of double crosslinked hydrogel films from carboxymethylchitosan and carboxymethylcellulose

Highlights

• Carboxymethylchitosan (CMCS)/carboxymethylcellulose (CMC) hydrogel film was prepared.

• Ionical crosslinking with CaSO₄ and covalent with genipin was combined to form film.

• The significant difference of mechanical properties of hydrogel films was analyzed.

• The biocompatibility of the crosslinked hydrogels was comparatively confirmed.

Abstract

A novel crosslinked hydrogel film was prepared from carboxymethylchitosan (CMCS) and carboxymethylcellulose (CMC) by ionical and covalent crosslinking with CaSO₄ and genipin, respectively. The swelling ratio of the crosslinked CMCS/CMC hydrogel films was investigated at different pH solutions (1–9), and the results indicated that the crosslinked hydrogels had the swelling–deswelling properties with two primary peaks of swelling ratio at pH 3 and 7. The surface morphologies of the crosslinked hydrogels at different pH values provided evidences of the swelling–deswelling properties. The mechanical properties of the hydrogel films were also examined. The ionical and covalent crosslinking were found to have the primary impact on the toughness and max load, respectively, of the crosslinked hydrogels. The cells comparatively cultured on the crosslinked hydrogels and the negative and positive controls suggested the biocompatibility of the crosslinked CMCS/CMC films. This kind of hydrogel films have potential application in drug delivery vehicles and skin tissue engineering.

Introduction

Hydrogels are polymers in three-dimensional network arrangement with the capability to absorb and retain large amount of water (Bakass et al., 2002, Jao et al., 2009, Peppas et al., 2000). The physically or chemically crosslinked polymer chains are usually crucial to keep the spatial structure of hydrogels (Hennink and van Nostrum, 2002, Sangeetha and Maitra, 2005). In recent decades, hydrogels based on natural polymers such as cellulose, chitosan, starch, alginate, dextrin, etc, and their derivatives have attracted much attention and shown great potential application in many fields including agriculture, medicine, pharmacy and biotechnology (Agnihotri et al., 2004, Gombotz and Wee, 1998, Nguyen and West, 2002)

Carboxymethylchitosan (CMCS) and carboxymethylcellulose (CMC), two natural polyelectrolytes derived from cellulose and chitosan by introducing carboxyl methyl group (–CH₂COOH), respectively, have attracted considerable interests in a wide range of biomedical applications (Chen et al., 2002, Chen et al., 2004, Lee and Chiang, 2004). Due to the unique biodegradability, biocompatibility, good film-forming property and pH sensitivity, they have been found potential applications as wound dressings, and artificial bone and skin (Chen et al., 2004, Muzzarelli, 1988, Thanou et al., 2001). Unfortunately, the application of these bio-based hydrogels was significantly hindered by the limited mechanical strength (Hoare & Kohane, 2008).

In recent years, considerable efforts have been focused on the development of novel methods to obtain hydrogels with extremely high mechanical strength. The interpenetrating polymer network (IPN) technology, which can be achieved by double crosslinking route, is one of the possible strategies to enhance the mechanical properties (Gong et al., 2003, Zhang et al., 2005a, Zhang et al., 2005b). Highly stretchable and tough hydrogel was prepared through combining chemical and physical crosslinking methods (Sun et al., 2012). This hydrogel could be stretched beyond 20 times their initial length and had fracture energies of 9000 J/m², which was almost comparable with 10,000 J/m² for natural rubbers (Lake, 1995) and higher than 1000 J/m² for cartilage (Simha, Carlson, & Lewis, 2004), suggesting a great potential for future application. Considering the possible application as biomaterials, the biotoxicity is a very important factor for hydrogel. Therefore, it is significantly necessary to use green chemical process to prepare hydrogel materials. Unfortunately, it is difficult for the traditional direct crosslinking or graft-polymerization to ensure the bio-nontoxicity of the final product.

Genipin, derived from an iridoid glycoside called geniposide present in fruit of Gardenia jasminoides (Akao, Kobashi, & Aburada, 1994), is an excellent natural crosslinker for proteins, collagen, gelatin, and chitosan (Muzzarelli, 2009). It has a low acute toxicity, much less (5000–10,000 times) toxic than glutaraldehyde and other commonly used synthetic crosslinking reagents. Therefore, genipin can be widely used in medical fields especially for biological tissue repair, and regulating agent for drug delivery (Sung et al., 1999, Tsai et al., 2000). In the present study, a novel CMCS/CMC hydrogel film was prepared by combining two types of crosslinking method, ionically by CaSO₄ and covalently by genipin. The properties of the prepared hydrogel films were characterized for the possible application.