Preparation and characterization of chitosan physical hydrogels with enhanced mechanical and antibacterial properties
Introduction
Chitosan, a linear biopolymer constituted by d-glucosamine and N-acetyl-d-glucosamine units, with the percent of d-glucosamine units exceeds 50%, has shown great potentials as a biomaterial for wound care due to its biocompatibility, nontoxicity, and antimicrobial properties (Boucard et al., 2007; Ong, Wu, Moochhala, Tan, & Lu, 2008; Ribeiro et al., 2009). Particularly, the chitosan hydrogels can follow the geometry of the wound to ensure a good superficial contact, enabling them a promising wound dressing for a variety of skin wounds, such as third-degree burns (Boucard et al., 2007, Jayakumar et al., 2011, Ribeiro et al., 2009; Tamura, Furuike, Nair, & Jayakumar, 2011).
In the past decade, there has been a growing interest in the pure physical hydrogels of chitosan without any residual cross-linkers due to their higher safety and lower cost. Physical hydrogels are held together by molecular entanglements or non-covalent bonds (Ostrowska-Czubenko & Gierszewska-Drużyńska, 2009). Generally, there are two ways to prepare chitosan physical hydrogels. One is to form rigid chitosan hydrogels by evaporating a chitosan acetate salt solution in a hydroalcoholic medium (Boucard, Viton, & Domard, 2005; Montembault, Viton, & Domard, 2005a). The other method first reported by Montembault, Viton and Domard (2005b) is to fabricate physical hydrogels of chitosan by the gelation of a chitosan solution under a gaseous ammonia atmosphere. Nie et al. (2015) proposed that the orientation in and formation of these chitosan hydrogels were due to the entanglements of polymer chains. Due to their good biocompatibility, the physical hydrogels of chitosan have great potentials in tissue engineering and wound dressing (Montembault et al., 2005b, Ribeiro et al., 2009). However, pure physical hydrogels of chitosan formed under a gaseous ammonia atmosphere (CTS/NH3 hydrogels) are limited by their poor mechanical properties. CTS/NH3 hydrogels are usually composited with rigid protective gels to form bi-layered physical hydrogels for its application as a wound dressing (Boucard et al., 2007), which makes the preparation more complicated and reduces the antibacterial activities of the hydrogels. Therefore, CTS/NH3 hydrogels with improved mechanical and antibacterial properties without composition needed is a promise, yet few studies carried out, for the applications of such hydrogels in biomedical field.
Ag+ is active against a wide range of pathogens including multi-drug resistant strains with a far lower propensity for resistance development (Ong et al., 2008) and has been widely used in many antibacterial biomaterials (Ma, Zhou, & Zhao, 2008; Raghavendra, Jung, kim, & Seo, 2016; Wei, Sun, Qian, Ye, & Ma, 2009; Yadollahi, Farhoudian, & Namazi, 2015). Recently, a hydrogel made with chitosan and AgNO3 was reported by Kozicki et al. (2016). However, the AgNO3 concentration had to be ten times as much as or even higher than that of chitosan to form a continuous hydrogel with adequate mechanical stability. These hydrogels could be used for the functional finishing of textiles but are inappropriate as wound dressings due to their high AgNO3 contents that could irritate the wound. In addition, the hydrogel shape is hardly controllable and simply adding AgNO3 into the solutions of chitosan to prepare hydrogel cannot guarantee a uniform composition.
In the current work, AgNO3 was added into chitosan solutions, followed by the gelation of the chitosan solution under an ammonia atmosphere, to achieve a novel chitosan-AgNO3 (CTS-Ag+/NH3) physical hydrogel with controllable shapes and visible sol-gel transitions. The ammonia was used as the gas phase to promote the formation of the chitosan-AgNO3 physical hydrogels. The addition of AgNO3 was aimed to enhance the mechanical properties of the hydrogels and endowed them a better antibacterial property. The strategy not only greatly reduced the AgNO3 content in the hydrogels but also resulted in more uniform composites.
Section snippets
Materials
Chitosan with a deacetylation degree of 90% and an average molecular weight of 2.3 × 106 was purchased from Zhejiang Aoxing Biotechnology Co., Ltd (Shanghai, China). Silver nitrate of analytical grade was purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Analytical grade ammonia solution and acetic acid were purchased from Xilong Chemical Co., Ltd (Guangdong, China).
Preparation of CTS-Ag+/NH3 hydrogels
Homogeneous chitosan solutions at various concentrations ranging from 0.5 wt.% to 3.0 wt.% were prepared by
Formation of CTS-Ag+/NH3 hydrogels
The chitosan-Ag+ mixture was a transparent and stable solution. When it was exposed to the ammonia gas, a transparent hydrogel immediately appeared at the interface between the solution and the ammonia gas and extended to the bottom of the Petri dish. Eventually, a transparent CTS-Ag+/NH3 hydrogel was formed. The color of the hydrogel changed from colorless to light brown with the increasing of Ag+ concentration. The higher Ag+ concentration was, the darker the color of the hydrogel was (Fig. 1
Conclusion
A novel method was developed for the preparation of transparent CTS-Ag+/NH3 hydrogels with improved mechanical properties and antibacterial activities. CTS-Ag+/NH3 hydrogels were prepared by the gelation of chitosan solutions in the presence of AgNO3 under a gaseous ammonia atmosphere. The formation mechanism of the hydrogels was deduced and demonstrated. The enhanced mechanical properties of the CTS-Ag+/NH3 hydrogels were attributed to the entanglement of chitosan chains and coordinate bonds
Acknowledgments
This work was supported by the Youth Talent Plan of Beijing City, the Combination Project of Guangdong Province and the “Yangfan” Innovative Research Team Project of Guangdong Province.
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