Preparation and characterization of environmentally friendly agar/κ-carrageenan/montmorillonite nanocomposite hydrogels
Graphical abstract
Introduction
Hydrogels, three dimensional cross-linked polymer networks, are able to absorb high capacity of water or physiological fluid because of their hydrophilic properties [1]. The hydrophilicity of the network is arised from the presence of chemical groups such as hydroxyl (OH), carboxyl (COOH), amide (CONH), sulphonic acid (SO3H) that can be found within the polymer chains. Besides the hydrophilicity, the swelling capacity of hydrogels increases with the the availability of large free volume between polymeric chains and high polymer chain flexibility. The swelling capacity of hydrogels is an important feature to absorp wound fluids by them in the case of the usage of hydrogels as a wound dressing. These polymeric systems have recently attracted considerable attention owing to their unique properties such as biocompatibility, tunable biodegradability and mechanical strength, flexibility, softness and porous structure [[2], [3], [4]].
Agar and κ-carrageenan are two important natural polysaccharides and potential candidate in the preparation of new hydrogel materials with non-toxicity, biodegradability and high water absorption capacity. Agar is derived from the cell wall of red seaweeds. It is a hydrophilic mixture of β-1,3-d-galactose and α-1,4-linked 3,6-anhydro-l-galactopyranose repeating units. Agar consists of agarose and agaropectin fractions. It has been used in the food industry for more than 350 years [5,6]. In addition, agar is used in pharmaceutical, cosmetic, medical, chemistry and biotechnology industries as gelling, thickening, water holding and stabilizing agents [7,8]. Carrageenans, a family of linear sulfated polysaccharides with high molecular weight, are extracted from red seaweeds. They composed of repeating d-galactose and 3,6-anhydro-d-galactose units [9,10]. Carrageenans possess the ability to form various types of gel structures depending on the the number and location of the ester sulfate groups on the repeating galactose units. Among the carrageenan types, κ-carrageenan has only one sulphate group per disaccharide, while iota-carrageenan has two and lambda carrageenan has three sulphates per disaccharide [11]. From meat and dairy products to baby foods, carrageenan has wide range of applications as thickening and stabilizing agent [12]. Furthermore, due to its good gelling property and mechanical strength, carrageenan is widely used in the cosmetic, personal care, textile industry, medical care products, pharmaceutical industries and controlled drug delivery systems [13,14].
The development of new composite materials with good mechanical, thermal and barrier properties from biodegradable and natural polymers is promising for the versatility of environmental friendly compounds in various fields. In literature, Derkach et al. [15] synthesized a series of hydrogels with different molar ratios of κ-carrageenan and gelatin. They observed that as the carrageenan content was increased in the hydrogel composition, gelation rate, viscoelastic parameter values, strength and melting temperature of hydrogels enhanced significantly. In another study carried out by Yin et al. [16], agar and alginate based pH-responsive composite hydrogels were prepared for drug delivery. They stated that the content of drug loading and mechanical stability of hydrogels increased by the addition of agar into the hydrogel structure. Nanocomposite materials prepared using the combination of clay and biodegradable natural polymers have become attractive in recent years because of their good mechanical, thermal and barrier properties and also environmental concern [17]. These ecofriendly and low cost materials are promising for all-round applications in various fields such as wound management [18], tissue engineering [19], drug delivery [20] and actuator systems [21].
Many research groups have reported that the addition of clay as nanofiller into the hydrogel matrix has improved some properties of nanocomposite hydrogels [22]. For example; Chang et al. [23] prepared PEG/laponite nanocomposite hydrogel for tissue engineering applications. They reported that as the concentration of clay in the hydrogel increases, strong physical interaction between PEG chains and laponite occurs and increases the overall crosslinking density in the hydrogel, which decreases the swelling of hydrogels by preventing water uptake due to the suppression of the osmotic effect of laponite. In another study conducted by Takeno et al. [24], the swelling properties of clay/sodium polyacrylate hydrogels were investigated. They observed that clay concentration within hydrogel matrix significantly change the swelling amounts of the hydrogel materials.
Montmorillonite (MMT) is naturally abundant, non-toxic and cheap reinforcing material. It belongs to the 2:1 type of phyllosilicate group and is the most widely used mineral with the general formula [(Na, Ca)0.33 (Al, Mg)2 (Si4O10) (OH)2. nH2O] in the preparation of polymeric nanocomposite materials [25,26]. In the preparation of nanocomposite structures, the modification of MMT is very important. In this study, a natural and non-toxic amino acid, phenylalanine, was used for the modification of MMT.
Until now, many polysaccharide-based hydrogel materials have been prepared. On the other hand, the preparation of new hydrogel materials with high absorption capacity from nontoxic and biodegradable agar and κ-carrageenan polysaccharides and the optimization of reaction conditions for the agar/κ-carrageenan hydrogels were not studied previously. In this study, for the first time, agar/κ-carrageenan and agar/κ-carrageenan/MMT hydrogels were prepared. Furthermore, the swelling behaviors of agar/κ-carrageenan hydrogels can be controlled by initiator and crosslinking agent concentrations, reaction temperature, polysaccharide ratio and MMT concentration. Therefore, the effect of these parameters on the swelling behavior and surface property of agar/κ-carrageenan hydrogel material was determined in detail due to the importance of swelling capacity for the absorption of wound fluids when the hydrogels would be used as a wound dressing.
Section snippets
Materials
Agar and κ-carrageenan used as polysaccharides, ammonium persulfate (APS) and N,N,N’,N’-tetramethylenediamine (TEMED) used as initiator-accelerator pair, tri (ethylene glycol) divinyl ether (TEGDE) used as a crosslinking agent in the synthesis of hydrogels were purchased from Sigma-Aldrich. Montmorillonite (MMT) used in the preparation of clay-containing hydrogels was provided by Süd-Chemie AG (München, Germany) under the trade name Nanofil 116. L-phenylalanine amino acid used in the
Synthesis mechanism of agar/κ-carrageenan hydrogels
Agar and κ-carrageenan were simultaneously crosslinked in a homogeneous medium using APS and TEMED as redox pair initiator and TEGDE as the crosslinker. The general reaction mechanism for crosslinking of agar and κ-carrageenan through free radical crosslinking reaction in the presence of TEGDE is shown in Fig. 1.
Crosslinking reaction is initiated by addition of APS which produces negatively charged sulphate radicals by decomposing with temperature. The sulfate anion radical takes hydrogen from
Conclusions
In this study, a series of agar/κ-carrageenan based-nanocomposite hydrogels were prepared by using a free-radical crosslinking reaction of agar and κ-carrageenan in the presence TEGDE. For the preparation of agar/κ-carrageenan hydrogel having the highest swelling capacity (2523 %), optimum reaction conditions were determined to be [APS] = 5 × 10−4 M, [TEGDE] = 5 × 10−4 M, reaction temperature = 70 °C and magar : mκ-carrageenan = 1: 4. Swelling degree of hydrogels decreased with increasing MMT
CRediT authorship contribution statement
Tülin Gürkan Polat: Investigation, Methodology, Data curation. Osman Duman: Conceptualization, Writing - review & editing. Sibel Tunç: Conceptualization, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
None.
Acknowledgements
This work was supported by the Scientific Research Projects Coordination Unit of Akdeniz University (Project Number: FDK-2015-398). We would like to thank to central laboratory of METU (Middle East Technical University) for SEM analysis. Furthermore, we thank Scientific and Technological Research Council of Turkey (TUBITAK) for the generous support under 2214/A fellowship. The authors thank Prof. Dr. Ozan Akkus (Case Western Reserve University, Department of Mechanical and Aerospace
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