The influence of UV-irradiation on thermal and mechanical properties of chitosan and silk fibroin mixtures
Introduction
Thermal and mechanical properties of materials are very important from the point of view of their potential applications. Natural polymers are widely applied in several fields, such as biomedical and cosmetic ones. They are also widely used in packaging and textile industry. Products made of natural polymer for biomedical applications need to be sterilized. For the sterilization of materials made of polymers both, UV radiation and high temperature can be used.
Silk is a biopolymer widely used in textile production and as biomaterial in medicine [1], [2], [3]. Raw silk consists of 75–83 wt% of silk fibroin, 17–25 wt% of glue-like coating sericine, wax and fats (1.5%), colorants and mineral components (1%), depending on the species, origin and culture conditions [4], [5]. Photochemical stability of silk fibroin was studied previously [6]. It was found that the absorption of silk fibroin in solution increased during UV-irradiation of the sample (most notably between 250 and 400 nm) and a wide peak emerged between 290 and 340 nm with maximum at about 305 nm. The new peak suggested that new photoproducts were formed during UV irradiation of regenerated silk fibroin [6]. SDS–PAGE chromatography showed the alterations of molecular weight of silk after UV exposure.
Chitosan is a natural polymer derived from chitin. Chitin is a polysaccharide, a homopolymer comprised of 2-acetamido-2-deoxy-β-D-glucopyranose units. Chitosan can be obtained from chitin by the deacetylation process. Some units in chitosan chains exist in the deacetylated form as 2-amino-2-deoxy-β-D-glucopyranose. Chitosan is a biodegradable natural polymer with great potential for pharmaceutical applications and cosmetic industry due to its biocompatibility, high charge density, non-toxicity and mucoadhesion [7], [8], [9], [10], [11], [12]. Chitosan itself is a very good material for biomedical applications, but after several modifications of its properties one can obtain versatile biomaterial for cell therapy, tissue engineering and gene therapy [13], [14], [15]. Covalent and ionic cross-linking of chitosan leads to formation of hydrogels which can be used as drug delivery system under pH-controlled conditions [16]. Chitosan derivatives can also be applied in various tissue engineering applications namely, skin, bone, cartilage, liver, nerve and blood vessel [17], [18], [19], [20], [21], [22]. In the scientific literature one can find information about blending of chitosan with other natural polymers and/or synthetic polymers. There are several methods of preparing polymer blends of cellulose, chitin and chitosan with natural and synthetic polymers [23]. Chitosan was blended with other proteins such as soy protein, collagen and silk fibroin [24], [25], [26], [27], [28], [29], [30]. From such the blends mainly biomaterials for applications in wound healing and skin tissue engineering scaffolding have been considered [30], [31].
The formation of scaffolds from chitosan/fibroin blends from aqueous solution is possible but it depends on the pH value [26], [27]. By electrospinning of the silk fibroin/chitosan blends with a chitosan content of up to 30% the continuous fibrous structure of material can be obtained [29].
Chitosan and silk fibroin due to the specific properties of each one may be used to produce synthetic mixture that confer unique structural properties [30], [31].
The aim of our work was to study the mechanical and thermal properties of chitosan and silk fibroin mixtures after treatment with UV-irradiation. We previously studied the influence of UV light on surface properties of thin films obtained by solvent evaporation from chitosan/silk fibroin mixture [31], [32]. Both, the contact angle measurements and the AFM investigations have proved that the surface of chitosan/silk fibroin blend is enriched in silk fibroin component. The contact angle measurements and values of surface free energy have shown that the wettability of chitosan/silk fibroin blended films was changed by UV-irradiation due to photooxidation process [32]. UV-irradiation caused the decrease of surface roughness of chitosan film, silk fibroin film and film made of the blend of chitosan and silk fibroin [32]. For our best knowledge the influence of UV light on the thermal and mechanical properties of chitosan/silk fibroin films has not been studied yet.
Section snippets
Materials and methods
Chitosan powder (CTS) (degree of deacetylation DD = 80% = 1.9 × 105 g/mol) was obtained from Aldrich and used without further purification. The viscosity average molecular weight of chitosan was measured with the Ubbelohde viscometer using 0.2 M sodium acetate and 2% acetic acid as a solvent [33] and calculated from the viscosity of solutions according to the Mark–Houwink–Sakurada equation [34]. The degree of deacetylation (DD) of chitosan was estimated by the conductometric of titration method
Results and discussion
Thermal properties of polymeric materials can be studied by means of thermogravimetric analysis and by differential scanning calorimetry. Thermogravimetric analysis is widely used for characterization of thermal stability of different polymeric materials. Thermal properties of polymers and their composites and/or blends provide valuable information regarding stiffness, toughness, stability and miscibility with other compounds [39], [40]. Miscibility investigations of chitosan (CTS) with silk
Conclusions
Mechanical properties of chitosan/silk fibroin films such as ultimate tensile strength, elongation at break and Young’s Modulus are sensitive to UV irradiation. The ultimate tensile strength and Young Modulus of chitosan films and chitosan/silk fibroin films decrease after UV-irradiation. For high concentration of chitosan in the blend we observed that ultimate tensile strength after 8 h of UV-irradiation is much bigger than for low concentration of chitosan in the blend. Thermal properties of
Acknowledgements
Financial support from the Rector of Nicolaus Copernicus University, Torun, Grant No. 501-Ch and COST Action TD 1305 EC is gratefully acknowledged.
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