Properties and applications of bacterial cellulose-alginate composite sponges

Abstract

Biodegradable materials have been utilized in many applications because of its environmental friendly and non-toxicity. On the other side, the replacement of fossil fuel by alternative energies and the implantation of culture cells in vivo have been widely concentrated in recent years. Bacterial cellulose (BC) and alginate are natural polymers, which have high potential to be used as support materials for microbial cell immobilization and as a scaffold for tissue engineering. However, there are still some limitations of both biopolymers. For example, without modification of the structure, hydrophilic gels of BC and gelatin have high mass transfer resistance of substrate and product transport. Alginate gel has rather low mechanical strength and it is not stable to be used over a long periods of time. On the other hand, the hydrogen bonds that occur between cellulose chains of BC are very strong and stable, which make BC very hard to dissolve in water or in general organic solvents. Consequently, modification of the structure of pure cellulose is not simple. To gain their beneficial properties and break through the limitations of both biopolymers, the composite sponge of bacterial cellulose-alginate (BCA) was developed. The BCA sponge was fabricated by cross-linking and freeze drying method in order to increase the substrate/product mass transfer and to enhance mechanical properties of the sponge. BCA sponge was applied in ethanol fermentation as a yeast cell carrier. It presented the heterogeneous structure which consisted of the thin layer on the outer and the interconnected porous structure in the interior. The BCA sponge exhibited several advantageous properties, such as high porosity, appropriate pore size, strong hydrophilicity and high mechanical, chemical and thermal stabilities. The results demonstrated that BCA sponge represented a highly effective and stable cell carrier for use in long-term (at least one month) ethanol fermentations. BCA sponge was enhanced its hydrophilicity by UV/ozone treatment to improve ethanol production. The advancing water contact angles of BCA sponge with and without UV/ozone treatment were measured. The hydrophilicity of BCA sponge was assessed using Washburn capillary rise (WCR) method. It was shown that WCR method could be used to estimate the treatment effect on the material by providing the advancing contact angle of the material before and after the treatment. It was shown that due to the etching effect in oxygen plasma application; the roughness of treated BCA was increased, resulting in a decrease of advancing water contact angle. However, oxygen plasma was unable to create hydrophilic functional groups on the surface of BCA. Thus, under batch fermentation, there was no significant difference in ethanol production obtained from the immobilized cell (IC) systems using carriers of treated BCA compared to the untreated one. Furthermore, for tissue engineering application, BCA sponge was fabricated with the adapted method, which was the reversal of the previous one. The highly open and interconnected porous structure throughout the modified BCA (N-BCA) sponge was obtained. The characterizations of N-BCA revealed that N-BCA should be suitable to be used as a scaffold for tissue engineering.