Chapter 2

Bacterial Cellulose-Based Nanocomposites: Roadmap for Innovative Materials

Ana R. P. Figueiredo, Carla Vilela, Carlos Pascoal Neto, Armando J. D. Silvestre and Carmen S. R. Freire*

Department of Chemistry, CICECO, University of Aveiro, Aveiro, Portugal

*Corresponding author: cfreire@ua.pt

Abstract

In the last decades there has been an increasing awareness in the search for biobased alternatives as sources of novel nanocomposites for application in several fields such as packaging, biomedical products and devices, as well as in high-technology domains. Nanocellulose forms like bacterial cellulose (BC), biosynthesized by several bacteria as a 3D network of nano- and micro-fibrils, have gained particular attention in this context because of their unique features, namely high purity, water-holding capacity, crystallinity, tensile strength and Young's modulus, that can be successfully exploited in the development of innovative nanostructured composite materials. In this chapter, a comprehensive overview on the production, processing, properties and applications of bacterial cellulose-based nanocomposites is compiled and discussed. A vast collection of BC nanocomposites such as those with other natural polymers, thermoplastic matrices and inorganic nanophases will be addressed, aiming to demonstrate the real potentialities of BC in this domain.

Keywords: Bacterial cellulose, nanocomposites, polymer composites, hybrid materials, inorganic nanoparticles

2.1 Introduction

Cellulose is the most abundant biological macromolecule on Earth, with about 1.5 × 1012 tons produced each year and a high economic importance in the pulp and paper as well as textile industries [1-3]. Most cellulose is obtained from plants, where it represents the main structural element of cell walls; but it is also produced by a family of sea animals called tunicates, several species of algae and some aerobic nonpathogenic bacteria, as well as through enzymatic and chemical methods [1]. Regardless of its origin, cellulose is a linear homopolymer of ß-D-glucopyranose units linked by ß-(14) glycosidic bonds, varying essentially on purity, degree of polymerization and crystallinity index [4]. Bacterial cellulose (BC) was first reported by Adrien Brown in 1886. While studying acetic fermentations, he noticed the formation of a white gelatinous pellicle on the surface of a liquid medium, which had the capability to grow to a thickness of 25 mm and proved to be very strong and tough. Brown also verified that this membrane was generated by a bacterium, initially named Bacterium xylinum, but later classified as Acetobacter xylinum and currently termed Gluconacetobacter xylinus. Further research studies showed that this material had the same chemical composition as the cellulose produced by plants, and until today bacterial cellulose remains as the most pure existing natural form of cellulose [5, 6].

Several other species of bacteria of the genera Gluconacetobacer, Sarcina and Agrobacterium have been reported as cellulose producers [4]. However, only Gluconacetobacter species can produce cellulose at commercial levels. Gluconacetobacter xylinus remains as a model strain and is used in research and commercial production [7]. It is a nonpathogenic, rod-shaped, obligate aerobic Gram-negative bacterium capable of producing relatively high amounts of cellulose from several carbon and nitrogen sources [4, 8]. Such bacteria are ubiquitous in Nature, being naturally present wherever the fermentation of sugars takes place, for example, on damaged fruits and unpasteurized juices, beers and wines [8]. Recently, we have also reported the production of BC by a Gluconoacetobacter sacchari strain using different carbon sources with yields comparable to those obtained with G. xylinum [9]. In the latter decades, the use of BC gained considerable attention in the global scenario of increasing awareness and demand for biobased environmentally-friendly functional materials because of its inherent abundance, renewability, biodegradability, biocompatibility and specific features (particularly the nanometric dimensions and nanostructured network). The creation of nanocomposites with diverse partners, as synthetic and natural polymers, bioactive compounds as well as inorganic NPs constitutes a wide field of BC research and development, as tentatively revised by some authors in the latter years [10-13]. For instance, Shah et al. [11] compiled representative methodologies for BC composites preparation, some classes of BC composites and their applications, Fu et al. [12] summarized the current investigation on BC-based materials for skin tissue repair and Hu et al. [13] collected relevant results on functionalized BC derivatives and composites. However, the domain of BC-based composites is exceptionally vast and in continuous innovation. Therefore, the aim of this chapter is to present a comprehensive, yet detailed, overview of most relevant results obtained on the production and properties of BC, and in particular on the design and applications of BC composites with different partners.

2.2 Bacterial Cellulose Production, Properties and Applications

2.2.1 Bacterial Cellulose Production

Bacterial cellulose synthesis by G. xylinus starts with the production of individual ß-(14) chains between the outer and plasma membranes of the bacterial cell. A single G. xylinus cell may polymerize up to 200000 glucose molecules per second into ß-(14) glucan chains, followed by their release outwards through pores in the cell surface [14]. BC chains then assemble into protofibrils, with approximately 2-4 nm of diameter, that further gather into microfibrils of approximately 3-15 nm thick and 70-80 nm wide [1, 4, 15, 16]. Microfibrils, in turn, entangle into a ribbon of crystalline cellulose whose interwoven produces the BC fibrous network (Figure 2.1) [5, 8, 17, 18]. The reason why these bacteria generate cellulose is still unclear, but it has been suggested that it is a mechanism that bacteria use to maintain their position close to the surface of the culture medium, where there is a high oxygen content; and also serves as a protective coating against ultraviolet radiation, prevents the entrance of enemies and heavy-metal ions whereas nutrients diffuse easily throughout the pellicle [1, 19].

Figure 2.1 Scanning electron microscopy (SEM) images of Gluconacetobacter xylinus and BC network of micro and nano fibrils; and schematic description of the formation of bacterial cellulose.

Reproduced with permission from [7].

BC has many applications which have triggered high interest in its production at a commercial scale. However, the main problems that hamper this process are the low yield and production costs, especially for low added value applications. Therefore, some attempts have been made in terms of optimization of culture conditions and medium composition, as well as the scaling-up process [14]. Bacterial cellulose is commonly produced using the Hestrin-Schramm (HS) medium, that uses glucose as the carbon source and a combination of peptone and yeast extract as nitrogen sources [14]. However, the use of glucose as carbon source for BC production is quite expensive and causes the formation of by-products such as gluconic acid that decreases the pH of the culture medium and ultimately declines the production of BC [4, 8]. Therefore, researchers have investigated the capability of G. xylinus to grow and produce BC using different carbon sources. Besides glucose and sucrose (the most commonly used), other carbohydrates such as fructose, maltose, xylose and starch, and polyols as glycerol have also been successfully tested [4].

In addition, other efforts have been also devoted to the identification of cheap feedstocks as alternatives to the expensive conventional culture media, with pure compounds, for the economically viable production of BC [20]. In this context, several industrial wastes have already been effectively explored for the production of BC, as for example tea infusions [21], wheat straw acid hydrolysate [22], grape bagasse and crude glycerol [23], beet molasses [24], sugar-cane molasses and corn steep liquor [20], Konjac powder [25], fruit juices, including orange, pineapple, apple, Japanese pear and grape [26, 27], grape skins aqueous extract and sulfite pulping liquor [28], and dry olive mill residue [29]. The development of culture media based on cheaper feedstocks will simultaneously allow the production of BC at lower price and the valorization of the residues themselves [29].

Enhancement of BC production has also been attempted through supplementation of the culture medium with different additives. Several chemicals including alcohols [30], vitamin C [31], lignosulfonates [32], water-soluble polysaccharides [33-35], thin stillage from rice wine distillery [36] have been investigated in this perspective. For instance, Lu et al. [30] investigated the stimulatory effects of six different alcohols, added at different concentrations, during static fermentation of G. xylinus 186. All alcohols tested improved BC production and could be ranked as n-butanol > mannitol > glycerol > ethylene glycol > methanol > n-propanol. However, results showed that n-butanol only improves BC production when added at concentrations lower than 1.5% v/v (maximum production of 132.6 mg/100 ml, 56.0% higher than the control), while mannitol stimulates BC production at any...