Characteristics and Applications of Bacterial Nanocellulose

Abstract

There is an increasing demand for products made from renewable and sustainable non-petroleum based resources. Cellulose, the most abundant polymer on Earth, is renewable, biodegradable, as well as non-toxic. In nature, cellulose is a ubiquitous structural polymer that confers its mechanical properties to higher plant cells. Cellulose is a linear polymer of ß-(1-4)-D-glucopyranose units in 4C1 conformation. Therefore the repeating unit of the cellulose polymer is known to comprise two anhydroglucose rings joined via a ß-(1?4) glycosidic linkage (called cellobiose). The two end-groups of this polymer are not chemically equivalent, since one bears the ?normal? C ?OH group (non-reducing end), whereas the other has a C1?OH moiety in equilibrium with the corresponding aldehyde function (reducing end). It is a very common component of all vegetables, as well as some algae and a few molds. Plants are the main source in combination with hemicellulose and ligning, as a ternary complex present in any cell wall. In addition, there are some strains of the prokaryotic, nonphotosynthetic organisms, which have the ability to synthesize high-quality cellulose organized as twisting ribbons of microfibril bundles. The number of the glucose units or the degree of polymerization (DP) is influenced by the origin and treatment of the raw material.Cellulose is an insoluble molecule consisting of between 2000 - 14000 residues. It forms crystals (cellulose I) where intra-molecular (O3-H?O5´ and O6?H-O2´) and intra-strand (O6-H?O3´) hydrogen bonds holds the network flat allowing the more hydrophobic ribbon faces to stack. Each residue is oriented 180° to the next with the chain synthesized two residues at a time. Although individual strands of cellulose are intrinsically no less hydrophilic, or no more hydrophobic, than some other soluble polysaccharides (such as amylose) this tendency to form crystals utilizing extensive intra- and intermolecular hydrogen bonding makes it completely insoluble in normal aqueous solutions (although it is soluble in more exotic solvents such as N-methylmorpholine-N-oxide (NMNO) or LiCl/N,N´-dimethylacetamide). Water molecules are considered the responsible of catalyze the formation of the natural cellulose crystals by aligning the chains through hydrogen-bonded bridging.Part of a cellulose preparation is amorphous between these crystalline sections. The overall structure is of aggregated particles with extensive pores capable of holding relatively large amounts of water by capillarity. The natural crystal is made up from metastable cellulose I with all the cellulose strands parallel. This cellulose I can be divided into two coexisting phases: cellulose Ia and cellulose Iß, with differing displacements of the chains relative to one another. If it can be recrystallized (e.g. from base or CS2) it gives the thermodynamically more stable cellulose II structure with an antiparallel arrangement of the strands and some intra-sheet hydrogen-bonding.Due to its hierarchical structure and semi crystalline nature, nanoparticles can be extracted from this natural polymer using a top-down mechanically or chemically induced deconstructing strategy or a bottom-up production of cellulose nanofibrils from glucose by bacteria. Cellulosic materials with one dimension in the nanometer range, such as whiskers, microfibrillated cellulose, nanofibrillated cellulose and cellulose nanofibrils or microfibrils are referred to generically as nanocelluloses. These materials combine important cellulose properties such as broad chemical-modification capacity, hydrophilicity, and the formation of versatile semi crystalline fiber morphologies. Nanocelluloses may be classified in three main subcategories: microfibrillated cellulose (MFC, delamination of wood pulp by mechanical pressure before and/or after chemical or enzymatic treatment, 5-60 nm diameter, several micrometers length), nanocrystalline cellulose (NCC, acid hydrolysis of cellulose from many sources, 5-70 nm diameter, 100-250 nm length), and bacterial nanocellulose (BNC, bacterial synthesis, 20-100 nm diameter). While the diameters of NCC and BNC are similar, the main difference between the two types of nanocellulose is their purity and crystal structure. BNC is essentially pure cellulose and NCC is usually a composite itself consisting of both celulose and hemicellulose.