Concept:
Bacterial cellulose is a form of cellulose resulting from biosynthesis performed by bacteria from the genera Glucanacetobacter, Agrobacterium, Rhizobium, Sarcina, Alcaligenes, etc., from which the most efficient and, consequently, more thoroughly studied is Glucanacetobacter xylinus. It was in the culture medium of these gram-negative, anaerobic bacteria, often found in fruits, vegetables, vinegar and fermented drinks, that A. J. Brown identified for the first time, in 1886, the jelly-like membrane nowadays known as bacterial cellulose and shown in Fig. 1.
Fig. 1 Bacterial cellulose membrane being removed from the culture medium
This biopolymer is synthesized inside bacteria, where enzymes link together individual glucose units, forming long cellulose chains with 2000 to 6000 monomers. These chains are expelled from the cell through about 100 specialized pores present in its cellular membrane and aggregate in groups of 37 units, held together by hydrogen bonds. Thus, elementary fibrils are formed, with an average diameter of 3.5 nm. 46 elementary fibrils align to form a ribbon from 40 to 60 nm wide. Ribbons curl up forming fibers whose width varies between 70 and 80 nm. The cellulose fibers dispersed in the culture medium aggregate and give rise to a gelatinous film that floats on the surface and acts as a support matrix for bacterial population growth. By altering the conditions of the growing medium (nutrients, additives, temperature, etc.) it is possible to control some characteristics of the final product, such as molar mass and supramolecular structure.
The chemical composition of bacterial cellulose is the same as that of vegetable cellulose. Both are linear polymers of β-D-glucopyranose units linked by β( 1 à4) glycosidic bonds, that is, the C-1 of one unit forms a single bond with an oxygen atom which is linked with the C-4 of the consecutive unit, as depicted in Fig. 2.
Fig. 2 Cellulose molecular structure
However, there are significant structural differences between the two: The polymer chains of bacterial cellulose are shorter than those of vegetable cellulose and also more spatially organized, which results in higher crystallinity. Besides, bacterial cellulose is obtained in a pure form, that is, free from other polymers, while vegetable cellulose forms a composite material with lignin and hemicelluloses. Visually, bacterial cellulose resembles a gel, since its water content is around 98% while vegetable cellulose, with a mere 60% water content, has a fibrous appearance.
Applications:
In the food industry
Bacterial cellulose is cooked with caramelized sugar to produce nata de coco, a dessert that is quite popular in some Asiatic countries.
In medicine
An interesting property of bacterial cellulose is that it blocks 99% the UV-A and UV-B radiation while allowing the passage of visible light. Consequently, it is transparent. This characteristic, along with its good biocompatibility, renders bacterial cellulose an attractive material as temporary artificial skin for burning wounds. It is already in the market as the bandage Nexfill®, sold by the Brazilian company Fibrocel. It is a nearly perfect bandage since it adheres perfectly to the exposed epidermis, eliminates pain in a few seconds (through a mechanism yet to be explained), allows the skin to breath, stops the entry of dangerous microbes, protects the wound from the sun and contrary to classical bandages, does not have to be changed since it dries and crumbles after the wound is completely healed. This is only an example among many other applications of bacterial cellulose in medicine, which may in the future include bone and dental implants, artificial blood vessels and molecular sieves for the isolation of DNA strands through electrophoresis.
In the development of new materials
In the search for new, resilient, cheap and sustainable materials able to replace those derived from fossil fuels, bacterial cellulose stands out as a powerful ally, mostly due to the following properties:
- High thermal resistance: it only starts burning at 325ºC;
- High tensile strengts, between 200 and 300 MPa (due to its high degree of crystallinity);
- Low density;
- Biodegradability;
- Material derived from renewable resources.
The list of publications mentioning bacterial cellulose as an excellent reinforcement in composites is long. Often, it is necessary to chemically modify its surface to enhance adherence between this highly hydrophilic material and hydrophobic polymeric matrices. Some of these composites are already being used for airplane and car parts. Bacterial cellulose is also used to produce new textiles, with a special interest in super-absorbent materials, as an additive in papermaking, as a stabilizer is cosmetic emulsions, etc.
Find out more:
Klemm D., 2005. Cellulose: Fascinating Biopolymer and Sustainable Raw Material, Angew.Chem. Int. Ed., 44 (22), 3358 – 3393
Belgacem M. N., Gandini A., Eds. 2008. Monomers, Polymers and Composites from
Renewable Resources; Elsevier: Amsterdam.
