Bacterial cellulose: a high-performance green new material synthesized by microorganisms

In nature, cellulose is the most abundant natural polymer material. The well-known plant tissues such as cotton, wood, and straw are rich in cellulose. But few people know that in addition to plants, certain microorganisms can also efficiently synthesize cellulose . This kind of cellulose secreted by microorganisms is called bacterial cellulose ( BC ) . Although it has the same molecular structural units as plant cellulose, it has shown irreplaceable value in many fields such as medical care, food, and new materials due to its superior performance and more flexible synthesis methods. It has become a research hotspot in the fields of materials science and biotechnology in recent years.

1. The nature and synthesis mechanism of bacterial cellulose

Bacterial cellulose is a general term for cellulose synthesized by microorganisms such as Acetobacter, Agrobacterium, and Rhizobium under specific conditions. The most representative one is Gluconacetobacter (formerly known as Acetobacter xylinum). It not only has the strongest cellulose production capacity, but is also an ideal model strain for studying the mechanism of cellulose synthesis. From a molecular level, bacterial cellulose is essentially a linear homogeneous polysaccharide composed of β-(1→4)-D- glucan chains, which is consistent with the molecular structure of plant cellulose. However, the synthesis process completely relies on the metabolic activities of microorganisms without relying on photosynthesis. It is a green material ' made by microorganisms ' .

The synthesis process is a multi-step reaction precisely controlled by the cellulose synthase complex: first, the microorganism uses water-soluble carbon sources such as glucose as raw materials to synthesize the cellulose precursor uridine diphosphate glucose; then, the cellulose synthase complex transfers glucose residues from the precursor to the newly generated polysaccharide chains to form β-(1→4)-D- glucan chains; these glucan chains are secreted extracellularly through the bacterial outer membrane, and finally form a unique supramolecular texture structure through self-assembly, crystallization and interweaving. It is worth noting that the movement of bacteria when synthesizing cellulose will control the accumulation and arrangement of microfibers. The free movement in the three-dimensional direction allows it to form a developed fine network structure, which is also the core source of many of its excellent properties.

Human beings have known bacterial cellulose for a long time. As early as 'Qi Min Yao Shu', there is a record of the formation of a gelatinous bacterial film on the surface of the fermentation liquid during the vinegar brewing process. This bacterial film is essentially bacterial cellulose. 1976In 2008, Brown and his collaborators first described the movement characteristics of acetic acid bacteria during cellulose synthesis. They found that the movement rate of bacteria when secreting cellulose microfibers at 25°C can reach 2.0microns / minute, which is equivalent to connecting 10⁸ glucan molecules into polysaccharide chains per hour. Its synthesis efficiency is amazing.

2. Core characteristics of bacterial cellulose

Compared with plant cellulose, bacterial cellulose does not need to go through plant extraction and chemical decontamination processes, and the synthesis process can be precisely controlled. It has a series of unique and excellent properties. These characteristics make it different from traditional cellulose materials and become the first choice for high-end applications:

• High purity and high crystallinity : Bacterial cellulose does not contain lignin, pectin, hemicellulose and other accompanying products in plant cellulose, and has extremely high purity; its crystallinity can reach 95% , which is much higher than the 65% of plant cellulose , which makes its structure denser and its mechanical properties better.

• Ultra-fine three-dimensional network structure : The single microfibers of bacterial cellulose are only 3 to 4 nanometers in diameter, and are combined into fiber bundles with a thickness of 40 to 60 nanometers. These fiber bundles are intertwined with each other to form a developed ultra-fine three-dimensional network structure with a large specific surface area, which provides the basis for its excellent water holding capacity and breathability.

• Excellent mechanical properties : Its elastic modulus is several times to more than ten times that of ordinary plant fibers. It has high tensile strength and can maintain good structural stability even in wet conditions. This characteristic makes it extremely advantageous in wet processing and application scenarios.

• Extremely strong water-holding capacity : The water-holding capacity of undried bacterial cellulose is as high as over 1000% . Even after freeze-drying, the water-holding capacity is still over 600%. After drying ；at 100°C , its reswelling ability in water is comparable to that of cotton linters, making it suitable for scenarios that require moisturizing and locking in water.

• Excellent biocompatibility and degradability : Bacterial cellulose is non-toxic and harmless to the human body and the environment, has excellent biocompatibility, can be completely degraded by microorganisms, does not produce microplastic pollution, and is in line with the concept of green and sustainable development.

• Synthesis controllability : By adjusting culture conditions and adding different water-soluble polymers or glucose derivatives, the microstructure, aggregation behavior and functional properties of bacterial cellulose can be adjusted to achieve ' customized ' synthesis.

3. Preparation method of bacterial cellulose

The preparation of bacterial cellulose mainly relies on microbial fermentation technology. The core is to select high-yielding strains, optimize culture conditions and post-processing processes. Currently, there are three main culture methods commonly used, each with its own emphasis and suitable for different application scenarios and production scales:

(1) Commonly used bacterial strains and culture conditions

In industry and laboratories, high-yielding strains such as Gluconacetobacter are mainly used to produce bacterial cellulose. The suitable culture conditions are: the temperature is controlled at 28~30°C and ，the pH value is maintained at 5.0~6.0 . The culture medium uses glucose, sucrose, fructose syrup, etc. as carbon sources, and corn steep liquor, yeast extract powder, etc. as nitrogen sources to provide sufficient nutrients for bacterial growth and cellulose synthesis.

(2) Main cultivation methods

1. Static culture method : This is the most classic preparation method. Add the inoculum solution to a shallow dish or Erlenmeyer flask and let it stand for 5 to 10 days. The bacteria will form a cellulose film at the interface between the culture solution and the air. The advantages of this method are high product purity and complete network structure, which is suitable for laboratory research and preparation of medical membrane materials; the disadvantages are low output and large floor space, making it difficult to achieve large-scale production.

2. Shaking / stirring culture method : Through shaking or mechanical stirring, sufficient oxygen is provided to the bacteria to promote their rapid reproduction. The bacterial cellulose produced by this method is in the form of particles or clusters, has high volume productivity, and is easy to scale up production; however, the shear force generated by stirring may inhibit the formation of cellulose, and stirring conditions need to be optimized to balance yield and quality.

3. Tank fermentation method : This is the current mainstream method of industrial production. The fermentation tank accurately controls the pH value (maintained at 5.0~5.5 ) and dissolved oxygen (maintained above 30% ), and adopts a feeding strategy to avoid sugar suppression, achieving efficient and continuous large-scale production. The advantages are high production efficiency and stable product quality; the difficulty is that the process is complex and requires high equipment and operation.

(3) Post-processing process

The harvested bacterial cellulose needs to undergo a series of treatments before it can be put into use: first, alkaline washing ( treatment with 0.5~1.0M NaOH at 80~95°C for 1~2 hours) to remove residual bacterial cells and impurities; then repeated washing with water to neutrality to remove residual alkali liquid; finally, according to application requirements, drying by natural air drying, freeze drying or supercritical CO₂ drying; if necessary, it can also be modified through cross-linking, compounding and other modifications to improve its mechanical properties and functional diversity.

4. Multi-field applications of bacterial cellulose

With its unique structure and excellent performance, bacterial cellulose has broken through the application boundaries of traditional cellulose and has been industrialized in many fields such as medical care, food, papermaking, and high-end materials, showing broad market prospects:

(1) Medical field: green and safe biomedical materials

The high biocompatibility, high wet mechanical strength and good air and liquid permeability of bacterial cellulose make it an ideal medical material. Currently, a number of bacterial cellulose products have been clinically applied, such as Biofill® and Gengiflex® . The former can be used as artificial skin for temporary dressing of second- and third-degree burns and ulcers to promote wound healing; the latter is used for the recovery of periodontal membrane tissue. In addition, based on its in-situ plasticity, researchers have also developed an artificial blood vessel material that can be used for microsurgery BASYC® , , and is expected 3Dto achieve breakthroughs in the fields of printing vascular stents and drug carriers in the future.

(2) Food field: natural and healthy functional base materials

Bacterial cellulose has good hydrophilicity, viscosity and stability. It can be used as a food thickener, molding agent, dispersant and dietary fiber, and is widely used in food processing. For example, Japan's popular dessert 'Nata de coco' (coconut) is a bacterial cellulose product made by fermenting acetic acid bacteria and rice flour sugar. In recent years, our country's scientific research team has also innovatively combined medicinal and edible ingredients such as red dates, ginger, and mugwort leaves with bacterial cellulose. Through enzymatic hydrolysis and two-step fermentation processes, new foods with both dietary fiber and health care functions have been prepared. It has antioxidant, lipid-lowering and sugar-lowering, intestinal conditioning and other functions, which meets consumers' demand for natural and healthy foods.

(3) Papermaking and high-end materials fields: key additives to improve product performance

In the papermaking industry, adding bacterial cellulose to pulp can significantly improve the strength, durability and water resistance of paper, and solve the problem of reduced fiber strength after waste paper recycling. Japan's Ajinomoto Company cooperated with Mitsubishi Corporation to use bacterial cellulose to develop special-grade paper for printing U.S. dollars. Its water-resistant and high-strength properties are widely recognized; the modified bacterial cellulose can also be used in high-end writing paper to improve ink absorption uniformity and adhesion. In addition, bacterial cellulose can also be used to make filter adsorption materials to increase the adsorption capacity of carbon fiber plates and reduce filler leakage.

(4) High-end audio and textile fields: new breakthroughs in innovative applications

The high purity, high crystallinity and high molecular orientation of bacterial cellulose give it excellent mechanical properties and sound conduction properties. After hot-pressing treatment, its Young's modulus can 30reach GPa , 4which is several times stronger than organic synthetic fibers. It is suitable for use in diaphragms of high-end speakers. Japanese company Sony cooperated with Ajinomoto to develop diaphragms for speakers, microphones and headphones made of bacterial cellulose. The sound transmission speed is fast and the internal consumption is high in a wide frequency range. The reproduced sound is clear and loud, and its performance is better than traditional aluminum and pine paper diaphragms. In the field of textiles, China's scientific research team has developed the world's first bacterial cellulose dress, which does not require traditional spinning and weaving processes. The production efficiency is 36twice that of traditional processes. The fabric is skin-friendly, breathable and biodegradable, attracting the attention of European and American luxury brands.

(5) Other emerging fields: Promoting green development

With the continuous advancement of technology, the application fields of bacterial cellulose continue to expand. In the field of new energy, it can be used as a battery separator to improve energy storage performance; in the field of agriculture, it can be used as an agricultural synergist to increase crop yields by more than 20% while reducing the use of chemical fertilizers and pesticides; in the field of environmental protection, it can be used to absorb oil spills at sea and help control marine pollution; in the field of plastic substitution, by compounding with materials such as polylactic acid, high-performance biodegradable plastics can be prepared, providing a green solution for the ' plastic ban ' policy.

5. Development status and future prospects of bacterial cellulose

At present, bacterial cellulose has achieved a certain scale of industrial production, and many countries around the world have deployed related industries. China 20has reached the international leading level in the fields of nano bacterial cellulose preparation and development of homologous functional products for medicine and food. It has the world's only 20nano biocellulose factory and has established a complete independent intellectual property system. But at the same time, the industrial development of bacterial cellulose still faces some challenges, such as high production costs, output needs to be improved, and some high-end application technologies are not yet fully mature.

In the future, the development of bacterial cellulose will focus on three core directions: First, optimize the production process, reduce production costs and increase output by screening high-yielding strains, developing low-cost carbon sources (such as using industrial waste as culture media), and improving fermentation technology; second, strengthen functional modification, through in-situ modification, complexation Combining technologies such as antibacterial, conductive, magnetic and other new functions, and expanding their applications in emerging fields such as flexible electronics and sensing materials; third, promoting the integration of multiple industries and deepening applications in medical, food, new energy, environmental protection and other fields to help achieve the goals of ' plastic substitution ' and ' carbon neutrality ' .

As a new green material with both natural properties and engineering properties, bacterial cellulose's ' microbial synthesis ' characteristics break the limitation of traditional cellulose relying on plant cultivation and achieve a ' green, efficient and sustainable ' production model. With the continuous breakthroughs in technology, this magical material ' woven ' by microorganisms will surely play an important role in more fields and provide new material support for the green development of human society.