Nanocellulose

In cellulose, crystalline regions coexist with non-ciystalline, amorphous domains statistically alternated along the fibril. Amount of amorphous phase in plant depends on the sample origin and, e.g., for Valonia it is 11%, while for primary walls of plants it reaches more than 70% [5,14]. 

Recently, bacterial cellulose has gained much attention. It is produced by a specific genera of bacteria and does not require any extra purification. What is more, it exhibits some exclusive properties that are not offered by plant cellulose [23]. This type of cellulose will be further discussed in the next subsection. 

Nanocelluloses combine in a very exciting manner important properties of cellulose with amazing features of nanoscale materials and give a start to completely new group of materials. Since interest in renewable raw materials, especially used as reinforcement in biocomposites, is still growing, nanocellulose perfectly fits this research wave and is a subject of many studies. 

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Because of the extraordinary supramolecular structure and exceptional product characteristics as high-molecular and high-crystalline cellulosics with a water content up to 99%, nanocelluloses require increasing attention. This review assembles the current knowledge in research, development, and application in the field of nanocelluloses through examples. The topics combine selected results on nanocelluloses from bacteria and wood as well as their use as technical membranes and composites with the first longtime study of cellulosics in the animal body for the development of medical devices such as artificial blood vessels, and the application of bacterial nanocellulose as animal wound dressings and cosmetic tissues. 

Keywords Nanocelluloses Membranes Composites Medical devices ... 

If cellulosics such as BC are composed of nanosized fibers and the nanofiber structuring determines the product properties, these polymers are described as nanocelluloses. 

In the last years, growing worldwide activity can be observed regarding extensive scientific investigation and increasing efforts for the practical use of the nanocelluloses. An overview of the increase of annually papers on BC since 2000 is presented in Fig. 1. 

As described before, one type of nanocellulose is formed directly as the result of biosynthesis of special bacteria. A very pure product with subsequently reported important properties is formed that necessitates challenging biosynthesis/biotechnological handling and the development of large-scale production. 

Another kind of nanocellulose can be prepared from the nearly inexhaustible source of feedstock wood using controlled mechanical disintegration steps to produce the favored product properties. 

Fig. 2 Shape and structure of BC. a molecular cellulose chain, b scanning electron microscopy (SEM) of freeze-dried nanofiber network (magnification 10000), c pellicle of bacterial nanocellulose from common static culture...

It should also be mentioned that the application of wood nanocellulose prepared by the described techniques - where the cell wall is further disintegrated by mechanical treatment - leads to lower-strength cellulose fiber-reinforced composites than in the corresponding BC materials [34]. 

Membranes and composites from cellulose and cellulose esters are important domains in the development and application of these polymer materials. The most important segment by volume in the chemical processing of cellulose contains regenerated cellulose fibers, films, and membranes, hi the case of the cellulose esters mainly cellulose nitrate and cellulose acetate as well as novel high-performance materials created therefrom are widely used as laminates, composites, optical/photographic films and membranes, or other separation media, as reviewed in [1], The previously specified nanocelluloses from bacteria and wood tie in with these important potentials and open novel fields of application. 

One recent example of the formation and application of foils/membranes of unmodified bacterial nanocellulose is described by George and coworkers [35]. The processed membrane seems to be of great relevance as a packaging material in the food industry, where continuous moisture removal and minimal-oxygen-transmission properties play a vital role. The purity, controllable water capacity, good mechanical stability, and gas-barrier... 

Compared to nanocellulose from wood, BC has the major advantage of modifiability during biosynthesis by simple addition of water-soluble compounds to the culture medium (in situ modification). 

Moreover, the nanosized fibers of the swollen nanocelluloses can be coated with different components and the pore system can be loaded with agents (post-modification). Further methods in this field use well-known procedures... 

The cultivation of BC in the presence of N-acetyl glucosamine (GlcNAc) causes a variation of the polymer formation by the insertion of GlcNAc units. It is possible to produce very thin membranes of the nanocellulose-chitin hybrid formed in this way [36]. 

There have been numerous investigations into the subsequent modification of bacterial and wood nanocelluloses. The additives range from other polysaccharides, albuminoids such as gelatine, different types of monomers and synthetic polymers, to metals, metal oxides, and inorganic fibers. On the... 

Yano and Nakahara [15] used accessory polysaccharides to form composites with wood MFC/nanocelluloses. The disintegrated wood celluloses were mixed with starch as a binder and then hot-pressed between porous metal plates. Using a starch content of 2 wt %, the bending strength reached 310 MPa, compared to 250 MPa for unmodified fibers. Concurrently, the Young s modulus decreased from 16 to 12.5 GPa. When the starch content was 20 wt %, the bending strength decreased to 270 MPa. This indicates that added starch may act not only as a binder but also as a plasticizer. 

To improve the thermal, mechanical, and viscoelastic properties of cellulose acetate butyrate, it was reinforced with nanocellulose crystals prepared from BC by acid hydrolysis. Using this nanosized cellulose (Sect. 1) a significant improvement in the properties of the composites was demonstrated [57]. 

Katagiri [62] investigated the combination of silica-alumina fibers and wood MFC/nanocellulose (95/5 wt %). A further development is oil-retaining sheets with good durability for cleaning rolls [63], which are made from 100 parts inorganic fibers to 5 parts wood MFC/nanocellulose. These sheets are characterized by a weight of 36.3 g/m2, thickness of 0.21 mm, void volume of 92%, and silicone and oil retention capacity of 0.74 g/cm3. 

The following subsections describe techniques for implantation, biological reactions in animal body as well as histological and ultrastructural investigations of incorporated BC. The first long-time study of nanocellulose in the... 

The wall of the BASYC tubes consists of BC loaded with water in the nanofiber network up to 99%. The hollow space of the material transports water, monovalent ions and small molecules, but not biopolymers or corpuscular blood constituents. The stored water not only stabilizes the cellulose network, but also contributes to the tissue- and hemocompatibility of the nanocellulose device. 

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