Dieter Klemm

Nanocelluloses: A New Family of Nature‐Based Materials

Cellulose fibrils with widths in the nanometer range are nature-based materials with unique and potentially useful features. Most importantly, these novel nanocelluloses open up the strongly expanding fields of sustainable materials and nanocomposites, as well as medical and life-science devices, to the natural polymer cellulose. The nanodimensions of the structural elements result in a high surface area and hence the powerful interaction of these celluloses with surrounding species, such as water, organic and polymeric compounds, nanoparticles, and living cells. This Review assembles the current knowledge on the isolation of microfibrillated cellulose from wood and its application in nanocomposites; the preparation of nanocrystalline cellulose and its use as a reinforcing agent; and the biofabrication of bacterial nanocellulose, as well as its evaluation as a biomaterial for medical implants.

Nanocelluloses as Innovative Polymers in Research and Application

Cellulose is a fascinating and almost inexhaustible and sustainable natural polymeric raw material characterized by exciting properties such as hydrophilicity, chirality, biodegradability, broad chemical-modifying capacity, and the formation of different semicrystalline fiber morphologies. If cellulosics such as bacterial cellulose or strongly disintegrated wood cellulose are composed of nanosized fibers and the nanofiber structuring determines the product properties, these polymers are described as nanocelluloses. 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 long-time 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.

About the structure of cellulose: debating the Lindman hypothesis

The hypothesis advanced in this issue of CELLULOSE [Springer] by Bjorn Lindman, which asserts that the solubility or insolubility characteristics of cellulose are significantly based upon amphiphilic and hydrophobic molecular interactions, is debated by cellulose scientists with a wide range of experiences representing a variety of scientific disciplines. The hypothesis is based on the consideration of some fundamental polymer physicochemical principles and some widely recognized inconsistencies in behavior. The assertion that little-recognized (or under-estimated) hydrophobic interactions have been the reason for a tardy development of cellulose solvents provides the platform for a debate in the hope that new scientific endeavors are stimulated on this important topic.

Formation, derivatization and applications of bacterial cellulose

Acetobacter xylinum produces highly crystalline cellulose extracellularly using glucose as a carbon source. The polymer formed is free of other biogenic compounds, separable in a simple way and characterized by its high water-absorption capacity. Stepwise solvent exchange from water to unpolar solvents leads to a drastic decrease of the water content of the bacterial cellulose without decrease of the highly swollen and activated state. Heterogeneous as well as homogeneous derivatizations, e.g. carboxymethylation, silylation and acetylation, were performed on the wet or dried biopolymer. Furthermore, different methods for formation of hollow fibres during biosynthesis were investigated. Such tubes may have applications as biocompatible material in medicine.

First Synthesis of 3-O-Functionalized Cellulose Ethers via 2,6-Di-O-Protected Silyl Cellulose

The present paper describes the synthesis of 2,6-di-O-thexyldimethylsilyl cellulose as a novel 2.6-di-O-protected cellulose derivative. This material was obtained by reacting cellulose in N,N-dimethylacetamide/ LiCl solution with thexyldimethylchlorosilane and imid azole for 24 h at 100°C. In a typical subsequent reaction the residual OH-group in position 3 could be completely etherified without loss of any protecting groups. Treatment with tetrabutylammonium fluoride leads to the novel compounds 3-O-allyl and 3-O-methyl cellulose. The structures of all polymers are revealed by means of one-( 1 H and 13 C) and two-dimensional (COSY and HMQC) NMR techniques.

Film-forming aminocellulose derivatives as enzyme-compatible support matrices for biosensor developments

Based on 6(2)-O-tosyl celluloses and 6(2)-O-tosylcellulose derivatives, it has been possible to synthesize a novel soluble aminocellulose type, P-CH2-NH-(X)-NH2 (P=cellulose, (X)=alkylene, aryl, aralkylene or oligoamine) with diamine or oligoamine residues at C6 and solubilizing groups (S) such as acetate, benzoate, carbanilate, methoxy and/or tosylate groups at C2/C3 of the anhydroglucose unit (AGU). Depending on the nature and degree of substitution (DS) of (S), the aminocelluloses are soluble either in DMA and DMSO or in water. They all form transparent films from their solutions. In the case of water-soluble aminocelluloses, for example, an enzyme-specific pH value can be adjusted by protonation of the NH2 end groups at C6. The aminocelluloses apparently form aggregates (on a scale of nanostructures) according to a structure-inherent organization principle. The nanostructures could be imaged on the aminocellulose film surface by atomic force microscopy (AFM) in the form of characteristic topographic structures – as a result of the aggregation of the aminocellulose derivative chains and their interaction with the functionalized film support. In this way, structural and environment-induced factors influencing the nanostructure formation were found. The aminocellulose films can be covalently coupled with biomolecules by bifunctional reaction via NH2-reactive compounds. With the aid of analytically relevant enzymes, e.g. glucose oxidase (GOD), horseradish peroxidase (HRP) and others, it was found that the enzyme parameters can be modified by the interplay of the aminocellulose and coupling structures. A number of new bifunctional enzyme coupling reactions, e.g. via L-ascorbic acid or benzenedisulfonyl chlorides, forming amide or sulfonamide coupling structures led to efficient enzyme activities and long-term stabilities in the case of GOD and HRP coupling to PDA cellulosetosylate films.

A novel soluble aminocellulose derivative type : its transparent film-forming properties and its efficient coupling with enzyme proteins for biosensors

By means of the reaction of 6-tosylcellulose derivatives with diamino compounds, it has been possible to provide a new soluble and film-forming aminocellulose-derivative type with a diamine residue at C-6 of the anhydroglucose unit (AGU) and with so-called solubilizing groups, such as acetate, benzoate, carbanilate, methoxy and/or tosylate groups at C-2/C-3. For example, aminocellulose derivatives were synthesized with aliphatic diamine residues of different alkyl chain lengths ((Ch 2 ) m with m = 2, 4, 6, 8, 12) at C-6. Other new aminocellulose derivatives were those with an aromatic diamine residue, e.g., 1,4-phenylenediamine or benzidine residue and others, at C-6. Depending on the structure of the diamine residue at C-6 and of the substituents at C-2/C-3, the aminocellulose derivatives were soluble in various solvents, mostly in N,N-dimethylacetamide (DMA) or dimethylsulfoxide (DMSO). Aminocelluloses with a diaminoethane or diaminobutane residue at C-6 and methoxy groups at C-2/C-3 were soblue in water. All the amino celluloses synthesized formed transparent films from their solutions. These aminocelluloses apparently form superstructures or so-called supramolecular architectures according to a structure-inherent organization principle, which became visible, for example, in the case of PDA cellulose tosylates (in DMA) as gel-like aggregates after approx. 1 week of storage at 4 °C. The superstructures or aggregates could be imaged on the aminocellulose film surface by atomic force microscopy (AFM) in the form of characteristic topographic structures, e.g. as hole structures. In this way, structural and environment-induced factors influencing the aggregate formation were found. The transparent aminocellulose films were excellently suited for covalent coupling with oxidoreductase enzymes such as glucose oxidase (GOD), lactate oxidase (LOD), peroxidase (POD) via bifunctional compounds. A number of new bifunctional enzyme coupling reactions, e.g. via L-ascorbic acid or benzenedisulfonic dichlorides forming amide or sulfonamide coupling structures led to functionally stable nanostructure building blocks with recognition patterns in the case of GOD and POD coupling to PDA cellulose tosylate films. The PDA cellulose derivatives proved to be promising cellulose structural units because the redox-chromogenic PDA residue at C-6 provides the derivatives with a wide range of reaction possibilities, e.g. diazo coupling reactions, NH 2 - reactive coupling reactions and oxidative coupling reactions to redox dyes.

A novel efficient enzyme‐immobilization reaction on NH2 polymers by means of L‐ascorbic acid

A new enzyme‐immobilization reaction by means of L‐ascorbic acid (ASA) is described using NH2 polymers based on cellulose or poly(vinyl alcohol) with the example of oxidoreductase enzymes. In this way, enzyme proteins such as glucose oxidase (GOD), glutamate oxidase, lactate oxidase, urate oxidase and peroxidase can be covalently fixed with a high surface loading to ultrathin and transparent NH2‐polymer films if their surfaces are previously treated with an ASA solution, in, for example, N,N‐dimethyl acetamide, DMSO or methanol. ASA then obviously reacts like a diketo compound with amino groups of the NH2‐polymer film and enzyme protein, forming dehydroascorbic acid derivatives with neighbouring Schiff’s‐base structures. In a subsequent fragmentation reaction, the latter presumably form stable oxalic acid diamide derivatives as coupling structures between enzyme protein and NH2‐polymer film, as suggested by results from investigations of the ASA reaction with n‐butylamine. The immobilized enzymes can be stored at 4 °C in bidistilled water for at least 1 month without becoming detached from the NH2‐polymer film and without diminished enzyme activity. The apparent Km values of the immobilized enzymes are in part clearly smaller than those of the dissolved enzymes or those found in other immobilization processes such as the diazo coupling or the bifunctional glutardialdehyde reaction. For example, the Km value of the immobilized GOD with different NH2 polymers as the matrix structure is smaller by a factor of approx. 20 than that of the dissolved enzyme.

White biotechnology for cellulose manufacturing—The HoLiR concept

A variety of approaches are available for generation of bacteria‐produced nanocellulose (BNC) in different forms. BNC production under static cultivation conditions usually results in fleeces or foils, characterized by a homogeneous, three‐dimensional network of nanofibers and a uniform surface. However, under static cultivation conditions in batch vessels, the widths and the lengths of the BNC sheets cultured are determined by the dimensions of the culture vessel. In this contribution, a novel, efficient process for a (semi‐)continuous cultivation of planar BNC fleeces and foils with a freely selectable length and an adjustable height is presented. By means of comprehensive investigations, the comparability of the BNC harvested to that gained from static cultivation under batch conditions is demonstrated. A first estimation of the production costs further shows that this type of processing allows for significant cost reductions compared to static cultivation of BNC in Erlenmeyer flasks. Biotechnol. Bioeng. 2010. 105: 740–747.

Determination of the substitution patterns of cellulose methyl ethers by HPLC and GLC-comparison of methods

SummaryThe substitution patterns of methyl cellulose as well as of a thexyldimethylsilyl cellulose after permethylation were determined by hydrolysis and separation of the resulting partially methylated glucoses without further derivatization by h.p.l.c.. On an amine-modified silica column the solutes get separated into glucose, 2,3,6-tri-O-methyl glucose and the groups of mono-O-methyl-and di-O-methyl glucoses. A chromatographic run on a reversed-phase column enables the identification of the single mono-O-methyl- and di-O-methyl glucoses. In this way, a determination of both the average degree of substitution and the substitution pattern of cellulose derivatives is possible. Comparison of the results with those obtained by standard methylation analysis including g.l.c.-m.s. proves the correctnes of the method employed.