Loss and non-linearity saturation effects on the bistability of non-linear distributed feedback structures

We investigate theoretically the effects of optical absorption and saturation of the non-linear index of refraction on the bistability of non-linear distributed feedback structures (NLDFBs). By assuming that the Kerr non-linearity saturates in an exponential fashion, we obtain for the first time closed-form expressions for the so-called self-phase and cross-phase modulation terms. Our investigation shows that both absorption and, in particular, saturation significantly affect the bistable properties of this structure and in many cases eliminate this response completely. In some cases, however, saturation alters the NLDFB's transfer characteristics in a potentially useful manner. We find that weak levels of saturation may increase the contrast ratio between the intensities of the high and low bistable states. At increasing levels of saturation, where bistability is no longer observed, we find regimes where the NLDFB structure could possibly be used for optical amplification. This revised version was published online in November 2006 with corrections to the Cover Date.     

The impact of cellulose structure on binding interactions with hemicellulose and pectin

Four cellulose substrates including highly crystalline cellulose nanowhiskers (CNWs) from Gluconacetobacter xylinus (cellulose Iα) or cotton (cellulose Iβ) and amorphous cellulose derived from CNWs (phosphoric acid swollen cellulose nanowhiskers, PASCNWs) were used to explore the interaction between cellulose and well-defined xyloglucan, xylan, arabinogalactan and pectin. The binding behavior was characterized by adsorption isotherm and Langmuir models. The maximum adsorption and the binding constant of xyloglucan, xylan and pectin to any CNWs were always higher than to PASCNWs derived from the same source. The binding affinity of xyloglucan, xylan and pectin to G. xylinus cellulose was generally higher than to cotton cellulose, showing that binding interactions depended on the biological origin of cellulose and associated differences in its structure. The surface area, porosity, crystal plane and degree of order of cellulose substrate may all impact the interactions.     

Roles of xyloglucan and pectin on the mechanical properties of bacterial cellulose composite films

Xyloglucan and pectin are major non-cellulosic components of most primary plant cell walls. It is believed that xyloglucan and perhaps pectin are functioning as tethers between cellulose microfibrils in the cell walls. In order to understand the role of xyloglucan and pectin in cell wall mechanical properties, model cell wall composites created using Gluconacetobacter xylinus cellulose or cellulose nanowhiskers (CNWs) derived there from with different amounts of xyloglucan and/or pectin have been prepared and measured under extension conditions. Compared with pure CNW films, CNW composites with lower amounts of xyloglucan or pectin did not show significant differences in mechanical behavior. Only when the additives were as high as 60 %, the films exhibited a slightly lower Young’s modulus. However, when cultured with xyloglucan or pectin, the bacterial cellulose (BC) composites produced by G. xylinus showed much lower modulus compared with that of the pure BC films. Xyloglucan was able to further reduce the modulus and extensibility of the film compared to that of pectin. It is proposed that surface coating or tethering of xyloglucan or pectin of cellulose microfibrils does not alone affect the mechanical properties of cell wall materials. The implication from this work is that xyloglucan or pectin alters the mechanical properties of cellulose networks during rather than after the cellulose biosynthesis process, which impacts the nature of the connection between these compounds.     

Biochemical localization of a protein involved in synthesis of Gluconacetobacter hansenii cellulose

Using subcellular fractionation and Western blot methods, we have shown that AcsD, one of the proteins encoded by the Acetobacter cellulose synthase (acs) operon, is localized in the periplasmic region of the cell. AcsD protein was heterologously expressed in Escherichia coli and purified using histidine tag affinity methods. The purified protein was used to obtain rabbit polyclonal antibodies. The purity of the subcellular fractions was assessed by marker enzyme assays.     

Synthesis of cationic bacterial cellulose using a templated metal phenolic network for antibacterial applications

Cellulose - Bacterial cellulose (BC) has been an attractive bio-based material for biomedical applications due to its biocompatibility, non-toxicity, ultrafine nanofibril network, robust mechanical...     

Characterization of water-soluble exopolysaccharides from Gluconacetobacter xylinus and their impacts on bacterial cellulose crystallization and ribbon assembly

Gluconacetobacter xylinus has the ability to produce different types of water soluble exopolysaccharides (EPS). Those EPS have different levels of association to bacterial cellulose (BC). At least a portion of the EPS can be released from the BC by 0.1 to 4 M NaOH solution treatments. Hard to extract EPS (HE-EPS) released by 4 M NaOH solutions have been characterized and contain approximately 75 % mannose and 25 % glucose. To study the effect of the EPS on BC synthesis, purified EPS were added to the medium at the start of cultivation and the BC produced was characterized. Results showed that the presence of HE-EPS in the culture medium interfered with the alignment of the BC crystals, but did not reduce crystal size. This is in contrast to similar studies performed using xyloglucan, xylan and glucomannan. The width of the average ribbon increased by 60 % when HE-EPS levels increased in the medium, which indicated that the HE-EPS could also modulate the bundling of cellulose ribbons. Based on the data we propose a mechanism for how HE-EPS alters cellulose formation and assembly. The addition of HE-EPS disturbs the preferential crystal orientation and increases the spacing of cellulose microfibrils without affecting crystallization by associating with ordered cellulose prior to physical aggregation or bundling.     

The impact of antibiotics on bacterial cellulose in vivo

This study investigated how antibiotics, to which Gluconacetobacter hansenii is naturally resistant, impact cellulose crystallinity, allomorph, aggregation into bundles and layers, cellulose yield, and cell morphology. G. hansenii was exposed to 100 μg/mL ampicillin, chloramphenicol, and kanamycin for 7 days, and cellulose structure was analyzed using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. Biomass and cellulose weights were also assessed. Ampicillin increased bundle thickness, and the bundles also showed nodular deposits indicative of non-cellulosic exopolysaccharide deposition. Ampicillin also yielded the lowest amount of cellulose per gram of biomass (p < 0.01) and induced significant filamentation behavior. Chloramphenicol inhibited biomass production (p < 0.01), increased the I-α allomorph content (p < 0.01), and also induced filamentation, though not as profusely as ampicillin. We hypothesize that defects in the peptidoglycan layer and in protein production lowered cellulose yield and promoted cells to undergo filamentation as a survival tactic. Additionally, we hypothesize that antibiotic stress caused additional exopolysaccharides to be produced and that they likely enhanced glucan chain aggregation into higher-order structures. Our findings have significant implications for downstream applications such as genetically engineering G. hansenii to produce bacterial cellulose with modified properties.     

Biosynthesis, production and applications of bacterial cellulose

Bacterial cellulose (BC) as a never-dried biopolymer synthesized in abundance by Gluconacetobacter xylinus is in a pure form which requires no intensive processing to remove unwanted impurities and contaminants such as lignin, pectin and hemicellulose. In contrast to plant cellulose, BC, with several remarkable physical properties, can be grown to any desired shape and structure to meet the needs of different applications. BC has been commercialized as diet foods, filtration membranes, paper additives, and wound dressings. This review article presents an overview of BC structure, biosynthesis, applications, state-of-the-art advances in enhancing BC production, and its material properties through the investigations of genetic regulations, fermentation parameters, and bioreactor design. In addition, future prospects on its applications through chemical modification as a new biologically active derivative will be discussed.     

Effect of different additives on bacterial cellulose production by Acetobacter xylinum and analysis of material property

Bacterial cellulose (BC) demonstrates unique properties including high mechanical strength, high crystallinity, and high water retention ability, which make it an useful material in many industries, such as food, paper manufacturing, and pharmaceutical application. In this study, different additives including agar, carboxymethylcellulose (CMC), microcrystalline cellulose, and sodium alginate were added into fermentation medium in agitated culture to enhance BC production by Acetobacter xylinum. The optimal additive was chosen based on the amount of BC produced. The produced BC was analyzed by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA). Among the evaluated additives, CMC yielded highest BC production (8.2 g/L) compared to the control (1.3 g/L). The results also indicated that CMC-altered BC production increased with CMC addition and reached saturation around 1%. The variation between replicates for all analysis was <5%. From XRD analysis, however, the crystallinity and crystal size decreased as CMC addition increased. FESEM results showed CMC-altered BC produced from agitated culture retained its interweaving property. TGA results demonstrated that CMC-altered BC had about 98% water retention ability, which is higher than BC pellicle produced with static culture. CMC-altered BC also exhibited higher T _max compared to control. Finally, DMA results showed that BC from agitated culture loses its mechanical strength in both stress at break and Young’s modulus when compared to BC pellicle. This study clearly demonstrated that addition of CMC enhanced BC production and slightly changed its structure.     

Structure characterization of native cellulose during dehydration and rehydration

The goal of this study is to investigate the hydration and dehydration induced structural changes of native cellulose. Never dried cotton, and never dried bacterial cellulose with and without added matrix polymer xyloglucan, are examined under the influence of dehydration and rehydration. Significant crystal structure changes were observed in the later stage of drying for both cotton and bacterial cellulose (BC). The 1 % lateral expansion in glucan chain spacing and 17 % decrease of calculated Scherrer dimension were detected for cotton due to the distortion of the structure possibly caused by mechanical stresses associated with drying. No detectable changes on average glucan chain spacings were observed for large BC crystals. However, an average width decrease by 4.4 nm was discovered in the (010) direction, which was more significant than that observed in the (100) and (110) directions. It is hypothesized that co-crystallized elementary fibrils preferentially disassociate along the (010) plane resulting in a significant reduction of crystal width. In the BC-xyloglucan model composite, the presence of xyloglucan does not interfere with the dehydration behavior. Rehydration leads to some structural changes but to a lesser extent than the initial drying. High temperature dehydration induced deformation and crystal size changes are found to be non-reversible due to the removal of the last hydration layer on the cellulose surface.