BC membrane

For the purification of the membranes, the common treatment with 0.1-2.0 N aqueous sodium/potassium hydroxide solutions has been compared with the purification of raw BC membranes using aqueous sodium/ potassium carbonate. Whereas hydroxide purification leads to a decrease of the tensile strength and an elongation of the material this effect is lower in the case of carbonate treatment. Moreover, the oxygen transmission rate is higher after carbonate washing. 

To open up new application fields, the structure and properties of BC membranes/composites can be modified by low-molecular organic and inorganic compounds, including monomers and metals/metal oxides, via carbohydrates and polysaccharides, up to different types of other natural and synthetic polymers. 

A survey of the large variety of technical uses of further BC membranes is shown in Table 1. A more detailed discussion of typical examples of these nano celluloses applications is given in the following. 

Because of its hydrophilic nature even unmodified BC shows great potential to separate azeotropes such as EtOH/FbO. It adsorbs seven times more water than ethanol. This selectivity and a reasonable flux increase with growing temperature and thinning of the membrane. In addition, the BC membranes also show a high water affinity in aqueous binary mixtures of organic solvents. 

Fig. 12 SEM of membrane surfaces, a unmodified BC, b to d poly(acrylic acid)-modified BC membranes formed by time-dependent UV irradiation (b 5 min. c 10 min. d 20 min). Reprinted with permission from [56]...

Later the same research group claimed that, in the first step of the in situ synthesis scheme, ionic functional groups as such are not necessary for the introduction of ferrous ions into a cellulose matrix [161,162], This suggestion was made based on a comparative study of ferrite synthesis between a case with anionically modified cellulose materials and the other case with non-ionic cellulose gels, which included a never-dried bacterial cellulose (BC) membrane and a never-dried cellulose wet-spun filament or cast film (Lyocell) using N-methylmorpholin-N-oxide as the solvent. SPM proper-... 

Dubey et al. [74] improved the BC membrane by the impregnation of chitosan [Mw 100-300 kDa]. The composite membrane was dried under vacuum. The potential of the composite membrane for the pervaporative separation of the ethanol/water azeotrope was comparable to that of a polyvinyl alcohol [PVA] membrane [74]. The normalized flux, selectivity, and PSI of the composite membrane were 42.8 kg pm m h , 9.2 and 350 kg pm m h , respectively. Compared with the PVA membrane, BC membrane impregnated with chitosan had excellent dimensional stability, better mechanical strength, and improved thermal stability. 

Static cultivation is the most common method, from which a highly hydrated BC membrane (or pellicle) on the air-culture medium interface is obtained (Figure 2.2) [19, 38]. As cellulose is synthesized, a membrane with increasing thickness is generated and, once oxygen is required for bacteria growth and cellulose production, it is assumed that the mature BC membrane is constantly pushed down as new cellulose is produced on the interface [8, 15]. 

BC is characterized by an ultrafine network structure composed of ribbon-shaped fibrils with an average diameter 100 times thinner than those of plant cellulose fibers (Figure 2.9) [4]. As a result, BC membranes are a highly porous material with substantial permeability for liquids and gases and high water uptake (water content >90%) [8]. 

Figure 2.9 SEM images of the surface (left) (reproduced from [56]) and cross-section (right) (reproduced with permission from [57]) of a BC membrane.

Bacterial cellulose has been also explored in a series of technical applications. For instance, SONY Corporation and Ajinomoto developed an audio speaker diaphragm membrane using a compressed low thickness ( 20 pm) BC membrane, that is currently utilized in audio headphones (Figure 2.10) [4, 58]. 

BC membranes are likewise promising nanostructured topical drug release systems for different drugs or active compounds, such as hdocaine hydrochloride, ibuprofen and caffeine, while at the same time serving as an efficient physical barrier against any external infection [76-78]. 

Lin et al. [110] developed porous BC/chitosan nanocomposite membranes prepared by immersing BC membranes in a chitosan solution followed by freeze-drying. Histological examinations revealed that wormds treated with these BC/chltosan membranes epithelized and regenerated faster than those treated with pure BC membranes and therefore are considered as potential candidates for wound dressing materials. 

Figure 2.14 General procedure for BC chemical grafting using aminopropylsilane. SEM images of BC membrane and its flexibility before (BC) and after (BC-NH ) chemical grafting. Reproduced with permission from [114],...

In a very creative fashion, Hu and Catchmark [140] developed bioabsorbable cellulose nanocomposites by integration of cellulases into BC membranes (Figure 2.17). Considering the harmless effect of the main product of the enzymatic degradation of cellulose, glucose, these composites may be perfect for specific wound care and tissue engineering applications where the bioabsorbable character is crucial. 

In a more fundamental vein, Zhang et al. [157] studied the confined crystallization behavior of PLA/acetylated BC nanocomposites prepared by compression molding. The results indicated that acetylated BC favored the crystallization of PLA at higher temperatures. In a similar mode, Quero et al. [158] investigated the micromechanical properties of laminated BC/PLA nanocomposites by Raman spectroscopy as a mean to understand the fundamental stress-transfer processes in these nanocomposites and as a tool to select appropriate processing and volume fraction of the fibers. Results showed that Young s modulus and stress at failure of PLA films were foimd to increase by 100 and 315%, respectively, for 18% volume fraction of BC and BC membranes cultured for 3 days exhibited enhanced interaction with PLA because of their higher total surface area. 

In order to avoid the laborious solvent exchanges of native BC membranes, required in this later studies, Trovatti et al. [187] investigated the preparation of BC/acrylic resin nanocomposites by casting water-based suspensions of two acrylic emulsions and BC nanofibrils. The excellent compatibility between the acrylic resins and BC, as observed by SEM, resulted in enhanced thermal stability (30 °C in the maximum degradation temperature for 10% BC) and mechanical properties. In this circumstance, the outstanding compatibility between the acrylic resins and BC nanofibrils was certainly favored by the presence of surfactants, typically used on the preparation of acrylic resins emulsions. 

The ex situ approach involves the previous preparation of the Ag NPs colloid and then the impregnation of the BC membrane [221], leading to a more heterogeneous distribution of the NPs, or even limited to the outmost surface of the membrane. For instance, the deposition of Ag NPs can be easily controlled by a previous layer by layer (LbL) sequential deposition of polyelectrolytes, such as poly(diallyldimethylammonium chloride) (PDDA) and poly(sodium 4-styrenesulfonate) (PSS), onto the BC nanofibers surface aiming at promoting a more homogeneous and adequate surface charge distribution [220]. 

Finally, BC/HAp hybrids can also be prepared by aggregation of the two phases in aqueous suspension, after previous disintegration of BC membrane [271, 275], or by regeneration of BC nanofibers from BC dissolved in 4-methylmorpholine-4-oxide... 

Figure 2.30 Typical FESEM aspect of BC BC/PVP and BC/HAp hybrids obtained by sequentially soaking BC membranes previously treated with poly(vinylpirrolidone), with Ca "" and PO solutions. Reproduced with permission from [276].

Lacerda et al. [63] prepared novel nanostructured composite materials from bacterial cellulose membranes (BC) and acrylate polymers by in-situ atom transfer radical polymerization (ATRP). The BC membranes were first functionalized with initiating sites by reaction with 2-bromoisobutyryl bromide (BiBBr), and then polymerization of methyl methacrylate (MMA) and n-butylacrylate (n-BA) was carried out in presence of catalysts copper (I) bromide and N, N, N, N", N"-pentamethyldiethylenetriamine (PMDETA), shown in Figure 5.8. 

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