Bacterial cellulose nanofiber membranes have been used as support for polyaniline (PANI) nanocomposites used as supercapacitor electrodes. Initial efforts on pro­ducing BC/PANI nanocomposites through in situ polymerization had limited suc­cess and a relatively low conductivity of 1.6 x 10—4 to 1.3 S/cm was obtained. Further studies by Wang et al. led to the development of nanocomposites having conductivity as high as 5.1 S/cm [12Wan]. A high supercapacitance of 273 F/g was obtained at 0.2 A g—1. The process of developing the nanocomposite is shown in Fig. 61.11. The surface area of the composites developed was about 33.9 m2/g, considerably higher than similar composites developed previously. It was reported that the properties of the composites could be easily controlled by varying the reaction conditions.

Similarly, BC nanofibers were implanted with oriented titanium dioxide (TiO2) nanoparticle arrays and the hybrid nanofiber arrays (Fig. 61.12) showed high photocatalytic activity exceeding that of the commercially available photocatalytic agent Degussa P25. Treating the hybrid nanofibers with nitrogen further increased the photocatalytic activity [10Sun]. Bacterial nanofibers were made into carbon nanofibers (annealing at 1,000 °C for 2 h in nitrogen) and treated with MnO2 to build a super capacitor with high energy and power density. In addition, the carbon fibers derived were also doped with nitrogen to improve capacitance. The 3D BC nanofibers coated with MnO2 were used as the positive electrode and the BC nanofibers doped with nitrogen were used as the negative electrode. The device was able to be reversible charged and recharged at 2 V to reach an energy density of

32.9 W h/kg and maximum power density of 284.6 kW/kg and also had good cycling durability with 95 % specific capacitance retained after 2,000 cycles [13Che2]. Figure 61.13 shows an image of the bacterial cellulose membrane,

Freeze-dried

PANI/BC nanocomposites

Fig. 61.11 Process of development of the bacterial cellulose nanocomposites [12Wan]. Reproduced with permission from American Chemical Society components of the supercapacitor, and a diode made using the supercapacitor [13Che2]. Other researchers have also shown that pyrolyzed bacterial cellulose can be used to develop porous 3D electrodes for high performance lithium ion batteries with the addition of tin oxide (SnO2) and/or germanium (Ge) nanoparticles [13Wan1]. The bacterial cellulose-germanium electrode had a very high specific capacity of 967 mAh g-1 and stable capacity of 230 mAh g-1 attributed to the homogenous distribution of active nanoparticles within the conducting cellulose nanofibrils that provide efficient electron conduction pathways and the interconnected voids facilitated the diffusion of lithium ions.

Bionanocomposites were manufactured by combining bacterial cellulose and starch and the properties of the composites developed were studied [10Woe]. Bacterial cellulose was treated (hydrolyzed) with enzymes (Trichoderma reesei) to improve dispersibility and properties of the thermoplastic blend. For the enzyme treatment, the bacterial cellulose fibers were hydrolyzed using 10 % enzyme, pH 4.8, citrate buffer at 45 °C for 20 to 240 min. Hydrolysis resulted in considerable changes to the morphology and the properties of the fibers. Degree of polymerization of the bacterial cellulose decreased considerably from 2,314 to

Fig. 61.12 SEM image of bacterial cellulose (a), TEM image of the nanofibers (b), TEM images of bacterial cellulose fibers at two different magnifications (c, d) containing TiO2 nanoparticles. Reproduced with permission from Royal Society of Chemistry [10Sun]

Fig. 61.13 Image of the bacterial cellulose nanofiber membrane and the supercapacitor formed using the membrane. A LED light glows when the bacterial cellulose supercapacitor had a closed circuit [13Che2]. Reproduced with permission from Wiley

430 when the hydrolysis was done for 240 min. In terms of morphology, the cellulose fiber bundles were found to be aggregated in 2-10 цш width before treatment and were rendered into short fibers and randomly distributed after the enzymatic treatment [10Woe]. Figure 61.14 shows AFM image of the bacterial cellulose before and after treatment. Reinforcing starch with the modified and unmodified bacterial cellulose led to substantial improvement in the properties of the composites. Elastic modulus increased from 4.3 to 141 MPa and strength

Fig. 61.14 Digital images of sisal fibers before and after growth of bacterial cellulose [08Pom]. Reproduced with permission from American Chemical Society

increased from 1.01 to 4.15 MPa when starch was reinforced with untreated bacterial cellulose. Further increase in strength up to 8.45 MPa and increase in modulus up to 576 MPa were observed when the starch was reinforced with hydrolyzed bacterial cellulose [10Woe]. Hierarchical nanocomposites were devel­oped by depositing bacterial cellulose onto natural fibers and improving the inter­facial adhesion [08Pom]. Sisal and hemp fibers were immersed in culture medium and used as substrate to grow cellulose from the strain Acetobacter xylinum. A weight gain of 5-6 % was observed on the fibers due to the growth of the cellulose. Figure 61.14 shows the digital images of the surface of the sisal fibers before and after bacterial growth. Natural sisal fibers had strength of 2.6 g/den and did not show any appreciable decrease in tensile properties whereas a drastic decrease in strength and modulus was observed for the hemp fibers after the growth of the bacterial cellulose [08Pom]. When used as reinforcement, the bacterial cellulose treated fibers showed substantially increased interfacial adhesion for poly(lactic acid) and cellulose acetate butyrate (CAB) matrices.

Bacterial cellulose was used as a binder for short sisal fibers to obtain preforms for composite with tensile strength of 13.1 kN/m. The BC treated biofiber performs were then mixed with acrylated epoxidized soybean oil and made into composites via resin transfer molding [12Lee]. Figure 61.15 shows SEM images of sisal fibers with bacterial cellulose as binder at different magnifications. It was estimated that the bacterial cellulose sheets had a high tensile strength of about 300 MPa. Properties of the composites obtained without and with the BC as reinforcement are shown in Table 61.5. As seen from the table, addition of the modified sisal fibers substantially increased both the tensile and flexural strength and modulus. Similar improvements were also observed in the viscoelastic properties.

To overcome the lack of antimicrobial activity in bacterial cellulose fibers, silver nanoparticles were in situ synthesized for potential wound dressing application [14Wu]. Commercially available bacterial cellulose membranes were purified and soaked in various concentrations of silver ammonia solution for 24 h. SEM image of the bacterial cellulose nanofiber membranes showed that the membranes had a

Fig. 61.15 SEM images of bacterial cellulose fibers binding sisal fibers seen at three (100x, 1,000x, and 2,500x) magnifications [12Lee]

Table 61.5 Some of the properties of the neat composites developed using acrylated epoxidized soybean oil (AESO) and those reinforced with the bacterial cellulose fibers [12Lee] Sample % BC Density (g/cm) Tensile properties Flexural properties Strength (MPa) Modulus (GPa) Strength (MPa) Modulus (GPa) Neat 0 1.09 ± 0.01 0.4 ± 0.1 4.1 ± 0.1 0.2 ± 0.1 9.0 ± 0.1 Sisal-AESO 40 1.17 ± 0.01 3.2 ± 0.2 18.4 ± 0.9 1.9 ± 0.2 28.9 ± 1.6 AESO-BC- Sisal-AESO 41 1.19 ± 0.01 5.6 ± 0.4 31.4 ± 0.5 4.6 ± 0.3 62.43.0

Fig. 61.16 SEM image and EDS analysis confirming the extensive deposition of silver nanoparticles on the fibers (a) and growth inhibition rings of bacterial cellulose containing silver nanoparticles (b) against E. coli (A), S. aureus (b), Pseudomonas aeruginosa (c) compared to commercial silver containing wound dressing (d) [14Wu]. Reproduced with permission from Elsevier

pore size in the 100 s nanometer range and were 3D. Such 3D structure allowed the diffusion of the nanoparticles into the inner spaces of the scaffolds. It was also found that silver nanoparticles were extensively adhered onto the surface (Fig. 61.16a) of the nanofibers indicating strong affinity between the silver and cellulose. A linear release rate was observed for the nanoparticles when the scaffolds were immersed in PBS solution but the total release of the nanoparticles was only about 16.5 % after 72 h. The developed BC membranes, especially the silver containing bacterial cellulose membrane, had excellent antimicrobial activity as seen from Fig. 61.16b.

Multiwalled carbon nanotubes were added into bacterial cellulose dissolved in an ionic solvent (1-allyl-3-methyl-imidazolium chloride) and electrospun into fibers. It was observed that the MWCNTs were well embedded into the cellulose and were aligned along the fiber axis [10Che]. A transformation of the cellulose from cellulose I to cellulose II was observed and the addition of the nanotubes led to

Blood vessels

Fig. 61.17 Depiction of the potential applications of bacterial cellulose fiber membranes [13Fu]. Reproduced with permission from Elsevier

Table 61.6 Some of the medical applications of bacterial cellulose-based materials [13Fu] Applications Materials Skin tissue repair Bacterial cellulose PVA-bacterial cellulose nanocomposites Collagen modified bacterial cellulose Silver loaded modified bacterial cellulose BC/hyaluronic acid loaded nanosilver composites Artificial dura mater PVA-bacterial cellulose Blood vessels Carboxymethyl cellulose-bacterial cellulose composite membrane Bacterial cellulose heparin composite Bone and connective tissue replacement Composite from BC, collagen, and hydroxyapatite Hydroxyapatite modified bacterial cellulose Antivirus mask BC treated with nanosilver BC with silver compounds BC with nanometer silver PVA and BC blends

an increase in strength and modulus by 290 and 280 %, respectively, and an improvement in the thermal stability and electrical conductivity was also observed.

Bacterial cellulose is considered to be one of the most suitable substrates for tissue engineering since it is biocompatible and contains functional groups required to modify the material or carry various substances for delivery in the body [13Fu]. Some of the potential medical applications for bacterial cellulose-based materials are shown in Fig. 61.17 and Table 61.6. In addition to the other unique features, BC has excellent conformability and is well suited to be applied on to

various parts of the body. Figure 61.18 shows BC dressing applied onto the face and torso demonstrating the excellent flexibility and conformability of the films [13Fu].