Regenerated cellulose films were developed by dissolving bacterial cellulose in NMMO solutions [12Sha]. Bacterial cellulose (5 %) was added into aqueous NMMO solution and stirred at 100 °C for 2 h and the solution obtained was cast into films with the addition of 30 % glycerol as plasticizer. During the dissolution, bacterial cellulose was transformed from cellulose I structure to cellulose II struc­ture and the % crystallinity decreased substantially from 79 to about 38 %. In a similar approach, bacterial cellulose was dissolved using NMMO solution to develop regenerated cellulose fibers [11Gao]. Fibers obtained had a striated surface,

Table 61.7 Some of the properties of bacterial cellulose modified using in situ and ex situ methods [12Ul] Modification Samples Total surface area (m2/g) Total pore volume (cc/g) Average pore diameter (A) Water holding capacity (g/g sample) In situ 1 178 0.505 309 106.4 2 168 0.144 57.12 100.4 3 135 0.124 58.06 91.8 4 104 0.091 49.48 85.3 Ex situ 1 285 0.728 191 121.2 2 370 0.613 74.8 84.4 3 223 0.314 116 125.8

physical form of cellulose II, and a degree of crystallinity of 61 %. Tenacity of the fibers at 0.6-1.7 g/den and elongation at 3-8 % was lower than that of other regenerated cellulose fibers. Bacterial cellulose films were used as reinforcement to protect vulnerable silk fabrics and the effect of BC on the light aging behavior was investigated [12Wu]. It was found that BC restored silk fabrics had 213 % increase in strength and improvements in crystallinity and thermal stability were also observed. BC could be removed from the silk fabrics without any degradation to the silk fiber properties. Presence of abundant hydroxyl groups on the surface of BC promotes good interfacial adhesion and therefore BC was capable of preserving the properties of the silk fabric [12Wu].

Although bacterial cellulose exhibits excellent moisture sorption and forms a hydrogel with good strength, bacterial cellulose films cannot be swollen after drying due to the formation of strong hydrogen bonds between the nanofibrils. Bacterial cellulose was hydroxypropylated using two modifier systems: sodium hydroxide/propylene oxide or sodium hydroxide/urea/propylene oxide. The equi­librium swelling ratio of the modified cellulose films could be controlled between 280 and 7,000 % by adjusting the NaOH concentration in the modifying system. Hydrogels systems suitable for medical applications were successfully developed from dried bacterial cellulose in this research [13Wan2]. Since the biomedical applications of bacterial cellulose are directly dependent on the water holding capacity and water release rate, attempts were made to increase the pore size, pore volume, and surface area using in situ and ex situ modifications [12Ul]. In situ modifications were done using a single sugar а-linked glucuronic acid-based oligosaccharide, and ex situ modifications were done using chitosan and montmo — rillonite. Table 61.7 provides some of the properties of the bacterial cellulose samples modified using different approaches.

Biohybrid fiber yarns as unique materials for tissue engineering were developed using microfibrils extracted from bacterial cellulose and used as reinforcement for poly(methyl methacrylate) (PMMA) by electrospinning [10Ols]. Cellulose mats obtained after 7 days of growth were harvested, boiled twice in 10 % NaOH for 20 min, and later with 50 % sulfuric acid. Finally, the cellulose sheets obtained were dispersed in dimethylformamide/tetrahydrofuran solution and mixed with PMMA in various ratios up to 20 %. Microfibrils developed had a diameter of 15-20 nm and

15 rpm/min

Fig. 61.19 Electrospinning setup used to produce the biohybrid fiber yams [10Ols]. Reproduced with permission from American Chemical Society

length varied between 0.3 and 8 ^m. Using the electrospinning setup shown in Fig. 61.19, the electrospun fibers were aligned by extruding the fibers onto a water surface and subsequently winding the fibers onto spools to form hybrid yarns. Figure 61.20 shows an SEM image of the multifilament-PMMA hybrid fibers obtained using 7 % cellulose microfibrils as reinforcement [10Ols].

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