3.2. Swelling properties of the hydrogels
The influence of the carboxymethylcellulose composition on the swelling ratio of cellulose/CMC hydrogels in distilled water at 37 C is shown in Fig. 6. The samples exhibited high equilibrium swelling ratio, indicating all of the samples were superabsorbent hydrogels. As expected,the equilibrium swelling ratio of the cellulose/CMC hydrogels increased rapidly with an increase in the CMC contents.
This confirmed further that highly hydrophilic carboxyl group of CMC could absorb a lot of water to enhance the space in the hydrogels. In the other hand, cellulose/ NaOH/urea aqueous solution could form irreversible gelation by heating [39], thus physical cross-linking in cellulose also played an important role in the formation of hydogels. So, the entanglements of cellulose chains through hydrogen bonds could occur easily in solutions of high cellulose concentration, leading to the decrease of the equilibrium swelling ratio with an increase of cellulose content. The maximum swelling ratio of the hydrogels was more than 1000, which was clearly higher than that prepared from cellulose derivative [32]. It is important for biodegradable materials to have high swelling ratio for wide application in the biomedical field. To evaluate the suitability of the cellulose/CMC hydrogels as biomaterials, we studied their swelling ratios in different simulated biological solutions. Fig. 7 shows the effects of the D-glucose, urea, physical saline water and synthetic urine solutions on the swelling phenomena of the different hydrogels. All of the hydrogels exhibited the same shrinking behaviors in a given solution, as a result of the inhibition of the electrostatic effects caused by the To evaluate the suitability of the cellulose/CMC hydrogels as biomaterials, we studied their swelling ratios in different simulated biological solutions. Fig. 7 shows the effects of the D-glucose, urea, physical saline water and synthetic urine solutions on the swelling phenomena of the different hydrogels. All of the hydrogels exhibited the same shrinking behaviors in a given solution, as a result of the inhibition of the electrostatic effects caused by the
The effect of salt concentration on the swelling ratio of the cellulose/CMC hydrogels is given in Fig. 8. In NaCl solution (Fig. 8a), the swelling ratio of hydrogels decreased
with an increase of the ionic strength of the solution. The hydrogels with higher CMC contents exhibited more significant decline of swelling ratio with the increase of the NaCl concentration. In CaCl2 aqueous solution, the swelling ratio decreased quickly because of the higher cationic charge of CaCl2 in comparison with NaCl, in accord with the Donnan equilibrium theory. In this case, the distinction in the concentration of mobile ions between the hydrogel and solution was reduced. Therefore, the osmotic swelling pressure of mobile ions inside the hydrogel decreased,and the hydrogel collapsed [38]. Fig. 9 shows the shrinking kinetics of the cellulose/CMC hydrogels in NaCl aqueous solution at 37 C. All of the swollen hydrogels tended to shrink and lose water once transferred into NaCl solution. However, the water retention of the hydrogels decreased from 53% for GEL55 to 28% for GEL91 after 3 h with an increase of CMC content,indicating that screening effect became more significant in the hydrogels. Thus, a faster shrinkage of the hydrogel occurred in the NaCl solution. In view of the above results,the hydrogels possessed smart behaviors of swelling and
shrinking in physical saline water, which will be very important for applications in biomaterials. Fig. 10 displays the reswelling behaviors of the dried cellulose/CMC hydrogels in distilled water at 37 C. The reswelling capabilities of the hydrogels decreased with the increasing CMC content. The water uptake of dried GEL55 reached 91%, whereas that of dried GEL91 exhibited a low value of 19%. These results indicated that it was more difficult for the higher swelling ratio samples to reach their initial swollen state. In this study, strong hydrogen bonding interactions between the COO groups of CMC and the hydroxyl groups of cellulose occurred during the desiccation process, greatly reducing the relaxation and expansion of the molecular chains. Therefore, the water uptakes of the hydrogels decreased with an increase of CMC content in the hydrogels from GEL55 to GEL91.