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water compared to non-porous ones, due to additional capillary retention [87, 99, 100]. Furthermore, they can rapidly swell or shrink in response to given stimuli, due to the convective water transport, taking place in the pore channels. Multiple synthesis methods have been proposed to produce porous hydrogels with given microstructure, which are generally based either on the use of a removable porogen/templating agent [121–123] or on the rapid expulsion of water by phase inversion in a non-solvent [87, 100]. Cryo-gelation, which consists of crosslinking a polymer solution at cryogenic temperatures, is a particular method where ice crystals are the interconnecting porogens, while the crosslinking reaction takes place in concentrated liquid microphases among them. Resulting cryogels exhibit a spongelike form with remarkable properties compared to other types of porous, soft gels. Indeed, in addition to a very quick swelling/deswelling response, cryogels may also show high elasticity and shape memory, which can make them even injectable for specific biomedical applications [124, 125]. So far several cellulose-based porous hydrogels, including different types of cryogels, have been proposed in the literature as smart materials for a wide range of applications, e.g., absorbents in the agriculture field [126], matrices for controlled drug release [127], stomach bulking

The investigation of hydrogel porosity is primarily performed, with different levels of resolution, via optical methods, such as scanning electron microscopy (SEM) and CLSM, usually followed by software-based image analysis [122, 129]. As discussed above, particular caution is needed in sample preparation to avoid artifacts, especially in cases where preliminary sample dehydration is required (e.g., in SEM). In this regard, cryo-SEM is particularly recommendable to visualize the structure of swollen hydrogels. Alternatively, freeze-drying may be exploited to preserve the gel structure as much as possible, before standard (i.e., under high vacuum) SEM observation. Other methods to analyze the hydrogel porosity include mercury intrusion porosimetry [121] and X-ray computed microtomography (μCT) [123, 130, 131]. While the former still necessitates preliminary sample drying and may not be suitable for the analysis of very soft materials such as hydrogels, μCT represents a powerful and versatile technique for the quantitative analysis of hydrogel porosity. In general, μCT allows the non-destructive visualization and reconstruction of the entire 3D structure of a given material. Then, proper analysis of acquired images/sections provides significant morphological information such as the pore volume fraction and the pore size distribution [123, 130, 131]. Although μCT does not require particular care in sample preparation, in the case of porous hydrogels it is worth mentioning that long scanning times are often required to obtain good quality images, due to the typical low density of the materials. The presence of water in swollen hydrogels may further increase the scanning time, thus implying the simultaneous desiccation of the sample under the X-ray beam. The μCT analysis of freeze-dried hydrogels is thus recommended and is usually reported [130, 131]. Although the μCT quantification of porosity may be particularly challenging for some hydrogel-based materials, e.g., cellulose-based ones, due to their very low density, the successful μCT characterization of various hydrogel types has been recently reported [123], thus suggesting the potential of the technique to be

agents [18] and scaffolds for tissue engineering [128].

Hydrogels - Smart Materials for Biomedical Applications

further refined for the analysis of a larger number of hydrogels.

Cellulose-based superabsorbent hydrogels are currently explored for a number of technological applications, which range from the traditional use of hydrogels as water absorbents in different contexts (e.g., personal care products, agriculture) to

8. Conclusions

48

Christian Demitri<sup>1</sup> \*, Marta Madaghiele<sup>1</sup> , Maria Grazia Raucci<sup>2</sup> , Alessandro Sannino<sup>1</sup> and Luigi Ambrosio<sup>2</sup>

1 Department of Engineering for Innovation, University of Salento, Lecce, Italy

2 Institute of Polymers, Composites and Biomaterials (IPCB), National Research Council of Italy, Naples, Italy

\*Address all correspondence to: christian.demitri@unisalento.it

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
