However, the superabsorbent polymer hydrogels currently available are almost exclusively acrylic-based products, and hence not biodegradable.0006Given the growing interest in environmental protection issues, over recent years a vast amount of interest has been focussed on the development of superabsorbent materials based on biodegradable polymers, having properties which are similar to those of the traditional superabsorbent polyacrylics.0007Examples of biodegradable polymers used to obtain superabsorbent polymer hydrogels are starch and cellulose derivatives.0008In 1990 Anbergen and Oppermann 1 proposed a method for the synthesis of a superabsorbent material made entirely from cellulose derivatives. In particular, they used hydroxyethylcellulose (HEC) and a carboxymethylcellulose sodium salt (CMCNa), chemically cross-linked in a basic solution with divinylsulphone. However, the absorption properties of such materials are not high compared to those of the acrylic-based superabsorbent materials.0009In 1996 Esposito and co-workers 2, studying the synthetic process proposed by Anbergen and Opperman, developed a method for increasing the absorption properties of the gel, acting mainly on the physical properties of the material. The basic idea was the induction of microporosity into the polymer structure, so as to promote absorption and retention of water by capillarity. Said microporosity was induced during the drying step, which was carried out by phase inversion in a nonsolvent for the polymer, and the absorption properties of the material thus obtained were markedly superior to those of the air-dried gel.0010CMCNa may be chemically cross-linked with any reagent which is bifunctional with respect to cellulose. Besides the divinylsulphone used in the synthetic process according to Anbergen and Opperman, epichlorhydrin, formaldehyde and various diepoxides have also been used as cross-linking agents. However, such compounds are highly toxic in their unreacted states 3. Some carbodiimides are known amongst the unconventional cross-linking agents. Particularly, the use of carbodiimides in order to cross-link salified or non-salified carboxymethylcellulose (CMC) was described in 4. Carbodiimides induce the formation of ester bonds between cellulose macromolecules without participating in the bonds themselves, instead giving rise to a urea derivative having very low toxicity 5. A superabsorbent polymer hydrogel obtained by cross-linking carboxymethylcellulose sodium salt and hydroxyethylcellulose with carbodiimide as the cross-linking agent is disclosed in the international patent application WO 2006/070337 6.0011However, the carbodiimide used as a cross-linking agent in WO 2006/070337 has the disadvantage of being extremely expensive. Moreover, during the cross-linking reaction with CMCNa, this substance turns into a slightly toxic urea derivative, which must be removed during the washing step, thereby further increasing the costs and the complexity of the production process. These drawbacks are extremely unfavourable, particularly in connection with those applications which require large scale production of the polymer hydrogels and which, consequently, involve high costs both with respect to the purchase of the starting materials and with respect to the disposal of the toxic substances which are produced during synthesis.0012Furthermore, the formation of substances having a certain degree of toxicity, although very low, is a key factor for ruling out the possibility of using such polymers in biomedical an