## 1 Introduction  
Hydrogels are three-dimensional networks of hydrophilic polymers capable of absorbing important quantities of water. The amount of water uptake may widely vary, starting from tens of grams per 1 g of dry gel to thousands of grams. By general definition, superabsorbent hydrogels are those that can retain an amount of liquid greater than 100 times its own weight in dry state. Both physically and chemically crosslinked hydrogels have been intensively studied in the last 50 years. As practically any water-soluble polymer can be turned into a hydrogel, the number of chemical compositions is vast and practical applications continue to grow. Starting from commercially available products such as diapers or contact lenses, due to their unique physical properties, hydrogels rapidly emerge into fields like tissue engineering [1; 2; 3; 4; 5], or other biomedical applications, including drug-delivery [6; 7], but also in the field of detectors, actuators, etc., [8; 9; 10; 11; 12; 13].  
By their ability to respond to external stimuli such as pH, temperature, light, solvent, ionic strength, concentration [14; 15; 16; 17; 18], hydrogels, more than any other biomaterials, can mimic different living tissues [2; 19; 20]. Acidic polymers, like poly(acrylic acid), PAA, or poly(methacrylic acid), PMA, which can form pH-responsive hydrogels are often grafted on different biopolymers [21] or copolymerized in the presence of other monomers or polymers [22] to obtain functional materials. At the same time, poly(2-hydroxyethyl methacrylate), PHEMA, employed either as homopolymer or copolymer, is another valuable candidate to be used for obtaining hydrogels. As a result of its peculiarities (good chemical and thermal stability, transparent to visible light, inert to biological environment or an excellent cytocompatibility) [23; 24; 25], such a hydrophilic component could be successfully utilized and chemically incorporated in tuning the swelling capacity of PAA-based hydrogels.  
The crosslinking of different systems using high energy radiation represents an alternative to chemical crosslinkers that may be many times toxic, especially when pharmaceutical materials were intended to be obtained. Thus, photoinduced crosslinking using UV and near-UV radiation [26], electron beam or \(\gamma\) radiation [27; 28; 29; 30] applied to a great variety of solid polymers or aqueous solutions of homopolymers, copolymers, or even monomers can lead to hydrogels with specific applications. While irradiation of solid polymers can be disadvantageous in terms of sample homogeneity and the requirement of high absorbed dose, aqueous solutions of polymers or monomers are successfully used in preparing hydrogels that are free of impurities and sterilized at the same time.  
In this paper we focused on preparing copolymers of acrylic acid and 2-hydroxyethyl methacrylate via solution polymerization and subsequently crosslinked them by \(\gamma\) irradiation to produce functional hydrogels with adjustable loading capacity. The presence of both carboxyl and hydroxyl groups in the same macromolecular chain favors intermolecular interactions leading to more thermally stable compounds compared to the corresponding homopolymers. The molar ratio between monomers was chosen so that the resultant copolymers remain soluble in water at a pH as low as 3. Aqueous solutions of the copolymers were subjected to \(\gamma\) irradiation to prepare hydrogels with different crosslinking densities. For comparison, mixtures of the same composition monomers in aqueous solutions were also subjected to \(\gamma\) irradiation and the hydrogels so obtained were studied in terms of their swelling equilibrium. Depending on the preparation protocol and/or hydrogel composition superabsorbent materials that can bind different compounds throughout side functional groups may be obtained.