**4. Thermal properties**

Even though hydrogels do not need to endure temperatures higher than that of the human body, the improvement of thermal properties can increase its long-term operation. Thus, for example, the incorporation of polyurethane into polyacrylamide network in the form of an interpenetrating polymer network enhanced the thermal properties due to higher crosslinking density imparted by the hard segment content [22]. Though silica can improve the mechanical properties of hydrogels, the differential scanning calorimetry results of PHEMA/SiO<sup>2</sup> hybrids are complicated, showing two glass transition temperatures (Tg) [77]. However, composite hydrogels with functionalized graphene sheets (FGNS) showed an unprecedented shift in Tg of up to 40 and 30°C in poly(acrylonitrile) with 1 wt. % of this nanomaterial [78].

Another strategy to improve the thermal properties of hydrogels is by nanoparticle filling. Thus, crosslinking metal nanoparticles added into the polymer backbone of PHEMA hydrogels enabled the preparation of thermally stable, soft, magnetic field-driven actuators with muscle-like flexibility [79]. Furthermore, thermal degradation can also be improved by this filling procedure. For example, the mechanical and thermal properties of a renewable and biocompatible hydrogel of gelatin were improved through cross-linking by cellulose nanowhiskers [80].

In the biomedical field, hydrogels are hydrophilic polymers, which are able to absorb large amounts due to contact with cells or tissue in the human body. Therefore, the thermal analysis of water and its influence on the swollen hydrogel properties becomes essential [12, 81, 82].
