Understanding the Properties of HEMA Hydrogels and Their Interaction with Water

Hydrogels have gained significant attention in various fields, including biomedical applications, due to their unique properties. One of the key types of hydrogels is HEMA (2-hydroxyethyl methacrylate) hydrogels, which are formed through a specific polymerization process. A common method involves using a redox initiator consisting of a 75% water solution of ammonium persulphate and sodium pyrosulfite. This initiates the crosslinking of HEMA monomers between silanized glass plates, resulting in transparent films after a heating process at 37 °C.

The interaction of HEMA hydrogels with water is crucial for their functionality. At equilibrium, the degree of swelling of these hydrogels varies between 30% and 70% in water and similar aqueous solutions. Interestingly, this degree of swelling is largely independent of the type or quantity of crosslinking agents used. In contrast, when exposed to DMSO (dimethyl sulfoxide), hydrogels exhibit swelling levels that are significantly higher, ranging from 100% to 400%. This profound difference is closely linked to the structural characteristics of the hydrogels.

In terms of water structure, it can be categorized into three types: bound, interfacial, and free water. Bound water is strongly associated with the polymer chains, typically through hydrogen bonding, while interfacial water interacts weakly with the polymer network. Free water behaves similarly to pure water and does not engage with the polymer chains. This classification helps in understanding how water molecules behave within the hydrogel matrix and their potential impact on drug release mechanisms.

Advanced analytical techniques such as differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) have been employed to study the nature of water within HEMA hydrogels more closely. DSC results indicate that the total water content and the percentage of non-freezing water decrease as the degree of crosslinking increases. Furthermore, NMR analysis reveals a biexponential decay in relaxation times, suggesting the presence of two distinct types of water molecules that interact differently with the polymer network.

Additionally, the glass-transition temperature (Tg) of HEMA hydrogels is influenced by both the polymer structure and the extent of hydration. A notable observation is that as hydration increases, the Tg decreases due to the plasticizing effects of water. The diffusion of water in these hydrogels follows a Fickian diffusion mechanism when assessed at 37 °C, highlighting the predictable nature of water absorption kinetics in these materials.

This comprehensive understanding of HEMA hydrogels and their interactions with water underscores their potential in drug delivery systems and other applications, paving the way for further research and development in this field.