# Hydrogel Tissue Expanders for Stomatology. Part II. Poly(styrene-maleic anhydride) Hydrogels
## 3 Results and Discussion
### Swelling Measurements  
The swelling phenomenon of macromolecular covalent network entails macroscopic change of sample volume: Mere swelling that is not complicated by simultaneous extraction is represented as volume growth to certain maximum equilibrium value. The equilibrium volume fraction of the given network-solvent system is a unique physico-chemical parameter that is related to the macromolecular network structure, namely its crosslink density, and to chemical composition of the system. The time-dependent swelling profiles of the polySMA hydrogels in PBS solution that represent aqueous environment containing ions (PBS osmolality is approx. 300 mOsm/kg aq) are shown in Figure 2 as the changes of the volume degree of swelling \(V_{t}/V_{0}\). In these gels, the physical swelling is accompanied by a chemical change of the macromolecular network: The MA groups undergo hydrolysis and the anhydride is converted into carboxyl groups. Although the volume growth of the polySMA samples seems to be quite fast, there is an initial \"induction\" period of approx. two days when the volume change is small for all tested samples (see insert in Figure 2). After this induction period that is related to both, the hydrolysis and also to the process of transition of the macromolecular network from its glassy to rubbery state, the swelling course becomes somewhat auto-accelerated-the linear portion of swelling volume change between the days 2 and 8. The fast solvent uptake is caused by more intensive hydrolysis enabled by the opening of the network structure and higher availability of hydrolyzing MA units at a time. The enhanced swelling rate is also related to the increasing segmental mobility of the network due to the drop of the mean temperature of glass transition (\(T_{\text{g}}\)) of the system polySMA-swelling liquid. The already hydrolyzed MA units further hydrophilic the polymer and also make the polymer chains anionic due to deprotonation of the formed carboxylates, which probably both contributes to the network expansion within this period by increased chain hydration and Coulombic repulsion among the polymer chains. Last but not least, as acid is formed during anhydride hydrolysis, mildly acidic environment inside the hydrolyzing gel probably autocatalytically accelerates hydrolysis of the remaining anhydride groups. This process is facilitated by the higher mobility of the network chains in the rubbery state. The volume degree of swelling reached its maximum after approx. 30 days for all the hydrogel compositions attaining the values more than 14, with the highest value being 15.7 for the polySMA-0.5 sample. The maximum swelling of the polySMA hydrogels is surprisingly high; the ratio of 15.7 corresponds to the content of gel only 6.4 vol % for the polySMA-0.5 composition. The other two gels swell somewhat less, their gel content being around 7 vol %, still remarkably low. The crosslink content was found to significantly influence the swelling course (\(P<0.05\)) within 1 to 4 days. At the initial stage (up to day 1) and after day 4, the differences between swelling ratios were not statistically significant (Figure 2). As we already pointed out, when ionized, the anhydride forms carboxylate groups and thus hydrophilicity of the whole structure increases. At a given ionic strength and pH, certain equilibrium degree of ionization is attained (see Figure 1b). All MA units in polySMA chains are ionized at pH as high as 12-14 [27]. In our case, the polySMA gels swell in the PBS solution of pH 7.4 and containing several anions and cations that participate in the hydrolysis. The detailed mechanism of the hydrolysis of MA units in these networks is still not fully understood and is under research interest [28]. From the practical application point of view, the approx. 2 days lasting induction period without rapid swelling would be particularly useful in the surgical application of these materials as tissue expanders.