# Hydrogel Tissue Expanders for Stomatology. Part II. Poly(styrene-maleic anhydride) Hydrogels
## 3 Results and Discussion
### Swelling Measurements
If the material expands immediately after placed in contact with tissue fluids, it brings a high risk of tissue rupture at the incision place. In case of postponed swelling, the wound has the opportunity to partly heal and the risk of overpressure damage by the swelling gel is noticeably lower. To better estimate the swelling process in living tissue, swelling behavior of polySMA-0.7 sample was measured in FBS and compared to swelling in PBS solution as shown in Figure 3. It is evident that until the day 5 the swelling profiles are comparable. With increasing time, the sample increased its volume more in PBS reaching the \(V_{\text{t}}/V_{0}\) value 14.5 compared to 9.7 in FBS having the osmolality as the PBS solution. The difference in swelling is probably caused by additional covalent crosslinking of the polymer chains by proteins from FBS (which typically contains several primary amino groups on the lysine residues per protein macromolecule able to react with MA units). The smaller volume increase in FBS is, however, still sufficient for the intended application and it can be stated that swelling profiles measured in PBS solution provide relevant information for the estimation of swelling in in vivo conditions. Another interesting feature of the prepared polySMA hydrogels is their anisotropic drying-swelling behavior, a kind of shape memory, that can be achieved by drying the swollen samples, obtained after polymerization in excess of ethyl acetate, that leads to fixing the sample shape by vitrification. Swelling of such dried samples leads to anisotropic increase in a particular dimension. In this study, the samples were prepared as cylinders with a circular cross-section (determined by the inner diameter and shape of the polyethylene tubes used as molds for polymerization). After washing and drying the samples after polymerization, the samples gained flattened cross-sections. Upon swelling, the former circular shape of the sample cross-section restored (Figure 4). The relative changes of the samples in particular dimensions, i.e., height, width and length, expressed as ratio \(\mathrm{a_{sw}/a_{0}}\) are given in Table 1, where \(\mathrm{a_{sw}}\) represents the dimension after 40 days of swelling and \(\mathrm{a_{0}}\) the dimension of the dry, extracted sample before swelling. The obtained values are the averages \(\pm\) standard deviations (\(n=3\)). Figure 3: The time-dependence of swelling volume ratio (\(V_{\mathrm{\mu}}V_{0}\)) of the hydrogel polySMA-0.7 in PBS and FBS. Figure 2: The time-dependence of swelling volume ratio (\(V_{\mathrm{\mu}}V_{0}\)) in PBS solution (pH 7.4) of the polySMA hydrogels prepared with different crosslinker content. Error bars were calculated using standard deviation (\(n=3\)). The values with \(\mathrm{\boldsymbol{{}^{*}}}\) represent the significant difference between the swelling ratios of individual hydrogels with \(P<0.05\). Height of the samples increased more than three times, while width and length increased less than twice. The possibility of setting up a specific volume change in selected direction is quite desired in restoration surgery.