# Hydrogel Tissue Expanders for Stomatology. Part II. Poly(styrene-maleic anhydride) Hydrogels
## 4 Conclusions  
We have shown that the covalently crosslinked polySMA alternating copolymer hydrogels are well-suited materials for TEs mainly due to their protracted swelling and excellent biocompatibility. The favorable swelling profile with protracted volume growth resulted from the ongoing hydrolysis of MA units in the polymer network chains during swelling. The swelling course and equilibrium can be further adjusted by \(p\)-divinylbenzene crosslinker content and thus the gels can be made to meet the requirements of given application. Hydrogels provide sufficient final swollen volume with around 15 times of their initial volume under the free swelling conditions and 12.7 times in constrained in vivo swelling conditions while still maintaining their structure integrity. In the free swelling in PBS solution, the gels exhibited a two-day initial induction period caused by transition from glassy state paralleled with gradual hydrolysis of MA units accompanied with only a small volume change. This feature is favorable for surgical applications because the initial delay in volume growth allows the wound to heal and thus diminishes the risk of rupture at the sutured site. The mechanical measurements showed that the polySMA hydrogels exhibited relatively high compressive stresses (380 to 550 kPa at 40% strain) and withstood compressive strains up to 50% without any mechanical failure, although their swelling was high (\(q_{2}\cong 0.065\)). In vitro tests with human fibroblasts demonstrated the biocompatibility of prepared hydrogels and, moreover, in vivo tests showed no tissue inflammation or necrosis, confirming that no toxic components released into the body after implantation into mice. Taken together, the results constitute the polySMA copolymers as promising soft hydrogel TEs for tissue restoring applications,  
Figure 6: The size of the connective tissue capsule around the implant Error bars were calculated using the standard deviation (\(n=6\)).  
Figure 7: Hematoxylin & Eosin stained tissue section of the connective tissue capsule around expander implanted for 7 days. (**A**) Connective tissue capsule formed around implant, (**B**) muscle tissue. Scale bar = 0.5 mm.  
not only in atomatology. However, swelling expansion determined after in vivo implantation seems to be rather fast so the future challenge remains the design of materials that exhibit a slower, close to linear volume expansion over the first 4 to 5 days. These aspects will be a topic for future research.  
Conceptualization, M.H., R.S. and P.L.; data curation, R.H.; formal analysis, M.D.-S.; funding acquisition, R.S.; investigation, J.H. ([PERSON]), E.C.K. and J.H. ([PERSON]); methodology, J.H. ([PERSON], R.H., M.D.-S. and J.M.; project administration, R.H., J.S. and P.L.; supervision, J.S. and P.L.; validation, M.D.-S.; writing--original draft, J.H. ([PERSON]; writing--review & editing, M.D.-S., R.H., J.S., M.H. and P.L.  
This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic within the National Sustainability Program II (Project BIOCEV-FAR LQ1604) and by the project \"BIOCEV\" (CZ.1.05/1.1.00/02.0109), and by the Grant Agency of the Czech Republic (project number 19-01602S).  
The authors thank Department of Biological Models, Institute of Macromolecular Chemistry AS CR, Prague for determination of in vitro biocompatibility and Asklepion-Lasercebtrum, Ltd., Prague for the financial support.  
The authors declare no conflict of interest.