# Hydrogel Tissue Expanders for Stomatology. Part II. Poly(styrene-maleic anhydride) Hydrogels
## 1 Introduction  
The soft tissue expanders (TE) prepared from swelling gels represents a useful modality in reconstructive surgery especially in the maxillofacial area. The surgical use of bone grafts (e.g., alveolar bone graft) is essential in the treatment of many pathologies, but it is often difficult due to periosteal deficiency that often leads to the necessity of grafting soft tissue prior to bone surgery. Keeping the living tissue under a constant tension created by TE leads to the growth of cells and tissue [1]. The periosteum is responsible for the bone growth and its presence on the surface is crucial for bone reconstruction [2]. The use of subperiodseal TE is able to produce excess periosteal tissue and lead to success in clinical setting [3]. TE have many advantages: They allow better wound closure without compromising the blood supply of the flap, they help to augment the bone and their transfer is safer due to higher vascularization [4; 5]. The history of TEs is dating back to 1957, to the first use of a rubber silicone balloon to expand skin for ear reconstruction [6]. Until today, TEs have undergone considerable development from balloon-type silicone TEs filled with air or saline solution requiring sequential inflating by injectionto self-inflating TEs filled by hypertonic solution enabling swelling due to osmotically driven water uptake [7]. The new area in tissue expansion began with the introduction of osmotically driven volume expansive synthetic hydrogel TE based on hydrophilic polymers such as poly(vinylpyrrolidone) or derivatives of poly(methacrylic acid) [8] which have already been tested in clinical practice [9; 10; 11; 12; 13; 14]. In 2011 the application of self-inflating osmotic hydrogel TE in 24 sites was shown to be successful for vertical ridge augmentation with regard to clinical and histological outcomes and complications [15]. Despite the fact that self-inflating hydrogel TE represents a promising technology, some limitations of its clinical applications exist. Typically, the hydrogel expansion rate is too fast at the initial stage of swelling process, reaching mostly 60%-80% of maximal expansion volume within few hours followed by a smaller gradual volume increase. However, change in the chemical composition of hydrogels mostly does not lead to a reduction of the initial swelling rate, but to an influence on final volume expansion. This was overcome by enclosing the hydrogel into semipermeable silicone membrane with various porosity to control the fluid rate and consequent TE expansion while providing the same swelling capacity [16]. However, the necessity of using an outer membrane constitutes a risk of membrane rupture or obstacles in sizing and shaping the samples as required by the application. So, the preparation of hydrogel TE without any perforated membrane with a suitable swelling pattern meeting the medicinal requirements of biocompatibility and capability to be easily handled still remains as a challenge. In our previous study [17] we presented the synthesis of TEs in form of a solid and uniform hydrogel blocks without any membrane cover based on poly(2-hydroxyethyl) methacrylate-\(co\)-methacrylic acid) copolymers. These hydrogels exhibited a close-to-linear volume expansion rates up to 3 to 4.5 times of their original volume using a combination of non-degradable and hydrolytically degradable crosslinkers, allowing the volume expansion even in a later stage due to the degradation of the polymer network accompanied by the additional increase in swelling. However, the initial expansion rate was still relatively fast concerning the surgical application. So, the aim of our present work is to prepare the hydrogel TE possessing the delayed and protracted initial swollen volume growth providing sufficient time to allow wound healing after TE implantation and to avoid the risk of rupturing the sutured site.