*Article* **Preparation of PVA–CS/SA–Ca2+ Hydrogel with Core–Shell Structure**

**Shuai Zhang 1,2, Yu Wan 1,3, Weijie Yuan 1,2,\*, Yaoxiang Zhang 1,2, Ziyuan Zhou 1,2, Min Zhang 1,3, Luzhen Wang <sup>4</sup> and Ran Wang 1,2**


**Abstract:** Hydrogels are highly hydrophilic polymers that have been used in a wide range of applications. In this study, we prepared PVA–CS/SA–Ca2+ core–shell hydrogels with bilayer space by cross-linking PVA and CS to form a core structure and chelating SA and Ca2+ to form a shell structure to achieve multiple substance loading and multifunctional expression. The morphology and structure of core–shell hydrogels were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The factors affecting the swelling properties of the hydrogel were studied. The results show that the PVA–CS/SA–Ca2+ hydrogel has obvious core and shell structures. The SA concentration and SA/Ca2+ cross-linking time show a positive correlation with the thickness of the shell structure; the PVA/CS mass ratio affects the structural characteristics of the core structure; and a higher CS content indicates the more obvious three-dimensional network structure of the hydrogel. The optimal experimental conditions for the swelling degree of the core–shell hydrogel were an SA concentration of 5%; an SA/Ca2+ cross-linking time of 90 min; a PVA/CS mass ratio of 1:0.7; and a maximum swelling degree of 50 g/g.

**Keywords:** core–shell structure; hydrogel; double load; response surface
