**1. Introduction**

Hydrogels are a class of hydrophilic polymers with a three-dimensional network structure formed by cross-linking with the action of covalent and hydrogen bonds [1–3], which have a large specific surface area [4], high carrier strength [5], controllability [6] and a wide range of physicochemical adjustability [7,8]. Since the 1960s, when the Czech scholar Wichterle first produced poly (2-hydroxyethyl methacrylate) hydrogels [9], hydrogels have been widely used in medicine, engineering, agriculture, forestry, environmental protection and the information industry due to their good biocompatibility, sensitivity to environmental changes and superb molecular designability [10–15].

Hydrogel is a material similar to the tissue of living organisms, which has a wide range of medical applications because of its excellent biological properties. It can both encapsulate drugs for slow release into the body and load dressings for contact with tissue wounds [16]. High-strength composite hydrogels can also be used as scaffold materials [17]. The high hydrophilicity of hydrogel can hold a large amount of water, play the role of water retention and drought prevention and promote the growth of crops and forests [18]; hydrogel has a strong adsorption effect and can adsorb heavy metals and pollutants in the

**Citation:** Zhang, S.; Wan, Y.; Yuan, W.; Zhang, Y.; Zhou, Z.; Zhang, M.; Wang, L.; Wang, R. Preparation of PVA–CS/SA–Ca2+ Hydrogel with Core–Shell Structure and Its Properties. *Polymers* **2022**, *14*, 212. https://doi.org/10.3390/ polym14010212

Academic Editors: Jingpeng Li, Yun Lu, Huiqing Wang and Vijay Kumar Thakur

Received: 16 November 2021 Accepted: 1 January 2022 Published: 5 January 2022

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soil [19]; hydrogel can also remove a wide range of aqueous pollutants containing toxic dyes and organic pollutants [20,21]. With the development of artificial intelligence, hydrogel strain sensors can be produced, which have enabled the extensive development of flexible wearable devices [22]. Due to its own properties, hydrogel can take on important roles. In addition, it can respond to changes in the external environment, for example, when factors such as temperature and humidity, pH and light intensity change, the three-dimensional network structure of the hydrogel will also change in response, which is why hydrogels are also known as smart polymers [23]. In addition, the composite of hydrogel and different media will expand its application areas, for example, the introduction of hydrogel into conductive media can produce conductive hydrogel [24].

The properties of hydrogels are determined by the polymer network structure. Singlematerial hydrogels are isotropic in terms of microstructure and macroscopic properties, lacking an ordered structure, and have poor mechanical properties, which in turn limit their application in some fields. A double network structure is usually used to solve the mechanical property problem, and a biocompatible material is used to solve the problem of poor biocompatibility. Double network hydrogels usually contain two kinds of networks: one is rigid, which is easy to break and can increase the tensile stress of the hydrogel by dissipating energy; and the other is flexible, which can increase the tensile strain of the hydrogel [25,26]. The polyvinyl alcohol (PVA) hydrogel is a class of highly hydrophilic, nontoxic, degradable elastomeric materials with good film formation and chemical stability, but a single PVA hydrogel is prone to swelling and adhesion and poor stability. Chitosan (CS) has good biocompatibility and tunability, but the intermolecular hydrogen bonding force is strong, and when it is solely used to prepare hydrogels, the products suffer from disadvantages such as their brittleness and poor mechanical properties. The cross-linking of PVA material with CS can make up for the deficiencies of both materials [27]. Sodium alginate (SA) is a green biomass sodium salt with strong affinity to dyes and metal ions because of its abundant hydroxyl and carboxyl groups, but the mechanical strength of a single SA hydrogel is weak, while cross-linking with Ca2+ through chelation can significantly enhance the stability of the structure [28].

Previous studies of hydrogels have shown them to be versatile but only able to perform a specific function. In this study, we propose to use polyvinyl alcohol (PVA) and chitosan (CS) to build the core of the hydrogel, and sodium alginate (SA) and Ca2+ to construct the shell, forming a core–shell structure with a bilayer space that does not affect each part of the structure in order to achieve the purpose of loading two substances, so that one hydrogel can achieve two different functions.

#### **2. Experimental Method**

#### *2.1. Raw Materials and Reagents*

Polyvinyl alcohol (PVA) was purchased from Sinopharm Group Chemical Reagent Co., (Shanghai, China, AR); chitosan (CS) and sodium alginate (SA) were purchased from Chengdu Kelong Chemical Reagent Factory, (Chengdu, China, AR); calcium chloride was purchased from Tianjin Tianli Chemical Reagent Co., (Tianjin, China, AR); citric acid was purchased from Tianjin Comio Chemical Reagent Co., (Tianjin, China, AR); dipotassium hydrogen phosphate was purchased from Tianjin Chemical Reagent Factory, (Tianjin, China, AR); sodium dihydrogen phosphate was purchased from Tianjin Fuchen Chemical Reagent Factory, (Tianjin, China, AR); and D-(+)-gluconic acid delta-lactone was purchased from Shanghai Maclean Biochemical Co., (Shanghai, China, AR).

Centrifuge (80-1), Shanghai Pudong Physical Optical Instrument Factory (Shanghai, China); freeze dryer (LGJ-10), Beijing Songyuan Huaxing Technology Development Co. (Beijing, China); scanning electron microscope (FEI Q45), FEI Inc. (Hillsboro, OR, USA); X-ray photoelectron spectroscopy (Vario EL III), Kratos Analytical Ltd. (Manchester, UK); Fourier transform infrared spectroscopy (VECTOR-22), Bruker Co. (Karlsruhe, Germany).
