The hydrogel is a hydrophilic polymer material. It is a cross-linked network formed by chemical bonds, hydrogen bonds, van der Waals forces, or physical entanglement. It is insoluble in water but can swell in a large amount of water and maintain a solid shape. Due to the presence of a cross-linked network, the hydrogel can swell and retain a large amount of water, and the amount of water absorbed is closely related to the degree of cross-linking. The higher the degree of crosslinking, the lower the water absorption. Hydrogels are mainly made of natural polymers and synthetic degradable biopolymer materials. Cross-linked polymers are polymers with a three-dimensional network structure. Hydrogels are a type of cross-linked polymers.
Figure 1. Structure and preparation of Poly (PEA-co-AAm) hydrogel
- Tissue engineering: Hydrogels, especially natural polymer hydrogels, are often used in tissue engineering scaffold materials, tissue engineering cartilage repair and other fields. Tissue engineering scaffolds can be divided into pre-plastic and injectable scaffold materials according to their plastic characteristics. Natural polymer hydrogels (chitosan, collagen, protein, etc.) are good injectable material. Because their gel network is filled with a large amount of water, they are similar in structure to the body tissues, and therefore has good biocompatibility. For example, a sodium alginate hydrogel scaffold with a 3D mesh structure can be prepared by using Ca2+ as a cross-linking agent and freeze-drying. The mesh structure of the scaffold shows isotropy, and different freezing treatment processes could significantly affect the scaffold mesh structure. Inoculating mouse liver cells on the scaffold material and culturing can obtain different forms of mouse liver cell lines.
Figure 2. Acrylic hydrogel
- Drug sustained release: The environmental sensitivity of the smart hydrogel makes it widely used as a drug carrier to achieve the controlled release of drugs. Some researchers introduced the in-situ polymerization of linear polymers to the interface of the hydrogel system to easily realize the strong adhesion between hydrogels of different materials and biological organs and hydrogel. Based on the introduction of different functional monomers, while ensuring the high mechanical strength of the hydrogel, it can further achieve the intelligent adjustment of the adhesion strength of the hydrogel interface. The research results further promote the application of hydrogel systems in the medical field.
Figure 3. Hydrogel lamination adhesion mechanism
- Adsorption treatment: Extensive attention has been paid to the treatment of water pollutants based on polysaccharide adsorbents. Because of the hydrogel adsorbents of natural polymer materials such as cellulose and chitosan, they are not only inexpensive but also have high-level sewage treatment capacity, some of which can also be recycled. Hydrogels have been widely reported for the adsorption of heavy metal ions and have been used in factories. For example, some researchers prepared starch-grafted acrylic hydrogels, and discussed their adsorption properties for Cd2+, and studied the effects of adsorbent concentration, adsorption time, grafting rate, and Cd2+ concentration on adsorption efficiency. Moreover, the adsorption process of Cd2+ by the hydrogel conforms to the two kinetic models of Langmuir and Freundlich, which has a better degree of agreement with the Langmuir isothermal model.
There are many types of hydrogels and there are various classification methods. According to different sources, they can be divided into natural polymer hydrogels and synthetic polymer hydrogels.
- Biomedical photonics: Natural polymers, such as starch, chitosan, alginate, agar and protein, can be cross-linked to form a hydrogel.
- Synthetic polymer hydrogel: It includes polyacrylic acid, polyacrylamide, polyethylene oxide, polyvinyl alcohol, and polyalkyl methacrylate.
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- Li Y, Rodrigues J, Tomas H.(2012) "Injectable and biodegradable hydrogels: gelation, biodegradation and biomedical applications." Chemical Society Reviews,41(6):2193-2221.