11.2.1　Introduction of hydrogel materials

Most organisms are composed of soft and water-containing gels.They can sense external environmental stimuli and generate real-time and fast responses.Among all artificial materials，hydrogels are the most similar materials to biological tissues.They are polymer networks formed by light cross-linking of hydrophilic polymers.Water molecules and some small molecules can move in the polymer network.Hydrogels can provide a microenvironment that mimics extracellular mechanisms and can regulate cell behavior and tissue function.In recent years，hydrogels based on synthetic or natural polymers have received extensive attention in the biomedical field.For example，contact lenses made of poly（hydroxyethyl methacrylate），bioadhesives made of recombinant protein or albumin for surgical bonding，wound dressings made of alginate polysaccharides，and hydrogels made of hyaluronic acid are widely used in clinical practice.Hydrogels have good biocompatibility，easy molding and processing，and controllable shape and size，and have broad application prospects in biomedical engineering fields，including three-dimensional tissue engineering matrices，drug carriers，and biocomposites.

According to their source，hydrogels can be classified into two types：natural polymer hydrogels and synthetic polymer hydrogels.Natural polymer hydrogels are mainly formed by natural polymer materials，including collagen，gelatin，fibrin，chitosan，sodium alginate，hyaluronic acid，chondroitin sulfate，agarose，etc.The compositions and structures of the natural polymer hydrogels are similar to the natural extracellular matrix，and they have the advantages of good biocompatibility and biodegradability.However，its mechanical properties are generally low and cannot meet the mechanical requirements for the repair of load-bearing tissues.Synthetic polymer hydrogels are mainly formed by cross-linking synthetic polymer materials，such as poly（ethylene glycol），poly（vinyl alcohol），poly（hydroxyethyl methacrylate），polyacrylamide，poly（acrylic acid），or their derivatives.Generally，synthetic polymer hydrogels have high mechanical properties，controllable structure and performance，and good repeatability.Yet，the lack of biological functions and poor biocompatibility should be considered for synthetic polymer hydrogels.(https://www.daowen.com)

According to the cross-linking method of hydrogels，they can be divided into physically crosslinked hydrogels and chemically crosslinked hydrogels.Physically crosslinked hydrogels refer to the formation of polymer chains through physical interactions（e.g.，chain entanglement）or weak supramolecular interactions（e.g.，ionic interactions，hydrogen bonds）.The reaction conditions for the formation of the physically crosslinked hydrogel are mild and no chemical cross-linking agent is required.Since the physical cross-linking is nonpermanent，the hydrogel may dissociate when the external environment（e.g.，pH，temperature，ion concentration）changes.The chemically crosslinked hydrogels are constructed using free radical polymerization，ultraviolet irradiation，and enzymatic crosslinking in which a stable covalent bond between polymer chains is generated to construct a hydrogel network.Compared with physically crosslinked hydrogels，their mechanical properties can be significantly improved，but residual chemical cross-linking agents，organic solvents，initiators，etc.may cause potential cytotoxicity.Therefore，the post-treatment for the materials is particularly important.