11.2.1.2　Injectable hydrogels

Injectable hydrogels refer to hydrogels that were fluid before injection，and after injecting into the subcutaneous tissue or muscle tissue through a syringe，they form gel insitu at the wound sites.The formation mechanism is based on the polymer material's response to external stimuli to change the state or conformation of the polymer under physiological conditions and complete the conversion process from solution to gel.The injectable hydrogels usually respond to external stimuli（e.g.，temperature，pH，enzyme）.Because these hydrogels have excellent biocompatibility and mechanical properties，they can quickly close wounds of any shape，bond with the contour of the wound to provide a physical barrier，also maintain the moist environment of the skin，and quickly stop bleeding and prevent wounds infection.These hydrogels are potentially useful in medical dressing.

Zhang et al.designed and developed a PEG2000-based injectable hydrogel by collagen（COL），chitosan（CS），and dibenzaldehyde through dynamic imine bond.COL/CS hydrogel has good thermal stability，injectability，pH sensitivity，and antibacterial capability.COL/CS hydrogel can adhere well to the tissue surface.After the COL/CS hydrogel acts on the moist wound surface，it can effectively promote wound healing and rapid wound hemostasis.Yin et al.introduced dopamine（DA）to aldehyde-modified alginate（ALG）skeleton to form bifunctional ALG and obtained injectable bioadhesive hydrogel（PGA/ALG-CHO hydrogel）with hydrazide-modified poly（L-glutamic acid）.The injectable bioadhesive hydrogel（PGA/ALG-CHO hydrogel）possessed various interactions（e.g.，hydrogen bond andπ-πstacking between catechol groups and tissues），endowing it with superior hemostatic capability.Liu et al.mimicked the characteristics of the extracellular matrix by using sulfhydryl bovine serum protein，KK polypeptide，and Ag+to build a protein-based injectable hydrogel that can promote the healing of infected wounds.In this protein-based hydrogel，Ag+，as a crosslinking agent，can not only provide a sterile microenvironment and strong antibacterial ability but also introduce K2（SL）6K2（KK）peptides to generate blood vessels.In addition，the results of in vivo experiments also showed that the protein-based hydrogel has considerable collagen deposition and vascularization ability in the early stage of wound healing so that new tissues are rapidly regenerated and new hair follicles appear（Figure 11-3）.(https://www.daowen.com)