Ca2+ induced silk protein hydrogels: structural characteristics, rapid water absorption and anti-compression behavior

Calcium is an essential element for regulating physiological activities and forming hard tissues, and it has significant value for the functional modification of biomedical materials. Based on the promising applications of two natural proteins silk sericin (SS) and silk fibroin (SF) in the field of biomaterials, a series of SS-based SS/SF hydrogels loaded with Ca²⁺ were developed and the effects of Ca²⁺ binding on the structure, rapid water absorption, and anti-pressure behavior were investigated. All hydrogels showed a regular porous network structure. After loading Ca²⁺, the SS/SF hydrogels maintained the original porous network structure, while the pore size decreased with the increase of loaded Ca²⁺, resulting in a decrease in the water absorption. However, the Ca²⁺-loaded SS/SF hydrogels still possessed notable rapid water absorption capacity, with over 480% water absorption rate within 5 s. Compared to the SF hydrogel or SS/SF hydrogels without Ca²⁺, the contents of α-helix and β-sheets in the SS/SF hydrogels increased after loading Ca²⁺, that was conducive to the molding of hydrogels. The compressive properties of the hydrogels also slightly increased by binding Ca²⁺, due to a slight increase in crystallinity. The study provided important guidance for the application of SS in areas such as hemostatic materials or bone repair scaffolds.