Ionically Cross-Linked Silk Microfibers/Alginate Tough Composite Hydrogels with Hierarchical Structures

Abstract

Developing hydrogels with enhanced mechanical properties has attracted broad attention in recent years. In this work, we propose a facile procedure to prepare tough composite hydrogels by incorporating silk microfibers (mSF) into alginate ionically cross-linked networks. The mSF gives rise to ionic bonds with Ca2+ and interfacial hydrogen bonds because of the carboxyl groups on the surface of mSF, and the neighboring alginate chains are interlinked by mSF, which synergistically led to efficient energy dissipation and prevention of stress concentration. The attained composite hydrogels show superior elastic modulus (1.58 MPa), tensile strength (1.60 MPa), and unique adaptive interface response. Moreover, the mechanical properties of the composite hydrogels can be tailored by the concentration of mSF and the time immersing the composite hydrogels in CaCl2 solution post-cross-linking. Intriguingly, the mechanical properties can be further improved through a prestretching methodology to align the alginate chains along the stress direction, where the oriented hierarchical structures are formed and well-retained in the prestretched composite hydrogels. We envisage that this study provides a general strategy for designing composite hydrogels with both excellent and tunable mechanical properties, which enriches the use of alginate hydrogels for promising applications where high loading is needed.