Hydrogels based on polysaccharide and polypeptide have been utilized in a wide range of biomedical applications because of the rich resources, good biocompatibility and biodegradability. However, their wide applications are always restricted by the relatively poor mechanical strength. This review aims to give an overview of strengthening of hydrogels based on these materials, and highlights four kinds of ways: strengthening via blending; strengthening via special topological structure; strengthening via crystallization; and strengthening via imitating robust hydrogels based on synthetic materials, in terms of their enhanced mechanical properties and corresponding strengthening mechanisms. Finally, we describe the main current achievements in the study of strengthening of hydrogels based on polysaccharide and polypeptide, and also provide some suggestions for further work particularly with regard to some unanswered questions and possible avenues for further enhancement of gel mechanical properties.

Contents
1 Introduction
2 Strengthening via blending
2.1 Blending via hydrogen bond
2.2 Blending via electrostatic interaction
3 Designing special topological structures
3.1 Dendritic topology structure
3.2 Star topology structure
4 Strengthening via crystallization
5 Strengthening via imitating robust hydrogels based on synthetic materials
5.1 Nanocomposite hydrogels
5.2 Double-network hydrogels
6 Conclusion and outlook