Hydrogels are a class of hydrophilic, crosslinked polymeric materials. They are valuable for drug delivery, biosensing and tissue engineering. Generally, hydrogels with nearly ideal network structures can be obtained via the click reactions which have advantages of fast reaction rate, modularity and less by-products. As the representative of click chemistry, Cu(Ⅰ)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction has been applied widely for the preparation of hydrogel materials. The copper metal contamination in the obtained hydrogels is a major concern because of the use of copper salts catalytic system. The drawback counteracts the applications of CuAAC reaction in the fields of synthesis of hydrogels. Copper-free click reactions, such as thiol-ene/yne reaction, furan/anthracene-maleimide (MI) Diels-Alder (D-A) reaction and strain-promoted alkyne-azide cycloaddition (SPAAC) reaction, have been widely used to prepare hydrogel materials. In this paper, the application of the copper-free click reaction in preparing the hydrogels and their functionalization are reviewed. In addition, the trend of their development is discussed.

Contents
1 Introduction
2 Synthesis of hydrogels via Cu-free click chemistry
2.1 Synthesis of hydrogels via thiol-ene click reaction
2.2 Synthesis of hydrogels via Diels-Alder click reaction
2.3 Synthesis of hydrogels via Cu-free azide-alkyne cycloaddition reaction
3 Outlook