Hydrogels—natural or synthetic polymer networks that swell in water—can be made mechanically,chemically and electrically compatible with living tissues.There has been intense research and development of hydrogels for medical applications since the invention of hydrogel contact lenses in 1960.More recently,functional hydrogel coatings with controlled thickness and tough adhesion have been achieved on various substrates.Hydrogel-coated substrates combine the advantages of hydrogels,such as lubricity,biocompatibility and anti-biofouling properties,with the advantages of substrates,such as stiffness,toughness and strength.In this review,we focus on three aspects of functional hydrogel coatings:(i)applications and functions enabled by hydrogel coatings,(ii)methods of coating various substrates with different functional hydrogels with tough adhesion,and(iii)tests to evaluate the adhesion between functional hydrogel coatings and substrates.Conclusions and outlook are given at the end of this review.