Double-network hydrogels with a gradient hydrophobic coating that prevents water evaporation and allows strong adhesion to a solid substrate

Hydrogels are soft and wet polymeric materials containing large amounts of water. The high water content gives hydrogels their unique characteristics, such as biocompatibility. However, the water in the hydrogels easily evaporates under atmospheric conditions. In addition, since hydrogels consist of mostly water, adhering them to solid substrates with commercial glues is difficult. To overcome these problems, we developed a method to apply a robust gradient hydrophobic coating to tough double-network (DN) hydrogels. Liquid hydrophobic monomers were dropped onto the DN gel precursor (the first network gel containing the second network precursor) and spontaneously spread to cover the gel surface. This led to the simultaneous polymerization of the hydrophobic monomer near the gel–air interface and the second network precursor of the bulk gel to yield a gradient hydrophobic coating. The resulting hydrophobic coating effectively prevents water evaporation from the coated DN gel. Moreover, strong adhesion of the coated gel to various solid substrates was achieved with commercial glues. We report a method for a gradient hydrophobic coating of tough double-network (DN) hydrogels. The robust gradient coating was easily achieved by simultaneously polymerizing the hydrophilic second network precursor in a bulk first network gel and the hydrophobic coating precursor near its surface. The resulting hydrophobic coating effectively prevents water evaporation from the coated DN gel. Moreover, strong adhesion of the coated gel to various solid substrates was achieved with commercial glues.