Negative swelling hydrogel enabled by osmotic pressure induced dehydration

Swelling positively in water is a common behavior of hydrogels, which may lead to reduced mechanical performance and stability. Enabling negative swelling represents a promising way to address those issues, but is extremely challenging to realize. Here real negative swelling hydrogels had been successfully prepared for the first time through a unique double network design. A transformable network confined by a rigid network was prone to self-assembly under osmotic pressure, which eventually caused dehydration of the entire hydrogel. Such gels could lose up to 35% of their weight underwater, and featured water-strengthened mechanical properties, enhanced structural responsiveness, underwater repair ability, resistance to deformation and swelling turn-off effect. Those unique properties allow future material development and applications to be carried out in much broader dimensions.