Osmosis-driven stiffening of structured hydrogels

Plant cells harness osmotic pressures to stiffen their leaves through strong turgor pressures. Key to this osmosisdriven stiffening is the confinement of liquids within semipermeable membranes that can regulate the transport of water molecules and ions. Inspired by the turgor effect in plants, we fabricate rather stiff hydrogels with inclusions and fill them with polyelectrolytes possessing a high degree of swelling. The swelling of these polyelectrolytes is spatially confined by the stiffer inert hydrogel surrounding, resulting in an up to three-fold increase in the stiffness of these structured hydrogels compared to that of the matrix with water-filled inclusions. We leverage osmotic pressure gradients to change the morphology of cm-sized hydrogel leaves by immersing their bottom parts in water, without the need for additional stimuli that consume energy or require a change in the environmental conditions. Similarly, we exploit the osmosis-driven stiffening to release a ball. We foresee these materials to open up new possibilities to actuate soft materials in a benign, energy-efficient manner.