Fig. 3: Cu²⁺ ions generate a transient osmotic swelling pulse upon rapid release by an acid stimulus.

a Schematic presentation of the mechanism, showing how acid delivered after Cu²⁺ has been removed from the external environment of the hydrogel protonates carboxylate groups and thereby releases the complexed Cu²⁺. Fast release would generate an osmotic swelling pulse (top) before acid contracts the hydrogel again, while slow addition of acid should lead to a slow Cu²⁺ release without transient swelling (bottom). b Experimental demonstration of the fast Cu²⁺ release (regime τ_H+ < τ_⊥), triggered by direct addition of concentrated 1 M HCl, which results in rapid disappearance of the blue color and transient reorientation of the microplates to an upright position. The dotted outlines indicate the change of the cross-sectional view of a single plate from nearly square (in the tilted state) to rectangular (in the upright state), and back to nearly square. c Stepwise addition of acid leads to a slow release of Cu²⁺, such that the gel remains contracted without transient swelling (regime τ_H+ > τ_⊥). Scale bar: 25 μm. d, e Time-dependent microplate tilt angle and r value (acid stimulus added at t = 0) for fast (d) and stepwise, slow (e) addition.