Fig. 1.
Conceptual scheme of dynamic memorizing–forgetting behavior from human to soft materials. (A) Memorizing–forgetting behavior in brain. Certain mediates such as dopamine and protein are involved in the process of forming memory through learning, and the memory decays gradually and spontaneously with time, exhibiting a dynamic forgetting. (B) Memorizing–forgetting curve of brain or soft materials. The memorizing–forgetting behavior of soft materials is based on its nonequilibrium process in response to stimuli. Stimuli-on induces quick formation of memory, which gradually decays with time after stimuli-off. (C) Design principle of dynamic memorizing–forgetting hydrogels based on thermal stimulus. We use a hydrogel containing abundant dynamic bonds. The gel has unique nonequilibrium features: It swells fast in a hot bath (learning temperature TL) and shrinks slowly in a cold bath (forgetting temperature TF). Moreover, the gel instantly turns from transparent to opaque (memory retravel) when being switched from the hot bath to the cold bath due to structure frustration. Consequently, the thermal history of the gel can be memorized and detected by its opaque appearance. With the shrinking progress toward the equilibrium, the gel gradually recovers to transparent, exhibiting a spontaneous and slow forgetting of the thermal history. The forgetting time (tF) in the cold bath is correlated to the thermal learning time (tL) in the hot bath as tF/tL = Dsw/Dsh, where Dsw and Dsh are the cooperative diffusion coefficients corresponding to swelling and shrinking, respectively. The fast water-absorbing and slow-releasing kinetics of the gel, corresponding to Dsw/Dsh >> 1, result in the fast learning and slow forgetting behaviors.