Ion-specific interactions provide a powerful route to regulate the phase behavior and transport properties of polyelectrolytes. Here, we present a hydrogel-based fluidic memristor constructed from a biomimetic cationic polyelectrolyte whose phase behavior is governed by the law of matching water affinity. In this system, weakly hydrated chaotropic anions selectively interact with the polycationic network, inducing local phase separation and transient densification through strong cation-π interactions, whereas strongly hydrated kosmotropic anions produce negligible memristive response. By coupling this anion-selective phase transition with asymmetric ion transport, the hydrogel device exhibits history-dependent ionic conductance and pinched current–voltage hysteresis. Under positive bias, anions migrate into the confined polycation region and decrease conductance through ion-mediated phase separation; under reverse or zero bias, ion redistribution and polymer relaxation gradually restore the initial state. This dynamic and reversible process enables short-term memory, pulse-dependent conductance modulation, and 4-bit temporal encoding. The device therefore functions as a soft physical reservoir that converts anion transport history into analog electrical states. Our results demonstrate that ion-specific polymer phase behavior can be harnessed as a design principle for chemically gated fluidic memristors, opening opportunities for soft neuromorphic systems that process both electrical and chemical information.
