Recent progress in proton involvement and coupling for bio-realistic synaptic devices

Neuromorphic systems aim to emulate the energy efficiency and adaptive learning of the human brain. Recent studies have suggested that ions such as Na⁺, K⁺, and H⁺ play key roles in biological decision-making, emphasizing ion-specific signaling in neural processes. Among emerging hardware approaches, proton-based synaptic devices have gained attention for their ability to replicate ion-mediated signaling in synapses. Owing to their high mobility arising from their low atomic mass, protons enable fast, low-power, analog switching, which is crucial for mimicking biological efficiency. This review classifies the switching mechanisms of protonic neuromorphic devices into two types: proton involvement and proton coupling. By analyzing two- and three-terminal architectures, we present a framework showing how each mechanism modulates resistance in different materials. Proton involvement typically involves field- or environment-driven ionic motion, whereas proton coupling refers to mechanisms in which protons interact with other ions to regulate redox activity and transport. In this context, we emphasize the importance of studying ion-mediated processes, particularly those involving protons, as windows into biological adaptability and intelligence. Ultimately, this review highlights that understanding proton-based switching mechanisms is crucial for realizing neuromorphic hardware that emulates the energy efficiency and adaptability of the brain.