Ions are among the most familiar and fundamental components of matter. All living organisms rely on ions to sustain essential biological functions. In humans, sodium and potassium ions regulate body-fluid balance and support the circulatory system, while calcium ions play key roles in sensing information such as pain. Ions are also involved in the repair and maintenance of biological functions, including the formation and regeneration of bones and teeth. In this sense, ions are deeply involved in almost all complex life processes.

Living organisms also possess sophisticated receptors, such as mechanoreceptors and photoreceptors. These receptors convert physical stimuli applied to the body surface into chemical signals, which are then transmitted to the central nervous system through sensory nerves. In other words, biological systems can convert mechanical and optical energy into chemical signals through highly controlled ionic processes. This demonstrates that living organisms possess remarkably advanced ion-control mechanisms, suggesting that the potential of ions is far more diverse than currently imagined.

However, despite this biological sophistication, artificial ion-control technologies remain limited. Practical applications of ions have so far been largely confined to energy-storage functions, such as batteries and capacitors, where electrochemical reactions and ion accumulation/release are utilized. Recently, we demonstrated a new function of ions by fixing ionic distributions to form an electric-double-layer electret, in which electric charge can be retained semi-permanently.

More recently, we have discovered that ions can be controlled by cross-correlated energies, such as light, mechanical force, and magnetic fields, which are conventionally considered unrelated to ion control. In these phenomena, the application of mechanical, optical, or magnetic energy to the electric double layer of ionic liquids induces rapid and soft changes in the molecular structures of the constituent cations and anions, as well as in their aggregates. These changes also modify the distribution of positive and negative ions within the electric double layer, resulting in variations in ionic density and, consequently, changes in the electric-double-layer capacitance.

In this talk, I will present emerging functionalities of ions enabled by cross-correlated energies and discuss their potential applications in future ion-based devices.