As biological signals are mainly based on ion transport, the differences in signal carriers have become a major issue for the intimate communication between electrical devices and biological areas. In this respect, an ionic device which can directly interpret ionic signals from biological systems needs to be designed. Particularly, it is also required to amplify the ionic signals for effective signal processing, since the amount of ions acquired from biological systems is very small. Here, we report the signal amplification in ionic systems as well as sensing through the modified design of polyelectrolyte hydrogel-based ionic diodes. By designing an open-junction structure, ionic signals from the external environment can be directly transmitted to an ionic diode. Moreover, the minute ionic signals injected into the devices can also be amplified to a large amount of ions. The signal transduction mechanism of the ion-to-ion amplification is suggested and clearly verified by revealing the generation of breakdown ionic currents during an ion injection. Subsequently, various methods for enhancing the amplification are suggested.

Iontronics is an artificial platform using ions or molecules as signal carriers in an aqueous environment and is inspired by biological systems and their operating principles. Applications of iontronics have been primarily developed to mimic the characteristics of biological systems or to form seamless biointerfaces for communication. This review provides a comprehensive description of such endeavors in iontronics over the recent decades, as well as demonstrations pertaining to biomimetic nonlinear behaviors and ionic chemical delivery devices. The research highlights and applications are discussed based on the types of charge-selective materials used and their underlying principles. As iontronics is still at the early stage of development and diversification, a brief overview of its historical aspects and origin is first provided, followed by theoretical discussions regarding each iontronic material and its related applications. Finally, the review is concluded with some perspectives regarding future developments of iontronics in relation to natural systems in living organisms.

In response to the extensive utilization of ionic circuits, including in iontronics and wearable devices, a new method for fabricating a hydrogel-based ionic circuit on a polydimethylsiloxane (PDMS) microchip is reported. Prolonged UV/ozone oxidation combined with proper surface functionalizations and a novel microchip bonding method using thiol-epoxy click reaction enable the robust attachment of the photopolymerized hydrogel to the microchannel surface for eventual operation in electrolytes as an ionic circuit. The stretchable ionic diode constructed on the PDMS microchip shows a superior rectification ratio even under tensile stress and long-term storage stability. Furthermore, the combination of the ionic circuit and unique material properties of PDMS allows us to maximize the versatility and diversify the functionalities of the iontronic device, as demonstrated in a pressure-driven ionic switch and chip-integrated ionic regulator. Its iontronic signal transmission mimicking the excitatory and inhibitory synapses also evinces the potential of the hydrogel-based iontronics on the PDMS microchip as developed toward an aqueous neuromimetic information processor while opening up new opportunities for various bioinspired applications.

Coordination of synapses onto electrodes with high specificity and maintaining a stable and long-lasting interface have importance in the field of neural interfaces. One potential approach is to present ligands on the surface of electrodes that would be bound through a protein–protein interaction to specific areas of neuronal cells. Here, we functionalize electrode surfaces with genetically engineered neuroligin-1 protein and demonstrate the formation of a nascent presynaptic bouton upon binding to neurexin-1 β on the presynaptic membrane of neurons. The resulting synaptically connected electrode shows an assembly of presynaptic proteins and comparable exocytosis kinetics to that of native synapses. Importantly, a neuroligin-1-induced synapse–electrode interface exhibits type specificity and structural robustness. We envision that the use of synaptic adhesion proteins in modified neural electrodes may lead to new approaches in the interfacing of neural circuity and electronics.

Ion current rectification (ICR), diodelike behavior in surface-charged nanopores, shows promise in the design of delivery probes for manipulation of neural networks as it can solve diffusive leakages that might be critical in clinical and research applications. However, it has not been achieved because ICR has restrictions in nanosized dimension and low electrolyte concentration, and rectification direction is inappropriate for delivery. Herein, we present a polyelectrolyte gel-filled (PGF) micropipette harnessing inverted ICR as a delivery probe, which quantitatively transports glutamate to stimulate primary cultured neurons with high efficiency while minimizing leakages. Since the gel works as an ensemble of numerous surface-charged nanopores, the current is rectified in the micro-opening and physiological environment. By extending the charge-selective region using the gel, inverted ICR is generated, which drives outward deliveries of major charge carriers. This study will help in exploring new aspects of ICR and broaden its applications for advanced chemical delivery.