Conventional ionic diode membranes typically achieve ion rectification through asymmetric geometries, functional group-modified nanochannels, or the integration of nanochannels with distinct geometric and charge distributions. However, these nanoscale rectification strategies are difficult to directly implement in hydrogel systems, as their intrinsically 3D macroporous networks feature pore sizes (1-100 μm) larger than the ionic Debye length (1-100 nm), thereby precluding effective electric double layer overlap [Figure 2A]. This mismatch in structural length scales fundamentally limits the implementation of conventional nanochannel-based rectification mechanisms in hydrogel ionic diodes and remains a major bottleneck for improving rectification performance. Yin et al. constructed a quasi‐solid‐state heterogeneous hydrogel membrane^[11] that achieves efficient unidirectional ion transport through the synergistic integration of intrinsic electrochemical gradients, asymmetric geometries and functional groups. By rationally engineering the hydrogel matrix, this strategy establishes a new route for designing hydrogel ionic diodes with enhanced rectification performance.

The ion transport and rectification efficiency of hydrogel ionic diodes are often limited by interfacial incompatibility, which may induce delamination and increased interfacial resistance. Therefore, reducing ion migration resistance and achieving stable, high-efficiency ion transport across heterogeneous hydrogel interfaces remains a critical challenge [Figure 2B]. Lin et al. developed an integrated dual-gradient heterogeneous hydrogel ionic diode through DC electric field-induced migration of charged 2-(methacryloyloxy)ethyl trimethylammonium chloride and 3-sulfopropyl methacrylate potassium salt monomers^[12]. The dual-gradient distribution generates numerous heterogeneous microstructures (microdiodes) with low interfacial resistance throughout the hydrogel bulk. These interconnected micro-heterojunctions bridge dissimilar domains and overcome ion transport limitations commonly observed in conventional asymmetric hydrogels. By combining electric-field-induced ion migration with in situ polymerization, this strategy effectively addresses the challenges of high interfacial resistance and poor interfacial stability. Furthermore, ion transport across asymmetric hydrogel interfaces can be substantially improved through the construction of continuous spatial gradients of cationic and anionic polymers via rapid reaction/diffusion^[13], or by incorporating gel layers with robust interfacial adhesion^[14].

Beyond ion transport within the hydrogel matrix, the ion-electron conversion efficiency at the hydrogel-electrode interface critically influences the overall rectification performance of hydrogel ionic diodes [Figure 2C]. As the key bridge connecting ionic transport with external electronic circuits, this interfacial process is governed by ion adsorption, migration kinetics, and selective ion transport. Therefore, precise regulation of ion and electron transport kinetics at the electrode interface is essential for enhancing ion rectification. Wang et al. reported a unipolar hydrogel ionic diode with giant and programmable rectification, achieved through voltage-driven dynamic modulation of the interfacial electric double layer (EDL)^[15]. By controlling EDL evolution and associated electrochemical reactions under an external electric field, this strategy enables dynamic control over carrier migration, thereby enhancing interfacial charge transport and storage. Consequently, both ion flux and power output can be significantly improved. Compared with conventional hydrogel ionic diodes, which primarily rely on structural or chemical asymmetry and typically exhibit rectification ratios of 10¹-10², such interface-engineered devices can achieve rectification ratios exceeding 10³ and even reaching 10⁴, highlighting the unique advantages of interfacial engineering in enhancing ionic signal selectivity and transport efficiency. Notably, beyond voltage stimulation, external stimuli such as mechanical stretching^[16] or compression^[17] also enable dynamic modulation of the interfacial EDL and ion-electron coupling.

The practical application of hydrogel ionic diodes is currently limited by the inherent characteristics of aqueous electrolytes, including solvent evaporation and narrow electrochemical window, both of which compromise long-term rectification stability [Figure 2D]. By judiciously selecting high-boiling-point, low-volatility organic solvents and incorporating them into hydrophobic polymer matrices, solvent evaporation can be effectively suppressed while maintaining excellent thermal stability. For example, Jiang et al. designed an ion-rectifying diode based on an organic gel polymer electrolyte^[18]. The use of poly(methyl methacrylate)/propylene carbonate and poly(vinylidene fluoride-co-hexafluoropropylene)/propylene carbonate matrices provides excellent thermal and electrochemical stability, thereby expanding the operational voltage window and enhancing device stability. In addition, incorporating electrochemically stable ionic liquids, salts, and organic solvents as electrolytes can also prevent undesired electrochemical redox reactions (or parasitic faradaic processes), further improving the operational stability of hydrogel ionic diodes.

Over the past decades, the rapid development of functional hydrogel systems has greatly expanded the application scope of hydrogel ionic diodes across multiple fields. In energy harvesting [Figure 2E], they convert environmental stimuli - including light^[19], mechanical pressure^[20], humidity^[21], and salinity gradients^[22] - into electrical outputs, enabling sustainable energy technologies such as solar, piezoelectric, hydrovoltaic, and osmotic energy harvesting. In biochemical sensing [Figure 2F], hydrogel ionic diodes serve as self-powered sensing platforms capable of harvesting energy from the environment and converting ion migration into readable electrical signals. This capability enables continuous signal output without external power sources^[23]. In artificial synapses and neuromorphic computing [Figure 2G], hydrogel ionic diodes exploit ion rearrangement within confined charged channels to generate history-dependent and tunable conductance, mimicking dynamic behaviors of biological synapses and enabling emulation of synaptic memory and computation^[24,25].

Despite these advances, the practical performance of hydrogel ionic diodes remains constrained by low ion-transport efficiency and interfacial incompatibility. In energy-harvesting systems, although power density has improved through rational hydrogel design and surface charge modulation, the overall energy conversion efficiency remains limited by ionic transport resistance within polymer networks, which impedes concentration-gradient-driven ion migration. For biochemical sensing, sensing performance - including sensitivity, response speed, and detection limit - is strongly dependent on ion transport across heterogeneous interfaces and ion-electron coupling at hydrogel-electrode interfaces. Insufficient transport efficiency not only retards signal response but also constrains rapid detection and ultrasensitive sensing at extremely low analyte concentrations. Moreover, in artificial synapses and neuromorphic computing, ion migration in synthetic synaptic devices is constrained by hydrogel networks and heterogeneous interfaces, yielding transport efficiencies far below biological synapses. Consequently, higher operating voltages are required to achieve comparable performance, while parasitic electrochemical reactions prolong response times and increase operational energy consumption by several orders of magnitude. From a bioinspired perspective, improving ion transport efficiency requires drawing on the structural and mechanistic features of biological ion channels. Precisely defined pore sizes, ordered surface charge distributions, and synergistic multi-channel effects can lower the energy barrier for ion migration, enabling highly efficient ion transport pathways approaching the diffusion limit.

Hydrogel ionic diode systems are gradually evolving from individual functional devices to large-scale integrated iontronic platforms, thereby imposing increasing demands on intelligent and multiscale manufacturing strategies. At the microscale, individual devices achieve controlled water transport, ion migration, and charge transfer through miniaturized design, printing techniques, and functional modulation, providing reliable pathways for stable signal transmission and efficient information processing [Figure 2H]. At the mesoscale, multiple units can be assembled into functional modules or arrays, forming controllably interconnected networks that enable multichannel signal response, parallel processing, and complex logic operations [Figure 2I]. At the macroscale, these modules can be seamlessly integrated into wearable electronics, biointerface devices, intelligent sensing platforms, and soft robotic systems, ultimately yielding fully integrated devices with system-level intelligent functionalities [Figure 2J]. These requirements have driven multi-module integration strategies combining micro/nanofabrication, 3D printing, and self-assembly techniques, supporting hydrogel iontronics applications across multiple fields.