Ionic superfluidics: a perspective on emerging frameworks for ion transport in confined channels

MAIN TEXT

Biological ion channels exemplify nature's exceptional solutions for achieving ultrafast directional ion transport, enabling critical physiological processes through remarkably efficient transport mechanisms^[1]. These sophisticated protein nanostructures benefit from structural features that facilitate ion transport rates approaching theoretical diffusion limits, with potassium channels demonstrating exceptional transport capabilities of up to 10⁸ ions per second - which is essential for rapid neural signal propagation and repolarization processes in excitable cells^[2]. In addition, voltage-gated sodium channels exhibit even faster gating characteristics, achieving activation within 100-500 μs to support action potential generation and propagation throughout the nervous system^[3]. Beyond these cation channels, anion channels, e.g., chloride channels, achieve a rapid ion transport rate exceeding 10⁶ ions per second to maintain cellular volume regulation, epithelial transport, and synaptic inhibition^[4]. Moreover, efficient ion transport in biological systems has found broad applications across multiple fields such as energy conversion, sensing, and signal transduction [Figure 1A-C]. A notable example is the electric eel, which could generate electric discharges exceeding 600 V through directional movement of sodium and potassium ions across specialized ion channels^[5]. In humans, sensory systems - including olfaction, gustation, and tactile sensation - are fundamentally regulated by ion flux through biological channels, a mechanism that has been recognized by several Nobel Prizes^[6]. Correspondingly, efficient ion transport plays an indispensable role in facilitating information transduction within the human body^[7]. The optimization of these biological transport systems, refined through billions of years of natural selection, provides compelling inspiration and fundamental design principles for bioinspired ion transport. However, these biological marvels face significant challenges in artificial environments - they require specific lipid membranes to maintain functionality, exhibit poor stability outside physiological conditions, suffer from limited operational lifetime, and cannot be easily integrated into synthetic devices^[8]. This limitation has driven the development of artificial ion channels that can mimic biological performance while offering superior robustness, tunability, and integrability for various technological applications.

Figure 1. Efficient ion transport in confined channels and applications. (A-C) High-efficiency ion transport in biological ion channels and relevant physiological function. This figure is quoted with permission from the authors^[5-7]; (D-F) Ion transport through confined artificial channels and their applications in energy conversion, sensing, and signal transduction. This figure is quoted with permission from the authors^[6,9,10]; (G) Achieving performance comparable to biological channels requires re-examining ion transport mechanisms in confined spaces.

Over the past decades, the artificial ion channels have witnessed remarkable breakthroughs with the development of diverse nanofluidic channels. The emergence of two-dimensional materials, block copolymers, metal-organic frameworks, and supramolecular polymers has created unprecedented opportunities for controlling ion transport within a confined space. These advanced material systems have enabled various applications across multiple technological domains [Figure 1B-F]. In blue energy harvesting, nanofluidic channels improve the conversion of salinity gradient energy into electrical power based on reverse electrodialysis technology^[9]. While power densities have progressively improved through optimized nanofluidic designs and surface charge engineering, the energy conversion efficiency remains limited by inherent transport resistances that restrict ion mobility under concentration gradients. In current systems, the power density remains below theoretical predictions based on single-pore measurements and scale-up calculations, primarily due to significant performance degradation observed when testing multi-channels. This scaling limitation suggests that conventional ion transport mechanisms fail to account for the critical role of synergistic ion transport in multi-channel configurations. For biosensing and analytical applications, engineered nanochannels provide exceptional platforms for detection of biomolecules through characteristic modulations in ionic current^[6]. The detection sensitivity and response time in these systems are directly governed by ion transport rates, which determine how quickly analyte molecules can be transported to the sensing region. Current limitations in transport rate impose fundamental constraints on detection speed and ultimately restrict the temporal resolution achievable in real-time monitoring applications. In neural signal transduction and neuromorphic computing, nanofluidic elements serve as crucial interfaces between electronic systems and ionic signals^[10]. The energy efficiency and signal fidelity in these hybrid systems are intimately tied to ion transport kinetics through the confined channels. Present artificial synaptic devices typically operate at energy consumption levels several orders of magnitude higher than biological synapses, primarily due to insufficient ion transport rates that require higher operating voltages and produce slower response characteristics.

Although substantial progress has been made in fabricating bioinspired channel materials with different nanostructures, further enhancement of ion transport rates faces fundamental theoretical limitations. Current theoretical frameworks, primarily based on classical electrokinetic models and modified Nernst-Planck equations, fail to account for the ultrafast ion transport phenomena observed in biological systems^[11]. This theoretical gap significantly hinders the rational design of next-generation artificial ion channels, whose performance remains far below that of biological systems. Consequently, the performance of bioinspired systems in energy conversion, sensing, and information processing is degraded. Therefore, we need to re-examine ion transport processes in biological systems and explore how ions traverse confined channels [Figure 1G]. Jiang et al. propose a macroscopic quantum state mechanism for ion transport^[12]. Drawing from the theoretical framework, we can gain a clear understanding of how ions can achieve ultrafast transport in confined spaces. This framework suggests that under confined channels, ions could form a collective quantum fluid that enables near-dissipation-free transport. The establishment of such macroscopic quantum states provides a fundamental explanation for exceptional transport phenomena beyond classical predictions and offers new principles for designing advanced bioinspired channel systems with performance approaching biological levels.

The 2025 Nobel Prize in Physics recognized groundbreaking advances in quantum tunneling effects, providing a crucial theoretical foundation for understanding how microscopic particles penetrate classically insurmountable energy barriers through their inherent wave-like properties^[13]. Complementing this fundamental understanding, Professor Jiang's group proposed that ion transport through biological channels exhibits quantum-state transmission characteristics with remarkably low energy dissipation, wherein nanoconfined ions exhibit coherent transport [Figure 2A and B]^[14]. The macroscopic quantum state refers to a macroscopic quantum coherent state in which ions undergo coherent transport within confined environments, as permitted by quantum mechanics^[12]. Under this state, ions form collectively coupled chains displaying distinct quantum wave characteristics - including phase coherence, interference, and resonant oscillation - while traversing energy barriers via quantum tunneling. This coherent transport demonstrates long-range phase correlation with minimal dispersion, enabling energy-efficient ultrafast transmission that fundamentally transcends classical diffusion mechanisms. This framework establishes a physical basis for ultrafast biological signal transfer, thereby bridging macroscopic quantum phenomena with ionic dynamics.

Figure 2. Ion transport frameworks in confined channels. (A) Schematic representation of ultrafast ion transport through the sub-nanochannel with an ordered strand, and the proposed “quantum tunneling fluid effect”. This figure is quoted with permission from the authors^[14]; (B) Schematic illustration of macroscopic quantum state of ion channels; (C) Summary of ion coherence-determined transport through the confined ion channel. This figure is quoted with permission from the authors^[15].

As shown in Figure 2C, this proposed framework finds strong support in recent multiscale simulations of biological ion channels^[15]. Critically, it has been demonstrated that confined ions can sustain a nanosecond-lived coherent oscillation state, which acts as the physical basis for high-flux transport. The process is further governed by picosecond-scale coherent ion transfers, directional translocation events that occur without perturbing the collective quantum coherence. The quantum-mechanical nature of this phenomenon is underscored by the fact that the estimated de Broglie wavelength of the confined ions is comparable to the dimensions of the molecular filter, creating a mechanism where quantum effects become dominant. Furthermore, the coherence and the resulting ionic conductance can be resonantly enhanced by applying external electromagnetic fields tuned to the characteristic frequency of the ion oscillation, providing a direct means of controlling transport efficiency. This convergence of evidence strongly suggests that ions within these biological nanopores can indeed behave as a correlated quantum fluid, thereby supporting the core principles of the macroscopic quantum transport model. These results not only challenge conventional transport theories but also open new avenues for quantum-inspired bioelectronics and neuromorphic computing systems, potentially paving the way for future energy conversion and information processing technologies. Experimentally validating the macroscopic quantum state faces distinct methodological hurdles: tracking synchronized phase correlation of hundreds of ions requires techniques with simultaneous nanoscale spatial and sub-picosecond temporal resolution, which is currently lacking. Isolating quantum coherence from classical collective transport effects remains challenging, as both can produce similar high-flux, low-dissipation phenomena. Direct quantification of the state’s “macroscopic” traits (long-range correlation, nanosecond lifetime) also lacks established experimental protocols, relying heavily on indirect transport measurements and simulation cross-validation.

The realization of ion superfluidics promises to break the trade-off that constrains the following domains. In energy conversion, bioinspired ion channels could potentially enable osmotic power generators with near-zero internal resistance, potentially doubling the energy conversion efficiency from salinity gradients and making blue energy a truly competitive renewable source. Three strategies may be possible to reduce transport resistances: constructing atomic-scale ordered channel structures to match ion de Broglie wavelengths, engineering bioinspired surface charge distributions, and designing synergistic multi-channel arrays. These approaches would leverage quantum coherence to overcome classical diffusion limitations, lowering inherent resistances under concentration gradients. In addition, mimicking biological ion channels - including precise pore sizes, ordered internal architectures, and periodic surface charge distributions - promotes the macroscopic quantum state. This bioinspired design strategy enhances ion coherence and transport efficiency, bridging the performance gap between artificial and biological channels. The combination of ultra-low transport resistance and highly selective transport could approach the theoretical maximum efficiency of osmotic energy conversion. For biosensing applications, the extreme sensitivity of artificial ion channels to single-ion interactions could enable detection limits approaching the ultimate quantum limit. The ultimate quantum limit in biosensing denotes the ability to detect single biomolecules by resolving their discrete perturbations to the quantum coherence of confined ions. Unlike classical limits constrained by thermal noise or diffusion, this quantum-enabled limit leverages the high sensitivity of coherent ion transport to subtle interfacial interactions, enabling detection of rare biomarkers or single-molecule binding events with minimal background interference. This would revolutionize medical diagnostics by enabling detection of rare biomarkers and early disease indicators that currently evade conventional detection methods. Most groundbreakingly, in neuromorphic computing, the creation of bioinspired ionic channels would enable the development of “iontronic synapses” that closely mimic the brain's own efficient, ion-based signaling while eliminating Joule heating effects. These artificial synapses could operate at energy costs approaching biological levels, potentially enabling brain-scale computing systems that are currently impossible with conventional electronics while maintaining the brain's remarkable ability to learn, adapt, and process information in parallel.

Looking ahead, the ionic superfluidics field stands at the threshold of a fundamental transformation - from classical nanofluidics to iontronics governed by quantum coherence. While classical frameworks have provided foundational insights, they are inherently constrained by diffusive transport mechanisms. The proposed framework of a macroscopic quantum state in ion channels transcends these limitations by establishing a physical basis for coherent ion transport. This paradigm shift, which bridges condensed matter physics with nanofluidics, opens a new chapter for iontronics. The continued validation and refinement of this quantum model are poised to establish a new cornerstone for bio-inspired technologies, ultimately unlocking their full potential in advanced energy conversion and neuromorphic computing.