Biomimetic action-potential transmission via mechano-gated potassium channels assembled by quadruple hydrogen-bonded crown ethers

Abstract

Biological potassium channels exemplify nature’s precision in ion discrimination, governing critical processes such as osmotic regulation and neuronal signaling. Developing artificial potassium channels with biological-level and dynamic selectivity is of fundamental importance for intelligent nanofluidic devices. Here, we present a biomimetic mechanoresponsive potassium channel using telechelic polymers with 18-crown-6 and 2-ureido-4-pyrimidinone terminals, achieving a record potassium/sodium selectivity of 104.7. Quadruple hydrogen-bonded supramolecular networks enable both membrane elasticity and pressure-responsive crown ether aggregation state modulation. Mechanical deformation induces a structural reconfiguration that decelerates potassium conduction while accelerating sodium transport, effectively mimicking action potential generation through the reversible inversion of sodium/potassium flux. This mechanoregulatable iontronic mechanism establishes foundational principles for dynamic single-ion selectivity membranes surpassing static biological analogs, highlighting its potential in ion separation, desalination, and renewable energy conversion.

The assembly of crown ether channels enables the mechanoregulation of K⁺/Na⁺ selectivity to mimic neural transmission.

INTRODUCTION

Potassium (K⁺) channels critically maintain cellular homeostasis through two defining functional attributes: extraordinary K⁺/Na⁺ selectivity and dynamic ion-gating mechanisms (1–3). These transmembrane conduits enable the selective permeation of K⁺ while excluding competing cations, thereby preserving osmotic equilibrium and orchestrating signal transduction cascades (4, 5). The molecular basis for the K⁺ discrimination resides in angstrom-scale selectivity filters where precisely arranged oxygen atoms within the pore lumen establish specific coordination geometries that favor K⁺ stabilization (6, 7). When stimulated by external signals, such as voltage and force, the geometric characteristics of the selective filter experience reversible changes, regulating K⁺/Na⁺ selectivity and converting environmental signals into ionic signals. Mimicking biological K⁺ channels with perfect ion discrimination, ultrafast ion permeation, and smart ionic gate mechanism could pave the way to wide fields of ion recovery (8, 9), desalination (10, 11), energy conversion (12–14), and so on (15, 16).

Extensive efforts have focused on engineering biomimetic K⁺ channels. Crown ethers, macrocyclic polyethers containing several oxygen-methylene units, emerge as primary building blocks due to their structural homology to biological selectivity filters. The incorporation of crown ethers featuring distinct cavity architectures into ion channel systems enables the selective discrimination among specific alkali ions (17, 18). There exist two design strategies for artificial K⁺ channels, which are divided into polymer-based systems (19–22) and nanofluidic architectures (23–27). Polymeric scaffolds incorporating 18-crown-6 moieties exhibit compromised selectivity due to swelling-induced network enlargement (28). Nanochannel surfaces with crown ether strategies enhance discrimination via nanoconfined ion-molecule binding. However, both approaches fall short of biological-level K⁺/Na⁺ selectivity and critically lack the stimulus-responsive gating mechanisms essential for dynamic ion regulation in living systems.

Here, we report a mechanoregulated K⁺ channel based on the aggregation of terminal crown ethers in a telechelic polymer, showing a record K⁺/Na⁺ selectivity of 104.7. The aggregation of two terminal crown units is induced by a quadruple hydrogen bond from the terminal 2-ureido-4-pyrimidinone (UPy) groups, thereby minimizing the content of hydrophilic crown ether groups and avoiding the loss of K⁺/Na⁺ selectivity. The elastic membrane experiences marked deformation under external pressure, regulating the structure of the crown ether channel. As a result, the slower K⁺ migration and faster Na⁺ transport contribute to the fabrication of mechanoactivated Na⁺/K⁺ pumps. The mechanism of mechanoregulated K⁺/Na⁺ selectivity is revealed by molecular dynamics (MD) simulation, as K⁺ is more easily captured by crown channels and exhibits a lower energy barrier for K⁺ migration. Moreover, the membrane with mechanoresponsive K⁺/Na⁺ selectivity is extended to simulate the transduction of nerve impulse, as the membrane potential reverses under mechanical activation.

RESULTS

Fabrication of the quadruple hydrogen-bond–induced crown-aggregate membrane

The quadruple hydrogen-bond–induced crown-aggregate (qHB-CA) membrane was fabricated by a telechelic polymer, synthesized by a sequential modification of 18-crown-6 and quadruple hydrogen-bond (UPy) units to carboxyl-terminated polybutadiene [PB-(COOH)₂] (Fig. 1A and fig. S1; see more details in Materials and Methods). From the ¹H nuclear magnetic resonance (NMR) spectra, the carboxyl signal disappears, while signals corresponding to 18-crown-6 and UPy groups emerge after modification (fig. S2). Fourier transform infrared analysis further confirms the loss of the carboxyl signal and shows new peaks for the crown ether (1138 and 1238 cm⁻¹) and UPy groups (1590 to 1660 cm⁻¹) (fig. S3). X-ray photoelectron spectroscopy reveals the presence of nitrogen in the qHB-CA membrane (fig. S4). Gel permeation chromatography confirms that the terminal group modification did not result in chemical coupling between polymer chains (fig. S5). The biterminal UPy groups function as physical linkers between polymer chains through quadruple hydrogen bonds (29, 30), forming alternating hard UPy-crown aggregates and soft PB chain structures. Thus, the fiber-like network structure is observed from the atomic force microscopy (AFM) height image. The bright part contributes to the stacking of the crown ether–UPy units, with an areal density of 10¹¹ cm⁻² (Fig. 1B). The continuous ion transport network is verified through AFM height images of the membrane surface at different depths (fig. S6). Because the alternating soft-hard chains experience microphase separation, the signal corresponding to a feature size of ~10 nm is seen in the small-angle x-ray scattering (SAXS) result. Meanwhile, the aggregation of crown ether–UPy is also confirmed by the wide-angle x-ray scattering (WAXD) curve and pattern, with a d-spacing of ~4.58 Å (Fig. 1C). The physical UPy cross-linkers between polymer terminals bestow the membrane with excellent elasticity, showing a strain of 250% and excellent deformation ability (Fig. 1D and fig. S7).

Fig. 1. Quadruple hydrogen-bond–induced crown ether aggregation with a telechelic polymer.

(A) The design strategy of the crown ether aggregate membrane for the K⁺ channel. (B) AFM height image of the qHB-CA membrane, showing crown ether–aggregate structure. (C) X-ray scattering curves of the membrane, showing crown ether aggregate self-assembling into a microphase separation structure. Inset: Corresponding two-dimensional x-ray scattering pattern. arb., arbitrary. (D) Strain-stress curve of the elastomeric membrane, showing excellent flexibility.

Ultrahigh K⁺/Na⁺ selectivity of the qHB-CA membrane

The crown ether aggregate induced by the quadruple hydrogen bond shows ultrahigh K⁺ selectivity toward Na⁺ due to the specific binding between K⁺ and 18-crown ether-6 (31, 32). Under external driving force such as an electric field or concentration gradient, K⁺ selectively migrates between adjacent crown ethers in a hopping mode (Fig. 2A). The K⁺/Na⁺ selectivity of the qHB-CA membrane is first investigated by current-voltage curves in 0.1 M KCl and NaCl electrolytes. As shown in Fig. 2B, the K⁺ transport current is more than one order of magnitude higher than the Na⁺ transport current under the electric field. To quantify the difference in ionic transport, we defined the K⁺/Na⁺ selectivity (S_E) as the ratio of ionic conductivity: S_E=G (K⁺)/G (Na⁺). The influence of ionic strength on K⁺/Na⁺ selectivity is investigated by varying the testing KCl and NaCl electrolyte concentrations, ranging from 0.1 mM to 3.0 M. The K⁺/Na⁺ selectivity increases steadily and reaches a maximum value of approximately 48.8 at a concentration of 0.1 M. Further increasing concentration leads to a notable decline in selectivity (Fig. 2C and figs. S8 and S9). This is because at lower concentrations, higher ion transport resistance across the membrane results in reduced K⁺/Na⁺ selectivity. Conversely, at elevated concentrations, the shortened Debye length diminishes the interaction between K⁺ ions and 18-crown-6, thereby further reducing selectivity. The excellent K⁺/Na⁺ selectivity is maintained over large areas (fig. S10). Moreover, a much higher selectivity is achieved toward Li⁺, Rb⁺, and Cs⁺ due to weaker bindings with 18-crown-6 and smaller ionic mobilities (fig. S11).

Fig. 2. Excellent K⁺/Na⁺ selectivity with crown ether aggregate.

(A) Illustrated diagram of the specific K⁺ transport through crown ether aggregates. (B) I-V curves of the qHB-CA membrane with 0.1 M KCl and NaCl solutions. (C) Summarized the K⁺/Na⁺ selectivity of the qHB-CA membrane in different concentrations. (D) Power densities as a function of external resistance under a 0.5 M/10 mM concentration gradient. (E) Electrochemical stability under a 0.5 M/10 mM concentration gradient. (F) I-V curve of an asymmetric concentration (0.1 M KCl/0.1 M NaCl), showing a diode-like property. Inset: Illustrated diagram of the ion transport under different bias potentials.

The K⁺/Na⁺ selectivity of the qHB-CA membrane is also characterized by ion diffusion tests under different concentration gradients. Along the direction of the concentration gradient, cations selectively migrate through the qHB-CA membrane, producing the diffusion potential. By subtracting the redox potential, the diffusion potentials of KCl and NaCl systems under different concentration gradients are shown in figs. S12 and S13 and tables S1 to S3. The selectivity (α) is calculated with the Goldman-Hodgkin-Katz equation (33, 34)

$$E=\frac{k_{\mathrm{B}}T}{e}\text{ln}\left(\frac{\mathrm{\alpha }{a}_{\mathrm{H}}+a_{\mathrm{L}}}{\mathrm{\alpha }{a}_{\mathrm{L}}+a_{\mathrm{H}}}\right)$$

where E, k_B, T, and e represent the diffusion potential, the Boltzmann constant, absolute temperature, and elementary charge, respectively. $a_{\mathrm{H}}$and $a_{\mathrm{L}}$ represent the activity of Cl⁻ for high concentration solution and low concentration solution. The K⁺/Na⁺ selectivity is estimated at ~104.7, approaching the biological K⁺ channel (fig. S14 and table S4). For binary ion diffusion experiments, the ion concentration at the permeate side is analyzed by inductively coupled plasma mass spectrometry (35). The qHB-CA membrane shows an ultrahigh Na⁺/K⁺ selectivity of 85.3 and a considerably high K⁺ flux of 119 mmol·m⁻²·hour⁻¹ under a concentration gradient–driven diffusion process (fig. S15 and table S5). Supramolecular interactions enable high-density channels by aggregating polymer terminal groups, yielding high selectivity and flux. We have also determined the energy barriers for K⁺ and Na⁺ transport by analyzing temperature-dependent ion flux with an Arrhenius-type equation (36), finding a markedly lower value for K⁺ (details in the Supplementary Materials and fig. S16). Moreover, the difference in diffusion potential is directly reflected by the performance of osmotic energy conversion, showing a much higher power density with KCl electrolytes (Fig. 2D and figs. S17 to S19). Because the content of the hydrophilic crown ether is extremely low (only two groups at the polymer terminals), the swell degree of qHB-CA membrane is only ~0.45%, ensuring excellent mechanical and electrochemical stability. The diffusion potential under a 0.5/10 mM concentration gradient for KCl and NaCl electrolytes remains almost unchanged with a 3-hour testing period (Fig. 2E). The different transport rates for K⁺ and Na⁺ have been extended to design nanofluidic devices. Figure 2F reveals a diode-like ion transport property under an asymmetric concentration of 0.1 M KCl/0.1 M NaCl. Under the positive bias, K⁺ preferentially transports across the membrane along the electric field direction, indicating a higher ionic current. Meanwhile, the migration of Na⁺ is blocked by the K⁺-selective membrane under negative bias, resulting in a notably lower ionic current (fig. S20).

Mechanoregulated K⁺/Na⁺ selectivity

Similar to biological K⁺ channels with regulatable K⁺/Na⁺ selectivity, the qHB-CA membrane exhibits mechanoresponsive K⁺/Na⁺ selectivity. The in situ detection of mechanoregulatable K⁺/Na⁺ selectivity of qHB-CA membrane is illustrated in fig. S21. The pressure is regulated by the height difference between the two sides of the membrane. The pressure-induced membrane deformation generates membrane tension, regulating crown ether aggregates and influencing the ion transport route (Fig. 3A). The qHB-CA membrane exhibits symmetric mechanoresponse toward the pressurized directions. As shown in Fig. 3B and figs. S22 and S23, the K⁺ transport current decreases with increased pressure. Meanwhile, the Na⁺ transport current exhibits an increasing trend. The variation of K⁺ and Na⁺ conductivity toward external pressures is depicted in Fig. 3C, indicating that the magnitude of the conductivity change becomes smaller with pressure increases. As a result, the K⁺/Na⁺ selectivity decreases with increasing pressure. When stepwise decreasing external pressure, the K⁺ and Na⁺ currents well recover their original value (figs. S24 and S25). The mechanoregulated K⁺/Na⁺ selectivity under concentration gradients is investigated by the performance of osmotic energy conversion. As the K⁺ selectivity and permeability decrease and the Na⁺ selectivity and permeability increase under an external pressure (fig. S26), the power density with KCl electrolytes decreases from 3.43 to 2.74 W·m⁻², and the power density with NaCl electrolytes increases from 0.37 to 0.49 W·m⁻² (Fig. 3E and figs. S27 and S28). Owing to the excellent elasticity of the qHB-CA membrane, the mechanoregulated K⁺/Na⁺ selectivity exhibits excellent stability, as confirmed by the 10-cycle pressurization-depressurization process. The variation of K⁺ current, Na⁺ current, and K⁺/Na⁺ selectivity remains almost unchanged in each cycle (Fig. 3F and figs. S29 and S30).

Fig. 3. Mechanoregulated K⁺/Na⁺ selectivity.

(A) Schematic of pressure-regulated K⁺ and Na⁺ diffusion based on the qHB-CA membrane. (B) I-V curves of 0.1 M KCl electrolyte under different pressures. (C) The variation of K⁺ and Na⁺ conductivities with different pressures. (D) Mechanoregulated K⁺/Na⁺ selectivity. (E) Mechanoregulated power density under a 0.5 M/10 mM concentration gradient. (F) Ten-cycle performance of K⁺ and Na⁺ transport current under 0- and 3-mbar pressure. Inset: Corresponding variation of K⁺/Na⁺ selectivity toward pressure.

MD simulations for K⁺/Na⁺ selectivity

The regulatable K⁺ selectivity of the qHB-CA membrane is analyzed by MD simulation. The process of ion transport could be simplified into two steps: ion absorption and ion transformation (Fig. 4A). When the ions approach the membrane surface, K⁺ is selectively absorbed to form the ion-crown complex. The potential of mean force (PMF) for the ion absorption process is calculated. Upon K⁺ absorption, PMF experiences a marked reduction, indicating a thermodynamically favorable process. In contrast, Na⁺ adsorption leads to an increase in PMF, suggesting an unfavorable binding (Fig. 4B). The difference in PMF variation results from the optimal size match between K⁺ and 18-crown-6. Spatial distribution analysis further confirms stronger K⁺ localization near the pore center, consistent with its preferential complexation (Fig. 4C). The deviation primarily arises from the alignment of PMF potential wells, as ions move away from the crown. Second, the ion migrates between adjacent crown ether aggregations until it goes through the whole crown channel. To simplify the computation, we choose two adjacent crown aggregates to simulate the ion transport process. The PMF of the whole ion migration process is captured. For K⁺, PMF goes up and down, as the K⁺ hops from the first crown to the next crown. In addition, the highest PMF is calculated to be 6.72 kcal·mol⁻¹ (Fig. 4D). Meanwhile, the PMF section of Na⁺ ions exhibits a similar variation tendency, but the highest relative free energy is ~6.83 kcal·mol⁻¹ (Fig. 4E), slightly higher than that of K⁺. On the basis of these two steps of ion absorption and ion migration, K⁺ transport through the qHB-CA membrane is much easier than Na⁺, indicating an excellent K⁺/Na⁺ selectivity. The mechanoregulatable K⁺/Na⁺ mechanism is also analyzed by the PMF variation through the ion hopping process. Because the PB soft chains are stretched under external pressure, the steric hindrance for the crown aggregation decreases, showing a decreased d-spacing of 4.38 Å (fig. S31). With the decreased d-spacing, the binding effect between K⁺ and crown center during the ion hopping increases, resulting in an increased energy barrier of 8.18 kcal·mol⁻¹. While for Na⁺, the decreased d-spacing indicated a shorter ion transport route, performing a lower energy barrier (Fig. 4F and fig. S32). As a result, the qHB-CA membrane exhibits a mechanoregulatable K⁺/Na⁺ selectivity. With increased external pressure, the K⁺/Na⁺ selectivity decreases.

Fig. 4. MD simulation for the mechanoregulated K⁺/Na⁺ selectivity.

(A) Illustrated diagram of the ion transport process through the qHB-CA membrane, consisting of ion absorption and ion hopping. (B) PMF section of the ion absorption process, along with the shortening of the ion-crown center distance. (C) Ion position distribution near the crown center. (D) PMF section of K⁺ hopping between adjacent crown aggregates. (E) PMF section of Na⁺ hopping between adjacent crowns. (F) The highest energy barrier for the ion hopping process with and without an external pressure.

Bioinspired Na⁺/K⁺ pump for simulating nerve impulse transmission

Inspired by the mechanism of biological Na⁺/K⁺ pumps, the qHB-CA membrane could achieve mechano-driven active ion transport. In the biological process of information transfer, K⁺ migrates from intracellular to extracellular, forming an externally positive and internally negative resting potential. Na⁺ channels open, and Na⁺ ions are transmitted from extracellular to intracellular when receiving external stimuli, forming an externally negative and internally positive action potential (37). Biological systems achieve impulse transmission across nerve fibers by dynamically adjusting K⁺ and Na⁺ ion selectivity (Fig. 5A). The qHB-CA membrane with regulatable K⁺/Na⁺ selectivity could simulate the nerve impulse transmission. Without external pressure, K⁺ diffusion along the concentration gradient generates a positive potential signal. The mechanical stimuli activate the Na⁺ channel, achieving Na⁺ active transport against the concentration gradient and producing a negative potential (Fig. 5B). As shown in Fig. 5C, the K⁺ diffusion produces a positive potential (resting potential) under a 0.2 mM KCl/0.1 mM NaCl system. The membrane potential decreases, as the external pressure on the NaCl side increases from 0 to 2 mbar. Further increasing the bias pressure to 3 mbar, the membrane potential changes from positive to negative. In this case, the ion transport channel for Na⁺ diffusion opens, realizing the active transport and inverting the potential (action potential). The process of mechano-inversed membrane potential shows excellent reversibility (Fig. 5D). The qHB-CA membrane also performs self-healing ability due to dynamic hydrogen bond networks, which is a basic property for cell membranes (fig. S33). The ultrahigh K⁺/Na⁺ selectivity and mechanoregulatable Na⁺/K⁺ pump properties of the self-healed membrane recover well (Fig. 5E and figs. S34 and S35). Thus, the qHB-CA membrane with mechanoregulatable K⁺/Na⁺ selectivity well reproduces the process of nerve impulse transmission, showing its potential in bioinspired ion informatics.

Fig. 5. Simulated nerve impulse transmission based on mechanoactivated Na⁺/K⁺ pump.

(A and B) Schematic of biological and bioinspired nerve impulse transmission. Mechanoactivated Na⁺ channel inverts the membrane potential. (C) The transformation of membrane potential during the pressurized process (0.2 mM KCl/0.1 mM NaCl). (D) Mechanoactivated Na⁺ channel for action potential and recovery of resting potential by releasing the pressure. (E) Stable K⁺/Na⁺ selectivity of the self-healed qHB-CA membrane.

DISCUSSION

In summary, the artificial K⁺ channel is achieved with the least crown ether content, achieving the highest reported K⁺/Na⁺ selectivity. Benefiting from the physical cross-linker between polymeric chains, the qHB-CA membrane shows excellent elasticity under external pressure, experiencing marked deformation and changing the crown ether channel structure. As a result, the K⁺/Na⁺ selectivity is regulated, and the mechanoactivated K⁺/Na⁺ pump is achieved. Similar to the electric signal transduced in the neurofibril, the membrane potential changes from positive (resting potential) to negative (action potential) under mechanical stimuli. With further exploration, the artificial channel system will reproduce the neuron’s complete message transmission.

MATERIALS AND METHODS

Modification of crown ether to biterminal carboxyl PB-(COOH)₂

The biterminal carboxyl PB-(COOH)₂ (1.0 g; M_w=4200), di(aminobenzo) [18]crown-6 [18-crown-6- (NH₂)₂] (0.93 g, 2.4 mmol), ethyldimethylaminopropyl carbodiimide (EDC; 0.15 g, 1.0 mmol), and 4-dimethylaminopyridine (DMAP; 0.06 g, 0.5 mmol) were dissolved in 25 ml of CH₂Cl₂. The mixture was reacted for 18 hours at 25°C. The rotary evaporation operation was used to remove excess CH₂Cl₂. Then, the concentrated mixture was precipitated two times into methanol to obtain the white product PB-(crown-NH₂)₂ (0.86 g of glassy solid, yielding 72.5%). From the ¹H NMR results, the disappearance of carboxyl signals indicates the complete modification.

Modification of 2-(1-imidazolylcarbonylamino)-6-methyl-4[1H]-pyrimidinone to PB-(crown-NH₂)₂

The synthesized PB-(crown-NH₂)₂ (0.5 g) and 2-(1-imidazolylcarbonylamino)-6-methyl-4[1H]-pyrimidinone (0.12 g, 0.5 mmol) were dissolved in 10 ml of N,N′-dimethylformamide. The mixture was reacted for 12 hours at 100°C and in an N₂ atmosphere. The mixture was precipitated two times into methanol to obtain a white product PB-(crown-UPy)₂ (0.36 g of solid, yielding 67.4%). From the ¹H NMR results, the signal for UPy units was observed.

Fabrication of the qHB-CA membrane

A total of 4 mg of PB-(crown-UPy)₂ sample was dissolved in 0.2 ml of tetrahydrofuran. The mixture was ultrafiltered with a 0.22-μm membrane and spin coated onto a 2×2 cm silicon wafer with a sacrificial poly(styrene sulfonate) (PSS) layer. Next, the membrane was annealed at 40°C in a vacuum-drying oven. Above the glass transition temperature (Tg; ~−80°C) of PB, the PB chains gain sufficient mobility to reorganize toward a state of thermodynamic equilibrium. Under the synergetic effect of inherent thermodynamic incompatibility and quadruple hydrogen bond, the terminal groups self-assembled into a crown ether aggregate structure. Last, the self-assembled membrane was immersed in water to remove the PSS layer, and the highly elastic and free-standing membrane was obtained.

MD simulation

Intermolecular interactions were modeled using the all-atom optimized potentials for liquid simulation (OPLS-AA) force field (38, 39). Partial atomic charges for the qHB-CA membrane units were assigned on the basis of the original OPLS-AA parameterization. Water molecules were represented by the rigid TIP4P model (40), modified for compatibility with a long-range Coulombic solver (41) in LAMMPS (42). All MD simulations were carried out using the LAMMPS v.29Aug2024 software (42). PACKMOL (43) and Moltemplate (44) software are used to construct the initial structures. Two adjacent crown aggregates were modeled and energy minimized.

Before the calculations of the PMF curves, all systems underwent isotropic NPT relaxation at T=300 K and atmospheric pressure for 1 ns with a timestep of 1 fs, under periodic boundaries in all directions. Each PMF simulation was performed in the NVT ensemble for 100 ns with a timestep of 2 fs, where each ion absorption or migration process was split into 100 solvated ion position configurations, with each sampled for 1 ns using the Colvars module (45). For ion absorption, the process was simulated along the perpendicular direction relative to the plane of the crown pore over a range from 0 to 1.5 nm. For ion migration, the process was modeled as lateral displacement along the parallel direction relative to the line connecting the centers of a fixed vertical separation of 1.5 nm. The crown aggregates were held fixed throughout all simulations. The Weighted Histogram Analysis Method was used to reconstruct the PMF curves.

Supplementary Materials

This PDF file includes:

Supplementary Text

Figs. S1 to S35

Tables S1 to S5

References