Long-lasting and stable anti-fog coating combined with active and passive strategy

Abstract

There is an urgent need to design long-term effective anti-fog coatings to ensure the stabilization of uniform water films on hydrophilic surfaces, considering the impacts of interfacial strength, cyclic drying, and contaminants. Here, we propose an anti-fog strategy that leverages manipulating free volume and hydrogen bonding of hydrophilic networks through the twisted non-coplanar structures, synergized with photothermal effects, to enhance anti-fog efficiency. The introduction twisted non-coplanar structures of 1(2H)-phthalazinone not only significantly enhances the coating’s interfacial stability but also increases the molecular free volume, thereby substantially improving its moisture absorption capacity and extending the anti-fogging duration. The developed coating simultaneously achieves high transparency and efficient solar light absorption in the UV/NIR regions, effectively enhancing photothermal fog prevention and removal. The insights into anti-fog strategy promise to translate the structural free-volume benefits measured in laboratories into real-world applications for long-lasting and stable anti-fog coatings.

Antifog coatings are of great interest, though it is challenging to balance interfacial strength, cyclic drying, and resistance to contaminants. Here the authors designed a coating using noncoplanar structures to enhance stability and anti-fogging efficiency.

Introduction

Surface fogging, a prevalent phenomenon occurring near or below the atmospheric dew point, arises from vapor condensation forming light-scattering microdroplets on surfaces^1,2. This process not only causes severe vision impairment and optical transparency loss in applications ranging from eyewear to solar panels, but also imposes energy inefficiencies in systems requiring clear optical interfaces, such as greenhouses and photovoltaic module^3–6. To address this challenge, current anti-fog strategies can be broadly categorized into passive and active approaches^7,8. Passive approaches rely on surface chemistry modifications to control water droplet behavior through wettability engineering, typically involving either hydrophobic or hydrophilic coatings. Active approaches employ external energy inputs (e.g., thermal, optical) to prevent condensation through environmental modulation. This synergy between surface-dependent passive methods and energy-driven active solutions directly impacts their sustainability profiles and alignment with global energy transition goals.

Current research predominantly focuses on passive hydrophobic coatings designed to repel water droplets via low surface energy. While these can remove droplets exceeding ~10 μm through gravity-assisted shedding, they suffer from inevitable fog accumulation during the critical initial condensation phase^9,10. Alternative passive hydrophilic coatings promote water film formation through rapid droplet spreading^11,12, but face inherent limitations: volumetric swelling during moisture absorption compromises optical transparency, while mechanical instability during drying-swelling cycles leads to wrinkle formation and interfacial failure^13–15. Notably, both passive strategies share fundamental constraints in long-term performance. Hydrophobic surfaces lose efficacy as nanoscale condensation breaches the Cassie-Baxter state, whereas hydrophilic systems become saturated under prolonged humidity^16–18. Hybrid passive approaches combining hydrophilic/hydrophobic domains partially address these issues but introduce manufacturing complexity and component compatibility challenges¹⁹. Crucially, neither strategy resolves the core limitation of transient effectiveness in real-world dynamic environments.

Emerging active anti-fog technologies offer complementary advantages by converting ambient energy (e.g., sunlight) into thermal gradients that suppress condensation^20,21. Photothermal coatings exemplify this approach, providing operational cost savings and reduced chemical pollution compared to conventional electric heating systems, features particularly aligned with sustainable development objectives in the energy transition era^22,23. However, the current implementation faces critical challenges: an exclusive focus on achieving high photothermal performance results in compromised transparency due to excessive coloration, while weather-dependent performance hinders their functionality under low-light conditions^24,25.

The fundamental challenge lies in reconciling the transient effectiveness of passive mechanisms with the environmental limitations of active systems. This requires revisiting the molecular basis of water-surface interactions. Hydrophilic functional groups (-OH, -NH₂, -COOH) enable hydrogen bonding with water molecules, but their efficacy depends critically on polymer architecture^26,27. A survey and analysis of the previously reported anti-fog strategies (active/passive) revealed inherent limitations in sustaining long-term anti-fog performance (Supplementary Table. 1, 2). Mechanistic studies further identified haze formation caused by excessive volumetric expansion as the primary failure mode in hydrophilic coatings. The innovation of this work transcends incremental improvements, revealing the critical influence of free volume within crosslinked networks on moisture sorption kinetics and equilibrium capacity, a parameter previously overlooked in anti-fog design^28,29. By strategically engineering molecular-scale voids rather than simply increasing coating thickness, we hypothesized that synergistic active-passive functionality could be achieved through: (i) enhanced passive moisture uptake via optimized free volume; (ii) active photothermal effects using sunlight-responsive nanomaterials; (iii) hydrogen-bond stabilization of swollen states during humidity cycling.

Herein, we demonstrate a long-term stable anti-fog strategy combining passive moisture regulation with active photothermal effects. A hydrogen-bonded polyvinyl alcohol/ sulfonated twisted non-coplanar polymers matrix provides tunable free volume and interfacial stability, while cesium tungsten bronze (CTB) nanoparticles synergize with twisted non-coplanar polymers to enable selective UV/NIR harvesting with maintained visible transparency. This dual mechanism ensures continuous fog prevention across day-night cycles and variable humidity, addressing the critical durability limitations of conventional approaches. The design principles established here, particularly the anti-fogging capability independent of coating thickness or additional energy consumption, open new avenues for sustainable optical surface engineering.

Results

Anti-fog coating design based on hydrogen bonding

In high-humidity environments, micrometer/sub-micrometer coatings often show limited anti-fog durability, underscoring the demand for stable surface designs. While increasing coating thickness is a common strategy to extend anti-fog duration by enhancing water absorption, it compromises optical clarity and transmittance stability. To address this, we used polyvinyl alcohol (PVA), a common anti-fog coating material, as an example and attempted to optimize its internal coating structure without altering its thickness. Using PVA as a cross-linked skeleton with hydroxyl (-OH) groups, we incorporated segmented polymer chains containing sulfonic acid (-SO₃H) groups to create hydrophilic coatings via hydrogen-bond crosslinking. The coatings demonstrated good processability across dip-coating, spin-coating, and blade-coating methods, enabling versatile fabrication (Fig. 1a and Supplementary Fig. 1). To systematically explore how molecular microstructure and chain flexibility affect performance, we compared flexible (poly (2-acrylanmido-2-methyl-1-propanesulfonic acid) (PAMPs)), rigid (sulfonated poly ether-ether-ketone (SPEEK)), and twisted non-coplanar segments (sulfonated poly phthalazinone ether sulfone ketone (SPPESK)) segments (Fig. 1b and Supplementary Fig. 2, 3). Infrared spectroscopy confirmed the presence of -OH (3270 cm⁻¹) and -SO₃H (1078/1017 cm⁻¹) groups in PVA-SPPESK, with a redshifted -OH peak indicating robust hydrogen bonding (Supplementary Fig. 4) ³⁰. These bonds enhance hydrophilicity, substrate adhesion, transparency, and physicochemical properties. Molecular dynamics simulations revealed that the PVA-SPPESK system exhibits the strongest hydrogen-bond network (shortest bond lengths, highest bond count) and highest solution viscosity (Fig. 1c and Supplementary Fig. 6, 7). The twisted SPPESK structure reduces steric hindrance by spatially separating hydroxyl groups, facilitating hydrogen-bond formation. Additionally, this conformation redistributes internal charges, positioning hydrogen atoms closer to negative charge centers, thereby amplifying electrostatic attraction and charge complementarity (Supplementary Fig. 5). These synergistic effects enable simultaneous multi-hydrogen-bond formation, significantly improving molecular stability and anti-fog performance.

Fig. 1. Design, fabrication and verification of hydrogen bonding anti-fogging system.

a Schematic illustration for fabricating the PVA-SPPESK-CTB coating. b Schematic illustration of the synthesis of the PVA-SPPESK strong hydrogen bonding crosslinking system. c Comparison of hydrogen bonding crosslinked systems and cohesive energies of PVA-PAMPs, PVA-SPEEK, and PVA-SPPESK using molecular dynamics simulations; simulate hydrogen bond types and bond lengths across various systems.

Incorporating twisted non-coplanar structures into PVA molecular chains boosted moisture-absorption capacity by 2.14 times and enhanced kinetics by 14.39%. Real-time hygroscopic analysis revealed rapid water diffusion into the coating via free-volume channels, with the amplified absorption capacity substantially extending anti-fog duration despite modest uptake acceleration (Fig. 2a, b and Supplementary Fig. 8). The PVA-SPPESK coating overcame water’s Laplace pressure barrier through strong polar interactions (total surface energy: 42.57 mN/m, polar component ratio: 76.30%, Supplementary Fig. 9), enabling a “high-capacity, moderate-rate” synergy critical for humidity-responsive applications like eyewear or windshields. In harsh conditions (85 ^oC water bath), pure PVA coatings failed within 5 min, while PVA-PAMPs degraded optically after 10 min due to interfacial flow (Fig. 2c and Supplementary Fig. 10). In contrast, the rigid biphenyl-enhanced PVA-SPPESK maintained near-transparency for 30 min at 25 μm thickness (Supplementary Figs. 11–13), outperforming most reported hydrophilic coatings. Moreover, cold fog tests (ΔT = 30 ^oC, RH = 95%) showed stable transmittance (> 90%) and minimal roughness increase (1.28 nm) after 10 cycles (Supplementary Figs. 14, 15), attributed to robust hydrogen-bond networks. And the absence of ice-melting peaks (Supplementary Fig. 16) confirmed water stabilization in nanoconfined non-freezing states, effectively suppressing ice nucleation and thermal delamination.

Fig. 2. Exploration of the anti-fogging mechanism.

a, b Comparison of moisture absorption capacity and kinetics of moisture absorption of coatings. (Each data point represents three technical replicates and error bars indicate the standard deviation. Data are presented as mean ± standard deviation.) c The average transmittance of PVA, PVA-PAMPs, PVA-SPEEK, PVA-SPPESK and PVA-SPPESK-CTB coated (25 μm) glass slides over time when exposed to hot water vapor (85 ^oC). d–f Fitted plot of the volume expansion signal-stress-strain relationship measured by FBG. g Molecular dynamics simulations of PVA-PAMPs, PVA-SPEEK and PVA-SPPESK hydrogen bonded cross-linking systems. h The weak adhesion and low strength of PVA and PVA-PAMPs polymer network coatings lead to interface failure, wrinkling, and even rupture; the rigid structure of PVA-SPEEK ensures robust anchoring, preventing interface failure; the higher water absorption rate in the twisted non-coplanar structure of PVA-SPPESK prolongs the anti-fogging duration.

The Fiber Bragg Grating (FBG) sensor³¹, an optical waveguide-based tool for precision temperature/stress monitoring in civil and aerospace engineering³², was innovatively applied to track real-time moisture-induced stress variations in coatings (Supplementary Fig. 17). Through advanced signal processing, we quantified microstrain evolution during water absorption. Moisture absorption-induced expansion elevated internal stress/strain (Fig. 2d–f), yet PVA-SPPESK showed lower peak stress and delayed response, confirming its good long-lasting anti-fog capabilities.

Free volume, unoccupied spaces enabling molecular transport in polymer networks, which governs chain mobility and water diffusion. Expanded free volume facilitates water penetration, boosting absorption capacity. Molecular dynamics simulations (Fig. 2g, and Supplementary Fig. 18, Table. 3, 4) preliminarily revealed that the PVA-SPPESK system possesses maximized free volume characteristics. This computational prediction was further corroborated by positron annihilation lifetime spectroscopy^33,34 (PALS), which not only validated the free volume trend but also resolved a distinct bimodal distribution within the material (Supplementary Fig. 19, and Table. 5). The larger pores arise from packing defects induced by the twisted non-coplanar structure, while smaller pores stem from polar repulsion between -HSO₃ groups and polymer chains (Fig. 2h). PVA-SPPESK’s bimodal pores (4.48 Å/2.00 Å) outperform PVA-PAMPs’ unimodal pores (3.78 Å) via hydrogen-bond-proton coupled transport orchestrated by size compartmentalization. 2.00 Å pores in PVA-SPPESK trap water via enhanced hydrogen bonding (red shift of O-H in FTIR), while sulfonic groups mediate proton-hopping that reduces migration barriers³⁵, synergizing with 4.48 Å diffusion channels (> 2.7 Å water diameter) to form a self-sustaining cycle: thermally activated release from small pores couples with proton-relayed transport into barrier-free large pores³⁶. Consequently, compared to the PVA-PAMPs system, the water vapor transmission rate increases by 17% (2.48 vs. 2.12 × 10⁻³g h⁻¹ cm⁻², Supplementary Fig. 20) and the swelling ratio improves by 81% (263.4% vs. 145.3%). This conclusively demonstrates the distinctive water dynamics enabled by the bimodal pore architecture in PVA-SPPESK. Combined with moisture sorption kinetics and dynamic contact angle analyses, these results validate that the hierarchical architecture synergistically enables rapid moisture diffusion (via large pores) and enhanced water retention (via small pores).

Molecular dynamics simulations (Supplementary Figs. 21, 22) of interfacial interaction energy between the coating surface and water molecules revealed that the enhanced hydrogen bond density, Coulombic forces, and van der Waals interactions between -SO₃H/-OH in PVA-SPPESK and water molecules maximize interfacial water adhesion. This originates from its distorted non-coplanar structural design, which disrupts rigid planar constraints, enabling high dispersion and full exposure of sulfonic acid groups, thereby completely eliminating steric hindrance to maximize hydrogen bond density and strength. Concurrently, the expanded free volume establishes continuous water-molecule channels, synergistically enhancing directional Coulombic adsorption forces. Strong interfacial interaction energy fundamentally enhances anti-fogging performance by suppressing discrete water droplet nucleation and promoting the formation of a continuous ultrathin water layer. Increased free volume improves stress redistribution and deformation resistance, enabling rapid recovery under external forces. Enhanced chain flexibility reduces internal stress accumulation during thermal/moisture fluctuations, critical for fatigue resistance. By structurally tuning free volume through twisted conformations, we achieved prolonged anti-fog stability and cyclic durability (Supplementary Fig. 23).

Evaluation of photothermal anti-fogging behavior

While the PVA-SPPESK coating extends anti-fog duration and interfacial stability without thickness changes, passive approaches remain vulnerable to fogging after moisture saturation. To address this, we integrated the system with photothermal effects to harness sunlight for surface heating, combining active/passive strategies to eliminate temperature gradients³⁷. Our design (Fig. 3a) prioritizes selective spectral absorption in the high UV/NIR absorption (maximizing heating) and > 90% visible transparency to avoid compromising substrate optics. Over 50% of solar energy resides in the NIR spectrum, enabling efficient harvesting of invisible radiation. Even minor surface temperature (T_s) increases suppress droplet nucleation exponentially, drastically reducing fogging risk. Building on prior work³⁸, we combined highly conjugated SPPESK (with UV/NIR absorption) with cesium tungsten bronze (CTB) nanoparticles to create a transparent photothermal coating. The PVA-SPPESK-CTB system achieved 92% visible transmittance alongside 93% UV and 51% NIR absorption (Fig. 3b, c and Supplementary Fig. 24). CTB’s wide bandgap ensures visible transparency while enabling NIR absorption via localized surface plasmon resonance (0.7 eV) and polariton absorption (1.4 eV), with Cs⁺ doping in WO₃ enhancing charge compensation for efficient heat generation^39,40. This dual mechanism allows simultaneous visible transmission and UV/NIR harvesting (56% of solar energy), enabling practical long-term anti-fog performance.

Fig. 3. Design and optical properties of the photothermal anti/defogging coating.

a Transparent coating photothermal mechanism: UV and NIR light are absorbed by the coating and dissipated into heat (photothermal effect), while largely transparent in the Vis range. The resulting temperature increase of the surface prevents fogging. b Transparency test of coatings (glass substrate). c Sunlight absorption spectrum of the coatings. The filler section represents the AM 1.5 Global solar reference spectrum: 49.9% of the solar irradiance is in the NIR, 45.5% in the Vis, and 4.6% in the UV range. d Infrared thermography to monitor changes in coating surface temperature (1 sun irradiation). e Curves of surface temperature response of each coating at 1 solar irradiance as a function of time. T_e = 26.3 ^oC. f Mean temperature increase of the surface under solar irradiance from 0.5 to 1.0 sun. g Qualitative image sequence under 1 sun illumination (T_e ≈ 26.3 ^oC, RH ≈ 65%). While the control is still covered with fog at t = 120 s, the PVA-SPPESK-CTB coating has completely regained its transparency. h Evolution of fog fraction f over time (1 sun illumination), indicating its mean value on the control sample (solid line) and the PVA-SPPESK-CTB coating (dashed line). The metamaterial coating starts to defog much earlier and faster (steeper slope) than the control sample. The coating reduces t_d by a factor of ≈ 3. i Comparison of the visible transmittance and coating surface temperature of our and reported photothermal coatings.

The energy dispersive spectroscopy (EDS) spectra of the composite coating reveal the presence of C, O, Cu, Cs, and W elements. Complementary X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations confirm the special octahedral structure of CTB NPs, their successful incorporation and uniform distribution within the composite structure (Supplementary Fig. 25, 26 and 27). Furthermore, the surface morphology exhibits a high degree of smoothness, indicating that the addition of CTB NPs did not significantly alter the coating’s roughness (Supplementary Fig. 28). The PVA-SPPESK-CTB coating was applied to optical lenses using a spraying technique. Under 1 sun irradiance (corresponding to 1 kW m⁻²), the left lens served as the control group and exhibited rapid fogging due to the formation of a condensation film, which significantly impaired visibility. Conversely, the right lens, designated as the experimental group, maintained a clear field of vision. Infrared thermographic analysis in Supplementary Fig. 29 revealed that the temperature of the right lens increased to ~57 ^oC, while the left lens remained at 28 ^oC. Additionally, the photothermal performance of the coating was assessed under more realistic conditions (Fig. 3d–f), specifically at irradiances of 0.5 and 0.8 kW m⁻², which correspond to 0.5 and 0.8 suns, respectively. Under identical conditions, solely the synergistic interaction between SPPESK and CTB NPs exhibited exceptional photothermal effects. Under 1 sun irradiance, the T_s increased by 28.5 ^oC within ~3 min, notably, the coating exhibited substantial heating even under reduced irradiances, with increases of 14.4 ^oC and 22.4 ^oC observed at 0.5 and 0.8 suns, respectively. These temperature elevations were sufficient to markedly reduce the rates of condensate nucleation by several orders of magnitude. The dual mechanism of continuous moisture absorption and photo-thermal defogging provides powerful anti-fogging effects in low-temperature, high humidity environments, offering a viable solution for frost-prone optical and automotive glazing systems. This exceptional performance surpasses that of numerous state-of-the-art polymeric anti-fog coatings reported in the literature^{19,20,24,25,41–49} (Fig. 3i and Supplementary Table. 2).

Defogging performance

We then assessed the defogging performance of the resultant coating, that is, the recovery of visibility once the surface is completely fogged. For this, we cooled the control samples and the PVA-SPPESK-CTB coating in a freezer to 2 ^oC for 2 min, and then transferred them to an environment with T ≈ 60 ^oC and a relative humidity of ≈ 90% until a fog layer formed. Following this, we focused simulated sunlight on the coatings to provide a power density of 1 kW m⁻² while recording the evolution of the surface fogging in real time. Figure 3g qualitatively illustrates how the PVA-SPPESK-CTB coating completely defogs within 120 s and to a significant extent within 90 s, whereas the control sample remains largely fogged during the same period. The small size of the condensed droplets results in significant scattering, producing a very white image of the sample, clearly recognizable in the control image at t = 0 s. Experiments under varying levels of illumination, accompanied by recorded photos of the backscattered light, provide a detailed quantification of the defogging dynamics. From these data, we extract the evolution of the fog fraction (f) over time. The defogging time (T_d), defined as the time required for the fog fraction to drop below 5% (f < 0.05), indicates a substantial improvement for the PVA-SPPESK-CTB coating compared to the control samples. Specifically, under the given experimental conditions, T_d = 120.0 s for the coated samples, representing a 3.5-fold enhancement in stark comparison to T_d ≈ 420.0 s for the control samples. Under weaker illumination, T_d is further reduced by factors of 3.0 (T_d ≈ 180.0 s vs 540.0 s) and 2.7 (T_d ≈ 240.0 s vs 655.0 s) at 0.8 and 0.5 sun illuminations, respectively (Fig. 3h).

Durability

In addition to selective photo-thermal anti-fog, other characteristics and durability of the coating during use are also critical. Large-scale preparation of the PVA-SPPESK-CTB anti-fog coating (1.0 m × 1.0 m) can be achieved through a simple casting method (Fig. 4a and Supplementary Fig. 30). Evaluation results regarding the applicability of the coating on flexible substrates show that, despite having a thickness of only 200 μm and a relatively large curvature radius due to bending deformation, folding into arbitrary shapes does not cause mechanical damage or affect optical properties (Fig. 4b, c). Consequently, this coating can be effectively deposited on any flexible, foldable materials and can be retroactively applied to existing surfaces, such as windows, protective shields, or glass. More notably, the biphenyl structure endows the coating with high hardness and Young’s modulus (Supplementary Table. 6). We also conducted adhesion tests according to the American Society for Testing Materials (ASTM) D3359 standard to examine the bonding capability of the fabricated PVA-SPPESK-CTB with the underlying glass substrate⁵⁰. Briefly, we created regular grids with a 1 mm interval on the PVA-SPPESK-CTB surface using multiple tap cutters. The coating was then compressed by 3 M tape under a uniform load of 2 kg, followed by peeling off (Fig. 4d). Our results showed that the coating exhibited no apparent detachment of fragments and maintained good structural integrity, meeting the ASTM 5B standard. In contrast, control samples without the layered design demonstrated limited adhesion strength, as evidenced by the ease with which the coating peeled off the glass substrate. The coating demonstrates exceptional ease of cleaning even when contaminated with oily substances (e.g., human fingerprints), as simple water rinsing or wiping restores its original state (Fig. 4e). Combined with its chemical stability and anti-fouling performance against proteins and bacteria (Supplementary Figs. 31–33), these attributes position it as an ideal material for contamination-prone applications demanding optical clarity and anti-fogging capabilities. Such dual functionality highlights its potential for implementation in medical devices (e.g., endoscopes) and marine optical sensors where surface integrity and maintenance efficiency are critical.

Fig. 4. The basic performance and long-term durability of the PVA-SPPESK-CTB coating.

a An image of the transparent selective photothermal coating (1.0 × 0.8 m²) prepared on a flexible substrate. b Bending tests: due to its thinness, the coating resists serious deformation and is readily applicable on flexible, foldable and portable substrates. c An origami crane folded from a 5 ×  5 cm² PVA-SPPESK-CTB coated sheet and its corresponding infrared thermal image. d A schematic of adhesion test and SEM images of PVA coating, PVA-PAMPs coating, and PVA-SPPESK coating after adhesion test, respectively. e Fingerprints on PVA-SPPESK-CTB coated and uncoated glass slides after rinsing with water. f Changes in contact angle and transmittance during sandblasting, wipe abrasion, and tape stripping experiments. g UV shielding performance test. h Absorption and transmittance testing of coatings exposed to UV radiation processes. i Comparison of this work with anti-fogging effect of the reported coatings over steam at 90  ^oC. j Comparison of the actual anti-fog effect of this work with commercially available anti-fog coatings in daily life (40%-60% humidity).

Considering the coating may be exposed to harsh environmental conditions such as UV radiation, water corrosion, or mechanical wear, which could degrade its anti-fog performance, we investigated the durability of the transparent selective photothermal coating through UV irradiation stability tests (30 d), water resistance tests, and abrasion tests. The results demonstrate that the coating maintains good stability even after undergoing 8 cycles of water absorption, swelling, and drying (Supplementary Figs. 34, 35). Under long-term usage conditions involving severe wear (such as sand abrasion and wiping), the coating retains its solar absorptance and visible light transmittance without any noticeable degradation (Fig. 4f). Additionally, it exhibits effective UV shielding and resistance to UV aging within 30 d (Fig. 4g, h). Most notably, PVA-SPPESK-CTB maintains its anti-fog performance for up to 30 min in high vapor harsh environments (~100% RH), even in the absence of light, by leveraging its large free volume network. This performance surpasses that of all previously reported coatings of the same thickness^11,51–53 (Fig. 4i). In everyday conditions with air humidity typically ranging from 40% to 60%, PVA-SPPESK-CTB demonstrates an anti-fog duration of nearly 60 d, far exceeding the performance of commercial anti-fog products. Furthermore, when exposed to sunlight, PVA-SPPESK-CTB can maintain long-term anti-fog properties even in high-humidity environments (Fig. 4j). Monitoring the internal stress of the anti-fog coatings at 7 and 30 d revealed that the stress levels had not yet reached their maximum (~363.9 kPa), indicating that the surface temperature under one sun can facilitate water evaporation and maintain internal moisture balance, thus achieving long-term or even permanent anti-fog effects. As such, the PVA-SPPESK-CTB coating emerges as an ideal candidate for advanced transparent photothermal anti-fog applications.

Real-world feasibility tests

Armed with the knowledge of this enhanced anti-fog/de-fog performance, we evaluated the real-world feasibility of our coating. For this, one lens of a pair of glasses was coated with PVA-SPPESK-CTB and placed outside on a winter day, where the solar irradiance levels were ~0.5 suns. After exposing the glasses to sunlight for 5 min, we wore them and exhaled while wearing a well-fitted mask, simulating a real-world condition that we recently experience daily. The warm breath acts as a supersaturated stream of vapor, which impinges on both the uncoated lens and the PVA-SPPESK-CTB coated lens (at an apparently increased temperature above T_e). Figure 5a, b demonstrate the results at two different locations. While the uncoated right lens completely fogs, the coated lens (left) retains its full visibility, even under these non-ideal conditions. Further experiments (from a cold outdoor environment to a warmer indoor, wear a mask and goggles at the same time) and movie recordings can be found in Supplementary Movie. 1-4.

Fig. 5. PVA-SPPESK-CTB-coated anti-fog function and multi-scenario application.

a The schematic illustration and test of fogging on eyeglasses when entering warm indoors (25 ^oC) from cold outdoors (−10 ^oC). b The schematic route and test of fogging on eyeglasses/protective glasses when wearing a mask. c Schematic diagram illustrating the anti-fog mechanism for automotive window glass and testing. d Schematic diagram of anti-fog coating for greenhouse films and images documenting the fog prevention performance after thermal fogging at 100  ^oC.

Moreover, applying the PVA-SPPESK-CTB coating to the inner surface of the car’s windshield can immediately resolve the issue of fog accumulation that drivers may encounter during their journeys (Supplementary Fig. 36). This not only enhances safety but also ensures optimal visibility at all times, thereby improving the overall driving experience (Fig. 5c). The rising water vapor wetted the greenhouse canopy layer, forming a water film (Fig. 5d). To simulate an agricultural greenhouse, a 0.15 curvature shed was constructed (4.0 m × 2.0 m), with half of its area uncoated for comparison^11,12,54. It was clearly observed that the uncoated film was covered with dense water droplets, whereas the coated PVA-SPPESK-CTB film still maintained more than 90% transparency (Supplementary Fig. 37). Even commercial anti-fog sprays have an effective usage period of only 6 h in continuous high steam environments, which is far less than the 200 h+ stability of PVA-SPPESK-CTB. Since the primary goal of creating antifogging films is to reduce light refraction without affecting photosynthesis, the intensity of light transmission before and after the introduction of water vapor was measured (Supplementary Fig. 38). The light intensity of the uncoated film decreased significantly, from 1.0 kW m⁻² to 0.6 kW m⁻², while the processed sample exhibited only a minor reduction, demonstrating its good applicability.

Overall, the anti-fogging performance of the coating demonstrates significant effectiveness across various real-world scenarios, promising to translate the advantages observed in laboratory settings into practical applications. We anticipate that our work is a starting point to utilize the full potential of anti-fogging properties of PVA-SPPESK-CTB materials.

Discussion

Inspired by free volume theory, we engineered an anti-fogging system using PVA-SPPESK, where robust hydrogen bonding synergizes with photothermal effects to achieve sustained fog prevention. The rigid biphenyl structure enhances interfacial stability, while the twisted non-coplanar configuration increases free volume, improving moisture uptake and extending anti-fog duration. The solution-processable coating combines scalable manufacturability with mechanical robustness, enabling direct integration into multi-layer systems for anti-fogging upgrades. SPPESK-CTB synergy achieves 92% visible transparency alongside UV-NIR photothermal activity, generating 28 ^oC thermal gradients under 1-sun illumination to accelerate defogging 3.5× faster than untreated surfaces. This performance persists under 0.2-sun irradiance with good humidity durability. Demonstrated in practical applications (glasses, car windows, greenhouses, etc.), our free volume engineering strategy establishes a framework for designing hydrogen-bond-optimized anti-fog materials.

Future efforts should prioritize coating optimization to enable durable deployment across infrastructure. While our hygroscopic-photothermal synergy demonstrates antifogging efficacy, scalable manufacturing, environmental resilience, and cost efficiency demand focused attention. The current hygroscopic-photothermal dual mechanism could be further enhanced through two strategic extensions: integration of self-healing capabilities to mitigate mechanical degradation; implementation of stimuli-responsive materials enabling environment-adaptive transitions between anti-fog and de-fog modes based on humidity/temperature triggers. Process-structure-property co-design, particularly engineering mechanically stable polymer networks with controlled free volume will accelerate industrial translation from lab prototypes to commercial anti-fog solutions.

Methods

Preparation of anti-fog coatings

A 5% PVA aqueous solution (stirred in a water bath at 90 ^oC) and a 10% aqueous solution of PAMPs/SPEEK/SPPESK (based on the mass of PVA) were prepared separately and then mixed homogeneously at 25 ^oC. Subsequently, 0.05% CTB nanoparticles were ultrasonically dispersed and added to ensure uniform mixing. The resulting series of complex solutions were homogeneous, transparent, and demonstrated stability for long-term storage at 25 ^oC.

Anti-fog coatings were applied to various substrates, including optical glass, PET flexible plastic, and eyeglass lenses. Initially, the substrates were sonicated in ethanol and deionized water for 10 min to remove impurities physically adhered to the surface, followed by drying with N₂. Subsequently, the substrates underwent surface treatment by immersion in a “piranha solution,” which introduced a significant number of hydroxyl groups. This increase in hydroxyl groups enhances hydrogen bonding interactions between the substrate and the polymer coating, thereby improving adhesion.

The preparation of anti-fog coatings using the lift-off method was executed with a pulley-set robotic arm. The treated substrates were vertically submerged in the complex solution at a constant speed of 0.5 cm/s for 10 min. Subsequently, the substrates were lifted vertically from the solution at a rate of 0.1 cm/s and allowed to dry at 25 ^oC for 12 h. The spin coating method was also employed, utilizing a spin coater to uniformly apply the composite solution onto the substrate at 2500 rpm for 120 s, followed by a drying period of 12 h at 25 ^oC. Additionally, a scraping method was utilized, in which the polymer solution was poured onto a glass plate positioned on a heated platform, allowing the solvent to evaporate and resulting in the formation of anti-fog coatings.

Measurement of anti-fogging performance

The anti-fogging performance was evaluated using a hot vapor testing method. Specifically, the samples were positioned 2 cm above a water bath maintained at a constant temperature of 85 ^oC. The fogging time of the coatings and their transmittance rates were recorded to comprehensively assess the anti-fogging effectiveness of the different samples.

In the fiber optic demodulator tests, we secured the fiber core within the anti-fog coating and connected its other end to the fiber optic demodulator and computer to elucidate the coating’s anti-fogging mechanism. As the coating absorbs moisture, its internal volume expands, and the fiber sensor detects changes in internal stress, reflecting the maximum stress achieved by different samples and the time required to reach this stress. Since fiber gratings also respond to temperature, we conducted the tests under constant conditions of temperature (25 ^oC) and humidity (90%) to eliminate the affect. The stress values can be derived from the following Eq. (1)⁵⁵:

$$\Delta \epsilon =\Delta \lambda /k_{\epsilon }$$ 1

where Δε represents the change in the microstrain, Δλ refers to the change in the reflection wavelength and k_ε refers to the coefficient related to the optical fiber, whose value is 1.2 pm με^-1. After formulaic signal processing, the trend in the variation of the microstrain can be calculated by Eq. (2).

$$\sigma =E\times {\Delta }_{\epsilon }$$ 2

The change in strain can be converted to stress according to this formula, in which σ refers to the stress of the materials and E denotes to the elastic modulus of the coatings. The trend in stress closely mirrors that of the strain.

Photothermal and defogging performance of anti-fogging coating

The coatings and their substrates were tested under a solar simulator (CEL-PF300-T8 xenon light source) and monitored for surface temperature using an infrared thermal imager (Fluke Ti401 Pro). The photothermal de-fogging performance was evaluated by assessing the extent to which visibility was restored after the coated surfaces became completely fogged. To induce fogging, both control samples and anti-fog materials were placed in a freezer set at 0 ^oC for 20 min, then transferred to a 25 ^oC environment until complete fogging occurred. Once fully fogged, the samples were exposed to the solar simulator, and the evolution of surface fog and the time required for complete evaporation were recorded. All images were binarized to quantify the dynamic changes in the fog fraction (f). In a dark-field optical setup, condensed droplets within the fogged area scatter incident light, appearing bright, while transparent non-fogged regions remain dark. A reasonable binarization threshold was manually set such that the fog-free region had f ≈ 0, and the fully fogged region had f ≈ 1²⁴.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Author contributions

Conceptualization: C.J.Z., B.T.L., Z.H.W. Methodology: C.J.Z., B.T.L., Z.H.W. Data Analysis: C.J.Z., B.T.L., S.Y.Y., J.H.L. Investigation: C.J.Z., S.Y.Y. Supervision: C.J.Z., Q.L., S.H.Z., X.G.J., Z.H.W. Writing-original draft: C.J.Z., Z.H.W. Writing-review and editing: C.J.Z., B.T.L., Z.H.W.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewers for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data that support the findings of this study are available within the main text and Supplementary Information. Source data are provided with this paper. All data are available from the corresponding author upon request or on Figshare with (10.6084/m9.figshare.28658657). Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-025-64055-0.