Printable polymer nanocomposites for scalable and architected radiative cooling

Domain: physics
Published: 2025-12-27
Editorial outcome: minor_revision

Abstract
Space cooling and lighting together consume 25% of global electricity, yet existing daytime radiative coolers are mostly limited to porous planar coatings that block visible light and lack durability. Here, we introduce rheology-optics coupling as a design principle that links polymer viscoelasticity to particle dispersion and optical scattering. Guided by this principle, we develop printable polydimethylsiloxane-zirconium oxide composites that achieve solar reflectance (~97.3%) and mid-infrared emissivity (~96.9%) comparable to the best reported values, despite a low filler loading of only ~4.5 vol.%. These scalable coatings provide up to 7.4 oC sub-ambient cooling and cut electricity use by 37% versus commercial paint in pilot-scale testing, while withstanding mechanical, thermal, and environmental stresses. Beyond planar coatings, the rheology-tunable polydimethylsiloxane-zirconium oxide ink enables direct ink writing of daylight-regulating architectures that deliver sub-ambient radiative cooling while admitting diffuse daylight for illumination, reducing both cooling and lighting demand. This work provides a practical and versatile platform for radiative cooling.

Introduction
Space cooling and lighting are among the largest drivers of global electricity demand, together accounting for nearly one quarter of total use, which impose heavy loads on power grids and drive greenhouse gas emissions 1–5. Air conditioning dominates residential demand, while in commercial buildings electric lighting often exceeds cooling loads 6,7. With rising global temperatures and accelerating urbanization, reducing both cooling and lighting energy use has become an urgent technological and societal priority 3–5.

Daytime radiative cooling offers a passive strategy to reduce cooling demand by reflecting solar heat while emitting thermal radiation to the cold sky, enabling sub-ambient cooling under direct sunlight 8–26. This strategy has shown broad potential across building envelopes 10,11, wearable fabrics 21–23,25, electronics 25,27, and industrial systems 28,29. Despite this promise, nearly all reported high-performance daytime radiative cooling materials are fabricated as opaque planar films 8–20. While effective at rejecting solar heat, these designs block visible light, preventing integration into windows, skylights, or other daylighting systems 30,31. In buildings, this limitation can force greater reliance on electric lighting, reducing potential net energy savings 32,33.

Beyond blocking light, existing materials face fundamental barriers that restrict their practical impact. Most high-performance radiative cooling materials are realized as porous coatings optimized for solar scattering (Table S1), resulting in flat, light-blocking formats 9–17, 20–25. However, high porosity inherently limits geometric versatility and mechanical robustness, leaving these films sensitive to humidity 34–36 and prone to environmental degradation and performance loss 36–38. Attempts to improve durability using fluorine-containing polymers 9,13,37,39 introduce additional constraints, as they depend on costly, specialized chemistries that pose growing environmental concerns 40–42. Nonporous polymer composites with particle fillers have also been explored, but they generally underperform in optical performance 19,43–49 and typically rely on either costly silver backing layers 19,45,46 or high particle loadings (>25 vol.%) 47–49. Such high loadings inevitably cause brittleness and poor processability 50,51, reflecting a deeper conceptual gap in processing control. Previous studies have largely treated particle-polymer composites as simple mixtures, focusing on optical metrics while overlooking how the polymer matrix and processing conditions govern particle dispersion and stability. Consequently, scattering efficiency at low filler content remains poorly controlled. Collectively, these limitations reveal the absence of a fundamental framework connecting material structure, processing, and optical performance. This critical gap has hindered the progress of radiative cooling toward broadly deployable systems.

Here, we uncover rheology-optics coupling as a design principle that fundamentally links polymer viscoelasticity to particle dispersion and optical scattering in daytime radiative cooling composites. Leveraging this principle, we formulate a scalable and printable polydimethylsiloxane-zirconium oxide (PZO) ink that achieves state-of-the-art solar reflectance (~97.3%) and mid-infrared emissivity (~96.9%) at a remarkably low particle loading of 4.5 vol.%, enabling up to 7.4°C sub-ambient cooling. Field tests show 37% lower cooling energy use compared to commercial white paint, corresponding to potential national-scale savings of 95–178 TWh annually and avoidance of 64–119 Mt CO2 emissions. Beyond scalable planar coatings, the rheology-tunable PZO ink also enables direct ink writing (DIW) of daylight-regulating radiative cooling architectures that unite sub-ambient cooling with controlled daylight transmission (Fig. 1a). This dual-function design directly addresses the trade-off between thermal regulation and natural illumination, providing a pathway to reduce both cooling and lighting electricity demand. In addition, the ink supports additive manufacturing of intricate, customized 3D geometries, from breathable cooling masks to patterned thermal facades and fluidic thermal systems, demonstrating broad multifunctionality and geometric freedom for diverse practical applications. The discovery of rheology-optics coupling as a fundamental design principle advances radiative cooling beyond conventional coatings, opening pathways to multifunctional systems with unprecedented performance and architectural versatility.

Results
Rheology-directed control of optical scattering
Efficient daytime radiative cooling requires materials that scatter sunlight strongly across the solar spectrum, while maintaining high mid-infrared emissivity. Our key finding is that this performance depends not only on the choice of high-index fillers, but also on how polymer rheology shapes particle dispersion and size distribution during processing. By tuning viscoelasticity, the matrix controls particle breakup, dispersion, and sedimentation, which in turn determines the collective scattering behavior of the composite 52,53.

To identify optimal fillers, we simulated scattering efficiencies of common dielectric nanoparticles in a polymer host (refractive index n=1.4). Zirconium oxide (ZrO2) was identified as uniquely effective (Fig. 1b), combining a high refractive index (n=2.1–2.2) with negligible UV absorption 54,55. In contrast, other high-index oxides like TiO2 and ZnO suffer from strong UV absorption 56,57, while low-index oxides including Al2O3, SiO2, MgO and BaSO4 (n=1.4–1.8) provide only weak scattering (Figs. S1 and S2). As a result, ZrO2 achieves broadband scattering efficiencies that not only surpass conventional pigments, but also far exceed those of air pores (Fig. 1c). This indicates that high-index ZrO2 can, in principle, enable highly reflective composites that outperform porous designs. Importantly, scattering efficiency strongly depends on particle size, e.g., submicron particles dominate in the UV-visible range (λ<0.8μm), whereas micron-sized particles enhance near-infrared scattering (λ>0.8μm). Thus, achieving broadband solar reflectance requires a hierarchical size distribution that can be stabilized within the polymer matrix.

Polydimethylsiloxane (PDMS) was selected as the host matrix due to its strong emissivity in the 8–13μm atmospheric window and its tunable viscoelasticity 45. We compared three formulations with distinct rheological properties (Fig. 1d), i.e., L-PDMS (low viscosity, hexane-diluted Sylgard 184), M-PDMS (medium viscosity, Sylgard 184), and H-PDMS (high viscosity, SE1700), and observed a direct coupling between rheology and microstructure. In L-PDMS, ZrO2 particles were finely dispersed with minimal aggregation, but the size distribution was narrow, with most particles ≤0.5μm (Figs. 1e and h and S3) 43,44. In M-PDMS and H-PDMS, reduced shear intensity limits agglomerate breakup, resulting in broader, more balanced distributions extending to 0.7–1.5μm (Figs. 1f, g, i and j and S3) 52,53.

Critically, L-PDMS and M-PDMS suffered severe sedimentation, with particles settling at the bottom during curing (Figs. 1e, f and S3). In contrast, H-PDMS suppressed sedimentation owing to its pronounced shear-thinning behavior and high zero-shear viscosity, which immobilized particles during processing (Fig. 1g and S3) 58,59. The resulting microstructure exhibited uniform spatial dispersion with a broad effective size distribution. This rheology-mediated stabilization strategy provides a fundamental design principle for tuning optical scattering in nonporous composites, enabling broadband solar reflectance and scalable manufacturing within a single material platform. Unlike prior studies that treated particle dispersion primarily as a processing variable, here we establish rheology–optics coupling as a mechanism that directly links polymer viscoelasticity to particle breakup, hierarchical size distribution, and collective scattering behavior. This coupling allows high reflectance at only ~4.5 vol.% filler, overcoming the long-standing need for either metallic backings or >25 vol.% particle loadings in radiative cooling coatings.

Broadband optical response and cooling performance
The effect of rheology on filler dispersion is evident in the optical response. Incorporating ZrO2 into PDMS converts the polymer from transparent to bright white (Fig. 2a), but the level of reflectance depends strongly on matrix viscoelasticity (Fig. 2b). L-PDMS, representative of the low-viscosity systems commonly used in prior radiative cooling coatings 43,44, achieved only 91.5% solar reflectance due to narrow size distribution and severe sedimentation. M-PDMS improved modestly but still suffered from settling of larger aggregates. In contrast, H-PDMS immobilized particles during curing, producing spatially uniform, stable dispersions with broad size distributions. This microstructure enabled superior optical properties, reaching a weighted average solar reflectance of ~97.3% over 0.3–2.0μm and a weighted average mid-infrared emissivity of ~96.9% over 8–13µm (Fig. 2b and c), comparable to state-of-the-art daytime radiative coolers (Fig. S4, Tables S1 and S2). Notably, this was achieved with only ~4.5 vol.% ZrO2 at a practical film thickness of 2 mm (Figs. S5 and S6), far below the typical 25–80 vol.% loadings required in prior nonporous systems 47–49, thereby reducing material cost and improving processability.

The superiority of ZrO2 as a filler is further confirmed by direct comparison with other oxides (Fig. 2c). Consistent with simulations (Figs. 1b, S1 and S2), ZrO2-based composites delivered the highest reflectance, outperforming both low-index scatterers (Al2O3, SiO2, MgO, and BaSO4) and UV-absorbing oxides (TiO2 and ZnO). These results establish ZrO2 as the optimal filler and show that rheology-optimized PDMS matrices can fully harness its broadband scattering for scalable, high-performance radiative cooling.

Beyond optical performance, the rheology-optimized PZO ink exhibits shear-thinning behavior that allows smooth extrusion during deposition and rapid viscosity recovery for shape retention, enabling advanced fabrication via DIW with minimum feature size of ~200µm 60. We highlight DIW here as a versatile demonstration platform for architected multifunctionality. Equally important, the low filler loading and rheology control also make the ink compatible with conventional scalable coating methods, such as slot-die coating, blade-casting, screen printing, and roll-to-roll processing (Table S3). Thus, rheology-optics coupling provides a unifying design principle that not only achieves state-of-the-art performance, but also supports both industrial-scale planar coatings and DIW fabrications of multifunctional architectures.

We evaluated the daytime radiative cooling performance of PZO by printing a 20 cm × 20 cm film and testing it outdoors on a building roof in Urbana, Illinois (Fig. 2d). The PZO film, along with a white commercial paint film and a mylar film as controls, was placed on a custom-built apparatus consisting of Styrofoam block wrapped in mylar (Fig. S7). As shown in Figs. 2e and S8, the PZO film achieved an average of 6.6°C sub-ambient cooling near noontime (10 AM to 2 PM) under peak solar irradiance of 868 W/m2 61, while both controls remained at or above ambient temperature. A 4-day continuous test confirmed the reliability of its cooling performance, consistently delivering an average sub-ambient reduction of 7.4°C (Fig. S9). Notably, PZO maintained strong cooling performance both day and night, highlighting its ability to effectively reject solar heat due to its high solar reflectance. The cooling power of PZO was experimentally measured at an average of 104 W/m2 (Figs. 2f and S10), comparable to the best daytime radiative coolers.

To probe real-world impact, we applied the PZO ink coating to the exterior of a shed and compared it against two control sheds, one uncoated and one coated with commercial paint (Fig. 2g), under identical conditions. Each shed was equipped with a portable air conditioner to maintain 25°C, and cooling energy consumption was monitored. Over a one-month testing period (September 24–October 22, 2024), the PZO-coated shed showed substantial reductions in cooling demand, cutting electricity use by 70% relative to the uncoated shed and 37% relative to the commercial paint-coated shed (Fig. 2h). We note that the sheds were not insulated to the degree of residential buildings, and thus the results should be interpreted as a proof-of-concept demonstration rather than a direct building-scale model. Nevertheless, extrapolating the relative reductions to U.S. residential cooling suggests possible annual electricity savings of 95.4–177.7 TWh and 64.0–119.4 million tons of avoided CO2 emissions (Fig. S1, 1 and Table S4) 62. While these values represent an order-of-magnitude estimate, they underscore the transformative potential of scalable PZO coatings to reduce both electricity use and emissions at national scale.

Architected structures for dual cooling and daylighting
While planar PZO films exhibit excellent radiative cooling performance, the unique advantage of this ink is its ability to form architected structures. A persistent challenge in building design is balancing thermal regulation with natural daylighting. To illustrate this capability, we used DIW to create a “LumiCool” structure by directly printing tilted PZO fin arrays onto transparent glass substrates (Fig. 3a, b). This geometry leverages the broadband solar reflectance of PZO to block high-angle mid-day sunlight, while still admitting low-angle and diffuse light. In this way, LumiCool integrates radiative cooling with controlled daylight transmission, illustrating a pathway to mitigate the long-standing trade-off between thermal regulation and natural illumination.

To quantify this effect, we simulated the daily direct sunlight duration (Tdsd) as a function of panel tilt angle (θ=θ1+θ3, assuming θ1=θ) and length-to-spacing ratio (R=Length/Spacing) (Fig. 3b,c). The results showed that increasing fin tilt or spacing ratio reduces Tdsd, leading to reduced daytime solar heat gain. When θ exceeds 95° and R>2.9, Tdsd fell below 1.5 h per day. While admitting low-angle light may slightly increase indoor temperatures during summer mornings or evenings, it provides a beneficial passive heating effect in cold seasons, particularly at high latitudes with long winters.

For experimental validation, we fabricated small-scale house models (200×150×150mm) with three rooftop configurations (Fig. 3a). The LumiCool module consisted of a glass panel (130×120mm) with DIW-printed PZO fins inclined at θ1≈35° and R≈3, mounted at a roof tilt angle of θ2=60° (Fig. 3a and d). Two control models were also prepared, one with fully transparent glass and another with a continuous planar PZO film. To isolate rooftop effects, all other surfaces were coated with PZO film.

Outdoor testing confirmed the dual functionality of LumiCool. During peak solar hours, it maintained indoor temperatures 2–3°C below ambient, comparable to the fully PZO-covered roof, whereas the transparent roof overheated by 3–10°C above ambient (Fig. 3e). At the same time, LumiCool preserved stable indoor illuminance near 1000 lux from 11 AM to 8 PM (Fig. 3f), providing comfortable and functional lighting. In contrast, the fully coated roof reduced illumination to near-darkness (<100 lux), while the transparent roof produced excessive brightness (>5000 lux), causing glare and visual discomfort. Importantly, the LumiCool architecture is tunable, as adjusting the width (W) and spacing (S) allows designers to tailor the balance between lighting comfort and thermal regulation. These results demonstrate the potential of printable PZO ink to create a new class of dual-function architectures and façade systems that unite broadband radiative cooling with optimized daylighting, capabilities beyond the reach of conventional planar coatings.

Expanding multifunctional radiative cooling systems
The versatility of the PZO ink extends radiative cooling into a wide range of multifunctional and geometrically complex applications. For building integration, DIW enables simplified fabrication of structured wall coatings with intricate surface profiles that can be used to enhance cooling performance on vertical facades. As a demonstration, we printed sawtooth-like gratings with ~1 mm pitch and ~33° tilt in a single step (Fig. 4a). These geometries replicate the core structural motifs used in previously reported angularly asymmetric thermal emitters, which traditionally require multi-step lithographic or molding processes 63,64. Note that our demonstration focuses on the geometric fabrication stage, without the final directional IR-reflective coating employed in those systems. By enabling these complex geometries through a single-step printing process, our approach substantially simplifies the manufacturing workflow and provides a practical foundation for incorporating directional IR-reflective coatings in future implementations.

In addition to performance optimization, DIW demonstrations show the potential for aesthetic and architectural customization. As shown in Fig. 4b, a variety of high-fidelity 2D patterns, including logos, tiled motifs, and pictorial illustrations, were printed with excellent resolution, expanding the design freedom for radiative cooling surfaces that are both functionally efficient and visually distinctive. These demonstrations showcase the design flexibility of the ink for creating architected building components while simplifying fabrication.

The adaptability of the ink also supports conformal and lightweight designs for wearables and electronics systems. Radiative cooling masks can be printed with tunable filament spacing, which combine breathability with passive cooling capability (Fig. 4c). Complex geometries such as honeycomb panels and cylindrical enclosures were also fabricated (Fig. 4d), demonstrating form-fitting cooling shells for irregular components in communication nodes, drones, or autonomous vehicles. These examples highlight the ability of DIW with PZO ink to deliver customized, high-performance passive cooling solutions for next-generation wearable and electronic platforms.

Finally, DIW of PZO ink facilitates seamless integration of radiative cooling with fluidic thermal management. Printed grooved arrays with shallow trenches can promote condensation by increasing the nucleation area and allowing continuous droplet drainage, sustaining high condensation flux. Channeled slabs, printed monolithically without interfacial joints, create enclosed pathways that retain and transport water while limiting heat gain from the surroundings, thereby preserving low temperatures for cold-water generation and storage (Fig. 4e). Together, these topographically tailored structures illustrate the capability of PZO ink via DIW to unite radiative cooling with fluid transport, advancing applications in steam condensation, dew harvesting, and cold-water production.

Durability for real-world deployment
Beyond multifunctional design flexibility, practical deployment also requires long-term durability. The PZO composite displayed exceptional mechanical resilience, withstanding up to 500% strain without failure (Figs. 5a and b and S12), owing to the flexible silicone matrix and the absence of stress-concentrating pores that are prone to fracture. In abrasion tests following ASTM standard D4060, PZO showed nearly half the mass loss of commercial paint after 2000 cycles, underscoring superior wear resistance for long-lasting performance (Fig. 5c, Fig. S13 and Table S5). Furthermore, to ensure versatility across architectural surfaces, the ink was directly printed onto diverse substrates, including wood, glass, acrylic, and metal, without pretreatment (Fig. S14). The ink adhered strongly to the substrates, forming uniform coatings with pull-off adhesion strength (ASTM D4541) comparable to commercial paints (Figs. 5d and S15). The composite also retained optical performance under repeated thermal cycling (–15°C to 50°C) and remained stable at extreme cold (–196°C; Fig. 5e and S16). Under accelerated UV exposure over a month 65, no degradation was observed (Fig. 5f and S17). Finally, the coating proved waterproof (Fig. 5g) and flame-retardant (Fig. S18), extending its applicability from buildings to outdoor electronics, space systems, and protective equipment.

Discussion
We establish rheology-optics coupling as a design principle for scalable, high-performance radiative cooling. This fundamental link between polymer viscoelasticity and optical scattering in particle composites enabled a printable PDMS-ZrO2 (PZO) ink with state-of-the-art solar reflectance (97.3%) and mid-infrared emissivity (96.9%) at only ~4.5 vol.% filler loading. This low loading avoids brittleness and poor adhesion in highly filled paints, while supporting both industrial-scale coating methods (e.g., blade coating, slot-die, screen printing, roll-to-roll) and direct ink writing (DIW). With DIW, the PZO ink enables architected multifunctional systems, including daylight-regulating radiative cooling architectures (LumiCool), patterned thermal facades, wearable cooling masks, conformal electronic enclosures, and fluidic thermal devices. The PZO composite also exhibits exceptional durability against mechanical, thermal, and environmental stresses, ensuring stability for real-world deployment. Pilot-scale field tests confirmed substantial reductions in cooling demand compared to conventional coatings, translating to gigawatt-hour electricity savings and tens of millions of tons of avoided CO2 emissions at national scale. Together, these results establish rheology–optics coupling as a key design framework that unites fundamental optical control, scalable manufacturing, and architectural versatility, advancing radiative cooling from simple coatings toward practical, multifunctional systems poised to address urgent global needs in sustainable buildings, electronics, wearables, and industrial systems.

Methods
Materials and fabrication methods
H-PDMS and M-PDMS refer to SE1700 base and Sylgard 184 base, respectively. L-PDMS was obtained by further diluting Sylgard 184 base with hexane at a weight ratio of 4:1 (hexane:Sylgard 184). For the PZO ink formulations, the weight ratios of PDMS to ZrO2 particle fillers were set at 10:2, 10:3, and 10:4, corresponding to volume fractions of approximately 3.0%, 4.5%, and 5.9%, respectively. The curing agent to polymer base ratio was kept at 1:10 by weight. All components were thoroughly blended using a centrifugal mixer (Speedmixer, FlackTek) at 2000 rpm for 2 min to remove the bubbles. The particle fillers were purchased (US Research Nanomaterials, Inc.) and used as received. Fig. S19 presents the Scanning electron microscope (SEM) image of the commercial ZrO2 particles. The ZrO2 sample consists mainly of fine particles with diameters ranging from 0.1 to 0.4µm. In addition to these primary particles, larger features (~2–3µm diameter) are observed, indicating significant particle aggregation.

Direct ink writing process
DIW fabrication was conducted at room temperature using a customized printing system (AGS-1000, Aerotech Inc). During the printing process, the gantry system, which carries the ink cartridge, moves along the X-Y-Z directions, while a pressure dispenser connected to the ink cartridge drives the ink through the tapered dispensing tip (200μm—1mm diameter) onto the substrate. Printing paths for the designed patterns were created using computer-aid design software (AutoCAD) and converted to G-code using commercial software (CADFusion). Post-printing curing was carried out either at room temperature for 24 h or in a heated oven (e.g., 6 h at 70°C) to enable thermal crosslinking and solidify the printed structure.

Rheology measurement
The viscosity of the composite ink was measured by a rheometer (DHR-3; TA Instrument, Inc.) equipped with a flat geometry plate (25mm diameter). The temperature was maintained at 20°C throughout the measurements. Prior to testing, the ink was carefully dispensed onto the plate to avoid introducing air bubbles, and any excess ink was removed. The measurement gap was set to 1000μm, and each composition was tested at least three times to ensure reliable results. All viscosity measurements were conducted within 1 hour of mixing.

Morphology and optical characterization
SEM images and energy-dispersive X-ray spectroscopy elemental mapping were acquired using a Hitachi S4800. UV–Vis-NIR reflectance spectra were measured using a Varian Cary 5G spectrometer equipped with an integrating sphere (DRA2500, Agilent). Infrared emissivity and reflectance were characterized by an FTIR spectrometer (Thermo Nicolet iS50 FTIR) equipped with a gold integrating sphere (PIKE Technology).

Micro-computed tomography (Micro-CT) analysis
Micro-CT was performed using a Zeiss Xradia 630 Versa system housed at the Beckman Institute for Advanced Science and Technology, University of Illinois Urbana–Champaign. Rectangular specimens (~5×2×0.5mm³) were cut from the cured PDMS–ZrO2 composites and mounted on the rotary stage for scans at an anisotropic voxel size of 500nm and a field of view of 1016μm × 1016μm. Scans were collected with a 25W Tungsten source X-ray beam at a tube voltage of 80kV and 1601 sequential projections with an exposure time of 13 s over 180° rotation. The radiographic projections were manually corrected by the alignment of center shifts and reconstructed using Xradia Reconstructor (Carl Zeiss). The reconstructed 3D volume of film composite was imported into Avizo version (version 2019.1, Thermo Fisher Scientific) for volume rendering to visualize the internal dispersion of ZrO2 particles within the PDMS matrix.

LumiCool house model test
The LumiCool rooftop tests were performed on a small-scale house model (200×150×150mm), as shown in Fig. 3. Each LumiCool module consisted of a 130×120mm soda-lime glass substrate, onto which an array of parallel, evenly spaced PZO panels was printed using direct ink writing (DIW). The panels featured an inclination angle of θ1≈35° and a length-to-spacing ratio of R≈3 (Fig. 3d). The modules were installed on both roof slopes of the test house at a fixed tilt angle of θ2=60°. For comparison, two additional small-scale house models of identical dimensions were prepared as control groups: one with a fully transparent glass rooftop (Fig. 3a, left), and the other with a rooftop uniformly covered by a PZO film of ~3mm thickness (Fig. 3a, center). All other exposed surfaces of the tested houses were covered with identical PZO films. K-type thermocouples were used to continuously monitor the temperature on the inner surface of each rooftop.

Water contact angle measurement
Water contact angle measurements were performed using a Ramé-Hart Model 250 Contact Angle Goniometer. Reported values were the average of measurements taken at four different locations on the same sample.

Abrasion and pull-off tests
The abrasion test was conducted according to ASTM D4060. Mass loss of the samples was measured at 500-cycle intervals throughout the 2000-cycle test using CS-10 calibrase with a 500g load. Adhesion strength was evaluated on various substrates using the pull-off test following ASTM D4541, Method E, with sample thickness kept below 2mm.

Outdoor thermal measurements
Outdoor thermal measurements were performed using a custom-built experimental apparatus made of Styrofoam boxes. The setup was placed on a wood table covered with Mylar film to minimize parasitic heat loss. The measurements were conducted on the roof of a building in Urbana, Illinois of the U.S. (40.11°N, 88.21°W; 225m altitude). The temperatures of the PZO film, commercial paint, and Mylar film were measured by fast-response T-type surface thermocouples (SA1-T-72; Omega Engineering Inc.), while the ambient air temperature was recorded by a sheathed T-type thermocouple (TTIN-116G-12; Omega Engineering Inc.). All temperature data were recorded with a data acquisition device (DAQ, USB 2408; Measurement Computing Corp.). The radiative cooling power measurement was performed using a similar setup. An electrical heater was placed beneath the testing sample, with its heating power adjusted in real-time using a LabVIEW program. The program monitored the temperatures of the PZO film and ambient air, compared their difference, and adjusted the heating power to ensure the film temperature closely matched the ambient air temperature. By maintaining equal temperatures for the PZO film and ambient air, convective and conductive heat transfer were eliminated. Under these conditions, the electrical heating power, fully dissipated through thermal radiation, equated to the net radiative cooling power.

Electricity consumption measurement
To evaluate large-scale applicability and energy-saving potential, three identical sheds were installed at the U.S. Army Construction Engineering Research Laboratory (CERL) in Champaign, IL. The sheds were assigned to three experimental groups: one coated with PZO radiative cooling ink, one with commercial white paint, and one left uncoated. Each shed was equipped with a portable air conditioner (LP1021BHSM, LG Electronics) and set to maintain an internal temperature of 25°C. The air conditioner units were connected to power data loggers (ELITEpro XC™, Dent Instruments) to record electricity usage at one-minute intervals. The collected data was analyzed to compare the power consumption of the PZO coated shed against that of uncoated and white-painted sheds.

Tensile tests
Tensile testing was performed using a dynamic mechanical analyzer (DMA, Q800, TA Instruments) on DIW-printed samples with approximate dimensions of 40×2×1mm. The samples were prepared by printing at room temperature, followed by thermal curing at 70°C for 6h. The DMA tests were conducted with a preload of 0.1N and a loading rate of 3N/min, up to a maximum possible load of 18N at 20°C.

Thermal stability tests
Thermal stability was evaluated through thermal cycling and extreme-cold tests. The thermal cycling test involved 15 consecutive cycles of 24-h heating, followed by 24-hour cooling. Heating was performed in a forced-convection oven at 50°C, while cooling was conducted in a freezer at −15°C. For the extreme cold test, the PZO samples were immersed in liquid nitrogen for 24h.

UV, water, and flame resistance tests
The UV exposure test was conducted using a custom-built setup. UV radiation, with a total power density of ~13.5 W/m2 in the UVA and UVB ranges, was provided by a UVB 150 lamp. The samples were exposed to UV irradiation for 30 days, simulating approximately 46 days of outdoor radiation in Florida. The temperature during the test was maintained at 21°C. The waterproof performance of the PZO coating was assessed by immersing a PZO-coated plywood sample in water for several days and measuring water absorption. An identical, non-coated plywood sample served as a control. For the flame resistance test, a free-standing PZO sample was vertically positioned and exposed to a butane lighter flame. The PZO sample exhibited no signs of ignition after 10 s of exposure.

Data availability
The data supporting the findings of this study are available within the paper and its supplementary information files. Source data for all plots and graphs are provided with the paper. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript by Zhou et al. reported an additive manufacturing process for radiative cooling materials, also termed as high-refractive-index materials. This work may be attractive to readers of Nature Communications as it addresses important challenges related to manufacturing and energy. The reviewer has the following comments:\n1. Unclear why AM or DIW approaches are needed to fabricate the ZrO2 materials. What is shown in Fig. 2(a) can also be made using conventional manufacturing processes such as tape-casting, which is a truly scalable process. In addition, using the AM process does not seem to improve cooling efficiency.\n2. Motivated by the current scalability limitation of existing materials, the authors proposed the AM process. However, the scalability of AM and DIW is not justified in the work. If using the small nozzle shown in Figure 1, it is not realistic to scale up production. In addition, how to eliminate air bubbles trapped in the viscous inks, and how to avoid particle sedimentation have to be addressed for large-scale production. Metrics such as zeta potential and surface energy will need to be characterized to support the potential for scalability.\n3. Are all the structures in Fig.2 designed for cooling purposes? It is not clear what those 3D structures are for.\n4. Does the thickness of the material also play a role in cooling?\n5. Plots in Figure 5 need error bars.\n6. In Figure 5, the “uncoated” control group appears to be black, while the other two groups are white. Black color is known for its high absorption of thermal energy, which consumes more power to dissipate the heat. White reflects more light than black. The three sets of results may not provide a parallel comparison.\nOverall, the reviewer agrees that this work provides interesting engineering findings. But it lacks scientific discoveries and validations. The AM-DIW process does not bring benefits for cooling and thus lacks novelty. Scalability has to be addressed if the authors want to claim it as a strength of the work.", "reviewer_2": "In this manuscript, the authors present a direct ink writing (DIW) approach to fabricate daytime radiative coolers. By incorporating ZrO2 particles into the polymer ink, they fabricated samples with high solar reflectivity and infrared emissivity, enabling sub-ambient cooling of up to 7.4 °C in outdoor settings and 37% electricity saving in a month-long field test. The work is pretty well done with comprehensive experiment demonstrations including optical, thermal, and mechanical characterizations, and the results are presented in a very clear and well-organized manner.\nThat said, there are several concerns regarding the novelty and scientific impact of this work. Specifically, the necessity and advantages of using DIW for radiative cooling applications are not convincingly established. Since only the surface layer contributes to solar reflectivity and infrared emission, it remains unclear why full 3D printing of the bulk object is needed, as opposed to simpler surface deposition methods like ZrO₂-based spray coating or painting with less time and material consumption. Furthermore, other 3D printing techniques have already been applied to fabricate radiative cooling structures with sub-ambient performance (e.g., S. Park, et al., Nanophotonics, 2024, pp. 611–620). To meet the publication criteria of Nature Communications, the work would need to clearly demonstrate its high level of scientific novelty and significance.\nThe reviewer suggests the following points for further consideration:\n1. What specific use cases (e.g., wearables, curved surfaces, microdevices) require the programmable patternability of DIW? Some demonstrations would help clarify the necessity of structured radiative coolers.\n2. The manuscript should include a quantitative comparison of the scalability of the DIW method (e.g., deposition rate, printing area, and material usage) with existing scalable methods such as spray coating, painting, or roll-to-roll fabricated films.\n3. The combination of ZrO2 particles and PDMS matrix have been proposed for achieving radiative coolers with sub-ambient cooling performance (e.g., Y. Zhang, et al., Solar Energy Materials and Solar Cells, 2021, 111129; X. Yang, et al., Inorganic Chemistry Communications, 2023, 110586). These relervant papers should be properly cited and discussed in the manuscript. As the DIW technique cannot enhance the cooling performance, what performance metrics justify the use of DIW over less complex techniques?\n4. What are the specific advantages of DIW over other 3D printing strategies previously used for radiative cooling materials? A direct comparison would clarify the unique value of the proposed method.\n5. The manuscript emphasizes mechanical durability, but how much of this is attributable to the DIW geometry vs. the inherent robustness of the PDMS-ZrO₂ composite? Can similar durability be achieved with sprayed or cast coatings using the same material?\nBelow are some minor comments:\n1. The acronym “PZO” appears throughout the manuscript without providing its definition.\n2. The spectral ranges used to define solar reflectivity and mid-infrared emissivity should be clearly specified.\n3. What is the minimum feature size (line width and thickness) that can be reliably achieved with this DIW technique?\n4. In Fig. 5c, why does the PZO film exhibit smaller mass loss than commercial paint? Is the PZO film 3D-printed or painted on a film substrate?", "reviewer_3": "The manuscript by Zhou et al. presents a 3D-printable polymer-nanoparticle ink designed for direct ink writing of non-porous radiative coolers, effectively addressing the scalability and durability challenges associated with traditional porous designs. The 3D-printed structures demonstrate high solar reflectivity and infrared emissivity, resulting in a cooling effect of 7.4 °C for energy-saving applications. This study has the potential to connect high-performance radiative cooling with scalable 3D printing for practical use, making it a strong candidate for further consideration for publication. However, several notable issues have been identified, as outlined below:\n1. The inks depicted in Figure 1c exhibit a morphology resembling high-viscosity fluids that appear to flow readily. This behavior deviates from the expected behavior of high-quality 3D printing inks. The authors should clarify this observation.\n2. To fully assess the ink’s printability, additional rheological characterizations beyond dynamic viscosity are necessary. Specifically, dynamic oscillatory shear measurements (e.g., storage/loss moduli) or step-strain tests would help determine whether the ink exhibits the necessary viscoelasticity for smooth extrusion while retaining structural fidelity post-deposition. Such analyses would clarify how the ink balances extrudability with shape retention to prevent uncontrolled spreading.\n3. The incorporation of fine nanoparticles (e.g., ZrO₂) often significantly modifies the rheological properties of polymeric inks, typically enhancing shear-thinning behavior or introducing yield stress via filler-network formation. However, the manuscript does not show a pronounced rheological effect from ZrO₂ addition. The authors should discuss possible reasons for this observation. A mechanistic explanation would strengthen the ink design rationale.\n4. Achieving and maintaining uniform ZrO₂ dispersion in PDMS from ink preparation through curing is critical for consistent material performance. The authors should elaborate on the strategies and mechanisms to prevent particle settling or aggregation. Currently, only the final cured structure is characterized, leaving the dispersion dynamics during printing and curing unexplored/unexplained.\n5. The PZO fiber displays impressive tensile strain tolerance of approximately 500% before fracture. It is important to evaluate the properties of these fibers—such as solar reflectance and infrared emissivity—under mechanically stretched conditions.Otherwise, the mechanical deformability may not be relevant to radiative cooler applications.\n6. The results in Figures 4 and 5 suggest that conventional coating or painting methods could produce similar radiative cooling samples. Since the manuscript emphasizes DIW in its title, the authors should clearly highlight the unique advantages that 3D printing offers, potentially through a proof-of-concept demonstration in this context."}, "author_response": "Point-to-Point Response to Reviewers' Comments\n(Manuscript ID: NCOMMS-25-16828-T)\nWe would like to thank the reviewers for their time and effort in evaluating our work. We greatly\nappreciate the constructive and insightful comments the reviewers provided. In response, we have\nsubstantially revised the manuscript. The main text has been comprehensively rewritten, with new\ndata, figures, and expanded discussions added to address every reviewer comment. Taken together,\nthe revised manuscript is substantially strengthened from the original submission, establishing\nboth new scientific understanding and significant technical advances with clear pathways to\npractical applications. A detailed point-to-point response is provided in the following pages. For\nclarify, a summary of the key revisions and advances is presented below:\n• Scientific Novelty: We introduce rheology–optics coupling as a new design principle that\nlinks polymer viscoelasticity to particle dispersion and optical performance. Prior studies\nlargely treated particle-polymer composites as simple mixtures, focusing on optical metrics\nwhile overlooking the critical influence of the polymer matrix on processing behavior and\noverall performance. Consequently, scattering efficiency at low filler content remains\npoorly controlled. We added micro-CT data and microstructure analysis in Fig. 1d-j, Fig.\n2b, and Fig. S3, confirming uniform particle dispersion, and expanded discussion on\nsedimentation suppression through rheology tuning in main text. These findings ground\nour radiative cooling ink design in a fundamental mechanism rather than material choice\nalone.\n• Record Performance: The revised manuscript demonstrates scalable and printable\npolymer composites with state-of-the-art solar reflectance (97.3%) and mid-infrared\nemissivity (96.9%) at only 4.5 vol.% filler loading, overcoming the long-standing\nlimitations of prior coatings requiring 25–80 vol.% fillers or metallic backings (see the\nupdated comparison in Fig. S4 and Table S1-S2).\n• Unique Value of DIW: We highlight DIW as a complementary fabrication strategy\nenabling multifunctional, architected systems beyond conventional coatings. New results\non proof-of-concept demonstrations with expanded discussion of specific use cases are\nnow presented in Fig. 3 and Fig. 4, including the LumiCool module that unites radiative\ncooling with daylighting, sawtooth-patterned façades with angularly selective emission,\nbreathable cooling masks, and fluidic-integrated devices for dew harvesting and cold-water\nstorage. These directly address reviewers’ concerns about DIW’s unique value.\n• Scalability and Practicality: We emphasize that the low-loading formulation is\ncompatible with industrial coating techniques (slot-die, blade, screen, roll-to-roll), while\nDIW supports multifunctionality. Pilot-scale tests confirm up to 7.4 °C sub-ambient\ncooling, 37% electricity savings compared to commercial paint, and strong durability\nagainst mechanical, thermal, and environmental stresses, highlighting both practicality and\nsocietal significance.\n• Differentiation from Prior Art: The revised text more explicitly distinguishes our work\nfrom prior studies (e.g., Park et al., Nanophotonics, 2024; Zhang et al., Solar Energy\nMaterials and Solar Cells, 2021). The comparisons to state-of-the-art works in terms of\nperformance, manufacturability, and durability are updated in Table S1-S2 and Fig. S4.\nReviewer #1 (Remarks to the Author):\nThe manuscript by Zhou et al. reported an additive manufacturing process for radiative cooling\nmaterials, also termed as high-refractive-index materials. This work may be attractive to readers\nof Nature Communications as it addresses important challenges related to manufacturing and\nenergy. The reviewer has the following comments:\n1. Unclear why AM or DIW approaches are needed to fabricate the ZrO2 materials. What is shown\nin Fig. 2(a) can also be made using conventional manufacturing processes such as tape-casting,\nwhich is a truly scalable process. In addition, using the AM process does not seem to improve\ncooling efficiency.\nResponse: We thank the reviewer for this valuable comment. In the revised manuscript, we have\nsubstantially revised the entire manuscript, the data presentation and the manuscript title to better\nillustrate the scientific insights and technical advances of our work.\nFirst, we now introduce rheology–optics coupling as a design principle that links polymer\nviscoelasticity to particle dispersion, hierarchical size distribution, and optical scattering. This\nmechanism allows us to achieve record optical performance (~97.3% solar reflectance, ~96.9%\nmid-IR emissivity) at only ~4.5 vol.% filler loading. Compared to prior nonporous coatings that\nrequired >25 vol.% loading or metallic backings, our low-loading formulation avoids brittleness,\npoor adhesion, and limited processability. Thus, even as a planar coating produced by conventional\nmethods, our approach provides a scalable and scientifically novel pathway that overcomes the\nchallenges of prior coatings.\nSecond, we clarify the role of DIW. The goal of DIW is not to increase intrinsic cooling efficiency,\nbut to demonstrate the architectural versatility of the ink. DIW enables programmable structures\nthat provide system-level advantages beyond the reach of planar coatings. For example, we present\nthe LumiCool module (Fig. 3), which for the first time unites sub-ambient radiative cooling with\ncontrolled daylight transmission. Additional demonstrations include sawtooth-patterned façades\n(Fig. 4a), breathable cooling masks (Fig. 4c), and fluidic-integrated devices (Fig. 4e), each\nhighlighting how DIW unlocks unique functionalities not achievable by tape casting or spray\ncoating.\nIn this way, the revised manuscript emphasizes that our contributions extend beyond fabrication.\nWe provide both a fundamental scientific advance (rheology–optics coupling) and a practical\nmaterials platform that supports scalable planar coatings and multifunctional DIW architectures.\nRevisions:\n1) Revised the main text, data presentation and manuscript title to better illustrate the scientific\ninsights and technical advances of our work. The edited text articulates clearly the\ncontributions for both scalable planar coatings and DIW architectures.\n2) Added a new section “Rheology-Directed Control of Optical Scattering” to discuss rheology–\noptics coupling as the key scientific principle enabling record performance at low filler loading.\nNew data is presented in Fig. 1d-j, Fig. 2b, Fig. S3.\nFigure 1. Ink formulation strategy by rheology-optics coupling. (a) Schematic of broadband-reflective\nPZO ink and its DIW-based applications, including daylight-regulating roofs, breathable masks, and\npatterned thermal surfaces. Scattering efficiency of (b) ZrO filler and (c) air pore within the PDMS\n2\nmatrix at different sizes. (d) Rheological measurements of different PDMS matrices: H-PDMS (SE1700,\nhigh viscosity), M-PDMS (Sylgard 184, medium viscosity), and L-PDMS (hexane-diluted Sylgard 184,\nlow viscosity). (e–g) X-ray micro-computed tomography (Micro-CT) images of cured PDMS–ZrO₂ films\nprepared with different PDMS matrices: (e) L-PDMS, (f) M-PDMS, and (g) H-PDMS. The scanned\nvolume was 785 μm × 994.5 μm × 500 μm (X × Y × Z). (h–j) Particle size distributions of ZrO₂ fillers\nin the cured PDMS films, derived from Micro-CT analysis: (h) L-PDMS, (i) M-PDMS, and (j) H-PDMS.\nFigure 2b. UV–Vis–NIR reflectance spectra of PDMS- ZrO₂ composite films prepared using H-\nPDMS, M-PDMS, and L-PDMS. All samples contain 4.5 vol% ZrO₂.\nFigure S3. SEM image showing the cross-section view of the matrix with different viscosity.\n3) Clarified in discussion and conclusion that the low-filler-loading ink is compatible with\nscalable planar coating methods (slot-die, blade, screen, roll-to-roll), with addition of the\nfollowing sentences in section “Broadband Optical Response and Cooling Performance”.\n“Equally important, the low filler loading and rheology control also make the ink compatible\nwith conventional scalable coating methods, such as slot-die coating, blade-casting, screen\nprinting, and roll-to-roll processing. Thus, rheology-optics coupling provides a unifying design\nprinciple that not only achieves record performance, but also supports both industrial-scale\nplanar coatings and DIW fabrications of multifunctional architectures.”\n4) Added new demonstrations of DIW-enabled multifunctional systems, including LumiCool (Fig.\n3), façades, and masks (Fig. 4) to demonstrate the impact of architected structures. Added more\ndetailed discussions in sections “Architected Structures for Dual Cooling and Daylighting”\nand “Expanding Multifunctional Radiative Cooling Systems” to clarify the specific use cases\nof DIW-fabricated structures.\nFigure 3. Optical design and performance of LumiCool module. (a) Photographs of house models with\nthree roof configurations: transparent, fully covered, and LumiCool (left to right). Insets illustrate the\nresulting interior lighting conditions. (b) Schematic illustration of the optical design utilizing the strong\nand broadband solar reflectance of the PZO material. Direct sunlight enters the interior only when the\nsolar incident angle is smaller than the critical angles α₁ (morning) or α₂ (afternoon), enabling controlled\ndaylighting while minimizing heat gain. (c) Simulated daily direct sunlight duration on architectures\nconstructed with LumiCool structure. (d) Top-down and side-views of a LumiCool module comprising DIW-\nprinted PZO fin array structures on glass, showing tilted fins with representative inclination and periodic\nspacing. (e) Indoor temperature profiles of test house modules with different roof configurations under\ndirect sunlight. (f) Indoor illuminance levels measured under different roof configurations.\nFigure 4. DIW-fabricated radiative cooling structures with diverse geometries and applications. (a) DIW-\nprinted sawtooth-like grating structure with angular asymmetry. (b) Representative 2D patterns printed\nusing high-viscosity DIW-PZO ink, including an “ILLINOIS” logo, periodic triangular tiling, and a panda\nillustration. (c) Radiative cooling mask fabricated via DIW. Insets show the detailed filament structure,\nwearer demonstration, and overall geometry. (d) Complex 3D structures fabricated via DIW, including a\nhoneycomb panel and cylindrical tube. (e) DIW-fabricated structures designed for thermal–fluidic\napplications, including grooved arrays and channeled slabs.\n2. Motivated by the current scalability limitation of existing materials, the authors proposed the\nAM process. However, the scalability of AM and DIW is not justified in the work. If using the\nsmall nozzle shown in Figure 1, it is not realistic to scale up production. In addition, how to\neliminate air bubbles trapped in the viscous inks, and how to avoid particle sedimentation have to\nbe addressed for large-scale production. Metrics such as zeta potential and surface energy will\nneed to be characterized to support the potential for scalability.\nResponse: We appreciate the reviewer’s thoughtful concerns about scalability. In the revised\nmanuscript, we have refined the scope of our work to clarify that two complementary scalability\npathways are enabled by our ink design:\nPlanar large-area deployment via conventional coating: One of the central scientific advances of\nthis work is the discovery of rheology–optics coupling, which allows us to achieve record optical\nperformance at only ~4.5 vol.% filler loading. This low-loading formulation avoids brittleness and\nprocessability challenges of prior high-loading coatings and is fully compatible with established\nlarge-scale techniques such as slot-die, blade, screen, and roll-to-roll coating. Thus, even without\nDIW, our ink provides a scalable and manufacturable pathway for planar radiative cooling films.\nArchitected demonstrations via DIW: We clarify that DIW is not intended to be the sole pathway\nfor scale-up of large-area coatings. Rather, it is a complementary fabrication strategy that enables\nprogrammable 3D geometries and multifunctional systems. In the revised manuscript, we present\nseveral such demonstrations: the LumiCool module (Fig. 3) that achieves simultaneous radiative\ncooling and daylighting; sawtooth-patterned façades (Fig. 4a) that enhance angularly selective\nemission on vertical walls; breathable cooling masks (Fig. 4c) that combine comfort with passive\ncooling; and fluidic-integrated devices (Fig. 4e) for dew harvesting and cold-water storage.\nThese examples highlight how DIW goes beyond planar films by unlocking system-level\ncapabilities and application-specific functionalities that cannot be realized with conventional\ncoating techniques. For such use cases, DIW scalability can be increased through multi-nozzle\narrays and parallel printing, as already adopted in industrial additive manufacturing.\nIn addition, we address bubble and sedimentation concerns by providing new data and discussion.\nOur method incorporates high-shear mixing and degassing, which reliably eliminates trapped\nbubbles prior to printing or coating. For sedimentation control, we added new micro-CT and SEM\ndata to demonstrate how the rheology control immobilizes particles during curing, preventing\nsedimentation and ensuring uniform dispersion (Fig. 1d–j and Fig. S3). By linking processing\nstability to rheology–optics coupling, we show how careful matrix viscoelasticity design\nsimultaneously improves optical performance and scalability.\nFinally, regarding the reviewer’s suggestion on zeta potential, we note that zeta potential is not\ndirectly relevant in our system, as it is primarily a measure of electrostatic stabilization in low-\nviscosity aqueous colloids. In contrast, our inks are based on a high-viscosity, nonpolar PDMS\nmatrix, where particle stability is governed less by electrostatics and more by rheological\nimmobilization and shear-thinning–driven dispersion. As demonstrated in our revised manuscript,\nby tuning polymer viscoelasticity we achieve uniform, stable dispersions without sedimentation,\nenabling scalable coatings and DIW fabrication without the need for zeta-potential optimization.\nOverall, the revised scope emphasizes that our work provides both (i) a scalable pathway for planar\ncoatings and (ii) a platform for architected multifunctional systems through DIW, which are\nenabled by the scientific principle of rheology–optics coupling.\nRevisions:\n1) Added clarification in “Broadband Optical Response and Cooling Performance” section and\nConclusion that low-loading rheology-optimized PZO can promote planar scale-up using\nconventional coating methods, such as slot-die, blade, screen, and roll-to-roll coating.\n2) Added new demonstrations (LumiCool, façades, masks) in Figs. 3–4, as shown above in\nResponse to Comment 1, and expanded discussion in main text to highlight DIW’s role in\nenabling multifunctional cooling architectures.\n3) Added a new section “Rheology-Directed Control of Optical Scattering” with micro-CT and\nSEM data, showing immobilization of ZrO₂ particles during curing, confirming suppression of\nsedimentation (Fig. 1d–j and Fig. S3, as shown in Response/Revision to Comment 1).\n4) Specified in Methods that high-shear mixing and degassing were used to remove bubbles.\n3. Are all the structures in Fig.2 designed for cooling purposes? It is not clear what those 3D\nstructures are for.\nResponse: We appreciate the reviewer’s request for clarification. In the original submission, we\nshowed several DIW-fabricated structures, but did not explicitly describe their intended functions.\nIn the revised manuscript, we now make clear that each printed architecture was designed to\nhighlight a specific functional use case for radiative cooling:\nLumiCool module (Fig. 3): demonstrates dual cooling and daylighting, addressing the long-\nstanding trade-off between thermal regulation and illumination.\nSawtooth-patterned façades (Fig. 4a): illustrate the one-step fabrication of angularly selective\nemitters for improving cooling performance on vertical building walls, for which previous work\nrequired multi-step, complex clean-room fabrication.\nBreathable cooling masks (Fig. 4c): combine sub-ambient cooling with wearer comfort for\nwearable applications.\nHoneycomb/cylindrical shells (Fig. 4d): provide conformal passive cooling for electronic\ncomponents and irregular geometries.\nFluidic-integrated architectures (Fig. 4e): enable dew harvesting and cold-water storage by\ncombining radiative cooling with fluidic control.\nPatterned/aesthetic coatings (Fig. 4b): demonstrate architectural customization, expanding\nradiative cooling surfaces beyond utilitarian coatings.\nBy explicitly linking each structure to an application scenario, we emphasize that the DIW\ndemonstrations go beyond proof-of-concept printing and illustrate the unique system-level\nopportunities unlocked by our rheology-optimized PZO ink.\nRevisions:\n1) Revised the Introduction part to add the discussion about dual-function of lighting and cooling\nas an example with global energy significance, highlighting the benefit of architected structures\nmade by DIW.\n2) Added and expanded the DIW demonstrations in Figs. 3–4, as shown in Response/Revision to\nComment 1.\n3) Added explicit explanations in the main text for the functional purpose of each printed structure.\n4. Does the thickness of the material also play a role in cooling?\nResponse: We thank the reviewer for pointing this out. Indeed, thickness plays an important role\nin determining solar reflectance and thus cooling performance. The thickness-dependent optical\ndata is shown in Figs. S5 in the Supporting Information.\nFigure S5. (a) Solar reflectance spectra of PZO at varying thickness and (b) plot of average solar\nreflectance values as a function of thickness.\n5. Plots in Figure 5 need error bars.\nResponse: We appreciate this suggestion. Error bars are included in tests that are statistically\nmeaningful (e.g., pull-off tests), and for tests where they are not appropriate, we instead provide\nthe corresponding measured optical spectra in the SI to support the presented results.\n6. In Figure 5, the “uncoated” control group appears to be black, while the other two groups are\nwhite. Black color is known for its high absorption of thermal energy, which consumes more power\nto dissipate the heat. White reflects more light than black. The three sets of results may not provide\na parallel comparison.\nResponse: We thank the reviewer for raising this point. We need to clarify that including both\nuncoated and commercial white paint-coated controls is necessary. The uncoated control represents\nthe baseline condition that is commonly encountered in the market, such as unpainted sheds, bare\nroofing, or untreated industrial surfaces. Many real-world structures remain in this state, making\nit an important and practical reference. The commercial white paint serves as the direct benchmark,\nrepresenting the widely adopted standard coating used for thermal management. Together, these\ntwo controls provide a complete picture: the uncoated sample illustrates the absolute benefit of\napplying a cooling coating compared to the common untreated state, while the commercial paint\nhighlights the specific advantage of PZO over current market alternatives.\nOverall, the reviewer agrees that this work provides interesting engineering findings. But it lacks\nscientific discoveries and validations. The AM-DIW process does not bring benefits for cooling\nand thus lacks novelty. Scalability has to be addressed if the authors want to claim it as a strength\nof the work.\nResponse: We sincerely thank the reviewer for this thoughtful summary. In revising the\nmanuscript, we made major changes to ensure that the scientific discoveries, validations, and\nscalability pathways are clearly articulated:\n• Scientific discovery: We now introduce rheology–optics coupling as a new design principle\nthat directly links polymer viscoelasticity to particle dispersion and optical scattering. This\nmechanism allowed us to achieve record optical performance (~97.3% solar reflectance, ~96.9%\nmid-IR emissivity) at only ~4.5 vol.% filler loading, which represents a fundamental\nimprovement over prior nonporous coatings that required >25 vol.% fillers or metallic backings.\nBy connecting rheology to optics, our work provides a scientifically novel framework that advances\nradiative cooling beyond trial-and-error material mixing.\n• Validations: We strengthened the manuscript with additional data and analyses, including 1)\nMicro-CT and SEM evidence of particle dispersion and sedimentation suppression (Fig. 1d–j);\n2) expanded the comparison to state-of-the-arts in Table S1-S2 and Figure S4, as shown below,\nvalidating the record performance; 3) Outdoor field testing and pilot-scale shed demonstrations\nwith quantified energy savings (Fig. 2e–h).\n• Novelty of AM-DIW: We clarify that DIW is not intended to increase intrinsic cooling efficiency.\nIts novelty lies in enabling programmable architectures that deliver system-level benefits\nbeyond planar coatings. The newly added demonstration of LumiCool module (Fig. 3)\nillustrates this by combining sub-ambient cooling with controlled daylighting, solving a long-\nstanding trade-off between cooling and illumination. Additional demonstrations (façades,\nmasks, fluidic devices) shown in Fig. 4 highlight multifunctionality that cannot be realized\nwith conventional coatings or tape-casting.\n• Scalability: We revised the scope to explain that scalability is addressed in two complementary\nways. 1) Planar coatings: our rheology-optimized, low-loading ink is fully compatible with\nslot-die, blade, screen, and roll-to-roll coating, making it scalable for large-area deployment.\n2) DIW architectures: for multifunctional modules, DIW scalability can be enhanced using\nmulti-nozzle arrays and parallel printing, as already practiced in industrial additive\nmanufacturing.\nOverall, we believe these revisions establish that our work makes a fundamental scientific\ncontribution (rheology–optics coupling), provides robust validations, and clearly addresses\nscalability for both planar and architected systems.\nFigure S4. Comparison of solar reflectance and infrared emissivity between this work and recent literature\n(summarized in Table S1). In the plots, colorful solid symbols denote nonporous structures without precious\nmetal, black hollow symbols denote porous structures, and colored half-filled symbols denote nonporous\ndesigns with a silver reflective layer.\nTable S1. Comparison of fabrication requirements for recent daytime radiative cooling materials\nFabrication limitations\nR_\nRef solar/ Method\nε_\nIR High temp. Toxic Noble Fluorine High load.\nVacuum\n(>600℃) solvent metal content (>25 vol%)\nPorous designs\n0.96/ Phase\n9 × ×\n0.97 inversion:\n0.96/ Delignification\n10 ×\n0.90 & hot press\n0.96/ Coating\n12 × × ×\n0.95 & sintering\n0.92/ Twin-screw\n13 ×\n0.95 mixing\n0.99/ Mixing\n15 × ×\n0.90 & sintering\n0.96/ Electrostatic\n16 ×\n0.79 spinning\n0.92/ Solvent\n20 ×\n0.80 & exchange\n0.97/ Electrostatic\n21 × ×\n0.85 spinning\n0.98/ Mixing\n24 × ×\n0.95 & coating\n0.93/ Mixing\n37 × × ×\n0.97 & spray\n0.95/ Ultrasonication\n39 × ×\n0.96 & spraying\nNonporous designs\n0.97/ Multi-layer\n8 × ×\n0.65 deposition\n0.96/ Extrusion\n19 × ×\n0.93 & E-beam\n0.90/ Mixing&\n43 ×\n0.95 blade coating\n0.91/ Mixing&\n44 ×\n0.93 blade coating\n0.96/ E-beam & spin\n46 × ×\n0.93 coating\n0.92/\n0.95\n0.93/\n0.96\nBlade coating\nThis 0.97/\nor direct ink None None None None None None\nwork 0.97\nwriting\nTable S2. Comparison of durability for recent daytime radiative cooling materials\nAdhesion\nR_ Wear index Water Thermal UV\nComposition solar/ strength\nε_ (mg/103cycle)* proof stability resistance\nIR (MPa)\nPorous materials\nPorous PVDF-HFP 0.96/\ncoating9 0.97\nDelignified and 0.96/\npressed wood10 0.90\nPorous glass-Al O 0.96/\n2 3 ✓ ✓\ncoating12 0.95\nPorous PTFE/TiO - 0.92/\nPorous α-alumina 0.99/\n✓\nceramic15 0.90\nPorous PEO fiber 0.96/\nfilm16 0.79\n0.92/\nPorous PE aerogel20\n0.80\n0.97/\nPorous PMP fabrics21 ✓\n0.85\n0.98/\nPorous BaSO paint24 150\n4 0.95\n0.93/\nPorous TiO paint37 ✓\n2 0.97\n0.95/\nPVDF/BasSO /SiO 39 ✓\n4 2 0.96\nNonporous materials\n0.97/\nSiO /HfO /Ag8 ✓\n2 2 0.65\n0.96/\nSiO -TPX/Ag film19 ✓\n2 0.93\n0.90/\nZrO /PDMS43\n2 0.95\n0.91/\nZrO /PDMS44\n2 0.93\nSiO /PDMS/ 0.96/\n2 ✓\nCr/Ag/Cr46 0.93\nAl O /PDMS- 0.92/\n2 3\nTiO /PDMS47 0.95\n0.934/\nfluorescent ✓ ✓\n0.96\npigment/PSA49\n0.97/\nThis work 1.04 14 ✓ ✓ ✓\n0.97\nReviewer #2 (Remarks to the Author):\nIn this manuscript, the authors present a direct ink writing (DIW) approach to fabricate daytime\nradiative coolers. By incorporating ZrO2 particles into the polymer ink, they fabricated samples\nwith high solar reflectivity and infrared emissivity, enabling sub-ambient cooling of up to 7.4 °C\nin outdoor settings and 37% electricity saving in a month-long field test. The work is pretty well\ndone with comprehensive experiment demonstrations including optical, thermal, and mechanical\ncharacterizations, and the results are presented in a very clear and well-organized manner.\nThat said, there are several concerns regarding the novelty and scientific impact of this work.\nSpecifically, the necessity and advantages of using DIW for radiative cooling applications are not\nconvincingly established. Since only the surface layer contributes to solar reflectivity and infrared\nemission, it remains unclear why full 3D printing of the bulk object is needed, as opposed to\nsimpler surface deposition methods like ZrO₂-based spray coating or painting with less time and\nmaterial consumption. Furthermore, other 3D printing techniques have already been applied to\nfabricate radiative cooling structures with sub-ambient performance (e.g., S. Park, et al.,\nNanophotonics, 2024, pp. 611–620). To meet the publication criteria of Nature Communications,\nthe work would need to clearly demonstrate its high level of scientific novelty and significance.\nThe reviewer suggests the following points for further consideration:\n1. What specific use cases (e.g., wearables, curved surfaces, microdevices) require the\nprogrammable patternability of DIW? Some demonstrations would help clarify the necessity of\nstructured radiative coolers.\nResponse: We thank the reviewer for this helpful suggestion. In the revised manuscript, we\nsignificantly expanded the demonstrations of DIW-enabled architectures to highlight specific use\ncases where programmable patternability is essential:\nLumiCool module (Fig. 3): A DIW-printed structure that combines sub-ambient cooling with\ndaylighting, addressing the long-standing trade-off between thermal regulation and illumination.\nSawtooth-patterned façades (Fig. 4a): One-step fabrication of architected surfaces that provide\nangularly selective emission, improving cooling performance on vertical building walls, for which\nprevious work required multi-step, complex clean-room fabrication.\nBreathable cooling masks (Fig. 4c): Mesh structures with tunable filament spacing that combine\nwearer comfort (breathability) with passive cooling, highlighting the potential for wearables.\nHoneycomb and cylindrical shells (Fig. 4d): Conformal coatings for irregular electronic\ncomponents, drones, and devices.\nFluidic-integrated architectures (Fig. 4e): Grooved and channeled slabs that integrate radiative\ncooling with condensation and cold-water storage, advancing fluidic thermal management.\nThese demonstrations illustrate that DIW goes beyond serving as a fabrication method. It enables\nsystem-level multifunctionality and application-specific adaptability that can be challenged with\nconventional planar coatings, tape casting, or spray deposition.\nRevision:\n1) Revised the Introduction part to add the discussion about dual function of lighting and cooling\nas an example with global energy significance, highlighting the benefit of architected structures\nmade by DIW.\n2) Expanded DIW demonstrations with explicit functional purposes: LumiCool (cooling +\ndaylighting, as shown in Fig. 3 below), sawtooth façades, breathable masks, conformal shells\nfor electronics, and fluidic devices (as shown in Fig. 4 below).\n3) Added new sections “Architected Structures for Dual Cooling and Daylighting” and\n“Expanding Multifunctional Radiative Cooling Systems” with expanded text to better highlight\nthe relevance of these use cases for buildings, wearables, and electronics.\nFigure 3. Optical design and performance of LumiCool module. (a) Photographs of house models with\nthree roof configurations: transparent, fully covered, and LumiCool (left to right). Insets illustrate the\nresulting interior lighting conditions. (b) Schematic illustration of the optical design utilizing the strong\nand broadband solar reflectance of the PZO material. Direct sunlight enters the interior only when the\nsolar incident angle is smaller than the critical angles α₁ (morning) or α₂ (afternoon), enabling controlled\ndaylighting while minimizing heat gain. (c) Simulated daily direct sunlight duration on architectures\nconstructed with LumiCool structure. (d) Top-down and side-views of a LumiCool module comprising DIW-\nprinted PZO fin array structures on glass, showing tilted fins with representative inclination and periodic\nspacing. (e) Indoor temperature profiles of test house modules with different roof configurations under\ndirect sunlight. (f) Indoor illuminance levels measured under different roof configurations.\nFigure 4. DIW-fabricated radiative cooling structures with diverse geometries and applications. (a) DIW-\nprinted sawtooth-like grating structure with angular asymmetry. (b) Representative 2D patterns printed\nusing high-viscosity DIW-PZO ink, including an “ILLINOIS” logo, periodic triangular tiling, and a panda\nillustration. (c) Radiative cooling mask fabricated via DIW. Insets show the detailed filament structure,\nwearer demonstration, and overall geometry. (d) Complex 3D structures fabricated via DIW, including a\nhoneycomb panel and cylindrical tube. (e) DIW-fabricated structures designed for thermal–fluidic\napplications, including grooved arrays and channeled slabs.\n2. The manuscript should include a quantitative comparison of the scalability of the DIW method\n(e.g., deposition rate, printing area, and material usage) with existing scalable methods such as\nspray coating, painting, or roll-to-roll fabricated films.\nResponse: We thank the reviewer for this important point. In the revised manuscript, we refined\nour scope to clarify that two complementary scalability pathways are enabled by our ink design:\nPlanar large-area coatings: A central scientific advance of this work is the discovery of rheology–\noptics coupling, which allows record reflectance/emissivity at only ~4.5 vol.% filler loading. This\nlow-loading formulation avoids brittleness and processing challenges of prior high-loading\nsystems and is fully compatible with established large-area manufacturing methods including slot-\ndie coating, blade coating, screen printing, and roll-to-roll processing. Thus, even without DIW,\nour ink provides a scalable and manufacturable pathway for planar radiative cooling films.\nArchitected modules via DIW: We clarify that DIW is not proposed as the sole pathway for scaling\nlarge-area coatings. Rather, it is a complementary fabrication strategy for applications where\nprogrammable 3D architectures add system-level benefits (e.g., LumiCool modules for cooling +\ndaylighting, sawtooth façades, breathable masks, fluidic devices). For such cases, DIW scalability\ncan be improved by adopting multi-nozzle arrays, parallel gantry systems, or hybrid\nadditive/subtractive workflows, which are already widely applied in industrial additive\nmanufacturing.\nWe therefore distinguish between: (i) industrial-scale coating processes for large-area deployment,\nand (ii) DIW-enabled multifunctional modules for application-specific architectures. This dual\nstrategy directly addresses scalability concerns and highlights the versatility of our approach.\nRevision:\n1) Revised the main text in the section “Broadband Optical Response and Cooling Performance”\nto clarify dual scalability pathways (planar coating compatibility + DIW architectures).\n2) Added explicit discussion of conventional scalable methods (slot-die, blade, screen, roll-to-\nroll) in the “Broadband Optical Response and Cooling Performance” section and clarified that\nour ink formulation is directly compatible with them.\n3) Updated the “Architected Structures for Dual Cooling and Daylighting” and “Expanding\nMultifunctional Radiative Cooling Systems” sections to emphasize DIW as a complementary\nplatform for multifunctional modules, not the sole scale-up route.\n4) Reflecting this broader scope and the clarified scientific novelty, we have updated the\nmanuscript title to explicitly capture its implications for scalable coatings and architected\nmultifunctional systems.\n3. The combination of ZrO2 particles and PDMS matrix have been proposed for achieving\nradiative coolers with sub-ambient cooling performance (e.g., Y. Zhang, et al., Solar Energy\nMaterials and Solar Cells, 2021, 111129; X. Yang, et al., Inorganic Chemistry Communications,\n2023, 110586). These relervant papers should be properly cited and discussed in the manuscript.\nAs the DIW technique cannot enhance the cooling performance, what performance metrics justify\nthe use of DIW over less complex techniques?\nResponse: We thank the reviewer for highlighting these important prior works. In the revised\nmanuscript, we now cite and discuss these studies explicitly to properly position our contribution\nin the context of PDMS–ZrO₂ composites. While previous reports demonstrated that ZrO₂ can\nserve as an effective scatterer in PDMS, our work advances the field in three key ways:\n• Scientific novelty on rheology–optics coupling: Prior studies largely treated composites as\nsimple particle–polymer mixtures. Here, we uncover and validate a new design principle,\nshowing how polymer viscoelasticity governs particle breakup, dispersion, and sedimentation,\nwhich in turn controls broadband scattering. This coupling enabled record optical performance\n(~97.3% reflectance, ~96.9% emissivity) at only ~4.5 vol.% loading, significantly\noutperforming the previous PDMS-ZrO works and overcoming the brittleness and processing\n2\nchallenges of high-loading systems (>25 vol.%).\n• Scalability and compatibility with industrial coatings: Unlike prior PDMS–ZrO₂ films,\nour low-loading formulation is directly compatible with scalable coating methods (slot-die, blade,\nscreen, roll-to-roll) without brittleness, cracking and poor adhesion, bridging the gap between\nlab demonstrations and manufacturability.\n• DIW-enabled multifunctionality: While DIW does not increase intrinsic cooling efficiency,\nit enables programmable architectures that provide system-level advantages beyond planar films.\nWe demonstrate this with LumiCool (cooling + daylighting), sawtooth façades, breathable\nmasks, conformal shells, and fluidic-integrated devices. These multifunctional, application-\nspecific structures cannot be achieved with spray coatings or tape casting. Thus, the\nperformance metric that justifies DIW is the creation of multifunctional, architected systems\nthat address real-world trade-offs (e.g., cooling vs. lighting) and expand radiative cooling into\nnew application spaces.\nRevision:\n1) Added a new section “Rheology-Directed Control of Optical Scattering” to highlight how our\napproach differs fundamentally by introducing rheology–optics coupling. Added additional\nMicro-CT and microstructure analysis in Fig. 1e-j, Fig. 2b, and Fig. S3, as shown below.\nFigure 1. (e–g) X-ray micro-computed tomography (Micro-CT) images of cured PDMS–ZrO₂ films\nprepared with different PDMS matrices: (e) L-PDMS, (f) M-PDMS, and (g) H-PDMS. The scanned\nvolume was 785 μm × 994.5 μm × 500 μm (X × Y × Z). (h–j) Particle size distributions of ZrO₂ fillers\nin the cured PDMS films, derived from Micro-CT analysis: (h) L-PDMS, (i) M-PDMS, and (j) H-PDMS.\nFigure 2b. UV–Vis–NIR reflectance spectra of PDMS- ZrO₂ composite films prepared using H-\nPDMS, M-PDMS, and L-PDMS. All samples contain 4.5 vol% ZrO₂.\nFigure S3. SEM image showing the cross-section view of the matrix with different viscosity.\n2) Clarified in “Broadband Optical Response and Cooling Performance” section that our ink\nformulation is directly compatible with industrial-scale coating methods.\n3) Added new sections “Architected Structures for Dual Cooling and Daylighting” and\n“Expanding Multifunctional Radiative Cooling Systems” with expanded discussions to show\nthat DIW justifies the novelty through multifunctionality and programmability.\n4) Added citations and discussion of prior PDMS–ZrO₂ studies, and expanded the state-of-the-art\ncomparison in Table S1-S2 and Fig S4, as shown below.\nFigure S4. Comparison of solar reflectance and infrared emissivity between this work and recent literature\n(summarized in Table S1). In the plots, colorful solid symbols denote nonporous structures without precious\nmetal, black hollow symbols denote porous structures, and colored half-filled symbols denote nonporous\ndesigns with a silver reflective layer.\nTable S1. Comparison of fabrication requirements for recent daytime radiative cooling materials\nFabrication limitations\nR_\nRef solar/ Method\nε_\nIR High temp. Toxic Noble Fluorine High load.\nVacuum\n(>600℃) solvent metal content (>25 vol%)\nPorous designs\n0.96/ Phase\n9 × ×\n0.97 inversion:\n0.96/ Delignification\n10 ×\n0.90 & hot press\n0.96/ Coating\n12 × × ×\n0.95 & sintering\n0.92/ Twin-screw\n13 ×\n0.95 mixing\n0.99/ Mixing\n15 × ×\n0.90 & sintering\n0.96/ Electrostatic\n16 ×\n0.79 spinning\n0.92/ Solvent\n20 ×\n0.80 & exchange\n0.97/ Electrostatic\n21 × ×\n0.85 spinning\n0.98/ Mixing\n24 × ×\n0.95 & coating\n0.93/ Mixing\n37 × × ×\n0.97 & spray\n0.95/ Ultrasonication\n39 × ×\n0.96 & spraying\nNonporous designs\n0.97/ Multi-layer\n8 × ×\n0.65 deposition\n0.96/ Extrusion\n19 × ×\n0.93 & E-beam\n0.90/ Mixing&\n43 ×\n0.95 blade coating\n0.91/ Mixing&\n44 ×\n0.93 blade coating\n0.96/ E-beam & spin\n46 × ×\n0.93 coating\n0.92/\n0.95\n0.93/\n0.96\nBlade coating\nThis 0.97/\nor direct ink None None None None None None\nwork 0.97\nwriting\nTable S2. Comparison of durability for recent daytime radiative cooling materials\nAdhesion\nR_ Wear index Water Thermal UV\nComposition solar/ strength\nε_ (mg/103cycle)* proof stability resistance\nIR (MPa)\nPorous materials\nPorous PVDF-HFP 0.96/\ncoating9 0.97\nDelignified and 0.96/\npressed wood10 0.90\nPorous glass-Al O 0.96/\n2 3 ✓ ✓\ncoating12 0.95\nPorous PTFE/TiO - 0.92/\nPorous α-alumina 0.99/\n✓\nceramic15 0.90\nPorous PEO fiber 0.96/\nfilm16 0.79\n0.92/\nPorous PE aerogel20\n0.80\n0.97/\nPorous PMP fabrics21 ✓\n0.85\n0.98/\nPorous BaSO paint24 150\n4 0.95\n0.93/\nPorous TiO paint37 ✓\n2 0.97\n0.95/\nPVDF/BasSO /SiO 39 ✓\n4 2 0.96\nNonporous materials\n0.97/\nSiO /HfO /Ag8 ✓\n2 2 0.65\n0.96/\nSiO -TPX/Ag film19 ✓\n2 0.93\n0.90/\nZrO /PDMS43\n2 0.95\n0.91/\nZrO /PDMS44\n2 0.93\nSiO /PDMS/ 0.96/\n2 ✓\nCr/Ag/Cr46 0.93\nAl O /PDMS- 0.92/\n2 3\nTiO /PDMS47 0.95\n0.934/\nfluorescent ✓ ✓\n0.96\npigment/PSA49\n0.97/\nThis work 1.04 14 ✓ ✓ ✓\n0.97\n4. What are the specific advantages of DIW over other 3D printing strategies previously used for\nradiative cooling materials? A direct comparison would clarify the unique value of the proposed\nmethod.\nResponse: We thank the reviewer for this important question. In the revised manuscript, we\nemphasize that the unique advantage of DIW is its ability to process functional particle–polymer\ncomposites at room temperature. Unlike stereolithography (which requires UV-curable resins) or\nthermoplastic extrusion (which requires high-temperature melts), DIW accommodates viscous,\nshear-thinning inks such as PDMS–ZrO₂, which cannot be processed by these other approaches.\nThis room-temperature capability enables us to retain the high optical performance of ZrO₂, avoid\nthermal degradation of the polymer matrix, and fabricate multifunctional architectures directly\nwithout additional curing or post-processing steps. Thus, while other 3D printing strategies have\nbeen applied to radiative cooling, DIW uniquely combines material compatibility and ambient\nprocessing, making it a promising route for developing architected radiative cooling systems.\n5. The manuscript emphasizes mechanical durability, but how much of this is attributable to the\nDIW geometry vs. the inherent robustness of the PDMS-ZrO₂ composite? Can similar durability\nbe achieved with sprayed or cast coatings using the same material?\nResponse: We appreciate this insightful question. In the revised manuscript, we clarify that the\nmechanical durability arises primarily from the intrinsic properties of the PDMS–ZrO₂ composite,\nrather than the DIW geometry. Specifically, the nonporous PDMS matrix provides high\nstretchability (up to 500% strain) and resilience to abrasion and thermal cycling. The low filler\nloading (~4.5 vol.%) enabled by rheology–optics coupling avoids brittleness and poor adhesion,\nwhich are common in high-loading composites. These attributes ensure that similar durability can\nindeed be achieved with sprayed or cast coatings using the same ink formulation. DIW’s unique\ncontribution is not to enhance durability, but rather to enable programmable architectures for\nmultifunctional systems (e.g., LumiCool modules, façades, masks, fluidic devices). In summary,\ndurability is an inherent strength of the material design and the resulting microstructure, while\narchitectural versatility is the strength of DIW.\nBelow are some minor comments:\n1. The acronym “PZO” appears throughout the manuscript without providing its definition.\nResponse & Revision: In the revised manuscript, we have specified it in its first appearance.\n2. The spectral ranges used to define solar reflectivity and mid-infrared emissivity should be\nclearly specified.\nResponse & Revision: We now specified the spectral ranges as follows “~97.3% reflectance\nacross 0.3 – 2.0 μm and ~96.9% mid-infrared emissivity over 8 – 13 µm”.\n3. What is the minimum feature size (line width and thickness) that can be reliably achieved with\nthis DIW technique?\nResponse: We thank the reviewer for this question. In our DIW system, the minimum reliable\nfeature size is determined by the nozzle diameter and the rheology of the ink. Using nozzles down\nto ~260 μm inner diameter, we achieved line widths of ~300 μm with good fidelity and continuity.\nFor film thickness, single-pass layers as thin as ~100 μm can be deposited uniformly.\n4. In Fig. 5c, why does the PZO film exhibit smaller mass loss than commercial paint? Is the PZO\nfilm 3D-printed or painted on a film substrate?\nResponse: We thank the reviewer for pointing this out. The smaller mass loss of PZO compared\nto commercial paint in the abrasion test arises from two factors: 1) Nonporous PDMS matrix.\nUnlike brittle, pigment-rich paints, the PDMS–ZrO₂ composite has a flexible elastomeric matrix\nthat resists crack initiation and particle detachment under abrasion. 2) Low filler loading (~4.5\nvol.%). Enabled by rheology–optics coupling, this avoids the brittleness common in high-loading\ncoatings, improving wear resistance. Regarding the specimen preparation, the PZO samples tested\nin Fig. 5c were planar coatings deposited on rigid substrates (cast films), not 3D-printed\nfreestanding structures. We have clarified this in the revised manuscript.\nReviewer #3 (Remarks to the Author):\nThe manuscript by Zhou et al. presents a 3D-printable polymer-nanoparticle ink designed for direct\nink writing of non-porous radiative coolers, effectively addressing the scalability and durability\nchallenges associated with traditional porous designs. The 3D-printed structures demonstrate high\nsolar reflectivity and infrared emissivity, resulting in a cooling effect of 7.4 °C for energy-saving\napplications. This study has the potential to connect high-performance radiative cooling with\nscalable 3D printing for practical use, making it a strong candidate for further consideration for\npublication. However, several notable issues have been identified, as outlined below:\n1. The inks depicted in Figure 1c exhibit a morphology resembling high-viscosity fluids that appear\nto flow readily. This behavior deviates from the expected behavior of high-quality 3D printing inks.\nThe authors should clarify this observation.\nResponse & Revision: We thank the reviewer for this pointing this out. In the revised manuscript,\nwe replaced the original image with photographs of viscous formulations in Fig. 2a that are suitable\nfor DIW printing. These images more clearly reflect the high-viscosity, shear-thinning behavior of\nthe inks that enable smooth extrusion while maintaining shape fidelity after deposition. This\nchange eliminates the ambiguity caused by the earlier photo and better represents the actual\nprintability of the developed inks.\nFigure 2a. Visual comparison of pure H-PDMS (without filler) and H-PDMS-ZrO₂ ink.\n2. To fully assess the ink’s printability, additional rheological characterizations beyond dynamic\nviscosity are necessary. Specifically, dynamic oscillatory shear measurements (e.g., storage/loss\nmoduli) or step-strain tests would help determine whether the ink exhibits the necessary\nviscoelasticity for smooth extrusion while retaining structural fidelity post-deposition. Such\nanalyses would clarify how the ink balances extrudability with shape retention to prevent\nuncontrolled spreading.\nResponse & Revision: We thank the reviewer for this valuable suggestion. In the revised\nmanuscript, we emphasize that the suitability of the ink for DIW was validated through a\ncombination of rheological measurements and structural demonstrations. Specifically, viscosity\nmeasurements (Fig. 1d–g, see Revision in Comment 4) confirm pronounced shear-thinning\nbehavior, which ensures smooth extrusion under pressure. Micro-CT and microscopy (Fig. 1h–j\nand Fig. S3, see Revision in Comment 4) show uniform particle dispersion without spreading or\nsedimentation, indicating sufficient viscoelasticity for post-deposition shape retention. The\nsuccessful printing of high-fidelity structures, including fin arrays, façades, breathable masks, and\nfluidic devices (Figs. 3–4), demonstrates the ink’s ability to maintain geometric integrity across\ndiverse architectures. Together, these results show that the inks strike the necessary balance\nbetween extrudability and structural fidelity.\n3. The incorporation of fine nanoparticles (e.g., ZrO₂) often significantly modifies the rheological\nproperties of polymeric inks, typically enhancing shear-thinning behavior or introducing yield\nstress via filler-network formation. However, the manuscript does not show a pronounced\nrheological effect from ZrO₂ addition. The authors should discuss possible reasons for this\nobservation. A mechanistic explanation would strengthen the ink design rationale.\nResponse: We appreciate this insightful observation. In the revised manuscript, we now discuss\nwhy ZrO₂ addition did not strongly alter the rheological curves. This outcome is primarily a\nconsequence of our low filler loading strategy, enabled by rheology–optics coupling. Record\noptical performance was achieved at only ~4.5 vol.% ZrO₂, far below the >25 vol.% typically\nrequired in other composites. At such low concentrations, particle–particle interactions are\nminimal, and a continuous filler network does not form, which explains the absence of pronounced\nyield-stress effects. Furthermore, the viscoelastic response is dominated by the high-viscosity\nPDMS matrix, whose rheological properties overshadow the more subtle contributions of sparsely\ndispersed nanoparticles. Finally, optimized mixing and dispersion protocols suppress particle\naggregation, preventing the formation of secondary particle networks that would otherwise\nenhance shear-thinning behavior. Collectively, these factors explain why ZrO₂ incorporation had a\nlimited effect on the rheological curves in our system.\nRevisions:\n1) Added mechanistic discussion in the new section “Rheology-Directed Control of Optical\nScattering” to explain why ZrO₂ had limited impact on rheology.\n2) Linked this explanation to the scientific advance of rheology–optics coupling, showing how\nviscoelastic control reduces the need for high filler content and associated rheological\ncomplications.\n4. Achieving and maintaining uniform ZrO₂ dispersion in PDMS from ink preparation through\ncuring is critical for consistent material performance. The authors should elaborate on the strategies\nand mechanisms to prevent particle settling or aggregation. Currently, only the final cured structure\nis characterized, leaving the dispersion dynamics during printing and curing\nunexplored/unexplained.\nResponse: We thank the reviewer for highlighting this important point. In the revised manuscript,\nwe elaborate on the strategies and mechanisms that prevent particle settling or aggregation during\nprocessing. The SE1700-based PDMS formulation exhibits high zero-shear viscosity, which\neffectively immobilizes particles during curing and suppresses sedimentation. At the same time,\nthe ink’s shear-thinning behavior allows efficient particle dispersion during mixing and extrusion,\nwith viscosity rapidly recovering after deposition to stabilize the distribution. To further ensure\nuniformity, we employed an optimized blending protocol involving high-shear mixing followed\nby degassing to eliminate bubbles and promote consistent dispersions. Finally, micro-CT and SEM\nanalysis (Fig. 1h–j and Fig. S3) directly confirm that the cured structures exhibit uniform particle\ndistributions with no evidence of settling or aggregation. Together, these results demonstrate that\nrheology–optics coupling not only enhances optical performance but also stabilizes dispersions\nthroughout processing and curing, ensuring reproducibility and scalability.\nRevisions:\n1) Added a new section “Rheology-Directed Control of Optical Scattering” with expanded\ndiscussion to describe mechanisms of sedimentation suppression.\n2) Added micro-CT and SEM data (Fig. 1e–j and Fig. S3) to directly show uniform particle\ndistribution in the cured composites.\n3) Clarified in Methods the mixing and degassing protocol used to ensure stable dispersions prior\nto printing.\nFigure 1. (e–g) X-ray micro-computed tomography (Micro-CT) images of cured PDMS–ZrO₂ films\nprepared with different PDMS matrices: (e) L-PDMS, (f) M-PDMS, and (g) H-PDMS. The scanned\nvolume was 785 μm × 994.5 μm × 500 μm (X × Y × Z). (h–j) Particle size distributions of ZrO₂ fillers\nin the cured PDMS films, derived from Micro-CT analysis: (h) L-PDMS, (i) M-PDMS, and (j) H-PDMS.\nFigure S3. SEM image showing the cross-section view of the matrix with different viscosity.\n5. The PZO fiber displays impressive tensile strain tolerance of approximately 500% before\nfracture. It is important to evaluate the properties of these fibers—such as solar reflectance and\ninfrared emissivity—under mechanically stretched conditions.Otherwise, the mechanical\ndeformability may not be relevant to radiative cooler applications.\nResponse: We thank the reviewer for this thoughtful suggestion. The purpose of presenting the\n~500% strain tolerance is not to suggest that radiative coolers will operate under such extreme\ndeformation, but rather to demonstrate the robustness and toughness of the ink formulation. In\nmost practical radiative cooling applications, such as building coatings, façades, sheds, or\nelectronic enclosures, the coatings are not expected to undergo large uniaxial stretching. Instead,\nthey may experience bending, thermal expansion, abrasion, or handling stresses, where elasticity\nis beneficial to prevent cracking and delamination. We therefore highlight the high stretchability\nas an indicator of material resilience, which supports durability across diverse real-world\nenvironments, even though the highly stretched state itself is not directly relevant to common\ncooling applications.\n6. The results in Figures 4 and 5 suggest that conventional coating or painting methods could\nproduce similar radiative cooling samples. Since the manuscript emphasizes DIW in its title, the\nauthors should clearly highlight the unique advantages that 3D printing offers, potentially through\na proof-of-concept demonstration in this context.\nResponse: We thank the reviewer for this important comment. In the revised manuscript, we\nclarify that our work provides two complementary pathways:\nPlanar coatings via conventional scalable methods: The rheology–optics coupling principle allows\nus to achieve record optical performance (~97.3% solar reflectance, ~96.9% emissivity) at only\n~4.5 vol.% filler loading. This formulation is fully compatible with slot-die coating, blade coating,\nscreen printing, and roll-to-roll processes, ensuring large-area scalability.\nArchitected modules via DIW: While conventional coatings can reproduce planar films, they\ncannot realize programmable 3D geometries with multifunctional system-level advantages. DIW\nuniquely enables facile fabrication of 1) LumiCool modules (Fig. 3) that combine radiative cooling\nwith daylight transmission. 2) Sawtooth-patterned façades (Fig. 4a) that enhance angularly\nselective emission for vertical walls. 3) Breathable masks (Fig. 4c) that merge breathability with\ncooling. 4) Fluidic-integrated devices (Fig. 4e) for dew harvesting and cold-water storage.\nThese examples serve as proof-of-concept demonstrations of functionalities that cannot be easily\nachieved with conventional painting or coating. Thus, the novelty of DIW mainly lies in expanding\nradiative cooling into multifunctional, architected systems.\nRevisions:\n1) Revised the Introduction part to better highlight the novelty and scope of the work.\n2) Expanded the section “Broadband Optical Response and Cooling Performance” to highlight\nplanar coating compatibility with industrial methods.\n3) Added and expanded DIW demonstrations with explicit functional purposes: LumiCool\n(cooling + daylighting, Fig. 3), sawtooth façades, breathable masks, conformal shells, and\nfluidic devices (Fig. 4).\n4) Added detailed explanations in “Architected Structures for Dual Cooling and Daylighting”\nand “Expanding Multifunctional Radiative Cooling Systems” sections of how DIW enables\nunique multifunctional applications.\n5) In line with this broader emphasis, we have revised the manuscript title so that the scope and\nnovelty are clear from the outset, highlighting its broad practical implications for both scalable\ncoating and architected fabrication.\nFigure 3. Optical design and performance of LumiCool module. (a) Photographs of house models with\nthree roof configurations: transparent, fully covered, and LumiCool (left to right). Insets illustrate the\nresulting interior lighting conditions. (b) Schematic illustration of the optical design utilizing the strong\nand broadband solar reflectance of the PZO material. Direct sunlight enters the interior only when the\nsolar incident angle is smaller than the critical angles α₁ (morning) or α₂ (afternoon), enabling controlled\ndaylighting while minimizing heat gain. (c) Simulated daily direct sunlight duration on architectures\nconstructed with LumiCool structure. (d) Top-down and side-views of a LumiCool module comprising DIW-\nprinted PZO fin array structures on glass, showing tilted fins with representative inclination and periodic\nspacing. (e) Indoor temperature profiles of test house modules with different roof configurations under\ndirect sunlight. (f) Indoor illuminance levels measured under different roof configurations.\nFigure 4. DIW-fabricated radiative cooling structures with diverse geometries and applications. (a) DIW-\nprinted sawtooth-like grating structure with angular asymmetry. (b) Representative 2D patterns printed\nusing high-viscosity DIW-PZO ink, including an “ILLINOIS” logo, periodic triangular tiling, and a panda\nillustration. (c) Radiative cooling mask fabricated via DIW. Insets show the detailed filament structure,\nwearer demonstration, and overall geometry. (d) Complex 3D structures fabricated via DIW, including a\nhoneycomb panel and cylindrical tube. (e) DIW-fabricated structures designed for thermal–fluidic\napplications, including grooved arrays and channeled slabs."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have addressed my concerns brought up in the first-round of review. However, the revised manuscript still needs more work. Authors should check all the plots and add error bars where necessary. Error bars are used to access the repeatability of all the results. All the photos should have scale bars. \"rheology–optics coupling\" is a new concept introduced in the revised manuscript. Authors need to add experimental results to quantitatively justify this new concept.", "reviewer_2": "The authors have substantially revised the manuscript, introducing a new conceptual framework of rheology–optics coupling that effectively addresses my previous concern about the lack of scientific novelty. With the new focus, the manuscript now presents deeper scientific insight and broader applicability. The revised data and new figures convincingly demonstrate its uniqueness and significance compared to state-of-the-art research. Overall, the responses are quite satisfactory, and the revision has greatly strengthened the manuscript. I can now recommend its publication in Nature Communications after the authors appropriately address the following comments.\n1. Will the fabrication method (e.g., printing, blade-casting) influence the optical performance? As the authors claim this ink is compatible with many different processing methods, it is important to demonstrate that high optical performance can be achieved using other methods.\n2. The authors should be careful claiming “record-high” performance. Figure S4 has shown some state-of-the-art works with even higher solar reflectance or mid-IR emissivity than values presented in this manuscript. Also, please clearly state in the main text that the solar reflectance value of 97.3% is a weighted average.\n3. For the sawtooth-like structure, is the oblique surface coated with an IR-reflecting layer? Without this coating, I assume it is still a wide-angle emitter. Can you provide angular thermal emission imaging using IR camera to demonstrate it is indeed an angular emitter?\n4. Fabricating radiative cooling wearable devices is an interesting application. I am just curious if this mask is safe enough. Are the ZrO2 nanoparticles stable within the matrix? Even though ZrO2 is biosafe itself, there might still be unexpected effects as nanoparticles can enter human cells if being inhaled. Besides, is the pore size of this mask comparable to that of commercial ones?", "reviewer_3": "The authors have made efforts to revise the manuscript, effectively addressing the previously raised concerns. The ink flow characteristics and filler dispersion stability have been better clarified. Some concrete examples of 3D-printed radiative cooling devices have enhanced the practical relevance of the work. This manuscript is now likely suitable for publication consideration.\nI have one minor suggestion: the strategy for creating a flexible, breathable textile-like material for radiative cooling masks (Fig. 4c) is particularly intriguing. To provide conclusive evidence of its permeable microstructure, which is not clearly visible in the current optical image, including a micrograph (either from optical microscopy or SEM) would be highly valuable."}, "author_response": "Response to Reviewers’ Comments (NCOMMS-25-16828A-Z)\nReviewer #1 (Remarks to the Author):\nAuthors have addressed my concerns brought up in the first-round of review. However, the revised\nmanuscript still needs more work. Authors should check all the plots and add error bars where\nnecessary. Error bars are used to access the repeatability of all the results. All the photos should\nhave scale bars. \"rheology–optics coupling\" is a new concept introduced in the revised manuscript.\nAuthors need to add experimental results to quantitatively justify this new concept.\nResponse: We thank the reviewer for the thoughtful suggestions. We have addressed the comments\nas follows:\n1) Error bars and measurement repeatability: In the revised manuscript, we have added error bars\nwhere applicable, such as in Figure 5(e). For data where error bars are not appropriate, we have\nprovided additional repeated measurement results to demonstrate repeatability. Specifically, for\nFigure 5(b), the repeated tensile test data are now included in Supplementary Information as Figure\nS12. For Figure 5(c), the abrasion tests were conducted by a certified industrial laboratory\n(Metallurgical Engineering Services, Inc) following standard testing protocols (ASTM D4060)\nand were performed across multiple loading cycles (500–2000 cycles), which supports the\ncredibility and robustness of the results. We have also added Table S5 in the Supplementary\nInformation to present the mass loss data obtained from the abrasion tests at different cycles.\n2) Scale bars: We have added scale bars to Figures 2(a), 2(g), 3(a), and 3(d) as requested.\n3) “Rheology-optics couple” concept: Regarding the concept \"rheology–optics coupling\", the\nexperimental results have been broadly discussed in the main text with title ‘Rheology-Directed\nControl of Optical Scattering’. We showed three formulations of PDMS with distinct rheological\nproperties (Figure 1d), i.e., L-PDMS (low viscosity, hexane-diluted Sylgard 184), M-PDMS\n(medium viscosity, Sylgard 184), and H-PDMS (high viscosity, SE1700), and observed a direct\ncoupling between rheology and microstructure (Figs. 1e-j and S3). We highlighted that this\nrheology-mediated stabilization strategy provides a fundamental design principle for tuning optical\nscattering in nonporous composites, enabling broadband solar reflectance and scalable\nmanufacturing within a single material platform. L-PDMS, representative of the low-viscosity\nsystems, achieved only 91.5% solar reflectance due to narrow size distribution and severe\nsedimentation. M-PDMS improved modestly, but still suffered from settling of larger aggregates.\nIn contrast, H-PDMS immobilized particles during curing, producing spatially uniform, stable\ndispersions with broad size distributions. This microstructure enabled superior optical properties,\nreaching ~97.3% reflectance across 0.3 – 2.0 μm and ~96.9% mid-infrared emissivity over 8 – 13\nµm (Figs. 2b and 2c). We believe these experimental results explicitly validate the concept\n\"rheology–optics coupling\".\nRevision: The following figures and table have been either modified or added in the main text and\nsupplementary information.\nFigure 5(e). Comparison of solar reflectance and infrared emissivity for as-fabricated PZO, PZO\nafter 30 days of thermal cycling (15 cycles of alternating daily heating at 50 oC and cooling at –15\noC), and PZO after a 24-hour extreme cold treatment at –196 oC (soaking in liquid nitrogen).\nFigure S12. Additional tensile test data of the PZO fiber with 0 vol.% and 4.5 vol.% of ZrO\n2\n.\nTable S5. Mass loss data of PZO film and commercial white paint during abrasion tests (ASTM\nD4060).\nSamples Weight (g) Weight Conditioned Wear Avg. wear\nWear Before After loss Temp RH index (mg index (mg\ncycles (mg) (°C) (%) loss/1000 loss/1000\ncycles) cycles)\nPZO film 500 24.45283 24.44593 6.90 13.8 14.8\n1000 24.44593 24.44113 4.80 9.6\n1500 24.44113 24.43260 8.53 17.1\n2000 24.43260 24.42331 9.29 18.6\n23 50\nCommercial 500 25.19240 25.18260 9.80 19.6 30.1\nwhite paint 1000 25.18260 25.16730 15.30 30.6\n1500 25.16730 25.15136 15.94 31.9\n2000 25.15136 25.13227 19.09 38.2\nReviewer #2 (Remarks to the Author):\nThe authors have substantially revised the manuscript, introducing a new conceptual framework\nof rheology–optics coupling that effectively addresses my previous concern about the lack of\nscientific novelty. With the new focus, the manuscript now presents deeper scientific insight and\nbroader applicability. The revised data and new figures convincingly demonstrate its uniqueness\nand significance compared to state-of-the-art research. Overall, the responses are quite satisfactory,\nand the revision has greatly strengthened the manuscript. I can now recommend its publication in\nNature Communications after the authors appropriately address the following comments.\nResponse: We sincerely thank the reviewer for the positive assessment of our revisions and for\nrecognizing the significance and broader impact of our work. We greatly appreciate the\nencouraging feedback.\n1. Will the fabrication method (e.g., printing, blade-casting) influence the optical performance? As\nthe authors claim this ink is compatible with many different processing methods, it is important to\ndemonstrate that high optical performance can be achieved using other methods.\nResponse: We thank the reviewer for suggesting clarification regarding the versatility in\nfabrication methods. To address this, we measured the solar reflectance and infrared emissivity of\nPZO films prepared using different techniques, including drop casting, blade-coating, and screen\nprinting. The resulting optical properties confirm that high performance is retained across these\ndifferent fabrication methods. The weighted averages of the solar reflectance and infrared\nemissivity for PZO films prepared with each method are reported in Table S3, validating the\ncompatibility of our ink with diverse processing techniques.\nRevision: The following table has been added in the supplementary information.\nTable S3. Weighted averages of the solar reflectance and infrared emissivity for PZO films\nfabricated by different techniques.\nMethod Average solar reflectance, % Average infrared emissivity, %\nDrop casting 97.2 96.9\nBlade-coating 96.9 97.1\nScreen printing 97.3 96.6\nDIW 97.3 96.9\n2. The authors should be careful claiming “record-high” performance. Figure S4 has shown some\nstate-of-the-art works with even higher solar reflectance or mid-IR emissivity than values\npresented in this manuscript. Also, please clearly state in the main text that the solar reflectance\nvalue of 97.3% is a weighted average.\nResponse: We thank the reviewer for pointing out this. We have changed the ‘record-high’ term\nto ‘comparable to state-of-the-art’ and specified the solar reflectance and mid-infrared emissivity\nvalues as ‘weighted average’ in the main text.\nRevision: The following sentences have been modified in the main text.\nIn abstract:\nGuided by this principle, we develop printable polydimethylsiloxane-zirconium oxide (PZO)\ncomposites that achieve solar reflectance (~97.3%) and mid-infrared emissivity (~96.9%)\ncomparable to the best reported values, despite at a low filler loading of only ~4.5 vol.%.\nIn results and discussion:\nThis coupling allows high reflectance at only ~4.5 vol.% filler, overcoming the long-standing need\nfor either metallic backings or >25 vol.% particle loadings in radiative cooling coatings.\nThis microstructure enabled superior optical properties, reaching a weighted average solar\nreflectance of ~97.3% over 0.3 – 2.0 μm and a weighted average mid-infrared emissivity of ~96.9%\nover 8 – 13 µm (Figs. 2b and 2c), comparable to state-of-the-art daytime radiative coolers (Fig.\nS4, Table S1 and Table S2).\nThus, rheology-optics coupling provides a unifying design principle that not only achieves state-\nof-the-art performance, but also supports both industrial-scale planar coatings and DIW\nfabrications of multifunctional architectures.\nIn conclusion:\nThis fundamental link between polymer viscoelasticity and optical scattering in particle\ncomposites enabled a printable PDMS-ZrO (PZO) ink with state-of-the-art solar reflectance\n2\n(97.3%) and mid-infrared emissivity (96.9%) at only ~4.5 vol.% filler loading.\n3. For the sawtooth-like structure, is the oblique surface coated with an IR-reflecting layer?\nWithout this coating, I assume it is still a wide-angle emitter. Can you provide angular thermal\nemission imaging using IR camera to demonstrate it is indeed an angular emitter?\nResponse: We thank the reviewer for the insightful comment regarding the sawtooth-like\nstructure. We should note that, in our demonstration, the surface was not coated with an IR-\nreflective layer. Our primary objective in this section was to illustrate that the DIW approach can\nsimplify and streamline the fabrication of such complex geometric features, reducing the\npreviously required multi-step lithographic or molding processes to a single-step, on-demand\nprinting process. This printed geometry provides the necessary structural foundation for\nsubsequent integration of directional IR-reflective coatings, which would enable asymmetric\nthermal emission as demonstrated in Refs. 63 and 64. Since our focus here is on process simplicity\nand structural manufacturability, we have clarified this point in the revised manuscript and noted\nthat adding a directional reflective coating is a straightforward subsequent step for achieving\nangular thermal emission.\nRevision: We have made revision to the relevant part in the main text for clarification.\n“The versatility of the PZO ink extends radiative cooling into a wide range of multifunctional and\ngeometrically complex applications. For building integration, DIW enables simplified fabrication\nof structured wall coatings with intricate surface profiles that can be used to enhance cooling\nperformance on vertical facades. As a demonstration, we printed sawtooth-like gratings with ~1\nmm pitch and ~33° tilt in a single step (Fig. 4a). These geometries replicate the core structural\nmotifs used in previously reported angularly asymmetric thermal emitters, which traditionally\nrequire multi-step lithographic or molding processes (63, 64). Note that our demonstration focuses\non the geometric fabrication stage, without the final directional IR-reflective coating employed in\nthose systems. By enabling these complex geometries through a single-step printing process, our\napproach substantially simplifies the manufacturing workflow and provides a practical foundation\nfor incorporating directional IR-reflective coatings in future implementations.”\n4. Fabricating radiative cooling wearable devices is an interesting application. I am just curious if\nthis mask is safe enough. Are the ZrO2 nanoparticles stable within the matrix? Even though ZrO2\nis biosafe itself, there might still be unexpected effects as nanoparticles can enter human cells if\nbeing inhaled. Besides, is the pore size of this mask comparable to that of commercial ones?\nResponse: We thank the reviewer for the thoughtful questions regarding the safety and stability of\nZrO₂ nanoparticles in the mask design. The ZrO₂ particles remain fully embedded within the\nPDMS matrix, as evidenced by SEM images (Figure S3). During our abrasion tests (Fig. 5c and\nTable S5), the total mass loss after 2000 abrasion cycles was only 9.29 mg, equivalent to 0.038%\nof the total mass, indicating that particle detachment is extremely limited. We therefore believe the\nrisk of particle inhalation under normal use is negligible, provided the mask is not mechanically\ndamaged or decomposed.\nRegarding pore size, we have now added an additional optical image in the inset of Figure 4c to\nbetter visualize the permeable structure. We note that our prototype is based on the design of the\nMoldex 2200 N95 mask (https://www.fishersci.ca/shop/products/moldex-type-n95-particulate-\nrespirators-9/19003246b), as shown in the image below. The mask is fabricated using multiple\nlayers of longitudinal and transverse printing passes, with controlled spacing between adjacent\nprinted lines to ensure breathability. Therefore, the air permeability can be tuned by adjusting the\nspacing between printed lines and the total number of printed layers.\n[Figure Redacted]\nSource: Thermo Fisher Scientific Inc.\nRevision: The following image has been updated in the main text.\n(c)\n3 mm\nFigure 4c. Radiative cooling mask fabricated via DIW. Insets show the detailed permeable\nstructure, wearer demonstration, and overall geometry.\nReviewer #3 (Remarks to the Author):\nThe authors have made efforts to revise the manuscript, effectively addressing the previously\nraised concerns. The ink flow characteristics and filler dispersion stability have been better\nclarified. Some concrete examples of 3D-printed radiative cooling devices have enhanced the\npractical relevance of the work. This manuscript is now likely suitable for publication\nconsideration.\nI have one minor suggestion: the strategy for creating a flexible, breathable textile-like material\nfor radiative cooling masks (Fig. 4c) is particularly intriguing. To provide conclusive evidence of\nits permeable microstructure, which is not clearly visible in the current optical image, including a\nmicrograph (either from optical microscopy or SEM) would be highly valuable.\nResponse: We thank the reviewer for the positive assessment of our revised manuscript. We also\nappreciate the helpful suggestion to provide clearer evidence of the material’s permeable\nmicrostructure. In response, we have added an optical image in the insets of Figure 4c to enhance\nthe visibility of pores within the radiative cooling mask. The mask was fabricated by combining\nmultiple longitudinal and transverse printing passes, with controlled spacing between adjacent\nprinted lines to ensure breathability. Therefore, its air permeability can be further tuned by\nadjusting the line-to-line spacing and the number of printed layers.\nRevision: The following image has been updated in the main text.\n(c)\n3 mm\nFigure 4c. Radiative cooling mask fabricated via DIW. Insets show the detailed permeable\nstructure, wearer demonstration, and overall geometry."}]

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