Skin-inspired scalable superhydrophobic coatings with strong mechano-chemical-thermal robustness

Designing environment-friendly, scalable superhydrophobic coatings with a wide spectrum of robustness in mechanochemical strength, interfacial adhesion, and ame retardancy is critical to low-carbon economy and green environment, especially for green buildings. Simultaneously achieving these preferred functionalities dominated by either interfacial or bulky properties has proved challenging. Herein, we propose an integral skin-inspired triple-layered coating (STC) that resolves conicting demands imposed by individual features on the structure, component, and thermal property of materials. The STC demonstrates sustained anti-wetting and anti-corrosion even under harsh environments, strong adhesion and anti-abrasion, and excellent ame retardancy, all of which enhance its durability and lifetime so as to eliminate the maintenance and repair, thereby reducing cost and energy consumption. Together with facile spraying fabrication process, our STC paves the way to construct a green and resource-saving society.

production and real-world applications 23,24 . Taken together, developing a co-design strategy that simultaneously achieves the integral functionalities by decoupling distinctive requirements on the structure, component, thermal property of coatings is still challenging.
Our design is inspired by human skin (Fig. 1a), featuring with a hierarchical structure made of three layers including epidermis, dermis, and subcutaneous tissue, respectively. The epidermis layer is the outermost layer of the skin, serving as a protective barrier over the body's surface. The middle layer, dermis, is mostly composed of collagen and elastin, in which collagen bers and elastic bers are intertwined and buried in the matrix to strengthen and support the skin. And the subcutaneous tissue consists of loose connective tissue and adipose tissue, which intimately link skin and muscles. Fig. 1b shows the schematic drawing of the skin-inspired triple-layered coating (STC) that exhibits collective performances by taking advantage of three levels of gradient in roughness/wetting, strength and ame retardancy along its triple layers. A common component within STC is polar epoxy resin matrix which can be chemically functionalized, physically mixed with llers, and molecularly modi ed to give rise to desired functionalities. Speci cally, the epidermis-mimicking top layer with superhydrophobicity and anti-abrasion is constructed by the choice of uorinated epoxy resin (Supplementary Fig. 1) with regularly dispersed uorinated cellulose/SiO 2 particles ( Supplementary Fig. 2). In the dermis-like middle layer, the amount of uorinated cellulose and SiO 2 in the uorinated epoxy resin is 50% reduced to act as a bridge between the top layer and bottom layer. To endow strong bonding with various substrates, we choose the utility of un-uorinated epoxy resin in the bottom layer, followed by molecular mixing with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to give rise to enhanced ame retardancy (Fig. 1b). One key innovation in our design lies in decoupling the distinctive requirements imposed by anti-wetting, anti-abrasion and anti-aming into three layers while preserving a continuous interface across the triple-layer. To ensure optimal performances in each layer and the entire STC coating, we rst compared its superhydrophobicity and anti-abrasion properties by tuning the mass fraction of epoxy matrix across triple-layer, the mass fraction of llers ( uorinated cellulose and SiO 2 ) in epoxy in the top layers ( Supplementary Fig. 3).
The STC with the optimized components is fabricated by a three-stage spraying/curing process, in which the temperature and the duration of each curing stage are carefully tailored to allow for conformal adhesion with underlying substrate and bonding within layers. In particular, the bottom layer was sprayed and cured at room temperature, a process distinct from existing in standard spraying coating. To impart an integral adhesion within layers, a soft-curing at a temperature of 45 o C for a duration of 30 mins was introduced after spraying the middle layer, followed by spraying the top layer with a complete curing (120 o C and 10 h). To demonstrate the superior properties of STC, a monolayer coating (Control 1) and a bilayer coating (Control 2), both of which have the same compositions as STC, were also fabricated by using similar preparation process ( Supplementary Fig. 4). Fig. 1c shows a representative STC sample fabricated on Al substrate with a size up to 20 inches. Its surface exhibits a water-repellant property with a water contact angle ~172.3°. This high contact angle is rendered by the presence of hierarchically micro/nanostructures as well as the low surface energy of uorine element, as evidenced by scanning electron microscopic (SEM) measurement and XPS measurement ( Fig. 1d and inset). The micro-sized celluloses in both the surface and bulk of STC can be re ected by confocal uorescence microscope measurement, which shows the formation of the 3D intertwined framework labelled in green (Fig. 1e, Supplementary Fig. 5 and 6). As shown in the crosssectional electron microprobe (EM) image of the fabricated STC (Fig. 1f), the boundary between each layer of STC can be clearly discerned. In particular, the bottom layer of STC shows the presence of a variety of polar groups ( Supplementary Fig. 7), which is in striking contrast to that of Control 1 and Control 2 ( Supplementary Fig. 8 and 9). On the basis of the EM mapping as shown in Fig. 1g and 1h, both silicon and uorine contents in the top layer are higher than those in the middle layer, and there is no apparent existence of uorine element in the bottom layer. Moreover, phosphorus element resulting from DOPO is found to be distributed in the bottom layer alone, as shown in Fig. 1i and XPS measurement ( Supplementary Fig. 10).
Practical applications of coatings, particularly in green buildings, involve various operating environments. At high humidity and high temperature, conventional superhydrophobic coatings are easy to collapse owing to the penetration of tiny water droplets into rough structures. When the STC was exposed to hot steam (~100 o C) using home-made hot steam device ( Supplementary Fig. 11), it still maintained a sustained anti-wetting property. As shown in Fig. 2a, high-temperature water vapor rst condensed into discrete small droplets with an average size of 400 μm, and then easily rolled off the surface by gravity or coalescence-induced jumping (Supplementary Movie 1), otherwise impossible to be achieved for control samples ( Supplementary Fig. 12). In the opposite condition, we put the STC into a refrigeration cycle machine with a programmable temperature ranging from 0 °C to -70 °C and examined the ice formation by depositing deionized water (2 mL) on its surface at different locations with 10 °C intervals. We found that the weight of ice on STC-Al sheet is only about 1 g at -30 o C, which is 6-fold smaller than that on the pristine Al sheet. Even at -70 o C, the weight of the ice on STC-Al sheet is about 58.4 % of that on pristine Al sheet, indicating that the STC-Al sheet has excellent anti-icing effect (Fig. 2b, Supplementary Fig. 13). In addition, by applying a small external force on the STC-Al sheet, the ice fell off as a whole piece easily owing to the presence of air barrier layer 25  In addition to maintaining a stable anti-wetting property, the presence of air barrier layer in STC also reduces its contact area with the corrosive medium and thus promotes anti-corrosion and durability 26-29 . Here, we combined the copper-accelerated acetic acid salt spray test and electrochemical measurement to assess the corrosion resistance of the coating (Supplementary Fig. 15). The STC-Al sheet yields the biggest E corr (-0.01 V) and the smallest I corr (2.337×10 -4 A·cm -2 ) compared with other samples (Fig. 2d), showing a strong corrosion resistance. In the Nyquist spectrum (Fig. 2e), the impedance value signi cantly increases from 2.7 Ω·cm -2 for pristine Al sheet to 590.1 Ω·cm -2 for STC-Al sheet, indicating that the STC can signi cantly protect Al metal from corrosion. The mass loss rate per unit area (△K MLR , see notes in Supplementary Information) was used to assess the corrosion resistance of the coating. As shown in Supplementary Fig. 16, after 120 h corrosion, the surface morphology of STC rarely changes whereas noticeable corrosion pits occur to control samples. The △K MLR of the STC-Al sheet decreases to below 1 g·m -2 h -1 , which is much lower than that of Control 1-Al sheet (14.8 g·m -2 h -1 ), Control 2-Al sheet (8.6 g·m -2 h -1 ) and the pristine sample (22 g·m -2 h -1 ) (Fig. 2f). Even after 300 h, the STC-Al sheet was slightly corroded while control samples are completely destroyed (Fig. 2g-j, Supplementary Movie 3). In addition, the STC still maintains robust superhydrophobicity and high corrosion resistance in corrosive environments such as after immersing in aqua regia for 40 h and in 12 M NaOH solution for 60 min (Supplementary Fig. 17 and 18). These results demonstrate the durable anti-wetting properties in various extreme conditions.
One limitation in conventional coatings lies in the di culty in solving the tradeoff between anti-wetting and anti-abrasion owing to the mechanical fragility and easy damage of micro/nanoscale roughness.
The abrasion resistance of our sample was tested using the Taber abrasion tester (Supplementary Fig.  19). Even after 3,000 cycles under a 250 g load, the θ C is larger than ~150° and θ R is less than 10°, a performance at least 10 times higher than our previous work 30 and other existing studies [31][32][33][34] (Fig. 3a-c, Supplementary Movie 4). Moreover, even under a 750 g load, our sample performed well up to ~1000 cycles, suggesting simultaneously preserving the anti-wetting and anti-abrasion property. In contrast, without the use of the layered-design, the superhydrophobic properties of Control 1 and Control 2 were collapsed after 300 and 1000 cycles' testing, respectively (Fig. 3a, 3b, Supplementary Movie 5 and 6). More importantly, the STC is also tolerant to various sharp objects (Supplementary Movie 7) such as steel-wool (width: 0.13±0.05 mm), screwdriver, steel-blade (feather-cut special: 58*13.35*0.15 mm), utility knife (blade angle: 30 o ) and hammer attack. This is different from the armor design reported by us before 30 where the microscale cavity is lled with mechanically weak resin which can be easily damaged by sharp tips. Thus, our STC manifests both anti-wettability and mechanical stability, a property extending from the surface to the middle layer. Without the optimal control of components fraction in triple-layer, i.e., mainly the mass ratio of llers in the top and middle layer, the STC samples are susceptible to sharp variation in structural integrity, and hence their mechanical robustness and antiwetting performances are compromised as opposed to their counterparts (Fig. 3d), demonstrating the essence of preserving gradually-varying and continuous interfaces across the triple-layer.
We also conducted adhesion test using the American Society of Testing Materials (ASTM) D3359 standard ( Supplementary Fig. 20) to examine the bonding capability of the as-fabricated STC with underlying glass substrate. Brie y, we used multiple tap cutters to create regular grids with an interval of 1 mm on the STC surface. Then the coating is compressed by 3 M tape under a uniform load of 2 kg, followed by peeling off (Fig. 3e). We found that the coating is free of any apparent detachment of fragments and maintains a superior structural integrity, exhibiting a performance that meets the ASTM 5B standard (Fig. 3f). In contrast, without the layered-design, control samples suffer from limited adhesion strength as evidenced by the easy peeling of coating from glass substrate (Fig. 3g and 3h). Such a strong adhesion can still be maintained even when the STC was brought to harsh treatment. As shown in Supplementary Fig. 21, after cutting and immersing in water at room temperature for 24 hours or in boiling water for 20 minutes and in aqua regia for 40 hours, there is no pronounced change in the adhesion property of the STC after being peeling-off using 3 M tape.
In addition to preserving an enhanced mechano-chemical robustness, our STC coating also demonstrates anti-aming capacity so as to extend its lifetime and avoid unnecessary repair, a property essential for safety at extremely high temperatures and dry conditions. We applied our coating to different models such as wooden house of different sizes and electric circuits. First, for a STC-coated model with a size of 15.6 cm×18.6 cm×23.3 cm, it was partially burned when being ignited using an alcohol blowtorch for 110 s, however, it spontaneously self-extinguished at 135 s (Fig. 4a, Supplementary  Movie 8). In contrast, the pristine wooden house model was ignited within 45 s and went into re within 180 s, reaching a ame temperature up to 726.5 o C as evidenced by thermal infrared camera measurement. When a large STC-coated wooden house with 1.2 m×1.2 m×1.4 m was brought to a jet gun with an outer ame temperature of 1300 o C for 30 mins, there was no igniting observed. As a comparison, without the application of STC, the pristine wooden house with the same size gradually became a pile of ash within 25 mins (Supplementary Movie 9). Similar trend was also found when spraying STC as a protective coating on supercapacitors in electrical circuits. As shown in Supplementary Fig. 22, the STCcoated supercapacitor can still light up LED even under 10 s of ignition, while the pristine supercapacitor is burned within 3 s (Supplementary Movie 10). All these experiments indicate that our coating has good ame retardancy and manifests its practical value after large-scale spraying process. Notable, these enhanced ame retardancy is achieved with only 2 wt% DOPO loading, a fraction low enough without sacri cing the mechanical property of the material [35][36][37] .
We then sprayed STC and control samples on polyurethane (PU) to quantitatively characterize the ame retardancy of STC using a cone calorimeter. First, the limiting oxygen index of STC-PU in our experiment is ~ 26.7 %, which is higher than a critical ammable value of 22% in air and also higher than that of Control 1-PU (24.7 %) and Control 2-PU (24.2 %) (Fig. 4b). Second, the time to ignition of STC-PU is also 75% longer than that of pristine PU (Fig. 4b). Third, the maximum heat release rate of STC-PU is about 79.6 kW·m -2 , which is 48.2 % of Control 2-PU and 33.4 % of Control 1-PU as well as 19.9 % of pristine PU, indicating that STC can effectively suppress heat release (Fig. 4c). Similarly, the total heat release of STC-PU is only one third of that of pristine-PU (Fig. 4d). The release of toxic gas (CO 2 , CO), speci c extinction area, effective heat of combustion and mass loss rate were also measured to prove the ame retardancy of STC (Supplementary Fig. 23). The enhanced anti aming robustness of STC directly results from the ame retardant imbedded in the bottom layer. Upon in contact with re, DOPO can generate orthophosphoric acid and release P containing free radicals (e.g. PO•), which can capture free radicals of H• and HO• to terminate combustion reaction. The orthophosphoric acid also burns into a coke layer to cut off oxygen, further hampering the combustion reaction ( Supplementary Fig. 24 and 25). Second, we also considered the effect of air barrier layer trapped in hierarchically micro/nanostructures due to the low thermal conductivity of air (~0.026 W/(m·K)). To do so, we deposited STC, Control 1 and Control 2 on steel substrates, respectively, all of which were then brought into heating using alcohol blowtorch ( Supplementary Fig. 26). The measured surface temperature of STC is 306 o C, which is 21.4% and 26.2% smaller than those of Control 1 (389.4 ℃) and Control 2 (414.7 ℃), suggesting the suppression of heat transfer owing to the presence of air layer (Fig. 4e-g).
In conclusion, we demonstrate a skin-inspired, triple-layered multifunctional superhydrophobic coating exhibiting superior mechanical-chemical-thermal properties to existing coatings ( Supplementary Fig. 27). Combining these desirous performances with the facile solution-processible fabrication process, the codesign strategy provides a new approach for engineering robust coatings that expand the application domains from green buildings to other scenarios such as vehicles, airplane wings, marine vessels, oil pipelines, and power lines.   Mechanical robustness. The changes in (a) water contact angle (θ C ) and (b) water roll-off angle (θ R ) of STC, Control 1 and Control 2 with Taber abrasion cycles under two different loads (250 g and 750 g). For STC, after 3,000 cycles abrasion under a 250 g load and 1,000 cycles under a 750 g load, θ C is ~150° and θ R is less than 10°, respectively. c, Comparison of the antiabrasion performance of STC with existing superhydrophobic surfaces. All the references use Taber abrasion test method. d, The effect of gradient mass ratio of llers in top and middle layers on the superhydrophobicity and mechanical robustness. e, A schematic of adhesion test. f-h, SEM images of STC, Control 1 and Control 2 after adhesion test, respectively. After peeling off, the retention of STC is signi cantly better than that of Control 1 and Control 2.

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