When exposed to favorable environments (i.e., moisture, sufficient air and fat), hydrophilic wood surfaces can be easily degraded and damaged due to the attacks of fungi and bacteria [16]. Traditionally, inorganic waterborne preservatives (i.e., chromated copper arsenate) [21] and organic mold inhibitors (i.e., 4,5-dichloro-2-octyl-isothiazolone) [196] have been used to protect wood surfaces. However, they suffer from harms to human beings and environments, degradation and decomposition [21,197]. Recently, an alternative strategy is to introduce SHSs, which can serve as water barriers and thus prevent SHWSs from being permeated by moisture [16]. Up to now, many SHWSs have been used in the fields of anti-fungi [16,21] and anti-bacteria [19,78,119,193].

For example, Yao et al. prepared SHWSs by dip-coating and brush-coating, and found that SHWSs can thoroughly prevent fungal attachment to wood surfaces compared to hydrophobic or hydrophilic wood surfaces [16]. This is because SHWSs work as water barriers and can isolate moisture with wood structures, thus limiting water resources for the growth of fungi and bacteria. Another reason is that SHWSs have very low adhesion, which makes the attachment of fungi and bacteria on SHWSs difficult.

The reason why SHWSs can effectively prevent fungi and bacteria is that SHWSs possess very low adhesion for both fungi and bacteria, and also show excellent water repellency to stop water moisture. These two advantages inhibit the living environments of fungi and bacteria, and thus can prevent the growing of fungi and bacteria for a long time. In other words, the anti-fungi and anti-bacteria properties of SHWSs can be maintained as long as the surface superhydrophobicity of SHWSs exists. As time elapses, however, the surface superhydrophobicity of SHWSs cannot be guaranteed under extremely humid environments towards various fungi and bacteria. A realistic roadmap towards protecting wood materials is to introduce SHWSs as well as the regular maintenance of SHWSs. In a word, self-cleaning and anti-fouling properties of wood surfaces can prevent wood materials from being molded, which will extend their real outdoor practical applications.

Oceanic oil spills and the discharge of oily wastewaters have caused significant threats to the ecological environment and human health [32,198,199]. To mitigate this issue, traditional methods have been utilized to remove toxic chemicals in oily wastewaters, including in situ burning, air flotation, centrifugation, chemical dispersion, and bioremediation [24,32]. These methods, however, suffer from complex operations, high cost, low separation efficiency, and secondary pollution [198,199]. Thus, adsorption and separation materials draw much attention for dealing with oily wastewaters. Recently, many materials have be used in the field of oil/water separation, such as papers [24], polymers (i.e., polyester fabrics) [200], metal meshes [201], cottons [202] and wood [203]. Among them, the sustainable superhydrophobic wood material with porous structures is an excellent candidate for oil/water separation or oil recovery (See Table 2) [203].

Although wood materials have many advantages (i.e., high strength, easy machinability, aesthetic characteristics) for indoor and outdoor construction and decorations, the combustion of wood materials is an unavoidable problem for the safe utilization of wood materials [5,17,133,179]. With wood materials have combustion characteristics, a possible solution is to endow wood materials with fire resistance. One traditional method is to treat wood materials with fire retardants; however, these fire retardants are harmful to the environments. With the assistance of adhesives, durable flame retardant coatings have been utilized for improving the fire resistance of wood materials [206]. Among various retardant coatings, superhydrophobic coatings consisting of metal or inorganic oxides (i.e., ZnO [5], Mg-Al-layered double-hydroxide [17] and SiO₂ [133,179]) have been successfully introduced onto wood surfaces, showing excellent fire-resistance properties. The fire-resistance property of SHWSs is possibly due to the combination of the fire resistance of metal oxides (or inorganic oxides) and the superhydrophobic coatings of wood materials acting as a protective layer.

To achieve the property of fire resistance, the design of wood materials may focus on properties of inorganic coatings when preparing SHWSs, considering the thickness of coatings, thermal stability, flammability and smoke production. To be realistic, SHWSs can prevent combustion for a certain time, while they cannot constantly stop the flame for a long time as time elapses and flame temperature increases. All in all, superhydrophobic coatings on wood materials can be used as retardant coatings, and the fire resistance property of SHWSs is enough for wood materials as applied in some indoor or outdoor practical applications.

When wood materials are exposed to external environments, anti-UV irradiation should be considered such that the longevity of wood surfaces can be estimated. Generally, anti-UV irradiation properties are investigated by monitoring the color changes of pristine wood and SHWSs over a period of time [152]. For example, Jnido et al. found that the color change in uncoated wood is clearly recognizable, while the color change in the polyester/TiO₂-coated wood is not immediately visible [152]. Li et al. showed that the color of the pristine wood became noticeably darker and yellow after 18 days of UV irradiation, while no visible color change on superhydrophobic wood surfaces was found [152]. By introducing nanomaterials (i.e., TiO₂ [107,152,176], ZnO [165] and CoFe₂O₄ [156]) with anti-UV properties, wood materials can be endowed with anti-UV properties and, thus, the utilization of wood materials can be enhanced in outdoor fields.

The color changes before and after UV irradiation are measured in accelerated aging tests to evaluate the anti-UV property of the pristine wood and SHWSs [156]. Usually, the change tendency of the chromaticity parameters (Δa* (a tendency to turn reddish), Δb* (a tendency to turn yellowish)), the lightness (ΔL*) and the overall color change (ΔE*) of the pristine wood and SHWSs is characterized. Wang et al. studied color fastness enhancement of dyed wood with a Si-sol@PDMS-based superhydrophobic coating [95]. Tuong et al. prepared superhydrophobic epoxy@ZnO coated wood, and found that the color stability can be improved by around 50% compared with that of the uncoated wood [74]. Thus, the superhydrophobic coatings are suitable to be utilized for the protection of wood materials for UV resistance.

In short, as for outdoor applications of wood materials, the anti-UV property is one of most important factors that determine the application of wood materials. Solar irradiation (i.e., UV irradiation) can be easily absorbed by the component lignin in wood materials, thereby gradually degrading wood materials as time elapses. To enhance anti-UV properties, delignified wood materials may be a good choice when used as wood substrates when preparing SHWSs. Overall, the introduction of superhydrophobic coatings with anti-UV properties is also a realistic approach towards protecting wood materials in outdoor environments.

As the use of portable electronic devices increases, massive electromagnetic wave pollutions significantly influence living environments, which has a detrimental impact on human health [207]. To mitigate this issue, EMI shielding and electromagnetic (EM) wave absorption materials have been developed such that public health can be protected. When preparing SHWSs, superhydrophobic coatings are suitable for use as a protective layer against EMI interference.

Now that the widespread use of portable electronic vehicles and devices are unavoidable in modern life, the mitigation of EM wave pollutions has become a huge and realistic challenge. As for indoor and outdoor construction and decorations, the introduction of SHWSs can be considered such that wood materials can simultaneously possess many excellent performances (i.e., self-cleaning and water repellency) as well as the abilities of EMI shielding and EM wave absorption.

Self-cleaning is an excellent property of SHSs, which facilitates automatically removing contaminants (i.e., dust, powder, wastewater and organic contaminants). Inorganic contaminants can be easily removed by SHSs, while organic contaminants may stay on SHSs and thus gradually destroy the superhydrophobicity of SHSs as time elapses. If SHSs possess the self-cleaning property as well as photocatalytic performance, the self-cleaning property of SHSs can be definitely enhanced in various different environments.

Recently, SHWSs have been developed to have a good photocatalytic functionality in the degradation of organic contaminants. For example, Wang et al. prepared thermally induced responsive superhydrophobic wood, and studied the performance of the TRS-wood on the degradation of oleic acid.

Of course, compared with many nanomaterials (i.e., Pd and Pt), the photocatalytic property of SHWSs is not good enough. This is because the photocatalytic property of SHWSs is not the main property of wood materials, which is an additional functionality that enhances the self-cleaning property. For example, wood-material-based house roofs can be functionalized with the photocatalytic property such that birds’ droppings can be gradually degraded under solar irradiation.

Anti-icing surfaces can be defined as follows: (1) repelling cooled water droplets, (2) suppressing ice nucleation, and (3) lowering ice adhesion strength [209,210,211], which are suitable for SHSs as well as SHWSs. Wood materials have some differences with other materials (i.e., ceramics, metals and polymers) when discussing the anti-icing property due to their water adsorption capability. As for SHWSs, the anti-icing property can be maintained as long as the surface superhydrophobicity exists.

In cold outdoor environments, the anti-icing property of wood surfaces is very important for the utilization of wood construction [212]. This is because the mechanical property of wood materials may decrease when water-adsorbing wood materials experience freeze–thaw cycles. Thus, SHWSs are necessary to be used in wood construction to prevent wood materials from being absorbed by moisture (or water vapor). Traditionally, water-proof paints are used for wood construction, which are good enough for regular construction. In highly humid environments, SHWSs are recommended to be introduce onto wood constructions such that the life span of wood materials can be extended, and thus the safety of wood constructions be improved in cold and humid environments.

Except for the above applications, SHWSs can also be utilized in many other fields, such as thermal energy storage [67,147], light-driven devices [151], photoluminescence [1,137], piezoresistive pressure sensors [49], moisture harvesting [61] and wood preservation [115], because of their excellent physiochemical properties.

When SHWSs are functionalized with light-responsive (or light-absorbing) layers, they can be utilized in photo-responsive fields. Kong et al. fabricated a polydivinylbenzene (PDVB)-nanotubes-based superhydrophobic thermal-energy-storage coating on wood substrate, and found that this coating shows a great loading capacity for IPW (78.29 wt.%).

Furthermore, SHWSs have also been developed for functional devices and protective applications. For example, Li et al. studied sandwich-structured photothermal wood for durable moisture harvesting and pumping [61]. Huang et al. investigated superhydrophobic and high-performance wood-based piezoresistive pressure sensors for detecting human motions [49]. David et al. studied superhydrophobic coatings based on cellulose acetate for pinewood preservation [115]. Although wood materials have been applied for the above utilizations, their performances are not as good as other specially designed substrates (i.e., ceramics, metals and polymers). In a word, low-cost wood materials become promising in various fields as long as they are functionalized with surface superhydrophobicity.