Superhydrophobic surfaces, generally characterized by a water contact angle (CA) over 150°, exist extensively in nature, for example, in the surface of lotus leaves and water-strider legs. Creating artificial superhydrophobic surfaces and exploring their versatile applications, such as waterproofing, selfcleaning, drag reduction, and selective absorption, have excited the research frontiers. 

However, good stability, flexibility, practicability, and universality of superhydrophobic coatings are needed for practical use, and multifunctionality has been a new focus of superhydrophobic coatings. Superhydrophobic smart coatings for wearable sensing applications have not yet been reported. 

Recently, ZHANG Ting’s group at the Suzhou Institute of Nano-Tech and Nano-Bionics of Chinese Academy of Sciences reported a novel superhydrophobic and piezoresistive coating with high flexibility and universal applicability to various substrate materials. Related results were published in Advanced Materials, and the article was selected as the inside cover paper. 

In this study, a highly flexible multifunctional smart coating was fabricated by spray-coating multiwalled carbon nanotubes (MWCNTs) dispersed in a thermoplastic elastomer (TPE) solution, followed by treatment with ethanol. TPE acted as both the matrix and surfactant for the MWCNTs so that the suspension had high stability with no MWCNT precipitation for at least three months. 

The introduction of MWCNTs, with high intrinsic electrical conductivity, 1D, high aspect ratios, and excellent mechanical properties, into the superelastic insulating TPE matrix not only induces micro-/nanostructured superhydrophobic surfaces, but also endows the composite coating with reversible elastic deformation and variable electrical sensing performance. 

When a cyclohexane suspension of MWCNTs and TPE was sprayed on a substrate, a solid–liquid MWCNT/TPE/cyclohexane film was initially formed. Then, impact forces from the continuous spraying of liquid droplets acted on the solid–liquid surface, generating large amounts of micrometer-sized features, which were then immobilized during the fast evaporation of cyclohexane. 

For the MWCNT/TPE film treated with ethanol, absolute ethanol partially dissolved the TPE and introduced air gaps between MWCNTs and the film surface formed a network with many pit-like features, which is the reason for the gradual transformation of the MWCNT/TPE film from hydrophobic to superhydrophobic with increasing treatment time in ethanol. 

The coatings show superior sensitivity (gauge factor of 5.4–80), high resolution (1° of bending), a fast response time (<8 ms), a stable response over 5000 stretching–relaxing cycles, and wide sensing ranges (stretching: over 76%, bending: 0°–140°, torsion: 0–350 rad m^-1). They not only endow various substrate materials with superhydrophobic surfaces, but can also respond to stretching, bending, and torsion—a property useful for flexible sensor applications. 

Multifunctional coatings with thicknesses of only 1 μm can be directly applied to clothing for full-range and real-time detection of human motions, which also show extreme repellency to water, acid, and alkali, helping the sensors to work under wet and corrosive conditions. 

This work was supported by the National Natural Science Foundation of China, and the Science Foundation for Distinguished Young Scholars of Jiangsu Province.