Polyacrylamide/sodium alginate double network hydrogel with easily repairable superhydrophobic surface for strain sensor resistant to fluid interference.

Constructing an easily repairable hydrophobic layer on the hydrogel surface that confers resistance to liquid interference remains a great challenge for hydrogel strain sensors. In this paper, superhydrophobic hydrogel sensors were prepared by driving hydrophobic organically modified silica (o-SiO) nanoparticles to the surface of polyacrylamide/sodium alginate (PAM/SA) double network hydrogels by a weak ultrasonic field in o-SiO/cyclohexane dispersion. The hydroxyl groups present on the surface of o-SiO are able to form hydrogen bonds with hydrogels, which in turn form a strong surface hydrophobic layer on its surface. The sensor exhibits superhydrophobic properties for different types of liquids, such as acids, salt solutions, etc., even in the stretched state. The broken o-SiO layers can be repaired by immersing in the o-SiO/cyclohexane dispersion. The SA significantly improved the mechanical properties as well as the strain response sensitivity of the hydrogels. The hydrogel sensor is characterized by low hysteresis to strain, wide detection range (0-894 %), low detection limit (1 %), high sensitivity (GF = 4.8), and good cyclic stability. The superhydrophobic surface allows the sensor to exhibit excellent anti-liquid interference. Salt solution droplets, prolonged contact with salt solution, and even short-term water immersion will not affect the sensor's response to strain. Moreover, repairing the broken hydrophobic layer enables the sensor to restore its resistance to liquid interference. The prepared hydrogel can be used for human motion monitoring in complex scenarios, including exercise sweating, rain, and short-time exposure to water.