Synthetic hydrogels with hydrophobic interactions, which show excellent mechanical performance and good anti-swelling ability in saltwater, have great potential in various industries, such as soft robots, 3D printing, and wearable sensors. Normally, hydrophobic molecules inside a hydrophobic hydrogel tend to aggregate to form a large hydrophobic domain, leading to a phase separation phenomenon because water is a poor solvent of the hydrophobic domain. This aggregation, however, inhibits the adhesion of the hydrophobic hydrogel to various dry materials and thus limits its application in device and sensor industries. In this study, we report the synthesis of hybrid hydrogels with ionically and hydrophobically cross-linked networks. This novel hybrid hydrogel can strongly adhere to various substrates, such as glass, polypropylene, silicone, wood, and polytetrafluoroethylene, with a maximum adhesion strength measured to be 100 kPa. Meanwhile, this hybrid hydrogel can be stretched beyond 8–10 times of its initial length. We attribute this observed strong adhesion and high toughness properties to the synergy of electrostatic interactions and hydrophobic associations. With the strong adhesion and excellent tensile performance, these hydrogels may serve as a model system to explore the strong adhesion mechanism of hydrophobic hydrogels and expand the scope of hydrogel applications.

Hydrophobic hydrogels with high strength and great stretchability hold immense potential in various fields, such as soft robots, 3D printing, and flexible sensors. However, the formation of large hydrophobic domains in a hydrophobic hydrogel can lead to a heterogeneous structure in the bulk hydrogel. This phenomenon will result in the hydrophobic hydrogel becoming opaque, having a large energy hysteresis during stretching, poor strain-sensitivity, and slow self-recovery. In this study, we successfully developed a series of transparent hydrophobic hydrogels that exhibit excellent mechanical properties (low hysteresis and high toughness of ∼1.8−2.5 MJ m −3 ) with a desirable strain-sensitivity. The key factor in achieving this was the ability to tune large, inhomogeneous hydrophobic structures into small, well-ordered domains at the scale of 16.50−52.08 nm by introducing a small number of electrostatic groups into the hydrophobic networks. The hydrophobic hydrogels were able to form strong dual physical interactions, including electrostatic interactions and hydrophobic associations, making them ideal materials for fabricating wearable sensors with both in air and underwater applications. This facile and effective approach provides a novel method to prepare hydrophobic hydrogels with good mechanical performance, low hysteresis, and good strain-sensitivity, opening up new potential for their applications in various fields.

Surface exchange coefficient (k) and bulk diffusion coefficient (D) are important properties to evaluate the performance of mixed ionic-electronic conducting (MIEC) ceramic oxides for use in energy conversion devices, such as solid oxide fuel cells. The values of k and D are usually estimated by a non-linear curve fitting procedure based on electrical conductivity relaxation (ECR) measurement. However, the rate-limiting mechanism (or the availability of k and D) and the experimental imperfections (such as flush delay for gaseous composition change, τf) are not reflected explicitly in the time–domain ECR data, and the accuracy of k and D demands a careful sensitivity analysis of the fitting error. Here, the distribution of characteristic times (DCT) converted from time–domain ECR data is proposed to overcome the above challenges. It is demonstrated that, from the DCT spectrum, the rate-limiting mechanism and the effect of τf are easily recognized, and the values of k, D and τf can be determined conjunctly. A strong robustness of determination of k and D is verified using noise-containing ECR data. The DCT spectrum opens up a way towards visible and credible determination of kinetic parameters of MIEC ceramic oxides.