Animal bodies are mainly composed of hydrogels—polymer networks infiltrated with water. Most biological hydrogels are mechanically flexible yet robust, and they accommodate transportations (eg, convection and diffusion) and reactions of various essential substances for life–endowing living bodies with exquisite functions such as sensing and responding, self-healing, self-reinforcing and self-regulating et al. To harness hydrogels’ unique properties and functions, intensive efforts have been devoted to developing various biomimetic structures and devices based on hydrogels. Examples include hydrogel valves for flow control in microfluidics [1], adaptive micro lenses activated by stimuli-responsive hydrogels [2], color-tunable colloidal crystals from hydrogel particles [3, 4], complex micro patterns switched by hydrogel-actuated nanostructures [5], responsive buckled hydrogel surfaces [6], and griping and self-walking structures based on hydrogels [7–9]. Entering the era of mobile health or mHealth, as unprecedented amounts of electronic devices are being integrated with human body [10–14], hydrogels with similar physiological and mechanical properties as human tissues represent ideal matrix/coating materials for electronics and devices to achieve long-term effective bio-integrations [15–17]. However, owing to the weak and brittle nature of common synthetic hydrogels, existing hydrogel electronics and devices mostly suffer from the limitation of low mechanical robustness and low stretchability. On the other hand, while hydrogels with extraordinary mechanical properties, or so-called tough hydrogels, have been recently developed [18–22], it is still challenging to fabricate tough hydrogels into stretchable electronics and devices capable of novel functions. The design of robust, stretchable and biocompatible hydrogel electronics and devices represents a critical challenge in the emerging field of soft materials, electronics and devices.