Electronic skin (e-skin) is expected to play a crucial role in the next generation of robotics and medical devices. However, existing e-skin-based sensing platforms primarily focus on monitoring physical parameters and rely on the power from the batteries or near field communication, which significantly hinders their broad use and sustainability toward continuous wireless sensing. Here I will introduce our recent works on flexible self-powered integrated electronic skin for multiplexed metabolic sensing in situ. These battery-free wearable sensors contain biosensors as well as highly efficient energy harvesters (enzymatic biofuel cells and triboelectric nanogenerators) that utilize a unique integration of 0 dimensional to 3 dimensional nanomaterials to achieve remarkably high power intensity and long-term stability. The wearable devices could selectively monitor key metabolites and electrolytes during prolonged physical activities, and wirelessly transmit the data to the user interface. Such battery-free soft wearable systems with highly efficient energy harvesting from the human body hold great promise for robotics and personalized healthcare applications.