Harvesting energy from multiple sources available in our personal and daily environments is highly desirable, not only for powering personal electronics, but also for future implantable sensor-transmitter devices for biomedical and healthcare applications. Here we present a hybrid energy scavenging device for potential in vivo applications. The hybrid device consists of a piezoelectric poly(vinylidene fluoride) nanofiber nanogenerator for harvesting mechanical energy, such as from breathing or from the beat of a heart, and a flexible enzymatic biofuel cell for harvesting the biochemical (glucose/O2) energy in biofluid, which are two types of energy available in vivo. The two energy harvesting approaches can work simultaneously or individually, thereby boosting output and lifetime. Using the hybrid device, we demonstrate a "self-powered" nanosystem by powering a ZnO nanowire UV light sensor. © 2010 American Chemical Society.