  This paper presents the design, implementation and evaluation of In-N-Out, a
software-hardware solution for far-field wireless power transfer. In-N-Out can
continuously charge a medical implant residing in deep tissues at near-optimal
beamforming power, even when the implant moves around inside the human body. To
accomplish this, we exploit the unique energy ball pattern of distributed
antenna array and devise a backscatter-assisted beamforming algorithm that can
concentrate RF energy on a tiny spot surrounding the medical implant.
Meanwhile, the power levels on other body parts stay in low level, reducing the
risk of overheating. We prototype In-N-Out on 21 software-defined radios and a
printed circuit board (PCB). Extensive experiments demonstrate that In-N-Out
achieves 0.37~mW average charging power inside a 10~cm-thick pork belly, which
is sufficient to wirelessly power a range of commercial medical devices. Our
head-to-head comparison with the state-of-the-art approach shows that In-N-Out
achieves 5.4$\times$--18.1$\times$ power gain when the implant is stationary,
and 5.3$\times$--7.4$\times$ power gain when the implant is in motion.
