The trend toward ubiquitous deployment of electronic devices demands novel low maintenance power schemes to decrease the burden of maintaining such a large number of devices. In this paper, we propose Interaction-based Power Transfer (IPT):a novel power scheme for power-as-needed devices (i.e., devices that only require power during interaction). IPT allows for the removal of built-in batteries on these devices, and to instead be powered up through direct contact interaction with the user (e.g. gripping a mouse, holding a pen). We prove the concept and show the potential of IPT through our TouchPower prototype. TouchPower transfers on-body power to off-body power-as-needed devices through contact between electrodes on a glove worn by the user and those on the target device during the interaction process. We design TouchPower to automatically detect the contact topology at runtime to supply power accordingly, and place electrodes on the glove so that TouchPower is compatible with various interactions with different objects. We also show the methodology of placing electrodes on the device-end, and evaluate it on a mouse and a remote controller. Results show that during interaction, TouchPower is able to provide stable power supply to these devices with only a small sacrifice in regards to interaction naturalness. At last we demonstrate six applications of TouchPower, and discuss the limitations and potential of TouchPower and IPT systems.