Battery-free, fully wireless neurostimulation via radiative RF power transfer in freely behaving mice

Wireless neurostimulators are essential for investigating causal relationships between neural circuit activity and behavior in freely behaving animals. However, most existing systems rely on onboard batteries, while near-field magnetic wireless power transfer (WPT) requires specialized environments such as large transmission coils. Radiative radio-frequency (RF) WPT enables longer transmission distances but has been considered unsuitable for freely moving conditions due to alignment sensitivity and output instability. Here, we present a battery-free, fully wireless transcranial direct current stimulation (tDCS) system based on radiative RF WPT that operates reliably in freely behaving mice. The system integrates a quasi-omnidirectional flexible printed circuit board-based monopole antenna with an RF-to-DC rectifier and a DC-DC converter, enabling robust power management despite continuous changes in position and orientation. Experiments in freely behaving mice showed that the neurostimulator maintained the regulated output voltage (~9.7 V) for the majority of the experimental period across a cage-scale environment, while transient voltage reductions occasionally occurred during abrupt posture and orientation changes. Furthermore, targeted tDCS of the M2 cortical region consistently induces circling behavior, demonstrating functional neurostimulation at the behavioral level. This work establishes the feasibility of radiative RF WPT-enabled fully wireless neurostimulation in freely behaving in vivo environments and demonstrates stable extended-duration electrical operation under continuous radiative RF powering conditions. The proposed platform provides a practical foundation for battery-free wireless neurostimulation without tethered connections or coil-embedded behavioral environments.