A Battery-Free Neural-Recording Chip Achieving 5.5 cm Fully-Implanted Depth by Galvanically-Switching Passive Body Channel Communication

Wireless fully implanted devices are widely adopted for long-term neural-recording applications, where the cable-induced infection risk can be avoided. Battery-free communication based on wireless power transfer (WPT) can eliminate the battery to reduce the size of a wireless implant, realizing minimally invasive surgery. However, conventional battery-free implants suffer from a short communication range, such as inductive coupling, near-infrared (NIR) transmission, and active body-channel communication (BCC), which cannot apply to deep brain zones. Ultrasonic power transfer and communication benefit from a low channel loss, but the low carrier frequency leads to a low data rate, which is not able to transfer full-span neural signals such as spikes and multichannel signals. In this work, a galvanically-switching passive-BCC technique is proposed for neural implants, to extend the effective range of both power transfer and wireless communication. The brain tissue is utilized to form a galvanic loop for power delivery, while the neural-recording data switch the loop current to conduct passive BCC. The proposed technique is implemented in a neural recording chip fabricated in a 55-nm CMOS process. Through-tissue measurement shows that the chip realizes a battery-free communication range of 5.5 cm, with a bit-error rate (BER) of $4.4\ttimes10^{-6}$ . In the in-vivo demonstration, a 5.9-mm $^3$ flexible prototype with the proposed chip inside is fully implanted into a Sprague–Dawley rat, where the neural signals are read battery-free through the passive-BCC technique.