A mm-Sized Free-Floating Wirelessly-Powered Implantable Optical Stimulation Device

This paper presents a mm-sized, free-floating, wirelessly-powered, implantable optical stimulation (FF-WIOS) device for untethered optogenetic neuromodulation. A resonator-based 3-coil inductive link creates a homogeneous magnetic field that continuously delivers sufficient power (>2.7 mW) at an optimal carrier frequency of 60 MHz to the FF-WIOS in the near field without surpassing the specific absorption rate (SAR) limit, regardless of the position of the FF-WIOS in a large brain area. Forward data telemetry carries stimulation parameters by on-off-keying (OOK) the power carrier at a data rate of 50 kbps to selectively activate a 4×4 μLED array. Load-shift-keying (LSK) back telemetry controls the wireless power transmission (WPT) by reporting the FF-WIOS received power level in a closed-loop power control (CLPC) mechanism. LEDs typically require high instantaneous power to emit sufficient light for optical stimulation. Thus, a switched-capacitor based stimulation (SCS) architecture is used as an energy storage buffer with one off-chip capacitor to receive charge directly from the inductive link and deliver it to the selected μLED at the onset of stimulation. The FF-WIOS system-on-a-chip (SoC) prototype, fabricated in a 0.35-μm standard CMOS process, charges a 10 μF capacitor up to 5 V with 37% efficiency and passes instantaneous current spikes up to 10 mA in the selected μLED, creating a bright exponentially decaying flash with minimal wasted power. An in vivo experiment was conducted to verify the efficacy of the FF-WIOS by observing light-evoked local field potentials (LFP) and immunostained tissue response from the primary visual cortex (V1) of two anesthetized rats.