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A perturbed gut microbiome has recently been linked with multiple disease processes, yet researchers currently lack tools that can provide in vivo, quantitative, and real-time insight into these processes and associated host-microbe interactions. We propose an in vivo wireless implant for monitoring gastrointestinal tract redox states using oxidationreduction potentials (ORP). The implant is powered and conveniently interrogated via ultrasonic waves. We engineer the sensor electronics, electrodes, and encapsulation materials for robustness in vivo, and integrate them into an implant that endures autoclave sterilization and measures ORP for 12 days implanted in the cecum of a live rat. The presented implant platform paves the way for long-term experimental testing of biological hypotheses, offering new opportunities for understanding gut redox pathophysiology mechanisms, and facilitating translation to disease diagnosis and treatment applications.

We present an analytical framework for optimizing the efficiency of ultrasonic wireless power links for implantable devices scaled down to sub-mm dimensions. Key design insights and tradeoffs are considered for various parameters including the operating frequency, the transmission depth, the size of the transmitter, the impedance and the aperture efficiency of the miniaturized receiver, and the interface between the receiver and the power recovery chain on the implant. The performance of spherically focused transducers as ultrasonic transmitters is analyzed to study the limits and the tradeoffs. Two optimization methods are presented: "Focal Peak" sets the focus of transducers at target depths, and "Global Maximum" maximizes the efficiency globally with offfocus operation. The results are also compared to phased array implementations. To investigate the efficiency of implants, miniaturized receivers made from single crystalline piezoelectric material, PMN-PT, are used as they have resonances in the derived optimal carrier frequency range (~ l-2 MHz). A methodology to achieve an efficient interface to the power electronics is then provided using an optogenetic stimulator as an example platform. The analytical results are verified through both simulations and measurements. Finally, an example ultrasonic link using a spherical transmitter with a radius of 2 cm is demonstrated; link efficiencies of 1.93-0.23% are obtained at 6-10 cm depths with sub-mm receivers for the optogenetic application.

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