Implanted medical devices improve human lives by treating and monitoring body conditions. But devices containing electronics require power, either from a battery or delivered through a wireless process like inductive coupling. Wireless power transfer is the preferred solution because it avoids the need for surgery to replace batteries, but it must adapt to challenges presented by the human body, such as motion, tissue change, and device migration within the body.

Existing wireless transfer systems perform this adaption by adding complicated communication circuits to the device or using indirect measurement methods. In new research, Tian et al. demonstrate a wireless power transfer system that adapts to changes in an implanted device by using harmonic feedback from a backscattered field.

The authors utilized the fact that bioelectronic devices have nonlinearities, such as the conduction threshold of a circuit’s components, which generate harmonics during powering. They modeled the harmonic generation of a wireless power transfer to a nonlinear load and showed that the load reflects power into the backscattered harmonic frequencies. Additionally, the model showed that increasing the incident power reveals a threshold, past which there is a sharp increase in the reflection spectrum. This threshold allowed for a direct measure of the power level without adding complexity to the circuit.

The team demonstrated the advantage of this method by controlling the light intensity from an LED implanted in the brain of a rat using wireless power transfer. They found that measuring the threshold value provided a way to maintain the brightness of the LED regardless of changes in the distance between the system’s source and receiver.

Source: “Control of wireless power transfer to a bioelectronic device by harmonic feedback,” by Xi Tian, Pui Mun Lee, and John S. Ho, AIP Advances (2018). The article can be accessed at https://doi.org/10.1063/1.5030600.