A Hybrid Inductive-Ultrasonic Link for Wireless Power Transmission to Millimeter-Sized Biomedical Implants

Ultrasound has recently been utilized for efficient wireless power transmission (WPT) to biomedical implants with millimeter (mm) dimensions and below. However, the power transmission efficiency (PTE) of ultrasonic links reduces significantly in mediums with different acoustic impedances. This brief presents a hybrid inductive-ultrasonic WPT link for powering mm-sized implants that utilizes two cascaded co-optimized inductive and ultrasonic links for WPT through bone/air and tissue, respectively. The ultrasonic link is first optimized based on the implant depth (<inline-formula> <tex-math notation="LaTeX">$\boldsymbol {d_{\mathrm{ us}}}$ </tex-math></inline-formula>) and load (<inline-formula> <tex-math notation="LaTeX">$\boldsymbol {R_{L}}$ </tex-math></inline-formula>) to find the optimal geometries for ultrasonic transducers and operation frequency (<inline-formula> <tex-math notation="LaTeX">$\boldsymbol {f_{p}}$ </tex-math></inline-formula>). Then, the inductive link is optimized at <inline-formula> <tex-math notation="LaTeX">$\boldsymbol {f_{p}}$ </tex-math></inline-formula> to drive the transmitter (Tx) transducer. A hybrid link was optimized and measured to transfer power to a receiver (Rx) transducer with the diameter of 1.1 mm, loaded by <inline-formula> <tex-math notation="LaTeX">$\boldsymbol {R_{L}}$ </tex-math></inline-formula> of 2.5 k <inline-formula> <tex-math notation="LaTeX">$\boldsymbol {\Omega }$ </tex-math></inline-formula> and located at <inline-formula> <tex-math notation="LaTeX">$\boldsymbol {d_{\mathrm{ us}} = 3}$ </tex-math></inline-formula> cm inside castor oil, as the tissue model. The Tx coil was placed in air and spaced from the Rx coil by 3 cm, resulting in a total powering distance (<inline-formula> <tex-math notation="LaTeX">${d}$ </tex-math></inline-formula>) of 6 cm. At the optimal <inline-formula> <tex-math notation="LaTeX">$\boldsymbol {f_{p}}$ </tex-math></inline-formula> of 1.1 MHz, the hybrid link achieved a considerable measured PTE of 0.16%.