Flexible Electromagnetic-Lens for Wireless, Implantable Brain Machine Interfaces

Wireless power transfer has emerged as an essential requirement in implantable brain-machine interfaces (BMIs), providing significant advantages over traditional wired interfaces. However, challenges such as poor wireless power transfer efficiency, limited coverage area and transmission distance, safety, and interference with wireless communication hinder the widespread adoption of wireless power transfer in implantable BMIs. This paper demonstrates that two types of flexible electromagnetic lenses (EM-lens), which is an array of relay resonators formed as metamaterials, can significantly improve power transfer efficiency (PTE). The circular split ring resonator (C-SRR) based EM-lens outperforms in ex-vivo experiments with a $\mathbf{1 2. 0 7} \mathrm{dB}$ PTE enhancement from $\mathbf{0. 0 6 \%}$ to $\mathbf{1. 0 3 \%}$. The square resonator based EM-lens outperforms in air, improving the PTE from $\mathbf{0. 6 6 \%}$ to $\mathbf{1 6. 5 8 \%}$. Additionally, EM-lenses placed at two different locations in the subcutaneous and epidural regions are simulated in HFSS to determine the optimal placement. The simulations indicate that epidural placement could achieve higher PTE $\mathbf{4. 5 3 \%}$ but with increased specific absorption rate (SAR) as a trade-off. Finally, measurements of PTE versus location misalignment show that the EM-lens can maintain power coverage up to 88.58% of the lens area; and improve the power transmission distance by a factor of $\mathbf{5 3. 8 5 \%}$ compared to a single transmitter configuration.