• Biomedical Engineering
• Dual-Frequency Coupling
• Electromagnetic Stimulation
• Frequency-Selective Stimulation
• Magnetic Resonance Coupling
• Metasurfaces
• Noninvasive Therapy
• Passive Contact Lens Receiver
• Power Transfer Efficiency (PTE)
• Retinal Neurostimulation
• Retinal Prosthesis
• Targeted Retinal Stimulation
• Visual Restoration.
• Wireless Power Transfer (WPT)

Abstract
This study presents a novel wireless power transfer (WPT) system for retinal therapies, which exploits metasurfaces to improve power transfer efficiency (PTE) and allow targeted stimulation. Designed for the treatment of retinal degenerative diseases, the system provides noninvasive, patient-specific electromagnetic stimulation through a dual-frequency WPT configuration. The system consists of glasses with lenses that integrate two planar spiral transmission coils (f_1 = 2.5 MHz, f_2 = 6 MHz) and a 5×5 double layer metasurface positioned between the lens and the eye. At the receiving end, two concentric resonant coils act as a passive contact lens covering the sclera preserving vision. Tuned to their respective frequencies, the receivers allow independent energy transfer for selective retinal stimulation via frequency coding. Numerical simulations show that metasurface incorporation increases PTE from 4% to 5% at f_1 and from 38% to 46% at f_2. These results highlight the ability of metasurfaces to improve WPT performance while providing accurate targeting. To validate these results, a prototype was fabricated and experimentally tested. Measurements confirmed the predicted behavior, showing an increase in PTE from 5% to 6.5% at 3MHz and from 43% to 53% at 6.75MHz upon metasurface integration. In conclusion, the proposed system improves PTE for retinal neurostimulation by enabling selective activation of specific retinal regions. Future work will focus on optimizing receiver geometry to ensure anatomical compliance and reduce invasiveness, further linking engineering innovations with therapeutic advances.
Introduction
Blindness and severe visual problems affect millions of people worldwide: conditions such as age-related macular degeneration (AMD), retinitis pigmentosa (RP) and glaucoma cause progressive retinal degeneration, [5]. Despite extensive photoreceptor loss, postmortem analysis reveals that inner retinal layers, including ganglion cells, often remain undamaged. This residual neural structure enables electrical stimulation strategies, as demonstrated by the perception of phosphenes in response to direct retinal stimulation, [6]. Retinal prostheses use this principle to restore vision by electrically stimulating the remaining neural components, with minimally invasive techniques showing promising safety and clinical potential. Wireless power transfer (WPT) has become essential for powering implantable medical devices, removing the risks associated with batteries, [4]. Among WPT methods, magnetic resonance coupling is particularly efficient and safe, using inductive coupling between transmitting and receiving coils, [3]. However, challenges like transmission losses, electromagnetic interference, and biosafety concerns still need addressing to optimize power transfer efficiency (PTE) and minimize tissue heating, [1]. The integration of metamaterials has greatly enhanced WPT performance. These engineered structures improve near-field coupling, reduce electromagnetic losses, and optimize energy delivery for biomedical applications. By adjusting their resonant frequencies, metasurfaces increase energy transfer efficiency without the downsides of traditional shielding. This integration allows for better power distribution control while ensuring safety, with an equivalent circuit model simplifying their analysis in WPT systems, [2].
This research proposes an innovative WPT-based system for electromagnetic stimulation of the retina, which comprises a passive contact lens and glasses equipped with transmission coils and a metasurface. The contact lens, designed for scleral compatibility, acts as a receiver, generating an electric current at exposure to the transmitted magnetic field to stimulate targeted retinal regions. The metasurface, integrated into the glasses, enhances energy transmission by allowing resonance at distinct frequencies. The dual-frequency transmission pattern enables precise targeting of retinal areas, supporting personalized treatment strategies. By fusing advanced WPT principles with personalized retinal stimulation, this work aims to establish a new framework for noninvasive, personalized retinal therapies, advancing the field of neurostimulation and vision restoration.
Methods and Results
Numerical Design - The system was designed and implemented for numerical simulations with FEKO, using a two-dimensional (2D) coils geometry for computational convenience. The set-up consists of two transmitters (TX1 and TX2) and two receivers (RX1 and RX2). The distances between transmitters and receivers were defined by considering the tradeoff between space between the eye and the glasses, user comfort, and system efficiency. In particular, the distance between receivers and their dimensions are constrained by the anatomical structure of the eye, [3]. The resonance frequencies of the system were set at f_1 for TX1 and RX1 and f_2 for TX2 and RX2. Given an undesirable coupling between TX1 and RX2 and vice versa, switch systems and trap circuits (parallel LC configurations, ensuring each receiver remained off at the unwanted frequency, presenting a high impedance) have been used to overcome this phenomenon. The metasurface was designed with two layers, each containing an array of 5×5 coils, and coupled to the transmitters and receivers. The first layer was matched to TX1 and RX1, while the second layer was paired to TX2 and RX2.
Numerical Results - Efficiency was analyzed by extracting Z-parameters for TX1-RX1 and TX2-RX2 configurations. The integration of the metasurface enhanced the efficiency by 25% at f_1 and by 21% at f_2. In addition, the proper behavior of the system was confirmed by the presence of relevant currents concatenated in the specific receiver at the desired frequency.
Prototype Fabrication - The prototype was fabricated with special attention to structural integrity, geometric accuracy, and functionality. A support structure was created to securely house the copper wire coils consisting of stands and boards with wire guides, (using utodesk Fusion 360, Anycubic Photon workshop and succisively printers with Anycubic Photon M3 Premium using Anycubic 3D Printer Resin). Printed circuit boards were used to allow the use of capacitors and inductors as coil loads for proper system tuning.
Experimental Results - Experimental measurements of the fabricated wireless power transfer system were performed using the Keysight E5080A Vector Network Analyzer. Analysis of S-parameters revealed a slight upward shift in resonance frequencies compared to simulations, corrected by adjusting transmitter-side capacitance. The first and second only two-coil configurations achieved efficiencies of 5% at 3 MHz and 43% at 6.75 MHz, respectively. The introduction of a metasurface between the coils further improved resonant coupling, increasing efficiencies to 6.5% and 53%, confirming its effectiveness in enhancing system performance.
Conclusions
The metasurface-enhanced wireless power transfer (WPT) system developed in this work demonstrates a significant improvement in power transfer efficiency (PTE) for retinal neurostimulation applications. By allowing frequency-selective activation, the system supports targeted stimulation of distinct retinal regions while keeping the implant components completely passive, thus reducing invasiveness. The design takes into account anatomical and ergonomic constraints and incorporates the use of metasurfaces to improve wireless coupling without compromising comfort or visual function. Simulation results and experimental validations confirmed efficiency improvements in selected frequency bands. The prototype was very similar to the simulated performance, highlighting the effectiveness of metasurfaces in enhancing resonant coupling. Future directions include adapting the geometry of the receiving coil to the ocular curvature, improving biocompatibility through flexible materials, and optimizing electrode distribution to better target areas of retinal degeneration. These advances underscore the system's potential as a minimally invasive and efficient platform for next-generation retinal prostheses.
Bibliography
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