Studies in RF power communication, SAR, and temperature elevation in wireless implantable neural interfaces

Area of Science:

• Biomedical Engineering
• Neuroscience
• Electromagnetics

Background:

• Implantable neural interfaces are crucial for advanced prosthetic systems, but wired connections limit their longevity and application scope.
• Radio Frequency (RF) wireless interfaces offer improved robustness and longevity, yet pose risks of tissue heating due to RF absorption.
• Understanding and mitigating RF power absorption and associated thermal effects are critical for safe wireless neural interface development.

Purpose of the Study:

• To assess power, heating, and specific absorption rate (SAR) for wireless RF transmission within the human head for neural interfaces.
• To optimize receiving antenna design (geometry, depth) for maximum power harvesting while adhering to safety regulations.
• To investigate the relationship between antenna size, frequency, and harvested power relative to SAR limits.

Main Methods:

• Numerical simulations coupled with analytical validations were used to model RF power absorption and tissue heating.
• Antenna geometries and implantation depths within the brain were systematically varied.
• Frequencies ranging from 1 GHz to 4 GHz were analyzed in conjunction with antenna dimensions.

Main Results:

• Longer receiving antennas (e.g., 15 mm dipole) and lower frequencies (e.g., 1.24 GHz) yield higher power availability before violating SAR limits.
• A 15 mm dipole antenna on the brain surface could harvest 730 uW at the FCC SAR limit; at 5 cm depth, it harvests 190 uW.
• 3D bio-heat simulations confirmed that FCC SAR limits are reached well before a 1°C temperature increase for all evaluated configurations.

Conclusions:

• Powering implantable neural interfaces via RF is feasible.
• Achieving functional RF-powered neural interfaces necessitates ultra-low power circuit design and sophisticated simulation techniques.
• Careful antenna design and frequency selection are essential to balance power harvesting with safety regulations.