Abstract
Implantable neural interfaces are designed to provide a high spatial and temporal precision control signal implementing high degree of freedom real-time prosthetic systems. The development of a Radio Frequency (RF) wireless neural interface has the potential to expand the number of applications as well as extend the robustness and longevity compared to wired neural interfaces. However, it is well known that RF signal is absorbed by the body and can result in tissue heating. In this work, numerical studies with analytical validations are performed to provide an assessment of power, heating and specific absorption rate (SAR) associated with the wireless RF transmitting within the human head. The receiving antenna on the neural interface is designed with different geometries and modeled at a range of implanted depths within the brain in order to estimate the maximum receiving power without violating SAR and tissue temperature elevation safety regulations. Based on the size of the designed antenna, sets of frequencies between 1 GHz to 4 GHz have been investigated. As expected the simulations demonstrate that longer receiving antennas (dipole) and lower working frequencies result in greater power availability prior to violating SAR regulations. For a 15 mm dipole antenna operating at 1.24 GHz on the surface of the brain, 730 uW of power could be harvested at the Federal Communications Commission (FCC) SAR violation limit. At approximately 5 cm inside the head, this same antenna would receive 190 uW of power prior to violating SAR regulations. Finally, the 3-D bio-heat simulation results show that for all evaluated antennas and frequency combinations we reach FCC SAR limits well before 1 °C. It is clear that powering neural interfaces via RF is possible, but ultra-low power circuit designs combined with advanced simulation will be required to develop a functional antenna that meets all system requirements.
Highlights
Neural interfaces provide a direct functional interface with the brain to monitor or initiate neural activity
The specific absorption rate (SAR) safety regulations regarding Radio Frequency (RF) power deposition in the head varies for different applications: the International Electrotechnical Commission (IEC) and the Food and Drug Administration (FDA) limits local SAR
At the same operational frequency of 2.07GHz, the maximum temperature elevations for an antenna at various brain-depths are similar. This could be explained based on the Equation 4: the temperature changes due to the RF radiation of the external antenna mainly depend on the SAR distribution; since the maximum SAR is limited to the same value (1.6W/kg averaged over every gram of tissue), the increased temperature is expected to be very similar
Summary
Neural interfaces provide a direct functional interface with the brain to monitor or initiate neural activity. Harrison et al [29] presents bench and in vivo experimental results from an integrated circuit designed for wireless implantable neural recording applications demonstrating wireless and inductively powered neural recordings from a cat and non-human primate using a single-chip system (INI3 chip) with a minimal number of off-chip components None of these studies examine tissue heating increases inside the human brain due to the wireless operation. Ibrahim et al provided an initial estimation of the amount of tissue heating under the SAR limitation with the operation of a wireless neural interface device [32] All these calculations were performed in two dimensions (2-D) finite difference time domain (FDTD) method and the peak temperature changes caused by electromagnetic absorption in the head were predicted using the 2-D bio-heat equation. The 3-D bio-heat equation was used to calculate the temperature changes in the head due to the external antenna
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