Electromagnetic Power Absorption and Temperature Changes due to Brain Machine Interface Operation

Abstract

To fully understand neural function, chronic neural recordings must be made simultaneously from 10s or 100s of neurons. To accomplish this goal, several groups are developing brain machine interfaces. For these devices to be viable for chronic human use, it is likely that they will need to be operated and powered externally via a radiofrequency (RF) source. However, RF exposure can result in tissue heating and is regulated by the FDA/FCC. This paper provides an initial estimate of the amount of tissue heating and specific absorption rate (SAR) associated with the operation of a brain-machine interface (BMI). The operation of a brain machine interface was evaluated in an 18-tissue anatomically detailed human head mesh using simulations of electromagnetics and bio-heat phenomena. The simulations were conducted with a single chip, as well as with eight chips, placed on the surface of the human brain and each powered at four frequencies (13.6 MHz, 1.0 GHz, 2.4 GHz, and 5.8 GHz). The simulated chips consist of a wire antenna on a silicon chip covered by a Teflon dura patch. SAR values were calculated using the finite-difference time-domain method and used to predict peak temperature changes caused by electromagnetic absorption in the head using two-dimensional bio-heat equation. Results due to SAR alone show increased heating at higher frequencies, with a peak temperature change at 5.8 GHz of approximately 0.018 degrees C in the single-chip configuration and 0.06 degrees C in the eight-chip configuration with 10 mW of power absorption (in the human head) per chip. In addition, temperature elevations due to power dissipation in the chip(s) were studied. Results show that for the neural tissue, maximum temperature rises of 3.34 degrees C in the single-chip configuration and 7.72 degrees C in the eight-chip configuration were observed for 10 mW dissipation in each chip. Finally, the maximum power dissipation allowable in each chip before a 1.0 degrees C temperature increase (most stringent standards as denoted in the FDA guidelines) is exceeded in the head was simulated and found to be 2.92 mW in the single-chip configuration and 1.25 mW in the eight-chip configuration. As thermal heating due to SAR was insignificant, this study suggests that wireless electromagnetics, i.e., RF may be a viable option for powering, and communicating with brain machine interfaces for clinical applications.

Numerical Thermal Analysis of a Wireless Cortical Implant with Two-Body Packaging

“…The computational method used in this study is based on the Pennes bio-heat equation [14], which is widely used in the literature for analyzing temperature distribution in living tissues [7][8][9][10][11][12]15]. The equation can be written as [12]:…”

“…It is also noted that brain temperature higher than 40.5 • C causes heat stroke in humans [6]. Although the former argument does not imply that 1 • C temperature rise in the brain will cause any damage, 1 • C is the precautionary choice for maximum allowable temperature elevation in the brain due to operation of a cortical implant [6][7][8].…”

“…The authors used finite-difference time-domain (FDTD) simulations with 2-D and 3-D head models in order to calculate the temperature elevation due to heat dissipation in the implant and exposure to electromagnetic (EM) fields. In another study, Ibrahim et al presented the FDTD simulation results for temperature changes resulting from a brain-machine interface operation by using an 18-tissue 2-D head phantom [7]. A more recent study investigated the thermal effects resulting from the operation of an active 3-D microelectrode array implanted in the brain [8].…”

“…The temperature increase in the head due to power dissipation in the implant is simulated by using an FDTD-based coupled EM-thermal simulation software, SEMCAD X v14.2, with a high-detail 3-D head phantom composed of 22 different tissues. This study focuses on only the temperature elevation resulting from the power dissipation in the cortical implant, as the temperature increase due to exposure to EM fields is relatively small [7,10,11]. The temperature elevations in the tissues are analyzed for different power consumption, chip size, and location of the implant.…”

Wireless Microstimulators for Neural Prosthetics

“…It is estimated that absorption of 10 mW of power at 1–5.8 GHz by the human head produces only a negligible temperature rise of 0.01–0.02°C. 28 Basal metabolic rate, ~1 W/kg in an adult person, 29 is commonly used as the safe limit for RF absorption in the tissue in order to avoid an uncomfortable warm sensation or heat-induced trauma in the body.…”