Electromagnetic limits to radiofrequency (RF) neuronal telemetry

Abstract

The viability of a radiofrequency (RF) telemetry channel for reporting individual neuron activity wirelessly from an embedded antenna to an external receiver is determined. Comparing the power at the transmitting antenna required for the desired Channel Capacity, to the maximum power that this antenna can dissipate in the body without altering or damaging surrounding tissue reveals the severe penalty incurred by miniaturization of the antenna. Using both Specific Absorption Rate (SAR) and thermal damage limits as constraints, and 300 Kbps as the required capacity for telemetry streams 100 ms in duration, the model shows that conventional antennas smaller than 0.1 mm could not support human neuronal telemetry to a remote receiver (1 m away.) Reducing the antenna to 10 microns in size to enable the monitoring of single human neuron signals to a receiver at the surface of the head would require operating with a channel capacity of only 0.3 bps.

Pushing the limits of radiofrequency (RF) neuronal telemetry

“…The impact this difference makes in the results of the previous study 1 can be understood in its simplest terms by comparing the two magnetic dipole radiators, one the conventional electric loop antenna, the other, the same loop antenna wrapped around a sphere of dispersive ferromagnetic (or ferrimagnetic) material and comparing their input resistance and radiation efficiency at resonance.…”

“…Before proceeding with this comparison it is worth mentioning the case of the insulated electric dipole, a case also not addressed in the original report 1 . Long linear electric dipoles are commonly used for communication under sea water, an extreme case of a low impedance medium.…”

“…Thus, the original conclusion from the first report 1 still holds: antennas operating in the magnetic dipole mode have a decided advantage. We show in this contribution that two identical microscopic true magnetic dipole antennas can communicate inside the lossy dielectric body medium with an efficiency that is over 8 orders of magnitude greater than could be attained by conventional electric dipoles or electric loop antennas of the same size.…”

“…In the rodent case, it was estimated that a 14.5 μm diameter loop antenna with a magnetic core (that raised its dipole moment by a factor of 3) could communicate at the reduced rate of 3 Kbps. Making the antennas any smaller increases the power dissipated into the proximate tissues above the accepted safety limits 1 . Assuming that the desired size of a sufficiently unobtrusive embedded telemetry node (antenna plus on-board transmitter) is 10 μm, these results rendered unfeasible the prospect of using such a microwave RF telemetry system to track neuronal activity in-vivo 2 3 4 .…”

“…In the reference paper 1 , the effect of a magnetic core for loop antennas was only modeled as an increase of the antenna’s dipole moment thus leading to a slight benefit in radiated gain. The reality, evident from recent work on magnetic conformal antennas 5 6 7 is that the magnetic core of a loop antenna can dramatically alter its input impedance, changing its behavior from that of a metal loop to that of a permeable dipole.…”