Pushing the limits of radiofrequency (RF) neuronal telemetry

Abstract

In a previous report it was shown that the channel capacity of an in vivo communication link using microscopic antennas at radiofrequency is severely limited by the requirement not to damage the tissue surrounding the antennas. For dipole-like antennas the strong electric field dissipates too much power into body tissues. Loop-type antennas have a strong magnetic near field and so dissipate much less power into the surrounding tissues but they require such a large current that the antenna temperature is raised to the thermal damage threshold of the tissue. The only solution was increasing the antenna size into hundreds of microns, which makes reporting on an individual neuron impossible. However, recently demonstrated true magnetic antennas offer an alternative not covered in the previous report. The near field of these antennas is dominated by the magnetic field yet they don’t require large currents. Thus they combine the best characteristics of dipoles and loops. By calculating the coupling between identical magnetic antennas inside a model of the body medium we show an increase in the power transfer of up to 8 orders of magnitude higher than could be realized with the loops and dipoles, making the microscopic RF in-vivo transmitting antenna possible.

Active Electric Dipole Energy Sources: Transduction via Electric Scalar and Vector Potentials

“…Moreover, the electrical engineering community have also shown that the dual of Maxwell's equation may be excited by non-conservative electromagnetic systems or voltage generators [1][2][3], without the need for monopoles to exist. Thus, from two-potential theory, and given there is no magnetic scalar field in the system we are describing, we may write the potential of the defined fields in Equations ( 3)- (11) as…”

“…Inside the active dipole, the polarisation field, P i e , exists without any applied electric field, with both vector and scalar potential components, with the scalar component exactly equal and opposite to the scalar potential of the E field, consistent with Equation (3). Meanwhile, E i e and E T have the same vector curl and thus the same component of electric vector potential, while satisfying the constitutive given by Equation (11).…”

“…In contrast, external to the dipole, the electric field can be described by either an electric scalar or vector potential, as the field is capable of doing work on a test charge, but also exist as a reactive near field (or fringing field) due to the unusual boundary condition between the outside and inside of an active electric dipole, where the tangential electrical field is maximum at the boundary [1][2][3][4][5]. Such active dipoles are usually configured with a balun, and can be used to generate or detect tangential electric fields, in particular they are used to characterise the near field of many systems, including antennas, materials and electrical fields in biological systems [6][7][8][9][10][11]. Both can be modelled by a voltage source with a capacitive output impedance [1] .…”

Radio Frequency Backscatter Communication for High Data Rate Deep Implants

“…The loop antenna has an intense magnetic near-field which dissipates much less power to the surrounding tissues than the dipole antennas with a strong electric field. Thus a small loop antenna results in better efficiency in the biological tissues [29]. The reason for improved efficiency with the loop antennas is the magnetic permeability of the biological tissues that is lossless (µr=1).…”