Medical Design: Under my skin

Transmitted power is limited both by regulatory restrictions and, for most implanted devices, by power source capacity. Dielectric losses and wave trapping in the body result in transmission losses much greater than seen in free space communications. The small antenna size required for physiological acceptability, combined with the high and differing permittivity of body tissues, cause detuning best addressed by active compensation.

Design optimization must trade antenna size, geometric complexity and material cost against efficiency, operating bandwidth and driving power.

Implantable medical devices combine functions of actuators such as defibrillators and neurostimulators, and sensors such as EKG monitors. The device may need to transmit physiological information, normal operation state data, and alarm signals. Received messages may carry control signals or configuration data. With the proper implanted antenna, an RF link does not require a transducer in close proximity to the outside of the body, an advantage not shared by other communication modalities such as magnetic coupling, acoustic coupling, or direct wiring through the skin.

Any antenna designer is concerned with controlling the far field radiation pattern to complete a communication link. To control the far field, the implanted antenna designer must be concerned with the influence of the body, particularly the interaction of the near field with the tissues of the body.

Consider the dipole and the loop, two basic antennas with complementary radiation patterns. Near the antenna, particularly if the major dimension is smaller than a half wavelength, the E field contains most of the energy for the dipole, while the H field contains most of the energy for the loop antenna. Moving a sufficient number of wavelengths from either antenna, the power in the E and the H fields equalize.

The dipole radiates most strongly in the plane perpendicular to the wire axis, while the loop antenna radiates most strongly in the direction perpendicular to the plane of the loop.

Antenna performance can be described by four parameters; Gain, Return Loss, Efficiency, and Operating Bandwidth.

Gain is defined as the ratio of the power radiated in a particular direction, to the total power accepted by the antenna. It combines loss and directionality into a measurable parameter describing radiation characteristics. As exemplified by the dipole and the loop, all antennas are directional; no realizable antenna has an isotropic radiation pattern. All antenna materials dissipate power; conductors exhibit resistance, dielectrics exhibit dielectric loss. We will see that, for an implanted antenna, the environment of the body dominates realized antenna gain through propagation loss and field confinement.

Return Loss, defined as the ratio of reflected power to power accepted by the load, gives a useful measure of electrical power transfer efficiency. For maximum power transfer, power source and load impedances must be matched. A mismatch sets up a standing wave pattern in the transmission line carrying the power, due to the superposition of the outgoing wave and the wave reflected from the load. Generally, a matching network is used to convert the output impedance of the driving source to the input impedance of an antenna. Since there is always some mismatch, we distinguish between power available to the antenna and power accepted by the antenna, hence the qualification of accepted power in the definition of antenna gain.

Efficiency, calculated by integrating realized gain over the surface of an enclosing sphere outside the body, indicates the RF power available outside the body. In free space, antenna geometry determines the operating bandwidth, by which we mean central frequency and bandwidth. For an implanted antenna, body tissues significantly perturb efficiency and operating bandwidth from their free space values