Medical Design: Under my skin - Embedded.com

Medical Design: Under my skin

Incorporating an RF communications link in an implanted medical device can increase its range of applicability and improve quality of life for the user. This article discusses the challenges of designing an implantable antenna for operation in the MICS (Medical Implant Communication Service) band, 402 to 405MHz, reserved for ultra low power communication with implanted medical devices. Developments in support electronics decrease design risk, but the implanted antenna remains a critical component of a communications link that operates at very low received power.

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

An implanted antenna radiates in an environment of skin, fat, blood, muscle and bone. The relative permittivity of these materials, also known to engineers as the dielectric constant, is much greater than that of air, resulting in propagation wavelengths much less than in air; see table 1 and equation 1.

Fig 1: Return loss variation with fat thickness.

Table 1: Material propagation characteristics at 403.5MHz

For physiological acceptance, an antenna must be small; for the designs under consideration in this article, under one quarter of a wavelength in the body for the MICS band. Such fractional wavelength antennas are either lossy or are relatively inefficient and highly resonant, with a narrow bandwidth.

Small resonant antennas are particularly sensitive to detuning, generally requiring active matching compensation using electrically variable capacitances. Controlling both the resonant frequency and maintaining impedance match generally requires two independent variable capacitors. Depending on the antenna design, the tuning capacitor may require a second connection to the driven element, apart from the driving connection.

The near field environment within a few centimeters of an antenna strongly influences its electrical characteristics; return loss, effective wavelength and operating bandwidth. Figure 1 shows the variation of return loss with implantation depth of a simulated 1mm thick patch antenna implanted in fat under skin. The antenna is tuned and matched for operation at 402MH in the relatively uniform environment of a 20mm thick fat layer.

With reduced fat thickness, the influence of the high permittivity skin layer increases the effective wavelength of the antenna, reducing its resonant frequency, which detunes it from its operating point, thereby increasing return loss above the optimal tuned condition.

Electromagnetic waves are refracted and reflected at interfaces of materials of differing permittivity. A wave propagating from a material of low permittivity, such as fat, to a material of high permittivity, such as skin, undergoes strong reflection. This tends to keep radiation confined inside the body. For an antenna implanted in the fat layer, the interface to the muscle layer further confines radiation to the fat layer.

The tissues of the body exhibit significant dielectric loss, primarily due to water content, converting power from the E field in to heat. The loss is greatest in the high E field region near the antenna. An optimized design configures the antenna to enhance the near H field over the near E field. With careful design, an implanted antenna can achieve efficiency of between 0.01 percent and 3 percent. In comparison, free space antennas easily achieve 95 percent efficiency. The Loss Budget therefore becomes critical in implanted design, increasingly so with communication power restrictions. Since confinement of the field inside the body is unavoidable, the design must optimize near field loss, directivity and matching.

A loop antenna offers a reduced local E field, but lacks directionality. The modified patch antenna reconfigures the basic patch antenna to confine the near E field in the antenna dielectric. The design situates the radiating element over a conductive plate that acts as a mirror, conventionally called a ground plane. This directs radiation out of the body.

A dielectric such as alumina or ceramic separates the driven element from the ground plane. Since the fields at the ground plane are out of phase with that of the driven element, the bandwidth and efficiency of the antenna decrease with decreasing antenna dielectric thickness, with near complete cancellation as thickness approaches zero. The resulting implanted antenna is a small, flat, layered structure. Typically, the metal device case of the implant serves as the ground plane.

Choosing an antenna dielectric with greater permittivity can increase the electrical thickness, reducing the required physical thickness of the antenna. This approach is limited by bandwidth reduction and decreased efficiency if implanted in relatively low permittivity material such as fat.

Moreover, if implanted in fat, the greater effective wavelength of the antenna restricts performance improvements achievable from modifying its shape. On the other hand, if implanted in a higher permittivity material such as muscle, increased design freedom comes at the expense of greater sensitivity to mechanical tolerance.

Material selection is critical for implant design. Biocompatible dielectrics suffer from poorer electrical characteristics and greater production variability than general electronic materials. To inhibit fibrotic growth, the implant can be coated with a polymer, but, due to fluid absorption by the polymer, materials not sealed in the case must be biocompatible.

Impedance variation over the operating band sets the matching conditions for an antenna operating in free space. In addition, an implanted antenna experiences variation due to variability of the local environment. Well developed techniques make matching network design a straightforward task.

However, an implantable antenna will require wider tuning and matching ranges than a free space design, forcing the adaptation algorithm to cope with more variable conditions.Here are shown results for two MICS band antennas. Both designs reduce the local E field to minimize dielectric loss. The loop antenna serves as a reference to a conventional design. The modified patch antenna represents the optimization of a specialized geometry.

Figure 2 illustrates a hypothetical incorporation of the two designs with a device case.

Fig 2: Implantable antennas for wireless implant; neurostimulator or pacemaker.

Table 2 shows power distribution results; most of the power is dissipated in the body.

Table 2: Simulated power dissipation of implanted loop and patch antennas.

Figure 3 shows both simulated and measured antenna gain. Both antennas achieve gain within 9db of isotropic in the horizontal plane. The modified patch antenna achieves 13db greater gain than the loop antenna.

Fig 3: Simulated and measured antenna gains

The antenna must be included among the components of an implantable device requiring rigorous design. Given the increasing capabilities of wireless electronic technology, the dominating influences on achievable system performance will be the properties of the tissues at the implant location, restrictions on antenna size and shape, and available power.

Since required antenna performance is likely to push the limit of what can be practicably realized, system requirement specifications should allow for performance trade offs between the components of the communication system.

A successful antenna design will balance development cost, complexity and production cost against the incremental value of performance improvements. The gain of the modified patch antenna demonstrated here significantly improves the communication loss budget over a simple loop antenna.

Jean-Daniel Richerd, R. Srinivasan and Matthew Reich are with Cambridge Consultants in the U.S. ” www.cambridgeconsultants.com.

This story appeared in the May 2009 print edition of Embedded Systems Europe

You can download a digital edition of this Embedded Systems Europe print edition here.

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