Global Sources
EE Times-India
Stay in touch with EE Times India
 
EE Times-India > Embedded
 
 
Embedded  

Design implantable antenna for medical devices

Posted: 29 Jun 2009     Print Version  Bookmark and Share

Keywords:RF  medical device  antenna 

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.

image name

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

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

image name

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.

image name

Figure 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
  Cambridge Consultants

View the PDF document for more information.


 First Page Previous Page 1 • 2 • 3



Comment on "Design implantable antenna for medic..."
Comments:  
*  You can enter [0] more charecters.
*Verify code:
 
 
Webinars

Seminars

Visit Asia Webinars to learn about the latest in technology and get practical design tips.

 

Go to top             Connect on Facebook      Follow us on Twitter      Follow us on Orkut

 
Back to Top