Implanting Antennas in Humans
Miniaturized Biocompatible Antenna for Implantable Telemetry Devices
Tech Area / Field
- PHY-RAW/Radiofrequency Waves/Physics
- MED-DID/Diagnostics & Devices/Medicine
3 Approved without Funding
VNIITF, Russia, Chelyabinsk reg., Snezhinsk
- University of Mississippi, USA, OH, Oxford\nZarlink Semiconductor, UK, Wiltshire
Project summaryGoal of the Project is to develop a miniaturized antenna for medical telemetry devices implanted in a patient organism.
Nowadays, downsizing of electronic devices offer increasingly more possibilities for monitoring a patient organism and correction of its certain functions. Being rather small, these devices are surgically introduced into an organism and independently monitor and, if necessary, correct organs functioning according to the program specified by a special built-in program module. Most common among them are implantable cardiac pacemakers. These pacemakers provide data, which can be stored in a special memory module and further analyzed by medical experts. They can be reprogrammed if patient condition changes.
But in reality, the above advantages are difficult for realization, since all commercially available pacemakers need to be taken out of a patient organism and then implanted again. And of course, this cannot be frequently repeated. Moreover, they turn to be useless when nonprogrammed urgent medical intervention is required.
Very critical for the present-day medicine is the transmittance of telemetAric data and control commands to various implantable systems: pacemakers, defibrillators, cochlea implants, neurostimulators, insulin pumps, and implantable physiological sensors. Inclusion of these sensors in the system Body Area Network (BAN) is also considered. Arrangement of the radio channel to transmit data from implanted sensors would be the best solution of the problem. But this requires integration of transceivers and antennas into the system. Development of this antenna is just the goal of the project.
Heart pacemakers are most well developed in medical telemetry. Originally, induction coils with magnetic or dielectric cores were used for communication with a basic station. But low-rate transmittance of data and other drawbacks of these systems is the reason of their current non-use. Therefore, solely independent devices are used nowadays.
Recently, the Canadian company “Zarlink Semiconductor” announced creation of the first-in-the history miniature transceiver ZL70100 intended for wireless communication between the implanted device and the basic station. Commercial version of this transceiver is designed for extracorporal application (on the shoulder). Development of the implantable version is scheduled for 2005. This device will have dimensions, which meet requirements posed to implantates. It will have channels for data transmittance and reception at frequencies 402-405 MHz and 433-434 MHz, as well as the channel to take the transceiver from the waiting mode at the carrier frequency 2.45 GHz. Channel 433-434 MHz is intended to integrate the receiver into BAN. The high-frequency section of the transceiver has a matching terminal to connect the transmitter-receiver antenna. But any data on the antenna design and developer are not available in publications, and this allows assumption that the developer has not yet identified the antenna type.
Great number of publications devoted to antenna downsizing demonstrates difficulty of this problem. Actually, the antenna for biotelemetry shall meet conflicting requirements:
- Antenna shall have the minimum possible size to be convenient for implantation in a patient body;
- Antenna shall have the maximum possible energy-transfer coefficient so that energy resources could be most efficiently used in order to have maximum continuous operation.
Dimensions of a high-efficient antenna are known to be proportional to the wavelength in the medium around the antenna and are approximately equal to its one fourth. Implantable medical devices can use allowed frequencies 402-405 MHz (standards MICS), 433-434 MHz (for BAN), or 2.45 GHz. So, antennas with the 2.45 GHz working frequency will obviously have minimal dimensions, but electromagnetic radiation of this range is strongly attenuating in a conducting medium, and human tissues are just this medium. So, this case requires increased transmitting power and hence reduces to zero advantages gained from downsizing. At the 400-450 MHz frequency, the size of a conventional microstrip antenna will be approximately 6-7 cm with the regard for the shortening in the medium [see e.g. P.Soontorpipit, C.Furse, Y.C.Chung, “Design of Implantable Microstrip Antenna for Communication with Medical Implants”, IEEE Trans. MMT, vol.52, no.8, pp1944-1951, August 2004] what is unacceptable for implantates.
So, the antenna size needs to be reduced with the radiation efficiency retained. Well-known are many methods how to reduce the antenna size and retain its efficiency. They are: identifying the optimal type of the antenna, its design, shape, and materials. But specificity of the antenna development for medical implantates is: this antenna is found in the absorbing medium, which significantly changes its parameters if compared with the same one in the air. Influence of the surrounding medium complicates investigations, since an actual shape and electrophysical parameters of a human body shall be taken into account. Publications describe some of these investigations but they were mainly focused on matching parameters required for the antenna. But efficiency characterized by the antenna gain and the directional radiation pattern is to be considered the most important factor. So, the task of this development is (i) simulating the implanted antenna in a human body, (ii) determining (through calculations, or experiments) its efficiency, and (iii) identifying the optimal type, geometry, and materials.
Key experts of RFNC-VNIITF will be involved in the development of the miniature implantable antenna within this project. For this purpose, they will use the 50-years long experience of the Institute on the market of the antenna equipment. The Institute possesses appropriate technical basis: an anechoic room, an automated test-bed for antenna measurements, as well as production and technological facilities for antennas manufacture. For preliminary theoretical development of antenna designs, the Institute has both commercial software to calculate antenna parameters, and software of our development to calculate electromagnetic fields by the FDTD method in areas having complicated configurations.
Theoretical development, physical modeling, and parameters measurement will identify the optimal version of the antenna having the best combination of size-efficiency parameters. Up-to-date measurement equipment, simulation electrodynamics’ methods (FDTD), and genetic algorithms (GA) will be used for this development.
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