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Monday, March 4, 2013

Antenna Implants-Possible Future Trends

Over the past two decades, the world has been able to benefit from its significant wealth in knowledge relating to telecommunications engineering. During this time, there has been an exponential growth in the field of mobile communications, proving beyond doubt that people love to talk. Coincident with this success there has been a massive increase in healthcare provision in the world combined with an associated revolution in how treatment is offered to the patient. The simplicity and utility of technologies like Global System for Mobile Communications (GSM) with voice, data, 3G with streaming video and 4G with its superior resource allocation all offer much to healthcare, particularly for non-secure medical telemetry. Discussed here is the future concept of 4G systems implanted into the body with bidirectional link to the cellular network. This is different from current systems that communicate with implanted devices over short range links (<410 m).

Given the right safeguards for implanted mobile phone technology, it would for example be possible to measure the properties of a heart attack in real time and perhaps monitor the effects of treatment subsequent to the event, whilst allowing the patient freedom of movement. What would be needed would be a system that could be implanted into a patient for short periods of time (perhaps several weeks) that could be used to transmit data out of the body and to a medical expert. In this context such a system would use data rather than voice, be non-real time with low isochronous application usage.

What is envisaged might be low SAR (Specific Absorption Rate) flexible antennas just beneath the skin surface for use with cellular systems and their vast networks of base stations. Such a system would comprise a small telecommunications module with integrated micro controller and power supply attached by cable to an antenna. The module, its battery and its associated sensors would lie inside the body. To minimize SAR the antenna would lie as close to the outside of the body as practical but not outside the skin. The system would be encapsulated and screened to reduce energy interactions with tissue. From the point of view of avoiding infection the proposal to have the whole system inside the body’s protective skin is of clear benefit. By not breaching the skin complications arising from infection; hygiene and painful snagging would be avoided. Furthermore such in-body systems would be invisible to other people and may allow patients an extended freedom of movement and much more privacy. All of the components except the antenna are state-of-the-art. In considering the size of such antennas we are helped a great deal by the permittivity of the surrounding tissue which is generally high. Therefore, such antennas would tend to be much smaller than their free-space counterparts (for example about 25mm long for a half wave dipole at 900 MHz).

It can be shown, for example with LTE wireless communications, that it would not currently be a problem sending 4G signals to a modem implanted inside a body cavity. However, because of the very strict legacy limits related to medical implants, the tricky part of such a system would be how to get 4G signals out of the body to a base station without exceeding SAR limits within the body. For a mobile handset power levels from a handset are limited to 2 Watts but are typically around 0.6 Watts split across several channels. However for medical implants the limit is 25 micro-Watts.

The standards germane to this discussion are the Medical Device Radiocommunications Service (MedRadio) and the Wireless Medical Telemetry Service (WMTS) . MedRadio has a spectrum between 401 and 457 MHz. The more common devices realized have been implanted cardiac pacemakers and defibrillators, and neuromuscular stimulators for physical mobility. WMTS has spectrum at around 0.6 GHz and 1.4 GHz and has been used for sending data about such things as pulse and respiration rates to close in receiving stations. A typical application would be a cardiac monitor wirelessly linked to a nurse’s station for post operative care.

The ability to communicate with an implant over a high bandwidth link would facilitate many new applications and enhance existing applications such as pacemakers, implantable cardioverter defibrillators, neuro-stimulators, hearing aids, robotic prostheses, artificial eyes, brain pacemakers to control Parkinsons disease, monitoring of blood glucose levels for diabetic patients, stimulation and recording of brain and muscle activity, swallowable pills for traversing the gastrointestinal tract and implantable drug delivery systems.
Implanted medical devices save lives, increase the quality of the user’s life, reduce the number of trips a patient has to make to a hospital and save billions of dollars in hospital beds, resources and doctors’ time. Lifesaving implants such as cardiac pacemakers, neuro-stimulators and pumps, have now become routine and do not attract the negative media attention that normally follows attempts to create so called bionic people. The cardiac pacemakers and defibrillators have grown into a multi-billion pound industry since the first implanted pacemaker in 1958. As implanted antennas are aimed at the same market, improving the quality of life of severely ill patients or others who are at high risk of illness, it is expected that it will be well received by both the medical community and patients alike.

After all, antennas are implanted inside the human body as a method of treating tumors using hyperthermia. Previously, battery power has been a limiting factor. A typical pacemaker uses less than 10mW and the battery lasts 10 years. However, a long range medical biotelemetry system consisting of a sensor(s), a battery, a 4G communications module and a low power subcutaneous antenna may only need to be in place for a few weeks.

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