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.
