This "TET" device could find a bigger market than the "VAD" device! Who knows what new gimmicks will be needed in future cyborgs. :-)
60 watts is a lot of juice---about 1/13 Horsepower!
According to a paper that I will append to this posting, the limit of safe transfer intensity is something like 10mw/cm2, or 10000x10mw/m2, or (100 watts/m2). To put this in perspective, the solar constant is about 1000 watts/square metre---at high noon, equatorial. So I find 60 watts to be a reasonable power transfer level.
Losses at 2mhz amount to 1.5%, apparently; the skin would absorb 1.5% of the 60 watts being transferred---0.9 watts, over 1m2.
Coil temperature of course is important. As is power conversion inside body cavities! How to step 2 Mhz (say) down to a more usable frequency? What is the efficiency (30% loss! see paper attached)?
These are some facts that seem relevant to TET:
--limit of 10mw/cm2 or 100W/m2; about 10% of solar flux levels
--TET frequency in 1-100 mhz range (higher=cooking; lower=too big)
--Absorption about 1.5% near 2 Mhz
However, finding no histological damage after a few weeks, in a pig, is not very convincing. Much longer (year) tests seem desirable.
-------------------------------------------------------------------
Fred Bealle (who never quite finished his biomedical eng. MSc Thesis!)
-----------------
attachment:
7.1.10.1 A High-Efficiency Biotelemetry System for Implanted Electronic Device
Hamici, Zoubir; Itti, Roland; Champier, Jacques. Univ. de Lyon, Lyon (France)
The development of an implanted micro-emitter having a considerable implantation depth needs a high-efficiency magnetic
transcutaneous link. This paper describes a new system for the biotelemetry of internal signals. The system associates the
highest performance class E power amplifier with the impedance modulation concept. The resistive load of the transmitter is
influenced by the variation of the internal equivalent resistance, modulating this latter between two rails permits to modulate the
amplitude of the external transmitter current and then transmitting internal digital data. Furthermore, the dependence of the
modulation index on coupling factor allows to find the external coil correct position using a position feedback loop.
INTRODUCTION
THE Medicine of the future will be intimately more and more associated with and dependent upon concepts and advances of electronics. In
order to transmit signals from the internal of the human or animal body, radio-frequency biotelemetry constitutes an ideal way to communicate
with implantable electronic devices without the risk of causing infections. The transcutaneous link is realized using two coils: the primary coil
driven by a power transmitter is positioned externally on the skin, the secondary coil implanted with a power receiver circuit (Fig. 1) energizes
the implant [1]-[3]. Their magnetic link allows data transfer through the biological tissue from the implant to a remote receiver. The efficiency of
the whole transmission system depends greatly on the efficiency of the transmission output stage and the implanted device bulk. This paper
presents a new system, which associates the transmission by Impedance Modulation with a class E power amplifier [1], [2], [4], [5].
METHODS
The inductive power signal is received by an anti-resonance circuit tuned to fr =2 MHz, the choice of this frequency was made by a compromise
between the compactness of the receiver coil which needs the use of the highest possible frequency in the range 1-100 MHz and the absorption
of radio-frequency signals by tissues which is reasonable at relatively low frequencies. At the frequency used, the percentage of absorption is
lower than 1,5 % of the transmitted energy at critical coupling. This choice permits to use the AM intermediate frequency of 455 KHz for the
digital bioelectric signal reception. The output impedance of the power amplifier controls the amplitude of the carrier frequency, this frequency
is influenced by the variation of the implanted equivalent resistance, modulating this latter between two rails (R2max, R2min) permits to
modulate the amplitude of the operating frequency and then to transmit internal data without the classical emitter design. Internal data
transmission using this concept allows a decrease of the internal electronic circuitry bulk and permits to obtain a high-efficiency energizing
device. The amplitude deviation of the driver signal is proportional to the relative position between coils, the attenuation of the transmitter
output power due to transmitter and receiver coils misalignments is eliminated using a position feedback loop, thus positioning the powering
system adequately. The resistive load seen by the power amplifier is given by:
[equation 1 omitted]
with [equation 2 omitted]
Rid is the ideal load of the class E power amplifier. Qr is the quality factor of the receiver parallel circuit, k the coupling factor, Lc the
transmission coil, L2 the receiver coil and R2 the implant equivalent resistance.
The resistance R which constitutes the transmitter resistive load is a linear function of the implant equivalent resistance at w=wr, where
equation (1) agrees with the corresponding result found in [2]. In the range where the biotelemetry system will be used, we must keep correct
operation of the class E power amplifier, so the resistive load R must vary within the interval [-37 %, +55 %] of the ideal value [5]. This range
allows the design of the biotelemetry system to be used in an implantation depth of 20-40 mm. The amplitude deviation is a function of the
lateral displacement, implantation depth and angular misalignment, it should be kept large enough to achieve a high signal-to-noise ratio. The
internal digital signal is received from the implanted unit by an Amplitude Shift Keying (ASK) remote receiver. The amplitude deviation, which
represents the modulation index, is converted to a voltage amplitude which is numerically displayed at the receiver system. The modulation
index is defined as the ratio between the equivalent resistive load maximum variation and the induced maximum resistive load: m=
(Rmax-Rmin)/Rmax. The data transmission is performed by the transmitter HF current variation caused by the modulation of the power amplifier
resistive load. The modulation index is given by equation (3):
[equation 3 omitted]
with the minimum critical coupling factor given by (4):
[equation 4 omitted]
[Figure 1 omitted]
The search of the correct position of the external coil relative to the implanted coil is obtained by varying gradually its position in several
directions until the obtainment of the highest value of the display system. This latter gives us the coupling degree of the inductive link: higher
is the voltage, better is the strength of the inductive link. This constitutes a position feedback loop where the sensor is the display system and
the tracking system is the operator. The maximum efficiency search permits to use an external coil with a diameter chosen in order to have the
highest coupling factor without tacking into account the misalignment tolerance between the coils, because this latter problem is resolved
using the manual feedback loop. In this way, we improve the power and data transmission performance of the overall system, since we track the
absolute transmission efficiency maximum.
This device is the main part of a new generation of diagnostic tools used for pharmacological studies in Nuclear Medicine. After injection of a
radioactive tracer, the pulses of the gamma rays in the left ventricle, are detected with a sensor constructed around a Cadmium Telluride
semi-conductor and then transmitted by radio-frequency [6].
RESULTS
Within a wide range of implantation depths, from 40 mm to 20 mm, the efficiency of the power transfer varies between 44 and 70 %. Concerning
the digital data transmission, a variation of R2 of 20 % produces a modulation index which varies between 10 % at 40 mm and 16 % at 20 mm.
The whole system is intended to eliminate the attenuation of efficiency due to lateral displacement between the power transmitting and
receiving coil. So by varying the position of the transmitter coil, the amplitude deviation N1-N0 (Fig. 1) of the transmitter carrier frequency
changes and reaches its maximum when the two coils are coaxial. This maximum is constant within a radius of 20 mm at 40 mm of implantation
depth, when using a transmitter and a receiver coils respectively with radius of 45 and 10 mm. The whole system allows to transmit internal data
without the classical implanted emitter design using the high-efficiency class E power amplifier. Data transmission based on this concept
permits to reduce the bulk of the internal electronic circuitry at an absolute minimum.
DISCUSSION AND CONCLUSION
The system is characterized by very high performances in term of power transfer efficiency, reduction of the implant bulk and data transmission
ability. Indeed, Zierhofer and Hochmair have presented a circuit which gives 60-70 %, but only in the range 1-8 mm depths [3]. Concerning data
transmission by the external remote receiver the modulation index used is sufficient for correct decoding. The transmitter designed for sending
power and receiving internal data leaves the biological tissue intact and transmits power with respect to the safety standards (10 mW/cm2).
REFERENCES
[1] S. Makay, ''Bio-Medical telemetry,'' IEEE press, 1992.
[2] N. de N. Donaldson, ''Passive signaling via inductive coupling, '' Med. Biol. Eng. Comput., vol. 24, pp. 223-224, Mar. 1986.
[3] C. M. Zierhofer and E. Hochmair, ''High-efficiency coupling-insensitive transcutaneous power and data transmission system via an inductive
link,'' IEEE Trans. Biomed. Eng., vol. BME-37, pp. 716-722, Jul. 1990.
[4] M. Kazimierczuk and K. Puczko, ''Exact analysis of class E tuned power amplifier at any Q and switch duty cycle,'' IEEE trans. Circuits Syst.,
vol. CAS-34, pp. 149-159, Feb. 1987.
[5] F. H. Raab, ''Effects of circuits variations on the class E tuned power amplifier,'' IEEE J. Solid-State Circuits, vol. SC-13, pp. 239-247, Apr. 1978.
[6] Z. Hamici and R. Itti, ''Preliminary study of an implanted device powered by inductive link for the telemetry of the epicardic electrocardiogram
and the radionuclide activity of the left ventricle,'' Phys. Med. Biol., vol. PMB-40, Apr. 1995. (to be published). |
