Proceedings of the 3rd International
IEEE EMBS Conference on Neural Engineering
Kohala Coast, Hawaii, USA, May 2-5, 2007
ThD1.20
Passive Backscatter Biotelemetry for Neural Interfacing
Bruce C. Towe, Member, IEEE
Abstract— Wireless telemetry of low level biopotentials from
bioelectrodes can be accomplished over decimeter-order
distances by using the biopotential to directly modulate the
characteristics of a simple passive resonant circuit consisting of
two varactor diodes and an inductor. Backscattered radio
frequency energy at or near the circuit resonance in the 300
MHz region is amplitude modulated by the biopotential and
detected by a remote receiver. The varactor circuit exhibits a
high input impedance suited to bioelectrodes and a sensitivity
to submillivolt levels. The circuit is miniaturizable to several
cubic millimeters and can be realized with conventional
commercially available surface mount components.
S
I. INTRODUCTION
ensitive wireless transmitting and recording of
bioelectrical events using miniature telemetry devices is
important to a variety of research related to neuroprosthetics
and to the relatively new field of man-machine interfaces.
Miniaturization of radio transmitters for implantation in
the human body has been of high interest but imposes
particularly stringent requirements on device design, size,
and powering. Depending on the biotelemetry application,
there needs to be a high sensitivity to sub-millivolt
biopotentials and desirably extending down to tens of
microvolts generated by the brain. Good waveform fidelity
of neural spikes desirably need bandwidths to 10 KHz.
Design issues have been the chip space required for low
noise amplification, device power dissipation and tissue
heating, and poor antenna efficiency in small device sizes.
There are also needs for signal conditioning, buffering for
electrode high impedances, offset potential compensation,
and space for an on-chip power supply.
Fitting these circuitry requirements into a compact device
is a daunting problem. Single channel devices as well as
some multichannel are typically centimeter-order sizes in
largest linear dimension [1]. If biotelemetry device sizes
could be further reduced to something for example, that
would fit through a syringe needle, then a larger number of
neuroprosthetic biopotential and biosensor applications
would appear possible on humans due to the reduced trauma
of implantation. Our present work is a step in this direction.
II. BACKGROUND
The basic idea of using an RF exciter to couple power to a
resonant circuit and then detecting the load-shift or reflected
impedance change is the basis for a large industry that
Manuscript received February 15, 2007. B. Towe is with the Harrington
Department of Bioengineering Department, Arizona State University.
(email:bruce.towe@asu.edu). Support for this project is pending.
1-4244-0792-3/07/$20.00©2007 IEEE.
makes modern RF identification (RF-ID) tags such as found
in card-key entry systems. The advantages of these devices
are their simple circuitry, passive operation, and potentially
compact size. An excellent review is presented by
Finkenzeller [2].
UHF and microwave RF-ID systems are often known as
backscatter systems for classification because common
designs modulate their resonance to an incoming
electromagnetic wave and so change the degree to which
they reflect or scatter the energy. This can be done, for
example, by shifting a tuned circuit’s resonant frequency
relative to that emitted by a local RF exciter.
Towe [3] in 1986 demonstrated a low power quasi-passive
technique of resonant frequency shifting to telemeter analog
bioelectrical waveforms. Investigators at the WIMS center
at the University of Michigan [4] have developed miniature
wireless neural interface biotelemetry devices. Tang et al.
[5] have analyzed resonant load-shift keying as a method of
digital data transmission. Troyk [6] has presented an
overview of wireless monitoring.
This present work is based on the observation that submillivolt level biopotentials can detectably modulate the L-C
resonance of a nonlinear varactor diode circuit in a way that
allows telemetry of analog biopotential waveform
information. Although digital information has reportedly
been telemetered by frequency keying of a passive resonant
circuit [5,7], we have not found reports where low level
biopotential information is simply and directly used for
analog modulation of a passive circuit.
This latter approach is remarkable in that there appears to
be little or no need for on-chip signal amplification or
processing of bioelectrical signals as usually required in
conventional biotelemetry. This design strategy appears
particularly appropriate for implantable wireless devices
where only very short range communication is needed to a
receiver outside the body.
III. THEORY
At frequencies greater than about 300 MHz, methods of
characterizing the performance of passive telemetry systems
can be treated with scattering theory [2]. The amount of
backscattered microwave power Sback from a distant and
small device is given by:
Sback = P σ / (4π)2 r4
where P is the emitted power, σ is the backscatter aperture,
and r is the distance [2]. In practice, for r distances that are
more in the near field this relationship is not as useful.
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The effective aperture σe is proportional to the antenna
gain G at its resonant frequency:
σe = λ0 2 G/4π.
As a resonant circuit moves in and out of resonance with
an applied RF carrier source, its gain at that frequency
changes, as defined by its Q curve, and so modulates the
amplitude of backscatter Sback. This is the principle of our
biotelemetry system described here.
In general, it is difficult for tuned structures of size far
smaller than the physical wavelength λ0 (tens of centimeters
in this case) to achieve high values of aperture. We want
small tuned antenna structures to minimize implant size. The
backscatter modulation is inefficient with small antennas,
but we find adequate for short communication distances.
As the operating frequency rises the aperture of small
antennas increase, suggesting that small wireless devices for
implants would best operate in the microwave frequency
region.
An issue that arises with microwave operation is RF
penetration through tissues. Electromagnetic skin depth is
smaller in the volume conductivity σ of tissue as frequency
rises. The electromagnetic skin depth δ is inversely
proportional to root frequency ω and given by:
δ = c/(2πσµω)0.5
Calculation of signal path losses shows that backscatter
telemetry systems for medical implants will probably need
to operate in the mid-UHF or the low end of the GHz
microwave spectrum in order to achieve ten centimeterorder penetrations in body tissue. As a compromise between
potential implant size versus operating depth, we chose
operating frequency in this work of about 300 MHz.
Resonant circuits using capacitances that are voltagevariable such as varactor diodes offer an unusual capability.
The diode value of C is given by C=C(V)m where m
indicates an exponent related to the type of varactor diode.
Changes in voltage across them by a modulating signal
voltage causes a shift in the operating point of the varactor
diode capacitance and hence shift in the LC resonant
frequency modulating the backscatter amplitude.
Figure 1. Schematic of the microtelemetry system. V1 models the
telemetered biosensor signal.
m=11 from 0.1 volt to 2.7 volts. This is a relatively large
change in capacitance with voltage and is one of the factors
responsible for the sensitivity of the circuit to low level
signals.
Although not a requirement, we have found
improvements in circuit performance can often be achieved
with some types of varactor diodes, by applying a small dc
bias of -0.1 to -0.5 volts in series with the signal input.
Electrode offset potentials from dissimilar electrode metals
have been used to supply this bias in some cases to avoid
complicating the circuit.
The wireless circuit antenna is a major design issue and
there is much room for improvement over what we report
here. We use a loop inductor about 7 mm in diameter. This
size was chosen to strongly RF-couple to the benchtop
exciter and is a factor in determining range.
Figure 2 shows a photomicrograph of a circuit built on a
perforated circuit board and used for bench testing.
Prototype versions used two turns of thin #28 gauge
enameled copper wire as an inductor for 300 MHz.
Excitation RF was provided by a HP 8640B signal
generator and a power amplifier BGD904 (Phillips Inc.) in
order to achieve excitation power up to 1 watt.
IV. METHODS
Figure 1 is a schematic diagram of the telemetry system
electrical circuit. It consists of antenna-inductor L1, two
back to back varactor diodes D1 and D2 whose capacitance
resonates L1 to a specific operating frequency. There is also
an isolating series input resistor R1 and a biopotential signal
source modeled by V1. In some configurations the
biopotential signal return path center taps the inductor.
Our designs use epoxy packaged hyper-abrupt
varactors MA4ST2000 series (Microwave Associates Inc.)
that are packaged in SC-79 carriers of 0.94 mm width.
These diodes have a capacitance of 4 pF with a Vr =1.0
volts. Capacitance ratio changes with voltage of typically
Figure 2. Photomicrograph of the passive biotelemetry device reported in
this work. The outer coil inductor is about 7 mm in diameter. The
biopotential electrode lead wires have been removed for clarity.
A phase synchronous direct conversion demodulation
system (Loral Inc) was used to achieve both amplitude and
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phase sensitive detection. The demodulator is constructed
around a Mini-Circuits Inc. (Brooklyn, New York, USA)
ZLW-1W frequency mixer. A reference signal from the RF
exciter introduced into the frequency mixer produces a
direct conversion receiver whose output is then amplified
and bandpass filtered.
Independent exciter and backscatter receive loops
approximately 7.5 centimeters in diameter were arranged
off-axis from each other and tilted to minimize their mutual
coupling to each other while maintaining coupling to the
wireless unit loop antenna. The separation distances of the
two loop antennas from the telemetry device was a variable
but in the range of 10 cm.
was filtered 1 Hz-100 Hz and displayed on an oscilloscope.
The observed ECG showed evidence of some motion
artifact and muscle tremor. But in comparison to a hardwired ECG system the noise was not distinguishable from
the telemetered waveform.
V. RESULTS
A. Biotelemetry
When an RF carrier wave is applied to the resonant circuit
at a drive frequency located on the skirt of its nominal LC
resonance curve, amplitude and phase of the RF voltage
induced across L1 changes with V1. This is due to the
shifting of the resonance frequency with respect to the driver
frequency and so causes a variation in backscatter aperture.
Figure 3 shows a plot of the demodulated signal amplitude
as a function of input signal to the telemetry circuit through
a 50K ohm series input resistor R1. The demodulated
waveform amplitude shows a sensitive linear response to
millivolt level signals that saturates at about 80 mV. Present
designs under optimum conditions, show the system noise
level at about 30 microvolts in 1kHz bandwidths.
The wireless device input port has relatively high
impedance. We have been connecting its input directly to
bioelectrodes with satisfactory performance.
Figure 4 shows a human ECG waveform detected by
commercial silver chloride electrodes placed over the
precordial region of the chest of a human volunteer. The
biopotential electrodes were directly applied to the telemetry
circuit shown in Figure 1. The telemetry circuit was excited
at 300 MHz and separated from the antenna loops by a
distance of approximately 10 cm. The demodulated signal
0.4
R em otely D etected R esponse (relative
units)
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
50
100
150
M illiv olts S ignal In put
Figure. 3 Plot showing the relative demodulated amplitude of a 10 KHz
sine wave applied to the circuit in Figure 1 as a function of input signal
level.
Figure 4. ECG waveform of about 1 mV amplitude from bioelectrodes
on a human subject telemetered over a 10 cm distance using circuit in
Figure 1.
B. Parametric Amplification
Under conditions of sufficient RF excitation, such that
there is a volt or so of induced RF across L1 and the
varactor diodes, we observe an apparent circuit voltage gain.
The applied test signal V1 is observed to cause AM
envelope modulation on L1 that is unexpectedly larger than
the test signal V1. Apparent voltage gains of up to ten have
been observed on the bench. Under some conditions of RF
drive and bias levels applied to the diodes, we have
observed the varactor circuit to break into spontaneous
oscillation. This is consistent with a system that has gain.
Why this should occur without active FETs or transistors is
unclear.
Our limited investigation of this suggests that this RFpumped time-varying capacitance device exhibits a form of
parametric amplification. If so, this process is known in the
literature [8,9]. The time varying parameter in this case is a
capacitive reactance. Time-varying capacitance circuits are
known to provide nearly noiseless amplification because
pure reactances have no Johnson noise. Parametric amplifier
circuits are not used much in semiconductor electronics
because they are more complex and require an RF pump
source and the use of carrier demodulators. These
components however are all part of our wireless system. We
speculate that parametric gain may be, in part, responsible
for the unexpected sensitivity to low level bioelectric events.
C. Telemetry Range
Preliminary results show a transmission range of about a
decimeter for a 7 cm receiving loop antenna but this can
extend to at least several decimeters with larger antennas.
The telemetry range enters into tradeoffs with other
functional aspects such as modulating signal amplitude,
bandwidth, exciter power, operating frequency, varactor
Q’s, and design of the demodulation system.
The performance and range of the system is most
sensitive to the exciter RF power. With AM demodulation
techniques the exciter is best located in a fixed position such
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as on the body surface relative to an implanted wireless
device. So far we have not developed techniques to
accommodate ambulatory applications in small animals.
D. Bandwidth
Figure 5 plots the overall measured wireless system
bandwidth including the demodulation system. This plot
was made at an exciter frequency of 300 MHz. A 90K ohm
resistor was in series with input signal lead to simulate an
electrode impedance.
D em o du lated O u tp ut (m V )
70
60
50
40
30
20
10
0
1.E + 00
1.E + 02
1.E + 04
Inp u t S ig n al F requ ency (H z )
1.E + 06
Figure 5. Experimentally measured bandpass of the wireless circuit
system in Figure 1 with the inclusion of the demodulator .system.
We observe that the modulation transfer frequency
response is flat from dc (not shown) to about 100 KHz. Our
test circuit exhibits response peaking due to an unknown
cause at 500 KHz and then cuts off over about 1 MHz.
VI. DISCUSSION
These preliminary studies suggest an RF-powered
biotelemetry design approach that is an unexpectedly simple
yet sensitive method for short-range communication of low
level bioelectrical events originating from bioelectrodes. Its
design has a number of advantages for short range
communication that includes potentially very small size, low
parts count, ready availability of miniature components, and
wide bandwidth. The design provides simplicity of the
telemetry circuit by trading-off size for a relatively higher
complexity in excitation and demodulation.
The present demodulation approach recovers AM
components of the backscatter signal. This makes the
system sensitive to relative motion between the exciter and
telemetry unit since separation distance will also amplitude
modulate the received backscatter. Although motion artifacts
can be filtered out, phase demodulation may be more
resistant to these forms of artifact [5].
The device power dissipation is not easily measured and
varies with excitation power. No device heating has been
detected and dissipation is expected to be relatively low with
high-Q LC components. Direct tissue heating by applied RF
is a safety concern but only at higher power levels.
The shown prototype system is relatively larger in physical
size than it needs to be for short range neural applications.
Commercial varactor diodes are submillimeter in package
size. The more important size-limiting factor is the design
of the antenna, which rapidly loses efficiency with smaller
size. Even so, we find that reduced antennas compared to
Figure 2, of about three millimeters in diameter operating
above 500 MHz can be functional. There are design
tradeoffs in bandwidth and excitation power that may enable
further reduced size.
Bench tests suggest that with good low impedance
biopotential electrodes and sufficient attention to
demodulator design, that telemetry of neural spike
waveforms in the 50-500 µV range may be directly possible
without additional pre-amplification.
Multichannel operation is conceivable by using an array of
small devices operating at multiple microwave frequencies.
But due to resulting demodulation complexity, its unclear as
to the practicality of high channel densities.
This work suggests to us of the development of a class of
ultra-miniature implantable biotelemetry devices for low
level bioelectrical signals; perhaps something that can be
introduced into tissue by simple injection. Among potential
applications are telemetry of signals from the brain, muscle,
and nervous system, to a small worn exciter-receiver on the
body surface.
VII. CONCLUSION
We have shown the potential for an unusual approach to
wireless biotelemetry of low level electrical signals from
bioelectric sources. This approach has a remarkable
simplicity in the circuit design and so appears a candidate
for biomedical applications where there is need for
exceptionally small size, short range, and low power
performance.
ACKNOWLEDGMENT
This is to acknowledge the help and contributions of Dr.
Darrin Rothe leading up to this work.
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