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Electro-Quasistatic Animal Body Communication for
Chronic Untethered Rodent Biopotential Recording
Shreeya Sriram1*, Shitij Avlani1, Matthew P Ward2,3, and Shreyas Sen1
1School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, 47906, USA
2Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, 47906, USA
3Indiana University School of Medicine, Indianapolis, IN, 46202, USA
*sriram11@purdue.edu, shreyas@purdue.edu
ABSTRACT
Continuous multi-channel monitoring of biopotential signals is vital in understanding the body as a whole, facilitating accurate
models and predictions in neural research. The current state of the ar t in wireless technologies for untethered biopotential
recordings rely on radiative electromagnetic (EM) fields. In such transmissions, only a small fraction of this energy is received
since the EM fields are widely radiated resulting in lossy inefficient systems. Using the body as a communication medium
(similar to a ’wire’) allows for the containment of the energy within the body, yielding order(s) of magnitude lower loss than
radiative EM communication. In this work, we introduce Animal Body Communication (ABC), which utilizes the concept of
using the body as a medium into the domain of chronic animal biopotential recording. This work, for the first time, develops
the theory and models for animal body communication circuitry and channel loss. Using this theoretical model, a sub-inch
3
,
custom-designed sensor node is built using off the shelf components which is capable of sensing and transmitting biopotential
signals, through the body of the rat at significantly lower powers compared to traditional wireless transmissions. In-vivo
experimental analysis proves that ABC successfully transmits acquired electrocardiogram (EKG) signals through the body with
correlation accuracy >99% when compared to traditional wireless communication modalities, with a 50x reduction in power
consumption.
Introduction
Chronic monitoring of biopotential signals has paved the way for a better understanding of neural pathways along with
improved therapeutic treatments. Recent proliferation in small form-factor wearables has enabled a new domain of continuous
health monitoring. This coupled with miniaturized biological sensors, both in the wearable and the implantable domain has
resulted in gathering valuable information regarding the body. Surface biopotential signals such as EKG, sEMG (Surface
Electromyography), EEG (Electroencephalogram) have been studied as a means of understanding the behavior of the body.
Neural recording systems interfaced with the peripheral nervous system have been extensively explored as a method to acquire
meaningful data that is used to predict and understand the motor, sensory, proprioceptive, and feedback functions of the brain.
The use of animals in biological research and medicine has been a longstanding practice given the similarity between
the animal and human anatomy and physiology. The current state of the art in animal signal recording includes miniaturized
implantable devices with stimulation and recording capabilities. These devices implanted inside the animal body, transmit data
to an external unit capable of receiving and processing this information. A vast majority of animal recording systems still rely
on tethered units especially in cases when continuous long-term information is a priority. Tethered systems are not limited by
the data rates and are a gold standard for reliable comprehensive information, this, however, does come with a few caveats.
Tethered systems are limited by the bias and irritation introduced by these devices on the subject. Long experimental duration
is possible, however, the experimental arena is hindered by the need for long wires which in turn results in restricted movement
of animals. Noisy systems result from the animals tucking and biting at the wires. Tethered systems require signal conditioning
electronics to be placed external to the body, long wires also act as a site for infection and require buffers to prevent signal
attenuation
1
. To reduce these effects, wireless telemetry of signal information is needed. Wireless recording systems have
since evolved from discrete modules to system on chip devices. These small form-factor wireless devices eliminate the bias
introduced by the tethered systems; however, it is limited by the high-power consumption due to the need for up-conversion of
the baseband signal to higher radio frequencies
2
and loss due to radiative communication. Animal studies, in particular, require
the animal to wear a heavy battery pack to meet this high-power requirement. The need for constant replacement of batteries
limits the experimental duration and causes undue stress on the animals which in turn corrupts the data
3
. On the other hand,
arXiv:2005.05370v1 [eess.SP] 11 May 2020
BLE
ABC
PC : Bluetooth and
ABC comparison
Received
signal to the
Data
Acquisition
System
Received bit stream
to the PC for
demodulation and
comparison
Wired
Connection to
the data
acquisition and
processing unit
Data Acquisition
System (DAQ)
DAQ and PC
Ground
Connected
Ground Plane
Rat cage
isolated
from the
ground
plane
0
5
10
15
20
25
30
BLUETOOTH ABC
29.5 mW
0.51 mW
POWER COMPARSION (mW)
Conductive
Surface
Right
Leg
Drive
OOK
Transmitted
Signal
Bluetooth
Transmission Animal Body
Communication
Transmission from the
body electrode
through the
subcutaneous tissues
to the ground isolated
conductive plane
Received
Signal
EKG Electrode
Figure 1. Animal Body Communication: a) Overview of Animal Body Communication on a Rodent Model. Custom
designed sensor node is placed on the back of the rat. This sensor node is capable of sensing and transmitting the surface
biopotential signals via Bluetooth and Animal Body Communication. The sensed signal is transmitted through the body to the
conductive surface in the form of OOK (On-Off Keying) sequences. The specially designed rat cage is isolated from the ground
surface. A conductive surface is placed on the base of the rat cage which is then connected to a Data Acquisition System (DAQ)
which receives the transmitted signals. The Bluetooth receiver and DAQ are connected to a PC for processing, with the DAQ
and PC ground referenced. In this model Bluetooth communication acts as a validity check for ABC. The rat model in a) was
created using Paint 3D.
perpetual recording could be achieved through wireless power transfer
4
, however, due to the inherent high-power consumption
of the sensing node with electromagnetic communication, high-power needs to be harvested, increasing the on-device harvester
size significantly.
To overcome these constraints there is a need for a low power communication modality capable of withstanding the
continuance of the experiment, along with having an unbiased model in which the animals are free to move in their natural
environments. In this work, we introduce a novel communication modality that uses the animal body as a medium to transmit
information. This system eliminates the bias introduced by the tethered systems and has a significant size, weight, area, and
power benefits compared to electromagnetic communication systems. We demonstrate this
Electro-Quasistatic Animal Body
Communication (EQS-ABC)
as a low loss, efficient channel which addresses the aforementioned drawbacks of both tethered
and wireless EM communication systems. Using the body of the animal as a ‘wire’ensures signal confinement resulting in
significantly lower losses when compared to traditional wireless technologies. Here, we demonstrate ABC using a rodent model
and explain the theory and biophysical models of ABC, followed by ABC demonstration with a sub-inch
3
, custom-designed
sensor node in the subsequent sections. Figure 1, describes the concept of the ABC setup, surface biopotential signals are
acquired by a custom-designed sensor node that then transmits the signal using ABC through the subcutaneous tissues of the
animal body using EQS-ABC. These signals are picked up by a receiver connected to the ground isolated conductive surface. In
this setup, we also transmit the signals using Bluetooth as a method to compare the ABC transmitted signal with an established
communication modality. In this pilot study, we use the concept of body communication and extend it into the animal domain
enabling long term benefits in energy consumption along with size, weight, and area benefits. The low power requirement
enables the use of smaller batteries or coils in the case of energy harvested nodes. We show the concept of ABC applied to
animal biomedical studies, this modality can be extended into the neuroscience domain, with implantable nodes to acquire the
signals and then communicate using the body as a medium in the future. This work presents the first validation of ABC as a
communication modality. Experiments were performed with EKG signals of the rat as the chosen surface biopotential signal.
2/15
State of the Art in Wireless Biopotential Recording
The first biopotential discovery dates back to 1666 when Francesco Redi measured the EMG from a specialized muscle
in the electric eel
5
, the field of animal biopotential recordings evolved from tethered systems to wireless systems in the year
1948 when Fuller and Gordon first used radio communication for biopotential signal transmission
6
. Presently, multi-channel
recording devices with wireless power transfer is being implemented with smart devices and arenas for in-sensor analytics. The
evolution and detailed comparison of the state of the art in biopotential recording has been described in a later section.
Biopotential signals, both non-invasive (skin surface) and invasive, have been studied as a means of building bio-electronic
medical devices. The central nervous system controls the body and this control can be observed by studying the changes in
the peripheral physiological factors such as changes in the heart rate, muscle activity, and breathing. To study these changes,
long term monitoring of these physiological signals is necessary
7
. EKG is one of the most widespread diagnostic tools in
medicine and the similarity between human and rat EKG
8
has permitted the study of various physiological conditions and
cardiac diseases
9,10
. Along with EKG signals other surface biopotentials such as sEMG and EEG are studied in rats, analysis
of these signals is used in sleep studies, epilepsy, locomotive analysis, and effect of spinal cord injuries11,12.
The study of the brain along with the body is essential in understanding the control mechanisms of the brain on physiology.
Sican Liu described a novel neural interface system for simultaneous stimulation and recording of EEG/EMG and ENG
(Electroneurogram) signals
13
. Along with surface biopotential signals, invasive recording allows for localized, high fidelity
signal analysis. Neural biopotential signal analysis is a topic of extensive research in experimental neuroscience, with the aim
of improving the quality of life of people with severe sensory and motor disabilities. Wireless neural recording systems have
been described in insects, rodents and non-human primates. In rodents particularly, various neural interface systems which
include bidirectional communication has been explored
14,15
. Application-specific integrated circuit (ASICs) for neuro-sensing
applications has been described for implantable neurosensors4,16–18.
Chronic multi-channel neural recording is a powerful tool in studying dynamic brain function. Multi-electrode arrays
permit recording of more than one channel simultaneously enabling neuroscientists to explore different regions of the brain in
response to a particular stimulus. Bandwidth constraints limit the number of channels that can be recorded simultaneously
resulting in a trade-off between the number of channels that can be simultaneously recorded, power requirements, and the
form factor of the device. For example, Borton et al designed an implantable hermetically sealed device that was capable of
sending neural signal information via a wireless data link to a receiver placed 1 meter away. This system permitted 7-hours of
continuous operation
17
. Chae et al. describes a 128 -channel 6mW wireless neural recording IC with on the fly spike detection
for one selected channel. A sequential turn-on method is used to minimize the power requirement19. Similarly, Miranda et al.
developed a 32-channel system that can be used for 33-hours continuously but requires two 1200 mAh batteries
20
. To achieve a
meaningful experimental duration, the power consumption is often >10mW, generally dominated by the communication (radio)
power. Thus, it is evident that wireless neural interfaces are power-hungry and there is a need for constant replacement of the
batteries or selective channel selection in a chronic setting. To overcome these constraints wirelessly powered neural interfaces
were developed, which eliminates the need for constant replacement of the batteries. Implantable devices, in particular, need
wirelessly powered devices to reduce the need for a battery at the implant site. Enriched experimental arenas allow for the
constant transmission of power facilitating chronic recordings. Yeager et al. developed a wireless neural interface, NeuralWISP
capable of sending neural information over a 1-m range
21
. Lee et. al describes an EnerCage-HC2 to inductively transfer
power to a 32-channel implantable neural interface
4
. Wireless power transfer though ensures longer experimental duration, one
has to take into account the exposure to high electromagnetic fields along with concerns regarding excessive heat dissipation.
Thus, it is evident that neural recordings are limited by size constraints and overall power consumption. This leads to the next
advancement in wireless biopotential recording with electro-quasistatic animal body communication.
Body Communication Basics
Body communication-based wearable technology has gained prominence over recent times as a communication modality for
sending real-time information.
Recent advances in using the human body as a channel for bio-physical communication has resulted in an energy-efficient
information exchange modality. HBC was first proposed as a method to connect devices on a Personal Area Network (PAN)
by Zimmerman et al.
23
, using a capacitively coupled HBC model where the return path is formed by the electrode to ground
capacitance. The transmitter capacitively couples the signal into the human body which is then picked up at the receiver
end. Galvanic coupling-based HBC introduced by Wegmueller et al.
24
, the signal is applied and received differentially by
two electrodes respectively. HBC utilizes the conductivity of the human body for a low transmission loss, high-efficiency
transmission modality making it ideal for energy-constrained devices. Traditional wireless body area networks (WBAN) use
EM signals that radiates outside the body all around us, resulting in only a fraction of the energy being received. This radiative
nature and high frequencies in WBANs are typically high energy and of the order of 10nJ/bit
25
. Now, if the body’s conductivity
is used, it provides a low loss broadband channel that is private (the full bandwidth is available for communication). This low
3/15
Animal Body Field Lines
Transmitter
Field Lines
Signal Plane
Ground Plane
Rs
CG_Tx
VIn CL
CCSG
+
Vo
a) b)
c) d)
Animal Body Circuit Model Human Body Circuit Model
Simplified ABC Model
HBC Transfer Function:
ABC Transfer Function:
Human Foot on Earth Ground
Animal Foot on
Conductive Plane
Rs
CSkin
CG_Tx
VIn
CL
RL
RSkin CB_CS
CFoot
RFoot CCS_G +
Vo
CF2
Rs
CSkin
CG_Tx
VIn
CLRL
RSkin
CBody
CSkin
RSkin
CG_Rx
+
Vo
Rs
Body Electrode :
Signal Plane
RFoot
CB_CS
CG_Tx
RLReceiver Unit
Load
Ground Isolated
Conductive
Surface
Wire
Connection to
Receiver
CL
Ground Plane
CCSG
DAQ: Receiver Unit
Signal
Earth Ground
CFoot
Vin
Figure 2.
Animal Body Communication model: a) Represents the field lines corresponding to the rat body and the transmitter
ground plane. b) The rat body couples to the conductive plane and the associated capacitances are depicted. The conductive
plane is ground isolated and forms the capacitance C
CSG
. The node consists of the ground plane which couples with the earth’s
ground to form the capacitive return path. c) The circuit model associated with the experimental setup is shown in the figure.
The simplified model of the animal body communication circuit shows that the output voltage is proportional to the conductive
plane to ground capacitance CCSG, return path capacitance CG_T X , and load capacitance CL. d) Simplified Human Body
Communication circuit model and transfer function as depicted by S. Maity22.
loss and wide bandwidth availability along with the low-frequency operation results in ultra-low power body communication
at 415 nW
26
as well as very low energy communication at 6.3pJ/bit
27
. Low-frequency HBC was not widely adopted due to
the high loss at these frequencies because of resistive (50
Ω
) termination
28
. Recently we demonstrated, by using capacitive
termination, the loss in the EQS region is reduced by a factor of >100, making it usable
27,29
. The first bio-physical model for
EQS-HBC was developed by S.Maity
22
and a detailed understanding of the forward path
30
and return path
31
was described.
EQS-HBC is presently the most promising low-power, low-frequency communication alternative for WBAN. It has also been
shown that the EQS-HBC adheres to the set safety standards32.
The state of the art in body communication has been restricted to human body communication. In this work we propose to
utilize the recent developments in the concept of body communication and apply it to the animal body for biopotential and
neural recordings, drastically reducing the size, weight, area, and power of the device. We propose a capacitive termination
EQS communication from a sensing node on the rat’s body and also device an experimental arena to pick up these EQS signals
most efficiently. This form of communication utilizes electro-quasistatic transmission through the conductive layers of the rat
below the skin surface. The skin is a high impedance surface while the inner tissue layers are conductive. The transmission of
the electro-quasistatic signals through the body with a capacitive return path at frequencies below 1 MHz ensures that the signal
is contained within the body.
Animal Body Communication - Biophysical Theoretical Model
As already established, Human Body Communication has been explored as a viable communication model, extending this
to an animal body allows for a low loss, efficient channel model, compared to the traditional wireless modalities currently
used. Figure 2
a
and
b
depicts the concept of Animal Body Communication, the rat body capacitively couples with the signal
4/15
SIMILAR
COMPONENTS
DESCRIPTION
Source Resistance (R
s)Source resistance of the
transmitter
Skin Layer Resistance
(
RSkin)
Rat Body Skin resistance varies
depending on the fur and other
factors
Skin Layer Capacitance
(
CSkin)
Depending on the skin layer
thickness, capacitance from the
signal electrode to the conductive
tissue layers of the rat body
Transmitter Ground to
Earth Capacitance
(
CG_Tx)
Return path capacitance from the
earth ground to the ground plane
on the transmitter
Load Capacitance (C
L)
Load capacitance of the receiver
probes
Load Resistance (R
L)
Load resistance of the receiver
probes
DISSIMILAR
COMPONENTS
ANIMAL BODY
COMMUNICATION
HUMAN BODY
COMMUNICATION
Capacitance to Conductive
Surface
(CB_CS)
Capacitance from the rat body to the
conductive surface
Does not exist in HBC
Body Capacitance
(
CBody)
Does not exist in ABC
Capacitance from the body of the
human to the earth ground
Foot Resistance (
RFoot)
Resistance of the rat foot to the
conductive surface
Does not exist in HBC
Foot Capacitance (
CFoot)
Capacitance between the foot of the rat
to the conductive surface, varies
depending on the distance of the foot
from the conductive surface
Does not exist
in HBC
Signal Plane Capacitance
(CCSG)
Capacitance from the conductive
surface to the earth ground
Does not exist in HBC
Receiver Ground to Earth
Capacitance (
CG_Tx)
Does not exist in ABC
Return path capacitance from the earth
ground to the ground plane of the
receiver
CIRCUIT MODEL COMPONENT COMPARISON BETWEEN ANIMAL BODY COMMUNICATION AND
HUMAN BODY COMMUNICATION
Figure 3. Comparison between Animal Body Communication and Human Body Communication circuit components.
plane. The transmitter placed on the body of the rat modulates this electric field to transmit OOK (On-Off Keying) sequences
corresponding to the sensed biopotential signal. The experimental arena is designed such that the animal moves around on
a conductive surface, which is isolated from the earth’s ground. This surface picks up the EQS signals coupled onto the
animal’s body and is received through ground-referenced receiver. Hence, the received voltage is inversely proportional to
the capacitance of the signal plane to ground (the less the capacitance the easier it is for the wearable device on the animal to
modulate the potential of the animal body and the surface). The circuit model for Animal Body Communication is described
in Figure 2
c
. At lower frequencies the skin impedance and the series body and foot impedance is negligible compared to
the capacitance between the signal plane and ground. Given the operation of ABC in the electro-quasistatic regime, these
impedances can be neglected in the computation of the channel loss. From the simplified circuit model, the output voltage
Vo
and the input voltage VIn are related as follows:
ZSkin,ZBod y,ZFoot 1
ωCCSG
Vo
VIn
=CG_T X
CG_T X +CCSG +CL
(1)
In Figure 2
c
and
d
we compare the animal body circuit model along with the established human body circuit model. The
output voltage is a function of
CCSG
as shown in equation 1for ABC while it a function on
CBody
in case of HBC. Body
communication-based systems heavily depend on the body surface and ground sizes. S. Maity describes the Bio-Physical
Model for HBC
22
, figure 3compares the ABC model with the HBC model. Traditional HBC systems have the human body
connected to a transmitter and a receiver placed on a different part of the body. The human body has a much larger surface
area when compared to an animal. In this ABC setup, the sensor node is placed on the body of the rat, while the receiver is
a large conductive plane. This large conductive plane ensures that the movement of the rat is not restricted, and data can be
continuously recorded. In contrast, in human body communication, the body is on the earth’s ground and there exists a trunk
path to ground. Due to this, the output voltage is affected by the body capacitance, unlike in the animal body setup. Figure
3illustrates the key components of HBC and ABC. The capacitance of the body varies from ABC and HBC due to the fact
that the ABC channel model consists of the additional conductive surface on which the rat is free to move. Another important
component is the rat foot impedance, in ABC the rat’s feet rest on the conductive surface.
CFoot
and
RFoot
change depending
on the position of the rat’s foot on the conductive plane.
In the human model, the received signal is collected from the body surface itself, thus the output voltage depends on the
capacitive return path of both the transmitter and the receiver. In the ABC model, the conductive surface is ground isolated
and connected to an oscilloscope which acts as the receiving unit. The transmitter couples to the floating body and the return
path capacitance
CG_T X
from the earth’s ground plane to the transmitter ground plane completes the loop, allowing for signal
5/15
transmission. The receiver in ABC is the oscilloscope signal probe, which can be modeled as the load capacitance
CL
in parallel
with the load resistance
RL
. This oscilloscope is earth ground referenced and hence eliminates the capacitive return path of
HBC. The low-loss in ABC coupled with low-carrier frequency communication (as a wire) enables ABC power consumption to
be much lower when compared to wireless communication modalities such as Bluetooth. This reduced power enables longer
duration experiments with small form factor devices.
Results
Animal Body Communication was explored as a new modality for the transmission of biopotential signals. The sensing and
transmitting devices are built using off the shelf components and consist of a communication module, a processing module,
a power source, and an interface to connect it to the rat body. Surface electrodes are placed on the skin surface of the rat,
after employing appropriate skin preparation techniques and then connecting the electrodes to the front end of the device.
Biopotential information is sensed and modulated for transmission, simultaneously transmitting the signal over Bluetooth
and through the body of the rat as Animal Body Communication. Bluetooth has long been used as a wireless communication
modality and widely cited in literature as a means to transmit biopotential information. In this work, we use this gold standard
of communication to compare the biopotential information received from the ABC transmitter and Bluetooth module. A
correlation analysis is performed to compare both signals. Experiments were performed on a rat to prove the feasibility of
Animal Body Communication.
Animal Body Communication Experimental Setup
The Animal Body Communication setup is tested on Sprague Dawley rats, experiments were performed on anesthetized rats.
In this study, capacitive coupling is used as a means to achieve Animal Body Communication. The details of the sensor node
are described in the methods section.
ABC AND BLUETOOTH RECEIVER UNIT ANESTHETIZING UNIT
SENSOR NODE
CONDUCTIVE PLANE
ANESTHETIZED RODENT
RA
Electrode
LA
Electrode
RL
Electrode
Experiment Setup Sensor NodeCasing
EXPERIMENTAL TEST SETUP
Figure 4.
Experimental Test Setup: The sensor node is placed on the rat skin surface and connected to the surface electrodes
(Right Arm (RA), Left Arm (LA), and Right Leg (RL) for EKG sensing. The rat’s feet are taped on a conductive copper plate,
the plate is then connected to a receiver for ABC transmission. The Bluetooth receiver and the ABC receiver are connected to
the computer for signal acquisition and processing. The setup also consists of the anesthetizing unit which delivers the
anesthetizing drug and oxygen to rat.
Anesthetized rats are placed on a non-conductive surface, the sensor node, in a casing, is placed on the rat skin surface
and patch connectors are used to connect to the surface electrodes. The feet of the rat are placed on a conductive copper plate,
6/15
signals are acquired using the sensing unit, then transmitted via Bluetooth to a receiver connected to a computer as shown in
Figure 4. Only the feet are connected to the conductive plane while leaving the body on a non-conductive surface. This depicts
a case when the rat moves in a cage with only the feet on the bottom plane. ABC happens through the transmission of OOK
sequences from the node through the body, to the conductive copper plate. These signals are picked up using an oscilloscope
connected to the conductive plane. The oscilloscope signal probe is connected to the conductive plane while the ground probe
is left floating. EKG signals are acquired using a three-electrode setup with the electrodes placed on the Right Arm (RA),
Left Arm (LA), and Right Leg (RL). The RL serves as the right leg drive, common to EKG recording systems. Additional
monitoring systems such as the anesthetizing setup and body vital measurement systems are present in this experimental setup
not part of the communication setup. This setup aims to mimic the setup as described in Figure 1. The copper plates act as the
conductive surface which in an awake recording setup will form the base on which the rat is free to move.
Time Division Multiplexing
Biopotential signal measurements require the body to be grounded to improve the CMR of the entire system. Grounding the
body eliminated the floating nature which is essential for body communication. Thus, to sense and transmit biopotential signals,
time-division multiplexing is used. Such multiplexing between each sensing cycle and transmission cycle ensures that surface
biopotentials can be sensed accurately and also transmitted via body communication.
SENSING TRANSMISSION SENSING TRANSMISSION
ABC Transmission
Bluetooth Transmission
5s 10s Cycle continues over time
RA : Right Arm
LA: Left Arm
RL: Right Leg
5s
10s
Single Lead
EKG Recording
Time (s) SENSING
Post Processed Sample: ABC Decoded EKG Signal
Post Processed Sample: Bluetooth Transmitted EKG Signal
Animal Body Communication Transmitted Data
-275128 -275128 -275128 -275128 -275128
Bluetooth Transmitted Data
a)
b)
Figure 5. Time multiplexed sensing and transmission cycles: a) Sensing cycles consists of acquiring the single-lead ECG
signal using three electrodes, Right Arm, Left Arm and Right Leg. ABC Transmission starts at the end of the sensing cycle and
has the same duration as the sensing cycle. Bluetooth transmission duration is almost twice the ABC transmission duration.
Upon the completion of both transmissions, the sensing cycle restarts, and this cycle repeats. b) Post-processing steps on ABC
transmitted sequence and Bluetooth transmitted sequence transmission
In the event of simultaneous sensing and transmission, given that the transmitter is placed on the surface of the body, the
sensing electrodes pick up the OOK sequences used in the transmission, resulting in a corrupted sensed signal. To avoid this,
sensing and transmission are time multiplexed. This technique is critical for body communication with surface biopotentials.
Figure 5
a
describes the time multiplexing cycles, data is sensed for a period of 5s followed by the transmission for 10s. The
transmission of ABC and Bluetooth occurs simultaneously, however Bluetooth sequences take longer to transmit due to packet
constraints resulting in a longer transmission time as compared to the sensing time. Following the transmission cycle, the
sensing cycle repeats. ABC data is sent as OOK sequences which are then demodulated and decoded to retrieve the EKG
sample as shown in Figure 5
b
. Bluetooth samples are transmitted as characters corresponding to the ADC codes, which are
then converted to corresponding samples to compare with the transmitted ABC signal.
Time Domain Correlational Analysis on Acquired EKG Signal
EKG signals are chosen for testing the animal body communication setup. The experiment was conducted on a total of 8
Rats over 2 months. This current set-up ensures continuous synchronized transmission of the biopotential signal from both the
Bluetooth module and the ABC transmitter.
As mentioned before, the signals are time-multiplexed allowing Animal Body Communication. The EKG signal is sensed
for a period of 5s followed by simultaneous transmission of ABC and Bluetooth. Figure 6shows the EKG sample comparison,
6
a
shows the Bluetooth and the ABC EKG data for a period of 0.6s, these two signals are overlaid in 6
b
, the PQRST peaks of
the characteristic EKG signal align, similarly, the data is compared for all 8 rats and correlation accuracies across each trial is
7/15
a)
b)
c)
d)
e)
f)
SENSING
PERIOD
ABC
TRANSMISSION
PERIOD
0.999
0.9961 0.9962
0.9999 0.9988
0.9965
0.9997 0.9993
0.98
0.985
0.99
0.995
1
CORRELATION COEFFICIENT
Figure 6.
Rat Electrocardiogram (EKG) Analysis: a) Bluetooth and ABC transmitted signal. b) Overlaid Bluetooth and ABC
transmitted signal data, depicting overlap of the EKG peaks. c) Complete 5s Bluetooth and ABC transmitted EKG Signal. d)
Time multiplexed ABC transmission cycles, 5s transmission time followed by a 10s wait time to allow for Bluetooth
transmission and next cycle sensing. e) Overlaid plots of EKG Signals from eight rats with correlational analysis between
Bluetooth and ABC transmitted signals. f) Time varying correlational analysis of one 5s sensing cycle of rat EKG signal.
depicted in 6
e
. The correlation accuracy for all the rats was seen to be > 99.5 %. In 6
c
, we can see the complete overlaid 5s
sample. Time multiplexing results in an ABC transmission period followed by a wait time for the completion of the Bluetooth
transmission and sensing. 6
d
depicts this time-multiplexed ABC data, with this cycle being continuous. 6
f
depicts the variation
of correlation across the entire 5s window, the correlation between Bluetooth and ABC is approximately 1 throughout the 5s
window depicting a reliable transmission system.
Effect of Distance of Foot from Conductive Surface to Received ABC Signal
A key component of animal body communication is the dependence of the received signal on body resistance and capacitance.
Variation of the distance of the foot from the conductive surface changes the magnitude of the received signal which then tests
the robustness of the system. Experimental analysis with only one foot on a conductive surface with varying distances shows
that even with the foot raised, OOK sequences can be picked up from the conductive strip. It is evident that as the distance
from the conductive surface reduces, the amplitude of the coupled signal increases. However, even at large distances, though
the signal amplitude is lower, the received Bluetooth and ABC signal can be decoded and display >97% correlation. When
the rat foot is raised above the conductive surface, the resistance becomes infinite, but the capacitance between the rat foot
and body, to the conductive surface exists as shown in Figure 2and this ensures the necessary path for transmission of the
signal. Since body communication works on capacitive coupling, even without complete contact with the conductive surface,
the OOK sequences couple to the conductive surface. It is highly unlikely that the rat would have all feet raised above the
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Reduction In Distance
Foot raised Foot Completely on Surface
(Foot Raised) Position 1 Position 6 (Foot Completely on Surface)
b)
a) c) d)
POSITION 1 POSITION 2 POSITION 3
POSITION 4 POSITION 5 POSITION 6
0.9783
0.9998
0.9931
0.9997
0.9911
0.9904
0.96
0.97
0.98
0.99
1
123456
VARIATION OF CORRELATION ACROSS DISTANCES
POSITION
Figure 7.
Effect of distance on received signal strength: a) Experimental setup to measure the effect of distance variation on
received OOK signal; In position 6, the rat foot is closest to the conductive surface and in position 1 the foot is furthest from the
conductive surface. b) Received OOK signals of different distance variations and the decoded signals for the different distances
c) Correlation between Bluetooth and ABC received signals as a function of distance, each depicting correlation accuracy
>97%. d) Variation in the amplitude of the received signal as a function of distance.
conductive plane, for a long time. In the event of improper contact with the conductive surface or when the rat jumps, it is
shown that the signals can still be received on the conductive plane and can be successfully decoded. For short burst errors, the
implementation of error-correcting codes can ensure robust transmission. In Figure 7the variation of the distance of the rat foot
from the conductive surface is shown, position 1 is furthest away from the conductive strip, while in position 6, the rat foot
is completed taped on the conductive surface. The amplitude of the received signal increases with the reduction in distance
however in all cases it can be seen that the sequences can be decoded and all show high correlation accuracies with Bluetooth.
Discussions
Capacitive coupling from the transmitter ground plane to the earth’s ground ensures the return path necessary for animal body
communication. The presence of a large conductive signal plate prevents the existence of such a capacitive path to ground. The
addition of a conductive plane connected to the receiver ground placed above the rat body provides the necessary return path.
The transmitter ground plane along with this floating ground plane forms the capacitance
CG_T X
. In the setup with a rat cage,
as shown in Figure 8
b
the top and bottom surfaces of the rat cage are made conductive, with the top plate connected to the
receiver ground, while the bottom plate, which acts as the signal plane is connected to the signal probe of the receiver. During
in vivo
tests, the ground plane consisted of a hand-held conductive plane above the anesthetized rat body. Only the feet of
the rat are connected to the signal plane, with a slot in the conductive plane to allow for the placement of the rat. Figure 8
a
describes the need for the addition of the conductive ground plane in a model rat cage. Similar to Figure 2
b
, the capacitive
coupling from the device ground plane to the external ground plane provides the necessary return path. The sensor node placed
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ABC Receiver
Ground Plane
Capacitive Return
Path CG_TX
Signal Plane
Signal Field
Lines
Transmitter
Field Lines
Receiver
Signal
Probe
Receiver
Ground
Probe
CB_CS
CG_Tx
CCSG
CG_Tx
Ground Plane
Ground
Isolated
Signal Plane
Transmitter
Ground
Plane
Wired Connection
to Receiver
Ground
Earth
Ground
CG_TX
Floating
Ground
Plane
Signal
Plane
Transmitter to
Floating
Ground
Plane
Capacitance
a) b)
Figure 8. Effect of an External Ground Plane: a) Experimental setup depicting the electric field lines from the transmitter
ground plane to the external ground plane; Capacitive return path from the transmitter ground to the external ground plane. b)
In vivo test setup with a hand-held external ground plane. The external ground plane is placed above the rat body similar to the
experimental setup, mirroring the top of a rat cage.
on the body of the rat has the transmitter ground plane on the top surface and the signal electrode touches the body of the rat.
The addition of this floating ground plane allows for the use of a large signal plane, providing a larger experimental arena for
the rat to move on without being limited by the loss of the signal return path.
Conclusion
To conclude, in this work we demonstrate a novel communication modality in the animal studies domain and show the use of
the animal body as a communication channel. Biopotential signals were acquired from the rat and transmitted using Animal
Electro-Quasistatic
Animal Body
Communication
Ultra Low Power
Transmission
50x reduction
Signal Transmission using
Wires
Tethered Systems
•Not limited by power
•Not limited by a battery,
limited by the stress on the
animal
•Long wires introduce bias
and irritation in animals
•High Bandwidth
•Small Devices but with long
wires
Signal Transmission using Radio Frequencies
EM/Radio Based Wireless Systems
•Large power consumption >10mW
•Duration typically several hours and with
large batteries, wireless power transmission
could increase the duration but with large coil
area
•Animals need to carry large battery packs,
causing stress
•Low Bandwidth
•Device size limited by battery and antenna
requirements. Large coils needed for wireless
power transmission increasing system size
Signal Transmission using Electro-Quasistatic
Body Communication
Animal Body Communication
•50x reduction in power
•Ultra low power permits perpetually recording
time with small form factor energy harvested
systems
•Small form factor devices and small batteries
limit the stress on the animals
•Low Bandwidth
•Very Small devices, small coils needed for
wireless power transmission due to low
power requirements
Parameters
•Power
•Duration of
Recording
•Bias and Stress
on Animals
•Bandwidth
•Size of Devices
1666
First EMG Signal
Recording
Francesco Redi - EMG in
electric eel [4]
1790
First Neural Recording
Luigi Galvani - Electrical
activity in the nervous system
of a frog [32]
1842
First EKG Signal
Recording
Carlo Matteucci –
Discovered the electrical
activity with each heart beat
in a frog [33] 1875
First EEG Signal
Recording
Richard Caton - EEG in
rabbits and monkeys [34]
1948
Wireless Radio Telemetry
Fuller and Gordon - Radio
inductograph for recording
physiological activity in
unrestrained animals [5]
1970
Wireless Power Transfer
Inductive power transfer for
biomedical devices and
implantables [35]
1986 - Present
Multi-Channel Recording
Multi-channel recording, radio
based wireless telemetry with
wireless power transfer in-sensor
analytics for smart devices [36,37]
Figure 9. Evolution of animal biopotential recordings5,6,33–38 ; Comparison of tethered, traditional wireless systems and
Animal Body Communication.
Body Communication. The theory and channel model for animal body communication was developed and a custom-designed
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sensor node was built and tested in vivo. The correlation accuracy between standard wireless transmission systems and ABC
was found to be > 99 % in these tests. The power consumption for Bluetooth transmission was observed to be 29.5 mW,
while the power consumption for ABC transmission was found to be 0.5 mW. This depicts a > 50x reduction in power. If a
custom-designed IC is built with only ABC transmission, the device size and power can be further significantly reduced, along
with the possibility to make these high bandwidth systems. The effect of variation of distance of the foot of the rat from the
receiver signal plane was observed and it is clear that reliable signals can be received even with improper contact or raised feet,
adding to the reliability of this communication channel. A modified test setup was explored as an additional technique to ensure
robust communication. While in this study, EKG was the chosen biopotential, it can be extended to neural signal acquisition
and transmission, where low power communication modalities are essential. In Figure 9the evolution of animal biopotential
recording was studied, the key differences between tethered, wireless and EQS-ABC was compared and it was found that
EQS-ABC can prove to be the next advancement in this domain, allowing for an ultra-low power, efficient channel model.
Methods
➢Sub-cubic inch sensor
node
➢Ultra Low power
Sensing
➢Ultra Low power ABC
Transmission
➢Bluetooth Transmission
for data comparison
a) System Architecture b) Detailed View c) Assembled View
Processing and
Communication Block Sensing Block
Matching
Network
ABC Electrode
NRF52840 SoC
Bluetooth
Tx
Bluetooth
Stack
Data
Buffer
SPI
Timer
PWM
Tx
Logic
Battery
EKG Electrode
ADS1298 SoC
Control
Logic
Analog Front End
SPI
Timer
Battery
Management
System
Buffer
ABC
Electrode
NRF52840
µC+BLE SoC
Antenna +
Matching Network
Battery
Management
ADS1298
AFE SoC
EKG
Connector
Figure 10. System Architecture of the custom-built node for biopotential acquisition through animal body communication
and Bluetooth Low Energy; a) Block diagram of the custom-built node, b) Functional blocks depicted on the actual device, c)
Custom node after stacking.
System Architecture
Size, weight, area, and power consumption of wireless recording devices have the potential to significantly affect animal
behavior and compromise the quality and length of recordings, thereby hindering scientific studies. Overcoming these obstacles
formed the core design objectives for the custom node for the acquisition of biopotential signals and wireless transmission
of data and resulted in the following initial specifications. Physical dimensions were constrained to one cubic inch, which
is sufficiently small to be placed on a rodent and large enough to house the various components. The net weight and power
consumption were capped at 50g, and 50 mW respectively. This posed a significant challenge since the analog front end for
sensing, micro-controller for computing, wireless communication for comparison purposes, power management, and animal
body communication had to be miniaturized and integrated into the device while meeting the power budget.
The system architecture as shown in Figure 10
a
can be broadly divided into three blocks, the custom-wireless signal
acquisition node, the Bluetooth receiver connected to the data logging system (computer), and the animal body communication
receiver. The custom node consisted of two vertically stacked custom-designed printed circuit boards (PCB) which were
populated with commercially available integrated circuits and discrete components. The top board in the stack contained the
micro-controller and Bluetooth System on Chip (SoC), along with the antenna and matching network on the top layer. The
bottom layer consisted of the power management system and charging connector. The analog front end was housed on the top
layer of the bottom stack, with the bottom layer serving as the electrode for animal body communication. The detailed view
and the assembled view of the sensor node is shown in as shown in Figure 10band 10crespectively.
A System on Chip (NRF52840, Nordic Semiconductors) which integrates an ARM Cortex-M4F micro-controller and a
Bluetooth 5.0 transceiver was selected to form the core of the node since it would minimize the device footprint and power
11/15
consumption. The on board 1MB flash memory and 256KB RAM was sufficiently large to store the sampled signals and
implement in-sensor analytics in the future. Power efficiency was further improved by utilizing the on-chip DC-DC converters.
The custom node collected the EKG signals from a zero-insertion force connector placed on the PCB. Signal conditioning
and sampling of the EKG signal was performed by another SoC (ADS1298, Texas Instruments). This analog front-end chip
incorporates a programmable gain differential amplifier and right-leg drive generation for conditioning EKG signals, which
were subsequently sampled at 500Hz by a 24-bit analog to digital converter. The SoC was programmed to optimize signal
acquisition quality and power consumption. The sampled signals were sent to the micro-controller through an on-chip Serial
Peripheral Interface.
The sampled data was stored in a buffer in the micro-controller until the transmission window started. The samples
were then converted to characters and transmitted as a string over Bluetooth after adding delimiters to differentiate between
subsequent samples. For Animal Body Communication, the sample was transmitted in its original 24-bit binary integer form
after creating packets by adding two bits (binary 1) at the start and end of the sample. Each bit in ABC was represented by
on-off keying, wherein a 500kHz, 50% duty cycle square wave was turned on (binary 1) or off (binary 0). ABC data was
transmitted at 25Kbps, which was significantly lower than the minimum required Bluetooth bandwidth of 45Kbps, which
excludes the overhead added by the Bluetooth stack.
The custom-designed node was packaged in a 3D-printed housing of dimensions 25mm x 25mm x 10mm, which is
equivalent to 0.39 cubic inches. It had a net weight of 20g and average power consumption of 28.5 mW (with Bluetooth
transmission for data comparison purposes) which resulted in approximately 20 hours of battery life. This is 19 times smaller
and has more than twice the battery life when compared to a commercial wireless unit (Bio-Radio). We expect a much longer
lifetime when the Bluetooth transmission is turned off and only ABC transmission is turned on. The power required for sensing
is typically orders of magnitude lower than the power required for communication, thus the system power is dominated by this
communication power. The ABC transmission power is 50x lower when compared to the Bluetooth transmission power and
this translates into an order of magnitude improvement in the device lifetime and reduction in the battery size.
The Bluetooth receiver was essentially another NRF52840 SoC connected via USB to the data logging system, which in
this case was a computer. This setup was used instead of the inbuilt Bluetooth device of the computer since it would be easier
to collate data from multiple transmitters.
The conductive signal plane is connected to the high impedance receiver probe. A computer-based oscilloscope, by
Pico Technologies, was used as the ABC receiver. The OOK sequences are sampled at 3.9 MSamples/s and collected for
post-processing.
Signal Processing
OOK sequences collected from the ABC receiver are sent to a computer for processing. Signals are first band-passed between
400kHz to 600 kHz with 80 dB attenuation software filters. Filtered sequences are demodulated using envelop detection
and thresholding. Sequences are then decoded using the start and stop bit followed by software error correction. Bluetooth
sequences in the form of ADC codes are converted to corresponding voltage values and compared to the received ABC signals.
Communication Protocols
•Time Multiplexed Data
As discussed earlier, a requisite for animal body communication especially while recording surface biopotential signals is
the need to time multiplex the sensing and transmission periods.
•Error-Correcting Algorithms
There is a possibility to bring in redundancy into the communication channel to ensure the robustness of this communica-
tion modality. We have shown that if the rat foot is lifted from the conductive surface, the received signal can still be
picked up by the receiver. The goal of this paper is to ensure that long term recordings of freely moving animals can be
obtained. To ensure that there is a successful transmission of data, error-correcting algorithms become a necessity.
Bi-modular Redundancy can be introduced by repeating packets over time. In the event of a jump or signal drop, repeated
packets ensure that the signal information is faithfully transmitted. This technique reduces the data rate due to the added
redundancy.
Block Codes a common error-correcting technique of encoding the data in blocks, such that the code is a linear
combination of the message and parity bits in a linear block code.
Surgery
All surgical procedures were performed under aseptic conditions at Purdue Animal Facility. 5% Isoflurane gas and oxygen
were used to anesthetize the rat in an induction chamber, followed by a continuous flow of 2.5 % Isoflurane gas with oxygen
12/15
delivered through a nose cone. The dosage of Isoflurane and the flow of oxygen is continuously monitored to ensure that the
rat does not respond to the toe pinch while still maintaining a steady breathing rhythm and observable pink extremities. A
heating pad is placed below the rat to maintain the body temperature and lubricating drops are added to the eyes of the rat to
prevent drying. The skin surface is shaved and cleaned for the placement of the surface electrodes. The device is placed on a
shaved surface on the belly of the rat with the signal plane touching the skin surface. The surface electrodes are connected to
the device using patch connectors.
All procedures were approved by the Institutional Animal Care and Use Committee (IACUC) and all experiments were
performed in accordance with the Guide for the Care and Use of Laboratory Animals. The experiments were closely monitored
and reviewed by Purdue Animal Care and Use Committee (PACUC).
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Acknowledgements
This work was supported by National Science Foundation CAREER Award (ECCS 1944602), Air Force Office of Scientific
Research YIP Award under Grant FA9550-17-1-0450 and by the NIH Stimulating Peripheral Activity to Relieve Conditions
(SPARC) program (OT2OD023847 and OT2OD028183). (The contents of this article do not necessarily represent the official
views of the NIH). The authors would like to thank Dr. Shovan Maity, graduated PhD student, Shramana Chakraborty, M S
Student, Jongcheon Lim, Umm E Hani Abdullah, David Yang, Arunashish Datta, Debayan Das, Mayukh Nath, PhD Students
as well as Visiting Scholar Gargi Bhattacharya at Purdue University for their immense co-operation and support during the
experiments.
Author Contributions
S. Sriram, S. Avlani, M P. Ward, S. Sen, conceived the idea, S. Avlani and S. Sriram designed the ABC sensor node. S.
Sriram conducted the theoretical analysis and performed the experiments under the guidance of M P. Ward and S. Sen. All
authors contributed to the drafting of this manuscript and have read and approved the final version of the manuscript.
Additional Information
Competing Interests: The authors declare no competing interests.
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