Download A Wireless ECG Recording System for Small Animal Models of Heart

A Wireless ECG Recording System for Small Animal Models of
Heart Regeneration
Hung Cao1, Yu Zhao2, Ammar B. Kouki1, Yu-chong Tai3 and Tzung K. Hsiai4
1
Department of Electrical Engineering, ETS Montreal, QC, Canada
2
SIAT, Chinese Academy of Sciences, Shenzhen, China
3
Department of Electrical Engineering, Caltech, Pasadena, CA, USA
4
Department of Bioengineering, UCLA, Los Angeles, CA, USA
Abstract — Heart failure afflicts the developed world, causing
mortality more than any other diseases. This is due to the fact that
humans’ heart possesses a very limited capacity to regenerate.
Heart attacks or myocardial infarction (MI) could result in an
irreversible loss of cardiomyocytes and consequently heart failure.
Besides, zebrafish and neonatal mice are well-known for their
magical capacity to recover after ventricular amputation, thus
becoming precious models for heart regeneration studies. In this
work, we report the first wireless electrocardiography (ECG)
recording system for small animal models of heart regeneration.
The system consists of a microelectrode array (MEA) and
electronic components for wireless powering, signal processing
and data communication. The MEA is based on a biocompatible
and flexible polymer so it could conform to non-planar anatomical
surfaces. The power transfer is achieved using inductive coupling
between two solenoids and the ECG signals are sent through an
optical link. The wireless operation can free the animal,
eliminating anesthesia during experiments and thus minimizing
unwanted side effects. The first generation of the device was
demonstrated successfully with neonatal mice, revealing awake
ECG signals with all features, thereby paving the way to
physiologically investigate heart regeneration in long-term
without disrupting the animals’ normal activities.
Index Terms — Wireless ECG, flexible MEA, zebrafish,
neonatal mice, inductive coupling, heart regeneration.
I. INTRODUCTION
Understanding heart regeneration in a vertebrate model
system is highly important to public health. Heart failure has
been the leading cause of morbidity and mortality in the
developed countries due to failure to adequately replace lost
ventricular myocardium from ischemia-induced infarct. Adult
mammalian ventricular cardiomyocytes have a limited capacity
to divide, and this proliferation is insufficient to compensate the
significant loss of myocardium from ventricular injury [1].
Unlike adult mammals, zebrafish (Danio rerio) are well known
for their regeneration capacity after 20% ventricular amputation
[2]. Further, Porrello et al. recently discovered the transient
regenerating capacity of 1-day-old neonatal mice after birth, but
this capacity is lost by 7 days of age [3]. Therefore, these
become precious models for drug discoveries and
investigations of heart regeneration.
Although imaging means were typically used to characterize
heart injuries and the regeneration process phenotypically and
genetically, electrocardiogram (ECG) recording using
microelectrodes could also be useful to elucidate changes in
cardiac-conduction functionalities of injured myocardium [4].
With the zebrafish model, it has been shown that ventricular
repolarization (ST intervals and T waves) failed to normalize
despite fully regenerated myocardium at 60 days post
ventricular amputation, suggesting further cardiac remodeling
may be required to fully integrate regenerating myocardium
with host myocardium [5]. However, the current ECG
recording methodology used with the animals requires
pharmacological sedation which affects the cardiac information
[6]. Therefore it is highly desirable to have a long-term
wearable and wirelessly-operated ECG recording system to
collect data continuously from non-anesthetized animal
models. Nevertheless, the small size and irregular movements
of the aforementioned small animals are challenging issues for
implementing such a system. Therefore, we aim to address and
target (1) Surface-conformable recording electrodes with
biocompatibility and flexibility; (2) A proper mechanical
fixture that can secure the device for stable and long-term
recording; and (3) A recording circuitry operated on wireless
power instead of battery for continuous and long-term
measurement. Furthermore, the integrated and packaged
system needs to be compact and light-weighted in order to bring
comfort to the animals and avoid unwanted side effects.
In this work, we implemented flexible polymer-based
membranes containing gold microelectrodes using traditional
micro-fabrication technologies to target continuous wireless
ECG acquisition in small animal models. The signal processing
circuitry was designed to be assembled within the membrane.
Wireless power transfer was done by inductive coupling
between 2 solenoids while wireless data communication was
optically achieved to accommodate the under-water situation in
the case of zebrafish. The block diagram and the conceptual
design of our system are illustrated in Fig. 1. A proof-ofconcept system was prototyped on a printed circuit board (PCB)
using off-the-shelf components for the demonstration with
Fig. 1. (a) The block diagram and (b) the conceptual design of the
wireless ECG system using 2 solenoids.
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neonatal mice. Awake ECG signals were obtained with
comparable signal-to-noise ratios and the rhythm was different
with that recorded with sedated animals as expected.
II. DESIGN AND FABRICATION
A. MEA fabrication
Parylene C was chosen as the base and insulating material
due to its well-suited combination of biocompatibility and low
permeability against water, gases and ions. Using conventional
micro-fabrication techniques, we fabricated gold electrodes
(0.2/0.02 μm of Au/Ti) sandwiched between two layers of
parylene C with exposed recording cites and connecting pads.
A MEA was picked to record ECG signals instead of one single
electrode in order to obtain the site-specific ECGs [4], similar
to the standard 12-lead ECG used for humans. Several electrode
configurations in shape, size and spacing were implemented in
order to optimize for a specific application. Using impedance
analyses, MEAs of four round recording electrodes were
chosen with diameters of 200-300 µm. The reference electrode
was placed apart from the recording electrodes. Low-power
oxygen plasma was applied to roughen the microelectrode
surface, allowing for an increase in the effective contact area
and a decrease in the interface impedance. To facilitate longterm operation, the membrane was coated with the
biocompatible silicone to match with the Young’s modulus of
the animal skin. The membrane was measured ~10 μm in
thickness and less than 0.5 mg in weight. In the case of
zebrafish, silicone wing components were added to support
long-term implantation by wrapping around the fish body and
applying medical epoxy at the two ends on the dorsal side (Fig.
2a and 2b).
zebrafish or a small cage housing a neonatal mouse within the
coil (Fig. 1b). A miniaturized and high-quality-factor receiver
power coil was constructed by winding the 30/48 Litz wire
around a Ni/Zn ferrite core to establish high magnetic
permeability and low electrical loss. It had 10 turns, an outer
diameter of 1.2 mm, an inner diameter of 0.7 mm, and a length
of 4 mm. At 10 MHz, an inductance of 143 nH and a quality
factor of 23.6 were achieved. The overall power transfer
efficiency was measured, revealing a constant efficiency of
1.2% in the frequency range 9-15 MHz (Fig. 2b); thus, the
operating frequency of 11.1 MHz was selected. The external
unit had an optical detector to acquire the ECG data which were
stored and displayed in a computer.
B. The signal processing circuitry and wireless power link
C. The first-gen device
The circuitry on the animal side was designed to be
assembled within the parylene membrane. Gold traces were
fabricated for circuit routing and conductive epoxy was used
instead of soldering. Packaging was achieved by casting
silicone. The signal amplification and filtering were critical to
address the small ECG signal strength (<150 mV) and frequency
range (2-125 Hz). The signal processing circuitry consisted of
a two-stage amplifier and a bandpass filter prior to transmitting
the signals to the data receiver. Signals with frequency below 2
Hz and above 125 Hz were filtered out. The circuit
communicated with the external unit via an Infrared Light
Emitting Diode (IRLED).
To operate the entire system, a DC voltage of 1.8 V or above
and an average power of 300 µW were required. A 2-coil
inductive link coupled in a solenoidal configuration was
designed to provide power continuously. This would guarantee
stable wireless powering, eliminating the misalignment issues
when using parallel planar coils [7]. The transmitter coil had 20
turns, a diameter of 3 cm and a length of 6 cm, resulting in an
inductance of 6.62 µH. With those design parameters, there
would be enough space to put a water tube containing a
The first generation of the system was implemented for uses
with neonatal mice. The signal processing and wireless
communication circuitry were prototyped on a PCB including
two-stage amplification, band-pass filtering, inductive power
transfer, and optical data transmission. Surface-mount
components were utilized instead of bare-dies in the case of
zebrafish. The ECG signals were passed through a first-stage
instrumental amplifier (INA333 from Texas Instrument) to
provide a high input impedance, and then to an operational
amplifier (OPA333 from Texas Instrument). The MEA was
manually assembled to a customized zero-insertion-force (ZIF)
cable end so that it could be connected to the PCB (Fig. 2d).
The solenoids and optical data transfer part were implemented
as mentioned in the previous section.
Fig. 2. (a) Zebrafish with an attached MEA membrane. (b) Power
transfer efficiency analysis of the receiver coil (inset). (c)
Conceptual design of the device. (d) First-gen PCB prototype.
III. EXPERIMENTS AND RESULTS
The animal experiments were performed in compliance with
the Guide for the Care and Use of Laboratory Animals of the
National Institutes of Health. For wireless ECG recording,
neonatal mice aged between 1 to 7 days old were positioned on
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a baseplate without sedation. The PCB was first gently fixed on
the abdomen of the mouse by medical tapes, followed by
adhering the MEA membrane to the chest. The mouse lying on
baseplate was then placed inside the power transmitting coil.
When the power was activated, real-time wireless ECG signals
were acquired (Fig. 3). The distance between the IRLED and
the photo-detector was about 3 cm.
Reproducible trials on neonatal mice were performed to
reveal distinct ECG signals in the presence and absence of
pharmacological sedation. In Fig. 3, a comparison in ECG
signals between the non-sedated and sedated conditions
highlights the distinct heart rates and ECG repolarization
patterns. A reduction in the heart rates by 50% was noted in the
presence
of
sedation
(80
µg/gr
body
weight
Ketamine/xylazine). With the mechanical interference during
recording and the signal attenuation during transmission, the P
waves, QRS complexes and T waves remained distinct despite
a reduction in SNR without sedation (Fig. 3b).
IV. DISCUSSIONS AND CONCLUSIONS
The small size and constant movements of the small animals
remain a challenge to implement a wireless ECG system. Here,
we developed conformable flexible sensors with dry-contact
MEA membrane, enabling secure ECG recordings in long term.
For the first time, with the wireless feature, we have provided
electrophysiological signals from a neonatal mouse model of
heart regeneration in the absence of sedation. The capability to
monitor intrinsic heart rates and electrical signals without
exogenous neurologic influence opens a new road for cardiac
and neurologic dug screening, disease modeling, and toxicity
studies. With further miniaturization, the validated wireless
ECG recording through the dry-attachable microelectrode
membrane and its variations can acquire electrical phenotypes
from conscious, freely-moving small animals without
interfering with the intrinsic heart rates, causing animal stress,
and predisposing to infection and injury.
The recent advances in stem cell-based therapy open longterm strategies to address heart diseases. Thus, our wearable
and wireless ECG monitoring system would further offer a noninvasive validation tool to address maturation and integration
of the stem cell-derived cardiomyocytes. With the development
of flexible and stretchable electronics as well as wireless
technology [8, 9], our electrode membrane could be further
utilized not only for pre-clinical animal models of regenerative
medicine but also in addressing health monitoring for
personalized medicine and telemedicine.
ACKNOWLEDGEMENT
The studies were supported by the National Institutes of
Health R01HL-083015 (T.K.H.) and R01HL111437 (T.K.H.).
Fig. 3. ECG signals of (a) a sedated mouse and (b) an awake
mouse using our system. The inset shows a full-feature ECG
obtained by our wireless system.
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978-1-4799-8275-2/15/$31.00 ©2015 IEEE