short-range remote powering for long-term implanted sensor...
TRANSCRIPT
Short-Range Remote Powering
for Long-Term Implanted Sensor Systems
in Freely Moving Small Animals
Enver G. Kilinc⋆, Franco Maloberti⋄, and Catherine Dehollain⋆
⋆ RF–IC Group, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland⋄ Integrated Microsystem Laboratory, Universita degli Studi di Pavia, Pavia, Italy
Email: [email protected], [email protected], [email protected]
Abstract—The paper presents comparison of different powersources for implantable biomedical systems. It is shown that nearfield wireless power transfer is a suitable technique for short-range remote powering compared to other options. The required2.8 mW is received wirelessly via an optimized inductive linkdriven by an efficient class-E power amplifier at 13.56 MHzfrom a 3 cm distance with 21% power efficiency including drainefficiency of power amplifier. An integrated rectifier and voltageregulator generate 1.8 V supply voltage from received AC signal.The integrated powering circuits are fabricated in a 0.18 umCMOS technology. The implantable micro-system weighs 1.05 gand the overall size is 12x12x2.3 mm3. Measurement results showthe capability of the implantable system in freely moving smallanimals.
I. INTRODUCTION
Technological advances in microelectronics enable produc-
tion of implantable patient monitoring devices for personalized
therapy [1]. These devices acquire the vital parameters through
sensors in a patient and transmit the data to a server, hence
an expert can access acquired data at any moment. In order to
achieve a fully-implantable monitoring device, transcutaneous
cables should be replaced with the remote powering systems.
The remote powering systems transmit the required power
from a source to an implanted device without infection unlike
the transcutaneous wires. Therefore, the remote powering is a
promising solution for implantable monitoring devices.
Most of the therapies are developed by using small lab-
oratory animals [2]. The small animals, especially rodents,
are preferred in research activities due to their small size
and reproducibility. However, the small size brings out dif-
ficult challenges for designing of implantable device. The
implantable device must be small and light-weight to be placed
in a small animal. Consequently, the implantable device should
be batteryless and remotely powered to reduce the overall size
and weight.
During monitoring phase, the animals must be in a natural
living space otherwise the monitoring data will be unreliable
due to the stressful environment. In addition, the animal must
be in conscious and move freely in order to obtain reliable
data [2]. The power transmission to a freely moving animal
in a natural living space is a demanding task. The power must
be transferred to the implanted device continuously according
to the animal position in the living space. The wireless power
A) Vibration
B) Light
C) Biofuel D) Thermal
Fig. 1. Power harvesting methods for biomedical systems. Images from [3],[4], [5], [6].
transmission can stop due to the misalignment of the external
and implanted units and the implanted unit cannot continue
its operation. Therefore, the suitable remote powering system
should be designed. This paper presents short-range remote
powering for batteryless fully-implantable sensor system for
long-term duration.
II. AVAILABLE POWER SOURCES FOR IMPLANTABLE
BIOMEDICAL SYSTEMS
There are several power harvesting or transfer methods
which are possible power sources for implantable systems. Ta-
ble I compares the performances and summarizes advantages
and drawbacks of different power sources for implantable
biomedical systems. The power harvesting methods use re-
newable energies such as solar energy, kinetic energy, thermal
energy, and biofuels. Therefore, these sources are used in
self-powered systems with long life. However, these sources
are insufficient for the implanted systems demanding high
power due to their low energy conversion efficiency per
area. Additionally, the conversion efficiency changes with
available environment conditions such as light conditions,
temperature, etc. The power harvesting using piezoelectric
materials is a solution for self-powered implantable systems
[3]. However, the power harvesting from vibration is sensitive
978-1-4673-4642-9/13/$31.00 ©2013 IEEE
A) Far field B) Ultrasound
C) Near field
Fig. 2. Power transfer methods for biomedical systems. Images from [7],[8], [9].
to amplitude of input force and needs electro-mechanical
devices and special fabrication which increases the cost. The
implantable systems need to be placed inside the body and
powered continuously for long duration. Although the thermal
and optical power harvesting methods are used as a power
source in the biomedical systems, they need high temperature
difference and intensive light [4], [5]. However, the body
temperature is almost constant and the light penetration on the
body tissues is weak. Therefore, the thermal and light power
sources are incapable for power harvesting and performing
the implantable system inside the body. In the biofuel cells,
the current is generated by the implanted electrodes during
oxidation of compounds in the body such as glucose [6].
Although the biofuel is a convenient source for self-powered
deeply implants, the generated power is limited with a few
micro-watts. In addition, the cell has low-voltage output and
is sensitive to the environment conditions.
The remotely powered implantable systems can also be
performed by power transfer. The power can be transferred to
long distance by using electromagnetic waves. Although the
receiver antenna can be small as mm range and suitable for
implantable sensor system [7], the power transfer efficiency
is very low and the body tissue lost is high compared to
other power transfer methods. Therefore, the transmitter output
power level increases up to a few watts. Ultrasound is a
suitable option for power transfer to deeply implants in the
body [8]. The power transfer efficiency of ultrasound systems
is very high in the water but not in the air. Therefore, the power
transferred from a patch transmitter which has a contact with
body. Near field power transfer is an appropriate method for
short-range remote powering systems. The power is transferred
with a high power transfer efficiency by magnetic field [9].
III. IMPLANTABLE SENSOR SYSTEM & BUILDING BLOCKS
Many treatments can be achieved by long-term monitoring
the several compounds on the body. Especially, the person-
alized therapy is becoming widespread due to advances in
12 mm
12m
m
Implanted
Coil
Electronics
& Magnet
Multi
Sensors
SKIN
DATA
869 MHz
Receiver Oscillator
VVDD
Power
AmplifierPowering Coil
13.56 MHz
Implanted CoilFull-Wave
RectifierVrect
VGND
Vreg
Voltage
Regulator
Sensor
SystemTransmitterTransmitter
Electronics
& Magnet
Multi
SensorsImplanted
Coil
POWER
E
X
T
E
R
N
A
L
I
M
P
L
A
N
T
Fig. 3. Block diagram of batteryless implantable multi-sensor system.
micro-nano-bio technologies. These advances allow to in-
tegrate different technologies and produce small and low-
power implantable devices for the patients. The small size is
important for the comfort of the patient. Additionally, small
size is more crucial for implanting the devices inside the
laboratory animals for test and development. The low-power
electronics are needed to perform the system with available
power sources without using battery.
In order to achieve the personalized therapy, long term mon-
itoring of several compounds and vital signals is required [1].
Therefore, multi-sensor system is essential to obtain different
measurements by using the same implanted device. Fig. 3
illustrates the conceptual design of the batteryless implantable
multi-sensor system. The implantable multi-sensor system is
a 3-layer platform. All the sensors are placed on the top of
the platform to have contact with body. The sensors are com-
posed of electrochemical sensors to measure amount of drug
and glucose. Additionally, temperature and pH sensors are
also needed to calibrate the electrochemical sensors properly.
The integrated electronics and some off-chip components are
placed in the middle layer. The electronics consist of remote
powering, power management, programmable analog front
end for chemical sensing, controller, and data communication
circuits. Moreover, a permanent magnet is also located in the
middle layer of the implanted device. The magnet helps to
track the animal in a living space. Finally, the implanted coil
is fixed at the bottom of the platform. The implanted coil
allows to induce current from available magnetic field.
The power is transferred by magnetic field over an opti-
mized inductive link for 30 mm distance. The powering coil is
driven by a class-E power amplifier with a high drain efficiency
at 13.56 MHz frequency. The generated magnetic field induces
current on the implanted coil. The integrated powering circuits
convert the received AC signal to a DC supply with high
efficiencies [10]. A passive full-wave rectifier is followed by
a high-speed voltage regulator to suppresses the ripples on the
TABLE IPOWER SOURCES FOR IMPLANTABLE BIOMEDICAL SYSTEMS
Source Year Performance Distance Size Advantages Drawbacks
(µW/cm2) (cm) (cm2)
Vibration [3] 2005 11.03 — 1 Self-powered, long life Sensitive to load & input, expensive
Light [4] 2011 12.59 0.2 2.1 Long life, MRI compatible High tissue loss, sensitive to light condition
Thermal [5] 2007 20 — 12.9 Self-powered, long life Low output voltage, sensitive to ambient cond.
Biofuel [6] 2012 30 — 0.25 Deeply implant, self-powered Sensitive to load & ambient conditions
Far Field [7] 2012 1631 5 0.03 Long distance, small antenna Low efficiency, high tissue loss
Ultrasound [8] 2012 10191 7 0.79 Deeply implant High loss in air
Near Field [9] 2012 45195 5 1.54 High performance Short distance
supply voltage and creates a clean 1.8 V DC supply.
The data is collected from the body by the electrochemical
sensors and transmitted by OOK modulated transmitter at 868
MHz frequency. The transmitter is composed of a free running
LC oscillator and an off-chip inductor served as transmitter
antenna for short-range communication. Although the power
consumption of the transmitter is reduced compared to a
transmitter with a PLL block, the oscillator has low-stability
in the frequency domain. The resonance frequency of the LC
tank fluctuates due to the temperature change and transistors
behaviors. Therefore, a custom design receiver is required
to detect the transmitted signal. The wide-band receiver is
designed for communication with implanted transmitter.
The implanted system should be also batteryless to reduce
the size and weight. In addition, the battery needs to be
replaced at the end of its life time which is insufficient to
obtain long-term application. Therefore, a servo-controlled x-
y rail system (IRPower system [11]) tracks the animal on the
living space and delivers the power to the implant efficiently
[12]. Fig. 4 shows the Intelligent Remote Powering (IRPower)
system for implantable systems in freely moving animals for
continuous long-term applications. IRPower system tracks the
magnet placed in the implanted system as shown in Fig. 3
by the magnetic field sensors and moves the powering coil
according to the position of the magnet. Accordingly, the
animal is tracked in the living space and the power is delivered
continuously to the batteryless implanted system.
The power demanded by the implanted system changes
due to the number of the sensors measuring simultaneously.
Therefore, the power level of the implanted system is tracked
to avoid interrupts during measurement and the transferred
power level to the implanted system should be adjusted by
a feedback loop. The rectifier output is compared with a
reference voltage and the power feedback data is transmitted
to the external unit [12]. If the rectifier output drops under
a certain voltage, the supply of power amplifier increases,
hence the transferred power level increases. Conversely, if the
rectifier output increases more than the reference voltage, the
power amplifier decreases the output level not to damage the
implanted system.
IV. MEASUREMENT RESULTS
The capability of the remotely powered implantable micro-
system is verified by the measurements. Most of the power is
lost during the wireless power transfer due to weak coupling
Y-axis rail
X-axis rail
Cage
Magnetic
Field Sensor
Implanted Unit
with Magnet
Powering Coil
Fig. 4. IRPower system for implantable systems in freely moving animals.
between the coils. Therefore, the remote powering link should
be optimized to achieve an efficient power transfer [13]. The
remote powering link is optimized and the power is transferred
over the inductive link is composed of 80x80 mm2 powering
and 12x12 mm2 implanted coils. The powering coil is driven
by an efficient class-E power amplifier at 13.56 MHz with 21%
power transmission efficiency including the drain efficiency
of power amplifier. The implantable system receives 2.8 mW
from 3 cm of separation distance.
The integrated powering circuits convert the induced AC
signal to 1.8 V supply voltage. The passive full-wave recti-
fier operates with 80% of power efficiency for 2 mW load
condition. The voltage regulator has -60 dB of power supply
rejection at DC and also operation frequency. The data rate
of the integrated OOK transmitter increases up to 1.5 Mbit/s.
The power consumption of the transmitter is 320 µW at 1.8
V supply voltage.
IRPower system moves the powering coil faster than the
TABLE IISHORT-RANGE REMOTE POWERING SYSTEMS FOR BATTERYLESS IMPLANTABLE BIO-SYSTEMS IN SMALL ANIMALS
Reference Year Application Freqency Efficiency Distance Power Size Weight
(MHz) (%) (cm) (mW) (cm3) (g)
Cong et al. [15] 2010 Blood pressure 4.0 N/A 1 0.3 0.17 0.33
Chang et al. [16] 2013 Neural recording 13.56 N/A N/A 20 mA 15 cm2 13.17
Hsieh et al. [17] 2013 Long-term monitoring 0.125 N/A N/A 12 8.4 8.33
This work 2013 Multi-bio sensors 13.56 21 3 2.80 0.33 1.05
Assembled Implantable
Micro-System
Implanted Coil
Permanent Magnet
Circuit Board 5 mm
2 mm
12 mm
12mm
12 mm
12mm
IC and SMCs
Fig. 5. Remotely powered implantable micro-system for freely moving smallanimals.
freely moving mice which allows to track the animals precisely
in the living space. Accordingly, the power is transferred to the
implanted sensor system continuously. In addition, the power
feedback control loop keeps the power level of the implanted
system at a certain level. The output of the rectifier is kept
around 2.27 V by the power feedback loop.
The overall weight and size of the implantable system is
crucial for small animals especially mice. The implant must be
10% or less of the body weight of the animal [14]. Fig.5 shows
the implantable micro-system for small animals. It weighs
1.05 g and consumes 0.33 cm3 (1.2x1.2x0.23 cm3) which is
applicable to implant in small animals especially mice. Table
II summarizes the remote powering performances of this study
and compares with recent studies.
V. CONCLUSION
Different power sources for implantable biomedical systems
are discussed. The pros&cons and performances of the sources
are presented. The most appropriate solution for short-range
remote powering is near field power transfer. The implantable
sensor system demands for 2.8 mW power. The power is
transferred by an optimized inductive link driven by an ef-
ficient class-E power amplifier at 13.56 MHz from a 3 cm
distance with 21% overall power efficiency. The integrated
powering circuits generate 1.8 V supply voltage from received
AC signal. The circuits are fabricated in a 0.18 um CMOS
technology. The implantable micro-system weighs 1.05 g and
the overall size is 1.2x1.2x0.23 cm3 compatible for implanting
inside to small animals.
ACKNOWLEDGMENT
The authors thank to M.A Ghanad and G. Conus for their
valuable feedback and help. Also, the authors are grateful to
X. Warot and R. Doenlen from Life Science department, EPFL
for their supports on rodent animals. This project is supported
by Swiss National Funding (SNF) through Sinergia Initiative.
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