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Charvet IEEE 2011 .pdf



Nom original: Charvet_IEEE_2011.pdf
Titre: WIMAGINE: A Wireless, Low Power, 64-Channel ECoG Recording Platform for Implantable BCI Applications
Auteur: Guillaume Charvet

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FrD1.4

Proceedings of the 5th International
IEEE EMBS Conference on Neural Engineering
Cancun, Mexico, April 27 - May 1, 2011

WIMAGINE: A wireless, low power, 64-channel ECoG recording
platform for implantable BCI applications
G. Charvet, M. Foerster, S. Filipe, J. Porcherot, J.F. Bêche, R. Guillemaud, P. Audebert, G. Régis, B.
Zongo, S. Robinet, C. Condemine, Y. Têtu, F. Sauter, C. Mestais, A.L. Benabid
Abstract_NER2011_MF-GC_V13.doc

Abstract— WIMAGINE is a wireless low power 64-channel
data acquisition system designed for Electrocorticogram
(ECoG) recording. The system is based on a custom integrated
circuit (ASIC) for amplification and digitization and
commercial components for RF transmission. It supports the
RF transmission of 32 ECoG recording channels (among 64
channels available) sampled at 1 kHz per channel with a 12-bit
resolution. The RF communications use the MICS band
(Medical Implant Communication Service) at 402-405 Mhz and
can achieve a data rate of 480 kbps with a bit-error rate of less
than 1.5 x 10-10. The device is powered wirelessly through an
inductive link at 13.56 MHz able to provide 100mW (30mA at
3.3V). This integration is a first step towards an implantable
device for BCI (Brain Computer Interface) studies and neural
prosthetics. Here, we present the platform architecture,
components and the first in vivo validations.
I.

T

INTRODUCTION

he term “BCI”, (Brain Computer Interface) refers to

methods and technological devices to bridge the central
nervous system with the external world, allowing the brain to
either receive sensory inputs from sensors or to send orders
to effectors. The global strategy is to record neuronal
activity, and to recognize specific patterns, which could be
Manuscript received December 17, 2010. The development of WIMAGINE
Plateform was supported by French government Carnot funding.
G. Charvet is with the CEA/LETI/CLINATEC, MINATEC Campus, Grenoble, France
(e-mail: guillaume.charvet@cea.fr).
M. Foerster is with the CEA/LETI, MINATEC Campus, Department DTBS, Grenoble,
France (e-mail: michael.foerster@cea.fr).
S. Filipe is with the CEA/LETI, MINATEC Campus, Department DTBS, Grenoble,
France (e-mail: sabine.filipe@cea.fr).
J. Porcherot is with CEA/LETI, MINATEC Campus, Department DTBS, Grenoble,
France (e-mail: jean.porcherot@cea.fr).
J.F. Bêche is with the CEA/LETI, MINATEC Campus, Department DTBS, Grenoble,
France (e-mail: jfbeche@cea.fr).
R. Guillemaud is with the CEA/LETI, MINATEC Campus, Department DTBS,
Grenoble, France (e-mail: regis.guillemaud@cea.fr).
S. Robinet is with the CEA/LETI, MINATEC Campus, Department DSIS, Grenoble,
France (e-mail: stephanie.robinet@cea.fr).
P. Audebert is with the CEA/LETI, MINATEC Campus, Department DSIS, Grenoble,
France (e-mail: patrick.audebert @cea.fr).
G. Regis is with the CEA/LETI, MINATEC Campus, Department DSIS, Grenoble,
France (e-mail: guillaume.regis@cea.fr).
B. Zongo was with the CEA/LETI, MINATEC Campus, Department DSIS, Grenoble,
France.
C. Condemine is with the CEA/LETI, MINATEC Campus, Department DSIS,
Grenoble, France (e-mail: cyril.condemine@cea.fr).
Y. Têtu is with the CEA/LETI, MINATEC Campus, Department DSIS, Grenoble,
France (e-mail: yann.tetu@cea.fr).
F. Sauter is with the CEA/LETI/CLINATEC, MINATEC Campus, Grenoble, France
(e-mail: fabien.sauter@cea.fr).
C. Mestais is with the CEA/LETI/CLINATEC, MINATEC Campus, Grenoble, France
(e-mail: corinne.mestais@cea.fr).
A.L. Benabid is with the CEA/LETI/CLINATEC, MINATEC Campus, Grenoble,
France (e-mail: alim-louis.benabid@cea.fr).

978-1-4244-4141-9/11/$25.00 ©2011 IEEE

used to command prostheses. These patterns must be
detected amongst complex neural activity, recorded in a
noisy and busy environment. For this purpose, neuronal
[1]
activity is being recorded using scalp electrodes , epidural
[2]
[3][4]
electrodes , subdural electrodes
or microelectrode
[5][6][7]
. Records are analyzed with a variety of
arrays
algorithms, most of them aimed at classifying data into two
classes of event-related electrical patterns, the detection of
which triggers effectors such as moving a cursor on a
[1][6]
[8].
screen
or activating a motorized device
Experiments
have been performed on rodents, non-human primates and in
human patients, taking advantage of already implanted
[4][9]
electrodes for diagnostic or therapeutic purposes
.
However, bringing a BCI out of the laboratory into real-life
application remains a challenging task. BCI systems used by
subjects suffering from severe motor disabilities and living in
a natural environment must be reliable, stable and
autonomous. Systems should be activated without any
external help, whenever needed by users and for lengthy
periods.
To address this challenge, CEA/LETI/CLINATEC® is
currently conducting a project to develop a neuroprosthesis,
based on electrocorticogram (ECoG) measurement in order
to compensate motor deficits in tetraplegic patients. The
cortical surface of the brain has been selected to record the
neuronal signals because electrocorticogram recordings
require less invasive electrodes than deep brain
measurements, and provides a better spatial resolution and a
higher signal to noise ratio than scalp EEGs.
The present study is a first step towards this BCI project
which aims to give a patient the ability to control a large
number of degrees of freedom. The next step is therefore the
development of an implantable medical device for real time
multi-electrode recording and wireless transmission of the
ECoG signals of each electrode to an external computer
housing the algorithms and control software.
II. THE WIMAGINE PLATEFORM
The WIMAGINE (WIreless Multi-channel Acquisition
system for Generic Interface with NEurons) platform was
developed as a proof of concept and first functional
prototype of an implantable device for recording ECoG
signals on a large number of electrodes (64). The design of
the WIMAGINE platform takes into account all the
constraints of an implantable medical device: ultra-low
power, miniaturization, safety and reliability.

356

A. WIMAGINE Architecture
The neurological signals (ECoG signals) are captured by an
array of 64 platinum electrodes (3mm²) at a pitch of 4.5mm.
After signal amplification and analog multiplexing, the
neural data from each channel is sampled at 1 kHz and
digitized with a resolution of 12 bits/sample using two
integrated circuit named CINESIC32 specially designed for
this application (ASIC). Each ASIC CINESIC32 performs an
ECoG signal amplification and analog to digital conversion
on 32 channels. The electronic environment around these
two ASICs is implemented using COTS (commercial-offthe-shelf) components. An ultra-low power microcontroller
(MSP430) provides the control of the different modules of
the WIMAGINE board. The wireless data link module is
implemented using the ZL70101, commercially available
from Zarlink, designed for use in implanted devices and
operating in the 402 - 405 MHz MICS (Medical Implantable
Communications Service) band. The ZL70101 transceiver
provides a high data rate with an ultra-low power
consumption
of typically 5mA during
TX/RX
communication. Power is supplied wirelessly to the
WIMAGINE board through an inductive link based on a
frequency of 13.56 MHz and providing up to 100mW.
The block diagram of the WIMAGINE platform is shown in
Figure 1.

Figure 1: The WIMAGINE Architecture

The experimental set-up used for BCI applications is made
up of the WIMAGINE board, an EM field generator as
power source, an RF receiver from Zarlink connected to a
computer running the control software and the data
processing algorithms controlling the effectors. The
Graphical user interface (GUI) allows the user to remotely
control the implant through the RF link.

Figure 3: The experimental set-up for WIMAGINE

The general specifications of WIMAGINE are summarized
in Table 1.
Acquisition analog front End
Number of channels
64 (based on 2 ASICs CINESIC32)
Variable gain
1, 5, 200 or 1000 (adjustable for each electrode)
Detection range
+/- 1.3 mV (gain 1000)
Bandwidth
0.5 Hz to 300 Hz
Resolution
12 bits - ADC architecture : SAR
Input referred noise
0.8 µV RMS (in max gain conditions and on BW [0.5Hz;300Hz])
Sampling frequency
600Hz, 1 kHz, 3kHz per channel (selectable)
Microcontroller and sensors
Microcontroller
MSP430 (Texas Instrument)
Embedded sensors
Temperature, accelerometer and humidity
RF link
COTS component
Transceiver ZL70101 (Zarlink)
Transmission
402-405 MHz (10 MICS channels)
480 kb/s (allowing a continuous transmission of 32 ECoG
High data rate
channels @ 1kHz chosen from the 64 electrodes available

The WIMAGINE board was designed to be fitted into a
titanium housing of 50mm in diameter with a thickness of
7mm. As shown in Figure 2 the board is made up of two
circular PCBs (diameter ~ 40mm) linked by a flex. The
ASICs CINESIC32 (dimensions: 13.5mm x 6.5mm) and
their external components are placed on one PCB (at the
bottom) while the other PCB (at the top) contains the
microcontroller MSP430, the wireless power transfer and RF
link components.

Figure 2: The 3D model of the WIMAGINE board

Power consumption
5 mA (continuous TX / RX )
Range
<2m
Inductive wireless power management
Frequency
13,56MHz
Wireless power supply ~100 mW (30mA @ 3.3V)
Table 1: General specifications of the WIMAGINE plateform

B. Integrated Electronic: ASIC CINESIC32
Interfacing electrodes using discrete electronics rapidly
limits the number of channels, creating the need for highly
integrated solutions to achieve sufficient spatial resolution
[10][11]
. For this purpose, the dedicated ASIC CINESIC32
(CIrcuit for NEuronal SIgnal Conversion 32 channels) is
being developed with the constraints of implantable
applications in mind: ultra low power consumption and
safety of the patient.
A first 8-channel version was developed and tested to
validate the future CINESIC32 architecture. The ASIC
filters, amplifies and digitizes the EcoG data acquired from
the electrodes. The architecture of CINESIC8 is shown in
Figure 4.

357

Figure 5: The ASIC CINESIC8

Figure 4: Architecture of the CINESIC8 ASIC

Each input channel is combined with an external capacitor
(1.5nF) in order to suppress the risk of leaking current in a
first default condition, which is essential for implantable
applications. The AC-coupled analog channel is comprised
of a fully differential low-noise amplifier, followed by a
voltage gain amplifier and a programmable low-pass filter.
The consumption of one analog channel is ~34µA.
Numerical peripherals such as configuration registers and an
SPI (Serial Peripheral Interface) controller are also
integrated on the chip. A special attention was paid on
configurability so as to target different applications. A
dedicated protocol was defined to address configuration
registers. Consequently, the user can turn each channel ON
or OFF, program the input switches in different mode and
put the amplification stages in different gain (four possible
values) and bandwidth settings (two possible bandwidths).
The analog filter cut-off frequency can be configured to
300Hz or 5kHz. For EcoG applications, the channels will be
configured to a [0.5Hz; 300Hz] bandwidth and a 60dB
voltage gain. Each analog data is digitized through a 12-bit
ADC. The CINESIC8 chip was designed in CMOS 0.35µm
technology (figure 5) It has been tested and its features are
listed in Table 2.
Process
Number of channels
Modularity
Topology
Power supplies
Amplification Gain

Features of CINESIC8 ASIC
AMS C35B4C3 0.35 µm CMOS
8
1 to 8
Fully-differential
0 – 3.3V
60dB; 46dB; 14dB; 0dB

(selectable for each channel)

Bandwidth

BW1 = [0.5Hz;300Hz]
BW2 = [0.5Hz ; 3kHz]
Input referred noise
0.8mVRMS in BW1 with 60dB gain
ADC Architecture and resolution 12-bit SAR, 10.7 ENOB
One channel power consumption 33.5µA
Safety capacitor
External (1.5nF)
Sampling Frequency (per channel) 600Hz, 1kHz, 3kHz per channel (selectable)
Digital Interface standard
SPI
Additional features (can be
Calibration / Impedance measurement /
turned off)
Internal Multiplexer for signal monitoring
Operating temperature range
35°C - 42°C
Area
6400 µm x 5000 µm
Table 2: Features of ASIC CINESIC8
(selectable for each channel)

C. Microcontroller module
The MSP430F2618-EP from Texas Instruments was chosen
for its ultra low power characteristics, its multiple
communication interfaces and because it belongs to the
HiRel series from TI's Enhanced Products program. The
MSP430 manages the remote power supply, controls the RF
link, the data acquisition from the CINESIC32 and the
embedded sensors. Three sensors have been embedded on
the WIMAGINE platform: a temperature sensor (TMP121EP from TI), a humidity sensor (SHT21 from Sensirion) and
an accelerometer (ADXL345 from Analog Devices).
D. RF link module
The RF communication module ZL70101 from Zarlink was
chosen for its ultra low power characteristics (5mA in
RX/TX), its high data rate (~ 800 kb/s raw data for ~480
kb/s effective data rate) and because it was designed
specially for implantable applications.
E. Wireless power management module
An implanted battery would require repeated recharge or
surgeries. Moreover, there is no commercially available
battery certified for implantable applications capable of
providing sufficient power to supply the WIMAGINE board.
Therefore an inductive link was developed using COTS
components in order to remotely supply the WIMAGINE
implant with the required power. It is dimensioned for
providing 30mA at 3.3V with an HF antenna of 10cm² made
of a biocompatible platinum wire. The RF front-end
combined to the implantable antenna comprises a rectifying
stage, a shunt regulator and ultra-low noise regulators. The
implanted antenna and the generator antenna were designed
together by taking into account the loading effect between
them. Moreover, this remote power link was designed in
compliance with European recommendations for inductive
applications
(ERC7003)
and
with
European
recommendations for human exposure to EM fields (CE519).
A specific absorption rate (SAR) of less than 0.01W/kg was
achieved (maximum allowed: 2W/kg).
F. User-friendly interface
The WIMAGINE System offers a user-friendly interface
allowing rapid and easy setting of acquisition parameters like
sampling frequency of the device and the gain of each
electrode depending on the measured electrical cell activity.
Furthermore, all data from the 64 channels can be saved and
reloaded with the WIMAGINE software or analyzed later

358

using Spike2 software (Cambridge Electronic Design
Limited, UK).
III. FIRST IN VIVO VALIDATIONS
Prior to the development of the WIMAGINE platform, the
different functional modules were tested and validated
individually on separate boards. A first in vivo test merged
these functional modules together using the CINESIC8
ASIC, a board for the wireless power management and a
board comprising the MSP430 and the RF link. The recorded
ECoG data was transmitted wirelessly and displayed in the
GUI of the WIMAGINE platform.
The goal of this first in vivo test was to record the cerebral
activity of a freely moving rat (ECoG). Screws were fixed in
the skull of the animal and used as electrodes. The
experimental procedures and animal care were carried out in
compliance with the European Community Council Directive
of 24th November 1986 (86/609/EEC). The electrodes were
connected by wire through a free-rotating connector to the
ASIC CINESIC8. Brain signals were acquired with a low
noise and in a bandwidth from 0.5Hz up to 300Hz for BCI
studies (Figure 6). A typical ECoG activity at 4-7Hz was
recorded on all electrodes implanted in the rat.

V. CONCLUSION
An innovative platform named WIMAGINE has been
developed in the context of the BCI project, currently lead
by Prof. A.L. Benabid in the context of the
CEA/LETI/CLINATEC® structure. WIMAGINE is a
wireless low power 64-channel data acquisition system
dedicated to electrocorticogram (ECoG) recordings. First in
vivo tests of the functional blocks of WIMAGINE were
successfully performed. The design of the WIMAGINE
platform takes into account the constraints of an implantable
medical device: ultra low power, miniaturization, safety and
reliability. Indeed, the next step is the development of an
implanted medical device for wireless multichannel ECoG
recording used in the development of a neuroprosthesis.
ACKNOWLEDGMENTS
This work was performed thanks to the close collaboration of
the technical staff of CEA/LETI/DTBS, CEA/LETI/DSIS
and CEA/LETI/CLINATEC®.
The authors wish also to thank C. Moro and T. Costecalde
(CLINATEC®) for their involvement in the in vivo
validations and S. Bonnet (DTBS) for signal analysis.
REFERENCES
[1]

Figure 6: Experimental set-up [A], First In Vivo ECoG recording on the
WIMAGINE user interface [B]

IV. TOWARDS AN IMPLANTABLE DEVICE
The development of the WIMAGINE platform is a first step
towards the development of an implantable multichannel
ECoG recording device for in vivo investigations or
therapeutic functionalities (prostheses). An important part of
the work to be done for transforming the proof-of-concept
into an actual implantable device lies in optimizations in
terms of safety and reliability. Related topics like the design
of a biocompatible housing, hermetic feedthroughs and
implantable antennas for RF communication and wireless
power supply are being addressed simultaneously within the
CEA/LETI.

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