Sensors and Actuators A 144 (2008) 275–279
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Sensors and Actuators A: Physical
journal homepage: www.elsevier.com/locate/sna
First human trials of a dry electrophysiology sensor using a carbon nanotube
G. Ruffinia,∗, S. Dunnea, L. Fuentemillab, C. Graub, E. Farr´ esa, J. Marco-Pallar´ esa,b,
P.C.P. Wattsc, S.R.P. Silvac
aStarlab, Ed. de l’Observatori Fabra, C. de l’Observatori s/n, 08035 Barcelona, Spain
bNeurodynamics Laboratory, Psychiatry and Clinical Psychobiology Department, University of Barcelona, Spain
cNanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guilford, UK
a r t i c l ei n f o
Received 24 January 2007
Received in revised form 22 February 2008
Accepted 3 March 2008
Available online 14 March 2008
EEG CNT nanotubes
a b s t r a c t
We report the results from the first human trials of a new dry electrode sensor for surface biopotential
applications. This sensor uses nanotechnology to improve performance and wearability. Standard elec-
trodes for demanding applications such as EEG require skin preparation and/or the use of electrolytic
substances to provide and effective electrical interface. The contact surface of the electrode described
here is covered with an array of carbon nanotubes designed for penetration of the outer layers of the
skin for improved electrical contact without the use of electrolytic gel. In previous papers we detailed
the design concept and reported the results of the initial tests of a first prototype—immersion in saline
solution and signal detection in animal skin. In this paper we describe the first human trials of the proto-
type, which indicate performance on par with state-of-the-art research-oriented wet electrodes. No side
effects have been observed 6 months after the tests, and the subject did not report any pain or unusual
sensations upon application of the electrode.
© 2008 Elsevier B.V. All rights reserved.
Fatigue, sleepiness and disturbed sleep are increasingly impor-
tant factors affecting health and safety in modern society and
ambulatory monitoring of related physiological indicators [1,2].
Electrophysiology, the measurement of electrical activity of biolog-
ical origin, is a key technique for the measurement of physiological
parameters in several applications, but it has been traditionally
difficult to develop sensors for measurements outside the labora-
tory or clinic with the required quality and robustness [3,4]. This
is in part due to the fact that electrodes used for high quality
low amplitude measurements (such as EEG) require skin prepara-
tion and the use of electrolytic gel, resulting in longer preparation
times (up to several minutes per electrode) and long stabiliza-
tion times (diffusion of the electrolytic gel into the skin). In this
paper we report the results from the first series of human tri-
als with a new electrophysiology sensor using multiwalled carbon
nanotube arrays (MWCNTs)  whose design goal is penetration
of the outer layer of the skin and improved electrical contact .
∗Corresponding author. Tel.: +34 932540362.
E-mail address: email@example.com (G. Ruffini).
These arrays were grown on highly doped silicon substrates using
plasma-enhanced chemical vapour deposition of acetylene over
an iron catalyst and mounted on a back end providing amplifi-
cation. The trials described here, which have included traditional
protocols for the analysis of the electrical activity of the brain –
spontaneous EEG and event-related potentials (ERPs) – indicate
performance on a par with state of the art research-oriented wet
2. Electrode design
The aim of the tested design was to eliminate skin preparation
and gel application requirements in order to reduce noise while
improving wearability . The key aspect of this prototype is the
electrode–skin interface, which is provided by a large number of
MWCNTs forming a brush-like structure —see Figs. 1 and 2. Due
to the excellent mechanical (high modulus of elasticity, tensile
strength ) and electrical properties  of MWCNTs, this nano-
structure design should provide a robust and stable low noise elec-
skin layer – the Stratum Corneum – resulting in a comfortable and
pain free interface. In addition, shallow penetration, together with
the small diameter of CNTs, should result in a lower infection risk
as compared to more invasive approaches using microtechnology.
0924-4247/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
G. Ruffini et al. / Sensors and Actuators A 144 (2008) 275–279
contact with the Stratum Corneum. The signals are locally amplified for downstream
stood because the mechanical properties of the skin at nanoscale
are difficult to estimate, but the baseline sensor paradigm relies
on MWCNTs penetrating the outer skin layer to a depth of
pling enhanced by the increase in surface contact area. Although
MWCNTs can be coated for improved transduction, a polariz-
able uncoated prototype has first been tested and is reported
mounted on state of the art commercial active electrodes (i.e., with
on-site amplification) and connected to commercial-off-the-shelf
research electrophysiology recording equipment for comparison
testing of a wireless sensor system with fully integrated MWCNTs
and electronics will be reported elsewhere.
here had the following interface characteristics: MWCNT arrays
with a nanotube diameter of ca. 50nm, MWCNT length of
10–15?m, Fe catalyst and no coating. The MWCNT arrays were
grown on highly doped silicon substrates (Charntec Electronics
?100? N type 0.8–0.15?cm) using plasma-enhanced chemical
vapour deposition (PECVD) of acetylene over an iron catalyst .
Generally, a ca. 10nm iron film was sputtered onto the silicon
wafer immediately after etching the native oxide by immersion in
Fig. 2. Electron microscope image of the MWCNT array.
strate to 650◦C for 20min in vacuum in order to break the Fe film
into small islands between 50 and 100nm in diameter. During the
growth process the substrate was maintained at 650◦C and acety-
lene was introduced to the chamber at a concentration of 5.0% with
H2as the carrier gas.
By controlling the temperature and growth time the MWCNTs
ment, length and diameter.
was cut into 1cm squares each supporting an array of MWCNTs. 4
Biosemi Active 2 flat electrodes were selected for the experiment.
The Ag/AgCl pellets used in these standard “wet” electrodes were
carefully removed from the electrode enclosure in order to expose
the lead wire. The silicon samples were then mounted on the elec-
trode casing using silver-loaded epoxy adhesive to hold the lead
wire in place and sealed using an epoxy resin. This provides an
electrode that differs only in the contact surface from those to be
used during the benchmark tests. This also has the advantage of
using a proven and known recording system for both the standard
and dry electrodes.
3. Preliminary tests
Prior to the human trials, a series of some simple tests were
carried out as a quality check of the electrodes. These tests are
described fully in Ref.  and we only provide an overview here.
A first requirement for an electrode is that it exhibit low noise in
the region of 1–2?V R.M.S. or less in the ∼0.1–100Hz range when
immersed in saline solution. We carried out this test both for the
prototype electrodes and the commercial electrodes in saline solu-
tion. The noise measured by the new electrodes is low and rather
similar to that of the commercial electrodes .
In order to test signal response in a semi-realistic situation, the
electrodes were placed on pig skin and a small test signal applied
beneath the skin. Pig skin is similar in structure to human skin
and provides a good starting point for prototype development. The
standard electrode was applied to pig skin using electrolytic gel
while the new electrode was applied without gel or other skin
preparation. The results from the comparison were again very
4. Human tests
For completeness, human tests were first carried out with con-
ventional electrodes without gel and with prototype electrodes
without CNT array interfaces (but with the same substrate). In both
cases, the measurements were contaminated by a large amount
of noise (1–2 orders of magnitude above those with wetted elec-
trodes), as expected.
one subject only (the tests were approved by the Ethical Commit-
tee of the University of Barcelona and the University of Surrey). The
experimental protocol was designed to mimic typical EEG mea-
surements conducted under both clinical and research practices.
The sensor was thus evaluated under normal brain state condi-
brain responses are required. Hence, the protocol encompassed
both spontaneous EEG and ERPs .
EEG measurements were collected simultaneously again by a
wet commercial electrode and the dry prototype electrode. The
two electrodes were placed near each other in the scalp area (near
G. Ruffini et al. / Sensors and Actuators A 144 (2008) 275–279
the F p2 position in the 10–20 system) and referenced to the tip
of the nose. Eye movements (EOG) were recorded by two com-
mercial wet electrodes placed at the outer canthus of each eye. A
EEG signals were sampled at 1024Hz with no band-pass on-line
filtering. The participant sat in a comfortable chair in a dimly lit
and electrically and acoustically shielded booth.
of alpha waves (8–12Hz) and the associated spectral peak when
a subject relaxes, which is clearly visible when the subject closes
the eyes. In the eyes-open condition, alpha EEG dominance is sup-
pressed and an increase in the beta (?) range frequency (15–30Hz)
ity can be similarly recorded by the new electrode and the wet
commercial electrodes, two protocols were employed. The first
one was based on recording a long period of eyes-open and eyes-
closed states, thus leading to the collection of long periods of alpha
and beta oscillatory activity. The second one featured short-time
periods of alternating eyes-open and -closed condition states. The
alternation of eyes opening and closing within brief time periods
allows for the evaluation of the reactivity of brain rhythms to light
stimulation. Spontaneous EEG recordings consisted of two periods
of 5min during which the subject did not perform any task. In the
eyes-open condition, the subject was instructed to focus the eyes
on a fixed point on a computer screen, thus avoiding eye-blinks. In
the eyes-closed condition, the subject was just required to keep the
eyes closed and to relax.
EEG transition recordings consisted of a 3-min EEG recording
with an alternation of 5s of eyes-open and eyes-closed conditions.
A pure tone (90db SPL, 1000Hz and 10ms rise/fall) was presented
binaurally with headphones to advise the subject that period con-
ditions had changed. The subject was required to open and close
the eyes following the auditory tone.
ERPs are other well-established events in the study of cerebral
electrical activity. ERPs are cerebral responses to the presentation
of a sensory stimulus. The cerebral response is recorded as an EEG
and is analyzed in the form of waves, characterized by their latency
Fig. 3. PSD results for each electrode (Biosemi and ENOBIO) for each EEG state condition. Beta (green arrow) and alpha (red arrow) frequency peaks are clearly observable in
each condition. A large 50Hz amplitude peak and its harmonics corresponding to the notch noise can be observed in both. A sample of eyes-closed raw data(with a band-pass
of 1–35Hz) and filtered to 8–12Hz is presented at the bottom.
G. Ruffini et al. / Sensors and Actuators A 144 (2008) 275–279
of appearance and amplitude. Here, the appearance of an auto-
matic auditory ERP component, the auditory N1, was evaluated.
N1 is reflected by prominent scalp fronto-central negativity EEG
deflection with a stimulus onset latency around 100–150ms. Audi-
tory N1 reflects the transitory auditory cortical areas (temporal
lobes) response to an auditory stimulus. Aural stimulation was
implemented by means of the stimulation software Presentation®
(Version 7.0) with the aid of portable equipment, presenting the
visual stimuli (i.e., fixation point) and auditory stimuli through
headphones. While the subject focused on a distracting task (read-
ing), a sequence of auditory stimuli of two different kinds, namely
a standard (p=0.8; 75ms; 90dB SPL, 1000Hz and 10ms rise/fall)
and a deviant one (p=0.2; 25ms; 90dB SPL, 1000Hz and 10ms
rise/fall) was presented binaurally with headphones. The sequence
consisted of stimuli-trains formed first by a standard or deviated
stimulus (at random) and followed by two standard stimuli. The
interval between the stimuli within trains was 300ms, while the
interval between the trains was 400ms.
The spontaneous EEG recording power spectral density (PSD)
was evaluated using Welch’s method for each condition separately.
The PSD in open and closed eye conditions can be observed in
Fig. 3 for each electrode type, revealing a high similarity of the
EEG data collected with each electrode. Moreover, the appearance
of an increased alpha frequency peak (8–12Hz) appearance in the
eyes-closed condition as compared with eyes-open one for each
electrode type can be clearly observed. Furthermore, a small but
clear beta frequency peak (around 20Hz) can be observed in each
electrode in the eyes-open condition.
With equivalent results, the periods of 5s recordings for eyes-
open and closed conditions were separated from the continuous
EEG recording and their PSDs were computed. Subsequently, the
PSDs of each 5s trail were averaged within conditions and sep-
arately for each electrode. Alpha activity was detected within
periods of 5s EEG recordings for both electrodes, thus reinforc-
ing the consistency of the results from the data collected by both
electrodes with a high similarity.
ERPs were obtained off-line by averaging 400ms EEG epochs,
digital band-pass filter was used just for ERP extraction. Due to the
fact that EEG electrodes were placed near the F p2 position, eye
ing. Hence, a strict artefact rejection process was conducted in the
EEG trial selection. Epochs were excluded if signal values exceeded
±100?V in EEG or ±10?V in EOG. Lastly, a visual inspection of the
remaining trials was conducted by two experienced researchers.
After artefact removal, a total of 67 trials were selected for further
analysis. Fig. 4A shows all selected trials recorded by conventional
and prototype electrodes. A large negativity between 100 and 200
ms from stimulus onset – the N1 ERP response at each electrode
– can be observed with each electrode. The repeated measure t-
time-frame recorded produced no significant differences through
the 400ms time period (P>0.01). Further, a prominent negativity
at 100–200ms can be observed from the all-trial averages, which
reflects the auditory N1 ERP (Fig. 4B).
The design of a CNT-based electrophysiology electrode is a
fascinating and challenging multi-disciplinary exercise involving
analysis of requirements, skin and CNT mechanical and electri-
cal properties at nanoscale, electrochemistry and biocompatibility
issues. In a previous publication, we analyzed requirements for
EEG/ECG/EOG applications and provided the logic for the electrode
design, starting from a careful analysis of all the noise sources and
measurement requirements and we proposed a of dry electrode
based on the use of MWCNTs to penetrate the outer skin cell lay-
ers and thus reduce the noise of measurements. The test results
reported here provide robust evidence that this dry electrode per-
forms rather similarly to state of the art research-oriented wet
electrodes, thanks to an interface exploiting the control of surface
features at nanoscales and the excellent properties of CNTs.
It is also important to note that the subject did not report any
pain or special sensations on application of the CNT-coated elec-
trode, and 6 months after the trials no side effects have appeared
(irritation, redness, itching or else). Nevertheless, as with other
nanotechnology research, further work is needed to explore safety
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EU projects on technology development in earth observation and biomedical appli-
cations. He graduated from UC Berkeley in mathematics and physics and obtained a
PhD in physics from UC Davis/Los Alamos in 1995. His current research includes the
development of novel sensors for brain/body monitoring exploiting new technolo-
gies, computational neuroscience – exploiting EEG data for studying, monitoring
sensors and algorithms.
Stephen Dunne is currently Applied Neuroscience R&D Manager at Starlab. He
began his studies in, what was then, the Galway Regional Technical College where
he obtained a National Certificate in instrument physics. From there he moved to
the University of Wales in Aberystwyth where in 1995 he obtained a BSc (Hon-
ours) in planetary and space physics followed by a Masters in optoelectronics and
information processing from Queens University in Belfast, graduating in 1997.
His current work includes the development of sensors and applications in the
area of brain and body monitoring as well as control systems for earth observation
where he also went on to receive his PhD in neuroscience in 2007. He currently
holds a post-doctoral position at the Institute of Cognitive Neuroscience, University
College London where his current field of interest is cognitive neuroscience.
Carles Grau Fonollosa obtained his PhD in Medicine from the UAB (Universitat
Psychobiology Department of the UB. He licensed in the following medical disci-
plines: clinical neurophysiology and psychiatry. He is the researcher responsible of
a Consolidated Research Group (SGR) called Cognitive and Clinical Neurodynamics
(NECOM) of the Generalitat de Catalunya. More than 20 researchers from different
countries of the European Community belong to that group.
Esteve Farr´ es Berenguer graduated in physics from the Universitat Aut` onoma de
Barcelona (UAB) in 1987. In 1987, he joined the Electronic Department of the UAB as
assistant professor. From 1991 to 1998 he was in charge of the services of character-
ization and systems of the National Center of Microelectronics of Barcelona (CSIC)
working in CMOS technologies characterization. In 1998, he joined to BONAL S.A.,
a control traffic company as R&D Manager. From 2000 to 2004 he was in charge of
Technologies Ltd. (Buffalo., NY) working in telecommunications. He joined STARLAB
BARCELONA in 2004 and since then he has been working as researcher on Remote
Sensing and Earth Observation projects, mainly funded by ESA and EU. As hardware
researcher in Starlab Instrument Department, he is working in signal conditioning
and real time processing applied to earth observation and bio potential character-
ization. In 2005 has been awarded with a “Torres Quevedo” fellowship from the
Spanish Ministry of Technology and Science.
in physics from the University of Barcelona in 2000 where he later received his PhD
Paul Watts worked as a research assistant for 2 years at the Advanced Technol-
ogy Institute. His research focussed on aligned carbon nanotube (CNT) growth,
functionalisation of CNTs, hydrogen storage and chemical sensors. He received his
PhD in materials chemistry (2004) from Sir Prof Harold Kroto’s group at the Uni-
versity of Sussex (UK) after submitting a thesis on the electrical properties and
applications of carbon nanotube–polymer composites. In 2007, Dr. Paul Watts took
up a new position at Shell Global Solutions Research and Technology Centre in
Ravi Silva (S.R.P. Silva) is the Director of the ATI and heads the Nano-Electronics
Centre (NEC), which is an interdisciplinary research activity. The NEC has over 50
research staff, and is considered to be one of the leading laboratories in carbon-
based electronics worldwide. His research has resulted in over 300 presentations
at international conferences, and over 250 journal papers. He is the inventor of 15
patents, including a key patent on low temperature growth of carbon nanotubes,
and one on the fabrication of large area nanotube-organic solar cells. One of the
companies, Surrey NanoSystems Ltd., won the spin-out company of the year 2007
award from the Engineer Magazine. In 2001, he was awarded the Charles Vernon
Boys Medal by the Institute of Physics, and in 2003 awarded the IEE Achieve-
ment Award by the Institute of Electrical Engineers. In 2003, he was awarded the
Albert Einstein Silver Medal and Javed Husain Prize by UNESCO. He is also mem-
ber of the EPSRC Nanotechnology Task Force and Technology Opportunities Panel