Flexible carbon nanotube nanocomposite sensor
for multiple physiological parameter monitoring
Item Type Article
Authors Nag, Anindya; Mukhopadhyay, Subhas Chandra; Kosel, Jürgen
Citation Nag A, Mukhopadhyay SC, Kosel J (2016) Flexible carbon
nanotube nanocomposite sensor for multiple physiological
parameter monitoring. Sensors and Actuators A: Physical 251:
148–155. Available: http://dx.doi.org/10.1016/j.sna.2016.10.023.
Eprint version Post-print
DOI 10.1016/j.sna.2016.10.023
Publisher Elsevier BV
Journal Sensors and Actuators A: Physical
Rights © 2016. This manuscript version is made available under the CC-
BY-NC-ND 4.0 license
Download date 09/08/2022 15:54:08
Item License http://creativecommons.org/licenses/by-nc-nd/4.0/
Link to Item http://hdl.handle.net/10754/621061
Accepted Manuscript
Title: Flexible carbon nanotube nanocomposite sensor for
multiple physiological parameter monitoring
Author: Anindya Nag Subhas Chandra Mukhopadhyay
J¨urgen Kosel
PII: S0924-4247(16)30707-5
DOI: http://dx.doi.org/doi:10.1016/j.sna.2016.10.023
Reference: SNA 9879
To appear in: Sensors and Actuators A
Received date: 30-5-2016
Revised date: 13-10-2016
Accepted date: 14-10-2016
Please cite this article as: Anindya Nag, Subhas Chandra Mukhopadhyay,
J¨urgen Kosel, Flexible carbon nanotube nanocomposite sensor for multiple
physiological parameter monitoring, Sensors and Actuators: A Physical
http://dx.doi.org/10.1016/j.sna.2016.10.023
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Flexible carbon nanotube nanocomposite sensor for
multiple physiological parameter monitoring
Anindya Nag
1*
, Subhas Chandra Mukhopadhyay
1
, Jürgen Kosel
2
1
Faculty of Science and Engineering
Macquarie University
Sydney, NSW, Australia.
*Corresponding author email: anindya1991@gmail.com
2
Computer, Electrical, Mathematical, Science and Engineering Division
King Abdullah University of Science and Technology (KAUST)
Saudi Arabia.
Highlights:
Novel flexible, strain-sensitive sensor prototypes were fabricated for multiple physiological
parameter monitoring.
The sensor patch was made of Polydimethylsiloxane (PDMS) as the substrate and a thin layer of a
nanocomposite comprising of PDMS and functionalized multi-walled carbon nanotubes (MWCNTs)
as electrodes.
Laser cutting was done to design the electrodes.
The novelty lies in the material and structural aspect.
The sensor patch was used for monitoring limb movements and respiration.
Abstract— The paper presents the design, development, and fabrication of a flexible and wearable sensor based on carbon nanotube
nanocomposite for monitoring specific physiological parameters. Polydimethylsiloxane (PDMS) was used as the substrate with a thin
layer of a nanocomposite comprising functionalized multi-walled carbon nanotubes (MWCNTs) and PDMS as electrodes. The sensor
patch functionalized on strain-sensitive capacitive sensing from interdigitated electrodes which were patterned with a laser on the
nanocomposite layer. The thickness of the electrode layer was optimized regarding strain and conductivity. The sensor patch was
connected to a monitoring device from one end and attached to the body on the other for examining purposes. Experimental results
show the capability of the sensor patch used to detect respiration and limb movements. This work is a stepping stone of the sensing
system to be developed for multiple physiological parameters.
Keywords— PDMS, carbon nanotubes, nanocomposite, sensor patch, respiration, limb movement.
1. Introduction
The development of a novel flexible sensor patch for sensing multiple parameters is described in this paper. The sensor patch
utilizes polydimethylsiloxane (PDMS) as the substrate and a nanocomposite of PDMS and carbon nanotubes (CNT) as electrodes.
PDMS had been substantially used [1-3] for the development of flexible sensors due to its low cost, non-toxicitiy, inertness and
hydrophobic nature. CNTs were preferred as the conducting material over other metallic nanowires because of their
biocompatibility, high flexibility, resistance towards temperature change, low stiffness, and high tensile strength.
Multi-walled carbon nanotubes (MWCNTs) were used for the experiments functionalized with carboxylic groups (-COOH). The
functionalized MWCNTs have a better dispersing capability inside a polymer compared to unfunctionalized or single-walled
carbon nanotubes (SWCNT). This leads to a better interfacial bonding between the nanotubes and the polymer resulting in a
higher conductivity. Interdigitated electrodes were patterned on the nanocomposite layer, allowing for a non-invasive and single-
sided strain measurement. The patterns were produced using CO
2
laser ablation [4, 5]. Compared to other fabrication techniques
like 3-D printing [6], photolithography [7], inkjet printing [8], etc., it excels at the ease of sample preparation without the need for
any templates or additional material. This method fabricates very thin and flexible materials and can cut smooth edges which are
approxmiately parperdicular to the surface. By attaching the sensor to the skin, respiration and limb movements were tested on
different people as shown in the experimental results section, to verify its functionality.
The concept of sensors to monitor people’s health and lifestyle has been capitalized since the past two decades [9, 10]. Different
types of sensors have been used to monitor the activities and physiological parameters of the individuals to understand and
generate a pattern for human behavior [11-13]. Sensors with flexible substrates are one sector where prominent research work
[14-17] has been done in recent times. Light weight, low cost of fabrication, long lasting capability are some of the reasons for
their increased usage over rigid substrates. The sensors developed for smart home usage are mainly dedicated for single parameter
monitoring purposes like PIR sensors [18], pressure sensors, etc.
Multiparameter monitoring is of great interest due to the disadvantage caused by sensors for individual applications. For example,
the cost is largely reduced in using a multi-functional sensor. The sensor patch development shown in this paper is much simpler
and easier to fabricate compared to previously developed sensors, which had been fabricated for multiple functions containing a
coil [19-21] operating on a magnetic principle.
Significant research work has also been done on the detection of joint and limb movements. The majority of them involves fixed
sensors [22] or the study of an artificial robot [23] to analyze the human behavior. Wearable sensors [24] and accelerometers [25]
are other techniques used to monitor human movement. Shoe sensors [26] and braces [27] are some types of wearable sensors
used for monitoring of physical activities involving limb movements. The existing concepts have distinct disadvantages. Some
would be wearable sensing devices required to be worn by the person at times; others would involve complicated gadgets working
on specific computational algorithms involving an expertise to operate them. Thus, there is a need for a simple, non-invasive,
sensing device which upon its attachment to the monitored region would precisely detect the movements, even on a smaller scale.
Research work to monitor the rate of respiration has been done previously using devices with and without flexible substrates. The
photoplethysmographic technique [28, 29] is widely used for the detection of respiratory rate. But this technique is complex and
requires technicians at the time of monitoring. Piezo-resistive [30], fabric attached sensing [31, 32] and optical sensors [33] are
other ways used to monitor respiratory rate. Technical complexity, cost and specific positioning of the subject during monitoring
are some of the demerits of these techniques. Monitoring of respiration and other physiological parameters has also been done
using PVDF-based piezoelectric sensors [34, 35]. But the disadvantages of using PDVF are the strong temperature depending
performance along with high hysteresis exerted by the sensors. There are different force sensors available in the market. Table 1
classifies them based on price, size and some applications related to physiological parameter monitoring. Typically, either the
price of the sensors is very high or the sensor size is large. In this paper, we show the change in capacitance of an interdigitated
electrode on a flexible sensor patch by simply attaching it to the lower part of the diaphragm of an individual. The inhalation and
exhalation rates were monitored based on the strain induced on the sensor patch. This could be used for applications like the
abnormality in the rate of respiration caused due to hypoxemia and hyperemia which can be analyzed by monitoring the change in
sensor capacitance between a healthy person and a patient.
2. Theory
The working principle of the sensor is based on the deformation of an interdigital electrode structure. The capacitance of any
parallel plate capacitive device can be generally expressed by,
(1)
where,
C is the capacitance of the interdigital sensor,
= 8.85 is the × 10
−12
F·m
−1
is the permittivity of vacuum,
is the relative permittivity,
A is the effective area, and
d is the effective spacing between electrodes of different polarity.
A change of d or A causes a change of the capacitance. This can be exploited to monitor a physiological event through the change
in capacitance based on the deformation-reformation of the sensor patch. The exertion of tensile stress on the patch via a
physiological event changes the capacitance with respect to its normal position [36, 37]. Fig. 1 depicts the notion. L and W stand
for the length and width of the sensor patch, respectively. ∆L, ∆W, and ∆d are the changes in length, width and interdigital
distance of the sensor patch, respectively, caused when deformed. Using equation (1), the change in capacitance can be calculated
as a function of change in length (∆L), width (∆W) and interdigital distance (∆d) as shown in equation (2).
(2)
3. Fabrication and Characterization of the Sensor Patch
The schematic diagram of the fabrication steps is given in fig. 2. PDMS (SYLGARD ® 184, Silicon Elastomer Base) was cast at
a ratio of 10:1 of base elastomer (pre-polymer) and curing agent (cross-linker) on a Poly (methyl methacrylate) (PMMA)
template. The template was patterned using a laser cutter (Universal Laser Systems). PMMA was chosen because of its
impassiveness towards PDMS and the cured material can be easily peeled off from the base without any additional steps. The
thickness of the cast PDMS was adjusted to 1 mm by a casting knife (SHEEN, 1117/1000 mm). The sample was then desiccated
for 2 hours to remove any trapped air bubbles.
The sample was cured at 80
0
C for 8 hours to form the substrate for the sensor patch. A mixture consisting of functionalized
MWCNTs (Aldrich, 773840-100G) and PDMS was then cast onto the cured PDMS. 4 % wt. of CNT was used after an
optimization between the conductivity and dispersion of CNT into PDMS. Followed by the adjustment of the thickness of the
nanocomposite layer by the casting knife to around 600 µm, the sample was again desiccated for 2 hours to remove any trapped
air bubbles. Then the nanocomposite layer was cured at 80
0
C for 8 hours. Laser induction (Universal Laser Systems) was then
