This document describes the development and testing of a printed flexible capacitive pressure sensor for detecting impacts and its integration onto a soccer headgear. Two sensor designs were tested - one using silver nanoparticle ink and one using carbon nanotube ink. The Ag NP sensor showed a larger capacitive response to lower pressures compared to the CNT sensor. An electronic readout circuit was developed and tested with the Ag NP sensor attached to the headgear. The sensor and circuit demonstrated the ability to detect and measure impacts sustained during play.

1. 1.01E-11
1.06E-11
1.11E-11
1.16E-11
1.21E-11
0 200 400 600 800 1000 1200
Capacitance(Farades)
Time (Sec)
Capacitive Response of Fully Printed Pressure Sensor
(Ag Np Ink)
6.5 kPa
1.6 kPa
23 kPa
4.9%
7.8%
12.7%
13.7%
14.7%
15.7%
0.4 kPa
47 kPa
67 kPa
150 kPa
275 kPa
308 kPa 514 kPa
732 kPa
97 kPa
2.0%
10.8%
16.7%
17.6%17.6%
ResultsResults
AbstractAbstract
Materials and MethodsMaterials and Methods
IntroductionIntroduction
.
ConclusionsConclusions
AcknowledgementsAcknowledgements
ReferencesReferences
Development of a Printed and Flexible Impact Detection
System for Use in Soccer Headgear
Savannah Crooks
Kalamazoo Area Math and Science Center
and Western Michigan University
6.47E-12
6.57E-12
6.67E-12
6.77E-12
6.87E-12
6.97E-12
7.07E-12
0 100 16 116 216 316 416 516 616 716 816 916 1016 1116 1216
Capacitance(Farads)
Time (sec)
Capacitance Response of Fully Printed Pressure Sensor
(CNT Ink)
1.4% 1.8%
2.3% 2.6%
3.2%
4.0%
6.3%
5.1%
8.2%
9.9%
.3.8 kPa
15.2 kPa
39.9 kPa
56.2kPa
74.1 kPa
110 kPa
150 kPa
199kPa
256 kPa
337 kPa
405 kPa
0.8%
Figure 2: Experiment setup for sensor tests.
Figure 1: Screen printed Ag NP sensor design on 1.15 mm
PDMS.
Figure 3: Screen printed CNT sensor design on 1.15 mm
PDMS.
Figure 5: Shows the results from the test with the 1.15 mm PDMS with CNT ink. The graph
displays the differences in percent increase and the different pressures applied.
Figure 4: Shows the results from the test with the 1.15 mm PDMS with Ag NP ink. The graph
displays the differences in percent increase and the different pressures applied.
Figure 6: Shows a comparison between the percent difference from the base capacitance of
Ag NP ink and CNT ink .
Figure 8: Experiment setup for sensor and circuit test. Figure 9: The electronic circuit showing the capacitance to
voltage converter on the left side and voltage amplification
circuit on the right side.
Figure 11: The response of the Ag based impact sensing sensor connected to
headgear showing the voltage change with respect to different displacements and
different pressures applied.
Figure 10: The response of the Ag based impact sensing sensor showing the
voltage change with respect to different displacements and different pressures
applied.
Figure 7: Shows the 1.15 mm pressure sensor attached to the
headgear. The sensor is under the front of the headgear and
the structure is on a stable platform under the moveable
platform and pressure sensor used in the experiment.
I would like to thank Dr. Atashbar for his support throughout the entire project
and his guidance in the development and manufacture of the sensor. I would also like
to thank Dr. Atashbar for his generous financial support of the project. The project would
also not be possible without the hard work of Dr. Binu Narakathu, Dr. Sai Guruva Reddy
Avuthu and Dinesh Maddipatla. Their guidance, insight, and experience were invaluable
to the success of the experiment. I would also like to acknowledge Western Michigan
University’s Engineering Department to allow us to work in their labs to develop our
project. Special thanks also goes to the Center for Advanced Smart Sensors and
Structures (CASSS) and the Center for Advancement of Printed Electronics (CAPE) in
Western Michigan University for housing the project and all the equipment and materials
for the experiment. A special thanks also goes out to Dr. John Goudie and Dr. Joe
Thorstenson and the Kalamazoo Area Math and Science Center (KAMSC) for allowing
us the opportunity to develop our research skills on the Research Team through
experiences out in the field. I would also like to thank them for all the hard work they
have done to place us in positive working environments and helping us with all aspects
of our research. Finally, I would like to thank the Kalamazoo Area Math and Science
Center (KAMSC) Parent Organization for their generous financial contribution to make
this experiment possible.
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3. B. B. Narakathu, A. Eshkeiti, A. Reddy, M. Rebrosova, M. Joyce, B. Bazuin, M. Atashbar, “A
Novel Fully Printed and Flexible Capacitive Pressure Sensor”, IEEE Sensors, pp. 1-4, 2012.
Retrieved November 15, 2014, from http://ieeexplore.ieee.org/xpls/abs_all.jsp?
arnumber=6411354
4. D. Janczak, M. Słoma, G. Wróblewski, A. Młożniak, M. Jakubowska, “Screen-Printed
Resistive Pressure Sensors Containing Graphene Nanoplatelets and Carbon Nanotubes”.
Sensors, vol. 14(9), pp. 17304-17312, 2014. Retrieved November 15, 2014, from
http://www.mdpi.com/1424-8220/14/9/17304/htm
5. Lichun, S., Palaniapan, M., & Wan, T. (2006). A continuous-time capacitance to voltage
converter for microcapacitive pressure sensors. J. Phys.: Conf. Ser. Journal of Physics:
Conference Series, 1014-1019. doi:10.1088/1742-6596/34/1/168
6. Lotters, J., Olthuis, W., Veltink, P., & Bergveld, P. (1999). A sensitive differential capacitance
to voltage converter for sensor applications. IEEE Trans. Instrum. Meas. IEEE Transactions
on Instrumentation and Measurement, 48(1), 89-96. doi:0018–9456/99
7. Wang, X., Li, T., Adams, J., & Yang, J. (2013). Transparent, stretchable, carbon-nanotube-
inlaid conductors enabled by standard replication technology for capacitive pressure, strain
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Retrieved November 15, 2014, from
http://pubs.rsc.org/en/Content/ArticleLanding/2013/TA/C3TA00079F#!divAbstract
A 130 µm thick transparent PET (Melinex®
ST 506) film from DuPont Teijin
Films was used as the substrate. Ag NP ink has an average particle size of 20-50 nm
(Inktec, TEC-PR-020). CNT ink has an average particle size of 97-125 nm. PDMS, a
soft polymer, was purchased as a two-part heat curable silicone elastomer kit Sylgard®
184 from Dow Corning. The Sylgard 184 pre-polymer was mixed in a 10:1 (w/w) ratio
with the included curing agent and stirred vigorously. The mixing introduces bubbles that
were removed by setting aside the mixture at room temperature for 30 minutes.
The prepared PDMS was poured into molds with the same thickness
(1.15 mm). Once in the molds the remaining bubbles were either allowed to degas or
manually poked out. The molds were then put in an oven for 40 minutes at 100-115 °C
and then allowed to cool. The screen printing press was calibrated and Ag NP ink was
used to print samples of the top and bottom electrode onto PET. The samples were
cured at 100 °C for 20 min in an oven and allowed to cool. The screen printing press
and screen were then thoroughly cleaned using ethylene glycol di-acetate. The sensor
was then placed between a force gauge and vertically moveable platform connected to
an Agilent E4980A LCR meter. The change in capacitance was measured using a
custom built LabVIEW™ program on a PC connected to the LCR meter via a USB
cable.
The sensor was fabricated by sandwiching a blank layer of PDMS dielectric
layer between electrodes printed on PET. A test was run to measure the capacitive
response of the sensor. Initially, the capacitance of the pressure sensor was recorded
for one minute, with no force applied to set a base capacitance. Then, the sensor was
subjected to the minimum detectable pressure for one minute, after which the
compressive force was released. The response of the sensor was again recorded for
another one minute. This cycle was continued for different increasing compressive
forces up to the maximum detectable compressive force. The CNT ink sensor was
printed directly onto the PDMS and underwent the same experiment. It was observed
that both sensors were rendered reversible, after each compressive force was released,
Connected via
Connecting wires
Connected via
Function Generator
Mark 10 Force Gauge
Digital Oscilloscope
Voltage Supply
Connecting wires
Connected via
Connecting wires
Connected via
Connecting wires
In this work, screen printing technique was successfully employed to fabricate a
flexible capacitive pressure sensor for monitoring concussion causing impacts sustained
in sports by using it on a Full 90 headgear. The sensor was screen printed onto PET.
Two sensor configurations were tested. One sensor was fabricated using Ag NP ink as
the metallization layer and the other was fabricated using CNT ink. Various compressive
forces were applied to the different sensor configurations for testing the sensors
capacitive responses. With the Ag NP ink sensor, a 17.6% capacitance increase was
achieved for a 0.4 kPa compressive force, when compared to the base capacitance.
With the CNT ink sensor, a 9.9% capacitance increase was achieved for a 405 kPa
compressive force, when compared to the base capacitance. When the results of the
two tests were compared, it was found that at a displacement of 3 mm the silver
produced a capacitance increase of 17.6%, from the base capacitance, while the CNT
ink sensor produced an increase of 9.9% from the base capacitance (a difference of
7.7%). It was determined that the Ag NP ink sensor produced the best capacitance
responses and hence the AG NP ink sensor was tested with the circuit. Various
compressive forces were applied to the sensor to view its voltage output response. The
Ag NP ink sensor without the headgear demonstrated a maximum decrease of 13% for
a 6.8 MPa compressive force, when compared to the base voltage. The Ag NP ink
sensor with the headgear demonstrated a maximum decrease of 12% for a 0.95 MPa
compressive force, when compared to the base voltage. The difference in the sensor
with and without the headgear was attributed to the effects of the headgear and the
difference in packaging. The results show the feasibility of the readout circuit and sensor
configuration as an efficient and cost effective way to monitor concussion causing
impacts sustained by athletes during play. Further studies include enhancing the
sensor’s and circuit’s sensitivity, improving the flexibility and packaging of the device,
testing the conformability and applicability of the device in real play, and use of these
sensors on other structures and in other fields.
The subject of concussions has become an important issue in recent years in
the sports world. This has created a large demand for devices with the ability to detect
whether a player has received a concussion. With the rapid evolution of the pressure
sensing field, new technology has become available to allow for the creation of fully
flexible pressure sensors using screen printing that gives accurate measurements, and
are cost effective to produce. The researcher in this study considered the use of a
screen printed Ag NP capacitive pressure sensor, a screen printed CNT capacitive
pressure sensor, and a capacitance to voltage circuit on a Full 90 concussion
prevention headgear for use on athletes during soccer games. The screen printing
process allows for various electronics to be printed at a low cost, with very little material
wastage and an end result that is extremely flexible. A capacitance to voltage circuit
allows capacitances to be converted into voltage response for uses in further circuitry.
In a previous work, a capacitive pressure sensor was designed, fabricated and
tested for use as an impact sensing device to be used for monitoring concussions in
athletes by testing different dielectric layer thicknesses. It was found that the thinner
sensor produced the best results. In this work, a Ag NP sensor and a CNT sensor were
tested and compared as viable options for use with the headgear. Ag ink is flexible but
is known to have cracking problems; CNT was used in hopes of eliminating the cracking
and getting comparable or better results. Capacitive sensors use the capacitance
measured between two opposing electrodes to determine how much pressure is applied
to a surface. Conventional screen printing processes were used to fabricate the fully
flexible capacitive pressure sensor. The non-conductive layer, or dielectric layer, was
made using PDMS. PDMS was chosen for this experiment due to its non-conductive
properties and its high flexibility. PET, a flexible film, was used as the substrate. The
practical use of the sensors as pressure sensors were then demonstrated by
investigating the capacitive response on the sensors under varying compressive forces.
An electronic readout circuit was designed and connected to the superior sensor to view
the capacitive response of the sensor in terms of voltage using a capacitance to voltage
circuit and an amplification circuit. The practical use of the circuit was then
demonstrated by investigating the voltage response on the sensors under varying
compressive forces.
Methods and Materials Cont.Methods and Materials Cont.
due to the fact that the capacitance always attained its base capacitance value.
An electronic readout circuit with capacitance to voltage converter and voltage
amplifier was designed on a breadboard. The Ag NP sensor with the headgear was then
connected to the readout circuit along with a voltage generator Agilent/HP Power Supply
E3620A (as supply voltage), an Agilent Function Generator 3311A 9 (to select desired
frequency) and Tektronix TDS 5054 Digital Phosphor Oscilloscope (to view output
waveforms). The sensor was subjected to the same test and voltage response was
recorded. It was observed that the sensor was rendered reversible, after each
compressive force was released due to the fact that the voltage always attained its base
voltage value.
A fully flexible silver (Ag) nanoparticle (NP) capacitive pressure (impact)
sensor, a fully flexible carbon nanotube (CNT) capacitive pressure (impact) sensor, and
electronic readout circuit were developed for use on a Full 90 concussion prevention
headgear. The sensors were successfully fabricated using the screen printing
technique. Polyethylene terephthalate (PET) was used as a substrate and
polydimethylsiloxane (PDMS) as a dielectric layer. The PDMS was prepared using a
PDMS pre-polymer and a curing agent in a 10:1 ratio. The electrode design was printed
using Ag ink onto PET and the sensor was assembled. The capacitive response of the
sensor was tested for varying compressive forces. The CNT ink sensor was printed
directly onto the PDMS and underwent the same experiment. With the Ag NP ink
sensor, a 17.6% capacitance increase was achieved for a 0.4 kPa compressive force
when compared to the base capacitance. With the CNT ink sensor, a 9.9% capacitance
increase was achieved for a 405 kPa compressive force when compared to the base
capacitance. When both sensors were compressed to 3 mm, the difference between
their capacitance change from their respective base capacitances was 7.7% (Ag NP –
CNT). Due to its superiority over the CNT ink, the Ag NP ink sensor was chosen for the
experiment continuation. An electronic readout circuit was designed and tested with the
Ag NP sensor to view the capacitive response of the sensor in terms of voltage using a
capacitance to voltage circuit and an amplification circuit. The Ag NP ink sensor without
the headgear demonstrated a maximum decrease of 13% for a 6.8 MPa compressive
force when compared to the base voltage. The Ag NP ink sensor with the headgear
demonstrated a maximum decrease of 12% for a 0.95 MPa compressive force when
compared to the base voltage. The response of the printed sensor and the electronic
readout circuit demonstrated the feasibility of the design as an efficient, flexible and cost
effective way to monitor sports concussions.