(536a) Impedance Spectroscopic Determination of Fabric Thermal Sensor, Temperature Detection Mechanism

Authors: 
Blasdel, N., The University of Akron

            A
temperature sensitive fabric using homemade electrospun nylon 6 (PA6) or Tecoplastª polyurethane (PU) nonwoven nanofiber membrane
materials, commercial multiwalled carbon nanotubes (MWCNTs), and chemical vapor
deposited (CVD) polypyrrole (PPy) was developed1. A simple sensor
construction procedure facilitates reproducibly producing the fabric sensors
using simple laboratory procedures. This advantageous procedure is completed
without needing toxic semiconductor device construction environments. The work illustrates
the viability of using polymers/carbon allotrope composites as functional
materials for sensor applications. This thermal sensor provides useful data
using Ohm's law and the overall equation for the temperature coefficient of
resistance, or α = 1/R0*dR/dT, which has been well defined previously and is common for
fitting the temperature dependence of the electrical resistance of materials2.
Sensor response is exactly linear and negative between 25C and 45C, which is a
useful range for epidermal temperature measurement3,4,
and displays a high resolution, approximately equal to 0.1C to 0.5C, being that
it is a mostly polymer, fabric thermal sensor. Fig. 1 illustrates the general
behavior of the sensor material that classifies it as a thermistor.

Sensor Schematic.jpg

Figure 1: Schematic illustrating the thermal sensor
response.

            This
thermal sensor was designed for biomedical issues in measuring temperature of
the epidermis in load bearing situations like prosthetic sockets or shoes, and
could be applied in military and sports applications. Research shows that
greater than a 4C difference between the same points for each foot of a
diabetic, can mean they are at a greater risk for foot ulceration, or could be
an indication to more severe conditions such as neuropathy if the feet do not
maintain relatively consistent temperatures regularly5-9. To measure
the same points on each foot is cumbersome, as one imagines. Diabetics remove
their shoes and socks, taking the temperature of each point on each foot using
an infrared gun or even worse, a basic home use thermometer, and recording and
reporting the information manually to their physician. This provides a
potential for inaccuraties. A thin fabric temperature
sensor could accommodate continuously monitoring the temperature between points
on each foot, making it easier for diabetics to tell if they need to maintain foot
temperatures. This would make it more efficient for reporting the results to
their physician if the sensor is integrated to work wirelessly with a computer
at home to log the data and possibly communicate directly with their
physician's equipment. In addition, amputees could benefit from measuring
temperature inside their prosthetic sockets to maintain a comfortable and
hygienic stump socket environmen4,10-14. Surveys
show that heat and sweat are the major contributors to whether an amputee is
happy with their prosthesis, which can determine whether the amputee will even
wear the prosthesis regularly3,15,16. Currently,
temperature-measuring devices are constructed using rigid metallic and
semiconductor materials2, so it would be beneficial in some of these
load-bearing situations where pressure points could be problematic, to have
thermal sensors constructed from thin and soft nanocomposite materials. Even
applications, where pressure points are not a main concern, like smart textiles
and sports clothing could benefit from these fabric thermal sensors integrated
into their increasingly more complex systems to obtain useful physiological
data during periods of exercise and extreme environmental stresses and
situations.

            The
procedure to construct these sensors starts by electrospinning PA6 or PU sheets
onto a homemade rotating drum. Commercial MWCNTs are vacuum filtered onto the
polymer membranes from aqueous Triton X-114 stabilized nanotube suspensions.
PPy is polymerized from pyrrole in a CVD chamber at Å 100 kilopascals of vacuum
and ambient temperatures for two days. The PPy overcoats on and throughout the MWCNTs
on the polymer membranes, using iron(III)chloride, or
ammonium persulfate as the oxidant, making a nanostructured composite material
that is electrically sensitive to temperature changes. The sensors are then cut
from the membrane in 1-cm2 geometries and tested in humidity and temperature
controlled environment using 2, 3, and 4 electrode
configurations17-20. Fig. 2 depicts the 4
electrode setup.

Figure
2: Design for sensor testing.

            This
material has unique electrical characteristics. The polymers generally have electrical
surface resistivities in the 1010 and 1013 to 15 Ohm-cm
ranges for PA6 and PU, respectively. The application of MWCNTs and PPy to the polymer
membranes has reduced those surface resistivities over seven orders of magnitude
to give resistivities in the kOhm range and
temperature sensitive super abilities. The material's electrical resistance
responds linearly and reproducibly to temperature changes between 25C and 45C
with a negative temperature coefficient of resistance ranging from -0.56%/C to
-0.14%/C, a 0.1C to 0.5C thermal resolution, and sensitivities ranging from 2
Ohms/¼C to nearly 30 Ohms/¼C. Fig. 3 represents the steady state calibration.

Full Calibration_Wuuuuuut.jpg

Figure
3: Steady state calibration curve showing the actual steady state temperature
and resistance versus time data.

            Bulk
DC resistivity, and hence bulk resistance, or R = ρl/a, where R is the resistance in Ohms, r is the resistivity in Ohm/cm, l is the length travelled by the
current, and a is
the cross sectional area perpendicular to the flow of current, is used as a
model to determine temperature. This does not really elucidate the true sensing
mechanism at the nano, or atomic, scale of the sensor material. Nor does it
provide any useful information regarding the nature of the interface between
the sensing material, the outside environment, and the sensor's electrical connections.

            Impedance
spectroscopy (IS) is an important scientific tool that can provide extremely
useful information about the interface between a sensor and its electrical
contacts, chemi- and physi-sorption
as well as any reactions and their kinetics at the material surface and within21,22. Ultimately IS elucidates a material's dielectric
structure under electrical stress, which also translates into mechanical, stress,
thereby providing a window into electronic transport through a material and its
interfaces23,24

            This
presentation will focus on the value and constraints of IS in determining
interfacial phenomena between this constructed temperature sensitive fabric,
its electrical contacts, and the surrounding environment. An expanded
equivalent circuit model for three and four electrode configurations will be
proposed. In addition a simplified circuit equivalent will be shared. A short
discussion on the differences in measuring techniques between solid-state
devices and electrochemical cells will finish the talk, while emphasizing the
use of common circuit analysis to determine and simplify "dry" and "wet" sensor
models.

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