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2 CORE BODY TEMPERATURE
ESTIMATION FROM BODY
SURFACE MEASUREMENTS
One of the earliest efforts to develop a sensor for
core body temperature measurement from surface
measurements has been based on zero heat flow
method (Fox, Solman, Isaacs & MacDonald 1973).
The method uses a heater to create a zone of zero
heat flux such that the skin temperature under the
sensor reaches the core temperature. Studies have
shown the effectiveness of this method except
during exceptional rapid cooling or heating as the
response time is around 15 to 20 minutes when
applied to forehead (Togawa 1985). There have been
other attempts to evaluate the effectiveness of this
scheme for different applications (Zeiner et.al. 2010)
(Teunissen et.al. 2011). However, the use of a
heating element creates problems for applications
which involve mobility and require low power
consumption. New technique which does not involve
the use of heater and is based on a double sensor has
been reported for monitoring heat strains (Gunga
et.al. 2008) and also for space applications (Gunga
et.al. 2009). The design uses two temperature
sensors separated by an insulating layer whose
thermal conductivity is known. By measuring the
skin temperature and the temperature at the upper
sensor and knowing the thermal conductivity of
human tissue, the core temperature can be
calculated. However, this technique still requires the
correct knowledge of the thermal conductivity of the
epidermal tissues where the sensor is placed.
Another solution has been proposed using two heat
flow channels in parallel (Kitamura, Zhu, Chen &
Nemoto 2010). This work avoids the dependence on
knowledge of the thermal properties of tissue below
the sensor. Its performance has been compared to the
zero heat flux method. Further work has been
presented to improve the response time (Sim, Lee,
Baek & Park 2012). These present interesting ideas
for use in continuous monitoring applications.
However, additional constraints on materials of the
device to conform to body contour have to be added
for it to be acceptable for long term use on neonates.
Also, correspondence between experimental
measurements and theoretical correctness and
assumptions has to be established. We explore these
aspects by considering sensors for neonates. We
study the accuracy under varying conditions and at
different sites of measurement using both
simulations and experiments.
2.1 The Device Structure
For the device to be easily acceptable to parents for
long term use, it has to be such that it does not leave
any mark on the neonate’s skin. Also, it cannot be
taped to the skin as regular application and removal
of tapes create redness. Using a hard metal contact at
the skin interface or an inflexible material for the
sensor cover, would create problems. Hence using a
soft material that bends to conform to the body
contour and also ensure that the sensor is in contact
with the skin is a good option.
PolyDimethylsiloxane (PDMS) can be used as the
material for the device. PDMS provides both
flexibility and biocompatibility. Hence it can be
placed on the delicate skin of neonates over long
periods of time. Another major advantage provided
by PDMS is that it is highly hydrophobic. This
makes it very good for usage where it is likely to be
exposed to water or urine.
We use Dow Corning’s Sylgard 184 elastomer
kit for the purpose. Two different ratios of base and
curing agent have been used for two different layers
of the sensor (shown in Fig 1). For the layer that is
contact with the skin, base elastomer to curing agent
ratio of 10:0.4 is used while and for the top layer, a
ratio of 10:1 is used. The top layer is prepared first
and the lower layer is added after partial curing of
the first layer. This ensures that the two layers bind
together. By following this method, the need for
complex surface modification techniques like use of
oxygen plasma to improve adhesion between the
layers is avoided. The lower layer provides adhesion
to the skin surface and the upper layer provides the
rigidity needed for the sensor. Also, the lower layer
is softer and does not leave any mark when placed
on the skin. Experiments were performed with
different ratios of base and curing agent. It was
observed that a ratio of 10:0.2 provides better
adhesion over a longer time. But the layer is too soft
and can get easily damaged. The ratio of 10:0.4
provides adhesion while being strong enough for
long term use. However, the adhesion of the sensor
is not enough to hold it for a long time over
prolonged removal and placement cycles especially
in the presence of oil or dirt on the skin surface.
Hence a belt has been provided on the side of sensor.
In the presence of a belt, the adhesion is enough to
ensure contact of the sensor with the skin to get
reliable measurements with low pressure from the
belt on the skin. Instead of using IC based
temperature sensors as used in prior work, we use
high precision thermistors (MF51E103E3950). Since
the device can be made in very controlled manner
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