Tech Briefs Magazine - May 2022 - 24

Sensors
Polymer Mixture Creates Ultra-Sensitive Heat Sensor
The flexible, transparent, and printable sensor has applications ranging from wound healing
and electronic skin to smart buildings.
Linköping University, Linköping, Sweden
A
n ultra-sensitive heat sensor was developed
that is based on the fact
that certain materials are thermoelectric.
The electrons in a thermoelectric
material move from the cold side to the
warm side when a temperature difference
arises between the two sides and a
voltage difference arises. In this project,
researchers developed a thermoelectric
material that uses ions as charge carriers
instead of electrons, and the effect
is 100 times larger.
A thermoelectric material that uses
electrons can develop 100 µV/K (microvolt
per Kelvin), compared with
10 mV/K from the new material. The
signal is thus 100 times stronger, and
a small temperature difference gives a
strong signal.
The new material is an electrolyte
that consists of a gel of several ionic
polymers. Some of the components are
polymers of p-type, in which positively
charged ions carry the current. A highly
conductive polymer gel of n-type was
discovered in which negatively charged
ions carry the current. Very few such
materials have been available until now.
With the aid of previous results from
work with electrolytes for printed electronics,
the researchers have now develThe
ultra-sensitive heat sensor. (Photo: Peter Holgersson AB)
oped the first printed thermoelectric
module to use linked n- and p-legs,
where the number of leg connections
determines how strong a signal
is produced. The scientists have used
screen-printing to manufacture a highly
sensitive heat sensor based on the different
and complementary polymers. The
heat sensor has the ability to convert a
tiny temperature difference to a strong
signal: a module with 36 connected legs
gives 0.333 V for a temperature difference
of 1 K.
For more information, contact Monica
Westman at monica.westman@liu.se; +46 13
286839.
Material for the Next Generation of Smart Sensors
Taking inspiration from the biomimicry of butterfly wings and peacock feathers, an opal-like
material could warn when an earthquake might strike.
University of Surrey, Guildford, United Kingdom
S
cientists have developed color-changing,
flexible photonic crystals that
could be used to develop sensors that
warn when an earthquake might strike
next. The wearable, robust, and low-cost
sensors can respond sensitively to light,
temperature, strain, or other physical and
chemical stimuli, making them a promising
option for cost-effective smart visual
sensing applications in a range of sectors
including healthcare and food safety.
24
The method to produce photonic
crystals containing a minuscule amount
of graphene results in a wide range of
desirable qualities with outputs directly
observable by the naked eye. Intensely
green under natural light, the sensors
change color to blue when stretched or
turn transparent after being heated.
The mechanically robust yet soft, freestanding,
and flexible polymer-based
opals contain solution-exfoliated priswww.techbriefs.com
tine
graphene. The scientists took inspiration
from the biomimicry abilities in
butterfly wings, peacock feathers, and
beetle shells where the color comes from
structure and not from pigments.
Other applications include time-temperature
indicators (TTI) for intelligent
packaging in which the sensors give a
visual indication if perishables, such as
food or pharmaceuticals, have experienced
undesirable time-temperature
Tech Briefs, May 2022
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