Clevertex - Grado Zero Espace Srl

2.7 Piezoelectric materials
2.7.1 Description
The word ‘piezo’ is derived from the Greek word for pressure. In 1880, Jacques and Pierre Curie
discovered that pressure, applied to a quartz crystal, creates an electrical charge in the crystal; they
called this phenomenon the piezo effect. Later they also verified that an electrical field applied to the
crystal would lead to a deformation of the material. This effect is referred to as the inverse piezo effect.
Piezoelectric materials can be used to convert electrical energy into mechanical energy and vice
versa. After the discovery it took several decades to utilize the piezoelectric phenomenon. The first
commercial applications were ultrasonic submarine detectors developed during World War I and in the
1940’s scientists discovered that barium titanate ceramics could be made piezoelectric in an electric
field.
Pressure sensors to provide information on pressure field distribution and thus enabling to monitor
seating or standing positions of the wearer over time can be based on piezoelectric and piezoresistive
substrates, but they are difficult to integrate into elastic fabrics because mechanical stresses, like
impact and compression, can seriously damage the sensors. Works and projects on pressure sensors
are already reported on the chapter conductive materials.
However, we would like to mention the fabric strain sensor developed at the University of Pisa at this
point to explain the sensing function. The sensing function of the textile material is realised in two
different ways:
1. yarn coated with polypyrrole
2. yarn coated with carbon loaded rubber
Polypyrrole is a π-electron conjugated conducting polymer that combines good properties of elasticity
with mechanical and thermal transduction. Lycra fabric is PPy-coated using a slightly modified method
developed by Milliken Research Corporation. In this way a fabric sensor is developed (US patent
4,803,096).
The sensors based on carbon filled rubber (CFR) are realized either by directly printing the
carbon/rubber mixture onto fabrics or by weaving CFR coated fibres.
Due to the piëzoresistive properties of the used materials, there is a relationship between an imposed
strain and the resistance of the material. In this way a wearable sensing system is developed to
measure biomechanical signals such as respiration, heartbeat and body movement.
Researcher in the US developed an in-shoe multisensory data acquisition system that monitors
temperature, pressure and humidity [203].
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