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Smart material performs better under pressure

Photo Credit: University of Wollongong

The development of a rubber that combines flexibility with high electrical conductivity may bring forth advanced robotics sensitive touch or the next-generation wearable devices that have sophisticated sensing capabilities.

As reported, researchers from the University of Wollongong’s (UOW) Faculty of Engineering and Information Sciences, in Australia, have developed a new smart composite material that increases in electrical conductivity as it is deformed, particularly when it is elongated.

Robotics and wearable technology need elastic materials such as rubbers because they are inherently flexible and can be easily modified to suit a particular need.

A conductive filler, like iron particles, is then added these elastic materials in order to produce a composite material that is electrically conductive.

The need for flexibility and conductivity

Researchers were initially hampered with finding a combination of materials that will produce a composite that overcomes the competing functions of flexibility and conductivity.

Typically, as a composite material is stretched, its ability to conduct electricity decreases as the conductive filler particles separate.

However, it is a vital requirement for the emerging spheres of robotics and wearable devices that despite being bent, compressed, stretched or twisted, conductivity will be retained.

The University researchers have developed a material that throws out the rule book on the relationship between mechanical strain and electrical conductivity.

They discovered a composite material, with the use liquid metal and metallic microparticles as conductive filler, which increases its conductivity as more strain is placed on it.

This discovery, which came about in an unexpected way, presents new possibilities in applications.

The first step in the process was a mixture of liquid metal, iron microparticles, and elastomer that, by a fortuitous accident, had been cured in an oven for much longer than normal.

The over-cured material had reduced electrical resistance when subjected to a magnetic field.

However, it took dozens more samples to find that the reason for the phenomena was an extended curing time of several hours longer than it would normally take.

Upon accidentally stretching a sample while its resistance was being measured, the team discovered that the resistance reduced dramatically.

Meticulous testing showed that the resistivity of this new composite could drop by seven orders of magnitude when stretched or compressed, even by a small amount.

The increase in conductivity, when the material is deformed or a magnetic field is applied, shows unprecedented properties.

Potential applications

The researchers demonstrated several interesting applications such as exploiting the composite’s superior thermal conductivity to build a portable heater that warms where pressure is applied.

Adding pressure to the area increases the heat while removing it reduces the heat. This is a feature that can be used for flexible or wearable heating devices, such as heated insoles.

The addition of electrical conductivity makes the materials ‘smart’ by being able to convert mechanical forces into electronic signals.

The discovery had overcome the key challenge of finding a flexible and highly conductive composite material.

Additionally, its unprecedented electrical properties could lead to innovative applications, such as stretchable sensors or flexible wearable devices that can recognise human motion.

Many scientific advances have come from unusual ideas. The exploration of unconventional fields and a lab culture that encourages innovation is more likely to bring unexpected discoveries.

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