A multipurpose material that can sense strain and temperature and harvest energy from temperature gradients has been developed by researchers in the UK and the Netherlands. The new material, which could be used to create smart human–machine interfaces and health monitoring devices, was created by Emiliano Bilotti and collaborators at Queen Mary University of London, Imperial College London, Eindhoven University of Technology and Loughborough University.
Current wearable sensors typically have limited mechanical flexibility or require a stiff battery to work. In this research, Bilotti and colleagues have discovered that commercially available Lycra yarns, a flexible material commonly used in textiles, can be modified to show thermoelectricity and strain sensitivity. This is done by adding the conductive copolymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS).
Using the new material, the team developed a device that can operate in three different modes to sense strain, measure temperature differences or harvest energy. The strain sensitivity can be useful for creating gloves that track hand movements and the thermoelectric property of the material could be used to power such a glove by harvesting energy from the difference between body temperature and the surrounding ambient temperature.
Low-cost fabrication
The Lycra yarns are given strain sensitivity and thermoelectric properties by immersing them in a solution containing PEDOT:PSS. Upon evaporation of the solvent, the conductive copolymer attaches to the surface of the Lycra yarns, conferring electrical conductivity on the material.
The researchers noticed that, by applying a high strain on the coated fibres, they could induce the formation of cracks across the surface of the PEDOT:PSS coating. These cracks increase the total surface area of the yarns, and provide the strain sensitivity. This is because they create interconnected patches of the conductive copolymer, which separate with the application of a strain.
Increasing the separation between the isolated patches decreases the electrical conductivity of the material, allowing it to be used as a strain sensor. As a result, the material exhibits a large change in resistance even when the sensors are deformed using a low strain of 1%. However, the team found that the thermoelectric properties of the coated yarns are not influenced by strain or the cracks – which do not interfere with the ability of the sensor to measure temperature differences.
Relative temperature
Since the temperature sensitivity of the yarns is based on thermoelectricity, they are not able to sense absolute temperature values. Instead, the material can measure temperature differences as small as 7 °C. This could be used to measure the relative temperature of the human body with respect to the surrounding environment. In addition, the temperature gradients generate a voltage difference due to the thermoelectric effect, which can be used to create electric power.
Carbon-nanotube yarn converts motion into electricity
As a proof of concept, the researchers sewed the yarns onto a glove, and used them to sense the temperature of an object relative to that of the hand. Due to the small temperature differences between a human hand and the environment (around 10 °C), the team calculates that a glove containing 1800 strands of yarn could power the necessary electronics for its practical use. The material could also be used to measure the strain of the glove. This would be useful for creating self-powered wearable devices where, for example, the hand position and its temperature could be measured autonomously.
The findings are reported in Materials Horizons.
