Described in the journal Advanced Science, the sensor could reduce the complexity and cost of IoT solutions and have applications across several industries, including robotics, healthcare and security. Its underlying material is made by combining cellulose nanofibres with the conducting polymer PEDOT:PSS and freeze-drying the mixture in a vacuum. Adding a substance known as polysilane causes the resulting aerogel to become elastic.
The material conducts ions and electrons, exploiting the subsequent thermoelectric effect whereby electrons move from the cold side of the material to the warm side. This creates a voltage difference that can be measured. Increased pressure causes a fall in resistance and electrons flow more readily. Similarly, the larger the temperature difference between the warm and cold sides, the higher the voltage developed.
Humidity also affects the material, determining how rapidly the ions move from the warm side to the cold one. If the humidity is zero, no ions are transported. A key innovation from the Linköping scientists was devising a method to determine the effect of each parameter independently in order to get clean readings.
"What is new is that we can distinguish between the thermoelectric response of the electrons (giving the temperature gradient) and that of the ions (giving the humidity level) by following the electrical signal versus time,” said principal author Xavier Crispin, professor in the university’s Laboratory of Organic Electronics. “That is because the two responses occur at different speeds. This means that we can measure three parameters with one material, without the different measurements being coupled.”
The technique for filtering out the individual signals for pressure, temperature and humidity was developed by doctoral student Shaobo Han and senior lecturer Simone Fabiano. According to Fabiano, the simplicity and cost-effectiveness of the multimodal cellulose sensor could see it adopted for a range of applications.
"Our unique sensor also prepares the way for the internet of things, and brings lower complexity and lower production costs,” she said. “This is an advantage not least in the security industry. A further possible application is placing sensors into packages with sensitive goods.”
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