The next generation of soft robotics, smart clothing, and biocompatible medical devices are going to need integrated soft sensors that can stretch and twist with the device or wearer. However, most of the components used in sensing are rigid.
Now, researchers at Harvard University’s John A. Paulson School of Engineering and Applied Sciences have developed a soft, stretchable, self-powered thermometer that can be integrated into stretchable electronics, smart clothing, and soft robots.
The stretchable thermometer consists of three simple parts – a self-containing hydrogel electrolyte, an electrode, and a dielectric material to separate the two. The ions accumulate at the dielectric material and the electrolyte interface, while the electrons accumulate at the dielectric/electrode interface. The ions and electrons at the two interfaces are usually not charge-neutral, and this charge imbalance sets up an ionic cloud in the electrolyte. The design functions as a charged temperature-sensitive capacitor.
When the temperature changes, the ionic cloud changes thickness, and a voltage is generated. The voltage is sensitive to temperature but is unaffected by the thermometer being stretched or compressed.
“Because the design is so simple, there are so many different ways to customize the sensor, depending on the application,” said Yecheng Wang, a postdoctoral fellow at SEAS and first author of the paper. “You can choose different materials, arranged in different ways and optimized for different tasks.”
The researchers developed four designs with different configurations for the stretchable temperature sensor by arranging the electrolyte, dielectric, and electrode in different configurations. To prove its effectiveness, they integrated the sensor into a soft gripper and measured the temperature of a hot hard-boiled egg. They found that the sensor is more sensitive than traditional thermoelectric thermometers and can respond to changes in temperature within about ten milliseconds.
Depending on the materials used, the thermometer can measure temperatures upwards of 200 degrees Celsius or as cold as -100 degrees Celsius.
“We have developed soft temperature sensors with high sensitivity and quick response time, opening new possibilities to create new human-machine interfaces and soft robots in healthcare, engineering, and entertainment,” said Zhigang Suo, senior author of the paper.