An electrolyte powers a stretchy thermometer
Solutions with dissolved ions make versatile temperature sensors. They’re inherently transparent and flexible, qualities that are often desirable for such applications as soft robotics and human-interfacing medical devices. Now a device developed by Yecheng Wang at Harvard University and colleagues has another useful feature: It’s self-powered.
The researchers’ so-called ionotronic thermometer is essentially a capacitor. In its simplest configuration, sketched in the figure below, an electrolyte bridges two conductive electrodes. Cations in the electrolyte accumulate at the interfaces with the electrodes. Electrons in the electrode respond by building up on the other side of the interface.
The distribution of ions near the interface is characterized by the Debye length L, which in turn characterizes how far the ions’ electrostatic influence extends beyond an interface. The value of L is proportional to the square root of the temperature, and it affects the amount of electron buildup at each interface. If one end of the device is warmer than the other, the difference in L values causes a proportional voltage drop between the electrodes.
Y. Wang et al., Proc. Natl. Acad. Sci. USA 119, e2117962119 (2022)
The ionic conductors in the experiments were polyacrylamide hydrogels containing sodium chloride; the electrodes were stretchable silver-plated fabric and gold. The components were sealed in a transparent stretchable elastomer. To calibrate the devices, the researchers measured both the temperature difference and voltage between the two ends. The response was linear over temperatures from about 20 °C to about 50 °C.
Wang and coworkers confirmed that the same temperature-sensing principle worked with other device configurations. One involved two ionic conductors with different NaCl concentrations, and another used mismatched electrodes. Tests of the devices showed that the measured voltage didn’t change when the components were stretched and that the sensitivities, which ranged from about 1 mV/°C to about 10 mV/°C, remained nearly constant over the course of a week.
Y. Wang et al., Proc. Natl. Acad. Sci. USA 119, e2117962119 (2022)
The researchers incorporated the ionotronic thermometer into a soft pneumatic gripper and demonstrated its performance by picking up a hot egg. The stretchy, transparent sensors could be particularly useful for temperature monitoring of engineering structures and biological tissues. (Y. Wang et al., Proc. Natl. Acad. Sci. USA 119, e2117962119, 2022 .)
Updated 18 February 2022: A previous version of this story erroneously cited the upper limit of the device’s linear temperature response as about 30 °C.