(Main image) Time-lapse photos of the autonomous wire crawling of the SL-PS. The SL-PS moves a small cargo (medicine capsule) while simultaneously transferring its on-board stored electrical energy from left to right, illuminating a red LED. (Inset) Photo of the printing process and different SL-PS devices.Soft electronics promise an alternative to conventional solid-state electronics for a new generation of soft robotic, wearable or implantable devices. However, these devices are constrained by bulky power sources or tethered by leads ultimately limiting their usefulness. Now, researchers have designed an power source for soft robotics that change shape [Lee et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2022.04.003].
“We demonstrated a so-called ‘transformer power source’, which can autonomously shape-morph,” says Sang-Young Lee of Yonsei University in Korea, who led the effort with colleagues at Ulsan National Institute of Science and Technology (UNIST) and Inha University.
The new power source is based on a photothermal supercapacitor and liquid crystalline polymer network (LCN), which contains thermoresponsive rod-like molecules. Electrodes, consisting of the conductive polymer PEDOT:PSS and multiwalled carbon nanotubes (MWNTs), are deposited on top of the LCN film using mask-assisted spray printing. Ultraviolet-assisted dispenser printing is used to add a quasi-solid-state electrolyte (QSSE) ink into the interdigitated spaces of the electrodes. Finally, the structure is sealed with a thin film of parylene-C. When exposed to light or heat, the shape-reconfigurable locomotive rechargeable power source (or SL-PS) repeatedly changes shape while maintaining an uninterrupted source of power.
The MWNTs act as a flexible scaffold and a conductive network within the polymer PEDOT:PSS matrix. Moreover, the PEDOT:PSS- MWNT electrodes exhibit a photothermal effect – rapidly heating up when exposed to near-infrared radiation (NIR) and cooling down when irradiation ceases. Because the rod-like molecules in the LCN expand and contract differentially in response to temperature changes, the device curls up – or shape morphs – in response to heat or light, returning to a flat configuration when cooled. The QSSE, meanwhile, exhibits stable ionic conductivity at both room and high temperature.
The researchers fabricated various SL-PSs to demonstrate some of the possibilities. A two-dimensional flat SL-PS strip twists into a helical coil when heated to 80°C, while kirigami is used to create other pop-up shapes. Despite changing their morphology, SL-PSs continue to behave as a capacitor and continuously supply power, to an LED, for example. The team also demonstrate an autonomous wire-crawling device from three connected unit cells. When irradiated with NIR, the devices repeatedly curve and straighten to inch along guide wires in a stick and slip motion, carrying a small load.
“We envision that the transformer power sources hold promise as a multidisciplinary technology platform that opens a new avenue for untethered soft robotic power sources,” says Lee.