Spiders use crack-shaped slit organs near their leg joints to detect minute mechanical vibrations. This inspired Mansoo Choi and his team from Seoul National University to design their own version of a mechanical sensor based on nanoscale crack junctions. As the researchers describe in their December 2014 Nature publication (DOI: 10.1038/nature14002), they fabricated their sensors by depositing a stiff 20-nm thin film of electrically conductive platinum on a flexible polymer (polyurethane acrylate). The material is then wrapped around glass rods of different diameters to induce controlled directional cracking with different spacing between the individual zigzag cracks. While signal transductions through the spiders’ neurons are responsible for the extraordinary vibration sensitivity of their slit organ, changes in electrical resistance as the zigzag crack edges of the platinum gaps pull apart or reconnect are the underlying cause of the artificial sensor’s operation. It is the gap geometry both devices have in common, which is crucial to achieve the remarkably high sensitivity to mechanical stress.
Nanoscale crack junction-based sensor attached to a violin for sound wave recognition (top left), recorded wave functions of E (yellow, fundamental frequency of 659 Hz), A (green, 440 Hz), D (blue, 294 Hz), and G (red, 196 Hz) strings (top right), measured sound waves of music playing (“Salut d’Amour”; excerpt shown in the bottom panel). Reproduced with permission from Nature 516 (7530) (2014), DOI: 10.1038/nature14002; p.178. © 2014 Macmillan Publishers Ltd.
Among other tests to prove the functionality and reversibility of their mechanosensor, the research team attached it to a violin (see Figure). Not only were they able to register sound waves, but they could also observe unique sensor responses to different frequencies. Additionally, the flexible sensor was mounted on skin and successfully detected speech or heartbeats with superior signal/noise ratios. The publication’s supplementary information includes a real time–peak spectrogram of Elgar’s “Salut d’Amour” played on the violin, a special treat for music lovers.
Throughout their study the researchers show that their sensor pixel array outperforms other mechanosensors in the 0–2% strain range, including recently reported sensors based on graphene platelets, carbon nanotubes, or organic semiconductors. “Precise engineering of crack formation” says Choi, “might be the future of ultrasensitive mechanosensing.”
