A biologically inspired propulsion system has been demonstrated with a series of microrobots. Locomotion is effected without the use of engines or batteries: the hygrobots move by means of water absorption and evaporation.

Seoul National University researchers observed that some seeds can propel themselves into the ground by An “inchworming” hygrobot. Credit: Shin et al., Science Roboticsmeans of outer coating layers, some of which absorb moisture from air. Moist air causes the outer coating to stretch and bend, resulting in motion.

This finding was applied to microbots sporting absorbent and non-absorbent outer coatings, a combination which induces motion based on moisture. The hygroots were designed in particular shapes so that only one end moves, causing the device to move slowly in that direction as it repeatedly bends and relaxes. A specific mix of nanofibers was found to enable hygrobot travel across a wet surface in a dry environment, using only the natural differences in humidity levels.

An inch-worm-like hygrobot, carrying a payload of antibiotics, moved across a bacterial biofilm-containing petri-dish . The researchers indicate such bots might one day prove useful in military applications, or as agents able to deliver drugs inside the body.

The research is published in the journal Science Robotics.