Tiny Robots Inspired by Pine Cones

Future of Bio-inspired Robotics Lesson Learned from Plants

According to a group of Seoul National University researchers, the future of bio-inspired engineering or robotics would be greatly beneficial from lesson learned from plants. At the time of the 68th Annual Meeting of the Division of Fluid Dynamics, of American Physical Society held on Nov 22-24, 2015, in Boston, they shared details regarding how studying plants enabled them to develop tiny robots powered solely through changes in humidity.

The simplicity of the method by which the pine cones as well as seeds tend to respond to changes in an environmental humidity with motion, is the focus of work of the group. Professor in the Department of Mechanical and Aerospace Engineering at Seoul National University, Ho-Young Kim has explained that some seeds comprise of a head which contains all its genetic information together with a long appendage known as `awn’ which is responsible for locomotion like an animals’ sperm.

Awns are composed of two layers of tissue, one that swells with humidity – active, and the other that is insensitive to humidity changes – inactive. If the humidity in the environment tends to increase, the bilayer bends from changes in length-wise swelling. The bilayer tends to bend and unbend repeatedly due to periodic humidity changes which mean that changes in environmental humidity could be converted to mechanical work. Kim states that we imitate the bilayer structure in making an actuator which can generate motion on utilising environmental humidity changes.

Legs Attached to Actuator Known as `Ratchets’

Plants tend to move slowly, one cycle of bending and unbending could take a whole day. In order to increase the response speed of bilayer, it was essential to develop a new way of fabricating the active layer. The response speed tends to increase with the surface-area-to-volume ration of the layer since humidity is absorbed quickly and so active nanoscale fibres are deposited onto an inactive layer.

While the main step in the development of a robot, repeated bending as well as unbending does not produce net locomotion, Kim states that `this motion of cycle needs to be converted into directional motion in order to create a robot which moves.

Hence legs were attached to the actuator which enables only one-directional locomotion and the legs are called `ratchets’, and are combined with an actuator to build the bio-inspired robot’. The work of the group seems to be significant since it tends to open the door for tiny robots with the potential of locomotion based mainly on changes in environmental humidity, where no electrical power supplies seems to be involved.


One can imagine a robot operating in the field where no electricity is available, since they operate on changes in humidity levels just the way seeds do. Kim states that making a bilayer for the robots is not difficult but making a fast one needs technical expertise.

The group had also created a mathematical model to discover the optimum design for the robot in achieving the fastest speed for any given robot size. The reason for envisaging a bright future for humidity change-powered micro robots by the groups was because humidity changes seem to be all around us. Kim pointed out that for the group’s future plans, human skin seems to be more humid than the atmosphere and this is the main humidity gradient which was essential to tap into.

The team would be exploring the option of placing a tiny robot on human skin directly, one which bends due to its humid near skin. He commented that the concept is that on bending, some part of the robot would move away from the skin to meet with the dry atmospheric air. When it dries, the robot would return to an upright position near the skin and the cycle begins again with the robot continuing to move based on the skin’s humidity changes.