Japanese engineers have created the Pentabot, a robot inspired by a relative of the starfish called the “brittle star”, which has the unique ability to keep going even when it has lost most of its limbs.

By copying the neurological and physical attributes of the brittle star (aka an “ophiuroid”), the Pentabot has successfully emulated the zombie-like advancement of the starfish in trials, even after four of its five appendages had been severed.

Call it creepy, but it holds massive promise for rescue, clean-up and salvage work at disaster sites, where equipment is prone to damage which can slow down or derail missions.

Secrets of the brittle star

The brittle star has five functionally interchangeable arms. Its radial shape lets it move in any direction without having to turn to “face” in a new goal direction. It can self-amputate limbs to escape from predators and without being deterred from going where it needs to.

Despite its remarkable adaptive capabilities, it lacks a central nervous system and instead has radial nerves along each arm connected to a central nerve ring. That might explain a bit about its regenerative magic, but how do we explain its incredibly resilient movement?

Engineers studied the movements of the starfish with and without limbs and described their findings at the hand of a mathematical model, as follows:

  • Movement and direction is determined by a chemical stimulus, e.g. food.
  • Each arm moves randomly, generates force in contact with the environment and gives sensory feedback to the nervous system in a decentralised way.
  • If the contact force persists in propelling the organism towards the food, the organism persists with long propulsion strokes and quick recovery strokes.
  • If the contact force impedes movement towards the food, no further action is generated.

This model was successfully tested on a robot built and controlled as follows:

  • Each robot arm has a horizontal and vertical rotational joint, actuated by servo motors producing lift and movement.
  • A decentralised reflex controller was implemented to simulate movement as described in the model.

The model was proved sound when the robot moved in remarkably similar ways to the brittle star, even after up to 80% of its limbs were amputated in various combinations – all without changing or retuning the controller program.

The successful results will pave the way for development of more resilient and adaptive robots that can assist in human-inaccessible areas, e.g. disasters, and to refine our study of the essential mechanisms underlying resilient animal locomotion.