The ultimate soft machine
By Iain Anderson
Octopus eyes stared back at me from a rocky wall beneath the Shakespeare Cliff, Mercury Bay, New Zealand.
I couldnt get a good photo for he was hiding behind a shell, so I tried moving the shell out of the way but this resulted in the octopus retreating deeper into its rocky home. I mischievously moved the shell onto a ledge below, and waited.
He unrolled one tentacle until it engulfed one side of the shell. The sucker pads then attached to the shell and the arm retracted, hauling up the shell enabling additional suckers to become attached. The creature distributed its suckers along the shelllike many fingers and at the same time hauled up the other side.The whole process was executed with great care and dexterity.
Octopus arms are not only dexterous, they are also strong and can shorten or lengthen, curl-up or straighten, and capture prey using the suckers that line the arms. I once fought a tug-of-war with an octopus that wrapped its arm around the rubber cord between camera and strobe, an event that resulted in damage to the cord. With no hard skeleton or joints to restrict movement, there are a huge number of ways that it can move its arm to complete a task such as reaching out and fetching a shell.
The octopus arm is what engineers call a hydrostat.
Hydrostats maintain the same volume no matter what shape they take. Muscles inside work together to bend, lengthen and twist the organ.
If all muscles contract in a hydrostat then it simply becomes very stiff. Our tongue is a hydrostat. You can make your tongue lengthen or shorten, bend and twist, stiffen or become flaccid.
A soft and flexible device like an octopus arm would present a formidable control problem to an engineer. The octopus also has eight arms to control along with a soft and flexible body and octopus can control their body and arms to fit into any cavity that has a big enough volume, such as an old tyre.
They can also use muscles to change skin texture and colour for camouflage or communication. This requires a lot of control.
Rather than controlling the arms exclusively from the brain the octopus puts control locally into each arm.
Nerve centres (ganglia) within the octopus arm control movements with some autonomy from the octopus brain. So the brain sends a request to an arm to perform a task. The arm takes over and actions this in a particular predetermined way.
Octopus arms can perform a number of different actions including unfurling and rolling up. They can also make their arm bend around (in a fake joint) like our lower arm bends about the elbow joint. It is speculated that this makes it easier for the octopus to control the arm. They can even stand up and walk along the sea bottom as demonstrated in a recent video posted on Youtube.
For the engineer the development of an octopus-like device could open a few doors. It would become possible to reach deep inside mechanisms for repair or inspection, particularly in applications involving hard to get at places, such as squeezing through small holes or navigating around corners. It could also provide a powerful new platform for robotic surgical manipulators. Engineers are also currently making good progress at building octopus-like manipulators but I expect they have a lot of work ahead before a practical working system is available.
It will be some time before we see a robot that can manipulate objects, walk, crawl, and jet around like the octopus!
References and further reading:
1) Yekutieli, Y, Sumbre G, Flash T and Hochner B. How to move with no rigid skeleton. Biologist 49 (6), pp. 1-5, 2002.
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