Not just Heads and Limbs

Glass Octopus: Courtsey: Wikipedia

A  prominent  head and the 8 limbs (variously called arms, legs or tentacles)  are the only conspicuous physical features  of an octopus. But  to  relegate them to  the family of   Cephalopods,  is to be frank,  a grave injustice.  With  Kephalē (κεφαλή) meaning "head" and podos meaning foot,   cephalopod  literally translates as  "head-foot".   In addition to the  the head and 8 feet they are  abundantly enabled  with  nine brains and  three hearts.  While  biologists are interested  in  unravelling the evolutionary lineage  of  octopus,  neuroscientists marvel  at their  highly distributed  nervous system and robotic engineers are stumped by the infinite  flexibility and dexterity of the tentacles.

Octopus is  blue blooded  because its  oxygen carrier is  copper based  hemocyanin which turns bright blue  upon binding with oxygen.  Since hemocyanin is a sluggish  oxygen carrier compared to hemoglobin, octopuses have 2 small  cardiac pumps  at the base of their gills  for   pumping deoxygenated  blood through the gills and  oxygenated blood to the systemic heart.  The systemic  heart situated at the centre of the body   ensures blood rich in   oxygen  and nutrients   reach  all the tissues. 

The nervous system is extremely simple  but admirably decentralized and has  a total of 500 million neurons almost as much as in a dog.  Of the  9 brains,   the  central  brain   is   housed  in the head  between the eyes.  This  unit  with 150-200 million neurons governs the octopus's intelligence and takes  major executive decisions. The remaining   neurons are distributed  in the peripheral nervous system.   A massive, segmented nerve cable  runs along each tentacle  with  mini brains of ganglia  at  each of the  individual suction cups called the suckers.  These   suckers  are also equipped with   thousands of chemoreceptors and mechanoreceptors, enabling  the tentacles    to move, think, taste and react autonomously.   So autonomous that even  an  amputated  tentacle   can twitch and  turn   for a short time.  

The tentacles are basically muscles and  exhibit  infinite degree of freedom through simple  mechanics of contraction, elongation, bending, twisting  and torsion.   Since muscles are mostly water which can't be compressed, octopus arm is in essence  a muscular hydrostat.  Though each tentacle  demonstrates  dexterity and flexibility  they   never   get  entangled or stick to each other because the  sensors in the suckers  can distinguish between "self"  and "other".     

If only these features can be replicated  in a bionic soft robotic arm!.   The major challenge in this endeavor  has been to mimic the simplicity and size.  Conventional   rigid jointed robotic arms  move with a "jerk" within defined geometrical angles whereas octopus' arms move elegantly, fluidly as that of a  ballerina.  To design  such a robotic arm   soft  materials with unique  mechanical characteristics and  miniature stimuli/sensors are not enough,  complex  algorithms to control shape, stiffness and motion are also needed.  

For materials  we   do have a wide spectrum of elastomers to choose from. Ranging from  very soft and pliable to moderately    flexible and to very   rigid and tough materials.  Mixing and matching these elastomers in  appropriate geometry  can yield  desired results.  Depending upon the material chosen the  stimuli could be  magnetic fields ( eg. elastomers   embedded with magnetic particles ), heat/temperature (for thermo-sensitive elastomers),   light (for photo sensitive elastomers ), electricity (for electroactive polymers), and fluid pressure (for hollow, tube like geometry).  For a given stimulus, the chosen materials would  respond in varying degrees enabling   bending, turning, twisting, stretching  and clutching. 

Recently a research team from Genoa, Italy  succeeded  in designing   a soft robotic tentacle  using  special grade of silicone elastomer.   The  arm  had a   conical shape  with 410 mm in length and 40 mm in diameter at the base. It  sported  ten  suction cups of decreasing  size from base to tip (from 20 mm to 12 mm). The suction cups were embedded with optoelectronic mechano-sensors minimizing the need for wiring.  The arm performed  with   high reliability,  and low power consumption.  

TAIL PIECE

Octopuses are loners, pairing only to mate and die.   The male dies soon after impregnating the female.  The  female  lays the eggs, cares for them meticulously even forfeiting food.    Once the eggs begin hatching,  she too succumbs perhaps from  starvation and exhaustion.   

REFERENCES:

1. How Octopus Arms Bypass the Brain

1. Learning from Octopuses: Cutting-Edge Developments and Future Directions

2. Peripheral control enabled by distributed sensing in an octopus-inspired soft robotic arm for autonomous underwater grasping