Mar. 18, 2015 | By Alec

The availability of affordable 3D printed prosthetics has been surging in recent years, and are currently changing the lives of thousands of people everywhere. Not only are these far more affordable than all traditionally-manufactured prosthetics available on the medical market, they're also far easier to customize and adjust. Of course, most of these are simple plastic prosthetics with a mechanical grip, but 3D printed bionic alternatives are on their way.

While several ongoing projects are working to develop 3D printed bionic arms - that register and function through electronic signals from the arm's muscles – it looks like the Greek OpenBionics initiative is one of the first to succeed. This team of students should not be confused with the Bristol, England-based startup of the same name. Their team consists of postdoctoral associate at Yale Minas Liarokapis and undergraduate and PhD students Agisilaos Zisimatos, Christoforos Mavrogiannis and George. Professor at the National Technical University of Athens, Kostas Kyriakopoulos acts as their adviser.

As the OpenBionics team proudly reveals, their new designs can be made for less than $200 each, made from 3D printed components and off-the-shelf materials that won’t weigh more than 300 grams – less than an actual hand would. But most important is their new approach to mechanical grasping, which will ensure that every finger can move separately and together facilitate 144 different grasp movements with a single actuator.  It should, in short, be every bit as functional as a traditionally made prosthetic, but only cost a fraction of its price. That number is reached by 16 different finger combinations (all of which are done with a single motor) and 9 discrete positions of the thumb.

As you can see in the clip above, this OpenBionics hand is very functional.

Their hand designs are, as they should, aiming to fully recreate the functionality of the human hand. 'We argue that anthropomorphism is an important feature in hand design, resulting in improved performance for a variety of every day life tasks. In our design, anthropomorphism is reflected in two specific design choices: 1) the use of an anthropomorphic kinematic model and 2) the use of a bio-inspired finger actuation and transmission system,’ they explain.

Part of that anthropomorphic is their ‘Whiffletree’ selectively lockable differential mechanism, which can block the motion of any of the fingers. ‘The differential allows the user to select in an intuitive manner the desired finger combinations, implementing different grasping strategies with only 1 motor. The top two bars of our whiffletree have appropriately designed holes and the palm accommodates a series of buttons that upon pressing are elongated,’ they explain. ‘The idea is that when the button is pressed the elongated part fills the appropriate finger hole and the motion of this particular finger is constrained.’

The Wiffletree (above) and Thumb (below) mechanisms in a CAD file.

Their design also incorporates soft-tipped fingers that should interact with objects in the same fashion as a regular hands. These are realized through Plexiglas figures covered with silicone sheets. The thumb that sets us apart from so many animals is imitated through a lockable toothed mechanism that can implement 9 different opposition configurations. ‘The proposed mechanism is completely stiff when it is locked and is not affected by torsional forces inherent in dynamic / unstructured environments. A separate tendon routing system is used for the thumb and its tendon is terminated to a separate servo pulley,’ they write.

All that is packaged in a very inexpensive set of materials: A combination of sponge-like materials with low thickness rubber, that offers a high friction coefficient for fingertips, Dyneema fishing line which is used for the cables, offering zero elasticity and handling of high forces, a series of low friction tubes and pulleys, silicone joints, and a number of rigid links that form the core of the hand. Alongside these parts, numerous components have been 3D printed.

That latter fact means this bionic prosthesis is easily adapted for individual users. ‘The only parameters that we need in order to derive the finger phalanges lengths and the personalized finger base frames positions and orientations, are the human hand length (HL) and the human hand breadth (HB),’ the Greek team explains.

They have therefore also decided to make this hand project as open source as possible and help as many people as possible. Interested people can contact them through a webform on their website, in which they can simply provide a number of specifications (is it the right hand or left hand? What are your hand’s desired length and breadth? Those sort of things). They are then more than happy to provide personalized design files that can be 3D printed and assembled at home with parts available in most hardware stores. Alternatively, you can also directly download their CAD designs from their website here, while the code necessary to run the custom operating system can also be downloaded from there. 

 

Posted in 3D Printing Applications

 

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