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 The lightweight plastic hand itself was designed and 3D-printed by
a research team from Saarland University. The muscle-like fibers are
made from strands of nickel-titanium wire, each about the width of a
human hair. The metal wire, known as shape-memory alloy, has the
highest energy density of all known actuation mechanisms, which
allows it to perform powerful movements in restricted spaces.
"This enables us to build particularly lightweight systems, and the
fact that they come in the form of wires enables us to use them as
artificial muscles, or artificial tendons. So we can build systems
with those that can be like bio-inspired, look-to-nature for a
successful prototype, and that's what we realized with this first
prototype of a robotic hand using shape-memory alloy wires,"
explained Professor Stefan Seelecke from Saarland University and at
the Center for Mechatronics and Automation (ZeMA).
The term 'shape memory' refers to the wire's ability to return to
its original shape after being deformed. In the case of the bionic
hand, an electrical charge transforms its lattice structure causing
it to contract like a muscle. When the charge is turned off, the
wire 'remembers' its original shape as it cools down.
 To demonstrate the effect, an early prototype model of a bat used
just two strands of shape-memory alloy to recreate the movement of
the beating of its wings.
Engineer Filomena Simone, a PhD student who co-developed the
prototype bionic hand, said they copied the structure of muscles in
the human body by grouping the fine wires into bundles to mimic
muscle fibers. This bundling of wires present a greater surface area
through which heat can be dissipated meaning they can undergo rapid
contractions and extensions, much like real human muscles.
"The movement of the hand is done by the wire. This wire, when
activated, they contract. And we are able to exploit this
contraction to make the finger move. And we can move each phalanx
independently," Simone said.
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She added that a single semiconductor chip controls the shape of the
smart wires, which use electrical resistance to function. This means
no external sensors are needed as the material itself has sensory
properties allowing the hand to perform extremely precise movements.
"We can monitor the position of the finger without adding any other
sensor; only exploiting this embedded feature of the wire. This
helps us to always preserve a very lightweight structure. This is a
big deal because normally prostheses until now are very heavy," she
said.
While the technology is still in the early stages of development,
the team is hopeful that the technology could eventually be used to
create prosthetic limbs that function and feel more like natural
ones. They say their design could reduce the need for bulky electric
motors and pneumatics that are inherent to most current robotic
prosthetics.
Seelecke also believes the technology could one day be fully
integrated into the human neurosystem.
"I think if you look down the road to future prostheses generations,
you'd like to see this integrated with the human body in a way that
you can actually sense the nerve stimuli and then can feed that into
a micro-controller which there will be translated to a corresponding
signal to activate the muscle. So, eventually you need to couple
nerves with proper electrodes and combine that with the actuation of
the muscles so you can create some integrated, biologically inspired
actuation system for prostheses," he said.
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