By Matthew Stock
Engineers in Germany have built a biologically inspired artificial hand with muscles made from bundles of 'smart' wires. An electric charge is all that's needed to make these wires tense or relax, meaning the hand can operate without the bulky & cumbersome electronics that often make artificial prosthetic hands impractical.
The lightweight plastic hand itself was designed & 3D-printed by a research team from Saarland University. The muscle-like fibers are made from strands of nickel-titanium wire, each approximately 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.
p> "This enables us to build particularly lightweight systems, & 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, & 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 & at the Center for Mechatronics & 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 & 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.
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 huge 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 & feel more like natural ones. They say their design could reduce the need for bulky electric motors & pneumatics that are inherent to most current robotic prosthetics.
Seelecke moreover 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 & 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 & combine that with the actuation of the muscles so you can create some integrated, biologically inspired actuation system for prostheses," he said.