Student-built exoskeleton mimics human knee

Mechanical engineers from ETH Zurich (Swiss Federal institute of Technology) are developing a new powered exoskeleton that they hope will improve the lives of paraplegics by allowing them to walk with greater freedom than possible with current assisted walking devices.

Metal exoskeleton suits worn outside the body, delivering energy for limb movement, are becoming more widespread, helping survivors of strokes, spinal cord injuries, and other lower extremity weaknesses to walk again. But one of the main limitations of current exoskeletons is restricted joint mechanisms that make anything other than walking in a straight line difficult.

The VariLeg team in Zurich believes they have hit upon the novel idea of adjustable softness in the knee that could make their exoskeleton the first model that does not struggle on uneven terrain. It mimics the natural behavior of knee movement during gait more accurately thanks to built in variable impedance actuators that continuously change the knee stiffness so it can adapt to irregular surfaces or obstacles.

"With the Varileg we implemented the mechanical variable impedance, which is something special that no other exoskeleton has implemented at the moment. And the advantage of this is that we can mimic the human-like stiffness adaptation of the human knee and this also allows us to adapt to unexpected obstacles because we can say how stiff the knee should behave," mechanical engineering student Patrick Pfreundschuh told Reuters.

He added: "This will give people the ability to adapt to unexpected obstacles which will also make them more flexible to walk and it's also possible to adapt this stiffness, and that's also what the human body does during the natural human gait cycle."

The first prototype VariLeg exoskeleton that uses the mechanical variable impedance is, admittedly, very bulky. However, the team has been working with paraplegics to work out how the design can be improved. Pfreundschuh explained that they are currently building the much more streamline VariLeg 2 exoskeleton. He added that the wearer would still need to use crutches with the VariLeg, but that these will act as a control to adjust the device for different situations.

"On the crutches we will have several buttons and with these buttons he (the wearer) can decide where he's going and also to change the modes; so we will have straight-walking mode, and stairs mode and sit-down mode. So he can change between the modes and then tell the device what it should do," said Pfreundschuh.

The VariLeg developers will be one of a number of teams participating in the first annual Cybathlon - a championship for physically disabled athletes who will use new assistive devices to compete to finish a variety of everyday tasks that are particularly difficult for their individual handicap. For instance, patients with prosthetic upper limbs will perform tasks such as carrying shopping bags or preparing a meal, while those with lower limb paralysis will be asked to undertake activities such as climbing stairs, using a robotic exoskeleton for assistance.

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According to the ETH Zurich website, "through the organization of the Cybathlon we want to help removing barriers between the public, people with disabilities and science."

The event takes place in October 2016.