Rehabilitation Technology - As Delicate As Human Hand

Every year, more than 12 million people worldwide suffer a stroke. Those affected often face long-term impairments and have to re-learn the simplest things, such as grasping. Not being able to grasp objects with the affected hand has a huge impact on everyday activities and quality of life. Exoskeletons, i.e. external movement aids worn on the body, can compensate for lost abilities. They help wearers to flex and extend their fingers and compensate for the loss of motor functions, thereby supporting recovery and independence.

An exoskeleton as light as a smartphone

Exoskeletons for the hand are usually very complex, as they have to support the delicate movements of the fingers. They are often bulky, consist of many mechanical components or break quickly and are uncomfortable to wear.

Natalie Tanczak, a doctoral student at ETH Zurich's Rehabilitation Engineering Laboratory (RELab), has developed an exoskeleton that requires no hydraulic or pneumatic drive and has a simple mechanical structure. Motors located on the forearm set it in motion. At 270 grams, the device, which has since been filed for patent, weighs just as little as a smartphone. With the exception of a USB port, there are no cables to interfere with the flow of motion, and the surface structure made of 3D-printed nylon is comfortable to wear. "Our exoskeleton allows for intensive, individual training after a stroke," explains Tanczak. "We can help patients regain their ability to move simply by doing their everyday tasks."

"It is the most elegant and compact design to enable such complex movements," says Roger Gassert. Together with Olivier Lambercy, he heads RELab, where several earlier prototypes of the hand exoskeleton were developed. However, the earlier models did not come close to this level of simplicity and robustness. "The beauty of the new exoskeletal fingers lies in their simplicity. Existing models consist of countless parts. Ours is made up of just two," says Lambercy.

One of the two components is a leaf spring made of stainless steel. It must be rigid enough to provide adequate resistance while remaining flexible at the same time. The biggest innovation, however, lies in the second component, an accordion-like exterior structure. This is the backbone that allows the leaf spring to convert a linear movement into a perpendicular grip force and controls bending and stretching in a natural way.

Technology to regain a more normal life

In search of a suitable design that supported flexion motion just like a human finger, Tanczak came up with the simple idea of taking inspiration from an accordion. From this, she developed the striking structure and, in collaboration with engineer Jay Song, produced a single component through which the leaf spring glides. 3D printing makes it possible to customise the exoskeleton so that it fits perfectly to each user's hand depending on their hand size and finger length. This further strengthens the supportive effect and comfort.

From finger to knee - potential for many other joints

Tanczak worked on the device for a total of three years. She began at the Future Health Technologies Laboratory at the Singapore-ETH Centre. For technical development, she came to RELab in Zurich, where she was able to draw on the experience from earlier developments. Meanwhile, Tanczak had the chance to test her product in a real environment. She followed eight stroke patients using the exoskeleton for 12 weeks and says about her experience: "As a mechanical engineer, I'm particularly pleased to see the impact the technology had on the users. That was huge. And their gratitude is priceless."

It was not only this that made Tanczak realise the decisive difference rehabilitation technology could make in regaining mobility. Her accordion-like structure design could also be used to support the elbow, knee or any other joint of the human body - and elegantly.