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Upgrading the Prosthetic Hand

A lightweight prosthetic hand uses hydraulics to achieve more natural finger movement.

A lightweight hydraulic hand with individually powered fingers could change the lives of amputees, say researchers in Germany. The Fluidhand, according to its developers, is lighter, behaves more naturally, and has greater flexibility than artificial hands that use motorized fingers.

Freedom to move: The Fluidhand (above) uses lightweight miniature hydraulics to enable the wearer to move each finger individually.

The Fluidhand prototype, developed by a team led by Stefan Schulz at the Research Center in Karlsrühe, in partnership with the Orthopedic University Hospital, in Heidelberg, Germany, has flexible drives located in each of its finger joints, enabling the wearer to move each finger independently. Lightweight miniature hydraulics are connected to elastic chambers that can flex the joints of the fingers. As sensors on the fingers and palm close around objects, nerves in the amputation stump pick up muscular sensations so that the amputee can use a weaker or stronger grip. The prosthetic provides five different strengths of grip.

“It is so intuitive that learning to use the device only takes about 15 minutes,” says Schulz.

Last September, 18-year-old Sören Wolf, who was born with only one hand, became the first person to use the Fluidhand. According to German press reports, Wolf was able to type on a keyboard with both of his hands for the first time in his life, and he told reporters that, when he’s wearing the Fluidhand, he doesn’t feel handicapped anymore.

International interest in the Fluidhand peaked late last month, when it was announced that the Orthopedic University Hospital is testing the device in comparison with the i-LIMB Hand. Wolf is the first amputee to use both prosthetics.

Produced by the Scottish company Touch Bionics, i-LIMB was the first prosthetic hand that enabled the movement of individual fingers. The prosthetic, released last summer, uses a different technical principle than the Fluidhand. With i-LIMB, movement is enabled by five small, battery-powered motors that are embedded in each finger. Schulz believes that the hydraulic system has some advantages over the motorized fingers. “In contrast to the movement with electric motors and transmissions, the Fluidhand remains soft and flexible,” he says. “Articles can therefore be seized more reliably, and the hand feels more natural.”

Both devices are significant improvements over conventional hand prostheses that only enable the wearer to pinch the thumb and forefinger to create a grip.

“There are many hand movements that require individual digit movements,” says Hugh Herr, director of the Biomechatronics Group at the MIT Media Lab. “The development of individual finger movements in a prosthetic is a remarkable step forward.”

One patient is currently wearing the Fluidhand to complete daily tasks, and a second is about to be fitted for the device. Some 250 people, including soldiers wounded in Afghanistan and Iraq, already use i-LIMB.

Stuart Mead, CEO of Touch Bionics, points out that the comparative study in Heidelberg is not a competitive one. “Many people have many different devices for different activities, and what works for one patient may not work for another,” he says.

Comparative studies of this nature do have value for determining how well the device can meet amputees’ needs. “They are probably testing each device’s strength, power, and versatility,” says Herr. “The prosthetics have to be able to pick up something very lightweight and fragile, like a piece of china, as well as something large and heavy.”

Soon, people requiring a prosthetic hand with movable digits will have more options. “The German-Austrian company OttoBock will probably present a new hand with movable fingers in 2009,” says Schulz.

Experts expect this rapid development in the field of prosthetic technologies to continue into the near future.

“I believe that there is a big push into wearable exoskeletons because the mechatronic technology has matured, becoming more cost effective, miniaturized, and powerful,” says Thomas Sugar of Arizona State University, who works in robotic prosthetics. “Batteries and motors are smaller and more powerful. Microprocessors have been very fast and cheap. Lastly, I do think there has been a big push by NIH [National Institutes of Health] and the DOD [Department of Defense] into medical robots for stroke therapy, powered exoskeletons, and powered prosthetics.”

The Biomechatronics Group’s Herr agrees. “Typically, when you plot prosthetic innovations against time, you see a spike in innovation after every war, and that is certainly true today,” he says. “In addition, we’re also seeing a number of disciplines such as robotics, mechanical engineering, and biomechatronics mature to the point [where] we can merge to create truly remarkable systems.”

There is still room for those remarkable innovations in prosthetic development.

“We find ourselves, as an industry, working to manage people’s expectations,” says Touch Bionics’ Mead. “A prosthetic doesn’t function like a real hand. We’re still only able to replicate 5 to 10 percent of what a real hand can do.”

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