Bionic leg avoids the pitfalls of traditional passive prosthetics

A new lower-limb prosthetic developed at Vanderbilt University allows amputees to walk without the leg-dragging gait characteristic of conventional artificial legs.

According to Vanderbilt, it is the first prosthetic with powered knee and ankle joints that operate in unison and it comes equipped with sensors that monitor its user’s motion. Microprocessors are programmed to use this data to predict what the person is trying to do and to operate the device in ways that facilitate these movements.

‘When it’s working, it’s totally different from my current prosthetic,’ said Craig Hutto, the 23-year-old amputee who has been testing the leg. ‘A passive leg is always a step behind me. The Vanderbilt leg is only a split-second behind.’

Prof Michael Goldfarb, right, with amputee Craig Hutto who is wearing the new bionic leg developed at Vanderbilt. (John Russell, Vanderbilt University)
Prof Michael Goldfarb, right, with amputee Craig Hutto who is wearing the new bionic leg developed at Vanderbilt. (John Russell, Vanderbilt University)

The bionic leg is the result of a seven-year research effort at the Vanderbilt Center for Intelligent Mechatronics, directed by Michael Goldfarb.

‘With our latest model, we have validated our hypothesis that the right technology was available to make a lower-limb prosthetic with powered knee and ankle joints,’ said Goldfarb. ‘Our device illustrates the progress we are making at integrating man and machine.’

The Vanderbilt prosthesis is designed for daily life, making it easier for an amputee to walk, sit, stand and go up and down stairs and ramps. Studies have shown that users equipped with the device naturally walk 25 per cent faster on level surfaces than when they use passive lower-limb prosthetics because it takes users 30 to 40 per cent less of their own energy to operate.

Recent advances have allowed the Vanderbilt engineers to produce a device that weighs about nine pounds — less than most human lower legs — and can operate for three days of normal activity, or 13 to 14km of continuous walking, on a single charge.

One of the latest capabilities that the engineers have added is an anti-stumble routine. If the leg senses that its user is starting to stumble, it will lift up the leg to clear any obstruction and plant the foot on the floor.

In order to incorporate all the improvements, the prosthetic’s hardware design has gone through seven versions and its electronics board has been redone 15 times.

According to Goldfarb, it was difficult to fit the powerful motors and drive train that they needed into the volume available but the biggest challenge was to develop the control system.

‘As you add greater capability, you are also adding greater liability,’ he said. ‘Not only does the controller have to perform individual operations reliably, but it also has to perform several operations at the same time and not get confused.’

Key aspects of the design have been patented by the university, which has granted exclusive rights to develop the prosthesis to Freedom Innovations, a developer and manufacturer of lower limb prosthetic devices.