Bionic ankle emulates nature

An amputee himself, Hugh Herr has been designing (and wearing) bionic leg prostheses that he says emulate nature – mimicking the functions and power of biological knees, ankles, and calves. Most of these prostheses have reached the world through Herr’s startup, BiOM (originally called iWalk). Since 2010, the company has brought the world’s first bionic foot-and-calf system to more than 900 patients worldwide, including some 400 war veterans.

Initially developed by Herr’s research group, BiOM’s prosthesis, dubbed the BiOM T2 System, simulates a biological ankle, delivering a natural ankle function during strides.

Using battery-powered bionic propulsion, two microprocessors and six environmental sensors adjust ankle stiffness, power, position, and damping thousands of times per second, at two major positions. First, at heel strike, the system controls the ankle’s stiffness to absorb shock and thrust the tibia forward. Then, algorithms generate fluctuating power, depending on terrain, to propel a wearer up and forward.

When fitting the prosthesis to patients, prosthetists can program appropriate stiffness and power throughout all the stages of a gait, using software created by Herr’s group – a process the company calls Personal Bionic Tuning.

Among other things, the system restores natural gait, balance, and speed; lowers joint stress; and drastically lowers the time required to acclimate to the prosthesis, which can take weeks or months with conventional models.

“Often, within minutes, a patient is walking around, even running around,” says Herr, BiOM’s CTO.
 

From bench to bedside

Herr, who lost both legs after a 1982 climbing accident, began researching the deficiencies of conventional prostheses throughout the 1990s and early 2000s and mathematically modeling how the ankle joint operates while walking.

Among other things, the ankle stiffens and provides propulsion in the trailing leg during stride, mitigating impact on the leading leg, and lessening strain on the leg joints and back. When amputees wear conventional prostheses – which rely on springs or hydraulics and don’t release more energy than they absorb – they walk more slowly, consume more metabolic energy, and experience greater musculoskeletal stress, which causes joint osteoarthritis.

The scientific and engineering research that ultimately led to today’s BiOM prosthesis was conducted by Herr’s research group within the MIT Media Lab. Since 2003, the group has designed and fabricated many prosthetic prototypes to test hypotheses on human-machine interaction.

Still today, Herr can remember stepping into the group’s first bionic leg prototype for the first time and then back to a traditional prosthesis.

“It was as profound as when you’re walking through the airport and you hit the moving walkway. When you get off and return to normal walking, you’re like, ‘Walking is really strenuous and slow,’” he says. “That’s what it was like going from our powered system to passive conventional systems. So I knew there was magic there clinically.”

By advancing prostheses, Herr says, the technology could also lead to innovation in a closely related field: humanoid robotics.

“Imagine a future where we’ll have bionic feet, ankles, knees, and hips that are technologically optimal. One could just bolt these pieces together to produce a humanoid hardware platform,” Herr says.

Ultimately, Herr says the work of both BiOM and his group at MIT aims to help revolutionize the idea of personal bionics, blurring the lines between electromechanics and the human body. For instance, his MIT group is working, among other things, on bionic limbs that can be controlled by the mind and attached to the body.

MIT
www.mit.edu

This article is adapted from a release from MIT News.

Photo credit: Bryce Vickmark