HealthTech Nexus with University of Twente

Technology that helps restore a natural step the bionic foot

In the HealthTech Nexus partnership between the University of Twente and Radboudumc, researchers are developing a bionic foot, known as the ‘Autonomous leg’. As the name suggests, it’s neither a passive prosthesis nor one limited to pre-set rules of motion. Instead, it’s a device that closely mimics the foot’s natural motion. The prosthesis is designed to help individuals recovering from a foot amputation regain a smooth, natural gait. 

Walking long distances at a constant speed over smooth terrain generally poses no difficulty. You automatically place one foot in front of the other, all while taking in the scenery or chatting with the person walking beside you. 'We want to know whether it’s possible to develop a system that mimics this ability after a foot has been amputated, allowing the user to walk without consciously managing every step,' says Massimo Sartori, Professor of Neuromuscular Robotics & Engineering, University of Twente.

This is the very essence of the ‘Autonomous leg’ research project: a bionic foot guided by a synergistic neuromechanical model. The aim is to help prosthetic foot users walk comfortably at different speeds, no matter what kind of prosthesis they have. The ‘Autonomous leg’ project forms part of the European SimBionics project, which explores how neuromechanical simulation and sensory feedback can be used to control a bionic leg. This project is a collaborative venture with the prosthetics company Ottobock (Germany), Aalborg University (Denmark), and rehabilitation centre The Roessingh (the Netherlands).

Left: Massimo Sartori, right: Ruud Leijendekkers

Mimicking the foot’s natural motion

'People who have undergone a foot amputation typically rely on prosthetic devices that are either passive or operate according to pre-set rules of motion,' says Prof. Sartori. 'We want to develop a model that mimics the foot’s natural motion. That motion is driven by the spinal cord, which contains specialized central pattern generators. Once activated, these produce ‘familiar’ rhythmic patterns, such as the walking motion. This is an ‘ingrained pattern’ – an automatic movement that demands very little conscious effort. In contrast, activities like playing the piano require the brain to stay continuously and actively engaged.'

To mimic that motor control, the researchers developed an operating system for a bionic foot. Massimo Sartori notes that: 'This system allows us to send targeted signals that rhythmically activate the muscles at the front and back of the leg, in a repeating pattern, helping the user walk comfortably and effortlessly.'

Treadmill test

In an initial test, a test subject fitted with a bionic foot of this kind was instructed to walk on a treadmill at two different speeds. The objective was to get a sense of how effectively the central pattern generators in the spine could guide the bionic foot’s movement. 'We want to see whether we can replicate the spinal cord’s more automated role in coordinating the legs during rhythmic and cyclic movements, such as walking', says Ruud Leijendekkers, associate professor and physical therapist at Radboudumc. 'If, indeed, the brain no longer needs to be involved, this could lessen user fatigue by significantly reducing the cognitive load.'

Vera Kooijman, a postdoctoral researcher in the rehabilitation department, reviews the results with the patient

This research project used the same hardware as the Mind Control study. 'So we already had a prototype', says Dr. Leijendekkers. 'In its current phase, the project still faces one key limitation – the ‘Autonomous leg’ model still needs to be tethered to a computer by a control cable. The next step is to eliminate this limitation, allowing the user to walk freely. This will require a compact computer that can be integrated into the prosthesis and track the user’s muscle movements. Other essentials include a reliable battery and the sensors required to personalize the model.'

Testing the system in a real home environment — a challenge?

So, the most critical remaining step is to eliminate the need for a tethered connection to the computer. 'We cannot begin testing the prosthesis outside the laboratory until it is able to operate in stand-alone mode,', says Prof. Sartori, 'whether at home, on the street, or perhaps even in the woods.” In other words, settings that do not involve controlled conditions.'

'Only then will it be possible to convincingly demonstrate the model’s genuine added value over passive prostheses', says Dr. Leijendekkers. 'The challenge here is that the model will need to comply with the requirements of the Medical Device Regulation', he says. 'After all, once we move beyond the laboratory setting and use it to help people move about comfortably in their homes, it will instantly be classified as a medical device. Moreover, because this is a robotic component linked directly to a person, a lengthy test period will be required. The test subjects will need time to integrate it into their system.' Massimo Sartori notes that: 'Only through day-to-day use can we determine whether the signaling system guiding the foot’s movement becomes increasingly efficient over time.'

About Health Tech Nexus

This research is part of HealthTech Nexus, the strategic collaboration between Radboudumc and the University of Twente. Together, they focus on addressing unmet healthcare needs: urgent challenges for which no viable solutions currently exist. The researchers leading the ‘Autonomous leg’ project are Massimo Sartori, Professor of Neuromuscular Robotics & Engineering, University of Twente, and Ruud Leijendekkers, associate professor and physical therapist at Radboudumc.

For more information about SimBionics, please go to: Simbionics – Neuromechanical Simulation and Sensory Feedback for the Control of Bionic Legs.