Building a Better Prosthetic Foot

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Soft robotics offers new possibilities to amputees

The design of prosthetic limbs has made incredible progress in the last few decades but, according to IIT SoftBots lab, two crucial elements have significant scope for improvement: the hand and the foot.

“The human foot is a complex system in which bones, muscles, ligaments and tendons work together, adapting the whole structure to all kinds of terrains and tasks, ensuring bodyweight support, impact absorption and user’s stability during daily activities. Despite all the efforts made in the last decades to improve the design of prosthetic feet and, thus, the quality of life of people with a lower-limb amputation, commercial prostheses can withstand the body weight while still lacking the flexibility and elasticity required for a stable ground adaptation,” explains Anna Pace, a Biomedical Engineer and Postdoctoral Researcher at the Soft Robotics for Human Cooperation and Rehabilitation laboratory (Istituto Italiano di Tecnologia, Genoa, Italy), led by Professor Antonio Bicchi.

“The existing benchmark for prosthetic feet is represented by feet with a stiff and flat sole, which obviously hinders ground adaptation,” she says. “Therefore, the prosthetic user’s stability is jeopardized when there are uneven terrains or obstacles, leading to possible falls that could be lethal for the user.”

While some manufacturers of prosthetic feet have incorporated features such as a split toe or actuators in the ankle into their designs in a bid to combat the problem, they’re still paired with a flat, rigid sole and come up short compared to the subtle reactive adjustments a human foot is able to make.

A bioinspired design

The SoftBots lab, with the help of funding from the ‘Natural BionicS’ ERC Synergy project, is working on another approach – soft robotic prostheses for an enhanced interaction with the environment. The lab already has a working soft prosthetic hand that can adapt its motion to task requirements and object shapes, and now Pace and the SoftBots team are working on a foot that is capable of adapting to different terrains in a manner that mirrors human lower limbs.

“Our robotic SoftFoot shows an anthropomorphic and intrinsically adaptive design with a longitudinal arch made of two rigid components (one anterior and one posterior) connected to each other at the ankle joint; a heel component; and a foot sole made of five elastic and flexible structures,” says Pace.

“This peculiar design allows the foot to vary its shape and stiffness depending on the load acting on it and the ground profile, leading to an overall improvement of the user’s stability on uneven grounds,” she goes on.

A version of the SoftFoot has already been tested on HRP-4, a humanoid robot, and the results showed an improved ability to navigate obstacles when compared to a rigid foot. Now, the team is working on a prosthetic version of the foot for people with a lower-limb amputation. 

The ambitious aim of the ongoing Natural BionicS project – in collaboration with Professor Oskar Aszmann (Medical University of Vienna, Austria), Professor Dario Farina (Imperial College London, UK), and their research groups – is the development of upper- and lower-limb prostheses directly interfaced with the central nervous system.

“The prosthetic user will be able to fully control the prosthesis as well as receive sensory feedback from it as a real part of the body. That’s possible thanks to bio-hubs that are surgically created at the residual limb, allowing a bidirectional flow of information between the prosthesis and the central nervous system,” explains Pace.

Motion analysis is fundamental to the design

Pace and the SoftBots team are using a Vicon system equipped with 12 Vero cameras and two Vue cameras at the Bergamo JOiiNT LAB, a technology transfer facility established by IIT and the Intellimech industry consortium, to test the foot.

The system is used for teleoperation of robots and for testing the ergonomics of human/robot interaction, and it has been crucial for performing gait analysis. “The system has been fundamental in running a preliminary experimental session whose results will drive the design of the new prosthetic soft foot,” explains Pace.

“Specifically, we recruited an able-bodied subject and a prosthetic user with a lower-limb amputation, and asked them to perform some daily-life tasks in our lab, including walking on ground with obstacles, while using different commercial prosthetic feet,” she says, adding that, “The able-bodied subject could walk with the prostheses thanks to some boots and ad hoc adapters.”

“The motion capture system was used to record the gait and analyze the biomechanics of the two subjects,” Pace says. “We got the subjects’ kinematics by recording the 3D motion of the reflective markers placed on their skin, as well as the dynamics, thanks to the two force plates embedded in the ground and an additional sensor synchronized with the rest of the Vicon data.”

The team is currently processing the data and plans to incorporate it into the design process for the foot soon.

“Our experience with the Vicon mocap system has been great so far!” Pace says. A number of features made it a strong fit for the SoftFoot project: “The possibility of synchronizing 3D marker trajectories with other data sources through the Vicon Lock Lab; the option to use a MATLAB package for post-processing in the Vicon Nexus software; the ability to record trials in which the sensor is considered as a second subject, so that its 3D motion can be captured and its orientation in each frame obtained by streaming the trial data in MATLAB; the option to display an overlay of the video from the Vue cameras with the 3D skeleton from the Vero cameras.

“Moreover, the creation of a custom skeleton template in Nexus was straightforward, the Nexus auto-labeling function worked well with the custom protocol, and the gap-filling function was very useful.”

There’s still plenty more motion analysis in the pipeline, too. “Motion capture systems represent the gold standard for an accurate gait assessment, so we’ll use our Vicon system for evaluating and comparing the performance of the new prosthetic SoftFoot with that of commercial prosthetic feet, and for analyzing the overall prosthetic user’s biomechanics while performing activities of daily living with different prosthetic feet,” says Pace.

“Quantifying the prosthesis and user’s performance through motion capture will help us to further optimize the design if necessary. We will also plan to assess the prosthetic SoftFoot performance with a larger number of prosthetic users,” Pace concludes.

While it may be a while before the SoftFoot is ready for use by amputees, Pace sees the SoftBots line of research into soft robotics as extremely promising: “It can truly exceed the performance of prosthetic feet currently on the market,” she says.