August 2023

A Human Lower Limb Mechanical Phantom for the Testing of Knee Exoskeletons

Authors: W. Sebastian Barrutia, James Bratt, Daniel P. Ferris

Affiliations: J. Crayton Pruitt Family Department of Biomedical Engineering, Human Neuromechanics Laboratory, Herbert Wertheim College of Engineering, University of Florida, Gainesville, FL, USA

Journal: IEEE Transactions on Neural Systems and Rehabilitation Engineering - May 2023, Volume 31, Pages 2497-2506 (DOI: 10.1109/TNSRE.2023.3276424)

The development of assistive lower-limb exoskeletons can be time-consuming. Testing prototype medical devices on vulnerable populations such as children also has safety concerns. Mechanical phantoms replicating the lower-limb kinematics provide an alternative for the fast validation and iteration of exoskeletons. However, most phantoms treat the limbs as rigid bodies and fail to capture soft tissue deformation at the human/exoskeleton interface. Human soft tissue can absorb and dissipate energy when compressed, leading to a mismatch between simulated and human exoskeleton testing outcomes.

We have developed a methodology for quickly testing and validating the performance of knee exoskeletons using a mechanical phantom capable of emulating knee kinematics soft-tissue deformation of the lower-limb. Our phantom consisted of 3D-printed bones surrounded by ballistic gel. A motorized hexapod moved the knee to follow a walking trajectory. A custom inverse dynamics model estimated the knee assistance moment from marker and load cell data. We applied this methodology to quantify the effects of soft-tissue deformation on exoskeleton assistance by loading the phantom knee with a torsional spring exoskeleton interfacing and bypassing the ballistic gel.

We found that including soft-tissue deformation led to a lower knee assistance moment and stiffness. Some but not all of this difference could be explained by the deflection of the exoskeleton relative to the knee angle, suggesting energy absorption within soft tissue. The direct measurements of exoskeleton assistance provide insight into increasing the assistive moment transmission efficacy. The phantom provided a relatively accurate framework for knee exoskeleton testing, aiding future exoskeleton design.


Keywords: ballistic gel, knee exoskeleton, lower limb, mechanical phantom, soft tissue deformation, stiffness

There are many advantages for testing an exoskeleton on a mechanical phantom. Being able to conduct medical device testing safely is crucial for product development, especially with vulnerable populations such as children. The phantom provides a risk-free environment for testing the mechanical integrity of the knee exoskeleton. The lack of human subject recruitment and testing can also speed up exoskeleton iteration. We can replicate most sagittal plane knee trajectories other than knee hyperextension due to a mechanical singularity at full knee extension. For example, we could use the phantom to test knee exoskeletons that assist children with crouch gait caused by cerebral palsy, the most common neuromuscular disorder in children. In addition to fast iteration, the phantom provides direct measurements of exoskeleton assistance that would otherwise be difficult to estimate from a subject as the total joint moment is a mixture of biological and exoskeleton components. Furthermore, device assistance metrics obtained from the phantom can provide insights to engineers on how to improve the efficacy of assistive moment transfer to the joint. Active exoskeletons relying on external power sources could benefit from prolonged battery life by reducing the energy lost at the interface. Passive devices using elastic energy storage and return could use a lower stiffness spring to provide the same assistive moment, reducing device weight. Ultimately, our phantom should accelerate the design process, provide a reliable means to assess mechanical integrity, and ultimately improve the design of lower limb exoskeletons.