Image Source: Pixabay
Coffee Table Science spoke to Dr. Tran (T) and Dr. Gabert (G) to learn more.
Dr. Minh Tran Dr. Lukas Gabert
CTS: Can you describe the novelty of the Utah Bionic Leg for our readers?
T&G: The Utah Bionic Leg is a powered prosthesis with motors and batteries that can actively assist amputee users during important activities of daily living such as walking, climbing stairs, and standing up from a chair. This is not possible with the commonly prescribed passive prostheses, which can only provide resistance but not assistance.
The most important feature of our bionic leg is that it is powerful but also very light, as light as passive prostheses and about half of other powered prostheses. With this device, we hope to improve the real-world mobility of above-knee amputees, many of whom can benefit from active assistance but cannot tolerate the large weight of existing powered devices.
Above-knee amputee participants ambulating with the Utah Bionic Leg
Image Source: https://www.science.org/doi/abs/10.1126/scirobotics.abo3996
CTS: What are the roles of our knee, ankle, and toe joints when we perform the actions you mentioned above (walking, standing up from a chair, etc.)?
T&G: During ambulation (ability to walk from place to place), humans actively control the movements of both the knee and ankle joints, and our muscles and tendons are always either absorbing or providing energy to propel our body. The toes are sometimes overlooked as lesser joints, but also play a crucial role in the energetics and positioning of the foot.
Passive prostheses (prosthetics with no motorized parts) cannot replicate these active control and energy injection functions. As a result, amputees have to use the rest of their body to partly compensate for the missing joints.
In the long term, this leads to injuries such as osteoarthritis and back pain. Many amputees even find the more challenging tasks such as stair climbing to be impossible after their surgery. With powered prostheses, this key limitation can be overcome.
CTS: Briefly, how does a torque-sensitive activator work? Why is it useful for active prosthetics?
T&G: A torque-sensitive actuator changes its transmission properties based on the load seen by the device. Its function is quite similar to that of cars with Continuously Variable Transmission (CVT) systems, but our design is completely different from those used in the automotive world because we have to work with the small space requirements of a prosthesis. We optimize our novel transmission system so that the knee can perform tasks with very high loads without over-heating, and also move very quickly when a responsive action is required.
This adaptive behavior means that our prosthetic knee can effectively assist users with multiple tasks of daily living, even though the requirements of these tasks are vastly different. Our innovation enables us to use a small yet extremely powerful and efficient transmission, which is essential to achieve our target weight and size requirements while not sacrificing performance.
CTS: What advantage does your custom sensors in Utah Bionic Leg serve over other active prosthetics?
T&G: The force/torque sensor informs the prosthesis of the interaction between the user and the ground. In a small form factor, not much larger and heavier than a standard prosthetic connector, the sensor can determine how much weight the user is putting through the prosthesis, and if they are leaning forward or backward on the device.
A sensing technology of such compactness that provides both torque and force measurements is of great value for prosthetic applications. It allows us and other researchers to develop small and lightweight prostheses, and also develop effective controllers for them with the rich information provided.
(A) A photo of the Utah Bionic Leg. (B) Partially sectioned view of the prosthesis model
highlighting the main electrical and mechanical components.
Image Source: https://www.science.org/doi/abs/10.1126/scirobotics.abo3996
CTS: In general, do you expect your design to be relatively easy to learn to use compared to passive prosthetics, and why?
T&G: Yes, we expect our design to be as easy to use as a passive prosthesis, and in fact, that is what we have seen from our testing. The ease of use of a prosthesis boils down to two main factors: weight of a device, and the intuitiveness of ambulating with the device.
The weight of a design has a huge impact on ease of use, as most people attach a prosthesis to their leg through a socket. The socket can be thought of as a cup on the bottom of the leg which holds the prosthesis up through friction or suction. If the prosthesis is heavy, it pulls so hard on the socket during ambulation that the socket may start to fall off the user, requiring the user to stop and readjust their socket.
Secondly, the device must be intuitive. This translates to the need for prosthesis software and controllers that are simple and almost effortless for users. An effective prosthesis must identify user intention in real-time, and quickly provide the correct response in all tasks of daily living and beyond. This needs to be done seamlessly, without users having to trigger the prosthesis with a complex movement of their remaining limb or reaching for their phone every time they want to change their task.
CTS: In your view, what barrier to prosthetics, if overcome, would have the biggest impact in your field?
T&G: The next step for us is to develop highly functional yet intuitive controllers for the Utah Bionic Leg. Now that there is a device that can provide the needed torque and power to assist amputees while being light enough, we need to make that device smarter.
For the near future, we plan to develop and explore different strategies. We will continue development in traditional methods, which use joint angle and position sensors to control the device in different tasks. We are also developing more advanced solutions which rely heavily on newer technologies such as machine learning and neural control. Once this control barrier is overcome, powered prostheses will move out of the lab and into the hands of the people who need them.
Resources:
1. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD013839.pub2/full
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