Lightweight Bionic Leg

Vision

Mobility is essential to health and independence. Unfortunately, current prosthetic technologies are not sufficient to enable people with lower-limb amputation to reach the mobility they want and need to live independently. By combining advanced mechatronics, AI, and neural control, we have developed the first prosthesis technology that has the lightness, comfort, and power to enable young and senior amputees regain their freedom.

Background

Millions of people worldwide are currently living with lower-limb amputations and rely on prosthetic products for mobility, and more than 75% of new amputees are seniors. Lower limb amputations have a negative effect on walking speed, walking effort, and stability. Limb loss also limits the tasks that most individuals are able to perform. Daily tasks such as standing up from a chair, climbing stairs, and walking on ramps can become burdensome or even impossible. As a result, amputees have reduced mobility, on average taking less than half the number of steps per day compared to able-bodied individuals. Reduced mobility can result in a rapid decline of health, quality of life, independence, and also exacerbates diseases like depression and arthritis. Thus, it is important to enable individuals with amputations to be as active and mobile as possible.

Limitations of Current Technology

The vast majority of prostheses available for purchase by amputees are passive. Although lightweight, passive devices cannot generate power, which is a critical component of mobility. For example, passive ankle prostheses cannot provide any power to propel the user forward, and passive knee prostheses cannot provide power to assist a user up steps, forcing the user to expend more effort with their hips and intact leg. This compensation makes many tasks more difficult, unstable, and tiring. Consequently, lack of power from a prosthesis is a major factor contributing to the decreased mobility seen in the amputee population.

Powered prostheses have long been considered a promising solution for restoring the function of lost limbs and improving mobility of amputees. Equipped with motors and batteries, they can overcome the biggest weakness of passive prostheses and actively provide power to users. Potential benefits are endless! Power can provide natural and stable movement, improved comfort and endurance, and can fight gravity to perform challenging tasks such as standing up and ascending stairs. This is particularly important for those who have significant trouble walking with passive prostheses, such as seniors and people with vascular diseases.

However, the results of powered prostheses have fallen far short of their expectations. The added components come with tradeoffs, most notably in weight, which has a huge effect on the stability, comfort, and effort required of the user. The addition of motors and batteries with powered prostheses have doubled their weight compared to passive solutions, nullifying most if not all of their promised benefits.

A New Solution

To remove this critical roadblock, we have developed the Utah Bionic Leg, a powered knee and ankle prosthesis. Our prosthesis weighs the same as commercial passive devices and has a build height compatible with more than 99% of amputees without taking shortcuts in performance. There is enough power to support a 330-lb user with all of the main tasks of daily life: walking, stairs, ramps, sitting/standing, and navigating obstacles. The leg can support all of this for a full day with a single battery charge. For the first time, amputees can experience the power of a motorized prosthesis with the light weight of a passive device.

In the Making

This breakthrough device has been the core of research and development effort at the University of Utah’s Bionic Engineering Lab since January 2017. Overcoming the long-standing weight roadblock of powered prostheses is not an easy task, and innovative design solutions are needed to realize an efficient and powerful, yet small and lightweight mechatronic system.

Within the knee module, there is an actively variable transmission (AVT), which matches the torque and speed of the motor to optimal values for each activity, such as walking, stairs, and standing up. This produces a similar effect as shifting gears on a bicycle or in a car. A miniature motor is responsible for modifying the linkage system of the AVT, which results in a lighter and more compact solution compared to traditional gearboxes. Direct contact with an aluminum frame and thermal paste increases the thermal performance of the motor and reduces the risk of overheating. Thanks to these innovations, we were able to use a smaller motor and transmission system, while also maximizing electrical efficiency and reducing battery weight.

The ankle module uses a novel polycentric design that accomplishes structural and transmission functions using the same mechanical elements to overcome the tight weight and space constraints. This mechanism consists of a seven-bar linkage with an optimal transmission ratio that reduces motor effort when the foot is propelled forward, a challenging task known as push-off. Due to this, the ankle uses a smaller motor and lighter transmission components than would otherwise be needed. The ankle module is 50% lighter than commercial powered devices while still fitting inside a normal shoe.

Alongside innovations in design, the Utah Bionic Leg automatically determines how much assistance to provide to the user for a wide range of tasks. This is done using data from multiple position, inertia, force, and torque sensors, which is then turned into commands for the motors through AI run on embedded electronics. The device thus functions as though it has a brain, allowing users to spend less effort on their movement and more on their journey.

Impact

Since his first visit with the Bionic Lab in October 2017, Alec Mcmorris has been integral in developing the leg and bringing it closer to a commercial product. He has used the device for walking, stairs, and ramps in the lab, and has taken the leg outdoors. The results of his tests, and his input on the design, have allowed the rest of the team to better tune both the hardware and software of the prosthesis. He believes that this leg is the future. To quote Alec: "The Utah Bionic Leg is on track to restore the freedom of all levels of lower extremity amputees".

As the Bionic Leg project matures, we have been able to put the device into testing with more than a dozen amputees of different physical and health conditions. Kerry Finn, a veteran who lost his leg 3 years ago due to diabetic complications, described walking with our device as “fantastic”. Using the device, he is able to climb stairs step-over-step, a daily task that’s been out of reach since his surgery. All of this was possible thanks to the added power of the Bionic Leg.

The Future

Using lessons learned along the way, and feedback from preclinical testing with the current prosthesis, we began the design of a new generation of the Utah Bionic Leg in summer 2019. This new generation will match the weight and power of the current generation while being much smaller, more robust, and more efficient. The new generation is designed to ISO standards, and has an emphasis on manufacturability and durability. Potential hazards for users, such as pinching points and exposed electronics are removed. Industrial-grade, custom embedded electronics improve system reliability. The Utah Bionic Leg will be ready to leave the lab and make a difference in the lives of amputees.