During the course of daily activity, people are constantly adapting their ankle joint stiffness based on the type of activity they are doing. This is a biological function that is not available to most people with lower limb amputation, as the vast majority use passive prosthetic devices that are designed for a fixed stiffness, usually targeted at steady, normal-pace walking. There is no commercially available prosthetic ankle-foot device that allows users to change the elastic stiffness of their prosthesis, and this design framework aims to target that gap in the market.
The basis of this design centers on a stack of sliding carbon leaf springs that can be variably engaged and disengaged, changing the effective thickness of the spring system. Engagement is achieved by implementing changing boundary conditions at the free end of each of the leaf springs, either preventing sliding entirely or allowing a finite amount of sliding before engagement. In this way, the base stiffness as well as the relationship between stiffness and ankle angle can be adapted to suit user needs. By changing the number of leaf springs, thickness of each spring, and the engagement boundary condition, the entire system behavior can be customized to suit an individual's preferences for many different types of walking activities.
The design allows for configuration in both passive and quasi-passive setups. In the prosthetic world, there are 3 main types of devices. Passive devices are the most basic, and do not have any electronics or actuation. They are the most common, and cover everything from basic wooden legs to carbon running blades. On the other end of the spectrum are active devices, which use motors, electronics, and sensing to mimic the biological function of the natural ankle joint by applying powered push-off or control multiple degrees of ankle rotation. There are very few of these devices available, and they trade off being more complex, heavier, and more expensive than passive devices in order to achieve their function. The middle ground is covered by quasi-passive devices. These use electronics to provide additional functionality over passive prostheses, but do not actively provide energy to the user in the way active prostheses do. Examples are devices that change ankle angle or use hydraulic dampers to lessen impact. This design framework targets quasi-passive devices as we see it as a promising area for innovation. The design concept can also be adapted to running devices and has other potential applications in orthoses and exoskeletons.