This research focuses on the design and evaluation of a fully passive variable damping transfemoral prosthesis, aiming to provide a low-cost alternative to existing microprocessor-controlled prosthetic knees (MPKs). Current MPKs, such as the Otto Bock C-Leg 4 and Össur Rheo Knee XC, offer advanced functionality by adjusting knee damping based on sensory input. However, their high cost make them inaccessible to many individuals, particularly in low-income regions. This study addresses this issue by developing a fully passive low-cost transfemoral prothesis with variable knee damping in the swing phase.

The design process involved defining functional requirements, identifying key subfunctions, and generating three concept designs.

The final design provides stability in the stance phase and controlled motion during the swing phase. The main innovation of this design is a centrifugal force-based damping mechanism. The foot’s centrifugal acceleration generates a force that is transmitted through a cable system to a friction brake in the knee joint, effectuating velocity-dependent damping. Additionally, knee stability in the stance phase is ensured by a spring latch lock mechanism, which engages at the end of the swing phase and disengages at the end of the stance phase through ankle dorsiflexion. An ankle spring mechanism provides ankle stability in the stance phase, and it provides energy for push-off. The final design is produced using laser-cut metal, 3D-printed components, and off-the-shelf parts to keep production costs low.

The evaluation phase included two key tests:

\textit{Swing phase test} – Assessed the velocity-dependent damping mechanism by suspending the prosthesis and measuring knee moment at various angular velocities. The results showed some velocity-dependent damping, but inconsistencies due to hysteresis were observed.

\textit{Ankle stiffness test} – Evaluated the rotational stiffness of the ankle joint using force and displacement measurements. The results showed slightly lower than expected ankle stiffness.

While the prototype shows the feasibility of a fully passive variable damping knee, several limitations were identified. The main limitation was that significant hysteresis was present the damping mechanism, leading to inconsistent results. Furthermore, the swing phase was performed only at walking speeds lower than occur at natural gait, so no conclusion can be drawn on its effectiveness in real world use.

Future research should focus on improving the robustness of the damping mechanism, reducing the hysteresis and conducting full gait cycle tests on human subjects. Additionally, implementing a delay between force sensing and damping activation could enhance the resemblance to natural gait.

In conclusion, this study demonstrates that a low-cost, fully passive variable damping prosthetic knee is possible, demonstrating a promising initial step towards the development of low-cost \textit{K3} prostheses. However, further development and testing is required before the design can be classified as a functional \textit{K3} prosthesis.