Advances in ankle-foot orthosis (AFO) technology have been trending toward more powerful and lightweight devices. A hydraulic series elastic actuator (HSEA) was explored to design a lightweight powered AFO that meets the high peak power demand of ankle gait. With its excellent power density and its ability to separate the power supply from the actuator using a hose, hydraulic power was used, combined with an SEA that takes advantage of the high-peak and low-average power profile of ankle gait to store energy and release it during the push-off stage of gait. The parameters required for the SEA were determined and validated using simulation. A gait pattern that would require 235W of motor power was able to be tracked using a motor rated at 95W. The actuator weight of the hydraulic ankle-foot orthosis (HAFO) at the ankle was 0.35, which is 43% of an equivalent electromechanical system. A novel design of an HSEA with a clutch capability is proposed for future HAFO applications.

In human walking, the ankle plays an important role of supplying power needed for the forward motion [1]. However, traditional transtibial (TT, a.k.a. below-knee, BK) prostheses are passive, lacking the ability of generating power output in the prosthetic ankle. Consequently, amputees fitted with such prostheses suffer from multiple issues (asymmetric gait, greater metabolic energy expenditure, etc.). To address such issues, researchers have explored various technical approaches to develop powered TT prostheses. Hydraulics and pneumatics have been attempted, leveraging the high power density with these actuators (e.g. [2]). Electromagnetic actuators were used more extensively with its technological maturity and convenience in packaging. Typical examples include the multiple prototypes developed by the MIT Biomechatronics Group (e.g., [3]), the SPARKy project, and the Vanderbilt Transtibial Prosthesis. The TT prostheses mentioned above all include powered ankle joints to provide power for the users’ locomotion. However, cost and complexity are often given lower priority than performance in the development of such devices. Powered TT prosthesis is a typical low-volume product from a commercial perspective, and the resulting high cost is a major hurdle for the large-scale adoption among amputee users. General robotic components (motors, gearsets, etc.), in contrary, are produced in large quantities with relatively low prices. Such contrast is the major inspiration for this work: the goal is to develop a modular powered TT prosthesis based on low-cost commercial robotic components while minimizing the complexity in manufacturing and assembly.

A numerical study is performed to translate the MRI induced ex-vivo heating in a stent measured in tissue mimicking gel phantoms to heating in vivo. The in vivo heating was simulated by solving the convective energy equation in a tissue perfused with a single blood vessel embedded with a stent. Appropriate power density function was determined by matching the ex-vivo heating results in the tissue with no blood flow. The effect of the blood flow on the in vivo heating was investigated by varying the flow. Results show that ≥3% of normal, mean physiologic flow in the blood vessel reduces the maximum temperature change to < 3 °C from ∼10 °C measured ex vivo for an MRI scan of ≤ 15 minutes. Local temperature change of up to 3 °C in the trunk is considered safe by the regulatory bodies. Also, the maximum temperature change was found to be ∼1 cm away from the stent ends — and not at the stent ends.