As fabrication processes improve, and portable power sources are scaled down, the more feasible it becomes to produce untethered powered orthoses, prostheses, and exoskeletons for human assistance. The advent of more-energy-dense batteries, such as lithium ion, has spurred the research, development, and commercialization of a number of electrically driven active orthoses, including direct motor drives [1], electrohydraulics, and “smart-material” actuators [2]. Advances in manufacturing have also made pneumatically driven actuators more lightweight through component integration [3]. Figure 1, however, illustrates the challenge in developing effective mobile assistive orthoses and prostheses—to operate for more than a few minutes, the requisite power-source mass must increase. For an hour, or more, of operation, electroactive polymers are shown to have the best mass-to-power ratio, with solenoids being a distant second (roughly 1 order of magnitude difference, but closing).

Reproducing the plantarflexion (step-off)...

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