Background: A self-contained, self-controlled, pneumatic power harvesting ankle-foot orthosis (PhAFO) to manage foot-drop was developed and tested. Foot-drop is due to a disruption of the motor control pathway and may occur in numerous pathologies such as stroke, spinal cord injury, multiple sclerosis, and cerebral palsy. The objectives for the prototype PhAFO are to provide toe clearance during swing, permit free ankle motion during stance, and harvest the needed power with an underfoot bellow pump pressurized during the stance phase of walking.

In this paper, we describe power and pressure characteristics of bellows designed for under-foot power harvesting during human walking for a single cycle. This single cycle corresponds to the typical human gait cycle (heel-strike and stance on the floor to toe-off and free swing to subsequent re-contact with the floor, with a total duration of about one second). A bellow can be placed in a shoe insole and compressed during initial heel strike or during the mid-stance portion of the gait cycle. The compressed fluid can then be used for power applications during the remaining portion of the cycle. A collapsible spring was placed inside the bellow to extend it when the foot is off the ground, yet allow the bellow to be compressed. Air is drawn into the bellow through a one-way valve allowing the bellow to recharge as it expands during the swing phase of the gait cycle. Thus, body weight is used as the power source for a self-contained pneumatic circuit. Experimental studies were conducted on two circular bellows with outside diameters of 1.625 in and 2.5 in (4.13 cm and 6.35 cm) and stroke lengths of approximately 1.4 cm. The pressure dynamics of the bellows placed under the heel of the foot or under the ball of the foot were investigated while walking on a treadmill. These pressure profiles were then reproduced on a compression testing machine to investigate the power generated per gait cycle. The results indicate that the bellows generated a peak power during normal walking of approximately 25–30 W and a maximum pressure of 450 kPa. The average power available over a single gait cycle is on the order of 1 W. This novel use of bellows demonstrates the ability to use these devices for regenerative fluid power harvesting capabilities during walking.

In this paper, we present a novel ankle-foot-orthosis (AFO) design that controls ankle motion by providing a plantarflexion stop with free dorsiflexion during gait. The biomechanical controls are accomplished with a unique application of a cam-follower design that uses pneumatic power harvested via an air bellow embedded into the insole of the AFO (Figure 1). This portable design is self-contained and does not require any external power source to provide for the plantarflexion stop locking mechanism. It is the first step in a series of untethered fluid-powered orthotic devices.

Pneumatic and hydraulic bellows were investigated for under-foot power harvesting during human walking. Placement under the heel allows the bellow to be compressed during the heel strike of the gait cycle, whereas placement under the metatarsal allows compression during the mid-stance and toe-off phases. In either case, body weight is used as the power source for a self-contained fluid power circuit. Once unweighted, air is drawn into the bellow through a one-way valve allowing the bellow to recharge as it expands during the swing phase of the gait cycle. A collapsible spring was placed inside the bellow to ensure full opened conditions for this phase. To evaluate this concept, experimental studies were conducted on two circular bellows with outside diameters of 4.13 cm and 6.35 cm placed under the heel or the metatarsal of the foot, on a person walking on a treadmill. These pressure profiles were then reproduced on a compression testing machine to investigate the power generated per cycle. During normal walking, the pneumatic bellows generated peak power levels of 20–25 W and maximum pressures of 450 kPa. The average power available over a single cycle was 1.5 and 4.5 W for the small and large bellows, respectively. This novel use of bellows demonstrates the ability to use these devices for regenerative fluid power harvesting capabilities during walking.