High strength bubble artificial muscles for walking assistance

We present a high strength artificial muscle for inclusion in an orthotic device that can deliver power at the knee joint with the aim of improving knee flexion in people with reduced mobility. The bubble artificial muscle (BAM) was developed by adapting traditional pleated pneumatic artificial muscles to achieve high contraction and tensile force. Initial gravimetric evaluation of a single BAM was performed, achieving a contraction of 23.7% while lifting a mass of 2.5 kg (over 100 times the mass of the actuator). To demonstrate the actuator's suitability for an orthosis, a device consisting of three parallel BAMs was tested with a physical pendular leg model based upon a human leg. A maximum knee flexion of 47.25 degrees was achieved, about 70% of the peak knee flexion of the average of young healthy subjects (68 degrees). 8.19 Nm of torque was delivered during testing, which is approximately 33% of the knee flexion moment during swing phase of a gait cycle. This high moment is achieved despite the low mass of the device (206.6 g). This work shows the suitability of BAMs for wearable orthotic devices such as soft assistive suits, where softness, lightness and high stress and strain are essential.