Soft Scissor: A Cartilage-Inspired, Pneumatic Artificial Muscle for Wearable Devices

Although rigid exoskeletons can strengthen human capabilities or provide full assistance to patients with disabilities, their rigidity may constrain natural movement, developing tissue damage in long-term usage. Soft and semi-soft exoskeletons and exosuits exhibit both compliance and comfort, and offer the potential to provide practical and widely-adopted assistance. Soft pneumatic muscles have been explored as a means to drive wearable assist devices for over a decade; however, their softness leads to compromises in terms of power output and the precision by which forces can be applied to the human body. In this article, we introduce a novel soft extending pneumatic actuator, which combines a compliant scissor structure inspired by human cartilage and soft pneumatic muscles. The structure behaves as a compliant skeleton to the force generating pneumatic muscle, guiding its actuation behaviour and maintaining high force transmission through its body. Different designs and dimensions of the actuator and structure were investigated to observe the effect of compliance on key performance parameters. A soft single-module actuator can deliver extending force over 100 N and achieve a maximum strain of 178% when inflated at 50 kPa. A slightly thicker, but still compliant, continuum two-module actuator exhibits twice the extension compared to a single-module actuator with the same design under the same load up to 4 kg, a significant and suitable force for comfortable wearable devices. Last, a wearable prototype of this novel actuator is demonstrated, exhibiting both extension and bending actuation behaviours.