Double Helix Actuators
Tensile actuators that reversibly generate large length change strokes are of great interest in many biomedical and robotic applications. Guest-host composite materials comprising volume change guests with multi-filament yarn hosts can be exploited as large stroke tensile actuators when the fiber host is formed into helices. Here, a new type of double helix actuator is introduced by plying two yarns together. The helical geometry suggests large length contractions are possible when the ply angle approaches the limit of 45° and when the yarn swells predominantly in the diameter direction as it occurs when reinforcing filaments are aligned in the yarn direction. Prototype double helix actuators are constructed from cotton yarn infused with a water-swellable hydrogel. A series of actuator samples are made with differing ply angles and the length changes monitored during repeated hydration/dehydration cycles. Water swelling causes length contractions of up to 9% and it increases in magnitude with an increase in ply twist. The experimental results are in good agreement with those calculated using the helical geometry model.