Thermally actuated hydrogel bead based braided artificial muscle

Braided (McKibben) artificial muscles are one of the most attractive biomimetic actuators since they exhibit similar static and dynamic performance to skeletal muscles. One disadvantage of these systems, when intended for portable applications, is the need for a compressor or pump to provide pressurised fluid for muscle activation. An alternative is to use an expandable material, such as a hydrogel. This work investigates the development of hydrogel-based braided artificial muscles which can produce mechanical work through controlled hydrogel swelling in the presence of water. Hydrogel bead-filled braided meshes were prepared where no internal bladder was needed; since the bead size was larger than the holes in the braided mesh. A spring test method was introduced to measure force generation and strain developed by the braided mesh when the thermo-responsive gel was cooled from 60 °C to 5 °C–8 °C. Reducing the temperature resulted in hydrogel swelling and braided mesh length contraction due to the internal pressure generated. Blocked forces and actuation free strokes of 5–6 N and 7%–8% were observed, respectively. A long response time was the limiting factor of these actuators due to the slow diffusion properties of the hydrogel.