A pH-activated artificial muscle using the McKibben-type braided structure

Abstract

Fluidic McKibben artificial muscle is one of the best biomimetic actuators, exhibiting static and dynamic behaviour in close analogy with skeletal muscle. It is also known that McKibben muscle can combine this analogical behaviour with a high force on mass and on volume ratio. This paper analyses the working possibility of a small-size McKibben muscle for which a chemical activation mode is substituted in place of pneumatic energy, with the hope of deriving an original actuator for new applications in robotics and medicine. A pH activation mode seems particularly well adapted to our approach due to a large availability of pH-sensitive materials and the ease of reversible control by acid and base flows. The use of ion-exchange resins is considered due to their high swelling ability and ball-like microscopic structure favourable to flow circulation through the inner chamber of the McKibben muscle. The paper reports experimental results of artificial muscles 70–100 mm in length and 8 mm in diameter under isometric and isotonic conditions and against loads between 0.5 and 3 kg. A maximum tension of about 100 N is generated by the use of 0.1N NaOH/HCl solutions, which corresponds to a maximum stress of approximately 500 kN/m 2 and is greater than corresponding vertebrate skeletal muscle. We propose adapting the pH-muscle to actuate via ion-exchange in a hydrogel environment in order to overcome the slow response time (>30 min), thus addressing one major drawback of their use in biomimetic robotics. Our preliminary results demonstrate the possibility of using synthesized hydrogel powders in order to generate quicker dynamic responses.