A macroporous smart gel based on a pH-sensitive polyacrylic polymer for the development of large size artificial muscles with linear contraction.

The physics of soft matter can contribute to the revolution in robotics and medical prostheses. These two fields require the development of artificial muscles with behavior close to biological muscles. Today, artificial muscles rely mostly on active materials, which can deform reversibly. Nevertheless transport kinetics is the major limit for all of these materials. These actuators are only made of a thin layer of active material and using a large thickness dramatically reduces the actuation time. In this article, we demonstrate that a porous material reduces the limit of transport and enables the use of a large volume of active material. We synthesize a new active material: a macroporous gel, which is based on polyacrylic acid. This gel shows very large swelling when we increase the pH and the macroporosity dramatically reduces the swelling time of centimetric samples from one day to 100 s. We characterize the mechanical properties and swelling kinetics of this new material. This material is well adapted for soft robotics because of its large swelling ratio (300%) and its capacity to apply a pressure of 150 mbar during swelling. We demonstrate finally that this material can be used in a McKibben muscle producing linear contraction, which is particularly adapted for robotics. The muscle contracts by 9% of its initial length within 100 s, which corresponds to the gel swelling time.