Biocompatible Ionogels for Electroactive Actuators

The development of soft robotics is challenging and exciting for wide range of field from industry to medicine. Its development is tied to the availability of efficient, flexible and soft actuators. Among actuators, electroactive actuators are the most promising and are considered as artificial muscles that gain more and more interest. Their light-weight, miniaturizable and stimuli-responsible are their major advantages. They are usually considered to be use in applications that are in interaction with human such as in haptics, e-textiles, biomedical device... Consequently, biocompatible materials are required in the development of electroactive actuators. Among the latter, those based on ionic motion mechanism such as ionic electroactive polymers, ionic polymer metal composites or carbon nanotubes (CNT)-based actuators allow large deformation under low stimulation voltage. In these actuators, ionic source plays an important role and in order to allow in-air actuation, the electrolyte must be kept in self-standing state and in contact with the electroactive materials. Ionogel is among the best solution for use as ionic source. They are polymer networks containing a continuous phase of ionic liquid. They inherit from the polymer networks their self-standing state, their softness and stretchability. On the other side, ionogels derive from ionic liquids the high ionic conductivity and their stability in air.<br/>Here we present the synthesis of a biocompatible ionogel suitable for use as ionic source in electroactive actuators. The ionogel was synthesized in one-pot photopolymerization using biocompatible liquid precursors mixture including acrylate-based monomers, ionic liquid and initiators, leading to materials that are highly ionically conductive reaching 1.9 mS/cm, soft and stretchable (Young modulus <i>&lt;</i>1 MPa and elongation at break ~ 75%). As a demonstration for the use of biocompatible ionogel, coiled CNT yarns actuators was fabricated. The working principle of the actuator is based on the accumulation of ionic species on CNT upon charging under the electrochemical double layer configuration. This accumulation of ions allows the swelling of the CNT coils leading to a linear contraction of the actuators. To be able to operate in air, two coiled carbon nanotube (CNT) yarns coated with the ionogels were fabricated and assembled together. They act as working and counter electrodes in a two-electrode electrochemical system of an electroactive actuator. The actuator exhibited a maximum contractile stroke of 1.78% under low voltage stimulation (4V) in open air. The actuation performance remained unchanged after 16h in-air actuation. The synthesized biocompatible ionogels has demonstrated their potential to be used as solid electrolyte under ionic coating form, suitable for open air operating electroactive actuators.