Electro-Actuators Based on Polycationic Hydrogel Networks

The development of actuators and robots made of soft materials has drawn considerable interest due to their adaptability and biomimetic nature. Unlike traditional hard materials, soft materials are compliant, environmentally and biologically compatible, and can be used for the manipulation of fragile objects or performance of smooth motions^1–3. Soft materials made of electro-active hydrogels (EAH) are great candidates for the development of soft actuators, due to their ability to convert a non-contact electrical stimulus into a programmable mechanical response. Particularly, the use of an electric field presents several advantages over other stimuli. The degree of actuation and direction can easily be controlled by tuning the strength and polarity of the electric field. The ability to modulate both speed and direction of soft-material actuation offers enormous potential for achieving programmable and autonomous movement⁴.<br/>While polyanionic EAH have been extensively used to create soft-actuators, the use of polycationic EAH has been constrained to a small number of examples and needs to be further explored. Herein, high aspect-ratio hydrogel strips based on ionic liquid monomers, quaternised ammonium monomers and amino-functionalised monomers with pKa values within the biological range were fabricated. The electro-induced actuation of all hydrogels was characterised as a function of hydrogel thickness, direction of the electric field, and response reversibility and reproducibility.<br/>It was found that thinner hydrogels of 0.10 mm thickness showed the greatest bending amplitude. Particularly, degrees of bending deformation up to 60% and actuation times of 100s were achieved.<br/>The inclusion of sugar-responsive boronic acid moieties in the amine monomer allowed for the realisation of multi-responsive hydrogel actuators (electric field and sugar solutions), where the actuation can be switched on or off in the absence or presence of fructose (100 mM). Following this finding, a hydrogel gripper was fabricated, and it was showed that upon application of an electric field, the gripper can close and catch a millimeter-sized alginate ball, whereas addition of fructose results in the inhibition of the electro-actuation and release of the ball.<br/>The results demonstrate a simple and effective way for reversible, unidirectional movement of soft matter and shows a proof of concept for the fabrication of a multi-responsive gripper. Current investigations focus on increasing the complexity of these electro-responsive hydrogel systems by employing their 3D fabrication using Digital Light Processing (DLP). By combining the materials presented herein with DLP-printing, a large variety of 3D structures of complex geometries are produced, to give rise to 4D soft actuators of programable response.<br/>(1) Yang, C.; Wang, W.; Yao, C.; Xie, R.; Ju, X.-J.; Liu, Z.; Chu, L.-Y. Hydrogel Walkers with Electro-Driven Motility for Cargo Transport. <i>Nat. Scie</i> <b>2015</b>, <i>5</i> (13622), 1–10.<br/>(2) Morales, D.; Palleau, E.; Dickey, M. D.; Velev, O. D. Electro-Actuated Hydrogel Walkers with Dual Responsive Legs. <i>Soft Matter</i> <b>2014</b>, <i>10</i> (9), 1235–1430.<br/>(3) Han, D.; Farino, C.; Yang, C.; Scott, T.; Browe, D.; Choi, W.; Freeman, J. W.; Lee, H. Soft Robotic Manipulation and Locomotion with a 3D Printed Electroactive Hydrogel. <i>ACS Appl. Mater. Interfaces</i> <b>2018</b>, <i>10</i>, 17512–12518.<br/>(4) Choi, M.-Y.; Shin, Y.; Lee, H. S.; Kim, S. Y.; Na, J.-H. Multipolar Spatial Electric Field Modulation for Freeform Electroactive Hydrogel Actuation. <i>Nat. Sci. eports</i> <b>2020</b>, <i>10</i> (2482), 1–8.