4D printing of patterned multimaterial magnetic hydrogel actuators

This paper demonstrates a new class of printable magnetic hydrogels that can be successfully used for multimaterial direct ink printing (4D printing) of soft actuators. To date, most reports on magnetic actuation have not considered biocompatibility issues associated with magnetic materials and synthetic polymers. For this reason, in this study, considerable attention was given to developing bionanocomposites that exhibit noncytotoxicity and biocompatibility and hence may be used in biomedical applications. Three inks with various concentrations of magnetic nanoparticles (MNPs) were used to print 3D objects, such as tubes (wheels), cubes, and cantilevers. The interactions between MNPs and hydrogel precursor network accounted for excellent shear thinning properties of the inks. Usually, hydrogel actuators move or change a shape upon anisotropic swelling and deswelling, possible only in an aqueous environment. Our study addresses this challenge by incorporating magnetic nanoparticles into the hydrogel, allowing for contactless in-air control of hydrogel motion. Because of the high structural integrity of the hydrogel, we can state that multimaterial direct ink printing is an excellent method for obtaining a 3D construct of high resolution, shape fidelity, tunable distribution of MNPs, and induced macroscopic anisotropy. The magnetic hydrogels are not only highly porous and noncytotoxic towards fibroblasts but also exhibit good mechanical stability and unique magnetic responsiveness. The simple approach allowed us to fabricate different magnetic actuators with various patterns, composed of magnetic and non-magnetic materials. The results demonstrate the interplay between magnetic and nonmagnetic hydrogels that influences the actuation performance of multimaterial objects, as illustrated by magnetically induced rolling, jumping, and bending. It was shown that programmable patterning of the hydrogels leads to the development of macroscopically anisotropic magnetic material. Our study confirmed that the intersection of 4D printing of magnetically responsive hydrogel materials and programmable patterning promises to fulfill future soft robotics' biocompatibility and functionality requirements.