3D Printable Magnetic Soft Actuators–Ink Formulation, Rheological Characterization, and Hydrogel Actuator Prototypes

Magnetic inks are presented for direct ink writing (DIW), a 3D printing technique, yielding magnetic hydrogel actuators. To obtain the shear-thinning and thixotropic behavior needed for DIW, the rheology modifier laponite is used. This additive ensures suitable rheological properties and dispersion of the magnetic iron oxide nanoparticles used. The base formulation of the ink consists of acrylamide as monomer, N,N’-methylenbisacrylamide as cross-linker, and ammonium persulphate as thermal initiator. The concentration of laponite varies from 1.5 to 6.9 mass%, and the effect on the ink viscosity, shear-thinning properties, and printability of the system is investigated. Starting at a concentration of 3.8 mass% laponite, the iron oxide nanoparticles are sufficiently stabilized to prevent sedimentation. The ink viscosity can be tuned over almost two orders of magnitude, with an optimum printability at 4.6 mass% laponite. The printed hydrogel precursors are cross-linked thermally at 50 °C. Thus, magneto-responsive prototypes for soft robotics applications are obtained. The advantages of the system are that a low mass percentage of rheology modifier is needed, that the number of polymeric components is reduced, and that the obtained hydrogels are mechanically stable. Laponite-containing ink is easy to handle and can therefore also be used in non-specialist laboratories.