4D Printing of polyoxometalate hydrogels from centrifuged inks for semi-solid lubricants

Polyoxometalate hydrogels, normally fabricated by linking polyanionic polyoxometalate particles with cationic ligands or multivalent ions, have found widespread potential applications in materials science and technology. Nevertheless, the binding with ligands/ions may adversely affect the intrinsic properties of polyoxometalates. And the prospects of polyoxometalate hydrogels in 4D printing and tribological fields have hardly been explored. Here we show that polyoxometalate hydrogels with favorable viscoelasticity, shear thinning, thixotropy, and thermal responsiveness, thus being capable of constructing various complex 4D printing geometries with shape reconfigurability and lubricating different types of solid tribopairs, can be created solely from high-speed centrifugation of aqueous dispersions of hybrid polyoxometalate nanosheets. By effectively preserving the inherent tribochemical activity and physical lubrication of the polyoxometalate nanosheet gelator using this approach, the hydrogels can provide potent lubrication for contacting surfaces for at least 200000 consecutive reciprocating friction cycles, featuring a high suitability for practical tribological applications. Centrifugation of aqueous dispersions of hybrid polyoxometalate nanosheets is used to create inks for the 4D printing of polyoxometalate hydrogels, exhibiting complex geometries and shape reconfigurability.