Tissue engineering has seen a surge of interest in recent years. Traditionally, it involves seeding cells onto biocompatible “scaffolds”, which biodegrade once tissues have assembled themselves into 3D organs. However, more flexible, scaffold-free approaches are also emerging, which enable cells to assemble themselves without the need for structural biomaterials. To do this, researchers use techniques including removable supports, and guiding forces from acoustic and electrostatic fields. One particularly promising approach involves magnetic levitation, through which strong field gradients can precisely guide tissue cells into place. To achieve strong enough gradients, however, cells must be suspended within a paramagnetic medium containing...