Directed colloidal assembly: Printing with magnets

Bartosz Grzybowski and collaborators investigated how to print a spectrum of colloidal objects, polymeric particles, silica particles, even live bacteria, suspended in a paramagnetic fluid. By using magnetic-field microgradients produced by metal grids embedded in a rubber layer a few hundreds of nanometers thick that is placed on a permanent magnet, the authors show that beautiful arrays of complex microstructures can be assembled with high yield over square centimeter areas. Grzybowski and co-authors refer to this assembly approach as 'magnetic molding' because solutions of paramagnetic salts regulate the response of the colloids to the magnetic fields; the solution becomes in fact part of the mould. Magnetic-mould techniques could potentially enable 3D printing of colloidal structures. The magnetic-template principle put to practice by Grzybowski and colleagues could be generalized to provide programmability and reconfigurability on demand.