Directed Assembly of Magnetic Nanoparticles into Centimeter Scale Wires for a 3D Cell Culture Platform

The bottom-up assembly of nanoparticles is utilized to construct hierarchical structures for various applications. Among the diverse inorganic nanoparticles, field-directed assembly with magnetic nanoparticles is important building blocks for forming anisotropic structures. Here, we report magnetic wires based on a magnetically directed assembly of uniform iron oxide nanoparticles based on a "brickand- mortar" concept and its potential application as a three-dimensional (3D) cell culture platform. The assembly of magnetic wires is controlled by the reactive bromo groups of iron oxide nanoparticles, the amount of crosslinking silane agents, assembly time, and strength of the magnetic field. Longer magnetic wires can be achieved by applying a uniform parallel magnetic field and can be extended up to the centimeter scale by employing an electromagnetic setup over a large area. The intrinsic celladhering property of magnetic wires allows their usage as a 3D cell culture platform, depicting enhanced secretion of FGF and IL-8 compared to the conventional 2D cell culture plate and 3D Matrigel. In the tumor growth study, tumor cells cultured with magnetic wires showed a faster tumor growth pattern compared to the conventional 2D cultured cells. These results represent a potential application of assembled magnetic wires with controllable physical properties as an efficient 3D cell culture platform.