Synthesis of Functionalized Microspheres with High Magnetic Concentration for Microscale Force Application

Abstract

Magnetic microspheres are commercially available in a wide range of diameters and with a variety of chemical functionalities for applications such as cell sorting, protein isolation, or microscale force spectroscopy. The most commonly-used products are magnetic spheres one micron in diameter or larger. These spheres generally consist of a polymer core which is swollen in a solvent and saturated with a solution of iron oxide nanoparticles. This top-down process tends to result in incomplete saturation of the polymer spheres and therefore a diminishing magnetic concentration as spheres grow larger since the magnetic content does not scale with volume.We present here a bottom-up approach to microsphere fabrication which begins with a high-permeability magnetic fluid consisting of magnetite nanoparticles complexed with poly(dimethyl siloxane-co-aminopropylmethyl siloxane). The magnetic content of the fluid may be adjusted smoothly from 0 - 50% wt. without any nanoparticle aggregation, resulting in a highly-magnetic silicone fluid which is homogenous at scales well below 100 nm. Using this material, we demonstrate the production of solid, spherical microbeads in diameters ranging from 2 - 30 microns. Since magnetic nanoparticles are distributed uniformly throughout the material, magnetic content scales directly with volume, resulting in a significant advantage over competitors in terms of magnetic force application at larger sizes. Controlling for diameter, the high magnetic content of these spheres results in nearly four times the force-generating capabilities of the leading 2.8-micron competitor, with advantages increasing in larger spheres. In addition, we use a cell-targeting assay to demonstrate our ability to functionalize the microsphere surface with a variety of ligands.