Microfabricated magnetophoretic focusing systems for the separation of submicrometer particles

Abstract

Magnetic separation is an actively researched field due to its broad applicability to the mineral, chemical, and biological industries. The objective of this work was to design, fabricate, and test systems to study magnetophoresis of particles in suspension. To achieve this goal, two system concepts were developed: an Alternating Field System and a Flow System. Both systems consisted of permanent magnets and miniaturized devices (separation chips), which integrated microfluidic channels with ferromagnetic core elements. The systems produced "sawtooth" magnetic fields that were combined with a long-range magnetic field or pressure- driven flow to bring about migration, focusing and trapping of nonmagnetic particles suspended in ferrofluid. A potential application of such systems is high-resolution, size-based separation of DNA, cellular organelles, viruses, and other like-sized biological entities. The systems were designed using finite element analysis and fabricated using IC/MEMS microfabrication techniques. The fabrication process for the separation chips realized a microfluidic channel and electroplating molds in a single layer of SU-8 photoresist on a glass substrate. Nickel core elements were electroplated into the molds, and a PDMS cover substrate was attached using a novel technique involving contact bond and heat cycling. The systems were tested via experiments using optical fluorescence methods to observe the concentration profiles of polydisperse suspensions of polystyrene beads.

(cont.) Alternating Field System involved simple migration under a long-range magnetic field, focusing under a sawtooth magnetic field, and attempted separation by combining the long-range and sawtooth fields. The most significant findings of the trials were the significant effect of particle- particle interactions and high sensitivity to the core design of the chip. The Flow System trials combined a sawtooth field with flow. The trials demonstrated size-based trapping of particles, where 840 nm beads were trapped earlier along a separation channel, while 510 nm beads were trapped further along. Moreover, the location along the channel at which particles of a given size were trapped was shown to be a function of flow rate. Size-based trapping in magnetic potential wells, as well as flow rate tuning, could form the basis of a high-resolution particle separation system.