Fabrication of a high aspect ratio (HAR) micropillar filter for a magnetic bead-based immunoassay

A microfluidic chip has been realized for investigating immune cell (U937) activation with lipopolysaccharide (LPS) and subsequent pro-inflammatory cytokine (Interleukin-6, IL-6) detection (Ruffert et al. Proc. EMBL Conference Microfluidics 2012a, p 184; Proc. NanoBioTech Montreux, Poster Sessions B 2012b, pp 17–18). The microfluidic chip comprises two compartments: one compartment for the on-chip cell cultivation, differentiation, and stimulation, while the second one hosts superpara-magnetic beads (Ø 2.8 μm) conjugated to anti-IL-6 antibodies for capturing the LPS-induced IL-6. The two compartments are separated by a micropillar-based filter with a spacing of 2 μm. This filter allows the induced cytokines to infuse into the bead compartment (i.e. the magnetic immunoassay compartment), while preventing the magnetic beads and cells to cross over to the other compartment. To fulfill this requirement, a high aspect ratio pillar array was demonstrated as key element of this study and functionally characterized. The pore size of the filter is given by the lateral distance between the single pillars, which are fabricated by molding microfluidic structures in polydimethylsiloxane, using a master mold made of the expoy-based photoresist SU-8?. An aspect ratio of 5:1 could be achieved with SU-8? bars featuring the dimensions 10 µm × 2 μm (height × width).