The emergence of digital immunoassays has advanced the sensitivity of protein analysis to ultrahigh sensitivity at the attomolar level. However, the background signal generated by the premixing of immunocomplexes and fluorogenic substrates can limit the precise quantification, especially in multiplexed assays. Herein, a bead-based SlipChip (bb-SlipChip) microfluidic device capable of massively parallel two-step sample loading is presented. The background signal can be suppressed through a two-step loading mechanism. Specifically, encapsulate the beads into the microwells first and then, through a slipping process, deliver the fluorogenic substrate in parallel into 281,200 microwells of 68 fL to perform the digital immunoassay. The quantification capability is demonstrated with a duplex assay of IL-6 and IL-10, achieving a limit of detection of 5.2 and 15.3 fg/mL, which is approximately 2-3 times improved compared to a commercial Simoa system. The bb-SlipChip provides a robust and universal method for digital immunoassay and can be extended to higher multiplexed detection as well as other biomedical applications involving microbeads.
Keywords: SlipChip; digital immunoassay; microbead; microfluidics; ultrasensitive detection.