title: A highly efficient bead extraction technique with low bead number for digital microfluidic immunoassay.
abstract: Here, we describe a technique to manipulate a low number of beads to achieve high washing efficiency with zero bead loss in the washing process of a digital microfluidic (DMF) immunoassay. Previously, two magnetic bead extraction methods were reported in the DMF platform: (1) single-side electrowetting method and (2) double-side electrowetting method. The first approach could provide high washing efficiency, but it required a large number of beads. The second approach could reduce the required number of beads, but it was inefficient where multiple washes were required. More importantly, bead loss during the washing process was unavoidable in both methods. Here, an improved double-side electrowetting method is proposed for bead extraction by utilizing a series of unequal electrodes. It is shown that, with proper electrode size ratio, only one wash step is required to achieve 98% washing rate without any bead loss at bead number less than 100 in a droplet. It allows using only about 25 magnetic beads in DMF immunoassay to increase the number of captured analytes on each bead effectively. In our human soluble tumor necrosis factor receptor I (sTNF-RI) model immunoassay, the experimental results show that, comparing to our previous results without using the proposed bead extraction technique, the immunoassay with low bead number significantly enhances the fluorescence signal to provide a better limit of detection (3.14 pg/ml) with smaller reagent volumes (200 nl) and shorter analysis time (<1 h). This improved bead extraction technique not only can be used in the DMF immunoassay but also has great potential to be used in any other bead-based DMF systems for different applications.
authors: Department of Mechanical Engineering, National Chiao Tung University , Hsinchu, Taiwan.
authors: Cheng-Yeh
authors: 0000-0001-7865-1020
authors: CY
authors: Huang
authors: Department of Mechanical Engineering, National Chiao Tung University , Hsinchu, Taiwan.
authors: Po-Yen
authors: PY
authors: Tsai
authors: Department of Applied Chemistry, National Chiao Tung University , Hsinchu, Taiwan.
authors: I-Chin
authors: IC
authors: Lee
authors: Department of Applied Chemistry, National Chiao Tung University , Hsinchu, Taiwan.
authors: Hsin-Yun
authors: HY
authors: Hsu
authors: Hong-Yuan
authors: HY
authors: Huang
authors: Department of Mechanical Engineering, National Taiwan University , Taipei, Taiwan.
authors: Shih-Kang
authors: SK
authors: Fan
authors: Da-Jeng
authors: 0000-0001-6631-780X
authors: DJ
authors: Yao
authors: Department of Power Mechanical Engineering, National Tsing Hua University , Hsinchu, Taiwan.
authors: Cheng-Hsien
authors: CH
authors: Liu
authors: Department of Mechanical Engineering, National Chiao Tung University , Hsinchu, Taiwan.
authors: Wensyang
authors: 0000-0002-0189-9578
authors: W
authors: Hsu