Design and fabrication of the microfluidic chip

Figure 1 presents the microfluidic device utilized in the study. It consists of 4 PDMS layers with 1 mm thick Poly(methyl methacrylate) (PMMA) bottom layer as support (Fig. 1B). The PDMS layers were fabricated using the conventional soft lithography technique with polycarbonate (PC) molds as pattern source. The PC molds, shaped by machine cutting process, was used to minimize the dimension variability in the fabricated PDMS as what was observed when using SU-8 molds (data not presented). Figure ^S1 contains the complete details of the measurements of the molds and the resulting PDMS layers. The first PDMS layer serves as the main inlet and cover of the second layer to prevent the liquid from spilling out of the chambers. The second layer secures and isolates the liquid inside the pentagonal prisms and prevents contamination. The tip of the base pentagon has a 2.4 mm square through-hole that serves as connecting channel to the third layer. The third layer or main layer contains the ring-structured channel with 1 mm width and an average circumference of 6 mm. The fluidic channel in this layer has a height of 0.7 mm and a height depression of 0.14 mm at the main-to-outlet channel junction (Fig. 1C). This functions as a physical barrier to keep the antibody carrying microbeads from escaping the main channel. A U-shaped channel was also utilized at the outlet to control the fluid discharge and the increasing width of the flow path at the end was done to minimize the capillary flow effect. In addition, the PMMA bottom layer has square holes aligned with the circular main channel to avoid obstructing the optical path and subsequently the measured absorption signal.

Fabricated microfluidic disk chip. (A) The assembled fluidic placed on the heater stage. Inset: microfluidic disk chip. (B) The disk consists of four layers of PDMS with PMMA bottom layer as support. (C) The ring channel served as the reaction chamber. Expanded image shows the height depression that prevents the (D) microbeads carrying the capture antibodies from escaping the main channel.