Microfluidic Chain Reaction of Structurally Programmed Capillary Flow Events

Abstract

Chain reactions are characterized by initiation, propagation and termination, are stochastic at microscopic scales and underlie vital chemical (e.g. combustion engines), nuclear and biotechnological (e.g. polymerase chain reaction) applications.1-5 At macroscopic scales, chain reactions are deterministic and limited to applications for entertainment and art such as falling domino and Rube Goldberg machines. Appositely, the microfluidic lab-on-a-chips (also called a micro total analysis system),6,7 was envisioned as an integrated chip, akin to microelectronic integrated circuits, yet in practice remain dependent on cumbersome peripherals, connections, and a computer for automation.8-11 Capillary microfluidics integrate energy supply and flow control onto a single chip by using capillary phenomena, but programmability remains rudimentary with at most a handful (eight) operations possible.12-19 Here we introduce the microfluidic chain reaction (MCR) as the conditional, structurally programmed propagation of capillary flow events. Monolithic chips integrating a MCR are 3D printed, and powered by the free-energy of a paper pump, autonomously execute liquid handling algorithms step-by-step. With MCR, we automated (i) the sequential release of 300 aliquots across chained, interconnected chips, (ii) a protocol for SARS-CoV-2 antibodies detection in saliva, and (iii) a thrombin generation assay by continuous subsampling and analysis of coagulation-activated plasma with parallel operations including timers, iterative cycles of synchronous flow and stop-flow operations. MCRs are untethered from and unencumbered by peripherals, encode programs structurally in situ, and can form frugal, versatile, bona fide lab-on-a-chip with wide-ranging applications in liquid handling and point-of-care diagnostics.

3D-printed capillaric ELISA-on-a-chip with aliquoting

“…32 For an IgG antibody with D = 35 µm 2 /s and DAB with D = 550 µm 2 /s, the time for diffusion across 10 mm is ∼16.5 days and ∼1 day, respectively. In our case the functional connection connecting the reservoir and the barrier forms a constriction, 24 hence the diffusion time would be further increased relative to the calculation.…”

“…It would be desirable to further simplify the operations and speed up the time-to-result to match LFAs, which could be facilitated by pre-drying reagents in the chip and rehydrating them with buffer. [20][21][22] Following these improvements, ELISA Chips could be deployed at the point-of-need and used by non-experts, and using a cell phone for imaging and quantifying the assay results, 21,22,24 quantitative, point-of-care test with the performance of a central laboratory ELISA become available for everyone.…”

“…In our case the functional connection connecting the reservoir and the barrier forms a constriction, 24 hence the diffusion time would be further increased relative to the calculation.…”

“…The ELISA is initiated by the user click-connecting the main capillary pump to the chip (Fig. 4A), which wicks liquid from the CC and the reservoirs, and initiates the MCR that propagates from one reservoir to the next via the capillary domino valves that chain the reservoirs, 24 (movie S2).…”

“…DAB is the substrate used here which is chromogenic and oxidized in the presence of HRP, forming a brown precipitate at HRP locations. Whereas in laboratory ELISA the assay produces a colored solution, here it forms a precipitate on the paper strip which can be read out by the naked eye or digitized with a scanner 24 . Although the read-out time for both ELISA and LFAs is limited to a window of a few minutes, the precipitate in the ELISA chip is stable and can be read out later, thus potentially also serving as an archival record.…”