Viscosity Independent Paper Microfluidic Imbibition

This work introduces capillary flow in paper microfluidics that features a flow rate Q that is constant in time, t, and independent of the viscosity of liquid sample, µ liquid : Q≠f(t, µ liquid). Compared to conventional paper microfluidics, we enclose the paper in solid walls and add a long and narrow air vent as outlet of the capillary pump, such that the flow rate is dominated by the downstream air resistance. Therefore, the flow rate depends on the viscosity of air rather than that of liquid. This significantly decreases the dependency of lateral flow biosensors on variations of sample fluid. INTRODUCTION Samples of interest for biosensors, e.g. urine and blood, show large person-to-person variations in viscosity: the relative standard deviation in healthy individuals is 10% and 33%, respectively [1, 2], while larger deviations occur in many unhealthy conditions (e.g. ~4× higher for polycythaemia). Moreover, whole blood behaves viscoelastic. Sample pumping time in capillary driven lateral flow devices (LFDs) depends linearly on viscosity (t fill ~µ liquid); as a result, variations in sample viscosity form a fundamental limitation to the quantitative performance of lateral flow biosensors. Capillary pumps with time independent flow rate, Q(µ liquid)≠f(t), can be constructed, e.g. by introducing a large upstream, liquid hydrodynamic resistance [3].