Evaporation-Driven Flow in Micropillar Arrays: Transport Dynamics and Chemical Analysis under Varied Sample and Ambient Conditions

Abstract

Microfluidic flow in lab-on-a-chip devices is typically very sensitive to the variable physical properties of complex samples, e.g., biological fluids. Here, evaporation-driven fluid transport (transpiration) is achieved in a configuration that is insensitive to interfacial tension, salinity, and viscosity over a wide range. Micropillar arrays ("pillar cuvettes") were preloaded by wicking a known volatile fluid (water) and then adding a microliter sample of salt, surfactant, sugar, or saliva solution to the loading zone. As the preloaded fluid evaporates, the sample is reliably drawn from a reservoir through the pillar array at a rate defined by the evaporation of the preloaded fluid (typically nL/s). Including a reagent in the preloaded fluid allows photometric reactions to take place at the boundary between the two fluids. In this configuration, a photometric signal enhancement is observed and chemical analysis is independent of both humidity and temperature. The ability to reliably transport and sense an analyte in microliter volumes without concern over salt, surfactant, viscosity (in part), humidity, and temperature is a remarkable advantage for analytical purposes.

Caged-Sphere Optofluidic Sensors: Whispering Gallery Resonators in Wicking Microfluidics

“…As previously reported, evaporation-driven flow that is insensitive to interfacial tension and viscosity of a sample fluid can be realised in micropillar arrays [25]. If the pillar array is long enough and the reservoir is inexhausted, a continuous flow can be achieved.…”

“…The gradients of the linear fits differ from the previous set of results from Figure 3c by up to 25% because the location of the sphere with respect to the reservoir differed. Since there is an approximately exponential increase in glucose concentration along the chip length [25], this means that the position of the sphere along the chip is important for calibration purposes and it influences the sensitivity of the chip. The longer pillar array has the advantage of the previous glucose solution being transported far from the sphere location, which ensures that the next measurement is not affected by the previous.…”

“…A fixed solvent matrix is, however, not an unrealistic assumption in many applications. The microfluidic approach can be used for sensing independent of temperature and humidity, as shown elsewhere [25]. It is envisioned that bioconjugation of the microspheres could allow for specific binding of a variety of target analytes including proteins, bacteria and DNA [8].…”

“…The thickness of the liquid film is precisely determined by the height of the pillars (in µm), and the volume required to fill the microfluidic chip is of the order of several µL [22]. The pillar array, similar to other open microfluidic systems, has advantages over traditional enclosed systems in terms of simple sample loading, rinsing and cleaning, and controllable evaporation can be used to drive a continuous flow between the pillars [25]. It was reported by Orlowska et al that by prefilling the pillar array with a known liquid, a predictable flow behaviour driven by evaporation of the liquid could be achieved, including for a biological fluid such as saliva or sweat [25].…”

Injection moulding of micropillar arrays: a comparison of poly(methyl methacrylate) and cyclic olefin copolymer

“…In previous studies, micropillar arrays with the same wicking effect were used as an ultrashort-pathlength (10-20 lm) cuvette for UV-Vis spectroscopic measurement of high-absorptivity chemicals (Holzner et al 2015), and as a platform to control the growth of orthorhombic crystals (Holzner et al 2017). Our recent work has further demonstrated the micropillar arrays as a useful analytical tool to generate spontaneous sample flow and achieve analyte concentration for high-sensitivity measurements (Orlowska et al 2020). Moreover, micropillar arrays are essential for sorting and separating cells and microparticles in microfluidic systems via the principle of deterministic lateral displacement (Inglis 2009;Chandna and Gundabala 2021).…”