Bio-inspired fluidic diode for large-area unidirectional passive water transport even against gravity

In this paper, we describe the design, fabrication and functional principle of microfluidic diodes - devices for directional liquid transport. They are passive (i.e., need no external energy input) and are capable of unidirectional water transport even vertically against gravity, i.e. they prevent backflow of liquid. In designing the devices with an area of several cm(2), we used a capillary channel network that was bio-inspired by the skin of the Texas horned lizard (Phrynosoma cornutum). By means of a CO2 laser, we engraved the microfluidic structures into plates made of poly(methyl methacrylate) (PMMA), a low-cost technical polymer commonly used in disposable microfluidics. We then characterized the topography of the fabricated 3D structures by optical coherence tomography (OCT). Theoretical analysis of the OCT data enabled us to determine their working principle which differs from that presented previously in the literature. We accessed the fluid transport properties of the device by measuring distance, velocity, wetted area and flow asymmetry. Biomimetic abstraction led to a simplified structure which was validated experimentally. All tested devices allowed unidirectional liquid transport at velocities in the range of a few mm/s in the forward direction while preventing backflow of volumes of about 0.5 ml in the backward direction on an area of approximately 28 mm x 14 mm. The transport velocity of about 1 mm/s was found to be nearly constant for a distance of approximately 2 cm, beyond which it decreased. The application spectrum ranges from biomedical microfluidics, lab-on-a-chip and micro-analysis devices to filtration, lubrication, cooling of electronics and e-ink displays. (C) 2018 Elsevier B.V. All rights reserved.