Microalgae concentration using a spiral microfluidic system based on inertial focusing

Microalgae are promising sources of high-value compounds and renewable biofuels. However, the economic sustainability of carbon-neutral biofuel production requires the development of more energy-efficient harvesting methods. Microfluidic inertial focusing has emerged as a compelling approach to address this challenge. In this study, the flow behavior of Chlorella vulgaris suspensions in a 6-loops spiral microchannel with rectangular cross-section is investigated. Absorbance measurements and in situ visualizations offer insights into the distributions of microalgae at the outlets and within the microchannel, respectively. From these distributions, equilibrium positions, focusing efficiencies, and migration times are identified under various flow conditions, thus offering deeper insight into the physical mechanisms governing algal cell migration. The spiral microfluidic device enables a two-order-of-magnitude increase in Chlorella vulgaris concentration, from 0.15% v/v to 15% v/v, corresponding to a transition from typical bioreactor concentrations to levels comparable with those achieved using conventional harvesting techniques, while offering potential for further energy optimization. At the lowest concentration tested (0.15% v/v), algal cells focus sufficiently rapidly near the inner wall of the spiral to allow bifurcation of the concentrated stream after just 4.75 of the 6 loops. At higher concentrations (6% v/v), the emergence of a microalgae-free region adjacent to the inner wall could enable the collection of a slightly more concentrated stream near the outer part of the channel right before the end of the first loop. This study demonstrates the potential of an optimized spiral microsystem for energy-efficient microalgae concentration.