High-Throughput Separation of Alexandrium Cells Based on Deterministic Lateral Displacement Arrays With Different Post Shapes
Industrial and agricultural wastes emptying in water bodies leads to the growth of harmful algae. The genus Alexandrium is one of the most concerning microalgae that cause dangerous algal blooms. Micropipettes, dilution, water droplet separation, antibiotic treatment, and plate coating are some of the conventional techniques for microalgae separation. However, these approaches are ineffective, time-consuming, and labor-intensive.
The researchers here introduce microfluidic chip technology for cell separation and enrichment. A deterministic lateral displacement (DLD) technology-based microalgae cell separation system is designed to sort cells according to size. The isolation of Alexandrium cells from other algal species was accomplished by designing and fabricating two microfluidic chips with two different micropillar designs, namely triangular and circular.
The DLD method is a passive separation technique that uses micropillars in a fluid flow channel. The fluid containing particles of different sizes is passed through the channel. After hitting micropillars, the particles larger than the separation threshold (DC) experience DLD, whereas smaller particles will flow in a zigzag pattern according to the micropillars placement without deflection. The value of the separation threshold (DC) depends on the size, geometry, and arrangement of the micropillars.
COMSOL Multiphysics was used to investigate the flow field of a designed chip containing circular and triangular micropillars. According to the results, the flow field distribution of triangular micropillars was asymmetrical, whereas circular micropillars were symmetrical. Due to its greater micropillar spacing, the triangular micropillar chip produced superior chip stability, fewer blockages, and higher throughput. Under the same critical value DC, the triangular micropillar chip's flow resistance was lower than that of the circular micropillar chip.
Microalgae cells and polystyrene particles were used to evaluate the separation chip experimentally. The microfluidic chips with circular and triangular micropillars have the same designs with two inlets and outputs. Inlet-1 introduces the mixed microalgae sample, and inlet-2 passes the sheath solution for the proper flow of the sample. The flow rate ratio between the sheath and the sample was optimized to 1:3 for the best separation effect. Here, the small-sized particles flowed out of side outlets, i.e., outlet-1, while the larger particles moved toward the middle zone to come out of the central outlet, i.e., outlet-2.
The chip is designed to have a DC value of 10 µm to separate 15-30 µm Alexandrium cells from other smaller cells and particles. First, the designed microfluidic chip was validated using polystyrene beads of sizes 20 μm and 30 μm to be separated from other small-sized beads. The outlet-2 collects 20 μm and 30 μm beads with purity levels of 89.1% and 93.52%, respectively. When tested under the same conditions with smaller Chlorella cells and larger T. weissflogii cells, the triangular micropillar chip's throughput, stability, and separation efficiency supersedes the circular one, which is also prone to flow blockage and lower efficiency.
The microfluidic chip introduced will be helpful in the future to improve the DLD separation technique in various applications, including Alexandrium algae research and red tide prevention for monitoring and protecting marine ecosystems.