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There are numerous filters to separate particles in liquid based on their size, which can be enough to isolate them; however, particle shape can be more important, as it distinguishes healthy red blood cells from those affected by sickle-cell disease or malaria. Shape can also be used to determine what stage a cell is in of the cell cycle, which would benefit researchers looking for dividing cells. Recent research by Dino Di Carlo of UCLA looks to separate particles of differing aspect ratios continuously, using inertial fluid-dynamics. His work, “Continuous Inertial Focusing and Separation of Particles by Shape,” featured in Physical Review X reminds me of his previous work to use inertial fluid-dynamics to continuously filter particles according to size.

Particle Separation

Existing methods to separate particles according to shape include hydro-dynamic filtration (HDF) and deterministic lateral displacement (DLD), along with a few others. DLD involves a grid of posts in a channel that is arranged in a way that effectively separates particles according to size. This can be enhanced by controlling particle orientation in order to filter by particle shape or shear stress to separate by particle deformability. HDF uses highly branched channels to separate particles by size according to the fluidic resistance of the side channels. Both of these methods are passive and continuous, but they have highly complex structures and require low flow rates (no greater than a few µL/min). Di Carlo’s method uses fluid inertia to focus particles in channels at high flow rates from 40 to 80 µL/min. This utilizes a shear-gradient lift force and a wall-effect lift force in order to shift particles across streamlines when the Reynolds number of the particle is on the magnitude of 1 or greater. The shear-gradient lift force is directed down the shear gradient and toward the wall, while the wall-effect lift force is caused by the wake of a particle near the wall and is directed away from the wall. With a Reynolds numbers on the order of 1, inertial lift forces dominate the particle behavior while viscous interactions dominate when the particle Reynolds number is << 1. As the particle Reynolds number increases, migration across streamlines, away from the center line, is observed.

Di Carlo attempted to separate spheres and ellipsoids of varying aspect ratios, while conserving volume. Spheres of 3 µm and 6 µm in diameter, and ellipsoids that conserved the volume at 1:3 and 1:5 aspect ratios were used. Spheres and ellipsoids are sorted using inertial fluid-dynamics. Particles with larger aspect radios reach equilibrium positions (Xeq) 4 cm downstream.

This work demonstrates that rod-like particles find equilibrium positions close to the center of the channel, while spheres of the same volume end up in streamlines close to the wall. When the major axis of the particles rotates perpendicular to the plane of the wall, the wall-effect lift increases and the particles is pushed away from the wall. Once the major axis has realigned with the direction of the flow, the wall-effect lift decreases and the particles move towards the wall one again. However, particles with higher aspect ratios experience a wall-effect lift greater than the shear-gradient lift and find equilibrium positions closer to the center of the channel. As the Reynolds number of the particle increases (increasing flow rate is one way to increase Reynolds number), the shear-gradient lift force increases faster than the wall-effect lift force. But the particles with higher aspect ratios rotate and experience a greater wall-effect lift force and return to the center. This relationship with increasing particle Reynolds number allows this method to scale with flow rate, while the previous methods do not.

Four different shape-activated particle-sorting (SAPS) devices were designed with varying numbers of outlets, outlet resistances, channel aspect ratios and flow rates. Three of the devices used 6 µm spheres and their derived ellipsoids, while the final device used the 3 µm particles. All of the devices had varying performances, but the researchers selected device C, which had 7 outlets and isolated 88% of the spheres with 87% purity, 49% of 1:5 rods with 78% purity and 77% of 1:3 rods with 80% purity, for sorting yeast cells. Yeast cells are normally spherical, but form a bispherical twin or aggregate when budding. This change in shape is similar to the varying aspect ratios examined previously. It is useful to synchronize cell cycle stages, but this may be achieved with chemicals that alter cell physiology, changes in temperature or size filtration. SAPS C was able to extract nondividing singles with high yield and purity up to 94% and 54% of budded yeast cells were recovered at 31% purity, which increased from 6.6% purity at the inlet. According to my rough measurements from the paper’s figures, the aspect ratio of budding yeast is less than 2:1 which may explain the difference in performance compared to the original ellipsoids. Previously, Sugaya et al. used a 5 outlet HDF system to separate budding yeast cells. In comparison, Sugaya achieved up to 69.4% purity of budding cells in one outlet, up from 39.4% at the inlet. This same outlet recovered 28.8% of budding cells while another outlet recovered 65.2%. There is still room for improvement of Di Carlo’s budding yeast yield, but this operated at 1500 cell/s, compared to 100 cell/s from previous work that utilized dielectrophoretic forces according to the opacity of dividing yeast.


Di Carlo has proposed that this work be used to sort shaped particles in other areas to improve cytometry that operates on spherical particles, alignment of barcoded particles, and identification of microalgae that vary in size and shape. Interestingly, he also introduced the capability of this setup for a non-biological process: improving cement. Cement strength and stability are affected by particle shape and size and could benefit from shape based separation. According to Dr. Di Carlo, “… [Cement] particles that are too large may not react completely in internal regions of the particle, while smaller particles with very high surface area to volume ratios can react too quickly and may not be stable.” I’m excited to see microfluidics expand into more established industries and further demonstrating real-world potential to be a more cost effective, accessible technology. Masaeli, M., Sollier, E., Amini, H., Mao, W., Camacho, K., Doshi, N., Mitragotri, S., Alexeev, A., & Di Carlo, D. (2012). Continuous Inertial Focusing and Separation of Particles by Shape Physical Review X, 2 (3) DOI: 10.1103/PhysRevX.2.031017

Di Carlo, D., Irimia, D., Tompkins, R., & Toner, M. (2007). Continuous inertial focusing, ordering, and separation of particles in microchannels Proceedings of the National Academy of Sciences, 104 (48), 18892-18897 DOI: 10.1073/pnas.0704958104

Sugaya, S., Yamada, M., & Seki, M. (2011). Observation of nonspherical particle behaviors for continuous shape-based separation using hydrodynamic filtration Biomicrofluidics, 5 (2) DOI: 10.1063/1.3580757




