Table 2.

Summary of fluid focusing methods in microfluidic cytometry.

Focusing Method	Principle	Advantage	Disadvantage
Sheath flow	2D and 3D Hydrodynamic focusing Ligler and Kim, 2010; Golden et al., 2012	Fabrication of fluidic channels without the need for active electrical circuitry	Production of large amounts of waste sheath buffer fluid Need for multichannel fluidic pumps to precisely position cells within the fluid Increased cell concentrations can result in the loss of a focused single cell stream resulting in erroneous cell counts
Sheath-less flow	Acoustic focusing Piyasena et al., 2012	High volumetric throughput Precise Spatial Positioning within 3D sheath flow	Requires integration of piezoelectric devices to generate acoustic waves
	Dielectrophoretic focusing Yu et al., 2005	Similar efficiency to acoustic focusing	Requires electrode integration within the channel Requires sample buffer conductivity to be adjusted Depends on particle polarizability
	Inertial focusing Gou et al., 2018	Passive method not requiring external driving power	Diminished performance at high cell concentrations similar to hydrodynamic focusing Results in pressure variations and consequently the shear stresses
	Magnetic focusing Zeng et al., 2012	Precise spatial positioning can be achieved by extrinsic magnetic bead labeling	Few biologicals particles are diamagnetic like erythrocytes and platelets Other cell types need to be tagged/ labeled using magnetic beads Requires the integration of strong magnets to produce intense field gradients