Impedance-based viscoelastic flow cytometry – A short review

INTRODUCTION TO IMPEDANCE-BASED VISCOELASTIC FLOW CYTOMETRY

Flow cytometry can be defined as the technique that enables high-throughput characterization and counting of particles, cells or molecules passing through a particle interaction region which is based on optical (light interaction) or electrical (electric field interaction) signals.

In the field of particle detection and characterization, particle focusing related to the alignment of suspended particles along the centerline of a conduit, is key to achieve high signal repeatability and low coefficient of variation in flow cytometry experiments.

To this purpose, several forces from externally applied electric [1, 2], acoustic [3, 4], magnetic [5], optical [6] as well as flow-induced inertial [7–9], elastic [10–13], and Dean-drag [14,15] were applied in microfluidic systems. Here the focus is given to flow-induced lift forces for passive manipulation of particles.

Flow-induced particle alignment in microfluidics can be achieved in two ways: with Newtonian fluids via inertial lift and drag forces [16,17] and with non-Newtonian (viscoelastic) fluids via flow-induced elastic lift arising from the normal stress differences. Viscoelastic over Newtonian fluid focusing allows for precise handling and focusing of particles even from very low (uL/h) to high flow rates (mL/s) [18,19,20,21].

Microfluidic impedance-based flow cytometry is used to measure cellular biophysical properties with electrical impedance measurements under flow [22]. The first step consists in dispersing particles in ionic buffers (e.g., PBS) or electrolyte solutions to ensure cell viability. Secondly, impedance signals are acquired at electrical detection region from individual cells aligned in a single stream at the channel center. Finally, ionic concentration of the solution is important and needs to be adjusted to generate a contrast between the impedance of the particles and the suspending liquid to ensure selectivity and high signal to noise ratio [23, 24]. Thus far, viscoelastic focusing has been effectively employed in optical cytometry [18, 25, 26]. Nevertheless, the use of viscoelastic solutions in impedance-based systems requires the study of the ionic buffer concentration effect on viscoelastic focusing due to potential combination of rheological and electrical properties of the medium.

Therefore, this work introduces an impedance-based microfluidic flow cytometry device employing polyethylene oxide (PEO) viscoelastic solutions.

AIM & OBJECTIVES

• To tune effectively PEO aqueous solutions properties for various conditions to ensure stable particle focusing.
• To achieve focusing of polystyrene particles and red blood cells for various flow regimes.
• To perform microfluidic impedance flow cytometry and analyse device performances.

KEY FINDINGS

The measurement principle described in Fig 1 and Fig 2, presents single inlet/outlet PDMS channel and three coplanar electrodes. Particles dispersed in viscoelastic solutions are sent through the inlet and collected at the outlet.

For specific flow conditions, due to the elastic lift or combination of both elastic and inertial lift forces, particles migrate to the center of the channel and a single train of particles is formed upstream the impedance sensing region.

PS bead and RBC suspensions are pumped through the inlet of the channel at varying pressures from 50 to 200 mbar for PS beads and from 50 to 400 mbar for RBCs via pressure-driven flow controller (Mk3 OB1, Elveflow).

The effect of ionic buffer concentration in viscoelastic focusing using both rheometer measurements (see Fig 3.) and focusing trajectories in square cross sectional microfluidic channels (see Fig 4.) was assessed for various flow rates.

Ionic concentration rate is critical for cell viability and impedance measurements. Shear viscosities and focused particle trajectories were found independent from concentration of PBS from 1X to 10X.

These results suggest that PEO viscoelastic solutions are good candidates for impedance based measurement applications. Elasto-inertial particle focusing at high Reynolds number was performed for PS particles. The focusing of RBCs was possible at considerably low Reynolds number. Additionally, parachute shape single RBC orientation was achieved at specific Reynold number as observed in video 1.

By tuning the properties of the viscoelastic solution, fixed particle orientation for non-spherical objects was obtained for a range of flow rates. This result is very critical in cytometry applications to get low signal variations.

Finally, viscoelastic particle focusing technique and impedance based microfluidic cytometry device were combined for the first time to perform impedance flow cytometry measurements for PS beads and RBCs.

This work presents a powerful tool for cell counting and sizing applications and more precisely for morphology-based characterization of disease-infected cells.

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References