Biological cells are known to respond to changes in their environment by changing their mechanical response. For several diseases (e.g. diabetes, spherocytosis) changes in mechanical properties of cells have been corelated with the progression of the disease. Changes in cells mechanical properties has also been corelated with their development and age. With the advent of advanced microfluidic characterisation techniques, the mechanical response of a large number of cells can be measured at the single cell level. These studies help in investigating fundamental questions in cell biology. These analysers are called microfluidic cell transit analysers. In this work we introduce a novel cell-transit analyser where an air-liquid interface is used as a mechanical element to induce deformability in cells. This technique enables both static and dynamic deformability measurements in live cells ex vivo.

The devices were fabricated using PDMS microchannels bonded to glass substrates. The air-water interface is formed using manually controlled syringes. A syringe pump was used to flow the cell containing samples. In this work, stiffer air-water interface (surface tension 72 mN/m) to deform more compliant Red Blood Cells (effective 20 mN/m) in micro-channels. As the interface stiffnesses are of similar order, the air-water interface also deforms. In the static mode, measurement of the air-water interface deformation allows us to measure the Laplace pressure drop and hence estimate the stiffness of the Red Blood Cells. Using our technique, we estimated the stiffness of a single RBC was measured to be 18±2 mN/m which is in good agreement with previous reports.

In the dynamic mode the RBC containing buffer was flowed from one side and air pressure was applied at the two perpendicular channels to create stable air-liquid interfaces with desired constriction widths. This allows us to perform high throughput measurements (>100 cells/s) of deformability index (DI) and transit time for different air-liquid interface constriction widths. The adjustments in constriction width can be made in real time and can be tuned to widths as small as 2µm by controlling the air-pressure. The technique also eliminates the cell-substrate interactions due to the use of a virtual wall.

In order to validate the ability of the technique to differentiate cell population based on mechanical response, DI and transit time for cells passing through the air-water interface constriction were measured for healthy and 0.01% Glutaraldehyde (GA) treated RBCs. The DI of the GA-RBCs was found to be smaller than healthy RBCs thereby confirming the effectiveness of the technique in deformability cytometry. Further, the mechanical response of cells to different constriction widths was measured. We found that for differentiating Red Blood Cells of different stiffness using DI and transit time, constriction widths < 4µm are optimum.