Hartmann, B.; Reichel, F.; Moeckel, C.; Guck, J.

The viscoelastic nature of biological cells has emerged as an increasingly important research subject due to its relevance for cellular functions under physiological and pathological conditions. Advancements in microfluidics have made this technology a promising tool to study the viscoelasticity of cells. However, significant challenges remain, including the complex distribution of stresses acting on cells depending on the channel geometry, and the difficulty of keeping cells in the focal plane for imaging. Here, we report a new approach using hyperbolic channels for measuring cell viscoelasticity. A channel height much larger than the typical cell size minimized shear stresses so that normal stresses in the hyperbolic region dominated the stress distribution. Reducing the complexity of the stress-strain relationship allowed us to use polyacrylamide microgel beads to calibrate the stress curve. Additionally, we introduced 3D hydrodynamic focusing which enabled us to focus cells and microgel beads in the center of the channel. Finally, Kelvin-Voigt and power-law rheology models were employed to extract the mechanical properties of microgel beads and human leukemia HL60 cells. The measurement technique described here will help establish the viscoelastic properties of cells as an important readout in biophysical research in health and disease.