Quantifying both viscoelasticity and surface tension: Why sharp tips overestimate cell stiffness.

Area of Science:

• Cellular mechanics
• Biophysics
• Biomaterials

Background:

• Cell mechanical properties influence cellular functions and can indicate disease.
• Models often use cell tension and viscoelasticity, but their relative importance is unclear.
• Microindentation is a technique to probe cell mechanics.

Purpose of the Study:

• To determine the most relevant mechanical features (tension vs. viscoelasticity) for modeling cell behavior.
• To investigate the relationship between microindenter tip radius and measured cell stiffness.
• To clarify how surface tension and viscoelasticity contribute to cell mechanical properties.

Main Methods:

• Performed microindentation experiments on various cell types (leukocytes, adherent and detached endothelial cells).
• Utilized microindenters with varying tip radii, including sharp microneedles.
• Developed a model relating contact stiffness to contact radius and derived cell tension.

Main Results:

• Cell contact stiffness showed an affine function of contact radius across different cell types.
• Deduced surface tension for leukocytes matched micropipette aspiration measurements.
• Microindentation with sharp tips accurately measured surface tension, even when neglecting viscoelasticity.

Conclusions:

• Cell surface tension is a dominant factor measurable via microindentation with sharp tips.
• This method allows direct surface tension measurement, bypassing complex viscoelastic relaxation analysis.
• Understanding cell mechanics aids in characterizing cellular states and potential pathologies.