Size Matters: Rethinking Hertz Model Interpretation for Cell Mechanics Using AFM

Area of Science:

• Biophysics
• Cell Biology
• Materials Science

Background:

• Cell mechanics serve as a biophysical indicator of cellular states, including cancer metastasis and cell cycle progression.
• Atomic force microscopy (AFM) is commonly used to measure cell mechanics, often employing the Hertz contact model to determine Young's modulus.
• The Hertz model relies on assumptions of elasticity, isotropy, and homogeneity, potentially overlooking factors like the cytoskeleton and cell dimensions.

Purpose of the Study:

• To investigate the impact of cell size on the Young's modulus estimation using liposomes as simplified cell models.
• To assess how variations in liposome size affect mechanical property measurements derived from the Hertz model.

Main Methods:

• Liposomes of varying sizes were prepared and filled with either phosphate-buffered saline (PBS) or hyaluronic acid (HA) to simulate cellular cytoplasm.
• Atomic force microscopy (AFM) was utilized to acquire force-indentation curves from the liposomes.
• The Hertz contact model was applied to fit the AFM data and calculate the Young's modulus for each liposome size.

Main Results:

• A clear inverse relationship was observed between liposome size and the estimated Young's modulus.
• This size-dependent effect on Young's modulus was consistent for both PBS-filled and HA-filled liposomes.
• The findings indicate that the Hertz model's Young's modulus is influenced by the dimensions of the measured object.

Conclusions:

• The Young's modulus derived from the Hertz model is not solely an intrinsic material property but is also dependent on cell dimensions.
• When interpreting cell mechanics data obtained via the Hertz model, it is crucial to consider and account for cell size variations.
• This study highlights the limitations of the Hertz model in accurately representing cell mechanics without considering geometric factors.