Feedback-tracking microrheology in living cells.

Area of Science:

• Cellular biophysics
• Active matter physics
• Soft matter physics

Background:

• Living cells are active materials where metabolism drives internal forces and self-organization.
• Understanding cell mechanics is complex due to material heterogeneity and internally generated forces.
• The fluctuation-dissipation theorem (FDT) typically describes equilibrium systems.

Purpose of the Study:

• To analyze the out-of-equilibrium mechanics of living cells.
• To investigate the interplay between material properties and nonthermal force fluctuations.
• To quantify cellular responses using adapted FDT.

Main Methods:

• Simultaneous active and passive microrheology using laser interferometry and optical trapping.
• Tracking microscopic probes in cells with vigorous cytoplasmic fluctuations using 3D feedback.
• Developing a theory adapting FDT for out-of-equilibrium systems with positional feedback.

Main Results:

• Observed polymer network viscoelastic response in fibroblasts (G* ∝ (-iω)3/4).
• Found glassy mechanics in epithelial cells (G* ∝ (-iω)1/2), attributed to cytosol dynamics.
• Quantified violations of FDT, indicating nonthermal force fluctuations.

Conclusions:

• Living cells exhibit distinct mechanical behaviors (viscoelastic vs. glassy) depending on their structure.
• The glassy state in cells differs from classical glasses and is unique to active materials.
• Cellular mechanics are governed by a combination of material properties and active, nonthermal fluctuations.