Real-Time Monitoring of Mass and Mechanical Properties in Single Cells and Multicellular Spheroids.

Cell state, growth, and mechanics are tightly connected properties underpinned by fundamental biological processes. Yet methods to quantify them simultaneously under physiologically relevant conditions, at resolutions sufficient to follow (sub)cellular processes, are still lacking. In particular, the correlation between cell state, mass, and mechanical properties during dynamic processes such as growth, differentiation, development, and disease remains poorly understood. Here, we develop experimental and theoretical approaches based on photothermally actuated microcantilevers integrated with optical microscopy to simultaneously monitor the morphology, mass, and quality (Q)-factor of single mammalian cells and larger cellular systems under culture conditions, with millisecond time resolution. We find that the Q-factor, a measure of mechanical energy dissipation, mirrors the mechanical properties of the cell, with the cytoplasm dominating as the largest and softest compartment. Our method reveals how cellular mechanical properties correlate with mass and depend on cell type, state, and growth. Applied to human spheroids, it further monitors how larger cellular systems regulate mechanical properties during development. Together, these measurements quantify mechanobiological parameters that have so far remained inaccessible, opening the way to a more complete characterization of how cellular systems regulate mechanics during fundamental processes of life.