Active and probe-free intracellular rheology via phase-sensitive thermoviscous flows

Determination of the rheological properties of cells is known to require active measurements, which largely depend on the internalisation of mechanical probes. Here we circumvent this problem via the introduction of Rheo-FLUCS: an active, yet probe-free approach that leverages light-induced flows to access mechanical changes in complex systems. While Rheo-FLUCS is facilitated by thermoviscous expansion phenomena rather than external forces, here we show equivalence in its ability to measure relative viscoelastic properties. Specifically, we demonstrate a phase-lag equivalence with probe-dependent active microrheology in a wide range of physically different, yet chemically identical materials. We exemplify the utility of Rheo-FLUCS in three distinctly different biological systems: compound-treated mouse fibroblasts (NIH-3T3), genetically modified human osteoblasts (U2OS) to elucidate the role of myosins in cytoplasmic mechanics, and early ascidian oocytes of Phallusia mammillata at fertilization stage. Our biological use-cases exemplify the application versatility of Rheo-FLUCS, which in the future may use phase information as a marker for developmental success.