A transition of dynamic rheological responses of single cells: from fluid-like to solid-like.

The mechanical properties of cells are crucial for elucidating various physiological and pathological processes. Cells are found to exhibit a universal power-law rheological behavior at low frequencies. While they behave in a different manner at high frequency regimes, which leaves the transition region largely unexplored. Here, we investigate single-cell rheological behaviors across different cell types (primary hematopoietic stem cells, the hippocampal neuronal cell line and human dental pulp stem cells) by atomic force microscopy (AFM)-microrheology method, uncovering a universal two-stage power-law rheological behavior. Cells behave fluid-like at shorter time scales and solid-like at longer scales. To characterize the transition region between these stages, we introduce a time-scale parameter, termed "transition time". Notably, for all the cell types under study, we find that the transition time decreases with increasing elastic moduli and increases for larger power-law exponent. Furthermore, based on our previous self-similar hierarchical model, we propose a theoretical method to determine the upper and lower bounds of the transition time range. Our experimental results exhibit an excellent agreement, consistently falling within the predicted theoretical limits. Furthermore, we present six crucial mechanical indices that depict both the dynamic and static mechanical properties of single cells. These parameters can effectively differentiate cell types and provide a comprehensive perspective on the mechanical states of cells. Our study may offer new insights into the viscoelastic transformation of cells from fluid-like to solid-like behaviors, and highlights the mechanisms underlying various time scales during biomechanical processes.