  Here we report on the viscosity of eukaryotic living cells as a function of
the time, and on the application of stochastic models to analyze its temporal
fluctuations. The viscoelastic properties of NIH/3T3 fibroblastic cells are
investigated using an active microrheological technique, where magnetic wires,
embedded into cells, are being actuated remotely. The data reveal anomalous
transient responses characterized by intermittent phases of slow and fast
rotation, revealing significant fluctuations. The time dependent viscosity is
analyzed from a time series perspective by computing the autocorrelation
functions and the variograms, two functions used to describe stochastic
processes in mathematical finance. The resulting analysis gives evidence of a
sub-diffusive mean-reverting process characterized by an autoregressive
coefficient lower than 1. It also shows the existence of specific cellular
times in the ranges 1 - 10 s and 100 - 200 s, not previously disclosed. The
shorter time is found being related to the internal relaxation time of the
cytoplasm. To our knowledge, this is the first time that similarities are
established between the properties of time series describing the intracellular
metabolism and statistical results from mathematical finance. The current
approach could be exploited to reveal hidden features from biological complex
systems, or determine new biomarkers of cellular metabolism.

Soft Condensed Matter;Biological Physics;Cell Behavior