Characterization of ferrofluid-based magnetic tweezers and extended analysis of creep experiments, related to Fig. 1 . (a) Calibrating force–distance relationship of the ferrofluid-based magnetic tweezers. The calibration was performed by tracking a ferrofluid sphere in a high-viscosity test fluid under a given magnetic field (Materials and methods). The solid lines show the fit with an exponential function. (b) Characterization of turn-on/off dynamics with high-temporal resolution recording. The time evolution of the magnetic force was recorded at 100 Hz. The time required to turn on/off the magnetic force was around 40 msec. Black shaded area indicates the period where the magnetic field was present. (c) A bright field image of a cell which has ferrofluid injected into the nucleus. Scale bar is 10 μm. (d and e) Mean compliance in the creep (d) and relaxation (e) phases. Error bar indicates standard deviation. (f) Compliance at t = 30 and 5 sec in the creep phase. (g) Characteristic timescale of the relaxation phase. (h–k) Four viscoelastic parameters k₀, k₁, η₀, and η₁ of the microtubule complex determined using the Burgers model.