FIGURE 8.

Typical movements of beads inside a cell that can be induced with the current magnetic tweezers. (a) Response of a bead that experiences a constant force of 60 pN. The bead trajectory is indicated by the white curve. (b) More complex movements are possible by changing the direction of the force. In this case, a triangular movement is induced. The shown trajectories in a and b are both to scale. (c) Measurements (circles) of the displacement of a 1.05-μm and a 0.35-μm bead versus time due to a force step from 0 pN to 60 pN for the larger bead and from 0 pN to 5 pN for the smaller bead. The response clearly indicates the viscoelastic behavior and is well described by the model (solid line). (d) Bead positions when the force is alternating between the three different poles in a clockwise fashion. Clearly a triangular movement is observed as expected. The viscoelastic model describes the observed movements very well (solid line).