(Color online) Schematic view of the setup. The -metal core of a solenoid is attached to a micromanipulator on a microscope equipped with a heated stage, antivibration table, motorized drive and shutter, and a CCD camera connected to a PC. Solenoid current and core tip position are controlled by the PC, and bead positions are tracked in real time.
(Color online) (A) force-distance curves are recorded at different currents (shown here: 100, 600, and ) for superparamagnetic beads (Invitrogen Dynabeads). (B) Fitting Eq. (4) to all data points yields a common intersection of the force-distance curves from multiple currents. Note that in a double-logarithmic plot, the force-distance curves form straight lines. (C) Equation (5) is fitted to the slopes of the double-logarithmically plotted force-distance curves (i.e., the power-law exponent of the force-distance relationship) at different currents. At high currents, both beads and pole tip are saturated and the slope flattens out. (D) coercitive currents for different magnetization currents are recorded and fitted by a fourth-order polynomial to compensate for the magnetic hysteresis of the solenoid core material.
(Color online) Force vectors point towards the needle tip. Force magnitude only depends on the bead-tip distance for all beads within a cone of about 120° around the tip (solid lines).
(Color online) Solenoid current, and hence force, is delayed by less than compared to the control voltage generated by the DA converter (DAC) board. This ensures that force application and image acquisition can be synchronized.
(Color online) (a) Magnetic beads were embedded into a polyacrylamide (PAA) gel or attached to the surface of a collagen-coated PAA gel, or attached to the apical membrane of cells via fibronectin (FN)-integrin linkages. (b) Displacement of beads embedded into the gel and attached to cells as a response to a stepwise increasing force. Note the purely elastic response of the gel and the viscoelastic creep of the cell. (c) Apparent stiffness of all three conditions vs applied force. While the gel-embedded beads show linear behaviour, surface-bound beads on PAA gel and cells show different amounts of nonlinearity at higher forces. (d) The amount of disrupted beads vs force is an indicator of bead binding strength.
Comparison of different types of beads in terms of force-to-volume ratio. Force is given for a driving current of and a bead-to-needle distance of .
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