Where the magnet sits sets the sign of the scaling law: membrane-anchored versus intracellular magnetic actuation of cells

Intracellular lysosome rods and membrane-anchored caps obey opposite scaling laws for step-out frequency, crossing at 184 particles and 4.04 Hz under matched single-cell magnetic moment.

• Why it matters: Treating these two magnetic geometries as interchangeable can mislead design and interpretation of cell-mechanics experiments, especially within the frequency window commonly used.
• What they did: The authors analyze single-cell magnetic actuation analytically, comparing step-out frequency versus assembly size for intracellular rods and surface-bound caps, and derive cap-tension results tied to magnetic texture.
• The result: A field-oriented cap reaches 2.69 mN m-1 versus 0.0033 mN m-1 for random easy axes, while a superparamagnetic cap transmits no torque; the model matches four published measurements without fitted parameters and predicts two zero outcomes that one experiment could falsify.

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