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. Measurement of cell microrheology by magnetic twisting cytometry with frequency domain demodulation. J Appl Physiol 91: [1152][1153][1154][1155][1156][1157][1158][1159] 2001.-Magnetic twisting cytometry (MTC) (Wang N, Butler JP, and Ingber DE, Science 260: 1124-1127, 1993) is a useful technique for probing cell micromechanics. The technique is based on twisting ligand-coated magnetic microbeads bound to membrane receptors and measuring the resulting bead rotation with a magnetometer. Owing to the low signal-to-noise ratio, however, the magnetic signal must be modulated, which is accomplished by spinning the sample at ϳ10 Hz. Present demodulation approaches limit the MTC range to frequencies Ͻ0.5 Hz. We propose a novel demodulation algorithm to expand the frequency range of MTC measurements to higher frequencies. The algorithm is based on coherent demodulation in the frequency domain, and its frequency range is limited only by the dynamic response of the magnetometer. Using the new algorithm, we measured the complex modulus of elasticity (G*) of cultured human bronchial epithelial cells (BEAS-2B) from 0.03 to 16 Hz. Cells were cultured in supplemented RPMI medium, and ferromagnetic beads (ϳ5 m) coated with an RGD peptide were bound to the cell membrane. Both the storage (GЈ, real part of G*) and loss (GЉ, imaginary part of G*) moduli increased with frequency as ␣ (2 ϫ frequency) with ␣ Ϸ 1 ⁄4. The ratio GЉ/GЈ was ϳ0.5 and varied little with frequency. Thus the cells exhibited a predominantly elastic behavior with a weak power law of frequency and a nearly constant proportion of elastic vs. frictional stresses, implying that the mechanical behavior conformed to the so-called structural damping (or constant-phase) law (Maksym GN, Fabry B, Butler JP, Navajas D, Tschumperlin DJ, LaPorte JD, and Fredberg JJ, J Appl Physiol 89: [1619][1620][1621][1622][1623][1624][1625][1626][1627][1628][1629][1630][1631][1632] 2000). We conclude that frequency domain demodulation dramatically increases the frequency range that can be probed with MTC and reveals that the mechanics of these cells conforms to constant-phase behavior over a range of frequencies approaching three decades. cell mechanics; cell viscoelasticity; complex elastic modulus; power law rheology; structural damping; magnetic tweezers MECHANICAL PROPERTIES OF THE CELL play an important role in essential cellular functions such as mechanotransduction, shape stability, motility, apoptosis and DNA synthesis (12-14, 19, 23). Techniques for studying cell mechanics include cell poking (6), atomic force microscopy (18), optical tweezers (28), laser tracking microrheology (27), magnetic bead microrheometry (2), and magnetic twisting cytometry (MTC) (23). MTC, in particular, has proved to be a useful tool for exploring force transmission across the cell membrane and for assessing cell stiffness and its changes (11,15,23,25,26). This technique was first introduced by Crick (4) and Crick and Hughes (5) and was further refined by Valberg (21), Wang et al. in 1993 (23), and, m...

We probed elastic and loss moduli in the adherent human airway smooth muscle cell through a variety of receptor systems, each serving as a different molecular window on cytoskeletal dynamics. Coated magnetic microbeads were attached to the cell surface via coating-receptor binding. A panel of bead coatings was investigated: a peptide containing the sequence RGD, vitronectin, urokinase, activating antibody against beta(1)-integrin, nonactivating antibody against beta(1)-integrin, blocking antibody against beta(1)-integrin, antibody against beta(1)-integrin, and acetylated low-density lipoprotein. An oscillatory mechanical torque was applied to the bead, and resulting lateral displacements were measured at baseline, after actin disruption by cytochalasin D, or after contractile activation by histamine. As expected, mechanical moduli depended strongly on bead type and bead coating, differing at the extremes by as much as two orders of magnitude. In every case, however, elastic and loss moduli increased with frequency f as a weak power law, f( x-1). Moreover, with few exceptions, data could be scaled such that elastic and frictional responses depended solely on the power law exponent x. Taken together, these data suggest that power law behavior represents a generic feature of underlying protein-protein dynamics.