Measurement of cell microrheology by magnetic twisting cytometry with frequency
domain demodulation.
Puig-De-Morales, Marina, Mireia Grabulosa, Jordi Alcaraz, Joaquim Mullol, Geoffrey N.
Maksym, Jeffrey J. Fredberg, and Daniel Navajas.
1Unitat Biofísica i Bioenginyeria, Facultat Medicina, Universitat Barcelona-IDIBAPS
and 2Hospital Clínic, 08036 Barcelona, Spain; 3School of Biomedical Engineering,
Dalhousie University, Halifax, Nova Scotia, Canada B3J 3J5; and 4Harvard School of
Public Health, Boston, Massachusetts 02115
APStracts 8:0287A, 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 ~10 Hz. Present demodulation approaches limit the MTC range to
frequencies <0.5 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 (~5 mum) coated with an RGD peptide were bound to the cell
membrane. Both the storage (G', real part of G*) and loss (G?, imaginary part of G*)
moduli increased with frequency as omegaalpha with alpha ˜ «1/4». The ratio G?/G' was
~0.5 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: 1619-1632, 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.
Received 14 February 2001; accepted in final form 23 April 2001
APS Manuscript Number A0151-1.
Article publication pending J Appl Physiol
ISSN 1080-4757 Copyright 2001 The American Physiological Society.
Published in APStracts on 18 June 2001