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* Unitat de Biofísica i Bioenginyeria, Facultat de Medicina, Universitat de Barcelona-IDIBAPS, 08036 Barcelona, Spain and Physiology Program, Harvard School of Public Health, Boston, Massachusetts 02115 USA
Correspondence: Address reprint requests to Daniel Navajas, Ph.D., Professor of Physiology, Unitat de Biofísica i Bioenginyeria, Facultat de Medicina, Casanova 143, 08036-Barcelona, Spain. Tel.: +34-93-402-4515; Fax: +34-93-402-4516; E-mail: dnavajas{at}medicina.ub.es.
Lung epithelial cells are subjected to large cyclic forces from breathing. However, their response to dynamic stresses is poorly defined. We measured the complex shear modulus (G*(
)) of human alveolar (A549) and bronchial (BEAS-2B) epithelial cells over three frequency decades (0.1–100 Hz) and at different loading forces (0.1–0.9 nN) with atomic force microscopy. G*() was computed by correcting force-indentation oscillatory data for the tip-cell contact geometry and for the hydrodynamic viscous drag. Both cell types displayed similar viscoelastic properties. The storage modulus G'() increased with frequency following a power law with exponent 
0.2. The loss modulus G''() was 2/3 lower and increased similarly to G'() up to 10 Hz, but exhibited a steeper rise at higher frequencies. The cells showed a weak force dependence of G'() and G''(). G*() conformed to the power-law model with a structural damping coefficient of 0.3, indicating a coupling of elastic and dissipative processes within the cell. Power-law behavior implies a continuum distribution of stress relaxation time constants. This complex dynamics is consistent with the rheology of soft glassy materials close to a glass transition, thereby suggesting that structural disorder and metastability may be fundamental features of cell architecture.
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