title: Resolving the stiffening-softening paradox in cell mechanics.
abstract: BACKGROUND
  Despite their notorious diversity, biological cells are mechanically well characterized by only a few robust and universal laws. Intriguingly, the law characterizing the nonlinear response to stretch appears self-contradictory. Various cell types have been reported to both stiffen and soften, or "fluidize" upon stretch. Within the classical paradigm of cells as viscoelastic bodies, this constitutes a paradox.
  
  
  RESULTS
  Our measurements reveal that minimalistic reconstituted cytoskeletal networks (F-actin/HMM) exhibit a similarly peculiar response. A mathematical model of transiently crosslinked polymer networks, the so-called inelastic glassy wormlike chain (iGwlc) model, can simulate the data and resolve the apparent contradiction. It explains the observations in terms of two antagonistic physical mechanisms, the nonlinear viscoelastic resistance of biopolymers to stretch, and the breaking of weak transient bonds between them.
  
  
  CONCLUSIONS
  Our results imply that the classical paradigm of cells as viscoelastic bodies has to be replaced by such an inelastic mechanical model.
authors: Institut für Theoretische Physik, Universität Leipzig, Leipzig, Germany.
authors: Lars
authors: L
authors: Wolff
authors: Pablo
authors: P
authors: Fernández
authors: Klaus
authors: K
authors: Kroy
mesh_terms: D000199:Actins / Q000378:metabolism; D001696:Biomechanical Phenomena; D002470:Cell Survival; D003599:Cytoskeleton / Q000378:metabolism*; D055595:Mechanical Phenomena; D008954:Models, Biological; D015879:Myosin Subfragments / Q000378:metabolism; D017711:Nonlinear Dynamics; D012212:Rheology; D013314:Stress, Mechanical
chemical_list: D000199:Actins; D015879:Myosin Subfragments