Complex Fluids: Probing Mechanical Properties of Biological Systems with Optical Tweezers

Byline: H. Daniel Ou-Yang, Department of Physics and Bioengineering University, Bethlehem, Pennsylvania 18015; email: hdo0@lehigh.edu, Department of Bioengineering, Lehigh University, Bethlehem, Pennsylvania 18015; email: miw206@lehigh.edu; Ming-Tzo Wei, Department of Bioengineering, Lehigh University, Bethlehem, Pennsylvania 18015; email: miw206@lehigh.edu Keywords: microrheology, viscoelasticity, nonequilibrium thermodynamics, motor proteins, DNA, actin filaments, focal adhesion complexes, cytoskeleton Abstract The mechanical properties of cells are crucial for cell sensing and reaction to mechanical environments. This review describes the basic principles of optical tweezers and their use as force sensors for studying the mechanical properties of biological systems. It covers experiments of four groups of biological systems arranged by increasing complexity: (a) packaging DNA into viral capsids by bacteriophage portal motors and the dynamical stiffness of DNA upon protein binding, (b) actin-coated giant vesicles and the myosin-II embedded actin polymer network, (c) suspension cells, and (d) adhesion cells. These examples demonstrate how optical tweezers have been used to improve the understanding of the mechanical properties of biological systems at subcellular and molecular levels.