Development and applications of an optical tweezer-based microrheometer: case studies of biomaterials and living cells

The investigation of mechanical properties of living biological cells and biomaterials is challenging because they are inhomogeneous and anisotropic at microscopic scales, and often time-dependent over a broad time scale. Through three case studies of biomaterials and living cells, we demonstrate that a novel, oscillating optical tweezer-based imaging microrheometer developed recently in our laboratory has overcome many technical barriers posed by the complexity of biological systems. In this paper, we present the working principle, system setup and calibration of the imaging microrheometer, and report the groundbreaking results of the three applications: gelation dynamics of cross-linkable hyaluronan acid (HA) hydrogels; Mechanical in-homogeneity and anisotropy in purified microtubule networks; and effects of drug treatment and temperature variation on the mechanical properties of in vitro human alveolar epithelial cells. In each case, micro beads inserted in the materials, or attached to the cell membrane were used as probes for optical trapping. The probe particle was set into a forced harmonic oscillation by oscillating optical tweezers. Position sensing optics and phase lock-in signal processing allow the determination of the amplitude and phase shift of the particle motion at high sensitivity. The complex mechanical modulus G^* is then calculated from the amplitude and the phase shift. The rheometer system is capable of measuring dynamic local mechanical moduli in the broad frequency range of 1.3-1000 Hz at a sampling rate of 2 data point per second across a wide dynamic range (1~20,000 dyne/cm²). Integration of the rheometer system with spinning disk confocal microscopy enables the study of micromechanical properties and the microstructure of the sample simultaneously. Combination of dual-axis, piezo-electric activated mirror and 2-D position sensing detector gives the rheometer system the capability of investigating mechanical anisotropy in highly structured biological samples.