Active Stochastic Microrheology using Optical Tweezers

Abstract

Cells are dynamic structures capable of generating and reacting to physical cues in their environment. Measuring mechanical properties is thus essential for elucidating cell or other material structure-function in particular during dynamic rearrangement of the cytoskeleton. Although a variety of rheological techniques have been developed using video microscopy, AFM, and magnetic traps, the measurable frequency range is limited by the time to obtain the measurement, and forcing conditions such as amplitude, direction, contact geometry, and probe location. Here, we developed active stochastic microrheology using optical tweezers to enhance the temporal resolution and precision of detection. A stochastic force is generated by moving the trap relative to the sample. Both bead displacement and trap position are monitored simultaneously by separate position sensitive devices. With this method, both storage and loss shear moduli of the extracellular matrices can be extracted over a wide frequency range of 10−2 – 103 Hz within a few minutes. Also, this method was used to probe the local mechanical environment of B-cell receptor using antigen specific interaction. We showed that the local mechanical properties are strengthened in response to antigen binding and repeated external excitation in a physiological range of 1–100pN. The mechanical responses can also be measured with respect to direction such as force applied normal and perpendicular to the cell membrane. This technique is useful in characterizing the mechanical properties at a user-defined location and magnitude, over a wide frequency spectrum, in a short time, and with a small deformation < 100nm. With these advantages, the method can also be applied to other cell processes, studies of complex fluids, fibril growth, and polymer solutions. Support from the NIGMS (GM-076689), an NSF Career Award (0643745), and the Singapore-MIT Alliance for Research and Technology (SMART-BioSyM) are gratefully acknowledged.