Optical Tweezers-based Microrheological Measurements Using a High-Speed Camera

Synopsis

Collagen, the most abundant protein in the body, assembles into an extra-cellular fibrillar gel, which has both viscous and elastic properties. These properties can be determined by using optical tweezers to hold a micron-sized bead within the sample. Measurement of the bead's thermally induced motion enables the determination of the frequency-dependent viscoelasticity. Rather than only probing response at a single location, holographic optical tweezers create multiple, independent traps, permitting simultaneous tracking of multiple embedded beads and characterization of their correlated motion. By using this technique in a collagen gel, we will be able to determine local and through-space viscoelastic properties, which vary at different locations during its formation. Implications of this research lie in the fields of health and biomaterials. 

The aim of this work is to devise and validate protocols for using holographic optical tweezers to measure local and through-space viscoelasticity. Rather than using laser deflection to track particle motion, I use of a high-speed camera and image analysis to track the simultaneous motion of multiple beads.  This approach provides nanometer-scale resolution of particle position at sampling rates of up to 2.5 kHz. I compare tracking data collected from the high-speed camera to those collected by the laser deflection method and find a discrepancy in the perceived motion of the bead. I perform many experimental tests to assess the root of this problem.

Additionally, I numerically represent bead motion measurements if collected using both methods (laser deflection method and high-speed camera method) and compare them to the idealized measurement results. In doing so, I learn about the limitations of each method, and how the viscous and elastic properties output from the data is affected by each measurement device.

Finally, based on my numerical representations, I suggest a simple procedure to gain more accuracy in the viscous and elastic properties for both simple fluids (such as water) and complex fluids (such as collagen solutions) when using each method. This procedure can be used in future holographic optical tweezers-based experiments to obtain an accurate representation of the local and correlated properties of collagen.