Viscoelasticity Imaging
Viscoelasticity is the property of a material that describes both viscous (liquid-like) and elastic (solid-like) behavior when a force is applied to it. Our research group is interested in measuring and imaging the viscoelastic properties of soft tissue, because the deviation from normal viscoelastic properties indicates pathological changes - a sign of diseases such as cancer. To measure viscoelasticity, we have developed a novel method that measures the dynamical behavior of small particles inside the material using near-infrared light.

Small particles (typically less than a micrometer) suspended in fluids perform a random walk, a phenomenon known as Brownian motion. This random walk is a combination of translational and rotational motion. We are interested in the rotational diffusion (tumbling) of nanorods typically 15 x 60 nm in size. Nanorods composed of gold are particularly useful for biomedical imaging because they are strong scatterers of near-infrared light due to a phenomenon called surface plasmon resonance. In addition, light scattered from plasmon resonant gold nanorods is highly polarized, and as such, gold nanorods are suitable as polarization-based optical probes in dynamic light scattering (DLS) experiments.

In our lab, we use polarization-sensitive optical coherence tomography (OCT) to study the rotational dynamics of gold nanorods diffused in various samples. The tumbling rate of the nanorods is directly tied to the cross-polarized intensity fluctuation of light scattered by these suspended nanorods. The knowledge of the tumbling rate of the nanorods allows us to determine their rotational diffusion coefficient in the solution. Finally, the viscosity of the solution can be determined by making use of a Stokes-Einstein relation to relate rotational diffusion coefficient with viscosity. In this way, we can infer viscosity based on the timescale of the intensity fluctuation due to the tumbling of nanorods in the solution. Since our technique is built upon the depth-ranging capability of OCT, it offers the unique ability to not only measure but also depth-range viscosity in a heterogeneous microenvironment.

Currently, our research is focused on extending our technique to probe the entire frequency dependent viscoelasticity of the medium.