Assessing the mechanical properties of tissue-mimicking phantoms at different depths as an approach to measure biomechanical gradient of crystalline lens.

We demonstrate the feasibility of using the dominant frequency of the sample surface response to a mechanical stimulation as an effective indicator for sensing the depthwise distribution of elastic properties in transparent layered phantom samples simulating the cortex and nucleus of the crystalline lens. Focused ultrasound waves are used to noninvasively interrogate the sample surface. A phase-sensitive optical coherence tomography system is utilized to capture the surface dynamics over time with nanometer scale sensitivity. Spectral analysis is performed on the sample surface response to ultrasound stimulation and the dominant frequency is calculated under particular loading parameters. Pilot experiments were conducted on homogeneous and layered tissue-mimicking phantoms. Results indicate that the mechanical layers located at different depths introduce different frequencies to the sample surface response, which are correlated with the depth-dependent elasticity of the sample. The duration and the frequency of the ultrasound excitation are also investigated for their influences on this spectrum-based detection. This noninvasive method may be potentially applied for localized and rapid assessment of the depth dependence of the mechanical properties of the crystalline lens.