Laser speckle Micro-rheology (LSM) is a novel optical tool for evaluating the viscoelastic properties of biomaterials. In LSM, a laser beam illuminates the specimen and scattered rays are collected through an objective by a high-speed CMOS camera. The self-interference of light rays forms a fluctuating speckle pattern captured by the CMOS sensor. Spatio-temporal correlation analysis of speckle images provides the intensity autocorrelation function, g2(t), for individual pixels. Next, the mean square displacements (MSD) of Brownian particles are deduced and substituted in the generalized Stokes-Einstein relation (GSER) to yield a 2D map of viscoelastic modulus, |G*(ω)|.
To compare the accuracy, sensitivity, and dynamic range of LSM measurements with standard mechanical testing methods, homogeneous polyethylene glycol (PEG), agarose, and polyacrylamide (PA) gels, of assorted viscoelastic properties were fabricated and evaluated using LSM, shear rheology, and indentation-mode atomic force microscopy (AFM). Results showed a statistically significant, strong correlation between G* values measured by LSM and shear rheology (R=0.94, p<5x10-6) (|G*|: 30 Pa - 30 kPa at ω = 1 Hz). Likewise, strong correlation was observed between G* values measured by LSM and indentation moduli of AFM (R=0.94, p,0.05). Next, polyacrylamide substrates with micro-scale stiffness patterns were tested using LSM. The reconstructed |G*| maps illustrated the high sensitivity of LSM in resolving mechanical heterogeneities below 100 microns. These findings demonstrate the competent accuracy and sensitivity of LSM measurements. Moreover, the non-contact nature of LSM provides a major advantage over mechanical tests, making it suitable for in vivo studies in future.
Zeinab Hajjarian Kashany, Shawn Ahn, Hadi Tavakoli Nia, Diane M. Tshikudi, Alan Grodzinsky, Rakesh K. Jain, and Seemantini K. Nadkarni, "Laser speckle micro rheology for micro-mechanical mapping of bio-materials
(Conference Presentation)," Proc. SPIE 9707, Dynamics and Fluctuations in Biomedical Photonics XIII, 970702 (Presented at SPIE BiOS: February 14, 2016; Published: 26 April 2016); https://doi.org/10.1117/12.2213761.4848677957001.
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