Spatiotemporal mapping of microscale stiffness during collagen polymerization and crosslinking by optical multifrequency time-harmonic elastography.

Abstract

Optical multifrequency time-harmonic elastography (OMTHE) was used for rapid mechanical characterization of extra-cellular matrix-derived collagen networks at micrometer resolution. OMTHE was optimized for point-wise shear wave excitation in small sample volumes and compared to tabletop magnetic resonance elastography (ttMRE) and optical intensity changes. Dynamic stiffening due to the fluid-gel transition during collagen polymerization and chemical crosslinking using glutaraldehyde was tracked by shear waves speed (SWS) at vibration frequencies between 3 and 10 kHz and frame rates up to 4 kHz. During collagen polymerization, after an initial lag phase, SWS increased on average 6 ± 3 min earlier than optical density, suggesting that a load-bearing percolating fiber network was established before fibril thickening enhanced light scattering. In contrast, chemical crosslinking showed a lag-free, SWS increase driven by the diffusion and subsequent action of the crosslinker from 1.7 ± 0.4 m s^-1 to 2.5 ± 0.5 m s^-1, matching the relative SWS change from ground-truth ttMRE. In conclusion, OMTHE provides a unique research tool that quantifies biomechanical property changes in small biological samples with spatiotemporal resolutions of micrometers and seconds.