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We use novel possibilities opened by compressional Optical Coherence Elastography (OCE) to characterize both natural interstitial gaps and laser-irradiation-produced porosity in collagenous tissues (corne and cartilage). Under increasing moderate compression up to strains ~several percent, the current Young modulus of cornea gradually increases from initial values below 100 kPa to values >MPa that are closer to Young's modulus of cartilages in which collagen fibers are much denser packed. The lower stiffness of cornea can reasonably be attributed to the initially looser packed collagenous fiber layers, so that initial high compressibility of cornea is dominated by interlayer voids (gaps). By analogy with geophysics, we apply a model describing the reduction in the tissue elastic modulus due to the presence of a system of nearly parallel, thin "crack-like" voids/pores between the collagenous fiber layers. Initially they are highly compressible, but with increasing compression are getting closed, so that the material gets stiffer. Characterization of such porous component in water-saturated packing of collagenous layers in the natural state is inaccessible to AFM and conventional microscopy, whereas OCE enables earlier unavailable possibility to non-invasively characterize such pores/gaps (their total volume and distribution over the aspect ratio) by analogy with crack characterization in geophysics. Also we apply OCE to characterize spatially-inhomogenous modification of pore characteristics by moderate IR-laser-irradiation in regimes typical of collagenous-tissue reshaping. The obtained results are important for obtaining better insight in the structural modificatons in collagenous packings in the context of the development of novel methods of laser-assisted non-surgical methods of cornea-refraction correction and biologically non-destructive reshaping of cartilaginous samples for fabrication of implants in otolaryngology and maxillofacial surgery.
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