MECHANICAL SIGNATURES OF BRILLOUIN SPECTROSCOPY

Abstract

Brillouin light spectroscopy (BLS) has recently emerged as a tool for noncontact, nonperturbative and label-free characterization of biomechanical properties. BLS probes the longitudinal modulus of material while traditional techniques for biomechanical characterization aim to quantify Young’s or shear modulus. However, empirical correlations between the different moduli have been observed in several biological materials, correlations that are not yet universally established. The objective of this thesis is to advance the understanding of these correlations and their limitations with controlled systematic comparisons of longitudinal modulus and gold-standard modulus of hydrogels and corneal tissue. First, using polymer hydrogels as model of study, experimental data and theoretical models were used to demonstrate that the correlation between longitudinal and shear moduli is due to their common dependence on underlying physico-chemical parameters of the polymer system. This dependence allowed to predict one modulus from the other when enough information of the system is available. Furthermore, the limitation of thiscorrelation was studied when hydrogels absorb water, finding that hydration affects both moduli but in different manner and thus, their correlation. Having established hydration as an important variable for biomechanical properties, the correlation between modulus in the corneal tissue and crosslinking procedure (CXL) was studied. CXL is the gold-standard treatment for corneal ectatic disorders, and its success is due to the strengthening of the mechanical properties of the cornea as a result of photochemical induced collagen crosslinking and dehydration of the tissue. However, most mechanical characterization ex vivo, does not factor in the tissue dehydration effect, overestimating the effect in the clinical situation. With experimental data obtained by gold-standard methods and established theoretical models, the modulus after hydration changes after the CXL was systematically characterized. Finally, another scenario where studying the correlation between moduli is important is the nonlinear mechanical behavior of the cornea. Effect that has been observed with different techniques, but BLS has failed to capture so far. This works proves that the reason of this discrepancy has to do with the mechanical anisotropy of the cornea and the nature of BLS, which is a purely uniaxial measurement of mechanical properties. Considering these factor, it is proven that BLS has the ability to measure the nonlinear mechanical properties of the corneal tissue.