Mechanical characterisation of biological materials using Brillouin microscopy

Biomechanics studies how biomaterials deform subjected to external loads. Most techniques used in biomechanics require direct contact or lack of subcellular resolution. By contrast, Brillouin microscopy is a contactless and label-free technique used to characterise mechanical properties of cells and tissues. Despite Brillouin microscopy measuring longitudinal modulus M , an empirical power law has been widely used to interpret Brillouin measurements as stiffness. In this thesis, we focused on the interpretation and relevance between the Brillouin microscopy measurements and quasi-static mechanical properties using hydrogels and cells. To investigate how Brillouin measurements relate to the mechanical properties of biological materials, we use hydrogels that approximate the mechanics of biphasic hydrated materials. By varying water content ε and Young’s modulus E in hydrogels, we found Brillouin measurements reflect changes in ε and the relationship between M and E arises due to their mutual dependence on ε. We further used binary mixture theory and polymer theory to explain the underlying physics. However, cells are neither passive nor homogeneous, we discussed the assumptions required to relate M and E to contextualise measurements made. We varied the osmotically active water content ε^* of cells by controlling the external osmotic stress whilst measuring M and E. We found both M and E depends on ε^* in a manner that can be explained by binary mixture theory and the ideal gas law. However, the correlation between M and E does not always exist when comparing different cellular components. Furthermore, we also assessed the potential of using Brillouin microscopy as an early diagnostic tool to detect the structural changes of ECM degradation in osteoarthritis (OA). To mimic OA, porcine cartilage was digested by enzyme and our results show that Brillouin microscopy can detect the structure change in OA and hence Brillouin microscopy could develop to a minimally-invasive arthroscope.Open Acces