Non-contact elastography methods in mechanobiology: a point of view

Abstract

In recent decades, mechanobiology has emerged as a novel perspective in the context of basic biomedical research. It is now widely recognized that living cells respond not only to chemical stimuli (for example drugs), but they are also able to decipher mechanical cues, such as the rigidity of the underlying matrix or the presence of shear forces. Probing the viscoelastic properties of cells and their local microenvironment with sub-micrometer resolution is required to study this complex interplay and dig deeper into the mechanobiology of single cells. Current approaches to measure mechanical properties of adherent cells mainly rely on the exploitation of miniaturized indenters, to poke single cells while measuring the corresponding deformation. This method provides a neat implementation of the everyday approach to measure mechanical properties of a material, but it typically results in a very low throughput and invasive experimental protocol, poorly translatable towards three-dimensional living tissues and biological constructs. To overcome the main limitations of nanoindentation experiments, a radical paradigm change is foreseen, adopting next generation contact-less methods to measure mechanical properties of biological samples with sub-cell resolution. Here we briefly introduce the field of single cell mechanical characterization, and we concentrate on a promising high resolution optical elastography technique, Brillouin spectroscopy. This non-contact technique is rapidly emerging as a potential breakthrough innovation in biomechanics, but the application to single cells is still in its infancy.

Keywords: Bioimaging; Brillouin spectroscopy; Elastography; Mechanobiology.

Fig. 1

A general breakdown of the techniques used to probe the mechanical properties of cells and tissues, discriminated on the basis of the need to come into contact with the sample (such as AFM) and the need to produce a mechanical stimulus. Spontaneous BS is unique in that it is based on sensing thermally activated acoustic modes

Fig. 2

Top: Correlative Second Harmonic Generation (SHG)-Brillouin axial optical sectioning analysis on corneal stroma. Comparison between a SHG images, b relative variations of Brillouin frequency, acquired at the specified depths below Bowman’s membrane. The reference frequency is ~ 9 GHz. Scale bars 10 μm (adapted from Mercatelli et al. 2019). Bottom: Brillouin imaging of bones. c Average mechanical properties, collected in 2D maps, with 3 μm steps, over the 4 main regions of the diaphysal ring of a human femur. d Distribution of point averaged longitudinal elastic moduli (adapted from Cardinali et al. 2020)