title: Biomechanics of fibrous proteins of the extracellular matrix studied by Brillouin scattering.
abstract: Brillouin light scattering (BLS) spectroscopy is a technique that is able to detect thermally excited phonons within a material. The speed of propagation of these phonons can be determined from the magnitude of the Brillouin frequency shift between incident and scattered light, thereby providing a measure of the mechanical properties of the material in the gigahertz range. The mechanical properties of the extracellular matrices of biological tissues and their constituent biopolymers are important for normal tissue function and disturbances in these properties are widely implicated in disease. BLS offers the prospect of measuring mechanical properties on a microscopic scale in living tissues, thereby providing insights into structure-function relationships under normal and pathological conditions. In this study, we investigated BLS in collagen and elastin-the fibrous proteins of the extracellular matrix (ECM). Measurements were made on type I collagen in rat tail tendon, type II collagen in articular cartilage and nuchal ligament elastin. The dependence of the BLS spectrum on fibre orientation was investigated in a backscattering geometry using a reflective substrate. Two peaks, a bulk mode arising from phonon propagation along a quasi-radial direction to the fibre axis and a mode parallel to the surface, depending on sample orientation relative to the fibre axis, could be distinguished. The latter peak was fitted to a model of wave propagation through a hexagonally symmetric elastic solid, and the five components of the elasticity tensor were combined to give axial and transverse Young's, shear and bulk moduli of the fibres. These were 10.2, 8.3, 3.2 and 10.9 GPa, and 6.1, 5.3, 1.9 and 8 GPa for dehydrated type I collagen and elastin, respectively. The former values are close to those previously reported. A microfocused BLS approach was also applied providing selection of single fibres. The moduli of collagen and elastin are much higher than those measured at lower frequency using macroscopic strains, and the difference between them is much less. We therefore believe, like previous investigators, that molecular-scale viscoelastic effects are responsible for the frequency dependence of the fibre biomechanics. Combining BLS with larger-scale mechanical testing methods therefore should, in the future, provide a means of following the evolution of mechanical properties in the formation of the complex structures found in the ECM.
authors.affiliation: School of Physics, University of Exeter, Exeter EX4 4QL, UK f.palombo@exeter.ac.uk.
authors.forename: Francesca
authors.identifier: http://orcid.org/0000-0001-6355-2601
authors.initials: F
authors.lastname: Palombo
authors.affiliation: School of Physics, University of Exeter, Exeter EX4 4QL, UK.
authors.forename: C Peter
authors.initials: CP
authors.lastname: Winlove
authors.affiliation: School of Physics, University of Exeter, Exeter EX4 4QL, UK.
authors.forename: Ryan S
authors.initials: RS
authors.lastname: Edginton
authors.affiliation: School of Physics, University of Exeter, Exeter EX4 4QL, UK.
authors.forename: Ellen
authors.initials: E
authors.lastname: Green
authors.affiliation: School of Physics, University of Exeter, Exeter EX4 4QL, UK.
authors.forename: Nick
authors.initials: N
authors.lastname: Stone
authors.affiliation: Istituto Officina dei Materiali del CNR (CNR-IOM) - Unità di Perugia, c/o Dipartimento di Fisica e Geologia, Perugia I-06100, Italy.
authors.forename: Silvia
authors.initials: S
authors.lastname: Caponi
authors.affiliation: Dipartimento di Fisica e Geologia, Università di Perugia, Perugia I-06100, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Madami
authors.affiliation: Dipartimento di Fisica e Geologia, Università di Perugia, Perugia I-06100, Italy Centro di Eccellenza su Materiali Innovativi Nanostrutturati (CEMIN), Università di Perugia, Perugia I-06100, Italy.
authors.forename: Daniele
authors.initials: D
authors.lastname: Fioretto
mesh_terms: D000818:Animals; D024042:Collagen Type I / Q000737:chemistry*; D024043:Collagen Type II / Q000737:chemistry*; D004549:Elastin / Q000737:chemistry*; D006736:Horses; D008027:Light; D060739:Phonons; D051381:Rats; D017207:Rats, Sprague-Dawley; D012542:Scattering, Radiation
chemical_list: D024042:Collagen Type I; D024043:Collagen Type II; D004549:Elastin
keywords: Raman; acoustic microscopy; fibrils; hexagonal crystals; mechanical moduli; protein fibres