A New Platform for Biomechanical Imaging Based on Brillouin Scattering

Mechanical changes in body tissues are widely associated with diverse clinical problems including many of the serious complications of diabetes, atherosclerosis and cancer, but their microstructural bases are poorly understood. In large blood vessels, stiffness is linked to increased risk of cardiovascular mortality - although the contribution of the stiffness from individual tissue constituents to this risk is unknown. Understanding the mechanisms linking stiffness to structure is limited by our ability to measure mechanical properties on length-scales which allow the relationships with tissue structure and composition to be established. BLS microscopy has the potential to fill this void, as demonstrated in a recent publication (Palombo, Madami, Stone and Fioretto, Analyst, 2014, 139, 729-733) where mechanical maps of structure and elasticity were generated based on Brillouin peaks corresponding to specific tissue components. New techniques of imaging the chemical composition of cells and tissues are making a major impact in many fields of biology and medicine. This proposal is concerned with the development of Brillouin light scattering (BLS) imaging, a new method for microscopic imaging of mechanical properties. The mechanical properties of the connective tissues, cartilage or blood vessels are central to their physiological function and changes in mechanical function are implicated in diseases ranging from diabetes to osteoarthritis. BLS imaging therefore offers the prospect of directly imaging a functionally important property, both in tissues and single constituents e.g. collagen and elastin fibres, whose biomechanics probed at multiple frequencies are almost completely unexplored, which will open a new window on tissue mechanics and offers the prospect of developing new diagnostic techniques.

The principle of the approach is to detect the change in frequency of inelastically scattered light arising from the generation of sound waves in the sample. Calculation of the speed of the wave provides a measure of the mechanical properties of the material. Preliminary studies have used confocal BLS microscopy to produce Brillouin frequency maps of an epithelial tissue, which can be combined with Raman micro-spectroscopic maps of chemical composition (Palombo et al. Analyst 2014). Complementary studies have investigated Brillouin scattering in collagen and elastin, the fibrous proteins of the extracellular matrix, and their relationships to fibre micromechanics (Palombo et al. J R Soc Interface, asap).

This work provides the basis for the proposed investigations. The methodology will be applicable to many problems of connective tissue physiology and pathology, but the immediate aim will be to investigate the mechanics of extracellular protein fibres - collagens, elastin, proteoglycans - and the effects of physiological and pathological conditions. These changes are widely associated with diverse clinical problems including many of the serious complications of osteoarthritis and diabetes, but their microstructural bases are poorly understood. The project will require construction of a system that will enable biomechanical imaging through Brillouin scattering with a Virtually Imaged Phase Array (VIPA) spectrometer. This will enable us to develop a significantly faster and more versatile technology for the analysis of fibres (and tissues) and to define functionally significant changes which can, in the longer term, become targets for monitoring or therapeutic intervention.

This proposal will develop novel techniques for mechanical testing and noninvasive mechanical imaging. Our immediate application is to fundamental problems in tissue biomechanics, which will have direct impact on our understanding of ageing and a wide range of diseases. However, in the longer term, the methodology will also be applicable in many areas of materials research and can form the basis of entirely new approaches to clinical diagnosis, which will generate impacts in many areas.
The team will develop an advanced Brillouin imaging microscopy to measure, identify and localise elasticity and composition in biomaterials. We propose studying important, fundamental open-problems related to tissue structure, mechanics and biophysics, for the academic and clinical communities. The programme is focussed at investigating protein fibres of the extracellular matrix and how physiological and pathological changes affect the mechanics at a molecular level. Ultimately, this new analytical tool may replace current techniques based on elastography and ultrasounds and provide a powerful clinical diagnosis of pathology. A pioneering aspect of the project is the application of the new imaging platform to the essential building blocks of biological tissues: the protein fibres of extracellular matrices. Brillouin imaging or mechanical microscopy offers a unique combination of advantages since it is contact-free, enables fast 3D scanning, has mechanical specificity and high spatial resolution, and relies on intrinsic properties of materials. This gives the new method an unprecedented potential for in vivo applications. The outputs of this research will be shared with clinical collaborators leading to improvements in the setup design and operation to realise a clinically viable multimodal device for possible commercial exploitation. Beyond this, efforts will be made to generate wider impact on the society and economy by advancing knowledge and improving healthcare and by exploring other technological applications of the methods. The specific pathways through which this multiple impact will be achieved are described below.

Societal impact: Impact on the society includes advancing knowledge and improving healthcare. Changes in the mechanical properties of the extracellular matrix are implicated in major diseases such as atherosclerosis, arthritis, disc degeneration and many forms of cancer which are enormous burdens to patients; the public health impact shows that atherosclerosis accounts for one in three of all deaths in the developed world, whilst more than one in three people in the UK develops cancer during their life. A better understanding of these mechanical changes and the development of methods of monitoring them will enhance understanding of disease processes and lead to new therapeutic approaches.

Economic impact: Advancing knowledge and improving healthcare through improved understanding of disease and novel imaging tools has a substantial impact on economy as well as people. Noninvasive clinical tests able to give reliable information and to replace current gold-standard techniques will make surgical interventions obsolete and unnecessary, hence greatly reducing the costs.
There is also a ubiquitous need for mechanical analysis in diverse systems e.g. polymers, synthetic metamaterials, auxetic materials and synthetic biomaterials to which our methodology can be applied. There is active and growing research in these fields in Engineering at Exeter and once our instrument is operational we shall engage with these groups to explore such applications.