Brillouin Spectroscopy: From Biomedical Research to New Generation Pathology Diagnosis

Birefringence-induced phase delay enables Brillouin mechanical imaging in turbid media

Acoustic vibrations of matter convey fundamental viscoelastic information that can be optically retrieved by hyperfine spectral analysis of the inelastic Brillouin scattered light. Increasing evidence of the central role of the viscoelastic properties in biological processes has stimulated the rise of non-contact Brillouin microscopy, yet this method faces challenges in turbid samples due to overwhelming elastic background light. Here, we introduce a common-path Birefringence-Induced Phase Delay (BIPD) filter to disentangle the polarization states of the Brillouin and Rayleigh signals, enabling the rejection of the background light using a polarizer. We demonstrate a 65 dB extinction ratio in a single optical pass collecting Brillouin spectra in extremely scattering environments and across highly reflective interfaces. We further employ the BIPD filter to image bone tissues from a mouse model of osteopetrosis, highlighting altered biomechanical properties compared to the healthy control. Results herald new opportunities in mechanobiology where turbid biological samples remain poorly characterized.

Silk fibroin increases the elasticity of alginate-gelatin hydrogels and regulates cardiac cell contractile function in cardiac bioinks

Silk fibroin (SF) is a natural protein extracted fromBombyx morisilkworm thread. From its common use in the textile industry, it emerged as a biomaterial with promising biochemical and mechanical properties for applications in the field of tissue engineering and regenerative medicine. In this study, we evaluate for the first time the effects of SF on cardiac bioink formulations containing cardiac spheroids (CSs). First, we evaluate if the SF addition plays a role in the structural and elastic properties of hydrogels containing alginate (Alg) and gelatin (Gel). Then, we test the printability and durability of bioprinted SF-containing hydrogels. Finally, we evaluate whether the addition of SF controls cell viability and function of CSs in Alg-Gel hydrogels. Our findings show that the addition of 1% (w/v) SF to Alg-Gel hydrogels makes them more elastic without affecting cell viability. However, fractional shortening (FS%) of CSs in SF-Alg-Gel hydrogels increases without affecting their contraction frequency, suggesting an improvement in contractile function in the 3D cultures. Altogether, our findings support a promising pathway to bioengineer bioinks containing SF for cardiac applications, with the ability to control mechanical and cellular features in cardiac bioinks.

Mechano-Chemistry across Phase Transitions in Heated Albumin Protein Solutions

The presence of certain proteins in biofluids such as synovial fluid, blood plasma, and saliva gives these fluids non-Newtonian viscoelastic properties. The amount of these protein macromolecules in biofluids is an important biomarker for the diagnosis of various health conditions, including Alzheimer's disease, cardiovascular disorders, and joint quality. However, existing technologies for measuring the behavior of macromolecules in biofluids have limitations, such as long turnaround times, complex protocols, and insufficient sensitivity. To address these issues, we propose non-contact, optical Brillouin and Raman spectroscopy to assess the viscoelasticity and chemistry of non-Newtonian solutions, respectively, at different temperatures in several minutes. In this work, bovine and human serum albumin solution-based biopolymers were studied to obtain both their collective dynamics and molecular chemical evolution across heat-driven phase transitions at various protein concentrations. The observed phase transitions at elevated temperatures could be fully delayed in heated biopolymers by appropriately raising the level of protein concentration. The non-contact optical monitoring of viscoelastic and chemical property evolution could represent novel potential mechano-chemical biomarkers for disease diagnosis and subsequent treatment applications, including hyperthermia.

Spectral resolution enhancement for impulsive stimulated Brillouin spectroscopy by expanding pump beam geometry

Brillouin microscopy has recently emerged as a powerful tool for mechanical property measurements in biomedical sensing and imaging applications. Impulsive stimulated Brillouin scattering (ISBS) microscopy has been proposed for faster and more accurate measurements, which do not rely on stable narrow-band lasers and thermally-drifting etalon-based spectrometers. However, the spectral resolution of ISBS-based signal has not been significantly explored. In this report, the ISBS spectral profile has been investigated as a function of the pump beam's spatial geometry, and novel methodologies have been developed for accurate spectral assessment. The ISBS linewidth was found to consistently decrease with increasing pump-beam diameter. These findings provide the means for improved spectral resolution measurements and pave the way to broader applications of ISBS microscopy.

Brillouin and Raman Micro-Spectroscopy: A Tool for Micro-Mechanical and Structural Characterization of Cortical and Trabecular Bone Tissues

Human bone is a specialized tissue with unique material properties, providing mechanical support and resistance to the skeleton and simultaneously assuring capability of adaptation and remodelling. Knowing the properties of such a structure down to the micro-scale is of utmost importance, not only for the design of effective biomimetic materials but also to be able to detect pathological alterations in material properties, such as micro-fractures or abnormal tissue remodelling. The Brillouin and Raman micro-spectroscopic (BRmS) approach has the potential to become a first-choice technique, as it is capable of simultaneously investigating samples’ mechanical and structural properties in a non-destructive and label-free way. Here, we perform a mapping of cortical and trabecular bone sections of a femoral epiphysis, demonstrating the capability of the technique for discovering the morpho-mechanics of cells, the extracellular matrix, and marrow constituents. Moreover, the interpretation of Brillouin and Raman spectra merged with an approach of data mining is used to compare the mechanical alterations in specimens excised from distinct anatomical areas and subjected to different sample processing. The results disclose in both cases specific alterations in the morphology and/or in the tissue chemical make-up, which strongly affects bone mechanical properties, providing a method potentially extendable to other important biomedical issues.

Mapping Tumor Spheroid Mechanics in Dependence of 3D Microenvironment Stiffness and Degradability by Brillouin Microscopy

Altered biophysical properties of cancer cells and of their microenvironment contribute to cancer progression. While the relationship between microenvironmental stiffness and cancer cell mechanical properties and responses has been previously studied using two-dimensional (2D) systems, much less is known about it in a physiologically more relevant 3D context and in particular for multicellular systems. To investigate the influence of microenvironment stiffness on tumor spheroid mechanics, we first generated MCF-7 tumor spheroids within matrix metalloproteinase (MMP)-degradable 3D polyethylene glycol (PEG)-heparin hydrogels, where spheroids showed reduced growth in stiffer hydrogels. We then quantitatively mapped the mechanical properties of tumor spheroids in situ using Brillouin microscopy. Maps acquired for tumor spheroids grown within stiff hydrogels showed elevated Brillouin frequency shifts (hence increased longitudinal elastic moduli) with increasing hydrogel stiffness. Maps furthermore revealed spatial variations of the mechanical properties across the spheroids' cross-sections. When hydrogel degradability was blocked, comparable Brillouin frequency shifts of the MCF-7 spheroids were found in both compliant and stiff hydrogels, along with similar levels of growth-induced compressive stress. Under low compressive stress, single cells or free multicellular aggregates showed consistently lower Brillouin frequency shifts compared to spheroids growing within hydrogels. Thus, the spheroids' mechanical properties were modulated by matrix stiffness and degradability as well as multicellularity, and also to the associated level of compressive stress felt by tumor spheroids. Spheroids generated from a panel of invasive breast, prostate and pancreatic cancer cell lines within degradable stiff hydrogels, showed higher Brillouin frequency shifts and less cell invasion compared to those in compliant hydrogels. Taken together, our findings contribute to a better understanding of the interplay between cancer cells and microenvironment mechanics and degradability, which is relevant to better understand cancer progression.