Consensus Statement on Brillouin Light Scattering Microscopy of Biological Materials.

Brillouin Light Scattering (BLS) spectroscopy is a non-invasive, non-contact, label-free optical technique that can provide information on the mechanical properties of a material on the sub-micron scale. Over the last decade it has seen increased applications in the life sciences, driven by the observed significance of mechanical properties in biological processes, the realization of more sensitive BLS spectrometers and its extension to an imaging modality. As with other spectroscopic techniques, BLS measurements not only detect signals characteristic of the investigated sample, but also of the experimental apparatus, and can be significantly affected by measurement conditions.

Predicting nanocarriers' efficacy in 3D models with Brillouin microscopy.

Thanks to their unique nanoscale properties, nanomedicines can overcome some of the shortcomings of conventional therapies. For better predictive screening, it is important to assess their performance in three-dimensional (3D) multicellular tumour spheroids (MCTS) that can recapitulate the physiological barriers found in real tumours. Today, the evaluation of drug delivery nanosystems in MCTS is mainly explored by means of microscopy techniques that are invasive and require fluorescent labels which modify the composition and fate of the carriers.

Probing molecular crowding in compressed tissues with Brillouin light scattering.

Volume regulation is key in maintaining important tissue functions, such as growth or healing. This is achieved by modulation of active contractility as well as water efflux that changes molecular crowding within individual cells. Local sensors have been developed to monitor stresses or forces in model tissues, but these approaches do not capture the contribution of liquid flows to volume regulation.

THESEUS1 modulates cell wall stiffness and abscisic acid production in .

Plant cells can be distinguished from animal cells by their cell walls and high-turgor pressure. Although changes in turgor and the stiffness of cell walls seem coordinated, we know little about the mechanism responsible for coordination. Evidence has accumulated that plants, like yeast, have a dedicated cell wall integrity maintenance mechanism.

Evaluation of commercial virtually imaged phase array and Fabry-Pérot based Brillouin spectrometers for applications to biology.

Measuring the complex mechanical properties of biological objects has become a necessity to answer key questions in mechanobiology and to propose innovative clinical and therapeutic strategies. In this context, Brillouin light scattering (BLS) has recently come into vogue, offering quantitative imaging of the mechanical properties without labels and with a micrometer resolution. In biological samples, the magnitude of the spectral changes are typically of a few tens of MHz, and the ability of modern spectrometers to monitor such subtle changes needs to be evaluated.

Micromechanical imaging of dentin with Brillouin microscopy.

The structure of teeth can be altered by diet, age or diseases such as caries and sclerosis. It is very important to characterize their mechanical properties to predict and understand tooth decay, design restorative dental procedures, and investigate their tribological behavior. However, existing imaging techniques are not well suited to investigating the micromechanics of teeth, in particular at tissue interfaces.

Brillouin Light Scattering Microspectroscopy for Biomedical Research and Applications: introduction to feature issue.

There has been a marked revival of interest in brillouin light scattering spectroscopy/microscopy over the last decade in regards to applications related to all optically studying the mechanical problems associated with systems of biological and medical interest. This revival has been driven by advancements in spectrometer design, together with mounting evidence of the critical role that mechanical properties can play in biological processes as well as the onset of diverse diseases. This feature issue contains a series of papers spanning some of the latest developments in the field of Brillouin light scattering spectroscopy and microscopy as applied to systems of biomedical interest.

High-Frequency Mechanical Properties of Tumors Measured by Brillouin Light Scattering.

The structure of tumors can be recapitulated as an elastic frame formed by the connected cytoskeletons of the cells invaded by interstitial and intracellular fluids. The low-frequency mechanics of this poroelastic system, dictated by the elastic skeleton only, control tumor growth, penetration of therapeutic agents, and invasiveness. The high-frequency mechanical properties containing the additional contribution of the internal fluids have also been posited to participate in tumor progression and drug resistance, but they remain largely unexplored.

Drug delivery to tumours using a novel 5-FU derivative encapsulated into lipid nanocapsules.

In this work, a novel lipophilic 5-fluorouracil (5-FU) derivative was synthesised and encapsulated into lipid nanocapsules (LNC). 5-FU was modified with lauric acid to give a lipophilic mono-lauroyl-derivative (5-FU-C12, MW of about 342 g/mol, yield of reaction 70%). 5-FU-C12 obtained was efficiently encapsulated into LNC (encapsulation efficiency above 90%) without altering the physico-chemical characteristics of LNC.

Opto-acoustic microscopy reveals adhesion mechanics of single cells.

Laser-generated GHz-ultrasonic-based technologies have shown the ability to image single cell adhesion and stiffness simultaneously. Using this new modality, we here demonstrate quantitative indicators to investigate contact mechanics and adhesion processes of the cell. We cultured human cells on a rigid substrate, and we used an inverted pulsed opto-acoustic microscope to generate acoustic pulses containing frequencies up to 100 GHz in the substrate.

In-depth phenotypic characterization of multicellular tumor spheroids: Effects of 5-Fluorouracil.

MultiCellular Tumor Spheroids (MCTS), which mimic the 3-Dimensional (3D) organization of a tumor, are considered as better models than conventional cultures in 2-Dimensions (2D) to study cancer cell biology and to evaluate the response to chemotherapeutic drugs. A real time and quantitative follow-up of MCTS with simple and robust readouts to evaluate drug efficacy is still missing. Here, we evaluate the chemotherapeutic drug 5-Fluorouracil (5-FU) response on the growth and integrity of MCTS two days after treatment of MCTS and for three colorectal carcinoma cell lines with different cohesive properties (HT29, HCT116 and SW480).

Optical tracking of picosecond coherent phonon pulse focusing inside a sub-micron object.

By means of an ultrafast optical technique, we track focused gigahertz coherent phonon pulses in objects down to sub-micron in size. Infrared light pulses illuminating the surface of a single metal-coated silica fibre generate longitudinal-phonon wave packets. Reflection of visible probe light pulses from the fibre surface allows the vibrational modes of the fibre to be detected, and Brillouin optical scattering of partially transmitted light pulses allows the acoustic wavefronts inside the transparent fibre to be continuously monitored.

Effect of refracted light distribution on the photoelastic generation of zero-group velocity Lamb modes in optically low-absorbing plates.

Zero-group velocity (ZGV) Lamb modes are associated with sharp local acoustic resonances and allow, among other features, local measurement of Poisson's ratio. While the thermoelastic generation of Lamb waves in metal plates has been widely studied, the case of materials of low-optical absorption remains unexplored. In materials such as glasses, the generation of bulk elastic waves has been demonstrated to be sensitive to the refracted light distribution.

Universality of the network-dynamics of the cell nucleus at high frequencies.

The interior of the cell nucleus is comparable to a solid network bathed in an interstitial fluid. From the extrapolation of low frequency data, it is expected that such network should dictate the response of the nucleus to mechanical stress at high frequencies, described by unique elastic moduli. However, none of the existing techniques that can probe the mechanical properties of cells can exceed the kHz range, and the mechanics of the nuclear network remain poorly understood.

Probing single-cell mechanics with picosecond ultrasonics.

The mechanical properties of cells play a key role in several fundamental biological processes, such as migration, proliferation, differentiation and tissue morphogenesis. The complexity of the inner cell composition and the intricate meshwork formed by transmembrane cell-substrate interactions demands a non-invasive technique to probe cell mechanics and cell adhesion at a subcell scale. In this paper we review the use of laser-generated GHz acoustic waves--a technique called picosecond ultrasonics (PU)--to probe the mechanical properties of single cells.

Nanoscale mechanical contacts mapped by ultrashort time-scale electron transport.

Mechanical contacts are crucial to systems in engineering, electronics and biology. The microscopic nature of the contacting surfaces determines how they mesh on the nanoscale. There is thus much interest in methods that can map the actual area of two surfaces in contact--the real contact area--during the loading or unloading phases.

Transverse mechanical properties of cell walls of single living plant cells probed by laser-generated acoustic waves.

Probing the mechanical properties of plant cell wall is crucial to understand tissue dynamics. However, the exact symmetry of the mechanical properties of this anisotropic fiber-reinforced composite remains uncertain. For this reason, biologically relevant measurements of the stiffness coefficients on individual living cells are a challenge.

Acoustic beam steering by light refraction: illustration with directivity patterns of a tilted volume photoacoustic source.

The symmetry of a thermoelastic source resulting from laser absorption can be broken when the direction of light propagation in an elastic half-space is inclined relatively to the surface. This leads to an asymmetry of the directivity patterns of both compressional and shear acoustic waves. In contrast to classical surface acoustic sources, the tunable volume source allows one to take advantage of the mode conversion at the surface to control the directivity of specific modes.

Remote opto-acoustic probing of single-cell adhesion on metallic surfaces.

The reflection of picosecond ultrasonic pulses from a cell-substrate interface is used to probe cell-biomaterial adhesion with a subcell resolution. We culture monocytes on top of a thin biocompatible Ti metal film, supported by a transparent sapphire substrate. Low-energy femtosecond pump laser pulses are focused at the bottom of the Ti film to a micron spot.

Effect of laser beam incidence angle on the thermoelastic generation in semi-transparent materials.

When a laser beam is absorbed in a semi-transparent material, a volume acoustic source is created owing to penetration of the laser beam inside the material and to thermoelastic transduction. Many experimental and theoretical studies have been conducted to better understand this ultrasound generation process with normal laser light incidence on the material surface. The purpose of this paper is to analyze the effects of the asymmetry caused by oblique incidence of a laser line source on the generation of acoustic waves in semi-transparent isotropic materials.

Beam distortion detection and deflectometry measurements of gigahertz surface acoustic waves.

Gigahertz acoustic waves propagating on the surface of a metal halfspace are detected using different all-optical detection schemes, namely, deflectometry and beam distortion detection techniques. Both techniques are implemented by slightly modifying a conventional reflectometric setup. They are then based on the measurement of the reflectivity change but unlike reflectometric measurements, they give access to the sample surface displacement.

Picosecond time scale imaging of mechanical contacts.

By means of an ultrafast opto-acoustic technique we study the nanoindentation of thin chromium films on sapphire substrates using a ceramic ball bearing. Acoustic pulses at approximately 40 GHz returning from the film-indenter interface allow the film indentation profiles to be probed to sub-nanometer resolution over contact areas approximately 25 microm in radius. The deformation of the films during loading is thereby revealed.