New insights into the mechanical properties of Acanthamoeba castellanii cysts as revealed by phonon microscopy

Parallel imaging with phonon microscopy using a multi-core fibre bundle detection

In this paper, we show a proof-of-concept method to parallelise phonon microscopy measurements for cell elasticity imaging by demonstrating a 3-fold increase in acquisition speed which is limited by current acquisition hardware. Phonon microscopy is based on time-resolved Brillouin scattering, which uses a pump-probe method with asynchronous optical sampling (ASOPS) to generate and detect coherent phonons. This enables access to the cell elasticity via the Brillouin frequency with sub-optical axial resolution. Although systems based on ASOPS are typically faster compared to the ones built with a mechanical delay line, they are still very slow to study real time changes at the cellular level. Additionally, the biocompatibility is reduced due to long light exposure and scanning time. Using a multi-core fibre bundle rather than a single channel for detection, we acquire 6 channels simultaneously allowing us to speed-up measurements, and open a way to scale-up this method.

Non-contact and label-free biomechanical imaging: Stimulated Brillouin microscopy and beyond

Brillouin microscopy based on spontaneous Brillouin scattering has emerged as a unique elastography technique because of its merit of non-contact, label-free, and high-resolution mechanical imaging of biological cell and tissue. Recently, several new optical modalities based on stimulated Brillouin scattering have been developed for biomechanical research. As the scattering efficiency of the stimulated process is much higher than its counterpart in the spontaneous process, stimulated Brillouin-based methods have the potential to significantly improve the speed and spectral resolution of existing Brillouin microscopy. Here, we review the ongoing technological advancements of three methods, including continuous wave stimulated Brillouin microscopy, impulsive stimulated Brillouin microscopy, and laser-induced picosecond ultrasonics. We describe the physical principle, the representative instrumentation, and biological application of each method. We further discuss the current limitations as well as the challenges for translating these methods into a visible biomedical instrument for biophysics and mechanobiology.

Sensitive impulsive stimulated Brillouin spectroscopy by an adaptive noise-suppression Matrix Pencil

Impulsive stimulated Brillouin spectroscopy (ISBS) plays a critical role in investigating mechanical properties thanks to its fast measurement rate. However, traditional Fourier transform-based data processing cannot decipher measured data sensitively because of its incompetence in dealing with low signal-to-noise ratio (SNR) signals caused by a short exposure time and weak signals in a multi-peak spectrum. Here, we propose an adaptive noise-suppression Matrix Pencil method for heterodyne ISBS as an alternative spectral analysis technique, speeding up the measurement regardless of the low SNR and enhancing the sensitivity of multi-component viscoelastic identification. The algorithm maintains accuracy of 0.005% for methanol sound speed even when the SNR drops 33 dB and the exposure time is reduced to 0.4 ms. Moreover, it proves to extract a weak component that accounts for 6% from a polymer mixture, which is inaccessible for the traditional method. With its outstanding ability to sensitively decipher weak signals without spectral a priori information and regardless of low SNRs or concentrations, this method offers a fresh perspective for ISBS on fast viscoelasticity measurements and multi-component identifications.

Polarization-Sensitive Super-Resolution Phononic Reconstruction of Nanostructures

In this paper, we show for the first time the polarization-sensitive super-resolution phononic reconstruction of multiple nanostructures in a liquid environment by overcoming the diffraction limit of the optical system (1 μm). By using time-resolved pump-probe spectroscopy, we measure the acoustic signature of nanospheres and nanorods at different polarizations. This enables the size, position, and orientation characterization of multiple nanoparticles in a single point spread function with the precision of 5 nm, 3 nm, and 1.4°, respectively. Unlike electron microscopy where a high vacuum environment is needed for imaging, this technique performs measurements in liquids at ambient pressure, ideal to study the insights of living specimens. This is a potential path toward super-resolution phononic imaging where the acoustic signatures of multiple nanostructures could act as an alternative to fluorescent labels. In this context, phonons also offer the opportunity to extract information about the mechanical properties of the surrounding medium as well as access to subsurface features.

Phonon imaging in 3D with a fibre probe

We show for the first time that a single ultrasonic imaging fibre is capable of simultaneously accessing 3D spatial information and mechanical properties from microscopic objects. The novel measurement system consists of two ultrafast lasers that excite and detect high-frequency ultrasound from a nano-transducer that was fabricated onto the tip of a single-mode optical fibre. A signal processing technique was also developed to extract nanometric in-depth spatial measurements from GHz frequency acoustic waves, while still allowing Brillouin spectroscopy in the frequency domain. Label-free and non-contact imaging performance was demonstrated on various polymer microstructures. This singular device is equipped with optical lateral resolution, 2.5 μm, and a depth-profiling precision of 45 nm provided by acoustics. The endoscopic potential for this device is exhibited by extrapolating the single fibre to tens of thousands of fibres in an imaging bundle. Such a device catalyses future phonon endomicroscopy technology that brings the prospect of label-free in vivo histology within reach.

3D phonon microscopy with sub-micron axial-resolution

Brillouin light scattering (BLS) is an emerging method for cell imaging and characterisation. It allows elasticity-related contrast, optical resolution and label-free operation. Phonon microscopy detects BLS from laser generated coherent phonon fields to offer an attractive route for imaging since, at GHz frequencies, the phonon wavelength is sub-optical. Using phonon fields to image single cells is challenging as the signal to noise ratio and acquisition time are often poor. However, recent advances in the instrumentation have enabled imaging of fixed and living cells. This work presents the first experimental characterisation of phonon-based axial resolution provided by the response to a sharp edge. The obtained axial resolution is up to 10 times higher than that of the optical system used to take the measurements. Validation of the results are obtained with various polymer objects, which are in good agreement with those obtained using atomic force microscopy. Edge localisation, and hence profilometry, of a phantom boundary is measured with accuracy and precision of approximately 60 nm and 100 nm respectively. Finally, 3D imaging of fixed cells in culture medium is demonstrated.

Correlative Imaging of Motoneuronal Cell Elasticity by Pump and Probe Spectroscopy

Because of their role of information transmitter between the spinal cord and the muscle fibers, motor neurons are subject to physical stimulation and mechanical property modifications. We report on motoneuron elasticity investigated by time-resolved pump and probe spectroscopy. A dual picosecond geometry simultaneously probing the acoustic impedance mismatch at the cell-titanium transducer interface and acoustic wave propagation inside the motoneuron is presented. Such noncontact and nondestructive microscopy, correlated to standard atomic force microscopy or a fluorescent labels approach, has been carried out on a single cell to address some physical properties such as bulk modulus of elasticity, dynamical longitudinal viscosity, and adhesion.

Apparent attenuation by opto-acoustic defocus in phonon microscopy

Understanding the mechanical properties of biological cells is a challenging problem for the life sciences partly because there are limited methods for mapping elasticity with high resolution. Phonon microscopy is a form of Brillouin light scattering which uses coherent phonons for imaging with elasticity-related contrast, phonon resolution and without labels. It can measure material properties such as sound velocity, acoustic impedance and attenuation. To use it as a contrast mechanism in microscopy, high numerical aperture (NA) lenses are key to high resolution. However, increasing NA induces apparent attenuation, a premature decay of the detected signal. To reduce signal decay and quantify the sound attenuation coefficient in cells, it is necessary to understand the mechanisms that affect signal decay. Here we define opto-acoustic defocus as a signal decay mechanism and propose methods to achieve quantitative sound attenuation measurements, and to optimise in-depth imaging at high resolution which is crucial for cell imaging.

Time resolved Brillouin fiber-spectrometer

This report introduces a novel time resolved Brillouin spectrometer, consisting of an opto-acoustic transducer which resides on the tip of a single-mode optical fiber of arbitrary length with 125 μm outer diameter and 5 μm sensing diameter. Demonstrated here are proof of concept spectroscopic measurements - shifts in Brillouin frequency - with sensitivities of 41±3MHz/%wt and 2.5±0.6 MHz/°C for changes in water-salinity and water-temperature, respectively, and an interpolated frequency resolution of 9±2 MHz. The technique benefits from low-cost raw materials, scalable fabrication, scalable pixel density, easy alignment, and data acquisition speeds down to 0.4 s: traits which make this compatible with in vivo applications.

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.