Opto-acoustic microscopy reveals adhesion mechanics of single cells

Principles and advances in ultrafast photoacoustics; applications to imaging cell mechanics and to probing cell nanostructure

In this article we first present the foundations of ultrafast photoacoustics, a technique where the acoustic wavelength in play can be considerably shorter than the optical wavelength. The physics primarily involved in the conversion of short light pulses into high frequency sound is described. The mechanical disturbances following the relaxation of hot electrons in metals and other processes leading to the breaking of the mechanical balance are presented, and the generation of bulk shear-waves, of surface and interface waves and of guided waves is discussed. Then, efforts to overcome the limitations imposed by optical diffraction are described. Next, the principles behind the detection of the so generated coherent acoustic phonons with short light pulses are introduced for both opaque and transparent materials. The striking instrumental advances, in the detection of acoustic displacements, ultrafast acquisition, frequency and space resolution are discussed. Then secondly, we introduce picosecond opto-acoustics as a remote and label-free novel modality with an excellent capacity for quantitative evaluation and imaging of the cell's mechanical properties, currently with micron in-plane and sub-optical in depth resolution. We present the methods for time domain Brillouin spectroscopy in cells and for cell ultrasonography. The current applications of this unconventional means of addressing biological questions are presented. This microscopy of the nanoscale intra-cell mechanics, based on the optical monitoring of coherent phonons, is currently emerging as a breakthrough method offering new insights into the supra-molecular structural changes that accompany cell response to a myriad of biological events.

Towards shaping picosecond strain pulses via magnetostrictive transducers

Using time-resolved x-ray diffraction, we demonstrate the manipulation of the picosecond strain response of a metallic heterostructure consisting of a dysprosium (Dy) transducer and a niobium (Nb) detection layer by an external magnetic field. We utilize the first-order ferromagnetic-antiferromagnetic phase transition of the Dy layer, which provides an additional large contractive stress upon laser excitation compared to its zero-field response. This enhances the laser-induced contraction of the transducer and changes the shape of the picosecond strain pulses driven in Dy and detected within the buried Nb layer. Based on our experiment with rare-earth metals we discuss required properties for functional transducers, which may allow for novel field-control of the emitted picosecond strain pulses.

Acoustic shock waves emitted from two interacting laser generated plasmas in air

We present an acoustic detection technique to study the interaction of two shock waves emitted by two nearby, simultaneous, laser-induced air-breakdown events that resembles the phenomenon of interaction of fluids. A microphone is employed to detect the acoustic shock wave (ASW) from the interaction zone. The experiments were done at various separation distances between the two plasma sources. The incident laser energy of the sources is varied from 25 to 100 mJ in ratios from 1:1 to 1:4. The peak sound pressure of the ASW was compared between the single and dual plasma sources, showing that the pressures are higher for the dual plasma source than that of the single plasma. The evolution of peak sound pressures is observed to depend on (a) the pulse energy of the sources and (b) the plasma separation distance, d. For the equal energy sources, the peak sound pressures increased linearly up to a certain plasma separation distance d, beyond which the pressures saturated and decayed. For the case of unequal energy sources, the peak sound pressures showed an interesting response of increase, saturation, decay, and further increase with plasma separation distance d. These observations indicate the dynamics of acoustic wave interactions across the interaction zone of the two sources depend on the input laser pulse energy as well as the plasma separation distance d.

A Review of Single-Cell Adhesion Force Kinetics and Applications

Cells exert, sense, and respond to the different physical forces through diverse mechanisms and translating them into biochemical signals. The adhesion of cells is crucial in various developmental functions, such as to maintain tissue morphogenesis and homeostasis and activate critical signaling pathways regulating survival, migration, gene expression, and differentiation. More importantly, any mutations of adhesion receptors can lead to developmental disorders and diseases. Thus, it is essential to understand the regulation of cell adhesion during development and its contribution to various conditions with the help of quantitative methods. The techniques involved in offering different functionalities such as surface imaging to detect forces present at the cell-matrix and deliver quantitative parameters will help characterize the changes for various diseases. Here, we have briefly reviewed single-cell mechanical properties for mechanotransduction studies using standard and recently developed techniques. This is used to functionalize from the measurement of cellular deformability to the quantification of the interaction forces generated by a cell and exerted on its surroundings at single-cell with attachment and detachment events. The adhesive force measurement for single-cell microorganisms and single-molecules is emphasized as well. This focused review should be useful in laying out experiments which would bring the method to a broader range of research in the future.

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.

Adhesion of biofilms on titanium measured by laser-induced spallation

Eradication of established implant-associated and bacterial biofilm-forming infections remains difficult in part because these biofilms remain well-adhered to the implant surface. Few experimental techniques are available to measure macro-scale strength of bacterial biofilm-implant adhesion. We have adapted the laser spallation technique to compare the macro-scale adhesion strength of biofilms formed on titanium. By using a rapid pressure wave (35 ns) to load the interface, we prevent disturbance of the biofilm surface prior to measurement, and preclude the time necessary for the biofilm to respond to and adapt under loading. Biofilms of Streptococcus mutans, a Gram-positive bacterium associated with human dental caries (cavities) were cultured directly on commercially pure titanium within our custom substrate assembly. Growth conditions were varied by adding sucrose to the Todd Hewitt Yeast (THY) broth: THY control, 37.5 mM, 75 mM, 375 mM, and 750 mM sucrose. Multiple locations on each biofilm were loaded using the laser spallation technique. Loading pressure wave amplitude was controlled by adjusting laser fluence, energy per area. Initially, addition of sucrose to the media increased biofilm adhesion to titanium. However, once a saturation concentration of 75 mM sucrose was reached, increasing the sucrose concentration further resulted in a decrease in biofilm adhesion. This study is the first demonstration of the adaptation of the laser spallation technique to measure bacterial biofilm adhesion. Establishment of this macro-scale biofilm adhesion measurement technique opens the door for many biofilm-surface adhesion studies. We anticipate further work in this area towards understanding the complex relationships among bacteria species, environmental factors, surface characteristics, and biofilm adhesion strength.

Label-free multi-parametric imaging of single cells: dual picosecond optoacoustic microscopy

Advances in microscopy with new visualization possibilities often bring dramatic progress to our understanding of the intriguing cellular machinery. Picosecond optoacoustic micro-spectroscopy is an optical technique based on ultrafast pump-probe generation and detection of hypersound on time durations of picoseconds and length scales of nanometers. It is experiencing a renaissance as a versatile imaging tool for cell biology research after a plethora of applications in solid-state physics. In this emerging context, this work reports on a dual-probe architecture to carry out real-time parallel detection of the hypersound propagation inside a cell that is cultured on a metallic substrate, and of the hypersound reflection at the metal/cell adhesion interface. Using this optoacoustic modality, several biophysical properties of the cell can be measured in a noncontact and label-free manner. Its abilities are demonstrated with the multiple imaging of a mitotic macrophage-like cell in a single run experiment.

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

The single cell eukaryotic protozoan Acanthamoeba castellanii exhibits a remarkable ability to switch from a vegetative trophozoite stage to a cystic form, in response to stressors. This phenotypic switch involves changes in gene expression and synthesis of the cell wall, which affects the ability of the organism to resist biocides and chemotherapeutic medicines. Given that encystation is a fundamental survival mechanism in the life cycle of A. castellanii, understanding of this process should have significant environmental and medical implications. In the present study, we investigated the mechanism of A. castellanii encystation using a novel phonon microscopy technique at the single cell level. Phonon microscopy is an emerging technique to image cells using laser-generated sub-optical wavelength phonons. This imaging modality can image with contrast underpinned by mechanical properties of cells at an optical or higher resolution. Our results show that the Brillouin frequency, a shift of the colour of light induced by phonons, evolves in three well defined frequency bands instead of a simple shift in frequency. These observations confirm previous results from literature and provide new insights into the capacity of A. castellanii cyst to react quickly in harsh environments.

Quantifying cellular forces and biomechanical properties by correlative micropillar traction force and Brillouin microscopy

Cells sense and respond to external physical forces and substrate rigidity by regulating their cell shape, internal cytoskeletal tension, and stiffness. Here we show that the combination of micropillar traction force and noncontact Brillouin microscopy provides access to cell-generated forces and intracellular mechanical properties at optical resolution. Actin-rich cytoplasmic domains of 3T3 fibroblasts showed significantly higher Brillouin shifts, indicating a potential increase in stiffness when adhering on fibronectin-coated glass compared to soft PDMS micropillars. Our findings demonstrate the complementarity of micropillar traction force and Brillouin microscopy to better understand the relation between cell force generation and the intracellular mechanical properties.

Where No Hand Has Gone Before: Probing Mechanobiology at the Cellular Level

Physical forces and other mechanical stimuli are fundamental regulators of cell behavior and function. Cells are also biomechanically competent: they generate forces to migrate, contract, remodel, and sense their environment. As the knowledge of the mechanisms of mechanobiology increases, the need to resolve and probe increasingly small scales calls for novel technologies to mechanically manipulate cells, examine forces exerted by cells, and characterize cellular biomechanics. Here, we review novel methods to quantify cellular force generation, measure cell mechanical properties, and exert localized piconewton and nanonewton forces on cells, receptors, and proteins. The combination of these technologies will provide further insight on the effect of mechanical stimuli on cells and the mechanisms that convert these stimuli into biochemical and biomechanical activity.