Viscoelastic properties of white and gray matter-derived microglia differentiate upon treatment with lipopolysaccharide but not upon treatment with myelin.

BACKGROUND: The biomechanical properties of the brain have increasingly been shown to relate to brain pathology in neurological diseases, including multiple sclerosis (MS). Inflammation and demyelination in MS induce significant changes in brain stiffness which can be linked to the relative abundance of glial cells in lesions. We hypothesize that the biomechanical, in addition to biochemical, properties of white (WM) and gray matter (GM)-derived microglia may contribute to the differential microglial phenotypes as seen in MS WM and GM lesions. METHODS: Primary glial cultures from WM or GM of rat adult brains were treated with either lipopolysaccharide (LPS), myelin, or myelin+LPS for 24 h or left untreated as a control. After treatment, microglial cells were indented using dynamic indentation to determine the storage and loss moduli reflecting cell elasticity and cell viscosity, respectively, and subsequently fixed for immunocytochemical analysis. In parallel, gene expression of inflammatory-related genes were measured using semi-quantitative RT-PCR. Finally, phagocytosis of myelin was determined as well as F-actin visualized to study the cytoskeletal changes. RESULTS: WM-derived microglia were significantly more elastic and more viscous than microglia derived from GM. This heterogeneity in microglia biomechanical properties was also apparent when treated with LPS when WM-derived microglia decreased cell elasticity and viscosity, and GM-derived microglia increased elasticity and viscosity. The increase in elasticity and viscosity observed in GM-derived microglia was accompanied by an increase in Tnfα mRNA and reorganization of F-actin which was absent in WM-derived microglia. In contrast, when treated with myelin, both WM- and GM-derived microglia phagocytose myelin decrease their elasticity and viscosity. CONCLUSIONS: In demyelinating conditions, when myelin debris is phagocytized, as in MS lesions, it is likely that the observed differences in WM- versus GM-derived microglia biomechanics are mainly due to a difference in response to inflammation, rather than to the event of demyelination itself. Thus, the differential biomechanical properties of WM and GM microglia may add to their differential biochemical properties which depend on inflammation present in WM and GM lesions of MS patients.

In vivo characterization of chick embryo mesoderm by optical coherence tomography-assisted microindentation.

Embryos are growing organisms with highly heterogeneous properties in space and time. Understanding the mechanical properties is a crucial prerequisite for the investigation of morphogenesis. During the last 10 years, new techniques have been developed to evaluate the mechanical properties of biological tissues in vivo. To address this need, we employed a new instrument that, via the combination of micro-indentation with Optical Coherence Tomography (OCT), allows us to determine both, the spatial distribution of mechanical properties of chick embryos, and the structural changes in real-time. We report here the stiffness measurements on the live chicken embryo, from the mesenchymal tailbud to the epithelialized somites. The storage modulus of the mesoderm increases from (176 ± 18) Pa in the tail to (716 ± 117) Pa in the somitic region (mean ± SEM, n = 12). The midline has a mean storage modulus of (947 ± 111) Pa in the caudal (PSM) presomitic mesoderm (mean ± SEM, n = 12), indicating a stiff rod along the body axis, which thereby mechanically supports the surrounding tissue. The difference in stiffness between midline and presomitic mesoderm decreases as the mesoderm forms somites. This study provides an efficient method for the biomechanical characterization of soft biological tissues in vivo and shows that the mechanical properties strongly relate to different morphological features of the investigated regions.

Indentation probe with optical fibre array-based optical coherence tomography for material deformation.

We present a new optomechanical probe for mechanical testing of soft matter. The probe consists of a micromachined cantilever equipped with an indenting sphere, and an array of 16 single-mode optical fibres, which are connected to an optical coherence tomography (OCT) system that allows subsurface analysis of the sample during the indentation stroke. To test our device and its capability, we performed indentation on a PDMS-based phantom. Our findings demonstrate that Common Path (CP)-OCT via lensed optical fibres can be successfully combined with a microindentation sensor to visualise the phantom's deformation profile at different indentation depths and locations in real time. LAY DESCRIPTION: This work presents a new approach to simultaneously perform micro-indentation experiments and OCT imaging. An optical fiber array-based sensor is used to develop a new hybrid tool where micro-indentation is combined with optical coherence tomography. The sensor is therefore capable of compressing a sample with a small force and simultaneously collecting OCT depth profiles underneath and around the indentation point. This method offers the opportunity to characterize the mechanical properties of soft materials and simultaneously visualize their deformation profile. The ability to integrate OCT imaging with indentation technology is promising for the non-invasive and precise characterization of different soft materials.

Immersion photoacoustic spectrometer (iPAS) for arcing fault detection in power transformers.

We report on the development of the first immersion photoacoustic spectrometer (iPAS) for arcing fault detection in power transformers. The spectrometer consists of a detection system and an all-optical photoacoustic sensing head mounted inside a small permeable chamber where dissolved C2H2 diffuses while the transformer oil is kept out. Our all-optical iPAS sensor can be placed directly inside an oil bath and measure dissolved C2H2 with the sensitivity and linearity needed for in situ arcing fault detection. Moreover, its fast response time holds great promise for extra-early fault diagnosis.

70 µm diameter optical probe for common-path optical coherence tomography in air and liquids.

We investigate and validate a novel method to fabricate ultrathin optical probes for common-path optical coherence tomography (CP-OCT). The probes are obtained using a 65 µm barium titanate microsphere inserted into an inward concave cone chemically etched at the end of a single-mode fiber. We demonstrate that the high refractive index (n=1.95) of the barium titanate microspheres allows one to maintain high sensitivity even while imaging in liquids, reaching a sensitivity of 83 dB. Due to its low cost, flexibility, and ease of use, the probe holds promise for the development of a new generation of ultrathin needle-based OCT systems.

Demonstration of a highly sensitive photoacoustic spectrometer based on a miniaturized all-optical detecting sensor.

We report on the development of a highly sensitive photoacoustic (PA) spectrometer based on a miniaturized all-optical detecting sensor. The sensor has a cell volume of less than 6 µL and relies on a cantilever-based acoustic transducer, which is equipped with an optical fiber interferometric readout. The spectrometer reaches a noise equivalent concentration of 15 ppb (300 ms time constant) for acetylene detection using a 23 mW excitation laser source, which corresponds to a normalized noise equivalent absorption coefficient of 7.7 × 10-10 W cm-1 Hz-1/2. The performance offered by this PA spectrometer is thus comparable to those reported for bulkier PA analyzers. Furthermore, because both the excitation and detection signals are brought to the PA cell via optical fibers, our spectrometer can be used in harsh environments, where electronic devices are prone to failure, and it is specially suitable for multiplexed remote detection applications. We believe that our study paves the way for the development of PA spectrometers that allow in-situ gas detection in space-limited circumstances.

Miniaturized fibre-top cantilevers on etched fibres.

Fibre-top probes are self-aligned, all optical devices obtained by carving a cantilever on top of a 125-µm diameter single-mode optical fibre. In this paper, we show that this design can be adapted to smaller fibres as well. We evaluated the performance of a 20-µm diameter probe in contact mode atomic force microscopy (AFM) and that of a 50-µm diameter probe in nanoindentation measurements. AFM images proved to be accurate both in air and water, although some distortion was observed because of the mechanical bending of the fibre during scanning. Indentation curves resembled those obtained with larger devices. The maximum indentation depth, however, is limited by the small dimensions of the cantilever.

Local dynamic mechanical analysis for heterogeneous soft matter using ferrule-top indentation.

There is a strong demand for nanoindentation methods to probe the heterogeneous viscoelastic properties of soft tissues. Important applications include diagnosis of early onset diseases such as arthritis and investigations into cellular mechanoresponse in tissue. Quantification of tissue mechanics at length and time scales relevant to biological processes, however, remains a technical challenge. Here, we present a new nanoindentation approach that is ideally suited to probe the viscoelastic properties of soft, hydrated tissues. We built a ferrule-top probe that uses wavelength modulation in a Fabry-Pérot cavity configuration to detect cantilever deflection and to drive a feedback-controlled piezoelectric actuator. This technique allows us to control the static load applied onto the sample using an all-optical mm-sized probe. We extract the local elastic and viscous moduli of the samples by superposing a small oscillatory load and recording the indentation depth at the frequency of oscillation. By using a set of silicone elastomers with a range of stiffnesses representative of biological tissues, we demonstrate that the technique can accurately determine moduli over a wide range (0.1-100 kPa) and over a frequency range of 0.01-10 Hz. Direct comparison with macroscopic rheology measurements yields excellent quantitative agreement, without any fitting parameters. Finally, we show how this method can provide a spatially-resolved map of large variations in mechanical properties (orders of magnitude) across the surface of soft samples thanks to high sensitivity over large (>µm) cantilever deflections. This approach paves the way to investigations into the local dynamic mechanical properties of biological soft matter.

Using ferrule-top opto-mechanical probes as a new tool in VCSEL reliability experiments.

Today, vertical cavity surface emitting lasers (VCSELs) are used in many high-end applications, for which the laser lifetime is a critical parameter. Changes in the spatial distribution of the various emission modes of the VCSEL can be used as an early sign of device degradation, enhancing the speed and detail of failure mode analysis. We have developed a ferrule-top combined atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) probe that can be used to analyze the transverse mode pattern of the 850 nm radiation at a <200 nm spatial resolution. During accelerated lifetime testing, the newly developed method shows that small local changes in the optical output can already be detected before any sign of device degradation is observed with conventional methods.

A Traumatic Impact Immediately Changes the Mechanical Properties of Articular Cartilage.

OBJECTIVE: To investigate whether and how a single traumatic impact changes the mechanical properties of talar articular cartilage. DESIGN: A marble was placed on the joint surface and a weight was dropped on both medial and lateral caprine talus to create a well-defined single focal impact. The mechanical properties of intact and impacted talar cartilage were measured with a micro-indenter. Elastic (storage) and viscous (loss) moduli were determined by oscillatory ramp and dynamic mechanical analysis protocols. RESULTS: We found significant differences between ankles and within the same ankle joint, with the medial talus having significantly higher storage- and loss moduli than the lateral talus. The storage- and loss moduli of intact articular cartilage increased with greater indentation depths. However, postimpact the storage- and loss moduli were significantly and consistently lower in all specimens indicating immediate posttraumatic damage. The deeper regions of talar cartilage were less affected by the impact than the more superficial regions. CONCLUSIONS: A single traumatic impact results in an immediate and significant decrease of storage- and loss moduli. Further research must focus on the development of non- or minimally invasive diagnostic tools to address the exact microdamage caused by the impact. We speculate that the traumatic impact damaged the collagen fibers that confine the water-binding proteoglycans and thereby decreasing the hydrostatic pressure of cartilage. As part of the treatment directly after a trauma, one could imagine a reduction or restriction of peak loads to prevent the progression of the cascade towards PTOA of the ankle joint.

Demonstration of a miniature all-optical photoacoustic spectrometer based on ferrule-top technology.

We present a miniaturized photoacoustic (PA) spectrometer obtained by carving a micromachined flexural pressure transducer directly at the top of a glass ferrule. The ferrule is equipped with two optical fibers, one for laser excitation of the gas and one for interferometric readout of the transducer. To demonstrate the working principle and assess the sensitivity of the device, we performed a set of measurements of C2H2 traces in an Ar buffer atmosphere. The data acquired show that our ferrule-top scheme allows one to increase the minimum detectable concentration by more than one order of magnitude with respect to the other miniaturized PA spectrometers reported in the literature, while decreasing the integration time by a factor of 10.

Collecting optical coherence elastography depth profiles with a micromachined cantilever probe.

We present an experimental setup that combines optical coherence elastography depth sensing with atomic force microscope indentation. The instrument relies on a miniaturized cantilever probe that compresses a sample with a small footprint force and simultaneously collects an optical coherence tomography (OCT) depth profile underneath the indenting point. The deflection of the cantilever can be monitored via optical fiber interferometry with a resolution of 2 nm. The OCT readout then provides depth profiles of the subsurface layer deformation with 15 nm resolution and depth range of a few millimeters.

Top-down approach to fiber-top cantilevers.

Taking inspiration from conventional top-down micromachining techniques, we have fabricated a low mass gold fiber-top cantilever via align-and-shine photolithography. The cantilever is characterized by measuring its resonance frequency and mechanical quality factor. Our results show that the device grants mass sensitivity comparable to that reported for similar standard cantilevers. This proof-of-concept paves the way to series production of highly sensitive fiber-top devices for remote detection of biochemical substances.

Viscoelastic mapping of mouse brain tissue: Relation to structure and age.

There is growing evidence that mechanical factors affect brain functioning. However, brain components responsible for regulating the physiological mechanical environment are not completely understood. To determine the relationship between structure and stiffness of brain tissue, we performed high-resolution viscoelastic mapping by dynamic indentation of the hippocampus and the cerebellum of juvenile mice brains, and quantified relative area covered by neurons (NeuN-staining), axons (neurofilament NN18-staining), astrocytes (GFAP-staining), myelin (MBP-staining) and nuclei (Hoechst-staining) of juvenile and adult mouse brain slices. Results show that brain subregions have distinct viscoelastic parameters. In gray matter (GM) regions, the storage modulus correlates negatively with the relative area of nuclei and neurons, and positively with astrocytes. The storage modulus also correlates negatively with the relative area of myelin and axons (high cell density regions are excluded). Furthermore, adult brain regions are ∼ 20%-150% stiffer than the comparable juvenile regions which coincide with increase in astrocyte GFAP-staining. Several linear regression models are examined to predict the mechanical properties of the brain tissue based on (immuno)histochemical stainings.

Mechanical alterations of the hippocampus in the APP/PS1 Alzheimer's disease mouse model.

There is increasing evidence of altered tissue mechanics in neurodegeneration. However, due to difficulties in mechanical testing procedures and the complexity of the brain, there is still little consensus on the role of mechanics in the onset and progression of neurodegenerative diseases. In the case of Alzheimer's disease (AD), magnetic resonance elastography (MRE) studies have indicated viscoelastic differences in the brain tissue of AD patients and healthy controls. However, there is a lack of viscoelastic data from contact mechanical testing at higher spatial resolution. Therefore, we report viscoelastic maps of the hippocampus obtained by a dynamic indentation on brain slices from the APP/PS1 mouse model where individual brain regions are resolved. A comparison of viscoelastic parameters shows that regions in the hippocampus of the APP/PS1 mice are significantly stiffer than wild-type (WT) mice and have increased viscous dissipation. Furthermore, indentation mapping at the cellular scale directly on the plaques and their surroundings did not show local alterations in stiffness although overall mechanical heterogeneity of the tissue was high (SD∼40%).

Mapping the mechanical properties of paintings via nanoindentation: a new approach for cultural heritage studies.

A comprehensive understanding of the behaviour of the heterogenous layers within the paint stratigraphies in historical paintings is crucial to evaluate their long term stability. We aim to refine nanoindentation as a new tool to investigate the mechanical behaviour of historical oil paints, by adapting the probes and the protocol already used in biomechanical research on soft tissues. The depth-controlled indentation profile performed with a spherical probe provides an evaluation of the non-linear viscoelastic behaviour of the individual layers in paint at local scale. The technique is non-destructive and guarantees the integrity of the surface after indentation. The mapping of elasticity demonstrates the properties' heterogeneity of the composite material within the paint layers, as well as between the individual layers and their interfaces.

Toward clinical elastography of dermal tissues: A medical device to probe skin's elasticity through suction, with subsurface imaging via optical coherence tomography.

The mechanical behavior of dermal tissues is unarguably recognized for its diagnostic ability and in the last decades received a steadily increasing interest in dermatology practices. Among the various methods to investigate the mechanics of skin in clinical environments, suction-based ones are especially noteworthy, thanks to their qualities of minimal invasiveness and relative simplicity of setups and data analysis. In such experiments, structural visualization of the sample is highly desirable, both in its own right and because it enables elastography. The latter is a technique that combines the knowledge of an applied mechanical stimulus and the visualization of the induced deformation to result in a spatially resolved map of the mechanical properties, which is particularly important for an inhomogeneous and layered material such as skin. We present a device, designed for clinical trials in dermatology practices, that uses a handheld probe to (1) deliver a suction-based, controlled mechanical stimulus and (2) visualize the subsurface structure via optical coherence tomography. We also present a device-agnostic data-analysis framework, consisting of a Python library, released in the public domain. We show the working principle of the setup on a polymeric model and on a volunteer's skin.

Dynamic indentation reveals differential viscoelastic properties of white matter versus gray matter-derived astrocytes upon treatment with lipopolysaccharide.

Astrocytes in white matter (WM) and gray matter (GM) brain regions have been reported to have different morphology and function. Previous single cell biomechanical studies have not differentiated between WM- and GM-derived samples. In this study, we explored the local viscoelastic properties of isolated astrocytes and show that astrocytes from rat brain WM-enriched areas are ~1.8 times softer than astrocytes from GM-enriched areas. Upon treatment with pro-inflammatory lipopolysaccharide, GM-derived astrocytes become significantly softer in the nuclear and the cytoplasmic regions, where the F-actin network appears rearranged, whereas WM-derived astrocytes preserve their initial mechanical features and show no alteration in the F-actin cytoskeletal network. We hypothesize that the flexibility in biomechanical properties of GM-derived astrocytes may contribute to promote regeneration of the brain under neuroinflammatory conditions.

Photoacoustic spectroscopy for gas sensing: A comparison between piezoelectric and interferometric readout in custom quartz tuning forks.

We report on a comparison between piezoelectric and interferometric readouts of vibrations in quartz tuning forks (QTFs) when acting as sound wave transducers in a quartz-enhanced photoacoustic setup (QEPAS) for trace gas detection. A theoretical model relating the prong vibration amplitude with the QTF prong sizes and electrical resistance is proposed. To compare interferometric and piezoelectric readouts, two QTFs have been selected; a tuning fork with rectangular-shape of the prongs, having a resonance frequency of 3.4 kHz and a quality-factor of 4,000, and a QTF with prong having a T-shape characterized by a resonance frequency of 12.4 kHz with a quality-factor of 15,000. Comparison between the interferometric and piezoelectric readouts were performed by using both QTFs in a QEPAS sensor setup for water vapor detection. We demonstrated that the QTF geometry can be properly designed to enhance the signal from a specific readout mode.

A fiber-tip photoacoustic sensor for in situ trace gas detection.

Most trace gas detection methods developed so far largely rely on active sampling procedures, which are known to introduce different kinds of artifacts. Here, we demonstrate sampling-free in situ trace gas detection in millimeter scale volumes with fiber coupled cantilever enhanced photoacoustic spectroscopy. Our 2.4 mm diameter fiber-tip sensor is free from the wavelength modulation induced background signal (a phenomenon that is often overlooked in photoacoustic spectroscopy) and reaches a normalized noise equivalent absorption coefficient of 1.3 × 10-9 W cm-1 Hz-1/2 for acetylene detection. To validate its in situ gas detection capability, we inserted the sensor into a mini fermenter for headspace monitoring of CO2 production during yeast fermentation. Our results show that the sensor can easily follow the different stages of the CO2 production of the fermentation process in great detail.