We report on a comparison between the piezoelectric and interferometric readouts of vibrations in quartz tuning forks (QTFs) when employed as sound wave transducers in quartz-enhanced photoacoustic trace gas sensors. We demonstrate the possibility to properly design the QTF geometry to enhance interferometric readout signal with respect to the piezoelectric one and vice versa. When resonator tubes are acoustically coupled with the QTFs, signal-to-noise ratio enhancements are observed for both readout approaches. These results open the way to the implementation of optical readout of QTF vibrations in applications where external electromagnetic field could distort the piezoelectric signal.
We present a new approach to optical fiber photoacoustic spectroscopy for gas trace detection. After an explanation of its working principle, we show the results of the measurements obtained with our first prototype. We then analyze the advantages that this approach may provide, and further discuss how the expertise developed by the optical fiber sensor community may contribute to the field of photoacoustic gas sensing.
There is a significant interest in characterizing mechanical properties of the brain tissue due to the role of mechanics in neurodevelopment and neurological disorders. Previous Scanning Force Microscopy studies have reported that brain tissue has highly heterogeneous mechanical properties. Yet, it is not known how the structural components of the brain such as neurons, glial cells, their axons and dendrites, and extracellular matrix contribute to these stiffness variations. To investigate the structure-stiffness relation in brain tissue and thus solve this issue, we have employed dynamic indentation-controlled mapping with a spatial resolution of ~50 µm to measure viscoelastic properties of hippocampus and cortex on isolated horizontal mouse brain sections. Nonlinear-viscoelastic nature of the brain tissue was observed by oscillatory ramp testing, where stiffness increased linearly with the strain (strain < 10 %); frequency sweeps revealed frequency-stiffening (1-10 Hz) with the phase delay in the range of 15-30˚. Viscoelasticity maps showed that regions with distinct mechanical properties correspond to morphological layers with the mean storage modulus varying from 779±77 Pa for granular layer to 3260±74 Pa for stratum lacunosum-moleculare (mean ± SE). Density of the nuclei was estimated for the measured regions and was found to negatively correlate with the stiffness, except for alveus, mostly composed of axonal fibers, being significantly softer than all other high-stiffness low-cell-density regions. Taken together, our study shows that our novel indentation method is able to map mechanical differences of the brain at the cellular level, leading to a better understanding of the relation between tissue composition and stiffness.
The majority of tissue related diseases are known to alter tissue structure. During the last 10 years, increasing efforts have been put into the development of new techniques that could provide in vivo information on tissue morphology. Optical coherence tomography (OCT) is known to provide structural information in situ. However, there is also a strong demand to evaluate the mechanical properties of biological tissues in vivo. To address this need, we combined microindentation with non-invasive OCT imaging to determine spatiotemporal distributions of mechanical properties of in vivo and formaldehyde fixed chicken embryos. The use of OCT allows us to quantify changes in tissue morphology and to localize indentation at specific regions. To measure viscoelastic properties of living tissue, indentation tests are simultaneously performed on in vivo HH8-HH12 chicken embryos using a cantilever based force transducer. After performing live tissue indentation, we probed the properties of formaldehyde fixed embryo. The same general contrasts of elasticity between different histological regions were found, but the average value was found to be higher for the fixed sample. Furthermore, with this technique it is possible to follow the remodeling of tissue elastic properties during embryonic development, measure viscoelastic properties of living tissue, and investigate correlations between local mechanical properties during cell migration and differentiation. This method is applicable to a wide variety of biological samples and can provide new insight to better understand the link between the mechanical response of tissue and its biological structure, and to compare diseased tissues with healthy one.
Mechanical properties are critical in regulating pathophysiological cell behavior via mechano-transduction. Although biological tissues are generally regarded as viscoelastic, mechanobiology studies mainly focused on characterizing tissue elasticity and investigating cell behavior as a function of substrate stiffness. Moreover, mechanical properties are often derived using bulk testing techniques, likely being poorly representative of the local biomechanical environment felt by cells. Aiming at characterizing micro-mechanical viscoelastic properties at typical cell length-scales in physiological-like conditions, we present here a new testing approach to perform dynamic nano-indentation measurements in liquid (e.g. PBS, culture medium) at controlled temperature (e.g. 37 °C).
Our method involves a customized version of Optics11’s PIUMA Nanoindenter, which is based on a unique ferrule-top opto-mechanical cantilever force transducer operated by a z-axis piezoelectric motor, and which has been modified to enable user-defined load- and displacement-controlled measurements (e.g. creep, stress-relaxation, DMA and constant strain rate tests). Moreover, a temperature (T) sensor has been integrated with that of the PIUMA sample stage to control the actual sample T via a new master-slave control loop. In this study, we characterized the viscoelastic properties of PDMS samples, gelatin hydrogels at different temperatures and concentrations, single smooth muscle cells, and healthy and infarcted myocardial tissue samples. Experimental data within the linear viscoelastic region were fitted to Generalised Maxwell models, deriving instantaneous and equilibrium elastic moduli, and characteristic relaxation times.
This method could be beneficial for better investigating soft tissues/(bio)materials mechanics and for designing new mechano-mimetic substrates for tissue engineering, disease modelling and cell mechanobiology studies.
Each year, in the Netherlands alone, more than 50.000 percutaneous procedures are performed for treatment or for removal of tissue from possibly diseased organs, of which 30% return non-diagnostic due to erroneous needle targeting, often as a result of non-homogeneity of the penetrated tissue. In this study, we aim to facilitate needle targeting by assessing the tissue in front of the needle based on its mechanical properties. A probe that can identify tissues via real-time measurements of their mechanical properties is placed at the tip of the needle. The probe, actuated by a remote system at the distal part of the needle, employs the bending of a micro-machined cantilever fabricated on top of an optical fiber. The displacement of the cantilever, imposed by pressing a micro-bead (r = 75 µm) glued at the tip of the cantilever against the tissue, is interrogated by Fabry-Pérot interferometry and converted to force acted on the tissue in real-time. The force transducer is able to perform in harsh environments due to its monolithic design and all-optical working principle. Using our setup, load-indentation curves were obtained during needle insertion in several gelatin-based specimens. We demonstrate the ability of our device to detect and quantify layers of varying stiffness and to successfully locate tissue boundaries in animal tissue embedded in gelatin. Furthermore, a diagnostic measurement can be made by quantifying intra-organ tissue stiffness at the needle target location.
A simple, yet effective, setup for the simultaneous interrogation of multiple ferrule-top-cantilever sensors for acoustic sensing is here presented and experimentally tested with two ferrule-top-cantilever sensors; results confirm the feasibility of the approach.
Two fiber optic sensors (FOSs) for detection of precursory acoustic emissions in rockfall events are proposed and
experimentally characterized. While both sensors are interferometric, the first one use a fiber coil as sensing element,
whereas the second sensor exploits a micro-machined cantilever carved on the top of a ferrule. Preliminary
experimental comparison with standard piezo-electric transducers shows the viability of such FOSs for acoustic
emission monitoring in rock masses.
Ferrule-top (FT) cantilevers are a new generation of all optical micromechanical sensors obtained by carving
microstructures on the top of ferrule terminated fibers. In this paper, we will demonstrate how this plug and play design
can be used for the development of a new generation of sensors and actuators for harsh environments, where
commercially available devices would be prone to failure. Ferrule-top sensors can work in two main modes - static and
dynamic. The static mode is based on recording elastic deflection of the cantilever; the dynamic mode relies on tracking
changes in its mechanical properties (resonance frequency, quality factor). Depending on the application, one can choose
which mode is most suitable or combine both to achieve best performance. We will illustrate the relation between
specific measured quantity (humidity, flow) and the behavior of the sensor. Further, we will show the setup in which the
sensor can be actuated using light, giving the possibility to excite the cantilever without any electronics on the sensing
head. This technique might by use for the development of fully optical beamsteering microdevices.
KEYWORDS: Near field scanning optical microscopy, Atomic force microscopy, Near field optics, Optical microscopy, Imaging systems, Photodiodes, Atomic force microscope, Liquids, Transmittance, Optical fibers
Fiber-top and ferrule-top cantilevers (FTC) are a new generation of all optical, monolithic, self-aligned microdevices.
They are obtained by carving a cantilever on the cleaved end of an optical fiber (fiber-top) or on a ferrule terminated
fiber (ferrule-top). FTCs rely on Fabry-Perot interferometry to measure the deflection of the cantilever with
subnanometer deflection sensitivity. FTCs specially developed for scanning probe microscopy are equipped with a sharp
tip that has the dual function of probing the topography and collecting/emitting light. We perform the scanning probe
microscopy using these probes in air, liquid and at low temperature (12°K). The light emission/collection functionality of
FTC probes also allows one to combine scanning near field optical microscopy (SNOM) and optical transmission
microscopy with contact and non-contact mode atomic force microscopy (AFM). This makes FTCs ideal for
AFM+SNOM on soft samples, polymers and biological specimen, where bent fiber probes and tuning fork based
systems would not be recommended because of the high stiffness of those probes. We demonstrate here the capability of
fiber-top cantilevers to measure deflection and collect near field optical signal, and also the capability of ferrule-top
cantilevers for simultaneous optical transmission microscopy and topography of SNOM gratings. Thanks to their unique
features, FTCs also open up possibilities for UV nanolithography and on-demand optical excitation at nanoscale.
We present an optical fiber ferrule-top sensor for humidity measurements. Changes in water content in a gaseous
environment are detected by monitoring the resonant frequency and the quality factor of a cantilever fabricated on top of
a ferruled optical fiber. Our experimental data demonstrate that the device offers high resolution and high repeatability.
We present the application of a ferrule-top cantilever optical fiber sensor to measure the velocity of low speed air flows.
The sensor is statically characterized against a Pitot tube in a small wind-tunnel and both probes are facing the flow in its
laminar region. The ferrule-top sensor is equipped with a single-wavelength interferometric readout and it shows good
sensitivity and short-term repeatability, despite the fact that the geometry and the read-out are not optimized.
Fibre-top devices (micromachined devices fabricated on top of a single mode optical fibre)
provide unprecedented opportunities for the development of user-friendly, all-optical
miniaturized sensors. Unfortunately, the impact of this technology has been hampered by the high
costs of fabrication. In this paper we introduce a new generation of fibre-top-like devices that can
be fabricated with cost-effective processes and still maintain all the advantages of fibre-top
technology.
At the beginning of 2009, our group has introduced a new technique that allows fabrication of
photolithographic patterns on the cleaved end of an optical fibre: the align-and-shine
photolithography technique (see A. Petrušis et al., "The align-and-shine technique for series
production of photolithography patterns on optical fibres", J. Micromech. Microeng. 19, 047001,
2009). Align-and-shine photolithography combines standard optical lithography with imagebased
active fibre alignment processes. The technique adapts well to series production, opening
the way to batch fabrication of fibre-top devices (D. Iannuzzi et al., "Monolithic fibre-top
cantilever for critical environments and standard applications", Appl. Phys. Lett. 88, 053501,
2006) and all other devices that rely on suitable machining of engineered parts on the tip of a
fibre. In this paper we review our results and briefly discuss its potential applications.
After a general review on fiber-top technology, we will present our recent achievements in the utilization of fiber-top
double-clamped cantilevers as refractometers and in the development of alternative techniques for the fabrication of
fiber-top devices.
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