A fast indentation system for 2D stiffness mapping of cartilage is presented. The goal of this technique, which has the potential to be deployed endoscopically, is to localize and to distinguish between regions of healthy and degenerated cartilage. In the system presented here, forces are measured by a fiber Bragg grating (FBG), which is located at the end of a fiber optic indenter probe. This approach enables small indenter dimensions, which are advantageous for endoscopic use. Differences between the stiffness parameters measured on healthy cartilage and on the same sample after artificial degeneration are observed. It is shown that the fast indentation system has the capability to distinguish healthy bovine cartilage from artificially degenerated one. The evaluation of this capability will make the conclusions of the work.
Laser-induced forward transfer (LIFT) has been used in recent years for the flexible and gentle 3D-bioprinting of cells with superior cell survival rates relative to other methods [1]. One drawback of the current state-of-the-art nanosecond laser based cell printing is the fact that material from an inorganic sacrificial layer, which is required for laser energy absorption, is transferred to the printed target structure, where it contaminates the printed construct [2]. Furthermore, existing LIFT based technologies transfer multiple cells at a time, i.e. with a single laser pulse. However, living cells in a functional in vitro microenvironment are exposed to multiple biophysical, biochemical and biological signals. Therefore, a 3D-bioprinting technique providing single cell resolution is desirable for a number of applications, e.g. the investigation of cell-cell interactions and cell niches, which has not yet been achieved using laser based bioprinting. Here we present a new femtosecond laser-based method for the efficient and precise single cell printing and sorting, which avoids the use of non-biological inorganic absorption layers. An ultrashort laser pulse (λ = 1030 nm, 600 fs, few μJ) is focused underneath a cell layer, which is suspended on top of a hydrogel reservoir. Nonlinear absorption leads to plasma ionization and rapid cavitation bubble expansion, which generates a jet of material, transferring cell-laden hydrogel from a gel/cell reservoir to an acceptor stage [3]. In addition, to the effective sacrificial-layer free transfer of multiple cells, individual cells can be selected based on their morphology and phenotype, and transferred to the acceptor slide, isolated from the remaining cells. Furthermore, laser-induced single cell printing efficiencies close to 100% were achieved for the first time.
Fiber optic microindentation sensors that have the potential to be integrated into arthroscopic instruments and to allow localizing degraded articular cartilage are presented in this paper. The indenters consist of optical fibers with integrated Bragg gratings as force sensors. In a basic configuration, the tip of the fiber optic indenter consists of a cleaved fiber end, forming a cylindrical flat punch indenter geometry. When using this indenter geometry, high stresses at the edges of the cylinder are present, which can disrupt the tissue structure. This is avoided with an improved version of the indenter. A spherical indenter tip that is formed by melting the end of the glass fiber. The spherical fiber tip shows the additional advantage of strongly reducing reflections from the fiber end. This allows a reduction of the length of the fiber optic sensor element from 65 mm of the flat punch type to 27 mm of the spherical punch. In order to compare the performance of both indenter types, in vitro stress-relaxation indentation experiments were performed on bovine articular cartilage with both indenter types, to assess biomechanical properties of bovine articular cartilage. For indentation depths between 60 μm and 300 μm, the measurements with both indenter types agreed very well with each other. This shows that both indenter geometries are suitable for microindentation measuremnts . The spherical indenter however has the additional advantage that it minimizes the risk to damage the surface of the tissue and has less than half dimensions than the flat indenter.
A fiber-based device for testing mechanical properties of cartilage is presented within this study. The measurement principle is based on stepwise indentation into the tissue and observing of corresponding relaxation of the stress. The indenter tip is constituted of a cleaved optical fiber that includes a fiber Bragg grating which is used as the force sensor. Stress relaxation measurements at 25 different positions on a healthy bovine cartilage sample were performed to assess the behavior of healthy cartilage. For each indentation step a good agreement was found with a viscoelastic model that included two time constants. The model parameters showed low variability and a clear dependence with indentation depth. The parameters can be used as reference values for discriminating healthy and degenerated cartilage.
A locally micro-structured fiber Bragg grating (LMFBG) was manufactured by forming a circumferential groove in the middle of a type I fiber Bragg grating (FBG). The groove was directly ablated using a fs-laser and had a length of 86μm, a depth of 27μm and steep side walls. Due to the precisely machined geometry of the structure the reflection spectra can be accurately described with a fairly simple theoretical model. At several constant temperatures in the range from 5°C to 45°C this structure was exposed to various compressive loads in the range from 0N to -1.42N. Here the force and temperature sensitivity of the LMFBG are presented. This structure can be used for miniaturized compressive force sensing at variable temperatures, which is of particular interest for many bio-medical applications.
An automated fs-laser machining procedure was developed to engrave circumferential grooves into the cladding of optical fibres. The grooves are positioned centrally to type I fibre Bragg gratings (FBG) and form locally micro structured FBGs. The grooves realized so far were ~30μm deep and were 48 μm to 200 μm long. These devices show the occurrence of a phase shifted spectrum when axial stress is applied. It is shown in this paper that this property can be used to achieve higher force sensitivities when compared to conventional FBGs. These devices are advantageous for the investigation of tissue by indentation-type elasticity measurements. An experimental and theoretical investigation of the dependence of the force sensitivity on the length of the structure is reported.
The production and characterisation of a micro-structured FBG force sensor is described. Employing femtosecond laser micro machinery a circumferential ditch of about 30 μm depth and 40 μm width is engraved in the clad of an optical fibre at the centre of a 3 mm long type I fibre Bragg grating (FBG). The purpose of the structure is the enhancement of the force sensitivity characteristics for the measurement of sub-mN forces. Phase-shift spectra occur when axial stress is applied to the fibre. Exploiting this phenomenon experimental tests show a 10% improvement in the sensitivity performance when compared to an unstructured FBG.
A novel kind of miniaturized, all optical probe concept to measure the elasticity of biological tissues is here presented. The probe is based on fibre Bragg grating sensors (FBG) inscribed in optical fibres. The measurement procedure exploits the high strain sensitivity of Bragg gratings. A study on the reproducibility, reliability, and resolution of the sensor is presented and a first measurement on bovine cartilage tissue is reported. A linear elastic model of the cartilage has been used to analyse the data. The results indicate a good agreement with previous values given in the literature for micro-indentation.
We take in consideration three applications which are strongly affected from atmospheric as well as artificial turbulence. Systems in development at the IOSB institute for correcting the turbulence in each application are reported. The set of problems we are interested in are related to the tracking of objects through the atmosphere, the improvement of a laser beam for laser communication or countermeasure and the imaging of objects distorted from turbulent layers. We propose the use of adaptive optics (AO) based on classical Shack-Hartmann sensors (SH) to improve the performances of tracking systems. The SH has the advantage to be sufficiently fast, robust and suited for coherent as well as incoherent point sources correction. In consequence it is also well suited for the purpose of laser beam correction in the atmosphere. The problem of improving a complex image or scene cannot be solved with wavefront sensors in an easy way. A correction of the wavefront based on quality metric estimation and the Stochastic Parallel Gradient Descent (SPGD) algorithm is then reported here to cover this aspect. In the final part two modifications of the SPGD algorithm to improve its performances are proposed.
The performance of optical systems is degraded by atmospheric turbulence. Over propagation distances that exceed
several kilometers, it is difficult to evaluate its impact because of terrain variability - a factor that should be taken into
account. However, to optimize performance, the turbulence characteristics and its effect on optical wave propagation
along the propagation path should be known. The understanding of turbulence impact is one of the main objectives of the
NATO group SET 165: "Adaptive Optics (AO) for laser beam delivery, passive and active imaging and turbulence
mitigation". In this paper we describe experiments performed by the NATO SET 165 research group, namely, a set of
atmospheric experiments over a 7 km distance, and discuss some preliminary results of data processing. The experiments
were conducted at the University of Dayton Intelligent Optics Laboratory (UD/IOL) in October 2011. It benefited
significantly from the available optical setups and the infrastructure on the UD/IOL site.
In this paper we introduce a technique to correct atmospheric turbulence. The characteristic of this promising procedure
is that the distorted wavefront is corrected without any information about the wavefront itself. This technique relies on
the combined use of a deformable mirror controlled by a Stochastic Parallel Gradient Descent (SPGD) algorithm and an
image quality measurement. Even though this procedure is in terms of time slower than a direct wavefront
reconstruction, it appears that compared to conventional AO systems the problems related to scintillations are noticeably
reduced.
Several studies on different adaptive optics concepts are presented in the article.
Each one of the procedures shows its peculiar advantages when considering different situations where the image
distortion, due to atmospheric or artificial turbulence, becomes problematic. A setup is presented based on
the usual wavefront reconstruction techniques using a Shack-Hartmann wavefront sensor in a closed loop with
a deformable mirror and a computer. The reconstruction method follows the modal approach which has been
demonstrated to be more robust and suitable than the zonal one when limited to the correction of the first Zernike
components1 . Some results deriving from the studies are reported. A second part in the article describes the
measurements and the characteristics of the atmospheric turbulence present in direct tests. Another procedure
based on the control of a deformable mirror by mean of a fast iterative procedure is also treated and the relative
results about the compensation of laser beams as well as extended images are shown.
A recently introduced approach to restore images distorted by atmospheric turbulence without a direct knowledge about
the wavefront is being discussed in this paper. This technique is based on the use of a deformable mirror controlled by a
Stochastic Parallel Gradient Descent (SPGD) algorithm applied to an image quality measurement. This procedure is now
being tested for the correction of extended sources as well as laser beams. Because the technique does not rely on
wavefront sensors, the problems related to scintillations are noticeably reduced. Preliminary results are presented.
The development of a system intended as a demonstrator for improving the tracking of distant point-like sources
through the correction of the atmospheric optical distortions is here reported. The demonstrator consists of a
motorized mirror, which can pursuit a moving light source, united to an adaptive optics setup to improve the
performances and the precision of the tracing of the object trajectory. The adaptive optics setup consists of
a closed loop between a quad cell sensor and a tip-tilt mirror for the atmospheric jitter compensation and of
another closed loop between a Shack-Hartmann sensor and a membrane deformable mirror for the compensation
of higher order aberrations. Atmospheric measurements of an incoherent source will be also presented. In the
case of our interest, where the atmospheric disturbances cannot be addressed only to a turbulent layer near the
pupil (near field approximation), the scintillation becomes an important part of the noise. Its effects will be
analyzed here, with particular attention to the influence on the wavefront sensor.
We describe two types of adaptive optics systems developed at FGAN-FOM and the progress in their realization
during the last year will be presented. The first system is based on classical adaptive optics scheme and is aimed
to resolve objects inside the isoplanatic angle of the wavefront sensor. The second system is based on an iterative
algorithm and on the evaluation of the quality of the image to correct the wavefront. This last system is intended
to resolve extended targets outside the isoplanatic limits. A constructed mobile system will be also introduced
as multipurpose system for measuring atmospheric characteristics as well as tracking and resolving point-like
sources in the isoplanatic angle. Finally some results of measurements and attempts to correct the centroid
movement of the image of an incoherent point source located at 2.5 km distance using the mobile system will be
discussed.
In the field on blind image deconvolution a new promising algorithm, based on the Principal Component Analysis
(PCA), has been recently proposed in the literature. The main advantages of the algorithm are the following:
computational complexity is generally lower than other deconvolution techniques (e.g., the widely used Iterative Blind
Deconvolution - IBD - method); it is robust to white noise; only the blurring point spread function support is required to
perform the single-observation deconvolution (i.e., a single degraded observation of a scene is available), while the
multiple-observation one is completely unsupervised (i.e., multiple degraded observations of a scene are available). The
effectiveness of the PCA-based restoration algorithm has been only confirmed by visual inspection and, to the best of our
knowledge, no objective image quality assessment has been performed. In this paper a generalization of the original
algorithm version is proposed; then the previous unexplored issue is considered and the achieved results are compared
with that of the IBD method, which is used as benchmark.
Hereby we present the idea of a new passive sensor intended to compensate atmospheric turbulence distortions
of object images. It is based on the applications of the already successful concept of adaptive optics. The
main application of this sensor will be the compensation of the trajectory jitter of flaring objects in the far
distance which will allow quicker identification and better tracking. The system consists of a wavefront sensor
and a deformable correcting mirror, both commercially available, keeping the overall costs and size in reasonable
limits. The research is divided into two main topics: the first is the characterization of the influence of the
atmospheric turbulence on the object image when the observer's line of sight is parallel to the ground. The
second is the development of the components and the software to achieve the required performances. First
progress have been made on determining the shape of the deformable mirror with good accuracy by means of a
modal reconstruction as well as in measuring the wave front distortions of a point-like source due to atmospheric
turbulence.
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