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There has been a significant transformation in the treatment of intracranial aneurysms (IAs) over the past century, with the most pivotal changes occurring in the past three decades. To characterize this evolution, we assessed the number of articles published on various procedures for the treatment of IA as a measure of their interest and usage over time. We separated our analysis into two main areas: surgical and endovascular approaches. We further subdivided these two main categories into clipping and bypass for surgery, and coiling, flow diversion, and liquid material embolization for endovascular approaches. We found 5956 publications on open surgical approaches in the 70-year period from 1947 to 2017, with papers on clipping (n = 4204), being the most common. We found 8602 endovascular publications beginning in 1964, with most of the activity taking place in the late 1990s and beyond. Coiling had the most publications of the endovascular approaches (n = 5436). In 1999, the number of annual publications on endovascular treatments surpassed those of open surgery, signaling a crossover point in the IA literature. The same trend continues to this date.
Growth and propagation of fish-infecting microsporidians within cell culture has been more difficult to achieve than for insect- and human-infecting microsporidians. Fish microsporidia tend to elicit xenoma development rather than diffuse growth in vivo, and this process likely increases host specificity. We present evidence that the fish microsporidian, Loma salmonae, has the capacity to develop xenomas within a rainbow trout gill epithelial cell line (RTG-1). Spore numbers increased over a 4 weeks period within cell culture flasks. Xenoma-like structures were observed using phase contrast microscopy, and then confirmed using transmission electron microscopy. Optimization of the L. salmonae-RTG-1 cell model has important implications in elucidating the process of xenoma development induced by microsporidian parasites.
The aim of this work is to improve bone-implant bonding. This can, potentially, be achieved through the use of an implant coating composed of fibre networks. It is hypothesised that such an implant can achieve strong peri-prosthetic bone anchorage, when seeded with human mesenchymal stem cells (hMSCs). The materials employed were 444 and 316L stainless steel fibre networks of the same fibre volume fraction. The present work confirms that hMSCs are able to proliferate and differentiate towards the osteogenic lineage when seeded onto the fibre networks. Cellular viability, proliferation and metabolic activity were assessed and the results suggest higher proliferation rates when hMSC are seeded onto the 444 networks as compared to 316L. Cell distribution was found uniform across the seeded surfaces with 444 showing a somewhat higher infiltration depth.
Investigations to determine the electrical contact
performance under repeated cycles at low force conditions for
carbon-nanotube (CNT) coated surfaces were performed. The surfaces under
investigation consisted of multi-walled CNT synthesized on a silicon
substrate and coated with a gold film. These planar surfaces were mounted on
the tip of a PZT actuator and contacted with a plated Au hemispherical
probe. The dynamic applied force used was 1 mN. The contact resistance
(Rc) of these surfaces was investigated with the applied force and with
repeated loading cycles performed for stability testing. The surfaces were
compared with a reference Au–Au contact under the same experimental
conditions. This initial study shows the potential for the application of
gold coated CNT surfaces as an interface in low force electrical contact
Wrinkling of thin sheets under strain is a universal phenomenon. The amplitude and period of the wrinkles formed in a thin sheet clamped at both ends are dependent on its strain and material parameters. In our study, wrinkling is observed in microscale for double clamped thin films (L>W>>t) consisting of 200nm deposited low stress silicon nitride bridges fabricated by bulk micromachining. A bilayer system is formed with 30nm aluminum evaporated on to these bridges. At room temperature the bridges are essentially flat. When an electrical current passes through the aluminum layer electrothermal, heating results in thermal expansion that wrinkles the bilayer. In addition we investigated various dimensions of the bridges and their correlation to the amplitude and the number of wrinkles. The observations are compared to existing wrinkling theory.
The failure strength of polycrystalline silicon is discussed in terms of activation of critical flaws, as well as the material microstructure and inhomogeneity. The Weibull probability density function parameters were obtained to deduce the scaling of material and component strength and to identify critical flaw populations, especially when two or more flaw sets are concurrently active. It was shown that scaling of strength changes for small feature sizes, which limits the applicability of strength data from large MEMS components to self-similar small MEMS components. On the other hand, the probability of failure of small components is described by a larger Weibull material stress parameter, which makes uniaxial strength data a conservative design approach. Furthermore, according to mode I and mixed mode I/II fracture studies for polysilicon, it is concluded that microstructural inhomogeneity alone accounts for 50% scatter in strength (with reference to the minimum recorded value). Thus, the conditions for applicability of the Weibull probability density function are rather weak in polycrystalline silicon, because flaws of the same length that are subjected to the same macroscopic stresses are not always critical.
Microelectromechanical systems (MEMS) are being used in many critical applications that require very high stress levels. To properly design MEMS components, mechanical properties should be characterized testing relevant sized samples that are fabricated with the same procedures as the final structure. In this paper we use atomic force microscopy (AFM) experiments to study the fracture strength statistics of polycrystalline SiC and SiN nanobeams, and compare their mechanical performance with the performance of previously tested Si nanostructures. Using the same AFM method and similar sample shape and sizes, allows a direct comparison to be made, which will be useful in determining the best material for different mechanical applications and also to validate the theoretical limits.
The tactile sensors for human support robots which can detect both normal stress and shear stress and have human-friendly surface have been proposed. Micro-cantilevers adequately inclined by Cr deflection control layer were fabricated by the surface micromachining on SOI wafer. The cantilevers were covered with the PDMS elastomer for human-friendly surface. When the stress is added to the surface of elastomer, the deformation of cantilevers along with elastomer is detected as piezoresistive layer in the cantilevers. The piezoresistive response of the cantilever is analyzed by FEM calculation. The response of the fabricated tactile sensor to normal stress and shear stress was measured by output from this resistance. The tactile sensor with PDMS elastomer can detect both normal stress and shear stress. On the other hand, it hardly has sensitivity to shear stress of orthogonal direction to the cantilever. It means that the tactile sensor can distinguish the direction of shear stress. The sensitivity of tactile sensor vary widely with cantilever pattern and relation between direction of cantilever and crystallite orientation of Si. It is suggested that the sensitivity of tactile sensor can be improved by using FEM estimation and selective ion implantation.
This paper reports on the systematic characterization of a deep reactive ion etching based process for the fabrication of silicon microneedles. The possibility of using such microneedles as protruding microelectrodes enabling to electroporate adherently growing cells and to record intracellular potentials motivated the systematic analysis of the influence of etching parameters on the needle shape. The microneedles are fabricated using dry etching of silicon performed in three steps. A first isotropic step defines the tip of the needle. Next, an anisotropic etch increases the height of the needle. Finally, an isotropic etch step thins the microneedles and sharpens their tip. In total, 13 process parameters characterizing this etching sequence are varied systematically. Microneedles with diameters in the sub-micron range and heights below 10 µm are obtained. The resulting geometry of the fabricated microneedles is extracted from scanning electron micrographs of focused ion beam cross sections. The process analysis is based on design-of-experiment methods to find the dominant etch parameters. The dependence of the needle profiles on process settings are presented and interpolation procedures of the geometry with processing conditions are proposed and discussed.
For MEMS technology, reliability is of major concern. The implementation of a protection and passivation layer, that may easily enhance reliability of capacitive Micromachined Ultrasonic Transducers (cMUTs) must be done without degrading device performance. In this work, realization, simulation and characterization of passivated cMUT are presented. Two materials, SiNx and Parylene C, were selected with regard to their mechanical and physical properties as well as their compatibility with device processing. Particular attention was paid on layer deposition temperature to avoid a structural modification of the top aluminium electrode and, hence, a membrane bulge. The characterization results are in good agreement with the simulations. The SiN passivation layer clearly impact device performance while Parylene C effectiveness is clearly pointed out even through ageing characterizations. If SiNx layer can be used for passivation with particular precautions, Parylene is definitely an interesting material for cMUT passivation and protection.
This paper outlines a simple method to fabricate a bilayer membrane consisting of a thin nanoporous gold layer infused with uncured polydimethylsiloxane. The fabrication technique offers excellent adhesion due to mechanical interlocking between porous layer and elastomer, and excellent electrical conductivity up to 25% strain, despite a very low effective elastic modulus (∼1.35 MPa) due to cracks in the embedded gold layer. Initially freestanding circular membranes displayed significant out of plane buckling, and created difficulties in extraction of membrane mechanical properties. The underlying mechanisms of compressive stress accumulation that lead to membrane buckling and remedies to prevent it are discussed.
This paper reports on recent improvements of the bulge and microtensile techniques for the reliable extraction of material parameters such as the Young's modulus E, Poisson's ratio ν, plane strain modulus Eps = E/(1–ν2), prestress σ0, fracture strength μ, Weibull modulus m and strain hardening coefficients n, and on the direct comparison between the two methods. The bulge technique is extended to full wafer measurements enabling throughputs of data with statistical relevance whereas key improvements of a previous fabrication process of microtensile specimens lead now to much higher yields, approaching 100%. Both techniques are applied to an extensive set of materials, brittle and ductile, typically used in MEMS applications. These include thin films of silicon nitride, silicon oxide, polycrystalline silicon and aluminum deposited by techniques such as thermal oxidation, LPCVD, PECVD and PVD.
This paper reports a transition in the fracture behavior of micron-sized single-crystal-silicon (SCS) film in an MEMS structure for various film thicknesses and ambient temperatures. The mean fracture toughness of 4-µm-thick SCS films was 1.28 MPa at room temperature (RT), and the value increased as the film thickness decreased, reaching 2.91 MPa for submicron-thick films. The fracture toughness of 4-µm-thick film did not change for ambient temperatures ranging from RT to 60ºC. However, it drastically increased at 70ºC and reached 2.60 MPa at 150ºC. Enhanced dislocation activity in the SCS crystal near the fracture surface was observed on 1-µm-thick film at RT and 4-µm-thick film at 80ºC by high-voltage electron microscopy. This change in dislocation activity seemed to correlated with the transition in fracture behavior.
The mechanical properties of polydimethylsiloxane (PDMS) were characterized by using uniaxial compression, dynamic mechanical analysis (DMA), and nanoindentation tests as well as finite element simulation methods. A five-parameter linear solid model was used to emulate the behavior of PDMS. The study results indicated that the effect of viscoelasticity affected the PDMS pillar arrays significantly. The traditional approach for calculating the cell force basing on the linear elastic mechanics could result in considerable errors.
Micromechanical structures were investigated nondestructively via laser-Doppler-vibrometry to determine defect structures. Therefore, silicon membrane structures were characterized by their measured resonant frequencies and mode shapes. The influence of defects on the micromechanical structures is shown on the measured dynamic properties. Defect samples were indentified on the basis of the ratios of measured resonant frequencies and the quantified comparison of mode shapes without an identification of unknown parameters. The investigations showed that a fast determination of defect structures is possible by measured dynamic properties.
A specific experimental setup combining nanoindentation and electrical inputs has been developed in order to determine the reliability and the performances of Micro-ElectroMechanical Systems (MEMS) which include thin free-standing elements like micro-switches applications. The evolution of the electrical resistance with respect to a mechanical solicitation applied on the contact, are henceforth available. The description of the set-up goes with a brief overview of the tests performed on a gold ohmic switch. A discussion is developed considering the mechanisms involved in the contact response. This is based on a confrontation among the experimental results, the analytical modeling and also finite-element analysis.
In this paper we report on our latest efforts to fabricate and characterize optically actuated deformable micro mirrors for wavefront correction in an adaptive optics system. The optically actuated DMM device consists of a Silicon Nitride (Si3N4) thin film patterned into a spring plate array, an SU-8 photoresist supporting structure that provides the space through which the mirror is allowed to deform, and a PIN photodiode which allows an optical control signal to actuate the DMM.
Metals with one or more dimensions in the submicron regime are widely used in MEMS devices. Device stresses often exceed the strength of the corresponding bulk material by an order of magnitude and can lead to a variety of mechanical failures. At moderate temperatures, high stresses occur because dislocations are unable to move to relax the stress. This is partly because of an elastic dimensional constraint, but complex dislocation behavior, such as junction formation, annihilation, and nucleation, are also observed. In this report, we present results from analytical models, cellular automata simulations, and large-scale dislocation dynamics simulations of submicron films to examine the relationship between dislocation interactions and material strength. Our results reveal a complex relationship between dislocation interactions and stress inhomogeneity that arises from the stress fields of the dislocations. We show that the stress inhomogeneity increases both the likelihood of interactions and acts to increase the strain hardening rate.