1,721,066 research outputs found
Magnetorheological elastomers characterization under shear loading up to failure: A magneto-mechanical multivariate analysis
Full text linkThis work analyses the shear behavior of magnetorheological elastomers (MRE), a class of smart materials which presents interesting magneto-mechanical properties. In order to determine the effect of several variables at a time, a design of experiment approach is adopted. A set of several samples of MRE was manufactured, by varying the weight fraction of ferromagnetic material inside the viscoelastic matrix and the isotropicity of the material, by adding an external magnetic field while the elastomeric matrix was still liquid. The mechanical behavior of each sample was analyzed by conducting cyclic tests at several shear rates, both with and without an external magnetic field. Moreover, in order to estimate the maximum shear stress, the specimens were loaded monotonically up to failure. Shear stiffness, maximum shear stress and specific dissipated energy were calculated on the basis of the experimental data. The results were analyzed using an Analysis of Variance (ANOVA) to assess the statistical influence of each variable. The experimental results highlighted a strong correlation between the weight fraction of ferromagnetic material in each sample and its mechanical behavior. Moreover, the dissipated energy of the MRE drops down when the magnetic field stiffens the behavior or the shear rate increases. The ultimate failure shear stress is strongly affected by the external magnetic field, increasing it by nearly 50%. The ANOVA on the results provides a simple phenomenological model is built for each output variable and it is compared with the experimental tests. These models produce a fast and fairly accurate prediction of each analyzed response of the MRE under various shear rates and applied magnetic fields
Control of oxygen affinity in mammalian hemoglobins: Implications for a system biology description of the respiratory properties of the red blood cell
No full textL'emoglobina e la mioglobin sono state considerate i modelli paradigmatici della funzione delle proteine, al punto da essere stati definiti "gli atomi di idrogeno della biologia" [Frauenfelder et al. Proc. Natl. Acad. Sci. USA, 2003; 100, 8615-8617]. Data questa posizione privilegiata e l'enorme mole di informazione disponibile su queste proteine, il globulo rosso potrebbe apparire come il modello e "l'atomo di idrogeno" della biologia dei sistemi cellulari. Infatti, essendo il trasporto dell'O2 la principale funzione del globulo rosso, la distanza che separa la proteina (emoglobina) e la cellula potrebbe apparire modesta. Ciononostante una grande quantità di informazione biochimica è necessaria per modellizzare le proprietà respiratorie dell'eritrocita. Questo problema è aumentato se la modellizzazione si prefigge lo scopo di scoprire o spiegare proprietà dell'emoglobine selezionate dall'evoluzione. La difficoltà principale risiede nel fatto che emoglobine dotate di proprietà funzionali e storie evoluzionistiche diverse potrebbero comportarsi in modo simile nel complesso ambiente del sangue, mentre emoglobine simili che condividono una sostanziale omologia di sequenza possono presentare importanti differenze funzionali a causa della mutazione di pochi residui critici. Quindi, la proprietà funzionali dell'emoglobina e del sangue possono riflettere più strettamente adattamenti ambientali recenti piuttosto che la storia evoluzionistica dell'animale. In questa review noi analizziamo il caso delle emoglobine di mammiferi in un tentativo di fornire un sommario della loro complessità che, noi speriamo, possa essere di aiuto per gli scienziati interessati alla esplorazione quantitativa dell'evoluzione della funzione respiratoria. Alla base di ogni modellizzazione di successo deve infatti trovarsi un grande corpus di informazione raccolta in faticose e spesso dimenticati studi sulle proprietà biochimiche dell'emoglobina condotti in un intervallo di tempo di oltre cento anni.Hemoglobin and myoglobin have been considered for a long time the paradigmatic model systems for protein function, to the point of being defined the “hydrogen atom[s] of biology” [Frauenfelder et al. Proc. Natl. Acad. Sci. USA, 2003; 100, 8615-8617]. Given this privileged position and the huge amount of quantitative information available on these proteins, the red blood cell might appear as the model system and“hydrogen atom” of system biology. Indeed, since the red cell's main function is O2 transport by hemoglobin, the gap between the protein and the cell may appear quite small. Yet, a surprisingly large amount of detailed biochemical information is required for the modelization of the respiratory properties of the erythrocyte. This problem is compounded if modelization aims at uncovering or explaining evolutionarily selected functional properties of hemoglobin. The foremost difficulty lies in the fact that hemoglobins having different intrinsic properties and relatively ancient evolutionary divergence may behave similarly in the complex milieu of blood, whereas very similar hemoglobins sharing a substantial sequence similarity may present important functional differences because of the mutation of a few key residues. Thus, the functional properties of hemoglobin and blood may reflect more closely the recent environmental challenges than the remote evolutionary history of the animal. We summarize in this review the case of hemoglobins from mammals, in an attempt to provide a reasoned summary of their complexity that, we hope, may be of help to scientists interested in the quantitative exploration of the evolutionary physiology of respiration. Indeed the basis of a meaningful modelization of the red cell requires a large amount of information collected in painstaking and often forgotten studies of the biochemical properties of hemoglobin carried out over more than a century
Magneto-mechanical characterization of magnetorheological elastomers
Full text linkThis work analyses the properties and the magneto-mechanical characteristics of magnetorheological elastomers, a class of smart materials not yet broadly investigated. First, set of several samples of this material was manufactured, each one characterized by a different percentage of ferromagnetic material inside the viscoelastic matrix. The specimens were manufactured in order to create isotropic and anisotropic configurations, respectively, with randomly dispersed ferromagnetic particles or with an aligned distribution, obtained through and external magnetic field. Then, the mechanical behaviour of each sample was analysed by conducting a compression test, both with and without an external magnetic field. Moreover, a three-point bending test was also performed on the same specimens. Stiffness, deformation at maximum stress and specific energy dissipated were calculated based on the experimental data. The results were analysed considering the mechanical responses, and an analysis of variance was carried out in order to assess the statistical influence of each variable. The experimental results highlighted a strong correlation between the percentage of ferromagnetic material in each sample and its mechanical behaviour. The anisotropicity of the material, aligned in columnar structures, also affects the stiffness measured in the compression test, while the external magnetic field’s main contribution is to reduce the samples’ maximum deformation. Using analysis of variance results as guidelines, we built a simple phenomenological model which produces quite reliable predictions regarding the mechanical response of the magnetorheological elastomers under compressive stress
Shear behaviour of magnetorheological elastomers: Viscoelastic and magnetorheological properties
No full textDesign and characterization of a magnetorheological elastomer linear actuator for low-frequency applications
Full text linkThis work presents the design and characterization of an innovative linear actuator for low-frequency applications based on a magnetorheological elastomer (MRE) disc coupled to an electromagnet. MREs are a class of smart materials in which micrometre-sized magnetic particles are suspended in an elastomeric matrix. Most research works study their applicability as semi-active systems, but less effort is devoted to their applicability in actuators, but their applicability in the field is possible and could lead to potential advantages in terms of integration of the system especially for microactuation. The proposed MRE device relies on a commercial electromagnet which provides linear motion of the MRE element. The stiffness of the elastomeric matrix is exploited to bring the system back to its initial position, so that the system is monostable. The magneto-mechanical behaviour is modelled both analytically and by means of finite element magneto-mechanical simulations, and the models are compared with the experimental tests. Two membrane thicknesses and two different gaps between the membrane and the electromagnetic actuator were manufactured and characterized. The results show the effect of the design variable on the actuator behaviour and confirm that the analytical model provided can predict the actuator's behaviour with a good approximation in all the configuration analysed. The dynamic range of the proposed system, regardless of the configuration selected, demonstrates that the magnetic contribution is always able to increase the actuator force by 50% and that the provided model can easily be used as a reliable design tool for this kind of smart system
Hemoglobin allostery and pharmacology
No full textThe oxygen demands of the human body require the constant circulation of blood carrying an enormous concentration of hemoglobin (Hb). Oxygen transport depends not only on the amount of Hb, but also on the control over the affinity of the protein for the gas, which can be optimized for the environmental conditions by changes in the concentration of effectors (hydrogen ions, chloride, CO2, and DPG) inside the red cell. Some pathological conditions affecting Hb may benefit from pharmacological interventions to increase or decrease its affinity for oxygen, or otherwise modify its properties, or alter its biosynthesis. Examples of such conditions include sickle cell anemia, thalassemias and inherited hemoglobinopathies. Effective and safe drugs such as voxelotor, bezafibrate and efaproxiral are available that significantly increase or decrease Hb oxygen affinity. Some medical conditions not directly affecting the blood or its oxygen carrying capacity may also be relieved by the manipulation of Hb. For example, the standard treatment of acute cyanide poisoning requires the oxidation of a fraction of the Hb in the bloodstream so that it efficiently scavenges cyanide. Tumors are often extremely hypoxic and therefore strongly resistant to radiotherapy; the sensitivity of cancerous tissue to X-rays may be increased by improved oxygenation through drugs binding Hb. This review attempts to provide a systematic exploration of the pharmacology of Hb, its molecular basis, and its intended and possible uses
Self-expanding Nitinol stents for endovascular peripheral applications: A review
Full text linkPeripheral arterial diseases affect a significant portion of the global population, fostering research to find innovative and effective solutions to improve people's life. A primary focus for researchers and manufacturers is the continuous improvement of the most important, non-surgical treatment for this pathology, the endovascular stent. This device is the main feature enabling a lifesaving technique: the percutaneous vascular interventions. Stents are vital for restoring blood flow and enhancing long-term vessel patency, they are available in various materials, shapes and typologies. Recent advancements in stent design, particularly through additive manufacturing, create new opportunities for optimizing the device performance and possibly opening new areas of intervention. This review provides a detailed quantitative analysis on the most widely used category of devices: self-expanding stents made of Nitinol, a nickel-titanium alloy that shows a superelastic behavior. A set of figures of merit related to stent design are described and analyzed, with a focus on the influence of geometry on mechanical performance. Additionally, a comprehensive comparative analysis of the commercial stents evaluates the geometry and performance of many commercial solutions, including both arterial and venous types. This analysis offers quantitative tools to assist surgeons and designers in selecting the most important features of a stent with respect to its main application. To conclude this work, an overview of future manufacturing possibilities is provided mainly focusing on the additive manufacturing technology. The freedom of shape given by this method opens up new paths in terms of global shapes, strut geometry and sizes, revealing new avenues which point strongly towards ad-hoc and specifically patient-customized stent design
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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