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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Left atrial ligation in the avian embryo as a model for altered hemodynamic loading during early vascular development(Journal of Visualized Experiments, 2023) Department of Mechanical Engineering; Sevgin, Börteçine; Çoban, Merve Nur; Karataş, Faruk; Pekkan, Kerem; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of EngineeringDue to its four-chambered mature ventricular configuration, ease of culture, imaging access, and efficiency, the avian embryo is a preferred vertebrate animal model for studying cardiovascular development. Studies aiming to understand the normal development and congenital heart defect prognosis widely adopt this model. Microscopic surgical techniques are introduced to alter the normal mechanical loading patterns at a specific embryonic time point and track the downstream molecular and genetic cascade. The most common mechanical interventions are left vitelline vein ligation, conotruncal banding, and left atrial ligation (LAL), modulating the intramural vascular pressure and wall shear stress due to blood flow. LAL, particularly if performed in ovo, is the most challenging intervention, with very small sample yields due to the extremely fine sequential microsurgical operations. Despite its high risk, in ovo LAL is very valuable scientifically as it mimics hypoplastic left heart syndrome (HLHS) pathogenesis. HLHS is a clinically relevant, complex congenital heart disease observed in human newborns. A detailed protocol for in ovo LAL is documented in this paper. Briefly, fertilized avian embryos were incubated at 37.5 degrees C and 60% constant humidity typically until they reached Hamburger-Hamilton (HH) stages 20 to 21. The egg shells were cracked open, and the outer and inner membranes were removed. The embryo was gently rotated to expose the left atrial bulb of the common atrium. Pre-assembled micro-knots from 10-0 nylon sutures were gently positioned and tied around the left atrial bud. Finally, the embryo was returned to its original position, and LAL was completed. Normal and LALinstrumented ventricles demonstrated statistically significant differences in tissue compaction. An efficient LAL model generation pipeline would contribute to studies focusing on synchronized mechanical and genetic manipulation during the embryonic development of cardiovascular components. Likewise, this model will provide a perturbed cell source for tissue culture research and vascular biology.Publication Metadata only Impedimetric antimicrobial peptide biosensor for the detection of human immunodeficiency virus envelope protein GP120(Cell Press, 2024) Department of Mechanical Engineering; Uygun, Zihni Onur; Taşoğlu, Savaş; Department of Mechanical Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); KU Arçelik Research Center for Creative Industries (KUAR) / KU Arçelik Yaratıcı Endüstriler Uygulama ve Araştırma Merkezi (KUAR); College of EngineeringThis study presents the design and implementation of an antimicrobial peptide -based electrochemical impedance spectroscopy (EIS) based biosensor system. The biosensor consists of a gold coated carbon electrode with MXene and silver nanoparticles (AgNPs) for the label -free detection of the human immunodeficiency virus (HIV) envelope protein gp120. Scanning electron microscopy was used to confirm the presence and distribution of MXene and AgNPs on the biosensor surface. The employment of the antimicrobial peptide on the electrode surface minimized the denaturation of the biorecognition receptor to ensure reliable and stable performance. The biosensor exhibited a linear range of 10-4000 pg mL-1 for gp120 detection, demonstrating good repeatability in real samples. The limit of detection (LOD) and limit of quantification (LOQ) were also calculated as 0.05 pg mL-1 and 0.14 pg mL-1, respectively. This biosensing platform has promising applications in the detection of HIV in clinical and point -of -care settings./Yönetim kurulu üyelerinin azlini zorlaştıran düzenlemeler, bir taraftan farklı bağlamlarda şirket tüzel kişiliğinin, azınlık pay sahiplerinin ve yönetim kurulu üyelerinin menfaatlerinin korunmasına hizmet edebilmekte; ayrıca ülkemizdeki gibi blok pay sahipliği yapısının yaygın olduğu hukuk sistemlerinde, çoğunluk ve azınlık pay sahipleri arasında kontrol paylaşımını da mümkün kılmaktadır. Diğer taraftan ise, genel kurulun yönetim kurulu üyelerini görevden alma konusundaki mutlak ve devredilemez yetkisini sınırlandırmakta; böylelikle yönetim kurulu üyesi ve şirket arasındaki güvenin sarsıldığı durumlarda ilgili üyenin görevine son verilebilmesi imkânını daraltmaktadır. Öte yandan Türk Hukukunda, genel kurulun yönetim kurulu üyelerini azil yetkisini sınırlandıran düzenlemelerin ortaklık düzeninde geçerli olup olmadığı hususunda bir belirsizlik vardır. Bu çalışmada esas sözleşmedeki bir düzenlemeyle, genel kurulun yönetim kurulu üyelerini görevden alma yetkisinin sınırlandırılmasının mümkün olup olmadığı konusunda iki soruya cevap aranmıştır: Bunlardan biri, böyle bir kararın alınmasının belirli bir süreye yahut haklı sebeplerin veya bazı şartların varlığına bağlanıp bağlanamayacağı; diğeri ise, genel kurul toplantısı ve kararı açısından, özel yetersayılar aranıp aranmayacağıdır. İlk soruya öğretide genellikle olumsuz cevap verilmesine karşılık, ikinci soruya verilen cevap ise olumlu yöndedir. Çalışmada savunduğumuz görüş çerçevesinde, yönetim kurulu üyelerinin azli imkânının haklı sebep, koşul, süre veya yaptırım gibi düzenlemelerle sınırlandırılması mümkün olmamalıdır. Benzer şekilde yönetim kurulu üyelerinin görevden alınması hususunda toplantı ve/veya karar yetersayılarının da kanuna göre ağırlaştırılması imkân dahilinde olmamalıdır. Anılan türdeki düzenlemelerin esas sözleşmeye eklenmesi hâlinde ise, bunların bâtıl olacağının kabulü gerekir.Publication Metadata only Measurement and characteristic analysis of ris-assisted wireless communication channels in sub-6 ghz outdoor scenarios(IEEE, 2023) Lan, Jifeng; Sang, Jian; Zhou, Mingyong; Gao, Boning; Meng, Shengguo; Li, Xiao; Tang, Wankai; Jin, Shi; Cheng, Qiang; Cuit, Tie Jun; Department of Electrical and Electronics Engineering; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; College of EngineeringReconfigurable intelligent surface (RIS)-empowered communication has recently drawn significant attention due to its superior capability in manipulating the wireless propagation environment. However, the channel modeling and measurement of RIS-assisted wireless communication systems in real environment has not been adequately studied. In this paper, we construct a channel measurement system using vector network analyzer (VNA) is used to investigate RIS-assisted wireless communication channel in outdoor scenarios at 2.6 GHz. New path loss (PL) models including angle domain information are proposed by refining the traditional close-in (CI) and floating-intercept (FI) models. In the proposed models, both influences of the distance from transmitter (TX) to RIS and the distance from receiver (RX) to RIS on the PL, are taken into account. In addition, the value of root mean square (RMS) delay spread of RIS-assisted wireless communication is found to be much smaller than that of the traditional non line-of-sight (NLOS) case, implying that RIS provides a virtual line-of-sight (LOS) link.Publication Metadata only Measurement-based characterization of physical layer security for ris-assisted wireless systems(IEEE, 2023) Kesir, Samed; Wikelek, Ibrahim; Pusane, Ali Emre; Gorcin, Ali; Department of Electrical and Electronics Engineering; Kayraklık, Sefa; Başar, Ertuğrul; Department of Electrical and Electronics Engineering; CoreLab; Graduate School of Sciences and Engineering; College of EngineeringThere have been recently many studies demonstrating that the performance of wireless communication systems can be significantly improved by a reconfigurable intelligent surface (RIS), which is an attractive technology due to its low power requirement and low complexity. This paper presents a measurement-based characterization of RISs for providing physical layer security, where the transmitter (Alice), the intended user (Bob), and the eavesdropper (Eve) are deployed in an indoor environment. Each user is equipped with a software-defined radio connected to a horn antenna. The phase shifts of reflecting elements are software controlled to collaboratively determine the amount of received signal power at the locations of Bob and Eve in such a way that the secrecy capacity is aimed to be maximized. An iterative method is utilized to configure a Greenerwave RIS prototype consisting of 76 passive reflecting elements. Computer simulation and measurement results demonstrate that an RIS can be an effective tool to significantly increase the secrecy capacity between Bob and Eve.Publication Metadata only Sculpture surface machining: a generalized model of ball-end milling force system(Elsevier Sci Ltd, 2003) N/A; Department of Mechanical Engineering; Lazoğlu, İsmail; Faculty Member; Department of Mechanical Engineering; College of Engineering; 179391A new mechanistic model is presented for the prediction of a cutting force system in ball-end milling of sculpture surfaces. The model has the ability to calculate the workpiece/cutter intersection domain automatically for a given cutter location (CL) file, cutter and workpiece geometries. Furthermore, an analytical approach is used to determine the instantaneous chip load (with and without runout) and cutting forces. In addition to predicting the cutting forces, the model also employs a Boolean approach for a given cutter, workpiece geometries, and CL file to determine the surface topography and scallop height variations alone, the workpiece surface which can be visualized in 3-D. The results of model validation experiments on machining Ti-6A1-4V are also reported. Comparisons of the predicted and measured forces as well as surface topography show good agreement.Publication Metadata only A computational study of drop formation in an axisymmetric flow-focusing device(Amer Soc Mechanical Engineers, 2006) Department of Mechanical Engineering; Department of Mechanical Engineering; Filiz, İsmail; Muradoğlu, Metin; N/A; Faculty Member; Department of Mechanical Engineering; College of Engineering; College of Engineering; N/A; 46561We investigate the formation and dynamics of drops computationally in an axisymetric geometry using a Front-Tracking/Finite-Difference (FT/FD) method. The effects of viscosity ratio between inner and outer liquids on the drop creation process and drop size distribution are examined. It is found that the viscosity ratio critically influences the drop formation process and the final drop distribution. We found that, for small viscosity ratios, i.e., 0.1 < lambda < 0.5 drop size is about the size of the orifice and drop distribution is highly monodisperse. When viscosity ratio is increased, i.e., 0.5 < lambda < I a smaller drop is created just after the main drop. For even higher viscosity ratios, the drop distribution is usually monodisperse but a satellite drop is created in some cases. The effect of the flow rates in the inner jet and the co flowing annulus are also studied. It is found that the drop size gets smaller as Q(in) / Q(out) is reduced while keeping the outer flow rate constant.Publication Metadata only Dimensional stability of 51CrV4 steel during bainitic phase transformation under tensile and compressive stresses(Elsevier, 2010) Lambers H.-G.; Maier H.J.; Department of Mechanical Engineering; Canadinç, Demircan; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433The effect of the prior austenitization treatment on the stress-strain response of a low alloy 51CrV4 steel in its supercooled austenitic state and its effect on the evolution of transformation plasticity strain during a subsequent isothermal bainitic transformation was investigated. One of the key findings is that the prior austenitization treatment strongly affects the evolution of transformation plasticity strains during the subsequent isothermal bainitic transformation under stress, such that higher transformation plasticity strains are present when the austenite grain size is increased. In addition, higher 0.2% offset yield strengths of the supercooled austenite are present following an incomplete austenitization treatment due to the existence of residual carbides. Overall, the current results clearly lay out the effect of the prior austenitization treatment on the stress-strain response of the supercooled austenite and the evolution of transformation plasticity during bainitic transformation.Publication Metadata only On the cyclic stability and fatigue performance of ultrafine-grained interstitial-free steel under mean stress(Trans Tech Publications Ltd, 2008) Niendorf, Thomas; Maier, Hans J.; Karaman, Ibrahim; Department of Mechanical Engineering; Canadinç, Demircan; Faculty Member; Department of Mechanical Engineering; College of Engineering; 23433This paper reports on the fatigue performance of an ultrafine-grained (UFG) interstitial-free (IF) steel deformed at various mean stress levels. The UFG microstructure was achieved using equal channel angular extrusion processing at room temperature (RT) and along an "efficient" route, giving way to the formation of high angle grain boundaries (HAGBs) with a high volume fraction. The current study not only confirms the previous finding that a high volume fraction of HAGBs promotes cyclic stability, but also inquires into the role of mean stress level on the cyclic stability. It is shown that the UFG IF steel exhibits a stable cyclic deformation response in the lowcycle fatigue regime within the medium applied mean stress range of -75 to 75 MPa. The corresponding fatigue lives can still be predicted with the Smith-Watson-Topper approach within this range. Furthermore, the present study demonstrates that the evolution of mean strains with cyclic deformation can be linked to the evolution of mean stresses in strain-controlled loading.Publication Metadata only Flow simulation and optimization of a left ventricular assist device(American Society of Mechanical Engineers (ASME), 2007) Sorguven, Esra; Ciblak, Namik; Okyar, A. Fethi; Akgun, Mehmet A.; Egrican, A. Nilufer; Safak, K. Koray; Ahn, Hojin; Kucukaksu, Suha; Department of Mechanical Engineering; Lazoğlu, İsmail; Faculty Member; Department of Mechanical Engineering; College of Engineering; 179391Artificial assist devices offer a promising treatment option for patients with congestive heart failure, especially when the patient is not eligible for heart transplantation. In order to develop a left ventricular assist device an interdisciplinary research, involving engineering and medical research teams, is conducted. The left ventricular assist device investigated in this study is the MicroMed DeBakey VAD [1], an axial blood pump that provides flow from the left ventricle to the aorta. The geometry of this baseline design is generated via parametric modeling. An optimization surface around the baseline design is formed by using the design of experiments method. Accordingly, eighty parameter sets and the corresponding CAD models are created. Flow through these pumps is simulated at the operation point. Flow data are evaluated to predict the pump performance, blood damage and bearing friction. An axial pump, closer to the optimum, is found that provides 8635 Pa pressure increase at a flow rate of 6 l/min and a rotational speed of 10000 rpm. Pressure head of the selected pump is 18% higher and blood damage is 4% less than the baseline design.Publication Metadata only Design methodology microelectromechanical systems. Case study: torsional scanner mirror(Asme-Amer Soc Mechanical Eng, 2007) N/A; N/A; Department of Mechanical Engineering; Meral, Faik Can; Başdoğan, İpek; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179940Future optical microsystems, such as microelectromechanical system (MEMS) scanners and micromirrors, will extend the resolution and sensitivity offered by their predecessors. These systems face the challenge of achieving nanometer precision subjected to various disturbances. Predicting the performance of such systems early in the design process can significantly impact the design cost and also improve the quality, of the design. Our approach aims to predict the performance of such systems under various disturbance sources and develop a generalized design approach for MEMS structures. In this study, we used ANSYS for modeling and dynamic analysis of a torsional MEMS scanner mirror. ANSYS modal analysis results, which are eigenvalues (natural frequencies) and eigenvectors (mode shapes), are used to obtain the state-space representation of the mirror. The state-space model of the scanner mirror was reduced using various reduction techniques to eliminate the states that are insignificant for the transfer functions of interest. The results of these techniques were compared to obtain the best approach to obtain a lower order model that still contains all the relevant dynamics of the original model. After the model size is reduced significantly, a disturbance analysis is performed using Lyapunov approach to obtain root-mean-square values of the mirror rotation angle under the effect of a disturbance torque. The magnitude levels of the disturbance torque are obtained using an experimental procedure. The disturbance analysis framework is combined with the sensitivity analysis to determine the critical design parameters for optimizing the system performance.