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Item Metadata only 3D-printed microrobots: translational challenges(MDPI, 2023) 0000-0003-4604-217X; 0000-0002-5295-5701; 0000-0003-0519-4513; Yetisen, Ali K.; Department of Mechanical Engineering; N/A; N/A; Taşoğlu, Savaş; Sarabi, Misagh Rezapour; Karagöz, Ahmet Agah; Faculty Member; PhD Student; PhD Student; KU Arçelik Research Center for Creative Industries (KUAR) / KU Arçelik Yaratıcı Endüstriler Uygulama ve Araştırma Merkezi (KUAR); Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Koç Üniversitesi İş Bankası Yapay Zeka Uygulama ve Araştırma Merkezi (KUIS AI)/ Koç University İş Bank Artificial Intelligence Center (KUIS AI); College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 291971; N/A; N/AThe science of microrobots is accelerating towards the creation of new functionalities for biomedical applications such as targeted delivery of agents, surgical procedures, tracking and imaging, and sensing. Using magnetic properties to control the motion of microrobots for these applications is emerging. Here, 3D printing methods are introduced for the fabrication of microrobots and their future perspectives are discussed to elucidate the path for enabling their clinical translation.Item Metadata only Bioprinting in microgravity(Amer Chemical Soc, 2023) 0000-0003-4604-217X; 0000-0002-5295-5701; Yetisen, Ali K.; Department of Mechanical Engineering; N/A; Taşoğlu, Savaş; Sarabi, Misagh Rezapour; Faculty Member; PhD Student; KU Arçelik Research Center for Creative Industries (KUAR) / KU Arçelik Yaratıcı Endüstriler Uygulama ve Araştırma Merkezi (KUAR); Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); College of Engineering; Graduate School of Sciences and Engineering; 291971; N/ABioprinting as an extension of 3D printing offers capabilities for printing tissues and organs for application in biomedical engineering. Conducting bioprinting in space, where the gravity is zero, can enable new frontiers in tissue engineering. Fabrication of soft tissues, which usually collapse under their own weight, can be accelerated in microgravity conditions as the external forces are eliminated. Furthermore, human colonization in space can be supported by providing critical needs of life and ecosystems by 3D bioprinting without relying on cargos from Earth, e.g., by development and long-term employment of living engineered filters (such as sea sponges-known as critical for initiating and maintaining an ecosystem). This review covers bioprinting methods in microgravity along with providing an analysis on the process of shipping bioprinters to space and presenting a perspective on the prospects of zero-gravity bioprinting.Item Metadata only Cardiac magnetic resonance T2* mapping in patients with COVID-19 pneumonia is associated with serum ferritin level?(Springer Science and Business Media B.V., 2023) 0000-0001-7637-4445; 0000-0002-2176-5278; Ciftci, Hatice Ozge; Keles, Nursen; Karatas, Mesut; Parsova, Kemal Emrecan; Kahraman, Erkan; Durak, Furkan; Kocogulları, Cevdet Ugur; Yiyit, Nurettin; Department of Mechanical Engineering; N/A; Pekkan, Kerem; Özkök, Serçin; Faculty Member; PhD Student; College of Engineering; Graduate School of Sciences and Engineering; 161845; N/AThe coronavirus disease of 2019 (COVID-19)-related myocardial injury is an increasingly recognized complication and cardiac magnetic resonance imaging (MRI) has become the most commonly used non-invasive imaging technique for myocardial involvement. This study aims to assess myocardial structure by T2*-mapping which is a non-invasive gold-standard imaging tool for the assessment of cardiac iron deposition in patients with COVID-19 pneumonia without significant cardiac symptoms. Twenty-five patients with COVID-19 pneumonia and 20 healthy subjects were prospectively enrolled.Cardiac volume and function parameters, myocardial native-T1, and T2*-mapping were measured. The association of serum ferritin level and myocardial mapping was analyzed. There was no difference in terms of cardiac volume and function parameters. The T2*-mapping values were lower in patients with COVID-19 compared to controls (35.37 [IQR 31.67–41.20] ms vs. 43.98 [IQR 41.97–46.88] ms; p < 0.0001), while no significant difference was found in terms of native-T1 mapping value(p = 0.701). There was a positive correlation with T2*mapping and native-T1 mapping values (r = 0.522, p = 0.007) and negative correlation with serum ferritin values (r = − 0.653, p = 0.000), while no correlation between cardiac native-T1 mapping and serum ferritin level. Negative correlation between serum ferritin level and T2*-mapping values in COVID-19 patients may provide a non-contrast-enhanced alternative to assess tissue structural changes in patients with COVID-19. T2*-mapping may provide a non-contrast-enhanced alternative to assess tissue alterations in patients with COVID-19. Adding T2*-mapping cardiac MRI in patients with myocardial pathologies would improve the revealing of underlying mechanisms. Further in vivo and ex vivo animal or human studies designed with larger patient cohorts should be planned. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.Item Metadata only Comparison of constant and variable discharge flow and force coefficients for reciprocating compressor(Elsevier Sci Ltd, 2023) 0000-0002-8316-9623; 0000-0002-7638-3189; 0000-0002-3511-3887; Karabay, Ahmet Yasin; Sahin, Caglar; Department of Mechanical Engineering; N/A; N/A; Lazoğlu, İsmail; Pashak, Pouya; Malik, Anjum Naeem; Faculty Member; PhD Student; PhD Student; Manufacturing and Automation Research Center (MARC); College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 179391; N/A; N/AEnhancing the mathematical models always have been a critical issue in every field, and the reciprocating compressors are not exceptional. Flow and force coefficients are two empirical factors that help to calculate the mass flow rate through the valve and gas force, respectively. These coefficients can be considered constant values or valve lift dependent. However, in the discharge process, the piston is close to the top dead center, which may affect the discharge flow field and, subsequently, the coefficients.These coefficients should be determined by experiments or numerical simulations such as computational fluid dynamics (CFD). Experiments are expensive, and the exact geometry may not always be available to perform the CFD analysis. Also, CFD analysis for various valve lifts and piston positions is computationally expensive.This study studied the effect of constant, single variable (valve lift dependent) and double variable (valve lift and piston position dependent) coefficients on compressor performance and discharge valve movement in three operating conditions. A mathematical model was developed for this purpose, and a 3-D CFD simulation was used to compute the coefficients. To validate the results, a strain gauge and a calorimeter were used as measurement tools.The single and double variable models captured the valve behavior, and pressure picks better than the constant model. However, the constant model estimated the indicated power, cooling capacity, and COP with good accuracy. In predicting the discharge losses, the double variable model was the most reliable among the others.Item Metadata only Experimental and numerical analysis of two different bottom pumps of a hermetic reciprocating compressor at low speeds(Elsevier Sci Ltd, 2023) 0000-0002-8316-9623; 0000-0002-7638-3189; 0000-0003-3891-3684; 0000-0002-3378-3350; Department of Mechanical Engineering; N/A; N/A; N/A; Lazoğlu, İsmail; Pashak, Pouya; Shahzad, Aamir; Abdülhamid, Farouk; Faculty Member; PhD Student; PhD Student; PhD Student; Manufacturing and Automation Research Center (MARC); College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 179391; N/A; N/A; N/AOperating at low speeds (< 2000 rpm) is critical in achieving optimal energy efficiency for inverter-type hermetic reciprocating compressors. However, ensuring the efficient delivery of lubrication oil to the compressor components at such low speeds is challenging. The hermetic reciprocating compressor crankshaft utilizes a machined helical channel on the outer surface to provide lubrication oil to the upper bearings. The lubrication oil is provided to this channel's inlet from the compressor's oil sump by an eccentric internal channel in the bottom of the crankshaft. However, this design cannot deliver the oil to the helical channel under low-speed conditions. This research investigates a screw pump, the smooth inner crankshaft wall with threaded pin inside (SC-TP) configuration, and compares its performance numerically and experimentally with a conventional double eccentric internal channel pump (DEC) design. For this purpose, computational fluid dynamics (CFD) is used. For the simulation validation, the pumps are fabricated from transparent plexiglass. Also, two sets of solvers for two-phase flow, i.e., the implicit volume of fluid model (VOF) with Compressive scheme and explicit VOF model with Geo-Reconstruct scheme, are used to examine the methods' accuracy. When running above 1400 rpm, the DEC pump delivers faster and more oil than the screw pump. However, while operating under 1400, the screw pump provides oil until 800 rpm, which the DEC pump cannot do. This research shows how shear force dominates centrifugal force during low-speed pumping situations and also investigates how different viscosities impact performance.Item Metadata only Simplified top-down fabrication of sub-micron silicon nanowires(IOP Publishing Ltd, 2023) 0000-0002-2712-1908; N/A; N/A; 0000-0001-5931-8134; N/A; N/A; N/A; Department of Mechanical Engineering; Karimzadehkhouei, Mehrdad; Akıncı, Seçkin; Zare Pakzad, Sina; Alaca, Burhanettin Erdem; Researcher; Master Student; PhD Student; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced Research; N/A; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; N/A; 115108Silicon nanowires are among the most promising nanotechnology building blocks in innovative devices with numerous applications as nanoelectromechanical systems. Downscaling the physical size of these devices and optimization of material functionalities by engineering their structure are two promising strategies for further enhancement of their performance for integrated circuits and future-generation sensors and actuators. Integration of silicon nanowires as transduction elements for inertial sensor applications is one prominent example for an intelligent combination of such building blocks for multiple functionalities within a single sensor. Currently, the efforts in this field are marred by the lack of batch fabrication techniques compatible with semiconductor manufacturing. Development of new fabrication techniques for such one-dimensional structures will eliminate the drawbacks associated with assembly issues. The current study aims to explore the limits of batch fabrication for a single nanowire within a thick Si layer. The objective of the current work goes beyond the state of the art with significant improvements to the recent viable approach on the monolithic fabrication of nanowires, which was based on a conformal side-wall coating for the protection of the nanoscale silicon line followed by deep etch of the substrate transforming the protected layer into a silicon nanowire. The newly developed fabrication approach eliminates side wall protection and thereby reduces both process complexity and process temperature. The technique yields promising results with possible improvements for future micro and nanofabrication processes.Item Metadata only Innovative MEMS stage for automated micromechanical testing(Institute of Electrical and Electronics Engineers Inc., 2023) 0000-0002-2712-1908; 0000-0003-2063-1566; N/A; 0000-0001-5931-8134; N/A; N/A; N/A; N/A; Department of Mechanical Engineering; N/A; Karimzadehkhouei, Mehrdad; Ali, Basit; Zare Pakzad, Sina; Alaca, Burhanettin Erdem; Çoban, Semih Berk; Researcher; PhD Student; PhD Student; Faculty Member; PhD Student; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced Research; N/A; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; Graduate School of Sciences and Engineering; N/A; N/A; N/A; 115108; N/AThis study introduces a comprehensive methodology for designing, fabricating, and testing a MEMS stage integrated into a commercial testing device, with a focus on enabling automated testing of multiple samples under in-plane loading conditions. Drawing inspiration from recent innovative MEMS stage designs, a new approach is developed to integrate micromanipulator tips into a commercial micro-mechanical testing machine, allowing for automated one-directional loading of micro-scale samples. To address challenges related to handling and alignment, a co-fabrication technique is employed, enabling the simultaneous fabrication of the micro-sample and MEMS stage within a single process flow. A novel fabrication method utilizing a silicon-on-insulator substrate is utilized. The calibration of testing method is conducted using both analytical and experimental methods to ensure accurate measurement of force and deflection within the in-plane testing protocol. The released micro-beam structures undergo repetitive loading to evaluate bending deformation. The developed approach is extended to multiple testing attempts on MEMS stage-micro-sample, combinations co-fabricated within a single chip, enabling precise statistical treatment of the measurements. © 2023 IEEE.Item Metadata only Nanomechanical modeling of the bending response of silicon nanowires(Amer Chemical Soc, 2023) 0000-0001-5931-8134; N/A; 0000-0002-0795-8970; Nasr Esfahani, Mohammad; Tasdemir, Zuhal; Wollschla''ger, Nicole; Li, Taotao; Li, XueFei; Leblebici, Yusuf; Alaca,; Department of Mechanical Engineering; N/A; N/A; Alaca, Burhanettin Erdem; Zare Pakzad, Sina; Yılmaz, Mustafa Akın; Faculty Member; PhD Student; PhD Student; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced Research; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 115108; N/A; N/AUnderstanding the mechanical behavior of silicon nanowiresis essentialfor the implementation of advanced nanoscale devices. Although bendingtests are predominantly used for this purpose, their findings shouldbe properly interpreted through modeling. Various modeling approachestend to ignore parts of the effective parameter set involved in therather complex bending response. This oversimplification is the mainreason behind the spread of the modulus of elasticity and strengthdata in the literature. Addressing this challenge, a surface-basednanomechanical model is introduced in this study. The proposed modelconsiders two important factors that have so far remained neglecteddespite their significance: (i) intrinsic stresses composed of theinitial residual stress and surface-induced residual stress and (ii)anisotropic implementation of surface stress and elasticity. The modelingstudy is consolidated with molecular dynamics-based study of the nativeoxide surface through reactive force fields and a series of nanoscalecharacterization work through in situ three-pointbending test and Raman spectroscopy. The treatment of the test datathrough a series of models with increasing complexity demonstratesa spread of 85 GPa for the modulus of elasticity and points to theorigins of ambiguity regarding silicon nanowire properties, whichare some of the most commonly employed nanoscale building blocks.A similar conclusion is reached for strength with variations of upto 3 GPa estimated by the aforementioned nanomechanical models. Preciseconsideration of the nanowire surface state is thus critical to comprehendingthe mechanical behavior of silicon nanowires accurately. Overall,this study highlights the need for a multiscale theoretical frameworkto fully understand the size-dependent mechanical behavior of siliconnanowires, with fortifying effects on the design and reliability assessmentof future nanoelectromechanical systems.Item Metadata only The role of native oxide on the mechanical behavior of silicon nanowires(Elsevier, 2023) 0000-0001-5931-8134; N/A; Esfahani, Mohammad Nasr; Department of Mechanical Engineering; N/A; Alaca, Burhanettin Erdem; Zare Pakzad, Sina; Faculty Member; PhD Student; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced Research; College of Engineering; Graduate School of Sciences and Engineering; 115108; N/AMolecular dynamics simulations are employed to study the effect of native oxide on the size-dependent mechanical properties of silicon nanowires. Despite their immense potential as essential building blocks in nanoelectromechanical systems, mechanical behavior of silicon nanowires still needs further attention for a full understanding. The leading source of ambiguity can be traced back to the fact that the presence of native oxide on silicon nanowire surfaces is ignored when interpreting nanomechanical test data, when it comes, for example, to converting force and deflection measurements to stress and strain. This problem needs immediate attention, because, first, nanowires have a significant surface area, and second, native oxide is the dominant surface state. With prior work reporting conflicting dimensional and computational viewpoints regarding the effect of native oxide on silicon nanowires properties, size dependence of nanowire mechanical properties is investigated here with great attention placed on critical size and atomistic simulation perspectives. For this purpose, Tersoff-Munetoh and modified Stillinger-Weber potentials are employed in this intensive computational study to address the influence of size and crystal orientation on nanowire elastic behavior and tensile strength. As a result, a striking set of differences is obtained. First, the presence of native oxide layer is observed to decrease both the modulus of elasticity and the ultimate strength. The reduction in the modulus of elasticity is observed to be as much as 30% and 40% for < 100 > and < 110 >-oriented nanowires, respectively. Similarly, the reduction in the ultimate strength is estimated to be as much as 20% using the modified Stillinger-Weber potential, which proved to be more suitable for strength analysis compared to Tersoff-Munetoh potential. Finally, the failure behavior is studied through the ductile failure probability calculations, where a higher size-dependent failure probability is observed for decreasing nanowire width upon oxidation. These results shed light on the background of existing inconsistencies between experimental and numerical findings in the literature, as opposing trends for silicon nanowire stiffness and strength were reported with decreasing size. The study provides a guideline to quantify the scale effect in silicon nanowire mechanical behavior as a combined outcome of oxide thickness, nanowire size and crystal orientation and thus to reduce the extent of uncertainties originating from inadequate interpretation of nanomechanical test data.Item Metadata only Surgical and transcatheter pulmonary valve replacement in patients with repaired tetralogy of fallot: cardiac magnetic resonance imaging characteristics and morphology of right ventricular outflow tract(Springer, 2023) 0000-0002-2176-5278; 0000-0001-7637-4445; Ciftci, Hatice Ozge; Kose, Kevser Banu; Yucel, Ilker Kemal; Sasmazel, Ahmet; Celebi, Ahmet; N/A; Department of Mechanical Engineering; Özkök, Serçin; Pekkan, Kerem; PhD Student; Faculty Member; Graduate School of Sciences and Engineering; College of Engineering; N/A; 161845BackgroundPulmonary valve replacement is recommended in patients with repaired tetralogy of Fallot based on cardiac magnetic resonance imaging (MRI) criteria. This procedure is performed by surgical or transcatheter approaches.ObjectiveWe aimed to investigate the differences in preprocedural MRI characteristics (volume, function, strain) and morphology of the right ventricular outflow tract and branch pulmonary arteries in patients for whom surgical or transcatheter pulmonary valve replacement was planned.Materials and methodsCardiac MRI of 166 patients with tetralogy of Fallot were analyzed. Of these, 36 patients for whom pulmonary valve replacement was planned were included. Magnetic resonance imaging characteristics, right ventricular outflow tract morphology, branch pulmonary artery flow distribution and diameter were compared between surgical and transcatheter groups. Spearman correlation and Kruskal-Wallis tests were performed.ResultsCircumferential and radial MRI strain for the right ventricle were lower in the surgical group (P=0.045 and P=0.046, respectively). The diameter of the left pulmonary artery was significantly lower (P=0.021) and branch pulmonary artery flow and diameter ratio were higher (P=0.044 and P = 0.002, respectively) in the transcatheter group. There was a significant correlation between right ventricular outflow tract morphology and right ventricular end-diastolic volume index and global circumferential and radial MRI strain (P=0.046, P=0.046 and P= 0.049, respectively).ConclusionPreprocedural MRI strain, right-to-left pulmonary artery flow, diameter ratio and morphological features of the right ventricular outflow tract were significantly different between the two groups. A transcatheter approach may be recommended for patients with branch pulmonary artery stenosis, since both pulmonary valve replacement and branch pulmonary artery stenting can be performed in the same session.