Research Outputs

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    A computational biomechanical investigation of posterior dynamic instrumentation: combination of dynamic rod and hinged (dynamic) screw
    (Asme, 2014) Kiapour, Ali; Goel, Vijay K.; N/A; N/A; Erbulut, Deniz Ufuk; Öktenoğlu, Bekir Tunç; Özer, Ali Fahir; Researcher; Faculty Member; School of Medicine, College of Engineering; School of Medicine; 37661; 220898; 1022
    Currently, rigid fixation systems are the gold standard for degenerative disk disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization (PDS) is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semirigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiskal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4-L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DSDR with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiskal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and RSDR) except in the RSDR model in extension where the motion was higher than intact by 9.7% at L3-L4 and 11.3% at L5-S1. The facet loads were insignificant, not exceeding 12N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.
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    A microstructure-sensitive model for simulating the impact response of a high-manganese austenitic steel
    (Asme, 2016) N/A; N/A; Department of Mechanical Engineering; Mirzajanzadeh, Morad; Canadinç, Demircan; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Engineering; N/A; 23433
    Microstructurally informed macroscopic impact response of a high-manganese austenitic steel was modeled through incorporation of the viscoplastic self-consistent (VPSC) crystal plasticity model into the ANSYS LS-DYNA nonlinear explicit finite-element (FE) frame. Voce hardening flow rule, capable of modeling plastic anisotropy in microstructures, was utilized in the VPSC crystal plasticity model to predict the micromechanical response of the material, which was calibrated based on experimentally measured quasi-static uniaxial tensile deformation response and initially measured textures. Specifically, hiring calibrated Voce parameters in VPSC, a modified material response was predicted employing local velocity gradient tensors obtained from the initial FE analyses as a new boundary condition for loading state. The updated micromechanical response of the material was then integrated into the macroscale material model by calibrating the Johnson-Cook (JC) constitutive relationship and the corresponding damage parameters. Consequently, we demonstrate the role of geometrically necessary multi-axial stress state for proper modeling of the impact response of polycrystalline metals and validate the presented approach by experimentally and numerically analyzing the deformation response of the Hadfield steel (HS) under impact loading.
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    A variable elastance-based mock circulation model for replicating human cardiovascular system
    (Wichtig Editore, 2011) Küçükaksu, S.; Şahin, Tuğcan; Department of Mechanical Engineering; Lazoğlu, İsmail; Faculty Member; Department of Mechanical Engineering; College of Engineering; 179391
    N/A
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    Access pattern-aware data placement for hybrid DRAM/NVM
    (TUBITAKScientific and Technical Research Council Turkey, 2017) Department of Computer Engineering; Erten, Didem Unat; Faculty Member; Department of Computer Engineering; College of Engineering; 219274
    in recent years, increased interest in data-centric applications has led to an increasing demand for large capacity memory systems. Nonvolatile memory (NVM) technologies enable new opportunities in terms of process-scaling and energy consumption, and have become an attractive memory technology that serves as a secondary memory at low cost. However, NVM has certain disadvantages for write references, due to its high dynamic energy consumption for writes and low bandwidth compared to DRaM writes. in this paper, we propose an access-aware placement of objects in the application code for two types of memories. Given the desired power savings and acceptable performance loss, our placement algorithm suggests candidate variables for NVM. We present an evaluation of the proposed technique on two applications and study the energy and performance consequences of different placements.
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    Analysis and pptimization on FlexDPDP: a practical solution for dynamic provable data possession
    (Springer-Verlag Berlin, 2015) N/A; Department of Computer Engineering; Department of Computer Engineering; Esiner, Ertem; Küpçü, Alptekin; Özkasap, Öznur; Master Student; Faculty Member; Faculty Member; Department of Computer Engineering; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; N/A; 168060; 113507
    Security measures, such as proving data integrity, became more important with the increase in popularity of cloud data storage services. Dynamic Provable Data Possession (DPDP) was proposed in the literature to enable the cloud server to prove to the client that her data is kept intact, even in a dynamic setting where the client may update her files. Realizing that variable-sized updates are very inefficient in DPDP (in the worst case leading to uploading the whole file again), Flexible DPDP (FlexDPDP) was proposed. In this paper, we analyze FlexDPDP scheme and propose optimized algorithms. We show that the initial pre-processing phase at the client and server sides during the file upload (generally the most time-consuming operation) can be efficiently performed by parallelization techniques that result in a speed up of 6 with 8 cores. We propose a way of handling multiple updates at once both at the server and the client side, achieving an efficiency gain of 60% at the server side and 90% in terms of the client's update verification time. We deployed the optimized FlexDPDP on the large-scale network testbed PlanetLab and demonstrate the efficiency of our proposed optimizations on multi-client scenarios according to real workloads based on version control system traces.
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    Assessment of oxidant susceptibility of red blood cells in various species based on cell deformability
    (IOS Press, 2011) Simmonds, Michael J.; Meiselman, Herbert J.; Marshall-Gradisnik, Sonya M.; Pyne, Michael; Kakanis, Michael; Keane, James; Brenu, Ekua; Christy, Rhys; Başkurt, Oğuz Kerim; Faculty Member; School of Medicine; N/A
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    PublicationOpen Access
    Attenuation constant measurements of clear glass samples at the low terahertz band
    (Wiley, 2020) Department of Electrical and Electronics Engineering; Yılmaz, Türker; Akan, Özgür Barış; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering
    The technical performance requirements from wireless communication networks are continuously rising. One method to satisfy the demands is increasing the carrier frequency to the millimetre wave or low terahertz band spectrum to utilise wider operation bandwidth. In order to facilitate the studies in this frequency range, the corresponding electromagnetic (EM) wave properties, channel attributes and material characteristics need to be analytically formulated. In line with these, this Letter initially presents the theoretical expressions governing the EM wave transmission across a conducting medium. Then, by using the relative S21 parameter quantities in the proposed attenuation constant (alpha) computation technique, the alpha results of the measurements performed between 260 and 350 GHz for the clear window glass samples of different thicknesses are given. This Letter concludes with the evaluation of the outcomes.
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    Biomechanical comparison of transdiscal fixation and posterior fixation with and without transforaminal lumbar interbody fusion in the treatment of l5-s1 lumbosacral joint
    (Sage Publications Ltd, 2018) Ozalp, Hakan; Ozkaya, Mustafa; Demir, Teyfik; N/A; Yaman, Onur; Doctor; N/A; Koç University Hospital; 219524
    Transdiscal screw fixation is generally performed in the treatment of high-grade L5-S1 spondylolisthesis. The main thought of the study is that the biomechanical performances of the transdiscal pedicle screw fixation can be identical to standard posterior pedicle screw fixations with or without transforaminal lumbar interbody fusion cage insertion. Lumbosacral portions and pelvises of 45 healthy lambs' vertebrae were dissected. Animal cadavers were randomly and equally divided into three groups for instrumentation. Three fixation systems, L5-S1 posterior pedicle screw fixation, L5-S1 posterior pedicle screw fixation with transforaminal lumbar interbody fusion cage insertion, and L5-S1 transdiscal pedicle screw fixation, were generated. Axial compression, flexion, and torsion tests were conducted on test samples of each system. In axial compression, L5-S1 transdiscal fixation was less stiff than L5-S1 posterior pedicle screw fixation with transforaminal lumbar interbody fusion cage insertion. There were no significant differences between groups in flexion. Furthermore, L5-S1 posterior fixation was stiffest under torsional loads. When axial compression and flexion loads are taken into consideration, transdiscal fixation can be alternatively used instead of posterior pedicle screw fixation in the treatment of L5-S1 spondylolisthesis because it satisfies enough stability. However, in torsion, posterior fixation is shown as a better option due to its higher stiffness.
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    PublicationOpen Access
    Blue TiO2 nanotube arrays as semimetallic materials with enhanced photoelectrochemical activity towards water splitting
    (TÜBİTAK, 2020) Department of Chemistry; Peighambardoust, Naeimeh Sadat; Aydemir, Umut; Researcher; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); College of Sciences; N/A; 58403
    In the past years there has been a great interest in self-doped TiO2 nanotubes (blue TiO2 nanotubes) compared to undoped ones owing to their high carrier density and conductivity. In this study, blue TiO2 nanotubes are investigated as photoanode materials for photoelectrochemical water splitting. Blue TiO2 nanotubes were fabricated with enhanced photoresponse behavior through electrochemical cathodic polarization on undoped and annealed TiO2 nanotubes. The annealing temperature of undoped TiO2 nanotubes was tuned before cathodic polarization, revealing that annealing at 500 degrees C improved the photoresponse of the nanotubes significantly. Further optimization of the blue TiO2 nanotubes was achieved by adjusting the cathodic polarization parameters. Blue TiO2 nanotubes obtained at the potential of -1.4 V (vs. SCE) with a duration of 10 min exhibited twice more photocurrent response (0.39 mA cm(-2)) compared to the undoped TiO(2 )nanotube arrays (0.19 mA cm(-2)). Oxygen vacancies formed through the cathodic polarization decreased charge recombination and enhanced charge transfer rate; therefore, a high photoelectrochemical activity under visible light irradiation could be achieved.
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    CMR may predict VT in ICM. how about in better hearts, does it really work?
    (Wiley, 2015) Yalın, Kıvanç; Aksu, Tolga; Gölcük, Şükriye Ebru; Doctor; School of Medicine; Koç University Hospital; N/A
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