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Publication Metadata only Effect of preservation period on the viscoelastic material properties of soft tissues with implications for liver transplantation(Asme, 2010) N/A; N/A; N/A; Department of Mechanical Engineering; Department of Mechanical Engineering; Öcal, Sina; Özcan, Mustafa Umut; Başdoğan, İpek; Başdoğan, Çağatay; Master Student; Master Student; Faculty Member; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; N/A; N/A; 179940; 125489The liver harvested from a donor must be preserved and transported to a suitable recipient immediately for a successful liver transplantation. In this process, the preservation period is the most critical, since it is the longest and most tissue damage occurs during this period due to the reduced blood supply to the harvested liver and the change in its temperature. We investigate the effect of preservation period on the dynamic material properties of bovine liver using a viscoelastic model derived from both impact and ramp and hold experiments. First, we measure the storage and loss moduli of bovine liver as a function of excitation frequency using an impact hammer. Second, its time-dependent relaxation modulus is measured separately through ramp and hold experiments performed by a compression device. Third, a Maxwell solid model that successfully imitates the frequency- and time-dependent dynamic responses of bovine liver is developed to estimate the optimum viscoelastic material coefficients by minimizing the error between the experimental data and the corresponding values generated by the model. Finally, the variation in the viscoelastic material coefficients of bovine liver are investigated as a function of preservation period for the liver samples tested 1 h, 2 h, 4 h, 8 h, 12 h, 24 h, 36 h, and 48 h after harvesting. The results of our experiments performed with three animals show that the liver tissue becomes stiffer and more viscous as it spends more time in the preservation cycle.Publication Metadata only 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; 1022Currently, 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.Publication Metadata only Noninvasive in vivo determination of residual strains and stresses(ASME, 2015) N/A; Department of Molecular Biology and Genetics; Department of Mechanical Engineering; Donmazov, Samir; Pişkin, Şenol; Pekkan, Kerem; PhD Student; Researcher; Faculty Member; Department of Molecular Biology and Genetics; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; N/A; 148702; 161845Vascular growth and remodeling during embryonic development are associated with blood flow and pressure induced stress distribution, in which residual strains and stresses play a central role. Residual strains are typically measured by performing in vitro tests on the excised vascular tissue. In this paper, we investigated the possibility of estimating residual strains and stresses using physiological pressure-radius data obtained through in vivo noninvasive measurement techniques, such as optical coherence tomography or ultrasound modalities. This analytical approach first tested with in vitro results using experimental data sets for three different arteries such as rabbit carotid artery, rabbit thoracic artery, and human carotid artery based on Fung's pseudostrain energy function and Delfino's exponential strain energy function (SEF). We also examined residual strains and stresses in the human swine iliac artery using the in vivo experimental ultrasound data sets corresponding to the systolic-to-diastolic region only. This allowed computation of the in vivo residual stress information for loading and unloading states separately. Residual strain parameters as well as the material parameters were successfully computed with high accuracy, where the relative errors are introduced in the range of 0-7.5%. Corresponding residual stress distributions demonstrated global errors all in acceptable ranges. A slight discrepancy was observed in the computed reduced axial force. Results of computations performed based on in vivo experimental data obtained from loading and unloading states of the artery exhibited alterations in material properties and residual strain parameters as well. Emerging noninvasive measurement techniques combined with the present analytical approach can be used to estimate residual strains and stresses in vascular tissues as a precursor for growth estimates. This approach is also validated with a finite element model of a general two-layered artery, where the material remodeling states and residual strain generation are investigated.Publication Metadata only Finite element modeling of a vibrating touch screen actuated by piezo patches for haptic feedback(Springer, 2012) N/A; N/A; Department of Mechanical Engineering; Baylan, Buket; Arıdoğan, Mustafa Uğur; Başdoğan, Çağatay; Master Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 125489The aim of our work is to design a touch screen for displaying vibrotactile haptic feedback to the user via piezo patches attached to its surface. One of the challenges in the design is the selection of appropriate boundary conditions and the piezo configurations (location and orientation) on the screen for achieving optimum performance within the limits of human haptic perception. To investigate the trade-offs in our design, we developed a finite element model of the screen and four piezo actuators attached to its surface in ABAQUS. The model utilizes the well-known Hooke's law between stress and strain extended by piezoelectric coupling. After selecting the appropriate boundary condition for the screen based on the range of vibration frequencies detectable by a human finger, the optimum configuration for the piezo patches is determined by maximizing the vibration amplitude of the screen for a unit micro Coulomb charge applied to each piezo patch. The results of our study suggest that the piezo patches should be placed close to the clamped sides of the screen where the boundary conditions are applied. © 2012 Springer-Verlag.Publication Metadata only 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; 219524Transdiscal 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.Publication Metadata only 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; 219274in 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.Publication Metadata only Electromechanical modeling of a novel moving magnet linear oscillating actuator(Korean Soc Mechanical Engineers, 2018) N/A; N/A; N/A; Department of Mechanical Engineering; Hassan, Adnan; Bijanzad, Armin; Lazoğlu, İsmail; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Manufacturing and Automation Research Center (MARC); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 179391This article presents a design of a novel moving magnet linear actuator (MMLa) for linear refrigerator compressor. a methodology to estimate the magnetic flux density as well as the magnetic force of MMLa is presented. Considering the simulation time of a 3D FEM software, A combination of 2D FEM with the analytical models makes this technique convenient and expeditious. Using the 2D FEM, the magnetic flux path is segregated into several loops which are divided into finite reluctance zones. the reluctance models for each zone incorporates the effect of armature position to predict the magnetic flux density and linear force incorporating the stator saturation. a 3D FEM simulation is executed to compare the presented models as well as computation times. Furthermore, An analytical approach for evaluating the effect of the excitation frequency and motor constant on the dynamic performance of a linear oscillating actuator is presented and validated experimentally.Publication Metadata only Do we need a transforaminal lumbar interbody fusion cage to increase the stability of functional spinal unit when comparing unilateral and bilateral fixation?(Sage Publications Ltd, 2018) Ulutas, Murat; Ozkaya, Mustafa; Demir, Teyfik; N/A; Yaman, Onur; Doctor; N/A; Koç University Hospital; 219524Transforaminal lumbar interbody fusion was an alternative to posterior lumbar interbody fusion for decompression surgeries. This study investigates the biomechanical responses of the unilateral and bilateral pedicle screw fixations with/without transforaminal lumbar interbody fusion cages under axial compression, flexion, and torsional loads. Ovine vertebrae were used in this study. Cadavers, randomly divided into five, were intact control group, bilateral pedicle screw fixation group, bilateral pedicle screw fixation group with transforaminal lumbar interbody fusion cage, unilateral pedicle screw fixation group, and unilateral pedicle screw fixation group with transforaminal lumbar interbody fusion cage. Axial compression, flexion, and torsion tests were performed on specimens. All study groups provided higher stiffness and yield load values than control group under axial compression. Addition of transforaminal lumbar interbody fusion cage to bilateral fixation increased the stiffness under axial compression. Moreover, additional use of transforaminal lumbar interbody fusion in unilateral fixation increased the yield load values under axial compression. Control group was the stiffest in flexion test. Placing a transforaminal lumbar interbody fusion cage to both unilateral and bilateral fixations did not significantly change the stiffness values. Additional transforaminal lumbar interbody fusion cage increased the yield moment of the bilateral fixation. In torsion test, control group had the highest stiffness and yield torque. The facet joints are the most important parts of the vertebrae on the stability. When comparing the bilateral and unilateral fixations with transforaminal lumbar interbody fusion addition, the more facet preserving approach has significantly higher stability under axial compression, flexion, and torsion. Unilateral fixation with transforaminal lumbar interbody fusion cage can be said biomechanically stable and advantageous fixation system because of the advantage on the less facet and soft tissue resection compared to bilateral fixation with transforaminal lumbar interbody fusion.Publication Metadata only Midterm results of implantable cardioverter defibrillators in children and young adults from a single center in Turkey(Wiley, 2016) Aykan, Hayrettin Hakan; Karagöz, Tevfik; Gülgün, Mustafa; Ertuğrul, İlker; Aypar, Ebru; Özer, Sema; Alehan, Dursun; Özkutlu, Süheyla; Çeliker, Alpay; Other; School of Medicine; N/ABackground: Despite concerns about complications with the implantable cardioverter defibrillator (ICD), it is effective for the prevention of sudden cardiac death (SCD). We aimed to analyze our midterm experience with ICD in children and young adults. Methods: This retrospective study included patients who were implanted with an ICD between 2001 and 2014. Demographic characteristics, clinical information, shock features, and complications for all patients with ICD were analyzed. The study population was divided into two groups: early-era patients implanted before 2008, and late-era patients implanted after 2008. Results: Sixty-nine patients (median age: 12 years, median follow-up: 52 months) were implanted with an ICD. Diagnostic categories were channelopathy (56.6%), cardiomyopathy (36.2%), congenital heart disease (5.8%), and other (1.4%). We performed implantation for primary prevention in 66.6% (39.3% in early-era patients and 85.4% in late-era patients). Thirty-one (44.9%) received 139 appropriate shocks (66% of total shocks) while 14 (20.2%) received 71 inappropriate shocks. However, there was no statistically significant difference in the use of appropriate shocks in the primary (66.7%) versus the secondary (72.2%) prevention groups. The incidence of appropriate and inappropriate shock was 66.7% and 33.3% in the primary prevention group, and 72.2% and 27.8% in the secondary prevention group, respectively. Two patients died, although only one death was the result of a lead problem. Conclusions: Although lead integrity problems, inappropriate shocks, and infections are significant issues, ICD therapy appears to be a safe, effective, and necessary option for the prevention of SCD in both children and young adults.Publication Metadata only Spontaneous smile intensity estimation by fusing saliency maps and convolutional neural networks(Spie-Soc Photo-Optical Instrumentation Engineers, 2019) Wei, Qinglan; Morency, Louis-Philippe; Sun, Bo; N/A; Bozkurt, Elif; PhD Student; Graduate School of Sciences and Engineering; N/ASmile intensity estimation plays important roles in applications such as affective disorder prediction, life satisfaction prediction, camera technique improvement, etc. In recent studies, many researchers applied only traditional features, such as local binary pattern and local phase quantization (LPQ) to represent smile intensity. To improve the performance of spontaneous smile intensity estimation, we introduce a feature set that combines the saliency map (SM)-based handcrafted feature and non-low-level convolutional neural network (CNN) features. We took advantage of the opponent-color characteristic of SMs and the multiple convolutional level features, which were assumed to be mutually complementary. Experiments were made on the Binghamton-Pittsburgh 4D (BP4D) database and Denver Intensity of Spontaneous Facial Action (DISFA) database. We set the local binary patterns on three orthogonal planes (LBPTOP) method as a baseline, and the experimental results show that the CNN features can better estimate smile intensity. Finally, through the proposed SM-LBPTOP feature fusion with the median- and high-level CNN features, we obtained the best result (52.08% on BP4D, 70.55% on DISFA), demonstrating our hypothesis is reasonable: the SM-based handcrafted feature is a good supplement to CNNs in spontaneous smile intensity estimation. (C) 2019 SPIE and IS&T
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