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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Influence of soft segment structure, hydrogen bonding, and diisocyanate symmetry on morphology and properties of segmented thermoplastic polyurethanes and polyureas(Tubitak Scientific & Technological Research Council Turkey, 2023) Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Department of Chemistry; College of SciencesA comprehensive review of the structure-morphology-property relations in segmented thermoplastic polyurethanes and polyureas (TPU) is provided. Special emphasis is given to the influence of the soft segment structure, polarity, and molecular weight, diisocyanate symmetry and the nature, extent, and strength of hydrogen bonding on the morphology and thermal and mechanical properties of TPUs. Experimental results obtained on composition-dependent TPU morphology and properties by various techniques were also compared by the morphology profiles generated by computational methods such as quantum mechanical calculations and molecular dynamics simulations.Publication Metadata only Effect of finger moisture on tactile perception of electroadhesion(Institute of Electrical and Electronics Engineers, 2024) Lefevre, Philippe; Martinsen, Orjan Grottem; Department of Mechanical Engineering; Aliabbasi, Easa; Muzammil, Muhammad; Şirin, Ömer; Başdoğan, Çağatay; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of EngineeringWe investigate the effect of finger moisture on the tactile perception of electroadhesion with 10 participants. Participants with moist fingers exhibited markedly higher threshold levels. Our electrical impedance measurements show a substantial reduction in impedance magnitude when sweat is present at the finger-touchscreen interface, indicating increased conductivity. Supporting this, our mechanical friction measurements show that the relative increase in electrostatic force due to electroadhesion is lower for a moist finger.Publication Metadata only Exploration of novel 6,8,9-trisubstituted purine analogues: synthesis, in vitro biological evaluation, and their effect on human cancer cells(Tubitak Scientific & Technological Research Council Turkey, 2024) Polat, Muhammed Fatih; Atalay, Rengul; Tuncbilek, Meral; N/A; Şahin, İrem Durmaz; Koç University Research Center for Translational Medicine (KUTTAM); School of MedicineCancer, a leading global cause of mortality, demands continuous advancements in therapeutic strategies. This study focuses on the design and synthesis of a novel series of purine derivatives, specifically 6 -(substituted phenyl piperazine)-8-(4-phenoxyphenyl)9-cyclopentyl purine derivatives (5-11). The motivation behind this endeavor lies in addressing acquired resistance mechanisms in cancer cells, a significant hurdle in current treatment modalities. The synthesis, starting from 4,6-dichloro-5-nitropyrimidine, involves a multi -step process, resulting in seven new purine derivatives. Biological evaluation against human liver, colon, and breast cancer cells (Huh7, HCT116, and MCF7, respectively) was performed using the SRB assay. Among the synthesized analogs, compounds 5 and 6, exhibited notable cytotoxic activity, surpassing clinically used positive controls 5-Fluorouracil and Fludarabine in terms of efficacy. This research underscores the potential of purine derivatives with a phenyl group at the C-8 position as a scaffold for developing compounds with improved anticancer properties. The findings offer insights for future exploration and development of novel agents in cancer pharmaceutical research.Publication Metadata only Switching the left and the right hearts: a novel BI-ventricle mechanical support strategy with spared native single-ventricle(Springer, 2023) Şişli, Emrah; Aka, İbrahim Başar; Tuncer, Osman Nuri; Atay, Yüksel; Özbaran, Mustafa; Department of Mechanical Engineering; Yıldırım, Canberk; Pekkan, Kerem; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of EngineeringEnd-stage Fontan patients with single-ventricle (SV) circulation are often bridged-to-heart transplantation via mechanical circulatory support (MCS). Donor shortage and complexity of the SV physiology demand innovative MCS. In this paper, an out-of-the-box circulation concept, in which the left and right ventricles are switched with each other is introduced as a novel bi-ventricle MCS configuration for the "failing" Fontan patients. In the proposed configuration, the systemic circulation is maintained through a conventional mechanical ventricle assist device (VAD) while the venous circulation is delegated to the native SV. This approach spares the SV and puts it to a new use at the right-side providing the most-needed venous flow pulsatility to the failed Fontan circulation. To analyze its feasibility and performance, eight SV failure modes have been studied via an established multi-compartmental lumped parameter cardiovascular model (LPM). Here the LPM model is experimentally validated against the corresponding pulsatile mock-up flow loop measurements of a representative 15-year-old Fontan patient employing a clinically-approved VAD (Medtronic-HeartWare). The proposed surgical configuration maintained the healthy cardiac index (3-3.5 l/min/m(2)) and the normal mean systemic arterial pressure levels. For a failed SV with low ejection fraction (EF = 26%), representing a typical systemic Fontan failure, the proposed configuration enabled a similar to 28 mmHg amplitude in the venous/pulmonary waveforms and a 2 mmHg decrease in the central venous pressure (CVP) together with acceptable mean pulmonary artery pressures (17.5 mmHg). The pulmonary vascular resistance (PVR)-SV failure case provided a similar to 5 mmHg drop in the CVP, with venous/pulmonary pulsatility reaching to similar to 22 mmHg. For the high PVR failure case with a healthy SV (EF = 44%) pulmonary hypertension is likely to occur as expected. While this condition is routinely encountered during the heart transplantation and managed through pulmonary vasodilators a need for precise functional assessment of the spared failed-ventricle is recommended if utilized in the PVR failure mode. Comprehensive in vitro and in silico results encourage this novel concept as a low-cost, more physiological alternative to the conventional bi-ventricle MCS pending animal experiments.Publication Metadata only Myocardial biomechanics and the consequent differentially expressed genes of the left atrial ligation chick embryonic model of hypoplastic left heart syndrome(Springer, 2023) Chan, Wei Xuan; Motakis, Efthymios; Ho, Sheldon; Yap, Choon Hwai; Lashkarinia, S. Samaneh; Department of Mechanical Engineering; Siddiqui, Hummaira Banu; Çoban, Mervenur; Sevgin, Börteçine; Pekkan, Kerem; Department of Mechanical Engineering; College of Engineering; Graduate School of Sciences and EngineeringLeft atrial ligation (LAL) of the chick embryonic heart is a model of the hypoplastic left heart syndrome (HLHS) where a purely mechanical intervention without genetic or pharmacological manipulation is employed to initiate cardiac malformation. It is thus a key model for understanding the biomechanical origins of HLHS. However, its myocardial mechanics and subsequent gene expressions are not well-understood. We performed finite element (FE) modeling and single-cell RNA sequencing to address this. 4D high-frequency ultrasound imaging of chick embryonic hearts at HH25 (ED 4.5) were obtained for both LAL and control. Motion tracking was performed to quantify strains. Image-based FE modeling was conducted, using the direction of the smallest strain eigenvector as the orientations of contractions, the Guccione active tension model and a Fung-type transversely isotropic passive stiffness model that was determined via micro-pipette aspiration. Single-cell RNA sequencing of left ventricle (LV) heart tissues was performed for normal and LAL embryos at HH30 (ED 6.5) and differentially expressed genes (DEG) were identified.After LAL, LV thickness increased by 33%, strains in the myofiber direction increased by 42%, while stresses in the myofiber direction decreased by 50%. These were likely related to the reduction in ventricular preload and underloading of the LV due to LAL. RNA-seq data revealed potentially related DEG in myocytes, including mechano-sensing genes (Cadherins, NOTCH1, etc.), myosin contractility genes (MLCK, MLCP, etc.), calcium signaling genes (PI3K, PMCA, etc.), and genes related to fibrosis and fibroelastosis (TGF-beta, BMP, etc.). We elucidated the changes to the myocardial biomechanics brought by LAL and the corresponding changes to myocyte gene expressions. These data may be useful in identifying the mechanobiological pathways of HLHS.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 Characterization of frequency-dependent material properties of human liver and its pathologies using an impact hammer(Elsevier, 2011) Dogusoy, Gulen; Tokat, Yaman; N/A; N/A; Department of Mechanical Engineering; Özcan, Mustafa Umut; Öcal, Sina; Başdoğan, Çağatay; Master Student; Master 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 current methods for characterization of frequency-dependent material properties of human liver are very limited. In fact, there is almost no data available in the literature showing the variation in dynamic elastic modulus of healthy or diseased human liver as a function of excitation frequency. We show that frequency-dependent dynamic material properties of a whole human liver can be easily and efficiently characterized by an impact hammer. The procedure only involves a light impact force applied to the tested liver by a hand-held hammer. The results of our experiments conducted with 15 human livers harvested from the patients having some form of liver disease show that the proposed approach can successfully differentiate the level of fibrosis in human liver. We found that the storage moduli of the livers having no fibrosis (F0) and that of the cirrhotic livers (F4) varied from 10 to 20 kPa and 20 to 50 kPa for the frequency range of 0-80 Hz, respectively.Publication Metadata only Novel fenestration designs for controlled venous flow shunting in failing fontans with systemic venous hypertension(Wiley, 2013) Albal, Priti G.; Menon, Prahlad G.; Kowalski, William; Undar, Akif; Turkoz, Riza; Department of Mechanical Engineering; Pekkan, Kerem; Faculty Member; Department of Mechanical Engineering; College of Engineering; 161845The Fontan procedure is employed as the final-stage palliation in single-ventricle congenital heart patients and results in diversion of venous blood directly to the pulmonary arteries. Fontan patients have been known to suffer from postoperative systemic venous hypertension, which in turn is associated with pleural effusions and protein losing enteropathy, leading to a decreased duration and quality of life. Despite the ongoing debate on its benefits, a circular fenestration hole (typically 4?mm) establishing a venous shunt to the common atrium is traditionally employed to relieve venous pressure in the Fontan conduit and improve early postoperative Fontan hemodynamics. However, these improvements come at the cost of reduced oxygen saturation due to excessive right-to-left shunting if the fenestration is permanent. The ideal selective fenestration would therefore limit or eliminate shunt flow at tolerable systemic venous pressures and allow increased flow at high pressures. The objective of this study is to introduce new fenestration designs that exhibit these desirable pressure-flow characteristics. Novel plus-shaped and S-shaped fenestration designs with leaflets are introduced as alternatives to the traditional circular fenestration, each having identical effective orifice areas at the fully open states. In vitro steady leakage flow tests were performed for physiological flow-driving pressures in order to obtain pressure-drop versus flow-rate characteristics. In addition, the leaflet opening kinematics of the plus-shaped fenestration was investigated computationally using finite element simulation. Fluid-structure interaction analysis was performed to determine leaflet displacements and pressure-flow characteristics at low pressures. Further, a lumped parameter model of the single-ventricle circuit was created to simulate pulsatile flow conditions For the plus-shaped fenestration, the flow rate was found to increase nonlinearly with increased driving systemic venous pressures at high physiological-pressure drops which did not cause the leaflets to fully open, and linearly for low driving pressures. These results indicate that leaflets of the plus-shaped fenestration design activated passively after a critical systemic venous pressure threshold. This feature is ideal for minimizing undesirable excessive venous shunting. A large variability in shunting flow rate may be obtained by changing the shape, thickness, size, and material of the fenestration to suit requirements of the patient, which can further limit shunt flow in a controlled manner.Publication Metadata only In silico analysis of modular bone plates(Elsevier, 2021) N/A; N/A; Department of Mechanical Engineering; N/A; Department of Mechanical Engineering; Subaşı, Ömer; Oral, Atacan; Noyan, Sinan; Tunçözgür, Orçun; Lazoğlu, İsmail; PhD Student; PhD Student; Undergraduate Student; Master 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; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; N/A; N/A; 179391Background: Inventory management or immediate availability of fracture plates can be problematic since for each surgical intervention a specific plate of varying size and functionality must be ordered. Modularization of the standard monolithic plate is proposed to address this issue. Methods: The effects of four different unit module design parameters (type, degree of modularization, connector screw diameter, sandwich ratio) on the plate bending stiffness and failure are investigated in a finite element four-point-bending analysis. A chosen, best-performing modular plate is then tested in silico for a simple diaphyseal tibial fracture scenario under anatomical compressional, torsional, and bending loads . Results: A modularization strategy is proposed to match the monolithic plate bending properties as closely as possible. With the best combination of design parameters, a fully modularized equivalent length plate with a 42.3% decrease in stiffness and 46.2% decrease in strength could be assembled. The chosen modular plate also displayed sufficient mechanical performance under the fracture fixation scenarios for a potentially successful osteosynthesis. Conclusions: Via computational methods, the viability of the modularization strategy as an alternate to the traditional monolithic plate is demonstrated. As a further realized advantage, the modular plates can alleviate stress shielding thanks to the reduced stiffness.Publication Metadata only Evaluation of nonablative fractional laser treatment in scar reduction(Springer London Ltd, 2017) N/A; Gökalp, Hilal; Doctor; N/A; Koç University Hospital; N/AFractional lasers have been used for the improvement of scar tissue in the recent years but there has not been extensive research on their impact. The purpose of this study was to evaluate the efficacy of nonablative fractional laser (NAFL) on acne, burn, and surgery/traumatic scar. The scars were also categorized as atrophic, hypertrophic, and keloid, and treatment efficacy was investigated accordingly. This is a retrospective, single-center study. Scar tissues were treated using a nonablative fractional 1550-nm Erbium glass laser in high-energy parameters at 4-week interval for 4-8 sessions. The scar regression score (SRS) was used to determine the decrease in scar appearance. Forty-six patients with acne (n:18), burn (n:13), or surgery/traumatic (n:15) scar were included. The number of sessions was higher for burn patients while SRS in burn patients was lower than in patients with acne or a surgical/traumatic scar. Evaluation according to scar types showed that atrophic scars had a significantly better response to NAFL treatment. This study indicates that NAFL treatment with the high-energy parameters has better outcomes in atrophic acne scars, while the success rate is considerably low in post-burn and keloid scars.