F.L. Degertekin

Georgia Institute of Technology, Atlanta, Georgia, United States

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Publications (204)268.22 Total impact

  • Karim G Sabra, Justin Romberg, Shane Lani, F Levent Degertekin
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    ABSTRACT: Monolithic integration of capacitive micromachined ultrasonic transducer arrays with low noise complementary metal oxide semiconductor electronics minimizes interconnect parasitics thus allowing the measurement of thermal-mechanical (TM) noise. This enables passive ultrasonics based on cross-correlations of diffuse TM noise to extract coherent ultrasonic waves propagating between receivers. However, synchronous recording of high-frequency TM noise puts stringent requirements on the analog to digital converter's sampling rate. To alleviate this restriction, high-frequency TM noise cross-correlations (12-25 MHz) were estimated instead using compressed measurements of TM noise which could be digitized at a sampling frequency lower than the Nyquist frequency.
    The Journal of the Acoustical Society of America 06/2014; 135(6):EL364. · 1.65 Impact Factor
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    ABSTRACT: A lattice Boltzmann (LB) formulation, which is consistent with the phase-field model for two-phase incompressible fluid, is proposed to model the interface dynamics of droplet impingement. The interparticle force is derived by comparing the macroscopic transport equations recovered from LB equations with the governing equations of the continuous phase-field model. The inconsistency between the existing LB implementations and the phase-field model in calculating the relaxation time at the phase interface is identified and an approximation is proposed to ensure the consistency with the phase-field model. It is also shown that the commonly used equilibrium velocity boundary for the binary fluid LB scheme does not conserve momentum at the wall boundary and a modified scheme is developed to ensure the momentum conservation at the boundary. In addition, a geometric formulation of the wetting boundary condition is proposed to replace the popular surface energy formulation and results show that the geometric approach enforces the prescribed contact angle better than the surface energy formulation in both static and dynamic wetting. The proposed LB formulation is applied to simulating droplet impingement dynamics in three dimensions and results are compared to those obtained with the continuous phase-field model, the LB simulations reported in the literature, and experimental data from the literature. The results show that the proposed LB simulation approach yields not only a significant speed improvement over the phase-field model in simulating droplet impingement dynamics on a submillimeter length scale, but also better accuracy than both the phase-field model and the previously reported LB techniques when compared to experimental data. Upon validation, the proposed LB modeling methodology is applied to the study of multiple-droplet impingement and interactions in three dimensions, which demonstrates its powerful capability of simulating extremely complex interface phenomena.
    Physical Review E 03/2014; 89(3-1):033311. · 2.31 Impact Factor
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    ABSTRACT: Capacitive Micromachined Ultrasonic Transducers (CMUTs) operating in immersion support dispersive evanescent waves due to the subwavelength periodic structure of electrostatically actuated membranes in the array. Evanescent wave characteristics also depend on the membrane resonance which is modified by the externally applied bias voltage, offering a mechanism to tune the CMUT array as an acoustic metamaterial. The dispersion and tunability characteristics are examined using a computationally efficient, mutual radiation impedance based approach to model a finite-size array and realistic parameters of variation. The simulations are verified, and tunability is demonstrated by experiments on a linear CMUT array operating in 2-12 MHz range.
    Applied Physics Letters 02/2014; 104(5):051914. · 3.79 Impact Factor
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    ABSTRACT: Intravascular ultrasound (IVUS) and intracardiac echography (ICE) catheters with real-time volumetric ultrasound imaging capability can provide unique benefits to many interventional procedures used in the diagnosis and treatment of coronary and structural heart diseases. Integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with front-end electronics in single-chip configuration allows for implementation of such catheter probes with reduced interconnect complexity, miniaturization, and high mechanical flexibility. We implemented a single-chip forward-looking (FL) ultrasound imaging system by fabricating a 1.4-mm-diameter dual-ring CMUT array using CMUT-on-CMOS technology on a front-end IC implemented in 0.35-μm CMOS process. The dual-ring array has 56 transmit elements and 48 receive elements on two separate concentric annular rings. The IC incorporates a 25-V pulser for each transmitter and a low-noise capacitive transimpedance amplifier (TIA) for each receiver, along with digital control and smart power management. The final shape of the silicon chip is a 1.5-mm-diameter donut with a 430-μm center hole for a guide wire. The overall front-end system requires only 13 external connections and provides 4 parallel RF outputs while consuming an average power of 20 mW. We measured RF A-scans from the integrated single- chip array which show full functionality at 20.1 MHz with 43% fractional bandwidth. We also tested and demonstrated the image quality of the system on a wire phantom and an ex vivo chicken heart sample. The measured axial and lateral point resolutions are 92 μm and 251 μm, respectively. We successfully acquired volumetric imaging data from the ex vivo chicken heart at 60 frames per second without any signal averaging. These demonstrative results indicate that single-chip CMUT-on-CMOS systems have the potential to produce realtime volumetric images with image quality and speed suitable for catheter-based clinical applications.
    IEEE transactions on ultrasonics, ferroelectrics, and frequency control 02/2014; 61(2):239-50. · 1.80 Impact Factor
  • D Torello, F Levent Degertekin
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    ABSTRACT: A new method of actuating atomic force microscopy (AFM) cantilevers is proposed in which a high frequency (>5 MHz) wave modulated by a lower frequency (∼300 kHz) wave passes through a contact acoustic nonlinearity at the contact interface between the actuator and the cantilever chip. The nonlinearity converts the high frequency, modulated signal to a low frequency drive signal suitable for actuation of tapping-mode AFM probes. The higher harmonic content of this signal is filtered out mechanically by the cantilever transfer function, providing for clean output. A custom probe holder was designed and constructed using rapid prototyping technologies and off-the-shelf components and was interfaced with an Asylum Research MFP-3D AFM, which was then used to evaluate the performance characteristics with respect to standard hardware and linear actuation techniques. Using a carrier frequency of 14.19 MHz, it was observed that the cantilever output was cleaner with this actuation technique and added no significant noise to the system. This setup, without any optimization, was determined to have an actuation bandwidth on the order of 10 MHz, suitable for high speed imaging applications. Using this method, an image was taken that demonstrates the viability of the technique and is compared favorably to images taken with a standard AFM setup.
    The Review of scientific instruments 11/2013; 84(11):113705. · 1.52 Impact Factor
  • Shane Lani, M Wasequr Rashid, Karim G Sabra, F Levent Degertekin
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    ABSTRACT: Capacitive micromachined ultrasonic transducer (CMUT) arrays are made up of microscale (10-100[micro sign]m wide) membranes with embedded electrodes for electrostatic excitation and detection of acoustic waves. While typically used for far-field imaging, CMUT arrays also support dispersive evanescent surface waves. These surface waves derive their dispersive properties not only from the periodic structure of the array, but also from the membrane resonance. One advantage of CMUTs as a metamaterial is that the dispersive qualities of the array can be tuned by changing the applied bias voltage to the membranes, which in effect changes the membrane stiffness. A second advantage is that the CMUT array elements can be used as receivers to record the acoustic waves with high spatial resolution, which make laser displacement measurement based characterization unnecessary. These properties allow the possibility of CMUTs to exploit these slowly propagating evanescent waves as a means for creating subwavelength resolution fields for high-resolution ultrasound imaging and sensing in the near field by appropriately tuning the physical characteristics of individual membranes. The dispersive behavior of these evanescent surface waves propagating along a CMUT array was quantified using a computationally efficient, boundary element method based model and validated with both finite element analysis and experimental data obtained from a 1 x 16 CMUT array with a membrane resonance tunable between 5 and 6.5 MHz.
    The Journal of the Acoustical Society of America 11/2013; 134(5):4102. · 1.65 Impact Factor
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    ABSTRACT: A shape coefficient is introduced to quantify droplet shape by measuring its similarity to a desired shape to enable the study of droplet shape evolution upon impingement on a solid surface. Parametric simulations are performed with an experimentally validated numerical model to determine the impact conditions to maximize the shape coefficient. Results show that the Weber number is the controlling factor that determines the maximum achievable shape coefficient and the time instant when it is achieved for small Ohnesorge numbers, whereas the Reynolds number becomes the key parameter defining the optimal shape when the Ohnesorge number is large. A regime map is also developed to define the regions where a desired droplet shape can be achieved without splash. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3071–3082, 2013
    AIChE Journal 08/2013; 59(8). · 2.58 Impact Factor
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    ABSTRACT: Effective intracellular delivery is a significant impediment to research and therapeutic applications at all processing scales. Physical delivery methods have long demonstrated the ability to deliver cargo molecules directly to the cytoplasm or nucleus, and the mechanisms underlying the most common approaches (microinjection, electroporation, and sonoporation) have been extensively investigated. In this review, we discuss established approaches, as well as emerging techniques (magnetofection, optoinjection, and combined modalities). In addition to operating principles and implementation strategies, we address applicability and limitations of various in vitro, ex vivo, and in vivo platforms. Importantly, we perform critical assessments regarding (1) treatment efficacy with diverse cell types and delivered cargo molecules, (2) suitability to different processing scales (from single cell to large populations), (3) suitability for automation/integration with existing workflows, and (4) multiplexing potential and flexibility/adaptability to enable rapid changeover between treatments of varied cell types. Existing techniques typically fall short in one or more of these criteria; however, introduction of micro-/nanotechnology concepts, as well as synergistic coupling of complementary method(s), can improve performance and applicability of a particular approach, overcoming barriers to practical implementation. For this reason, we emphasize these strategies in examining recent advances in development of delivery systems.
    Journal of the Association for Laboratory Automation 06/2013; · 1.46 Impact Factor
  • S. Satir, J. Zahorian, F.L. Degertekin
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    ABSTRACT: A large-signal, transient model has been developed to predict the output characteristics of a CMUT array operated in the non-collapse mode. The model is based on separation of the nonlinear electrostatic voltage-to-force relation and the linear acoustic array response. For modeling of linear acoustic radiation and crosstalk effects, the boundary element method is used. The stiffness matrix in the vibroacoustics calculations is obtained using static finite element analysis of a single membrane which can have arbitrary geometry and boundary conditions. A lumped modeling approach is used to reduce the order of the system for modeling the transient nonlinear electrostatic actuation. To accurately capture the dynamics of the non-uniform electrostatic force distribution over the CMUT electrode during large deflections, the membrane electrode is divided into patches shaped to match higher order membrane modes, each introducing a variable to the system model. This reduced order nonlinear lumped model is solved in the time domain using commercial software. The model has two linear blocks to calculate the displacement profile of the electrode patches and the output pressure for a given force distribution over the array. The force-to-array-displacement block uses the linear acoustic model, and the Rayleigh integral is evaluated to calculate the pressure at any field point. Using the model, the time-domain transmitted pressure can be simulated for different large drive signal configurations. The acoustic model is verified by comparison to harmonic FEA in vacuum and fluid for high- and low-aspect-ratio membranes as well as mass-loaded membranes. The overall software model is verified by comparison to transient 3-D finite element analysis and experimental results for different large drive signals, and an example for a phased array simulation is given.
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 01/2013; 60(11):2426-2439. · 1.82 Impact Factor
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    ABSTRACT: We designed and fabricated a dynamic receive beamformer integrated circuit (IC) in 0.35-μm CMOS technology. This beamformer IC is suitable for integration with an annular array transducer for high-frequency (30-50 MHz) intravascular ultrasound (IVUS) imaging. The beamformer IC consists of receive preamplifiers, an analog dynamic delay-and-sum beamformer, and buffers for 8 receive channels. To form an analog dynamic delay line we designed an analog delay cell based on the current-mode first-order all-pass filter topology, as the basic building block. To increase the bandwidth of the delay cell, we explored an enhancement technique on the current mirrors. This technique improved the overall bandwidth of the delay line by a factor of 6. Each delay cell consumes 2.1-mW of power and is capable of generating a tunable time delay between 1.75 ns to 2.5 ns. We successfully integrated the fabricated beamformer IC with an 8-element annular array. Experimental test results demonstrated the desired buffering, preamplification and delaying capabilities of the beamformer.
    IEEE Transactions on Biomedical Circuits and Systems 10/2012; 6(5):454-67. · 2.74 Impact Factor
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    ABSTRACT: An intravascular ultrasound (IVUS) catheter that can provide forward viewing volumetric ultrasound images would be an invaluable clinical tool for guiding interventions. Single chip integration of front-end electronics with capacitive micromachined ultrasonic transducers (CMUTs) is highly desirable to reduce the interconnection complexity and enable miniaturization in IVUS catheters. For this purpose we use the monolithic CMUT-on-CMOS integration where CMUTs are fabricated directly on top of pre-processed CMOS wafers. This minimizes parasitic capacitances associated with connection lines. We have recently implemented a system design including all the required electronics using 0.35-µm CMOS process integrated with a 1.4-mm diameter CMUT array. In this study, we present the experimental volumetric imaging results from an ex-vivo chicken heart phantom. The imaging results demonstrate that the single-chip forward looking IVUS (FL-IVUS) system with monolithically integrated electronics has potential to visualize the front view of coronary arteries.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2012; 2012:3195-8.
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    Hamdi Torun, Ofer Finkler, F Levent Degertekin
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    ABSTRACT: The authors describe a method for biomolecular force clamp measurements using atomic force microscope (AFM) cantilevers and micromachined membrane-based electrostatic actuators. The actuators comprise of Parylene membranes with embedded side actuation electrodes and are fabricated on a silicon substrate. The devices have a displacement range of 1.8μm with 200V actuation voltage, and displacement uncertainty is 0.8nm, including the noise and drift. The settling time, limited by the particular amplifier is 5ms, with an inherent range down to 20μs. A force clamp measurement setup using these actuators in a feedback loop has been used to measure bond life-times between human IgG and anti-human IgG molecules to demonstrate the feasibility of this method for biological experiments. The experimental findings are compared with a molecular pulling experiment and the results are found to be in good agreement.
    Ultramicroscopy 07/2012; 122C:26-31. · 2.47 Impact Factor
  • Hasan Giray Oral, Zehra Parlak, F Levent Degertekin
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    ABSTRACT: We present an in-depth analysis of time-resolved interaction force (TRIF) mode imaging for atomic force microscopy (AFM). A nonlinear model of an active AFM probe, performing simultaneous topography and material property imaging on samples with varying elasticity and adhesion is implemented in Simulink®. The model is capable of simulating various imaging modes, probe structures, sample material properties, tip-sample interaction force models, and actuation and feedback schemes. For passive AFM cantilevers, the model is verified by comparing results from the literature. As an example of an active probe, the force sensing integrated readout and active tip (FIRAT) probe is used. Simulation results indicate that the active and damped nature of FIRAT provides a significant level of control over the force applied to the sample, minimizing sample indentation and topography error. Active tip control (ATC) preserves constant contact time during force control for stable contact while preventing the loss of material property information such as elasticity and adhesive force. Simulation results are verified by TRIF mode imaging of the samples with both soft and stiff regions.
    Ultramicroscopy 06/2012; 120:56-63. · 2.47 Impact Factor
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    Gokce Gurun, Michael Hochman, Paul Hasler, F Levent Degertekin
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    ABSTRACT: When capacitive micromachined ultrasonic transducers (CMUTs) are monolithically integrated with custom-designed low-noise electronics, the output noise of the system can be dominated by the CMUT thermal-mechanical noise both in air and in immersion even for devices with low capacitance. Because the thermal-mechanical noise can be related to the electrical admittance of the CMUTs, this provides an effective means of device characterization. This approach yields a novel method to test the functionality and uniformity of CMUT arrays and the integrated electronics when a direct connection to CMUT array element terminals is not available. Because these measurements can be performed in air at the wafer level, the approach is suitable for batch manufacturing and testing. We demonstrate this method on the elements of an 800-μm-diameter CMUT-on-CMOS array designed for intravascular imaging in the 10 to 20 MHz range. Noise measurements in air show the expected resonance behavior and spring softening effects. Noise measurements in immersion for the same array provide useful information on both the acoustic cross talk and radiation properties of the CMUT array elements. The good agreement between a CMUT model based on finite difference and boundary element methods and the noise measurements validates the model and indicates that the output noise is indeed dominated by thermal-mechanical noise. The measurement method can be exploited to implement CMUT-based passive sensors to measure immersion medium properties, or other parameters affecting the electro-mechanics of the CMUT structure.
    IEEE transactions on ultrasonics, ferroelectrics, and frequency control 06/2012; 59(6):1267-75. · 1.80 Impact Factor
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    ABSTRACT: Ultrasound contrast agents (UCAs) have tremendous potential for in vivo molecular imaging because of their high sensitivity. However, the diagnostic potential of UCAs has been difficult to exploit because current UCAs are based on pre-formed microbubbles, which can only detect cell surface receptors. Here, we demonstrate that chemical reactions that generate gas forming molecules can be used to perform molecular imaging by ultrasound in vivo. This new approach was demonstrated by imaging reactive oxygen species in vivo with allylhydrazine, a liquid compound that is converted into nitrogen and propylene gas after reacting with radical oxidants. We demonstrate that allylhydrazine encapsulated within liposomes can detect a 10 micromolar concentration of radical oxidants by ultrasound, and can image oxidative stress in mice, induced by lipopolysaccharide, using a clinical ultrasound system. We anticipate numerous applications of chemically-generated microbubbles for molecular imaging by ultrasound, given ultrasound's ability to detect small increments above the gas saturation limit, its spatial resolution and widespread clinical use.
    Annals of Biomedical Engineering 05/2012; 40(9):2059-68. · 3.23 Impact Factor
  • F Levent Degertekin
    The Journal of the Acoustical Society of America 03/2012; 131(3):2343. · 1.65 Impact Factor
  • J. Zahorian, S. Satir, F.L. Degertekin
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    ABSTRACT: Finite Element Analysis (FEA) of CMUT arrays offers flexibility with membrane geometry, but it can become computationally expensive as the transducer array and fluidic space increases in size. The objective is to combine the modeling flexibility of 3D FEA with the computational effectiveness of 2D numerical methods to analyze and design CMUT arrays with arbitrary membrane shapes. Effective numerical methods for CMUT arrays combine finite difference (FD) approximations of thin plate equations with fluid coupling through the use of the Boundary Element Method. As such, the array can be modeled with a 2D surface mesh, but the thin plate approximations limit the analysis to CMUTs with high aspect ratios and uniform surfaces. A hybrid method using static FEA (COMSOL 4.2a) of an individual CMUT membrane was used to generate an equivalent stiffness matrix associated with the mesh density of the 2D surface, eliminating the dependence on the thin plate approximation. This FEA need only be performed once for a given geometry and mesh density as it is reused for an entire array. The resonant frequency of square CMUTs with 2 μm thickness in vacuum was evaluated using FD and hybrid methods. The hybrid method shows good agreement with full FEA, while the FD method deviated for low aspect ratios. Hybrid analysis of mass-loaded CMUTs with an aluminum electrode for operation at ~40 MHz shows good agreement FEA analysis while reducing the computation time by a factor of 20 for a simple array simulation.
    Ultrasonics Symposium (IUS), 2012 IEEE International; 01/2012
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    ABSTRACT: An intravascular ultrasound (IVUS) catheter that can provide forward-viewing volumetric ultrasound images would be an invaluable clinical tool for guiding interventions in coronary arteries for treatment of chronic total occlusions (CTOs) and for helping stent deployment. Single-chip integration of front-end electronics with CMUTs is highly desirable for reduced interconnect complexity, miniaturization, and flexibility of IVUS catheters. For this purpose we use the monolithic CMUT-onCMOS integration where CMUTs are fabricated directly on top of pre-processed CMOS wafers. We have previously demonstrated the characterization and imaging performance of monolithically integrated single-chip CMUT-on-CMOS Forward looking IVUS array. Here, we present 3-D real-time volumetric imaging results on vessel mimicking and relevant biological phantoms. The fabricated single chip CMUT-on-CMOS device has 56 transmit and 48 receive array elements operating at 20 MHz with a fractional bandwidth of 50%. The imaging results demonstrate that the single-chip forward looking IVUS (FLIVUS) system with monolithically integrated electronics has potential to visualize the front view of coronary arteries.
    Ultrasonics Symposium (IUS), 2012 IEEE International; 01/2012
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    ABSTRACT: Ink-jet deposition enables more efficient, economic, scalable manufacturing for a wide variety of materials than other additive techniques. The impact of droplets onto a substrate is critical for accuracy control and optimisation of the droplet deposition process. However, most previous research about droplet impact focused on the spreading radius of the droplet, which does not provide enough information for manufacturing purposes. This paper presents a new measure of droplet shape characterisation so that the droplet deposition process can be optimised to build desired geometries. A dimensional analysis is conducted to reduce the number of parameters of the impact conditions. Then the shape evolution of a wide range of impact conditions is simulated with a validated numerical model. The effects of the dimensionless numbers on the shape evolution are examined and analysed. Successive multiple droplets impact is also investigated.
    Virtual and Physical Prototyping 01/2012; 7(1):49-64.
  • S. Satir, J. Zahorian, F.L. Degertekin
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    ABSTRACT: A transient transmit model has been developed to investigate and optimize output pressure characteristics of a CMUT array element in non-collapse mode. The model uses SIMULINK to calculate transient output pressure from a CMUT array element given the input voltage waveform. The model has a nonlinear block for electrostatic actuation, and two linear blocks for force to membrane displacement and force to pressure calculations. Force to displacement transfer functions are obtained using the boundary element method which accounts for membrane dynamics including acoustic cross coupling in the array and provides surface velocity distribution for pressure calculation more efficiently as compared to FEA. Using the model, the transient transmitted pressure can be simulated for various pulse amplitude and DC bias configurations for CMUT optimization and phased array operation. The model is verified on a 2 by 2 CMUT array which is simulated both with transient FEA and the SIMULINK model. This type of model would be useful for designing a CMUT element optimized for maximum output pressure given input signal constraints and a desired frequency response. As an example, the optimal pulse width and DC bias to maximize the output pressure of a CMUT-on-CMOS dual-ring array element are calculated for given device geometry and constrained drive pulse amplitude of 25 V.
    Ultrasonics Symposium (IUS), 2012 IEEE International; 01/2012

Publication Stats

1k Citations
268.22 Total Impact Points

Institutions

  • 2000–2014
    • Georgia Institute of Technology
      • • School of Mechanical Engineering
      • • School of Electrical & Computer Engineering
      • • School of Chemistry and Biochemistry
      Atlanta, Georgia, United States
    • The International Society for Optics and Photonics
      International Falls, Minnesota, United States
  • 2011–2012
    • Bogazici University
      • Department of Electrical and Electronic Engineering
      İstanbul, Istanbul, Turkey
    • Bahçeşehir University
      İstanbul, Istanbul, Turkey
  • 2009–2011
    • Isik University
      İstanbul, Istanbul, Turkey
    • University of South Florida
      Tampa, Florida, United States
  • 2006–2008
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
  • 1993–2001
    • Stanford University
      • Department of Electrical Engineering
      Stanford, CA, United States
  • 1998
    • Bilkent University
      • Department of Electrical & Electronic Engineering
      Ankara, Ankara, Turkey
  • 1994
    • Texas Instruments Inc.
      Dallas, Texas, United States