C. Negreira

National Polytechnic Institute, Villa Gustavo A. Madero, The Federal District, Mexico

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Publications (57)26.65 Total impact

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    ABSTRACT: Development of successful small-diameter vascular grafts constitutes a real challenge to biomaterial engineering. In most cases these grafts fail in-vivo due to the presence of a mechanical mismatch between the native vessel and the vascular graft. Biomechanical characterization of real native vessels provides significant information for synthetic graft development. Electrospun nanofibrous vascular grafts emerge as a potential tailor made solution to this problem. PLLA-electrospun nanofibrous tubular structures were prepared and selected as model bioresorbable grafts. An experimental setup, using gold standard and high resolution ultrasound techniques, was adapted to characterize in vitro the poly(l-lactic acid) (PLLA) electrospun structures. The grafts were subjected to near physiologic pulsated pressure conditions, following the pressure-diameter loop approach and the criteria stated in the international standard for cardiovascular implants-tubular vascular prostheses. Additionally, ovine femoral arteries were subjected to a similar evaluation. Measurements of pressure and diameter variations allowed the estimation of dynamical compliance (%C, 10(-2)mmHg) and the pressure-strain elastic modulus (EPε, 10(6)dyncm(-2)) of the abovementioned vessels (grafts and arteries). Nanofibrous PLLA showed a decrease in %C (1.38±0.21, 0.93±0.13 and 0.76±0.15) concomitant to an increase in EPε (10.57±0.97, 14.31±1.47 and 17.63±2.61) corresponding to pressure ranges of 50 to 90mmHg, 80 to 120mmHg and 100 to 150mmHg, respectively. Furthermore, femoral arteries exhibited a decrease in %C (8.52±1.15 and 0.79±0.20) and an increase in EPε (1.66±0.30 and 15.76±4.78) corresponding to pressure ranges of 50-90mmHg (elastin zone) and 100-130mmHg (collagen zone). Arterial mechanics framework, extensively applied in our previous works, was successfully used to characterize PLLA vascular grafts in vitro, although its application can be directly extended to in vivo experiences, in conscious and chronically instrumented animals. The specific design and construction of the electrospun nanofibrous PLLA vascular grafts assessed in this work, showed similar mechanical properties as the ones observed in femoral arteries, at the collagen pressure range. Copyright © 2014 Elsevier B.V. All rights reserved.
    Materials Science and Engineering C 12/2014; 45:446-54. · 2.74 Impact Factor
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    ABSTRACT: In the manufacturing of cheese, the cutting of the curd is an essential step which depends on the firmness of the curds and significantly affects the yield of the cheese and its quality. In this work, we present a technique to measure elastic properties of the curd during coagulation that could be used to quantitatively determine the cutting time. The technique uses ultrasound to generate and measure shear waves. These waves do not propagate in liquids and their velocity of propagation depends on the viscoelastic characteristics of the medium. Hence, they can be used to identify the beginning of coagulation and subsequently to monitor the evolution of the coagulum firmness. Our results showed this technique is sensitive to changes of the medium structure during coagulation. It also proved reproducible and sensitive to different coagulation conditions. Therefore this technique can be used to develop a system suitable for the dairy industry.
    Journal of Food Engineering 09/2014; 136:73–79. · 2.58 Impact Factor
  • 2014 UIA Symposium; 01/2014
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    ABSTRACT: The quantification of fluid-structure interactions in arterial walls (from a biomechanical standpoint) requires a complete characterization of blood flow, shear stress in the interface between blood and endothelium, wall elasticity and wall stresses distribution. These interactions play an important role in pathogenic mechanisms of cardiovascular diseases, such as atherosclerosis and arteriosclerosis. A specific hemodynamic work bench simulator is used for an in-vitro characterization of the biomechanics and hemodynamics of a custom constructed physical model of arterial bifurcation, under near-physiologic pulsated flow conditions. The model was constructed using fresh porcine aorta. Some experimental in-vitro results are shown. In-vitro results are compared with in-silico results obtained from a simple CFD model of the abovementioned bifurcation. Experimental data allow construction of meshes and parameter estimation of the biomechanical properties of the arterial wall, as well as boundary conditions, all suitable to be employed in CFD and FSI numerical simulation. In-silico simulations enables the estimation of some parameters that cannot be easily obtained by means of in-vitro experimental measurements (for example wall shear stress).
    Journal of Physics Conference Series 12/2013; 477(1).
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    ABSTRACT: To quantify fluid-structure interactions in arterial walls, from a biomechanical standpoint, a complete characterization of blood flow, shear stress in the interface between blood and endothelium, wall elasticity and wall stresses distribution are needed.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 07/2013; 2013:735-738.
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    ABSTRACT: In a lossless system, the causal and acausal Green's function for elastic waves can be retrieved by cross-correlating the elastic field at two positions. This field, composed of converging and diverging waves, is interpreted in the frame of a time-reversal process. In this work, the near-field effects on the spatio-temporal focusing of elastic waves are analyzed through the elastodynamic Green's function. Contrary to the scalar field case, the spatial focusing is not symmetric preserving the directivity pattern of a simple source. One important feature of the spatial asymmetry is its dependency on the Poisson ratio of the solid. Additionally, it is shown that the retrieval of the bulk wave speed values is affected by diffraction. The correction factor depends on the relative direction between the source and the observed field. Experimental verification of the analysis is carried out on the volume of a soft-solid. A low-frequency diffuse-like field is generated by random impacts at the sample's free surface. The displacement field is imaged using ultrasound by a standard speckle tracking technique. One important application of this work is in the estimation of the shear elastic modulus in soft biological tissues, whose quantification can be useful in non-invasive diagnosis of various diseases.
    The Journal of the Acoustical Society of America 05/2013; 133(5):2755-66. · 1.65 Impact Factor
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    ABSTRACT: Abstract –– Research looking for the achieving of an accurate measurement of thickness changes in thin biological walls (e.g. of blood vessels), is a promising work line in the medical area, because it would provide the bases to analyze the possibility of attaining early diagnoses of some diseases such as hypertension or atherosclerosis. But, to obtain a non-invasive estimation of these parameters on internal tissues, currently presents many difficulties that must be overcome. The use of high-frequency ultrasonic systems appears to offer a possible solution. In fact, the application of conventional ultrasonic imaging has shown this, but the spatial resolution related to this commercial option is not sufficient for a thickness evaluation with sufficient clinical significance, which would require accuracies of few microns. In this paper, some preliminary results of applying a new broadband ultrasonic procedure, recently developed by the authors for thickness measurement purposes, are analyzed for sub-millimeter layers made of materials similar (phantoms) to that of the biological tissues to be encountered into the artery walls. Two optional algorithms for estimating the power spectral density of the multi-pulse signals are assessed with some experimental echoes. Their potential resolutions and capabilities to provide accuracies around a micron are comparatively analyzed, for walls thickness estimation.
    PANAMER. HEALTH CARE EXCH. (PAHCE 2013). IEEE., Medellin; 01/2013
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    ABSTRACT: The spectral analysis techniques has been widely used to extract information about certain signals and the process in which those are generated. Ultrasonic (US) signal acquired from biological tissues are a particular case of that. Spectral analysis has been used to extract information from US signal related to biological tissue condition, mainly related with the internal scattering structure distribution and regularity. High-resolution spectral analysis techniques are applied to determine pathological characteristics that could indicate the presence of disease in the tissue. Some small changes in harmonics location could be drastically affected by the presence of noise in the analyzed signal. In this work, an evaluation of two parametric HR spectral analysis techniques is performed, first applied to signals acquired from a biological tissue phantom in a temperature range from 21°C to 45°C. A second evaluation was performed, considering simulated US signals, which were added with three different levels of SNR, and then they were analyzed with two parametric methods, which present variations in the error estimated for different noise level conditions. But the general behavior is a smaller error when noise decreases.
    PANAMER. HEALTH CARE EXCH. (PAHCE 2013). IEEE, Medellin; 01/2013
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    44th Congreso Nacional / EAA Eur. Sym. / 6° Iberico de Acustica, Valladolid; 01/2013
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    E. Budelli, N. Pérez, P. Lema, C. Negreira
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    ABSTRACT: Experimental determination of time of flight and attenuation has been proposed in the literature as alternatives to monitoring the evolution of milk coagulation during cheese manufacturing. However, only laboratory scale procedures have been described. In this work, the use of ultrasonic time of flight and attenuation to determine cutting time and its feasibility to be applied at industrial scale were analyzed. Limitations to implement these techniques at industrial scale are shown experimentally. The main limitation of the use of time of flight is its strong dependence with temperature. Attenuation monitoring is affected by a thin layer of milk skin covering the transducer, which modifies the signal in a non-repetitive way. The results of this work can be used to develop alternative ultrasonic systems suitable for application in the dairy industry.
    IOP Conference Series Materials Science and Engineering 12/2012; 42(1):2041-.
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    ABSTRACT: El cambio en la velocidad de propagación de ondas ultrasónicas ha sido propuesto como un método para el monitoreo en línea del proceso de coagulación enzimática de leche para la fabricación de queso. El objetivo de este trabajo fue estudiar la influencia de la condición de contorno de temperatura sobre las medidas de tiempo de vuelo (tv) de ondas de compresión durante este proceso. La coagulación se realizó utilizando leche pasteurizada entera a 35 ºC adicionada de CaCl2 y quimosina. Se realizaron medidas de textura instrumental y ultrasónicas en paralelo. Se registró la fuerza en función del tiempo mediante ensayos de extrusión inversa con una sonda cilíndrica plana en un texturómetro TA.XT2i. Para las medidas ultrasónicas se utilizó una celda prismática, en la cual se introdujo un transductor de 3.5 MHz en una de las paredes laterales, registrándose el primer eco reflejado en la pared opuesta. Para este eco se calculó la variación en el tv durante el proceso de coagulación. Durante el proceso de coagulación la fuerza máxima y el tv aumentaron, mientras que la temperatura dentro de la celda de ultrasonido descendió. Se obtuvo una buena correlación entre el tv y la fuerza máxima (R2=0.94) y entre el tv y la temperatura (R2=0.93). Cuando la temperatura de contorno se mantuvo igual que la del medio se registró un aumento en la fuerza máxima pero variaciones no significativas en el tv. Estos resultados permiten concluir que las variaciones en el tv responden a variaciones en la temperatura dentro de la celda de coagulación y no a cambios en la estructura durante el proceso. Esto limita la aplicación de esta técnica para el monitoreo de la coagulación de la leche. http://aiquruguay.org/congreso/download/P83.pdf
    XXVI Congreso Interamericano de Ingeniería Química; 11/2012
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    ABSTRACT: Some biological tissues, such as skeletal muscle, have regular or quasi-periodic structures. Periodicity of these structures can be investigated by spectral methods based on ultrasonic backscattered signals. These methods estimate the Mean Scatterer Spacing (MSS). In this work we analyzed the MSS dependence with temperature in bovine skeletal muscle tissue samples. Through spectral analysis we found that the relative variation of the MSS, in the temperature range 20 - 48°C was of ~ 2% /°C. For the same sample and under the same experimental conditions, we analyzed the propagation velocity of shear waves by transient elastography. It was determined that the relative variation of shear elastic modulus was ~ 3% /°C in the same temperature range. The repeatability of the results was tested in three tissue samples, with a set of 6 measurements on each. The relative variation is therefore similar for both parameters. Thus, this study shows that both techniques can be used as a method of noninvasive thermometry for small temperature range. In addition we emphasize the conceptual differences between both methods. As temperature changes, MSS estimation varies due to two physical phenomena. An apparent variation in mean spacing is produced because the sound speed is temperature-dependent, while tissue expansion produces a real change. In practice it is difficult to discriminate the weight of each component in the MSS variation. On the other side, transient elastography is less sensitive to thermal expansion. Thus it measures the actual shear wave speed change with temperature. This differentiation between both methods could be used to construct useful thermoelastic models for soft tissues in the future.
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    ABSTRACT: Knowledge of muscle mechanical properties is important in clinical practice and biomechanical research. In usual clinical practice the muscle's Young modulus (Y) is qualitatively inferred through manual palpation and others valuation techniques of joint range. Although these methods are sensitive, they have predictable variations between different examiners, which reveals the need for methods capable of providing reliable quantitative values of muscular elasticity. Previous studies have shown that transient elastography (TE) is a useful method for assess locally and in vivo the Young modulus of muscle. In these works, experiments using TE were carried out on biceps brachii of healthy volunteers, testing the muscular elasticity changes under three load conditions: 0, 1 and 2 Kg. However, due to its high cost, this method is currently unavailable in many countries for standard clinical practice. In this work, taking into account the existing background with TE, we propose a low-cost method based on surface wave (SW) propagation for the in vivo assessment of the muscle's global mechanical properties. To validate the method, we repeated the experiments with TE, and compared the quantitative values and the general behavior exhibited by both methods. The results showed a good agreement between TE and SW methods, explaining the differences mainly by diffraction effects. We think that the in the current context, this method could be used widely in the rehabilitative and sports medicine field, both as a monitoring tool as well as research.
    Ultrasonics Symposium (IUS), 2012 IEEE International; 01/2012
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    ABSTRACT: Abstract. In time reversal focalization of acoustic waves, the identification of the system dynamics is usually made in the time domain by sending a short temporal pulse and acquiring the impulse response signal at the focus point. In the case of metallic acoustic cavities, the focalization can be obtained using only one transducer, by putting in phase all the frequency components of the impulse response. In several practical applications, the set of possible focus points must be pre-calibrated, forming a “library” of impulse responses. At the start of these experiments two parameters must be determinate, the sampling frequency and the temporal duration of the acquiring window. In this work, the dependence of the focus amplitude with the sampling frequency and the temporal window is investigated. The practical goal is to obtain the impulse response that allows the time reversal focalization using the minimum memory size and without appreciable loss of amplitude. The results show that there exist a maximum useful sampling frequency and a maximum useful temporal window from which there are not significant increases in the focus amplitude. In that sense, a minimal impulse response is obtained.
    AIP Conference Proceedings 01/2012; 1433:173.
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    ABSTRACT: To achieve a precise noninvasive temperature estimation, inside patient tissues, would open promising research fields, because its clinic results would provide early-diagnosis tools. In fact, detecting changes of thermal origin in ultrasonic echo spectra could be useful as an early complementary indicator of infections, inflammations, or cancer. But the effective clinic applications to diagnosis of thermometry ultrasonic techniques, proposed previously, require additional research. Before their implementations with ultrasonic probes and real-time electronic and processing systems, rigorous analyses must be still made over transient echotraces acquired from well-controlled biological and computational phantoms, to improve resolutions and evaluate clinic limitations. It must be based on computing improved signal-processing algorithms emulating tissues responses. Some related parameters in echo-traces reflected by semiregular scattering tissues must be carefully quantified to get a precise processing protocols definition. In this paper, approaches for non-invasive spectral ultrasonic detection are analyzed. Extensions of author's innovations for ultrasonic thermometry are shown and applied to computationally modeled echotraces from scattered biological phantoms, attaining high resolution (better than 0.1 °C). Computer methods are provided for viability evaluation of thermal estimation from echoes with distinct noise levels, difficult to be interpreted, and its effectiveness is evaluated as possible diagnosis tool in scattered tissues like liver.
    Computational and Mathematical Methods in Medicine 01/2012; Vol. 2012:paper-275405, pp 1-14. · 0.79 Impact Factor
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    ABSTRACT: Parametric algorithms previously developed by authors for spectral evaluation of biological multi-echo waveforms are adapted and improved here in order to achieve a more elevated frequency resolution. These results permit to undertake viability studies around the possible application of a new auto-regressive spectral technique to estimate physical properties like wall thickness changes in blood vessels, with accuracy enough. These difficult and sophisticated measurements in vessels have an increasing interest as tools to estimate basic parameters for calculating elastic properties in the vessel walls. Laboratory thickness data obtained for a latex phantom (mimicking vessel properties) are shown, giving a promising expectative for this improved estimation technique in blood vessels characterization, a diagnostic tool nowadays of growing attention by the researches. In fact, the results suggest clear improvement in spatial resolution, over the classic cross-correlation and non-parametric techniques, to estimate delays between pulsed signals. But, still further efforts and rigorous analyses of ultrasonic echo-signals acquired from well-controlled sanguineous tissues phantoms are needed in order to optimize the potential resolution of this new thickness measurement procedure and also to evaluate its possible clinic limitations.
    Confer. PAHCE-2012. 01/2012; IEEE Catalog number: CFP1218G-PRT:111-116..
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    ABSTRACT: Paper invitado
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    ABSTRACT: Inspired by seismic-noise correlation and time reversal, a shear-wave tomography of soft tissues using an ultrafast ultrasonic scanner is presented here. Free from the need for controlled shear-wave sources, this passive elastography is based on Green's function retrieval and takes advantage of the permanent physiological noise of the human body.
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 07/2011; · 1.50 Impact Factor
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    ABSTRACT: The use of test and calibration systems has great importance during the design & development of medical ultrasonic devices. In the particular case of analyzing in a laboratory new transit-time designs (for flow estimation in cardiac vessels) using phantoms (artificial tissues), it is necessary to confirm the existence of a certain linearity in the measurements, and also to achieve accurate calibrations of those in absolute value. However, the distinct acoustic and elastic properties encountered in the walls of the artificial blood vessels, commonly used in laboratory (based on plastic o silicone materials), during the design and development phases, can significantly alter these results, leading en many cases to wrong conclusions in laboratory evaluations. This work is devoted to study the influence that the characteristics of artificial vessels, respect to the real blood vessels, can exert in the measures acquired by an electronic broadband Transit-Time system, which is due to the distinct propagation times through the walls of the phantom devices.
    IEEE Cat. number CFP1118G-PRT. PROCEEDINGS PHACE. 01/2011; IEEE – PANAMER. HEALTH CARE EXCH. (PAHCE 2011):391-395.

Publication Stats

125 Citations
26.65 Total Impact Points


  • 2012
    • National Polytechnic Institute
      Villa Gustavo A. Madero, The Federal District, Mexico
  • 2007–2012
    • Universidad de Montevideo
      Ciudad de Montevideo, Montevideo, Uruguay
  • 2011
    • University Joseph Fourier - Grenoble 1
      • Institut des Sciences de la Terre (ISTerre)
      Grenoble, Rhône-Alpes, France
  • 2009–2010
    • University of the Republic, Uruguay
      • Facultad de Ciencias
      Montevideo, Departamento de Montevideo, Uruguay
  • 2006
    • Universidade do Algarve
      • Faculdade de Ciências e Tecnologia (FCT)
      Faro, Distrito de Faro, Portugal
  • 2005
    • The Police Academy of the Czech Republic in Prague
      Praha, Praha, Czech Republic
  • 2002
    • National Institute of Industrial Technology
      Buenos Aires, Buenos Aires F.D., Argentina
  • 2000
    • Università degli Studi di Salerno
      Fisciano, Campania, Italy