Estudo experimental de escoamentos fisiológicos em microcanais fabricados por xurografia

Este projecto de mestrado em Tecnologia Biomédica é de natureza experimental e possui como principais pontos de interesse a fabricação de microcanais em polidimetilsiloxano (PDMS) a baixo custo e a visualização do escoamento no interior dos microcanais produzidos. Constituem como principais objectivos deste projecto a fabricação de microcanais em PDMS com recurso a tecnologias aplicadas à indústria gráfica e efectuar a sua validação. Propôs-se desenvolver/melhorar uma metodologia de microfabricação de baixo custo, conhecida por xurografia, efectuar a visualização e quantificação de vários fenómenos associados ao escoamento sanguíneo em microcanais. A técnica desenvolvida neste projecto possui como referencia a litografia suave, que por sua vez é a técnica mais utilizada na área da microfabricação. No entanto esta técnica é muito dispendiosa e requer uma sala limpa. Devido à inexistência de uma sala limpa no Instituto Politécnico de Bragança, neste trabalho foi utilizada uma plotter de corte e diversos materiais utilizados na indústria gráfica (vinil, papel adesivo, entre outros) para a produção de um molde e microcanais em PDMS. As geometrias utilizadas para o estudo possuíam larguras com dimensões a variar entre os 150μm e os 1000μm. Estas possuíam ramificações com zonas de bifurcações e consequentes zonas de confluências. Apos a fabricação dos microcanais procedeu-se ao estudo do escoamento de fluidos fisiológicos nesses microcanais. Os fluidos fisiológicos utilizados foram sangue ovino com percentagens de hematócrito entre 1% e 15% em dextrano 40. Foram também testados caudais variáveis entre 5 e 15μL/min. Procedeu-se à visualização do escoamento dos fluidos utilizando um sistema de microscópica e captaram-se várias imagens dos microcanais, nomeadamente antes da bifurcação e depois da confluência. As imagens foram tratadas num software informático, quantificando a espessura da camada livre de células formada junto das paredes e a jusante da confluência. Os resultados obtidos demonstram que a técnica designada por xurografia pode ser utilizada para estudar vários fenómenos fisiológicos existentes na microcirculação. This project of the master's degree in Biomedical Technology is an experimental work and includes as the principal interests the fabrication of microchannels of polydimethylsiloxane (PDMS) at low cost and the ability to study blood flow phenomena within the microchannels produced. The main objective of this project is to fabricate microchannels in PDMS using the technologies applied to the printing industry and validation of their performance. It has been proposed to develop/improve a low cost method of microfabrication, to acquire know how about xurography and to visualize and quantify several physiological phenomena associated with blood flow in microchannels. The technique used as a reference in this project was soft lithography, which is the most used technique in the field of microfabrication. However this technique is very expensive and requires a clean room. Due to the lack of a clean room at the Polytechnic Institute of Bragança, a cutting plotter and various materials used in the printing industry (technique known as xurography) were used to produce the molds and the correspondent microchannels in PDMS. The geometries used in this study had widths with dimensions ranging from 150μm up to 1000μm. These channels had ramifications with bifurcations and confluences regions. After the fabrication of the microchannel the physiological fluid flow in these microchannels were studied. The physiologic fluids used were sheep blood with dextran 40 of which hematocrit percentages were between 1% and 15%. The flow rates varying between 5 and 15μL/min were also tested. The fluid flows were visualized using a microscopic system and captured images of the various microchannels, including especially before the bifurcation and after the confluence. The images were processed with a computer software to quantify the thickness of the cell free layer formed adjacent to the microchannel walls and downstream of the confluence. The results demonstrate that the technique known as xurography can be applied to investigate blood flow phenomena happening in microcirculation

Predictive Optimal Matrix Converter Control for a Dynamic Voltage Restorer with Flywheel Energy Storage

This paper presents a predictive optimal matrix converter controller for a flywheel energy storage system used as Dynamic Voltage Restorer (DVR). The flywheel energy storage device is based on a steel seamless tube mounted as a vertical axis flywheel to store kinetic energy. The motor/generator is a Permanent Magnet Synchronous Machine driven by the AC-AC Matrix Converter. The matrix control method uses a discrete-time model of the converter system to predict the expected values of the input and output currents for all the 27 possible vectors generated by the matrix converter. An optimal controller minimizes control errors using a weighted cost functional. The flywheel and control process was tested as a DVR to mitigate voltage sags and swells. Simulation results show that the DVR is able to compensate the critical load voltage without delays, voltage undershoots or overshoots, overcoming the input/output coupling of matrix converters

Fast optimum-predictive control and capacitor voltage balancing strategy for bipolar back-to-back NPC converters in high-voltage direct current transmission systems

Multilevel power converters have been introduced as the solution for high-power high-voltage switching applications where they have well-known advantages. Recently, full back-to-back connected multilevel neutral point diode clamped converters (NPC converter) have been used inhigh-voltage direct current (HVDC) transmission systems. Bipolar-connected back-to-back NPC converters have advantages in long-distance HVDCtransmission systems over the full back-to-back connection, but greater difficulty to balance the dc capacitor voltage divider on both sending and receiving end NPC converters. This study shows that power flow control and dc capacitor voltage balancing are feasible using fast optimum-predictive-based controllers in HVDC systems using bipolar back-to-back-connected five-level NPC multilevel converters. For both converter sides, the control strategytakes in account active and reactive power, which establishes ac grid currents in both ends, and guarantees the balancing of dc bus capacitor voltages inboth NPC converters. Additionally, the semiconductor switching frequency is minimised to reduce switching losses. The performance and robustness of the new fast predictive control strategy, and its capability to solve the DC capacitor voltage balancing problem of bipolar-connected back-to-back NPCconverters are evaluated

HVDC transmission systems: Bipolar back-to-back diode clamped multilevel converter with fastoptimum-predictive control and capacitor balancing strategy

Voltage source multilevel power converter structures are being considered for high power high voltage applications where they have well known advantages. Recently, full back-to-back connected multilevel neutral diode clamped converters (NPC) have been used in high voltage direct current (HVDC) transmission systems. Bipolar back-to-back connection of NPCs have advantages in long distance HVDC transmission systems, but highly increased difficulties to balance the dc capacitor voltage dividers on both sending and receiving end NPCs. This paper proposes a fast optimum-predictive controller to balance the dc capacitor voltages and to control the power flow in a long distance HVDCsystem using bipolar back-to-back connected NPCs. For both converter sides, the control strategy considers active and reactive power to establish ac grid currents on sending and receiving ends, while guaranteeing the balancing of both NPC dc bus capacitor voltages. Furthermore, the fast predictivecontroller minimizes the semiconductor switching frequency to reduce global switching losses. The performance and robustness of the new fast predictive control strategy and the associated dc capacitors voltage balancing are evaluated. (C) 2011 Elsevier B.V. All rights reserved

Cell-free layer analysis in a polydimethysiloxane microchannel: A global approach

The cell-free layer (CFL) is a hemodynamic phenomenon that has an important contribution to the rheological properti es of blood flowing in microvessels. The present work aims to find the closest function describing RBCs flowing around the cell depleted layer in a polydimethysiloxane (PDMS) microchannel with a diverging and a converging bifurcation. The flow behaviour of the CFL was investigated by using a high-speed video microscopy system where special attention was devoted to its behaviour before the bifurcation and after the confluence of the microchannel. The numerical data was first obtained by using a manual tracking plugin and then analysed using the genetic algorithm approach. The results show that for the majority of the cases the function that more closely resembles the CFL boundary is the sum of trigonometric functions.The authors acknowledge the financial support provided by PTDC/SAU-ENB/116929/ 2010 and EXPL/EMS-SIS/2215/2013 from FCT (Science and Technology Foundation), COMPETE, QREN and European Union (FEDER). R.O. Rodrigues, D. Pinho and V. Faustino acknowledge respectively, the PhD scholarships SFRH/BD/97658/2013, SFRH/BD/89077/2012 and SFRH/BD/99696/2014 granted by FCT.info:eu-repo/semantics/publishedVersio

On the Problem of Balancing the DC Capacitor Voltage Divider in Back-to-Back Multilevel Converters

This paper presents a new generalized solution for DC bus capacitors voltage balancing in back-to-back m level diode-clamped multilevel converters connecting AC networks. The solution is based on the DC bus average power flow and exploits the switching configuration redundancies. The proposed balancing solution is particularized for the back-to-back multilevel structure with m=5 levels. This back-to-back converter is studied working with bidirectional power flow, connecting an induction machine to the power grid

A rapid and low-cost nonlithographic method to fabricate biomedical microdevices for blood flow analysis

Microfluidic devices are electrical/mechanical systems that offer the ability to work with minimal sample volumes, short reactions times, and have the possibility to perform massive parallel operations. An important application of microfluidics is blood rheology in microdevices, which has played a key role in recent developments of lab-on-chip devices for blood sampling and analysis. The most popular and traditional method to fabricate these types of devices is the polydimethylsiloxane (PDMS) soft lithography technique, which requires molds, usually produced by photolithography. Although the research results are extremely encouraging, the high costs and time involved in the production of molds by photolithography is currently slowing down the development cycle of these types of devices. Here we present a simple, rapid, and low-cost nonlithographic technique to create microfluidic systems for biomedical applications. The results demonstrate the ability of the proposed method to perform cell free layer (CFL) measurements and the formation of microbubbles in continuous blood flow.The authors acknowledge the financial support provided by PTDC/SAU-BEB/105650/2008, PTDC/SAU-ENB/116929/2010, EXPL/EMS-SIS/2215/2013 and scholarship SFRH/BD/89077/2012 and SFRH/BD/97658/2013 from FCT (Science and Technology Foundation), COMPETE, QREN and European Union (FEDER).info:eu-repo/semantics/publishedVersio

Flow of red blood cells in microchannel networks: in vitro studies

Human blood is a multiphase biofluid primarily composed by the deformable red blood cells (RBCs) suspended in plasma. Because the complex structure of RBCs, blood exhibits unique flow characteristics on micro-scale level, due to their complex biochemical mechanisms and their response to both shear and extensional flow, which influence the rheological properties and flow behaviour of blood [1,2]. In the past years in vitro blood studies have been extensively performed and some important physiological phenomena, such as Fahraeus and Fahraeus-Lindqvist effect, were revealed [1,3]. This pioneer studies performed by Fahraeus and Fahraeus-Lindqvist in straight glass microchannels [4] revealed that for narrow tubes (diameter<300 μm), the apparent viscosity of blood declines with decreasing diameter. More recently, due to the developments in microscopy, computers and image analysis techniques, several researchers have used new measuring methods to obtain deeper quantitative understanding of the blood flow dynamics, in vitro [5-8] and in vivo experiments [9-10]. The increasing interest by the microfluidic and biomedical communities has also played a key role in several recent developments of lab-on-chip devices for blood sampling, analysis and cell culturing, aimed in a near future, the development of blood diagnostic devices, as an alternative tool to the traditional diagnostic strategies. However, the blood flow in microvascular networks phenomena remains incompletely understood. Thus, it is important to investigate in detail the behaviour of RBCs flow occurring in a microchannel network, such as, with divergent and convergent bifurcations, which mimics the irregular vessel segments linked by numerous diverging and converging bifurcations. Previously, we made in vitro studies in microchannels with a simple divergent and convergent bifurcation, that showed a pronounced cell-free layer (CFL) immediately downstream of the apex of the convergent bifurcation [1,4]. This interesting result led us to the present work, where the CFL in a microchannel network is investigated by using a high-speed video microscopy system in order to further understand the blood flow behaviour in microvessels networks

Visualization of the cell-free layer (CFL) in a PDMS microchannel with a micro-stenosis

Red blood cells (RBCs) have a tendency to undergo axial migration due to the parabolic velocity profile which results in a high shear stress around wall that forces the RBC to move towards the center induced by the tank treading motion of the RBC membrane. As a result there is a formation of cell-free layer (CFL) with extremely low concentration of cells. Based on this phenomenon several works have proposed microfluidic designs to separate the suspending physiological fluid from whole in vitro blood. However, most of these studies have the aim of the complete extraction of cells from plasma which is not the case of the present study. The biomedical device that is present in this work aims to obtain a CFL with a low enough RBC propose a combination of image analysis techniques able to measure automatically the CFL thickness before and after micro-stenosis is used