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- Blood flow in microchannels manufactured by a low cost technique: xurographyPublication . Pinto, Elmano; Taboada, Bruna Rafaela Pereira; Faustino, Vera; Cidre, Diana; Rodrigues, Raquel Oliveira; Miranda, João Mário; Garcia, Valdemar; Dias, Ricardo P.; Lima, Rui A.The xurography is a technique that has been used to make molds to produce microchannels. In contrast to soft lithography [1, 2], xurography uses equipments and materials commonly used in the printing industry, such as cutting plotters, vinyl and other materials. The main advantage of this technique is to fabricate microchannels at a reduced cost [3, 4]. The Fahraeus-Lindqvist effect is a well know phenomenon that happens in microcirculation, where red blood cells (RBCs) have tendency to migrate toward the centre of the microtube resulting in a marginal cell-free layer (CFL) at regions adjacent to the wall [5]. Recently several studies showed strong evidence that the formation of the CFL is affected by the geometry of the microchannel [1, 6, 7] and the physiological conditions of the working fluid, such as the hematocrit (Hct) [2, 8]. The main objective of the present work is to fabricate polydimethysiloxane (PDMS) microchannels by using a soft xurography technique in order perform blood flow studies. Additionally, a high-speed video microscopy system is used to measure the CFL thickness in two different geometries, i. e., bifurcations and confluences.
- Laminar blood flow in stenotic microchannelsPublication . Calejo, Joana A.C.; Garcia, Valdemar; Fernandes, Carla S.In this work, Newtonian and non-Newtonian laminar blood flow in rectangular microchannels with symmetric and asymmetric atheroma were numerically studied. It was observed that the impact of symmetry of the atheroma is almost negligible and the non-Newtonian properties of blood leads to higher pressure drops and wall shear stresses than the ones obtained for Newtonian flows.
- In vitro blood flow behaviour in microchannels with simple and complex geometriesPublication . Garcia, Valdemar; Dias, Ricardo P.; Lima, Rui A.Over the years, various experimental methods have been applied in an effort to understand the blood flow behaviour in microcirculation. The development of optical experimental techniques has contributed to obtain possible explanations on the way the blood flows through microvessels. In recent years, due to advances in computers, optics, and digital image processing techniques, it has become possible to combine a conventional particle image velocimetry (PIV) system with an inverted microscope and consequently improve both spatial and temporal resolution. The present review outlines our most relevant studies on the flow properties of blood at a microscale level by using current micro-PIV and confocal micro-PIV techniques. In this chapter, our recent studies about in vitro blood flow behaviour in microchannels both in straight and with complex geometries are presented. In straight microchannels we present some phenomena such as Fahraeus effect and Fahraeus-Lindqvist effect, the flow of particles and red blood cells (RBCs) in diluted suspensions, the flow of RBCs in concentrated suspensions, the cell-free layer and sedimentations effects. The most recent studies in blood flow through complex geometries such as bifurcations, confluences and stenosis are also reviewed. By using a chromatographic method, the flow of RBC s through a network of microcapillaries is presented.
- In vitro blood flow and cell-free layer in hyperbolic microchannels: visualizations and measurmentsPublication . Rodrigues, Raquel Oliveira; Lopes, Raquel; Pinho, Diana; Pereira, Ana I.; Garcia, Valdemar; Gassmann, Stefan; Sousa, Patrícia C.; Lima, Rui A.Red blood cells (RBCs) in microchannels has 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 centre induced by the tank treading motion of the RBC membrane. As a result there is a formation of a 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. This study aims to characterize the CFL in hyperbolic-shaped microchannels to separate RBCs from plasma. For this purpose, we have investigated the effect of hyperbolic contractions on the CFL by using not only different Hencky strains but also varying the series of contractions. The results show that the hyperbolic contractions with a Hencky strain of 3 and higher, substantially increase the CFL downstream of the contraction region in contrast with the microchannels with a Hencky strain of 2, where the effect is insignificant. Although, the highest CFL thickness occur at microchannels with a Hencky strain of 3.6 and 4.2 the experiments have also shown that cells blockage are more likely to occur at this kind of microchannels. Hence, the most appropriate hyperbolic-shaped microchannels to separate RBCs from plasma is the one with a Hencky strain of 3.
- A influência da temperatura e da radiação solar na capacidade de armazenamento térmico de elementos construtivosPublication . Almeida, Manuela; Garcia, Valdemar; Maldonado, Eduardo; Bragança, LuisA capacidade de armazenamento térmica dos diversos elementos construtivos depende de vários factores, sendo a radiação solar incidente e as amplitudes térmicas verificadas nos espaços adjacentes, dois dos factores que mais fortemente a afectam e que não têm sido levadas directamente em conta na sua quantificação. Neste trabalho apresenta-se a metodologia que foi usada para o cálculo da quantidade de calor armazenada diariamente por esses elementos, QARM, descrevendo-se a forma como foram estabelecidas as dependências deste parâmetro com a radiação solar incidente e com a amplitude térmica diária dos perfis das temperaturas ambientes dos espaços confinantes com o elemento. Mostra-se como foram desenvolvidas correlações entre estes parâmetros e apresentam-se os resultados obtidos para alguns dos elementos construtivos mais utilizados nos edifícios em Portugal.
- Fabricação de microcanais utilizando o método de xurografia: visualização da camada livre de célulasPublication . Pinto, Elmano; Pinho, Diana; Bento, David; Correia, Teresa Montenegro; Garcia, Valdemar; Dias, Ricardo P.; Miranda, João MárioNo presente estudo, pretendeu-se desenvolver/melhorar uma metodologia de microfabricação de baixo custo, conhecida por xurografia e efectuar a visualização e quantificação da camada livre de células para diferentes escoamentos sanguíneos em microcanais.
- Visualization and measurement of red blood cells flowing in microfluidic devicesPublication . Rodrigues, Raquel Oliveira; Pinho, Diana; Faustino, Vera; Yaginuma, Tomoko; Bento, David; Fernandes, Carla S.; Garcia, Valdemar; Lima, Rui A.Several experimental techniques were performed in the past years using in vitro environments, in an attempt to not only understand the blood flow behaviour in microcirculation but also develop microfluidic devices as an alternative clinical methodology to detect blood diseases. Hence, the visualization and measurement of red blood cells (RBCs) flowing in a microfluidic device are important to provide not only essential information about hydrodynamic characteristics of the blood but also vital information to diagnose the initial symptoms of diseases during clinical investigations. For instance, RBC rigidity has been correlated with myocardial infarction, diabetes mellitus, hypertension, and also other haematological disorders and diseases that affect RBC deformation more directly, such as, hereditary spherocytosis, sickle cell anaemia and malaria. Regarding a better understanding of the RBCs deformation and motion, we present in this paper a compilation of studies made in our research group, using several microfluidic devices with different microchannel geometries and fabrication techniques (i.e., soft-lithography, xurography and hybrids) that focus in the shear and extensional flow behaviour, either in healthy or chemically stiffed RBCs.
- Gas-liquid flow dispersion in a vertical pipePublication . Garcia, Valdemar; Sobrinho Teixeira, João AlbertoThe main purpose of this work was to analyse the transport f a solute dissolved in a flowing liquid in upward direction at low velocity along a vertical column, in the bottom of which slugs are injected. These experiments were carried out in a column with an internal diameter of 19 mm and height of 344.2 cm. Liquid flowrates of 1.0, 2.5 and 5.5 cm3/s were utilized, and an enlarged variety of gas flowrates were injected; in these conditions the slugs were normally formed at a frequency between 0.78 and 88.1 s-1. The scientific techniques used in this study based on the measurement, at the top of the column, of a tracer solution injected in its bottom. A simplified physical model was developed to explain the results obtained, which was based on two hypotheses: (i) the flow of the liquid between two slugs is laminar, being the respective profile of velocity approximate to the Poiseuille law; (ii) the stirring action provoked by each slug in the liquid is limited to a small region, the wake of slug, which can be taken as a “perfectly mixed tank”. One of the most relevant conclusions derived from this work and which at first analysis would appear paradoxical, is that, it is possible to proceed in such a way that the bubbling of gas through a liquid will provoke less dispersion in the residence time of liquid than which would be obtained in the absence of the introduction of gas.
- Blood flow visualization and measurements in microfluidic devices fabricated by a micromilling techniquePublication . Singhal, Jaron; Pinho, Diana; Lopes, Raquel; Sousa, Patrícia C.; Garcia, Valdemar; Schütte, Helmut; Lima, Rui A.; Gassmann, StefanThe most common and used technique to produce microfluidic devices for biomedical applications is the soft-lithography. However, this is a high cost and time-consuming technique. Recently, manufacturers were able to produce milling tools smaller than 100 μm and consequently have promoted the ability of the micromilling machines to fabricate microfluidic devices capable of performing cell separation. In this work, we show the ability of a micromilling machine to manufacture microchannels down to 30 μm and also the ability of a microfluidic device to perform partial separation of red blood cells from plasma. Flow visualization and measurements were performed by using a high-speed video microscopy system. Advantages and limitations of the micromilling fabrication process are also presented.
- Flow focusing technique to produce PDMS microparticles for blood analogue fluidsPublication . Silva, S.F.; Vega, E.J.; Pinho, Diana; Garcia, Valdemar; Lima, Rui A.; Montanero, J.M.The study of the blood flow behaviour through microchannels is crucial to improve our understanding about blood flow phenomena happening in the human microcirculatory system. However, the difficulties associated with the use of in vitro blood, such as coagulation and sample storage, have promoted the increasing interest to develop fluids with rheological properties similar to real blood [1]. Polydimethysiloxane (PDMS), due its remarkable properties such as good optical transparency, biocompatibility and permeability to gases, is widely used to fabricate microfluidic devices for in vitro blood experiments [2]. Recently, this inert elastomer has been used to produce monodisperse PDMS microbeads through a microfluidic approach [3]. Jiang et. al. have proposed a flow-focusing technique where a PDMS precursor was dispersed into microdroplets within an aqueous continuous phase [3]. By using this method they were able to produce PDMS microbeads with an average dimension of 80 microns. However, to develop blood analogue fluids it is essential to have PDMS microparticles with dimensions more close to the blood cells, i. e., the microparticles should have dimensions smaller than 20 microns. Hence, in this study a novel flow focusing technique was used to produce PDMS microparticles with dimensions more close to real blood cells. This technique was recently proposed to produce jets, droplets, and emulsions with sizes ranging from tens of microns down to the submicrometer scale [4]. This procedure is also based on the flow focusing principle which the above mentioned method relies on. Nevertheless, our technique makes use of the breakage of a steady jet to form the microparticles, which can lead to much higher production rates. In our technique, liquid is injected at a constant flow rate through a hypodermic needle to form a film over the needle’s outer surface. This film flows toward the needle tip until a liquid ligament is steadily ejected. Both the film motion and the liquid ejection are driven by the viscous and pressure forces exerted by a coflowing fluid stream. The outcome is a capillary jet which breaks up into droplets.