Rheology of vitreous humour and pharmacological substitutes used in eye surgery

The Vitreous Humour (VH) is a transparent gel that fills the vitreous cavity in the Human eye. VH is mainly composed of water (99%) and includes a network of protein fibrils (collagen) and hyaluronan (1%). It is known that VH becomes progressively liquefied with age, or as a consequence of some diseases, and consequently the rheological, biomechanical and structural properties of the fluid also change. For the normal operation of the eye, when vitreoretinal pathologies appear, effective treatments must be performed. A common treatment for these diseases is the injection of a VH substitute in the vitreous cavity. The major vitreous substitutes commercially available are silicone oils and perfluorocarbon liquids, but currently there is little information about their rheological properties, which are known to be intimately linked to their functionality. The aim of this work is to measure the rheological properties of the VH and five commercial vitreous substitutes currently used in vitreoretinal surgeries. Six fluids were analysed: VH collected from rabbit eyes, two silicone oils (RS-Oil1000 and RS-Oil5000, Alchimia), two perflurocarbon liquids (HPF8 and HPF10, Alchimia) and a mixture of silicone oil with perfluorocarbon (Densiron68, Fluoron). The rheological characterization of the fluids was done using a shear rheometer (DHR-2, TA Instruments) to perform steady shear, small amplitude oscillatory shear (SAOS) and creep measurements. Extensional measurements using the Slow Retraction Method were performed with a capillary break-up extensional rheometer (Haake CaBER1, ThermoElectron) and an in-house extensional rheometer. Our results show that VH separates into two different phases (gel and liquid) and both phases exhibit viscoelastic behaviour. The liquid phase rheology is independent of time but the gel properties are time dependent, with the VH gel phase exhibiting storage (G’) and loss (G’’) moduli that increase with time after dissection (e.g. the maximum value of G’ and G’’ increase from 3.2 Pa and 0.9 Pa right after dissection to 11.4 Pa and 2.3 Pa a few hours after dissection). Despite being used for the same purpose, the pharmacological fluids show significantly different rheological properties between them and when compared with the VH. All pharmacological fluids tested display a Newtonian behaviour in steady shear flow and their viscosities vary from 0.49 mPa s to 4.57 Pa s, at 37°C

Multiple inlet sudden expansion flow of power-law fluids

The flow of power-law fluids through two-dimensional planar sudden expansion geometries with multiple inlets was simulated numerically using an in-house finite volume code. The fluids are modelled using the power-law model, and a range of power-law index values (0.4 ≤ n ≤ 1.4) was investigated to cover shear-thinning, Newtonian and shear-thickening inelastic behaviour. The effect of the generalised Reynolds number and the spacing between inlets on the flow behaviour and stability was analysed. The main characteristics of the vortices formed near the wall are similar to those found in single inlet sudden expansions, with symmetric wall vortices forming at low Reynolds numbers, with transitions to steady asymmetric, third eddy flow regimes and time-dependent flow at higher generalised Reynolds numbers. In some cases, the steady asymmetric and third eddy regimes are absent and the flow transitions directly from symmetric flow to time dependent flow (as seen for the geometry with the largest spacing). The stabilising nature of shear-thinning behaviour was observed and the opposite effect was seen for shear-thickening behaviour. In addition, intermediate vortices were seen to form between inlets which grow in size within the symmetric regime as the generalised Reynolds number is increased. New scalings which help collapse the data are introduced and a strong effect of inlet spacing on all flow transitions is shown, with the ratio of the wall distance to inlet height being a determining factor on the corner vortices length when the spacing between inlets is sufficiently large

Regime mapping and the role of the intermediate region in wall-coated microreactors

Operation of a wall-coated microreactor can occur in several mass transfer-reaction regimes. We define these regimes analytically in several planes of a multi-parametric map, taking into account the different degrees of concentration profile development, as well as the influence of non-unity orders of reaction and reactant inhibition in the kinetic law. It was found that the regions where conversion can be calculated from simplified mass transfer models are not discriminated by common results for entrance-length. We also illustrate the trade-offs that exist across this operating map concerning the catalyst design (costs associated with loading and volume) and overall system performance (evaluated in terms of reactant conversion, flow efficiency and microreactor effectiveness). It is shown that under certain conditions, the existence of moderate mass transfer resistance can be advantageous (even if internal limitations cannot be avoided), clarifying the role of the intermediate transport-reaction region

Hyperbolic contraction measuring systems for extensional flow

In this paper an experimental method for extensional measurements on medium viscosity fluids in contraction flow is evaluated through numerical simulations and experimental measurements. This measuring technique measures the pressure drop over a hyperbolic contraction, caused by fluid extension and fluid shear, where the extensional component is assumed to dominate. The present evaluative work advances our previous studies on this experimental method by introducing several contraction ratios and addressing different constitutive models of varying shear and extensional response. The constitutive models included are those of the constant viscosity Oldroyd-B and FENE-CR models, and the shear-thinning LPTT model. Examining the results, the impact of shear and first normal stress difference on the measured pressure drop are studied through numerical pressure drop predictions. In addition, stream function patterns are investigated to detect vortex development and influence of contraction ratio. The numerical predictions are further related to experimental measurements for the flow through a 15:1 contraction ratio with three different test fluids. The measured pressure drops are observed to exhibit the same trends as predicted in the numerical simulations, offering close correlation and tight predictive windows for experimental data capture. This result has demonstrated that the hyperbolic contraction flow is well able to detect such elastic fluid properties and that this is matched by numerical predictions in evaluation of their flow response. The hyperbolical contraction flow technique is commended for its distinct benefits: it is straightforward and simple to perform, the Hencky strain can be set by changing contraction ratio, non-homogeneous fluids can be tested, and one can directly determine the degree of elastic fluid behaviour. Based on matching of viscometric extensional viscosity response for FENE-CR and LPTT models, a decline is predicted in pressure drop for the shear-thinning LPTT model. This would indicate a modest impact of shear in the flow since such a pressure drop decline is relatively small. It is particularly noteworthy that the increase in pressure drop gathered from the experimental measurements is relatively high despite the low Deborah number range explored

Investigating the stability of viscoelastic stagnation flows in T-shaped microchannels

We investigate the stability of steady planar stagnation flows of a dilute polyethylene oxide (PEO) solution using T-shaped microchannels. The precise flow rate control and well-defined geometries achievable with microfluidic fabrication technologies enable us to make detailed observations of the onset of elastically driven flow asymmetries in steady flows with strong planar elongational characteristics. We consider two different stagnation flow geometries; corresponding to T-shaped microchannels with, and without, a recirculating cavity region. In the former case, the stagnation point is located on a free streamline, whereas in the absence of a recirculating cavity the stagnation point at the separating streamline is pinned at the confining wall of the microchannel. The kinematic differences in these two configurations affect the resulting polymeric stress fields and control the critical conditions and spatiotemporal dynamics of the resulting viscoelastic flow instability. In the free stagnation point flow, a strand of highly oriented polymeric material is formed in the region of strong planar extensional flow. This leads to a symmetry-breaking bifurcation at moderate Weissenberg numbers followed by the onset of three-dimensional flow at high Weissenberg numbers, which can be visualized using streak-imaging and microparticle image velocimetry. When the stagnation point is pinned at the wall this symmetry-breaking transition is suppressed and the flow transitions directly to a three-dimensional time-dependent flow at an intermediate flow rate. The spatial characteristics of these purely elastic flow transitions are compared quantitatively to the predictions of two-dimensional viscoelastic numerical simulations using a single-mode simplified Phan-Thien–Tanner (SPTT) model.Swiss National Science Foundation (Grant PBEZ2- 115179)Fundação para a Ciência e a Tecnologia (FCT) (Grant PTDC/EQU-FTT/71800/2006)Fundação para a Ciência e a Tecnologia (FCT) (Grant PTDC/EQU-FTT/70727/2006

Profile and regulation of annexin II expression during early embryogenesis in cattle Perfil e regulação da expressão da anexina II durante a embriogênese em bovinos

The presence of annexin II (Ann-II) during the initial stages of bovine embryo development and the regulation of Ann-II expression by retinol and insulin-like growth factor I (IGF-I) were studied. Bovine embryos at different stages of development were produced in vitro on Synthetic Oviductal Fluid (SOF) medium (control group), SOF supplemented with retinol (retinol group; 0.1ng/ml), or IGF-I (IGF-I group; 10ng/ml). The embryos were processed for mRNA extraction, cDNA production and polymerase chain reaction (PCR) using Ann-II-specific oligonucleotides. Ann-II was detected in all stages of early embryo development, except for the 16-cell stage. The blastocyst rates were significantly higher (P<0.05) in the group supplemented with retinol (37.8%, 45/119) during in vitro embryo culture (IVC) than in those cultured in SOF (20.5%, 24/117) or SOF with IGF-I (25.8%, 24/93). Semiquantitative analysis of Ann-II expression in embryos produced in medium supplemented with IGF-I or retinol revealed a lower expression of this gene when compared with embryos cultured in SOF (P<0.05). The Ann-II expression was not different in embryos cultured in the presence of retinol and IGF-I. The presence of retinol increased the production of embryos in vitro by decreasing the expression of Ann-II in early-stage of bovine embryo.<br>Foram estudadas a presença de anexina II (Ann-II) durante a fase inicial do desenvolvimento embrionário bovino e sua regulação pelo retinol e pelo fator de crescimento semelhante à insulina (IGF-I). Embriões bovinos em diferentes estádios de desenvolvimento foram produzidos in vitro em fluido sintético de oviduto (SOF) sem suplementação (grupo-controle) ou suplementado com retinol (grupo retinol; 0,1ng/ml medium) ou IGF-I (grupo IGF-I; 10 ng/ml de meio). Os embriões foram processados para extração de mRNA, produção de cDNA e posterior análise por reação em cadeia da polimerase (PCR) com oligonucleotídeos específicos para Ann-II. Em todos os estádios de desenvolvimento embrionário, Ann-II foi detectada, exceto no estádio de 16 células. Os índices de blastocisto foram significativamente maiores (P<0,05) no grupo suplementado com retinol (37,8%, 45/119) durante o cultivo in vitro de embriões (PIV) que aqueles obtidos no grupo controle (20,5%, 24/117) ou no IGF-I (25,8%, 24/93). Análise semiquantitativa da expressão de Ann-II em embriões produzidos em meio suplementado com IGF-I ou retinol revelou uma menor expressão desse gene quando comparado com embriões cultivados somente em SOF (P<0,05). A expressão de Ann-II não foi diferente em embriões cultivados na presença de retinol e IGF-I. A presença de retinol aumentou a produção de embriões in vitro, e diminuiu a expressão de Ann-II em estádios iniciais do desenvolvimento embrionário bovino

Efeito do tamanho da presa e do acréscimo de ração na larvicultura de pacamã Effect of prey size selection and feeding on the larviculture of pacamã

Objetivou-se estudar o efeito do tamanho da presa, com ou sem adição de ração na larvicultura do pacamã (Lophiosilurus alexandri), espécie nativa da bacia do Rio São Francisco ameaçada de extinção. As larvas foram cultivadas em aquários por 16 dias e alimentadas com zooplâncton natural, selecionado em malhas de 650 e 1.300 µm, com ou sem ração. A oferta de zooplâncton de maior tamanho resultou em melhor desenvolvimento das larvas de pacamã, comprovando a vantagem do fornecimento de partículas alimentares maiores para o potencial de crescimento da espécie. O aumento na quantidade de ração não influenciou o desempenho das larvas, evidenciando a baixa ou falta de capacidade das larvas em digerir a ração ofertada. O fornecimento de ração diminuiu o pH e aumentou a condutividade elétrica, prejudicando a qualidade da água, o que não afetou o desempenho das larvas.<br>The effect of prey size selection and feeding on pacamã Lophiosilurus alexandri larvae culture (native specie of San Francisco basin classified as threatened of extinction) was evaluated. The larvae were cultivated in aquariums during 16 days and fed with natural zooplankton, screened in 650 and 1300 µm sieve, with and without ration. The offer of zooplankton of largest size provided a better development of the pacamã larvae, demonstrating the need of alimentary particles larger for the growth potential of the species. The increase of the amount of ration did not affect the performance of the larvae, evidencing low or lack of capacity in digesting the offered ration. The ration fed reduced the pH and increased the electric conductivity, worsening water quality, which did not affect the performance of the larvae