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Marques Mendes Almeida da Rosa Leal, Catarina
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- Real-time rheology of actively growing bacteriaPublication . Portela, R.; Almeida, Pedro L.; Patricio, Pedro; Cidade, T.; Sobral, R. G.; R. Leal, CatarinaThe population growth of a Staphylococcus aureus culture, an active colloidal system of spherical cells, was followed by rheological measurements, under steady-state and oscillatory shear flows. We observed a rich viscoelastic behavior as a consequence of the bacteria activity, namely, of their multiplication and density-dependent aggregation properties. In the early stages of growth (lag and exponential phases), the viscosity increases by about a factor of 20, presenting several drops and full recoveries. This allows us to evoke the existence of a percolation phenomenon. Remarkably, as the bacteria reach their late phase of development, in which the population stabilizes, the viscosity returns close to its initial value. Most probably, this is caused by a change in the bacteria physiological activity and in particular, by the decrease of their adhesion properties. The viscous and elastic moduli exhibit power-law behaviors compatible with the "soft glassy materials" model, whose exponents are dependent on the bacteria growth stage. DOI: 10.1103/PhysRevE.87.030701.
- Living S. aureus bacteria rheologyPublication . Portela, R.; Franco, J. M.; Patricio, Pedro; Almeida, Pedro L.; Sobral, R. G.; Leal, Catarina R.The rheological characterization of Staphylococcus aureus cultures has shown a complex and rich viscoelastic behavior, during the bacteria population growth, when subject to a shear flow [1,2]. In particular, in stationary shear flow, the viscosity keeps increasing during the exponential phase reaching a maximum value (∼30x the initial value) after which it drops and returns close to its initial value in the stationary phase of growth, where the cell number of the bacterial population stabilizes. These behaviors can be associated with cell density and aggregation patterns that are developed during culture growth, showing a collective behavior. This behavior has no counterpart in the bacterial growth curve obtained by optical density monitorization (OD620nm and cfus/ml measurements). In oscillatory flow, the elastic and viscous moduli exhibit power-law behaviors whose exponents are dependent on the bacteria growth stage. These power-law dependencies of G’ and G’’ are in accordance with the Soft Glassy Material model [3], given by: G’~ ωx and G’’~ ωx To describe the observed behavior, a microscopic model considering the formation of a dynamic web-like structure was hypothesized [1], where percolation phenomena can occur, depending on the growth stage and on cell density. Recently, using real-time image rheology was possible to visualize the aggregation process associated with these dramatic changes in the viscoelastic behavior. In particular, the formation of web-like structures, at a specific time interval during the exponential phase of the bacteria growth and the cell sedimentation and subsequent enlargement of bacterial aggregates in the passage to the stationary phase of growth. These findings were essential to corroborate the microscopic model previously proposed and the main results of this study are compiled and presented in this work, see Fig.1.
- Bacterial cellulose: a versatile biopolymer for wound dressing applicationsPublication . Portela, Raquel; R. Leal, Catarina; Almeida, Pedro L.; Sobral, RitaAlthough several therapeutic approaches are available for wound and burn treatment and much progress has been made in this area, room for improvement still exists, driven by the urgent need of better strategies to accelerate wound healing and recovery, mostly for cases of severe burned patients. Bacterial cellulose (BC) is a biopolymer produced by bacteria with several advantages over vegetal cellulose, such as purity, high porosity, permeability to liquid and gases, elevated water uptake capacity and mechanical robustness. Besides its biocompatibility, BC can be modified in order to acquire antibacterial response and possible local drug delivery features. Due to its intrinsic versatility, BC is the perfect example of a biotechnological response to a clinical problem. In this review, we assess the BC main features and emphasis is given to a specific biomedical application: wound dressings. The production process and the physical-chemical properties that entitle this material to be used as wound dressing namely for burn healing are highlighted. An overview of the most common BC composites and their enhanced properties, in particular physical and biological, is provided, including the different production processes. A particular focus is given to the biochemistry and genetic manipulation of BC. A summary of the current marketed BC-based wound dressing products is presented, and finally, future perspectives for the usage of BC as wound dressing are foreseen.
- Aging effects on the rheology of LC solutions of hydroxypropylcellulosePublication . Leal, Catarina R.; Godinho, M. H.; Martins, A. F.; Fried, F.We apply the recently developed continuum theory of liquid crystalline polymers1 to interpret the behavior of the shear viscosity η(γ.) and the first normal-stress difference N1(γ.) measured for liquid crystalline solutions (c = 37%, c > c*) of HPC (Aldrich, Mw = 100,000) in acetic acid (AA) with different ages: 1 day, 1 month and 2 years. η(γ.) and N1(γ.) were measured over four decades in γ. N1(γ.) is observed to change from positive to negative and again to positive, as the shear rate γ. increases. η(γ.) shows a small newtonian plateau at low shear rates and a strong shear-thinning at higher values of γ. The rate of decrease of η(γ.) in this region is not monotonous, as usual, but shows an “hesitation” similar to one previously observed in a different system2. The aging effect promotes a “depolymerization”3 of HPC. This, in turn, should have a strong influence on the behavior of Nl(γ.) and η(γ.) which is indeed observed4. All these observations can be rationalized within the framework of the theory1.
- Rheo-NMR velocimetry characterisation of PBLG/m-cresolPublication . Leal, Catarina R.; Feio, Gabriel; Almeida, Pedro L.NMR spectroscopy was used to characterise the velocity profile in nematic solutions of poly(gamma-benzyl-L-glutamate) (PBLG) in m-cresol during the application of a simple shear-flow at constant shear-rate values. In this study, the rheo-NMR technique was used that allows an insight over the fluid dynamics during the application of a shear flow in the presence of an external magnetic field, in a perpendicular direction to the shear gradient. Using this Magnetic Resonance Imaging (MRI) based technique, the velocity profile developed inside the system is accessed.
- NMR study of flow and viscoelastic properties of PBLG/m-Cresol lyotropic liquid crystalPublication . Veron, Alain; Gomes, Alexandre E.; Leal, Catarina R.; Klink, Jacques Van Der; Martins, Assis F.Director distortion patterns and backflow effects in transient flows of nematic solutions of PBLG/m-cresol are studied by deuteron NMR. The simulation of the observed NMR spectra as a function of time allows to characterise the director field pattern and to evaluate most of the viscoelastic properties of the system. A good agreement with classical rheometry is observed.
- Rheology of living cellsPublication . Portela, R.; Franco, Jose M.; Almeida, Pedro L.; Patricio, Pedro; Sobral, Rita; R. Leal, CatarinaThe mechanical behavior of living cells, during planktonic growth, has been thoroughly explored combining common biological techniques with rheology and rheo-imaging measurements. Under a shear flow, bacterial cultures of Staphylococcus aureus revealed a complex and rich rheological behavior not usually accessed in biological studies. In particular, in stationary shear flow, the viscosity increased during the exponential phase and returned close to its initial value at the late phase of growth, accompanied by the stabilization of the bacterial population. In oscillatory flow, the elastic and viscous moduli exhibited power-law behaviors whose exponents are dependent on the bacteria growth stage, and can be associated to a Soft Glassy Material behavior. These behaviors were framed in a microscopic model that suggests the formation of a dynamic web-like structure, where specific aggregation phenomena may occur, depending on growth stage and cell density. Furthermore, systematic measurements combining optical density and dry weight techniques presented new evidences, which confirmed that the observed cell aggregation patterns developed during growth, under shear, can not only be cell density dependent.
- Temperature dependence of the rheological properties of acetoxypropylcellulose in the thermotropic chiral nematic phasePublication . Cidade, M. T.; Leal, Catarina R.; Godinho, M. H.; Navard, P.The rheological behaviour of two different molecular weights of a thermotropic liquid crystalline cellulose derivative, acetoxypropylcellulose (APC), at T=I2OoC, has already been reported [ l]. In this work we present the temperature dependence of the rheological properties of two molecular weights of APC. The shear viscosity, q, and first normal stress difference, N,, were measured at temperatures T=120, 130 and 140°C. for Mciw = 94000 g/mole, and T=120 and 140°C for Mw = 129000 g/mole, and for shear rates, +, between 0.01 and 10 s-’. The shear viscosity q(+) decreases with increasing T, for both samples, showing a strong shear thinning behaviour at all temperatures and over the whole range of + studied, except for shear rates between about 0.2 and 1 s-’ (depending on IGiw and T), where a quasi-Newtonian plateau is observed. The first normal stress difference N1 (+) also decreases with increasing T at a given +. It increases with shear rate over the whole + range studied, and shows an inflection at + values slightly above + = 1/r, where z is the relaxation time of the polymer memory function. The temperature dependence of the shear viscosity was fitted to an Arrhenius law, giving an apparent activation energy (E,) in the order of 8-15 kcaUmole, depending on molecular weight and shear rate. The activation energy was found to increase with molecular weight (at a given + ). The variation of E, with +, , for Mw = 94000 g/mole, shows a minimum at ;i x 2 s-l. Using a continuum theory for nematic polymers, proposed by Martins [2], some fundamental parameters were obtained from the fit of the theory to the experimental data. Using these parameters it was possible to construct a “master curve” for the viscosity, q(+), in good agreement with the experimental data.
- An electro-rheological study of the nematic liquid crystal 4-n-heptyl-4'-cyanobiphenylPublication . Cidade, M. T.; Leal, Catarina R.; Patricio, PedroAn experimental and theoretical study of the electro-rheological effects observed in the nematic phase of 4-n-heptyl-4'-cyanobiphenyl has been conducted. This liquid crystal appears to be a model system, in which the observed rheological behaviour can be interpreted by the Leslie-Ericksen continuum theory for low molecular weight liquid crystals. Flow curves are illustrated at different temperatures and under the influence of an external electric field ranging from 0 to 3 kV mm-1, applied perpendicular to the direction of flow. Also presented is the apparent viscosity as a function of temperature, over similar values of electric field, obtained at different shear rates. A master flow curve has been constructed for each temperature by dividing the shear rate by the square of the electric field and multiplying by the square of a reference value of electric field. In a log-log plot, two Newtonian plateaux are found to appear at low and high shear rates, connected by a shear-thinning region. We have applied the Leslie-Ericksen continuum theory, in which the director alignment angle is a function of the electric field and the flow field boundary conditions are neglected, to determine viscoelastic parameters and the dielectric anisotropy.
- Rheology of the cytoskeleton as a fractal networkPublication . Patricio, Pedro; R. Leal, Catarina; Duarte, Jorge; Januário, CristinaWe model the cytoskeleton as a fractal network by identifying each segment with a simple Kelvin-Voigt element with a well defined equilibrium length. The final structure retains the elastic characteristics of a solid or a gel, which may support stress, without relaxing. By considering a very simple regular self-similar structure of segments in series and in parallel, in one, two, or three dimensions, we are able to express the viscoelasticity of the network as an effective generalized Kelvin-Voigt model with a power law spectrum of retardation times L similar to tau(alpha). We relate the parameter alpha with the fractal dimension of the gel. In some regimes ( 0 < alpha < 1), we recover the weak power law behaviors of the elastic and viscous moduli with the angular frequencies G' similar to G" similar to w(alpha) that occur in a variety of soft materials, including living cells. In other regimes, we find different power laws for G' and G".