A possible unique ecosystem in the endoglacial hypersaline brines in Antarctica.

Here, we present the results related to a new unique terrestrial ecosystem found in an englacial hypersaline brine found in Northern Victoria Land (Antarctica). Both the geochemistry and microbial (prokaryotic and fungal) diversity revealed an unicity with respect to all the other known Antarctic brines and suggested a probable ancient origin mainly due a progressive cryoconcentration of seawater. The prokaryotic community presented some peculiarities, such as the occurrence of sequences of Patescibacteria (which can thrive in nutrient-limited water environments) or few Spirochaeta, and the presence of archaeal sequences of Methanomicrobia closely related to Methanoculleus, a methanogen commonly detected in marine and estuarine environments. The high percentage (35%) of unassigned fungal taxa suggested the presence of a high degree of undiscovered diversity within a structured fungal community (including both yeast and filamentous life forms) and reinforce the hypothesis of a high degree of biological uniqueness of the habitat under study.

Impedance spectroscopy of conductive commercial hydrogels for electromyography and electroencephalography.

In biomedical applications such as the electroencephalogram, electrocardiogram and electromyogram, usually conductive hydrogels are applied to the skin providing a skin-electrode layer, with the aim of lowering contact impedance and improving the signal/noise ratio. It is important to know the electrical behavior of these materials, obtaining reliable values of their conductivity. It can be extracted by the whole cell impedance, which is affected not only by the bulk but also by the surface properties. The aim of the present work is to characterize the impedance of some currently available conductive hydrogels in the frequency range 5 Hz-1 MHz. For this purpose we have designed a novel cell, with silver electrodes, optical detection of the gap and guard ring. In this paper the results are compared with those obtained by means of a commercial cell with stainless steel electrodes, without a guard ring. In order to extract the different contributions of the bulk and the interface, we fitted the experimental data with a very simple model: the bulk properties are described by a single RC-parallel, whereas the interface is represented by a constant phase element.

Twist defects in helical sonic structures.

We analyze theoretically both the acoustic wave propagation in periodic media made of anisotropic materials whose stiffness tensor is uniformly rotating along a given axis x(3) and the defect mode produced by twisting about x(3) one part of the helical structure with respect to the other. Within the Bragg band of the periodic structure, the twist defect gives rise to a resonant mode that is a superposition of two standing waves: one localized with exp(-gamma|x(3)|) dependence centered at the defect and the other extended over the whole sample. The ratio between the amplitudes of the localized and nonlocalized waves depends sharply on both the twist angle and the elastic anisotropy, and can assume huge values. The defect mode and the resonance frequency omega(0) are defined by fully analytical and very simple expressions. Finally, we discuss how around omega(0), a finite sample acts as a frequency filter for circularly polarized shear waves, whose bandwidth can be changed by many orders of magnitude by varying the sample thickness, the twist angle, or the elastic anisotropy.

Open problems in the optics of crystals: the role of multiple scattering.

The thickness b of the transition boundary layer, always present in crystals and giving still unsolved problems for the boundary conditions, is shown to be essentially determined by the multiple scattering of light, due to the inhomogeneity of any periodic structure. The parameter b depends on the orientation theta of the boundary plane with respect to the crystal lattice, and diverges for some critical orientations where strong macroscopic effects are found, which cannot be interpreted by any macroscopic model based on bulk and boundary equations. Our analysis exhaustively defines the limits of validity of macroscopic models for periodic nanoscale structures and solid crystals.

Bloch wave approach to the optics of crystals.

The Bloch wave method is used to find the effective permittivity tensor epsilon of periodic liquid crystals and artificial structures whose period p is short with respect to the light wavelength lambda and whose optical properties are defined by a permittivity field epsilon(r). The main role of the multiple scattering within the periodic medium is evidenced, and very general expressions of epsilon, based on expansions in ascending powers of the ratio p/lambda and of the light wave vector k, are found. Such expansions allow to discuss the general properties of epsilon, to clarify the role of the spatial dispersions, i.e., to separate the part of epsilon explicitly depending on k from its k-independent part, and to find some interesting properties of crystals that are (i) periodic in only one direction, or (ii) locally isotropic. Finally, the limits of validity of the macroscopic model are discussed. Within these limits only a few terms of the power expansions are required, and their expressions are explicitly given. The obtained results are also useful to better understand the macroscopic optical properties of solid crystals.

Acoustic wave propagation in structurally helical media.

A theoretical analysis is given of the acoustic wave propagation in periodically nonhomogeneous media made of a solid material whose stiffness tensor is uniformly rotating along a given axis. In the last years, such media have been studied theoretically as well as experimentally, in particular for what concerns sample preparation and possible applications. A detailed analysis of their acoustical properties is given here, based on fully analytic and simple propagation equations. For axial propagation: (i) the dispersion curves of media where the transversal field components and the longitudinal ones are not coupled show only one forbidden band, that gives selective Bragg diffraction; in the opposite case they show at least a second forbidden band, that involves the quasilongitudinal and one of the quasitransversal eigenmodes; (ii) in the first case (absence of coupling), the medium gives pure acoustical rotation for p<>lambda; (iii) in the presence of the coupling, regions of mode exchange between the longitudinal component and a transversal one are generally present. The cases of lossy media and of quasiaxial propagation are also considered, and the analogies between optical and acoustical properties discussed.

Polyurethane-maleamides for cardiovascular applications: synthesis and properties.

Several polyurethane-maleamides (PUMAs) containing polyether or polycarbonate soft segments, and aromatic or aliphatic hard segments were synthesized by solution or bulk polymerization, using maleic acid (MA) or a mixture of MA and butanediol as chain extenders. Using this process, activated double bonds are introduced into the polymer chains and the base polyurethanes may undergo further modification via specific grafting, thus improving their tissue compatibility. PUMAs chemicophysical properties were evaluated by gel permeation chromatography (GPC), intrinsic viscosity analyses, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and tensile mechanical tests. Polycarbonate diol (PCU)-based PUMAs showed higher molecular weights than polyether diol (PEU)-based ones. The use of butanediol in mixture with maleic acid led to an increase of molecular weights. FT-IR confirmed the presence of the bands related to the amide groups and to the conjugated double bond, yet more evident for the polymer obtained in solution. The higher crystallinity shown by this polymer was also indicative of a better phase separation. All the PCU-PUMAs exhibited similar tensile properties with a higher stiffness than PEU-PUMAs. Among the PEU-PUMAs, the highest tensile properties were shown by the polymer obtained in solution, and by the one derived from a mixture of maleic acid and butanediol.