Influence of boundary conditions on the order and defects of biaxial nematic droplets

We employ Monte Carlo simulations to study the defects occurring in a nematic droplet formed by biaxial molecules. The simulations are carried out using a lattice model based on a dispersive orientational biaxial potential previously employed to establish the rich phase diagram of the system. The focus of the present investigation is on the molecular organization inside the droplet when bipolar and toroidal anchoring conditions at the surface are considered. In both cases, we describe how the defect structure arises in the system, and we analyze the behavior of the defect core region in connection with the elastic properties of the phase in a continuum theory perspective.

On the Defect Structure of Biaxial Nematic Droplets

We present a detailed Monte Carlo study of the effects of molecular biaxiality on the defect created at the centre of a nematic droplet with radial anchoring at the surface. We have studied a lattice model based on a dispersive potential for biaxial mesogens [Luckhurst et al., Mol. Phys. 30, 1345 (1975)] to investigate how increasing the biaxiality influences the molecular organisation inside the confined system. The results are compared with those obtained from a continuum theory approach. We find from both approaches that the defect core size increases by increasing the molecular biaxiality, hinting at a non universal behaviour previously not reported.

Rotational diffusion of shape switching particles in nematic liquid crystals

The theory of rotational diffusion of particles of various symmetry embedded in a liquid crystal host, essential to interpret a variety of spectroscopic observables, has been available for some time, but only for the case of rigid molecules. Here we generalize the treatment and present a theory to describe the rotational diffusion of shape-changing particles dispersed in nematic liquid crystals. The interaction of the particles with the environment is modeled by an effective field potential, while the particles are allowed to assume an arbitrary discrete number of shapes. The transition between shapes is modeled by a Markovian process which is combined with rotational diffusion. Our model is applied to the simple case of a particle that can exchange between three shapes: a rod, a disk, and a sphere. We consider in detail the effect of shape transitions in some selected correlation functions which are relevant for experiments.

Molecular dynamics of anhydrous glycolipid self-assembly in lamellar and hexagonal phases

The molecular dynamics of a synthetic branched chain glycolipid, 2-decyl-tetradecyl-β-d-maltoside (C_14-10G₂), in smectic and columnar liquid crystal phases.

Nematic liquid crystals in planar and cylindrical hybrid cells: Role of elastic anisotropy on the director deformations

Nematic samples filling a flat cell or the annular region between two concentric cylinders with hybrid anchoring conditions at the boundaries are investigated by setting up and minimizing their Frank elastic free energy. The coupling with the surfaces is taken to be strong on one side and weak on the other. The equations are numerically solved and the conditions for which the molecular organization inside the cell becomes uniform are analyzed. The classical calculation performed by G. Barbero and R. Barberi [J. Phys. 44, 609 (1983)] is reproduced and investigated from a different point of view, in order to compare the results of planar and cylindrical geometries. The results suggest that the cylindrical cell presents some unusual features deserving a more complete investigation. Although most part of the transitional phenomena are found for K(11)>K(33), a case not common for ordinary (lyotropic and thermotropic) liquid crystals, it is possible to find a completely uniform cell even for K(11)<K(33) in both the geometries considered here.

Molecular organization of nematic liquid crystals between concentric cylinders: role of the elastic anisotropy

The orientational order in a nematic liquid crystal sample confined to an annular region between two concentric cylinders is investigated by means of lattice Monte Carlo simulations. Strong anchoring and homeotropic orientations, parallel to the radial direction, are implemented at the confining surfaces. The elastic anisotropy is taken into account in the bulk interactions by using the pair potential introduced by Gruhn and Hess [T. Gruhn and S. Hess, Z. Naturforsch. A 51, 1 (1996)] and parametrized by Romano and Luckhurst [S. Romano, Int. J. Mod. Phys. B 12, 2305 (1998); Phys. Lett. A 302, 203 (2002); G. R. Luckhurst and S. Romano, Liq. Cryst. 26, 871 (1999)], i.e., the so-called GHRL potential. In the case of equal elastic constants, a small but appreciable deformation along the cylinder axis direction is observed, whereas when the values of K(11)/K(33) if K(22)=K(33) are low enough, all the spins in the bulk follow the orientation imposed by the surfaces. For larger values of K(11)/K(33), spontaneous deformations, perpendicular to the polar plane, increase significantly. Our findings indicate that the onset of these deformations also depends on the ratio K(22)/K(33) and on the radius of the cylindrical surfaces. Although expected from the elastic theory, no tangential component of the deformations was observed in the simulations for the set of parameters analyzed.

Exploring the energy landscape of the charge transport levels in organic semiconductors at the molecular scale

The extraordinary semiconducting properties of conjugated organic materials continue to attract attention across disciplines including materials science, engineering, chemistry, and physics, particularly with application to organic electronics. Such materials are used as active components in light-emitting diodes, field-effect transistors, or photovoltaic cells, as a substitute for (mostly Si-based) inorganic semiconducting materials. Many strategies developed for inorganic semiconductor device building (doping, p-n junctions, etc.) have been attempted, often successfully, with organics, even though the key electronic and photophysical properties of organic thin films are fundamentally different from those of their bulk inorganic counterparts. In particular, organic materials consist of individual units (molecules or conjugated segments) that are coupled by weak intermolecular forces. The flexibility of organic synthesis has allowed the development of more efficient opto-electronic devices including impressive improvements in quantum yields for charge generation in organic solar cells and in light emission in electroluminescent displays. Nonetheless, a number of fundamental questions regarding the working principles of these devices remain that preclude their full optimization. For example, the role of intermolecular interactions in driving the geometric and electronic structures of solid-state conjugated materials, though ubiquitous in organic electronic devices, has long been overlooked, especially when it comes to these interfaces with other (in)organic materials or metals. Because they are soft and in most cases disordered, conjugated organic materials support localized electrons or holes associated with local geometric distortions, also known as polarons, as primary charge carriers. The spatial localization of excess charges in organics together with low dielectric constant (ε) entails very large electrostatic effects. It is therefore not obvious how these strongly interacting electron-hole pairs can potentially escape from their Coulomb well, a process that is at the heart of photoconversion or molecular doping. Yet they do, with near-quantitative yield in some cases. Limited screening by the low dielectric medium in organic materials leads to subtle static and dynamic electronic polarization effects that strongly impact the energy landscape for charges, which offers a rationale for this apparent inconsistency. In this Account, we use different theoretical approaches to predict the energy landscape of charge carriers at the molecular level and review a few case studies highlighting the role of electrostatic interactions in conjugated organic molecules. We describe the pros and cons of different theoretical approaches that provide access to the energy landscape defining the motion of charge carriers. We illustrate the applications of these approaches through selected examples involving OFETs, OLEDs, and solar cells. The three selected examples collectively show that energetic disorder governs device performances and highlights the relevance of theoretical tools to probe energy landscapes in molecular assemblies.

Computer simulations of the ordering in a hybrid cylindrical film of nematic liquid crystals

We present an investigation of the ordering in a nematic liquid-crystal film confined between two cylindrical surfaces with antagonistic (radial and planar) anchoring alignments. A Monte Carlo study of a Lebwohl-Lasher model with suitable boundary conditions has been performed to calculate the ordering and the molecular organization for different film thicknesses. The simulation results are compared with some theoretical predictions obtained with the elastic continuum approach. The agreement between theory and simulation is improved as the thickness decreases.

Theoretical characterization of the structural and hole transport dynamics in liquid-crystalline phthalocyanine stacks

We present a joint molecular dynamics (MD)/kinetic Monte Carlo (KMC) study aimed at the atomistic description of charge transport in stacks of liquid-crystalline tetraalkoxy-substituted, metal-free phthalocyanines. The molecular dynamics simulations reproduce the major structural features of the mesophases, in particular, a phase transition around 340 K between the rectangular and hexagonal phases. Charge transport simulations based on a Monte Carlo algorithm show an increase by 2 orders of magnitude in the hole mobility when accounting for the rotational and translational dynamics. The results point to the formation of dynamical structural defects along the columns.

Controlling surface defect valence in colloids

We perform large-scale Monte Carlo simulations of orientational ordering in nematic shells and study the type and position of topological defects when an external electric field (homogeneous or quadrupolar) is applied. The field-induced variation of the defect number (and strength) can be used to change the valence of colloidal particles coated with a nematic layer.

Biaxial liquid-crystal elastomers: a lattice model

We present a simple coarse-grained lattice model for monodomain biaxial liquid-crystal elastomers and perform large-scale Monte Carlo simulations in the proposed model system. Orientational ordering--uniaxial or biaxial--reflects in sample deformations on cooling the system. The simulation output is used to predict calorimetry data and deuterium magnetic resonance spectra.

Insulin glargine improves glycemic control and health-related quality of life in type 1 diabetes

BACKGROUND AND AIMS

Glargine improves glucose control and reduces the risk of nocturnal hypoglycemia compared with neutral protamine Hagedorn (NPH) insulin. To date, only one study has measured the effects of glargine on health-related quality of life (HRQL); the aim of this study was therefore to confirm the beneficial effects of glargine on disease-specific HRQL in type 1 diabetes.

METHODS AND RESULTS

Forty-seven patients (mean age, 46 range, 25-74; males, 54%) with diabetes of at least 1-year duration, and with suboptimal glucose control under intensive insulin treatment (IIT), were switched from NPH to glargine. Forty patients maintained on IIT were used as controls. Diabetes-related HRQL was assessed using the Well-being Enquiry for Diabetics (WED), before and after a 6- to 8-month switch to glargine. An 11-item questionnaire based upon diabetes-specific issues was used to assess treatment satisfaction and perceived changes after switching. On glargine, the mean glycosylated hemoglobin decreased by 0.7% (treatment vs. baseline, P<0.0001) and several WED scores improved (discomfort, P=0.020; impact, P=0.0002; total score, P=0.0005). WED changes were associated with a lower perceived risk of hypoglycemia and less problems in daily life on glargine.

CONCLUSIONS

The results of this study show that the beneficial effect of glargine is not limited to better metabolic control; the burden of type 1 diabetes mellitus on everyday life is also reduced.

Memory effects in nematics with quenched disorder

We present a combined experimental and Monte Carlo study of a nematic phase in the presence of quenched disorder. The turbidity of a nematic liquid crystal embedded in a porous polymer membrane is measured under different applied field conditions for field-cooled and zero-field-cooled samples. We find that a significant permanent alignment of the nematic can be induced by fields as low as 0.1 V/microm applied during the isotropic to nematic transition. An analogous effect and dependence on sample history is found by studying the order parameter of a sprinkled disorder Lebwohl-Lasher spin model, indicating that dilute quenched randomness is sufficient to produce memory effects in nematics. The large memory induced by field cooling appears to be written in the system during the transition as a result of the field action on freely oriented nematic nuclei. At lower temperature the nuclei consolidate into permanent nematic textures developed from the interaction with quenched disorder.

External field-induced switching in nematic elastomers: a Monte Carlo study

We present a Monte Carlo study of external field-induced switching in nematic elastomers, employing a coarse-grained shearable lattice model. In large enough systems a full-wavelength Fréedericksz effect is observed --as opposed to the half-wavelength effect seen in ordinary nematics-- that clearly reflects in simulated polarized light textures, as well as in deuterium magnetic resonance spectra. The reorientation of mesogenic units is accompanied by pronounced shear deformations.

An empowerment-based educational program improves psychological well-being and health-related quality of life in Type 1 diabetes

Educational programs are reported to improve metabolic control and well-being in Type 1 diabetes mellitus (DM), but the effects of newly- structured interventions, aimed at promoting empowerment in educated patients in active selfcare, have received little attention. Ninety patients with Type 1 DM in intensive insulin treatment were invited to an empowerment-based educational intervention. Changes in quality of life and psychological well-being in the 54 patients participating in the program (median age, 44 yr) were compared with those measured in patients who refused. The following questionnaires were administered at baseline and 12 months later: Psychological General Well-Being (PGWB), Medical Outcome Survey Short-Form 36 (SF-36), and Well-Being Enquiry for Diabetics (WED). Baseline values were indicative of moderate, but significant, psychological distress in the whole cohort. At follow-up, the experimental group had a better metabolic control {glycosylated hemoglobin, -0.4% [time x treatment analysis of variance (ANOVA), p = 0.005 vs controls]}, and a general improvement in comprehensive indices and most scales of PGWB and SF-36. Vitality (p = 0.042) and Social Functioning (p = 0.039) were no longer different from population norm. Similarly, the Symptoms (p = 0.005), Discomfort (p = 0.043) and Impact scales (p = 0.032) of WED, reflecting physical functioning, diabetes-related worries and familial relationships, role functioning and social network, improved significantly in treated patients. An educational empowerment-based intervention significantly improves the psychosocial aspects of diabetes and quality of life also in patients in active and effective self-care. Repeated educational interventions are the way towards a normal life with Type 1 DM.

High-resolution 3D scaffold model for engineered tissue fabrication using a rapid prototyping technique

Rapid prototyping, automatic image processing (computer-aided design (CAD)) and computer-aided manufacturing techniques are opening new and interesting prospects for medical devices and tissue engineering, especially for hard tissues such as bone. The development of a bone high-resolution scaffold prototype using these techniques is described. The results testify to the fidelity existing between microtomographic reconstruction and CAD. Furthermore, stereolithographic manufacturing of this scaffold, which possesses a high degree of similarity to the starting model as monitored by morphological evaluations (mean diameter 569 +/- 147 microm), represents a promising result for regenerative medicine applications.

Nematics with quenched disorder: pinning out the origin of memory

Memory effects and glassy behavior have been repeatedly observed in disordered nematic liquid crystals but the connection between these effects and the system topology remained unrevealed. We present an analysis of the local and global topology of the nematic ordering in the presence of quenched disorder and we show that nematics with quenched disorder can be mapped into a system of pinned defect lines and that the memory of the system stems from the pinning of these strings.

Non-alcoholic fatty liver and insulin resistance: a cause-effect relationship?

The role of insulin resistance in non-alcoholic fatty liver disease is suggested by laboratory data (hyperinsulinemia and decreased sensitivity to endogenous and exogenous insulin). The clinical association with features of the metabolic syndrome, particularly in the most aggressive stages of the disease, further confirms a causative role. Fat accumulation in the liver may stem either from genetic defects, primarily responsible for insulin resistance, or excessive calorie intake and visceral obesity, and is mediated by adipocytokines (leptin, adiponectin, tumour necrosis factor-alpha). Progression of fatty liver to steatohepatitis may be the result of an imbalance between pro-inflammatory and anti-inflammatory cytokines, triggering the formation of reactive oxygen species and intrahepatic lipid peroxidation. This process may also be promoted or accelerated by pro-oxidant xenobiotics or environmental factors. Insulin resistance provides a target for specific treatment of non-alcoholic fatty liver, and insulin-sensitising agents (metformin or thiazolidinediones) as well as lifestyle changes to reduce visceral adiposity are the most promising therapeutic options. Future trials need to be performed in order to test the long-term effectiveness of these treatments on the basis of clinically relevant histological outcomes.

Non-alcoholic steatohepatitis in patients cared in metabolic units

The clinical significance of liver disease is frequently underestimated in patients with metabolic disorders. In patients followed up in a metabolic unit for diabetes, obesity or hyperlipidemia (n=147), we studied the prevalence and the severity of liver disease, and its relationship with the metabolic syndrome (MS). Cases cared for in a liver unit (n=179) were used as controls. Patients in the metabolic series were older and had a higher prevalence of coronary heart disease. Criteria for the metabolic syndrome were fulfilled in 64% and 22% of cases, respectively (P<0.0001). Liver biopsy was obtained in 44 and 66% of cases. Metabolic patients had a more severe steatosis score (P<0.0001), whereas the scores of fibrosis and necroinflammation were less severe (P=0.0059 and 0.0007, respectively). Histological criteria for non-alcoholic steatohepatitis (NASH) were present in 82% of metabolic cases and 68% cases in the liver series (P=0.057). Liver disease in patients routinely cared for in metabolic units is similar to that observed in patients cared for in liver units, and potentially may progress to terminal liver failure. Liver biopsy is recommended for diagnostic and prognostic purposes, as well as for testing treatment effects in controlled trials.

Computer simulations of hard pear-shaped particles

We report results obtained from Monte Carlo simulations investigating mesophase formation in two model systems of hard pear-shaped particles. The first model considered is a hard variant of the truncated Stone-expansion model previously shown to form nematic and smectic mesophases when embedded within a 12-6 Gay-Berne-like potential [R. Berardi, M. Ricci, and C. Zannoni, ChemPhysChem 7, 443 (2001)]. When stripped of its attractive interactions, however, this system is found to lose its liquid crystalline phases. For particles of length to breadth ratio k=3, glassy behavior is seen at high pressures, whereas for k=5 several bi- layerlike domains are seen, with high intradomain order but little interdomain orientational correlation. For the second model, which uses a parametric shape parameter based on the generalized Gay-Berne formalism, results are presented for particles with elongation k=3, 4, and 5. Here, the systems with k=3 and 4 fail to display orientationally ordered phases, but the system with k=5 shows isotropic, nematic and, unusual for a hard-particle model, interdigitated smectic A2 phases.

Structures and transitions in thin hybrid nematic films: a Monte Carlo study

We confirm by Monte Carlo simulations of a Lebwohl-Lasher lattice spin model the existence of a biaxially ordered nonbent structure in a liquid-crystalline cell subject to opposing boundary conditions. We report on the observation of the bending transition from the biaxial to the bent-director structure when the temperature of the system is lowered. The structural transition is monitored both by the change of the order parameters and by heat capacity. We discuss the thickness dependence of the transition temperature by means of wetting-induced phenomena and elastic deformations. We propose the correspondence to the phenomenological description, which agrees well without any fitting parameters.

Nematics with dispersed polymer fibrils: a Monte Carlo study of the external-field-induced switching

We present a Monte Carlo study of molecular ordering in nematics with dispersed regular and random arrays of straight and distorted polymer fibrils. We focus on the collective molecular reorientation--the switching--resulting from the competing aligning effects of fibrils and of a progressively applied transversal external field, and for straight fibrils identify structural Fréedericksz and saturation transitions. The role of fibril topography in the switching is monitored by simulating electric capacitance Slightly distorted fibrils are shown to give a sharper switching at a lower threshold.

Topological defects in schlieren textures of biaxial and uniaxial nematics

Monte Carlo and theoretical studies of thin 3D films of biaxial and uniaxial nematics with tangential boundary conditions show distinct differences in structure and evolution of topological defects. In the uniaxial films, defects of strength k=+/-1 are point defects that bear no bulk singularity and disappear by annihilation with each other. In the biaxial films, k=+/-1 defects are true singular bulk disclinations that split into pairs of k=+/-1/2 lines; the latter disappear by annihilation processes of the type +1/2-1/2=0. These observed differences are of relevance for the current debate on the existence of biaxial phases.

Nematics with quenched disorder: how long will it take to heal?

Nematics with quenched disorder have been repeatedly predicted to form glass phases. Here we present turbidity experiments and computer simulations aimed at studying glass key features such as dynamics and history dependence in randomly perturbed nematics. Electric field-cooling alignment has been employed to prepare samples in suitably oriented starting states. Remarkable remnant order and slow dynamics are found both by experiment and simulations, indicating that random disorder can, by itself, induce a nematic glass state even without perturber restructuring.

Polymer network-induced ordering in a nematogenic liquid: a Monte Carlo study

In this Monte Carlo study we investigate molecular ordering in a nematogenic liquid with dispersed polymer networks. The polymer network fibers are assumed to have rough surface morphology resulting in a partial randomness in anchoring conditions, while the fiber direction is assumed to be well defined. In particular, we focus on the loss of long-range aligning capability of the network when the degree of disorder in anchoring is increased. This process is monitored by calculating relevant order parameters and the corresponding 2H nuclear magnetic resonance spectra, showing that the aligning ability of the network is lost only for completely disordering anchoring conditions. Moreover, above the nematic-isotropic transition temperature surface-induced paranematic order is detected. In addition, for perfectly smooth fiber surfaces with homeotropic anchoring conditions topological line defects can be observed.

Border-tracing algorithm implementation for the femoral geometry reconstruction

In some orthopaedic applications such as the design of custom-made hip prostheses, reconstruction of the bone morphology is a fundamental step. Different methods are available to extract the geometry of the femoral medullary canal from computed tomography (CT) images. In this research, an automatic procedure (border-tracing method) for the extraction of bone contours was implemented and validated. A composite replica of the human femur was scanned and the CT images processed using three different methods, a manual procedure; the border-tracing algorithm; and a threshold-based method. The resulting contours were used to estimate the accuracy of the implemented procedure. The two software techniques were more accurate than the manual procedure. Then, these two procedures were applied to an in vivo CT data set in order to determine to most critical region for repeatability. Only for the images located in this region, the repeatability measurement was carried out for six in vivo CT data sets to evaluate the inter-femur repeatability. The border-tracing method was found to achieve the highest repeatability.

HIDE: a new hybrid environment for the design of custom-made hip prosthesis

This technical note describes a new software environment (HIPCOM design environment, HIDE) for the design of custom-made total hip replacements. These devices are frequently designed using general-purpose mechanical computer-aided design (CAD) programs using a set of bone contours extracted from the computer tomography (CT) images as anatomical reference. On the contrary, the HIDE system was developed to let the operator directly design the stem shape onto the CT images in a single-step operation. The operator can directly import CT data in DICOM format or use special functions to reconvert to a digital stack, the CT images printed on a radiological film. Once the stack of CT images is loaded, the operator can design the implant shape by imposing control sections directly on the CT images. The interpolation of these control sections produces the basic 3D shape of the custom-made stem. The shape is then exported to the CAD-computer-aided manufacturing (CAM) program to refine the design and to generate the part program to manufacture the implant with a CNC tooling machine. Using HIDE, the duration of design steps it affected was reduced by more than 50% with respect to the standard method in use at the manufacturer site. HIDE also improved the accuracy and the repeatability of the whole procedure. The learning curve became flat after only ten cases. These good results were achieved because of the integration of the vectorial description of the prosthetic component with the raster description of the CT data that allowed the designer to use all details available in the CT images.