Synthesis and degradation of agar‐carbomer based hydrogels for tissue engineering applications

Hydrogels studied in this investigation, synthesized starting from agarose and Carbomer 974P, were chosen for their potential use in tissue engineering. The strong ability of hydrogels to mimic living tissues should be complemented with optimized degradation time profiles: a critical property for biomaterials but essential for the integration with target tissue. In this study, chosen hydrogels were characterized both from a rheological and a structural point of view before studying the chemistry of their degradation, which was performed by several analysis: infrared bond response [Fourier transform infrared (FT‐IR)], calorimetry [differential scanning Calorimetry (DSC)], and % mass loss. Degradation behaviors of Agar‐Carbomer hydrogels with different degrees of crosslinkers were evaluated monitoring peak shifts and thermal property changes. It was found that the amount of crosslinks heavily affect the time and the magnitude related to the process. The results indicate that the degradation rates of Agar‐Carbomer hydrogels can be controlled and tuned to adapt the hydrogel degradation kinetics for different cell housing and drug delivery applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A nanoscale model for characterizing the complex pore structure of biochars

The development of a novel nanoscale model that can accurately describe the reactivity of solids consisting of multiple components and having ordered and random pores is presented. Domains of multiple solid phases are distributed on a computational grid to simulate reactants with different specific reactivities and dispersions. Sub‐nanometer slit pores and larger cylindrical pores with given size distributions are also distributed on the grid in regular and random arrangements respectively. The generated solids are then eroded using rules that simulate a gas‐solid, non‐catalytic reaction occurring in the kinetic control regime. A parametric study is first carried out to demonstrate how key pore structural parameters affect the reactivity patterns. Model predictions are found to be in excellent agreement with experimental thermogravimetric data for the combustion of biochars, both when the slit and random cylindrical pores are fully accessible to the reactant and when diffusional limitations appear in the smaller slit pores. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3412–3420, 2013     

Calculated optical transitions in a silicon quantum wire modulated by a quantum dot

The photoluminescence lifetimes of Si quantum wires and dots have been previously calculated within a continuum model that takes into account the anisotropy of silicon band structure. Here, we present our calculations on the optical transitions in Si quantum wires modulated by a quantum dot. The geometrical parameters of the buldged wire are appropriate for porous Si and the ground state is localized. The photoluminescence lifetimes are calculated and compared with those of straight wires and dots. The magnitude of the lifetime is sensitive to the structural parameters of the nanostructures. Lifetimes varying from nanoseconds to milliseconds have been obtained. The results of the calculations provide insight to the optical properties of Si nanostructures.     

Accelerating dislocations: Wave-front behavior

The wave-front behavior of accelerating dislocations and the singularities involved are discussed (a) for screw and edge dislocation starting from rest and moving with constant velocity, (b) for screw dislocations accelerating from rest with constant acceleration, (c) for supersonically moving screw dislocations and (d) for circular dislocation loops at rest which start expanding with constant radial velocity.     

Efficient thermoelectric energy conversion on quasi-localized electron states in diameter modulated nanowires

It is known that the thermoelectric efficiency of nanowires increases when their diameter decreases. Recently, we proposed that increase of the thermoelectric efficiency could be achieved by modulating the diameter of the nanowires. We showed that the electron thermoelectric properties depend strongly on the geometry of the diameter modulation. Moreover, it has been shown by another group that the phonon conductivity decreases in nanowires when they are modulated by dots. Here, the thermoelectric efficiency of diameter modulated nanowires is estimated, in the ballistic regime, by taking into account the electron and phonon transmission properties. It is demonstrated that quasi-localized states can be formed that are prosperous for efficient thermoelectric energy conversion.     

Ag-doped Bioactive Glass-Ceramic 3D Scaffolds: Microstructural,Antibacterial, and Biological Properties

Ag-doped sol-gel derived bioactive glass-ceramic particles (Ag-BG) were used to fabricate highly porous scaffolds exhibiting advanced antibacterial properties and formation of an apatite-like layer. The applied heat treatment for the development of the 3D Ag-BG scaffolds was selected after the characterization of the thermal behavior of Ag-BG particles using differential thermal analysis (DTA), thermogravimetric analysis (TGA), and hot stage microscopy (HSM). The structural characteristics of the scaffolds were studied using optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), micro-computerized tomography (Micro-CT), X-ray diffraction (XRD), Fourier-transformed infrared (FTIR), magic angle spinning nuclear magnetic resonance (MAS-NMR), and transmission electron microscopy (TEM) to correlate how the characteristics in the hierarchal structure of the Ag-BG scaffolds affected their antibacterial performance and apatite forming ability. Methicillin-resistant Staphylococcus aureus (MRSA) was used to evaluate the antibacterial response of the Ag-BG scaffolds. The observed characteristics make these Ag-BG scaffolds attractive candidates for biomedical applications.     

Band Structure Engineering in Geometry-Modulated Nanostructures for Thermoelectric Efficiency Enhancement

The energy subband structure of nanowires with periodically modulated cross-section has been calculated within a continuum model and the effective mass approximation. A characteristic structure of minibands and resonances has been found. This leads to a remarkable enhancement of the Seebeck coefficient compared with that of nonmodulated nanowires of the same dimensions. The Seebeck coefficient enhancement depends on the interplay between the thermal broadening and the quantum confinement. It is pointed out here that the modulation geometry and material parameters can provide design tools for Seebeck coefficient enhancement in nanowires.     

Sintering and crystallisation of 45S5 Bioglass® powder

The sintering process of 45S5 Bioglass^® powder (mean particle size < 5 μm) was investigated by using different thermal analysis methods. Heating microscopy and conventional dilatometry techniques showed that bioactive glass sinters in two major steps: a short stage in the temperature range 500–600 °C and a longer stage in the range 850–1100 °C. The optimal sintering temperature and time were found to be 1050 °C and 140 min, respectively. Differential thermal analysis (DTA) showed that Bioglass^® crystallises at temperatures between 600 and 750 °C. The characteristic crystalline phases were identified by Fourier Transformed Infrared Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). The crystallisation kinetics was studied by DTA, using a non-isothermal method. The Kissinger plot for Bioglass^® powder heated at different heating rates between 5 and 30 °C/min yielded an activation energy of 316 kJ/mol. The average value of Avrami parameter determined using the Augis–Bennett method was 0.95 ± 0.10, confirming a surface crystallisation mechanism. After sintering at 1050 °C for 140 min, the main crystalline phase was found to be Na₂Ca₂Si₃O₉. The results of this work are useful for the design of the sintering/crystallisation heat treatment of Bioglass^® powder which is used for fabricating tissue engineering scaffolds with varying degree of bioactivity.     

Evolution equation of moving defects: dislocations and inclusions

Evolution equations, or equations of motion, of moving defects are the balance of the “driving forces”, in the presence of external loading. The “driving forces” are defined as the configurational forces on the basis of Noether’s theorem, which governs the invariance of the variation of the Lagrangean of the mechanical system under infinitesimal transformations. For infinitesimal translations, the ensuing dynamic J integral equals the change in the Lagrangean if and only if the linear momentum is preserved. Dislocations and inclusions are “defects” that possess self-stresses, and the total driving force for these defects consists only of two terms, one expressing the “ self-force” due to the self-stresses, and the other the effect of the external loading on the change of configuration (Peach–Koehler force). For a spherically expanding (including inertia effects) Eshelby (constrained) inclusion with dilatational eigenstrain (or transformation strain) in general subsonic motion, the dynamic J integral, which equals the energy-release rate, was calculated. By a limiting process as the radius tends to infinity, the driving force (energy-release rate) of a moving half-space plane inclusion boundary was obtained which is the rate of the mechanical work required to create an incremental region of eigenstrain (or transformation strain) of a dynamic phase boundary. The total driving force (due to external loading and due to self-forces) must be equal to zero, in the absence of dissipation, and the evolution equation for a plane boundary with eigenstrain is presented. The equation applied to many strips of eigenstrain provides a system to solve for the position/ evolution of strips of eigenstrain.