Poroviscoelastic characterization of particle-reinforced gelatin gels using indentation and homogenization.

Hydrogels are promising materials for bioengineering applications, and are good model materials for the study of hydrated biological tissues. As these materials often have a structural function, the measurement of their mechanical properties is of fundamental importance. In the present study gelatin gels reinforced with ceramic microspheres are produced and their poroviscoelastic response in spherical indentation is studied. The constitutive responses of unreinforced gels are determined using inverse finite element modeling in combination with analytical estimates of material parameters. The behavior of composite gels is assessed by both analytical and numerical homogenization. The results of the identification of the constitutive parameters of unreinforced gels show that it is possible to obtain representative poroviscoelastic parameters by spherical indentation without the need for additional mechanical tests. The agreement between experimental results on composite gelatin and the predictions from homogenization modeling show that the adopted modeling tools are capable of providing estimates of the poroviscoelastic response of particle-reinforced hydrogels.

Face-based Selection of Corners in 3D Substructuring

In most recent substructuring methods, a fundamental role is played by the coarse space. For some of these methods (e.g. BDDC and FETI-DP), its definition relies on a 'minimal' set of coarse nodes (sometimes called corners) which assures invertibility of local subdomain problems and also of the global coarse problem. This basic set is typically enhanced by enforcing continuity of functions at some generalized degrees of freedom, such as average values on edges or faces of subdomains. We revisit existing algorithms for selection of corners. The main contribution of this paper consists of proposing a new heuristic algorithm for this purpose. Considering faces as the basic building blocks of the interface, inherent parallelism, and better robustness with respect to disconnected subdomains are among features of the new technique. The advantages of the presented algorithm in comparison to some earlier approaches are demonstrated on three engineering problems of structural analysis solved by the BDDC method.

Size effects in indentation of hydrated biological tissues

Fluid flow in biological tissues is important in both mechanical and biological contexts. Given the hierarchical nature of tissues, there are varying length scales at which time-dependent mechanical behavior due to fluid flow may be exhibited. Here, spherical nanoindentation and microindentation testings are used for the characterization of length scale effects in the mechanical response of hydrated tissues. Although elastic properties were consistent across length scales, there was a substantial difference between the time-dependent mechanical responses for large and small contact radii in the same tissue specimens. This difference was far more obvious when poroelastic analysis was used instead of viscoelastic analysis. Overall, indentation testing is a fast and robust technique for characterizing the hierarchical structure of biological materials from nanometer to micrometer length scales and is capable of making quantitative material property measurements to do with fluid flow. © 2011 Materials Research Society.

Room temperature all-silicon photonic crystal nanocavity light emitting diode at sub-bandgap wavelengths

Silicon is now firmly established as a high performance photonic material. Its only weakness is the lack of a native electrically driven light emitter that operates CW at room temperature, exhibits a narrow linewidth in the technologically important 1300-1600 nm wavelength window, is small and operates with low power consumption. Here, an electrically pumped all-silicon nano light source around 1300-1600 nm range is demonstrated at room temperature. Using hydrogen plasma treatment, nano-scale optically active defects are introduced into silicon, which then feed the photonic crystal nanocavity to enhance the electrically driven emission in a device via Purcell effect. A narrow (Δλ=0.5 nm) emission line at 1515 nm wavelength with a power density of 0.4mW/cm2 is observed, which represents the highest spectral power density ever reported from any silicon emitter. A number of possible improvements are also discussed, that make this scheme a very promising light source for optical interconnects and other important silicon photonics applications. © 2012 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Enhancement of room temperature sub-bandgap light emission from silicon photonic crystal nanocavity by Purcell effect

We report the enhancement of sub-bandgap photoluminescence from silicon via the Purcell effect. We couple the defect emission from silicon, which is believed to be due to hydrogen incorporation into the lattice, to a photonic crystal (PhC) nanocavity. We observe an up to 300-fold enhancement of the emission at room temperature at 1550 nm, as compared to an unpatterned sample, which is then comparable to the silicon band-edge emission. We discuss the possibility of enhancing this emission even further by introducing additional defects by ion implantation, or by treating the silicon PhC nanocavity with hydrogen plasma. © 2011 Elsevier B.V.

Electrical and optical properties of ion implanted SOI-based photonic crystals

We investigate the electrical properties of Silicon-on-Insulator photonic crystals as a function of doping level and air filling factor. A very interesting trade-off between conductivity and optical losses in L3 cavities is also found. © 2011 IEEE.