Limited Range Fractality of Randomly Adsorbed Rods

Multiple resolution analysis of two dimensional structures composed of randomly adsorbed penetrable rods, for densities below the percolation threshold, has been carried out using box-counting functions. It is found that at relevant resolutions, for box-sizes, $r$, between cutoffs given by the average rod length and the average inter-rod distance $r_1$, these systems exhibit apparent fractal behavior. It is shown that unlike the case of randomly distributed isotropic objects, the upper cutoff $r_1$ is not only a function of the coverage but also depends on the excluded volume, averaged over the orientational distribution. Moreover, the apparent fractal dimension also depends on the orientational distributions of the rods and decreases as it becomes more anisotropic. For box sizes smaller than the box counting function is determined by the internal structure of the rods, whether simple or itself fractal. Two examples are considered - one of regular rods of one dimensional structure and rods which are trimmed into a Cantor set structure which are fractals themselves. The models examined are relevant to adsorption of linear molecules and fibers, liquid crystals, stress induced fractures and edge imperfections in metal catalysts. We thus obtain a distinction between two ranges of length scales: $r$ where the internal structure of the adsorbed objects is probed, and $< r < r_1$ where their distribution is probed, both of which may exhibit fractal behavior. This distinction is relevant to the large class of systems which exhibit aggregation of a finite density of fractal-like clusters, which includes surface growth in molecular beam epitaxy and diffusion-limited-cluster-cluster-aggregation models.Comment: 10 pages, 7 figures. More info available at http://www.fh.huji.ac.il/~dani/ or http://www.fiz.huji.ac.il/staff/acc/faculty/biham or http://chem.ch.huji.ac.il/employee/avnir/iavnir.htm . Accepted for publication in J. Chem. Phy

Apparent Fractality Emerging from Models of Random Distributions

The fractal properties of models of randomly placed $n$-dimensional spheres ($n$=1,2,3) are studied using standard techniques for calculating fractal dimensions in empirical data (the box counting and Minkowski-sausage techniques). Using analytical and numerical calculations it is shown that in the regime of low volume fraction occupied by the spheres, apparent fractal behavior is observed for a range of scales between physically relevant cut-offs. The width of this range, typically spanning between one and two orders of magnitude, is in very good agreement with the typical range observed in experimental measurements of fractals. The dimensions are not universal and depend on density. These observations are applicable to spatial, temporal and spectral random structures. Polydispersivity in sphere radii and impenetrability of the spheres (resulting in short range correlations) are also introduced and are found to have little effect on the scaling properties. We thus propose that apparent fractal behavior observed experimentally over a limited range may often have its origin in underlying randomness.Comment: 19 pages, 12 figures. More info available at http://www.fh.huji.ac.il/~dani

Filtros solares para proteger de la radiación solar

Traducción de Patente Europea E98900657 (fecha de solicitud, 15/01/1998).-- Prioridad: IL1997011697120022.-- Titulares: Yissum Research Development Company of the Hebrew University of Jerusalem, Consejo Superior de Investigaciones Científicas (CSIC).La invención se refiere a materiales sol-gel dopados de filtro solare, útiles para proteger los tejidos del organismo como la piel, las uñas, el cabello y otras superficies. Las matrices sol-gel son transparentes a la radiación UV más allá de 250 nm y los agentes de filtro solar dopados son filtros solares químicos o físicos capaces de absorber la radiación UV más allá de 250 nm. Cualquier molécula, fracción o partícula puede utilizarse en la presente invención. Las matrices sol-gel pueden ser partículas de cualquier forma, de un diámetro comprendido entre 0,01 y 100 micras, capas finas o revestimientos delgados, o presentarse en forma de monolito. La invención se refiere también a un procedimiento que permite preparar materiales sol-gel dopados de filtro solar, según el cual se polimeriza por condensación al menos un monómero elegido entre los alcoxidos metálicos, los alcoxidos semimetálicos, los ésteres metálicos, los ésteres semimetálicos y los monómeros de fórmula (M(R)n(P)m, en la que M es un elemento metálico o semimetálico, r es un substituyente hidrolizable, n un número enetero comprendido entre 2 y 6, P un substituyente no polimerizable o una fracción o un derivado de filtro solar, y m es un número entero comprendido entre 0 y 6, en presencia de al menos un principio activo de filtro solar, para atrapar los principios de filtro en el interior de la matriz sol-gel formado.Peer reviewe

Sustained release from biodegradable metallic matrix—The entrapment of drugs within iron

Iron and its alloys have been widely used for variety of medical implants. These are used for long term applications as cheap implants with high inertness and low corrosion rate, and also as implants with high biocompatibility (the fourth-generation type). Such degrading implants can provide a temporary scaffold while the body heals. In addition to the needed mechanical support, it is highly desirable to provide local drug therapy, providing antibacterial properties, preventing rejection of the implant, and more. So far, the combination of a degradable metallic implant which serves also as a three-dimensional matrix for drug release, remained un-answered. Here we present, we believe for the first time realization of this concept: Entrapment of drugs within a 3D degradable metal matrix—iron—from which the entrapped drugs are sustain-released. This new type of material is based on the molecular metals entrapment materials methodology, resulting in drugs@Fe. Two drugs have been successfully entrapped and released: chlorhexidine - an antiseptic drug, and rapamycin—used for avoiding transplant rejection. The delivery profiles of the composites were studied in two forms—powders and pressed discs showing two different types of drug release profiles. The release of the drugs from the powder hasa first order release profile, while the pressed disk is a slower, zero-order release profile, which is highly desirable due to the constant rate of the release. Full characterization of the metallic biomaterials is provided, including XRD, SEM, TGA, elemental analysis, and surface area/porosity analysis