3,070 research outputs found
A Direct Method for the Fabrication of Macro-Porous SiOC Ceramics from Preceramic Polymers
No full textCellular ceramics, possessing both open or closed porosity,
find use in several demanding engineering applications because
of their favorable set of properties.[1] Several processing
methods have been proposed for their fabrication, including
the replication of the structure of polymeric foams, direct
blowing, the use of sacrificial fillers, extrusion through special
dies (for honeycombs), solid freeform techniques, the
mimicking of natural templates (e.g. wood) or the assemblage
of fibers or hollow bodies.[2,3]
Preceramic polymers, in particular silicones, have
been successfully used for obtaining ceramic components
(such as foams and membranes) possessing a large
amount of porosity, in the micro-, meso- and macro-size
scale.[4, and references therein] However, some of the fabrication
methods have some limitations: for instance, direct foaming
techniques often lead to a gradient in the porosity amount
and pore size along the main expansion axis;[5,6] the infiltration
of a silicone resin within organic sacrificial fillers
requires a burn out step that has to be carried out in a very
controlled fashion in order to produce components without
defects (besides often requiring warm pressing – depending
on the rheological characteristics of the preceramic polymer –
to obtain a well controlled morphology), thus limiting the size
and shape of the component that can be produced;[7] the use
of supercritical CO2 is regulated by the diffusion within the
solid polymer, and works well only for components of
limited thickness.[8] In recent years, it has been shown that
blending preceramic polymers with different characteristics (molecular weight, molecular architecture, ceramic yield)
allows to produce cellular ceramics.[9,10] This paper further
explores this possibility, with the specific aim of directly
developing a large amount of porosity within the resulting
ceramic body during a one-step pyrolysis treatment
Ceramic coatings by ion irradiation of polycarbosilanes and polysiloxanes. Part I: Conversion mechanism
No full textCeramic coatings by ion irradiation of polycarbosilanes and polysiloxanes. Part II: hardness and thermochemical stability
No full textEngineering porosity in polymer-derived ceramics
No full textBy employing carefully controlled processing methods, a large amount of porosity (>70 vol%) was introduced in ceramic materials derived from
preceramic polymers (silicone resins) after pyrolysis at 1000–1200 ◦C in inert atmosphere. The resulting components have a bulk density ranging
from ∼250 to 950 kg/m3. Three main fabrication methods have here been employed: (1) direct foaming of a solution of a thermosetting silicone
resin in a suitable solvent (with or without the addition of polyurethane precursors), acting also as a blowing agent; (2) the use of sacrificial fillers
that decompose during pyrolysis, consisting in polymeric microbeads; (3) the mixing of preceramic polymers possessing different characteristics,
in particular ceramic yield, depending on their molecular structure. In addition to that, several methods for developing micro- or meso-pores
within the resulting SiOC macro-porous ceramics were explored, with the aim of fabricating components with hierarchical porosity. These include
a controlled thermal treatment, the addition of fillers with a high specific surface area (SSA), the deposition of zeolites or meso-porous silica
coatings, the infiltration with aerogels, selective etching of the SiOC material and the in situ formation of C-based nanostructures. Depending on
the fabrication procedure adopted, cells with an average size ranging from the micrometer to the millimeter were obtained. All these processes
are simple, economical and versatile, and large bodies with various shapes (tubes, plates, blocks) can be produced, possessing a wide range of
morphologies and properties. Compression strength, flexural strength and Young’s modulus vary with the morphology and density of the porous
components. It is also possible to add to the preceramic polymers some filler powders, for instance possessing electrical conductivity or magnetic
properties, leading to the production of functional cellular ceramics
Geometric release systems: principles, release mechanisms, kinetics, polymer science and release-modifying material
No full textGeometrical features such as shape and surface area of a drug-releasing matrix affect drug release kinetics by changing diffusion rates across the matrix, lengthening the diffusion pathway through drug–matrix composition or simply presenting a different surface area to the dissolution medium. Such variables can change during the dissolution process, due to drug depletion or erosion or dissolution of release-modifying components.
Such phenomena can help the development of novel dosage units since they present opportunities to capitalize on shape and surface effects designing a matrix that optimally delivers drug at the required rate. The historical development and state-of-the-art of geometrically designed dosage forms are presented in this chapter
Single layer dermal patch: skin accumulation assessed by tape stripping
No full textThe aim of this work was to assess the lidocaine skin accumulation after application of single-layer dermal patches, compared to a commercial gel formulation. The tape stripping technique was used. The model drug was lidocaine hydrochloride, applied at the dose of 0.3 mg/cm2 as lidocaine base.
Methods. Lidocaine containing patches were applied on the forearm of 6 volunteers for 30 min. (both sex, 24-27 years old), in the presence or absence of iontophoresis. After removal of the patch, or of the excess formulation, the skin was submitted to tape stripping. Tapes were extracted with acetonitrile: pH 4 buffer (14: 86) and lidocaine content was determined by HPLC. In the current assisted experiments, anodal iontophoresis was applied (current density 0.5 mA/cm2 for 30 min.).
Results. The results obtained show that lidocaine accumulated in human stratum corneum after single-layer patch application. Drug concentration in skin strips was higher in the more superficial layers and decreased in the deeper layers. The total amount of lidocaine recovered in the tape strips was 7.97 +/- 0.84 and 4.78 +/- 0.78 mcg per mg of stratum corneum for patch and commercial gel, respectively. The application of the patch without water, was less effective, giving rise to a drug content of 2.44 +/- 0.61 mcg/mg. When iontophoresis was applied on the patch (current density 0.5 mA/cm2 for 30 min.), the amount recovered was 13.07 +/- 0.85 mcg/mg.
Conclusions. The single-layer dermal patches show higher lidocaine accumulation with respect to reference gel formulation, when compared in vivo using the tape stripping technique. The application of current directly on the patch further increased the amount of drug recovered in the stratum corneum
Advanced oxide ceramics from a preceramic polymer and fillers
No full textAdvanced oxide ceramics (mullite and wollastonite) have been obtained using a silicone resin plus inorganic or organo‐metallic fillers. Highly dense and crack‐free mullite samples were prepared, in the temperature range 1200–1500°C, by oxidation of the silicone resin, mixed with alumina nano‐particles. A large mullite yield was achieved even at low firing temperature (1250°C), due to the high reactivity of nanometric inclusions towards the silica provided by the polymer. The addition of calcined clay as secondary filler allowed mullitization already at 1200°C. Biocompatible wollastonite ceramics, based on mixtures of both low temperature phase (β‐CaSiO3) and high temperature phase (α‐CaSiO3) were developed from the pyrolysis in nitrogen of silicone filled with Ca‐acetate. The phase assemblage could be tailored by varying the pyrolysis heat treatment. The use of filled silicones for the development of advanced ceramics appears particularly promising, since shaping of complex components can be obtained using polymer processing techniques
Experimental evaluation of the efficacy of the efficacy of cleaning methodologies for low porosity natural stones
No full text- …
