Artificial porous structures

Pores can be created by a variety of techniques. In keeping with the sintering methodology, pores can be created by control of crystallite morphology (Nakahira et al. 

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In order to achieve the firm fixation of the artificial cornea to host tissues, composites of collagen-immobilized poly(vinyl alcohol) hydrogel with hydroxyapatite were synthesized by a hydroxyapatite particles kneading method. The preparation method, characterization, and the results of corneal cell adhesion and proliferation on the composite material were studied. PVA-COL-HAp composites were successfully synthesized. A micro-porous structure of the PVA-COL-HAp could be introduced by hydrochloric acid treatment and the porosity could be controlled by the pH of the hydrochloric acid solution, the treatment time, and the crystallinity of the HAp particles. Chick embryonic keratocyto-like cells were well attached and proliferated on the PVA-COL-HAp composites. This material showed potential for keratoprosthesis application. Further study such as a long-term animal study is now required [241]. 

For a few decades now cellular and porous systems have been classified in morphological terms by simulating the real systems by one or another imaginary, and always simplified, geometrical or stereometrical scheme using an artificially ordered-structure model. Such classifications have always been based on the concept that in any cellular or porous system it is possible to isolate a structural element (cell or pore). However, the diversity of pore and cell types even in small-sized real foamed systems does, in most cases, not permit a definition by only one single geometrical structural parameter, as for other types of solids (type and volume of elementary cell, interplanar or interatomic distances, etc.)... 

Coatings can form naturally by reaction with the surrounding atmosphere aluminum is quickly covered by an aluminum oxide and hydroxide layer in the presence of oxygen and water. This inert and protective layer can be formed artificially, as is done for window and door frames. A porous layer of alumina can be formed by reaction with oxygen. Pigments can be included in the pores for decorative purposes. After inclusion of the pigments, a suitable treatment of the layer transforms its porous structure to a continuous nonporous one (sealing). These coated frames can withstand relatively harsh conditions for years. 

The definition of the different types of pores is based on their width, which represents the distance between the walls of a slit-shaped pore or the radius of a cylindrical pore. This classification, which is not entirely arbitrary, is now widely accepted and used. It takes into account differences in the behaviour of molecules adsorbed in micropores and in mesopores. It appears that for pore widths exceeding 1.5-2.0 nm, the gaseous adsorbate condenses in a liquid-like state and a meniscus is formed. As a consequence, a hysteresis loop appears on desorption and its interpretation can lead to the distribution of the mesopores in the adsorbent [23]. The limit between mesopores and macropores at 50 nm is more artificial, and corresponds to the practical limit of the method for pore-size determination based on the analysis of the hysteresis loop. As a rule, the porous structure of the usual types of activated carbons is tridisperse, i.e. they contain micropores, mesopores and macropores. Micropores are of the greatest significance for adsorption owing to their very large specific surface area, and their large specific volume. At least 90-95% of the total surface area of an activated carbon can correspond to micropores. 

Molecular sieves (zeolites) are artificially prepared aluminosilicates of alXali metals. The most common types for gas chromatography are molecular sieve 5A, a calcium aluminosilicate with an effective pore diameter of 0.5 nm, and molecular sieve 13X, a sodium aluminosilicate with an effective pore diameter of 1 nm. The molecular sieves have a tunnel-liXe pore structure with the pore size being dependent on the geometrical structure of the zeolite and the size of the cation. The pores are essentially microporous as the cross-sectional diameter of the channels is of similar dimensions to those of small molecules. This also contrilsutes to the enormous surface area of these materials. Two features primarily govern retention on molecular sieves. The size of the analyte idiich determines whether it can enter the porous... 

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