Hydrogels hydrophilic materials

Polyethylene glycol (PEG) and the very similar polyethylene oxide (PEO) are used as biocompatible coating agents and hydrogel forming materials, often as block or graft copolymers with other materials (Eig. IF). They are often bound to polyurethanes to form hydrophilic foams such as Biopol (Metabolix Inc.). 

Polymeric soft contact lenses came into existence in the 1950s (12]. Otto Wich-terle discovered the hydrogel poly(hydroxyethyl methacrylate) (HEMA), a transparent, soft, hydrophilic material that could be used to prepare contact lenses, Wichterle utilized a free radical polymerization of the HEMA monomer (including cross-linker, solvent, initiator, and stabilizer) with either thermal or ultraviolet initiation of the reaction. Initially, the len.ses were produced via spin casting, which involved the use of a concave mold that is spun at a particular rate. The rate of the mold spin determines the resultant lens power (13). After production of the lens in the mold, the lens would be hydrated from the mold in a warm water solution. Once hydrated, the lens would float free from the mold. Each lens is inspected for rips, tears, and clarity. Finally, the lens is packaged, sterilized, and boxed for shipping. The surface quality of the mold determines the surface chemistry and morphology on the anterior surface of the lens produced. 

PEG (also known as poly(ethylene oxide)) is one of the most widely used materials in tissue engineering (Drury and Mooney 2003). As a result of its extreme hydrophilicity, PEG is highly resistant to protein adsorption and therefore works well as a non-fouling, non-immunogenic surface in a biological environment. This makes PEG materials a blank slate so that cellular interactions can be precisely controlled through added features (e.g., peptides). Eurthermore, because it is one of the few synthetic hydrophilic materials available, it has become a prime candidate for use in many soft tissue applications, where hydrogels are preferable to hard materials. 

To control the relative wettability of the MF reactor walls with two liquids, it is also important to choose appropriate materials for the fabrication of the MF reactor. For example, hydrophilic materials such as glass or silicon can be used in the formation of hydrophobic prepolymer droplets (i.e., direct oil-in-water emulsions), whereas hydrogel particles derived from inverse (water-in-oil) emulsions can be produced in MF reactors fabricated in hydrophobic materials [5]. Alternatively, one can modify the surface of the MF reactor to render its wettability suitable for... 

Hydrogels, ie, gelatin and agar, have been known for a long time. In the late nineteenth century, Herschel proposed the use of jelly materials on the cornea for the correction of vision (108). In 1960, the use of synthetic hydrogels for contact lenses was proposed and several U.S. patents were obtained for the invention of cross-linked hydrophilic polymers, eg, systems based on 2-hydroxethyl methacrylate [868-77-9] (HEMA) (5) (109—112). 

Because of the many choices of hydrophilic monomers, cross-linkers, and hydrophobic monomers, a large number of formulations have been developed and manufactured into hydrogel lenses. The water content of these hydrogel lenses ranges from about 38%, for HEMA-based lenses, to 80%, for poly(vinyl alcohol) and partially hydrolysed acrylonitrile lenses. Table 2 gives a representative Hst of FDA approved hydrogel materials available to the consumer in the early 1990s. 

Electric-field-driven transport in media made of hydrophilic polymers with nanometer-size pores is of much current interest for applications in separation processes. Recent advances in the synthesis of novel media, in experimental methods to study electrophoresis, and in theoretical methodology to study electrophoretic transport lead to the possibility for improvement of our understanding of the fundamentals of macromolecular transport in gels and gel-like media and to the development of new materials and applications for electric-field-driven macromolecular transport. Specific conclusions concerning electrodiffusive transport in polymer hydrogels include the following. 

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