Hydrogels date back to 1960 when Wichterle and Lim first proposed the use of hydrophilic networks of poly(2-hydroxyethylmethacrylate) (PHEMA) in contact lenses (Wichterle and Lim 1960).Since then, the use of hydrogels has extended to various biomedical (Hoffman 2002; Peppas et al. 2006; Kopeceka 2007) and pharmaceutical (Peppas 2000) applications. In particular, due to their physical properties similar to those of human tissues (water content, soft and pliable consistence) hydrogels have been used for different administration routes such as oral, rectal, ocular, epidermal and subcutaneous (Peppas 2000; Guy 1996; Jatav et al. 2011). Hydrogels are composed of hydrophilic macromolecules forming three-dimensional insoluble networks able to imbibe large amounts of water or biological fluids (Peppas and Mikos 1986). Commonly, the polymers utilized to make hydrogels are insoluble due to the presence of permanent or reversible crosslinks (Berger 2004). Permanent crosslinked hydrogels (Wichterle and Lim 1960; Xiao and Zhou 2003; Brasch and Burchard 1996) are characterized by covalent bonds forming tie-points or junctions, whereas reversible crosslinked hydrogels (Watanabe et al. 1996; Wang et al. 1999; Qu et al. 1999) present ionic, hydrophobic, or coiled-coil physical interactions. These kinds of crosslinks in the polymer structure yield insoluble materials able to swell in aqueous environments retaining a significant fraction of water in their structure, up to thousands of times their dry weight in water. Hydrogels can be divided into homopolymer or copolymers based on the preparative method, but they can also be natural polymers, synthetic polymers or derivatives. In nature hydrogels can be found in plants (pectin, pullulan), various species of brown seaweed (alginic acid, agar, carrageenan), crustaceans (chitin) and animal tissue (hyaluronic acid, collagen, fibrin). Typical simple synthetic materials applied for general-purpose hydrogels are poly(ethylene oxide), poly(vinyl alcohol), poly(vinyl pyrrolidone), poly(hydroxyethyl methacrylate) and poly(N-isopropyl acrylamide). Moreover, the synthetic pathway offers more possibilities to create hydrogels with modified functional properties. In fact, several physiologically-responsive hydrogels are obtained from chemical or physical modifications of natural and synthetic polymers and tested for use in the so-called "intelligent biomaterials" (Hoffmann 1991; Miyata et al. 2002; Murdan 2003; Chen et al. 2004) because they are capable of reacting to various environmental stimuli (temperature, pH, ionic strength, solute concentration, electric radiation, light, sound, etc.). Hydrogels can be homogeneous, when the pores between polymer chains are the only spaces available for mass transfer and the pore size is within the range of molecular dimensions (a few nanometers or less), or porous when the effective pore size is over 10 nm. In homogeneous hydrogels the transfer of water or other solutes is achieved by a pure diffusional mechanism, which restricts the rate of absorption and to some extent the size of species that are absorbed. Porous hydrogels can be made by different polymerization methods in the presence of dispersed porosigens (ice crystals, oil, sucrose crystals) which can be removed later to leave an interconnected meshwork, where the pore size depends on the size of the porosigens (Hickey and peppas 1995). The introduction of a porosigen reduces mechanical strength significantly making porous hydrogels weaker than homogeneous hydrogels. In medical, engineering and pharmaceutical technology, hydrogel degradation is considerable important. In fact, investigators have focused on controlling degradation behavior of hydrogels to design polymers able to be cleared from the body once they complete their roles (Anderson and Shive 1997; Timmer et al. 2002): for this reason labile bonds are frequently introduced in the gels. These bonds can be present either in the polymer backbone or in the crosslinks used to prepare the gel. The labile bonds can be broken under physiological conditions either enzymatically or chemically, in most cases by hydrolysis (Damink et al. 1996; Eliaz and Kost 2000; Lee et al. 2004).

Hydrogel vehicles for hydrophilic compounds / T. Cerchiara; F. Bigucci; B. Luppi. - STAMPA. - (2015), pp. 285-297. [10.1007/978-3-662-45013-0]

Hydrogel vehicles for hydrophilic compounds.

CERCHIARA, TERESA;BIGUCCI, FEDERICA;LUPPI, BARBARA
2015

Abstract

Hydrogels date back to 1960 when Wichterle and Lim first proposed the use of hydrophilic networks of poly(2-hydroxyethylmethacrylate) (PHEMA) in contact lenses (Wichterle and Lim 1960).Since then, the use of hydrogels has extended to various biomedical (Hoffman 2002; Peppas et al. 2006; Kopeceka 2007) and pharmaceutical (Peppas 2000) applications. In particular, due to their physical properties similar to those of human tissues (water content, soft and pliable consistence) hydrogels have been used for different administration routes such as oral, rectal, ocular, epidermal and subcutaneous (Peppas 2000; Guy 1996; Jatav et al. 2011). Hydrogels are composed of hydrophilic macromolecules forming three-dimensional insoluble networks able to imbibe large amounts of water or biological fluids (Peppas and Mikos 1986). Commonly, the polymers utilized to make hydrogels are insoluble due to the presence of permanent or reversible crosslinks (Berger 2004). Permanent crosslinked hydrogels (Wichterle and Lim 1960; Xiao and Zhou 2003; Brasch and Burchard 1996) are characterized by covalent bonds forming tie-points or junctions, whereas reversible crosslinked hydrogels (Watanabe et al. 1996; Wang et al. 1999; Qu et al. 1999) present ionic, hydrophobic, or coiled-coil physical interactions. These kinds of crosslinks in the polymer structure yield insoluble materials able to swell in aqueous environments retaining a significant fraction of water in their structure, up to thousands of times their dry weight in water. Hydrogels can be divided into homopolymer or copolymers based on the preparative method, but they can also be natural polymers, synthetic polymers or derivatives. In nature hydrogels can be found in plants (pectin, pullulan), various species of brown seaweed (alginic acid, agar, carrageenan), crustaceans (chitin) and animal tissue (hyaluronic acid, collagen, fibrin). Typical simple synthetic materials applied for general-purpose hydrogels are poly(ethylene oxide), poly(vinyl alcohol), poly(vinyl pyrrolidone), poly(hydroxyethyl methacrylate) and poly(N-isopropyl acrylamide). Moreover, the synthetic pathway offers more possibilities to create hydrogels with modified functional properties. In fact, several physiologically-responsive hydrogels are obtained from chemical or physical modifications of natural and synthetic polymers and tested for use in the so-called "intelligent biomaterials" (Hoffmann 1991; Miyata et al. 2002; Murdan 2003; Chen et al. 2004) because they are capable of reacting to various environmental stimuli (temperature, pH, ionic strength, solute concentration, electric radiation, light, sound, etc.). Hydrogels can be homogeneous, when the pores between polymer chains are the only spaces available for mass transfer and the pore size is within the range of molecular dimensions (a few nanometers or less), or porous when the effective pore size is over 10 nm. In homogeneous hydrogels the transfer of water or other solutes is achieved by a pure diffusional mechanism, which restricts the rate of absorption and to some extent the size of species that are absorbed. Porous hydrogels can be made by different polymerization methods in the presence of dispersed porosigens (ice crystals, oil, sucrose crystals) which can be removed later to leave an interconnected meshwork, where the pore size depends on the size of the porosigens (Hickey and peppas 1995). The introduction of a porosigen reduces mechanical strength significantly making porous hydrogels weaker than homogeneous hydrogels. In medical, engineering and pharmaceutical technology, hydrogel degradation is considerable important. In fact, investigators have focused on controlling degradation behavior of hydrogels to design polymers able to be cleared from the body once they complete their roles (Anderson and Shive 1997; Timmer et al. 2002): for this reason labile bonds are frequently introduced in the gels. These bonds can be present either in the polymer backbone or in the crosslinks used to prepare the gel. The labile bonds can be broken under physiological conditions either enzymatically or chemically, in most cases by hydrolysis (Damink et al. 1996; Eliaz and Kost 2000; Lee et al. 2004).
2015
Percutaneous Penetration Enhancers Chemical Methods in Penetration Enhancement
285
297
Hydrogel vehicles for hydrophilic compounds / T. Cerchiara; F. Bigucci; B. Luppi. - STAMPA. - (2015), pp. 285-297. [10.1007/978-3-662-45013-0]
T. Cerchiara; F. Bigucci; B. Luppi
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/488168
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