Hydrogel vehicles for hydrophilic compounds.

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)

Microencapsulation Strategies for Essential Oils - A Review

Throughout history the main aims of microencapsulation of essential oils has been to protect them against degradation caused by environmental factors, improve their solubility end efficacy, mask or enhance their taste, turn them into stable compounds and ensure their release with specific mechanisms suggested by the microcapsule shell and core characteristics. Essential oil microencapsulation processes are commonly based on the principle of oil-in-water (o/w)-emulsion formulation and subsequent conversion into a solid form by different technological methods. With this aim, various technologies have been examined for microcapsule preparation including complex coacervation, spray-drying and interfacial polycondensation. This article reviews the current state of the art in essential oil microencapsulation techniques focusing on process-related aspects of both well-established and more advanced technologies

Chitosan-based hydrogels for nasal drug delivery: from inserts to nanoparticles

Importance of the field: Chitosan represents a multifunctional polymer, featuring both mucoadhesive and permeation-enhancing properties and therefore is a widely studied excipient for mucosal drug delivery. As regards nasal administration, chitosans have been used for the preparation of gels, solid inserts, powders and nanoparticles in which a three-dimensional network can be recognized. Areas covered in this review: This review provides a discussion of the different nasal dosage forms based on chitosan hydrogels. In the first section intranasal delivery is discuss as a useful tool for non-invasive administration of drugs intended for local or systemic treatments. Then chitosan-based hydrogels are described with a focus on their mucoadhesive and permeation-enhancing ability as well as their capacity of controlled drug release. Finally, a detailed discussion regarding several examples of the different nasal dosage forms is reported, including considerations on in vitro, ex vivo and in vivo studies. What the reader will gain: Summary and discussion of recent data on the different pharmaceutical forms based on chitosan hydrogels could be of interest to researchers dealing with nasal drug delivery. Take home message: The aim of this review is to stimulate further investigations in order to achieve the collection of harmonized data and concrete clinical perspectives

pH-sensitive microcapsules of ketoprofen and mint essence for colon targeting.

Microcapsules containing ketoprofen were obtained by a spray drying process starting from a O/A emulsion in presence of different pH-sensitive materials (Eudragit\uae L100, Eudragit\uae S100 and stearic acid) dissolved in the external phase. The influence of formulation factors (oily phase employed for drug solubilisation, type of coating) on the morphology, particle size distribution, drug loading capacity, in-vitro release and ex-vivo permeation characteristics were investigated