الفهرس 
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Abstract
The skin is the largest single organ of the body. It provides the pharmaceutical formulator with a vast and easily accessible area for drug application. The target areas of treatment after topical application to skin include surface, stratum corneal and viable epidermal-dermal treatments in addition to systemic treatment. The later is termed transdermal drug delivery and can be achieved only after delivering therapeutic amounts of the drug to the systemic circulation.
Transdermal drug delivery offers many advantages over other traditional routes of drug administration (Chein, 1987; Weissinger, 1993). These include the avoidance of the hepatic first-pass metabolism which can reduce the bioavailability of many drugs although skin may metabolize some drugs, possible development of controlled drug delivery system, a reduction in the side effects, rapid termination of therapy when required by removal of the system from the skin surface and improved patient compliance. Unfortunately, the barrier nature of the skin made it difficult for most drugs to be delivered into and through the skin. This barrier has to be breached so as to develop transdermal drug delivery systems (Scheuplein and Blank, 1971; Barry, 1983). Various techniques have been adopted to enhance skin drug absorption. The most common methods include chemical penetration enhancement (Goodman and Barry, 1988; El Maghraby et al., 2009; Bavaskar et la., 2015), adjustment of chemical potential via supersaturation (Megrab et al., 1995), employing iontophoresis, electroporation or sonophoress (Miller et al., 1990; Kost et al., 1996; Banga et al., 1999) and using colloidal drug delivery systems such as liposomes and niosomes(Manosroi et al., 2008; El Maghraby and Williams, 2009; Khindri et al.,2015). Microemulsion provides another promising alternative to improve dermal and transdermal drug delivery. They have been claimed to have high ability to deliver both polar and non-polar drugs into and through the skin (Mehta et al., 2000).
This thesis evaluates chemical enhancer, microemulsion and microemulgel systems for simultaneous transdermal delivery of kojic acid from different formulations in vitro. We will present a brief description of the structure and functions of the human skin, the routes of permeation through skin, and various techniques for enhancing transdermal delivery and a review for chemical enhancers, microemulsion and microemulgel as skin drug delivery systems. Finally the aim and organisation of the thesis will be presented.