State-of-art on gel technologies for transdermal delivery of bacitracin

Abstract

Gels have consistently been studied for their role in topical and transdermal drug delivery systems as a non-invasive technique, for pharmaceutical and cosmetics application. These formulations are semi-solid three-dimensional structures, porous, with unique characteristics, such as rigidity and elasticity at the same time. Because of their high aqueous phase content, gels permit a greater dissolution of drugs through the skin and enhance skin hydration by retaining a significant amount of transepidermal water, in contrast to creams and ointments. Conventionally, gels are differentiated into two different types according to the nature of their liquid phase: hydrogels, which contain a polar solvent (water) and organogels, which contain an organic/non-polar solvent, as external phase. Hydrogels consist of polymeric materials that exhibit the ability to swell and retain a large amount of water or other biofluids in its structures. Despite its great affinity for water, they only possess a swelling behavior without dissolving in water. This proves its high flexibility, similar to natural tissue. Organogels consist of a network of self-assembled molecules which forms a thermally reversible gel upon cooling, immobilizing a non-aqueous liquid. They are mainly composed by lipids (organic phase), so they easily interact with the lipid skin surface and enhance the drug permeation through the skin. The most widely used lipids are based in edible oils due to their high biocompatibility, such as soybean oil, sunflower oil, sesame seed oil or olive oil. Organogels form viscoelastic structures through non-covalent associations with gelling agents in low concentrations. The commonly used organogelators include sorbitan monostearate or sorbitan monopalmitate. These superstructures, often long fibers or needle-shaped structures, which entangle or form pseudocrystalline regions, immobilizing the liquid largely by surface tension and forming a gel of variable consistency. Lecithin organogels are a special type of organogels that do not require addition of any additional surfactant or penetration enhancer, as lecithin serves both the purposes. Recent studies have reported other types of gels for dermal drug application, such as proniosomal gels, emulgels, bigels and aerogels, combining features of conventional hydrogels and organogels. In conclusion, further studies in gel technologies are essentials to overcome the drawbacks of each gel system and for developing cost effective delivery systems for transdermal applications.