Hydrogels for dermal and transdermal drug delivery.

A painless and non-invasive method to deliver drugs using dermal and transdermal administration routes has been expanding for more than 30 years as it reduces the risk of drug overdoses that can be associated with oral administrations or injections. To understand the particularities of this drug delivery pathway, we will present a rapid review of the skin, including its structure and the parameters that influence drug diffusion into it, and then discuss the strategies that improve dermal drug delivery. Of the multitude of existing systems used for topical dermal and transdermal applications, this review will focus on the breakthroughs in drug delivery systems made of hydrogels. Specifically, we will firstly present the use of hydrogels as innovative drug delivery vehicles to carry the active ingredient and penetrate the skin barrier. We will discuss the structure of hydrogels and the physicochemical parameters to master for improving drug delivery, as well as the drug encapsulation and release processes from hydrogels. In the last part, we will review the use of hydrogels as pharmaceutical forms associated with other vehicles - as emulsions, lipid nanoparticles, vesicles, capsules and polymeric or inorganic nanoparticles - suitable for skin penetration enhancement and drug protection, as well as side effects that may limit their use.